Session W14: Applications of Nuclear Physics and Nuclear Techniques

New neutron capabilities for the Berkeley Accelerator Space Effects (BASE) Facility at the 88-Inch Cyclotron at LBNL

The Berkeley Accelerator Space Effects (BASE) Facility provides heavy ions and protons for radiation effects testing by government laboratories (Defense, Energy and NASA) and contractors, private U.S. companies and international companies and laboratories. The combination of state-of-the-art ion sources for heavy ion running and relatively high intensities (up to 10 microamps) for protons makes it a very versatile `one-stop-shop' for the radiation testing community. To add to this capability, a fast neutron capability has been developed using the d(Be,n) reaction in stopping targets. The choice of deuteron energy, ranging from 5 -- 65 MeV, gives a broad energy spectra with some tunability. The commissioning of this facility will be discussed including energy and flux measurements, dosimetry and initial experiments. In the future, two off-line neutron generators will also be in operation at the BASE facility, providing thermal neutrons as well as monoenergetic neutrons at 2.5 and 14 MeV. These sources, running independently of the Cyclotron, will complement the broad spectra neutrons at higher energies, providing a unique and versatile neutron capability.   

Benchmarking Radiation Transport Codes for Space Missions

For long duration and/or deep space human missions, protection from severe space radiation exposure is a challenging design constraint and may be a potential limiting factor. The space radiation environment consists of galactic cosmic rays (GCR), solar particle events (SPE), trapped radiation, and includes ions of all the known elements over a very broad energy range. These ions penetrate spacecraft materials producing nuclear fragments and secondary particles that damage biological tissues, microelectronic devices, and materials. Accurate risk assessments critically depend on the accuracy of the input information about the interaction of ions with materials, electronics and tissues and the radiation transport codes. Due to complexity of the problem and paucity of huge amount of experimental data, it is prudent to benchmark leading radiation transport codes to build increasing confidence in exposure estimates. The deterministic code HZETRN2006, and the Monte Carlo Codes HETC-HEDS and FLUKA, are used for benchmarking efforts. The SPE Webber spectrum and 1977 GCR radiation environments has been taken to make radiation dose exposure studies on aluminum shield followed by water target.   

Investigation of the use of high-pressure xenon detectors in ion beam analysis.

Ion beam analysis (IBA) provides nondestructive compositional information. Three major requirements for detectors used in IBA are high efficiency, great resolution, and high signal to noise ratio (SNR). The standard detector used in $\gamma $-photon based IBA techniques is the thallium-doped sodium iodide - NaI(Tl) -- scintillator. High-pressure xenon detectors (HPXe) present certain advantages over NaI(Tl) and other detector types for IBA conditions. The performance of a 1.5-inch diameter by 3-inch long high-pressure xenon (HPXe) detector is investigated at energies useful to IBA. The performance is compared to theoretical predictions. Recommendations are then made for a physically larger HPXe system for IBA.   

Neutron-Induced Partial Gamma-Ray Cross-Section Measurements on Uranium at TUNL

Precision measurements of (n,n') and (n,2n) reaction cross sections have been performed on $^{235,238}$U targets at Triangle Universities Nuclear Laboratory using a pulsed and monoenergetic neutron beam. The excitation function has been studied with incident neutron energies between 5 and 14 MeV and beam flux of 10$^{4}$ n s$^{-1}$ cm$^{-2}$ at target position. Multiple partial cross sections have been determined observing gamma rays in clover and planar HPGe detectors. The results will be compared with calculations using Hauser-Feshbach model.   

Measurement of Delayed Neutron Production in a Tungsten Spallation Neutron Target

We use neutron reflectometry to determine the contribution of delayed neutrons to the spallation spectrum at the Manuel Lujan Jr. Neutron Scattering Center at the Los Alamos National Laboratory. The delayed neutron fraction is $1.1(1)\times 10^{-4}$ in conformance with rough theoretical expectations. Within the accuracy of the measurement, the delayed neutrons have the same spectrum as the prompt neutrons and display no time structure on the order of $\sim $100~ms. While this measurement describes the delayed neutron production from a Tungsten spallation neutron target, it provides guidance for the expected delayed neutron production in other targets including the Mercury target at the Oak Ridge National Laboratory Spallation Neutron Source.   

