Prototype of Compton camera using high resolution Si/CdTe detectors Si/CdTe Compton camera as a polarimeter

We are developing a Compton telescope based on high resolution Si and CdTe imaging devices in order to obtain a high sensitivity astrophysical observation in sub-MeV gamma-ray region. In this paper, recent results from the prototype Si/CdTe semiconductor Compton telescope are reported. The Compton telescope consists of a double-sided Si strip detector (DSSD) and CdTe pixel detectors, combined with low noise analog LSI, VA32TA. With this detector, we obtained Compton reconstructed images and spectra from line gamma-rays ranging from 81 keV up to 356 keV. The energy resolution is 3.8 keV and 7.9 keV at 122 keV and 356 keV, respectively, and the angular resolution is 9.9° and 5.7° at 122 keV and 356 keV, respectively.

Single Photon Emission Computed Tomography (SPECT) is at present one of the major techniques for non-invasive diagnostics in nuclear medicine. Almost the whole clinical routine is based on collimated cameras, originally proposed by Anger. Due to the presence of mechanical collimation, detection efficiency and energy acceptance are limited by thesystem geometrical features. In order to overcome these limitations, the application of Compton cameras for SPECT is being investigated for several years. A Compton camera prototype is at present under development by our collaboration, for high energy gamma detection in ion beam therapy monitoring and nuclear medicine. We propose in this study to compare our detector to a commercial Anger device, the GE Healthcare Infinia system, through Monte Carlo simulations (GATE v7.1 and Geant4 9.6 respectively). Given the possible introduction of new radio-emitters at higher energiesintrinsically allowed by the Compton camera detection principle, the detectors are exposed to point-like sources at increasing primary gamma energies, from actual isotopes already proposed for SPECT applications. The detector performances are studied in terms of radial event distribution, detection efficiency and final image, obtained by gamma transmission analysis for the Anger system, and with an iterative LM-MLEM algorithm for the Compton reconstruction. The Compton camera prototype is also characterized in terms of rate of random coincidences and at different energy resolutions. Although the rate of random coincidences appears to be close to 50% at clinical source activities, preliminary results show for the Compton camera a detection efficiency increased of a factor greater than an order of magnitude, associated with an enhanced spatial resolution for energies beyond 500 keV. We discuss then the proven advantages of Compton camera application with particular focus on dose delivered to the patient, examination time and spatial uncertainties.

The Focusing Optics X-ray Solar Imager (FOXSI) is a NASA sounding rocket mission which will study particle acceleration and coronal heating on the Sun through unprecedented high-resolution imaging in the hard X-ray energy band (5-15 keV). Energy release occurring in the quiet region of the Sun may potentially play an important role in the coronal heating mechanism. With a combination of high-resolution focusing X-ray optics and fine-pitch imaging sensors, FOXSI will achieve superior sensitivity; two orders of magnitude better than that of the RHESSI satellite. FOXSI requires the spectral capability down to 5 keV, which requires a development of a new ASIC and detector with a better energy resolution. We plan to use a Double-sided Si Strip Detector (DSSD) with a low-noise front-end ASIC as the FOXSI focal plane detector, which will fulfill the scientific requirements on the spatial resolution, energy resolution, lower threshold energy and time resolution. We have designed and fabricated a DSSD with a thickness of 500 ¿m and a dimension of 9.6 mm×9.6 mm, containing 128 strips separated by a pitch of 75 ¿m, which corresponds to 8 arcsec at the focal length of 2 m. The DSSD was successfully operated in a laboratory experiment. Under a temperature of -20°C and a bias voltage of 250 V, we obtained spectra from both sides of the electrodes. The energy resolution was measured to be 980 eV and 2.4 keV for the p-side and n-side at 14 keV gamma-ray, sufficient for the FOXSI mission requirement.

A Compton camera for nuclear medicine applications using 113mIn 1

Comparison of reconstructed prompt gamma emissions using maximum likelihood estimation and origin ensemble algorithms for a Compton camera system tailored to proton range monitoring

Characterization of a Compton camera based on the TOFPET2 ASIC

We propose a novel Silicon Drift Detector (SDD) that can fulfill the demanding requirements of the ideal scatter detector for a high-resolution Compton Camera for medical imaging. This kind of detector demonstrates excellent performance in terms of energy and position resolution, maximum readout rate and the more relevant results are reported. The SDD proposed here is based on a multilinear architecture [controlled drift detector (CDD)] in which the backside has been segmented and instrumented to pick up the signal induced by the electron-hole (e-h) pairs generated by the interaction thus providing a fast coincidence signal between the recoil electron signal and the scattered gamma-ray signal. Several measurements have been carried out to study the shape of the induced signals and the achievable time resolution. Experimental tests with annihilation photons of a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22 </sup> Na source in a time coincidence setup showed best time resolution of 6 ns FWHM. These results confirm that SDD technology with fast pickup on the back electrodes and optimized design has the potential to develop a Compton telescope with Doppler limited performance for medical imaging

