Session BK: Applications of Nuclear Physics I

Proton-induced population of the isomeric state in Zr-89 using short-pulse, high-energy laser systems

Short-pulse (~ps), high-energy laser systems can be used to accelerate electrons, protons and ions to high energies via laser-plasma interactions. Such protons are then capable of causing nuclear reactions within target materials, the subsequent decay of which may be measured using scintillation detectors. During one such experimental campaign, AWE's HELEN laser system was used in chirped-pulse amplification (CPA) mode to produce individual laser pulses of $\sim$50 J energy at an irradiance of 1E19 W/cm$^2$. Protons were accelerated by these pulses from a thin aluminium foil target via the target-normal sheath acceleration (TNSA) mechanism, and were incident upon a target of Y-89. The Y-89(p,n)Zr-89* reaction was observed via the direct measurement of decay gammas emitted at 587 keV during the isomeric transition between the excited and ground states of Zr-89. Half- life measurements add further confirmation of the source of these gamma rays against the background of gamma- and X-rays emitted during the laser-plasma interaction.   

Laser-Induced Nuclear Activation Studies

A series of experimental campaigns, each designed to activated carefully selected materials, have been conducted with high- power short-pulse laser systems. These relatively new CPA laser systems can produce large bursts of X-rays, electrons, protons and other ions. Characterising the nature of these mixed radiation fields is neccessary for both physics experiments and facility safety. Three campaigns, two with the HELEN laser faility at AWE and one with the Vulcan Petawatt laser at the Rutherford Appleton laboratory, were designed to accelerate protons. These protons irradiated secondary activation targets of pure foils and various optical glasses, typically those used in target chamber environments such as those found at NIF, Omega and AWE's Orion laser facility. This talk discusses these experiments and covers the production of laser-produced radiation fields, the selection of activation targets, the interpretation the radioactive decay signals, the current status of the analysis and the future applications of this research.   

Alternative Neutron Detection Technology for Homeland Security

Neutron detection is an essential aspect of interdiction of radiological threats for homeland security purposes since plutonium is a significant source of fission neutrons. Radiation portal monitoring (RPM) systems, of which there are thousands deployed for homeland security and non-proliferation purposes, currently use $^{3}$He gas-filled proportional counters for detecting neutrons. Due to the large increase in use of $^{3}$He for homeland security, the supply has dwindled, and can no longer meet the demand. Consequently, a replacement technology for neutron detection is required in the very near future. In addition to alarming on the presence of actual neutron sources, homeland security applications also have a strict requirement for limiting neutron false alarms produced by a detector. This constrains any possible replacement neutron detection technology not to generate false neutron counts in the presence of a large gamma ray-only source. Of the currently available neutron detection technologies, BF$_{3}$-filled proportional detectors, boron-lined proportional detectors, $^{6}$Li-loaded scintillating glass fiber, or non-scintillating coated plastic fiber detectors are the possible replacements for $^{3}$He detector technology---if they are proven to have appropriate capabilities.   

Physics Applications for Nuclear Incident Response

Radiation detection plays a significant role in global security and nuclear incident response. Gamma-ray and neutron measurements are key elements in this capability, greatly improving interpretations of real-world situations. An overview of nuclear incident response efforts and applications will be presented. This work is performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.   

Nuclear Resonance Fluorescence Measurements on $^{237}$Np for Security and Safeguards Applications

The smuggling of nuclear material and the diversion of fissile material for covert weapon programs both present grave risks to world security. Methods are needed to detect nuclear material smuggled in cargo, and for proper material accountability in civilian fuel re-processing facilities. Nuclear resonance fluorescence (NRF) is a technique that can address both needs. It is a non-destructive active interrogation method that provides isotope-specific information. It works by using a $\gamma$-ray beam to resonantly excite levels in a nucleus and observing the $\gamma$-rays emitted whose energy and intensity are characteristic of that isotope. $^{237}$Np presents significant safeguard challenges; it is fissile yet currently has fewer safeguard restrictions. NRF measurements on $^{237}$Np will expand the nuclear database and will permit designing interrogation and assay systems. Measurements were made using the bremsstrahlung beam at the HVRL at MIT on a 7 g target of $^{237}$Np with two incident electron energies of 2.8 and 3.1 MeV. Results will be presented with discussion of the relevant nuclear structure necessary to predict levels in other actinides.   

Nuclear Resonance Fluorescence from Uranium above 2 MeV

The detection of special nuclear materials is critical to the nation's efforts to counter serious threat from nuclear terrorist attacks. A research program has been initiated at TUNL to address the need for new nuclear data on the actinides using the High-Intensity Gamma-Ray Source (HI$\gamma$S). The high-intensity nearly monoenergic and 100\% polarized $\gamma$-ray beams from H$\gamma$S were utilized to search for dipole states in $^{235}$U and $^{238}$U above 2 MeV. This information is necessary for developing technologies using Nuclear-Resonance Fluorescence (NRF) to nonintrusively scan cargo for specific nuclei. The existence of strong nuclear dipole transitions in the actinides above 2 MeV is important for nuclear forensics, because interrogation photons using NRF are the most penetrating at these energies. Results from our experiments at E$_{\gamma} >$ 2.0 MeV on uranium will be presented.   

