Measured Neutron Energy Research Articles

Diagnosing up-scattered deuterium-tritium fusion neutrons produced in burning plasmas at the National Ignition Facility (invited).

In the push to higher performance fusion plasmas, two critical quantities to diagnose are α-heat deposition that can improve and impurities mixed into the plasma that can limit performance. In high-density, highly collisional inertial confinement fusion burning plasmas, there is a significant probability that deuterium-tritium (DT) fusion products, 14.1MeV neutrons and 3.5MeV α-particles, will collide with and deposit energy onto ("up-scatter") surrounding deuterium and tritium fuel ions. These up-scattered D and T ions can then undergo fusion while in-flight and produce an up-scattered neutron (15-30MeV). These reaction-in-flight (RIF) neutrons can then be uniquely identified in the measured neutron energy spectrum. The magnitude, shape, and relative size of this spectral feature can inform models of stopping-power in the DT plasma and hence is directly proportional to α-heat deposition. In addition, the RIF spectrum can be related to mix into the burning fuel, particularly relevant for high-Z shell and other emerging National Ignition Facility platforms. The neutron time-of-flight diagnostic upgrades needed to obtain this small signal, ∼10-5 times the primary DT neutron peak, will be discussed. Results from several gain > 1 implosions will be shown and compared to previous RIF spectra. Finally, comparisons of experimental data to a simplified computational model will be made.

The initial measurement of a compact D-T neutron spectrometer based on a single-crystal chemical vapor deposition diamond stack for fusion plasma diagnostic.

For developing and characterizing a novel compact D-T neutron spectrometer based on a single-crystal chemical vapor deposition diamond stack for plasma diagnostics toward future D-T fusion reactors, the initial measurement was performed using the accelerator-based D-T neutron sources OKTAVIAN at Osaka University. This neutron spectrometer was designed for the detection of 3-17MeV neutrons and operated in the proton recoil telescope configuration by installing a polyethylene converter in front of the diamond stack. The measured neutron energy spectra were obtained by summing the energy of the recoil protons deposited in the diamond stack after the coincidence of the recoil protons identified by the time coincidence analysis. The neutron energy peaks measured by the compact D-T neutron spectrometer were almost in agreement with those obtained by the Monte Carlo N-Particle transport (MCNP) simulation. The energy resolution of the compact D-T neutron spectrometer was emulated to be about 4%-5% in D-T neutron measurement. In future work, the design of the compact D-T neutron spectrometer would be optimized to measure the fusion neutron for plasma diagnostics.

Preliminary study on the application of fluorescence intensity ratio method in fast neutron energy measurement

Preliminary study on the application of fluorescence intensity ratio method in fast neutron energy measurement

Design, modeling, and initial testing of a multi-volume neutron spectrometer

A detector system for neutron spectroscopy and imaging was modeled in Monte Carlo N-Particle transport code (MCNP) to examine the possibility of incident neutron energy measurement (spectroscopy) from an incident direction (imaging) in a hand-held detector system. Experiments using a Pu–Be (plutonium beryllium) neutron source were also conducted. The system is composed of three separate scintillation materials; a hydrogenous neutron scintillation material (BC-501A) and Cs2LiYCl6:Ce (CLYC) separated by Lu2(1-x)Y2xSiO5(LYSO). Light production location and magnitude would be collected using multiple silicon-based photo-multiplier arrays, allowing for spectroscopy and imaging of incident neutrons. Modeled neutron interaction criteria are collected and cataloged through the particle tracking supplemental code (PTRAC) of MCNP 6.2 for analog interactions. Implementing classical neutron kinematics and capture-gated time-of-flight, simulations show that neutron energy is measurable in experiment and that there is position sensitivity to be realized in experiment with the proposed detection architecture. The modeled design was experimentally evaluated with a Pu–Be source, confirming that the spectroscopic capabilities of the detector are functional and closely match the modeled expectation. In this work, we direct our attention to neutron spectroscopy, but the imaging capabilities will be addressed in further work.

Target mass dependence of photoneutron spectrum for 16.6 MeV photons on medium-heavy mass targets

We measured neutron energy spectra for the photonuclear reactions of 16.6 MeV linearly polarized photons on Pb, Au, Sn, Cu, Fe and Ti. The spectra for each target show dependence on the target mass, as well as on the angle between the directions of neutron emission and polarization. The components show angular dependence were extracted from the spectra for each target and compare with the excited levels of residual nuclei. Based on the data, we discuss a plan for further experiments to clearly observe the features of the spectra.

