Master Thesis. Panagiotis C. Tsopelas

Transcription

1 APPLYING GRIDPIX AS A 3D PARTICLE TRACKER FOR PROTON RADIOGRAPHY Master Thesis of Panagiotis C. Tsopelas Supervisors: prof. dr. ir. Els Koeman dr. Jan Visser prof. dr. Sytze Brandenburg Utrecht, 21 November 2011

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3 Abstract Proton therapy is one of the possible treatments in the ght against cancer, mostly applied to patients with tumors located near sensitive organs as these organs can be better spared with proton beam irradiation compared to X-ray irradiation. X-ray computed tomography images are used to provide information about the proton range inside the human body with an error of approximately of 4%. Miscalculations in the proton range can lead to delivering a high amount of dose in healthy tissue and not in the tumor so reducing these errors is essential. Acquiring images with proton computed tomography, the precision in range can be improved by a factor of 2.5[1]. To successfully use protons for radiography the construction of a 3D particle tracker is needed. This apparatus should provide us with precise information about the proton tracks and the proton energy. The detector R&D group of Nikhef, in Amsterdam is collaborating with the KVI, in Groningen in research for proton computed tomography. The Grid- Pix, a gas-lled detector combined with a CMOS pixel chip used at Nikhef for high energy physics experiments, is an ideal candidate for the needs of proton radiography. A 3D particle tracker consisting of GridPix detector and a BaF 2 scintillating calorimeter was designed and irradiated with MeV protons with the proton beam of the AGOR cyclotron in KVI. A number of samples was also tested in dierent congurations. The 3D reconstruction of proton tracks from the GridPix detector and the measurement of the energy of the protons from the BaF 2 are presented and compared with simulations in GEANT4. III

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5 Contents 1 Introduction 1 2 Hadrons in Medicine Hadron Therapy History & Perspectives Hadrons and the physics behind them Facilities Proton CT Motivation for Proton Radiography Image reconstruction Proton CT set-up Introduction GridPix based TPC The Timepix chip Field Cage Ingrid D Track Visualization Gas Mixtures Hit Resolution Read-out BaF 2 scintillating detector Trigger Experiment at the KVI AGOR cyclotron Experimental set-up Monitoring the Set-Up Calibration of the BaF 2 detector Synchronizing GridPix and the BaF 2 detector Collected Data Sets Overview Results & Analysis GridPix Data Projection on the XY plane Y-coordinate distribution of all hits Time Measurement & Z reconstruction Energy dependence of Ionization V

6 Contents Intensity eects Calorimeter Data Calibration Shifted Energy spectrum during Runs Correlation of GridPix and Calorimeter data Simulations in GEANT Interaction of GridPix with the proton beam Simulation of the Copper samples Conclusions Performance of the 3D particle tracker Outlook Bibliography IX A B C D XI A.1 Principles of a Time Projection Chamber XI A.2 Field Cage XII XIII B.1 Binary resolution XIII XV C.1 Trigger XV XVII D.1 Measurements with Cosmic Rays XVII VI

7 Chapter 1 Introduction In cancer treatment X-ray irradiation is the most common technique, causing in many cases serious side eects on the patient. During an X-ray irradiation there is an interaction probability between photons and tissue of about 3% per cm with an exponential reduction of the dose deposited in the tissue. To deliver the highest total dose to the tumor in cancer treatment with X-Rays, one cannot irradiate from one direction only as the tissue before and after the tumor along that direction will be severely damaged. Therefore multiple irradiations from dierent angles must be done, making it impossible to exclude areas near the tumor from getting irradiated (Fig. 1.1(a)). In proton therapy, the energy deposition at the beginning and along the track is low while the proton energy is close to the initial energy. The highest relative dose is deposited at the end of the proton range in a small and well dened area avoiding further damage in the body (Fig. 1.1(b)). This property of protons can be eectively used by matching the area where protons mainly deposit their energy with the location of the tumor. In cancer therapie, the desired radiation dose should have a broad, at peak, where the at section corresponds to the extent of the tumor being treated. In order to do so, the dosage is delivered in a broad, at distribution dened as Spread Out Bragg Peak 1 (SOBP) when the radiation is naturally deposited in narrow peaks as illustrated in Fig Miscalculating the range of protons through the body of the patient will lead to a high amount of dose delivered in healthy tissue and a too low or no amount of dose delivered in part of the tumor. A similar deviation in the case of X-rays has much less implications. To successfully use protons for cancer treatment detailed information about the patient is needed. Most of this information can be obtained by X-ray computed tomography imaging. However, the quality of data from X-ray computed tomography (CT) is not sucient. By using X-ray CT, the Hounseld numbers are converted to proton stopping power relative to water 2 using a calibration curve. This conversion can lead to errors in the calculation of proton range in 1 The SOBP can be produced in several ways: by using a range modulation wheel, a degrader and energy selection system or a syncrhotron that directly delivers the beam at the desired energies. 2 Human body is simulated as water. 1

8 Chapter 1 Introduction (a) Treatment of spinal cancer using X-rays. While the tumor is irradiated the most, surrounding organs are signicantly aected. Radia- (b) Same treatment using protons. tion is concentrated on the tumor. Figure 1.1: X-rays vs protons. Courtesy of Francis H. Burr, proton Therapy Center, Massachussets. Figure 1.2: X-ray and proton energy deposition as a function of the body depth. 2

9 patients of approximately of 4%. Using protons in making a CT image, one can reduce these errors by a factor of 2.5 as stated in [1]. The concept of using protons for imaging has started since Since then, in research for proton radiography and CT irradiation of samples of simple geometries have been made ([2],[3] and [4]) as well as more complex geometries representing a human head[5]. Silicon strip and pixel detectors have been used as part of a 3D particle tracker for protons in order to obtain information about the proton beam. The use of a gaseous detector for a 3D particle tracker is also an option that can oer advantages over the solid state detectors. A gaseous detector is radiation hard and has low interference with the beam. A GridPix based time projection chamber is a gaseous detector used in high energy physics experiments that oers the possibility to reconstruct charged particle tracks in 3D. The aim of this project is to demonstrate that a 3D particle tracker based on a GridPix detector for tracking a proton beam for radiography purposes is possible. The rst chapter presents the current status in cancer treatment with protons and introduces the concept of proton radiography and CT. The second chapter focuses on the physics behind the therapy and the radiography and explains the need for constructing a 3D particle tracker. In chapter 3 the principles of the GridPix based time projection chamber and the BaF 2 scintillating detector used as a 3D particle tracker are explained. The next two chapters describe the testing of the 3D particle tracker with a proton beam, the results obtained and comparison with simulations made in GEANT4. The thesis ends with conclusions and suggestions for future experiments. 3

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11 Chapter 2 Hadrons in Medicine 2.1 Hadron Therapy History & Perspectives The rst treatments with protons of patients with cancer were performed at Berkeley Radiation Laboratory in 1954 and at Uppsala (Sweden) in 1957[6]. Until 2010, about 75,000 patients have been treated according to [7] mainly in USA, Japan and also in Germany, Switzerland, Russia while the current annual treatment capacity is about 7,500 patients. Proton therapy is applied to patients with tumors located near sensitive organs that need to be treated such that radiation of these organs is avoided[8]. One of the countries that is interested in adopting proton therapy is the Netherlands. The insurance package in the Netherlands plans to cover future proton therapy since there appear to be around 7,000 patients annually who could benet from such treatment, so the construction of even 3 new facilities is under investigation. The KVI in Groningen in collaboration with Nikhef in Amsterdam are working on this eld of research. The KVI institute is involved in a project of constructing a clinical proton therapy facility and is collaborating with Nikhef in the research of proton CT and radiography Hadrons and the physics behind them A hadron is a particle with rest mass much larger than that of an electron[9]. A charged particle will lose kinetic energy through coulomb interactions with the charge of the electrons and the nuclei when traversing a material. Since electrons are more plentiful than charged nuclei, interactions with electrons are dominant[10]. During an interaction between the two particles, most of the energy is transfered from the heavier hadron to the lighter electron. Among the hadrons, the ones that are used in cancer therapy are protons, neutrons and ions (positively charged atoms containing both protons and neutrons). Although ions are not literally hadrons, they are also included in the particles of hadron therapy. Charged particles and photons interact dierently with matter and the 5

12 Chapter 2 Hadrons in Medicine dierences and properties of these physical interactions make up the basis for hadron therapy. The mean loss rate per unit thickness de/dx of charged particles in [MeV g 1 cm 2 ] is well described by the Bethe-Bloch 1 equation[11]: de dx = Kz 2 Z A 1 β 2 [ 1 2 ln 2m ec 2 β 2 γ 2 T max β 2 δ(βγ) ] I 2 2 Where K/A = MeV g 1 cm 2, A and Z the atomic mass and atomic number of the absorber, z the charge of the incident particle, β the velocity, γ the Lorentz factor, m e c 2 the rest mass of the electron, T max the maximum kinetic energy that can be transfered to a free electron with a single collision, I the mean excitation energy in ev and δ(βγ) the density eect correction to the ionisation energy loss. Important information we extract from the Bethe-Block formula is that the linear energy loss of a particle through a material depends on the nature of the material and the velocity of the particle. As a charged particle travels through a medium, it's stopping power follows the Bethe-Bloch curve. Since the human body is made of water for about 80%, simulations and calculations of the energy loss and the range of protons in water are used as a reference. The Bethe-Bloch curve for a proton traversing through water is shown in Fig. 2.1(a). The particle loses energy in small steps and gradually moves to low energy range in the Bethe- Bloch curve(fig. 2.1(a)). As the energy of the particle decreases the stopping power increases and a peak occurs called the Bragg peak. This results in the energy deposited or dose delivered in the patient to be well localized in space with only a part of it before the Bragg peak and another part higher than a factor of 3-4 inside the Bragg peak region before the particle stops(fig. 2.1(b)). Note that by changing the proton energy, the depth of the Bragg peak is shifted. Some of the dierences in radiation therapy between protons and ions lie in the higher biological eectiveness of the heavier ions in the Bragg peak region and the advantage that ions seem to have in the treatment of certain types of cancer (like bone and soft tissue sarcomas, lung and prostate cancer[14]). As shown in Fig. 2.2, the protons have a wider spread and stop almost immediately after the peak. On the other hand ions, although having a sharper shape, continue to ionize beyond the peak due to nuclear fragmentation of ions so the dose is not decreased immediately to zero[16]. Ions have an increased energy loss towards the Bragg Peak thus a signicant increase in irreparable damage is observed which yields a higher relative biological eciency[15]. A successful hadron therapy treatment aims to match the Bragg peak where the tumor is located, so the tumor cells are destroyed by generating irreparable DNA damage. 1 Additional corrections to the Bethe-Bloch formula exist, see [12]. 6

