Compton cameras have become an instrument of interest for long-range localization of radioactive materials in nuclear-nonproliferation applications. In this work, a specialized simulation tool was developed for the optimization of a Compton camera through realistic Monte Carlo simulations. This tool can be used for Compton cameras with different geometries. The MCNPX-PoliMi code was used to simulate Compton scatters and photoelectric absorptions in the camera's detectors. The imaging capability is derived from the physics of Compton scattering and the full-energy information obtained in photoelectric absorptions. The simulated system is a 1 x 1 m(2) Compton camera consisting of two planar arrays of photon detectors. Several scintillators were evaluated: LaBr3, CaF2, and NaI(Tl) (all inorganic scintillators), and C9H10 (plastic organic scintillator). The investigation was carried out with a Cs-137 source. A minimum detectable activity (MDA) was defined and used to assess the performance of cameras based on different detectors. The simulation results show that C9H10 is a reliable, low-cost, scatter-plane material, increasing the MDA by similar to 0.2 mCi when compared to a CaF2 scatter plane. On the other hand, the intrinsic background of LaBr3 undermines its above-average energy resolution as the MDA increased by similar to 0.5 mCi when compared to a NaI(TI) absorption plane. (C) 2015 Elsevier Ltd. All rights reserved.

Passive radiation detection systems have been developed to screen passengers, vehicles, and cargo for illicit radioactive sources by measuring gamma and neutron signatures with separate, specialized sensors. The paper introduces a novel concept combining neutron and gamma sensing in a single detector, thus reducing the overall expense. Low-cost converter media capture thermal neutrons and commute neutron flux in energetic gammas, which are then detected by a common gamma detector. Energy signals above 3 MeV indicate the neutron captures. Two prototype systems are presented: (1) The NCD-BGO, a segmented 655 ml BGO scintillator with embedded Cd absorber, demonstrated an intrinsic thermal-neutron detection efficiency of about 50%. (2) The PVTNG, comprising 75 l of PVT scintillator complemented with PVC panels, exhibited a neutron sensitivity of 1.9 cps/ng of 252Cf, thus almost meeting the corresponding requirement for Radiation Portal Monitors. Moreover, an unconventional construction of scintillator and light readout, combined with innovative electronics and proper detector stabilization, improved the gamma detector performance noticeably and enabled nuclide identification.

The neutron capture detector (NCD) is introduced as a novel detection scheme for thermal and epithermal neutrons that could provide large-area neutron counters by using common detector materials and proven technologies. The NCD is based on the fact that neutron captures are usually followed by prompt gamma cascades, where the sum energy of the gammas equals to the total excitation energy of typically 6–9MeV. This large sum energy is measured in a calorimetric approach and taken as the signature of a neutron capture event. An NCD consists of a neutron converter, comprising of constituents with large elemental neutron capture cross-section like cadmium or gadolinium, which is embedded in common scintillator material. The scintillator must be large and dense enough to absorb with reasonable probability a portion of the sum energy that exceeds the energy of gammas emitted by common (natural, medical, industrial) radiation sources. An energy window, advantageously complemented with a multiplicity filter, then discriminates neutron capture signals against background. The paper presents experimental results obtained at the cold-neutron beam of the BER II research reactor, Helmholtz-Zentrum Berlin, and at other neutron sources with a prototype NCD, consisting of four BGO crystals with embedded cadmium sheets, and with a benchmark configuration consisting of two separate NaI(Tl) detectors. The detector responses are in excellent agreement with predictions of a simulation model developed for optimizing NCD configurations. NCDs could be deployed as neutron detectors in radiation portal monitors (RPMs). Advanced modular scintillation detector systems could even combine neutron and gamma sensitivity with excellent background suppression at minimum overall expense.

PVT scintillators provide maximum gamma sensitivity at minimum cost. The low price makes PVT an attractive detector in spite of poor spectroscopic performance. In this context the paper introduces an inexpensive but very effective technique for characterizing and calibrating PVT detectors, called Backscatter Gating. This technique has been used for evaluating prototype detectors of FLIR Radiation's PVT-based STRIDE detection units, and for calibrating or linearizing the complete, commercial STRIDE DU 601.1 modules. Selected results are presented and discussed.

Recent efforts in nonproliferation and homeland security areas have focused on designing systems to accurately detect and locate radioactive material. This task is especially challenging when trying to locate material at large distances. One solution that has been under investigation is the Compton camera. In this work, the performance of a large-scale, two-plane Compton camera is investigated using different imaging reconstruction methods on simulated data. The simulation methodology is being validated by measurements conducted using a small-scale prototype of the system in the laboratory environment. The geometry consists of two planar arrays of scintillation detectors.

