Encapsulation of Plastic Scintillator to Prevent Degradation (#1015)
R. T. Kouzes1, P. E. Keller1, J. M. Shergur1, P. J. Smith1
1 Pacific Northwest National Laboratory, Richland, Washington, United States of America
It has been observed that polyvinyl toluene-based and polystyrene-based gamma ray detectors can suffer internal “fogging” when exposed to certain environmental conditions over long periods of time. When observed, this change results in reduced light collection from the plastic scintillator. Investigation of the physical cause of these changes has been explored, and a root cause identified. Mitigation methods are being investigated. One mitigation approach is encapsulation of the scintillator in materials impermeable to water vapor. At the same time, any encapsulation approach must be thin enough to allow low energy gamma rays to be detected.
This talk will present the results from a study of various encapsulation approaches. Water vapor, which causes the fogging of plastic scintillators, is exceedingly difficult to exclude from penetrating any enclosure, so it is a real challenge to sufficiently encapsulate plastic scintillator to maintain consistent performance. Results of experimental measurements that have been made on many samples of plastic scintillator to demonstrate the effectiveness of a few encapsulation solutions will be presented.
Keywords: PVT, plastic scintillator, degradation, polystyrene
Optimization of Spatial Resolution and Crosstalk in a High Energy Active Interrogation System (#3948)
P. B. Rose Jr1, J. Harms1, J. Nattress2, I. Jovanovic2, A. S. Erickson1
1 Georgia Institute of Technology, Nuclear and Radiological Engineering, Atlanta, Georgia, United States of America
Cargo containers are transported into the country at a staggering and increasing rate, resulting in an expanded vulnerability to nuclear terrorism via smuggling devices hidden inside. Alarmingly few of these containers are inspected for nuclear threats and the current methods of scanning do not provide information on the material, but merely the shape via imaging with high energy x-rays from bremsstrahlung sources. We have designed a proof on concept system to interrogate cargo containers using discrete energy gamma rays produced from low energy nuclear reactions generated by an compact ion accelerator. These multiple, monoenergetic gamma rays can be used to create the same images as the bremsstrahlung methods, but also provide a means to estimate the Zeff of the material in each pixel of the image via differential attenuation techniques.
In this research, we present an update to the design and performance of the imaging array detectors and characteristics as a whole system to investigate the effects of detector cross talk, pile up, and image spatial resolution as well as detector type and geometry. We evaluate two main detector types, typical NaI detectors currently used in nuclear security applications as well as nontraditional Cherenkov detectors with crude spectroscopic capabilities. This work compares and discusses validated computational results and experimental data to optimize the imaging detector array when using gamma ray energies up to the 15.1 MeV photons from the 11B(d,n-Ɣ)12C reaction.
Keywords: Active Interrogation, low energy nuclear reaction, imaging, cross talk, optimization
SrI2(Eu) Handheld Gamma Spectrometer Radionuclide Identification Performance and Silicon Photomultiplier Readout (#2714)
P. R. Beck1, B. M. Wihl1, N. J. Cherepy1, K. E. Nelson1, B. S. Seilhan1, E. L. Swanberg1, S. E. Fisher1, S. L. Hunter1, S. A. Payne1, S. E. Labov1
1 Lawrence Livermore National Laboratory, Livermore, California, United States of America
Quickly and accurately identifying distant, shielded, mixed, or masked radionuclides is a major challenge in the search operations critical to national security. With high photoelectric absorption cross-section (high effective atomic number, Z=49) and high energy resolution (3% FWHM at 662 keV), Europium-doped Strontium Iodide, SrI2(Eu), is well suited to the low signal-to-noise regime of search operations. To demonstrate the capabilities of this material, we have constructed a handheld gamma spectrometer based on SrI2(Eu), named MrID (mobile radioisotope identifier), with digital readout electronics and advanced radionuclide identification (RID) software onboard, the LLNL-developed Radionuclide Analysis Kit (RNAK). Efficient software design enables rapid identification on a mobile platform of over 60 radionuclides, alone and in mixtures, using full-spectral fitting, rather than only peak-finding. We will describe the results of our experimental measurement campaign comparing the ability of SrI2(Eu) and NaI(Tl) to identify mixed sources with low signal-to-noise using RNAK. With identically sized crystals, 1.5” x 1.5”, Strontium Iodide yields a significant decrease in the time to correctly identify weak and mixed gamma sources, as well as increased ability to detect and identify masked gamma sources. Building on our experience developing the PMT-based MrID, we are developing a next generation of handheld spectrometer utilizing silicon photomultiplier (SiPM) arrays to take advantage of increased ruggedness and photon detection efficiency. We will present recent progress in reading out SrI2(Eu) with SiPM arrays, a technology well matched to the bright but slower emission of SrI2(Eu).
