DESIGN OF THE INTERLOCK SYSTEM OF A TEST-BENCH FOR SILICON DETECTORS

Transcription

1 Università degli Studi di Ferrara FACOLTÀ DI SCIENZE MATEMATICHE, FISICHE E NATURALI Dipartimento di Fisica DESIGN OF THE INTERLOCK SYSTEM OF A TEST-BENCH FOR SILICON DETECTORS Tesi di Laurea Specialistica in Fisica Nucleare e Subnucleare Relatore: Dott. Paolo Lenisa Laureanda: Greta Guidoboni Correlatore: Dott. Sergey Michirtytchiants Sessione estiva Anno Accademico

2 a papá e alla Mary alla mia tatona alla zia Elena e alla nonna Margherita al mio nipotino G.W.

3 Contents Introduction III 1 The PAX proposal Why polarized antiprotons? Transversity distribution Nuclear form factors Accelerator setup for FAIR How to polarize antiprotons: a short history Depolarization measurement at COSY COSY ring Proton beam polarization Depolarizing cross section Spin-Filtering experiments at COSY and AD Spin-Filtering experiment at AD The AD ring Proposed measurement AD machine properties Experimental setup Polarized Internal Target Storage cell Target Gas Analyzer and Breit-Rabi Polarimeter.. 25 I

4 II CONTENTS 2.5 Detector system Detector configuration Detector performance: acceptance and event rate estimate Interlock system The STT interlock STT equipment States and normal operations Anomalous events and STT interlock actions Signals and logical structure Future development: Software Interlock Monitoring The vacuum interlock system Vacuum equipment States and normal operations Normal operations Venting procedures Starting procedure for the vacuum interlock Anomalous events and vacuum interlock actions Signals Logical functions Conclusions 73 Bibliography 75

5 Introduction The present thesis work has been developed inside the PAX (Polarized Antiproton experiment) experiment and prepared in collaboration with the Institut für Kernphysik of the Forschungszentrum at Jülich (Germany). The PAX collaboration aims to study double-polarized antiproton-proton interactions at HESR (High Energy Storage Ring), inside the new GSI facility, FAIR (Facility for Antiproton and Ion Research), in Darmstad (Germany). The study of polarized antirpoton-proton interaction will allow the access to some fundamental physics observables as the transversity distibution and the proton form factors, and the study of hadron spectroscopy. The method proposed for the production of polarized antiproton beams is the spin-filtering of a stored beam in a storage ring. Because of the spin dependence of the strong interaction, antiprotons in different spin states will have different cross sections in the interaction with a polarized proton target. After several interactions of an unpolarized stored antiproton beam circulating in the ring with a polarized hydrogen internal target, a polarized antiproton beam is obtained. In order to gain a complete understanding of the spin-filtering method, experiments with protons are planned at COSY (COoler SYnchrotron, Jülich) for next year. As the experimental basis for predicting the polarization buildup for a stored antiproton beam is pratically III

6 IV Introduction non-existent, it will be necessary for the PAX collaboration to perform experiments at AD (Antiproton Decelerator, CERN) to start with the firstever antiproton polarization buildup studies. The experimental setup required for spin-filtering experiments is composed by: a Polarize Internal Target (PIT); a low-β section; a detector system. The PIT consists in an ABS (Atomic Beam Source) to produce the polarized gas target and a storage cell in the target chamber. The polarization of the target gas is measured with the Breit-Rabi Polarimeter (BRP). The low-β section is necessary to pass the antiproton (or proton) beam through the storage cell. The detection system of the PAX experiment will be realized by three series of four telescopes, each one composed of three silicon detectors, in a diamond configuration around the target cell. In order to guarantee safe working conditions for the detection system an interlock system is needed. The aim of my thesis is the design of this system. It will be used first in the test-bench to calibrate the detectors, then for the experiments at COSY, AD and HESR. This interlock has to be composed of two parts: the STT and vacuum interlock, because only in the test-bench an overall control of pumping devices, gauges and valves is needed, while a vacuum interlock system is already existing in these accelerators. In particular I took care about the design of the logic for the vacuum interlock system of the test-bench, cooling and power supply interlock system

7 Introduction V of the detectors. First I defined safe working conditions and the standard operations to use the vacuum and the detection systems; then, I predicted problems and dangerous events that could happen and the respective reactions. Finally I defined the logical functions in order to implement the logic control. The vacuum interlock system has been completed while for the STT interlock a further work is needed. The thesis is devided in five chapters: Chapter 1: the possible measurements with polarized antiprotons and the experimental plans for the production of the first intense beam of polarized antiprotons are presented. Chapter 2: the AD experimental apparatus used for the spin-filtering experiments with antiprotons is described with particular attention to the detection system. In this chapter the work of my thesis is introduced. Chapter 3: the STT interlock system is described, from the devices to the structure of the logic control that will be used. Chapter 4: the vacuum interlock system is described, presenting the vacuum system apparatus, signals and the logical functions that have been implemented. Chapter 5: a summary is presented, describing what has been completed and which will be the future developments.

8

9 Chapter 1 The PAX proposal The PAX collaboration (Polarized Antiprotons experiment) aims to study double-polarized antiproton-proton interactions at HESR (High Energy Storage Ring), inside the new GSI facility, FAIR 1 (Facility for Antiproton and Ion Research), in Darmstadt (see Figure 1.1). Figure 1.1: The Facility FAIR in Darmstadt. 1 The completation is scheduled for

10 2 1. The PAX proposal 1.1 Why polarized antiprotons? The answer to this question is inside the importance of the spin. Spin discovery permitted to understand structure and interaction potentials of particles. The most relevant experiments on particle Physics have shown a strong dipendence from development of polarized particles sources and beams of high quality but unfortunately, there is still no any polarized antiprotons source or beam. At the moment, the only way to buildup polarized antiprotons beams seems to be the spin-filtering method (see Section 1.3) but it needs to be confirmed. The experimental basis for predicting the polarization buildup in a stored antiproton beam is pratically non-existent. The confirmation of the polarization buildup of antiprotons would pave the way to high luminosity double-polarized antiproton-proton colliders, which would provide the oppurtunity to study transverse spin physics in the hard QCD regime. Such a collider has been proposed by the PAX collaboration for the new facility FAIR, aiming at high luminosity of cm 2 s 1. The polarized antiproton-proton interactions at HESR will allow a unique access to a number of new fundamental physics observables described in the following sections Transversity distribution The Transversity distribution h 1 q (x, Q2 ) describes the quark transverse polarization inside a transversely polarized proton. With the quark momentum distribution q(x, Q 2 ) and the helicity distribution 2 q(x, Q 2 )), it is the last leading-twist missing piece of the QCD (Quantum Cromo Dynamics) description of the partonic structure of the nucleon. Using Deep Inelastic scattering (DIS), experimental and theoretical studies let to improve the knowledge of q(x, Q 2 ) and q(x, Q 2 ): the last peace to complete the description of the partonic structure of the nucleon is the transversity. The transversity cannot be reconstructed from the knowledge of q(x, Q 2 ) and q(x, Q 2 ) and it cannot be accessed in DIS of leptons off nucleons because it is a chiral-odd function. Electroweak and strong interactions conserve chirality, so in DIS events, h 1 q (x, Q2 ) is invisible because it cannot 2 The helicity distribution describes the quark longitudinal polarization inside a longitudinally polarized proton.

11 1.1 Why polarized antiprotons? 3 occur alone but has to be combined to a second chiral-odd quantity. Thus it is necessary to use one of the following reactions: Semi-Inclusive Deep Inelastic Scattering (SIDIS), where h 1 q (x, Q2 ) is coupled to a chiral-odd function called Collins function ; transverse Single Spin Asymmetries (SSA) in inclusive process like p p πx, where h 1 q(x, Q 2 ) is coupled to the Collins function; polarized Drell-Yan processes, where the product of two transversity functions is present. Recently Anselmino and collaborators have extracted h 1 q (x, Q2 ) with a simultaneous analysis of DIS experiments (HERMES and COMPASS) and the annihilation experiment BELLE [1]. The value obtained for h 1 q (x, Q2 ) was only an indirect measurement of the transversity, based on many phenomenological assumptions as the indipendence and universality of the structure functions. A direct measurement of the transversity can be done with Drell-Yan events, produced in the annihilation of protons and antiprotons, both polarized (for more details see [2]) p p e + e X. Measuring the double transverse spin asymmetry A TT, a direct measurement of transversity can be obtained. It should be noted that it is necessary to use a p beam because a proton beam would give a much lower useful event-rate. The proton beam would probe the see quarks, instead of the valence quarks, therefore leading to a much lower cross section Nuclear form factors Reproducing the structure of the proton is one of the defining problems of QCD. The electromagnetic structure can be expressed in terms of the electric and magnetic form factors, G E and G M, which depend only on the 4-momentum transfer squared, q 2. In the spacelike region (q 2 < 0, scattering process) they are analitical real functions, while in the timelike region (q 2 > 0, annihilation or reversed channel) they are complex functions. The form factors are related to the spatial distributions of the charge (G E ) and magnetization (G M ) in the proton, and in the non-relativistic limit are simply the Fourier transformations of these distributions.

