- Email: [email protected]
High rate proton detection with single photon counting hybrid pixel detector
Journal Pre-proof High rate proton detection with single photon counting hybrid pixel detector Anna Koziol, Artur Apresyan, Pawel Grybos, Ryan Heller, Piotr Maj, Mohd Meraj Hussain, Robert Szczygiel, Si Xie
PII: DOI: Reference:
S0168-9002(19)31564-5 https://doi.org/10.1016/j.nima.2019.163333 NIMA 163333
To appear in:
Nuclear Inst. and Methods in Physics Research, A
Received date : 18 July 2019 Revised date : 19 December 2019 Accepted date : 22 December 2019 Please cite this article as: A. Koziol, A. Apresyan, P. Grybos et al., High rate proton detection with single photon counting hybrid pixel detector, Nuclear Inst. and Methods in Physics Research, A (2019), doi: https://doi.org/10.1016/j.nima.2019.163333. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
*Manuscript Click here to view linked References
Journal Pre-proof
1 2
High rate proton detection with single photon counting hybrid pixel detector
3 4
Department of Measurement and Electronics, AGH University of Science and Technology, Mickiewicza av. 30, Cracow, 30-059, Poland b
9
Fermi National Accelerator Laboratory, Wilson Street and Kirk Road, Batavia, 60510, Il, USA c
10 d California
11
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a
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Anna Koziola,*, Artur Apresyanb, Pawel Grybosa, Ryan Hellerb, Piotr Maja, Mohd Meraj Hussainb,c, Robert Szczygiela, Si Xied
5 6
University of Chicago, Ellis Ave, Chicago, 5801, Il, USA
Institute of Technology, 1200 E California Blvd, Pasadena, CA 91125, USA
12
Abstract
14
We present the design and measurements of a proton detection system built using a single photon counting hybrid pixel array detector. The system uses the UFXC detector designed for operating with very high photon fluxes. We demonstrate, that with appropriately modified data acquisition firmware, the UFXC detector is capable of operating in the frame-triggered zero dead-time mode, which captures every frame containing desired information, at a rate of up to 50 kfps. The detector consists of a 128 x 256 matrix of square-shaped pixels with a pitch of 75 μm, which makes it suitable for particle tracking applications in test beam environments. We estimate the position resolution achieved with a single layer of the UFXC detector to be around 40 μm.
23 Keywords
24
Hybrid Pixel Array Detector; Single Photon Counting; Tracking, UFXC 1. Introduction
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Test beam measurements performed at the Fermilab Test Beam Facility (FTBF) [1] provide a unique opportunity to characterize prototype detectors for collider experiments. A typical application is to place a device under test (DUT) in the high energy beam, and measure the DUT’s response to the beam particles that pass through its active area. Precise tracking of beam particles is desirable in test beam experiments in order to precisely map the response of the DUT to the position through which the particle passed the detector. Low amount of detector material and high trigger rate capability are
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Corresponding author: [email protected]
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key requirements in the case of high energy particle tracking. While UFXC-based detectors are typically used for 2-D X-ray imaging [2], such as in biomedical or material science applications, the detector meets the requirements for tests at FTBF due to its fine pixel granularity, low material budget and high trigger rate capability, and was therefore incorporated into the setup operating at the FTBF beamline.
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The UFXC detector was tested in an experimental setup specifically constructed for the purpose of characterizing low-gain avalanche diodes (LGAD) to be used for the timing detector upgrade of the CMS experiment [3]. The FTBF facility is equipped with a telescope tracking system that provides precision measurements of particle trajectories. However, due to mechanical constraints, the telescope tracking system at FTBF is located about one meter downstream from the LGAD experimental station, and the extrapolation of track trajectories to the DUT area results in a poor resolution of position measurement. Due to the very compact size of the system it is simple to integrate the UFXC detector into the LGAD setup and thus to provide additional precision to particle tracking for the LGAD characterization. The UFXC position resolution measurements were done using the 120 GeV proton beam and triggered by an LGAD sensor placed about 10 cm upstream of the UFXC detector.
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The paper is organized as follows: the UFXC detector is described in Section 2, the experimental setup is described in Section 3, beam test results are described in Section 4, followed by conclusions in Section 5.
