Photon Instrumentation. First Mexican Particle Accelerator School Guanajuato Oct 6, 2011

Transcription

1 Photon Instrumentation First Mexican Particle Accelerator School Guanajuato Oct 6, 2011

2 Outline The Electromagnetic Spectrum Photon Detection Interaction of Photons with Matter Photoelectric Effect Compton Scattering Pair production Instrumentation Solid State Devices Gas Filled Devices Scintillation Counters Charge Coupled Devices

3 Outline Beam Line Measurements Beam Position Beam Profile Beam Intensity Summary

4 The Electromagnetic Spectrum Another way of looking at the spectrum

5 The Electromagnetic Spectrum Velocity of light (c) = Wavelength ( ) x Frequency( ) Photon Energy E = h h is Planck s constant ( 4.136*10 15 ev s) Wavelength ( ) nm Frequency ( ) THz Energy (ev) infrared ~ ~ ~ *10-3 Visible ~ ~ ~ Ultraviolet ~ ~3* ~ X-rays ~ ~3*10 7 3*10 4 ~124* ϒ-rays < 0.01 >3*10 7 >124*10 3

6 Light Sources Light sources provide photon beams over a wide spectrum The wide variety of instrumentation does not yield to even a cursory description We will limit ourselves to a small set of techniques and devices

7 Detection requires that Photon Detection the photons interact with the material in the detector the interaction generates some identifiable signal Example: Light detection by your eye The eye can do more than just detection, it can distinguish colors and also intensity Limitations of eye as a detector (among other things): Range, which is the visible spectrum

8 Photon Detection (Hamamatsu Photomultiplier Basics and Applications)

9 Photon Detection A detector s ability is related to the photon wavelength To state it another way, a detector s response is dependent on the photon energy It is not sufficient for us to detect photons We want to measure with some desired precision beam properties because experiments depend on knowing the characteristics of the photon beam The interactions of the beam with targets

10 Interactions of Photons with Matter We will consider the following mechanisms by which photons lose energy Photoelectric effect Compton Scattering Pair production

11 Photoelectric Effect Kinetic Energy of the electron = h W, where W is the binding energy of the electron Photoelectric effect is the dominant process at low photon energies with high Z materials Probability of photoelectric interaction Z n /(h ) 3, n is between 3 and 4 (picture credit:

12 Compton Scattering Energy transfer from photon to electron increases with photon energy. Probability of Compton scattering is approximately proportional to Z (picture credit

13 Pair Production Nucleus e + Photon e - Pair production probability increase when the incident energy is greater than 2*electron mass and approximately as Z 2

14 Photon Interactions with matter In the energy regime below 1 MeV, the dominant processes are photoelectric effect and Compton scattering

15 Instrumentation The information on energy dependent photon interactions with matter guides the choice of detectors Solid state devices can work at very low photon energies (<10 ev) Gas filled detectors are suitable when photon energies are around 30 ev Scintillation detectors cover a large range from around 10 ev to very high energies

16 Solid State Devices The energy deposited by photons solid state devices creates electron/hole pairs The electrons move from valence bond to conduction band The migration of electrons creates holes in the valence bond The number of electron/hole pairs is proportional to the energy deposited Application of an electric field generates a pulse

17 Solid State Devices Hamamatsu

18 Solid State Devices Signal to Electronics Current The contribution of statistics to the energy resolution is given by ΔE/E = 2.35 (Fε/E) 1/2 F is called the Fano Factor, E is the photon energy in ev and ε is the energy needed to create an electron/hole pair

19 Gas Filled Devices Ground While in the semiconductor devices, electron/hole pairs are created by radiation, in the gas counters electron/ion pairs are created. The anode is kept at a positive potential and the walls are at ground. As the electrons drift towards the cathode, an avalanche can form and the signal is collected from the anode

20 Pulse height is given by Gas Filled Devices A*N*e/C, where N is the number of electrons, e is the electron charge, C is the capacitance of the device and A is the amplification factor Depending on the applied voltage, gas filled counters can work as ionization counters, proportional or Geiger-Mueller counters Similar to solid state detectors, energy resolution is given by (note that F will have a different value) ΔE/E = 2.35 (Fε/E) 1/2

21 Scintillation Counters Scintillation counters are widely used in Nuclear and Particle physics A basic scintillation counter consists of a scintillator optically coupled to a photomultiplier tube Large selections of the scintillating material and photomultiplier tubes are available for applications Some uses of scintillation counters can be used are Particle Counting Measuring Particle Energy Triggering Time of flight

22 Scintillation Counters Common scintillating materials are inorganic crystals, plastics liquids. Many specialty materials such as lead tungstate are available for specific applications Scintillation Mechanism Photomultiplier Tubes Electron Multiplication Alkali Photocathode (Most of the material on scintillation counters is taken from Hamamatsu Photomultiplier Handbook)

