DT Neutron Research Articles

High detection efficiency scintillating fiber detector for time-resolved measurement of triton burnup 14 MeV neutron in deuterium plasma experiment.

The behavior of the 1 MeV triton has been studied in order to understand the alpha particle confinement property in the deuterium operation of toroidal fusion devices. To obtain time evolution of the deuterium-tritium (D-T) neutron emission rate where the secondary DT neutron emission rate is approximately 1012 n/s, we designed two high detection efficiency scintillating fiber (Sci-Fi) detectors: a 1 mm-diameter scintillation fiber-based detector Sci-Fi1 and a 2 mm-diameter scintillation fiber-based detector Sci-Fi2. The test in an accelerator-based neutron generator was performed. The result shows that the directionality of each detector is 15° and 25°, respectively. It is found that detection efficiency for DT neutrons is around 0.23 counts/n cm2 for the Sci-Fi1 detector and is around 1.0 counts/n cm2 for the Sci-Fi2 detector.

Determination of fast neutron spectrum using recoil-proton track imaging method

High sensitivity and good neutron/gamma discrimination, as well as online determination, are goals that researchers are pursuing for fast neutron spectrum measurements (FNSM). For this purpose, we are attempting a novel approach based on the recoil-proton track imaging (RPTI) method to do FNSM. The RPTI neutron spectrometer has been designed and built, and it mainly consists of a polyethylene foil for converting neutrons to protons, a gas scintillation chamber for generating photons along the recoil proton track, and a scientific camera system for recording the track photons in real time. Proof-of-principle experiments have been performed with the RPTI neutron spectrometer at a DT neutron generator at China Institute of Atomic Energy. As a result, numerous recoil proton track images at the single-particle level were successfully acquired and revealed in real time. Also, the proton tracks can be clearly distinguished from the background in the track images, indicating good signal-to-background ratio of the system. Particularly, both the proton ranges and the Bragg curve peak positions can be extracted online from the track images, which can lead to online generation of DT neutron spectra. These results show promising performance of the novel fast neutron spectrometer, which will bring significant advantages in various applications.

Modelling of the neutron production in a mixed beam DT neutron generator

Compact DT neutron generators based on accelerators are often built on the principle of a mixed beam operation, meaning that deuterium (D) and tritium (T) are both present in the ion beam and in the target. Moreover, the beam consists of a mixture of ions and ionized molecules (D, T ions, and ionized D-D, T-T and D-T molecules) so the relevant source components come from T(d, n), D(t, n), D(d, n) and T(t, 2n) reactions at different ion energies.The method for assessing the relative amplitudes of different source components (DD, DT, TT) is presented. The assessment relies on the measurement of the neutron spectrum of different DT components (T(d, n) and D(t, n) at different energies) using a high resolution neutron spectrometer, e.g. a diamond detector, fusion reaction cross-sections, and simulations of neutron generation in the target. Through this process a complete description of the neutron source properties of the mixed beam neutron generator can be made and a neutron source description card, in a format suitable for Monte Carlo code MCNP, produced.

Characterisation of neutron generators and monitoring detectors for the in-vessel calibration of JET

A calibration of the JET neutron detectors was carried out prior to the upcoming deuterium-tritium experimental campaign. Two Compact DT neutron generators (NGs) were purchased for this purpose from VNIIA, Russia. These generators are capable of producing approximately 2 × 108 neutrons/s with a DT fusion energy spectrum. Preceding the in-vessel calibration measurements, these compact generators were tested and fully characterised at the UK’s National Physical Laboratory (NPL). In order to support the characterisation measurements, detailed neutronics models were developed of the NGs, monitoring detectors and remote handling (RH) apparatus. Neutron spectra calculated from these models have been used to help determine NPL long counter efficiencies and effective centres, as well as NPL reference iron and aluminium activation foil reaction rates. The neutron emission rate has been measured for both generators as a function of angle using absolutely calibrated long counters and the relative emission rate by monitoring single crystal diamond detectors. The measured anisotropy profile is shown to be reproducible with a detailed NG MCNP model. Consequently, the neutron source routine and the MCNP model of the NGs can be reliably used for the analysis of the in-vessel calibration at JET.

