Neutron Production Rate Research Articles

The measurement and calculation of the correlated neutron production rate from [formula omitted] sources

The production of time-correlated neutrons from six 241 AmO 2 Be(α, n) neutron sources has been measured using a high level neutron coincidence counter (HLNCC-II). Five of the sources were contained in standard Amersham X.2 capsules while the sixth was in the larger X.3 format. The results quantitatively support the hypothesis that the predominant source of time-correlated neutrons arises from the Be(n, 2n) process initiated by the primary 241AmO 2/Be(α, n) neutrons. On this basis, analysis of the experimental data yields an (n, 2n) (α, n) ratio of ≈ 1.8% for the X.2 sources and ≈ 2.1% for the X.3 source. This work suggests that passive neutron coincidence counters could be used to good effect in the measurement and testing of integral (n, 2n) cross section data. Tentative conclusions are that the EFF-2 evaluation of the Be(n, 2n) double differential cross section is to be preferred over the ENDF/B-VI evaluation and that the integrated-over-angle Be(n, 2n) cross sections appearing in these evaluations may be approximately 20% low.

Performance simulations of the medusa neutron detector array (abstract)

A 960-channel neutron detector array is under construction to measure various neutron reaction products from direct-drive laser-driven inertially confined fusion experiments. Analytical and Monte-Carlo model simulations have been completed which demonstrate the usefulness of this diagnostic for broad classes of fusion experiments. The modeling accounts for neutron production rate and spectra from the target and detector and acquisition electronics response. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement No. DE-FC03-92SF19460 and the University of Rochester. The support of DOE does not constitute an endorsement by DOE of the views expressed in this article.

The measurement and calculation of the correlated neutron production rate from 241AmO 2/Be (α, n) sources

The production of time-correlated neutrons from six 241AmO2Be(α, n) neutron sources has been measured using a high level neutron coincidence counter (HLNCC-II). Five of the sources were contained in standard Amersham X.2 capsules while the sixth was in the larger X.3 format. The results quantitatively support the hypothesis that the predominant source of time-correlated neutrons arises from the Be(n, 2n) process initiated by the primary 241AmO2/Be(α, n) neutrons. On this basis, analysis of the experimental data yields an (n, 2n)(α, n) ratio of ≈ 1.8% for the X.2 sources and ≈ 2.1% for the X.3 source. This work suggests that passive neutron coincidence counters could be used to good effect in the measurement and testing of integral (n, 2n) cross section data. Tentative conclusions are that the EFF-2 evaluation of the Be(n, 2n) double differential cross section is to be preferred over the ENDF/B-VI evaluation and that the integrated-over-angle Be(n, 2n) cross sections appearing in these evaluations may be approximately 20% low.

Heat transport in PBX-M high beta p plasmas

High poloidal beta discharges in PBX-M routinely enter the H mode regime: typically, a quiescent phase followed by an MHD-active phase characterize the H mode period. An analysis of the energy transport during these phases is conducted using the experimental data and the code TRANSP; effective thermal diffusivities are computed. The quiescent H phase is characterized by a decrease of the thermal ion energy transport and a flattening of the associated effective diffusivity profile. Enhanced fast ion losses are observed during the MHD-active phase; particles in the lower end of the fast-ion energy spectrum with a large perpendicular velocity component are predominantly affected. Folding these losses into the analysis permits to reproduce the measured stored energy and time evolution of the neutron production rate during the MHD-active phase. During the MHD-active phase, the ion thermal diffusivity maintained a profile (magnitude and shape) similar to that obtained at the end of the quiescent H phase, suggesting, within the uncertainty of this analysis, that the MHD activity does not affect the thermal plasma energy transport. The observed stored energy (beta) saturation results from the enhanced losses in the fast-ion population. An analysis without inclusion of enhanced fast ion losses fails to reproduce the experimental measurements. Throughout the H mode period the electron effective diffusivity remains lower than the ion effective diffusivity. An error analysis is presented

Hydrogen/deuterium recycling and initial results of helium ash study in JT-60U

Reduction of particle recycling is an important technique in present plasma devices to improve energy confinement characteristics. Besides obtaining long pulse DT burning plasmas, controlled fuelling of the plasma with the primary species and removal of the helium ash are important issues. In the JT-60U tokamak, about 90% of the inner surface is covered with graphite tiles. Because recycling becomes more important with graphite surfaces, recycling characteristics of an all graphite first wall is important. In this paper, experimental results of JT-60U on recycling and helium ash removal are presented. The effect of wall temperature on recycling was observed. Significant hydrogen dilution was observed during deuterium discharges. Recycling characteristics in the ELMy H-mode discharges are also described where the neutral pressure increases with the electron density similar to L-mode discharges. In high βρ/hot ion discharges, a highest neutron production rate of 1.3×1016 s−1 was obtained with the lowest recycling. Initial results of helium ash experiment with strong He gas puffings in ohmically and neutral beam heated discharges are reported.

