Hard Xray photons with energies in the range of hundreds of keV typically undergo Compton scattering when they are incident on a detector. In this process an incident photon deposits a fraction of its energy at the point of incidence and continues onward with a change in direction that depends on the amount of energy deposited. By using a pair of detectors to detect the point of incidence and the direction of the scattered photon we can calculate the scattering direction and angle. The position of a source in the sky can be reconstructed using many Compton photon pairs from a source. We demonstrate this principle in the laboratory by using a pair of Cadmium Zinc Telluride detectors sensitive in the energy range of 20200 keV. The laboratory setup consists of the two detectors placed perpendicular to each other in a leadlined box. The detectors are read out by a customprogrammed Xilinx PYNQ FPGA board and data is then transferred to a PC. The detectors are first calibrated using lines from 241mathrmAm 155mathrmEu and 133mathrmBa sources. We irradiated the detectors with a collimated 133mathrmBa source and identified Compton scattering events for the 356 keV line. We run a Compton reconstruction algorithm and correctly infer the location of the source in the detector frame. This comprises a successful technology demonstration for a Compton imaging camera in the Hard Xray regime. We present the details of our setup the data acquisition process and software algorithms and showcase our results.
['Astrophysics - Instrumentation and Methods for Astrophysics']
Development of a Compton Imager Setup
{'Development of a Compton Imager Setup': 'Anuraag Arya a , Harmanjeet Singh Bilkhu a , Sandeep Vishwakarma b , Hrishikesh Belatikar a , Varun Bhalerao a , Abhijeet Ghodgaonkar c , Jayprakash G. Koyande b , Aditi Marathe a , N. P. S. Mithun d , Sanjoli Narang e,f , Sudhanshu Nimbalkar f , Pranav Page f , Sourav Palit g , Arpit Patel d , Amit Shetye f , Siddharth Tallur f , Shriharsh Tendulkar b , Santosh Vadawale d , Gaurav Waratkar a  \n- a Department of Physics, IIT Bombay, Powai, Mumbai - 400076, India\n- b Department of Astronomy and Astrophysics, TIFR, Colaba, Mumbai - 400005, India\n- c Department of High Energy Physics, TIFR, Colaba, Mumbai - 400005, India\n- d Astronomy and Astrophysics division, PRL, Ahmedabad - 380054, India\n- e Computer Science & Artificial Intelligence Laboratory, MIT, USA\n- f Department of Electrical Engineering IIT Bombay, Powai, Mumbai - 400076, India\n- g ICSP, Netai Nagar, Kolkata - 700099, India  \nAbstract. Hard X-ray photons with energies in the range of hundreds of keV typically undergo Compton scattering when they are incident on a detector. In this process, an incident photon deposits a fraction of its energy at the point of incidence and continues onward with a change in direction that depends on the amount of energy deposited. By using a pair of detectors to detect the point of incidence and the direction of the scattered photon, we can calculate the scattering direction and angle. The position of a source in the sky can be reconstructed using many Compton photon pairs from a source. We demonstrate this principle in the laboratory by using a pair of Cadmium Zinc Telluride detectors sensitive in the energy range of 20-200 keV. The laboratory setup consists of the two detectors placed perpendicular to each other in a lead-lined box. The detectors are read out by a custom-programmed Xilinx PYNQ FPGA board, and data is then transferred to a PC. The detectors are first calibrated using lines from 241 Am , 155 Eu and 133 Ba sources. We irradiated the detectors with a collimated 133 Ba source and identified Compton scattering events for the 356 keV line. We run a Compton reconstruction algorithm and correctly infer the location of the source in the detector frame. This comprises a successful technology demonstration for a Compton imaging camera in the Hard X-ray regime. We present the details of our setup, the data acquisition process, and software algorithms, and showcase our results.  \nKeywords: X-ray imaging, Hard X-rays, CZT detectors, X-ray telescopes.  \n- * Anuraag Arya, arya.a@iitb.ac.in', '1 Introduction': "High Energy photons in the energy range from hundreds of keV to tens of MeV typically undergo Compton Scattering in a detector - depositing a fraction of their energy at the interaction site, and creating a secondary photon, which may in turn be detected by the same or another detector. 1, 2 The scattered photon has a lower energy, and typically deposits its full energy in the detector via photoelectric interactions. The locations of the first and second interactions, and the energy deposits at both places, can be used to constrain the direction of the photon source. This concept is utilized for 'Compton Imaging'. 