SuperFGD prototype time resolution studies

The SuperFGD detector will be a novel and important upgrade to the ND280 near detector for both the T2K and Hyper-Kamiokande projects. The main goal of the ND280 upgrade is to reduce systematic uncertainties associated with neutrino flux and cross-section modeling for future studies of neutrino oscillations using the T2K and Hyper-Kamiokande experiments. The upgraded ND280 detector will be able to perform a full exclusive reconstruction of the final state from neutrino-nucleus interactions, including measurements of low momentum protons, pions and for the first time, event-by event measurements of neutron kinematics. Precisely understanding the time resolution is critical for the neutron energy measurements and hence an important factor in reducing the systematic uncertainties. In this paper we present the results of time resolution measurements made with the SuperFGD prototype that consists of 9216 plastic scintillator cubes (cube size is 1 cm3) readout with 1728 wavelength-shifting (WLS) fibers along the three orthogonal directions. We used data from a muon beam exposure at CERN. A time resolution of 0.97 ns was obtained for one readout channel after implementing the time calibration with a correction for time-walk effects. The time resolution improves with increasing energy deposited in a scintillator cube, improving to 0.87 ns for large pulses. Averaging two readout channels for one scintillator cube further improves the time resolution to 0.68 ns implying that signals in different channels are not synchronous. In addition the contribution from the time sampling interval of 2.5 ns is averaged as well. Most importantly, averaging time values from N channels improves the time resolution by ∼ 1/√(N). For example, averaging the time from 2 scintillator cubes with 2 fibers each improves the time resolution to 0.47 ns which is much better than the intrinsic electronics time resolution of 0.72 ns in one channel due to the 2.5 ns sampling window. This indicates that a very good time resolution should be achievable for neutrons since neutron recoils typically interact with several scintillator cubes and in addition produce larger signal amplitudes than muons. Measurements performed with a laser and a wide-bandwidth oscilloscope in which the contribution from the electronics time sampling window was removed demonstrated that the time resolution obtained with the muon beam is not far from the theoretical limit. The intrinsic time resolution of a scintillator cube and one WLS fiber is about 0.67 ns for signals of 56 photo electrons which is typical for minimum ionizing particles.

Simultaneous measurement of energy spectrum and fluence of neutrons using a diamond detector

Due to the excellent radiation hardness and high–temperature endurance, diamond detectors are suitable for intense neutron measurements and promising for neutron diagnostics of scientific fusion devices. In the present work, simultaneous measurement of energy spectrum and fluence of neutrons using a diamond detector was realized for the first time. The absolute response matrix of the diamond detector was simulated based on detailed analysis of the nuclear reactions and the proper selection of nuclear reaction data. Neutron energy spectra as well as neutron fluences for 5.0, 5.5, 8.5, 9.5 and 10.5 MeV neutrons from d–d reaction were measured using the diamond detector based on the absolute response matrix. The measured neutron energy spectra and neutron fluences are reasonable compared with those detected using a EJ-309 liquid scintillator and a 238U fission chamber, respectively, which verifies the reliability of the present work. Furthermore, the energy spectrum and fluence of a 14.2 MeV d–t neutron source were also measured using the diamond detector. The present work demonstrates the ability of simultaneous measurement of energy spectrum and fluence as well as for both d–d and d–t neutrons using a diamond detector, which is of great significance for neutron diagnostics of scientific fusion devices.

Time Projection Chambers instrumented with resistive MicroMegas for the SAND near detector of DUNE

The Deep Underground Neutrino Experiment (DUNE) is a next-generation long-baseline neutrino accelerator experiment aiming for precise measurements of the neutrino oscillation parameters. DUNE will include a near detector complex regrouping three different detectors among which SAND (System for on-Axis Neutrino Detection) that will be the only one permanently on the neutrino beam axis in charge of monitoring in detail the emitted neutrino beam and its stability through time, a crucial characteristic to realize accurate oscillation measurements at the percent level. SAND will reuse the superconducting magnet and the electromagnetic calorimeter of the KLOE experiment. We will describe in the following the proposal of using, as inner tracker of SAND, a large 3D matrix of 1.5cm side scintillator cubes (3DST) surrounded by 3 gaseous Time Projection Chambers. This setup allows to realize accurate beam monitoring combining the 3DST unprecedented capability of neutron detection and energy measurement with the high precision momentum resolution for charged particles offered by the TPCs. The proposed TPC design allows to reach spatial resolutions of a few hundreds of micrometers using 1 cm pads by deploying the resistive MicroMegas technology for the charge readout.