The intensive DT neutron generator of TU Dresden

TU Dresden operates an accelerator-based intensive DT neutron generator. Experimental activities comprise investigation into material activation and decay, neutron and photon transport in matter and R&D work on radiation detectors for harsh environments. The intense DT neutron generator is capable to produce a maximum of 1012 n/s. The neutron source is a solid-type water-cooled tritium target based on a titanium matrix on a copper carrier. The neutron yield at a typical deuteron beam current of 1 mA is of the order of 1011 n/s in 4Π. A pneumatic sample transport system is available for short-time irradiations and connected to wo high-purity germanium detector spectrometers for the measurement of induced activities. The overall design of the experimental hall with the neutron generator allows a flexible setup of experiments including the possibility of investigating larger structures and cooled samples or samples at high temperatures.

Calibration of ITER Instant Power Neutron Monitors: Recommended Scenario of Experiments at the Reactor

Instant power is a key parameter of the ITER. Its monitoring with an accuracy of a few percent is an urgent and challenging aspect of neutron diagnostics. In a series of works published in Problems of Atomic Science and Technology, Series: Thermonuclear Fusion under a common title, the step-by-step neutronics analysis was given to substantiate a calibration technique for the DT and DD modes of the ITER. A Gauss quadrature scheme, optimal for processing “expensive” experiments, is used for numerical integration of 235U and 238U detector responses to the point sources of 14-MeV neutrons. This approach allows controlling the integration accuracy in relation to the number of coordinate mesh points and thus minimizing the number of irradiations at the given uncertainty of the full monitor response. In the previous works, responses of the divertor and blanket monitors to the isotropic point sources of DT and DD neutrons in the plasma profile and to the models of real sources were calculated within the ITER model using the MCNP code. The neutronics analyses have allowed formulating the basic principles of calibration that are optimal for having the maximum accuracy at the minimum duration of in situ experiments at the reactor. In this work, scenarios of the preliminary and basic experimental ITER runs are suggested on the basis of those principles. It is proposed to calibrate the monitors only with DT neutrons and use correction factors to the DT mode calibration for the DD mode. It is reasonable to perform full calibration only with 235U chambers and calibrate 238U chambers by responses of the 235U chambers during reactor operation (cross-calibration). The divertor monitor can be calibrated using both direct measurement of responses at the Gauss positions of a point source and simplified techniques based on the concepts of equivalent ring sources and inverse response distributions, which will considerably reduce the amount of measurements. It is shown that the monitor based on the average responses of the horizontal and vertical neutron chambers remains spatially stable as the source moves and can be used in addition to the staff monitor at neutron fluxes in the detectors four orders of magnitude lower than on the first wall, where staff detectors are located. Owing to low background, detectors of neutron chambers do not need calibration in the reactor because it is actually determination of the absolute detector efficiency for 14-MeV neutrons, which is a routine out-of-reactor procedure.

Testing of a Code for the Calculation of Spectra of Neutrons Produced in a Target of a Neutron Generator

The correctness of calculations performed with the SRIANG code for modeling the spectra of DT neutrons is estimated by comparing the obtained spectra to the results of calculations carried out with five different codes based on the Monte Carlo method.

Feasibility of the Precise Energy Calibration for Fast Neutron Spectrometers

Computational studies aimed at improving the accuracy of measurements performed using neutron generators with a tritium target were performed. A measurement design yielding an extremely narrow peak in the energy spectrum of DT neutrons was found. The presence of such a peak establishes the conditions for precise energy calibration of fast-neutron spectrometers.

