Neutron Yield Measurements Research Articles

Neutron-producing gas puff Z-pinch experiments on a fast, low-impedance, 0.5 MA linear transformer driver

A study on the neutron production from single and double gas puff Z-pinches on the CESZAR linear transformer driver with ∼0.45 MA current and 170 ns rise time is presented. Total neutron yield measurements made with a LaBr activation detector are compared for three configurations, using a double nozzle setup. When a single, hollow, deuterium gas shell was used, reliable implosions could only be attained at higher load mass than the optimal value to match implosion time with the driver rise time, with neutron yields of ∼106 per pulse. The use of a double gas puff configuration with a deuterium center jet allowed a reduction in the shell density and operation closer to machine-matched conditions, recording up to (4.1 ± 0.3) × 107 neutrons/pulse when either Kr or D2 was used in the shell. For a comparable mass and implosion time, using a higher atomic-number gas in the outer shell results in more unstable plasma surface and smaller plasma radius at the location of instability bubbles, which, however, do not seem to consistently correlate with a higher neutron yield. Comparing implosion dynamics with models and neutron yields with literature scaling suggests that the machine current is not well coupled to the plasma during the final stages of compression. Optimizing current and energy coupling to the pinched plasma is critical to improving performance, particularly in low-impedance drivers.

The next-generation magnetic recoil spectrometer (MRSnext) on OMEGA and NIF for diagnosing ion temperature, yield, areal density, and alpha heating.

The next-generation magnetic recoil spectrometer (MRSnext) is being designed to replace the current MRS at the National Ignition Facility and OMEGA for measurements of the neutron spectrum from an inertial confinement fusion implosion. The MRSnext will provide a far-superior performance and faster data turnaround than the current MRS systems, i.e., a 2× and 6× improvement in energy resolution at the NIF and OMEGA, respectively, and 20× improvement in data turnaround time. The substantially improved performance of the MRSnext is enabled by using electromagnets that provide a short focal plane (12-16cm) and unprecedented flexibility for a wide range of applications. In addition to being able to measure neutron yield, apparent ion temperature, areal density, and plasma-flow velocity over a wide range of yields, the NIF MRSnext will be able to directly, uniquely assess the alpha heating of the fuel ions through measurements of the alpha knock-on tail in the neutron spectrum. The goal is to implement a radiation-hard electronic detection system capable of providing rapid data acquisition and analysis. The development of the MRSnext will also set the foundation for the more advanced, time-resolving MRSt and serve as a testbed for its implementation on the NIF.

Characteristics of a neutron source based on an electron accelerator LINAC-200

As part of the commissioning work at the linear electron accelerator LINAC-200 (JINR), we conducted experiments to study the characteristics of a pulsed neutron source obtained by irradiating a converter target with a 140 MeV electron beam. To estimate neutron source parameters and neutron energy spectra in lead and tungsten targets with different sizes numerical simulations were performed. Additionally, we have conducted experiments to measure the energy spectra of neutrons and gamma rays using high-resolution time-of-flight method. It has been found that the ratio of integral estimates between the calculated and average measured neutron yields (for tungsten and lead targets) did not exceed 30%. The fluence of resonant and thermal neutrons in the target is estimated at 2.8×1013 neutrons per second, which corresponds to the requirements for modern pulsed neutron sources.

Pulsed fast neutron yield measurements based on 79mBr activation in LaBr3(Ce)