CHARACTERIZATION AND COMPARSION OF NEUTRON GENERATORS OF IEC AND LINEAR D-T BY THE SPECTROMETRIC SYSTEM NGA-01

This article focuses on description of two different neutron fields from linear and cylindrical Inertial Electrostatic Confinement (IEC) neutron generators. Both of these generators are well defined and commonly used. They use a deuterium-tritium reaction that produces neutrons with energies in the range 13 – 16 MeV, depending on the direction and the energy of the incoming deuterium nucleus. Two-parametric spectrometric system for neutron/gamma mixed fields NGA-01 was used to characterize neutron spectra in the proximity of generators. The cylindrical 45x45 mm stilbene scintillator was connected to this device using an active voltage divider. This way, we were able to measure neutron energies in the range 1 - 15 MeV while filtering out gamma radiation, even when counts per second is high. For the neutron spectrum calculation recoil spectra using deconvolution through maximum likelihood estimation was used. Measured neutron spectra have been compared with simulations realized via MCNP6. According to the theoretical prediction, these two types of generators produce different neutron fields. In case of the linear generator the target is very close to point located tritium bombarded by deuterons. Thus the neutron spectrum varies depending on the angle between the detector axis and the axis of the generator. Both experimental results and simulation show a shift of the neutron energy peak in pulse height histogram. For IEC type generators the neutron spectrum is more complicated. The shape and the position of the neutron energy peak depend heavily on the position of the detector. The most prominent effect is in the position in the plane perpendicular to the generator axis. In this case, the peak splits into two peaks that can be measured and distinguished. These results were verified by the diamond detector which was also used for characterization of the IEC type generator.

Plasma focus neutron energy and anisotropy measurements using zirconium–beryllium​ pair activation detectors

Zirconium and beryllium fast-neutron activation detectors are used to investigate the fusion neutron emission from the NX3 Plasma Focus (PF) device operated in deuterium gas, with 7.2 kJ bank energy. Both Zr and Be activation cross-sections increase with energy, but with different trends, enabling an effective neutron energy Eneff to be inferred from the Zr/Be count ratio. The relationship between Eneff and Zr/Be count ratio is established by MCNP5 simulation. Compact Zr/Be detector pairs were positioned at 0° and 90° to the PF axis, permitting measurements of neutron yield, energy- and fluence-anisotropy to be made for each shot. Series of NX3 shots were performed for D2 gas pressures ranging from 1.5 to 10 mbar. Typical effective neutron energies Eneff for the 0° and 90° directions are found to be ∼2.8 MeV and ∼2.5 MeV, respectively. The highest neutron yields of ∼109 neutron/shot were observed for 5 mbar D2 gas pressure. Neutron fluence-anisotropy AnBe for individual shots ranged from ∼2.5 to ∼4.5. The mean value AnBe exhibits a steady decline with increasing D2 gas pressure. By contrast, neutron energy-anisotropy ΔEn remains almost constant as the D2 gas pressure is varied. The effect of blocking the forward D+ ion beam with an obstacle plate positioned 6 cm in front of the anode tip is also studied. Marked reductions in both neutron yield and fluence-anisotropy are observed, whilst the effective neutron energy Eneff increases slightly at both 0° and 90° directions. Fusion contributions from thermonuclear or gyrating-particle processes are found to be negligible. All results are completely consistent with a straightforward beam–target model of PF fusion.

Characterization of two extraordinary AmLi neutron sources

Two legacy AmLi sources have been actively used across several projects within Pacific Northwest National Laboratory, however, applicability of source measurement data has been substantially limited due to the lack of original manufacturer documentation and cross comparison to other sources. This paper summarizes measurements conducted to better quantify source radionuclide content, neutron energy spectrum and emission rate of the AmLi sources. The measured neutron energy spectra of AmLi sources produced by Monsanto Research Corporation have not been found in open literature before and therefore the focus of the study was to obtain the high definition energy distribution of the sources. The initial gamma spectrometry measurements, aimed to estimate the radionuclide content and potential impurities, revealed a significant contamination of the source material with beryllium. Consequent neutron spectrum measurements confirmed beryllium contamination through the presence of a high energy tail contributing up to 17% to the total neutron emission rate. The obtained lithium-only portion of the measured source energy distribution was compared against other AmLi spectra discussed in the past and recent literature. Determination of the total neutron yields of the sources was made with measurements in a neutron well counter. Detection efficiency of the well counter was calculated specifically for these sources via detailed Monte Carlo simulations based on the obtained neutron energy distribution.