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2. UFXC detector
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The UFXC is an integrated readout circuit designed for hybrid pixel array detectors (HPAD) [4], used for X-ray photon detection. The chip is capable of working with different types of semiconductor sensors such as Si, CdTe, and others, allowing for the detection of photons in a wide energy range. The pixel matrix is designed as an array of 128 x 256 square pixels with a 75 µm pitch. Each pixel contains analog and digital blocks dedicated to signal processing and fast data readout comprising of a charge sensitive amplifier, a shaper and two independent discriminators followed by two independent 14-bit depth counters. The use of discriminator blocks allows for separating signals generated in the sensor from electronics noise yielding noiseless counting of incoming photons.
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The UFXC can be configured to operate in the following modes [5]:
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Standard – two independent counters are connected to two independent discriminators allowing imaging within an energy window;
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High Dynamic Range – two independent 14-bit counters configured into one 28-bit counter connected to one discriminator;
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Zero Dead-Time mode – two counters alternately counting single discriminator events with zero dead-time of switching between the counters; a time required for the counter’s readout defines a frame rate of the ZDT mode;
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Burst mode – two 14-bit counters are virtually split into fourteen 2-bit sub-counters, allowing exceptionally high-speed acquisition of 14 successive images.
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The feature that distinguish the UFXC from other single photon counting HPADs is its exceptionally high frame rate exceeding 50 kfps continuous acquisition in zero dead-time mode and 1.2 Mfps in burst mode [6]. This unique feature of a high frame rate allows the UFXC detector to be considered for use in time-resolved experiments at synchrotron facilities but also a potential candidate for particle tracking detection systems.
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3. Experimental setup
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3.1 LGAD and Telescope tracker systems at FTBF
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Test-beam measurements were performed at FTBF, which provides a 120 GeV proton beam from the Fermilab Main Injector accelerator. The FTBF beam is resonantly extracted in a slow spill for each Main Injector cycle delivering a single 4.2 sec long spill per minute, tuned to yield approximately 60 000 protons per single spill. The primary beam (bunched at 53 MHz) consists of 120 GeV protons. All measurements presented in this paper were taken with the primary beam particles.
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The LGAD DUTs and the UFXC-detector were mounted on a remotely operated motorized stage, placed inside an environmental chamber with controlled temperature and humidity. A schematic diagram of the experimental setup are shown in Fig. 1, which presents the arrangement of five LGAD detectors and the UFXC detector. The relative alignment was mechanically constrained to be around one mm, and we estimate the slant angle to be less than 5 degrees. LGAD detectors were placed upstream of the UFXC detector. The first LGAD after UFXC detector was the HPK 50D [7], while the rest were manufactured by Fondazione Bruno Kessler (FBK). The HPK 50D is manufactured by Hamamatsu and is composed of 4 square channels each with area of 3x3 mm2. The trigger to the telescope, to the UFXC detector, and to the LGAD DAQ system was provided by a signal from the HPK 50D detector output going into a NIM discriminator module. The DAQ system for the LGAD DUTs is based on a CAEN V1742 digitizer board [8], which provides digitized waveforms sampled at 5 GS/s, and with one ADC count corresponding to 0.25 mV. A trigger signal was generated when any of the four channels on the HPK 50D sensor registered signal. The trigger rate was tuned to be about 6 000 triggers per single spill, in order to ensure that the CAEN V1742 and pixel telescope maintained full synchronization.
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Fig. 1 A schematic diagram of the test beam setup of the LGAD characterization experimental station. The FTBF is equipped with two silicon telescopes aligned along the beam line and configured to operate synchronously [9]. It has a pixel telescope assembled from eight planes and a telescope with strip modules made up of fourteen detector planes. Each microstrip plane consists of 639 microstrips, each 60 μm wide, placed orthogonally to each other, and the pixel telescope’s cell sizes are 100x150 μm. The strip telescope increases the coverage of the pixel telescope and improves its tracking performance. The Data Acquisition (DAQ) hardware is based on the CAPTAN (Compact And Programmable daTa Acquisition Node) system developed at Fermilab. The CAPTAN is a flexible and versatile data acquisition system designed to meet the readout and control demands of a variety of pixel and strip detectors for high energy physics applications. At the control room PC, the data from each CAPTAN node are saved in a binary file for each Run. Data from the UFXC, the silicon telescope, and DUTs were merged offline by matching the trigger counters of each system.
