Total Neutron Yield 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.

High repetition-rate 0.5 Hz broadband neutron source driven by the Advanced Laser Light Source

Neutron beams are an essential tool to investigate material structure and perform nondestructive analysis, as they give unique access to element composition, thus ideally complementing density analysis allowed by standard x-rays investigation. Laser-driven neutron sources, though compact and cost-effective, currently have lower average flux than conventional neutron sources, due to the limited repetition rate of the lasers used so far. However, advancements in laser technology allow nowadays to address this challenge. Here, we report results obtained at the Advanced Laser Light Source characterizing stable production of broadband (0.1–2 MeV) neutrons produced at a high repetition rate (0.5 Hz). The interaction of laser pulses of 22 fs duration and 3.2 J on-target energy with 2-μm-thick tantalum targets produced protons in the Target Normal Sheath Acceleration (TNSA) regime up to 7.3 MeV. These protons were subsequently converted into neutrons by (p,n) reactions in lithium fluoride (LiF). Activation measurements and bubble detectors were used to characterize neutron emissions, with a neutron fluence of up to ∼1.4×105 neutrons/shot/sr and energies mainly between a few hundred of kilo-electron volt and 2 MeV. The total neutron yield was ∼5×105 neutrons/shot. This paves the way for numerous applications, e.g., in homeland security, materials science, or cultural heritage.

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.

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.

The effect of dominance ratio on the statistical convergence of sensitivity in Monte Carlo codes

Sensitivity computation in Monte Carlo-based codes is widely used and has been enhanced significantly. Although sensitivity converges in most reactor configurations, it encounters difficulties in large core designs. We suspect that this issue might be linked to the dominance ratio. To test this hypothesis, we develop a simple benchmark to validate whether statistical convergence of sensitivity depends on dominance ratio, and if it may be linked to other factors, such as neutron energy spectrum. The benchmark’s simplicity enables us to calculate the dominance ratio analytically and use eigenmodes decomposition to compute sensitivity to total fission neutron yields, substantially lowering computational costs. This method results in a good match when compared to the direct method which serves as a reference. We clearly see regular sensitivity convergence speed behavior linking it with the number of latent generations and the dominance ratio. Therefore, we establish a formula to recommend the number of latent generations required for sensitivity to converge, thus significantly saving computational resources.

Fast Beam Driven Neutron Yield in Thermonuclear Neutron Source Plasmas

The thermonuclear fusion between fast (super-thermal) particles injected in plasma as a neutral beam and the ions of the background plasma is expected to be the main source of fusion neutrons in FNS (fusion neutron source) design based on tokamak. Neutral beam contribution in fusion reactivity and in the total neutron yield depends on the high-energy ion fraction in the integral energy distribution. NESTOR code [1] calculates nuclear fusion rates in the FNS plasma volume, taking into account an external source of high-energy fast ions. Neutral beam model reproduces in detail the actual beam structure in phase space at the injection port plane; while the fast ion distributions in magnetically confined plasma are calculated using a combination of slowing-down classical formulae and magnetic field topology in the tokamak chamber. Here we discuss the issues relevant to the overall neutron production and the contribution of fast ions to the neutron output in plasma.

Neutron yield using a semi-Monte Carlo simulating method for D-T fusion neutron sources

Neutron yield and spatial distribution are crucial parameters of a gaseous target D-T neutron source. The exact calculation of these parameters in large-size geometries with complex fluid dynamics is very important for gaseous target neutron source design and subsequent applications. The gaseous target on operating conditions put more requirements on the Multiphysics simulating method. The fusion neutron yield simulating method has been developed using semi-Monte Carlo based on the finite element analysis for Multiphysics. The method has been verified using the model of a D-T fusion neutron source with a windowless gas target and compared with experimental results and the GEANT4 simulations. Compared with the experimental results, the simulated neutron yield error is 8.31%, and compared with the result of GEANT4, the relative error of total neutron yield is 3.34%. The spatial distributions of neutron yield are in good agreement with the results of GEANT4. In particular, the computing time is reduced from ∼200 core-hours with GEANT4 to ∼50 core-hours with the developed semi-Monte Carlo method, greatly improving the calculation efficiency.

