Neutron Capture Time Research Articles

Second gadolinium loading to Super-Kamiokande

The first loading of gadolinium (Gd) into Super-Kamiokande in 2020 was successful, and the neutron capture efficiency on Gd reached 50%. To further increase the Gd neutron capture efficiency to 75%, 26.1 tons of Gd2(SO4)3⋅8H2O was additionally loaded into Super-Kamiokande (SK) from May 31 to July 4, 2022. As the amount of loaded Gd2(SO4)3⋅8H2O was doubled compared to the first loading, the capacity of the powder dissolving system was doubled. We also developed new batches of gadolinium sulfate with even further reduced radioactive impurities. In addition, a more efficient screening method was devised and implemented to evaluate these new batches of Gd2(SO4)3⋅8H2O. Following the second loading, the Gd concentration in SK was measured to be 333.5±2.5 ppm via an Atomic Absorption Spectrometer (AAS). From the mean neutron capture time constant of neutrons from an Am/Be calibration source, the Gd concentration was independently measured to be 332.7 ± 6.8(sys.) ± 1.1(stat.) ppm, consistent with the AAS result. Furthermore, during the loading the Gd concentration was monitored continually using the capture time constant of each spallation neutron produced by cosmic-ray muons, and the final neutron capture efficiency was shown to become 1.5 times higher than that of the first loaded phase, as expected.

Evaluation of the response of plastic scintillator bars and measurement of neutron capture time in non-reactor environment for the ISMRAN experiment

We present a detailed study on detector response to different radioactive sources and the measurements of non-reactor environmental backgrounds with the Indian Scintillator Matrix for Reactor Anti-Neutrinos (ISMRAN) detector setup consisting of 9 × 10 Plastic Scintillator Bars (PSBs) array at BARC, Mumbai. These measurements are useful in the context of the ISMRAN detector setup, which will be used to detect the reactor anti-neutrinos (ν¯e) and measure its energy spectra through the inverse beta decay (IBD) process. A GEANT4 based Monte Carlo (MC) simulation is used to understand the optical transmission, energy resolution and energy non-linearity of the ISMRAN detector. A detailed analysis procedure has been developed to understand the natural radioactive, cosmogenic and cosmic muon-induced backgrounds with the ISMRAN detector setup in a non-reactor environment, based on their energy deposition, number of bars hit as well as topological event selection criteria in position and time for triggered events. Data and MC simulated distributions of reconstructed sum energy and number of bars hit has been compared for the radioactive γ + positron source such as 22Na placed at the center of the ISMRAN array. Fast neutron energy response and capture time distribution in ISMRAN array has been studied using a novel technique involving Time of Flight (TOF) of the measured fast neutrons. The observed characteristic neutron capture time (τ) of 68.29±9.48μs is in good agreement with ∼65μs obtained from MC simulation. These experimentally measured results will be useful for discriminating the correlated and uncorrelated background events from the true IBD events in reactor ON and OFF conditions inside the reactor hall.

First gadolinium loading to Super-Kamiokande

In order to improve Super-Kamiokande’s neutron detection efficiency and to thereby increase its sensitivity to the diffuse supernova neutrino background flux, 13 tons of Gd2(SO4)3⋅8H2O (gadolinium sulfate octahydrate) was dissolved into the detector’s otherwise ultrapure water from July 14 to August 17, 2020, marking the start of the SK-Gd phase of operations. During the loading, water was continuously recirculated at a rate of 60 m3/h, extracting water from the top of the detector and mixing it with concentrated Gd2(SO4)3⋅8H2O solution to create a 0.02% solution of the Gd compound before injecting it into the bottom of the detector. A clear boundary between the Gd-loaded and pure water was maintained through the loading, enabling monitoring of the loading itself and the spatial uniformity of the Gd concentration over the 35 days it took to reach the top of the detector. During the subsequent commissioning the recirculation rate was increased to 120 m3/h, resulting in a constant and uniform distribution of Gd throughout the detector and water transparency equivalent to that of previous pure-water operation periods. Using an Am–Be neutron calibration source the mean neutron capture time was measured to be 115±1 μs, which corresponds to a Gd concentration of 111±2 ppm, as expected for this level of Gd loading. This paper describes changes made to the water circulation system for this detector upgrade, the Gd loading procedure, detector commissioning, and the first neutron calibration measurements in SK-Gd.