Activation measurements in support of the 14 MeV neutron calibration of JET neutron monitors

In preparation for the upcoming deuterium-tritium campaign at the JET tokamak, the ex-vessel fission chamber neutron diagnostics and the neutron activation system will be calibrated in absolute terms at 14MeV neutron energy, to a required accuracy of less than 10%. Two nominally identical DT neutron generators were chosen as the calibration sources, both of which were fully calibrated and characterized at the UK’s National Physical Laboratory. The neutron activation method was adopted as a complementary method for the purpose of determining the absolute value of the neutron yield from the neutron generators and to provide a means of cross check for the active detection methods being employed. The work being presented here shows the derivation of the neutron emission rate from the neutron generators based upon experimental activation foil measurements.

Neutron Generation in CANDOR, an Advanced-Fuel Fusion Experiment

CANDOR is a high-field advanced fusion fuel cycle experiment based on Ignitor, but with larger dimensions and higher fusion power: it is a feasibility study of a high-field Deuterium-Helium-3 (D3He) experiment of larger dimensions and higher fusion power than Ignitor, still based on the core Ignitor technologies. Results of investigations on the feasibility of D3He burning and side neutrons production in D3He plasmas and specifically in CANDOR show that, with the initial use of DT triggering, the need for an intense auxiliary heating would be considerably alleviated. The total released 14 MeV neutron energy during the 16-second burning sums to about 210 MJ. DT and DD neutron currents incoming in the CANDOR plasma chamber wall and the Neutron Wall Loads have been computed. D3He ignition could be studied in CANDOR, with modest and conservative developments of the present technology. CANDOR has a low neutron wall loading, softer neutron spectrum, low radiation damage, and - consequently - lower neutron induced activation and radioactive inventory.

Calculated differential and double differential cross section of DT neutron induced reactions on natural chromium (Cr)

Chromium is an important alloying element of stainless steel (SS) and SS is the main constituent of structural material proposed for fusion reactors. Energy and double differential cross section data will be required to estimate nuclear responses in the materials used in fusion reactors. There are no experimental data of energy and double differential cross section, available for neutron induced reactions on natural chromium at 14 MeV neutron energy. In this study, energy and double differential cross section data of (n,p) and (n,α) reactions for all the stable isotopes of chromium have been estimated, using appropriate nuclear models in TALYS code. The cross section data of stable isotopes are later converted into the energy and double differential cross section data of natural Cr using the isotopic abundance. The contribution from compound, pre-equilibrium and direct nuclear reaction to total reaction have also been calculated for 52,50Cr(n,p) and 52Cr(n,α). The calculation of energy differential cross section shows that most of emitted protons and alpha particles are of 3 and 8 MeV respectively. The calculated data is compared with the data from EXFOR data library and is found to be in good agreement.

Identification of Landmines and IEDs Using Compact Fusion Neutron Sources on Drones

This paper describes a system to detect landmines or IEDs by the use of small DD or DT neutron sources carried by a drone. The neutron source is powered by beaming RF or laser energy, at a distance of up to a km from the target, to a relay drone high (≈ 100 meters) above the neutron drone that converts the RF energy to electricity. The relay drone uses the electricity to generate another set of RF waves, and sends the energy down to the neutron drone to power the Inertial Electrostatic Confinement (IEC) fusion neutron generator. The neutrons emitted by the IEC generator interrogate the ground below the mobile neutron drone through neutron activation and the orbiting detector drones collect the gamma ray signals to determine the composition and location of the objects below. When the N/C/O signal is close to known chemical explosives signatures, the object is tagged for further investigation.

Lead Benchmark Experiment with DT Neutrons at JAEA/FNS

A new benchmark experiment on lead with DT neutrons was designed and carried out with a large lead assembly covered with Li2O blocks at JAEA/FNS to validate nuclear data of lead for measurement of reaction rates without impact of background neutrons. The experiment was analyzed by using the MCNP5-1.40 code with the latest nuclear data libraries, ENDF/B-VII.1, JEFF-3.2 and JENDL-4.0. The calculated reaction rates underestimated the measured ones with the depth. Moreover, the tendencies of C/Es were different among the nuclear data libraries. In order to find out the reasons of the differences, we examined reaction cross-sections of lead in the nuclear data libraries in detail. The potential reactions to cause the underestimation issue of the calculated reaction rates were indicated through this study.