Numerical Simulations of the Acceleration of Fast Protons and of the Excitation of Nuclear Reactions 11B(p, 3a) and 11B(p, n)C11 at the Intensities of Picosecond Laser Radiation in the Range 1018-1019 W/cm2

Results of numerical simulations for acceleration of proton beams at the irradiation of Al target by a superintense laser pulse are presented. There is a good agreement with the experimental data in a broad range of laser intensities from I = 1018 W/cm2 to I = 1019 W/cm2 at the fixed laser pulse duration. The obtained parameters of proton beams were used for calculation of the total yield of a particles and neutrons for the nuclear reactions 11B(p, 3a) and 11B(p, n)C11C at the collisions of proton beams with boron targets. It is shown that the number of a particles escaping boron target and arriving at track detectors is less than 5of the total amount of a particles, because the majority of these particles remain inside the target owing to ionization losses. The derived values of the yield ofparticles’ which arrive at detectors are in good agreement with the experimental data. We also calculate the total yield of neutrons in the reaction 11B(p, n)C11C. It is found that, at the intensity I = 1019 W/cm2 of the picosecond laser pulse, the yield is equal to Nn = 1.4 × 108 , this value is approximately of 3% of the total yield of a particles.

Influence of ICRF-NBI synergy on fast ion distribution and plasma performance in second harmonic heating experiments with deuterium NBI at EAST

Ion Cyclotron Range of Frequencies (ICRF) heating and Neutral Beam Injection (NBI) can have synergy due to the acceleration of NBI beam ions by ICRF wave fields at their harmonics. To understand the influence of ICRF-NBI synergy on fast ion distribution and plasma performance, dedicated experiments and TRANSP simulations have been carried out on EAST. The simulation results are consistent with the experimental results. They show that the ICRF-NBI synergy not only accelerates the NBI beam ions with energy lower than 80 keV to energy larger than 300 keV, but also generates fusion neutrons with energy larger than 3 MeV. Moreover, ICRF-NBI synergy improves the plasma performance by increasing the poloidal beta, plasma stored energy, core ion temperature, total neutron yield and kinetic pressure. In a typical H-mode plasma with 1.0 MW NBI and 1.5 MW ICRF power, it was observed that ICRF-NBI synergy increases the poloidal beta, plasma stored energy, core ion temperature and neutron yield by ∼35%, 33%, 22% and 80%, respectively. Various parameter scans show that the ICRF-NBI synergetic effects can be enhanced by decreasing the minority ion concentration or the distance between the harmonic resonance and magnetic axis, or by increasing the ICRF heating power or NBI beam energy. Consequently, this leads to a generation of fast ions with higher energy. For instance, the maximum energy of the fast ion tail increases from 300 to 600 keV as n(H) decreases from 5% to 0.1%.

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.

1D/2D benchmark experiments of aluminum alloy irradiated by 14 MeV neutrons for ICF facilities

Aluminum alloy (5083) is one of the most important structural materials used in laser-driven Inertial Confinement Fusion (ICF) facilities, which will be exposed to high-yield 14 MeV neutrons and highly activated after the DT explosion. Reliable estimation of shutdown dose rate levels is of great importance for machine operation planning and to ensure the safety of people entering into target bay. Both 1D and 2D benchmark experiments were performed on a DT neutron accelerator at the Institute of Nuclear Physics and Chemistry (INPC) to validate the calculation codes and cross section libraries (JMCT with FENDL-3.1d and FISPACT with EAF-2007) used in the simulations of neutron transport and activation for aluminum alloy 5083. The total neutron yield was about 6 × 1014 in the experiments. Both radioactivity of foils placed inside the assembly and air kerma rate at given positions out of the assembly have been measured in 1D/2D experiments. Monte-Carlo code JMCT with FENDL-3.1d was used in both neutron and gamma-ray transport, and the inventory code FISPACT with EAF-2007 was used to calculate neutron activation. Results show that the relative error for air kerma rate in simulations and experiments is less than 20%, and radioisotopes give different contribution ratios on dose levels for 1D/2D experimental assemblies after DT neutron irradiation, especially for 24Na, 27Mg and 56Mn at the time within 16 h after neutron irradiated. Besides, simulations of radioactivity give good agreement with experimental results obtained for radioisotopes 24Na, 196Au, 57Ni and 58Co in both 1D and 2D experiments, and the relative error is less than ±16%.

