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Studies of the spectral and angular distributions of transition radiation using a silicon pixel sensor on a Timepix3 chip
Abstract
X-ray transition radiation detectors (TRDs) are used for particle identification in both high energy physics and astroparticle physics. In most of the detectors, emission of the X-ray transition radiation (TR) starts at Lorentz factors above γ∼500 and reaches saturation at γ∼2÷3⋅103. However, many experiments require particle identification up to γ∼105, which is very difficult to achieve with conventional detectors. Semiconductor pixel detectors offer a unique opportunity for precise simultaneous measurements of spectral and angular parameters of TR photons. Test beam studies of the energy and the angular distributions of TR photons emitted by electrons and muons of different momenta crossing several types of radiators were performed at the CERN SPS with a 480 μm thick silicon detector bonded to a Timepix3 chip. High resolution images of the energy−angle phase space of the TR produced by different radiators were obtained and compared with MC simulations. The characteristic interference patterns are in agreement with the theoretical models with an unprecedented level of details. The studies presented in this paper also show that simultaneous measurements of both the energy and the emission angles of the TR X-rays could be used to enhance the particle identification performances of TRDs.
... The probability density function of TR is a fairly complex function of γ, radiator parameters, photon energy (ω) and its emission angle (θ). For well defined radiator parameters a measured two-dimensional distribution of photon energy vs its reconstructed emission angle is in very good agreement with the theory predictions [176]. ...
... For a single foil the largest part of the TR energy is emitted around the most probable angle θ = (1/γ 2 + ω 2 2 /ω 2 ) 1/2 , where ω 2 is the plasma frequency of the gas surrounding the radiator material elements. However, in case of multiple interfaces, interference effects may significantly change this angle and more realistic expression for the angle which corresponds to the last interference maximum of the energy spectra is θ ≈ 1.4π 2 /γ 2 sat − 1/γ 2 [176]. The higher is the gamma-factor, the larger is the angle of the first interference maximum. ...
... It reaches almost its asymptotic limit at γ = γ sat . This effect is illustrated in Fig. 35.17 [176] which shows two-dimensional distribution of the TR photon energy versus the reconstructed production angle obtained in 20 GeV electron beam with the radiator containing a stack of foils of 15.5 µm thickness spaced by 210 µm (the left plot) using a Si-pixel detector. TR produced by 20 GeV electrons is emitted mostly around θ ∼ 0.9 mrad. ...
Prog. Theor. Exp. Phys. 2020, 083C01 (2020) and 2021 update.
... We have designed an experimental setup to measure the energy spectra and the angular distributions of the TR X-rays emitted by fast electrons and positrons crossing different radiators. Similar measurements were performed in the past at the CERN SPS with beams of 20 GeV/ electrons and of 120, 180 and 290 GeV/ muons, using silicon strip detectors [6], silicon pixel detectors [7,8] and GaAs pixel detectors [8,9]. Parallel to the measurements, an effort to develop accurate Monte Carlo simulations of the TR process is being carried out [10]. ...
We have designed and implemented an experiment to measure the angular distributions and the energy spectra of the transition radiation X-rays emitted by fast electrons and positrons crossing different radiators. Our experiment was selected among the proposals of the 2021 Beamline for Schools contest, a competition for high-school students organized every year by CERN, and was performed at the DESY II Test Beam facility area TB21, using a high-purity beam of electrons or positrons with momenta in the range from 1 to 6 GeV/ c . The measurements were performed using a 100 μm thick silicon pixel detector, with a pitch of 55 μm. Our results are consistent with the expectations from the theoretical models describing the production of transition radiation in multilayer regular radiators.
... For each radiator the beam particle, their momenta and the distance between the radiator and the X-ray detector are reported. detectors [7,8] and GaAs pixel detectors [8,9]. Parallel to the measurements, an effort to develop accurate Monte Carlo simulations of the TR process is being carried out [10]. ...
