The shortage of He-3 has triggered the search for effective alternative neutron detection technologies for national security and safeguards applications. Any new detection technology must satisfy two basic criteria: (1) it must meet a neutron detection efficiency requirement, and (2) it must be insensitive to gamma-ray interference at a prescribed level, while still meeting the neutron detection requirement. It is the purpose of this paper to define measureable gamma ray sensitivity criteria for neutron detectors. Quantitative requirements are specified for: intrinsic gamma ray detection efficiency and gamma ray absolute rejection. The gamma absolute rejection ratio for neutrons (GARRn) is defined, and it is proposed that the requirement for neutron detection be 0.9 < GARRn < 1.1 at a 10 mR/h exposure rate. An example of the results from a He-3 based neutron detector is provided showing that this technology can meet the stated requirements. Results from tests of some alternative technologies are also reported. (C) 2011 Elsevier B.V. All rights reserved.

One of the main uses for 3He is in gas proportional counters for neutron detection. Large radiation detection systems deployed for homeland security and proliferation detection applications use such systems. Due to the large increase in use of 3He for homeland security and basic research, the supply has dwindled, and can no longer meet the demand. This has led to the search for an alternative technology to replace the use of 3He-based neutron detectors. In this paper, we review the testing of currently commercially available alternative technologies for neutron detection in large systems used in various national security applications.

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The advanced Large-Area Plastic Scintillator (ALPS) Project at Pacific Northwest National Laboratory investigated possible technological avenues for substantially advancing the state-of-the-art in gamma-ray detection via large-area plastic scintillators. The three predominant themes of these investigations comprised the following: * Maximizing light collection efficiency from a single large-area sheet of plastic scintillator, and optimizing hardware event trigger definition to retain detection efficiency while exploiting the power of coincidence to suppress single-PMT "dark current" background; * Utilizing anti-Compton vetoing and supplementary spectral information from a co-located secondary, or "Back" detector, to both (1) minimize Compton background in the low-energy portion of the "Front" scintillator's pulse-height spectrum, and (2) sharpen the statistical accuracy of the front detector's low-energy response prediction as impelmented in suitable energy-windowing algorithms; and * Investigating alternative materials to enhance the intrinsic gamma-ray detection efficiency of plastic-based sensors.

The authors constructed a prototype Time-Encoded Signature (TES) system, complete with automated detection algorithms that can be used to detect point-like gamma-ray sources in search applications where detectors observe large variability in background count rates beyond statistical (Poisson) noise. The person-carried system consists of two cesium iodide scintillators placed on opposite sides of a lead shield. This geometry mitigates systematic background variation and induces a unique signature upon encountering point-like sources. This manuscript focuses on the development of detection algorithms that identify point-source signatures while remaining computationally simple. The latter constraint derives from the instrument's mobile (and thus low power) operation. The authors evaluated algorithms using both simulated and field data. The results of this analysis demonstrate the capability to detect sources at a wide range of source-detector distances using computationally simple algorithms.

Recent ice core analyses suggest that the Carrington event of 1859 may have been the largest solar energetic particle event in the past several hundred years. Previous analyses of potential doses to humans and electronics from such an event suggested that a Carrington-like event, with a hard spectrum similar to that of the event of September 1989 could be catastrophic. Subsequent analyses of the 10Be concentration in the ice core data suggest that the spectral hardness of the Carrington event was softer and similar to the August 1972 event. In this work we review the earlier estimates of doses from a Carrington event, and present updated dose estimates for deep space crews and electronics using the Carrington event proton fluence ⩾30 MeV in combination with an event spectrum similar to that of the August 1972 event. Potential ramifications of these doses for humans and electronics on deep space missions are discussed.

Detection of radioactive point sources is inherently divided into two regimes encompassing stationary and moving detectors. The two cases differ in their treatment of background radiation and its influence on detection sensitivity. Stationary detectors are limited by the statistical fluctuation of the background, while moving detectors may be subjected to widely and irregularly varying background radiation as a result of geographical and environmental variation. This significant systematic variation, in conjunction with the statistical variation of the background, requires a very conservative threshold in order to yield the same false-positive rate as the stationary detection case. This manuscript discusses a novel detector geometry that induces a unique time-encoded signature (TES) when exposed to point sources. The identification of temporal signatures for point sources using TES has been demonstrated and compared with the canonical method. This work demonstrates that temporal signatures mitigate systematic background variation and thus increase point-source detection in a moving detector system.

Over the past two decades, hypothetical models of "worst-case" solar particle event (SPE) spectra have been proposed in order to place an upper bound on radiation doses to critical body organs of interplanetary crews on deep space missions. These event spectra are usually formulated using hypothetical extrapolations of space measurements for previous large events. Here we take a different approach. Recently reported analyses of ice core samples indicate that the Carrington flare of 1859 is the largest event observed in the past 500 years. These ice core data yield estimates of the proton fluence for energies greater than 30 MeV, but provide no other spectrum information. Assuming that the proton energy distribution for such an event is similar to that measured for other recent, large events, interplanetary crew doses are estimated for these hypothetical worst case SPE spectra. These estimated doses are life threatening unless substantial shielding is provided.

A variety of recent applications have led to a great interest in the development and application of sensor networks with the goal of providing more effective detection of moving radioactive sources. This paper endeavors to analyze and evaluate the costs and benefits associated with the use of a network of radiation detectors for applications involving the detection of a moving radioactive source. This analysis is restricted to the one-dimensional case, i.e., to the case where the moving source is constrained to move along a single path. It is found that the relative advantage resulting from sensor dispersal depends upon the goals, objectives, and constraints of the measurement scenario. The dispersal of sensors into a network may be advisable or required for operational reasons, but from a statistical perspective does not directly lead to improved performance in terms of detection efficiency and false detection rate.