Homeland security applications demand high performance Compton-camera systems, with high detector efficiency, good nuclide identification and able to operate in-field conditions. A low-Z scintillator has been proposed and studied as a promising candidate for use in the scattering plane of a scintillator-based Compton camera: CaF2(Eu). All the relevant properties for the application of this scintillator in a mobile Compton camera system have been addressed: the energy resolution and the non-linearity at room temperature and in the temperature range of -20 degrees C to + 55 degrees C, the photoelectron yield and the relative light yield in the relevant temperature range. A new method of inferring the relative light output of scintillators as a function of temperature has been proposed.

The primary issue regarding the proliferation of radioactive materials is their possible ill-intentioned use. Depending on the material, it could enable the construction of dirty bombs or even nuclear devices. Several detection systems have been engineered to help control the transport of these materials and to provide efficient detection capabilities. Compton cameras have been used in fields such as medical or astronomical imaging for nearly 40 years. The existing research on the Compton-camera concept is unfortunately not applicable to these applications: the energies and distances of interest are very different. We have designed a new way to simulate a two-plane Compton camera for nuclear nonproliferation applications using the MCNP-PoliMi code. The simulations include accurate background models and detector properties such as time and energy resolutions, and pulse-generation time. Energy spectra can be obtained for both planes, along with the back-projection images. In this work, we present a study on the sensitivity of various large-scale Compton-camera configurations using this simulation tool. The Compton-camera materials investigated are PVT and CaF2 for the scatter plane, and NaI and LaBr3 for the absorption plane. The planes optimized in previous work; the voxels of 2 inch × 2 inch were used throughout this work. Results are presented for the weakest detectable source (1.4 mCi for CaF2/NaI) at a 100-m standoff in a 60-s measurement for the various Compton-camera configurations.

This paper presents a novel technique developed for linearizing the energy spectra of radiation detectors in commercial radioisotope identification devices. Based on few spectrum measurements with standard radio-nuclide sources, this method allows generation of individual nonlinear calibration functions at minimum expense in the routine instrument setup. Instead of fitting peak positions, the measured raw data are compared with simulated spectrum templates, and local gain factors providing the best correspondence are taken as reference points for the calibration function. This approach avoids the problem of fitting multiple peaks with intensity ratios influenced by absorbing layers and assures an accuracy of 1% in the energy range of 30 keV to 3 MeV.

A unified method for representation and calculation of the system matrix for Compton cameras has been proposed. The method defines first a photon interaction scheme in form of a logic photon state machine and then calculates the system matrix elements based on the transition probabilities for each state. Additionally uncertainties of the measurement can be taken into account in form of probability density distributions or, in case of simplifications, as scalars. The method provides a structured approach for the system matrix calculation which simplifies the software development process and the optimization of the computational performance. Additionally, the system matrix calculation is completely independent of the reconstruction algorithm itself. Therefore, the same code can be used for the reconstruction of the measured data collected with the different hardware and for different applications (i.e. medical imaging or homeland security).

Compton cameras are of general interest in various fields of operation. Because of the ability to locate and identify remote sources, homeland security supports the development of such devices in a rugged and reliable form. The decisions upon appropriate materials for the scatter- and absorber plane depend on performance and economical trade-offs. In order to estimate the expected performance of the Compton camera, simulations are necessary. Certain experimentally determined parameters have to be fed into simulations, such as the energy resolution of the detector.Two materials with low effective atomic number (Z(eff)), CaF2 and plastic, promise to be good candidates for the scattering plane. Those scintillators are known for quite some time, but not very well characterized with respect of energy resolution and nonlinearity. A modified Compton coincidence technique using a high purity Germanium (HPGe) detector in coincidence with the investigated scintillator is discussed in this paper: The wide-angle Compton-coincidence (WACC) setup provides a fast and reliable means for characterization of low-Z scintillators. For quality control purposes, the actual scatter detector can be monitored inhouse using the WACC technique. This work presents results of different scintillator materials and sizes for validation and exploration of this method.