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3.2 UFXC detector setup
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A dedicated UFXC-based setup was developed for the purpose of the experiment at FTBF. The detector module consists of a single UFXC bump-bonded to a 320 µm thick silicon sensor. The detector is controlled by a National Instruments (NI) 7935R FlexRIO controller with a Kintex-7 FPGA, and an NI 6589 high-speed digital I/O adapter module. The UFXC operates in a non-standard zero dead-time mode controlled by FPGA with an additional functionality implemented allowing by-trigger single image separation from the 50 kfps continuous stream. The FPGA is used for chip control, data acquisition, image reconstruction and reduction of data with the use of a zerosuppression algorithm. Carrying pre-processing in the FPGA improves data throughput and reduces further post-processing of the collected frames. As an additional functionality, the FPGA can single out one certain frame when the trigger is received, and assign a time stamp to this frame.
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One of the basic principles of UFXC detector operation is the use of signal level discrimination blocks to distinguish the event from the electronics noise. The UFXC detector is optimized for 8 keV X-ray photons generating of around 2200 electrons in a silicon sensor. Inside this very same 320 µm thick silicon sensor, a single proton generates more than 10 times bigger charge.
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In order not to saturate the front-end, a non-standard, low-gain mode is used enforced by changing the active feedback blocks of the shaper blocks of the UFXC readout circuit. As it was not possible to characterize the detector with such a high signal generated from X-ray photons, the front-end behavior was verified prior to the experiment using protons from the beam. Acquiring the events while changing the discriminator level from the level of the noise to above the level of the signal, a so-called threshold scan is drawn (Fig. 2). Each point of the scan was acquired during different proton-beam spills and so small fluctuations of about 10% are visible. Despite those fluctuations the threshold scan taken prior to the experiment clearly shows a wide valid range of the discriminator level, where the signal generated by proton particles is separated from front-end electronics noise. In the experiment the discriminator was set to the constant value of 220 DAC LSB.
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The charge generated when a proton passes through the UFXC detector is collected by all pixels along the particle path. As the charge generated by the particle is large, it can be collected by more than a single pixel, as seen in a few examples in Fig. 3. Fig. 4 shows the distribution of the number of pixels induced by a single event, and this histogram corresponds to data collected within a single spill. The histogram indicates that the precision of the location of the event can be improved by clustering the pixels registered in the event. A simple center of gravity algorithm was used for clustering in the analysis.
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Fig. 2 Distribution of the UFXC event counts as a function of the discriminator level.
Fig. 4 Histogram of induced pixels registered in a single trigger
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Fig. 3 Example pixel patterns collected by UFXC after proton transition. The red pixel is the one determined as the center of gravity.
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4. Test beam results
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4.1 Verification of triggered zero dead-time mode
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One of the most demanding requirements for the UFXC detector-based system is to record every event triggered by the signal generated by the LGAD sensor. To achieve that, the FPGA controlling the detector assigns a timestamp to the currently acquiring frame each time the trigger signal arrives. In this section, we present the result of the experimental verification of the detector operation.
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Fig. 5 (a) presents signals generated by the protons from the FTBF beam which were collected during 30 minutes of data acquisition. During this period the trigger to the UFXC was provided by the HPK 50D sensor. The trigger signal was raised when any of the four channels in HPK 50D sensor registers a signal. In Fig. 5 (a), distinct square shapes can be observed matching the location and dimensions of the four channels of the HPK 50D detector. More than that, a certain gradient of events can be drawn from the center to the edges. In order to assure this gradient is caused by the shape of the proton beam, the UFXC detector operation was set to a self-triggered mode collecting every frame containing an event. This mode allows for a proton beam profile registration with higher statistics (Fig. 5 (b)). The gradient of counts visible in Fig. 5 (a) reflects the shape and intensity of the beam and prove the gradient visible in the registration of triggered events is solely related to the beam profile.
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(a) (b) Fig. 5 Protons detected by the UFXC detector in trigger mode (a) and in continuous mode (b). Axis scales are expressed in pixels of the UFXC and narrowed down to the area of interests.
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Fig. 6 presents the horizontal and vertical projections of the plots shown in Fig. 5. In the region covered by the LGAD channels the data taken in both modes (the externally triggered by the HPK 50D detector and the self-triggered mode) overlap proving that the external trigger from LGAD detector operates properly. The sharp drop in the middle of both plots indicates the inactive area separating the channels called the “inter-channel gap”.
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(a) (b) Fig. 6 Horizontal (a) and vertical (b) total counts recorded by the UFXC.