12 4 1. The PAX proposal Traditionally, spacelike form factors have been experimentally accessed by making use of the so-called Rosenbluth separation for ep elastic scattering. This tecnique is based on the assumption that interaction between electron and proton occurs via a single-photon exchange (Born approximation). Measuring the corss section in ep elastic scattering at constant momentum transfer, it is possible to deduce G E and G M from the Rosenbluth formula: dσ = σ [ Mott τg 2 dω e ε(1 + τ) M (q 2 ) + εg 2 E(q 2 ) ] (1.1) where τ = q 2 /4Mp 2, ε is the longitudinal polarization of the exchange virtual photon, ε 1 = 1 + 2(1 + τ)tan 2 (θ e /2), M p is the proton mass and θ e is the electron scattering angle. The Rosenbluth separation is performed by varying the incident electron energy and electron scattering angle to keep q 2 constant while varying ε. Rosenbluth separations of G E and G M have been reported from 1960 to the present day (see refs.[3],[4]). Fits to these data yield µ p G E /G M 1, implying similar charge and magnetization distributions. At large q 2 values, G M dominates the cross section at all ε values (contributing more than 90% for q 2 > 4 GeV 2 /c 2 ), and thus while a Rosenbluth separation can yield a precise extraction of G M, the uncertainty in G E increases with increasing q 2. The high q 2 behavior of the electric form factor can be more precisely determined in polarization transfer experiments, where longitudinally polarized electrons are scattered from unpolarized protons and both transverse and longitudinal polarization are transferred to the struck proton G E G M = P t P l E e + E e 2M p tan 2 ( ) θe. 2 During the last few years, polarization transfer experiments have been performed at Jefferson Lab with measured G E /G M up tp q 2 = 5.6 GeV 2 /c 2. These measurements show that the ratio µ p G E /G M ( 1 q 2), decreases with the incresing q 2 in strong contrast to the approximate scaling observed in the Rosenbluth measurements.

13 1.1 Why polarized antiprotons? 5 The discrepancy between the proton form factor measurements from the Rosenbluth separation technique and from polarization transfer experiments has led to different possible explanation. Most approches aim to explain the discrepancy in terms of the contribution from the two-photon exchange (TPE) diagrams. Because of the sensitivity of the Rosenbluth separation technique to correction depending on TPE, several experiments in the spacelike region are planned in order to isolate and measure the TPE contribute. From the theoretical point of view, perturbative QCD and analicity relate timelike and spacelike form factors, predicting a continous transition and spacelike-timelike equality at high q 2. In order to verify it, experiments in both regions are needed but most present experiments aim at investigatig in the spacelike region. In the timelike region, the mo-duli of G E and G M can be extracted from the measurements of the cross sections from e + e p p. Due to the low value of the cross sections and the consequent statistic, experiments performed till now could not determine G E and G M separately from the analysis of the angular distributions, but extracted G M using the (arbitrary) assumption G E = G M. The magnetic form factor has been derived in this way by e + e and p p experiments while the timelike electric form factor is basically unknown. The future PANDA and PAX experiments are planning to measure the form factors in the timelike region in two different ways: PANDA will measure G E and G M in the interaction p p e + e with the Rosenbluth separation technique. PAX will measure G E, G M and the relative phase between the two form factors using the spin correlation observables in the interaction p p ( ) e + e. The single-spin and the double-spin asimmetries will allow the first measurement of the complex phase and will provide a test of the different theoretical models and of the Rosenbluth separation in the time-like region. Other measurements. The elastic process of hard scattering p p, both polarized, is an interesting field and the possibility to control the polarization of the initial state will allow hadronic spectroscopy analysis.

14 6 1. The PAX proposal 1.2 Accelerator setup for FAIR The PAX collaboration submitted a spin Physics experiment program with polarized antiproton beams of high intensity at FAIR [5]. The goal of achieving the highest possible polarization of antiprotons and optimization of the Figure of Merit (FoM) dictates that one polarizes antiprotons in a dedicated low-energy ring (APR). The transfer of polarized low-energy antiprotons into the HESR ring requires pre-acceleration to about 1.5 GeV/c in a dedicated booster ring (CSR). Simultaneously, the incorporation of this booster ring into the HESR complex opens up the possibility of building an asymmetric antiproton-proton collider, in which polarized protons with momenta of about 3.5 GeV/c collide with polarized antiprotons with momenta up to 15 GeV/c. The main features of the accelerator (see Figure 1.2) are: APR (Antiproton Polariser Ring), built inside the HESR area with the crucial task of polarising antiprotons at kinetic energies around 50 MeV to be accelerated and injected into the other rings. A second Cooler Synchrotron Ring (CSR) in which protons and antiprotons can be stored with a momentum up to 3.5 GeV/c. This ring shall have a straight section, where a PAX detector could be installed, running parallel to the experimental straight section of HESR. By deflection of the HESR beam into the straight section of the CSR, both the collider or the fixed-target mode become feasible. The predicted luminosity of HESR is about cm 2 s 1. Figure 1.2: Setup of the accelerator HESR at FAIR.

15 1.3 How to polarize antiprotons: a short history How to polarize antiprotons: a short history When some 25 years ago, intense antiproton beams became a new tool in nuclear and particle physics, there was an immediate demand to polarize these antiprotons. At the time, there was no shortage of rough ideas on how this might be accomplished but, up to now, the only polarized antiprotons available for use were in a secondary beam facility, which made use of the decay of hyperons, and which was operating at Fermilab in the 1990s. Since the first workshop in 1985 at Bodega Bay (California, USA), organized by O.Chamberlain and A. Krisch [6], about 12 methods to polarize p s stored in a ring were identified and discussed. Conventional methods like ABS (Atomic Beam Source), appropriate for the production of polarized protons and heavy ions couldn t be used since antiprotons annichilate with matter. Polarized antiprotons have been produced from the decay in flight of Λ at Fermilab. The intensities achieved with antiproton polarizations P > 0.35 never exceed s 1 [7]. Scattering of antiprotons off a liquid hydrogen target could yield polarizations of P 0.2, with beam intesities of up to s 1 [8]. Polarization by Channeling through magnetized ferromagnetic foils was also discussed. Recently, the idea of Channeling by bent crystal as polarizer received some attention [9]. Unfortunately, all the above mentioned approaches did not allow efficient accumulation in a storage ring, which would have greatly enhanced the luminosity. Only two ideas were studied in depth afterwards: The Spin-Splitting based on repeated Stern-Gerlach kicks resulting in a spatial separation of two antiproton spin states. It has been discussed theoretically in a number of papers (e.g. [10]) and workshop contributions, but not been demonstrated experimentally yet. The Spin-Filtering based on the nuclear cross section spin dependence. Already in 1968 Csonka [11] suggested to use the Spin-Filtering method for the ISR (Intersecting Storage Ring) at CERN. The nuclear cross section spin dependence can be used in nucleon-nucleon reaction, as p p and p p. Because of the spin dependence of the strong interactions, antiprotons (protons) in one spin state will have a bigger scattering angle than antiprotons (protons) in the opposite spin state. The strong interaction works like a filter (see Figure 1.3).

16 8 1. The PAX proposal After several interactions of an unpolarized antiproton (proton) beam 3 with a polarized atomic hydrogen gas target, a polarized antiproton (proton) beam is obtained, even if the intensity of the final beam is much lower than the initial one. A convincing experimental prove of this method was given by FILTEX experiment at TSR (Test Storage Ring), Heidelberg, in Figure 1.3: How the Spin-Filtering method works. A detailed description of the FILTEX experiment can be found in [12], in which a theoretical calculation of the buildup polarization is based only on the strong interaction proton-proton. The result of this experiment is reported in Figure 1.4. The problem was to understand the discrepancy of the 50% between experimental and theoretical data. In 1994 Horowitz and Meyer [13] interpreted the mechanism of the spin filtering as due to three contributes: the strong interaction, the interference of electromagnetic and strong amplitudes and the interaction between proton beam and target polarized electrons. During the workshop in 2007 at Daresbury (UK), the attention was focused on the processes involved in the polarization bildup through filtering but there were two contradicting explanations of the FILTEX result from two different groups: 1. in 2005, new theoretical activities by a Novosibirsk group (Milstein and Strakovenco) [14] and a Jülich group (Nikolaev and Pavlov) [15] shed some doubts on the original Horowitz-Meyer approach. Both 3 Unpolarized antiproton (proton) beam means that the two polarization spin states are equally populated.

