Neutron Yield Research Articles

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.

Measurement of cross-section of the 6Li(d,α)4He, 6Li(d,p)7Li, 6Li(d,p)7Li*, 7Li(d,α)5He, and 7Li(d,nα)4He reactions at the deuteron energies from 0.3 MeV to 2.2 MeV

The interaction of a deuteron beam with lithium is characterized by a high yield of neutrons, high energy of neutrons and a large variety of reactions. The reliable data of the reactions cross-sections are important for many applications, including radiation testing of advanced materials and equipment. The experimental data on the cross-sections differ greatly from one author to another; for a number of reactions there is no data on the cross-section in the databases. Measurements of the reactions cross-sections were carried out at the accelerator-based neutron source VITA at Budker Institute of Nuclear Physics (Novosibirsk, Russia) using an α-spectrometer. The 6Li(d,α)4He, 6Li(d,p)7Li, 6Li(d,p)7Li*, 7Li(d,α)5He, and 7Li(d,nα)4He reactions cross-sections at deuteron energies from 0.3 MeV to 2.2 MeV have been measured. The obtained data are presented tabular form.

NEBOAS: A Neutron yiElds Based On AcceleratorS application

The web-based application NEBOAS was developed to calculate the neutron yield of the accelerator-based neutron source MONNET (MONo-energetic NEutron Tower) at JRC-Geel. Neutrons are produced with p and d-induced reactions on particular targets. NEBOAS provides double differential neutron yields, as a function of the angle of neutron emission and energy. It provides also integrated neutron yields, and neutron energy spectra. Any projectile energy may be utilized, as long as it is covered by the nuclear data available on thin targets (that just degrade the projectiles' energy) or thick targets (that stop the projectiles). The calculation employs reaction cross-section evaluations from Evaluated Nuclear Data File (ENDF) or compilations of experimental measurements from the Experimental Nuclear Reaction Data (EXFOR), and stopping powers as recommended in the National Institute of Standards and Technology (NIST) or the Stopping and Range of Ions in Matter (SRIM-2013) databases.

Optimized laser production of thermonuclear neutrons from plasma of submicron-sized clusters

The concept of maximizing the D-D fusion neutron yield from the laser-heated large volume of cluster medium by matching the focal spot size and cluster plasma structural scales to the laser pulse intensity was confirmed. For this purpose, the three-dimensional particle-in-cell GEANT4 simulations have been performed by zoning of the large interaction domain. While considering a small domain of the entire interaction volume, which is partitioned into successive zones along laser propagation direction, a special algorithm was proposed allowing to reconstruct the integral spectrum of deuterons and D-D neutron yield. We demonstrate that it makes possible to specify high-performance laser–cluster neutron source following this concept. For example, for the submicron heavy water droplets heated by femtosecond laser pulse of the intensity 3×1019 W/cm2 a D-D neutron yield may reach 107 neutrons per 1 J of deposited laser energy if the intensity contrast ratio prevents premature cluster destruction. Such yield is considerably higher than achieved to date for microstructured targets.

Mitigation strategies for time-resolved x-ray diagnostic data degradation due to high neutron yield of ICF experiments at the NIF.

Inertial confinement fusion experiments taking place at the National Ignition Facility are generating ever increasing amounts of fusion energy, with the deuterium tritium fusion neutron yield growing a hundredfold over the past ten years. Strategies must be developed to mitigate this harsh environment's deleterious effects on the operation and the performance of the time-resolved x-ray imagers deployed in the National Ignition Facility target bay to record the dynamics of the implosions. We review the evolution of these imagers in recent years and detail some of the past and present efforts undertaken to maintain or improve the quality of the experimental data collected on high neutron yield experiments. These include the use of a dump-and-read electronic backend, the selection of photographic film with a low background sensitivity, and the optical filtering of Cherenkov radiation.

A modular, high dynamic range passive neutron dosimeter and imaging diagnostic.

The multi-decade neutron dosimeter and imaging diagnostic (MDND) is a passive diagnostic that utilizes the polyethylene (n, p) nuclear reaction to enhance the diagnostic's sensitivity for time and energy integrated neutron measurements in the range of 2.45-14.1MeV. The MDND utilizes a combination of radiochromic film, phosphor image plates, and solid-state nuclear track detectors, with the goal of providing several orders of magnitude of dynamic range in terms of measured neutron fluence. The diagnostic design was guided by simulations in the Monte Carlo N-Particle (MCNP) transport code to determine the optimum thickness of the polyethylene convertor for maximum proton fluence incident on the detection medium as a function of incident neutron energy. In addition, the simulation results of complete diagnostic assemblies, or "stacks," were used to determine the total dynamic range of an MDND in terms of measured neutron source yield, which was found to be between around 107 and 1015 emitted into 4π with the detector located 1m away from the source. Complimentary to these simulations, individual detectors within a stack were simulated and analyzed to determine response as a function of neutron energy and yield. This work presents the diagnostic design, MCNP simulation results, and analysis of expected signals for varying neutron sources.

