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Obtaining directional information is required in many applications such as nuclear homeland security, contamination mapping after a nuclear incident and radiological events, or during the decontamination work. However, many directional radiation detectors are based on directional shielding, made of lead or tungsten collimators, introducing two main drawbacks. The first is the size and weight, making those detectors too heavy and irrelevant for utilization in handheld devices, drone mapping, or space applications. The second drawback is the limited field of view (FOV), which requires multiple detectors to cover the whole required FOV or machinery to rotate the detector’s narrow FOV detector. We propose a novel $4\pi $ directional detector based on a segmented hollow cubic detector, which uses the Compton effect interactions with no heavy collimators. The symmetrical cubical design provides both higher efficiency and $4\pi $ detection ability. Instead of the traditional two types of detectors (scatterer and absorber) structure, we use the same type of detector, based on Gd3Al2Ga3O12 (GAGG) (Ce) scintillator coupled to silicon photomultiplier. An additional advantage of the proposed detector is obtained by locating the photon sensors inside the detector, behind the scintillators, which improves the radiation hardness required for space applications. Furthermore, such an arrangement flattens the temperature variation across the detector, providing better gain stability. The main advantage of the proposed detector is an efficient $4\pi $ radiation detection for high-energy gamma rays without the use of heavy collimators. [ABSTRACT FROM AUTHOR]

The development of a new generation of soft gamma-ray telescopes should allow polarimetric measurements of astrophysical sources to be associated with the results from spectroscopy and imaging due to the promising developments in polarimetry achieved over the last decade. In this perspective, the advanced surveyor of transient events and nuclear astrophysics mission which includes a narrow field telescope (NFT) that comprises a wide band Laue lens (60–600 keV) associated with a high performance thick and stacked CdZnTe focal plane operating as spectrometer, imager, and scattering polarimeter has been submitted for the ESA Voyage 2050 long term scientific planning preparation. In this context, and with the aim of optimizing the focal plane configuration of an instrument such as NFT, several studies are underway. In particular, with this work we study some performance issues such as polarimeters of segmented semiconductor detectors at room temperature, such as CdZnTe and cadmium telluride (CdTe), by means of a prototype based on two planar CdTe spectroimagers operating in coincidence. The two CdTe detectors are 2-mm-thick crystals with the anode segmented in $8\times8$ pixels with 2-mm pitch. This prototype configuration allows the Compton polarimetric performance of a spectroimager to be assessed by changing the distance between the two detection layers while operating in the 100–600-keV energy range, thereby allowing the analysis of Compton polarimetry 3-D dependence. The results obtained provide relevant input to the design of large sensitive volume multilayer and 3-D sensitive focal planes. [ABSTRACT FROM AUTHOR]

With the growing number of facilities applying nuclear technology in China, a high-performance portable gamma-ray camera that uses well-developed gamma-ray imaging techniques is increasingly required. To fulfill this requirement, we have designed a camera using a self-supported mask, a CsI(Na) array coupled to a silicon photomultiplier (SiPM) rather than the traditional position-sensitive photomultiplier tube (PSPMT) and a highly integrated readout circuit. In addition, with the elimination of multi-Compton events in the crystal array based on a row/column readout circuit incorporated into the data processing method, the peak-to-valley ratio of the flood source map and the image quality quantified by the contrast-to-noise ratio index of 137Cs were improved significantly by 25.8% from 9.8 to 12.3 and 14.8% from 38.7 to 45.0, respectively. [ABSTRACT FROM AUTHOR]

In order to improve the image quality of a coded-aperture-based gamma-ray imaging apparatus, a method for removing Compton scattered and other multipixel events is presented that removes inaccurate position estimations from the data stream. In coded-aperture gamma-ray imaging detectors, scattering events that are shared between different partitions in the active medium can result in the inaccurate determination of the initial gamma-ray interaction position, the result of which is distortion in the angular reconstruction mapping. For coded-aperture system in which the rapid localization and identification of the emitting source is important, the distortion due to scattered radiation can increase the integration time of the system. We show, in both software and hardware implementations using a cerium-doped gadolinium aluminum gallium garnet pixelated scintillator array, that filtering the data to remove multipixel events can improve the image quality despite the efficiency loss associated with not utilizing all of the interactions within the detector. [ABSTRACT FROM AUTHOR]