17 1.3 How to polarize antiprotons: a short history 9 Figure 1.4: FILTEX measurement: the beam polarization as a function of the time. The two curves represent two different polarization states of the target and demonstrate that if the target polarization is inverted, then the induced polarization on the beam is inverted too. groups concluded that the buildup is entirely due to spin-dependent removal of beam particles by scattering out of the acceptance. 2. in 2007, a new method was proposed by T.Walker, H. Arenhövel et al. (Mainz) [16] with the promising feature of being virtually loss-free. The idea was to use spin transfer by free electrons to co-moving ions at a very low relative velocity (Spin-Flip). The proposal was based on numerical calculations of this process by Arenhövel indicating that for attractive system pe or pe + the corresponding cross section may be as large as barn = cm 2, resulting in very short buildup times. At the 409 th Wilhelm und Elise Heraeus Seminar (Bad Honnef, Germany) in 2008, the aim was to clarify the interpretation of the Spin- Filtering method. During this meeting, the first results from the depolarization experiment (see Section 1.4) at COSY (Cooler Synchrotron, Jülich) were presented and they clearly indicated no significant depolarization of the stored protons by the electrons of the cooler, which were periodically detuned to right velocity difference predicted by Mainz calculations. The sensitivity was such that a possible spin transfer cross section should be very small compared to the predicted barn. A more advance status of the analysis has been presented at Spin 2008 by F. Rathmann [17]. The con-

18 10 1. The PAX proposal clusion has not been changed, confirming that the result of the Novosibirsk- Jülich group was correct. The result has important implications: The possibility to use an electromagnetic and thus calculable process as polarizer does not exist. Spin-Filtering based on the spin dependence of the -partly unknown- pp interaction seems to be the only way. This make pp filtering test at COSY, and in particular pp filtering test at AD (Antiproton Decelerator, CERN) an essential mile stone on the way to polarize antiprotons. 1.4 Depolarization measurement at COSY The experimental task was to distinguish between the two theoretical scenario that described equally well the result of the spin filtering of a stored proton beam in the FILTEX experiment. The first explanation by Meyer included a transfer polarization from polarized electron in the polarized Hydrogen gas target to the orbiting protons, while the theoretical approch by Milstein/Strakhovenko and Nikolaev/Pavlov was based only on proton-proton scattering. In order to investigate the role of electrons and to distinguish between the two explanations, an approach of depolarizing an initially polarized beam with unpolarized electron of the electron cooling has been proposed. The goal of the measurement is to determine the spin-exchange cross section COSY ring COSY is a so colled COoler SYnchrotron that provides unpolarized and transversely polarized proton and deuteron beams in a momentum range fro 300 MeV/c to 3.7 GeV/c. Electron cooling is available up to momentum of 600 MeV/c while for high momentum beams in the range from 1.5 GeV/c to 3.7 GeV/c stochastic cooling is possible. The Figure 1.5 shows COSY and the injector JULIC together with some of internal and external experiments. The polarized or unpolarized beam particles originating from particle source underneath the cyclotron are injected into the cyclotron where

19 1.4 Depolarization measurement at COSY 11 they are accelerated up to the COSY injection energy of 45 MeV. From there the particles are transported into COSY. For an unpolarized beam, the maximum number of particles in the ring is in the order of The polarized beam have roughly 15 times less intensity for protons ans 20 times less particle for deuterons Proton beam polarization In this experiment, two are the fundamental components: electron cooler, as unpolarized electron target; beam polarimeter, composed by an internal polarized target (cluster target of D 2 ) and a detector system for the pruducts from elastic p d reaction. The proton beam polarization is measured through the right/left asimmetry (proportional to the analyzing power A y ) of the deuteron in the final state of the elastic scattering p d. The detectros has to be mounted in the region where the Factor of Merit (FoM) of this reaction has its maximum. The relation used to determine the FoM is: FoM = dσ dω(θ cm ) A y(θ) 2 From the experimental data corresponding to proton kinetic energy T p = 46.3 MeV, the maximum value of the FoM is θ p lab = 110 for polar angle in the center of mass frame, which corresponds to θ p lab = 80. The final setup of the polarimeter consisting of two telescope, each of them containing two 300 µm thick layers od silicon, is depicted in Figure 1.6. The energy deposit in first detector layers is plotted as a function of the energy deposit in the second layer in Figure The upper band corresponds to deuterons and the lower on to protons from p d elastic scattering Depolarizing cross section The depolarization cross section is determined from the ratio of the measured beam polarizations P detuned and P tuned. These polarizations correspond to well defined changes of the electron velocity with respect to the protons, which were achieved by detuning the accelerating voltage in the electron cooler by a specific amount.

20 12 1. The PAX proposal Figure 1.5: COSY and the injector JULIC together with some of internal and external experiments.

21 1.4 Depolarization measurement at COSY 13 Figure 1.6: Using events from elastic pd scattering, two silicon detector telescope mounted near the deuterium beam (D 2 ) of the ANKE cluster jet target measure the change of the proton ( p) beam polarization induced by the depolarizing e p spin flip in COSY electron cooler. Figure 1.7: Identification of the p d elastic scattering in the detector system.

22 14 1. The PAX proposal The depolarization cross section is plotted in Figure 1.8 as a function of the magnitude of the electron velocity in the proton rest frame. The measurement shows that the predictions by Mainz group were too large by at least six orders of magnitude. It should be noted that recently, the Mainz group has submitted two errata to their theoretical estimates were too large by about 15 orders of magnitude. Figure 1.8: The depolarizing cross section is plotted as a function of the magnitude of the electron velocity in proton rest frame, indicating an upper limit of a few 10 7 b. Prior to the experiment, the two theoretical estimates of this cross section differed by about 16 orders of magnitudes at a relative velocity of v/c = The experiment ruled out the higher estimate. 1.5 Spin-Filtering experiments at COSY and AD In order to get a complete understanding of the spin-filtering process and commission the setup for the experiments at AD, a set of polarization buildup measurements is forseen at COSY [19], using transverse and longitudinal polarized stored protons. At COSY, the build-up process itself can be studied in detail, because in this situation the spin dependence of the pp interaction is completely known. The polarized internal target required for these investigations was previously used at HERMES experiment at HERA/DESY. Including the

23 1.5 Spin-Filtering experiments at COSY and AD 15 target polarimeter, it has meanwhile been relocated to Jülich. At the same time,a large-acceptance detector system for the beam polarization is being developed. In contrast to the pp system, the experimental basis for predicting the polarization buildup in a stored antiproton beam by spin-filtering is practically non-existent. Therefore, it is of highest priority to perform, subsequently to the COSY experiments, a series of dedicated spin-filtering experiments using stored antiprotons. The AD ring at CERN is a unique facility at which stored antiprotons in the appropriate energy range are available with characteristics that meet the requirements for the first-ever antiproton polarization buildup studies. As already mentioned, the equipment required for the dedicated spin-filtering experiments at AD, must be extensively commissioned and tested at COSY, prior to the experimental investigations at CERN. Recently, a technical proposal for Measurement of the Spin-Dependence of the pbar-p interaction at the AD-ring [20] has been submitted to the SPS committee of CERN.

