Array Of Liquid Scintillators Research Articles

Correlations between fission fragment and neutron anisotropies in neutron-induced fission

Several sources of angular anisotropy for fission fragments and prompt neutrons have been studied in neutron-induced fission reactions. These include kinematic recoils of the target from the incident neutron beam and the fragments from the emission of the prompt neutrons, preferential directions of the emission of the fission fragments with respect to the beam axis due to the population of particular transition states at the fission barrier, and forward-peaked angular distributions of pre-equilibrium neutrons which are emitted before the formation of a compound nucleus. In addition, there are several potential sources of angular anisotropies that are more difficult to disentangle: the angular distributions of prompt neutrons from fully accelerated fragments or from scission neutrons, and the emission of neutrons from fission fragments that are not fully accelerated. In this work, we study the effects of the first group of anisotropy sources, particularly exploring the correlations between the fission fragment anisotropy and the resulting neutron anisotropy. While kinematic effects were already accounted for in our Hauser-Feshbach Monte Carlo code, $\mathtt{CGMF}$, anisotropic angular distributions for the fission fragments and pre-equilibrium neutrons resulting from neutron-induced fission on $^{233,234,235,238}$U, $^{239,241}$Pu, and $^{237}$Np have been introduced for the first time. The effects of these sources of anisotropy are examined over a range of incident neutron energies, from thermal to 20 MeV, and compared to experimental data from the Chi-Nu liquid scintillator array. The anisotropy of the fission fragments is reflected in the anisotropy of the prompt neutrons, especially as the outgoing energy of the prompt neutrons increases, allowing for an extraction of the fission fragment anisotropy to be made from a measurement of the neutrons.

First comparison of GEANT4 hadrontherapy physics model with experimental data for a NUMEN project reaction case

Gamma-ray and neutron spectra from the \(^{18}\hbox {O }{+}^{76} \hbox {Se}\) reaction at 15.3 MeV/u were measured with the EDEN array of liquid scintillators at the LNS. The results were compared to GEANT Hadrontherapy physics list simulations in order to assess the reliability of this model for the development of the NUMEN project. A good agreement with the shape of the experimental gamma-ray spectra and a reasonable agreement with the total count rates were obtained. The gamma spectra originated from the nuclear reactions were selected by time coincidence with the Superconducting Cyclotron radio-frequency reference signal. The random coincidence background rate was appropriately described only when the Faraday Cup, the material and geometry of the experimental hall and its contents were included in the simulation with sufficient detail. The information on the radiation spectra is important for the adequate development of the project of the detector arrays and electronic equipment for the advanced phase of NUMEN. Since orders of magnitude larger beam intensities are planned for this phase, the random coincidence rate is also of significant importance, particularly for the performance of the G-NUMEN gamma calorimeter array.

Prompt-fission-neutron spectra in the Pu239(n,f) reaction

Prompt-fission-neutron spectra from $^{239}\mathrm{Pu}$ ($\mathit{n},\phantom{\rule{0.16em}{0ex}}\mathit{f}$) were measured with respect to $^{252}\mathrm{Cf}$ spontaneous fission for incident neutron energies from 0.7 to $700\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ at the Weapons Neutron Research facility (WNR) of the Los Alamos Neutron Science Center. A newly designed high-efficiency fission chamber was coupled to the highly segmented Chi-Nu neutron liquid scintillator array to detect neutrons emitted in fission events. The double time-of-flight technique was used to deduce the incident neutron energies from the spallation target and the outgoing-neutron energies from the fission chamber. Prompt-fission-neutron spectra (PFNS) were measured with respect to $^{252}\mathrm{Cf}$ spontaneous fission down to $200\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$ and up to about $12\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ for all the incident neutron energies with typical total uncertainties well below $2%$ up to about 7-MeV outgoing-neutron energy. The general trend of PFNS is well reproduced by JEFF3.3 and ENDF evaluations, although a better agreement is found with JEFF3.3. Discrepancies were observed for the low-energy part of the spectra, especially around the opening of the second-, third- and fourth-chance fission. Neutron average kinetic energies as a function of incident neutron energy are obtained experimentally with reported total uncertainties below $0.5%$. The measured values agree with the most recent data. The trend is fairly well reproduced by the JEFF3.3 evaluation, although it fails to reproduce the experimental values within their uncertainties.

