C.L. Hollas

Comparison of active interrogation techniques

Active interrogation is necessary to detect highly enriched uranium in large cargo containers and trucks. To achieve sufficient penetration, most nuclear techniques first interrogate an object with high-energy photons or neutrons, to produce fission in any highly enriched uranium that may be present, and then detect the gamma rays or neutrons that are produced. Pulsed beams allow the detection of delayed gamma rays or neutrons from fission products between pulses. Many different systems have been proposed recently. This report presents the results for several techniques, both from measurements and from extensive simulations with the MCNPX code

Analysis of fissionable material using delayed gamma rays from photofission

The energetic gamma-ray spectra from the fission products of photofission have been investigated to determine whether photofission can identify heavily shielded fissionable material. Target samples of natural thorium, 93% enriched /sup 235/U, natural uranium, and 93% enriched /sup 239/Pu were irradiated with bremsstrahlung gamma rays produced by 10-MeV electrons from a small linear accelerator. The gamma-ray spectra for each of the four isotopes studied reveals a distinctive intensity distribution. For example, the intensity ratio of the pair of gamma rays at 1436 keV (/sup 138/Cs) and 1428 keV (/sup 94/Sr) is 1.9 for /sup 235/U, 2.4 for /sup 238/U, 1.7 for /sup 232/Th and 1.4 for /sup 239/Pu. 6 refs., 2 figs., 1 tab.

Linear Accelerator‐Based Active Interrogation For Detection of Highly Enriched Uranium

Photofissions were induced in samples of highly enriched uranium (HEU) with masses up to 22 kg using bremsstrahlung photons from a pulsed 10‐MeV electron linear accelerator (linac). Neutrons were detected between pulses by large 3He detectors, and the data were analyzed with the Feynman variance‐to‐mean method. The effects of shielding materials, such as lead and polyethylene, and the variation of the counting rate with distance for several configurations were measured. For comparison, a beryllium block was inserted in the beam to produce neutrons that were also used for interrogation. Because both high‐energy photons and neutrons are very penetrating, both approaches can be used to detect shielded HEU; the choice of approach depends on the details of the configuration and the shielding.

Photon and Neutron Active Interrogation of Highly Enriched Uranium

The physics of photon and neutron active interrogation of highly enriched uranium (HEU) using the delayed neutron reinterrogation method is described in this paper. Two sets of active interrogation experiments were performed using a set of subcritical configurations of concentric HEU metal hemishells. One set of measurements utilized a pulsed 14‐MeV neutron generator as the active source. The second set of measurements utilized a linear accelerator‐based bremsstrahlung photon source as an active interrogation source. The neutron responses were measured for both sets of experiments. The operational details and results for both measurement sets are described.

Effects of proton beam quality on the production of gamma rays for nuclear resonance absorption in nitrogen

We describe a method for performing nuclear resonance absorption with the proton beam from a radio frequency quadrupole (RFQ) linear accelerator. The objective was to assess the suitability of the pulsed beam from an RFQ to image nitrogen relative to that of electrostatic accelerators. This choice of accelerator results in tradeoffs in performance and complexity, in return for the prospect of higher average current. In spite of a reduced resonance attenuation coefficient in nitrogen, we successfully produced 3D tomographic images of real explosives in luggage the first time the unoptimized system was operated. The results and assessments of our initial laboratory measurements are reported.

A method for in vivo determination of carbon and oxygen using prompt gamma radiations induced by 14.7-MeV neutrons.

Quantification of body fat is important in studying obesity and other diseases involving nutritional assessment.1 The in vivo determinations of total-body carbon and oxygen con be used to obtain clinical information about a person’s fat content. Biggin and Morgan2 showed that the ratio of total-body oxygen to total-body carbon is a sensitive indicator of the percentage of fat tissue in the human body. Kyere et al.,3 measured total-body carbon by counting the number of 4.438-MeV gamma rays emitted from the 12C nuclei after excitation by 14-MeV neutrons. At Brookhaven National Laboratory, Kehayias et al.,4 developed a similar technique for measuring total-body carbon with a beam of 14-MeV neutrons from a pulsed neutron generator. We present here a variation on these two methods that permits the simultaneous determination of both carbon and oxygen, with the potential for a significantly reduced dose of neutron radiation.

Prototype explosives detection system based on nuclear resonance absorption in nitrogen

A prototype explosives detection system that was developed for experimental evaluation of a nuclear resonance absorption technique is described. The major subsystems are a proton accelerator and beam transport, high-temperature proton target, an airline-luggage tomographic inspection station, and an image-processing/detection-alarm subsystem. The detection system performance, based on a limited experimental test, is reported.

Analysis of Biological Samples Using Prompt Gamma Radiations Induced by 14.7-MeV Neutrons

We investigate a method for determining the elemental composition of biological samples that uses prompt gamma rays induced by 14,7-MeV neutrons. Alpha particles are produced simultaneously with the neutrons, which exit opposite the alpha detector through the vacuum chamber wall The sample under investigation is irradiated and emits gamma radiations in a spectral energy distribution characteristic of the material Barium-fluoride (BaF2) and high-purity germanium (HPGe) gamma detectors view the sample and record the spectrum of gamma radiation.

Data acquisition system developed for the resonance absorption project

Minimizing signal errors and losses in high-rate nuclear-imaging systems places demands on the signal-processing and data acquisition electronics. We will describe both the data acquisition system being developed for the resonant absorption project and techniques used to minimize signal errors and losses. The data acquisition system acquires pulse-height spectra from an array of gamma-ray detectors. The data is made available to multiple processors by using the VMEbus standard to provide concurrent data analysis. In addition, we use the VxWorks real-time operating system in conjunction with a SUN workstation to develop the application software. We have designed a pulse-height-analysis board that is optimized for low dead time. This board has eight independent signal channels, each consisting of a charge integrator, a fast analog-to-digital converter, and a first-in/first-out memory. This board also contains a 68020 CPU that performs the initial data compression and stores digitized data into dual-ported memory. By using an independent high-speed signal channel for each detector, we are able to improve performance over the standard multiplexed techniques commonly in use.