Thick target neutron yields from LiF, C, Si, Ni, Mo, and Ta bombarded by 6.7 MeV/u deuterons

Double-differential thick target neutron yields from the bombardment of LiF, C, Si, Ni, Mo, and Ta targets by 6.7 MeV/u deuterons were measured at forward angles using an EJ-301 liquid organic scintillator. The neutron energy spectra were derived via an unfolding method using the FORIST code with the detector response function calculated using the SCINFUL-QMD code. The neutron energy spectra and total neutron yields at 0∘ from the deuteron incidences were compared with the previous data of triton incidences on LiF, Si, Ni, Mo, and Ta targets measured at the same incident energy per nucleon. The comparison revealed that deuterons yield more neutrons for low-Z targets than tritons, and less neutrons for high-Z targets. The measured neutron energy spectra were compared with model calculations using the DEUteron-induced Reaction Analysis Code System (DEURACS). DEURACS successfully reproduced the neutron spectra from all the targets. For the triton incidence, some calculation codes were benchmarked, with JQMD/GEM and CCONE yielding good agreement with the measured neutron spectra at 0∘.

Neutron transmission studies for concrete used in the nuclear industry

Progress in the establishment of a fast neutron beam reference facility for the non-destructive testing of concrete and other materials used in the nuclear industry is described. An additional area of interest is the development of methods for the independent verification of the alignment to regulatory codes of the constituent materials used in concrete mixes for nuclear facilities, both existing and planned. The project is based on the principle of analyzing the distributions in energy of the neutrons transmitted through the sample. Modern methods of spectrum unfolding allow such analyses to be undertaken without the need for the neutron beam to be ns-pulsed in order to measure neutron energies by time-of-flight. First measurements and analyses of mortar (concrete without large aggregate) using continuous beams of fast neutrons are presented.

Study of Secondary Scattering/Albedo Neutron Fields and Their Impacts on SER as Function of Scene Topologies

This article describes a series of experimental measurements and calculations designed to quantify the scattering and albedo effects on neutron spectra and their consequences on single-event effect (SEE) sensitivities. An experimental approach was proposed to investigate the scattering and albedo neutron spectra using a 252Cf facility and for several scene topologies. A highly sensitive neutron spectrometer based on a Bonner sphere system (BSS) is used, allowing to measure neutron energy from thermal to tens of megaelectron-volts. To complete previous SEE measurements on 45 nm, the soft error rate (SER) was recorded on 28-nm technology. A second part considers complex topologies including concrete walls and high-density polyethylene (HDPE) blocks to represent a design facility and the shielding to protect some parts of the SEE test board, respectively. The results showed that devices are more impacted by the presence of materials, especially HDPE. Indeed, almost 25% of the events are due to the secondary neutrons. Orders of magnitude are not critical, nevertheless, they warn about taking into account the albedo contributions and how to add materials in the vicinity of measurement devices.

Conceptual design and first results for a neutron detector with interaction localization capabilities

A new high-precision detector for studying neutrons from beta-delayed neutron emission and direct reaction studies is proposed. The Neutron dEtector with Xn Tracking (NEXT) array is designed to maintain high intrinsic neutron detection efficiency while reducing uncertainties in neutron energy measurements. A single NEXT module is composed of thin segments of plastic scintillator, each optically separated, capable of neutron-gamma discrimination. Each segmented module is coupled to position sensitive photodetectors enabling the high-precision determination of neutron time of arrival and interaction position within the active volume. A design study has been conducted based on simulations and experimental tests leading to the construction of prototype units. First results from measurements using a $^{252}$Cf neutron source and accelerator-produced monoenergetic neutrons are presented.

Feasibility study on neutron energy spectrum measurement utilizing prompt gamma-rays

Abstract A new approach based on Prompt Gamma-ray Neutron Activation Analysis (PGNAA) was proposed for measuring neutron energy spectra . A system, which consisted of a high purity germanium (HPGe) detector and a multi-layer sample, was used to validate the method. The composite layers of the sample comprised of lead, polymethyl methacrylate, sodium chloride powder and borated polyethylene . Prompt gamma-rays emitted from Pb, H, O, C, Cl and B were selected to determine the incident neutron energy spectrum. The response function for the production of gamma rays from these elements by monoenergetic neutrons was obtained through Monte Carlo simulations. Then the neutron fields of Cf-252 neutron source and D-T neutron generator were investigated with this approach. With the measured prompt gamma-rays and the response matrix, the neutron spectra were obtained through the PSO algorithm. The unfolded results have a good agreement with the simulation spectrum.