Many radionuclide identification algorithms use statistical inference to collect a variety of features from gamma-ray spectra to deduce the presence of particular radionuclides. More modern algorithms require large amounts of data to learn and use latent features from spectra for classification. Both approaches are computationally expensive, which is reflected in their power consumption, and require large amounts of user intervention to prepare. In this article, we introduce a low-power, neuromorphic algorithm for the real-time identification of radionuclides which simultaneously considers the entire shape of a gamma-ray spectrum. Utilizing the output of a traditional gamma-ray detector, our spiking, locally competitive algorithm uses sparse coding optimization to compare global patterns in a gamma-ray spectrum with a dictionary of radionuclide templates. This approach allows us to model informative global features resulting from both photoelectric absorption and Compton scattering. For the purpose of radiation threat reduction, the dictionary consists of data from the Nuclear Wallet Cards, a list of radionuclides and their properties compiled by the National Nuclear Data Center. To test our algorithm, we use a variety of gamma-ray spectra created using radionuclides measured under laboratory conditions with varying durations, distances, activity levels, and backgrounds, resulting in a wide range of signal-to-noise ratios. We have created test sets for three different gamma-ray detector types, with 57Co, 137Cs, 152Eu, 60Co, 239Pu, and 235U sources, to quantify the effect of resolution, efficiency, and background on the accuracy of the algorithm. We demonstrate a true positive accuracy of 91% with a high-resolution detector and 89% with a low-resolution detector on the corresponding test sets. Experimenting with the same radionuclides included in the test sets in a variety of special nuclear material (SNM) masking configurations, we show that our algorithm is capable of correctly identifying both SNM and mask even when the activity level of the mask is several times higher than that of the SNM. We also determine that our algorithm achieves over a 99% reduction in power consumption over other radionuclide identification software applications, which is critical for long-term, independent monitoring and is the goal of this research. [ABSTRACT FROM AUTHOR]

Y2SiO5: Ce (YSO) scintillation with characteristics of high light yield, fast decay time, and high Compton scattering fraction shows good application potential for Compton imaging. In this article, we propose a 3-D scintillator detector, which is segmented by the YSO monolithic rods and pixelated by subsurface laser engraving (SSLE) in the depth direction. Two arrays of silicon photomultipliers are optically coupled to the YSO array at each end, in which the pixels are designed for a one-to-one match. By identifying the location number of array pixels and measuring the depth of interaction (DOI) of pixelated scintillator rods, the interaction coordinates of gamma photon can be obtained accurately. After energy-consistent calibration, the energy resolutions of the pixels in the 3-D detector were evaluated. The imaging test results indicate that the detector has the capability to locate a 0.1-μSv/h 137-Cs gamma-ray source within 90 s in 4π field of view (FOV). The efficiency of the imaging events is 2.2%. By using the maximum-likelihood expectation–maximization (MLEM) algorithm with ten iterations, the full width at half maximum (FWHM) of the reconstructed hotspot decreases to less than 9.0°. This configuration mainly simplifies the calculation of interaction coordinates compared to the center of the gravity method. Moreover, the higher segment precision for DOI estimation is realized due to the SSLE technique. The 3-D scintillator detector achieves 4π Compton imaging with high detection efficiency and high localization accuracy. [ABSTRACT FROM AUTHOR]