A detection system for accurate ([formula omitted], n) neutron counting measurements of low-rate ([formula omitted], n) neutron sources

The determination of the (α, n) neutron yield of (α, n) neutron sources with low absolute total neutron emission rates is a challenging task given the complexity of the (α, n) neutron production mechanism, experimental limitations involved with measurements of weak neutron sources, and, for sources containing special nuclear material, concurrence of the production of (α, n) and fission neutrons. However, (α, n) neutron sources are prevalent in a variety of nuclear engineering disciplines – such as nuclear fuel cycle engineering, nuclear safeguards and nonproliferation, and nuclear waste management – and as such, characterization of physical (α, n) neutron sources and accurate representation of (α, n) neutron production in neutron transport codes are critical aspects of these disciplines. In this work, a moderated 3He neutron detector array with high fidelity listmode data acquisition capability was developed for measurements of low-rate (α, n) neutron sources to support (α, n) reaction nuclear data validation needs for nonproliferation applications as well as to provide experimental data for benchmarking (α, n) neutron source modeling methodologies in Monte Carlo neutron transport codes. Measurements were performed of a calibrated Eckert & Ziegler AmBe (α, n) neutron source individually and simultaneously with 252Cf sources of different strengths to simulate pure (α, n) and mixed (α, n) + fission source conditions. Total neutron counting and coincidence analysis methods developed for system data analysis were applied to the measured data, and deduced (α, n) neutron yield values agreed to within 1% of the with the vendor-calibrated AmBe source (α, n) neutron yield traceable to international standards for all measurements. Total (α, n) neutron yield uncertainties for source conditions dominated by (α, n) neutron emission did not exceed 3%.

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.

The Spectrometric Monitor of the TRT-Tokamak Total Neutron Yield

The Spectrometric Monitor of the TRT-Tokamak Total Neutron Yield

Feasibility assessment of heat pipe space reactor passive start-up without external neutron source

Space reactors start-up with on external neutron sources increase both the structural complexity and the potential risk of core criticality in a launch failure. Fortunately, cosmic rays offer the possibility of in-orbit passive start-up. To evaluate the secondary neutrons produced by cosmic rays interacting with reactor materials, the neutron yields and distributions were studied by FLUKA code for a 200 kWe space nuclear reactor with an integrated honeycomb core design. The results show that the neutron yields in the reactor core can reach 106n/s for GCRs, and the total neutron yields can even reach 108n/s for some ERB and SPE. These secondary neutrons are more uniformly distributed than those from 252Cf as an external neutron source. The results shows that the secondary neutrons produced by cosmic rays can meet the basic source requirement for space reactor passive start-up.