Gamma-radiation detection limits for electrochemically induced deuterium cold-fusion rates

An experiment was performed in order to detect cold fusion in an electrochemical set-up similar to that described by Fleischmannet al. The detection of gamma-rays from deuteron formation was made with a NaI(Tl) scintillator. Upper bounds for the neutron production rate of 0.53 c/s and 0.23 c/s for the stationary and the transient processes, respectively, were determined.

A low background neutron measuring system and its application to the detection of neutrons produced by the D 2O electrolysis

A low background neutron measuring system has been constructed. The background neutron counting rate is 1.4 counts/h and the long term stability is within 0.1 counts/h over one year. The detection efficiency for fission neutrons from 252Cf is 4.4%. The system is capable of measuring low rate neutron emission events with a half life of 10 20 yr. The rate of neutron production from copper and lead by cosmic-rays was measured in an underground laboratory. The system was used for an experimental test of neutron emission from the electrolysis of D 2O. The results show no enhancement of neutron emission. No tritium production was observed.

A Simple Plasma Model for the Description ofd-dFusion in Condensed Matter

AbstractA simple plasmalike model that describes the time behavior of the deuteron-deuteron (d-d) fusion reaction rate as a function of charging time is presented. When used to describe the experimental shape of d-d neutron production rates averaged over broad time intervals, the model gives reasonable agreement. The fusion rates obtained from this comparison are of the order of the magnitude of effects that could be expected by the combination of electron screening and fluctuation enhancement. The model allows predictions of the conditions under which d-d fusion neutrons in condensed matter might be observed and explains why, in many cases, no effects are observed.

High-dose neutron generation from plasma ion implantation

Based on a theoretical estimation, a simple new scheme to generate high-dose, steady-state neutrons is presented. This new high-dose neutron source makes use of the plasma ion implantation, which consists of a cylindrical palladium rod saturated with deuterium atoms and immersed in a deuterium or tritium plasma. The plasma is contained by a grounded conducting chamber. The palladium rod is connected to a high-power modulator, which provides a series of negative-voltage pulses. During these negative pulses, deuterium or tritium ions fall into the palladium rod and collide with the deuterium atoms in it, initiating fusion reactions and emitting neutrons. This neutron device will be portable, reliable and almost free from radiation hazards except for the neutrons themselves. For reasonable system parameters allowed by present technology, we find from a theoretical calculation that the average neutron production rate 〈dN/dt〉 = 2.3 × 1012 s−1 for D-T reaction, which may be more than enough for most applications. The average neutron production rate for D-D reaction is two order of magnitude less than that for D-T reaction.

H-mode confinement in JET with enhanced performance by pellet peaked density profiles

The combination of two regimes of enhanced performance, the H-mode and the pellet enhanced performance (PEP) mode, has been achieved in JET. The strong enhancement of the central plasma parameters, obtained with pellet injection and subsequent auxiliary heating, is found to persist well into the H-mode phase. A characteristic of the PEP regime is that an improvement of the fusion reactivity over non-pellet discharges is obtained under the condition of nearly equal electron and ion temperatures. A maximum neutron production rate of 0.95 × 10l6 s−1 was obtained in a double-null X-point discharge with 2.5 MW of neutral beam heating and 9 MW of ion cyclotron resonance heating, with central ion and electron temperatures of about 10 keV and a central deuterium density of 8.0 × 1019 m−3. The corresponding fusion product nD(0)τETi(0) is between 7.0 and 8.6 × 1020 m−3·s·keV. The enhanced neutron production is predominantly of thermonuclear (Maxwellian) origin. The compatibility of these regimes is an important issue in the context of tokamak ignition strategies. Several technical developments on JET have played a role in the achievement of this result: (1) the use of low voltage plasma breakdown (0.15 V/m) to permit pellet injection in an X-point configuration before the formation of a q = 1 surface; (2) the elimination of ICRH specific impurities with antenna Faraday screens made of solid beryllium; (3) the use of a novel system of plasma radial position control that stabilizes the coupling resistance of the ion cyclotron heating system.