Keywords: Strontium Iodide, SrI2, high energy resolution, RNAK, radionuclide identification, RID, RIID, SiPM
Applications of Innovative SiPM-based PVT Scintillator Detectors (#2959)
M. Meshkian1, C. Allwork2, U. Gendotti1, M. Ellis2, P. Schotanus3
1 Arktis Radiation Detectors Ltd., Zürich, Switzerland
Recently, a novel plastic scintillator detector has been developed for detection of illicit trafficking of radionuclides. Silicon Plastic Readout Gamma Detection Technology (SiPR), has been developed jointly by Arktis and other European customers for efficiently detecting gamma radiation from 100 keV up to 3000 keV. SiPR utilizes a plastic scintillator with an embedded Silicon Photomultiplier (SiPM)-based light read-out solution. Due to its scalability and modularity, SiPR may have applications ranging from hand-held contamination systems up to radiation portal monitors (RPMs). In this study, a comparison is made between the performance of SiPR and a PMT-based detector with the same PVT form factor. In addition, the current study includes optimization of light collection efficiency as a function of the PVT form factor. The latter study has been necessary in order to increase the detection efficiency of lower gamma energies. As a result of the geometry optimization, the detector illustrates source categorization properties, which are unique for its type and size. This property is of particular importance for radiation portal monitors, where the source categorization is crucial in order to stop illicit trafficking of radionuclides.
Keywords: PVT, SiPM, SiPR, Gamma Detection, Light Collection, Source Categorization
Depth-of-Interaction Correction for Coded-Aperture, Gamma-Ray Images (#1333)
K. - P. Ziock1, M. A. Blackston1
1 Oak Ridge National Laboratory, NSITD, Oak Ridge, Tennessee, United States of America
The image scale of coded-aperture images is determined by the mask hole size, the focal length, and the distance to the scene being imaged. If the detector thickness is significant compared to the focal length, then this leads to radially blurred images because events at different detector depths have different effective focal lengths. However, with a detector that records the depth at which events occur, this blurring can be removed by rebinning the images to a common image scale. This complicates calculating the uncertainties associated with the image because the neighboring pixels in oversampled images are correlated. This paper shows both the effectiveness of depth-corrected images and how the uncertainties (including covariance) can be handled based on an “EventImage” construct used to generate images online.
Keywords: gamma-ray imaging, coded-aperture imaging, HPGe, depth of interaction
Real-time Temporal Gamma Spectroscopy in a Field-Programmable Gate Array (#2108)
M. Mannino1, E. Becker1, A. T. Farsoni1
1 Oregon State University, Nuclear Science and Engineering, Corvallis, Oregon, United States of America
Accurately measuring the quantity of special nuclear material quantity in a sample is a key capability for organizations engaged in nuclear non-proliferation and nuclear forensics efforts. The continuous temporal ratio method developed at Oregon State University mitigates the effect of nuclear data uncertainty in quantification estimation. This technique was previously demonstrated using mixtures of HEU and Pu at Idaho National Laboratory using an accelerator and Silver and Dysprosium using the OSU TRIGA reactor and offline data processing. Both the data acquisition system and the confidence level of the material estimates have been further improved using a newly developed real-time digital pulse processing. A multi-channel, 14-bit, 125 MSPS digital pulse processor, the DPP8, is being developed to replace the current conventional MCA and list-mode measurements. The DPP8 can perform all pulse processing digitally in real-time as well as control the operation of the pneumatic transfer system using an on-board FPGA. In addition to typical pulse processing modules such as trapezoidal shapers and triggers, a new “listogram” mode has been implemented that generates continuous time-stamped histograms, significantly reducing the amount of data that needs to be analyzed. The new system is validated and compared to the old system in terms of detection performance and data analysis.
Keywords: multi-channel, FPGA, pulse processor