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A Monte Carlo model has been developed for interrogation of fissionable material embedded in thick cargos when high-energy β-delayed γ-rays are detected following neutron-induced fission. The model includes the principal structural components of the laboratory, the neutron source and collimator assembly in which it resides, the assembly that represents cargo of given characteristics, a target of highly enriched uranium (HEU) and large external plastic scintillators for photon detection. The ability of this model to reproduce experimental measurements was tested by comparing simulations with measurements of the number of induced fissions and the number of detected photons when the HEU target was irradiated with 14.25-MeV neutrons in the absence of any cargo and while embedded in assemblies of plywood and iron pipes. The simulations agreed with experimental measurements within a factor of about 2 for irradiation of the bare target and when the areal density of intervening cargo was 33 g cm−2 (wood) and 61 g cm−2 (steel pipes). This suggests that the model can permit exploration of a large range in parameter space with reasonable fidelity.

There is an urgent need to improve the reliability of screening cargo containers for illicit nuclear material that may be hidden there for terrorist purposes. A screening system is described for the detection of fissionable material hidden in maritime cargo containers. The system makes use of a low-intensity neutron beam for producing fission and the detection of the abundant high-energy /spl gamma/ rays emitted in the /spl beta/-decay of short-lived fission products and /spl beta/-delayed neutrons. The abundance of the delayed /spl gamma/ rays is almost an order of magnitude larger than that of the delayed neutrons normally used to detect fission, and they are emitted on about the same time scale as the delayed neutrons, i.e., /spl sim/1 min. The energy and temporal distributions of the delayed /spl gamma/ rays provide a unique signature of fission. Because of their high energy, these delayed /spl gamma/ rays penetrate low-Z cargoes much more readily than the delayed neutrons. Coupled with their higher abundance, the signal from the delayed /spl gamma/ rays escaping from the container is predicted to be as much as six decades more intense than the delayed neutron signal, depending upon the type and thickness of the intervening cargo. The /spl gamma/ rays are detected in a large array of scintillators located along the sides of the container as it is moved through them. Measurements have confirmed the signal strength in somewhat idealized experiments and have also identified one interference when 14.5-MeV neutrons from the D, T reaction are used for the interrogation. The interference can be removed easily by the appropriate choice of the neutron source.

Conventional laser-induced front ablation at metal/vacuum interfaces is a well-known and successful technique used in processes such as pulsed laser deposition (PLD).

The goal of our research was to address the problem of detection of weapons of mass destruction (WMD) materials within containers in common use on commercial cargo trafficking. LLNL has created an experimental test bed for researching potential solutions using (among other techniques) active interrogation with neutrons. Experiments and computational modeling were used to determine the effectiveness of the technique. Chemical weapons materials and high explosives can be detected using neutron activation and simple geometries with little or no intervening material. However in a loaded container there will be nuisance alarms from conflicting signatures resulting from the presence of material between the target and the detector (and the interrogation source). Identifying some elements may require long counting times because of the increased background. We performed some simple signature measurements and simulations of gamma-ray spectra from several chemical simulants. We identified areas where the nuclear data was inadequate to perform detailed computations. We concentrated on the detection of SNM in cargo containers, which will be emphasized here. The goal of the work reported here is to develop a concept for an active neutron interrogation system that can detect small targets of SNM contraband in cargo containers, roughly 5 kg HEU ormore » 1 kg Pu, even when well shielded by a thick cargo. It is essential that the concept be reliable and have low false-positive and false-negative error rates. It also must be rapid to avoid interruption of commerce, completing the analysis in minutes. A potentially viable concept for cargo interrogation has been developed and its components have been evaluated experimentally. A new radiation signature unique to SNM has been identified that utilizes high-energy, fission-product gamma rays. That signature due to {gamma}-radiation in the range 3-6 MeV is distinct from normal background radioactivity that does not extend above 2.6 MeV. It's short half-life of 20-55 sec makes it distinct from neutron activation due to the interrogation that is typically much longer lived. This work spawned a collaboration with LBNL where experiments verified the abundance and other characteristics of this new signature [24]. Follow-on work funded by DoE/NA22 led to the development of a detailed system concept and evaluation of its impact on operating personnel and cargos [60] and characterization of one important interference that was identified [61]. The follow-on work led to two patent applications at LBNL and LLNL. The signature flux, while small, is 2-5 decades more intense than delayed neutron signals used and facilitates the detection of SNM even when shielded by thick cargo. The actual benefit is highly dependent on the type and thickness of cargo, with modest benefit in the case of metallic cargos of iron, lead, or aluminum, but maximum benefit in the case of hydrogenous cargo. In addition, unwanted collateral effects of the interrogation, such as neutron activation of the cargo, were analyzed [60] and one significant interference due to oxygen activation was characterized. This interference can be eliminated by lowering the energy of interrogating neutrons [60] and no others have yet been identified. The neutron source technology required exists commercially. Follow-on work to produce a laboratory prototype and to engage a commercial partner for development of a prototype to be fielded at a port was initially funded by DOE/NA-22 is currently supported by DHS. That support is expected to continue through FY06.« less