We have designed and implemented an experiment to measure the angular distributions and the energy spectra of the transition radiation X-rays emitted by fast electrons and positrons crossing different radiators. Our experiment was selected among the proposals of the 2021 Beamline for Schools contest, a competition for high-school students organized every year by CERN, and was performed at the DESY II Test Beam facility area TB21, using a high-purity beam of electrons or positrons with momenta in the range from 1 to 6 GeV/c. The measurements were performed using a 100 um thick silicon pixel detector, with a pitch of 55 um. Our results are consistent with the expectations from the theoretical models describing the production of transition radiation in multilayer regular radiators.
... At accelerators, most applications have used TRDs to identify electrons (as examples, see refs. [8,9]), and the ATLAS group has recently conducted an extensive series of beam exposures that have enabled detailed comparisons of the experimental results with GEANT predictions [10,11,12]. ...
Transition radiation detectors (TRDs) have been used to identify high-energy particles (in particular, to separate electrons from heavier particles) in accelerator experiments. In space, they have been used to identify cosmic-ray electrons and measure the energies of cosmic-ray nuclei. To date, radiators have consisted of regular configurations of foils with fixed values of foil thickness and spacing (or foam or fiber radiators with comparable average dimensions) that have operated over a relatively restricted range of Lorentz factors. In order to extend the applicability of future TRDs (for example, to identify 0.5 - 3 TeV pions, kaons, and protons in the far forward region in a future accelerator experiment or to measure the energy spectrum of cosmic-ray nuclei up to 20 TeV/nucleon or higher), there is a need to increase the signal strength and extend the range of Lorentz factors that can be measured in a single detector. A possible approach is to utilize compound radiators consisting of varying radiator parameters. We discuss the case of a compound radiator and derive the yield produced in a TRD with an arbitrary configuration of foil thicknesses and spacings.
Transition radiation detectors (TRDs) have been used to identify high-energy particles (in particular, to separate electrons from heavier particles) in accelerator experiments. In space, they have been used to identify cosmic-ray electrons and measure the energies of cosmic-ray nuclei. To date, radiators have consisted of regular configurations of foils with fixed values of foil thickness and spacing (or foam or fiber radiators with comparable average dimensions) that have operated over a relatively restricted range of Lorentz factors. In order to extend the applicability of future TRDs (for example, to identify 0.5–3 TeV pions, kaons, and protons in the far forward region in a future accelerator experiment or to measure the energy spectrum of cosmic-ray nuclei up to 20 TeV/nucleon or higher), there is a need to increase the signal strength and extend the range of Lorentz factors that can be measured in a single detector. A possible approach is to utilize compound radiators consisting of varying radiator parameters. We discuss the case of a compound radiator and derive the yield produced in a TRD with an arbitrary configuration of foil thicknesses and spacings.
Hybrid pixel semiconductor detectors find more and more applications in modern experimental physical setups. In particular, pixelated detectors based on GaAs:Cr sensor and Timepix3 chip are used in R&D for a state-of-the-art Transition Radiation Detector prototype at CERN. Motivation and usage aspects for GaAs:Cr-Timepix3 device in the experiment are covered in the talk. Authors” contribution to the work of the research group is a study of charge collection and transportation processes in the sensor including fluorescence and so-called charge sharing effect. These are able to significantly affect both spatial and energy resolution of the system. Estimates for the effects are obtained via numerical modelling and then used to analyze the impact on the detector performance.
New developments of pixel detectors based on GaAs sensors offer effective registration of the transition radiation (TR) X-rays and perform simultaneous measurements of their energies and emission angles. This unique feature opens new possibilities for particle identification on the basis of maximum available information about generated TR photons. Results of studies of TR energy-angular distributions using a 500 |j.m thick GaAs sensor attached to a Timepix3 chip are presented. Measurements, analysis techniques and a comparison with Monte Carlo (MC) simulations are described and discussed.
Many modern and future accelerator and cosmic ray experiments require identification of particles with Lorentz γ-factor up to 10 ⁴ and above. The only technique which reaches this range of Lorentz factors is based on the transition radiation detectors (TRD). This paper describes the development of a TRD based on straw proportional tubes. A prototype of such kind of detector was built and tested at the CERN SPS accelerator. Monte Carlo simulation model of the detector which matches well the experimental data was developed. This program was used for the simulation of a full-scale TRD for hadron identification at TeV energy scale.