Neutronics study on HCCB blanket for CFETR

Chinese Fusion Engineering Testing Reactor (CFETR) is aimed to obtain the technologies to fill the gaps between ITER and DEMO. The helium cooled ceramic breeder (HCCB) blanket is one of the candidates for CFETR. Ceramics Li4SiO4, beryllium and helium of 8MPa were selected as tritium breeding material, neutron multiplier and coolant, respectively. CLF steel, one of reduced activation ferritic/martensitic (RAFM) steels, was applied as structural materials of blanket. Neutron and gamma transport was simulated with MCNP code and FENDL/MC-2.1 neutron cross section data. Activation calculation was performed with FISPACT-2007 code and EAF-2007 activation file. In this study, neutronics characteristic of HCCB blanket such as tritium breeding ratio (TBR) and shielding were assessed. The arrangement of Be, Li4SiO4 and cooling plate in blanket module was optimized to improve TBR. The sensitive study for First Wall on TBR was also performed. The radioactivity inventories of HCCB blanket were estimated. The dominant radionuclides in structural materials, Li4SiO4 and Beryllium were discussed, respectively. The damage of plasma facing components such as First Wall and divertor due to DT neutron irradiation was analyzed in terms of DPA value and gaseous product.

Use of 41Ar production to measure ablator areal density in NIF beryllium implosions

For the first time, 41Ar produced by the (n,ϒ) reaction from 40Ar in the beryllium shell of a DT filled Inertial Confinement Fusion capsule has been measured. Ar is co-deposited with beryllium in the sputter deposition of the capsule shell. Combined with a measurement of the neutron yield, the radioactive 41Ar then quantifies the areal density of beryllium during the DT neutron production. The measured 1.15 ± 0.17 × 10+8 atoms of 41Ar are 2.5 times that from the best post-shot calculation, suggesting that the Ar and Be areal densities are correspondingly higher than those calculated. Possible explanations are that (1) the beryllium shell is compressed more than calculated, (2) beryllium has mixed into the cold DT ice, or more likely (3) less beryllium is ablated than calculated. Since only one DT filled beryllium capsule has been fielded at NIF, these results can be confirmed and expanded in the future.

The effects of magnetic field topology on secondary neutron spectra in Magnetized Liner Inertial Fusion

The Magnetized Liner Inertial Fusion (MagLIF) concept involves the compression of a magnetized fuel such that the stagnated fuel contains a magnetic field that can suppress heat flow losses and confine α particles. Magnetic confinement of α particles reduces the fuel ρR required for ignition. Recent work [1,2] has demonstrated that the magnitude of the magnetic field in deuterium fuel can be inferred from the yields and spectra of secondary DT neutrons. In this work we investigate the potential for using the shape of the secondary neutron spectra to diagnose the magnetic field topology in the stagnated fuel. Three different field topologies that could possibly occur in MagLIF experiments are studied: (1) a cylindrical fuel column containing axial and azimuthal magnetic field components, (2) a fuel column which is pinched at the ends to form a magnetic mirror and (3) a fuel column that has a helical tube shape with magnetic field lines following the helical path of the tube’s axis. Each topology is motivated by observations from experimental or simulated MagLIF data. For each topology we use a multi-physics model to investigate the shapes of the secondary neutron spectra emitted from a steady-state stagnated fuel column. It is found that the azimuthal and helical topologies are more suitable than the mirror topology for reproducing an asymmetry in the axial spectra that was observed in experiments. Gorgon MHD simulations of the MagLIF implosion in 1D are also carried out. These show that sufficient azimuthal magnetic field can penetrate from the liner into the fuel to qualitatively reproduce the observed spectral asymmetry.