Fissionable materials can be distinguished from normal materials by their emission of fast neutrons and neutrons moderated by surrounding materials. Radioisotopes can often be identified by their gamma-ray spectra. In this work, we use the nuclear physics Monte Carlo package MCNP6.2 to compare the detection efficiencies of different detector designs for the triple-mode detection of fast and moderated neutrons, and gamma-ray spectroscopy. For each design, we tabulate the detection efficiencies for neutrons from 0.1 eV to 5 MeV and for the full absorption of gamma rays from 0.1 to 5 MeV. All designs use 100 scintillators 1 cm × 1 cm × 5 cm. Of these, 36 are Cs2 LiLaBr6:Ce (90% 6Li) (CLLB) and 64 are polyvinyl toluene (PVT). The first design uses separate arrays of 6 × 6 CLLB and 8 × 8 PVT scintillators. Three other designs are 10 ×10 composite arrays using all 100 scintillators combined in different ways. PVT is a fast scintillator that can be used to detect fast neutrons by time-separated multiple proton recoils and discriminate against gammas. PVT in the three composite designs serves to moderate neutrons of all energies for capture by the 6Li in the CLLB. Gamma rays interact in the PVT and CLLB by Compton scattering and pair production; full energy absorption requires photoelectric absorption in the CLLB. The energy deposited by each gamma ray can be determined from a weighted sum of the CLLB and PVT signals from separate photodetectors. We find that for a fixed number of neutrons/cm2, one of the composite arrays has much higher 6Li detection efficiencies than the design that uses separate CLLB and PVT arrays, and has similar detection efficiencies for fast neutrons and gammas. We also find that in all cases, the average energy of the neutrons at capture by 6Li is well above the 25-meV thermal energy. [ABSTRACT FROM AUTHOR]

In this paper, we examine the history of detection efficiency measurements of photons (soft to hard X-rays and beyond) with microchannel plates (MCPs). We investigate the detection efficiency as a function of photon energy over a wide energy range, from a few hundred eVs up to 20 MeV. We also investigate detection efficiency as a function of incident angle onto the MCP. We interpreted the published efficiency data measured with a variety of MCPs and the use of MCP coatings for enhancement of X-ray quantum detection efficiency, and discuss theoretical expectations based on the main photon interactions with matter: the photoelectric effect, Compton scattering, and pair production. Contributing to the published literature at the highest end of the energy range, we also discuss our photon detection efficiency experiments from 2.5 to 20 MeV and theoretical implications. [ABSTRACT FROM AUTHOR]

A Neutron/Gamma-ray combined inspection system for hidden special nuclear materials (SNMs) in cargo containers has been developed under a program of Japan Science and Technology Agency in Japan. This inspection system consists of an active neutron-detection system for fast screening and a laser Compton backscattering gamma-ray source in coupling with nuclear resonance fluorescence (NRF) method for precise inspection. The inertial electrostatic confinement fusion device has been adopted as a neutron source and two neutron-detection methods, delayed neutron noise analysis method and high-energy neutron-detection method, have been developed to realize the fast screening system. The prototype system has been constructed and tested in the Reactor Research Institute, Kyoto University. For the generation of the laser Compton backscattering gamma-ray beam, a race track microtron accelerator has been used to reduce the size of the system. For the NRF measurement, an array of LaBr3(Ce) scintillation detectors has been adopted to realize a low-cost detection system. The prototype of the gamma-ray system has been demonstrated in the Kansai Photon Science Institute, National Institutes for Quantum and Radiological Science and Technology. By using numerical simulations based on the data taken from these prototype systems and the inspection-flow, the system designed by this program can detect 1 kg of highly enriched 235U (HEU) hidden in an empty 20-ft container within several minutes. [ABSTRACT FROM PUBLISHER]

To accurately characterize shielded special nuclear materials (SNM) using passive gamma-ray spectroscopy measurement techniques, the effective atomic number and the thickness of shielding materials must be measured. Intervening materials between the source and detector may affect the estimated source isotopics (uranium enrichment and plutonium grade) for techniques which rely on raw count rates or photopeak ratios of gamma-ray lines separated in energy. Furthermore, knowledge of the surrounding materials can provide insight regarding the configuration of a device containing SNM. The described method was developed using spectra recorded using high energy resolution CdZnTe detectors, but can be expanded to any gamma-ray spectrometers with energy resolution of better than 1% FWHM at 662 keV. The effective atomic number, Z, and mass thickness of the intervening shielding material are identified by comparing the relative attenuation of different gamma-ray lines and estimating the proportion of Compton scattering interactions to photoelectric absorptions within the shield. While characteristic \text K_\alpha x-rays can be used to identify shielding materials made of high Z elements, this method can be applied to all shielding materials. This algorithm has adequately estimated the effective atomic number for shields made of iron, aluminum, and polyethylene surrounding uranium samples using experimental data. The mass thicknesses of shielding materials have been estimated with a standard error of less than 1.3 g/cm2 for iron shields up to 2.5 cm thick. The effective atomic number was accurately estimated to 26 ± 5 for all iron thicknesses. [ABSTRACT FROM PUBLISHER]