In recent years, developments of gaseous detectors based on a combination of electron multiplication gap in the gas and pixel readout chips as a part of the anode plane (GasPixel detectors) reached a level where they can offer unique opportunities for particle detection. Transition radiation (TR) detectors based on this technology can be one of the possible applications. In this work, measurements of energy spectra and angular distributions of transition radiation photons produced by particles with different gamma factors made with a GridPix detector prototype are presented. The observed results are compared with theoretical predictions.
Measurements of hadron production in the TeV energy range are one of the tasks of the future studies at the Large Hadron Collider (LHC). The main goal of these experiments is a study of the fundamental QCD processes at this energy range, which is very important not only for probing of the Standard Model but also for ultrahigh-energy cosmic particle physics. One of the key elements of these experiments measurements are hadron identification. The only detector technology which has a potential ability to separate hadrons in this energy range is Transition Radiation Detector (TRD) technology. TRD prototype based on straw proportional chambers combined with a specially assembled radiator has been tested at the CERN SPS accelerator beam. The test beam results and comparison with detailed Monte Carlo simulations are presented here.
The SPIDR (Speedy PIxel Detector Readout) system is a flexible general-purpose readout platform that can be easily adapted to test and characterize new and existing detector readout ASICs. It is originally designed for the readout of pixel ASICs from the Medipix/Timepix family, but other types of ASICs or front-end circuits can be read out as well. The SPIDR system consists of an FPGA board with memory and various communication interfaces, FPGA firmware, CPU subsystem and an API library on the PC . The FPGA firmware can be adapted to read out other ASICs by re-using IP blocks. The available IP blocks include a UDP packet builder, 1 and 10 Gigabit Ethernet MAC's and a "soft core" CPU . Currently the firmware is targeted at the Xilinx VC707 development board and at a custom board called Compact-SPIDR . The firmware can easily be ported to other Xilinx 7 series and ultra scale FPGAs. The gap between an ASIC and the data acquisition back-end is bridged by the SPIDR system. Using the high pin count VITA 57 FPGA Mezzanine Card (FMC) connector only a simple chip carrier PCB is required. A 1 and a 10 Gigabit Ethernet interface handle the connection to the back-end. These can be used simultaneously for high-speed data and configuration over separate channels. In addition to the FMC connector, configurable inputs and outputs are available for synchronization with other detectors. A high resolution (≈ 27 ps bin size) Time to Digital converter is provided for time stamping events in the detector. The SPIDR system is frequently used as readout for the Medipix3 and Timepix3 ASICs. Using the 10 Gigabit Ethernet interface it is possible to read out a single chip at full bandwidth or up to 12 chips at a reduced rate. Another recent application is the test-bed for the VeloPix ASIC, which is developed for the Vertex Detector of the LHCb experiment. In this case the SPIDR system processes the 20 Gbps scrambled data stream from the VeloPix and distributes it over four 10 Gigabit Ethernet links, and in addition provides the slow and fast control for the chip.
This biennial Review summarizes much of particle physics. Using data from previous editions, plus 2158 new measurements from 551 papers, we list, evaluate, and average measured properties of gauge bosons, leptons, quarks, mesons, and baryons. We also summarize searches for hypothetical particles such as Higgs bosons, heavy neutrinos, and supersymmetric particles. All the particle properties and search limits are listed in Summary Tables. We also give numerous tables, figures, formulae, and reviews of topics such as the Standard Model, particle detectors, probability, and statistics. Among the 108 reviews are many that are new or heavily revised including those on neutrino mass, mixing, and oscillations, QCD, top quark, CKM quark-mixing matrix, V-ud & V-us, V-cb & V-ub, fragmentation functions, particle detectors for accelerator and non-accelerator physics, magnetic monopoles, cosmological parameters, and big bang cosmology.
Formulae for the X-ray transition radiation produced in an irregular stack of plates are obtained, taking into account the absorption and presented in a form convenient for numerical calculation. The transition radiation generated in a regular stack is also considered. For the last case a procedure for calculation of the radiation frequency spectrum depending on the stack parameters and the particle Lorentz-factor is given.
