Joseph A. Caggiano

Three-dimensional simulations of National Ignition Facility implosions: Insight into experimental observablesa)

We simulate in 3D both the hydrodynamics and, simultaneously, the X-ray and neutron diagnostic signatures of National Ignition Facility (NIF) implosions. We apply asymmetric radiation drive to study the impact of low mode asymmetry on diagnostic observables. We examine X-ray and neutron images as well as neutron spectra for these perturbed implosions. The X-ray images show hot spot evolution on small length scales and short time scales, reflecting the incomplete stagnation seen in the simulation. The neutron images show surprising differences from the X-ray images. The neutron spectra provide additional measures of implosion asymmetry. Flow in the hot spot alters the neutron spectral peak, namely, the peak location and width. The changes in the width lead to a variation in the apparent temperature with viewing angle that signals underlying hot spot asymmetry. We compare our new expectations based on the simulated data with NIF data. We find that some recent cryogenic layered experiments show appreciable temperature anisotropy indicating residual flow in the hot spot. We also find some trends in the data that do not reflect our simulation and theoretical understanding.

Scatter in cargo radiography

As a complement to passive detection systems, radiographic inspection of cargo is an increasingly important tool for homeland security because it has the potential to detect highly attenuating objects associated with special nuclear material or surrounding shielding, in addition to screening for items such as drugs or contraband. Radiographic detection of such threat objects relies on high image contrast between regions of different density and atomic number (Z). Threat detection is affected by scatter of the interrogating beam in the cargo, the radiographic system itself, and the surrounding environment, which degrades image contrast. Here, we estimate the extent to which scatter plays a role in radiographic imaging of cargo containers. Stochastic transport simulations were performed to determine the details of the radiography equipment and surrounding environment, which are important in reproducing measured data and to investigate scatter magnitudes for typical cargo. We find that scatter plays a stronger role in cargo radiography than in typical medical imaging scenarios, even for low-density cargo, with scatter-to-primary ratios ranging from 0.14 for very low density cargo, to between 0.20 and 0.40 for typical cargo, and higher yet for dense cargo.

Potential Applications of Nuclear Resonance Fluorescence

Nuclear resonance fluorescence (NRF) is a photon‐based active interrogation approach that provides isotope‐specific signatures that can be used to detect and characterize samples. Photon energies are in the range of a few MeV, so that penetration through significant material is possible. Unlike other active interrogation techniques that are based on inducing fission, NRF is sensitive to a wide range of isotopes: for example 11B, 12C, 13C, 14N, 16O, 27Al, 208Pb, 235U, 238U and 239Pu just to name a few. NRF is most likely to outperform existing technologies for applications requiring isotopic information of sealed samples. Pacific Northwest National Laboratory is conducting a review of potential applications that could be addressed by NRF techniques. These applications cover a wide range of topics, from geo‐location, to material assay, to safeguarding the nuclear fuel cycle. The objective of the project is to search for potential applications, define technical requirements, identify physical limitations, co...

Nuclear Resonance Fluorescence of /sup 235/U

Nuclear resonance fluorescence is a physical process that provides an isotopic-specific signature that can be used for the identification and characterization of materials. The technique involves the detection of prompt discrete-energy photons emitted from a sample which is exposed to photons in the MeV energy range. Potential applications of the technique range from detection of high explosives to characterization of special nuclear materials. One isotope of significant interest is 235U. Pacific Northwest National Laboratory and Passport Systems have collaborated to conduct measurements searching for nuclear resonance fluorescence signatures of 235U below 3 MeV using a 220 g sample of highly enriched uranium. Nine 235U resonances between 1650 and 2010 keV were identified in the preliminary analysis. Analysis of the measurement data to determine the integrated cross sections of the resonances is in progress.

A High-Efficiency Fieldable Germanium Detector Array

Historically, large germanium arrays for field applications have consisted of multiple detectors each housed in their own cryostat. In order to ruggedize the detectors for field use these cryostats have had additional support material introduced that significantly impacted cryogenic performance. This paper presents the development of a new cryostat design suitable for deployment of ~10 kg of high purity germanium detectors that achieves outstanding cryogenic performance (~5 W at 80 K) while providing the high detection efficiency required for stand-off measurements and the ruggedization required for use in a variety of field applications.