Novel deuterium-tritium (DT) neutron activation diagnostics for increased neutron yield operations at Sandia's Z-facility (invited).

As inertial confinement fusion experiments at Sandia National Laboratories' Z-facility begin using deuterium-tritium (DT) fuels, the expected DT-neutron yields are expected to increase many orders of magnitude. This brings about the need for diagnostics that have a significantly higher dynamic range of operation and ability to obtain yields where there will be significant radiation holds. Using the current activation diagnostic infrastructure at the Z-facility, a pair of activation diagnostics are being developed. The first is fluorine activation, which will be used in a similar manner to current copper activation diagnostics that are fielded in a rabbit for quick removal from the experiments without opening the vacuum experiment containment. The second is a prompt DT-neutron detector that exploits the activation of oxygen with similar operation characteristics as the current Be-probes fielded at Z. We will discuss the design of this pair of instruments, along with calibration work performed at Sandia's Ion Beam Laboratory in support of the development and anticipated instrument performance.

Preliminary study of a compact pulsed deuterium-deuterium neutron generator with a vacuum arc ion source and a linear induction accelerator

The pulsed neutron beam combined with the time-of-flight (TOF) technique has shown great value in many scientific research and application fields. Most of the neutron-based material identification and interrogation systems require an intense pulsed neutron beam. Time resolved prompt gamma neutron activation analysis (T-PGNAA) is an effective analytical method to greatly suppress interference from the other elements or the neutron shields of the detectors. To develop T-PGNAA method in a compact accelerator neutron source, a novel intense pulsed deuterium-deuterium (D-D) neutron generator based on a linear induction accelerator (LIA) was introduced. The generator has four major components: a vacuum arc ion source with deuterated electrode, a diode accelerator structure, a self-biased voltage secondary electron suppressed structure and a deuterated metal target. The present study characterizes the performance of the neutron generator with respect to neutron yield, the ionic current, beam profile, and beam composition as a function of the acceleration voltage at various discharge currents. Monoenergetic neutrons (2.5 MeV) are produced with a yield of ∼104 n/p and a pulse width of ∼100 ns using about 200 kV of induction acceleration voltage at 10−4 Pa vacuum environment. In this study, we will discuss various physical and technical issues related to the components of the neutron generator and the operation and merits of the novel neutron generator.

Compression and acceleration of ions by ultrashort, ultraintense azimuthally polarized light.

An efficient plasma compression scheme using azimuthally polarized light is proposed. Azimuthally polarized light possesses a donutlike intensity pattern, enabling it to compress and accelerate ions toward the optical axis across a wide range of parameters. When the light intensity reaches the relativistic regime of 10^{18}W/cm^{2}, and the plasma density is below the critical density, protons can be compressed and accelerated by the toroidal soliton formed by the light. The expansion process of the soliton can be well described by the snowplow model. Three-dimensional particle-in-cell simulations show that within the soliton regime, despite the ion density exceeding ten times the critical density, the ions' energy is insufficient for efficient neutron production. When the light intensity increases to 10^{22}W/cm^{2}, and the plasma density reaches several tens of times the critical density, deuterium ions can be compressed to thousands of times the critical density and simultaneously accelerated to the MeV level by tightly focused azimuthally polarized light during the hole-boring process. This process is far more dramatic compared to the soliton regime and can produce up to 10^{4} neutrons in a few light cycles. Moreover, in the subsequent beam-target stage, neutron yield is estimated to exceed 10^{8}. Finally, we present a comparison with the results obtained using a radially polarized light to examine the influence of light polarization.

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.