3, 4 This technique forms the core of astrophysical missions starting with COMPTEL 5 to the upcoming COSI (Compton Spectrometer and Imager). 6 However, past missions have typically focussed on the Gamma-ray band, and operated in the MeV range. 7 In this work, we demonstrate a hard X-ray Compton Imaging setup that operated in the hard X-ray energy range of hundreds of keV. We develop a setup using 5 mm thick Cadmium Zinc Telluride detectors in an orthogonal configuration. We irradiate the setup with radioactive sources and can successfully reconstruct the source position using observed data.  \nThis paper is organised as follows. In §2 we describe our experimental setup. §3 deals with detector calibration and data acquisition. We present our analysis and source position reconstruction in §4, and conclude with a summary and future prospects in §5.", '2 Experimental setup': 'Our experimental setup consists of three main components: the detectors (§2.1), power and readout electronics (§2.2), and the enclosure (§2.3).', '2.1 Detectors': 'Fig 1 A Cadmium Zinc Telluride (CZT) detector used in our experiments. These detectors are manufactured by GE Medical (formerly Orbotech).  \n<!-- image -->  \nOur setup is based on 5 mm thick CZT detectors (Figure 1) manufactured by GE Medical (formerly Orbotech Medical Solutions). The detectors consist of a 16 × 16 grid of square pixels with a total size of 39 mm on a side. The average detector composition is Cd 0 . 9 Zn 0 . 1 Te 1 . 0 . They operate in a nominal energy range of 20-200 keV, with an energy resolution of 10-12% at 60 keV. The typical band gap for room temperature for this composition is 1.57 eV, 8 and as per the vendor data sheet, the average electron-hole pair creation energy for these detectors is 4.43 eV.  \nThe connections to these detectors are low voltage power, a high voltage negative bias (we use -600 V), and communication lines. The detector module includes circuitry to detect onboard triggers, amplify them with a charge-sensitive pre-amplifier, and digitize the output. The digital output is communicated via Low Voltage Differential Signaling (LVDS) format on Serial Peripheral Interface (SPI) lines.  \nSuch detectors have been used in the Cadmium Zinc Telluride Imager on board AstroSat , and further details can be found in Bhalerao et al. (2017). 9', '2.2 Electronics': "The electronics have three parts: a detector board, the PYNQ board, and standard laboratory equipment like power supplies and a PC (Figure 2).  \nFig 2 The electronics and data streaming setup is illustrated in a schematic diagram that includes the PYNQ board. The diagram shows how data is streamed through the board. The readout systems are managed by the Programmable Logic (PL) section and controlled via the Programmable System (PS) section of the PYNQ board (for more details: Ramagond et al. (2017) 10 ). The board's IP address is used to operate the data acquisition process.  \n<!-- image -->  \nFig 3 Top: The native event data from the detector consists of 26-bits in length. The first 8 bits (Most Significant Bit, MSB) give the pixel number ('channel address' using data sheet terminology) while the next 12 bits give the event energy. This is followed by 5 reserved bits, and the Least Significant Bit (LSB) is a parity bit. Bottom: We extend this to a 64-bit format, adding a 32-bit timestamp at system clock resolution, a 2-bit detector ID and few reserve bits are added to the native data frame to make the 64-bit event data frame.  \n<!-- image -->  \nThe detectors are mounted on the detector board, which the appropriate mounting pins for the high voltage (HV) and low voltage (LV) contacts of the detector, and connects these to appropriate external lines. The board also accepts 5 V input power, and converts it to the required 3.3 V and 1.8 V levels for the detector module.  \nData acquisition is handled by a Xilinx FPGA PYNQ-Z2 board 1 . It is a Zynq 7000 development board with ZYNQ XC7Z020-1CLG400C SoC (System on Chip). It has dual core ARM Cortex-A9 processors clocked at 650 MHz connected to various peripherals such as Ethernet, SDIO, UART, etc, as well as an Artix-7 equivalent FPGA. The SoC thus has two distinct hardware parts: the Processing System (PS) consisting of the ARM processors and its peripherals, and the Programmable Logic (PL). All the codes (C/C++ in standalone mode, or the Embedded Linux) run on the PS, while all the Custom IPs are created on the PL. The board also has 512 MB DDR3 memory with a 16-bit bus operating at 1050 Megabits per second. This is the main lower-speed buffer for all the data. Embedded Linux is installed on an SD card which accesses the RAM via the AXI-DMA (Advanced Extensible Interface, Direct Memory Access) python code.  \nWeuse small add-on circuits that convert the LVDS signal from the detector board into the Low Voltage Transistor-Transistor Logic (LVTTL) levels used by the PYNQ board. A 10 MHz clock signal generated on the PYNQ board is supplied to the detector board for clocking the detector. Detector data are read in the native 26-bit format as shown in Figure 3 (top), which requires 7.5 µ s time for each event. This is the native time resolution of our setup in the low event rate case (count rate ≪ 13,000 events/sec). We add a 32-bit integer timestamp that counts the number of clock ticks that have elapsed since power-on. Our experiment does not require accurate absolute time, hence this suffices our requirements. This clock loops over in about 7 minutes, but this is fixed in post-processing of data by detecting the wrap-around and adding an appropriate offset.  \nWe split and pad the detector data and add the timestamp to save it in a 64-bit data format (Figure 3, bottom) for storing relevant photon information: the detector PHA 2 , pixel number (PixID), detector number (DetID), and timestamp (number of clock cycles).  \nWe run Python Jupyter notebooks in the Embedded Linux on the SoC PS, and process the data with our custom codes (§3.1). For archival and further processing, data are read out from the PYNQ board over ethernet by a laboratory computer. The PYNQ board can be powered via a USB cable from the computer, or from a table-top power supply. Lastly, the high voltage required by the detectors was supplied from a laboratory power supply.", '2.3 Enclosure': "The detectors are sensitive to optical photons which can create noise events in edge pixels of the detector by creating electron-hole pairs in CZT, hence the entire setup has to be light-tight. As a first step, we do not put any indicator LEDs on any of the circuit boards. Next, we mount the detectors and the radioactive source in a 30 cm × 30 cm × 60 cm aluminium box, which is further covered with a black cloth. The box is lined with 5 mm lead sheets for safety of the operator. The detectors are mounted at right angles to each other, with a vertical detector ('Det V', detector ID 21065) serving as the scatterer and a horizontal detector ('Det H', detector ID 17177) serving as the absorber of scattered radiation (Figure 4).  \nRadioactive sources are enclosed in a thick brass shield with a cylindrical hole of 5 mm diameter and 24 mmdepth for collimating the radiation. The source holder can be mounted at different heights so that photons are incident on Det V at varied angles. The collimation is sufficient to  \nhtml  \n.  \nFig 4 The experimental setup consisted of a holder containing a radioactive source to test the localization of hard X-rays at different positions and two perpendicular CZT detectors assembled on detector boards to stream the onboard data from the detector to the Xilinx FPGA controller for further data analysis.  \n<!-- image -->  \nensure that direct incidence of photons on Det H is small. Exact source location is measured mechanically to validate the source positions reconstructed from data.", '3.1 Detector Initialisation': "Like most electronic detectors, CZT detectors have lower noise when cooled. However, since it was not practical for us to cool the entire setup including the enclosure, we conducted our experiments at room temperature. This meant that we had more noise at low energies, and a higher number of noisy pixels in the detectors. As will be discussed in §3.2, we needed the ability to detect 31 keV photons. Hence, we set the Lower Level Discriminator (LLD, also known as threshold) for both detectors to 20 keV. This resulted in an acceptable level of noise in the detector even at room temperature.  \nNext, we disabled the noisy pixels in each detector. This was done iteratively by taking short acquisitions of background data and disabling noisiest pixels till a small subset of pixels no longer dominated the count rate. In most data acquisition 'runs', typically less than ten pixels were disabled. The typical background count rate in each detector was finally around 8-10 counts s -1 . If the detectors are cooled, this rate can drop down further.  \nThe highest PHA channel for the detector is used to denote any events with energy greater than or equal to the upper energy scale limit for the detector. Since exact energy information is not available for such events, we ignore them from our analysis.", '3.2 Gain-offset calibration': 'Lab measurements show that the PHA measured by the detector is linearly related to the energy ( E ) of the incident photon for these detectors. We model this as E = G × PHA + O where G is the detector gain, and O is the offset in units of energy. The values of G and O depend on operating temperature, high voltage, and vary from pixel to pixel, and need to be determined experimentally. We irradiate the individual detectors sequentially with 133 Ba , 155 Eu , 241 Am , which have multiple lines in the 20-200 keV range of interest (Figure 5). The reference information about line energies was obtained from Kinsey et al. (1996) 11 via the online interface 3 . Despite setting low values of the LLD, we find that the 31 keV line complex is contaminated by a sharp cut as well as electronic noise at low energies. Hence, we exclude this line from gain-offset calculations. We use pixel-wise spectra for each source, measure the PHA of the peak, and then calculate gain and offset values. These values are then tabulated and used in all subsequent analyses. To calculate the energy resolution of the detectors, we irradiate the detectors with 241 Am source on them, and create an average detector spectrum (Figure 5) by correcting each pixel data with its own gain and offset. We measure an energy resolution of ∼ 12 % at 59.6 keV at room temperature for both detectors, on par with values obtained in AstroSat CZTI. 9', '3.3 Data collection at various source positions': 'Final experimental data sets were obtained by placing the 133 Ba source at four different locations in the enclosure. 133 Ba has a prominent radioactive decay lines at 356 keV and 81 keV, with decay yields of 62% and 34% respectively. The source also has weaker lines at 276 keV, 303 keV, 384 keV, etc. Of all these, the 356 keV is most suitable for our analysis due its high yield, relatively high probability of Compton scattering as compared to photoelectric absorption, and because the energy deposited in both the scatterer and absorber is likely to be within our detector energy range. The 2.5 µ Ci source strength corresponds to about 9.25 × 10 4 disintegrations per second, of which a much smaller fraction exits through the collimator. We typically detect 35-65 counts s -1 on our detector. Note that there are several factors to consider in this number. It includes the 31 and 81 keV lines from Ba. Photons of 356 keV which are directly absorbed in the detector get discarded as discussed in §3.1. However, if these photons are Compton scattered by the detector and the energy deposit is in the 20-200 keV range, such events will be registered in the scatterer (Det V). The scattered photons head in random directions, and a fraction of them are incident on the absorber (Det H). If the energy of the scattered photon is also in the 20- 200 keV range, then these register as events in Det H.  \nThe final count rates were typically around 35-65 counts s -1 in Det V depending on the source position, and around 15 counts s -1 in Det H respectively. This includes background rates of 810 counts s -1 in each detector. We find that a few detector pixels occasionally become noisy during the observations, increasing the count rates to as much as few hundred counts per second. These events are stored along with the data, but discard all data from such pixels in post-processing. In our test runs, we typically acquired data for 4-5 hours to get sufficient statistics for analysis.  \n<!-- image -->  \nFig 5 Spectra for various radioactive sources for both detectors. The spectra were separately acquired for each detector for each radioactive element, but have been plotted together for clarity. They are normalized such that the peak is at 1. The top and bottom figures are for detectors H and V respectively. The green curves show 155 Eu which produces three lines at 45.3 keV, 86.6 keV and 105.3 keV. The orange curve is for 241 Am , showing a line at 59.6 keV. Lastly, blue curves show 133 Ba data with lines at 30.9 keV and 81 keV. Note that the 31 keV line is close to the detector LLD, and is likely impacted by the cutoff as well as electronic noise.  \n<!-- image -->', '4 Compton Event Reconstruction': 'There are various methods to infer the source position and other characteristics in a Compton camera starting from Simple Back-Projection of event circles onto the sky to iterative imaging reconstruction approaches like list-mode likelihood image reconstruction algorithm (LM-ML-EM) and the Maximum Entropy Method (MEM). 12,13 Our focus in this work is the demonstration of a Compton camera in sub-MeV range, hence in our analysis we stick to the straight forward backprojection method, which we discuss in this section. We discuss some of the limitations for this in §4.4.', '4.1 Event circles and source position': "Consider a photon incident on Det V at a position r V , where it undergoes Compton scattering in Det V, depositing energy E V at the point of incidence (Figure 6, top). The scattered photon lands on Det H at position r H and is fully absorbed, depositing its entire energy E H at that position. One can calculate the scattering angle ψ by applying energy and momentum conservation, leading to the well-known Compton equation:  \ncos ψ = 1 -m e c 2 E H + m e c 2 E V + E H (1)  \nCombining this with the scattering positions r V and r H , we can constrain the source location to a cone (Figure 6, top). The intersection of this cone with a large spherical surface (the 'sky') centred on the detector gives a circle known as the Event Circle. The intersection of circles created by multiple events gives us the location of the source in the sky (Figure 6, lower left).  \nIn practice, the event circles have finite widths, due to uncertainties in measurements. We account for these by modeling the event circle as a Gaussian band on the sky.  \nThe first key factor is the 'energy uncertainty' ( ∆ ψ E ) stemming from the finite energy resolution of the detector, given by the following equation: 12  \n∆ ψ E = m e c 2 sin ψ [ ( 1 E 2 H -1 ( E H + E V ) 2 ) 2 ∆ E 2 H + ( 1 E H + E V ) 4 ∆ E 2 V ] 1 / 2 (2)  \nThe second key factor is the positional uncertainty ( ∆ ψ P ) which arises from the 2.46 mm pixel size, which is not negligible as compared to the few cm length that the scattered photon travels: resulting in an uncertainty in the cone axis r V -r H . For this calculation, we use the average uncertainty corresponding to a pair of pixels near the center of the horizontal and vertical detectors, ∆ ψ P = 2 . 53 · (one-sigma). For each photon, we calculate the net uncertainty ( ∆ ψ ) by adding these terms in quadrature. The event circle is then constructed as a gaussian with standard deviation equal to ∆ ψ , and truncated at ± 3 σ .", '4.2 Compton Event Selection': "Our detector timing resolution is 0.1 µ s, while the light travel time for a scattered photon between the two detectors is sub-nanosecond. Hence the simple criterion for selecting Compton events is that photons should have the same timestamp in both the detectors. We note that our codes on  \nFig 6 Top: A incident photon hits Det V at position r V , deposits part of its energy as E V , scatters off and then hits Det H at position r H , depositing its remaining energy as E H via photoelectric absorption. The scattering angle ψ determines the half angle of the event circle cone (obtuse scattering angle reverses the cone's direction). Bottom left: When multiple event circles are back-projected on the sky, the common overlap position determines the location of the source, as is visualized for six event circles. Bottom right: The geometrically measured Compton scattering angle is given by the angle between the initial photon direction g 0 and the scattered gamma-ray direction g 1 . The difference between the kinematic and geometric Compton scatter angle gives the Angular Resolution Measure (ARM), ∆ φ ARM for each event. Based on a figure in. 14  \n<!-- image -->  \nthe PYNQ board PL run in parallel acquiring data from both detectors simultaneously, thereby ensuring that such photon pairs would indeed get the same timestamp.  \nAll such event pairs from one of our data sets are shown in Figure 7. The histograms on the sides show the spectra for such events in the individual detectors. We see that the individual detector spectra show a peak at low energies, consistent with being dominated by electronic noise.  \nTable 1 133 Ba sources were placed at predefined positions in four different cases. For these measurements we used a coordinate system origin at the centre of the surface of Det V. In Figure 9, these source positions are marked with a bold cross mark.  \nSince our background noise is relatively higher, we need another filter to limit our data to true Compton event pairs. We expect that the 133 Ba source photons undergoing Compton scattering must all be from the 356 keV decay line. By constructing a spectrum of the total deposited energy ( E V + E H ), we can detect the 356 keV line (Figure 8). We see a prominent drop on the high energy side of the line, indicating an absence of higher energy photons from the source. Fitting the line profile with a Gaussian, we measure an energy resolution of 3.2% (FWHM). As a cross-check, we divided the event data into two halves, and measured the line energy from both. We find that the measurements agree to better than 0.1 keV. To select photons from this line for our analysis, we apply a conservative filter, selecting all photon pairs where the total energy is within ± 5% of the line energy: | E V + E H -356 keV | < 17 . 8 keV.  \nThe source is collimated such that photons should be incident on Det V. However the collimation is not perfect and some photons may be incident on Det H. There is no fool-proof way to determine the scatterer and absorber for a given Compton photon pair at our time resolution. Instead, we use the concept of the Compton edge: the maximum energy that can be deposited in a scatterer for a source photon of a given energy [14, Equation 20]. For 356 keV photons from 133 Ba , this corresponds to 206 keV. Hence, if we find a Compton photon pair where E V > 206 keV, we conclude that the photon must have been scattered from Det H and absorbed in Det V. Such events are shown as green dots in Figure 8, while orange dots show events where we have considered Det V to be the scatterer.", '4.3 Results': 'The results were obtained using four different coordinates of the 133 Ba source, as listed in observation Table 1. We then use the back-projection algorithm and map each event to a ring on the sky as discussed in §4.1. The final sky maps are created by adding all event rings from the selected 356 keV events. The source location corresponds to the high probability region resulting from this process (Figure 9). We can see that in all four cases, our reconstructed position is consistent with the true source position (marked with a bold cross).  \nGiven the knowledge of the true source position, we can now evaluate the angular resolution of our setup. The best-fit position deviates from the true position due to various factors like the native angular resolution of the setup, geometric measurement uncertainties, and simplifying assumptions made in our analysis. We now evaluate the Angular Resolution Measure of our setup, and discuss some of the other limitations in §4.4.  \nThe angular resolution of a Compton camera is defined as the full width at half maximum (FWHM) of the Angular Resolution Measure (ARM) distribution. 14 For each Compton pair, we can calculate the Compton scatter angle ψ in two ways. The geometric Compton scatter angle  \nFig 7 3351 Compton event pairs detected in a test run. The scatter plot shows the energy deposited in detectors V and H respectively. The gray band denotes the region of the plot where we expected to get 356 keV 133 Ba photons, with a width set to the ± 5% energy resolution of that line. All blue dots therefore denote chance coincidence events. The orange and green dots denote events where the scatterer was considered to Det V and Det H respectively. The plots at the top and right show spectra of Compton events in Det H and Det V respectively. We can see the low-energy peaks in both, which correspond to electronic noise. Note that the 81 keV line is not clearly seen in these spectra.  \n<!-- image -->  \nψ geo is calculated from the actual positions of the source, scattering pixel, and absorbing pixel (Figure 6) as a dot product of the initial photon direction g 0 and the measured scattered gamma-ray direction g 1 . The kinematic Compton scatter angle ( ψ kin ) is determined from the measured energy deposits using equation 1, and gives us a ring on the sky. The ARM for each event is defined as the minimum distance between the ring and the geometrically calculated source direction (Figure 6,  \nFig 8 Compton event spectrum from the test run shown in Figure 7. This spectrum is created by using the total energy of photons in each Compton pair, without applying any selection for the 356 keV line. We see a small peak at the expected line position, and a prominent drop on its high energy side. A small number of photons seen at higher energies may be from the 384 keV 133 Ba line, and there are also hints of the 303 keV line. Fitting the 356 keV line profile with a Gaussian gives a resolution of 3.2% (FWHM).  \n<!-- image -->', 'lower right):': '∆ ψ ARM = ψ geo -ψ kin (3)  \nThe histogram of ∆ ψ ARM values is shown in Figure 10 for our four experiments. For an ideal detector, we expect a sharp peak at ∆ ψ ARM = 0 , with a very narrow width. In practice, we expect to see a distribution centered close to zero. The FWHM of this distribution is the angular resolution of the Compton Imager. We see that our setup has a direction-dependent energy resolution, ranging from ∼ 16 · to ∼ 30 · .', '4.4 Limitations': 'Our setup can satisfactorily recover the position of a 133 Ba source. However, there are certain limitations which impact the overall performance of the system, which we discuss below.', '4.4.1 Limited absorber coverage': "Source photons incident on Det V can be scattered in any direction. Only a fraction of these photons will hit Det H, while those traveling in other directions will not be detected. This leads  \nFig 9 All-sky maps displaing event circles with a finite width based on the detector's energy and position resolution uncertainties. These circles overlap, forming a high-probability region where the source is likely to exist. The actual source location (marked by a cross) was found to be within these regions. Upper left: Case 1 ( θ = -3 · , ϕ = 173 · ). Upper right: Case 2 ( θ = 20 · , ϕ = 184 · ). Lower Left: Case 3 ( θ = 38 · , ϕ = 184 · ). Lower Right: Case 4 ( θ = 24 · , ϕ = 190 · ).  \n<!-- image -->  \n<!-- image -->  \n<!-- image -->  \nFig 10 A histogram of the ∆ φ ARM values for the four tests from Table 1. Upper left: Case 1 ( θ = -3 · , ϕ = 173 · ). Upper right: Case 2 ( θ = 20 · , ϕ = 184 · ). Lower Left: Case 3 ( θ = 38 · , ϕ = 184 · ). Lower Right: Case 4 ( θ = 24 · , ϕ = 190 · ). A Gaussian fit to the main peak in each histogram gives the FWHM of the ARM, corresponding to the resolution of the Compton imaging system. As expected for our setup, we find that the value varies with direction. We obtain ARM in the range from ∼ 16 · to ∼ 30 · .  \n<!-- image -->  \n<!-- image -->  \nto the measured Compton rate being lower than the actual rate of Compton scattering. To mitigate the problem, one can add more detectors to the system.  \nThe addition of more detectors can also allow a more diverse range of scatterer-absorber vectors ( g 1 , Figure 6). A simple interpretation of this is an increased spread in the centers of the event rings, thereby improving source localization.", '4.4.2 Geometrical Error': 'The individual pixels of our CZT detectors are 2 . 46 × 2 . 46 × 5 mm 3 in size. We cannot infer the exact position of the photon interaction in this volume. Coupled with the relatively small (tens of mm) distance between the scattering and absorbing pixels, this leads to a few-degree uncertainty in the scattering angle.  \nThis error can be reduced within the current setup by increasing the separation between the detectors. But in that scenario the absorber subtends a smaller solid angle from the scatterer compounding the problem in §4.4.1.', '4.4.3 Chance Coincidences': 'As discussed in §4.2, the energies most of the temporally coincident photon pairs do not add up to any line emitted by the source. Some of these may arise from Compton scattering of lower energy lines of 133 Ba , but most are likely to be chance coincident photons rather than being true Compton event pairs. In our four tests, only about 4-7% of the temporally coincident photons came from the 356 keV line. A consequence of this is the need to select photons of a known source energy, which might not always be feasible.  \nThere are two methods to mitigate this: decreasing the background rate, and decreasing the chance coincidence probability. The spectra for individual detectors (Figure 7) are consistent with being dominated by background noise near the LLD. This can be mitigated by cooling the detectors to a lower temperature. The detectors were operated with a 10 MHz clock speed in our tests, but they support higher operations speeds upto 30 MHz. Tripling the readout rate will decrease the chance coincidence probability by a factor of three, further improving the fraction of real Compton events. These two are the most important steps in converting our setup into a broad application Compton Imager where we can relax the source energy assumption.', '4.4.4 Energy Resolution': 'Our detectors operating at room temperature have a resolution of 12% at 59.6 keV, and about 3% at 356 keV. Poorer energy resolution increases the width of the event rings through Equation 2, thereby limiting angular resolution. For the 356 keV line in our configuration, this value is about 2 · , comparable to the geometrical error ∆ ψ P discussed §4.1 and §4.4.2. This can be improved upon by either cooling the detectors, or by switching to other detectors. For instance, NuSTAR detectors have a resolution of 1% at 59.6 keV 15 and would give a much lower error. However, they are thinner (2 mm) and would not be sensitive to the higher energy photons used in this experiment.', '4.4.5 Source Position': "In our experiment, the source was placed at distances of 22-28 cm from the 4 cm-wide scatterer. This causes a divergence of ∼ 10 · in the source beam. However, our reconstruction algorithms assume incoming photons are parallel to each other. This 'near field source error' is likely the largest contributor to the high ARM seen in Figure 10. Taking the source further away decreases the source count rate, and was not feasible within the practical constraints of a laboratory setup.", '4.4.6 Doppler broadening': 'Equation 1 was derived based on the assumption that the target electron is at rest. However, in the detector the electrons are bound to a nucleus and have finite momentum. This introduces another uncertainty in the calculated energies which fundamentally limits the achievable angular resolution. This effect, called as Doppler broadening, is highly dependent on detector atomic number and incident photon energy. For our case, it constitutes a fundamental limit of around 2 · to 4 · for the angular resolution. 12', '5 Conclusion and Future Work': "We have successfully demonstrated a laboratory setup of a Compton imager in the hard X-ray energy range.  \nWe developed a new system based on simple custom PCBs and an off-shelf PYNQ board to operate the detectors and read out data into an FPGA or a PC. With this setup, we achieved detector performance on par with those of AstroSat CZTI. Our system operates the detectors at a time resolution of 7 . 5 µ s, significantly faster than the 20 µ s resolution of CZTI. We demonstrated the ability to concurrently read out two detectors with this setup, eliminating the need to serially poll them.  \nWe tested the imager by irradiating it with 133 Ba from four different positions. In each case, we could successfully recover the source position from the recorded event data. In our tests, we obtain a direction-dependent angular resolution in the range of ∼ 16 · to ∼ 30 · . The resolution is primarily limited by the 'near field' location of the source, which is not factored into the reconstruction algorithms. The overall angular resolution and reconstruction performance can also be improved by using more advanced position reconstruction algorithms.  \nWe have discussed a few limitations of this system in §4.4, and are now working towards resolving some of them. In the future, we will try to operate the setup at a cooler temperature, and at a higher clock speed. The system can also be expanded to include more detectors in tandem, increasing the absorber coverage as well as increasing the overall effective area of the imager.  \nThis successful technology development paves the way for the development of future complex CZT detector systems. The experience gained and lessons learnt from this experiment have aided the development of the Medium Energy Detector Package for the proposed Daksha space telescope. 16", 'Code and Data Availability': 'Data and relevant codes pertaining to this project will be made available on request.', 'Acknowledgements': 'This work was supported by the joint IIT Bombay - Indian Space Research Organisation (ISRO) Space Technology Cell (STC). We thank the staff of IIT Bombay and the Tata Institute of Fundamental Research (TIFR) for their help in setting up and running the experiments. We thank Ayush Nema for useful discussions.', 'References': "- 1 A. H. Compton, 'The spectrum of scattered x-rays,' Phys. Rev. 22 , 409-413 (1923).\n- 2 G. F. Knoll, Radiation detection and measurement (2000).\n- 3 P. von Ballmoos, R. Diehl, and V. Schoenfelder, 'Imaging the gamma-ray sky with Compton telescopes.,' A&A 221 , 396-406 (1989).\n- 4 S. E. Boggs and P. Jean, 'Event reconstruction in high resolution Compton telescopes,' A&AS 145 , 311-321 (2000).\n- 5 V. Schoenfelder, H. Aarts, K. Bennett, et al. , 'Instrument Description and Performance of the Imaging Gamma-Ray Telescope COMPTEL aboard the Compton Gamma-Ray Observatory,' ApJS 86 , 657 (1993).\n- 6 J. Tomsick, A. Zoglauer, C. Sleator, et al. , 'The Compton Spectrometer and Imager,' in Bulletin of the American Astronomical Society , 51 , 98 (2019).\n- 7 C. Kierans, Detection of the 511 keV positron annihilation line with the Compton Spectrometer and Imager . PhD thesis, University of California, Berkeley (2018).\n- 8 Z. Li, J. Cheng, F. Liu, et al. , 'Research on the technological progress of czt array detectors,' Sensors 24 (3) (2024).\n- 9 V. Bhalerao, D. Bhattacharya, A. Vibhute, et al. , 'The Cadmium Zinc Telluride Imager on AstroSat,' Journal of Astrophysics and Astronomy 38 , 31 (2017).\n- 10 S. Ramagond, S. Yellampalli, and C. Kanagasabapathi, 'A review and analysis of communication logic between pl and ps in zynq ap soc,' in 2017 International Conference On Smart Technologies For Smart Nation (SmartTechCon) , 946-951 (2017).\n- 11 R. R. Kinsey et al. , 'The nudat/pcnudat program for nuclear data.' Paper submitted to the 9th International Symposium of Capture Gamma-Ray Spectroscopy and Related Topics, Budapest, Hungary, October 1996. Data extracted from the NUDAT database, version (Aug 2024) (1996).\n- 12 A. C. Zoglauer, First light for the next generation of Compton and pair telescopes . PhD thesis, Munich University of Technology, Germany (2005).\n- 13 M. Frandes, B. Timar, and D. Lungeanu, 'Image reconstruction techniques for Compton scattering based imaging: An overview,' Current Medical Imaging 12 (2), 95-105 (2016).\n- 14 C. Kierans, T. Takahashi, and G. Kanbach, 'Compton Telescopes for Gamma-Ray Astrophysics,' in Handbook of X-ray and Gamma-ray Astrophysics , 18 (2022).\n- 15 T. Kitaguchi, B. W. Grefenstette, F. A. Harrison, et al. , 'Spectral calibration and modeling of the NuSTAR CdZnTe pixel detectors,' in Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series , Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series 8145 , 814507 (2011).\n- 16 V. Bhalerao, S. Vadawale, S. Tendulkar, et al. , 'Daksha: on alert for high energy transients,' Experimental Astronomy 57 , 24 (2024)."}