This work aims to test the concept of an activation detector for pulsed DD fusion neutron sources, based on the production of metastable Br79m within a LaBr3(Ce) scintillator crystal via (n,n′) inelastic scattering. The pulsed neutron source employed is the NX3 Plasma Focus (PF) device operated in deuterium gas, which yields about 109 neutrons per shot. A range of D2 gas pressures, from 1 to 13 mbar are used to vary the test conditions. For the sake of comparison, a beryllium fast-neutron activation detector is used simultaneously with the LaBr3(Ce), and for each NX3 PF shot we derive neutron yield values from both Be and LaBr3(Ce) detectors, denoted YnBe and YnLaBr. The two detectors are positioned in the equatorial plane (θ=90°) of the NX3 to expose them to bursts of neutrons with energies close to 2.5 MeV, to simulate a thermonuclear DD fusion source. Overall, the shot-to-shot values of YnBe and YnLaBr obtained compare reasonably well. At each D2 gas pressures the 10-shot averaged values ⟨YnBe⟩ and ⟨YnLaBr⟩ are mostly within 10% of one another; for the worst case (10 mbar) ⟨YnLaBr⟩ is 25% higher than ⟨YnBe⟩. Overall, it is concluded that LaBr3(Ce) scintillation detectors can function as a capable and readily obtainable fast-neutron activation detector for measuring neutron yields from pulsed DD fusion sources.

Development of a neutron yield measurement system utilizing BF_3 detectors on the HL-3 tokamak

A neutron yield measurement system, comprising five BF3 proportional counters with excellent n/γ discrimination capabilities, has been meticulously developed and implemented on the HL-3 tokamak to provide time-resolved assessments of neutron emission rates. The size of moderator, optimized for the flat energy responses, was skillfully simulated and designed using the MCNP code. An in-depth analysis of measurement errors stemming from pile-up phenomena determined that the upper limit of the count rate, under the current electronics design, is 131 kcps with a relative error of less than 20%. Drawing from the pulse amplitude spectrum obtained from a 252Cf neutron source and experimentation on the HL-3 tokamak, the amplitude threshold for the BF3 neutron detector systems was judiciously established. Furthermore, it was conclusively affirmed that the neutron yield measurement system possesses the capability to effectively discriminate between neutron and gamma signals. During HL-3 discharges, deuterium-deuterium (DD) neutrons were reliably detected with a temporal resolution of 10 ms, and the variations in neutron count rates was found to be in alignment with the neutral beam injection. Ultimately, the total neutron yield was successfully estimated through the utilization of count rates from four BF3 detectors, thereby validating the robust performance of the neutron yield measurement system.

Feasibility study of an XPCI diagnostic to observe the evolution of micro-voids in an ICF target

Bulk perturbations (voids or crystalline structure) inside the ablator of a capsule used for inertial confinement fusion are seeds for instabilities that can hinder the ignition. The study of these defects and their evolution during the implosion is one of the steps needed to achieve fusion. The current methods used by the field are to infer these effects indirectly with measurements of implosion velocity and neutron yield, among others. Observing them directly with an x-ray imaging diagnostic is difficult due to the small scale length of these defects. In this work we study the feasibility of a new diagnostic based on x-ray phase-contrast imaging. This technique has been demonstrated to perform better than standard x-ray absorption techniques in critical situations like this. By using a synthetic diagnostic we show the capabilities of this new possible approach and the limits in relation to the parameters of currently available laser facilities.

Overview of interpretive modelling of fusion performance in JET DTE2 discharges with TRANSP

In the paper we present an overview of interpretive modelling of a database of JET-ILW 2021 D-T discharges using the TRANSP code. The main aim is to assess our capability of computationally reproducing the fusion performance of various D-T plasma scenarios using different external heating and D-T mixtures, and to understand the performance driving mechanisms. We find that interpretive simulations confirm a general power-law relationship between increasing external heating power and fusion output, which is supported by absolutely calibrated neutron yield measurements. A comparison of measured and computed D-T neutron rates shows that the calculations’ discrepancy depends on the absolute neutron yield. The calculations are found to agree well with measurements for higher performing discharges with external heating power above ∼20 , while low-neutron shots display an average discrepancy of around +40% compared to measured neutron yields. A similar trend is found for the ratio between thermal and beam-target fusion, where larger discrepancies are seen in shots with dominant beam-driven performance. We compare the observations to studies of JET-ILW D discharges, to find that on average the fusion performance is well modelled over a range of heating power, although an increased unsystematic deviation for lower-performing shots is observed. The ratio between thermal and beam-induced D-T fusion is found to be increasing weakly with growing external heating power, with a maximum value of 1 achieved in a baseline scenario experiment. An evaluation of the fusion power computational uncertainty shows a strong dependence on the plasma scenario type and fusion drive characteristics, varying between ±25% and 35%. D-T fusion alpha simulations show that the ratio between volume-integrated electron and ion heating from alphas is 10 for the majority of analysed discharges. Alphas are computed to contribute between ∼15% and 40% to the total electron heating in the core of highest performing D-T discharges. An alternative workflow to TRANSP was employed to model JET D-T plasmas with the highest fusion yield and dominant non-thermal fusion component because of the use of fundamental radio-frequency heating of a large minority in the scenario, which is calculated to have provided ∼10% to the total fusion power.

