Measured Neutron Energy Spectra Research Articles

Nuclear diagnostic techniques at Nova (abstract)

Nuclear diagnostic techniques play an ever increasing role in the characterization of final fuel conditions for ICF implosions as target sizes increase. With today’s high neutron yields, many neutron-based measurements can be used to determine final core conditions such as fuel density and temperature, fuel and pusher areal densities, hot spot size and shape, and implosion characteristics such as neutron emission time and burn history. Experiments can be designed to emit neutrons or other nuclear reaction products from which measurements allow the study of processes such as charged particle slowing in plasmas or fuel-pusher mixing. An overview of work in progress at the Lawrence Livermore National Laboratory Nova Laser Fusion Facility will be presented, with particular emphasis on techniques involving the measurement of secondary and tertiary neutron energy spectra. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract No. W-7405-ENG-48.

Neutron Energy Spectra in Lithium: Measurements and Calculations

This paper reports a 15-MeV clean neutron field used to investigate various problems encountered in fusion reactor blanket designs. In particular, the adequacy of the {sup 7}Li tritium production neutron cross section was analyzed by comparing measured neutron energy spectra in a lithium tank with detailed Monte Carlo calculations. New evaluations are needed that would consider the various mechanisms that contribute to tritium production in{sup 7}Li and determine the shape of the secondary neutron energy spectrum. It is also found that uncertainties in the alignment of the source-target-detector configuration in integral experiments may result in considerable discrepancies in the analyses of such experiments.

Measurement of the fractional thermonuclear neutron yield during deuterium neutral-beam injection into deuterium plasmas

Two 3He ionisation chambers were used at the Joint European Torus (JET) to measure neutron energy spectra for discharges in which deuterium neutral beams were injected into deuterium plasmas. By comparing the measured spectra with theoretical calculations, the fraction of the total neutron yield due to thermonuclear reactions was deduced. Calculated thermonuclear fractions agree well with the experimental results.

Neutron energy spectrum measurements of neutron sources with an NE-213 spectrometer

The neutron energy spectra of the following sources were measured using a fast-neutron spectrometer with NE-213 liquid scintillator: 252Cf, Am-Be and T(d, n) 4He from a Van de Graaff accelerator (400 keV). The measured proton recoil pulse-height data were unfolded using the FANTI code to obtain the neutron energy spectrum. The spectrometer gives neutron spectra in the range of 2–16 MeV, with 6% intrinsic efficiency and a resolution between 4% and 11%. The 252Cf neutron energy spectrum was measured and the results obtained showed good agreement with the spectrum usually published in the literature, which can be fitted by the expression N(E) =√E exp(− E T ) with the constant T = 1.42 MeV.

An experimental setup for measurement of neutron energy spectra in lithium with collimated 14.7 MeV neutrons

Neutron energy spectra in an 88 cm diameter, 88 cm long lithium tank were measured with the Ben Gurion University experimental setup. In this setup, the lithium tank is separated from the DT neutron generator by a 120 cm thick paraffin wall with a 6 cm diameter collimator through it, along the axis of the neutron generator and the lithium tank. This enables unidirectionality and monoenergeticity of the neutrons penetrating the lithium tank. A neutron energy spectrum is obtained by unfolding with the code FORIST of proton-recoil spectra measured by an NE213 liquid scintillator. The important features of the spectrometry system, comprised of the NE213 scintillator and the attached electronic system, are the high pulse shape discrimination capability of the NE213 scintillator, which enables the separation of neutron and gamma events, relatively high energy resolution, and the system linearity. Also the simultaneous measurement of the low gain and high gain proton-recoil spectra prevents a distortion of the unfolded neutron spectrum. The neutron energy spectra are absolutely normalized and internormalized to each other by an absolutely calibrated, second NE213 scintillator, placed close to the neutron generator. The measured neutron energy spectra inside the lithium tank were compared to some preliminary calculations of the spectra, carried out with the discrete-ordinates transport code DOT4.2. Both spectra are in poor agreement. These discrepancies are assigned mainly to the inadequancy of the transport calculations. Finally, the distribution of the tritium production in the lithium tank, with the same experimental configurations, was calculated with the code DOT4.2 as well. The results indicate that the collimated neutron beam configuration is inappropriate for the purpose of tritium breeding ratio measurements.