Evaluation of the ion temperature in the WEST tokamak with ICRF heating

Accurate measurements of the ion and electron temperatures in tokamaks are essential for understanding the heating and transport properties of the plasma. While electron temperature measurements are readily available in most devices (e.g. with electron cyclotron emission or Thomson scattering diagnostics), ion temperature estimates usually require more sophisticated procedures such as charge-exchange recombination spectroscopy [1] or crystal spectroscopy [2]. In the absence of such dedicated ion temperature measurements, a technique often adopted in tokamak plasmas is to estimate the ion temperature by reverse engineering of the total neutron yield, assuming Maxwellian distributions for the ions. In the absence of external ion heating, this procedure is satisfactory since it mainly depends on the nuclear reaction cross sections and the bulk ion density, usually inferred from a combination of the electron density and the plasma dilution measurements. On the other hand, when Ion-Cyclotron Resonance Heating (ICRH) or Neutral-Beam Injection (NBI) is applied, the ion distribution functions are distorted and the Maxwellian representation used for the ion temperature estimate is not necessarily justified, typically leading to an overestimate of the thermal component of the ion temperature. By using a coupled wave / Fokker-Planck numerical solver to calculate the actual ion distribution functions under the influence of external heating and the resulting neutron yield, the fast and thermal components of the bulk ion distributions can be disentangled and a ‘thermal-only’ ion temperature consistent with the neutron measurements can be extracted. An example of this procedure for the WEST tokamak with ICRF heating will be presented and the differences between the results obtained with respect to the simplified Maxwellian-based procedure will be discussed.

Analysis and Modeling Study on the Effect of Working Parameters of Portable Neutron Tubes on Target Current

Abstract As an important parameter index of sealed neutron tubes, their high-pressure side target current directly affects neutron yield. The working parameters of a neutron tube affect the target current. Using self-developed neutron tubes, the influence of operating parameters of neutron tubes on target current was studied. The effect curves of gas pressure, anode high pressure, and target high pressure on neutron yield were obtained, and the secondary electron suppression characteristics of the target current were obtained. Further, we analyze and fit a multi-parameter coupling model for the target current, to provide a theoretical basis for establishing the working state and improving the performance of sealed neutron tubes.

Modeling ablator defects as a source of mix in high-performance implosions at the National Ignition Facility

Recent indirect drive inertial confinement fusion implosions on the National Ignition Facility (NIF) [Spaeth et al., Fusion Sci. Technol. 69, 25 (2016)] have crossed the threshold of ignition. However, performance has been variable due to several factors. One of the leading sources of variability is the quality of the high-density carbon (HDC) shells used as ablators in these experiments. In particular, these shells can have a number of defects that have been found to correlate with the appearance of ablator mix into the hot spot and a degradation in nuclear yield. These defects include pits on the ablator surface, voids in the ablator bulk, high-Z debris from the Hohlraum wall that adheres to the capsule surface, and finally the inherent granular micro-structure of the crystalline HDC itself. This paper summarizes high-resolution modeling of each of these mix sources in two recent high-performance NIF implosion experiments. The simulated impact from a range of individual capsule defects is found to be broadly consistent with the trends seen in experiment, lending credence to the modeling results and the details of the mixing process that they reveal. Interestingly, modeling of the micro-structure inherent to HDC shows that this perturbation source results in considerable mixing of the deuterium–tritium fuel with ablator material during the implosion. The reduction in fuel compression from this mix results in an approximately factor of two reduction in neutron yield in current implosions and emphasizes the importance of mitigating this significant performance degradation.

Global characterization of a laser-generated neutron source

Laser-driven neutron sources are routinely produced by the interaction of laser-accelerated protons with a converter. They present complementary characteristics to those of conventional accelerator-based neutron sources (e.g. short pulse durations, enabling novel applications like radiography). We present here results from an experiment aimed at performing a global characterization of the neutrons produced using the Titan laser at the Jupiter Laser Facility (Livermore, USA), where protons were accelerated from 23 $\mathrm {\mu }$ m thick plastic targets and directed onto a LiF converter to produce neutrons. For this purpose, several diagnostics were used to measure these neutron emissions, such as CR-39, activation foils, time-of-flight detectors and direct measurement of $^7{\rm Be}$ residual activity in the LiF converters. The use of these different, independently operating diagnostics enables comparison of the various measurements performed to provide a robust characterization. These measurements led to a neutron yield of $2.0\times 10^{9}$ neutrons per shot with a modest angular dependence, close to that simulated.

Outer shell symmetry for double shell capsules with aluminum ablators

Double shell targets are a promising potential avenue to obtain robust neutron yield at current laser facilities. Similar to single shell designs, double shells require the symmetric implosion of an ablator in order to uniformly compress and heat a fuel volume, with the goal of achieving thermonuclear burn. Significant differences between double and single shells include the usage of an aluminum ablator as well as a reverse ramp laser pulse. In addition, double shells require a different convergence than single shells for fuel ignition. Numerical implosion studies at various energies with comparisons to experimental outcomes are required to gain confidence that simulations can capture the ablator shape from subscale to full scale. The current work builds on previous implosion experiments conducted at 1-MJ laser energy to confirm achieved ablator symmetry at 1.25 and 1.5 MJ. Average ablator P2 and P4 shapes measured in these experiments are within 5% of the simulated shape, which merits the platforms for further experimental studies.

