Capture Γ-rays Research Articles

Capture γ-rays from 60Co as multi γ-ray efficiency standard for prompt γ-ray neutron activation analysis

The absolute emission probabilities of 49 γ-rays from thermal neutron capture in 59Co were measured in the energy range of 150–7500 keV. Two independent approaches were used to arrive at the number of captured neutrons. It has been proposed to use cobalt as a standard for absolute efficiency calibration of the detector over a wide energy range and as a suitable comparator in the k 0-standardisation method of prompt γ-ray neutron activation analysis work.

Limits on Radiative Capture γ-Ray Lines and Implications for Energy Content in Flare-Accelerated Protons

Measurements of radiative-capture γ-ray lines can provide information on both the energy content of nonthermal protons below 1 MeV and the temperature in the region where they interact. We have derived upper limits on the fluences in three proton-capture lines of 12C and 13C in the flare-averaged gamma-ray spectrum from 19 X-class flares observed with the Solar Maximum Mission (SMM). The most significant limit comes from the 2.37 MeV line that is excited by 0.46 MeV protons. We estimate an upper limit on the energy content in the accelerated protons by extrapolating the power-law spectrum derived at higher energies down to the resonant energy. The derived upper limit on the temperature, ∼ 5–10×107 K, is higher than measured in the flaring region with other techniques, even for this optimistic energy content. It is possible that NASA's High Energy Solar Spectroscopic Imager (HESSI) will be sensitive enough to detect the 2.37 MeV line if temperatures exceed ∼2×107 K.

Apparatus for parity-violation study via capture γ-ray measurements

The Time Reversal and Parity at Low Energy (TRIPLE) Collaboration uses a short-pulsed longitudinally polarized epithermal neutron beam at the Los Alamos Neutron Science Center to study spatial parity violation (PV) in the compound nucleus. The typical PV experiment measures the longitudinal cross-section asymmetry by the neutron transmission method through thick samples. Neutron capture γ-ray measurement provides an alternative method for the study of PV, which enables the use of smaller amounts of isotopically pure target material. In 1995 TRIPLE commissioned a new neutron-capture detector consisting of 24 pure CsI scintillators arranged in a cylindrical geometry around the neutron beam. The characteristics and the performance of the detector and spin transport are described.

Measurements of keV-Neutron Capture Cross Sections and Capture Gamma-Ray Spectra of 161 162 163Dy

The neutron capture cross sections and capture γ-ray spectra of 161 162 163Dy were measured in the neutron energy region of 10 to 90 keV and at 550 keV. A neutron time-of-flight method was adopted with a ns-pulsed neutron source by the 7Li(p, n)7Be reaction and with a large anti-Compton NaI(Tl) γ-ray spectrometer. A pulse-height weighting technique was applied to observed capture γ-ray pulse-height spectra to derive capture yields. The capture cross sections were obtained with the error of about 5% by using the standard capture cross sections of 197Au. The present results were compared with previous measurements and the evaluated values of ENDF/B-VI. Capture γ-ray spectra were obtained by unfolding observed capture γ-ray pulse-height spectra. An anomalous bump was observed around 2.5 MeV in the γ-ray spectra of each isotope, and the energy position of the bump was consistent with the systematics obtained in our previous work.

Measurements of keV-Neutron Capture Cross Sections and Capture Gamma-Ray Spectra of 161,162,163Dy.

The neutron capture cross sections and capture γ-ray spectra of 161 162 163Dy were measured in the neutron energy region of 10 to 90 keV and at 550 keV. A neutron time-of-flight method was adopted with a ns-pulsed neutron source by the 7Li(p, n)7Be reaction and with a large anti-Compton NaI(Tl) γ-ray spectrometer. A pulse-height weighting technique was applied to observed capture γ-ray pulse-height spectra to derive capture yields. The capture cross sections were obtained with the error of about 5% by using the standard capture cross sections of 197Au. The present results were compared with previous measurements and the evaluated values of ENDF/B-VI. Capture γ-ray spectra were obtained by unfolding observed capture γ-ray pulse-height spectra. An anomalous bump was observed around 2.5 MeV in the γ-ray spectra of each isotope, and the energy position of the bump was consistent with the systematics obtained in our previous work.

Rayleigh scattering of E = 465–2842 keV neutron capture γ-rays from Ta, In and Cu

Gamma rays in the energy range 465–2842 keV, produced by the Ni( n, γ reaction using thermal neutrons from a nuclear reactor, are used to measure Rayleigh scattering at θ = 1.02° from targets of Ta, In and Cu having a wide range of Z values. Very good agreement is obtained with a theoretical approximation based on modified relativistic form factors.