13 2.1 Hadron Therapy (a) Bethe-Bloch curve for a proton through water. (b) Ranges and Bragg peaks for dierent proton energies. Figure 2.1: Bethe-Bloch curve and corresponding energy loss per unit thickness in water. 7

14 Chapter 2 Hadrons in Medicine Figure 2.2: Attenuation of protons, ions and photons in water. Figure 2.3: A gantry rotating around the patient Facilities The construction of a proton therapy facility is a huge project where dierent elds like physics, biology, engineering, medicine, law, management and nance come together. In a modern proton therapy center the accelerator is connected to several treatment rooms by means of beam transport lines. The energies required for therapy of deep-seated tumors with hadrons are typically in the range 70 to 250 MeV for protons and 120 to 400 MeV/nucleon for carbon ions. These beam energies can be achieved with synchrotrons, cyclotrons or linacs as main accelerators. In existing proton treatment centers, the dose calculations are currently performed based on X-ray computed tomography and the patient is positioned with the help of X-ray radiographs. The accuracy of X-ray CT for proton treatment planning is limited due to the dierence in physical interactions between photons and protons, which partially obviates the advantage of proton therapy[17]. The need to improve the quality of the data for proton therapy and to determine the safety margin around the tumor volume led to proton CT[1]. 8

15 2.2 Proton CT Figure 2.4: The famous "lamb chop", the rst proton radiograph taken by Koehler at Harvard. The picture taken with protons (down) appears to show more detail than that with X rays (up), however general conclusions about comparison between X-ray radiography and proton radiography should not be drawn. 2.2 Proton CT Motivation for Proton Radiography The history of heavy charged particle radiography began in 1968 when Koehler showed (Fig. 2.4) that with parallel-sided objects with a thickness nearly equal to the path length or range of an incident 160 MeV proton beam, proton radiographic lms could be produced with much greater image contrast than that of X-ray radiographs taken under the same conditions[18]. Although a number of publications on the subject exist since then([2],[3] and [4]), not a lot of progress has been made as the use of X-ray radiography dominated. Due to the progress of proton therapy, the specic needs of the treatment in terms of imaging and the development of proton gantries (Fig. 2.3), proton radiography is being studied and is of great interest. Using the beam of proton therapy as an additional tool for radiography and CT would oer a great advantage in an eective treatment against dicult cases where the tumor is located close to radiation sensitive organs like regions close to the head or the neck[17]. This is now recognized as the major motivation for developing proton CT and also the major clinical application of proton beams for imaging. When using X-ray CT, the Hounseld numbers (the unit system of measuring attenuation coeecients of tissues in CT) are converted to proton stopping power relative to water using a calibration curve. This translation can lead to errors in the calculation of the proton range in patients in the order of a few mm for a depth of 100 mm. Proton CT can provide more accurate information about the proton energy loss and the straggling of protons through the dierent tissues. 9

16 Chapter 2 Hadrons in Medicine A proton CT system containing a proton gantry and fast image reconstruction techniques has not yet been developed. The main challenges lie in the development of techniques for image reconstruction and of a detector system that can measure all the information needed for the imaging Image reconstruction In proton therapy the aim is to stop the protons exactly in the tumor. In proton CT we are interested in irradiating the patient with a proton beam and measure precisely the protons before entering and after exiting the body. Looking back at the physics of the proton interaction with matter, although in therapy we want to match the Bragg peak region with the location of tumor, in radiography we have to stay well above that region in terms of the incoming proton energy. Thus, in order to avoid stopping the protons in the patient, a beam of higher energy than the one used for therapy is required. Extracting information from the proton tracks for imaging purposes is done by: (a) measuring the linear energy loss through the material and (b) detecting the lateral and angular displacements of the proton from its incident position and direction. These constitute the basis for proton radiography. Given the resolution of the existent detectors with which we can detect the energy and the straggling of the protons through matter, there is a limit in using higher relativistic protons. The reason for this is that the divergences in terms of energy loss and straggling through the material, the key elements that will provide the information we need, will become too small to detect. The optimum can only be determined by detailed simulations including the detector characteristics. The energy regime for proton CT, as dened by the characteristics of the accelerators that are based on the requirements for therapy, is shown in Fig. 2.1(a). A detailed description of the principles of proton CT can be found in [17]. To perform the above measurements and to retrieve all the necessary data leads to the design of the proton CT scanner or 3D particle tracker which is the subject of this project. 10

17 Chapter 3 Proton CT set-up 3.1 Introduction Over the years Nikhef has been involved in many projects in high energy physics in which solid state and gaseous detectors have been used in tracking particles. A new type of gaseous detector, a GridPix based TPC is being developed and was selected for this project. This detector is an ideal candidate for monitoring a proton beam oering some major advantages. Since it is a gas detector it is radiation hard and has low interference with the high ionizing protons due to its low thickness 1. The main advantage is that it oers the possibility to obtain 3D reconstruction of tracks of passing charged particles. The principle of a proton CT set-up is shown in Fig It's function is based on tracking single-proton events, stopping them to measure their energy and reconstructing their trajectory. The energy measurement can be done by using a calorimeter while the intersecting points, dened A and B in Fig. 3.1(b), are used to estimate a straight path (black line). That is not however the trajectory the proton has followed. By using the angle (θ 1 ) dened by the exiting proton track (red line), additional constraints on the reconstruction of the most likely path the proton has followed (blue line) can be put leading to a higher spatial resolution. Moreover, since the uncertainty in the length becomes smaller, a more accurate estimation of the average stopping power along the trajectory can be made. Based on the principle shown in Fig. 3.1, a 3D particle tracker using the detectors available at Nikhef was designed. The components used, both hardware and software, as well as the dierent parameters that are important for operating it are described in the following sections. 3.2 GridPix based TPC GridPix is a gas-lled detector in which a Micro Pattern Gas Detector is combined with a CMOS pixel chip. It follows the principles of a TPC (Time 1 Very important especially for tracking the protons that have traversed the body of the patient. 11

18 Chapter 3 Proton CT set-up (a) Schematic of a 3D particle tracker. exit energy is stored. The (b) Path reconstruction of single-proton event. Figure 3.1: 3D particle tracker and principle of use. Projection Chamber 2 ) with the readout being done by a Timepix chip[19]. The GridPix detector used in this project is shown in Fig. 3.2(c). It consists of a Timepix chip (section 3.2.1) with a layer of 1 µm thick aluminum with holes etched in it called Ingrid (section 3.2.3) attached on a NEXT board with a eld cage surrounding it (section 3.2.2) as presented in Fig. 3.2(a) and (b). The board is connected with the readout via a SCSI cable The Timepix chip The Timepix chip has 256 x 256 square pixels with a pitch of 55 µm. Each individual element of the pixel matrix is connected to a preamplier, discriminator and digital counter integrated on the readout chip[21]. Time is measured by a reference counting clock, which is generated by an external clock that can be set to various frequencies up to 100 MHz. The Timepix can be set in three operation modes: 1. Time over Threshold: measures the time that the signal in the pixel is above a certain threshold energy. This is an indirect measurement of the amount of charge deposited in the pixel. 2. Time of Arrival: the clock counter is incremented from the moment the signal goes over threshold until the shutter is closed or until it reaches counts. 3. Medipix: single photon counting mode, which increments a 14-bit counter by one for each hit above a set threshold. 2 Information about the principles of a TPC can be found in Appendix A 12

19 3.2 GridPix based TPC (a) Anatomy of a GriPix. The Timepix chip lies at the bottom of the eld cage with the Ingrid on top. (b) The GridPix used in this project. A blob of SU-8 is visible. (c) The GriPix mounted on a NEXT board with a eld cage on top. Figure 3.2: The GridPix detector. 13

20 Chapter 3 Proton CT set-up (a) A charged particle ionizes the gas and the free electrons drift towards the anode. (b) Avalanche evolution in the amplication gap. (c) A charged particle track through GridPix. Figure 3.3: Principles of a TPC. 14

21 3.2 GridPix based TPC Field Cage A eld cage is placed on top of the NEXT board in order to contain the gas in the system and provide a homogeneous electric eld. The walls are made of Kapton foil 50 µm thick surrounded by 17 µm thick Copper strips that serve as electrodes to shape the drift eld vertically 3. The dimensions of the eld cage t the Timepix chip (14 mm x 14 mm x 16 mm). The top is sealed by a copper cathode a few mm thick while at the bottom lies the Timepix chip with the Ingrid on top (Fig. 3.2(a)) Ingrid Since the charge produced in the individual ionization is too small to be detected by the bare Timepix chip, it has been extended with an integrated grid to create a charge avalanche with sucient charge to be detectable. This integrated grid (called Ingrid) is a layer of 1 µm thick aluminum with holes etched in it (bottom right corner of Fig. 3.2(a)). The Ingrid is attached to the chip by 50 µm tall spacers. The individual drift electrons from the ionizations in Fig. 3.3(a) are focused into the Ingrid-holes. Between the Ingrid and the chip the electric eld of the order of 10 4 V/cm is large enough to create avalanches. A Si 3 N 4 protection layer is placed over the pixels to avoid any damage from possible discharges that may occur. These avalanches are detected by the pixels of the chip (blue area at the bottom). In Fig. 3.3(b) the process is simulated D Track Visualization The 3D positions of single electrons from ionization by charged particles passing through the detector volume are measured in a drift region with a depth of 16 mm. The pixel hit gives us the (x,y) coordinates of the electron. The time that it takes for electrons to arrive at the Ingrid is measured by an internal counter in the Timepix chip started by an external trigger. Such a trigger is described in section 3.4. Knowing the frequency of the clock, the electric eld applied and the type of gas mixture in the TPC the drift time is converted into distance representing the z-coordinate of the electron generated in the ionization process[22]. By gathering information about all pixels hit and their drift time, a track can be visualized (Fig. 3.3(c)). In this project only one GridPix detector was used for proton tracking. In order to succesfully reconstruct the trajectory of a particle, at least one additional tracking detector is required. Thus, it was not possible to reconstruct particle tracks in the analysis but only to visualize them. For a detailed method of using tracking detectors in a telescope to t charged particle tracks one could look at [23]. 3 More on the electric eld conguration near the electrodes in Appendix A. 15