Measurement of neutron-proton capture in the SNO+ water phase

The SNO+ experiment collected data as a low-threshold water Cherenkov detector from September 2017 to July 2019. Measurements of the 2.2-MeV $\gamma$ produced by neutron capture on hydrogen have been made using an Am-Be calibration source, for which a large fraction of emitted neutrons are produced simultaneously with a 4.4-MeV $\gamma$. Analysis of the delayed coincidence between the 4.4-MeV $\gamma$ and the 2.2-MeV capture $\gamma$ revealed a neutron detection efficiency that is centered around 50% and varies at the level of 1% across the inner region of the detector, which to our knowledge is the highest efficiency achieved among pure water Cherenkov detectors. In addition, the neutron capture time constant was measured and converted to a thermal neutron-proton capture cross section of $336.3^{+1.2}_{-1.5}$ mb.

Aging study of Gd concentration in LAB-based Gd loaded liquid scintillator exposed to passivated stainless steel

Stainless steel is a candidate of the inner surface of the storage container for LAB-based Gadolinium loaded liquid scintillator (Gd-LS) to be used in the JSNS2 neutrino detector. Aging effect on Gd concentration of Gd-LS was investigated with the Gd-LS sample stored in the passivated stainless steel bottle by two independent methods. The direct comparison of the neutron capture time measurement showed that there is no significant degradation of the capture time after 617 days aging. Titration was performed to measure Gd concentration, and the result after 466 days aging is consistent with the result before aging within the uncertainty of the measurement. Both results lead to a conclusion that stainless steel is usable for Gd-LS storage for JSNS2 experiment.

Performance of a segmented 6Li-loaded liquid scintillator detector for the PROSPECT experiment

This paper describes the design and performance of a 50 liter, two-segment 6Li-loaded liquid scintillator detector that was designed and operated as prototype for the PROSPECT (Precision Reactor Oscillation and Spectrum) Experiment. The two-segment detector was constructed according to the design specifications of the experiment. It features low-mass optical separators, an integrated source and optical calibration system, and materials that are compatible with the 6Li-doped scintillator developed by PROSPECT. We demonstrate a high light collection of 850±20 PE/MeV, an energy resolution of σ = 4.0±0.2% at 1 MeV, and efficient pulse-shape discrimination of low dE/dx (electronic recoil) and high dE/dx (nuclear recoil) energy depositions. An effective scintillation attenuation length of 85±3 cm is measured in each segment. The 0.1% by mass concentration of 6Li in the scintillator results in a measured neutron capture time of τ = 42.8±0.2 μs. The long-term stability of the scintillator is also discussed. The detector response meets the criteria necessary for achieving the PROSPECT physics goals and demonstrates features that may find application in fast neutron detection.

Measurement of sin2(2θ13) via neutron capture on hydrogen at Daya Bay

An independent measurement of neutrino oscillation angle θ13 using samples based on neutron captured on hydrogen (nH) was performed at the Daya Bay Reactor Neutrino Experiment. To deal with the challenges due to larger backgrounds, longer neutron capture time and lower detection efficiency, the nH analysis developed several data-driven techniques to precisely measure backgrounds and to control systematic uncertainties. This statistically independent and largely systematically uncorrelated independent analysis provides a firm confirmation of the nGd result and improves the overall uncertainty of sin2(2θ13). With 621 days of data and two newly installed antineutrino detectors, nH analysis yields sin2(2θ13) = 0.071 ± 0.011 in the three neutrino oscillation framework.