The uses of neutron capture γ-rays in environmental pollution measurements

A neutron capture γ-ray spectroscopy facility using an isotopic neutron source, 252Cf, has been installed and used for investigation of some environmental samples. The facility is designed and calibrated for measurement of the prompt γ-ray spectra due to thermal neutron capture. Qualitative analysis studies of some local environmental samples have been carried out using some developed analytical programs. The experimental results of the environmental pollutant analysis are discussed.

Test of parity violation and time reversal invariance in slow neutron absorption reactions

Measurements of helicity asymetries in slow neutron absorption reactions have been performed by detecting capture γ-rays. Large enhancements of parity-violation effect observed in these reactions are explained by interference between s- and p-wave resonances. The weak matrix element in hadronic reaction has been deduced from the results. Violation of time reversal invariance is being searched for in reactions in which large enhancement of the parity violation effect occurs. The same order of enhancement is expected in violation of time reversal invariance. The difference between the probability for helicity flip (+ → −) and that for (− → +) during transmission through a transversely polarized target is being measured. Recently, 139La nuclei in LaAlO 3 crystal have been polarized successfully. The crystal is useful for the target, since the highest enhancement of the parity violation effect has been found for the 139La target. The perspective of the experiment is shown.

Modified Method for Efficiency Calibration of High Energy γ Detector

A modified method of absolute efficiency calibration for germanium detector has been developed based on the fact that the intensities of chlorine capture γ-rays can be determined by the 1368 keV γ-rays from sodium activation when a NaCl sample is irradiated by thermal neutrons. As a verification of this method, the intensity of the 4.44 MeV γ-ray from an Am-Be neutron source was determined.

Intense positron sources by pair creation with neutron capture γ-rays

An intense source of positrons in the MeV range is described. The positrons were produced by pair production of high energy prompt γ-rays from neutron capture reactions. The source consisted of layers of titanium and platinum and it was placed in the high thermal neutron flux (3.3 × 10 14 n/cm 2s) at the BILL beta spectrometer of the High Flux Reactor, Grenoble. The beta spectrometer served as a tunable monochromator. The system yielded 2.5 × 10 6 e +/s in 10 cm of the focal plane, with a momentum dispersion of Δp/ p = 1.5 × 10 −3 cm −1. A model for the numerical simulation of this positron production method was developed and the data are compared with the calculated spectra. A future application of the present positron production technique is presented, using the layers of cadmium and tungsten to produce a beam of moderated positrons.

Longitudinal asymmetry in a neutron radiative capture reaction of 139La

An asymmetry in the cross section for the neutron-induced reaction of 139La was examined by a simultaneous measurement of the capture γ-ray and the neutron transmission. Sizable asymmetry was observed only in the p-wave resonance at E n = 0.734 eV. No significant asymmetry was observed in the other reactions. The asymmetry in the p-wave resonance was roughly constant as a function of E n. The average asymmetries in the resonance were A 1, γ = (9.5±0.3)% and A L,n = (9.7±0.5)% in the γ-ray and the transmission measurements, respectively.

Absolute cross section of the3He(?,?)7Be reaction

The absolute cross section σ(E) of the3He(α, γ)7Be reaction has been measured fromE c.m. =195 to 686 keV. The studies employed both a supersonic jet gas target and an extended gas target, and were carried out by the observation of the prompt capture γ-ray transitions as well as the7Be residual nuclei. The absolute cross sections deduced from the capture γ-rays are in good agreement with most previous work and remove a discrepancy with other earlier work. In comparison, the σ(E) values obtained from the7Be residual nuclei are systematically higher, suggesting a small production of additional7Be by contaminant reactions.

Radiative capture γ-rays arising from iron fibre additions to ilmenite limonite concrete shields

Iron fibres of different concentrations were added to ilmenite limonite concrete shielding material in different concentrations using the developments of mixed designs to improve its structural properties and to reduce the energy and the flux of the emitted radiation to acceptable levels. The capture γ-rays generated from the interaction of thermal neutrons with the iron fibres added were studied as a function of iron fibre additional concentration to ilmenite limonite concrete samples and sample thicknesses. It was found that the capture γ-ray intensity increases with the increase of the iron fibre additional concentration from 0 to 4% then decreases slightly on increasing the iron fibre additional concentration up to 6% for each sample thickness. Also, it was found that the capture γ-ray decreases with the increase of the sample thickness for the different iron fibre additional concentration. An empirical formula has been derived which relates the capture γ-ray flux with the sample thickness, and iron fibre additional concentration to ilmenite limonite concrete shields. Good agreement was obtained between the measured values of the capture γ-ray fluxes and those calculated using the derived empirical formula within ≈ 1.15.