Neutron-neutron angular correlations in spontaneous fission of Cf252 and Pu240

Background: Angular anisotropy has been observed between prompt neutrons emitted during the fission process. Such an anisotropy arises because the emitted neutrons are boosted along the direction of the parent fragment.Purpose: To measure the neutron-neutron angular correlations from the spontaneous fission of $^{252}\mathrm{Cf}$ and $^{240}\mathrm{Pu}$ oxide samples using a liquid scintillator array capable of pulse-shape discrimination. To compare these correlations to simulations combining the Monte Carlo radiation transport code MCNPX with the fission event generator FREYA.Method: Two different analysis methods were used to study the neutron-neutron correlations with varying energy thresholds. The first is based on setting a light output threshold while the second imposes a time-of-flight cutoff. The second method has the advantage of being truly detector independent.Results: The neutron-neutron correlation modeled by FREYA depends strongly on the sharing of the excitation energy between the two fragments. The measured asymmetry enabled us to adjust the FREYA parameter $x$ in $^{240}\mathrm{Pu}$, which controls the energy partition between the fragments and is so far inaccessible in other measurements. The $^{240}\mathrm{Pu}$ data in this analysis was the first available to quantify the energy partition for this isotope. The agreement between data and simulation is overall very good for $^{252}\mathrm{Cf}(\mathrm{sf})$ and $^{240}\mathrm{Pu}(\mathrm{sf})$.Conclusions: The asymmetry in the measured neutron-neutron angular distributions can be predicted by FREYA. The shape of the correlation function depends on how the excitation energy is partitioned between the two fission fragments. Experimental data suggest that the lighter fragment is disproportionately excited.

Isovector excitations in 100Nb and their decays by neutron emission studied via the [formula omitted] reaction at 115 MeV/u

Spin–isospin excitations in Nb100 were studied via the Mo100(t,He3) charge-exchange reaction at 115 MeV/u with the goal to constrain theoretical models used to describe the isovector spin response of nuclei. The experiment was performed with a secondary beam of tritons, and 3He particles were analyzed in the S800 magnetic spectrometer. Decay by neutron emission from excited states in Nb100 was observed by using plastic and liquid scintillator arrays. Differential cross sections were analyzed and monopole excitations were revealed by using a multipole decomposition analysis. The Gamow–Teller transition strength observed at low excitation energies, which is important for estimating the electron-capture rate in astrophysical scenarios, was strongly fragmented and reduced compared to single-particle and spherical mean-field models. The consideration of deformation in the theoretical estimates was found to be important to better describe the fragmentation and strengths. A strong excitation of the isovector spin giant monopole resonance was observed, and well reproduced by the mean-field models. Its presence makes the extraction of Gamow–Teller strengths at high excitation energies difficult. The branches for statistical and direct decay by neutron emission were identified in the spectra. The upper limit for the branching ratio by direct decay (integrated over all observed excitations) was determined to be 20±6%. Even though the statistical uncertainties in the neutron-coincident data were too large to perform detailed studies of the decay by neutron emission from individual states and resonances, the experiment demonstrates the feasibility of the method.

Time Evolving Fission Chain Theory and Fast Neutron and Gamma-Ray Counting Distributions

We solve a simple theoretical model of time evolving fission chains due to Feynman that generalizes and asymptotically approaches the point model theory. The point model theory has been used to analyze thermal neutron counting data. This extension of the theory underlies fast counting data for both neutrons and gamma rays from metal systems. Fast neutron and gamma-ray counting is now possible using liquid scintillator arrays with nanosecond time resolution. For individual fission chains, the differential equations describing three correlated probability distributions are solved: the time-dependent internal neutron population, accumulation of fissions in time, and accumulation of leaked neutrons in time. Explicit analytic formulas are given for correlated moments of the time evolving chain populations. The equations for random time gate fast neutron and gamma-ray counting distributions, due to randomly initiated chains, are presented. Correlated moment equations are given for both random time gate and triggered time gate counting. Explicit formulas for all correlated moments are given up to triple order, for all combinations of correlated fast neutrons and gamma rays. The nonlinear differential equations for probabilities for time dependent fission chain populations have a remarkably simple Monte Carlo realization. A Monte Carlo code was developed for this theory and is shown to statistically realize the solutions to the fission chain theory probability distributions. Combined with random initiation of chains and detection of external quanta, the Monte Carlo code generates time tagged data for neutron and gamma-ray counting and from these data the counting distributions.