22 Chapter 3 Proton CT set-up Gas Mixtures The gas lling the eld cage is a mixture of two gases, one with large primary ionization statistics (called "counting" gas) and the other an organic molecule with large photoabsorption coecients up to large wavelengths (called "quencher" gas) in order to prevent secondary avalanches due to photons emitted in the ionization of the counting gas. The key factors for acquiring clear and well dened tracks with GridPix are a sucient number of ionizations in the TPC and low diusion of the free electrons produced from ionization on their way to the grid. The parameters that aect these two mechanisms are the gas mixture and the electric eld applied on the cathode and on the grid. Let D c be the diusion constant, u d the drift velocity, t the drift time, T the temperature, k the Boltzmann constant and µ e the electron mobility. From Einstein's formula[32], the mean deviation in any direction i is given by: with σ i = 2D c t (3.1) D c = kt e µ e (3.2) and u d = µ e E = eτ m E = L t (3.3) where e the electron charge, m the electron mass, τ the mean free time between collisions, E the electric eld and L the drift distance. By solving (3.3) for t and applying it in (3.1) space diusion RMS becomes: σ i = 2D c L u d (3.4) In order to achieve the minimum diusion, a gas mixture with high drift velocity is required. The distance L is also important as the higher an electron starts drifting, the more it will deviate from drifting straigth to the Grid. When the diusing body has thermal energy ɛ = (3/2)kT, D c has the form: D c = 2ɛ 3e µ e (3.5) By rewritting (3.3) as t = L/µ e E and subsituting t and (3.5) in (3.1) we get: σ i = 4ɛL 3eE (3.6) The mean deviation in any direction can be written: 16

23 3.2 GridPix based TPC where D is the diusion coecient for that direction: σ i = D L (3.7) D = 2D c µe (3.8) To detect a particle, the gas mixture chosen is such such that multiple scattering is minimized. For low mass gas mixtures it is known that ethane(c 2 H 6 ), Isobutane(iC 4 H 10 ) and dimethyl ether (C 2 H 6 O) are good quenchers in combination with helium (He), which help increasing the number of primary and total ion pairs per cm for a given density[33]. Two dierent gas mixtures were tested, (He 90%, ic 4 H 10 10%) and (DME 50%, CO 2 50%). The reason for this selection was the expected dierence in the behavior of these two gases. The values for D and u d (Fig. 3.4 and 3.5) as well as the expected number of ionizations (Fig. 3.6) were calculated in GARFIELD[23], a computer program for the detailed simulation of 2D and 3D drift chambers and gas detectors. In Fig. 3.4 and 3.5, the diagrams of the drift velocity and diusion(longitudinal and transversal) as a function of the electric eld applied show the dierences between the two gas mixtures. The properties for a good gas mixture for the TPC are high drift velocity and small diusion properties according to (3.4) and (3.7). Diusion in DME/CO 2 is an order of magnitude smaller than He/iC 4 H 10. Also, the drift velocity of DME/CO 2 is an order of magnitude larger. The reason why drift velocity diers between the two gas mixtures is due to the fact that the mean free time τ between collisions depends on the density of the electrons in the gas[24] and as shown in (3.3), u d is proportional to τ. DME/CO 2 appears to be appropriate for our measurements. If we choose our electric eld to be around 2,000 V/cm, the diusion is at its lowest value( 65 µm/ cm) and the drift velocity 1 cm/µsec. So, for an electron drifting from the middle and the highest point of the Field Cage 4, the deviation due to diusion according to (3.7) will be σ L/2 = 65 L/2 [µm] = 58 µm and σ L = 82 µm respectively. This agrees well with the experimental results obtained for the same drift heights with σ L/2 = 56 µm and σ L = 84 µm. For proton radiography, the YZ-plane of the tracking detector should be large enough to cover a typical eld of the proton beam which is about 10 cm x 10 cm. The depth of the detector (X-coordinate) should be determined by analysing what trajectory length you need to get sucient accuracy on the angles of the proton path with the Y and Z axis. Typically one would like to have about 1 mrad, so 0.1 mm per 100 mm[25]. The YZ-plane of the GridPix based TPC used in this project is 1.4 cm x 1.6 cm while the depth is 1.4 cm. There are various factors that play a role in constructing larger GridPix detectors. The X and Y-coordinates are limited by the dimensions of the Timepix chip (1.4 cm x 1.4 cm). It is possible to "tile" together Timepix chips with 4 the height of the Field Cage in the GridPix based TPC used in this project was L=1.6 cm 17

24 Chapter 3 Proton CT set-up (a) (b) Figure 3.4: Drif velocity of (a) He 90%, ic 4 H 10 10% and (b) DME 50%, CO 2 50% for T=300 K and p=1 atm. 18

25 3.2 GridPix based TPC (a) (b) Figure 3.5: Diusion coecients of (a) He 90%, ic 4 H 10 10% and (b) DME 50%, CO 2 50% for T=300 K and p=1 atm. The green line indicates the transversal and the orange the longitudinal diusion coecient. 19

26 Chapter 3 Proton CT set-up (a) (b) Figure 3.6: Distributions of number of electrons produced from protons through DME 50%, CO 2 50% for T=300 K and p=1 atm. The plots are made in GARFIELD. Ingrids on top in 2 x N congurations. A detector with 2 x 4 GridPix chips has been constructed in University of Bonn[26]. For making a higher drift gap (Z-coordinate), a higher voltage between the cathode and the grid is required in order to sustain the electric eld at 2000 V/cm for DME/CO 2. GridPix based TPCs with a drift gap of even 10 cm have been constructed at Nikhef. The electrons generated in the ionization process by low energy protons transversing the DME/CO 2 mixture follow the distributions of Fig The energies of 190 and 65 MeV are plotted with the rst corresponding to the highest energy of the AGOR cyclotron and the second to the remaining energy after 20 cm of water, the maximum distance a proton would travel traversing a common human body 5. As the energy of the incoming protons is decreasing, more ionizations are expected in the TPC Hit Resolution The error for a single hit in a coordinate i is of the form i 2 = σ 2 pitch + σ2 drift where σ drift is (3.7). On the x, y and the reconstructed z coordinates, the errors are given by (3.9) - (3.11): x 2 = d2 pitch 12 + D2 T z (3.9) 5 For patients with larger body mass a higher energetic beam is required. 20

27 3.2 GridPix based TPC Figure 3.7: The time walk eect. y 2 = d2 pitch 12 + D2 Lz (3.10) z 2 = (t binu drift ) D 2 Lz + t 2 timewalk (3.11) with d pitch = mm being the pixelpitch, t bin =10 ns the clockcycle 6, u drift the drift velocity. The values for the longitudinal D L and transversal D T diusion are extracted from Fig The denominator equal to 12 appears from the relation between the strip pitch and the detector resolution for binary read-out (Appendix B). t timewalk is an additional error in the z coordinate due to time walk, one of the eects that worsens ToT and ToA measurements. The rise of a weak signal has a smaller slope than the rise of a strong signal as shown in Fig Thus, the time it takes for a signal to cross the threshold (green horizontal line) varies depending on the charge cloud that produced the signal. This eect is called timewalk. Timewalk can be prevented in three ways: a) by raising the grid voltage, so that the amplied signal produced in the avalanche is strong enough and will have a fast rise time, b) by lowering the threshold, minimizing the pulseheight gap so weak signals will cross the threshold sooner or c) by measuring the time over threshold which gives the strength of the signal (the stronger the signal the less timewalk). However, each solution has a drawback. Raising the grid voltage might lead to another unwanted eect called crosstalk, where the amplied signal is so strong that hits adjacent pixels making it hard to connect the hit to a single pixel. Also, lowering the threshold increases the possibility of hits due to noise to appear along with the actual hits produced from ionization. The appearance of noisy pixels is unwanted cause it will aect the reconstruction of a track when a t is applied. Lastly, the measurement of time over threshold cannot be done simultaneously when the the time of arrival is measured. This limitation can be overrun with the forthcoming version of Timepix chip that allows simultaneous measurement of time over threshold and time of arrival. 6 The clockcycle depends on the frequency the clock is set. In this case the clock was set at its maximum frequency (100 MHz) thus each clockcycle is 10 ns. 21

28 Chapter 3 Proton CT set-up (a) The RelaxD board. (b) The Pixelman UI. Figure 3.8: Harware and software components for the GridPix Read-out. 22

