Neutron rem meters are routinely used for real-time field measurements of neutron dose equivalent where neutron spectra are unknown or poorly characterized. These meters are designed so that their response per unit fluence approximates an appropriate fluence-to-dose conversion function. Typically, a polyethylene moderator assembly surrounds a thermal neutron detector, such as a BF3 counter tube. Internal absorbers may also be used to further fine-tune the detector response to the shape of the desired fluence conversion function. Historical designs suffer from a number of limitations. Accuracy for some designs is poor at intermediate energies (50 keV-250 keV) critical for nuclear power plant dosimetry. The well-known Andersson-Braun design suffers from angular dependence because of its lack of spherical symmetry. Furthermore, all models using a pure polyethylene moderator have no useful high-energy response, which makes them inaccurate around high-energy accelerator facilities. This paper describes two new neutron rem meter designs with improved accuracy over the energy range from thermal to 5 GeV. The Wide Energy Neutron Detection Instrument (WENDI) makes use of both neutron generation and absorption to contour the detector response function. Tungsten or tungsten carbide (WC) powder is added to a polyethylene moderator with the expressed purpose of generating spallation neutrons in tungsten nuclei and thus enhance the high-energy response of the meter beyond 8 MeV. Tungsten's absorption resonance structure below several keV was also found to be useful in contouring the meter's response function. The WENDI rem meters were designed and optimized using the Los Alamos Monte Carlo codes MCNP, MCNPX, and LAHET. A first generation prototype (WENDI-I) was built in 1995 and its testing was completed in 1996. This design placed a BF3 counter in the center of a spherical moderator assembly, whose outer shell consisted of 30% by weight WC in a matrix of polyethylene. A borated silicone rubber (5% boron by weight) absorber covered an inner polyethylene sphere to control the meter's response at intermediate energies. A second generation design (WENDI-II) was finalized and tested in 1999. It further extended the high-energy response beyond 20 MeV, increased sensitivity, and greatly facilitated the manufacturing process. A 3He counter tube is located in the center of a cylindrical polyethylene moderator assembly. Tungsten powder surrounds the counter tube at an inner radius of 4 cm and performs the double duty of neutron generation above 8 MeV and absorption below several keV. WENDI-II is suitable for field use as a portable rem meter in a variety of work place environments, and has been recently commercialized under license by Eberline Instruments, Inc. and Ludlum Measurements, Inc. Sensitivity is about a factor of 12 higher than that of the Hankins Modified Sphere (Eberline NRD meter) in a bare 252Cf field. Additionally, the energy response for WENDI-II closely follows the contour of the Ambient Dose Equivalent per unit fluenc...
The usual Bonner sphere set consists of six to eight high density polyethylene spheres with diameters varying from 3 to 12 inches. Either a BF, counter, LiI scintillator, or ,He detector is located at the center of each sphere to detect the moderated neutrons. The responses of these spheres for high energy neutrons are very low even for the largest 12 inch sphere. To increase the response for high energy neutrons, high Z material such as Pb, W, TI, or Au can be added to the sphere to utilize the (n,xn) reaction of these materials. Monte Carlo Simulations for the Pb-added case with a 3He detector as an example will be presented. The response at high energy for this case is enhanced by a factor of 5 to 8 depending upon the sphere diameter.The use of the Bonner sphere sets for neutron spectra measurements above 20 MeV has been limited because of small neutron interaction cross sections in the sphere materials. Recent calculations on the use of 3He detectors in Bonner spheres for neutrons up to 20 MeV are given in [3]. The addition of fissile materials such as 235U would be ideal because of their production of induced-fission neutrons by the higher energy neutrons.However, such materials were not used because of their inherent radioactivity. Enhanced response can be obtained through the use of other high Z materials, such as W, Au, T1, or Pb which have large (n,xn') cross sections. Lead was chosen for these calculations.This paper describes calculations done to investigate the use of lead to enhance the high energy response.
A new deuterium-tritium (D-T) fusion gamma-to-neutron branching ratio [3H(d,γ)5He/3H(d,n)4He] value of (4.2 ± 2.0) × 10−5 was recently reported by this group [Y. Kim et al. Phys. Rev. C (submitted)]. This measurement, conducted at the OMEGA laser facility located at the University of Rochester, was made for the first time using inertial confinement fusion (ICF) plasmas. Neutron-induced backgrounds are significantly reduced in these experiments as compared to traditional beam-target accelerator-based experiments due to the short pulse nature of ICF implosions and the use of gas Cherenkov γ-ray detectors with fast temporal responses and inherent energy thresholds. It is expected that this ICF-based measurement will help resolve the large and long-standing inconsistencies in previously reported accelerator-based values, which vary by a factor of approximately 30. The reported value at ICF conditions was determined by averaging the results of two methods: (1) a direct measurement of ICF D-T γ-ray and neutron emissions using absolutely calibrated detectors and (2) a separate cross-calibration against the better known D-3He gamma-to-proton branching ratio [3He(d, γ)5Li/3He(d,p)4He]. Here we include a detailed explanation of these results, and introduce as a corroborative method an in-situ γ-ray detector calibration using neutron-induced γ-rays. Also, by extending the established techniques to two additional series of implosions with significantly different ion temperatures, we test the branching ratio dependence on ion temperature. The data show a D-T branching ratio is nearly constant over the temperature range 2–9 keV. These studies motivate further investigation into the 5He and 5Li systems resulting from D-T and D-3He fusion, respectively, and result in improved ICF γ-ray reaction history diagnosis at the National Ignition Facility.
The D-T gamma-to-neutron branching ratio (3 H(d,γ) 5 He/ 3 H(d,n) 4 He) has been determined at inertial confinement fusion (ICF) conditions, where the center-of-mass energy of 14-24 keV is lower than in previous accelerator-based experiments. A D-T branching ratio value of (4.2 ± 2.0)×10-5 was determined by averaging the results of two methods: 1) a direct measurement of ICF D-T γ-ray and neutron emissions using absolutely-calibrated detectors, and 2) a separate cross-calibration against the D-3 He gamma-to-proton branching ratio (3 He(d,γ) 5 Li/ 3 He(d,p) 4 He). Neutron-induced backgrounds are significantly reduced as compared to traditional beam-target accelerator-based experiments due to the short pulse nature of ICF implosions and the use of gas Cherenkov γ-ray detectors with fast temporal responses and inherent energy thresholds. These measurements of the D-T branching ratio in an ICF environment test several theoretical assumptions about the nature of A = 5 systems, including the dominance of the 3/2 + resonance at low energies, the presence of the broad first excited state of 5 He in the spectra, and the charge-symmetric nature of the capture processes in the mirror systems 5 He and 5 Li.
We report the first gamma-ray-based measurements of the areal density of ablators in inertial-confinement-fusion capsule implosions. The measurements, made at the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)], used observations of gamma rays arising from inelastic scattering of 14.1-MeV deuterium-tritium (DT) neutrons on 12C nuclei in the compressed plastic ablators. The emission of 12C(n,n′γ) gamma rays from the capsules is detected using the Gamma Reaction History instrument [H. W. Herrmann et al., J. Phys.: Conf. Ser. 244, 032047 (2010)] operating at OMEGA. From the ratio of a capsule's 12C(n,n′γ) emission to the emission from the same processes in an in situ reference graphite “puck” of known mass and geometry [N. M. Hoffman et al., in IFSA 2011 proceedings (submitted)], we determine the time-averaged areal density of 12C in the capsule's compressed ablator. Measured values of total ablator areal density for thirteen imploded capsules, in the range 23 ± 10 to 58 ± 14 mg/cm2, are comparable to values calculated in 1D radiation-hydrodynamic simulations, and measured by charged-particle techniques.
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