Image-reconstruction with Compton-scattered photons is a nonlinear problem, because it involves both the linear process of scattering and the exponential pre- and post-scatter attenuation. For thin and light objects and with high-energy photons, the exponential terms can be ignored, rendering the problem linear. We show that when attenuation is significant, linearity can also be accomplished by considering scattering as a scaling factor and with subsequent logarithmic transformation, the image-reconstruction problem is converted into a transmission-like problem along the travel lines of incident and scattered radiation. It is shown that this formulation produces images qualitatively reflecting object features in a manner similar to that of backprojection in conventional computed tomography. This is done without the need for calibration or scaling of measurements. The theoretical aspects of the introduced mathematical formulation are developed for idealized pencil-beams, adapted approximately to fan-beam configurations, and applied to imaging with laboratory-acquired measurements. The introduced transmission-like formulation and calibration-free aspects should promote wider use of scatter imaging. [ABSTRACT FROM AUTHOR]

To improve the quantitative accuracy of linear attenuation coefficients measured by computed tomography (CT), we used a single scatter model to estimate the Compton scatter distribution and then a polychromatic image reconstruction algorithm, namely the iterative maximum-likelihood polychromatic algorithm for CT (IMPACT), was implemented to include scatter correction (SC). To perform the IMPACT, the X-ray spectra of a tube were estimated via an expectation-maximization (EM) algorithm with SC. To test the accuracy of the estimated spectra, the projection images of cubic phantoms containing different depths of polymethylmethacrylate (PMMA) were acquired. The percentage of root mean square errors (%RMSE) of the measured transmission data and calculated transmission values were used to evaluate the accuracy of the estimated spectra. In addition, a hydroxylapatite (HA) phantom was used to study streak artifacts and evaluate the accuracy of the linear attenuation coefficients estimated using the IMPACT with SC. The %RMSE of the EM-with-SC estimated spectra were all lower than 1% and were also smaller than that without SC. The error in the quantification of the HA linear attenuation was only about 3% after SC. Our results showed that the quantitative accuracy of the linear attenuation coefficients measured with a cone beam CT was improved when the IMPACT with SC was used. [ABSTRACT FROM AUTHOR]

The Rotating Scatter Mask (RSM) system is an inexpensive retrofit that provides imaging capabilities to scintillating detectors. Unlike traditional collimator systems that primarily absorb photons in order to form an image, this system primarily scatters the photons. Over a single rotation, there is a unique, smooth response curve for each defined source position. Testing was conducted using MCNPX simulations. Image reconstruction was performed using a chi-squared reconstruction technique. A simulated 100 uCi, Cs-137 source at 10 meters was detected after a single, 50-second rotation when a uniform terrestrial background was present. A Cs-137 extended source was also tested. The RSM field-of-view is 360 degrees azimuthally as well as 54 degrees above and 54 degrees below the horizontal plane. Since the RSM is built from polyethylene, the overall cost and weight of the system is low. The system was designed to search for lost or stolen radioactive material, also known as the orphan source problem. [ABSTRACT FROM PUBLISHER]

Using \gamma -ray beams produced in the inverse Compton scattering between \CO_2 laser photons and relativistic electrons, we have calibrated electron beam energies in the nominal energy range 550–974 MeV at the synchrotron radiation facility NewSUBARU. The laser Compton-scattering (LCS) \gamma -ray beams were produced at energies from 561  to 1728 keV and detected with a high-purity germanium detector. The electron beam energies were determined by reproducing the full energy peaks of the \gamma -ray beams by Monte Carlo simulations. The accuracy of the calibration is (5.5 - 9.4) \times 10^ - 5. The reproducibility of the electron beam energy is excellent in an independent injection and deceleration. The present energy calibration of the electron beams offers a standard for the energy calibration of high-energy LCS \gamma -ray beams produced with a \Nd:YVO_4 laser. As applications of the energy calibration, we investigated the energy linearity of a 3.5^\prime\prime \times 4.0^\prime\prime \LaBr_3(\Ce) detector in the response to \gamma -rays at energies up to 10 MeV and the energy profile of the high-energy LCS \gamma -ray beams. [ABSTRACT FROM AUTHOR]