Neutron crosstalk between liquid scintillators

A method is proposed to quantify the fractions of neutrons scattering between liquid scintillators. Using a spontaneous fission source, this method can be utilized to quickly characterize an array of liquid scintillators in terms of crosstalk. The point model theory due to Feynman is corrected to account for these multiple scatterings. Using spectral information measured by the liquid scintillators, fractions of multiple scattering can be estimated, and mass reconstruction of fissile materials under investigation can be improved. Monte Carlo simulations of mono-energetic neutron sources were performed to estimate neutron crosstalk. A californium source in an array of liquid scintillators was modeled to illustrate the improvement of the mass reconstruction.

Digital fast neutron radiography of steel reinforcing bar in concrete

Neutron imaging has previously been used in order to test for cracks, degradation and water content in concrete. However, these techniques often fall short of alternative non-destructive testing methods, such as γ-ray and X-ray imaging, particularly in terms of resolution. Further,thermal neutron techniques can be compromised by the significant expense associated with thermal neutron sources of sufficient intensity to yield satisfactory results that can often precipitate the need for a reactor. Such embodiments are clearly not portable in the context of the needs of field applications. This paper summarises the results of a study to investigate the potential for transmission radiography based on fast neutrons. The objective of this study was to determine whether the presence of heterogeneities in concrete, such as reinforcement structures, could be identified on the basis of variation in transmitted fast-neutron flux. Monte-Carlo simulations have been performed and the results from these are compared to those arising from practical tests using a 252Cf source. The experimental data have been acquired using a digital pulse-shape discrimination system that enables fast neutron transmission to be studied across an array of liquid scintillators placed in close proximity to samples under test, and read out in real time. Whilst this study does not yield sufficient spatial resolution, a comparison of overall flux ratios does provide a basis for the discrimination between samples with contrasting rebar content. This approach offers the potential for non-destructive testing that gives less dose, better transportability and better accessibility than competing approaches. It is also suitable for thick samples where γ-ray and X-ray methods can be limited.

Development of an Array of Liquid Scintillators to Measure the Prompt Fission Neutron Spectrum at LANSCE

Higher quality measurements of outgoing prompt neutron spectra from neutron-induced fission as a function of the incoming neutron energy are needed. These data can be used in designing new fast reactors, predicting criticality for safety analyses, and developing techniques for global security applications. As part of the program to measure the prompt fission neutron spectra (PFNS) from the fission of 239Pu at the Los Alamos Neutron Science Center, we are developing a new array of liquid-scintillator detectors. This array will be used to measure the PFNS over a range of outgoing neutron energies from approximately 600 keV to 12 MeV and incident neutron energies from 0.5 to 30 MeV. A complete characterization of the detectors and the array as a whole will be carried out, targeted at understanding the light-output curves, efficiencies, and the neutron multiple-scattering backgrounds.

Prompt Fission γ-rays Measured Using Liquid Scintillators

The prompt γ-ray spectra from 235U(n,f) at incident energies of 1 to 20 MeV and for 235Cf(s.f.) were measured up to 4 MeV in coincidence with two parallel plate avalanche counters and the liquid scintillator array FIGARO. The unfolded γ-ray spectra from 1–4 MeV using the single value decomposition and iterative Bayesian techniques for the incident neutron energy regions corresponding to the first, second and third chance fissions were found to nearly identical and similar to the distribution from the spontaneous fission of 252Cf. General agreement with the γ-ray distributions from fission was also found with previous measurements.

Systematic Study of (d,n) Reactions at Ed= 16 MeV Using a Deuterated Scintillator Array

We have developed a multi-element deuterated liquid scintillator array for the study of reactions involving neutrons utilizing high-speed digital signal processing. A systematic study of (d,n) reactions at Ed = 16 MeV on 9 Be, 11 B, 13 C, 14 N, 15 N, and 19 F has been conducted from 10 o to 160 o (lab). In addition to previously unmeasured back-angle cross sections, this data can complement existing and future ( 3 He,d) measurements as an analog to (p,γ) for astrophysics applications.

