With its variety of radioactive sources, available facilities, and wealth of in-house technical expertise, the RSEL provides an ideal environment for testing of methods, materials, and instruments. The high intensity neutron generator and radioisotopes in the RSEL can be utilized for research, development, and testing of detectors, new detector development, unique material studies, assessments in gamma and/or neutron radiation fields, and a wealth of related applications ranging from instrument calibrations to biotechnology. As part of an educational institution, the RSEL has a mandate to provide for the education of Georgia Tech students, and to seek opportunities to educate the surrounding community.
Development and Testing of a New Zinc Oxide Neutron Detector
N. Hertel, C. McGahee, J. Wycuff
Preventing dangerous radioactive material from illegally entering and exiting the country is central to national security. While the extensively deployed 3He-based systems used by the Department of Homeland Security and other federal agencies have proven highly effective at informing operators of the possible presence of nuclear materail, these sytems can be prohibitively expensive, limiting where and when they can be used and potentially leaving key areas unmonitored.
As a potential replacement for 3He-based detection systems, we are exploring the use of zinc oxide (ZnO), a semiconductor crystal. Use of ZnO for neutron detection requires that neutrons are first incident on a precursor medium, termed a radiator. Exposure of a radiator to ionizing radiation, such as a neutron, triggers the emission of ionizing radiation. Our experiment uses lithium fluoride (LiF) as this radiator. Ionizing radiation emitted from the LiF causes scintillation events in the ZnO crystals. Scintillation light emitted from the ZnO crystals as a result of this secondary radiation can then be detected using a photomultiplier tube (PMT).
With the growing shortage and increasing price of 3He, a large benefit this compound (LiF-ZnO-PMT) system is cost. Growth of a ZnO crystal is ~ $10 while LiF radiators cost under $30 to build. The most expensive component of this system is the PMT, which ranges from $800-$2000, depending on the model. This comparatively low production cost allows the flexibilty to overcome the largest advantage of 3He-based systems: neutron detection efficiency. Specifically, far more detectors could be deployed, potentially spreading beyond border monitoring applications to other high population checkpoints, including airports or stadiums.
Testing of a Newly Designed Gd-doped Scintillator System for Neutron Detection
A. Grice, G. Peacock, J. Maddocks, K. Lee, P. Rose
Gas-based 3He proportional counters remain one of the most commonly used tools for neutron detection in homeland security and nonproliferation detection applications. With the increased needs for 3He by DHS and for basic research, demand has far outpaced supply. This has led to the search for alternative technologies capable of delivering comparable performance to 3He at a fraction of the cost. One research area of interest to accomplish this is the use of scintillator systems. Utilizing a Gd-doped water-based scintillator mixture, this new detector is low cost and can be used in high activity applications, or in large detectors used to search for rare events, such as neutrino experiments.
The detector and electronics (excluding signal processing hardware/software) are constructed as one unit. The detector is approximately 6 in. in diameter and approximately 3-3.5 ft long, including the PMTs (see below). It is filled with about 11.25 L of scintillator mixture.
The entire detector system was tested against neutron sources of varying energies to ensure that the positive initial results can be replicated in its designed environment. This included exposure to a 252Cf source, PuBe source, and D-T neutron generator. Efficiency and spatial dependence assessments were made at varying distances and placements relative the source and detector. Variable moderation was also employed assess the thermal vs. fast neutron response. The main goal is to ensure proper function of the detector, signified by any observed neutron response. Testing of this system used a variety of high activity neutron sources, as well as a D-T neutron generator system.
Development and Validation of Advanced Gamma Ray Spectrosopy Algorithms
J. Paul, G. Sjoden, F. DuBose
With the growing accountability and regulatiory needs of the nuclear industry, particularly regarding the special nuclear material produced as a byproduct of commercial and research reactor operation, more emphasis has been placed on the development of nondestructive methods to qualitatively and quantitatively characterize used fuel content. This is coupled with the need for methods that are fast, reliable, inexpensive, and minimally impacting to normal reactor operation.
Although many new detector designs have been developed and implemented, scintillation detectors, such as sodium iodide, remain a popular choice for in-field applications. Scintillation detectors are inexpensive, are available in a range of sizes, and can operate efficiently at room temperature. While this flexibility lends itself to easy field deployment, these systems suffer from inherently low energy resolution.
The SmartID-XP code was specifically developed to yield more meaningful data from low resolution spectra.
As a proof of concept, natural uranium fuel elements are being irradiating and used to study the decay of short and long-lived fission products. During and after irradiation, gamma-ray data will be collected using a NaI detector. These data then undergo post processing using the SmartID-XP analysis algorithms, which will be updated to extract and identify unique photopeaks represented in the underwater environment for pool cooled used fuel. The resulting spectral data will be post-processed using the updated SmartID algorithm folded with deterministic adjoint results to render qualitative and quantitative fuel content and irradiation estimates.
A Novel Gamma Ray Spectroscopy Method Based on Coincident Measurement of Two Detectors
D. Zabriskie, E. Chen, J. Nguyen, C. Wang
Spectrum measurement for megavoltage bremsstrahlung x-rays has been a long-standing challenge. This is because high-energy photons mostly undergo Compton scattering interactions in a detector, and consequently rarely fully deposit their energies in the detector. Most published results of the megavoltage bremsstrahlung photon spectra were obtained based on reconstruction (or unfolding) from the measured data1. These results are unsatisfactory because the process of spectrum unfolding involves the use of the detector’s response function which itself often contain large errors.
In this experimental study, we developed and tested a new gamma spectrum measurement method that is based on coincident measurement of two detectors. The short-term goal of this study was to eliminate the spectrum unfolding process and thus to be able to obtain the photon spectrum more accurately. Longer term, our aim is to apply this method to character.
The diagram below depicts the basic layout of our detection system. The current experiment uses a LaBr detector in the Detector 1 position and a NaI for Detector 2.
Photons incident on Detector 1 have a high probability of partial energy deposition and subsequent scattering out of the detector. A fraction of these scattered photons can then be detected by Detector 2. A time coincidence window is used to correlate events detected by both detectors. For events detected by both detectors within this window, the Compton electron energy (measured with Detector 1) and the given scattering angle (θ) together can be used to derive the energy of the incident photon. The optimal angle θ is chosen based on the trade-off between two quantities: the slope of the electron energy vs. photon energy curve and the Klein-Nishina cross section,. This slope dependence creates competing effects as relates to detection scheme optimization. Small angles (θ) negatively impact energy resolution as the uncertainty in electron energy is inversely related to photon energy. However, counting statistics favor a smaller angle.
Using a 0.7 Ci Cs-137 source and 60 degree scattering angle, preliminary results have been positive. Subsequent testing will involve use of two LaBr detectors before moving on to MV source measurements.