23 Scintillation Counters I k is the cathode current and B is the bandwidth of the measurement system

24 Fano Factor Fano factor is an adjustment factor introduced to relate the variance of the observed distribution to the mean of the distribution If <N> is the average number of electron/ion or electron/hole pairs due to ionization, the fluctuation in the ionization is given by σ 2 <N> = F <N> Where F is the Fano factor

25 Fano Factor

26 Energy Resolution: Some Detectors

27 (Imaging) Charge Coupled Devices CCDs are based on Metal Oxide Semiconductor (MOS) capacitors Charge stored on one area of the CCD can be transferred to another area The area where the charge is stored is called a potential well Referring to the figure when a voltage is applied to the gate electrode P2, (with P1 and P3 at zero volts), a potential well is created

28 Charge Coupled Devices By adjusting the voltages in a time sequence on the gate electrodes, the charge can be sequentially transferred, somewhat like a shift register Groups of electrodes form a pixel In the figure the three electrodes form a pixel Thus, charge is created by photoelectric effect, the charge is transferred sequentially by applying differential voltage at some frequency and the charge is converted to a voltage CCDs have very high quantum efficiency (~80%) Require cooling to reduce noise

29 Beam Line Measurements We consider four types of measurements in the beam line Beam Position Beam Profile Beam Intensity

30 Beam Position The following table gives examples of processes used for measuring beam position (S. Hustache-Ottini,, Proceedings of CERN Accelerator School)

31 Beam Position Blade type Beam position monitors Tungsten Blades (H. Aoyagi, T. Kudo, H. Kitamura, Nuclear Instruments and Methods) (S. Hustache-Ottini,, Proceedings of CERN Accelerator School)

32 Beam Position The x and y positions are given by x [(I ur + I dr ) - (I ul + I dl )]/ [(I ur + I dr ) + (I ul + I dl )] y [(I ur + I ul ) - (I dr + I dl )]/ [(I ur + I ul ) + (I dr + I dl )] Where I is the current and u,d,r & L represent up, down, Right and Left (I ur represents current in the upper right blade) Current is converted to Voltage using an I to V converter and the voltage is digitized by an ADC. The proportionality constants which have to be determined through calibration

33 Beam Position A harp (wire scanner) works on the same principle of generating current in the wire due to photon interaction with the electron in the material of the wire Four quadrant photodiodes provide another way to measure beam position (Hamamatsu)

34 Beam Profile Beam profile may be measured by imaging the beam, i.e. converting the beam into a visible image using a fluorescent screen The light from the screen can be viewed by a CCD camera using suitable optical elements The image can then be processed using a commercial or custom hardware/software systems CCD camera and Optical elements Screen

35 Beam Profile Of critical importance in deciding the choice of a fluorescent screen is the beam power density. The thermal characteristics of the screen should allow power dissipation in the form of heat without damaging the screen

36 Beam Intensity (S. Hustache-Ottini,, Proceedings of CERN Accelerator School)

37 Beam Intensity A thin material (suited to the energy of the photon beam) is inserted in the beam path The scattered or fluorescent photons are detected by, for example, a scintillation counter The beam intensity is proportional to the number of detected photons Counter Scatterer

38 Beam Intensity Responsivity is a measure of the detector s sensitivity to radiant energy It is the ratio of number of electrons generated per incident photon to photon energy. Its units are amps/watt R (amps/watt) = Y/h = exp(-μt)/w Y is the quantum yield h is the photon energy μ is the thickness of the surface oxide layer of the photodiode t is the thickness of the oxide layer

39 Beam Intensity W is the electron/hole pair creation energy This could be rewritten as i = e*n*a*h /W i is the current in the photodiode e is the electron charge N is the number of photon/s A is the fraction of x-rays absorbed by the diode Beam intensity can be obtained by measuring i (E. M.Gullickson, R. Korde, L. R. Canfield and R. E. Vest Journal of Electron Spectroscopy and Related Phenomenon. Vol. 80, ,1996) )X. Zhang, H. Fujimoto and A. Waseda IOP Conference series, Materials and Engineering, Vol. 24, 2011)

40 Beam Intensity Responsivity and transmissivity of a 5 μm thick photodiode made by IRD

41 Summary The field of beam diagnostic instrumentation is huge Physicists and engineers have a vast array of materials, detectors and data acquisition systems from which to choose when designing a detector system The references given at the end are a good starting point for in-depth knowledge and understanding of detection systems

42 References e4.pdf plete.pdf pdf Review of Particle Properties, Particle Data Group, Journal of Physics G, Vol. 37,No. 74, July

43 References S. Hustache-Ottini,, Proceedings of CERN Accelerator School on Beam Diagnostics, Dourdan, France, May 28-6June 2008 H. Aoyagi, T. Kudo, H. Kitamura, Nuclear Instruments and Methods in Physics Research, Section A. Vol July 2001, Signal.pdf E. M.Gullickson, R. Korde, L. R. Canfield and R. E. Vest Journal of Electron Spectroscopy and Related Phenomenon. Vol. 80, ,1996 X. Zhang, H. Fujimoto and A. Waseda IOP Conference series, Materials and Engineering, Vol. 24,