A transition radiation detector using a method of cluster counting measurement has been tested. The performance is considerably better than with the usual method of total charge measurement, as well as offering advantages in simplicity of construction and operation.
An electronic detector has been built which measures the angle of emission of transition radiation photons, as well as the energy deposit. A significant gain in the efficiency of particle identification is obtained for γ⋍103.
A transition radiation (TR) detector has been built which measures the distribution of charge along the particle track in the gas of a drift chamber. Charge clusters are observed arising from track ionization and the detection of TR photons. A cluster counting method based on these measurements is shown to have a considerable advantage with respect to the normal measurement of the total deposited charge.
Features of the multiplayered transition radiation detector for ultrarelativistic particles are studied, including especially details of: X-ray spectrum, gamma dependence, ( gamma =Lorentz factor) interference effects, saturation at high gamma , effects of irregularities, and multiple scattering. Three important dimensionless variables are introduced: a scaled Lorentz factor Gamma , a scaled frequency nu , and the ratio tau =foil spacing/foil thickness. One of the predictions is of large threshold effects due to interference, which can be exploited in a threshold detector for measuring a material's X-ray refractive index. Theoretical curves are given, along with a short discussion of photon statistics, optimisation of such a detector, and associated computational methods. (16 refs).
Growing energies of particles at modern or planned particle accelerator experiments as well as cosmic ray experiments require particle identification at gamma-factors (γ) of up to ∼10 ⁵ . At present there are no detectors capable of identifying charged particles with reliable efficiency in this range of γ. New developments in high granular pixel detectors allow one to perform simultaneous measurements of the energies and the emission angles of generated transition radiation (TR) X-rays and use the maximum available information to identify particles. First results of studies of TR energy-angular distributions using gallium arsenide (GaAs) sensors bonded to Timepix3 chips are presented. The results are compared with those obtained using a silicon (Si) sensor of the same thickness of 500 μm. The analysis techniques used for these experiments are discussed.
This work is dedicated to the study of a technique for hadron identification in the TeV momentum range, based on the simultaneous measurement of the energies and of the emission angles of the Transition Radiation (TR) X-rays with respect to the radiating particles. A detector setup has been built and tested with particles in a wide range of Lorentz factors (from about 10 ³ to about 4×10 ⁴ crossing different types of radiators. The measured double-differential (in energy and angle) spectra of the TR photons are in a reasonably good agreement with TR simulation predictions.
In this work, we construct for the first time the theory of small-angle transition radiation from multilayered structures. The theoretically obtained spectral and angular distributions of radiated photons are compared with those predicted by Geant4, a very popular package used today for numerical simulation of different physical processes. We demonstrate that, while spectral distributions ideally coincide, the angular ones differ. We argue that transition radiation from the multilayered structure must contain sharp spikes having the interference nature and caused by the effect of merging two maximum frequencies in dispersive media, and thus Geant4 needs improving in this respect. The transition radiation theory developed here for the small-angle case can play a vital part for the possible future Small Angle Spectrometer at the LHC, other experiments of this kind, and detectors for hadrons of the tera-electron-volt energy range.
X-ray Transition radiation detectors (TRDs) are used for particle identification in both high energy physics and astroparticle physics. Particle identification is often achieved based on a threshold effect of the X-ray transition radiation (TR). In most of the detectors, TR emission starts at γ factors above ∼500 and reaches saturation at γ∼2−3⋅10³. However, many experiments require particle identification up to γ∼10⁵, which is difficult to achieve with current detectors, based only on the measurement of the photon energy together with the particle ionization losses. Additional information on the Lorentz factor can be extracted from the angular distribution of TR photons. TRDs based on pixel detectors give a unique opportunity for precise measurements of spectral and angular distributions of TR at the same time. A 500 μm thick silicon sensor bump bonded to a Timepix3 chip was used in a test beam measurement at the CERN SPS. A beam telescope was employed to separate clusters produced by the primary beam particles from the potential TR clusters. Spectral and angular distributions of TR were studied with high precision for the first time using beams of pions, electrons and muons at different momenta. In this paper, the measurement and analysis techniques are described, and first results are presented.
