Investigation of the γ-rays accompanying thermal-neutron capture by certain rare-earth nuclei

Using a scintillation spectrometer measurements have been made of the spectra of -rays accompanying thermal-neutron capture in a number of nuclei.A number of intense lines have been found below 300 kev in the -ray spectra for thermal-neutron capture in europium, gadolinium, dysprosium, holmium, erbium, thulium, hafnium and tantalum. Lines corresponding to 4⁺2⁺ and 2⁺0⁺ transitions between rotational levels'of Er¹⁶⁸ and Hf¹⁷⁸ were found in the erbium and hafnium spectra. The intensity of these transitions corresponds to 0.5–0.8 photons per capture event.The authors are indebted to Academician I. V. Kurchatov for his interest in the work and to Professor L. V. Groshev, V. M. Strutinskii and D. P. Grechukhin for a number of valuable comments and Professor I. A. Zaozerskii for kindly furnishing the rare-earth samples.We wish to express our gratitude to G. P. Mel'nikov for providing reliable operation of the electronic apparatus.     

Spatial distribution of γ-rays and moderated neutrons in the graphite column of the RFT reactor

Measurements have been made of the spatial distributions of neutrons of various energies and the -rays in the graphite thermal column of the RFT reactor. The fluxes of thermal, resonance, and fast neutrons were measured using the activity induced in various indicators. The decay in the intensity of the /gg-radiation was determined by small condenser type ionization chambers. At distances from 80 to 160 cm the fluxes of resonance and fast neutrons fall off approximately exponentially with a decay length of /R~ 13 cm and 15.7 cm, respectively. At large distances the fluxes of fast and resonance neutrons are in equilibrium. The flux of slowed-down neutrons in this region falls off exponentially with a decay length of 18.2 cm as determined by the penetrating fast-neutron component with an energy of approximately 6.6 Mev. The intensity of the -radiation in the graphite column falls off in an almost exponential manner with an attenuation coefficient = 3.78 · 10^–2 cm^–1. The theoretical predictions are found to be in satisfactory agreement with the experimental data.     

Production of the Isomeric State of 138Cs in the Thermal Neutron Capture Reaction 137Cs(n, γ)138Cs

In order to obtain precise data of the neutron capture cross section of the reaction ¹³⁷Cs(n, γ)¹³⁸Cs, the production probability of isomer state ^138mCs was measured in this work. Targets of about 0.37MBq ¹³⁷Cs were irradiated for 3 min in. the pneumatic tube facility (Pn-3) of Kyoto University Reactor (KUR). The 1,436 keV gamma;-ray emitted from both of ^138gCs and ^138mCs was measured. A ratio of the production probability between ^138gCs and ^138mCs was deduced from time dependence of peak counts of 1,436 keV γ-ray by making use of difference of half-lives of ^138gCs (33.41 min) and ^138mCs (2.91 min). The production probability of ^138mCs was obtained as 0.75plusmn;0.18 and this value revised the effective cross section upwards by 9plusmn;2percnt;. The effective cross section ô and the thermal neutron capture cross section σo were obtained as ô=0.29±0.02 b and σ=0.27±0.03b with taking into account the production of ^138mCs.     

Measurement of Thermal Neutron Cross Section of 137Cs(n, γ)138Cs Reaction

A calculational model for a modified diffusion coefficient has been developed to incorporate the neutron streaming effect in heterogeneous low-density channels accurately into diffusion theory calculations. The model uses a supercell, and the axial and radial diffusion coefficients of the heterogeneous inner cell are so defined that they can reproduce Benoist's axial and radial diffusion coefficients of the supercell when the diffusion coefficient of the outer cell is given as 1/3 Σ_tr . In the case of the axial diffusion coefficient, the axial buckling effect is taken into account by modifying the neutron path length within the streaming channel in calculating the collision probabilities. This model has been applied to an RZ fast reactor Core model with a gas expansion module (GEM). By using the axial diffusion coefficient obtained with the presented model, calculational error of GEM worth was reduced to less than 1/7 compared to the formula of Rowlands and Eaton.     

Measurement of Thermal Neutron Capture Cross Section and Resonance Integral of the 237Np(n, γ)238Np Reaction

The thermal neutron capture cross section (σ₀) and the resonance integral (I ₀)of ²³⁷Np have been measured by an activation method to supply basic data for the study of transmutation of nuclear waste. The neutron irradiation of ²³⁷Np samples have been done at the Research Reactor Institute, Kyoto University (KUR). Samples of ²³⁷Np were irradiated between two Cd sheets or without a Cd sheet. Since ²³⁷Np has a strong resonance at the energy of 0.49 eV, the Cd cutoff energy was adjusted at 0.358 eV (thickness of the Cd sheets: 0.125 mm). A high purity Ge detector was employed for activity measurement. The reaction rate to produce ²³⁸Np from ²³⁷Np was analyzed by the Westcott's convention. Results obtained were 141.7±5.4 barns for σ₀ and 862±51 barns for I ₀ above 0.358 eV of ²⁷³Np. By setting the Cd cut-off energy at 0.358 eV considering the resonance at 0.49 eV, a smaller value of σ₀ was obtained in this work than the values reported by the previous authors.     

Measurement of Thermal Neutron Capture Cross Section and Resonance Integral of the 129I(n, γ) 130I Reaction

To obtain fundamental data for research on the transmutation of nuclear wastes, the thermal neutron cross section and the resonance integral of the ¹²⁹I(n, γ)¹³⁰I reaction have been measured using an activation method. The neutron cross sections for the formation of the ground (5⁺) state and the isomeric (2⁺) state of ¹³⁰I were measured separately.