INO-ICAL is a proposed underground experiment to study the oscillation parameters of neutrinos by detecting those produced in the atmospheric air showers. The Iron CALorimeter (ICAL) is to have 151 layers of iron plates stacked vertically corresponding to a height of ∼14.5 m, with active detector elements inserted between the iron layers. The iron layers will be magnetized with a maximum magnetic field of 1.5 T to enable the measurement of the charge and momentum of the μ - (or μ +) produced by ν μ (or ν̅ μ ) interactions inside the detector throughout the operation. Resistive Plate Chambers (RPCs) have been chosen as the active detector elements as they are amenable to large area coverage, as well as due to their high particle detection efficiency, long-term performance and low cost of production. The major factors that weigh in on the physics potential of the ICAL detector are the efficiency, position resolution and time resolution of the large area RPCs. A prototype detector called mini-ICAL (with 11 iron layers) was commissioned in order to understand the engineering challenges in building the large-scale electromagnet and its ancillary systems, and also to study the performance of the RPC detectors and readout electronics developed by the INO collaboration. As part of the performance study of the RPC detectors, an attempt is made to improve their position and time resolutions. The designed position resolution for the ICAL detector is of the order of 1 cm and the required time resolution is ∼1 ns. Even a small improvement in the position and time resolution will help to improve the measurements of momentum and directionality of the neutrinos in the ICAL detector. In ICAL detector simulation, where muons traverse in all directions with on average 20 RPC layers, a 30 % improvement in position resolution results in ∼10 % improvement in the momentum resolution at 3 GeV. Due to large iron materials in between two RPCs, the resolution is dominated by the effect of multiple scattering at this momentum. Also for a muon with 10 layer hit, the charge ambiguity reduces from 0.04 % to 0.001 % when the time resolution improves from 1 ns to 0.7 ns. The Time-over-Threshold (ToT) of the RPC pulses is recorded by the readout electronics. ToT is a measure of the pulse width and consequently the pulse amplitude. This information is used to improve the time and position resolution of RPCs and consequently INO's physics potential.

Improving Compton camera imaging of multi-energy radioactive sources by using machine learning algorithms for event selection

The design and performance of a 1.2 liter liquid xenon chamber equipped with 7 two-inch photomultiplier tubes, with the purpose of studying the scintillation response of xenon to /spl gamma/-rays and neutrons, is described. Measurements with /spl gamma/-rays indicate a high VUV light collection efficiency resulting in /spl sim/5.5 photoelectrons per 1 keV of deposited energy. The energy resolution (FWHM) is 18% and 22%, for 122 keV and 511 keV /spl gamma/-rays, respectively. An algorithm for the reconstruction of the scintillation coordinates in (x,y) plane was developed and tested. The position resolution is estimated to be /spl sigma/=6.9 mm for 122 keV /spl gamma/-rays.

Performance study of Si/CdTe semiconductor Compton telescopes with Monte Carlo simulation

This report proposes a high-accuracy Compton camera (CC) designed to improve gamma-ray visualization, which addresses the limitations of size and complexity of standard CCs. As a single-layer CC has low sensitivity, a stack of numerous single-layer CCs is used. Therefore, simplicity is as important as accuracy. There are two types of CCs. The first is the conventional CC with a simple structure, which detects the incident-ray direction using multiple gamma rays but suffers from high background and noisy signals. The second is the electron-tracking CC, which detects the incident-ray direction using a single gamma ray. However, it requires measuring the three-dimensional (3D) trajectory of recoil electrons generated by Compton scattering, which necessitates complex equipment such as gas chambers or dedicated semiconductor detectors. Instead of measuring the 3D trajectory of recoil electrons, we developed a method to analytically determine the 3D direction from a two-dimensional recoil-electron image captured by a charge-coupled device (CCD). We classified Compton scattering based on scattering conditions and formulated equations that describe incident directions for all scattering conditions. As this method uses a simple commercial CCD with a small pixel size, it enables simple and high-accuracy measurements. Our simulation using Geant4 indicated that for 511 keV gamma rays with a 10 μm pixel size, the full width at half maximum of the scatter plane deviation was less than 50°. Additionally, this method was applicable when the recoil-electron energy was 50 keV or more. Furthermore, we discuss the adaptation limits of this method induced by measurement errors.

Applications and Imaging Techniques of a Si/CdTe Compton Gamma-Ray Camera

A compact imaging system with a CdTe double-sided strip detector for non-destructive analysis using negative muonic X-rays

In this paper, a Compton camera capable of estimating depth of interaction (DOI) in the detector was designed and optimized using Geant4. The proposed DOI Compton camera consists of two pixelated CsI(Tl) scintillators coupled with digital silicon photomultipliers [Digital Photon Counter (DPC)-3200-22]. The parameters affecting the performance of a Compton camera were evaluated for optimization in terms of image sensitivity and image resolution with a measure of figure of merit. The considered parameters were: 1) type of the reflector surface; 2) thickness of the scintillator; and 3) the distance between the scatter and the absorber detectors. The simulation results showed that a ground white surface is better than a polished white one. The optimal thickness and the scatter-absorber distance were found to be 15 mm and 4 cm, respectively. With the optimized parameters, performance of the DOI Compton camera was estimated for a <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">137</sup> Cs source at 1 m using Geant4. The image resolutions were 6.06° full-width at half-maximum with the list-mode maximum likelihood expectation maximization algorithm. It took 20 s to take Compton images for the gamma ray sources of 0.1 μSv/h intensity with the DOI Compton camera. It was shown that there was a clear improvement by employing DOI estimation algorithm in combination with the use of thick scintillators for a Compton camera.