B.D. Rooney’s research while affiliated with Los Alamos National Laboratory and other places

NE-213 liquid scintillation detectors are currently used in radiation fields consisting of both gamma rays and fast neutrons and are an excellent tool for differentiating between each type of radiation via their respective interactions. In this experiment, an analysis was performed on an NE-213 liquid scintillation detector to investigate the effects of temperature changes on the light output. The two effects measured were the amount of the individual light decay components have to the total pulse height and the total gain of the system as a function of temperature.

The figure-of-merit for a gelled NE213 scintillator cell from Bubble Technology Industries is determined for a variety of digitizing speeds and vertical resolutions using the digital-charge-integration-comparison method for pulse shape discrimination. If a digitizing board allows for an increase in the vertical resolution at the expense of the digitization rate, then there is an optimal combination of the two. For the Gage Board 82G, a board that does allow this tradeoff to be made, the best pulse shape discrimination occurred for digitization rates and vertical resolutions of 1 GS/s and 8 bits, 500 MS/s and 9 bits, and 250 MS/s with 9 bits.

Various digital methods were examined for determining the relative arrival times of pulses from to 5.08×2.54cm BaF2 scintillators. In this study, pulses from the photomultiplier tubes were digitized by a 1Gs/s analog-to-digital converter and post processed with multiple techniques. These techniques include: (1) leading edge discrimination, (2) moment-analysis, (3) constant-fraction discrimination, (4) digital constant-fraction discrimination, (5) triangular pulse shaping with a leading edge linear regression, and (6) pulse-shape fitting. Average timing resolutions of 456+/-8ps were obtained with constant-fraction discrimination, which is slightly higher than the analog average resolution of 419+/-7ps. This study explores the application of these digital techniques for pulse-timing applications and their potential advantages and limitations.

At Los Alamos National Laboratory, Los Alamos, NM, we are constructing a multilayer prototype camera using silicon pixel detectors. Each pixel detector is 5.80 cm×6.30 cm×200 μm. The pixels are 3 mm×3 mm in size. With this prototype, we intend to study electron-tracking techniques along with the three-plane Compton-imaging technique. In this paper, we present results of the initial simulation studies of the three-plane Compton-imaging technique using our prototype detector geometry for monoenergetic gamma sources with energies from 100 keV to 500 keV. The GEANT simulation package was used to carry out these studies.

Gd2SiO5(Ce3+) (GSO) and BaF2 electron responses were measured using the Compton Coincidence Technique (CCT). The CCT has previously been used to characterize several scintillators and has proven to be an accurate and reliable technique. The measured GSO electron response was observed to increase by 28% as the electron energy increased from 5 to 445 keV. The measured BaF2 electron response increased 23% as the electron energy increased from 18 to 436 keV. Both electron responses become linear at higher electron energies (above about 200 keV). These observations made with GSO and BaF2 in this study are consistent with the general trend reported for previous CCT characterized non-alkali halide scintillators. To validate the GSO and BaF2 measured electron responses, respective photon responses were calculated and subsequently compared to measured photon response data. MCNP4C together with simplified electron cascade sequences for GSO and BaF2 were used in these photon responses calculations. Calculated photon responses for both crystals are in good agreement (within 10%) with measured photon responses. This agreement confirms the accuracy of the GSO and BaF2 measured electron responses.

Gd₂SiO₅(Ce³⁺) (GSO) and BaF₂ electron responses were characterized using the Compton Coincidence Technique (CCT). The CCT has been used previously to characterize several scintillators and has proven to be an accurate and reliable technique. The GSO measured electron response was observed to increase by 28% as the electron energy increased from 5 keV to 445 keV. BaF₂ measured electron response increased by 22% as the electron energy increased from 18 keV to 436 keV. The observations made with GSO and BaF₂ in this study are consistent with the general trend reported for previously characterized non-alkali halide scintillators. The GSO and BaF₂ measured electron responses were further used to calculate their respective photon responses. MCNP4C together with simplified electron cascade sequences for GSO and BaF₂ were used in these photon responses calculations. Calculated photon responses for both crystals are in good agreement with measured photon responses. This agreement confirms the accuracy of the GSO and BaF₂ measured electron responses.