29 3.2 GridPix based TPC Figure 3.9: Measurement of the ToA. a) The shutter (set by the trigger) is dened between the left dashed line and the right dashed line, b) an analog signal crosses the threshold and c) the clock of the Timepix starts counting until the end of the shutter. By substracting the clock counts from the shutter time we acquire the Time of Arrival (red ellipse) Read-out GridPix is read out by the RelaxD board which is connected to a PC via a 1 Gb/s Ethernet connection. There is the possibility to connect 1, 2 or 4 Timepix chips to the same board and operate them separately. The current maximum rate of reading data from the RelaxD board is 120 Hz per chip. Increasing the reading speed of the Relaxd is not possible with the current Timepix chip. This is because the time needed to read out the chip is about 8ms, a dead-time during which the detector cannot collect data. So, reading out more than 120 frames per second events will result in a number of events being lost (not measured). The data acquisition, calibration/equalization and triggering of GridPix was made by the Pixelman software[27]. The user has various options for these tasks like: dierent acquisition modes(individual frames, integral of all frames etc.) selecting the acquisition parameters(counts, time) select the chip mode (Medipix, ToT, ToA) set the triggering (start/stop by software, start/stop by external or a combination of them) By setting the triggering at start/stop by hardware and applying a trigger on the RelaxD board (details of the trigger in the Appendix), tracks through the GridPix can be recorded. Each track is a collection of hits and each hit is described by three numbers X,Y and C. X stands for the row of the pixel hit, Y for the column of the pixel hit and C is the number of counts of the pixel. The 23

30 Chapter 3 Proton CT set-up Figure 3.10: The BaF 2 detector. procedure of converting the clock counts to time of arrival is explained in Fig BaF 2 scintillating detector For measuring the energy of the protons a calorimeter is needed. In our case we used a scintillating detector consisting of a BaF 2 -crystal. The crystal is wrapped with Teon and an additional layer of aluminum foil as UV-reectors and coupled optically to the quartz window of the photomultiplier tube (Hamamatsu R ). Since no documentation or manual of the specic detector was found, information from [28] was used as suggested by [34]. By shaping the signal of the photomultiplier tube (PMT) in two dierent ways, a fast and a slow signal are generated. The calorimeter has 3 outputs, giving 1) a fast scintillation component (600 ps) used for triggering the GridPix, 2) a slow scintillation component (630 ns) used for measuring the energy and 3) the signal of the last dynode which was not used. The contribution of the fast scintillation component (λ = 220 nm) to the total light output (dominated by the slow scintillation component at λ 315 nm) diminishes with the increase of the energy density deposited by the ionizing particles(fig. 3.11(a) left). The ratio of both contributions remains constant over the full dynamic range up to relativistic and even ultra-relativistic energies[28]. In principle, both signals should be used for determining the deposited energy of a particle but since the slow component dominates, only the slow component signal was recorded and used for estimating the energy. A typical output of this signal is shown in Fig The purpose of this scintillating detector is to measure the energy of the scattered/outgoing protons. The particles are stopped in the detector and the energy they deposit is converted into scintillation light. The light emitted is captured by the PMT which produces the three signals mentioned. The scintillating detector 24

31 3.4 Trigger (a) Typical signal shape of BaF 2 for photons and charged particles(left), Particle identication based on the correlation of the fast and total light output(right) from [28]. (b) Scintillation intensity of the 315 nm (slow component) emission peak as a function of temperature from [30]. Figure 3.11: Characteristics of the BaF 2 crystal. must be sensitive enough to distinguish dierences in the energy of the incident protons in order to determine successfully whether or how they interacted. The BaF 2 is not the most appropriate candidate but was the only available scintillating detector. A better detector in terms of resolution would be a LaBr 3 crystal that gives about 60,000 photons of 390 nm[29]. The BaF 2 scintillating detector will also be referred in this project as calorimeter. An important eect that may aect our measurements is the temperature dependence of the scintillation intensity. The slow 315 nm peak has a temperature dependence of -1.1% per C as reported in [30] while the fast 220 nm peak does not change signicantly. As shown in Fig. 3.11(b), compared to the room temperature intensity a factor of 3 in light yield can be gained by cooling down the crystal. Since determining the energy of the protons is based on the measurement of the slow component, this eect may smear our energy resolution. The BaF 2 detector should be temperature stabilized in order to avoid this eect to appear. 3.4 Trigger The GridPix and BaF 2 detector were used in order to visualize a charged particle track and measure its energy. To do so, the data of the two dierent detectors should be synchronized to successfully reconstruct the events. A small scintillator 1.5 cm x 1.4 cm (not covering completely the surface dened by the side of the GridPix TPC) attached to a Hamamatsu H5783 PMT was combined with the signal of the fast output of the calorimeter to form the trigger. The order in which the components were placed is shown in Fig. 3.13(a). A track that passes through the scintillator and deposits energy in the calorimeter will have inevitably passed through the TPC. The scintillator signal and the calorimeter fast output are combined to form a coincidence. By this coincidence two time windows are opened(fig. 3.13(b)). 25

32 Chapter 3 Proton CT set-up Figure 3.12: Measured signal with cosmic rays recorded by an Agilent oscilloscope. The rst window, the shutter, is triggering the RelaxD board, which activates the GridPix. The second window, the VETO, is keeping the coincidence inactive so no other events trigger the RelaxD while it is busy. However, during the time the shutter is open more than one charged particles may pass through the set-up. This results in multiple tracks being reconstructed with some of them appearing to be outside the detector volume in the Z-coordinate. To minimize the appearance of such tracks, the length of the shutter should be equal to the largest drift time. The length of the VETO should be equal to the time that the RelaxD and the DAQ PC are busy processing the data in order to increase the count rate of the particles recorded. The logic modules used with all the parameters set can be found in Appendix C. The set-up needed to be tested with charged particles with enough energy to traverse the scintillators and the plastic cap of the BaF 2 detector and deposit energy in the calorimeter. In order to do so, the set-up of Fig was rotated facing towards the sky to detect cosmic rays. Cosmic rays are minimum ionizing particles meaning that their deposited energy in a material is at the minimum in the Bethe-Bloch curve. Therefore, the detection of such particles would prove that the 3D particle tracker set-up could well be used in detecting the low energy and more ionizing protons planned for a proton CT. Over a period of operation in the lab, MIPs were detected and recorded successfully 7 showing us that we are 7 3D visualizations of cosmic rays are presented in Appendix D. 26

33 3.4 Trigger (a) Placement of the GridPix TPC, scintillator and calorimeter. Components and distances among them not in scale. (b) Triggering on coincidences. Coincidences arriving late as well as ones rejected by the VETO are also displayed. Figure 3.13: Triggering on the coincidence of a scintillator and the calorimeter. 27

34 Chapter 3 Proton CT set-up ready to move to the next stage of the project, testing the set-up with a proton beam. 28

35 Chapter 4 Experiment at the KVI The KVI institute in Groningen is an international institute for atomic and subatomic physics, focusing on nuclear, theoretical and accelerator research. The KVI is collaborating with Nikhef in the research for a proton CT. The proton CT scanner of Chapter 3 was prepared and tested with cosmic rays at Nikhef in Amsterdam. It was transfered to Groningen where an experiment with protons from the KVI cyclotron was planned. The experiment at KVI is presented here containing the set-up, the calibration of the calorimeter with the proton beam, the runs taken at dierent energies and dierent intensities and the samples tested. 4.1 AGOR cyclotron The AGOR cyclotron (Accelerateur Groningen-ORsay) in KVI/Groningen depicted in Fig. 4.1(a) produces proton beams used for irradiations with primary energies of 90, 150 and 190 MeV. The ux can be from a few particles up to particles s 1 cm 2. The intensity is up to particles s 1 but mostly around particles s 1. Particles are delivered in bunches with the number of particles per bunch following the Poisson distribution. The properties of the beam are presented in Table 4.1. The initial narrow particle beam from the accelerator is broadened and attened with scatter foils, while collimators to stop protons that have been scattered over too large angles can also be placed. The initial beam energy can be degraded to lower energies by a compact ensemble of 9 aluminum plates of various thicknesses. Table 4.1: Properties of the KVI cyclotron beam. Maximum Energy σ Energy Frequency σ x,y σ beam 190 MeV 300 KeV 55 MHz 2 mm 1.5 mrad 29

36 Chapter 4 Experiment at the KVI (a) The cyclotron. (b) Beam line on breadboard tables. Figure 4.1: The AGOR facility for irradiations of materials. 4.2 Experimental set-up The 3D particle tracker described in Chapter 3 was placed in the irradiation area of AGORFIRM (Fig. 4.3). Samples and beam line components were aligned by using a 3D laser positioning system. An XY table (Fig. 4.1(b)) contains a large mounting rack that allows movement of samples through the beam with a range of 600 mm in the horizontal and 300 mm in the vertical direction with a relative accuracy of 0.01 mm. Special predrilled plates for sample mounting are used that can be quickly tted to the mounting rack. The proton primary energy used was 150 MeV (not the maximum energy that can be achieved). The intensity was monitored by converting the signal of the fast output of the calorimeter into a digital one with a discriminator and keeping track of the number of particles with a counter. After the particles leave the vacuum of the beam pipe through the exit foil, the following components were placed: a single scattering foil (1.44 mm Pb) to produce a homogeneous eld with a diameter of about 3 cm, a degrader (series of aluminum plates) to change the energy and a 2 x 2 cm collimator limiting the beam divergence to < 3 mrad. The GridPix based TPC was placed about 3 m downstream from the scatter system, the BaF 2 detector was mounted downstream of the TPC and between them the Figure 4.2: Side view of all the components of the experimental set-up. 30

37 4.3 Monitoring the Set-Up Figure 4.3: The 3D particle tracker. small scintillator used as part of the trigger (Fig. 4.2). The beam is traveling on the X-axis traversing the YZ-plane of the TPC as drawn in Fig The intersection point of the lasers used for the alignment mark the (0,0,0) of our reference system located at the center of the GridPix. 4.3 Monitoring the Set-Up The PC for the data acquisition for the GridPix containing the Pixelman software and the Agilent oscilloscope recording the signals from the BaF 2 detector were remotely controlled in the counting room. The data from GridPix were stored on the DAQ PC while the signals from the calorimeter were stored in the Agilent oscilloscope, with a limit in the maximum number of waveforms that could be stored in the segmented memory of the oscilloscope. The number of triggers of the GridPix were compared to the number of recorded signals in the oscilloscope. For low trigger rates, tested in the lab with cosmic rays and radioactive sources, these two numbers coincided indicating synchronization between the two detectors. The set-up had not been irradiated with high rates like the ones scheduled for the experiment thus synchronization at those rates was not tested. 4.4 Calibration of the BaF 2 detector The calibration of the BaF 2 detector was done by placing it directly into the proton beam, changing the beam energy and recording all the signals of the slow output. The energies, the corresponding output voltages and the thicknesses of 31