Measurement of the Neutron Capture Time on Hydrogen at the RENO

Measurement of the Neutron Capture Time on Hydrogen at the RENO

NSTAR—A capture gated plastic neutron detector

NSTAR (Neutron Sandwich Transmuter/Activation- γ Radiator) prototypes were developed and their performances were evaluated using radioactive sources and a pulsed neutron beam. The NSTAR operating principle is similar to that of Gd-loaded liquid scintillation detectors, where the scintillator has dual functions as neutron moderator and sensor of delayed capture γ -rays , but spatially separates scintillator from neutron converter components. The time dependent NSTAR response to neutrons consists of a prompt, energy related light flash followed by a delayed signal characteristic in both light output and delay time. This feature allows one to discriminate on average between neutrons and γ -rays and provides the basis for multiplicity determination. The detectors are scalable, economic to construct of environmentally benign components, and can be ruggedized. Prototype detector modules consist of 12×20×(50 or 100) cm 3 stacks of plastic scintillator slabs (Saint Gobain BC-408) alternating with thin Gd converter films viewed by fast photomultipliers (Philips XP2041). The effective Gd/scintillator ratio is 0.5 wt%. Results of tests of NSTAR with 252Cf and neutrons from the D(d,n) 3He reactions are in good agreement with theoretical estimates based on neutron transport simulations. Characteristics of the detector module include an average neutron capture time of 〈 t c 〉 = 21.7 ± 0.2 μ s and a detection efficiency of ɛ = 26 ± 3 % for DD neutrons. The NSTAR has been applied to determine the multiplicity distribution of neutrons produced in D(d,n) 3He reactions by a neutron generator.

Performances and stability of a 2.4 ton Gd organic liquid scintillator target for ν̄e detection

In this paper we report the performance and the chemical and physical properties of a 2 × 1.2 ton organic liquid scintillator target doped with Gd up to ∼ 0.1%, and the results of a 3 year long stability survey of the target. In particular we have measured and monitored the optical and fluorescent properties of the Gd-doped liquid scintillator (LS), the amount of both Gd and primary fluor in solution, and the performance of the two Gd doped targets as neutron detectors, namely neutron capture efficiency and average capture time. The experimental survey is ongoing, the targets being continuously monitored. From the spectrophotometric measurements performed on samples periodically extracted along the three years, we can exclude, at 99% C.L. level, a degradation of the light transmittance of the Gd-doped liquid scintillator larger than 1% y-1; from the in-tank measurements no significant decrease of the neutron capture efficiency and neutron capture time is observed. This is the largest stable Gd-doped organic liquid scintillator target ever produced and continuously operated for a long period.

Digital techniques for n /γ pulse shape discrimination and capture-gated neutron spectroscopy using liquid scintillators

We have investigated the use of digital pulse processing techniques for n/γ pulse shape discrimination (PSD) in liquid scintillators, and the application of these techniques to a capture-gated fast neutron monitor developed using the enriched 10B-loaded liquid scintillator BC523A. The motivation for this study has been to develop computationally fast digital PSD algorithms that can be implemented in a compact portable fast neutron monitor. The PSD separation achieved with a digital pulse risetime algorithm has a Figure of Merit comparable with previously reported analogue techniques, and provides extremely sensitive fast neutron detection in the presence of a strong gamma background. The capture-gated performance of a 685 cm 3 BC523A liquid scintillator cell was investigated using fast neutrons from an Am–Be source, and the application of digital pulse processing techniques to the capture-gated detection mode is discussed. The measured mean neutron capture time in BC523A was 470±80 ns which is a factor of five shorter than that reported for liquid scintillators loaded with natural boron.

The measurement of passive neutron multiplicity counter gate utilisation factors and comparisons with theory

In a recent paper, a new method of calculating the doublets and triplets gate utilisation factors f 2 and f 3 for passive neutron multiplicity counters has been developed resulting in the MCF computer code. In this paper we arrange the basic multiplicity equations and generate nuclear data parameter combinations to enable f 2, f 3, f 2/ f 3 and f 2 2/ f 3 to be derived from measurements taken using a calibrated 252Cf source and multiplicity shift-register electronics. The two approaches have been compared with reference to Harwell Instruments Ltd. N95 neutron coincidence counting measurement chamber operated both with and without an internal cadmium liner. The Monte Carlo code MCNP™ was used to calculate neutron capture time distributions and these were fed into the MCF code to produce theoretical gate utilisation factors to compare with the experimental data. Pre-delays of 2 and 4 μs and coincidence gate widths between 16 and 240 μs were investigated. The predicted values agreed with those measured within 1.5% and 5% for the f 2 and f 3 results, respectively. These results confirm the power of the new theoretical treatment in accurately predicting gate utilisation factors in an absolute sense and with confidence at the design stage. Relative predictions can be made with greater accuracy.