Gamma imaging based on Compton scattering was first proposed approximately 25 years ago as a replacement for mechanically collimated imaging systems. The advantages of such instruments over mechanically collimated systems are a wider field of view, higher efficiency (more source photons are used in the image construction), source localization, use in high-background environments, and non-tomographic three-dimensional imaging of near-field sources. One can also image multi-energy photons by selecting events based on the summed energy deposited in multiple detectors. The traditional example of such imaging systems is a Compton camera. Until recently, limitations with associated hardware have resulted in Compton imaging seeing few applications. However, with advances in high spatial resolution detectors, and further developments in the physical principles there has been a renewed interest in gamma imaging based on Compton scattering in many areas including astronomy and nuclear medicine. In this paper we present an evaluation of a three plane Compton imaging concepts, the three plane Compton imaging technique. Such a technique, if valid, could lead to many useful applications.

A comparison study of pulse-shape analysis techniques was conducted for a BC501A scintillator using digital signal processing (DSP). In this study, output signals from a preamplifier were input directly into a 1 GHz analog-to-digital converter. The digitized data obtained with this method was post-processed for both pulse-height and pulse-shape information. Several different analysis techniques were evaluated for neutron and gamma-ray pulse-shape discrimination. It was surprising that one of the simplest and fastest techniques resulted in some of the best pulse-shape discrimination results. This technique, referred to here as the Integral Ratio technique, was able to effectively process several thousand detector pulses per second. This paper presents the results and findings of this study for various pulse-shape analysis techniques with digitized detector signals.

To facilitate airborne radioxenon monitoring, a xenon concentration method with potential advantages over current technology in simplicity, size, and cost has been developed. The concentration technique is based on the preferential absorption of heavy noble gases (krypton, xenon, and radon) by certain organic fluids. To implement this concentration technique, a radioxenon monitoring system requires three integrated sub-systems: (1) an absorption sub-system; (2) a degassing sub-system; and (3) a radiation detection sub-system. This study is focused on the characterization and optimization of the first two sub-systems. Measurements using a small prototype absorption tower have indicated a xenon removal factor of approximately 50% and the specific concentration at saturation of certain organic fluids to be about 2.5 times the specific concentration in the sampled air. Various techniques for degassing have been investigated, including heating, purging, agitation and vacuum. Ultrasonic agitation of a thin film in a strong vacuum has been shown to be an effective means of degassing the transfer fluid continuously. Various schemes for integrating all of the sub-systems are considered. Combining the small prototype absorption and degassing sub-systems should result in a transfer efficiency of about 33%. Each stage of an optimized concentration system should be able to increase the radioxenon concentration by approximately one order of magnitude.

The NaI:Tl scintillation response has been studied for over four decades. Two of the significant conclusions that have been made from these studies (with respect to using NaI(Tl) to detect gamma rays and X-rays) are: (1) the NaI(Tl) scintillation response is nearly linear (especially over limited energy ranges) and (2) the NaI(Tl) scintillation response has significant nonproportionalities; while these conclusions may at first appear to be inconsistent with each other, upon further consideration these conclusions are not mutually exclusive. In fact, these conclusions are fully consistent with each other. Published results typically confirm only one of these conclusions (as it was likely the intention to study either linearity or proportionality, but not both) and are not always presented such that direct comparisons can be made with other results. Consequently, it is critical that both the intent of the study and the means with which the results are presented be appropriately considered. Otherwise, it is possible to make inaccurate conclusions about both linearity and proportionality. To facilitate accurate conclusions and meaningful comparisons, this study defines a fully consistent nomenclature, examines some of the subtleties which must be considered when comparing scintillation response results, and uses a review of the literature to demonstrate both the definitions and the subtleties