Neutron Capture Γ-rays Research Articles

Measurements of the neutron capture cross section of 243Am around 23.5 keV

ABSTRACT The neutron capture cross section of 243Am was measured by the time-of-flight method with a pulsed neutron beam from a spallation neutron source of the Japan Proton Accelerator Research Complex. An Fe neutron beam filter was used to make the incident neutron beam quasi-monoenergetic around 23.5 keV. The neutron capture γ-rays were detected with a NaI(Tl) detector. The pulse height weighting technique was employed to derive the neutron capture cross section from the pulse height spectrum. The cross section was determined relative to the capture cross section of 197Au of JENDL-4.0. The obtained neutron capture cross section of 243Am was 2.52 ± 0.14 b, determined with a much smaller uncertainty than those of past measurements.

Proposal for selective isotope transmutation of long-lived fission products using quasi-monochromatic γ-ray beams

ABSTRACTWe have proposed a new selective isotope transmutation method using photonuclear reactions with quasi-monochromatic γ-ray beams. This method is based on the fact that the particle threshold of a long-lived fission product (LLFP) such as 93Zr, 107Pd, or 79Se is lower than those of stable isotopes of the same chemical element. Therefore, this method has the excellent advantage that LLFPs cannot, in principle, be produced newly even if the target materials include stable isotopes in addition to LLFPs. Furthermore, this method is effective for 126Sn, 135, 137Cs, 90Sr, and 3H. The nuclear data involved and suitable γ-ray sources are discussed. Laser Compton scattering γ-ray sources and neutron capture γ-rays in nuclear reactors are candidates for this method.

Thermal neutron capture cross sections forO16,17,18andH2

Author(s): Firestone, RB; Revay, Z | Abstract: Thermal neutron capture γ-ray spectra for O16,17,18 and H2 have been measured with guided cold neutron beams from the Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II) reactor and the Budapest Research Reactor (BRR) on natural and O17,18 enriched D2O targets. Complete neutron capture γ-ray decay schemes for the O16,17,18(n,γ) reactions were measured. Absolute transition probabilities were determined for each reaction by a least-squares fit of the γ-ray intensities to the decay schemes after accounting for the contribution from internal conversion. The transition probability for the 870.76-keV γ ray from O16(n,γ) was measured as Pγ(871)=96.6 ±0.5% and the thermal neutron cross section for this γ ray was determined as 0.164±0.003 mb by internal standardization with multiple targets containing oxygen and stoichiometric quantities of hydrogen, nitrogen, and carbon whose γ-ray cross sections were previously standardized. The γ-ray cross sections for the O17,18(n,γ) and H2(n,γ) reactions were then determined relative to the 870.76-keV γ-ray cross section after accounting for the isotopic abundances in the targets. We determined the following total radiative thermal neutron cross sections for each isotope from the γ-ray cross sections and transition probabilities; σ0(O16)=0.170±0.003 mb; σ0(O17)=0.67±0.07 mb; σ0(O18)=0.141±0.006 mb; and σ0(H2)=0.489±0.006 mb.

Detector-Response Correction of Two-Dimensionalγ-Ray Spectra from Neutron Capture

The neutron-capture reaction produces a large variety of γ-ray cascades with different γ-ray multiplicities. A measured spectral distribution of these cascades for each γ-ray multiplicity is of importance to applications and studies of γ-ray statistical properties. The DANCE array, a 4π ball of 160 BaF2 detectors, is an ideal tool for measurement of neutron-capture γ-rays. The high granularity of DANCE enables measurements of high-multiplicity γ-ray cascades. The measured two-dimensional spectra (γ-ray energy, γ-ray multiplicity) have to be corrected for the DANCE detector response in order to compare them with predictions of the statistical model or use them in applications. The detector-response correction problem becomes more difficult for a 4π detection system than for a single detector. A trial and error approach and an iterative decomposition of γ-ray multiplets, have been successfully applied to the detector-response correction. Applications of the decomposition methods are discussed for two-dimensional γ-ray spectra measured at DANCE from γ-ray sources and from the 10B(n, γ) and 113Cd(n, γ) reactions.

Measurement of neutron capture cross sections and neutron capture gamma-ray spectra of 93Nb, 103Rh and 115In in keV-energy region

The neutron capture cross sections of 93Nb, 103Rh and 115In were measured by the time-of-flight method in the energy region from 15 keV to 100 keV. A pulse-height weighting technique was employed to derive the neutron capture cross sections. The neutron capture γ-ray spectra were derived by unfolding the pulse-height spectra with the detector response functions.