Hybrid pixel detectors were developed to meet the requirements for tracking in the inner layers at the LHC experiments. With low input capacitance per channel (10-100 fF) it is relatively straightforward to design pulse processing readout electronics with input referred noise of ∼100 e-rms and pulse shaping times consistent with tagging of events to a single LHC bunch crossing providing clean 'images' of the ionising tracks generated. In the Medipix Collaborations the same concept has been adapted to provide practically noise hit free imaging in a wide range of applications. This paper reports on the development of three generations of readout ASICs. Two distinctive streams of development can be identified: the Medipix ASICs which integrate data from multiple hits on a pixel and provide the images in the form of frames and the Timepix ASICs who aim to send as much information about individual interactions as possible off-chip for further processing. One outstanding circumstance in the use of these devices has been their numerous successful applications, thanks to a large and active community of developers and users. That process has even permitted new developments for detectors for High Energy Physics. This paper reviews the ASICs themselves and details some of the many applications.
We review the basic features of transition radiation and how they are used for the design of modern Transition Radiation Detectors (TRD). The discussion will include the various realizations of radiators as well as a discussion of the detection media and aspects of detector construction. With regard to particle identification we assess the different methods for efficient discrimination of different particles and outline the methods for the quantification of this property. Since a number of comprehensive reviews already exist, we predominantly focus on the detectors currently operated at the LHC. To a lesser extent we also cover some other TRDs, which are planned or are currently being operated in balloon or spaceborne astro-particle physics experiments.
A 300 μm thick thin p-on-n silicon sensor was connected to an energy sensitive pixel readout ASIC and exposed to a beam of highly energetic charged particles. By exploiting the spectral information and the fine segmentation of the detector, we were able to measure the evolution of the transverse profile of the charge carriers cloud in the sensor as a function of the drift distance from the point of generation. The result does not rely on model assumptions or electric field calculations. The data are also used to validate numerical simulations and to predict the detector spectral response to an X-ray fluorescence spectrum for applications in X-ray imaging.
The Timepix3, hybrid pixel detector (HPD) readout chip, a successor to the Timepix \cite{timepix2007} chip, can record time-of-arrival (ToA) and time-over-threshold (ToT) simultaneously in each pixel. ToA information is recorded in a 14-bit register at 40 MHz and can be refined by a further 4 bits with a nominal resolution of 1.5625 ns (640 MHz). ToT is recorded in a 10-bit overflow controlled counter at 40 MHz. Pixels can be programmed to record 14 bits of integral ToT and 10 bits of event counting, both at 40 MHz. The chip is designed in 130 nm CMOS and contains 256 × 256 pixel channels (55 × 55 μm²). The chip, which has more than 170 M transistors, has been conceived as a general-purpose readout chip for HPDs used in a wide range of applications. Common requirements of these applications are operation without a trigger signal, and sparse readout where only pixels containing event information are read out.
A new architecture has been designed for sparse readout and can achieve a throughput of up to 40 Mhits/s/cm². The flexible architecture offers readout schemes ranging from serial (one link) readout (40 Mbps) to faster parallel (up to 8 links) readout of 5.12 Gbps. In the ToA/ToT operation mode, readout is simultaneous with data acquisition thus keeping pixels sensitive at all times. The pixel matrix is formed by super pixel (SP) structures of 2 × 4 pixels. This optimizes resources by sharing the pixel readout logic which transports data from SPs to End-of-Column (EoC) using a 2-phase handshake protocol.
To reduce power consumption in applications with a low duty cycle, an on-chip power pulsing scheme has been implemented. The logic switches bias currents of the analog front-ends in a sequential manner, and all front-ends can be switched in 800 ns. The digital design uses a mixture of commercial and custom standard cell libraries and was verified using Open Verification Methodology (OVM) and commercial timing analysis tools. The analog front-end and a voltage-controlled oscillator for 1.5625 ns timing resolution have been designed using full custom techniques.