The goal of the work reported here is to develop a concept for an active neutron interrogation system that can detect small targets of SNM contraband in cargo containers, roughly 5 kg HEU or 1 kg Pu, even when well shielded by a thick cargo. It is essential that the concept be reliable and have low false-positive and false-negative error rates. It also must be rapid to avoid interruption of commerce, completing the analysis in minutes. A new radiation signature unique to SNM has been identified that utilizes high-energy (E{sub {gamma}} = 3-7 MeV) fission product {gamma}-ray emission. Fortunately, this high-energy {gamma}-ray signature is robust in that it is very distinct compared to normal background radiation where there is no comparable high-energy {gamma}-ray radiation. Equally important, it has a factor of 10 higher yield than delayed neutrons that are the basis of classical interrogation technique normally used on small unshielded specimens of SNM. And it readily penetrates two meters of low-Z and high-Z cargo at the expected density of {approx} 0.5 gm/cm{sup 3}. Consequently, we expect that in most cases the signature flux at the container wall is at least 2-3 decades more intense than delayed neutron signals used historicallymore » and facilitates the detection of SNM even when shielded by thick cargo. Experiments have verified this signature and its predicted characteristics. However, they revealed an important interference due to the activation of {sup 16}O by the {sup 16}O(n,p){sup 16}N reaction that produces a 6 MeV {gamma}-ray following a 7-sec {beta}-decay of the {sup 16}N. This interference is important when irradiating with 14 MeV neutrons but is eliminated when lower energy neutron sources are utilized since the reaction threshold for {sup 16}O(n,p){sup 16}N is 10 MeV. The signature {gamma}-ray fluxes exiting a thick cargo can be detected in large arrays of scintillation detectors to produce useful signal count rates of 2-4 x 10{sup 4} cps. That is high enough to quickly identify SNM fission by its characteristic high energy {gamma}-ray emission and characteristic fast decay time. Fortunately, the fission product {gamma}-radiation decays with a distinctive T{sub 1/2} = 20-30 sec lifetime that is well matched to cargo scan speeds of about one minute per container. Experimental characterization of the {gamma}-ray fluxes exiting thick cargos has not yet been undertaken. The work reported here leads to definite requirements for the interrogation neutron source that can be met with neutron commercially available source technology. A small (6-20 ft) deuteron accelerator producing about {approx} 1 mA, 2-5 MeV deuteron beam on a deuterium or beryllium target is required. Neutrons produced by such an accelerator are kinematically collimated in the forward direction, reducing shielding requirements while increasing the neutron flux on target to meet the intensity requirement even when there is thick intervening cargo. In addition, this technology provides a very penetrating beam in the energy range 4-8 MeV while remaining below the oxygen activation threshold. Maximum counting statistics and lowest error rates in the identification occur when the beam is pulsed with a 50 % duty cycle. The period for this pulsing must be comparable to the half-lives of the species that make up the signature, i.e. 10-60 sec. This is readily achieved with commercially available equipment and is well suited to rapid scanning of cargo containers.« less

We extend Seaton's semi-classical, impact parameter formulation of Coulomb excitation to treat a variety of inelastic ion-ion collisions. Comparisons with quantal calculations suggest that our algorithm provides an accurate as well as rapid estimate of these cross sections; comparisons involving two other semi-classical formulae show them to be less satisfactoory. We then investigate the importance of inelastic ion-ion collisions for applications in plasma spectroscopy and diagnostics; in particular we examine excitations by heavy ions as well as by protons. In one appendix, we consider the numerical evaluation of Coulomb trajectory functions, for a wide range of collision parameters. In another, we derive simple, approximate formulae for electron-impact excitation cross sections, for use in comparing ion-ion and electron-ion excitation rates and in determining (via scaling) reliable values of ion-ion cross sections at high collision velocities.