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4.2 Position resolution
In this section we present the measurements of the position resolution achieved with the UFXC detector. We measure the UFXC position resolution by measuring the particle detection efficiency across the HPK 50D sensor surface, including the inactive region in the transition region between two neighboring pixels, called the inter-channel gap. The transition from the active pixel area to the inactive area is smeared by the position resolution of the measuring device - the UFXC in this case. We estimate the shape of this efficiency curve in the transition region by an error function, shown
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in Eq. 1, from which we extract the parameter σ, which corresponds to the position resolution. The efficiencies in the region to the left and right of the inter-channel were both fitted to error functions, as shown in Fig. 7.
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𝑓(𝑥) =
1 2
𝑥−𝜇
𝐴 (1 + erf (
√2𝜎
)) + 𝑝
The sigma extracted from the region to the left and right of the inter-channel gap are 37 and 39 µm, respectively. Charge sharing improves the resolution over the pixel pitch size [10] but the measurement is potentially impacted by a relative rotation between the UFXC detector and the HPK 50D LGAD. We infer that the measured resolution of about 40 µm is an upper limit on the UFXC position resolution. The inter-channel gap width is measured as a distance between the centers of both fits at 50% of their amplitude. The measured width is 107 µm and is in good agreement with the measurement performed using the tracker telescope and with the HPK 50D sensor placed at the center of the telescope [7].
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Eq. 1
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Fig. 7 LGAD inter-channel gap fit
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4.3 Detection efficiency
In particle detection devices, an important parameter is the detection efficiency calculated as the ratio of the number of events registered by the detector to the total number of particles generated. In the presented analysis, telescope event counts were used as a reference to estimate detection efficiency of the UFXC detector. The events were compared trigger by trigger between the telescope and UFXC detector systems. Fig. 8 presents calculated detection efficiency for data collected during five different measurements. The average parameter value equals to 96%.
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Detection efficiency 100.00% 90.00% 80.00%
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70.00% 60.00% 50.00% 40.00% 20.00% 10.00% 0.00% Meas A
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Fig. 8 Detection efficiency of the UFXC detector calculated in regards to the events registered by the telescope. The different measurements represent different data batches called “runs” taking at different times.
5. Conclusions
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We performed studies of high energy particle detection with the UFXC single photon counting hybrid pixel array detector using the 120 GeV proton beam at the Fermilab Test Beam Facility, and demonstrated that the UFXC detector is capable of operating in the frame-triggered zero dead-time mode, and can be used as part of a system for charged particle tracking. The position resolution achieved by the UFXC detector was measured to be less than 40 μm. These results prove the successful operation of the UFXC detector system in the zero dead-time mode at high frame rate of 50 kfps, and represent a promising first step in establishing a pixelated tracking detector for test beam setups. We intend to expand the system to use two or more planes of the UFXC modules, combining several measurements of the beam particles for improved position resolution.
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Acknowledgements
This work was supported by the National Science Center, Poland, under contract no. UMO-2018/29/N/ST7/02786, and FNAL LDRD award 2017-027.
215 References
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[1]
"Fermilab Test Beam Facility," [Online]. Available: https://ftbf.fnal.gov.
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[2]
P. Grybos, P. Kmon, P. Maj, R. Szczygiel, "32k Channel Readout IC for Single Photon Counting Pixel Detectors with 75 μm Pitch, Dead Time of 85 ns, 9 e- rms Offset Spread and 2% rms Gain Spread", IEEE Trans. Nucl. Sci. 63 (2016) 1155–1161. doi:10.1109/TNS.2016.2523260.
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[3]
CMS Collaboration, "A MIP Timing Detector for the CMS Phase-2 Upgrade", CERNLHCC-2019-003, 2019, https://cds.cern.ch/record/2667167
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[4]
R. Ballabriga et. al., "Review of hybrid pixel detector readout ASICs for spectroscopic Xray imaging", J. Instrum. 11 (2016) P01007–P01007. doi:10.1088/1748-0221/11/01/P01007.
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[5]
A. Koziol, P. Maj, "High speed systems for time-resolved experiments with synchrotron radiation", J. Instrum. 13 (2018) C02049.
228 229 230
[6]
Q. Zhang et al., "Sub-microsecond-resolved multi-speckle X-ray photon correlation spectroscopy with a pixel array detector", J. Synchrotron Radiat. 25 (2018) 1408–1416. doi:10.1107/S1600577518009074.
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[7]
A. Apresyan et al., "Studies of uniformity of 50 μm low-gain avalanche detectors at the Fermilab test beam", Nucl. Instrum. Methods A, 895 (2018) 158–172. doi:10.1016/j.nima.2018.03.074.