This paper presents new developments in Monte Carlo simulation for test beam measurements of a silicon transition radiation detector based on DEPFET—a silicon active pixel detector. The test of DEPFET with fiber radiator has been carried out at the DESY 5 GeV electron beam. Monte Carlo simulation of the test beam setup is based on Geant4. A comparison of Geant4 simulation with test beam data is presented.
Transition radiation detectors can be used for the measurements of the high energy cosmic ray nuclear composition up to Lorentz factors ∼105∼105 if the X-ray yields can be increased beyond those of present designs. The state of the art of current and recently proposed detectors is briefly reviewed, and the considerations are discussed governing the dependence of the transition radiation signal on particle energy. It is shown that the incident particle energy range accessible to transition radiation, and the total intensity produced, can be increased using foils with large plasma frequency. Graphene radiator foils are proposed as a potential high energy radiator.
The possibilities of the characteristic radiation method for constructing XTRD are explored with the help of Monte Carlo program, and it is shown that one can extend the values of Lorentz factors of detected particles up to the region 104-105. The proposed XTRDs can find wide application at present and future accelerators and colliders such as LEP, Tevatron, UNK, SSC, CLIC etc.
The characteristic dependence of the intensity of transition radiation (TR) on the Lorentz factor γ=E/mc2 of a primary particle is key to a number of practical applications. In particular, one may use TR detectors for energy measurements of heavy cosmic-ray nuclei in a region where alternate techniques are difficult to apply. However, a serious constraint can be the saturation of the TR yield at high γ-values. We investigate how the onset of saturation can be pushed to as high a Lorentz factor as possible. We then describe the results of test measurements at CERN, which demonstrate the possibility of practical configurations for measurements over the Lorentz factor range of a few hundred to about 105.
A project of a hadron identifier for secondary beams of Eh ≥ 0.25 Tev is proposed. The XTR quanta detection is carried out due to their Compton scattering in the radiator made of a light substance. So the charged particles do not pass through the quantum detector and do not produce a background due to ionization losses. The lateral XTR quanta are registered by gaseous-xenon detector made in the form of a cylinder with almost 4π geometry. The Monte-Carlo calculation of the beam identifier has been carried out for a LiH radiator with the number of 0.05 mm-thick layers of 3360 at b = 1 mm air gaps.
Transition and Cherenkov radiation arising when a charged particle moves
in succession through two media differing in respect to dielectric and magnetic
properties are considered. The cases when the particle enters the medium from
vacuum or vacuum from the medium are discussed in detail. (tr-auth)
The semiconductor pixel detector Timepix (256×256 pixels with pitch of 55 μm) is a successor of the Medipix2 device. Each Timepix pixel can be independently operated in one of three possible modes: (1) counting of the detected particles; (2) measurement of the particle energy; and (3) measurement of the time of interaction. The energy measurement in the second mode is performed via the determination of the “time-over-threshold” (TOT). The energy measurement with the Timepix detector in TOT mode requires knowledge of the energy calibration of each pixel of the matrix. Such calibration is very nonlinear in the low energy range and can be described by a surrogate function depending on four parameters. The determination of all these parameters can be performed by measurement and evaluation of the response of each pixel in at least four calibration points. The procedure is extremely demanding: it requires the analysis of at least 250 thousand spectra and the performance of 330 thousand least-squares fits. In this article, it is demonstrated that even better result can be achieved with only two or three calibration points halving the number of least-squares fits needed. The method is based on precise analysis of the shape of spectral peaks. The article also discusses the performance of energy calibrated device for spectrometry of heavy charged particles.