Demonstration of indirectly driven implosion experiments with cryogenic pure deuterium layered capsules on the Shenguang Laser Facility

Abstract To achieve ignition in a laboratory via inertial confinement fusion, a spherical capsule containing a frozen layer of deuterium and tritium (DT) fuel will be imploded on an MJ-class laser facility. However, if pure deuterium fuel can be used in place of DT fuel for tuning shots, we may speed up the process of ignition experiments while maintaining the surrogacy by significantly reducing the level of radioactivity. Unfortunately, it has long been assumed that neither the approach of symmetrical infrared irradiation used in the Omega direct-drive experiments nor the method of beta-layering used in the NIF experiments can be used to smooth the D layered capsule in cylindrical hohlraums. The difficulty in smoothing the D ice layer prevents us from taking advantage of cryogenic D-layered capsules in indirect-drive experiments. In this work, we established a procedure to form a uniform D-ice layer for capsules held in cylindrical hohlraums and carried out indirect-drive cryogenic D-layered implosion experiments using a squared laser pulse on the Shenguang Laser Facility in China. The quality of the D ice layer is characterized by phase-contrast imaging. The root-mean-square of the power spectrum in modes 2–100 is about 2.2 μm. The implosion performance of the D-layered capsules is close to the prediction of one-dimensional simulations. The measured neutron yield and areal fuel density are 1.2 × 1011 and 80 mg cm−2, respectively.

Impact of supporting substrate on the measured neutron yields from natLi (p, n) system: A case study with natC and natTa stopping targets

Abstract The high energy neutron generation facilities worldwide uses the Li(p,n) system for production of the quasi mono-energetic neutrons from epithermal to few hundreds of MeV. Among the facilities either self-standing thick Li target or Li foil supported on a target assembly is used. In case of a supported Li target, either low or high mass supporting substrates like Al, C, Au, Ta etc. can be used. However, the neutron emission from the supporting substrate can interfere with the Li(p,n) reaction neutrons. In the present study neutron yields from a thin natural Li target supported on the thick natural Ta (high Z) and C (low Z) targets were measured at incident proton energies between 8-20 MeV. The proton irradiation of targets were performed at the Pelletron accelerator Facility, Mumbai and the emitted neutrons were measures at both forward (0°) and lateral (90°) directions with respect to the incident beam. The neutron yield was estimated using the CR-39 detectors and dual scintillator based active neutron spectrometer. A neutron dose equivalent (NDE) meter was also used to measure the neutron ambient dose equivalent at the measurement point. Experimentally measured neutron yields and ambient neutron dose equivalent estimates indicated higher Li(p,n) neutron yield with carbon as the supporting substrate. This is due to fewer self-emissions from natC compared to natTa target. Carbon supporting substrate also ensures improved spectral features of the Li(p,n) emission neutrons and lesser neutron fluence around the source location. This ensures efficient implementation of radiation protection practices with carbon target due to lower neutron fluence around the source compared to tantalum target. Carbon is a favorable supporting substrate compared to high Z- metal targets like Ta for emission neutron studies on Li target at proton energies between 8-20 MeV.