38 Chapter 4 Experiment at the KVI Table 4.2: Energy and Voltage values. Energy [MeV] Al thickness [mm] Voltage [mv] σ V [mv] Noise the aluminum plates are presented in Table 4.2 with the last entry being the noise. The primary energy is not 150 MeV but MeV due to energy loss when the beam passes the scatter foil and travels through the air. Note also the values of σ V as the voltage is decreasing. Since the light output (number of photons) is proportional to the energy, then we expect σ V V. The reason why σ V is not following this rule is due to the method the primary beam is degraded to lower energies. The energy spread of the particles is becoming larger with lower energy due to straggling in the Al degraders. Thus, the observed width is a convolution of the intrinsic resolution and the energy spread[25]. 4.5 Synchronizing GridPix and the BaF 2 detector Synchronization between the GridPix and the BaF 2 was lost in the initial runs due to the ineciency of the trigger to keep up with the intensity of the beam. Synchronization is important for our experiment to correlate the data between the the two detectors in order to match a track in the GridPix with the energy recorded in the calorimeter. After lowering the intensity the two detectors and by reducing the number of events per run, the data from the two detectors were nally synchronized. However, the cost is that the amount of data taken in those runs is limited. 4.6 Collected Data Sets A number of data sets was collected with the beam at maximum energy (144 MeV), the beam at low energy (55 MeV) and with two dierent kind of samples placed between the collimator and the GridPix (Fig. 4.4 and 4.5). The beam intensity was 500-7,000 protons per second. The samples used were a number of copper plates and two copper wedges of dierent size. The plates were 2.80 mm thick each and the wedges had a base of 30 mm x 30 mm while the maximum height of the small one was 12 mm and of the big one 24 mm. 32

39 4.6 Collected Data Sets (a) Set-up of Fig. 4.3 with the Cu plates. (b) Beam view of the set-up. Figure 4.4: The set-up with the Copper plates. (a) The small wedge used in the experiment. (b) Set-up of Fig. 4.3 with a wedge. Figure 4.5: The set-up with the wedge. 33

40 Chapter 4 Experiment at the KVI Table 4.3: Parameters and properties of the Runs. Runs Plates Wedges Congurations Initial Degrader 1 4 Small Big Full Energy Intensity [protons khz] No samples Sample Full YZ cover Sample Half YZ cover Num. of GriPix hits 267, ,296 68,882 66, , ,112 Num. of GriPix events 3,911 2,018 1,024 1,024 4,864 2,048 Num. of Calo events 3,026 1,056 1,026 1,025 4,864 2,048 Synchronization 4.7 Overview The Runs and the dierent congurations are summarized in Table 4.3. Around 15,000 tracks were recorded during the runs that lasted for a day. Since one of the main goals of the experiment was to investigate the correlation between the GridPix and the calorimeter data, synchronization between the two detectors was the key part. To achieve that however, the intensity of the beam had to be low which means less events per unit time. 34

41 Chapter 5 Results & Analysis The data collected from the experiment at KVI are presented in this chapter. First the data collected with the GridPix will be discussed and then the data from the BaF 2 detector. Finally, we will combine the data of the two detectors. 5.1 GridPix Data Projection on the XY plane To start with the analysis of the data collected at KVI, we looked at the XY plane information. The XY plane is the pixel matrix of the GridPix. We compared the XY data obtained from the proton irradiation with data collected from radioactive sources and cosmic rays at Nikhef. Looking only at the XY values of the GridPix data, an integral plot of the pixels hit is shown in Fig. 5.1(a). In Fig. 5.1(b) the same plot is reproduced by irradiating GridPix with a Sr-90 source in the lab. In both pictures we notice a number of artifacts with most obvious a dead area in the top right quadrant. The grid of a GridPix detector is supported by pillars of SU-8. In the production process, the SU-8 between the pillars is removed after the grid is made. In the case of our detector, not all the SU-8 was removed successfully. As a result electrons drifting to the grid over the area covered with SU-8 will not proceed into the multiplication gap. Therefore no signal will be induced on the pixels making this part of the detector inactive. In addition to the artifact due to the SU-8 there is an inactive column in the right side as well as some inactive areas on all four sides of the pixel matrix. This is due to the fact that when the eld cage was placed and glued on the GridPix, a portion of the glue covered parts of the grid. Electrons close to the walls of the Field Cage drifting towards the Grid will stop in the glue and no pixels will appear to be hit. The white areas in Fig. 5.1(a) and (b) represent inactive pixels. Looking at the top of the XY planes of Fig. 5.1(a) and (b), we notice a dierence in the hit density above Y = 200. Many pixels in Fig. 5.1(a) above Y = 200 have zero or a small number of hits recorded in contrast to Fig. 5.1(b), where this part of the detector appears to behave similar to the rest of the active 35

42 Chapter 5 Results & Analysis (a) Irradiation with protons. (b) Irradiation with Sr-90 source. Figure 5.1: Integral plots of pixels hit. In white are all the inactive pixels. (a) Histogram of the Hits per pixel. (b) Mapping of "good" pixels. Only 41,818 pixels (in red) out of 65,536 are considered reliable for our measurements. Figure 5.2: Discrimination of eective pixels through the hit spectrum. area. We conclude that the scintillator was not covering the whole XY plane but only until Y = 200, thus any hits above this line are caused by scattered protons or delta-rays. By integrating over all pixel hits, the hit spectrum of the initial runs is shown in Fig. 5.2(a). The peak at zero indicates that around 20,000 pixels (almost 1/3 of the total pixels) remain silent appearing not having been hit. Most of the rest of the pixels appear to have been hit around 6 times during the experiment. From Fig. 5.2(a) a low cut value at 2 (for silent pixels) and a high cut value at 20 (in case of noisy pixels) from the pixel hits can be extracted. Applying these cuts to the pixels of Fig. 5.1(a), a map of the "good" pixels, meaning the pixels inside the cut limits that should be considered as eective ones, is drawn in Fig. 5.2(b). The number of "good" pixels of this chip (around 64%) agrees with other GridPix detectors[23] which varies from 60-75%. In our experiment, 41,818 pixels 36

43 5.1 GridPix Data Figure 5.3: 3D view of the samples placed before GridPix. (marked with red in Fig. 5.2(b)) of the total 65,536 can be considered eective with 23,718 silent (marked with white in Fig. 5.2(b)) and 0 noisy pixels. No noisy hits appeared since the noisy pixels were masked before the experiment. Comparing to the number of pixels hit from the electrons of the Sr-90, there appear to be 17% more pixels hit in the case of the irradiation with the source. This is reasonable since the integral plot of Fig. 5.1(b) was made by pointing the radioactive source perpendicular to the pixel matrix for about 5 minutes in contrast to the proton irradiation where a part of the pixel martix was not active (due to the fact that the trigger scintillator was not covering the entire detector) Y-coordinate distribution of all hits Since the samples were placed perpendicular to the beam as shown in Fig. 5.3, the distribution of the Y-coordinate of the pixels hit can provide us with some interesting information. By using the 3D Display 1, proton tracks can be visualized in 3D (Fig. 5.4(a)). The proton beam is perpendicular to the YZ-plane so the histogram of the Y-coordinate of a single track will appear as a peak (Fig. 5.4(b)). Fitting the distribution with a gaussian, we can have an idea for the diusion. The σ from the t is 1.2 pixels which corresponds to 66 µm while, from section 3.2.5, the expected σ 0.75cm = 56 µm. The 15% divergence from the expected value of σ is due to the fact that the electric eld in the eld cage is not homogeneous so electrons drifting towards the grid will not diuse uniformly. In the runs with no samples, the hits are mainly distributed between pixels in the 1 The 3D Display and the Histogram Algorithm used to present the tracks of GridPix were implemented and kindly shared by Wilco Koppert. Additions to the Histogram were made by the author. Algorithm 37

44 Chapter 5 Results & Analysis (a) Track visualized with the 3D Display. (b) Y distribution of the hits forming the track. Figure 5.4: A proton track and the Y distribution of the hits forming it. 38

45 5.1 GridPix Data rows (Fig. 5.5(a) and (b)). The peaks and valleys correspond to the eect of adding a small number of individual tracks. This means that the summing of the individual tracks causes the appearance of peaks in Fig. 5.5 while their absence appears as valleys. In the case of the copper plates covering half of the YZ surface (Fig. 5.5(c) and (d)), the part of the detector not blocked by the sample (pixel Y: roughly) should be similar to Fig. 5.5(a). Although the events in the runs with the copper plates are less, the shape of both histograms appear to be the same. A sharp drop in the number of events around pixel row 100 with 1 plate and pixel row 70 with the 4 plates indicates that less hits are recorded in GridPix from the side covered by the plates. As protons traverse the plates, the copper reduces their energy and causes signicant scattering. This results in less tracks traversing the plates to be detected since these protons will not have traversed the triggering scintillator. This eect is more clear in the case of the 4 plates as the drop is sharper than the case with 1 plate. Although there appear to be less hits we cannot draw conclusions from this plot about the proton tracks. The assumption that a number of protons is scattered due to the copper plates could be easily veried by placing a second GridPix based TPC before the samples. By doing this, one could compare the tracks recorded before the sample and the tracks recorded after the sample and know precisely the ux, the ratio of the scattered protons over the total number of protons and the eect of the dierent thickness of the samples on this ratio. For the runs with the wedges (Fig. 5.5(e) and (f)), the same eect due to scattering described in the case of the copper plates is observed. The wedges were placed with the base parallel to the (X=0, Y=0) - (X=0, Y=256) of the GridPix as shown back in Fig. 5.3, with the tip of the wedge starting at (0, 0) and the thickness increasing towards (0, 256). The distributions have no major dierences except the region after pixel row 190 where the distribution of the big wedge appears to drop with a constant rate Time Measurement & Z reconstruction The main advantage of the GridPix based TPC, the reconstruction of the z coordinate, was already described in Fig The time of arrival is calculated by subtracting the clock counts of a single pixel from the shutter time. Knowing the drift velocity, this time is converted to distance. This process is repeated for all the hits in one event and eventually a track is reconstructed. By looking at the tracks through the 3D Display, many hits and in some cases even whole tracks appear to be higher than 1.6 cm - the height of the drift volume of the GridPix. These are what we call "late" electrons. The denition of "late" electrons or tracks is based on the way the time of arrival measurement is being done. The electric eld applied is 2000 V/cm so the drift velocity, as calculated from Fig. 3.4(c), is 10 µm/ns. The time an electron needs to drift from the highest point of the eld cage is 1,600 ns. The Timepix clock was set 39