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[8]
Caen V1742, [Online]. Available at https://www.caen.it/products/v1742/
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[9]
S. Kwan et al., "The pixel tracking telescope at the Fermilab Test Beam Facility", Nucl. Instrum. Methods A, 811 (2016) 162–169. doi:10.1016/j.nima.2015.12.003.
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[10] K. Mathieson et al., "Charge sharing in silicon pixel detectors", Nucl. Instrum. Methods A, 487 (2002) 113–122. doi:10.1016/S0168-9002(02)00954-3.
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*Author Contributions Section
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CRediT author statement Anna Koziol: Software, Investigation, Visualization, Writing - Original Draft, Funding acquisition
Pawel Grybos: Resources Ryan Heller: Investigation
Mohd Meraj Hussain: Investigation Robert Szczygiel: Resources
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Si Xie: Investigation, Writing - Review & Editing
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Piotr Maj: Software, Supervision, Writing - Review & Editing
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Artur Apresyan: Conceptualization, Methodology, Writing - Review & Editing, Funding acquisition
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*Declaration of Interest Statement
Declaration of interests
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☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
PII: DOI: Reference:
S0168-9002(19)31564-5 https://doi.org/10.1016/j.nima.2019.163333 NIMA 163333
To appear in:
Nuclear Inst. and Methods in Physics Research, A
Received date : 18 July 2019 Revised date : 19 December 2019 Accepted date : 22 December 2019 Please cite this article as: A. Koziol, A. Apresyan, P. Grybos et al., High rate proton detection with single photon counting hybrid pixel detector, Nuclear Inst. and Methods in Physics Research, A (2019), doi: https://doi.org/10.1016/j.nima.2019.163333. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
*Manuscript Click here to view linked References
Journal Pre-proof
1 2
High rate proton detection with single photon counting hybrid pixel detector
3 4
Department of Measurement and Electronics, AGH University of Science and Technology, Mickiewicza av. 30, Cracow, 30-059, Poland b
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Fermi National Accelerator Laboratory, Wilson Street and Kirk Road, Batavia, 60510, Il, USA c
10 d California
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Anna Koziola,*, Artur Apresyanb, Pawel Grybosa, Ryan Hellerb, Piotr Maja, Mohd Meraj Hussainb,c, Robert Szczygiela, Si Xied
5 6
University of Chicago, Ellis Ave, Chicago, 5801, Il, USA
Institute of Technology, 1200 E California Blvd, Pasadena, CA 91125, USA
12
Abstract
14
We present the design and measurements of a proton detection system built using a single photon counting hybrid pixel array detector. The system uses the UFXC detector designed for operating with very high photon fluxes. We demonstrate, that with appropriately modified data acquisition firmware, the UFXC detector is capable of operating in the frame-triggered zero dead-time mode, which captures every frame containing desired information, at a rate of up to 50 kfps. The detector consists of a 128 x 256 matrix of square-shaped pixels with a pitch of 75 μm, which makes it suitable for particle tracking applications in test beam environments. We estimate the position resolution achieved with a single layer of the UFXC detector to be around 40 μm.
23 Keywords
24
Hybrid Pixel Array Detector; Single Photon Counting; Tracking, UFXC 1. Introduction
27 28 29 30 31 32 33
Test beam measurements performed at the Fermilab Test Beam Facility (FTBF) [1] provide a unique opportunity to characterize prototype detectors for collider experiments. A typical application is to place a device under test (DUT) in the high energy beam, and measure the DUT’s response to the beam particles that pass through its active area. Precise tracking of beam particles is desirable in test beam experiments in order to precisely map the response of the DUT to the position through which the particle passed the detector. Low amount of detector material and high trigger rate capability are
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Corresponding author: [email protected]
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key requirements in the case of high energy particle tracking. While UFXC-based detectors are typically used for 2-D X-ray imaging [2], such as in biomedical or material science applications, the detector meets the requirements for tests at FTBF due to its fine pixel granularity, low material budget and high trigger rate capability, and was therefore incorporated into the setup operating at the FTBF beamline.
39 40 41 42 43 44 45 46 47 48 49
The UFXC detector was tested in an experimental setup specifically constructed for the purpose of characterizing low-gain avalanche diodes (LGAD) to be used for the timing detector upgrade of the CMS experiment [3]. The FTBF facility is equipped with a telescope tracking system that provides precision measurements of particle trajectories. However, due to mechanical constraints, the telescope tracking system at FTBF is located about one meter downstream from the LGAD experimental station, and the extrapolation of track trajectories to the DUT area results in a poor resolution of position measurement. Due to the very compact size of the system it is simple to integrate the UFXC detector into the LGAD setup and thus to provide additional precision to particle tracking for the LGAD characterization. The UFXC position resolution measurements were done using the 120 GeV proton beam and triggered by an LGAD sensor placed about 10 cm upstream of the UFXC detector.