Measurement of the cosmogenic neutron yield in Super-Kamiokande with gadolinium loaded water

Measurement of the cosmogenic neutron yield in Super-Kamiokande with gadolinium loaded water

Measurement of neutron yield for a medical linear accelerator below 10 MV.

The recent trend toward 10MV for volumetric radiotherapy treatment such as volumetric modulated arc therapy (VMAT), stereotactic radiosurgery (SRS), and stereotactic ablative body radiotherapy (SABR) introduces photoneutron production, with implications for non-therapeutic patient dose and additional shielding requirements for treatment room design. The sharply nonlinear drop-off in photoneutron production below 10MV to negligible at 6MV has scarcely been characterized quantitatively, yet can elucidate important practical insights. To measure photoneutron yields in a medical linac at 8MV, which may strike a reasonable balance between usefully increased beam penetration and dose rate as compared to 6MV while reducing photoneutron production which is present at 10MV. A Varian iX linear accelerator undergoing decommissioning at our clinic was made to operate over a range of photon energies between 6 and 15MV by calibrating the bending magnet and adjusting other beam generation parameters. Neutron dose within the treatment room was measured using an Anderson-Braun type detector over a continuum of intermediate energies. The photoneutron production for energies below 10MV was measured, adding to data that is otherwise scarce in the literature. Our results are consistent with previously published results for neutron yield. We found that the photoneutron production at 8MV was about 1/10 of the value at 10MV, and about 10 times higher than detector background at 6MV. Photoneutron production drops off below 10MV, but is still present at 8MV. An 8MV beam is more penetrating than a 6MV beam, and may offer a suitable tradeoff for modern radiotherapy techniques such as VMAT, SRS, and SABR. Further studies are needed to better understand the impact on treatment plan quality between 8 and 10MV beams considering the benefits to facility requirements and non-therapeutic patient dose.

Measurement of thick target neutron yield from 80.5 MeV/u 12C incidence on Be, C, W, and Pb targets

Thick target neutron yield produced from 80.5 MeV/u 12C ions stopping in the thick Be, C, W, and Pb targets were measured at 0°, 37°, 60° using liquid scintillation detector and the time of flight method. The experimental data were compared with those calculations using the Liege IntraNuclear-Cascade (INCL) implemented in Geant4 and the JAERI Quantum Molecular Dynamics 2.0 (JQMD-2.0) model followed by the Generalized Evaporation Model (GEM) implemented in PHITS, and default model implemented in FLUKA. The results of the three simulation codes with different physics models are similar for beryllium and carbon targets and in good agreement with the experimental data. For tungsten and lead targets, the calculation results from PHITS and FLUKA successfully reproduced the neutron spectra. While the results from GEANT4 overestimate the experimental data above 50 MeV at 0° and significantly underestimated experimental data in the 15–60 MeV energy range at 0°, 37° and 60°. In general, the simulation results of PHITS with the JQMD-2.0 physical model are in good agreement with the experimental data for all targets.

Neutronic simulations of in-vessel neutron calibrations for ITER neutron diagnostics by using simplified ITER model

The absolute calibration of the detection efficiency for the measurement of the total neutron yield in the whole plasma is one of the most important issues in neutron diagnostics. In ITER, a compact deuterium-tritium (D-T) neutron generator will be used as a neutron source for the neutron calibration prior to the D-T plasma experiments, where the neutron source will be moved in the vacuum vessel. Neutronic simulations of neutron diagnostic calibrations have been carried out for the strategy and scheduling of the calibration experiments. We have established a simplified 360° ITER model for the simulation, which includes neutron flux monitors (NFMs) in an equatorial port (EQ), micro fission chambers, diverter neutron flux monitors, and a neutron activation system. At first, angular neutron spectra of the compact D-T neutron generator have been evaluated by MCNP calculations. We evaluate the discrepancies among the detection efficiencies to be obtained by the neutron calibration experiment, those by idealistic D-T ring source, and actual detection efficiencies for the plasma neutron source. Point efficiency measurement using the compact D-T neutron generator facing NFM in EQ#1 is an effective method for the NFM calibration.