In order to measure atomic number (Z) of materials, we present a Z identification method based on MV X-ray scattering spectra analysis in this paper. Scattering photons, which include positron annihilation photons, bremsstrahlung photons and Compton scattered photons, are generated by interactions of X-rays with matters and carry their Z information. A LaBr3(Ce) detector is used for its short resolving time (<100 ns) to measure scattered photons to alleviate pulse pileup problem in the 5 \mus X-ray pulse duration. ADC with high sampling rate of 120 MHz and high resolution of 14-bit is used to digitize signal waveform from the preamplifier of the LaBr3(Ce) detector. An offline algorithm is designed to reconstruct scattering spectra. Appropriate shielding structure is set up to reduce the number of transmission photons from accelerator target and scattering photons in environment that enter the detector. These designs ensure effective acquisition of scattering photons in the intense pulsed radiation field. We investigated 10 materials with Z from 13 to 82. Preliminary experimental results show that Z of different materials can be determined successfully. [ABSTRACT FROM PUBLISHER]

New results from testing an array of 6\,\times\,6\,\times\,15 mm^3 virtual Frisch-grid CdZnTe (CZT) detectors with common-cathode readout for charge signals correction and rejection of incomplete charge collection events (ICC) are presented. The array employs parallelepiped-shaped crystals of a large geometrical aspect ratio with two planar contacts on the top and bottom surfaces (anode and cathode) and an additional shielding electrode placed on the sides to create the virtual Frisch-grid effect. The detectors are arranged in 2\,\times\,2 or 3\,\times\,3 detector modules with the common cathode readout by a single electronic channel. Because of the common cathode, the length of the shielding electrode can be further reduced with no adverse effects on the device performance. By implementing a novel technique for rejecting ICC events caused by the extended defects, we can achieve good spectral responses from ordinary CZT crystals, which can be produced with higher yield and at lower cost. For such crystals, the resolution of individual detectors is expected to be in the range of 0.8–1.5% FWHM at 662 keV with an average value of 1.3%. Arrays of virtual Frisch-grid detectors offer a robust and low-cost approach for making large-area detection modules that can potentially substitute for more advanced, but also more expensive and less available, pixel detectors in applications with slightly relaxed requirements on position- and energy-resolution (e.g., for coded aperture telescopes). In addition, such virtual Frisch-grid arrays will require a comparably smaller number of readout channels, which allows for lower power consumption. [ABSTRACT FROM AUTHOR]

Non-proportionality of light yield and energy resolution of Compton electrons in three scintillators (LaBr3:Ce, LYSO:Ce and CsI:Tl) were studied in a wide energy range from 10 keV up to 1 MeV. The experimental setup was comprised of a High Purity Germanium detector and tested scintillators coupled to a photomultiplier. Probing the non-proportionality and energy resolution curves at different energies was obtained by changing the position of various radioactive sources with respect to both detectors. The distance between both detectors and source was kept small to make use of Wide Angle Compton Coincidence (WACC) technique, which allowed us to scan large range of scattering angles simultaneously and obtain relatively high coincidence rate of 100 cps using weak sources of about 10 \muCi activity. The results are compared with those obtained by direct irradiation of the tested scintillators with gamma-ray sources and fitting the full-energy peaks. [ABSTRACT FROM PUBLISHER]