THE USE OF FAST NEUTRON DETECTION FOR MATERIALS ACCOUNTABILITY

For many years at LLNL, we have been developing time-correlated neutron detection techniques and algorithms for applications such as Arms Control, Threat Detection and Nuclear Material Assay. Many of our techniques have been developed specifically for the relatively low efficiency (a few percent) inherent in man-portable systems. Historically, thermal neutron detectors (mainly 3 He ) were used, taking advantage of the high thermal neutron interaction cross-sections, but more recently we have been investigating the use of fast neutron detection with liquid scintillators, inorganic crystals, and in the near future, pulse-shape discriminating plastics that respond over 1000 times faster (nanoseconds versus tens of microseconds) than thermal neutron detectors. Fast neutron detection offers considerable advantages, since the inherent nanosecond production timescales of fission and neutron-induced fission are preserved and measured instead of being lost in the thermalization of thermal neutron detectors. We are now applying fast neutron technology to the safeguards regime in the form of high efficiency counters. Faster detector response times and sensitivity to neutron momentum show promise in measuring, differentiating, and assaying samples that have modest to very high count rates, as well as mixed neutron sources (e.g., Pu oxide or Mixed Cm and Pu ). Here we report on measured results with our existing liquid scintillator array and promote the design of a nuclear material assay system that incorporates fast neutron detection, including the surprising result that fast liquid scintillator becomes competitive and even surpasses the precision of 3 He counters measuring correlated pairs in modest (kg) samples of plutonium.

Neutron Spectroscopy Without Time-of-Flight Measurement: A DSP-Based Deuterated Scintillator Array

Development and evaluation of a large-volume multi-element digital-signal-processing (DSP)-based deuterated (C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> ) liquid scintillator array for the study of reactions involving neutrons without the need for measurement of neutron time-of-flight (ToF) is described. In-beam testing was conducted at the University of Notre Dame (UND) 10 MV FN tandem Van de Graaff accelerator using a deuteron beam at E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</sub> =16.0 MeV. The energy response and detector efficiency of 100 mm dia. ×; 150 mm long cylindrical deuterated liquid scintillator (Eljen 315) detectors was determined in the range E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> =4.8 to 8.7 MeV and E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</sub> =6.8 to 21.2 MeV, respectively. In addition, we determined that optimized digital pulse-shape discrimination (DPSD) in liquid scintillator detectors using enhanced liquid (Eljen 315MOD) and other methods can yield improved recoil-particle tagging. As we demonstrate, this is particularly important in deuterated scintillators at neutron energies above 15 MeV. This is especially advantageous for study of nuclear reactions involving radioactive beams, since these often have large, positive reaction Q values.

Prompt energy distribution of 235U(n,f)[formula omitted] at bombarding energies of 1–20 MeV

The distributions of prompt γ rays from the spontaneous fission of 252Cf and neutron-induced fission of 235U were measured up to ∼4MeV using a liquid scintillator array. The unfolding of measured fission γ rays is presented using the Single Value Decomposition and iterative Bayesian methods. General agreement was found with comparisons made with previous measurements. The energy dependence of the prompt γ-ray distributions for the spontaneous fission of 252Cf and the neutron-induced fission of 235U from bombarding energies of 1-2, 5-10, and 10–20MeV were found to be almost identical in the γ-ray energy region 1–4MeV.

Monitoring Spent or Reprocessed Nuclear Fuel Using Fast Neutrons

Over the past few years a number of experiments have been carried out at LLNL with a scintillator array that has the ability to count individual MeV neutrons and γ-rays with nanosecond timing. It has been demonstrated that this array can be used to measure the statistical properties of the neutrons emitted in single fission chains. The multiple time scales over which these fission neutrons are correlated allow one to deduce quite a lot regarding the nature of the fissile assembly. In this paper we will describe how neutron correlations measured with a liquid scintillator array can be used to assay the amounts of fissile elements in reprocessed and spent nuclear fuels.