24 Chapter 2 Spin-Filtering experiment at AD 2.1 The AD ring The Antiproton Decelerator (AD) is the only facility of its kind anywhere in the world. It is sort of an antimatter factory, uniquely capable of trapping antiprotons and slowing them. At the end of the 70 s CERN built an antiproton source called the Antiproton Accumulator (AA). Its task was to produce and accumulate high energy antiprotons to feed into the Proton Syncrothron (PS) in order to transform it into a proton-antiproton collider (see figure 2.1). As soon as antiprotons became available, physicists realized how much cuold be learned by using them at low energy, so CERN decide to build a new machine: LEAR, the Low Energy Antiproton Ring. Antiprotons accumulated in the AA were extracted, decelerated in the PS and then injected into the LEAR for further deceleration. In 1986 a second ring, the Antiproton Collector (AC) was built in order to improve the antiproton production rate by a factor 10. In 1996 CERN s antiproton machines (AA, AC and LEAR) were closed down in order to free resource for LHC. However, a strong comunity of users wished to continue their LEAR experiments with very slow antiprotons. The PS division was asked to investigate if a low-cost way could be found to provide the necessary low-energy beam. A study was made during 1996 and a solution was found resulting in a design report (ref. [21]): the AD project was approved in 7 February 1997 and the new facility became operational 16

25 2.2 Proposed measurement 17 Figure 2.1: The original facility at CERN before the construction of the AD. in July 2000 (see Figure 2.2). There are three experiments underway in the AD, and each of them will trap antimatter and study it. Two of the experiments, ATHENA and ATRAP, will compare properties of hydrogen and antihydrogen. ASACUSA will create atomcules, hybrid atoms containing a helium nucleus and orbited by an electron and an antiproton. 2.2 Proposed measurement At this time, spin-filtering is the only known method that stands a risonable chance of succeding in the production of a stored beam of polarized antiprotons. In addition to a test of the feasibility of polarizing antiprotons by filtering, the proposed AD experiment will provide a measurement of the spin-dependent cross section σ 1 and σ 2 as function of energy from 50 to 450 MeV. σ 1 and σ 2 are the two double-spin observables in the parametrization of the total hadronic cross section σ TOT σ TOT = σ 0 + σ 1 ( P Q) + σ 2 ( P ˆk) (2.1) where σ 0 is the total spin-indipendent hadronic cross section and σ 1 is the total spin-dependent cross section for transverse orientation of beam polarization P and target polarization Q. σ 2 denotes the total spin dependent

26 18 2. Spin-Filtering experiment at AD Figure 2.2: Schematic design od the Antiproton Decelerator. Tha Antiproton Collector (AC) has been transformed into all in one machine which can, in addition, decelerate, cool and eject antiprotons at low energies (100 MeV). cross section for longitudinal polarization of beam and target polarization and ˆk = k/ k is the unit vector along the collision axis. The two spin-dependent total cross sections can be extracted from the polarization buildup of the beam which is possible to express as a function of filter time t ( ) t P(t) = tangh (2.2) where τ 1 is the polarizing time constant. The induced beam polarization is parallel to the target polarization. It can be shown that the time constant for transverse ( ) or longitudinal ( ) are given by respectevely τ 1 = 1 σ 1 Qd t f τ 1 = τ 1 1 ( σ 1 + σ 2 )Qd t f (2.3) where Q is the target polarization, d t is the target thickness in atoms/cm 2 and f is the revolution frequency of the particles in the ring. The filtering

27 2.3 AD machine properties 19 cross sections σ 1 and σ 2 are closely related to the spin-dependent cross sections σ 1 and σ 2 defined in equation 2.1. In order to extract both spin-dependent total cross sections σ 1 and σ 2 from the observed time constants (see equation 2.3), a measurement with transverse and longitudinal target (and therefore beam) polarization is required. In addition, the acceptance angle θ acc has to be known (tipically mrad), because the spin-dependent cross section σ 1 and σ 2 depend on θ acc. For protons, where the interaction is purely elastic, the spin-dependent cross section is already known and a NN scattering database exists (SAID and Nijmegen databases). For antiprotons such a database does not exist yet, and since both elastic scattering and all annihilation channels contribute, the correct cross section need to be calculated within a specific model. Recently, a calculation was published of the spin-dependent cross sections in pp, pn and pd at intermidiate energy, based on NN interaction models developed by the Jülich group [22]. 2.3 AD machine properties The AD is a relatively small ring (only 188 m in circumference, compared with 27 km in the LHC). A beam of protons at 26 GeV/c is produced in the PS and bombards a metallic target. The resulting antiproton beam has a momentum of 3.57 GeV/c and it is decelarated into AD to 5 MeV/c. During the deceleration, the beam is cooled first with stochatic cooling (till 300 MeV/c) and then with electron cooling, in order to reduce the spread in energy and the transverse emittance of the beam. At present, the AD provides about stored antiprotons but it could be able to increase this number by about a factor of five by development of stacking tecniques. With about 10 8 stored antiprotons, a luminosity of L = N p f d t = s atoms/cm 2 = cm 2 s 1 (2.4) will be achievable. Keeping in mind that after spin filtering for a few beam lifetimes a substantial remaining beam intensity is needed in order to measure the beam polarization, it is fairly important to dedicate some development effort towards increasing the number of antiprotons stored in AD.

28 20 2. Spin-Filtering experiment at AD A careful study of depolarizing resonances will be also needed, in order to guarantee that the polarization lifetime is much longer than the buildup time. An estimate of the polarization buildup by filtering is shown in figure 2.3. For the spin-dependent pp interaction, two different models are used (models A and D of ref. [22]). The solid line are calculated for different cell diameters and correspond to the curves in the right panel; the dashed line is for the teoretical acceptance angle limit of 25 mrad. Figure 2.3: Estimated beam polarization after two beam lifetimes of filtering (corresponding to a factor of about 7 decrease in the number of stored antiprotons) as function of the beam energy. The panels on the left and on the right are for the transverse and longitudinal polarization, respectively. The panels in top and bottom rows are for models A and D, respectively. The solid lines corrispond to different cell diameters.

29 2.4 Experimental setup Experimental setup At present, the AD at CERN is the only place world wide, where the proposed measurement can be performed. The new components that need to be installed in the AD will be tested and commissioned at COSY in Jülich and they are described in the following paragraphs. Figure 2.4: Floorplan of the AD showing the forseen locations of the polarized internal target, the upgraded electron cooler and the Siberian snake Polarized Internal Target The measurement requires implementing a Polarized Internal storage cell Target (PIT) in the straight section between injection and electron cooling of the AD (see Figure 2.4). The target density depends strongly on the transverse dimension of the storage cell. In order to provide a high target density, the β function at the storage cell has to be minimize at the cell: a special insertion is proposed, which includes additional quadrupoles around the storage cell (see Figure 2.5). The low-β section should be designed in such a way that the storage cell limits the machine aceptance only marginally.

30 Figure 2.5: Full installation forseen at AD for the straight dection between injection and electron cooling. (The beam moves from right to left. Shown in red are the existing AD quadrupole magnets that define the up- and downstream boundaries of the low-β insertion. The atomic beam source is mounted above the target chamber that houses the detector system and the storage cell. The Breit-Rabi target polarimeter and the target gas analyzer are mounted outwards of the ring Spin-Filtering experiment at AD

31 2.4 Experimental setup 23 The operation of the polarized target requires transoprting the stored beam through the narrow storage cell. Since at injection into AD at 3.57 GeV/c, the beam is not yet cooled, the apertures in the target region shall not be resticted by the storage cell. For the measurements proposed here, the beam is ramped down to the energies of interest ( MeV). There, the machine optics is squeezed by appling stronger focusing using the additional quadrupole magnets and only after this is accomplished, the storage cell is closed. The PAX target is depicted in figure 2.6. It consists of an ABS, a storage cell and a Breit-Rabi polarimeter (BRP). H or D atoms in a single hyperfinestate are prepared in the ABS and injected into the thin-walled storage cell. Inside the target chember, the cell is sorrounded by the detector system. Figure 2.6: Left panel: the PAX target at AD. The atomic beam source (ABS) is mounted on the top of the target chamber which houses the storage cell and the detector system. The antiproton beam passed through the target from behind. The Breit-Rabi polarimeter (BRP) on the right is fed by a small sample beam extracted from the storage cell. Right panel: the ABS and the BRP with a small test chamber setup in Jülich. As it shown in figure 2.7, a small sample of the taget gas propagates from the center of the cell into the BRP where the atomic polarizatin is measured. Simultaneously, the sample gas enters the Target Gas Analyzer (TGA) where the ratio of atoms to molecules in the gas is determined. A weak magnetic holding field around the storage cell provides the quantization axis for the target atoms; it can be oriented along the transverse (x), vertical (y) or longitudinal(z) direction.

32 24 2. Spin-Filtering experiment at AD Figure 2.7: Schematic view of the target setup with the sextupole magnet systems and the radio-frequency transition units Storage cell In order to be compatible with the antiproton beam operation at the AD, a dedicate storage cell has been developed that can be opened and closed at different times during the AD cycles. In particular at injection energy, a free space of 100 mm diameter is required at the cell position. As a compromise between density and acceptance angle, the closed cell has a square cross section of 10 x 10 mm 2 and a length of 400 mm. In order to allow the detection of low-energy recoil particles, the cell walls are made from a thin (5 µm) Teflon foil, a technique developed for the same reason for experiments at the Indiana Cooler [23]. A prototype of the cell has been designed and produced in the mechanical workshop of Ferrara University. At this moment, a test of the cell is carried out at Jülich. Photographs of the closed and open cell are shown in Figure 2.8 and a CAD view of the storage cell arrangement inside the target chamber is also shown in Figure 2.9.