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The paper is organized as follows: the UFXC detector is described in Section 2, the experimental setup is described in Section 3, beam test results are described in Section 4, followed by conclusions in Section 5.
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2. UFXC detector
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The UFXC is an integrated readout circuit designed for hybrid pixel array detectors (HPAD) [4], used for X-ray photon detection. The chip is capable of working with different types of semiconductor sensors such as Si, CdTe, and others, allowing for the detection of photons in a wide energy range. The pixel matrix is designed as an array of 128 x 256 square pixels with a 75 µm pitch. Each pixel contains analog and digital blocks dedicated to signal processing and fast data readout comprising of a charge sensitive amplifier, a shaper and two independent discriminators followed by two independent 14-bit depth counters. The use of discriminator blocks allows for separating signals generated in the sensor from electronics noise yielding noiseless counting of incoming photons.
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The UFXC can be configured to operate in the following modes [5]:
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Standard – two independent counters are connected to two independent discriminators allowing imaging within an energy window;
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High Dynamic Range – two independent 14-bit counters configured into one 28-bit counter connected to one discriminator;
69 70 71
Zero Dead-Time mode – two counters alternately counting single discriminator events with zero dead-time of switching between the counters; a time required for the counter’s readout defines a frame rate of the ZDT mode;
72 73
Burst mode – two 14-bit counters are virtually split into fourteen 2-bit sub-counters, allowing exceptionally high-speed acquisition of 14 successive images.
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The feature that distinguish the UFXC from other single photon counting HPADs is its exceptionally high frame rate exceeding 50 kfps continuous acquisition in zero dead-time mode and 1.2 Mfps in burst mode [6]. This unique feature of a high frame rate allows the UFXC detector to be considered for use in time-resolved experiments at synchrotron facilities but also a potential candidate for particle tracking detection systems.
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3. Experimental setup
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3.1 LGAD and Telescope tracker systems at FTBF
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Test-beam measurements were performed at FTBF, which provides a 120 GeV proton beam from the Fermilab Main Injector accelerator. The FTBF beam is resonantly extracted in a slow spill for each Main Injector cycle delivering a single 4.2 sec long spill per minute, tuned to yield approximately 60 000 protons per single spill. The primary beam (bunched at 53 MHz) consists of 120 GeV protons. All measurements presented in this paper were taken with the primary beam particles.
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The LGAD DUTs and the UFXC-detector were mounted on a remotely operated motorized stage, placed inside an environmental chamber with controlled temperature and humidity. A schematic diagram of the experimental setup are shown in Fig. 1, which presents the arrangement of five LGAD detectors and the UFXC detector. The relative alignment was mechanically constrained to be around one mm, and we estimate the slant angle to be less than 5 degrees. LGAD detectors were placed upstream of the UFXC detector. The first LGAD after UFXC detector was the HPK 50D [7], while the rest were manufactured by Fondazione Bruno Kessler (FBK). The HPK 50D is manufactured by Hamamatsu and is composed of 4 square channels each with area of 3x3 mm2. The trigger to the telescope, to the UFXC detector, and to the LGAD DAQ system was provided by a signal from the HPK 50D detector output going into a NIM discriminator module. The DAQ system for the LGAD DUTs is based on a CAEN V1742 digitizer board [8], which provides digitized waveforms sampled at 5 GS/s, and with one ADC count corresponding to 0.25 mV. A trigger signal was generated when any of the four channels on the HPK 50D sensor registered signal. The trigger rate was tuned to be about 6 000 triggers per single spill, in order to ensure that the CAEN V1742 and pixel telescope maintained full synchronization.
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Fig. 1 A schematic diagram of the test beam setup of the LGAD characterization experimental station. The FTBF is equipped with two silicon telescopes aligned along the beam line and configured to operate synchronously [9]. It has a pixel telescope assembled from eight planes and a telescope with strip modules made up of fourteen detector planes. Each microstrip plane consists of 639 microstrips, each 60 μm wide, placed orthogonally to each other, and the pixel telescope’s cell sizes are 100x150 μm. The strip telescope increases the coverage of the pixel telescope and improves its tracking performance. The Data Acquisition (DAQ) hardware is based on the CAPTAN (Compact And Programmable daTa Acquisition Node) system developed at Fermilab. The CAPTAN is a flexible and versatile data acquisition system designed to meet the readout and control demands of a variety of pixel and strip detectors for high energy physics applications. At the control room PC, the data from each CAPTAN node are saved in a binary file for each Run. Data from the UFXC, the silicon telescope, and DUTs were merged offline by matching the trigger counters of each system.