Transport Analysis of Measured Neutron Energy Spectra in a Graphite Stack with a Collimated Deuterium-Tritium Neutron Beam

The Ben-Gurion University measurements of neutron energy spectra in a graphite stack, resulting from the scattering of 14.7-MeV neutrons streaming through a 6-cm-diam collimator in a 121-cm-thick paraffin wall, have been used as a benchmark for the compatability and accuracy of discrete ordinates, P/sub n/, and transport calculations and as a tool for fusion reactor neutronics. The transport analysis has been carried out with the DOT 4.2 discrete ordinates code and with cross sections processed with the NJOY code. Most of the parameters affecting the accuracy of the flux and L system scattering cross sections in the P/sub n/ approximation, the quadrature set employed, and the energy multigroup structure. First, a spectrum calculated with DOT 4.2, with a detector located on the axis of the system, was compared with a spectrum calculated with the MCNP Monte Carlo code, which was a preliminary verification of the DOT 4.2 results. Both calculated spectra were in good agreement. Next, the DOT 4.2 calculations were compared with the measured spectra. The comparison showed that the discrepancies between the measurements and the calculations increase as the distance between the detector and the system axis increases. This trend indicates that when the flux is determined mainlymore » by multiple scatterings, a more divided multigroup structure should be employed.« less

Improvements of the Electric Field Correction in Recoil-Proton Detectors

An improved correction for distortions of the electric field in recoil-proton detectors, applied in neutron energy spectrum measurements, was developed. It utilizes a simple analytical expression for the representation of the response function for the entire interval of the electric charge, thus avoiding the previous separate treatment of the upper 20 to 30% of the charge range. The simplicity of the functional expression for the response function allowed the integral equation of the unfolding problem to be solved analytically. This refined unfolding technique apparently provides a better resolution, as it shows the spectrum deformation due to the large scattering resonances to be more pronounced. This highly accurate analytical unfolding procedure is likely to be applicable in other areas as well.

Inertial confinement fusion ion temperature measurements using a single-hit detector array

Neutron time-of-flight techniques have been used to diagnose fuel ion temperatures of inertial confinement fusion plasmas. A new technique for making this measurement using an array of ‘‘single-hit’’ detectors operating in single-particle counting mode is described. This technique has some potential advantages over previous methods (faster timing, better energy resolution, good sensitivity) and can possibly be further developed to allow detailed measurements of neutron energy spectra to give other diagnostic information, such as fuel areal density (〈ρR〉).

Helium production in HFIR-irradiated pure elements

Helium measurements have been made for Ni, Fe, Ti, Nb, Cr, and Cu samples from several different fusion materials irradiations in the mixed-spectrum High Flux Isotope Reactor (HFIR). The results are compared with helium predictions based on neutron energy spectrum measurements and the ENDF/B-V Gas Production File. The comparisons indicate good agreement for Ni, Fe, and Cr, but cross-section discrepancies were observed for helium production by fast neutrons in Ti, Nb, and Cu. Nickel and copper exhibit enhanced helium production at high neutron fluences, and there may also be a small enhancement for iron. This work demonstrates the importance of measuring helium production in materials of interest over the full range of neutron fluences used in fusion materials tests. The combination of measurements and predictions from well-characterized neutron fields can then be used to provide accurate helium estimates for materials irradiations.

Neutron energy spectra produced from thick targets by light-mass heavy ions

The measured neutron energy spectra produced from thick targets by light-mass heavy ion beams were analyzed with the phenomenological hybrid model of equilibrium and pre-equilibrium emissions. The neutron energy spectra are fitted to two (in the energy region of incident projectile energy lower than 100 MeV) and three (in the energy region of incident projectile energy higher than 100 MeV) Maxwellian-type components. The lowest energy component corresponds to the evaporation neutrons from a compound nucleus having a nuclear temperature independent of the neutron emission angle and the higher two components to the pre-equilibrium neutron emission having a nuclear temperature depending on the angle.