Development of a Clinical Neutron Source for Boron Neutron Capture Therapy

<h3>Purpose/Objective(s)</h3> In boron neutron capture therapy (BNCT), a source of epithermal (1 eV to 30 keV) neutrons activates high concentrations of ¹⁰B, selectively accumulated in tumor cells from infusion of a non-radioactive pharmaceutical, whose high LET reaction products fully deposit their energy within 10 microns of the reaction site. Historically, only nuclear reactors provided sufficient neutron intensity and beam quality for clinically acceptable BNCT treatments, but hospital-based BNCT is now feasible using compact accelerator sources. The present work has designed a system to produce an epithermal neutron beam using a tandem electrostatic accelerator, proton beam line, neutron-producing targets, neutron beam shaping assembly (BSA) and collimators. Testing was required to verify that this design can produce a clinically viable neutron source for BNCT in terms of total neutron yield, output reproducibility and operational stability. <h3>Materials/Methods</h3> The present neutron source uses neutrons from the ⁷Li(p,n)⁷Be reaction at nominal proton energy and current of 2.5 MeV and 10 mA. A negative hydrogen ion source is pre-accelerated to 200 keV and injected into the tandem accelerator, which increases the negative ion kinetic energy to half the goal proton energy at its center. A charge exchange target strips the electrons to create positively-charged protons that are further accelerated to the full operational energy at the tandem exit. The proton beam is focused and may be directed to up to three treatment rooms, each with a copper-backed target of natural lithium metal under vacuum. The maximum neutron energy from 2.5 MeV protons slowing down in lithium is 800 keV and requires moderation to the desired epithermal range. A static beam BSA surrounds the target and provides efficient neutron moderation and reflection using carefully chosen materials. The BSA's 25 cm circular exit port may be collimated to small circular epithermal beam diameters as small as 5-8 cm. <h3>Results</h3> A tandem accelerator, proton beam line and lithium target have been installed and began initial testing. Proton energy and current up to 2.3 MeV and 7 mA have been achieved for continuous operating periods exceeding 30 minutes with no energy or current degradation. Raster patterns have been designed to scan the 1-cm diameter proton beam over a 10-cm circular area on the lithium target, and water cooling of the copper backing has maintained maximum target temperatures below 150°C. The measured lithium deposition uniformity is expected to translate to a neutron yield uniformity better than 5%. <h3>Conclusion</h3> This BNCT neutron source has demonstrated operational stability and reproducibility of proton energy, proton current and neutron production. These measurements support an expectation that the equipment currently undergoing evaluation, in combination with the planned BSA, will meet the necessary requirements for a hospital-based, clinical BNCT system.

Compact neutron source from head-on collision of high energy density plasma jets

We propose a new scheme to generate a neutron source with a short duration, and the scheme is validated in two-dimensional radiation hydrodynamic simulations. When a laser beam irradiates a deuterated polystyrene cone with a half-opening angle of 60°, high energy density plasma jets are produced due to the converging effect. As two laser-driven counter-propagating jets collide in a head-on configuration, the kinetic energy converts into internal energy efficiently, inducing a significant increase in density and temperature. Then, deuteron–deuteron thermonuclear reactions are triggered, and plenty of neutrons are released. The simulation results show that the total neutron yield is as high as 6.6×108 with a duration of 103 ps, providing a new way to achieve ultrafast diagnosis with high resolution in experiments.

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.

Калибровка нейтронных счетчиков токамака Глобус-М2

The paper discusses the results of the calibration of two corona neutron counters used to measure the total neutron yield from the plasma of the Globus-M2 tokamak. The calibration was carried out in the experimental hall of the Globus-M2 facility using an AmBe source. During the calibration, the source moved uniformly around the central solenoid in the equatorial plane of the vacuum chamber, and one of the detectors was gradually moved away from the tokamak along a line with a constant toroidal angle. The values of the calibration coefficient obtained depending on the distance of the detector from the tokamak axis are presented. The calibration technique made it possible to separate in the detector signal the contributions from the direct neutron flux emitted by the plasma and from the flux of neutrons scattered on the elements of the experimental hall.

Calibration of neutron counters at the Globus-M2 tokamak

The paper discusses the results of the calibration of two coronaneutron counters used to measure the total neutron yield from the plasma ofthe Globus-M2 tokamak. The calibration was carried out in the experimentalhall of the Globus-M2 facility using an AmBe source. During thecalibration, the source moved at a constant speed around the central solenoid in theequatorial plane of the vacuum chamber, and one of the detectors wasgradually moved away from the tokamak along a line with a constant toroidalangle. The dependence of the calibration coefficient obtained depending distance of the detector from the tokamak is presented. Thecalibration technique made it possible to separate the contributions from the direct neutron flux emitted by the plasma andfrom the flux of neutrons scattered on the elements of the experimentalhall in the detector signal. Keywords: neutron diagnostics, spherical tokamak, neutron yield, neutron counters, thermonuclear fusion\