Calculation of [formula omitted] and [formula omitted] thick targets neutron yields from particle accelerator-induced nuclear reactions using PHITS code

The double-differential cross-sections, and neutron yields of the 9Be(4He,n)12C and 12C(d,n)13N reactions at various energies, have been calculated. The cross-sections for the consider nuclear reactions were obtained using PHITS code based on Monte Carlo method. The calculated data were validated against the available experimental data with varying agreement up to 30MeV. We have suggested the most suitable reaction, at certain energies, for better neutron yield and hence radiotherapy applications.

Evaluating optimal quality assurance and quality control conditions of activation measurements at the accelerator-based boron neutron capture therapy system employing a lithium target

Evaluating neutron output is important to ensure proper dose delivery for patients in boron neutron capture therapy (BNCT). It requires efficient quality assurance (QA) and quality control (QC) while maintaining measurement accuracy. This study investigated the optimal measurement conditions for QA/QC of activation measurements using a high-purity germanium (HP-Ge) detector in an accelerator-based boron neutron capture therapy (AB-BNCT) system employing a lithium target. The QA/QC uncertainty of the activation measurement was evaluated based on counts, reproducibility, and standard radiation source uncertainties. Measurements in a polymethyl methacrylate (PMMA) cylindrical phantom using aluminum-manganese (Al–Mn) foils and aluminum-gold (Al–Au) foils and measurements in a water phantom using gold wire with and without cadmium cover were performed to determine the optimal measurement conditions. The QA/QC uncertainties of the activation measurements were 4.5% for Au and 4.6% for Mn. The optimum irradiation proton charge and measurement time were determined to be 36 C and 900 s for measurements in a PMMA cylindrical phantom, 7.0 C and 900 s for gold wire measurements in a water phantom, and 54 C and 900 s at 0–2.2 cm depth and 3,600 s at deeper depths for gold wire measurements with cadmium cover. Our results serve as a reference for determining measurement conditions when performing QA/QC of activation measurements using HP-Ge detectors at an AB-BNCT employing a lithium target.

Investigation of breakup process in 9Be(α,n)12C reaction

Neutron energies and angular distributions were measured in 9Be(α,n)12C reaction for α energies of 5.5 and 6.5 MeV. Three major neutron groups were observed in the spectrum, which correspond to the ground and first two excited states of 12C. Measured data could only be explained by the TALYS calculation if reaction at more than one location within the target is considered for a given beam energy. The preferred locations are driven by the resonance energy levels existing in 13C. Neutron yield due to the 9Be breakup process was determined which is found to be 12.6 ± 0.2% and 18.4 ± 0.5% of the total reaction cross-section for 5.5 and 6.5 MeV respectively.

Influence mechanism of lithium surface oxidation on neutron yield of lithium target for accelerator-based neutron source

Lithium is generally adopted as the target to generate epithermal neutrons based on 7Li(p,n)7Be nuclear reaction for the application of accelerator-based boron neutron capture therapy (AB-BNCT). The stability of neutron yields and neutron energy spectrum are key factors for the therapeutic effects of the AB-BNCT. Owing to the active chemical properties of lithium, the surface oxidation formation of lithium may influence the soundness of the target system and the stability of neutron yields. Experimental and simulation methods were performed for a better understanding of the passivation layer formation after air exposure and the influence of the passivation layer on the neutron yield and energy spectrum. On one hand, X-ray photoelectron spectroscopy was adopted to study the chemical states of lithium after air exposure. On the other hand, particle and heavy ion transport code system (PHITS) was used to evaluate the effect of surface chemical states variation on neutron yield and energy spectrum. The simulation results indicate that the formation of Li2O with a thickness of 2 μm could mainly influence the neutron yields at the neutron emission angle of 20–90° with a maximum reduction of ∼ 10.4%.

Calculating the stopping power and neutron yield of 133CS (A,2N) 135LA reaction for the energies between (18.5-47)

In the present work, calculated the cross sections of 133CS (A,2N) 135LA reaction By using interpolated and take the cross-sections published in the international literature for the selection of the appropriate energies ground level interaction in a computer–based program (MATALAB-17.0), in steps of energies (0.5 MeV). The quasi-experimental equation was derived from tabulated results linking a specific cross-section with energies, accounting for the neutron yield in accordance with the Zeigler neutron interactions formula using the program (SRIM 2013), and accounting for the stopping power which was also drawn and tabulated by measuring the yield at two closely spaced energies E1 and E2, one can determine the average value of the integrand over this energy interval and it turns out that the values are directly proportional until the alpha energy values reach 40 million electron volts. We notice that the change in the values is very slight. The neutron yield values begin to increase until the alpha energy reaches 40 million electron volts, after which the neutron yield becomes almost constant.