A novel approach for the readout of a TPC at the future linear collider is to use a CMOS pixel detector combined with some kind of gas gain grid. A first test using the photon counting chip Medipix2 with GEM or Micromegas demonstrated the feasibility of such an approach. Although this experiment demonstrated that single primary electrons could be detected the chip did not provide information on the arrival time of the electron in the sensitive gas volume nor did it give any indication of the quantity of charge detected. The Timepix chip uses an external clock with a frequency of up to 100 MHz as a time reference. Each pixel contains a preamplifier, a discriminator with hysteresis and 4-bit DAC for threshold adjustment, synchronization logic and a 14-bit counter with overflow control. Moreover, each pixel can be independently configured in one of four different modes: masked mode: pixel is off, counting mode: 1-count for each signal over threshold, TOT mode: the counter is incremented continuously as long as the signal is above threshold, and arrival time mode: the counter is incremented continuously from the time the first hit arrives until the end of the shutter. The chip resembles very much the Medipix2 chip physically and can be readout using slightly modified versions of the various existing systems. This paper presents the main features of the new design, electrical measurements and some first images.
A large area transition radiation detector has been proposed for the European Hybrid Spectrometer at CERN. Its construction is based on experience with a smaller size model detector which has been tested in hadron beams between 40 and 140 GeV/c momentum. The detector allows identification of pions versus kaons and protons at momenta beyond ∼90 GeV/c.
The use of transition radiation (TR) as a means of identifying high energy particles has now become a subject of intensive experimental investigations and applications. Our intention is first to study the physics of these phenomena and to describe ways of building detectors which can efficiently identify particles.
A prototype particle tracking telescope has been constructed using Timepix
and Medipix ASIC hybrid pixel assemblies as the six sensing planes. Each
telescope plane consisted of one 1.4 cm2 assembly, providing a 256x256 array of
55 micron square pixels. The telescope achieved a pointing resolution of 2.3
micron at the position of the device under test. During a beam test in 2009 the
telescope was used to evaluate in detail the performance of two Timepix hybrid
pixel assemblies; a standard planar 300 micron thick sensor, and 285 micron
thick double sided 3D sensor. This paper describes a detailed charge
calibration study of the pixel devices, which allows the true charge to be
extracted, and reports on measurements of the charge collection characteristics
and Landau distributions. The planar sensor achieved a best resolution of 4.0
micron for angled tracks, and resolutions of between 4.4 and 11 micron for
perpendicular tracks, depending on the applied bias voltage. The double sided
3D sensor, which has significantly less charge sharing, was found to have an
optimal resolution of 9.0 micron for angled tracks, and a resolution of 16.0
micron for perpendicular tracks. Based on these studies it is concluded that
the Timepix ASIC shows an excellent performance when used as a device for
charged particle tracking.
The generation and detection of transition radiation have been studied in a series of experiments with electrons from 1 to 15 GeV at SLAC and at the Cornell Synchrotron. Periodic radiators, consisting of thin plastic foils stretched in air at constant spacings, were used, and proportional chambers filled with krypton or xenon served as detectors. A detailed discussion of the theoretical predictions is given, and the measurements are systematically compared with the predictions by varying the most critical parameters, such as configuration of radiators and detectors, and energy of the electrons. In general, good agreement between theory and experiment has been found. On the basis of these results, the criteria are summarized under which transition radiation can readily be observed.
X-ray transition radiation can be used to measure the Lorentz factor of relativistic particles. Standard transition radiation detectors (TRDs) typically incorporate thin plastic foil radiators and gas-filled x-ray detectors, and are sensitive up to \gamma ~ 10^4. To reach higher Lorentz factors (up to \gamma ~ 10^5), thicker, denser radiators can be used, which consequently produce x-rays of harder energies (>100 keV). At these energies, scintillator detectors are more efficient in detecting the hard x-rays, and Compton scattering of the x-rays out of the path of the particle becomes an important effect. The Compton scattering can be utilized to separate the transition radiation from the ionization background spatially. The use of conducting metal foils is predicted to yield enhanced signals compared to standard nonconducting plastic foils of the same dimensions. We have designed and built a Compton Scatter TRD optimized for high Lorentz factors and exposed it to high energy electrons at the CERN SPS. We present the results of the accelerator tests and comparisons to simulations, demonstrating 1) the effectiveness of the Compton Scatter TRD approach; 2) the performance of conducting aluminum foils; and 3) the ability of a TRD to measure energies approximately an order of magnitude higher than previously used in very high energy cosmic ray studies. Comment: 10 pages, 4 figures, To be published in NIM
Transition radiation effects in particle energy losses
- Garibian