In this work we propose a parallel beam approximation for the computation of the detection efficiency of crystals in a PET detector array. In this approximation the detection efficiency of a crystal is estimated using the distance between source and the crystal and the pre-calculated detection cross section of the crystal in a crystal array which is calculated for a uniform parallel beam of gammas. The pre-calculated detection cross sections for a few representative incident angles and gamma energies can be used to create a look-up table to be used in simulation studies or practical implementation of scatter or random correction algorithms. Utilizing the symmetries of the square crystal array, the pre-calculated look-up tables can be relatively small. The detection cross sections can be measured experimentally, calculated analytically or simulated using a Monte Carlo (MC) approach. In this work we used a MC simulation that takes into account the energy windowing, Compton scattering and factors in the “block effect”. The parallel beam approximation was validated by a separate MC simulation using point sources located at different positions around a crystal array. Experimentally measured detection efficiencies were compared with Monte Carlo simulated detection efficiencies. Results suggest that the parallel beam approximation provides an efficient and accurate way to compute the crystal detection efficiency, which can be used for estimation of random and scatter coincidences for PET data corrections. [ABSTRACT FROM AUTHOR]

The detection of radioactive contraband is a critical problem in maintaining national security for any country. Gamma-ray emissions from threat materials challenge both detection and measurement technologies significantly. The development of a sequential, model-based Bayesian processor that captures both the underlying transport physics of gamma-ray emissions including Compton scattering and the measurement of photon energies offers a physics-based approach to attack this challenging problem. The inclusion of a basic radionuclide representation of absorbed/scattered photons at a given energy along with interarrival times is used to extract the physics information available from noisy measurements. It is shown that this representation leads to an “extended” physics-based structure that can be used to develop an effective sequential detection technique. The resulting model-based processor is applied to data obtained from a controlled experiment in order to assess its feasibility. [ABSTRACT FROM AUTHOR]

Defence R&D Canada has an active research and development program on detection of explosive devices using nuclear methods. One system under development is a coded aperture-based X-ray backscatter imaging detector designed to provide sufficient speed, contrast and spatial resolution to detect antipersonnel landmines and improvised explosive devices. The successful development of a hand-held imaging detector requires, among other things, a light-weight, ruggedized detector with low power requirements, supplying high spatial resolution. The University of California, San Diego-designed HEXIS detector provides a modern, large area, high-temperature CZT imaging surface, robustly packaged in a light-weight housing with sound mechanical properties. Based on the potential for the HEXIS detector to be incorporated as the detection element of a hand-held imaging detector, the authors initiated a collaborative effort to demonstrate the capability of a coded aperture-based X-ray backscatter imaging detector. This paper will discuss the landmine and lED detection problem and review the coded aperture technique. Results from initial proof-of-principle experiments will then be reported. [ABSTRACT FROM AUTHOR]

Recent developments of large-area Si(Li) orthogonal-strip detectors have revealed their capability for applications in Compton-effect-based instruments. Some inherent advantages of silicon such as the dominance of Compton scattering in photon interactions and operation at room or somewhat lower temperature combined with the availability of large-volume Si(Li) detectors could stimulate the development of powerful Compton instruments. Several diodes 10 mm in thickness with a diameter of 102 mm were fabricated. Two 10 mm thick diodes were cut to form a 74 mmtimes74 mm square with slightly rounded corners. The same position-sensitive structure, 32 strips with a pitch of 2 mm, was produced on the thin Li-diffused n-contact and boron-implanted p+-contact by means of photolithography and plasma etched grooves. The position-sensitive area of 64 mmtimes64 mm is surrounded by a 5 mm wide guard-ring. One of these 10 mm thick Si(Li) orthogonal-strip detectors has been mounted in a cryostat prepared at Lawrence Livermore National Laboratory (LLNL). The detector will be extensively tested there with the goal of being integrated into the Compact Si+Ge Compton camera system consisting of this Si(Li) and a HPGe orthogonal-strip detector [ABSTRACT FROM PUBLISHER]

When a polarized gamma ray photon undergoes a Compton scatter, the probability for the photon to scatter at a fixed scattering angle depends on the azimuthal angle. The polarization of incident gamma rays can be measured using this effect by observing the angular distribution of scatters. Room-temperature 3-dimensional position-sensitive CdZnTe detectors can provide good energy resolution as well as positions of interactions, allowing these detectors to perform Compton imaging and polarization detection simultaneously. In this paper, we demonstrate that an 11 × 11 pixellated 15 × 15 × 10 mm³ CdZnTe detector with 3-D position sensing capability can be an effective instrument to detect the polarization of incident gamma rays. [ABSTRACT FROM AUTHOR]