33 2.4 Experimental setup 25 Figure 2.8: View along the beam direction of the prototype of the openable storage cell with thin Teflon walls when opened (left panel) and closed (right). Figure 2.9: Left panel: sketch of the silicon detector sorrounding the target cell. The Teflon cell walls are shown in violet, the alluminium target frame in sky blue and the sensitive modules in yellow. Right panel: the complete system with the openable cell support and gear, silicon detectors frames and kapton connection to the front-end electronic (Only half of detectors are shwon) Target Gas Analyzer and Breit-Rabi Polarimeter The Target Gas Analyzer (TGA) measures the atomic and molecular content of the gas extracted from the storage cell through the sample tube. The TGA arrangement consist of a chopper, a 90 off-axis quadrupole mass spectrometer (QMS) with a cross beam ionizer and a channel electron multiplier (CEM) for single ion detection. The TGA is integrated into the sextupole chamber of the Breit-Rabi polarimeter (BRP) and it is mounted 7 off-axis respect to the BRP, in order not to interfere with the beam entering the polarimeter. A chopper rotating at a frequency of 5.5 Hz is periodically blocking the sample beam in order to allow sustraction of the

34 26 2. Spin-Filtering experiment at AD residual gas signal. Particles entering the detector are ionized by electrons, mass filtered with the QMS and finally detected by the CEM. A detailed description of the hardware and woking principle of the HERMES target gas analyzer can be found in ref [24]. The Breit-Rabi Polarimeter measure the relative population n i of the hyperfine states of hydrogen (or deuterium) atoms contained in the sample beam. From this measurement the absolute atomic polarizations can be calculated by applying the known field strength at the target. A schematic view of the BRP is shown in figure Figure 2.10: Schematic view of the BRP/TGA vacuum system. The sample beam exits the storage cell and travels from the left to the right Two transition units are used to exchange the populations between pairs of hyperfine states: a strong field transition unit (STF) with tilted resonator that can be tuned for both π and σ transitions and a medium field transition (MFT) unit that can induce various π transitions according to the static field strenght and gradient setting used. The sextupole system is composed of two magnets and spinfilters the sample from the storage cell by focusing atoms with m s = + 1 toward the 2 geometrical axis of the BRP and defocusing atoms with with m s = 1. A 2 beam blocker of 9 mm diameter placed in front of the first sextupole magnet ensures that no atoms in m s = 1 state can reach the detector. 2 The detector stage is identical to the one employed for the TGA: a cross beam ionizer, a quadrupole mass spectrometer (QMS) and a chan-

35 2.5 Detector system 27 nel electron multiplier (CEM). In contrast to the TGA, only hydrogen (or deuterium) atoms are detected bye the BRP. A detailed description of the hardware and working principle of the HERMES Breit-Rabi polarimeter is given in ref [25]. 2.5 Detector system The PAX collaboration is developing a detector system to efficiently determine the polarizations of beam (or target) by measuring the polarization observables in pp elastic scattering. Moreover, the requirement of having particle identification together with a precise energy determination and tracking with a vertex resolution of 1 mm suggests the use of several layers of double-sided silicon detectors installed inside the accelerator vacuum and arrange in a telescope structure. The detector has been designed to meet the following requirements: provide a measurement of polarization observables in both pp (at COSY [26]) and pp elastic scattering at AD [27]. operate in vacuum to track low-momentum particles in the kinetic energy range from a few to a few tens of MeV. allow antiproton beam envelope at injection and allow for the storage cell to be opened during operation at AD. provide a large coverage of solid angle and luminous volume. provide a trigger signal within 100 ns of the particle passage. This allows one to use the detection system stand-alone and to set timing coincidence with other other detector components. The self-triggering capabilities of the telescope will be placed close to the side and forward direction of the target and used to identify the polarimetry reaction. Moreover, the trigger signal in coincidence with the trigger from other detectors will significantly reduce the amount of background events. depending on the field of operation and the range of kinetic energy that the telescope has to cover, it is desiderable to have a modular system that allows an easy exchange of detectors and electronics. In addition this will make the mantainance of the telescope less difficult.

36 28 2. Spin-Filtering experiment at AD Detector configuration The detector system is based on silicon microstrip detectors and the design follows closely the one recently developed at IKP for the ANKE experiment at Jülich. A detailed description of the ANKE detector system can be found in [28]. The PAX detector is based on two layers of double-sided silicon-strip sensors (see Figure 2.11) of large area (97 x 97 mm 2 ) and standard thickness of 300 µm. Eventually, the number of silicon layers can be increased to three to provide redundancy of the obtained track information. On P doped-side the detector has 1023 strips and on the N side it has 631 strips, all of which are capacitively coupled to the bond pads that are used to connect the strip to the electronics. On the detector the strip are combined to groups of four strips with the first group consisting of thress strips. On the Kapton foil that is used to connect the detector to the front-end electronics these 256 groups are again combined to 128 segments by bonding two groups on one segment on the Kapton foil. On th N side of the detector the strips are combined in groups of two strips with the first strip being a single group resulting in 316 groups of strips. On the Kapton foil the first 10 groups are combined to the first segment, followed by 149 segments made up of two gropus. A pitch of 0.76 mm provides the require vertex resolution of about 1 mm, while minimizing the number of channels to be read out. The Figure 2.11: Layer of the PAX detector with double-sided silicon-strip sensors and Kapton foils. detector setup, shown in Figure 2.9, is placed around the 400 mm long and 10 x 10 mm 2 cross section of the storage cell. Three adjacent detector layers along the beam direction cover the central and forward sections of the storage cell in order to maximize the acceptance for elastic scattering. Events occur mainly in the central region of the cell where the target density has its maximum. The minimum radial distance of the inner silicon layer

37 2.5 Detector system 29 is defined by the space required for the injection tube and the movement of the cell at injection. The 10 mm radial distance between two silicon layers is chosen to maximize the azimuthal acceptance while preserving the resolution on the vertex. The read-out electronics (see Figure 2.12) are based on a scheme that has been developed for the ANKE detector at COSY. The in-vacuum board carries the red-out chips with 11 MeV linear range and a time resolution of better than 1 ns, sufficient to provide a fast signal for triggering. The interface card outside the vacuum provides power supplies, control signals, trigger pattern threshold and calibrationpulse amplitudes to the front-end chips. The vertex board developed at Jülich comprises a sequencer together with a 12 bit ADC with 10 MHz sampling; it allows common-mode correction for hardware zero-suppression to reduce the output flow to 0.1 MByte/s with less than 50 µs dead-time. A programmable trigger and prescaler board have been developed to provide a flexible trigger logic. Figure 2.12: The developed electronic chain: In-vacuum boards with self-triggering chips (left panel), front-end interface board (center) and complete board with 12 bit ADC and sequencer (right panel). Detector simulation For the detector optimization and performance estimate, i.e. regarding theuncertinty in the measured kinematical parameters and the background rejection capability, the original sftware based on GEANT4 [29] was used. The simulation has been done with the following assumption: a 400 mm long target cell of 10 x 10 mm 2 cross-section and 10 µm thick Teflon walls is assumed;

38 30 2. Spin-Filtering experiment at AD the simulation accounts for the cell frames and the injection tube for the polarized gas from the atomic beam source; the vertex is randomly generated, depending on the target gas density distribution and the transverse beam size; three beam energies have been choosen as representative for the anticipated measurement at the AD. In order to estimate the accepted event rate, the total cross section σ pp tot has been taken from ref. [30]: it ranges from 250 mb at 220 MeV beam energy. The primary interaction of the p beam with the proton target into three sub-processes: elastic, inelastic (annihilation) and charge-exchange. Each of these are separately simulated with relative intensities taken from ref. [30]. From the measured count rates (or yields) at different beam and/or target polarizations, the beam or target polarization can be extracted using the available analyzing powers (or spin correlations, once measured) or vice versa Detector performance: acceptance and event rate estimate An AD luminosity equal to cm 2 s 1 = 1 mb 1 s 1 is assumed here at all energies. (During spinfiltering for two beam lifetimes, the initial luminosity, given in Section 2.2 has decreased by about a factor 7). The acceptance for elastic events is about 15%, resulting in an event rate of the order of 10 s 1. The background due to antiproton annihilations is estimated to be negligible (less than 10 4 ), since no background event survives out of primary annihilations. In Figure 2.13 the acceptance is shown as a function of the vertex coordinate (z, along the beam) at 120 MeV beam energy. The triangular distribution of all primary generated events corresponds to the density profile of the target gas. Proton and antiproton electromagnetic interactions with matter are the same. Therefore, it is impossible to distinguish these two particles detected by silicon detectors if antiprotons do not annihilate. This leads to pp mixing: events withe antiproton scattering angle θ lab > π/4 are accepted through detection of forward protons. Thereby, the measured cross section and analyzing power will be degenerated. On the other hand, the scattering cross section dominates in the forward hemisphere which makes possible