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

We had pointed out that the elastic scattering and capture reaction data of the recent nuclear data libraries for copper had included some problems in the resonance energy region, which had caused a large underestimation of reaction rates of non-threshold reactions. In order to corroborate this issue, we carried out a new benchmark experiment on copper in the neutron field with more low energy neutrons. We measured reaction rates using the activation foils and analyzed the experiment with MCNP5.140 and ENDF/B-VII.1, JEFF-3.2 and JENDL-4.0. As a result, the calculated reaction rates related to low energy neutrons excessively underestimated the measured ones as in the previous experiment. The calculated reaction rates with the modified copper nuclear data in the previous study reproduced the measured ones well. However, it was revealed that the modified copper data needed more revision.

Thought Experiment to Examine Benchmark Performance for Fusion Nuclear Data

There are many benchmark experiments carried out so far with DT neutrons especially aiming at fusion reactor development. These integral experiments seemed vaguely to validate the nuclear data below 14 MeV. However, no precise studies exist now. The author’s group thus started to examine how well benchmark experiments with DT neutrons can play a benchmarking role for energies below 14 MeV. Recently, as a next phase, to generalize the above discussion, the energy range was expanded to the entire region. In this study, thought experiments with finer energy bins have thus been conducted to discuss how to generally estimate performance of benchmark experiments. As a result of thought experiments with a point detector, the sensitivity for a discrepancy appearing in the benchmark analysis is “equally” due not only to contribution directly conveyed to the deterctor, but also due to indirect contribution of neutrons (named (A)) making neutrons conveying the contribution, indirect controbution of neutrons (B) making the neutrons (A) and so on. From this concept, it would become clear from a sensitivity analysis in advance how well and which energy nuclear data could be benchmarked with a benchmark experiment.

Development of An Epi-thermal Neutron Field for Fundamental Researches for BNCT with A DT Neutron Source

Boron Neutron Capture Therapy (BNCT) is known to be a new promising cancer therapy suppressing influence against normal cells. In Japan, Accelerator Based Neutron Sources (ABNS) are being developed for BNCT. For the spread of ABNS based BNCT, we should characterize the neutron field beforehand. For this purpose, we have been developing a low-energy neutron spectrometer based on 3He position sensitive proportional counter. In this study, a new intense epi-thermal neutron field was developed with a DT neutron source for verification of validity of the spectrometer. After the development, the neutron field characteristics were experimentally evaluated by using activation foils. As a result, we confirmed that an epi-thermal neutron field was successfully developed suppressing fast neutrons substantially. Thereafter, the neutron spectrometer was verified experimentally. In the verification, although a measured detection depth distribution agreed well with the calculated distribution by MCNP, the unfolded spectrum was significantly different from the calculated neutron spectrum due to contribution of the side neutron incidence. Therefore, we designed a new neutron collimator consisting of a polyethylene pre-collimator and boron carbide neutron absorber and confirmed numerically that it could suppress the side incident neutrons and shape the neutron flux to be like a pencil beam.

Detectors for Active Interrogation Applications

Active interrogation creates an environment that is particularly challenging from a radiation-detection standpoint: the elevated background levels from the source can mask the desired signatures from the SNM. Neutron based interrogation experiments have shown that nanosecond-level timing is required to discriminate induced-fission neutrons from the scattered source neutrons. Previous experiments using high-energy bremsstrahlung X-rays have demonstrated the ability to induce and detect prompt photofission neutrons from single target materials; however, a real-world application would require spectroscopic capability to discern between photofission neutrons emitted by SNM and neutrons emitted by other reactions in non-SNM. Using digital pulse-shape discrimination, organic liquid scintillators are capable of reliably detecting neutrons in an intense gamma-ray field. Photon misclassification rates as low as 1 in 106 have been achieved, which is approaching the level of gaseous neutron detectors such as 3He without the need for neutron moderation. These scintillators also possess nanosecond-timing resolution, making them candidates for both neutron-and photon-driven active interrogation systems. We have applied an array of liquid and NaI(Tl) scintillators to successfully image 13.7kg of HEU interrogated by a DT neutron generator; the system was in the direct presence of the accelerator during the experiment.