A large, low-energy neutrino detector for neutrino oscillations and supernovae watch

Abstract We describe a large, low-energy and low background Gd-loaded liquid scintillation detector with a fiducial mass of 1000 ton to be installed 200 m underground at a distance of 10–15 km from a power reactor. The detector will serve to explore neutrino oscillations down to mass parameters Δm2 of 10−4 eV2 and mixing angles sin22θ larger than 0.1 and also as a low-energy neutrino observatory for supernovae and other astrophysical events. The detector reaction νep → e+n provides a correlated e+, n signature with the neutron yielding an 8 MeV gamma cascade following capture in Gd. Some of the optimized parameters are: light transmission length 6.1 m, Gd concentration 0.05%, mean neutron capture time 46 μs. With thresholds for the positron kinetic energy and capture gamma rays of 1 and 3.5 MeV, respectively, we expect a detector efficiency of 50% and a signal rate of 16 neutrino events per day per 7.5 GW reactor power with a background rate of 2 per day. A 20 t prototype detector will serve for feasibility studies. Installed 1 km from a reactor it will be sensitive to mass parameters down to 10−3 eV2.

Neutron capture time scale of the s-process, estimated from s-process krypton in a meteorite

A krypton fraction enriched in s-process isotopes was extracted from a mineral fraction of the Murchison C2 chondrite. The (Kr-86)/(Kr-84) ratio is enhanced by 6 standard deviations, showing that significant amounts of Kr-86 were made in the s-process, despite the short, 10.8 yr beta-decay half-life of its precursor, Kr-85. Judging from this sample, the mean neutron capture time in the s-process was on the order of 5-100 yr for nuclei with cross sections of 125 mb.

S-PROCESS Studies: Branching and the Time Scale

The theory of S-process heavy-element formation is reformulated to allow for competition between beta decay and neutron capture at various nuclei along the path. Solutions to the resulting branching network equations are presented (under the assumption of constant temperature and neutron flux) that do not require steady flow for the neutron current. Using the exponential exposure distribution rho (tau) =G exp(-tau/tau/sub 0/) and recently calculated temperature-dependent beta-decay rates, comparison of several key branches yields the following average conditions for the solar-system S-process environment: Tapprox. =3.1 x 10/sup 8/ K, n/subn/approx. =1.6 x 10/sup 7/ neutrons cm/sup -3/. For tau/sub 0/=0.25 n mb/sup -1/ we find that about 4.8 neutron captures per exposed iron seed are required over a time of the order of a few thousand years for synthesis of the bulk of the solarsystem S-process material, with an average neutron capture time approx.10 years (for sigmaapprox.500 mb). (AIP)

A consideration of the neutron capture time scale in the s-process

A consideration of the neutron capture time scale in the s-process

Determination of the s-PROCESS Neutron Capture Time

The relative abundance that an isotope or element attains during the s-process depends critically, in many cases, upon the mean neutron capture time, T . The latter is very poorly known, however, with estimates ranging over a factor of 1O for a particular isotope. We show that it may be possible to determine T for a given isotope and star frotn observations. We have calculated the relative abundances of all the isotopes that participate in the s-process as a function of T under steady- state conditions. The assumption of steady state is not critical as long as we confine our attention to ratios of abundances of elements which are adjacent in the periodic table. We have also included in our calculations processes which may alter the effective fl-decay lifetime, such as the photo-beta process, and find that the effects on elemental abundance ratios are negligible. The calculations can be applied in a variety of ways. For example, in some highly evolved stars, material processed in the interior is apparently transported to the surface in such quantities as to completely dominate the atmospheric abundances of the heavy elements. In principle, one should be able to determine r,' for a given isotope for such a star by comparing observations with these calculations. With few exceptions, this does not appear possible as yet since the spectra of such stars are notoriously difficult to interpret. For R Cyg, however, Zr isotopic abundance observations rule out both very small and very large values of r,,. For solar-system material, examination of the abundance of isotopes produced only by the s-process indicates that r was probably on the order of 10 years. Subject headings: abundances, solar - abundances, stellar - Ba ii stars - interiors, stellar - nuclear reactions - S-type stars