On the Search for Nuclear Resonance Fluorescence Signatures of

Nuclear resonance fluorescence is a physical process that provides an isotope-specific signature that could be used for the identification and characterization of materials. The technique involves the detection of prompt discrete-energy photons emitted from a sample that is exposed to MeV-energy photons. Potential applications of the technique range from detection of high explosives to characterization of special nuclear materials such as 235U. We conducted a pair of measurements to search for a nuclear resonance fluorescence response of 235U above 3 MeV and of 238U above 5 MeV using an 8 g sample of highly enriched uranium and a 90 g sample of depleted uranium. No new signatures were observed. The minimum detectable integrated cross section for 235U varies from 4 eV b at 3 MeV up to 120 eV b at 8 MeV.

^{235}{\rm U}

Pacific Northwest National Laboratory and Passport Systems have collaborated to perform nuclear resonance fluorescence (NRF) experiments using several high quality high-explosive simulant samples. These measurements were conducted to determine the feasibility of finding and characterizing high explosive material by NRF interrogation. Bremstrahlung photon beams, produced from electron beams of 5.1, 5.3, 8.3, 9 and 10 MeV, irradiated the samples. Gamma-rays emitted from the samples were detected using high-purity germanium detectors. Nitrogen-to-carbon ratios of the high-explosive simulants were extracted from the 5.1 and 5.3 MeV data and compare favorably with accepted values. Analysis of the 8, 9 and 10 MeV data is in progress; preliminary isotopic comparisons within the samples are consistent with the expected results.

and

A low-power cryostat was designed and built for the Multi-sensor Airborne Radiation Survey (MARS) project for the purpose of housing a close-packed high-purity germanium (HPGe) detector array of 14 HPGe detectors. The power consumption of the cold mass in the cryostat was measured to be 4.07(11) watts, sufficient for 5.5 days of continuous operation using only 8 liters of liquid nitrogen. Temperatures throughout the cryostat were measured by platinum resistance temperature detectors. These measurements were used to determine the emissivity of the copper used in the floating radiation shield and outer cryostat wall, which was constructed using chemically cleaned and passivated copper metal. Using a PNNL-developed passivation process, an emissivity of 2.5(3)% was achieved for copper.

^{238}{\rm U}

The discovery and optimization of new scintillators has traditionally been a rather slow process due to the difficulties of single crystal growth. This paper discusses the production of polycrystalline scintillator thin films which were tested in order to determine what characterizations could be made concerning a materials ultimate potential as a scintillator prior to pursuing crystal growth. Thin films of a few microns thickness of CaF2(Eu), CeF3, and CeCl3, all known scintillators, were produced by vapor deposition. The hygroscopic CeCl3 was coated with multiple polymer-aluminum oxide bi-layers. Emission wavelengths and decay times agreed with values from single crystals. The films were too thin to measure gamma photopeaks, but using alpha energy deposition peaks, one could compare the relative photon yield/MeV between materials. The values obtained appear to give a relevant indication of a materials light yield potential. The technique may also be useful for quickly indicating the proper dopant amount for a given material.

Above 3 MeV

Nuclear resonance fluorescence is a physical process that provides an isotope-specific signature that could be used for the identification and characterization of materials. The technique involves the detection of prompt discrete-energy photons emitted from a sample that is exposed to photons in the MeV energy range. Potential applications of the technique range from detection of high explosives to characterization of special nuclear materials such as 235U. Pacific Northwest National Laboratory and Passport Systems have collaborated to conduct a a pair of measurements to search for a nuclear resonance fluorescence response of 235U above 3 MeV and of 238U above 5 MeV using an 8 g sample of highly enriched uranium and a 90 g sample of depleted uranium. No new signatures were observed. The minimum detectable integrated cross section for 235U is presented.