Neutron diagnostic experiment for Alcator C-Mod

The neutron production rate on Alcator C-Mod is expected to peak at 1×1016 n/s for the highest performance experimental pulses. Measurement of the total fusion yield is important for both assessing the experiment and for evaluation of radiological data. The neutron detectors must therefore be capable of accurate measurement over a wide operating regime (1011–1016 n/s from the source). This can be achieved with a series of fission detectors (U235 and U238), capable of operating in both a pulsed and current mode. Computer calculations optimizing the placement of these detectors and also the moderator design are detailed. Calibration of this system will be achieved using a Cf 252 source which will be moved on a track continuously around the major axis. It will also be placed discretely at a number of locations in the torus. MCNP calculations will complement the calibration data. Details of the experimental setup and calibration scheme will be presented.

Issues connected with cold fusion: A room temperature mechanism for the production of X-rays

It has been suggested that the reaction d+d to 3He+n has been observed at room temperature in electrolytically deuterated Ti and Pd. The reported neutron production rates cannot be explained on conventional grounds, and the possibility that neutrons are produced by fusion reactions in a small nonthermal population of fast deuterons has been raised. The K and L X-ray production which should result from these putative fast deuterons is calculated, and it is concluded that neutron production is a more sensitive probe of fast deuterons than X-rays. Estimates of the fusion rate/fast deuteron, as a function of deuteron energy are also made.

Can solid-state effects enhance the cold-fusion rate?

RECENTLY two groups1,2 have reported finding experimental evidence for an unexpectedly high rate of nuclear fusion at room temperature during the process of electrolytic deposition of deuterium on palladium1,2 and titanium2. To achieve the rate of neutron production (∼10−23s−1 per deuteron pair) cited in ref. 2 (and a fortiori, that inferred in ref. 1) requires the solid-state environment to produce either an unusual enhancement of the fusion reaction rate or a large suppression of the Coulomb barrier between deuterons. The latter would presumably arise from some kind of sophisticated many-body screening effect in Pd or Ti, perhaps associated with quasiparticles of large effective mass2. Here we point out that within the framework of the lowest-order Born-Oppenheimer approximation, a very severe constraint is imposed on all such screening mechanisms in solids in equilibrium by the observable binding affinity of 4He atoms for the metal in question. Unless the latter is quite anomalous, the Coulomb barrier penetration in a solid in equilibrium cannot be enhanced anywhere near the magnitude required to explain the fusion rates inferred from the experiments.

Measurements of radial profiles of the ion temperature and the plasma rotation velocity with the TFTR vertical x-ray crystal spectrometer

The TFTR vertical x-ray crystal spectrometer has now been operating with three crystals and position-sensitive detectors according to the original design specifications. The observed spectra of heliumlike Ti xxi, Cr xxiii, Fe xxv, and Ni xxvii have permitted a detailed comparison with the predictions from atomic theories, and they have provided data on the radial profiles of the ion temperature and toroidal rotation velocity, as well as the radial ion charge-state distribution in TFTR discharges. Central ion temperatures of 20 keV and central plasma rotation velocities of 5×105 m/s have been recorded from plasmas with auxiliary neutral beam heating. These experimental results are presented. Also discussed are further instrumental improvements, such as the installation of two additional crystals and detectors and the installation of γ and neutron shielding, which will make it possible to measure under full DD and DT operation with 27 MW of neutral beam injection where neutron production rates of 1019 neutrons/s are expected.

Measurements of radial profiles of the ion temperature and the plasma rotation velocity with the TFTR vertical x-ray crystal spectrometer (abstract)

The TFTR vertical x-ray crystal spectrometer has now been operating with three crystals and position-sensitive detectors according to the original design specifications. The observed spectra of heliumlike Ti xxi, Cr xxiii, Fe xxv, and Ni xxvii have permitted a detailed comparison with the predictions from atomic theories, and they have provided data on the radial profiles of the ion temperature and toroidal rotation velocity, as well as the radial ion charge-state distribution in TFTR discharges. Central ion temperatures of 20 keV and central plasma rotation velocities of 5×105 m/s have been recorded from plasmas with auxiliary neutral beam heating. These experimental results are presented. Also discussed are further instrumental improvements, such as the installation of two additional crystals and detectors and the installation of γ and neutron shielding, which will make it possible to measure under full DD and DT operation with 27 MW of neutral beam injection where neutron production rates of 1019 neutrons/s are expected.

