Yong Kong

A technique for measuring the energy resolution of low-Z scintillators

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.

Concept study of a two-plane Compton camera designed for location and nuclide identification of remote radiation sources

The concept of a two-plane planar Compton camera, consisting of scintillation detector elements, is presented. Several materials as C<inf>9</inf>H<inf>10</inf>, CaF<inf>2</inf>, YAlO<inf>3</inf>, NaI, and LaBr<inf>3</inf> 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 C<inf>9</inf>H<inf>10</inf> or CaF<inf>2</inf> for the scatter plane and NaI or LaBr<inf>3</inf> 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.

Energy resolution and nonlinearity of NaI(Tl), CaF 2 (Eu), and plastic scintillators measured with the wide-angle Compton-coincidence technique

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 (Zeff), 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 in-house using the WACC technique. This work presents results of different scintillator materials and sizes for validation and exploration of this method.

Linearization of Gamma Energy Spectra in Scintillator-Based Commercial Instruments

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 technique for estimating detection limits of radio-nuclide identifying detectors by means of computer simulations

A simulation technique has been developed to study the performance of nuclide identifying gamma detector units operating under a variety of different conditions. The studies are related to nuclide identification based on a template matching algorithm which has been well established, in particular, for the analysis of low statistics measurements. Results are presented for hand-held devices equipped with NaI(Tl) and LaBr3(Ce3+) scintillation detectors, respectively

Evolutionary ensembles that learn spectroscopic characteristics of scintillation and CZT detectors

A method is described to automatically generate spectrum reference data for radioisotope identification devices respecting a detectors physical individuality. It extracts the peak shape and non-proportionality characteristics of scintillation and CZT detectors. The representation of these quantities is done with evolutionary ensembles, groups of N-dimensional autonomous points, which are propagated within a constrained space. Each ensemble member is used as parametrical input for describing peak shape and position within a simulation framework based on Geant4. Each subsequent generation of the ensemble iteratively converges the simulation result towards an optimised match with the measurement. Examples for the scintillator show that the shape convergence is straightforward due to the gaussianity of the peak, while the correction of the non-proportionality is within the quantity of up to 10%. Contrarily, our CZT example yielded nearly no non-proportionality along the energy scale, but required a complex, multi-parametrical shape definition with learning curves for kurtosis, skewness and resolution to establish an adequately peak reproduction. A metric is presented to calculate the distance between the experimental data and the calculated result. The described system is suited to establish a production line with a fully automatised acquisition of spectral characteristics to support the deployment of detector individual reference data for nuclide identification instrumentation.

A Prototype Compton Camera Array for Localization and Identification of Remote Radiation Sources

A functional prototype two-plane Compton camera array for localization and identification of remote radiation sources, consisting of four PVT and four NaI(Tl) scintillation detectors with PMT readout, is presented. The large-volume, 76 × 76 ×76 mm scintillators provide a broad field of view for scattered photons and facilitate maximum efficiency at moderate cost. Each detector is equipped with a voxelSPEC, a compact electronic module that provides high voltage for the PMT, signal processing, detector stabilization, and an Ethernet communication interface. The voxelSPEC delivers list-mode event data with nanosecond precision timing over non-proprietary Ethernet and makes a system extension very easy. A software package has been developed for real-time data processing and image reconstruction. Advantages in the hard- and software allow stable, unattended operation of the camera array for many days, and provide easy-to-read information on the radiation source in real time. Measurements with the prototype array have been performed for a few standard scenarios and geometries to verify the model predications obtained by Monte-Carlo simulations. Simulations have been further performed to explore larger camera arrays with 2 × 4 × 4, 2 × 8 × 8, 2 × 10 × 10, 2 × 13 × 13 and 2 × 16 × 16 detectors.

Characterization and calibration of large-volume PVT detectors by Backscatter Gating

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 Radiations PVT-based STRIDE detection units, and for calibrating or linearizing the complete, commercial STRIDE DU 601.1 modules. Selected results are presented and discussed.

Comparison of different Cs 2 LiYCl 6 :Ce crystals: Energy resolution and pulse shape dependences on temperature

Portable radiation detection systems will greatly benefit from the use of the same detector for gamma and neutrons, on the roadmap towards miniaturization, provided such devices are capable of robust discrimination of gammas against neutrons. One of the most promising and yet commercially available inorganic scintillator, delivering different pulse shapes for thermal neutrons and gammas is the Cs2LiYCl6:Ce (CLYC) [1,2]. The detection for thermal neutrons follows through the 6Li(n,alpha)3H capture reaction, and with the appropriate levels of 6Li enrichment, which have currently reached 95%, the CL YC crystals could become very efficient neutron detectors. In order to successfully integrate this new scintillator in our digital devices, the variability of the energy resolution across various crystals, the dependence of the energy resolution on energy, shaping time and temperature, the pulse shape parameters variability across the crystals and the thermal neutron sensitivity were investigated.

Anomalous gain drop effects in Hamamatsu 3998-01 photomultiplier

Currently, in our IdentiFINDER LGH instruments, commercially available 30S30_B380 (LaBr3) detectors, purchased from Saint-Gobain Crystals, are deployed. They are equipped with R3998-01 photomultipliers from Hamamatsu Photonics. With this type of photomultiplier, we recently encountered a new issue in our devices related to a unprecedented gain drop/recovery, occurring at a certain temperature and count rate. The experiments will be outlined and the results presented.