skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Experimental study of variations in background radiation and the effect on Nuclear Car Wash sensitivity

Abstract

Error rates in a cargo screening system such as the Nuclear Car Wash [1-7] depend on the standard deviation of the background radiation count rate. Because the Nuclear Car Wash is an active interrogation technique, the radiation signal for fissile material must be detected above a background count rate consisting of cosmic, ambient, and neutron-activated radiations. It was suggested previously [1,6] that the Corresponding negative repercussions for the sensitivity of the system were shown. Therefore, to assure the most accurate estimation of the variation, experiments have been performed to quantify components of the actual variance in the background count rate, including variations in generator power, irradiation time, and container contents. The background variance is determined by these experiments to be a factor of 2 smaller than values assumed in previous analyses, resulting in substantially improved projections of system performance for the Nuclear Car Wash.

Authors:
; ; ; ;
Publication Date:
Research Org.:
Lawrence Livermore National Lab. (LLNL), Livermore, CA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
902608
Report Number(s):
UCRL-TR-229157
TRN: US0702979
DOE Contract Number:
W-7405-ENG-48
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
98 NUCLEAR DISARMAMENT, SAFEGUARDS AND PHYSICAL PROTECTION; 46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY; 73 NUCLEAR PHYSICS AND RADIATION PHYSICS; AUTOMOBILES; BACKGROUND RADIATION; CARGO; CONTAINERS; FISSILE MATERIALS; IRRADIATION; PERFORMANCE; RADIATIONS; SENSITIVITY

Citation Formats

Church, J, Slaughter, D, Norman, E, Asztalos, S, and Biltoft, P. Experimental study of variations in background radiation and the effect on Nuclear Car Wash sensitivity. United States: N. p., 2007. Web. doi:10.2172/902608.
Church, J, Slaughter, D, Norman, E, Asztalos, S, & Biltoft, P. Experimental study of variations in background radiation and the effect on Nuclear Car Wash sensitivity. United States. doi:10.2172/902608.
Church, J, Slaughter, D, Norman, E, Asztalos, S, and Biltoft, P. Wed . "Experimental study of variations in background radiation and the effect on Nuclear Car Wash sensitivity". United States. doi:10.2172/902608. https://www.osti.gov/servlets/purl/902608.
@article{osti_902608,
title = {Experimental study of variations in background radiation and the effect on Nuclear Car Wash sensitivity},
author = {Church, J and Slaughter, D and Norman, E and Asztalos, S and Biltoft, P},
abstractNote = {Error rates in a cargo screening system such as the Nuclear Car Wash [1-7] depend on the standard deviation of the background radiation count rate. Because the Nuclear Car Wash is an active interrogation technique, the radiation signal for fissile material must be detected above a background count rate consisting of cosmic, ambient, and neutron-activated radiations. It was suggested previously [1,6] that the Corresponding negative repercussions for the sensitivity of the system were shown. Therefore, to assure the most accurate estimation of the variation, experiments have been performed to quantify components of the actual variance in the background count rate, including variations in generator power, irradiation time, and container contents. The background variance is determined by these experiments to be a factor of 2 smaller than values assumed in previous analyses, resulting in substantially improved projections of system performance for the Nuclear Car Wash.},
doi = {10.2172/902608},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Wed Feb 07 00:00:00 EST 2007},
month = {Wed Feb 07 00:00:00 EST 2007}
}

Technical Report:

Save / Share:
  • The influence of incident neutron attenuation on signal strengths in the Nuclear Car Wash has been observed experimentally for both wood and steel-pipe mock cargos. Measured decay curves are presented for {beta}-delayed high-energy {gamma}-rays and thermalized neutrons following neutron-induced fission of HEU through varying irradiation lengths. Error rates are extracted for delayed-{gamma} and delayed-n signals integrated to 30 seconds, assuming Gaussian distributions for the active background. The extrapolation to a field system of 1 mA deuterium current and to a 5 kg sample size is discussed.
  • A large majority of US imports arrive at seaports in maritime cargo containers. The number of containers arriving is nearly 10 million per year, each with a cargo of up to 30 tons of various materials. This provides a vulnerable entry point for the importation of a nuclear weapon or its components by a terrorist group. Passive radiation sensors are being deployed at portals to detect radioactive material and portable instruments are carried by port personnel to augment detection. Those instruments can detect the neutrons and g-rays produced by {sup 240}Pu that is normally present in weapons grade plutonium inmore » cases where cargo overburden is not too great. However, {sup 235}U produces almost no neutron output in its normal radioactive decay and its principal {gamma}-radiation is at 186 keV and is readily attenuated by small amounts of wood or packing materials. Impurities such as {sup 232}U, often present in reactor irradiated material at the 100-200 ppt level, can provide a detectable signal through significant cargo overburden but the wide variations among samples of HEU make this an unreliable means of detecting SNM. High quality radiography may be useful in determining that the majority of containers are clearly free of SNM. However, some containers will lead to ambiguous results from radiography and passive radiation sensing. For these reasons active neutron interrogation is proposed as a means to produce fission and thus greatly amplify the radiation output of fissionable material to facilitate its reliable detection even when well shielded by large cargo overburden. Historically, the fission signature utilized as the unique identifying feature of fissionable materials is the detection of delayed neutrons. However, these neutrons have very low yield {approx} 0.017 per fission in {sup 235}U, and their low energy results in very poor penetration of hydrogenous materials such as fuels, water, wood, or agricultural products. That signature alone does not provide reliable detection in thick cargos. A new signature has been identified and has been developed within the current project for the detection of well shielded SNM. This SNM signature is based on high-energy {beta}-delayed {gamma}-radiation produced by fission products following neutron or photon induced fission. These {gamma}-rays are high enough in energy (E{sub {gamma}} > 3 MeV) to be readily distinguished from any natural background radioactivity since the latter does not extend above 2.6 MeV. Their abundance is nearly a decade greater than delayed neutrons and their short half-lives deliver nearly all of the signature radiation on time scales of one minute or less and thus facilitate rapid scanning. Finally, for this {gamma}-radiation in the 3-6 MeV range attenuation occurs only by Compton scattering and is in the range where minimum attenuation occurs in all materials. Even the thickest cargos of any material attenuate these {gamma}-rays by only a factor of 10-30X so that the signature is readily detected even with the most challenging shield materials. The goals of the current program are to detect significant quantities (much less than IAEA ''significant'' amounts) of well-shielded SNM, and to do so with detection probability P{sub d} {ge} 95% and with false alarm rates P{sub fp} {le} 0.001. It is the goal to meet these requirements in a scan that requires less than one minute to complete and does so without damage to the cargo or to people who may be hidden inside. We intend to meet these requirements even when the cargo overburden is up to {rho}L {le} 150 g/cm{sup 2} of any material ranging from fuels and agricultural products to steel and lead.« less
  • The goal of any alarm algorithm should be that it provide the necessary tools to derive confidence limits on whether the existence of fissile materials is present in cargo containers. It should be able to extract these limits from (usually) noisy and/or weak data while maintaining a false alarm rate (FAR) that is economically suitable for port operations. It should also be able to perform its analysis within a reasonably short amount of time (i.e. {approx} seconds). To achieve this, it is essential that the algorithm be able to identify and subtract any interference signature that might otherwise be confusedmore » with a fissile signature. Lastly, the algorithm itself should be user-intuitive and user-friendly so that port operators with little or no experience with detection algorithms may use it with relative ease. In support of the Nuclear Car Wash project at Lawrence Livermore Laboratory, we have developed an alarm algorithm that satisfies the above requirements. The description of the this alarm algorithm, dubbed ALARMA, is the purpose of this technical report. The experimental setup of the nuclear car wash has been well documented [1, 2, 3]. The presence of fissile materials is inferred by examining the {beta}-delayed gamma spectrum induced after a brief neutron irradiation of cargo, particularly in the high-energy region above approximately 2.5 MeV. In this region naturally occurring gamma rays are virtually non-existent. Thermal-neutron induced fission of {sup 235}U and {sup 239}P, on the other hand, leaves a unique {beta}-delayed spectrum [4]. This spectrum comes from decays of fission products having half-lives as large as 30 seconds, many of which have high Q-values. Since high-energy photons penetrate matter more freely, it is natural to look for unique fissile signatures in this energy region after neutron irradiation. The goal of this interrogation procedure is a 95% success rate of detection of as little as 5 kilograms of fissile material while retaining at most .1% false alarm rate. Plywood is used to simulate hydrogenous cargo material and steel (pipes) is used to simulate metallic cargo. The wood consists of 120 x 240 cm sheets and has approximately .65 g/cm{sup 3}. The steel pipes have approximately 10 cm diameters x 6.4 mm wall thickness are .6 g/cm{sup 3}. Fissile sources consist of a ''large'' (380 g) and ''small'' (250 g) sample of HEU (U{sub 3}O{sub 8} 94% enriched). Note that the masses of the HEU sources used in our experimental runs are at least an order of magnitude smaller than 5 kilograms. Runs are done with either wood or steel cargoes stacked at various heights and the HEU sources placed at various depths within the cargo.« less
  • A bolometer cooled by a He-3 refrigerator has been built to detect small anisotropies in the cosmic background radiation. This bolometer can be tested on ground-based instruments and is proposed to be eventually used on a shuttle-borne system.