Development of a neutron source for NIF diagnostics

Neutron time-of-flight is a key diagnostic to determine the neutron spectrum from inertial confinement fusion at the National Ignition Facility (NIF). The down scattered fraction of the neutron spectrum with neutron energies between 10 and 13 MeV is proportional to the time-weighted areal density of the fuel, which is an important quantity for obtaining ignition. To detect down scattered neutrons after the initial 14 MeV neutrons from DT fusion, a fast neutron scintillator is required. To test different scintillator material for decay time and efficiency, a deuteron breakup neutron source is being developed at the 88-Inch cyclotron of Lawrence Berkeley National Laboratory (LBNL). The commissioning of this facility will be discussed including neutron energy and flux measurements, dosimetry and results from testing the neutron scintillators.   

Neutron Activation Diagnostic at the National Ignition Facility

A new cost-effective implementation of a Neutron Activation Diagnostic at the National Ignition Facility (NIF) will complement the Magnetic Recoil Spectrometer (MRS) and neutron Time-of-Flight (nToF) diagnostics by measuring the spatial distribution of downscattered neutrons ($\sim$10-13 MeV) in the NIF chamber. This helps quantify the angle-to-angle ``sampling error'' in those devices due to their single-position insensitivity to capsule implosion asymmetries. It will also provide a high-accuracy ($<$2$\%$ uncertainty) absolute measurement of the primary DT neutron yield. Activation samples will be mounted on three roughly-orthogonal DIMs (Diagnostic Instrument Manipulators), including one near the MRS for normalization. Only reactions with long half-lives (several hours to days) will be used and the samples will be removed manually. The ratio of the neutron-induced activities of downscatter-sensitive to downscatter-insensitive reactions allows determination of angular variation in the downscattered fraction in the chamber with most systematic uncertainties minimized.   

Diagnosing Implosion Velocity and Ablator Dynamics at NIF

An enhanced understanding of the environment in a burning NIF capsule is of interest to both astrophysics and thermonuclear ignition. In this talk we introduce a new diagnostic idea, designed to measure dynamic aspects of the capsule implosion that are not currently accessible. During the burn,the NIF capsule ablator is moving relative to the 14.1 MeV dt neutrons that are traversing the capsule. The resulting neutron-ablator Doppler shift causes a few unique nuclear reactions to become sensitive detectors of the ablator velocity at peak burn time. The ``point-design'' capsule at the NIF will be based on a $^9$Be ablator, and the $^9$Be(n,p)$^9$Li reaction has an energy threshold of 14.2 MeV, making it the ideal probe. As discussed in detail below, differences in the ablator velocity lead to significant differences in the rate of $^9$Li production. We present techniques for measuring this $^9$Li implosion velocity diagnostic at the NIF. The same experimental techniques, measuring neutron reactions on the ablator material, will allow us to determine other important dynamical quantities, such as the areal density and approximate thickness of the ablator at peak burn.   

Reaction-in-Flight Neutrons as a Probe of Hydrodynamical Mixing at NIF

At the National Ignition Facility (NIF) reaction-in-flight (RIF) neutrons above the main 14 MeV peak make up about 0.5\% of the neutrons production. In this talk we present calculations that show the sensitivity of the RIF neutron production to hydrodynamical mixing of the outer shell of the NIF capsule into the main dt fuel. This mixing generally quenches the dt burn and could be a serious mode of ignition failure. These calculations suggest that a time-of-flight measurement or radiochemical measurement of the RIF neutrons could be used as a robust indicator of the degree o mix taking place in an imploded NIF capsule.   

Gamma-ray Spectroscopic Performance of a $\sim$10 kg Array of High Purity Germanium Crystals

The gamma-ray spectroscopic performance of a single-cryostat, close-pack array of high purity germanium crystals is presented. The unit design is intended to provide high detection efficiency ($\sim$1000\% relative efficiency) for standoff gamma-ray detection in field measurement applications. However, the array design shares much in common with design concepts proposed by the Majorana Collaboration to search for neutrinoless double beta decay of Ge-76. The presentation will focus on those topics of relevance to both the field application and basic scientific research. Specifically this will include array operation, data collection, and data reduction that elucidate the unique features of a $\sim$10 kg compact array of high purity germanium gamma-ray spectrometers.   

Observation of stress effect on iron diffusion in Si by M\"ossbauer spectroscopy

A silicon wafer may contain metallic impurities and crystal defects such as vacancies and dislocations. Such a defect must cause stress fields, which are considered to affect the atomic diffusion and segregation properties. Although such the stress-induced diffusion must play an important role in the metallic impurities diffusion as well, the diffusion of metallic impurities in Si has never been studied under external stress until now. In the present study, in order to investigate the influence of a stress on the iron diffusion in Si matrix, M\"ossbauer spectra for $^{57}$Fe doped Si sample were measured at room temperature as a function of the external stress up to 44 MPa using an Instron-type tensile testing machine. The M\"ossbauer spectra were analyzed in terms of two Lorentzian components each corresponding to substisutional and interstitial Fe. The interstitial line gets broader when the external stress is applied presumably due to a high Fe jump rate of about 10$^{6}$ s$^{-1}$. More details of the experiments will be presented.