46 Chapter 5 Results & Analysis (a) Full beam. (b) With degrader. (c) With 1 copper plate. (d) With 4 copper plates. (e) With small wedge. (f) With big wedge. Figure 5.5: Projections on Y. 40

47 5.1 GridPix Data Figure 5.6: Secondary "late" track appearing higher than the eld cage volume. to 100 MHz, meaning that one clock count from a pixel is equal to 10 ns. Thus, the maximum drift time or time of arrival in terms of clock counts is 160. Our shutter was 20 µs so 2000 clock counts. Subtracting the maximum drift time from the shutter clock counts, the limit of 1840 clock counts is set. This means that if the clock counts of a hit or of a collection of hits that form a track are less than 1840, then these hits will appear higher than the Field Cage volume. An example of this scenario is displayed in Fig. 5.6, where the upper proton track is around Z=10 cm while the height of the eld cage is only 1.6 cm. Many hits below 1840 counts were recorded. By excluding these late hits, integral scatter plots of the YZ-plane of GridPix can be drawn (Fig. 5.7). Looking at these plots, we notice: 1. the absence of hits at the edges of the chips (below Y=10 and above Y=235) due to the eect of the glue and shown also in Fig This is more obvious in (a), (b) and (d). 2. a small gap in the drift volume from Z=0 mm to Z=1 mm. This means that no hits occurred at this region, a fact that is highly unlikely though. The best explanation would be that there is a signicant delay from the trigger which results in missing the electrons generated close to the grid. 3. the majority of the hits are above Z=4 mm, marking the shadow of the scintillator. This is more obvious in Fig 5.7 (a), (c) and (d) where a large number of events was collected. Since the scintillator was not covering completely the YZ-plane of the eld cage as mentioned in section 3.4, not all of proton tracks entering GridPix could have been detected. By using also the information for the eective area in the XY plane obtained from Fig. 5.2(a) and (b), we have a 3D aspect of the active volume of the GridPix based TPC due to the scintillator. 41

48 Chapter 5 Results & Analysis (a) Initial runs at 144 MeV. (b) Runs with the Degrader at 55 MeV. (c) Runs with the 4 copper plates. (d) Runs with the small wedge. Figure 5.7: Integral scatter plots of the YZ-plane. 42

49 5.1 GridPix Data Energy dependence of Ionization Comparing the scatter plots of Fig. 5.7(a) and (b), some general conclusions about the energy dependence on ionization can be drawn. The expectancy of more ionizations as energy decreases was already introduced in section and depicted in Fig The GridPix based TPC is designed to detect single electrons ionized in the eld cage and drifting towards the grid. This way we can connect a hit of the Timepix chip with a single electron. Therefore, looking at the hits recorded we have an estimation of the ionization. In Fig. 5.8 two tracks of dierent energies are presented. Both tracks traverse the GridPix at the same height so diusion is expected to be the same. The number of electrons is larger in the track with the lower energy however the divergence of the Z coordinate remains the same in both tracks. The number of electrons per cm for each energy in Fig. 5.8 is within the limits of the distributions expected like the ones introduced in Fig In the initial runs at 144 MeV, 3,911 events were recorded producing 267,455 hits. Taking into account the count clock limit of the previous subsection, 203,265 of these hits are inside the drift volume of the eld cage. In the runs with the degrader, 2,018 events were recorded producing 260,296 total hits with 208,976 of these hits inside the drift volume. So, although in the runs with the degrader the number of events were two time less than the events in the initial runs, the number of hits (which corresponds to the number of ionized electrons) is not only in the same order of magnitude but almost equal. This is a rough estimate for the energy dependence of ionization made by looking at the total number of hits. A more accurate method would involve the application of an algorithm looking at the hits of single tracks and discriminating slow hits and double proton tracks. Then distributions like Fig. 3.6 can be plotted for each energy and comparison with the expected number of electrons per cm can be made in more detail. During the writing of this thesis the development of such an algorithm was not made Intensity eects The intensity of the proton beam was changed in order to achieve synchronization between the GridPix and the calorimeter. Looking at the histograms of the pixel clock counts (Fig. 5.9), it shown that the intensity aects the number of late hits. The probability of having a second proton within the 20 µs at 7,000 Hz is 3 x 10 4 while at 700 Hz the probaility is 3 x 10 5, a factor of 10 lower. In both histograms the red line and arrow indicate the limit of 1840 counts. In Fig. 5.9(a) many hits are distributed below 1840 while in Fig. 5.9(b) the late hits are signicantly reduced. The main cause for this discrimination is the reduction of late proton tracks arriving when the shutter is already open as described in Fig. 3.13(b). The cyclotron frequency is 55 MHz and protons are delivered in bunches so every second there are 55 x 10 6 bunches[34]. With a beam intensity of 7 khz, most 43

50 Chapter 5 Results & Analysis Figure 5.8: Energy dependence of Ionization. of these bunches are empty and the protons are distributed in the rest following Poisson statistics. With a beam intensity of 0.7 khz, even more bunches are empty so the probability that a second proton traverses the GridPix while the shutter is open is really low. That is why in Fig. 5.9(b) the appearance of late hits is suppressed. 5.2 Calorimeter Data Calibration The process of the calibration was described in 4.4. The histogram of Fig. 5.10(a) contains the oscilloscope signals of the calibration. The ve dierent beam energies from MeV can be seen as peaks in the histogram with the noise creating an extra high peak right after zero. However, only four of the peaks corresponding to energies were taken into account and tted as the fth one (around 450 mv) was considerably smeared. An additional t was applied to the noise to have an estimation on the mean value of the noise level. The values of the energy and the voltage are plotted and tted with a 3 nd order polynomial curve (Fig. 5.10(b)) in order to nd the relation between them. This curve (V (E) = E 0.15E 2 + ( E 3 ) will be used to convert the signals of the BaF 2 to energy. 44

51 5.2 Calorimeter Data (a) Initial runs at 144 MeV and 7 khz. (b) Runs with the Degrader at 55 MeV and 0.7 khz. Figure 5.9: Pixel clock counts. 45

52 Chapter 5 Results & Analysis (a) Histogram of the signals from the dierent beam energies. (b) Voltage - Energy correlation. Figure 5.10: Calibration of the BaF 2. 46

53 5.2 Calorimeter Data Table 5.1: Shifted Voltage values and corresponding Energies from the runs. Run Energy[MeV] Voltage [mv] Deviation from calibration [%] Full Energy 144 1, Degrader Plates Wedges (a) Largely shifted peak in the initial runs. (b) Partially shifted peak in the runs with the degrader. Figure 5.11: Calorimeter data. The dashed line represents the value obtained at the calibration Shifted Energy spectrum during Runs For the calorimeter data of the initial runs at 144 MeV and the runs with the degrader at 55 MeV, we expect to see a peak for each energy at the same voltage as was recorded in the calibration and plotted in Fig By looking at the histograms of Fig we notice that this is not the case. In the initial runs (Fig. 5.11(a)) there is a large shift of 90 mv to lower energy compared to the corresponding 144 MeV peak of the calibration. In the runs with the degrader, the shift is smaller being 15 mv below the corresponding peak of 55 MeV of the calibration(fig. 5.11(b)). One possible explanation could be the temperature dependence of the scintillation intensity mentioned in 3.3. Although the temperature in the irradiation area was monitored and stayed constant at 18 C, the temperature of the BaF 2 detector was not measured. The shifted values of the calorimeter output voltage are presented in Table 5.1. The shifted peak at 144 MeV is also observed in the two runs with the Copper plates. Since half of the detector was covered with 1 plate in the rst run and with 4 plates in the second, we expect two peaks in each histogram - one common peak indicating the full energy of the beam and a second peak due to the presence of the plates. The results in Fig. 5.12(a) verify our prediction. The common peak rises at 1,220 mv, while the secondary peak is located at 1,160 mv in the run with 1 plate and at 860 mv in the run with the 4 plates. In the runs with the wedges, the wedges were covering all of the YZ-plane so the even larger shift in Fig. 5.12(c) is due to the energy loss as the beam traverses the tip of the 47

54 Chapter 5 Results & Analysis (a) Energy spectrum with the copper plates. (b) Energy spectrum with the wedges. Figure 5.12: Calorimeter data of the runs with the samples. 48

55 5.3 Correlation of GridPix and Calorimeter data Figure 5.13: Voltage - Energy correlation with shifted voltage values. wedge. An attempt to estimate the energy loss due to the copper plates from the output voltage of the calorimeter fails. Protons of 144 MeV should deposit 10 MeV when traversing 2.80 mm of copper and 43 MeV when traversing 11.2 mm of copper so their nal energy should be 134 MeV and 101 MeV respectively. However, using the calibration plot of Fig. 5.10(b), the secondary peaks of Fig. 5.12(a) correspond to 115 MeV and 75 MeV (14% and 25% o of the expected energy). We conclude that since the whole energy spectrum is shifted after the calibration, we cannot use the curve of Fig. 5.10(b) to convert voltage to energy. Using the new voltage values for 144 MeV and 55 MeV and adding the voltage values from the secondary peaks due to the copper plates, a new Voltage - Energy curve (V (E) = E 0.11E 2 + ( E 3 ) can be drawn (Fig. 5.13). 5.3 Correlation of GridPix and Calorimeter data So far the data of the GridPix and the BaF 2 detector were presented individually. The synchronization between the two detectors achieved during the runs with the wedges allows us to combine the two dierent data sets in order to draw additional conclusions. In Fig the successful result of combining information for the tracks with the voltage they induced in the calorimeter is presented. The Y-coordinate of tracks that traversed the GridPix are plotted versus their corresponding voltage output (which is correlated to their energy). In the point close to the thin part of the wedge, tracks have their maximum energy. Moving along the Y-coordinate represents a thicker part of the wedge. It is clear that for tracks traversing the thicker parts of the wedges, their energy is decreasing. For the big wedge, this 49