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3.2 UFXC detector setup
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A dedicated UFXC-based setup was developed for the purpose of the experiment at FTBF. The detector module consists of a single UFXC bump-bonded to a 320 µm thick silicon sensor. The detector is controlled by a National Instruments (NI) 7935R FlexRIO controller with a Kintex-7 FPGA, and an NI 6589 high-speed digital I/O adapter module. The UFXC operates in a non-standard zero dead-time mode controlled by FPGA with an additional functionality implemented allowing by-trigger single image separation from the 50 kfps continuous stream. The FPGA is used for chip control, data acquisition, image reconstruction and reduction of data with the use of a zerosuppression algorithm. Carrying pre-processing in the FPGA improves data throughput and reduces further post-processing of the collected frames. As an additional functionality, the FPGA can single out one certain frame when the trigger is received, and assign a time stamp to this frame.
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One of the basic principles of UFXC detector operation is the use of signal level discrimination blocks to distinguish the event from the electronics noise. The UFXC detector is optimized for 8 keV X-ray photons generating of around 2200 electrons in a silicon sensor. Inside this very same 320 µm thick silicon sensor, a single proton generates more than 10 times bigger charge.
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In order not to saturate the front-end, a non-standard, low-gain mode is used enforced by changing the active feedback blocks of the shaper blocks of the UFXC readout circuit. As it was not possible to characterize the detector with such a high signal generated from X-ray photons, the front-end behavior was verified prior to the experiment using protons from the beam. Acquiring the events while changing the discriminator level from the level of the noise to above the level of the signal, a so-called threshold scan is drawn (Fig. 2). Each point of the scan was acquired during different proton-beam spills and so small fluctuations of about 10% are visible. Despite those fluctuations the threshold scan taken prior to the experiment clearly shows a wide valid range of the discriminator level, where the signal generated by proton particles is separated from front-end electronics noise. In the experiment the discriminator was set to the constant value of 220 DAC LSB.
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The charge generated when a proton passes through the UFXC detector is collected by all pixels along the particle path. As the charge generated by the particle is large, it can be collected by more than a single pixel, as seen in a few examples in Fig. 3. Fig. 4 shows the distribution of the number of pixels induced by a single event, and this histogram corresponds to data collected within a single spill. The histogram indicates that the precision of the location of the event can be improved by clustering the pixels registered in the event. A simple center of gravity algorithm was used for clustering in the analysis.
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Fig. 2 Distribution of the UFXC event counts as a function of the discriminator level.
Fig. 4 Histogram of induced pixels registered in a single trigger
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Fig. 3 Example pixel patterns collected by UFXC after proton transition. The red pixel is the one determined as the center of gravity.
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4. Test beam results
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4.1 Verification of triggered zero dead-time mode
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One of the most demanding requirements for the UFXC detector-based system is to record every event triggered by the signal generated by the LGAD sensor. To achieve that, the FPGA controlling the detector assigns a timestamp to the currently acquiring frame each time the trigger signal arrives. In this section, we present the result of the experimental verification of the detector operation.
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Fig. 5 (a) presents signals generated by the protons from the FTBF beam which were collected during 30 minutes of data acquisition. During this period the trigger to the UFXC was provided by the HPK 50D sensor. The trigger signal was raised when any of the four channels in HPK 50D sensor registers a signal. In Fig. 5 (a), distinct square shapes can be observed matching the location and dimensions of the four channels of the HPK 50D detector. More than that, a certain gradient of events can be drawn from the center to the edges. In order to assure this gradient is caused by the shape of the proton beam, the UFXC detector operation was set to a self-triggered mode collecting every frame containing an event. This mode allows for a proton beam profile registration with higher statistics (Fig. 5 (b)). The gradient of counts visible in Fig. 5 (a) reflects the shape and intensity of the beam and prove the gradient visible in the registration of triggered events is solely related to the beam profile.
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(a) (b) Fig. 5 Protons detected by the UFXC detector in trigger mode (a) and in continuous mode (b). Axis scales are expressed in pixels of the UFXC and narrowed down to the area of interests.