Measurements of the ion temperature and plasma rotation from Kα emission with the TFTR horizontal x-ray crystal spectrometer (abstract)

The TFTR horizontal x-ray crystal spectrometer has been improved by installation of γ and neutron shielding and is now operating in the TFTR Test Cell up to the presently maximum neutron production rates of 2×1016 neutrons/s. Further optimization by using a modified detector may assure operation of the spectrometer during the DT phase when TFTR is covered by an igloo. The instrument is set up for simultaneous observation of the entire satellite spectrum of Ni xxvii, and it provides data on the time evolution of the central ion temperature and the central plasma rotation velocity. Ion temperatures of 20–30 keV have been measured under ‘‘supershot’’ conditions. The highest rotation velocity observed with unidirectional injection was 1.1×106 m/s.

Neutron emission time measurements for ICF targets

The neutron emission time for laser-driven inertial confinement fusion targets is determined from data recorded with fast neutron and optical detector systems. Two types of neutron detectors are used. Radiation-induced conductivity devices have a 130-ps FWHM response and are sensitive to targets with yields greater than 5×1010 DT neutrons. They measure the average neutron emission time with a precision of ±50 ps and are fast enough to measure the neutron production rate within a target core as a function of time. Plastic scintillators coupled to microchannel-plate photomultiplier tubes have a 1.2-ns FWHM response and measure the average neutron emission time with a precision of ±75 ps for targets with yields greater than 106 neutrons. Streak cameras record the incident laser power. Optical fiducial signals that are injected into each detector are used to cross time between the detector systems. Measurements made on 1-mm-diam spherical targets irradiated with 23 kJ of 0.35-μm light delivered in a temporally square 1.1-ns pulse and yielding 1013 neutrons show a burn duration greater than 300 ps and an average neutron emission time of about 850 ps.

Fusion-neutron production in the TFTR with Deuterium neutral-beam injection

We report measurements of the fusion reaction rate in the Tokamak Fusion Test Reactor (TFTR) covering a wide range of plasma conditions and injected neutral-beam powers up to 6.3 MW. The fusion neutron production rate in beam-injected plasmas decreases slightly with increasing plasma density ne, even though the energy confinement parameter neτE generally increases with density. The measurements indicate and Fokker-Planck simulations show that with increasing density the source of fusion neutrons evolves from mainly beam-beam and beam-target reactions at very low ne to a combination of beam-target and thermonuclear reactions at high ne. At a given plasma current, the reduction in neutron source strength at higher ne is due to both a decrease in electron temperature and in beam-beam reaction rate. The Fokker-Planck simulations also show that at low ne, plasma rotation can appreciably reduce the beam-target reaction rate for experiments with coinjection only. The variation of neutron source strength with plasma and beam parameters is as expected for beam-dominated regimes. However, the Fokker-Planck simulations systematically overestimate the measured source strength by a factor of 2–3; the source of this discrepancy has not yet been identified.

Giant sawtooth oscillations in the Doublet III tokamak

Large-amplitude sawtooth oscillations have been observed in the soft-X-ray emission, central electron temperature, neutron production rate, radiated power, Dα emission, fast-neutral flux and one-turn voltage during beam-heating experiments in the Doublet III tokamak. The necessary conditions for the appearance of giant sawteeth seem to be operation at low qa and high βT. Many features of the giant sawteeth have been simulated with a transport code to which a sawtooth mixing model and several diagnostic models were added. Substantial redistribution of particles and energy evidently occurs over the plasma interior. Global energy confinement, however, is only moderately degraded by the sawteeth.

Analysis of Neutron Yields from High-Energy Proton Bombardment of Uranium Targets

A high-energy fission model is incorporated into the nucleon-meson transport code, NMTC, which has been used for predicting high-energy neutron yields from high-energy nucleon and pion collisions with nuclei. The experiments of Vasil'kov et al., Russel et al., and Fraser et al. to measure fissile material production rates from fertile material and to determine neutron production rates, are analyzed. Evaluations are made of the plutonium production rate from the infinite medium uranium block. The calculation including the high-energy fission process gives a more reasonable agreement with the experiments, than the process without high-energy fission. A possible refinement of the model, taking into account the rotational motion in the excited state, is discussed.