Improving symmetry tuning with I-raum in indirect-driven implosions

Indirectly driven inertial confinement fusion implosions using a three-step-shaped pulse are performed at a 100 kJ laser facility. At late time of the pulse, deposition of laser energy and distribution of X-ray radiation are significantly disturbed by motion of gold plasma in the original gas-filled cylindrical hohlraum with gold wall. As a result, owing to the lack of X-ray drive at the equator of the capsule, an unacceptable oblate implosion is produced. In the I-raum modified from the above cylindrical hohlraum, the initial positions of outer laser spots and gold bubbles are appropriately shifted to modify the disturbed radiation distribution due to plasma evolution, resulting in a spherically symmetric drive on the capsule. In the implosion shots with almost the same drive pulse, owing to improved symmetry, an spherical hotspot is observed in the new I-raum, and YOS (the ratio of measured neutron yield over simulated one) is up to 30%, while an oblate hotspot is observed in the cylinder, and YOS is only 13%. The simulation calculations and experimental measurements show that the I-raum can be used to significantly reduce the impact of gold bubble expansion in the three-step-shaped pulse driven implosion, which helps to tune the drive and implosion symmetry, and to improve its over-all performance.

Neutronic analysis and measurement performance assessment of ITER neutron flux monitor system

The ITER Neutron Flux Monitor (NFM) system consists of four subsystems located at ITER Equatorial Ports 1#, 7#, 8#, and 17#. NFM system carries out the measurement of total neutron yield and fusion power by employing 235U fission chambers. The total neutron yield to be measured spans a range of 7 orders of magnitude — from 1014 to 3.2 × 1020 n/s. Suitable moderator and detectors were selected to cover such a wide range of neutron yield and meet the requirements of time resolution and accuracy. Moderators and detectors’ sensitivities of NFM system are reasonably designed. The detectors’ sensitivities of the NFM#01 system under different moderators have been calculated with MCNP code, and graphite serves as the optimal moderator. The detectors’ sensitivities of the other three NFM subsystems have also been simulated simultaneously. Based on the required sensitivities of detector, the appropriate mass of 235U was selected for each fission chamber. According to the analysis of the measurement range of each NFM subsystem, it is finally confirmed that the measurement performance of the NFM system can meet the requirements well.

A new neutron time-of-flight detector for yield and ion-temperature measurements at the OMEGA Laser Facility.

A new neutron time-of-flight (nTOF) detector for deuterium-deuterium (DD)-fusion yield and ion-temperature measurements was designed, installed, and calibrated for the OMEGA Laser Facility. This detector provides an additional line of sight for DD neutron yield and ion-temperature measurements for yields exceeding 1 × 1010 with higher precision than existing detectors. The nTOF detector consists of a 90-mm-diam, 20-mm-thick BC-422 scintillator and a gated Photek photomultiplier tube (PMT240). The PMT collects scintillating light through the 20-mm side of the scintillator without the use of a light guide. There is no lead shielding from hard x rays in order to allow the x-ray instrument response function of the detector to be measured easily. Instead, hard x-ray signals generated in implosion experiments are gated out by the PMT. The design provides a place for glass neutral-density filters between the scintillator and the PMT to avoid PMT saturation at high yields. The nTOF detector is installed in the OMEGA Target Bay along the P8A sub-port line of sight at a distance of 5.3m from the target chamber center. In addition to DD measurements, the same detector can be used to measure the neutron yield and ion temperature from deuterium-tritium (DT) implosion targets in the 5 × 1010 to 2 × 1012 yield range. The design details and the calibration results of this nTOF detector for both D2 and DT implosions on OMEGA will be presented.