39 2.5 Detector system 31 to measure the analyzing power (which is also slightly degenerated). The Figure 2.13: Left panel: Event distribution as a function of the vertex z coordinate at 120 MeV for generated and reconstructed events. Right panel: Reconstructed event distribution as a function of the scattering angle lab at 120 MeV. The filled histogram (yellow) shows the ideal case of truly identified antiprotons. The empty histogram (blue) shows the real case when all tracks with θ lab < 45 are considered as scattered antiprotons. This leads to a small pp mixing effect at low beam energy. It should ne noted that the distribution reflects also the nonuniformity of the geometrical acceptance in the θ m interval (θ m is the measured scattering angle of a forward particle in the c.m. system). antiproton beam polarization can be measured using the degenerated analyzing power values for the forward scattered particles. In any case, the existing experimental data on the pp elastic scattering cross section and the analyzing power allow to account correctly for the influence of the mixing effect. Since multiple scattering dominates the resolution at the considered energies, almost no difference in the detector performance has been detected by reducing the strip pitch from mm to 0.5 mm. The most important features of detector system can be summarized in following list: the same detector can be used for measurements at COSY (pp elastic) and at the AD ( pp elastic), since the reconstruction of elastic scattering in pp and pp does not differ; the acceptance of the detector for elastic events is of the order of 15%, resulting in an expected rate for reconstructed pp pp elastic events around 10 s 1 ;

40 32 2. Spin-Filtering experiment at AD antiproton annihilation in the target environment as well as in the detector materials does not produce significant background and can be ignored; the detector resolution is mainly defined by multiple scattering, and a strip pitch of 0.76 mm is adequate; with increasing energy the acceptance slightly reduces but p versus p ambiguities are suppressed due to the dominating p scattering at small angles, at higher energies, there is a smaller annihilation rate and less track spread due to multiple scattering Interlock system The task of the interlock system is to guarantee safe working conditions for all the experimental apparatus and in particular for the STT s (Silicon Tracking Telescope). It has to help the operator to accomplish the right sequence of actions in the switching on/off of devices or opening/closing valves, e.g. by means of an external interface with LED s. The final decision to confirm the safety of working condition for STT s and to switch on/off cooling and power supply for STT s and front-end electronics (see section 3.1) has to be taken by the operator. The STT s will be calibrated in a test-bench and then used for the experiment first in COSY ring (COoler SYncrothron, Jülich) and afterward in AD ring (Antiproton Decelerator, CERN). In the test-bench, an overall control of pumping devices, valves and gauges is necessary. In case of COSY and AD, a vacuum interlock system is already existing to guarantee safe operations and vacuum condition. Thus, the designed interlock system is divided in two parts: the vacuum interlock which will be used only in a test-bench; the STT interlock which will be used besides in the test-bench, also in COSY and AD. The vacuum interlock system installed in the accelerators generates only one signal on the vacuum condition (e.g. safe vacuum) and receives an external signal used as VETO, which forbids vacuum operations. In order to get the same condition for the test-bench, the vacuum interlock receives a VETO signal and generates two output signals. One for safe vacuum

41 2.5 Detector system 33 condition and one for safe atmosphere condition, because STT can work also at atmospheric pressure. The STT interlock will generate a signal when STT s are in operation mode and it will be sent to the vacuum interlock as VETO. STT can be in operation mode only when safe atmosphere or safe vacuum signal is active. To design a reliable interlock, the first step is to make an inventory of the different devices used for vacuum system, cooling system and power supply. After that, input and output signals of each device have to be collected, to determine the communication protocol between the interlock system and each device (input/output to and from the device). The main functions of the interlock can be summarized in: 1. allow active actions as switch on/off devices and open/close valves (input to the device). 2. monitor status of devices and value of parameters as temperature and pressure (output from the device). In order to minimize the complexity and the cost of the system, it has been decided to use a hardware component that performs the function 1 and a software component to execute the function 2. In the following chapters, the STT and vacuum interlock system will be described in detail, starting from the equipment till the logic implementation.

42 Chapter 3 The STT interlock Silicon Tracking Telescope (STT) can work either at atmospheric pressure or in vacuum condition. A necessary condition for the proper functioning of the detectors is the absence of any light 1, which presence can be detected by an increasing in the detector currents. For this reason, cold cathode gauge will be used. In order to avoid water condensation on the STT surface or discharge between the STT layers (biased with high voltage), different precautions have to be applied: at atmospheric pressure detectors must not be cooled below dewpoint and in vacuum condition detectors has to be cooled 20 C. The STT hardware interlock is called to accomplish the following tasks: 1. Guarantee safe operations in Safe Atmosphere either Safe Vacuum conditions in which the STT can work. 2. Guarantee the right sequence of actions to switch on/off the cooling system, electronics and bias power supply. 3. Protect detectors in case of anomalous events as lost safe atmosphere or safe vacuum condition, broken vacuum, malfunction device, unconnected device to the STT interlock and power failure. 1 Light can be generated by hot cathode gauges or windows, but these elements are not present in the vacuum equipment for the STT test-bench. 34

43 3.1 STT equipment STT equipment To guarantee STT operation, different devices are needed: Cooling systems for detectors and front-end electronics in vacuum. It is realized by means of the integral T process thermostats, provided by LAUDA 2 (see Figure 3.1 (a) and (b)). Detector cooling in range going from room temperature ( 20 C) to 20 C is provided by an individual LAUDA for each telescope. Electronics of all telescopes are cooled by one common LAUDA. Low voltage power supplies (see Figure 3.1 (c)) for the complete front-end electronics, inside and outside the chamber (EPS). Bias voltage power supplies (see Figure 3.1 (c)) for the detector biases (BPS). To switch off bias voltage there are two possibilities: slow off in which the bias will be ramped down with a defined speed. fast off in which all channels must be set to zero voltage as fast as possible. This is an extreme choice because if detectors are switched off without ramping, they could be irreversible damaged. It can be used ONLY in case of broken vacuum which could happen inside COSY 3 because of a broken window in the vacuum system. 3.2 States and normal operations After describing the STT equipment, it is necessary to define the standard procedures to use detectors in the proper way. Normal operations are the sequences of actions to switch on/off the STT system (cooling and power supply). Starting from the state in which all devices are off (STT OFF), the final state is reached when all devices are working (STT ON), or viceversa. The normal operations for the STT system are shown in Figure 3.2: following that order, it is possible to define all the intermediate 2 From now, I will call this device simply LAUDA. 3 If we leave V0 chamber venting valve as manual valve, this situation could happen also in the test bench because of one crazy person who opens the valve during vacuum operation.

44 36 3. The STT interlock (a) Integral T process thermostat (LAUDA). (b) STT cooling system. (c) Mpod crate, providing low and bias voltage. Figure 3.1

45 3.2 States and normal operations 37 STT system states (see Table 3.1) determined by the status of each device (on/off). By definition, anomalous event are not considered in the procedure for normal operations. To define normal operations, it is necessary to know the hierarchy of devices: LAUDA for the detectors and electronics (lowest priority). It can be switched ON only if one of the signals from the vacuum interlock is active ( Safe Atmosphere or Safe Vacuum ). It can be switched OFF only when the EPS and the BPS are off (see below). Electronics Power Supply, EPS (middle priority). It can be switched ON only if electronics and detectors are cooled. It can be switched OFF only if the BPS is off. Bias Power Supply, BPS (highest priority). It can be switched ON/OFF only if electronics and detectros are cooled, and electronics is powered. Devices STT OFF States Intermediate states State A State B State C STT ON LAUDA el. OFF ON ON ON ON LAUDA det. OFF OFF ON ON ON EPS OFF OFF OFF ON ON BPS OFF OFF OFF OFF ON Table 3.1: States of STT system. The definition of these states is independent from the normal operations considered (switching on or off the STT system). In case of normal operations, the STT hardware interlock system can only enable to switch on/off devices: after the permission, this operation has to be done manually by the operator. In case of anomalous events (see Section 3.3), the STT interlock system can automatically switch off devices.

46 38 3. The STT interlock Safe_Atm VACUUM Safe_Vac T_Room Electronic COOLING LOWEST priority T_Room Detector COOLING T_Low ON OFF Low Voltage MIDDLE priority ON OFF Bias (slow ramping) HIGHEST priority enable to switch on low voltage enable to start cooling enable to switch on bias enable to stop pumping enable to switch off cooling enable to switch off low voltage Figure 3.2: Scheme of the order to switch on/off cooling and power supply devices.