Measurements of keV-Neutron Capture Cross-Sections and Capture γ-rays of 105Pd

The neutron capture cross-sections and capture γ-rays of 105Pd were measured for keV-neutron using the 3-MV Pelletron accelerator of the Research Laboratory for Nuclear Reactors at the Tokyo Institute of Technology. Pulsed neutrons were produced from the 7Li(p,n)7Be reaction, and the incident neutron spectrum on a capture sample was measured by neutron time-of-flight method with a 6Li-glass detector. The capture γ-rays were detected with a large anti-Compton NaI(Tl) γ-ray spectrometer. A pulse-height weighting technique was applied to measure the capture γ-ray pulse-height spectra to derive capture yields. The capture cross-sections were obtained by using the standard capture cross-section of 197Au. And we obtained the γ-ray energy spectrum of capture γ-ray by the pulse-height unfolding method. The present results were compared with the previous measurements and the evaluated data of ENDF/B-VII, JEFF-3.1, and JENDL-3.3.

Wide energy range efficiency calibration method for Ge detectors

A new method is proposed for the relative efficiency calibration of HPGe detectors in the energy range from 50 keV to 11 MeV. By simultaneously fitting all separately measured data sets from calibrated and uncalibrated multigamma sources as well as neutron capture γ-rays with a sum of orthogonal polynomials on a log–log scale, accuracies better than 0.5% between 100 and 3500 keV and better than 1% up to 6000 keV could be achieved for relative full energy peak efficiencies. Dividing the energy range and fitting piecewise with lower-order polynomials does not offer any advantage. Moreover, none of the popular non-linear empirical functions are able to fit the full range. The polynomial fit is nearly indistinguishable from a semiempirical fit. If at least one calibrated γ-ray source is included, the method can be used for absolute efficiencies as well.

Measurements of Rayleigh scattering and relativistic calculations of incoherent scattering functions

Recent measurements of Rayleigh scattering employing neutron capture γ-rays are presented. Experimental conditions are achieved such that the Rayleigh contribution is dominant and much larger than the other competing coherent processes. A detailed comparison with the modified relativistic form factor (MRFF) approximation is made and it is concluded that the latter overestimates the cross-section by 3–4%. New calculations of S, the incoherent scattering function, are presented in the relativistic treatment of Ribberfors and Berggren, using multiconfiguration Dirac–Fock relativistic wavefunctions. Tables of S, for Z=1–110, are shown on a momentum transfer mesh identical to previous non-relativistic calculations. S has been calculated at a representative angle θ=60° and energies compatible with the presentation mesh. For other scattering angles, the values presented in the tables are accurate to within 1–2% for momentum transfers larger than 0.1 Å −1. In the region below 0.1 Å −1 the accuracy worsens with decreasing momentum transfer, reaching 6% at 0.01 Å −1 and 10% at 0.005 Å −1. The same multiconfiguration wave functions were used to evaluate new MRFFs. The new elastic scattering cross sections differ by 3–6% compared with calculations based on single configuration wave functions.

On-line reconstruction of low boron concentrations by in vivoγ-ray spectroscopy for BNCT

Boron neutron capture therapy (BNCT) is a radiationtherapy in which the neutron capture reaction of 10B is used forthe selective destruction of tumours. At the High Flux Reactor (HFR) inPetten, a therapy facility with an epithermal neutron beam has been built. Inthe first instance, patients with brain tumours will be treated. The dosesdelivered to the tumour and to the healthy tissue depend on the thermalneutron fluence and on the boron concentrations in these regions. Anaccurate determination of the patient dose during therapy requiresknowledge of these time-dependent concentrations. For this reason, aγ-ray telescope system, together with a reconstructionformalism, have been developed. By using a γ-ray detector in atelescope configuration, boron neutron capture γ-rays of 478 keVemitted by a small specific region can be detected. The reconstructionformalism can calculate absolute boron concentrations using themeasured boron γ-ray detection rates. Besides the boronγ-rays, a large component of 2.2 MeV γ-rays emitted atthermal neutron capture in hydrogen is measured. Since the hydrogendistribution is almost homogeneous within the head, this component canserve as a measure of the total number of thermal neutrons in theobserved volume. By using the hydrogen γ-ray detection rate fornormalization of the boron concentration, the reconstruction tooleliminates the greater part of the influence of the inhomogeneity ofthe thermal neutron distribution. MCNP calculations are used as a toolfor the optimization of the detector configuration.Experiments on a head phantom with 5 ppm 10B in healthy tissueshowed that boron detection with a standard deviation of 3% requires aminimum measuring time of 2 min live time. From two position-dependentmeasurements, boron concentrations in two compartments(healthy tissue and tumour) can be determined. The reconstruction ofthe boron concentration in healthy tissue can be done with a standarddeviation of 6%. The γ-ray telescope can also be used forin vivo dosimetry.