Modification of apparent fission yields by Chemical Fractionation following Fission (CFF)

Grain-by-grain studies of the 2 billion year old Oklo natural reactor, using laser micro-extraction$^{1,2}$, yield detailed information about Oklo, a water-moderated pulsed reactor, cycle times, total neutron fluence and duration, but it also demonstrates Chemical Fractionation following Fission. In the CFF process, members of an isobaric yield chain with long half-lives are subject to migration before decay can occur. Of particular interest is the 129 isobar where 17 million $^{129}$I can migrate out of the host grain before decay, and iodine compounds are water soluble. This is amply demonstated by the variation of Xe spectra between micron-sized uranium-bearing minerals and adjacent uranium-free minerals. Fission 129 yields for the spontaneous fission of $^{238}$U generally come from measured $^{129}$Xe in pitchblend$^{2}$, ores emplaced by aqueous activity, and are incorrect due to the CFF process. $^{238}$U yields for the 131 and 129 chains, reported in Hyde$^{3}$, as 0.455 +- .02 and $<$ 0.012, respectively, the latter being anomalously low. $^{1}$A Meshik, C Hohenberg and O Pravdivtesva, PRL 93, 182302 (2004); A Meshik Sci. Am. Nov (2005), 55; $^{2}$E K Hyde, Nucl Prop of Heavy Elements III (1964).   

Fission xenon in trinities from the first nuclear test

Trinitites, greenish glassy remnants found in the crater of the first nuclear test, refer to the molten material of the desert where the Trinity test was conducted. Recently the Los Alamos Lab$^{1}$ suggested that the sand was first vaporized by the fireball and then precipitated onto a cooler desert surface forming trinitites. We measured the Xe mass-spectra during stepped pyrolysis of two trinitites and found an unusual Xe isotopic structure, dominated by $^{132}$Xe and $^{131}$Xe compared to the nominal fission yield spectra, which cannot be due to n-capture or any other nuclear processes. This structure is caused by the chemical separation of the immediate neutron-rich fission products, a process similar to CFF observed in the Oklo natural reactor$^{2}$. When quantitatively applied to our observations it suggests that 17 min after the test one of the samples had a temperature of 1390$^{o}$C, while 5 min after the test the other was at 1320$^{o}$C. These results contribute to a reconstruction of the cooling history of the trinities and a demonstration of which formation scenario is the more likely. $^{1}$V. Montoya et al, Denver X-ray Conf. (2007), $^{2}$A. Meshik, C. Hohenberg and O. Pravdivtseva, PRL 93, 182302 (2004).   

The three photon yield from e+ annihilation in biological liquids

Positrons annihilate either by the emission of 2-511 keV photons or 3-photons (from the decay of positronium in the triplet state.) The fraction of the 3-photon decay depends on the chemical environment and notably on the concentration of O$_{2}$. Consequently, 3-photon event detection has been proposed as a mean to measure hypoxia, a condition prevailing in cancer.~ The (delayed) three-photon yield in various fluids, at both high and low O$_{2}$ levels, has been extracted by fitting the time dependence of the two-photon yield to a set of coupled differential equations. The differential equations, in a simple and satisfactory fashion account for the e+ capture to form positronium and the decay and interconversion of the two forms. The total fraction of three photon events (both direct and delayed), which could be used for event-by-event position localization in PET-like imaging, is estimated to be $\sim $ 0.5 {\%} with the measured (from our work) delayed component of no more than 0.25{\%} in water (or blood-like) samples. There is no (or an exceedingly small) dependence on the dissolved oxygen content in aqueous solutions.   

A Neutron Scattering Kernel of Solid Methane in phase II

A neutron scattering cross section model of solid methane was studied for the cold neutron moderator of Low Energy Neutron Source (LENS) at IUCF/Indiana University especially in temperature range of 20.4 ~4K. The analytical scattering kernel was adapted from Ozaki.et al .[1][2] to describe molecular rotation in this temperature range. This model includes a molecular translation and intra-molecular vibration as well as the rotational degree of freedom in effective ways. For more broad applications into monte carlo simulations, neutron scattering libraries for MCNP were produced from the frequency spectrums using NJOY code. We have tested this newly- developed scattering kernels for phase II solid methane by calculating the neutron spectral intensity expected from the methane moderator at the LENS neutron source using MCNP. The predictions are compared to the measured energy spectra. The simulations agree with the measurement data at both temperatures. The simulation results show good agreement with measurement data in different temperatures. \newline [1] Y. Ozaki, Y. Kataoka, and T. Yamamoto, The Journal of Chemical Physics 73, 3442 (1980). \newline [2] Y. Ozaki, Y. Kataoka, K. Otaka, and T. Yamamoto, Can. J. Physics. 59, 275 (1981).