47 3.2 States and normal operations 39 Normal operation: STT OFF STT ON 1. It is allowed to change mode only if one of the two signals from vacuum interlock (Safe Atm or Safe Vac) is active. Press the button START to reach STT ON. The signals STT Run and STT Opr are generated and VETO for vacuum interlock is activated. 2. Measure room temperature RT and set T0=RT through the SIM process 4 ( Software Interlock Monitoring, see Section 3.5). 3. Switch on electronics and detector cooling to T0 after the interlock authorization. 4. Internal subroutine (see Figure 3.4): if Safe Atmosphere is active, electronics and detectors must be kept at T0. If Safe Vacuum is active, electronics and detectors must be cooled at the working temperature, respectively EWT 10 C and DWT 20 C. 5. When electronics and detectors have reached the right temperature, switch on the low voltage (EPS) after the interlock authorization. 6. When electronics is powered and detectors are cooled, it is allowed to switch on bias power supply (BPS). Set the bias voltage and switch it on. The STT ON state is reached. The normal operation to go from STT OFF to STT ON is summarized in Figure In this case the SIM process is used to set the cooling temperature but this operation could be done also at hardware level without using software and choosing the right threshold for signals on temperature.

48 40 3. The STT interlock Figure 3.3: Normal operation to go from STT OFF to STT ON.

49 3.2 States and normal operations 41 Figure 3.4: The scheme of the internal subroutine. This structure shows the actions to set and control the cooling temperature for detectors and electronics, during the normal operation to switch on the STT system.

50 42 3. The STT interlock Normal operation: STT ON STT OFF 1. Press the button STOP to reach STT OFF. 2. Switch off the BPS after the interlock authorization. 3. When the BPS status is inactive (all BPS channels are off), switch off the EPS after the interlock authorization. 4. When the EPS status is inactive (all EPS channels are off), start warming up electronics and detectors after the interlock autorization. 5. When electronics and detectors reach room temperature, switch off LAUDA after the interlock authorization. 6. When all LAUDA s are off, the STT OFF state is reached. 7. To deactivate the STT Opr signal which is the VETO for vacuum interlock, a responsible person has to unlock RED BUTTON by key. The normal operation to go from STT ON to STT OFF is summarized in Figure 3.5.

51 3.2 States and normal operations 43 Figure 3.5: Normal operation to go from STT ON to STT OFF.

52 44 3. The STT interlock 3.3 Anomalous events and STT interlock actions During normal operations, anomalous events can happen and the interlock action will depend on the STT system state. To save the detectors, it is extremely important to follow the SAME ORDER to switch off the system explained in the previous section. Before describing the STT interlock actions, it is necessary to analyze all the possible anomalous events: Lost safe vacuum or safe atmosphere condition in which safe vacuum condition is lost because of a leak in the vacuum chamber or problems with pumping system. In both of these situations there is enough time to switch off bias with ramp. Broken vacuum in which safe vacuum condition is lost in a little fraction of a second because of a broken window. The only chance to save detectors (at least some of them) is choosing bias fast off. To recognize this case, a particular kind of sensor can be installed, generating a signal which directly activates the bias fast off. LAUDA electronics alarm which is activated when the cooling liquid is almost finished or in case of malfunction device. There is only one LAUDA to cool all the electronics in vacuum. LAUDA detector alarm which is activated in the same way explained before but there is only one LAUDA for each telescope. All LAUDA detector alarms (in case of more than one telescope) are connected to the same line: if one of these cooling devices generates an alarm signal, all LAUDA detector will be switched off. EPS and BPS malfunction in which EPS and BPS will be automatically switched off with ramp by the internal interlock system of the MPOD crate. In this case, the STT interlock does not react. Lost connection device which means that the connection with the STT interlock is lost. It could happen because the cable is broken or only disconnected. Power Failure in which case EPS and BPS are automatically switched off. The MPOD crate and the STT interlock can still work because they are connected to an uninterruptable power supply.

53 3.3 Anomalous events and STT interlock actions 45 In case of anomalous events, the STT interlock actions typologies are three (see Table 3.2): 1. Switch off devices. Only in case of fast vacuum brake, the bias fast off will be activated, otherwise only bias slow off will be used. 2. Start warming up procedures for electronics and detectors. 3. Forbid to continue with normal operations till the device is connected again. More than one anomalous event can happen at the same time: the reaction of the STT interlock will be the sum of the actions for each event, executed in the order described in normal operation to switch off the system.

54 ANOMALOUS EVENTS STT OFF STATES State A State B State C STT ON Safe Atm lost off EPS - slow off BPS Safe Vac lost - Broken vacuum LAUDA el. alarm LAUDA det. alarm - X X - warm up el. - warm up - off EPS - slow off BPS el. and det. - warm up - off EPS el. and det. - warm up el. and det. - off LAUDA el. - off LAUDA el. - off EPS - fast off BPS and det. - off LAUDA el. - off LAUDA el. - off EPS - slow off BPS - off LAUDA el. - off EPS - off LAUDA el. - off LAUDA det. - off LAUDA det. - off EPS - slow off BPS - off LAUDA det. - off EPS EPS and BPS alarm Lost connect. - not allowed to - not allowed to - not allowed to LAUDA elect. - reach state B reach state C reach STT ON Lost connect. - not allowed to - not allowed to - not allowed to LAUDA det. - reach state B reach state C reach state STT ON Lost connect. - not allowed to - not allowed to - not allowed to EPS and BPS - reach state B reach state C reach STT ON - off LAUDA det. X X X Power failure The STT interlock Table 3.2: Actions of the STT interlock system are described in case of one anomalous event (rows) during one STT system state (columns). The symbol-means that STT interlock does not react in that situation and the operator can choose what to do. The symbol X means that the particular anomalous event cannot happen in that STT system state.

55 3.4 Signals and logical structure Signals and logical structure As already pointed out in section 3.2, cooling system, EPS and BPS have different priority levels. Thus, it is important to follow the proper order to switch on/off these devices, in order to safely run silicon detectors. For this reason, a chain of dependent device is constructed: lower priority device cannot change state till higher priority device is active. Before describing this structure, all the input and output signals for the STT interlock have to be analyzed (see Table 3.3). They can be devided in three categories: status of the device (input for the interlock); switch on/off the device (input for the interlock); alarm signal (input for the interlock). With this kind of signals it is possible to construct the same logic unit for each device, as it can be seen in Figure 3.6. For each unit there are three inputs: an external alarm (from another device, power failure or safe working condition lost), notice to switch on/off the device and VETO which does not allow to change the status of that device. There are also two outputs: the status and the alarm signal of that device. This description represent only a scheme but the real logical structure has not been implented yet. The complete logical structure with the SIM control will be like in Figure 3.6, where it is added the possibility to switch on/off devices through manual or remote control.

56 SIGNALS Device Function I/O Name Level (active) Bias Power Supply Electronic Power Voltage on channel outputs I BV Status High/Low(zero voltage on all channels) Bias slow ramp O BV ROff High(enable ramp up)/low(ramp down) Bias fast ramp O BV FOff High/Low(bias cut off as fast as possible) Voltage on channel outputs I LV Status Supply Switch on/off low voltage O LV On Electronic Cooling Detector Cooling External signals High(at least one FPS is ON)/ Low(all FPS are OFF) High(all FPS are OFF)/ Low(FPS enable to switch ON) Electronic at working temperature I LE W T High/Low(T = 20 ± 2C) Alarm I LE Alarm High(normal operation)/low(alarm) Switch on/off LAUDA O LE ON High(OFF)/Low(ON) Detector below room temperature I LD RT High/Low(T < 20C) Detector at working temperature I LD W T High/Low(T W ± 2C) Alarm I LD Alarm High(normal operation)/low(alarm) Switch on/off LAUDA O LD ON High(OFF)/Low(ON) Set temp to Room or Working O LD ST High(Room)/Low(Working) safe atmosphere condition I SAFE ATM High/Low(safe atm) safe vacuum condition I SAFE V AC High/Low(safe vac) STT in operation O STT OPR High/Low(STT in operation) Internal signal STT goes to operation or off STT Run High (to operation)/low(to off) Program interface (IC2C) switch on/off LAUDA E I ILE ON High(ON)/Low(OFF) switch on/off LAUDA D I ILD ON High(ON)/Low(OFF) switch on/off FPS I ILV ON High(ON)/Low(OFF) switch on/off BPS I IBV ON High(ON)/Low(OFF) The STT interlock Table 3.3: Input and Output signals.

57 3.4 Signals and logical structure 49 Figure 3.6: The complete structure for the STT interlock with SIM control.