Measurement of muon-induced neutron yield at the China Jinping Underground Laboratory * *Supported in part by the National Natural Science Foundation of China (11620101004, 11475093, 12127808), the Key Laboratory of Particle & Radiation Imaging (TsinghuaUniversity), the CAS Center for Excellence in Particle Physics (CCEPP), and Guangdong Basic and Applied Basic Research Foundation (2019A1515012216). Portion of this work performed at Brookhaven

Solar, terrestrial, and supernova neutrino experiments are subject to muon-induced radioactive background. The China Jinping Underground Laboratory (CJPL), with its unique advantage of a 2400 m rock coverage and long distance from nuclear power plants, is ideal for MeV-scale neutrino experiments. Using a 1-ton prototype detector of the Jinping Neutrino Experiment (JNE), we detected 343 high-energy cosmic-ray muons and (7.86 3.97) muon-induced neutrons from an 820.28-day dataset at the first phase of CJPL (CJPL-I). Based on the muon-induced neutrons, we measured the corresponding muon-induced neutron yield in a liquid scintillator to be μ−1g−1cm2 at an average muon energy of 340 GeV. We provided the first study for such neutron background at CJPL. A global fit including this measurement shows a power-law coefficient of (0.75 0.02) for the dependence of the neutron yield at the liquid scintillator on muon energy.

Neutron yield measurement system of HL-2A tokamak

This research presents the development of HL-2A neutron yield measurement which includes 235U fission chamber and BF3 and 3He proportional counters. Equivalent noise formula of the radiation detection signal amplification system was derived to guide the development of the signal amplification system. Then all detectors were calibrated in situ by using the 252Cf neutron source. The neutron yield of the HL-2A during neutral beam heating was analyzed. These results indicate that the developed neutron flux diagnostic system can obtain neutron yield results under various experimental conditions of the HL-2A tokamak, and can provide information on neutron yield.

Thick target neutron spectral yield and dose measurements with [formula omitted]C (p, n) system at intermediate proton energies between 8–20 MeV

The study of neutron parameters from a thick natural graphite target is of immense importance for obtaining the background neutron energy and yield corrections in case of proton irradiation of low Z targets, where graphite is used predominantly as the supporting target material. There are few measurements with thick graphite target reported in literature at proton energies up to 12 MeV and some at higher energies beyond 20 MeV. The present study focuses on the thick target neutron yield, dose and spectra measurements from thick natC(p,n) system at intermediate proton energies between 8–20 MeV. The experimental measurements were performed at 0°and 90°with respect to the incident beam direction. The generated data will complement the discontinuity in the neutron yield and spectral measurements, in the proton energy range of 12–20 MeV. These dosimetric estimates in the proton energy range of 8–20 MeV will be significant towards the personal radiation protection of occupational workers and neutron shielding evaluations.

Neutron diagnostic system at the Globus-M2 tokamak

A neutron diagnostic system was developed at the Ioffe Institute as part of the Globus-M2 tokamak to optimize NBI heating conditions and evaluate heating efficiency. The system contains two compact neutron spectrometers based on the liquid organic scintillator BC-501A and two gas-discharge counters based on a 10B isotope. The BC-501A spectrometers were calibrated by measuring neutron emission produced in a 9Be(α,n)12C nuclear reaction on the cyclotron facility at the Ioffe Institute. In addition, in situ calibrations of the system, including the neutron spectrometers and the gas-discharge counters, was carried out using an Am–Be neutron source to provide accurate measurements of the total neutron yield from the plasma of the Globus-M2 tokamak. During the plasma experiments at the Globus-M2 tokamak, a deuterium beam was injected into the deuterium plasma that causes a yield of the DD-neutrons with ∼2.45 MeV energy. The neutron spectrometry diagnostic system was used to provide neutron measurements and detect the DD-neutrons in these experiments. The neutron yield and the DD-reaction rate during plasma discharges were evaluated. The energy distributions of neutrons emitted from plasma during discharges with neutron beam injection were reconstructed from the measured neutron spectra.