ICF Plasma Diagnostics by Thermonuclear Reaction Particles

Thermonuclear reaction particle measurements and diagnostics of ICF Plasma are reviewed. Measurements of density-radius product by the neutron activation and knock-on praticle methods are dicussed as well as the measurements of neutron yield and energy spectrum. Charged particle measurements are also addressed.

Studies of neutron emission during the start-up phase of the Alcator C tokamak

Alcator C operations commenced with discharge cleaning and tokamak operation using hydrogen filling gas. Before and during these experiments no deuterium gas was allowed into the device. The earliest operation resulted in dosimeter readings of a few roentgen per shot in the vicinity of the limiter and a localized source of neutron emission of up to 109 neutrons per shot which were subsequently identified as having photonuclear origin. After seven months of operation, conditions were achieved that resulted in substantially lower photonuclear activity. Subsequently, deuterium fill gas was allowed into the device and measurement of neutron flux and energy spectra indicated that the majority of neutron emissions in Alcator C high-density deuterium discharges were consistent with having thermonuclear origins.

Neutron Streaming through a Slit and Duct in Concrete Shields and Comparison with a Monte Carlo Analysis

A series of measurements of ∼14-MeV deuterium-tritium neutrons streaming through a slit and a duct in concrete shields has been carried out using a Cockcroft-Walton-type neutron generator. Measured neutron energy spectra are compared with calculations in six configurations of the shields. The configurations are the simplified geometries of streaming paths of tokamak reactors, such as a divertor throat and a neutral beam injection port. The measured data were obtained with an NE-213 liquid scintillator using pulse shape discrimination methods to resolve neutron and gamma-ray pulse height data and using a spectral unfolding code to convert these data to energy spectra. The experiments were analyzed by a Monte Carlo code. The calculated neutron energy spectra slightly underestimate the measured data, especially in the range of 6 to 8 MeV. The agreement between the calculated and measured integral flux above 2.2 MeV ranges from 87.5 to 72.7% depending on the configurations.

Comparison of measured and calculated neutron and gamma ray energy spectra behind an in-line shielded duct

Integral experiments that measure the transport of ∼ 14 MeV neutrons through a 0.30-m-diameter duct having a length-to-diameter ratio of 2.83 that is partially plugged with a 0.15 m diameter, 0.51 m long shield comprised of alternating layers of stainless steel type 304 and borated polyethylene have been carried out at the Oak Ridge National Laboratory. Measured and calculated neutron and gamma ray energy spectra are compared at several locations relative to the mouth of the duct. The measured spectra were obtained using an NE-213 liquid scintillator detector with pulse shape discrimination methods used to simultaneously resolve neutron and gamma ray events. The calculated spectra were obtained using a computer code network that incorporates two radiation transport methods: discrete ordinates (with P3 multigroup cross sections) and Monte Carlo (with continuous point cross sections). The two radiation transport methods are required to account for neutrons that singly scatter from the duct to the detectors. The calculated and measured neutron energy spectra above 850 keV agree within 5–50% depending on detector location and neutron energy. The calculated and measured gamma ray energy spectra above 750 keV are also in favorable agreement, ∼ 5–50%, depending on detector location and gamma ray energy.

Spectroscopy ofF16from theO16(p,n)F16reaction at 99 and 135 MeV

We measured neutron energy spectra and extracted angular distributions for eleven separate transitions for the /sup 16/O(p,n)/sup 16/F reaction at 99.1 and 135.2 MeV. Several new spin and parity assignments are obtained for states in /sup 16/F by comparison of the excitation energy spectra with known analog states in /sup 16/O and with a shell-model prediction and by analysis of the neutron angular distributions. The most strongly-excited are two 2/sup -/ states at E/sub x/ = 0.40 +- 0.05 and 7.6 +- 0.1 MeV, a 4/sup -/ states at 6.37 +- 0.05 MeV, and two broad 1/sup -/ states at 9.4 +- 0.1 and 11.5 +- 0.1 MeV. These states are analogs of known 2/sup -/ (M2) states, a 4/sup -/ ''stretched'' state, and 1/sup -/ (E1) strength, respectively, in /sup 16/O. Three weakly-excited 1/sup +/ states are observed at E/sub x/ = 3.75 +- 0.05, 4.65 +- 0.05, and 6.23 +- 0.05 MeV. These states are analogs of known 1/sup +/ (M1) states in /sup 16/O and directly indicate correlations in the ground state of /sup 16/O. A weakly excited 4/sup -/ state is seen at 5.93 +- 0.05 MeV in good agreement with a 4/sup -/ state observedmore » in /sup 16/O (e,e') measurements. All of the most strongly excited states align (to within +- 200 keV) with known T = 1 analog states in /sup 16/O for a common net displacement energy of 12.6 MeV. The (p,n) reaction at medium energies is shown to be an important spectroscopic tool.« less