ICx Radiation has developed a novel method to determine the direction of radiation with a device containing only two detectors by comparing and analyzing the paired-energy distributions of coincident events. The method does not require complex image reconstruction but rather extracts the directionality from the means and skewness of the two coincidence spectra. All energy data contribute to the energy spectrum, while events that occur within the coincidence time window are also added to a separate time-dependent buffer that represents the “reduced” spectra from each detector. These spectra contain counts only from true Compton events. The mean and skewness of each detector's reduced spectra subset is then calculated. Equal skewness and means correspond to the source being in front of the device. Opposite polarity skewness correspond to the source being located to the left or right of the device. For low count rates due to weak sources or sources located far away from the detectors, the skewness comparison gives fast indication for the hemisphere in which the source is located, while increased count rates or increased acquisition times reduce the uncertainties and allow a detailed angular detection of the source position. The simulations, experimental results and the angular resolution as a function of the strength, source energies and distance of the radiation source are discussed.

Homeland security applications demand high performance Compton-camera systems, with high detector efficiency, good nuclide identification and able to operate in-field conditions. A low-Z scintillator has been proposed and studied as a promising candidate for use in the scattering plane of a scintillator-based Compton camera: CaF 2 (Eu). All the relevant properties for the application of this scintillator in a mobile Compton camera system have been addressed: the energy resolution and the non-linearity at room temperature and in the temperature range of −20°C to +55°C, the photoelectron yield and the relative light yield in the relevant temperature range. A new method of inferring the relative light output of scintillators as a function of temperature has been proposed.

Compton cameras have been used for astronomical and medical imaging applications as early as the 1970s. Recent interest in their potential for the detection and localization of special nuclear material (SNM) has led to increasing investigations. In this work, a specialized algorithm was developed for the optimization of a two-plane Compton camera. The MCNP-PoliMi code was utilized to simulate photon interactions within the detectors and coupled with an analytical technique used to estimate angular uncertainties. Using our specialized algorithm, a large area (approximately 1 m2) Compton camera consisting of two planar arrays of photon detectors was evaluated for several scintillators: LaBr3, CaF2, NaI, and plastic (C9H10). Optimization of plane thickness and voxel size were conducted for Compton camera efficiency and angular uncertainty at source energies of 100-400 keV.

The concept of a two-plane planar Compton camera, consisting of scintillation detector elements, is presented. Several materials as C9H10, CaF2, YAlO3, NaI, and LaBr3 are considered for operation in the scatter and/or absorption plane. The performance of the Compton camera is optimized by means of Monte Carlo simulations to meet the requirements for Homeland Security applications. For a low-threshold detector system we propose to utilize C9H10 or CaF2 for the scatter plane and NaI or LaBr3 for the absorption plane. Particular effort must be focused to achieve low energy thresholds in particular for the detectors of the scatter plane if photons of incident energies below 200 keV are to be detected with reasonable efficiencies.

Scintillator-based Compton cameras for remote localization and identification of radio nuclides require scatter detectors made of low-Z materials. The energy resolution of such detectors in a range dominated by Compton scattering is a crucial parameter. It has to be known for performance estimates, and it must be quantified and optimized for detector designs to be used in real systems, but it is hard to measure. The Compton Coincidence Technique (CCT) appears to be the best method for reliable and direct measurements, but appropriate facilities are expensive. This paper suggests and investigates a modified CCT which provides less expensive means for qualifying of scatter detectors in a reasonable time frame. The assembly consists of a single HPGe detector, the scatter detector to be investigated, and one or more common gamma sources in close geometry. Pulse height and timing information from both detectors is gathered by multi-parameter data acquisition. Coincidences of both detectors are due to a plurality of Compton scattering angles and corresponding energy transfers. A thorough data analysis then allows extracting the detector resolution as well as the non-linearity as a function of energy from data sets measured within hours. Results obtained for NaI and plastic scatter detectors will be presented and discussed.

Homeland security applications demand compact, high resolution detectors for gamma and X rays identification. Most commercial instruments use scintillating crystals or CdZnTe detectors. The identification performance is therefore intrinsically linked with the progress in developing crystals with a very high light output (LaBr3Ce or SrI2) and with the manufacturing of the CdZnTe detectors. A new detector concept is proposed, which essentially improves on the existing resolution limits. It combines two distinct detection mechanisms in a compact, state-of-the-art solid-state detector, providing complementary information in different but overlapping energy ranges. The dual-range detector consists of a LaBr3(Ce3+) scintillator coupled to a silicon drift detector (SDD). The SDD serves as a high-resolution X-ray on its own, and in parallel as a light readout device for the LaBr3(Ce3+) scintillator ensuring best possible resolution at higher energies.