Neutron Capture Reactions for Stockpile Stewardship and Basic Science

Parker, W ; Agvaanluvsan, U ; Becker, J ; Wilk, P ; Wu, C ; Bredeweg, T ; Couture, A ; Haight, R ; Jandel, M ; O'Donnell, J ; Reifarth, R ; Rundberg, R ; Ullmann, J ; Vieira, D ; Wouters, J ; Sheets, S ; Mitchell, G ; Becvar, F ; Krticka, M

关键词: BARIUM FLUORIDES;    BINDING ENERGY;    CAPTURE;    COMPOUND NUCLEI;    CROSS SECTIONS;    ELASTIC SCATTERING;    ENERGY SPECTRA;    ENERGY-LEVEL DENSITY;    EXCITED STATES;    HEAVY METALS;    HEAVY NUCLEI;    LINEAR ACCELERATORS;    LITHIUM HYDRID;   

DOI  :  10.2172/950093
RP-ID  :  UCRL-TR-234820
PID  :  OSTI ID: 950093
Others  :  TRN: US0901992

学科分类：核物理和高能物理

美国|英语

来源: SciTech Connect

【 摘 要 】

The capture process is a nuclear reaction in which a target atom captures an incident projectile, e.g. a neutron. The excited-state compound nucleus de-excites by emitting photons. This process creates an atom that has one more neutron than the target atom, so it is a different isotope of the same element. With low energy (slow) neutron projectiles, capture is the dominant reaction, other than elastic scattering. However, with very heavy nuclei, fission competes with capture as a method of de-excitation of the compound nucleus. With higher energy (faster) incident neutrons, additional reactions are also possible, such as emission of protons or emission of multiple neutrons. The probability of a particular reaction occurring (such as capture) is referred to as the cross section for that reaction. Cross sections are very dependent on the incoming neutron's energy. Capture reactions can be studied either using monoenergetic neutron sources or 'white' neutron sources. A 'white' neutron source has a wide range of neutron energies in one neutron beam. The advantage to the white neutron source is that it allows the study of cross sections as they depend on neutron energies. The Los Alamos Neutron Science Center, located at Los Alamos National Laboratory, provides an intense white neutron source. Neutrons there are created by a high-energy proton beam from a linear accelerator striking a heavy metal (tungsten) target. The neutrons range in energy from subthermal up to very fast - over 100 MeV in energy. Low-energy neutron reaction cross sections fluctuate dramatically from one target to another, and they are very difficult to predict by theoretical modeling. The cross sections for particular capture reactions are important for defense sciences, advanced reactor concepts, transmutation of radioactive wastes and nuclear astrophysics. We now have a strong collaboration between Lawrence Livermore National Laboratory, Los Alamos National Laboratory, North Carolina State University and Charles University in Prague. In this paper, we report neutron capture studies that are of particular interest to Lawrence Livermore National Laboratory. In addition to determining neutron capture cross sections, we are also interested in the nuclear properties of the excited state compound nuclei created in the capture reactions. One model that describes the behavior of the nucleus is the statistical model. Our statistical studies included measuring the photon strength function, resonance parameters, level density and gamma-ray ({gamma}-ray) cascade multiplicity. The DANCE array allows the separation of cascades by the number of transitions (multiplicity) in the cascade, and this makes it possible to study detailed properties of the statistical cascade such as the relationship between multiplicity and energy distributions. The work reported here includes reaction on molybdenum targets, europium targets, gadolinium targets and the first americium-242m target. Our goal is to improve the accuracy and provide new measurements for stable and radioactive targets. We are especially interested in energy-dependent neutron capture cross sections. In all of our experiments, the photons emitted in the capture reactions are gamma rays, and they are detected by the barium fluoride crystal array named the Detector for Advanced Neutron Capture Experiments (DANCE) shown in Fig. 1. The detector array is made of 160 crystals arranged in a sphere around the target. There are four different crystal shapes, each of which covers an equal solid angle. This array was specifically designed to measure neutron capture cross sections with targets that were milligram sized or smaller, including radioactive targets. The barium fluoride crystals are scintillation (light generating) detectors with very fast response time, and are therefore suitable for high count rate experiments. Actual neutron capture events must be reliably distinguished from background {gamma}-rays, which are always present in neutron induced reactions. To reduce the background of scattered neutrons, a lithium hydride shell is placed inside the array. The purpose of using the spherical array of detectors is to cover all possible directions of emitted {gamma} rays, so we will come as close as possible to complete detection of all the prompt {gamma}-ray cascades emitted in a capture reaction. The sum of the energy of the {gamma} cascades is a measure of the binding energy of the capture neutron. The binding energy is the energy required to remove a bound neutron from the nucleus. The measured mass of the nucleus is smaller than the masses of the target nucleus plus the captured neutron, and the difference (converted to energy) is the binding energy of the capture neutron. Because the detector is segmented into a large number of independent detectors, additional information on event multiplicities (number of {gamma} rays emitted) and other properties can be determined.