Micah Johnson

Isotope-specific detection of low-density materials with laser-based monoenergetic gamma-rays.

What we believe to be the first demonstration of isotope-specific detection of a low-Z and low density object shielded by a high-Z and high-density material using monoenergetic gamma rays is reported. The isotope-specific detection of LiH shielded by Pb and Al is accomplished using the nuclear resonance fluorescence line of L7i at 478 keV. Resonant photons are produced via laser-based Compton scattering. The detection techniques are general, and the confidence level obtained is shown to be superior to that yielded by conventional x-ray and gamma-ray techniques in these situations.

Transmission-based detection of nuclides with nuclear resonance fluorescence using a quasimonoenergetic photon source

We provide a detailed experimental validation of the concept of transmission-based isotope detection. The dominant background processes in this class of systems were measured by studying the detection of U238 with a quasimonochromatic (ΔE∕E∼3%) photon beam. A notch develops in the spectrum transmitted through our test objects due to the preferential attenuation of photons with an energy that resonantly excites a bound nuclear state in U238 near 2 MeV. The notch was measured downstream of our test objects by means of resonant photon scattering from a secondary U238 target. The dominant backgrounds measured in the notch detector due to radioactive decay and elastic scattering of the transmitted beam are presented. Processes that refill the notch with off-resonance photons will obscure the signal and result in a higher probability of false negatives. A measurement of the refill process produced a null result, and we report an upper limit on the magnitude of the notch fill factor.

Development of ORRUBA: A Silicon Array for the Measurement of Transfer Reactions in Inverse Kinematics

The development of high quality radioactive beams has made possible the measurement of transfer reactions in inverse kinematics on unstable nuclei. Measurement of (d,p) reactions on neutron-rich nuclei yield data on the evolution of nuclear structure away from stability, and are of astrophysical interest. Experimentally, (d,p) reactions on heavy (Z=50) fission fragments are complicated by the strongly inverse kinematics, and relatively low beam intensities. Consequently, ejectile detection with high resolution in position and energy, a high dynamic range and a high solid angular coverage is required. The Oak Ridge Rutgers University Barrel Array (ORRUBA) is a new silicon detector array optimized for the measurement of (d,p) reactions in inverse kinematics.

Using Quasi-Monoenergetic Photon Sources to Probe Photo-Fission Resonances

We present preliminary results of photo‐fission measurements of uranium isotopes with the quasi‐monoenergetic gamma‐ray source, HIGS. The measurements were performed to search for photo‐fission resonances. We discuss potential applications to use photo‐fission resonances to identify special nuclear material in cargo containers. We discuss the importance of quasi‐monoenergetic gamma‐ray sources for this kind of application.

Neutron Transfer Reactions: Surrogates for Neutron Capture for Basic and Applied Nuclear Science

Neutron capture reactions on unstable nuclei are important for both basic and applied nuclear science. A program has been developed at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory to study single-neutron transfer (d,p) reactions with rare isotope beams to provide information on neutron-induced reactions on unstable nuclei. Results from (d,p) studies on {sup 130,132}Sn, {sup 134}Te and {sup 75}As are discussed.

Livermore Accelerator Source for Radionuclide Science (LASRS)

The Livermore Accelerator Source for Radionuclide Science (LASRS) will generate intense photon and neutron beams to address important gaps in the study of radionuclide science that directly impact Stockpile Stewardship, Nuclear Forensics, and Nuclear Material Detection. The co-location of MeV-scale neutral and photon sources with radiochemical analytics provides a unique facility to meet current and future challenges in nuclear security and nuclear science.

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The production of 26Al in novae is uncertain, in part, because of the uncertain rate of the 25Al(p,g)26Si reaction at novae temperatures. This reaction is thought to be dominated by a longsought 3+ level in 26Si, and the calculated reaction rate varies by orders of magnitude depending on the energy of this resonance. We present evidence concerning the spin of a level at 5.914 MeV in 26Si from the 28Si(p,t)26Si reaction studied at the Holifield Radioactive Beam Facility at ORNL. We find that the angular distribution for this level implies either a 2+ or 3+ assignment, with only a 3+ being consistent with the mirror nucleus, 26Mg. Additionally, we have used the updated 25Al(p,g)26Si reaction rate in a nova nucleosynthesis calculation and have addressed the effects of the remaining uncertainties in the rate on 26Al production.

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We report detailed spectral and spatial characterization of a 0.1-MeV-0.8 MeV tunable ultra-bright laser-based Compton scattering source. Nuclear Resonance Fluorescence experiments with the source are also presented.

Al

Neutron transfer (d,p) reaction studies on the N = 50 isotones, 82Ge and 84Se, and A≈130 nuclei, 130,132Sn and 134Te, have been measured. Direct neutron capture cross sections for 82Ge and 84Se (n,γ) have been calculated and are combined with Hauser‐Feshbach expectations to estimate total (n,γ) cross sections. The A≈130 studies used an early implementation of the ORRUBA array of position‐sensitive silicon strip detectors for reaction proton measurements. Preliminary excitation energy and angular distribution results from the A≈130 measurements are reported.

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National security programs have expressed interest in developing systems to isotopically map shipping containers, fuel assemblies, and waste barrels for various materials including special nuclear material (SNM). Current radiographic systems offer little more than an ambiguous density silhouette of a container’s contents. In this paper we will present a system being developed at LLNL to isotopically map containers using the nuclear resonance fluorescence (NRF) method. Recent experimental measurements on NRF strengths in SNM are discussed.