56 Chapter 5 Results & Analysis Figure 5.14: Y-coordinate - Voltage correlation of tracks through the two wedges. drop is sharper indicating that the proton tracks have lost more energy thus they have traversed more distance though the material. The density of copper is about 9 times larger than the density of water. A 145 MeV proton would deposit around 7 MeV when traversing 2 mm of copper and would require 1.2 cm of water to deposit the same amount of energy. The use of thin copper samples (plates and wedges) introduces to the incident particles comparable scattering and energy loss with water samples of an order of magnitude thicker. 5.4 Simulations in GEANT4 Simulations of the set-up including the GridPix, the scintillator used for the trigger and the calorimeter were made in GEANT4. GEANT4 is a Monte- Carlo program operated in C++, a powerful tool used for simulating the passage of particles through matter that can provide information about the trajectory of the particles, the creation of secondary ones and the energy deposition of a particle. The 3D particle tracker set-up implemented in GEANT4 is presented in Fig. 5.15(a). By irradiating the set-up of Fig. 5.15(a) with 145 MeV protons we extract this information. The blue lines in Fig. 5.15(b) correspond to positive charged particles (the protons), the red lines to negative charged particles (a 50

57 5.4 Simulations in GEANT4 (a) Simulated set-up. (b) Simulated run (collection of events). Figure 5.15: Simulation made in GEANT4. delta ray in this case) and the yellow dots to steps (the points where a particle interacted with the volume it is traversing) Interaction of GridPix with the proton beam A number of simulations was developed to examine the interaction of GridPix with the protons and compare it with the beam divergence and energy spectrum of the proton beam. The information we are interested in is the energy of protons deposited in the GridPix and the divergence of the protons tracks after interacting with the gas and the walls of the eld cage. The 145 MeV proton beam has an angular displacement with a σ beam of 1.5 mrad (Fig. 5.16(c)). When the beam traverses the 57 mm degrader, the σ beam increases to 30 mrad due to the straggling of protons through the thick Al plate (Fig. 5.16(a)). Looking at the simulated data of the GridPix detector, Fig. 5.16(e) shows the divergence of the point of exit from the point of entrance of all proton tracks due to the walls and gas of the eld cage. The order of magnitude is in the order of 2 mrad with a σ GridP ix = 0.40 mrad proving that the interaction of protons with the GridPix causes a signicant small divergence from their initial trajectory comparing with the σ beam of Fig. 5.16(a) and (c). The plot of Fig. 5.16(f) shows the distribution of the energy deposited by 145 MeV protons in the GridPix. By applying a Landau t the most probable value is 70 kev. The energy loss through the GridPix appears to be too small to lead to errors when calculating the proton energy loss in case samples are placed before the 3D particle tracker. An additional simulation of the 145 MeV proton beam traversing a water sample of 10 cm thickness was developed (Fig. 5.16(g) and (h)). The divergence of the protons due to scattering in the water is about times larger than the divergence of the proton beam due to the gas and the walls of the eld cage. The most probable value of the proton energy loss through the GridPix is 120 kev. This larger energy deposition is expected since the protons, after having traversed the water, will have lost around 68 MeV. Thus, the energy deposition 51

58 Chapter 5 Results & Analysis (a) Divergence of the 145 MeV proton beam through the 57mm Al degrader. (b) Energy distribution of the 145 MeV proton beam through the 57mm Al degrader. (c) Divergence of the initial 145 MeV proton beam. (d) Energy distribution of the initial 145 MeV proton beam. (e) Divergence of 145 MeV protons due to the gas and the walls of the eld cage. (f) Energy depostition in GridPix of 145 MeV protons. (g) Divergence of 145 MeV protons after traversing 10 cm of water. (h) Energy depostition in GridPix of 145 MeV protons after traversing 10 cm of water. Figure 5.16: Simulated eect of GridPix on the proton beam. 52

59 5.4 Simulations in GEANT4 in the GridPix of these lower energetic protons will be larger as predicted by the Bethe-Bloch Simulation of the Copper samples The results of Fig. 5.7(c) showed that in the part of the eld cage covered with the copper, less hits are recorded. This is probably due to the fact the a number of the protons traversing the copper is scattered and not triggering the scintillator, therefore not detected in the GridPix. A simulation of the set-up with the copper samples covering half of the YZ-plane was made in order to obtain additional information of the eect of the copper plates on the proton beam (Fig. 5.17). An estimation of the number of ionizations cannot be made by GEANT4 so this process was not simulated. According to Fig. 5.18(a) and (b), the ux through GridPix is aected by the copper. In the simulation, all the protons of the beam were generated to traverse randomly within the boundaries of the GridPix YZ-plane. In the case of one plate, 34% of the protons detected traversed the sample and 33.5% belong to the initial beam. The rest 32.5% of the particles is not detected since they are scattered from the copper plates. In the case of the four plates, 30% of the protons traversed the samples, 39.3% belong to the initial beam and 30.7% is not detected due to the scattering through the copper. The simulated energy spectra in Fig. 5.18(c) and (d) contain two peaks, one representing the energy of the protons that traversed the copper and the other representing the energy of the particles that did not interact with the samples. The lower energetic peak in both histograms of Fig. 5.18(c) and (d) is wider due to the energy straggling of the protons through the copper. The simulated energy spectra agree with what was recorded in the experiment and was was expected, the appearance of the two dierent peaks and the wider spread of the secondary peak due to energy straggling of the lower energetic protons. According to the simulations, the number of scattered protons is not aected by the dierent thickness of the copper. However, the statistics taken is quite low (only 1024 events in each case 2 ) in order to draw denite conclusions. 2 The number of simulated protons traversing the copper plates was the same as the number of protons recorded in the experiment. 53

60 Chapter 5 Results & Analysis Figure 5.17: Simulation of protons traversing a copper plate and the 3D particle tracker. (a) Simulated scatter plot of the YZ-plane with 1 copper plane. (b) Simulated scatter plot of the YZ-plane with 4 copper planes. (c) Simulated energy spectrum with 1 copper plate. (d) Simulated energy spectrum with 4 copper plates. Figure 5.18: Simulation of the 3D particle tracker interacting with the Copper plates. 54

61 Chapter 6 Conclusions 6.1 Performance of the 3D particle tracker The application of the GridPix based TPC as a 3D particle tracker with 145 MeV protons for the proof of principle was investigated. Proton tracks were detected and visualized and their energy was recorded with a BaF 2 scintillating detector. A number of samples was also tested to examine how the GridPix and the BaF 2 correspond to the the uctuations introduced in the beam by the samples. The loss of synchronization between the two detectors prevented us from correlating each track in GridPix with a signal from the calorimeter except the data collected with the wedges. Nevertheless, by carefully analyzing the data the point of the syncronization loss can be detected and a correlation between the two dierent data sets is possible. The limitations of the RelaxD readout rate and the ineciency of the trigger to keep up with the intensity of the beam make data acquisition slow. The constant progress by people working at Nikhef in increasing the RelaxD readout spead can lead to a faster acquisition. The capability of using the 3D particle tracker in order to examine how protons interact with matter is very promising. By combining the GridPix and the calorimeter data, the density of the sample changing with respect to the distance can be detected as shown in the case of the wedges. By using only the data from the calorimeter, conclusions can still be drawn as in the case of the copper plates with the appearance of two dierent peaks in the energy spectrum. However, the shifted energy values during the runs prevented us to use the energy-voltage curve made by the calibration. The temperature dependence of the scintillation intensity cannot have caused this eect since it would take a temperature change of around 20 degrees for the deviations recorded. Two possible causes might be the degradation of the optical couplant in time (the coupling must be renewed every few years[35]) or the saturation of the photomultiplier (the fairly large non-linearity in the energy-voltage correlation points in that direction since nonlinearity is one of the eects caused by saturation[34]). The major advantage that GridPix oers in visualizing tracks in 3D was presented. The late hits and secondary proton tracks are discriminated in the 55

62 Chapter 6 Conclusions analysis but can also be reduced by setting the shutter time equal to the maximum drift time. The active area of the GridPix is currently limited due to the size of the chip. It is feasible to tile chips in a 2xN conguration (with most common a 2x2 area), increasing the detection area in the XY plane. To succesfully increase the drift volume, a higher voltage must be applied in order to sustain the electric eld. Thus, the construction of a larger GridPix based TPC is possible. 6.2 Outlook The 3D particle tracker designed and tested in this project constituted of only one GridPix based TPC. In proton CT the beam has to be monitored before it traverses the patient and after it exits. Thus, the addition of a second GridPix based TPC along the beam is neccessary (formation of a telescope with two GridPix detectors). With such a set-up, track reconstruction will be possible and the trajectory of a track after it exits a sample can be examined in order to dene the divergence from the incoming trajectory as explained in 3.1. In terms of analysis, the implementation of new algorithms is important. Such algorithms could be used to t the hits of each event in order to reconstruct a track, calculate the divergence in Z-coordinate and Y-coordinate to investigate eciently how diusion aects the hits, study the ionization distribution in different energies. The behaviour of the BaF 2 detector should be thoroughly examined to detect how the shift in the energy spectrum occurs. In order to look at the scenario of the BaF 2 /photomultiplier combination, the crystal should be repackaged with a new optical coupling to a photomultiplier with a well known behavior. For the saturation scenario, the proton energy versus the output voltage spectrum should be checked more thoroughly on line. Also, by irradiating the detector with protons as well as gamma sources one could show whether the eect is time dependent and how it corresponds to the nature of the incident particle. The option of using an other scintillating detector should be also taken into account since the BaF 2 crystal might not be the ideal candidate for this project. For example, the properties of LaBr 3 look very promising in terms of better energy resolution and the fact that it has only one light component as an output (instead of the two output scintillation components of the BaF 2 ). A new design of the trigger is essential. The expected new version of the Timepix chip on 2012 will allow a faster acquisition by the Relaxd readout. More simulations in GEANT4 will give an insight in how the experiment compares with theory especially in cases of sample testing. Research in the Nikhef R&D Group for improving GridPix is ongoing to improve the detector. For constructing a 3D particle tracker for proton CT and Radiography, that is not enough. A combination of detector technology and acquisition system, programming (analysis & simulations) and a close contact 56