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.

Unusual neutron-capture gamma-ray cascade in 124Te: A fingerprint of octupole-coupled multiphonon states

The properties of the special levels in 124Te that are strongly populated by thermal neutron-capture γ-rays have been determined with 123Te(n,γ) and 124Te(γ,γ′) experiments. The unusual decay pattern can be consistently explained within the quasiparticle-phonon model assuming a three-phonon |2 1 + ⊗ 2 1 + ⊗ 3 1 −; 2 −〉 configuration of the upper intermediate level at 3.101 MeV which exclusively decays to a close-lying two-phonon |2 1 + ⊗ 3 1 f-; 1 f-f> state at 2.747 MeV. The (γ,γf') reaction strongly excites the latter and also indicates the 1 f- member of the three-phonon 2 1 + ⊗ 2 1 + ⊗ 3 1 f- septuplet at 3.091 MeV.

Neutron polarization for P- and T-violation experiments

The polarization of neutron spin, target nucleus spin and He nuclear spin was developed at KEK for measuring the T-odd term in polarized neutron transmission through a polarized nuclear target. A method to measure the neutron spin rotation was developed for the T-violation experiment. This apparatus has been found to be quite useful for the P-violating neutron spin rotation experiment. The angular distribution of neutron-capture γ-rays was measured for the study of the enhancement mechanism of the P-violation. The results are also discussed.

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.

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.

Multi-element analysis of coal during borehole logging by measurement of prompt γ-rays from thermal neutron capture

The performance of borehold logging equipment is described which is based on a 252Cf neutron source and measurement of the spectra of thermal neutron capture γ-rays from most of the important elements in coal. It is shown that although a high statistical counting accuracy can be obtained for several elements in a measuring period of about 5 min, the derived concentrations are limited by the low capture cross-section of carbon. It is concluded that it is strongly preferable to employ a high energy neutron source, such as 241Am-Be, so as to include oxygen in the analysis and to obtain a higher precision of measurement for all the other elements.

Precise intensity measurement of primary γ-rays from the 197Au(nth, γ)198Au reaction

Abstract Absolute intensities are reported for primary neutron capture γ-rays with energies between 6.1 and 6.5 MeV from the 197Au(nth, γ)198Au reaction, using thermal neutrons from a neutron guide. These intensities per captured neutron, with total uncertainties as small as 3.6%, are of use as accurate standards for the efficiency calibration of gamma or electron spectrometers relative to the neutron capture flux, as a low (411 keV) to high (6 MeV) energy calibrator and as an on-line measure of the epithermal neutron flux at the target site.

10MV医療用直線加速器からの漏洩放射線について : 第1報 使用室迷路からの異常漏洩とその原因

As the result of the survey of leakage X-ray from the treatment room, unusual high leakage value is found at the entrance to the treatment room. The leakage values at the other measurement locations are less than those expected from a shielding calculation. Layout of the treatment room is an ordinary construction, and no constructional defects are found. As a consequence of tracing the origin of this unusual leakage, it is concluded that neutrons produced in Linac leak from the treatment room through that the maze, and that the main contribution to the measured leakage radiation is that from neutron capture γ-ray in materials composing the maze, the door etc.. For the purpose of reducing the unusual leakage, a reconstruction of the room is performed by reducing an open area at the part of the entrance from the main room to the maze. At the entrance to the treatment room, initial measured X-ray dose equivalent rate is 5.8 mrem/h in the condition of the beam direction vertically downward, the fully closed field and output 4.65 Gy/min, but, after the reconstruction, its value reduces to 1.8 merm/h and measured neutron dose equivalent rate is 3.9 mrem/h in the same condition. The X-ray and neutron leakage values in the fully opened field are less than the above values. From X-ray leakage values before and after the reconstruction, it is deduced that initial neutron leakage dose equivalent rate is about 12 mrem/h in the fully closed field. When a treatment room to set up a Linac with the accelerative energy exceeding 10 MV is designed, it is suggested from the above facts that some plans to reduce neutron leakage through the maze must be taken into consideration. Also, in survey of leakage radiation from the treatment room, not only X or γ-ray but neutron survey is demanded.