The nuclear data program at rensselaer

The nuclear data program at the Rensselaer Polytechnic Institute (RPI) is centered around a 60 MeV pulsed electron Linear Accelerator (LINAC) configured to deliver pulsed neutron beams. The LINAC electron pulse width can vary between 5 ns and 5 us, a repetition rate of 1–500 pulses per second and neutron yield of up to 1013 n/s. Over the years several experimental setups and techniques were developed to enable a variety of measurement capabilities. The experiments cover the energy range from 0.001 eV to 20 MeV and included neutron transmission, capture, scattering and fission measurements. The facility is also equipped with a Lead Slowing-Down Spectrometer (LSDS) producing a high neutron flux that is used for simultaneous measurements of the fission cross section and fission fragment mass and energy distributions. Detectors for (n, alpha) and (n, p) cross section measurements using the LSDS were also developed and measurements of (n, alpha) cross section were completed. The high neutron flux inside the LSDS enables measurements on small samples (sub micrograms) or samples with small cross section (sub millibarns).In order to fully utilize the capabilities of the facility, several measurement techniques were developed; high accuracy (<1%) total cross sections were measured using iron and uranium filtered neutron beams. A system for fast neutron scattering measurements using an array of liquid scintillators and a digital data acquisition system was recently developed and used for several measurements and is now being adapted for fission neutron studies.Methods for simultaneous measurements of fission and capture cross sections using the RPI multiplicity detector are under development and new modular detectors for both fast and resonance region neutron detection were developed and deployed.These developments result in numerous measurements on different materials and provide input to improved evaluated nuclear data libraries and thus help improve the accuracy of calculations of nuclear reactors and other nuclear systems.

Measurement and simulation of the neutron response of the Nordball liquid scintillator array

The response of the liquid scintillator array Nordball to neutrons in the energy range 1.5 < T n < 10 MeV has been measured by time of flight using a 252Cf fission source. Fission fragments were detected by means of a thin-film plastic scintillator. The measured differential and integral neutron detection efficiencies agree well with predictions of a Monte Carlo simulation of the detector which models geometry accurately and incorporates the measured, non-linear proton light output as a function of energy. The ability of the model to provide systematic corrections to photoneutron cross-sections, measured by Nordball at low energy, is tested in a measurement of the two-body deuteron photodisintegration cross-section in the range E γ = 14 – 18 MeV . After correction the present H 2 ( γ , n ) p measurements agree well with a published evaluation of the large body of H 2 ( γ , p ) n data.

Near-threshold measurement of the 4He[formula omitted] reaction

A near-threshold 4He(γ,n) cross-section measurement has been performed at MAX-lab. Tagged photons from 23<Eγ<42 MeV were directed toward a liquid 4He target, and neutrons were detected by time-of-flight in two liquid-scintillator arrays. Seven-point angular distributions were measured for eight photon energies. The results are compared to experimental data measured at comparable energies and Recoil-Corrected Continuum Shell Model, Resonating Group Method, and recent Hyperspherical-Harmonic Expansion calculations. The angle-integrated cross-section data are peaked at a photon energy of about 28 MeV, in disagreement with the value recommended by Calarco, Berman, and Donnelly in 1983.

First identification of excited states in the $T_z\! = \!1/2$ nucleus 93 Pd

The first experimental information about excited states in the N = Z + 1 nucleus 93Pd is presented. The experiment was performed using a 205 MeV 58Ni beam from the Vivitron accelerator at IReS, Strasbourg, impinging on a bismuth-backed 40Ca target. Gamma-rays, neutrons and charged particles emitted in the reactions were detected using the Ge detector array Euroball, the Neutron Wall liquid-scintillator array and the Euclides Si charged-particle detector system. The experimental level scheme is compared with the results of new shell model calculations which predict a coupling scheme with aligned neutron-proton pairs to greatly influence the level structure of $N\approx Z$ nuclei at low excitation energies.

Quasifree compton scattering from the deuteron and nucleon polarizabilities

Cross sections for quasifree Compton scattering from the deuteron were measured for incident energies of E(gamma) = 236-260 MeV at the laboratory angle straight theta(gamma(')) = -135 degrees. The recoil nucleons were detected in a liquid-scintillator array situated at straight theta(N) = 20 degrees. The measured differential cross sections were used, with the calculations of Levchuk et al., to determine the polarizabilities of the bound nucleons. For the bound proton, the extracted values were consistent with the accepted value for the free proton. Combining our results for the bound neutron with those from Rose et al., we obtain 1-sigma constraints of alpha;(n) = 7.6-14.0 and beta;(n) = 1.2-7.6.