58 50 3. The STT interlock 3.5 Future development: Software Interlock Monitoring The Software Interlock Monitoring (SIM) has different functions: launching programs to monitor front-end electronics and cooling system. controlling BPS and EPS, checking if all these programs are running. predicting unsafe situations and react on them analyzing data from all subsystem. It will be installed on a Linux platform and it will communicate with hardware via I2C bus. This interface will be connected to Linux PC by RS- 232 adapter. The best choise is to use such RS I2C adapters (which support ASCII interface) because some devices (e.g. LAUDA) have serial interface. To have enough serial channels, it is planned to use an insudtrial USB to Serial Adapter which will be connected by USB to host with running SIM. This software hasn t been designed yet but the first draft is shown in Figure 3.7.

59 3.5 Future development: Software Interlock Monitoring 51 Figure 3.7: The Software Interlock Monitoring (SIM).

60 Chapter 4 The vacuum interlock system After the STT interlock, the vacuum interlock system has to be described. The aim of this system is to guarantee safe working conditions for STT s which can work at atmospheric pressure or in vacuum condition. It will be used only in the test-bench and the vacuum equipment is described in next section. 4.1 Vacuum equipment The vacuum equipment for the STT test-bench includes the following devices (Figure 4.2). TurboCube (TC) including turbomolecular pump TPU-521, Display Unit Control (DCU) and control system (TC-600). The TVF 005 (VV) is the turbo venting valve and PKR 261 is a cold cathode/pirani combined, a full range gauge with a raugh precision around 30 %. Ion Getter Pump (IGP) used to keep good pressure conditions inside the STT chamber in case of safety valve closing (e.g. power failure). To this aim, the pump is powered by uninterruptable power supply (UPS). Safety Gate Valve VAT (VS) used to separate the STT chamber from the TurboCube in case of power failure or malfunction of the turbopump. This valve has a pneumatic actuator with solenoid and contacts for the position indication. The pressure difference at opening has to be 30mbar. 52

61 4.1 Vacuum equipment 53 Cold Cathode Pressure Gauge IKR-070 providing the pressure measurement inside the STT chamber and connected to the controller TPG- 300 (Pfeiffer). It can be switched on only when pressure is less than 10 4 mbar. Differential pressure switch VSC150 used to measure the difference between the pressure inside the vacuum chamber and the atmospheric pressure. Using this device, it is possible to define when the venting procedure is ended and thus when the safe atmosphere condition is attained. Chamber Venting Valve (V0), a variable leak valve used to vent the STT chamber in air. Manual Valves (V1 and V2) used to select the forepumping line. During normal operation, V1 is opened and V2 is closed. The optional pumping line through V2 is used to leak chase the vacuum system. All the abbreviations used for the names of devices are summarized in Table 4.1. Device TurboCube Ion Getter Pump Safety Valve Cold Cathode gauge Chamber Venting Valve Turbo Venting Valve Manual Valves Abbraviation TC IGP SV CC V0 VV V1 and V2 Table 4.1: Definition of abbreviations for the names of vacuum equipment.

62 54 4. The vacuum interlock system Figure 4.1: Vacuum chamber used for the test-bench. CC IGP TPG 300 Control Unit UPS LEGEND Devices connected to the vacuum interlock Pumping line V0 STT Vacuum Chmaber PKR 261 DCU TC 600 Venting Patm TurboCube SV VSC150 P=1.1 Patm TPU 521 V1 VV N2 optional pumping line V2 He leak chaser Figure 4.2: Scheme of the vacuum equipment for the SST test-bench.

63 4.2 States and normal operations States and normal operations As already pointed out, STT can work either at atmospheric pressure or in vacuum condition. In order to guarantee safe operations with detectors, it is necessary to define the meaning of: Safe Atmosphere and Safe Vacuum states; normal operation, which is the right sequence of actions to go from Safe Atmosphere to Safe Vacuum and viceversa. Anomalous events as power failure, malfunctioning of the device or lost connection between interlock and device are not considered. Each state of the vacuum system (see Table 4.2) is identified by TurboCube (TC) and Ion Getter Pump (IGP) status, the pressure value inside the chamber and the valve position. Safe Atmosphere state is attained when the Ion Getter pump (IGP) and the TurboCube (TC), composed by a turbopump and a forepump, are off. Safety Valve (SV) and turbo Venting Valve (VV) must be open because the venting of the chamber is done through the line of the turbopump. If venting is in air, the chamber venting valve (V0) has to be open while if the venting is in nitrogen, V0 has to be closed. To guarantee atmospheric pressure condition inside the chamber, a differential pressure switch is used. Safe Vacuum state is attained when TC and IGP are working, in particular the turbopump is at full speed (833 Hz). SV must be open but VV and V0 must be closed. The pressure P inside the chamber is less than 10 5 mbar and it is measured by the Cold Cathode gauge (CC). The states different from Safe Atmosphere and Safe Vacuum can be devided in two categories: intermediate states, which are attained during normal operation; undefined states, which are attained only in case of anomalous event. Two intermediate states exist for the in the case of normal operation to switch on the vacuum system and only one when the system is switched off. The different number of intermediate states between the two normal operations is due to the fact that the IGP can be switched on only when P< 10 5 mbar (that is, after TC is already working) while can be switched off at the same time of TC.

64 56 4. The vacuum interlock system In case of normal operation, the vacuum interlock can only enable to switch on/off devices. After the authorization, this operation has to be done by the operator. Devices States Safe Intermediate states Safe Atmosphere State A State B Vacuum TurboCube OFF ON ON ON Turbo venting valve VV Open Closed Closed Closed IGP OFF OFF ON ON Pressure (mbar) P=Patm Patm<P< <P< 10 6 P< Safety valve SV Open Open Open Open Chamber venting valve V0 Close/Open Closed Closed Closed V1 Open Open Open Open V2 Closed Closed Closed Closed Table 4.2: Definition of the vacuum system states.

65 4.2 States and normal operations Normal operations Safe Atmosphere Safe Vacuum Starting from Safe Atmosphere, the operator presses the button START to enable pumping operations (see Figure 4.3). If there is no VETO, the internal signal Run is activated and Safe Atm signal is deactivated. Then, the vacuum interlock system enables the operator to switch on TC. When TC is on, an automatic procedure is initialized: the forepump starts working and only when the pre-vacuum pressure is less than 10 1 mbar, the turbopump is activated. In this moment, the pressure P inside the chamber is monitored by the Pirani gauge and only when P is less than 10 4 mbar, the CC can be switched on. From this point, P is monitored by the CC and when P< 10 5 mbar the vacuum interlock allows to switch on IGP and the operator can execute this operation. If TC and IGP are working, the turbopump has reached the full speed and P< 10 6 mbar,the vacuum interlock will display that Safe Vacuum is attained with a led. The operator will confirm it pressing the button Safe Vacuum. Only in this case, the output signal Safe Vac will be generated from the vacuum inerlock and sent to the STT interlock system. The normal operation to go from Safe Atmosphere to Safe Vacuum is summarized in the following list. 1. Safe Atm signal is active. If there is no VETO and devices are connected, it is allowed to change mode. Press START to reach Safe Vacuum and check that Safe Atm signal is deactivated (visible through a led). 2. Switch on the TurboCube after the vacuum interlock authorization. 3. When P 10 5 mbar, switch on IGP the after the vacuum interlock authorization. 4. When P< mbar, operational pressure signal is generated. 5. When the pumping station and IGP are working and the operational pressure is attained, the Safe Vacuum state is reached. To confirm this state, it is necessary to press the button safe vacuum.

66 58 4. The vacuum interlock system Safe Vacuum Safe Atmosphere Starting from Safe Vacuum, the operator has to press the button STOP (see Figure 4.4). If there is no VETO, interlock system deactivates Safe Atm signal (check it) and allows first to switch off IGP and then TC. The venting procedure of turbopump will be automatically started by TC (see Section 4.2.2). SV must be open and thus the chamber will be vented too. When TC and IGP are off, SV and VV are open and P is equal to atmospheric pressure, Safe Atmosphere state is attained. The interlock will display this condition through a led. Only when the operator will press the button Safe Atmosphere the signal will be generated and sended to the STT interlock. The normal operation to go from Safe Vacuum to Safe Atmosphere is summarized in the following list. 1. Safe Vac signal is active. Press the button STOP to swtch off pumping devices. If there is no VETO, it is allowed to change mode Safe Vac signal is deactivated (check it). 2. Switch off the TurboCube and then the IGP after the vacuum interlock authorization. 3. Start venting procedure. 4. When the pressure P=P atm, the Safe Atmosphere state is attained. To confirm this state, it is necessary to press the button Safe Atmosphere.

67 4.2 States and normal operations 59 Figure 4.3: Normal operation to go from Safe Atmosphere to Safe Vacuum.

68 60 4. The vacuum interlock system Figure 4.4: Normal operation to go from Safe Vacuum to Safe Atmosphere.