Radiation portal monitors are commonly used at international border crossings to detect illicit transport of radioactive material. Most monitors use plastic scintillators to detect gamma rays, but next-generation monitors may contain NaI(Tl). In order to directly compare the performance of the two types of detectors, a prototype NaI(Tl) monitor was tested at two international border crossings adjacent to a comparable plastic scintillator monitor. The NaI(Tl) monitor housed four large detectors, each 10.2 cm /spl times/ 10.2 cm /spl times/ 41 cm. The empirical data set from the two field tests contains approximately 3800 passages with known cargo loads for each vehicle. For a small subset of the vehicles, high purity germanium detector spectra were also collected. During the survey period several vehicles containing commercial products with naturally occurring radioactive material (NORM) passed through the monitor. Typical NORM cargo included pottery, large granite slabs, rock-based floor tiles, construction stone blocks, abrasive material, and fertilizer. Non-NORM sources included a large source of /sup 60/Co (200,000 GBq) and a shipment of uranium oxide, both items being legally transported. The information obtained during the tests provides a good empirical data set to compare the effectiveness of NaI(Tl) and plastic-scintillator portal monitors. The capability to be sensitive to illicit materials, but not alarm on NORM, is a key figure of merit for portal monitors.

For over two decades, hypothetical models of "worst case" solar particle event (SPE) spectra have been proposed in order to place an upper bound on radiation doses to crews on space missions. These event spectra are usually formulated using hypothetical extrapolations of space measurements for previous large events, which have occurred during or just prior to the space era. In this work we take a different approach. Recently reported analyses of ice core samples indicate that the Carrington flare of 1859 is the largest event observed in the past 500 years. These ice core data yield estimates of the proton fluence for energies greater than 30 MeV, but provide no other spectrum information. Assuming that the proton energy distribution for such an event is similar to that measured for other recent, large events, critical body organ doses for crews in representative LEO missions are estimated for these hypothetical worst-case SPE spectra. If the event is accompanied by a substantial geomagnetic storm, these estimated doses are large and could be mission threatening unless substantial shielding is provided.

Recent analyses of ice core samples indicate that the Carrington flare of 1859 was the largest event observed in the past 500 years. These ice core data yield estimates of the proton fluence for energies greater than 30 MeV, but provide no other spectrum information. Assuming that the proton energy distribution for such an event is similar to that measured for other recent, large events, total ionizing doses in deep space are estimated for these hypothetical worst-case spectra. These estimated doses, as large as 50 krad (Si), could be catastrophic for sensitive electronic devices unless substantial shielding is provided.

The DOE tasked PNNL to investigate possible technological avenues for substantially advancing the state-of-the-art in gamma detection via large-area plastic scintillators. This report describes progress on this project as of the conclusion of FY 2003. The primary focus of the report is on experimental tests conducted with a single large-area plastic scintillator outfitted with a variety of photomultiplier tube configurations. Measurements performed to date incude scintillator response under broad-area exposure to various point-like gamma sources, and light-output uniformity mappings obtained by varying the position of a collimated beta-source over the surface of the scintillator. Development of a Monte Carlo program for modeling the response of a large-area scintillator sensor to ionizing radiation, explicitly including resolution-broadening effects of scintillation light generation, propagation, and collection is also described.

Deep-space manned flight as a reality depends on a viable solution to the radiation problem. Both acute and chronic radiation health threats are known to exist, with solar particle events as an example of the former and galactic cosmic rays (GCR) of the latter. In this experiment Iron ions of 1A GeV are used to simulate GCR and to determine the secondary radiation field created as the GCR-like particles interact with a thick target. A NASA prepared food pantry locker was subjected to the iron beam and the secondary fluence recorded. A modified version of the Monte Carlo heavy ion transport code developed by Zeitlin at LBNL is compared with experimental fluence. The foodstuff is modeled as mixed nuts as defined by the 71st edition of the Chemical Rubber Company (CRC) Handbook of Physics and Chemistry. The results indicate a good agreement between the experimental data and the model. The agreement between model and experiment is determined using a linear fit to ordered pairs of data. The intercept is forced to zero. The slope fit is 0.825 and the R2 value is 0.429 over the resolved fluence region. The removal of an outlier, Z=14, gives values of 0.888 and 0.705 for slope and R2 respectively.

The problems of high energy heavy ion (HZE) transport in shielding and biological samples are important to the field of space radiation protection. In this experiment secondary fluence fragment spectra are determined for NASA targets. The experiment was performed at the Alternating Gradient Synchrotron (AGS) of the Brookhaven National Laboratory as E-898 by Dr. Jack Miller et al from the Lawrence Berkeley National Lab-oratory. The AGS provided a beam of Fe-56 with a target entry energy of 1.05 +/- 0.005 GeV/nucleon. The post target fragment fluence spectra of several materials were experimentally determined and compared to a Monte Carlo model resulting in a good fit to the data.The experimental data are compared with the model results by plotting measured elemental fluence verses calculated elemental fluence on the x - y axes. For complete agreement the expected slope of the best fit line is unity. A linear regression is applied to the data and the resulting slope is compared to unity by means of a percent error. For the 5 and 10 g/cm(2) graphite-epoxy targets a slope error value of 10.6% and 7.8% were calculated respectively. For the 3.5g/cm(2) Heat Shield A target a slope error value of 31.2% was calculated. These results suggest good agreement between the model and the experiment for the given targets.