Calibration of a neutron time-of-flight detector with a rapid instrument response function for measurements of bulk fluid motion on OMEGA.

A newly developed neutron time-of-flight (nTOF) diagnostic with a fast instrument response function has been fielded on the OMEGA laser in a highly collimated line of sight. By using a small plastic scintillator volume, the detector provides a narrow instrument response of 1.7 ns full width at half maximum while maintaining a large signal-to-noise ratio for neutron yields between 1010 and 1014. The OMEGA hardware timing system is used along with an optical fiducial to provide an absolute nTOF measurement to an accuracy of ∼56 ps. The fast instrument response enables the accurate measurement of the primary deuterium-tritium neutron peak shape, while the optical fiducial allows for an absolute neutron energy measurement. The new detector measures the neutron mean energy with an uncertainty of ∼7 keV, corresponding to a hot-spot velocity projection uncertainty of ∼12 km/s. Evidence of bulk fluid motion in cryogenic targets is presented with measurements of the neutron energy spectrum.

Neutron energy measurement for practical applications

Industrial demand for neutrons constrains careful energy measurements. Elastic scattering of monoenergetic \(\alpha \)-particles from neutron collision enables neutron energy measurement by calculating the amount of deviation from the position where collision takes place. The neutron numbers with specific energy is obtained by counting the number of \(\alpha \)-particles in the corresponding location on the charged particle detector. Monte Carlo simulation and COMSOL Multiphysics5.2 are used to account for one-to-one collision of neutrons with \(\alpha \)-particles.

Level density and thermodynamics in the hot rotating Tc96 nucleus

Evaporated neutron energy spectra have been measured in coincidence with low-energy discrete $\ensuremath{\gamma}$ rays in the reaction $^{4}\mathrm{He}+^{93}\mathrm{Nb}$ at $E(^{4}\mathrm{He})=28$ MeV. The low-energy light-ion beam provides the scope of extracting the experimental nuclear level density (NLD) in the compound nuclear reaction. Angular-momentum gated NLDs have been extracted in the excitation energy range of ${E}^{*}\ensuremath{\sim}5--15$ MeV from the measured neutron energy spectra. The extracted NLDs have been compared with different theoretical calculations such as the exact pairing plus independent particle model at finite temperature (EP+IPM), Hartree-Fock plus BCS (HFBCS), and Hartree-Fock-Bogoliubov plus combinatorial method (HFBC). Interestingly, the experimental NLDs are in good agreement with the results of the EP+IPM, whereas the HFBCS and HFBC fail to describe these data. Consequently, the thermodynamic properties of $^{96}\mathrm{Tc}$ at finite angular momentum have been extracted using the EP+IPM NLDs. Through the analysis of the calculated thermodynamic quantities, it is shown that no pronounced bump is seen in the heat capacity of $^{96}\mathrm{Tc}$, in opposition with the earlier results of $^{96}\mathrm{Mo}$, which showed a prominent bump at $T\ensuremath{\sim}0.7--1$ MeV. This difference is understandable since pairing in the even-even system ($^{96}\mathrm{Mo}$) is always stronger than that in the odd-odd one ($^{96}\mathrm{Tc}$).

Correlations in prompt neutrons and gamma-rays from Cf-252 spontaneous fission

New event-by-event fission models have prompt neutrons and gamma-rays that are correlated in time, energy, and multiplicity, however there is limited measurement data available to validate these models. Measurement of high-order fission neutron and gamma-ray coincidences is difficult and there has previously been little motivation to measure properties of both particle types simultaneously. High-order Cf-252 spontaneous fission neutron and gamma-ray coincidences were measured with a cylindrical array of 22 liquid organic and 8 NaI(Tl) scintillation detectors, 50 cm from a central axis. Waveforms were acquired and saved for post-processing using four time-synchronized CAEN V1720 digitizers. Liquid organic scintillator waveforms were analyzed with off-line pulse shape discrimination techniques to categorize neutron and gamma-ray detections. Detected multiplicity was compared with MCNPX-PoliMi simulation results, where built-in fission models and event-by-event fission models, CGMF and FREYA, have been implemented. Additionally, measured neutron energy by time-of-flight and gamma-ray energy correlated by detected multiplicity were compared to simulated results.