Neutron capture gamma-rays from the low-lying resonances of 54Fe

Neutron capture γ-rays have been observed from the low-lying resonances of 54Fe using the Harwell electron linear accelerator HELIOS as a neutron source. Relative values for the partial radiative widths of resolved lines were obtained by area analysis of the γ-rays peaks over neutron resonance regions, and absolute values were obtained by a shape analysis fit to the yield deduced by comparison with a 10B(n, αγ) 7Li measurement. For the 7.8 keV s-wave resonance, a comparison of the partial widths with the predictions of the valence model has been made. In both relative and absolute senses, there is good agreement.

Systematics of the pygmy resonance in keV neutron capture γ-ray spectra of nuclei with N ≈ 82–126

We have measured capture gamma-ray spectra of Pr, Tb, Ho, Lu, Ta and Au at neutron energies of 10 to 800 keV with an anti-Compton NaI(Tl) detector, employing a time-of-flight technique. An anomalous bump, so-called the pygmy resonance, was observed in all these spectra. Remarkable features of the pygmy resonance were found to be that the resonance energy and the electric-dipole strength exhausted in the resonance increase with neutron number but these quantities decrease precipitously around the neutron magic number of N = 82. Comparison with theoretical calculations suggests that the pygmy resonance is mainly made up of neutron particle-hole states decoupled from the giant electric-dipole resonance and is excited in a collective mode.

A Doppler filter technique to measure the hydrogen content of planetary surfaces

A new technique to measure the hydrogen content of planetary surfaces is suggested. It relies on the Doppler energy shift of planetary albedo neutrons in the frame of a planar neutron sensor aboard a low-altitude orbiting spacecraft. It can be realized by a small modification to the current design of the anticoincidence shield enclosing a possible Mars Observer gamma-ray spectrometer. The resultant sensitivity to hydrogen near the surface of the Moon is estimated to be nearly three orders of magnitude greater than the sensitivity inherent in the detection of 2.223 MeV neutron capture γ-rays by the spectrometer.

Neutron Resonance Parameters of Tungsten-18 3 up to 1.1 keV

Neutron transmission, scattering and capture experiments on the separated isotope 183W were carried out in the resonance region, using a time-of-flight spectrometer of the Japan Atomic Energy Research Institute linear accelerator. A 6Li-glass and a 7Li-glass detectors were used in pair-wise to determine the spins by detecting scattered neutrons and capture γ-rays. Resonance parameters E and 2gγn were deduced up to 1.1 keV by analyzing the transmission data. Average level spacing <D> and s-wave strength function S0 were derived to be <D> = 13.3±leV, S0= (1.24±0.22) x 10−4 below 1.1 keV. Spins of 41 resonances were determined up to 900 eV from the capture and scattering experiments. The result on spin values supports the (2J+1) law on level density. The dependence of γgamma; on spin values is inferred. Frequent appearence of spacing of 173 eV between two arbitrary levels is discussed.

Measurement of the absolute cross section of the 3He(4He, γ)7Be reaction at Ec.m. = 525 keV

Abstract The cross section for the radiative capture reaction 3 He( 4 He, γ) 7 Be has been measured at 525 keV in the centre-of-mass by detection of prompt capture γ- rays. The targets were 3 He-implanted Nb foils that allowed us to circumvent the experimental difficulties inherent in the use of extended gas cells for absolute measurements. The results give an inferred zero-energy cross-section factor of S 34 (0) = 0.47 ± 0.04 keV · b. The present result is compared with results from previous capture γ-ray yield and 7 Be-activity methods of measuring the cross-section factor.

A new experimental study of the circular polarization of np capture γ-rays

An installation using a light-water neutron trap in the reactor core as a proton target is described. Results of the main and control measurements are presented which permit one to conclude that the parity-violating circular polarization of the γ-rays from the np → d γ reaction is P γ = (1.8 ± 1.8) × 10 −7.

Neutron resonance parameters of tungsten-183 up to 1.1 keV.

Neutron transmission, scattering and capture experiments on the separated isotope 183W were carried out in the resonance region, using a time-of-flight spectrometer of the Japan Atomic Energy Research Institute linear accelerator. A 6Li-glass and a 7Li-glass detectors were used in pair-wise to determine the spins by detecting scattered neutrons and capture γ-rays. Resonance parameters E and 2gγn were deduced up to 1.1 keV by analyzing the transmission data. Average level spacing <D> and s-wave strength function S0 were derived to be <D> = 13.3±leV, S0= (1.24±0.22) x 10−4 below 1.1 keV. Spins of 41 resonances were determined up to 900 eV from the capture and scattering experiments. The result on spin values supports the (2J+1) law on level density. The dependence of γgamma; on spin values is inferred. Frequent appearence of spacing of 173 eV between two arbitrary levels is discussed.