63 6.2 Outlook with the latest research in proton CT is needed to successfully implement protons in imaging. 57

64

65 Acknowledgements The making and writing of a research project is a procedure that demands a great deal of work, constant questioning of ones actions and sometimes a bit of luck. The results of such a task are not restricted only in the project itself but bear fruit for future research. While working on my Master thesis, I had the opportunity to enjoy my work in an environment that supplied me with everything I needed. Without the support, help, suggestions, comments I received during the last year this project would be completely dierent. The least thing I can do is spent the following lines to mention the people that I owe my gratitude. I would like to thank prof. Els Koeman for giving me the chance to work on this project. I wish her all the best and hope to see her soon back at Nikhef. The leader of the R&D group Nield van Bakel for supplying me with a GridPix detector in no time when the one I was working with suddenly broke down. Martin van Beuzekom and Bas van der Heijden for spending many hours helping me in the lab with the RelaxD, the trigger and the calorimeter. Matteo and the PhD students of the R&D group, Martin, Francesco, Enrico, Marten and Rolf for their continuous support. A special thanks to Wilco Koppert for sharing many useful ideas and algorithms and his patience for having to put with my endless questions and problems. Last but not least, I would like to thank dr. Jan Visser and prof. Sytze Brandenburg for the time they spent in making corrections, comments and suggestions to help me improve not only this thesis but also myself. The scientic character and sincerity of their remarks will guide my future steps as a physicist. I would like to end this thesis with my wish that more eort will be put on this project so it may prove to be benecial in research on proton radiography and CT. The best of luck to Brent Huisman in improving the 3D particle tracker set-up and the people in the R&D group working on the GridPix detector.

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67 Bibliography [1] U. Schneider, E. Pedroni, Proton Radiography as a tool for quality control in Proton Therapy Med. Phys (1995) [2] K. M. Hanson et al, Computed tomography using proton energy loss Phys. Med. Biol. Vol. 26, Los Alamos Scientic Laboratory (1981) [3] J. Seco, N. Depauw, Proof of principle study of the use of a CMOS active pixel sensor for proton radiography Med. Phys. DOI: / , Francis H. Burr Proton Therapy Center (2010) [4] C. Talamonti et al, Proton radiography for clinical applications Nuclear Instruments and Methods in Physics Research A, Vol. 612 Issue 3 (2010) [5] PSI website, Proton Radiography on the PSI gantry radmed.web.psi.ch [6] M. Regler, M. Benedikt, K. Poljanc, Medical Accelerators for Hadrontherapy with Protons and Carbon Ions CERN Accelerator School - Seville, Spain (2002) [7] Particle Therapy Co-Operative Group website [8] Saverio Braccini, Scientic and Technological Development of Hadron Therapy arxiv: v1 (2010) [9] Donald H. Perkins, Introduction to High Energy Physics Cambridge University Press (1972) [10] Edward L. Alpen, Radiation Biophysics Academic Press (1998) [11] Particle Data Group (2010) [12] J. F. Ziegler, The Stopping of Energetic Light Ions in Elemental Matter Rev. Appl. Phys., 85, (1999) [13] Jan Visser, Particle Detection Lectures (2010) [14] Daniela Schulz-Ertner et al., Results of Carbon Ion Radiotherapy in 152 patients DOI: Elsevier (2004) [15] G. Kraft, Tumortherapy with ion beams Nuclear Instruments and Methods in Physics Research A 454 (2000) [16] Advanced Cancer Therapy center website [17] M. A. Hayat, Cancer Imaging: Instrumentation and Applications Vol. 2 Academic Press (2007) IX

68 [18] A.M. Koehler, Proton Radiography Journal DOI: / science New York (1968) [19] Harry van der Graaf, GridPix: An integrated readout system for gaseous detectors with a pixel chip as anode Nuclear Instruments and Methods in Physics Research A, Vol. 580 Issue 2, Nikhef (2007) [20] F.W.N. de Boer et al, Delft Internal Conversion Experiment 1 st International Conference on Micro Pattern Gaseous Detectors, JINST 4 P11021 (2009) [21] Xavier Llopart, TIMEPIX Manual v1.0 (2006) [22] Wilco Koppert et al, High precision 3D Measurements of Single Electrons with GridPix Detectors Poster, Nikhef (2010) [23] Wilco Koppert, Testbeam Data Analysis Report, Nikhef (2011) [24] W. Blum, W. Riegler, L. Rolandi, Particle Detection with Drift Chambers 2 nd edition, Springer (2008) [25] Sytze Brandenburg, Needs for Proton Radiography and Proton CT Private discussion (2011) [26] M. Lupberger, Ethernet-driven readout system for gaseous detectors, Presentation, University of Bonn (2011) [27] [28] R.Novotny, Performance of the BaF 2 -calorimeter TAPS Nuclear Physics B, Vol. 61 Issue 3, University Giessen (1998) [29] E.V.D. van Loef et al., Scintillation properties of LaBr 3 :Ce 3+ crystals: fast, ecient and high-energy-resolution scintillators, Nuclear Instruments and Methods in Physics Research A, 486: , (2002) [30] P. Schotanus et al, Temperature dependence of BaF 2 scintillation light yield Nuclear Instruments and Methods in Physics Research A, Delft University of Technology (1985) [31] KVI, Groningen AGOR Facility for IRradiations of Materials Brochure [32] Claude Leroy, Pier-Giorgio Rancoita, Principles of Radiation Interaction in Matter and Detection 2 nd edition, World Scientic (2009) [33] Archana Sharma, Properties of some gas mixtures used in tracking detectors SLAC-JOURNAL-ICFA-16-3, Darmstadt (1998) [34] Reint Ostendorf, Properties of the AGOR cyclotron beam Private discussion (2011) [35] Paul Schotanus, Temperature dependence of BaF 2 Private discussion (2011) X

69 Appendix A A.1 Principles of a Time Projection Chamber Drift chambers have in common that the drift of the ionization electrons in the gas is used for a coordinate determination by measurement of the drift time. In a drift chamber, the position of the passing particle is determined by the time dierence between the passage of the particle and the arrival of electrons at the detection element(wire, CMOS detector etc.). Detectors with long drift distances perpendicularly to a multi-anode prportional plane provide three-dimensional information are called time projection chambers (TPCs). Such a TPC is the Micromegas detector which has an operational principle similar to the operational principle of the GridPix based TPC used in this project. In a Micromegas detector a thin metal grid is placed on top of a CMOS pixel detector and is enclosed by a drift chamber. When a charged particle traverses the detector, it ionizes the gas enclosed in the drift chamber (Fig. A.1). The drift chamber consists of a eld cage with a cathode placed on top. By applying a high voltage in the cathode, an electric eld between the cathode and the grid is induced. Thus, electrons produced in the ionization process drift towards the grid. By applying a high voltage in the grid, when an electron passes through a hole of the grid an avalanche is formed which induces a signal in a pixel of the CMOS detector. Figure A.1: Operation of a Micromegas detector. XI

70 (a) Field cage on top of a GridPix based TPC. The electrodes are pointed by the arrow. (b) Electric eld con guration near the electrodes of a eld cage. Figure A.2: Field cage. A.2 Field Cage The electric eld in the drift gap has to be as uniform as possible and ideally similar to that of an in nitely large parallel-plate capasitor. A very good approximation can be obtained covering the inner surface of the eld cage with a regular set of conducting strips (electrodes) perpendicular to the electric eld, with a constant potential di erence V between two adjacent strips V = E, where E the electric eld and the pitch of the electrode system. In Fig. A.2(a) these electrodes can be seen as thins strips surrounding the eld cage. Fig. A.2(b) shows the electric eld lines (full lines) and the equipotentials (broken lines) near the strips in a drift space D. The electric eld very near the electrodes is not uniform. The transverse component decays as exp( 2πt/ ) where t is the distance from the eld cage. XII

71 Appendix B B.1 Binary resolution Figure B.1: Strip pitch p and detector resolution. The relation between the strip pitch p and the detector resolution (Fig. B.1) for binary read-out follows: RMS = 1 x 2 x 1 and by setting x=0, x 1 = p/2, x 2 = +p/2 x2 x 1 (x x) 2 dx (B.1) RMS = 1 3p ( 1 p 3 = p = +p/2 p/2 24 p3 24 p 2 12 = p 12 ) (B.2) (B.3) (B.4) XIII

72

73 Appendix C C.1 Trigger Figure C.1: Schematic for the trigger used in the experiment at the KVI. XV

74

75 Appendix D D.1 Measurements with Cosmic Rays Figure D.1: The cosmic ray set-up. The GridPix based TPC was tested in the lab before the irradiation at the KVI. The cosmic ray set-up of Fig. D.1 was assembled. The set-up consists of two scintillators forming a coincidence and the GridPix based TPC. The scintillators have been placed such that their eective area covers the YZ-plane of the GridPix. When a particle traverses both scintillators, their coincidence triggers the GridPix and the shutter opens. Using the 3D Display, cosmic rays can be visualised. In Fig. D.2 two tracks are shown in the two dierent congurations of the set-up. (a) YZ-plane perpendicular to cosmics. (b) YZ-plane parallel to cosmics. Figure D.2: 3D visualization of cosmic rays in two dierent congurations of the cosmic ray set-up. XVII