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Fig. 6 presents the horizontal and vertical projections of the plots shown in Fig. 5. In the region covered by the LGAD channels the data taken in both modes (the externally triggered by the HPK 50D detector and the self-triggered mode) overlap proving that the external trigger from LGAD detector operates properly. The sharp drop in the middle of both plots indicates the inactive area separating the channels called the “inter-channel gap”.
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(a) (b) Fig. 6 Horizontal (a) and vertical (b) total counts recorded by the UFXC.
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4.2 Position resolution
In this section we present the measurements of the position resolution achieved with the UFXC detector. We measure the UFXC position resolution by measuring the particle detection efficiency across the HPK 50D sensor surface, including the inactive region in the transition region between two neighboring pixels, called the inter-channel gap. The transition from the active pixel area to the inactive area is smeared by the position resolution of the measuring device - the UFXC in this case. We estimate the shape of this efficiency curve in the transition region by an error function, shown
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in Eq. 1, from which we extract the parameter σ, which corresponds to the position resolution. The efficiencies in the region to the left and right of the inter-channel were both fitted to error functions, as shown in Fig. 7.
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𝑓(𝑥) =
1 2
𝑥−𝜇
𝐴 (1 + erf (
√2𝜎
)) + 𝑝
The sigma extracted from the region to the left and right of the inter-channel gap are 37 and 39 µm, respectively. Charge sharing improves the resolution over the pixel pitch size [10] but the measurement is potentially impacted by a relative rotation between the UFXC detector and the HPK 50D LGAD. We infer that the measured resolution of about 40 µm is an upper limit on the UFXC position resolution. The inter-channel gap width is measured as a distance between the centers of both fits at 50% of their amplitude. The measured width is 107 µm and is in good agreement with the measurement performed using the tracker telescope and with the HPK 50D sensor placed at the center of the telescope [7].
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Fig. 7 LGAD inter-channel gap fit
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4.3 Detection efficiency
In particle detection devices, an important parameter is the detection efficiency calculated as the ratio of the number of events registered by the detector to the total number of particles generated. In the presented analysis, telescope event counts were used as a reference to estimate detection efficiency of the UFXC detector. The events were compared trigger by trigger between the telescope and UFXC detector systems. Fig. 8 presents calculated detection efficiency for data collected during five different measurements. The average parameter value equals to 96%.
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Detection efficiency 100.00% 90.00% 80.00%
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Fig. 8 Detection efficiency of the UFXC detector calculated in regards to the events registered by the telescope. The different measurements represent different data batches called “runs” taking at different times.
5. Conclusions
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We performed studies of high energy particle detection with the UFXC single photon counting hybrid pixel array detector using the 120 GeV proton beam at the Fermilab Test Beam Facility, and demonstrated that the UFXC detector is capable of operating in the frame-triggered zero dead-time mode, and can be used as part of a system for charged particle tracking. The position resolution achieved by the UFXC detector was measured to be less than 40 μm. These results prove the successful operation of the UFXC detector system in the zero dead-time mode at high frame rate of 50 kfps, and represent a promising first step in establishing a pixelated tracking detector for test beam setups. We intend to expand the system to use two or more planes of the UFXC modules, combining several measurements of the beam particles for improved position resolution.
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Acknowledgements
This work was supported by the National Science Center, Poland, under contract no. UMO-2018/29/N/ST7/02786, and FNAL LDRD award 2017-027.
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P. Grybos, P. Kmon, P. Maj, R. Szczygiel, "32k Channel Readout IC for Single Photon Counting Pixel Detectors with 75 μm Pitch, Dead Time of 85 ns, 9 e- rms Offset Spread and 2% rms Gain Spread", IEEE Trans. Nucl. Sci. 63 (2016) 1155–1161. doi:10.1109/TNS.2016.2523260.
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*Author Contributions Section
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CRediT author statement Anna Koziol: Software, Investigation, Visualization, Writing - Original Draft, Funding acquisition
Pawel Grybos: Resources Ryan Heller: Investigation
Mohd Meraj Hussain: Investigation Robert Szczygiel: Resources
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Si Xie: Investigation, Writing - Review & Editing
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Piotr Maj: Software, Supervision, Writing - Review & Editing
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Artur Apresyan: Conceptualization, Methodology, Writing - Review & Editing, Funding acquisition
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*Declaration of Interest Statement
Declaration of interests
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☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: