DT Neutron Research Articles

Benchmark experiment on molybdenum with graphite by using DT neutrons at JAEA/FNS

In our previous benchmark experiment on Mo at JAEA/FNS, we found problems of the (n,2n) and (n,γ) reaction cross sections of Mo in JENDL-4.0 above a few hundred eV. We perform a new benchmark experiment on Mo with a Mo assembly covered with graphite and Li2O blocks in order to validate the nuclear data of Mo in lower energy region than in the previous experiment. Several dosimetry reaction and fission rates are measured and compared with calculated ones with the MCNP5-1.40 code and the recent nuclear data libraries, ENDF/B-VII.1, JEFF-3.2, and JENDL-4.0. It is suggested that the (n,γ) reaction cross section of 95Mo should be larger in the tail region below the large resonance of 45eV in these nuclear data libraries.

Benchmark integral neutron experiments for Fe, Be and C with DT neutron by liquid scintillation detector

The measurements of iron, beryllium and carbon sphere neutron leakage spectra using BC501A detector by DT neutron source are presented. The experiments were carried out in Institute of Nuclear Physics and Chemistry (INPC), China. Neutron leakage spectra in a wide energy range at angles of 0° and 30° from the direction of incident deuteron beam were obtained. The results show the leakage neutron flux decreases notably with the spherical shell increasing when neutron energy >10MeV. When neutron energy <6MeV the leakage neutron distribution tends to lower energy side with the shell increase. The total uncertainty evaluation of the neutron spectra is also provided: in the high-energy parts the uncertainty is about 5–7%, while in the low-energy parts the uncertainty is about 7–9%. In order to compare the experiment results a Monte-carlo calculation was made using the MCNP5 Code with the ENDF/B-VII.1 nuclear data files.

Characterization of the scintillation anisotropy in crystalline stilbene scintillator detectors

This paper reports a series of measurements that characterize the directional dependence of the scintillation response of crystalline melt-grown and solution-grown trans-stilbene to incident DT and DD neutrons. These measurements give the amplitude and pulse shape dependence on the proton recoil direction over one hemisphere of the crystal, confirming and extending previous results in the literature for melt-grown stilbene and providing the first measurements for solution-grown stilbene. In similar measurements of liquid and plastic detectors, no directional dependence was observed, confirming the hypothesis that the anisotropy in stilbene and other organic crystal scintillators is a result of internal effects due to the molecular or crystal structure and not an external effect on the measurement system.

A neutron activation spectrometer and neutronic experimental platform for the National Ignition Facility (invited).

At the National Ignition Facility, the diagnostic instrument manipulator-based neutron activation spectrometer is used as a diagnostic of implosion performance for inertial confinement fusion experiments. Additionally, it serves as a platform for independent neutronic experiments and may be connected to fast recording systems for neutron effect tests on active electronics. As an implosion diagnostic, the neutron activation spectrometers are used to quantify fluence of primary DT neutrons, downscattered neutrons, and neutrons above the primary DT neutron energy created by reactions of upscattered D and T in flight. At a primary neutron yield of 1015 and a downscattered fraction of neutrons in the 10-12 MeV energy range of 0.04, the downscattered neutron fraction can be measured to a relative uncertainty of 8%. Significant asymmetries in downscattered neutrons have been observed. Spectrometers have been designed and fielded to measure the tritium-tritium and deuterium-tritium neutron outputs simultaneously in experiments using DT/TT fusion ratio as a direct measure of mix of ablator into the gas.

Indications of flow near maximum compression in layered deuterium-tritium implosions at the National Ignition Facility.

An accurate understanding of burn dynamics in implosions of cryogenically layered deuterium (D) and tritium (T) filled capsules, obtained partly through precision diagnosis of these experiments, is essential for assessing the impediments to achieving ignition at the National Ignition Facility. We present measurements of neutrons from such implosions. The apparent ion temperatures T_{ion} are inferred from the variance of the primary neutron spectrum. Consistently higher DT than DD T_{ion} are observed and the difference is seen to increase with increasing apparent DT T_{ion}. The line-of-sight rms variations of both DD and DT T_{ion} are small, ∼150eV, indicating an isotropic source. The DD neutron yields are consistently high relative to the DT neutron yields given the observed T_{ion}. Spatial and temporal variations of the DT temperature and density, DD-DT differential attenuation in the surrounding DT fuel, and fluid motion variations contribute to a DT T_{ion} greater than the DD T_{ion}, but are in a one-dimensional model insufficient to explain the data. We hypothesize that in a three-dimensional interpretation, these effects combined could explain the results.

Sensitivity of chemical vapor deposition diamonds to DD and DT neutrons at OMEGA and the National Ignition Facility.

The particle-time-of-flight (pTOF) detector at the National Ignition Facility (NIF) is used routinely to measure nuclear bang-times in inertial confinement fusion implosions. The active detector medium in pTOF is a chemical vapor deposition diamond. Calibration of the detectors sensitivity to neutrons and protons would allow measurement of nuclear bang times and hot spot areal density (ρR) on a single diagnostic. This study utilizes data collected at both NIF and Omega in an attempt to determine pTOF's absolute sensitivity to neutrons. At Omega pTOF's sensitivity to DT-n is found to be stable to within 8% at different bias voltages. At the NIF pTOF's sensitivity to DD-n varies by up to 59%. This variability must be decreased substantially for pTOF to function as a neutron yield detector at the NIF. Some possible causes of this variability are ruled out.

High-resolution measurements of the DT neutron spectrum using new CD foils in the Magnetic Recoil neutron Spectrometer (MRS) on the National Ignition Facility.

The Magnetic Recoil neutron Spectrometer (MRS) on the National Ignition Facility measures the DT neutron spectrum from cryogenically layered inertial confinement fusion implosions. Yield, areal density, apparent ion temperature, and directional fluid flow are inferred from the MRS data. This paper describes recent advances in MRS measurements of the primary peak using new, thinner, reduced-area deuterated plastic (CD) conversion foils. The new foils allow operation of MRS at yields 2 orders of magnitude higher than previously possible, at a resolution down to ∼200 keV FWHM.

Application of the coincidence counting technique to DD neutron spectrometry data at the NIF, OMEGA, and Z.

A compact neutron spectrometer, based on a CH foil for the production of recoil protons and CR-39 detection, is being developed for the measurements of the DD-neutron spectrum at the NIF, OMEGA, and Z facilities. As a CR-39 detector will be used in the spectrometer, the principal sources of background are neutron-induced tracks and intrinsic tracks (defects in the CR-39). To reject the background to the required level for measurements of the down-scattered and primary DD-neutron components in the spectrum, the Coincidence Counting Technique (CCT) must be applied to the data. Using a piece of CR-39 exposed to 2.5-MeV protons at the MIT HEDP accelerator facility and DD-neutrons at Z, a significant improvement of a DD-neutron signal-to-background level has been demonstrated for the first time using the CCT. These results are in excellent agreement with previous work applied to DT neutrons.

Validation of large-angle scattering data via shadow-bar experiment

An experiment emphasizing the influence of large-angle scattering on nuclear data was conducted, in which a Fe shadow bar and a Fe slab target were placed before a deuterium–tritium fusion (DT) neutron source. Two Nb foils were set on both sides of the shadow bar in order to monitor the neutron source intensity and to measure the neutrons scattered from the slab target. The 93Nb(n,2n)92mNb reaction rate of the foil was measured following the DT neutron irradiation and calculated using the MCNP5 Monte Carlo radiation transportation code. The 92mNb production rates calculated using data from the JEFF-3.1 and JENDL-4.0 libraries agreed with that measured in the experiment, while the result calculated using data from the ENDF/B-VI library exhibited a 57% overestimation. Because the sensitivity of the 92mNb production rate to the scattering angular distribution was large in the angular region between scattering direction cosines of −0.9 and −0.4, the scattering angular distribution was adjusted in that region. This adjustment resulted in a calculation-to-experiment ratio close to 1, but had little influence on the existing integral benchmark experiment.

Evaluation of tritium release behavior from Li2TiO3 during DT neutron irradiation by use of an improved tritium collection method

The accurate measurement of behavior of bred tritium released from a tritium breeder is indispensable to understand the behavior for a design of a tritium extraction system. The tritium collection method combined a CuO bed and water bubbles was not suitable to measure transient behavior of tritium released from Li2TiO3 during neutron irradiation because tritium released behavior was changed to be delayed due to adsorption of oxidized tritium on the CuO. Hence, the tritium collection method with hydrophobic catalyst instead of the CuO was demonstrated and succeeded the accurate release measurement of tritium from Li2TiO3. With the method, we assessed the behavior of tritium release under the various conditions since tritium should be released from Li2TiO3 as the form of HT as much as possible from the view point of the fuel cycle. Our results indicated; promotion of isotopic exchange reaction on the surface of Li2TiO3 by addition of hydrogen in sweep gas is mandatory in order to release tritium smoothly from Li2TiO3 irradiated with neutrons; the favorable sweep gas to release as the form of HT was hydrogen added inert gas; and the temperature of Li2TiO3 was the dominant parameter to control the chemical form of tritium released from the Li2TiO3.

Investigating the Anisotropic Scintillation Response in Anthracene through Neutron, Gamma-Ray, and Muon Measurements

This paper reports a series of measurements that characterize the directional dependence of the scintillation response of crystalline anthracene to incident DT neutrons, DD neutrons, Cs-137 gamma rays, and, for the first time, cosmic ray muons. The neutron measurements give the amplitude and pulse shape dependence on the proton recoil direction over one hemisphere of the crystal, confirming and extending previous results in the literature. In similar measurements using incident gamma rays, no directional effect is evident, and any anisotropy with respect to the electron recoil direction is constrained to have a magnitude of less than a tenth of that present in the proton recoil events. Cosmic muons are measured at two directions, and no anisotropy is observed. This set of observations indicates that high dE/dx is necessary for an anisotropy to be present for a given type of scintillation event, which in turn could be used to discriminate among different hypotheses for the underlying causes of the anisotropy, which are not well understood.

Fusion-neutron measurements for magnetized liner inertial fusion experiments on the Z accelerator

Several magnetized liner inertial fusion (MagLIF) experiments have been conducted on the Z accelerator at Sandia National Laboratories since late 2013. Measurements of the primary DD (2.45 MeV) neutrons for these experiments suggest that the neutron production is thermonuclear. Primary DD yields up to 3e12 with ion temperatures ∼2-3 keV have been achieved. Measurements of the secondary DT (14 MeV) neutrons indicate that the fuel is significantly magnetized. Measurements of down-scattered neutrons from the beryllium liner suggest ρRliner∼1g/cm2. Neutron bang times, estimated from neutron time-of-flight (nTOF) measurements, coincide with peak x-ray production. Plans to improve and expand the Z neutron diagnostic suite include neutron burn-history diagnostics, increased sensitivity and higher precision nTOF detectors, and neutron recoil-based yield and spectral measurements.

Nuclear Diagnostics at the National Ignition Facility, 2013-2015

The National Ignition Facility (NIF) relies on a suite of nuclear diagnostics to measure the neutronic output of experiments. Neutron time-of-flight (NTOF) and neutron activation diagnostics (NAD) provide performance metrics of absolute neutron yield and neutron spectral content: spectral width and non-thermal content, from which implosion physical quantities of temperature and scattering mass are inferred. Spatially-distributed flange- mounted NADs (FNAD) measure, with nearly identical systematic uncertainties, primary DT neutron emission to infer a whole-sky neutron field. An automated FNAD system is being developed. A magnetic recoil spectrometer (MRS) shares few systematics with comparable NTOF and NAD devices, and as such is deployed for independent measurement of the primary neutronic quantities. The gas-Cherenkov Gamma Reaction History (GRH) instrument records four energy channels of time-resolved gamma emission to measure nuclear bang time and burn width, as well as to infer carbon areal density in experiments utilizing plastic or diamond capsules. A neutron imaging system (NIS) takes two images of the neutron source, typically gated to create coregistered 13-15 MeV primary and 6-12 MeV downscattered images. The radiochemical analysis of gaseous samples (RAGS) instrument pumps target chamber gas to a chemical reaction and fractionation system configured with gamma counters, allowing measurement of radionuclides with half-lives as short as 8 seconds. Solid radiochemistry collectors (SRC) with backing NAD foils collect target debris, where activated materials from the target assembly are used as indicators of neutron spectrum content, and also serve as the primary diagnostic for nuclear forensic science experiments. Particle time-of-flight (PTOF) measures compression-bang time using DT- or DD-neutrons, as well as shock bang-time using D3He-protons for implosions with lower x-ray background. In concert, these diagnostics serve to measure the basic and advanced quantities required to understand NIF experimental results.

Fast neutron and gamma-ray spectra measurements with a NE-213 spectrometer in the FNG Copper Benchmark Experiment

A neutronics benchmark experiment on a pure Copper assembly was performed at the Frascati Neutron Generator. The work aimed at testing of recent nuclear data libraries. This paper focuses on the measurement of fast neutron and gamma-ray flux spectra in the Copper assembly under DT neutron irradiation in two selected positions with a spectrometer based on the organic liquid scintillator NE-213. The measurement results were compared with Monte Carlo radiation transport calculations using MCNP and nuclear data from the JEFF-3.1.1 library. Calculations have been done with Cu data from JEFF-3.1.1, JEFF-3.2, FENDL-3 and ENDF/B-7.0. Discrepancies appear in the intermediate neutron energy range between experiment and calculation. Large discrepancies were observed in the gamma-ray spectra calculated with JEFF-3.2.

Capture-gated Spectroscopic Measurements of Monoenergetic Neutrons with a Composite Scintillation Detector

We report on the measurements of monoenergetic neutrons from DD and DT fusion reactions by use of the capture gating method in a heterogeneous plastic-glass composite scintillation detector. The cylindrical detector is 5.08 cm in diameter and 5.05 cm in height and was fabricated using 1-mm diameter Li-doped glass rods (GS20) and scintillating polyvinyl toluene (EJ-290). Different scintillation decay constants are used to identify energy depositions in two materials constituting the composite scintillator. Geant4 simulations of the neutron thermalization and capture process were conducted, finding a mean capture time of approximately $2.6 \mu\hbox{s}$ for both DD and DT neutrons. A capture gating time acceptance window based on simulation results was used to identify the neutron thermalization pulses. The total scintillation light yield produced in neutron thermalization was measured and found to show consistency on event-by-event basis despite the variety of neutron thermalization histories prior to capture. The ratio of light yields from thermalization of 14.1 MeV and 2.45 MeV neutrons in the EJ-290 scintillator was determined to be 14.6, and the light output from 2.45 MeV neutrons was also correlated to its electron equivalent, obtaining a value of $0.58 \pm 0.05 \hbox{MeVee}$ .

Comparison of DT neutron production codes MCUNED, ENEA-JSI source subroutine and DDT

As the DT fusion reaction produces neutrons with energies significantly higher than in fission reactors, special fusion-relevant benchmark experiments are often performed using DT neutron generators. However, commonly used Monte Carlo particle transport codes such as MCNP or TRIPOLI cannot be directly used to analyze these experiments since they do not have the capabilities to model the production of DT neutrons. Three of the available approaches to model the DT neutron generator source are the MCUNED code, the ENEA-JSI DT source subroutine and the DDT code. The MCUNED code is an extension of the well-established and validated MCNPX Monte Carlo code. The ENEA-JSI source subroutine was originally prepared for the modelling of the FNG experiments using different versions of the MCNP code (−4, −5, −X) and was later extended to allow the modelling of both DT and DD neutron sources. The DDT code prepares the DT source definition file (SDEF card in MCNP) which can then be used in different versions of the MCNP code. In the paper the methods for the simulation of the DT neutron production used in the codes are briefly described and compared for the case of a simple accelerator-based DT neutron source.

Development of a polar direct drive platform for mix and burn experiments on the National Ignition Facility

Capsules driven with polar drive [1, 2] on the National Ignition Facility [3] are being used [4] to study mix in convergent geometry. In preparation for experiments that will utilize deuterated plastic shells with a pure tritium fill, hydrogen-filled capsules with copper- doped deuterated layers have been imploded on NIF to provide spectroscopic and nuclear measurements of capsule performance. Experiments have shown that the mix region, when composed of shell material doped with about 1% copper (by atom), reaches temperatures of about 2 keV, while undoped mixed regions reach about 3 keV. Based on the yield from these implosions, we estimate the thickness of CD that mixed into the gas as between about 0.25 and 0.43 μm of the inner capsule surface, corresponding to about 5 to 9 μg of material. Using 5 atm of tritium as the fill gas should result in over 1013 DT neutrons being produced, which is sufficient for neutron imaging [5].

A new blanket tritium recovery experiment with intense DT neutron source at JAEA/FNS

We have performed the tritium release experiment on the fusion reactor blanket at JAEA/FNS since 2009, and then clarified the ratio of tritium release and the recovered tritium chemical form. In order to acquire the detailed tritium recovery performances, we have started a new blanket tritium recovery experiment with ionization chamber (IC) at JAEA/FNS. For the appropriate tritium measurement with IC, we improved the experimental container and carried out with an intense DT neutron source at JAEA/FNS. From our new experiment, the tritium recovery radioactivity from the LSC measurement corresponds with the calculation within 6%. However, it was pointed out that further improvement in the quantitative tritium measurement by IC method was needed.

Scattering chamber facility for double-differential cross-section measurement with 14 MeV DT neutron generator at IPR

Measurement of double-differential cross-sections of 14 MeV neutron-induced charged-particle productions is very important for estimating the nuclear heating and radiation damage of a fusion reactor. Only a few experimental data are available even though the nuclear reaction cross-section data of structural materials are important in fusion nuclear technology. In this context, general purpose scattering chamber facility has been developed for accelerator-based 14 MeV DT neutron generator to measure double-differential nuclear reaction cross-section at Fusion Neutronics Laboratory, IPR. It has been designed for experiments using silicon surface barrier detectors for the online detection of charged particles. It offers flexibility in the arrangement of silicon surface barrier detectors.

Integral experiment on molybdenum with DT neutrons at JAEA/FNS

An integral experiment on molybdenum is performed with a DT neutron source at JAEA/FNS. A Mo assembly is covered with lithium oxide blocks in order to reduce background neutrons inside the assembly. Several reaction rates and fission rates are measured along the central axis inside the assembly and compared with calculated ones with the Monte Carlo transport code MCNP5-1.40 and recent nuclear data libraries of ENDF/B-VII.1, JENDL-4.0, and JEFF-3.2. The calculated results generally show underestimation. From our detailed analysis, it is concluded that the (n,2n) cross section data of all the Mo stable isotopes in JEFF-3.2 are more suitable than those in JENDL-4.0 and the (n,γ) cross section data of 92Mo, 94Mo, 95Mo, 96Mo, 97Mo, and 100Mo in JENDL-4.0 are overestimated.