Neutron spectrum measurements using miniature proton recoil proportional spectrometers

A system has been developed for measurements of neutron energy spectra in the energy range from approximately 1 keV to 2 MeV, for use in the Purdue University Fast Breeder Blanket Facility. This energy range covers the neutron energy range of interest in fast reactor blankets. The neutron spectroscopy system uses hydrogen- and methane-filled cylindrical proportional counters for the measurements, and two-parameter analysis for discrimination against gamma-ray events in the low-energy hydrogen-filled detector. A detector and preamplifier can be inserted into the blanket with the removal of a single fuel rod having a diameter of 15.3 mm (0.60″).

Neutron source characterization for fusion materials studies

Neutron flux and energy spectrum measurements are conducted for all major fusion materials irradiation facilities, including fission reactors and accelerators. Dosimetry characterization experiments and integral cross section measurements have been performed. Multiple activation and helium production measurements are performed routinely to provide materials experimenters with neutron exposure parameters including fluence, spectrum, displacements, gas production, and transmutation with typical accuracies of 10–15%. Such data are crucial to the fusion materials program in order to correlate materials property changes between irradiations and facilities and to confidently predict the performance of materials in fusion reactors.

Digital timing walk correction and its application to a time-dependent fast neutron energy spectrum measurement

Timing walk is the pulse height dependent shift in the mean time at which a discriminator triggers. If there is any such walk, when a pulse height spectrum is sampled over short time intervals, as is the case in time-dependent fast neutron spectroscopy, severe distortions to the pulse height spectrum can occur. Results are given of measurements of walk and consequent pulse height spectrum distortions. A procedure which uses an on-line computer to compensate for walk is described and its effectiveness evaluated. A study is made of distortions of time-dependent spectra, resulting from noise which broadens time resolution at low pulse amplitudes. The reduction of timing walk to ±35 ps is illustrated for a time-dependent neutron energy spectrum measurement.

The effect of pulse pile-up on discrimination between neutrons and gamma rays

Pulse pile-up lengthens the rise-time of pulses. With an organic scintillator such as NE 213, pile-up can cause a short rise-time pulse originating from gamma rays to be interpreted by a rise-time analyser as a neutron. The degradation of pulse shape analyser performance at high count rates is shown to be directly related to pulse pile-up. Using this relationship, the contribution of piled-up gamma rays and neutrons to count rate related errors is calculated for a time-dependent fast neutron energy spectrum measurement. Errors of a few per cent occur even when the probability of a count per burst is as low as 0.01.

New6Li Sandwich Counter for Measurement of Neutron Spectra in Fast Neutron Systems

A new form of 6Li sandwich counter has been developed for use in measurements of neutron spectra in fast critical assemblies. The counters have two large surface area diodes approached very close to each other, on one of which is evaporated the target 6Li. This arrangement proves to provide a ten-fold increase of detection efficiency, while the resolution is improved by approximately ½ compared with currently used counters. The new counters were first tested on mono-energetic neutrons, after which neutron spectrum measurements were performed on natural uranium blocks, in the fast source reactor “YAYOI”, in the FCA IV-l-P and in the FCA V-3 core. The resulting spectra are compared with the measurements made with other types of counter and with calculated data. Proof was obtained that the new counter can serve practically and usefully for in-core measurements of fast neutron energy spectrum between 0.8 and 5 MeV.