Six ¹²⁹I targets were irradiated for 10 min with thermal reactor neutrons; three of them containing 2.55- 2.61 kBq of ¹²⁹I were irradiated within a Cd capsule, and the other three targets containing 259–261 Bq of ¹²⁹I were irradiated without it. The Co/Al and Au/Al alloy wires were used to monitor the neutron flux and the fraction of the epithermal part (Westcott's epithermal index). The gamma-ray spectra from the irradiated samples were measured with a Ge detector.

The thermal neutron capture cross section (the 2,200 m/s neutron cross section) and the resonance integral of the ¹²⁹I(n, 7)¹³⁰I reaction were determined to be 12.5±0.5b and 15.6±0.7b for the formation of the ground state ^130gI(5+), 17.8±0.7b and 18.2±0.8b for the formation of the isomeric state ^130mI(2⁺), and 30.3±1.2b and 33.8±1.4b for the formation of ¹³⁰I (the sum of the 2⁺ and the 5⁺ states), respectively. The sum of the thermal neutron capture cross sections forming the 2⁺ and the 5⁺ states was 12% larger than the evaluated values of JENDL-3.2 and ENDF/B-VI and that reported by Roy et al. This discrepancy is explained by the population of the isomeric level.     

Study on the performance of a large-size Cs I detector for high energy γ-rays

The measurement of high-energy γ-rays is an important experimental method to study the giant resonance in a nucleus, c reaction in nuclear astrophysics, and so on. The performance of a large-size Cs I detector for crays detection is studied by comparison between the experimental measurements and GEANT4 simulation. The reliability of the simulated efficiency for low-energy γ-rays is verified by comparing with the experimental data. The efficiency of the Cs I detector for high-energy γ-rays was obtained by the GEANT4 simulation. The simulation shows that the detection efficiency of 20 Me V γ-rays can reach 3.8%.     

Measurements of the Thermal Neutron Cross Sections of the Reaction of ~(126)Sn(n,γ)~(127)Sn~ (g m)

For the measurement of the thermal neutron cross section of the FP nuclide, the thermal neutron across sections of some long-lived nuclides, whose half-lives are more than 20 a and fission yields are little more than 0.01%, are not fully measured hitherto…     

Analysis of Transmitted Gammaγ-rays by the Multiple Scattering Method, (II)

Dose buildup factors and number spectra of γ-rays transmitted through stratified slabs have been estimated by the multiple scattering method, taking scattering into consideration up to the fourth order and including back scattering.

Further, the method proposed in the previous paper for estimating the contribution to the total dose buildup factor by γ-rays of the fifth and higher orders of scattering was applied to the present case of stratified slabs made up of different materials.

The calculations covered the range of source energy from 1 to 10 MeV, and various arrangements of strata made up of combinations of two of the three materials water, iron and lead, making up slabs of total thicknesses up to 6 mfp. Calculations were also performed for ⁶⁰Co γ-rays normally incident on stratified slabs of water-lead-water.

The results of the above calculations were found to be in good agreement with those of other calculations, such as by Monte Carlo method and by numerical integration of the photon transport equation. The present results were also found to agree well with the dose buildup factors calculated by our own empirical formula. Comparisons with experimental results too have proved good.     

Properties and morphology of PMMA/ABN blends obtained via MMA in situ polymerisation through γ-rays

Methylmethacrylate polymerisation in the presence of 4 wt% butadiene-acrylonitrile rubber was carried out at fixed dose-rate and temperature. The effect of irradiation was investigated with respect to the mechanical and dynamic-mechanical properties in the solid state. A general increase of the “compatibilisation” on irradiation is obtained. In fact both rubber and polymethylmethacrylate glass transition temperatures, determined through dynamic-mechanical analysis, reveal the presence of strong interactions between the components. Furthermore an improvement of the mechanical tensile properties of the blends is observed when irradiation is continued after polymerisation, at least until a certain absorbed dose.     

Measurement of Thermal Neutron Capture Cross Section and Resonance Integral of the Reaction 133Cs(n, γ)134m,134gCs

The measurements of the thermal neutron (2,200 m/s neutron) cross section (σ₀) and the resonance integral (I ₀) of the ¹³³Cs(n, γ;) reaction were performed by an activation method to obtain fundamental data for research on the transmutation of nuclear wastes. The cross sections for the formations of the isomeric state ^134mCs and the ground state ^134mCs were measured respectively by following the behavior of the γ-ray counting rate after the irradiation.

The thermal neutron capture cross sections and the resonance integrals of the ¹³³Cs(n, γ) reaction were determined to be 2.70±0.13 b and 23.2±1.8 b for the formation of the isomeric state ^134mCs, and 26.3±1.0 b and 275±16 b for the formation of the ground state of ^134gCs. The results for the reaction ¹³³Cs(n, γ)^134m+gCs were 29.0±l.0 b and 298±16 b, respectively. As for the thermal neutron capture cross section for the formation of ^134m+gCs, the evaluated value (29.00 b) of JENDL-3.2 agreed with the present result. The reported value by Baerg et al. was in good agreement with the present result within the limits of error on the thermal neutron capture cross section for ^134mCs. On the other hand, the resonance integral for ^134m+g Cs was 32% smaller than the experimental value by Steinnes et al.     

Transmission electron microscopy characterization of irradiated U–7Mo/Al–2Si dispersion fuel

The plate-type dispersion fuels, with the atomized U(Mo) fuel particles dispersed in the Al or Al alloy matrix, are being developed for use in research and test reactors worldwide. It is found that the irradiation performance of a plate-type dispersion fuel depends on the radiation stability of the various phases in a fuel plate. Transmission electron microscopy was performed on a sample (peak fuel mid-plane temperature ～109 °C and fission density ～4.5 × 10²⁷ f m^?3) taken from an irradiated U–7Mo dispersion fuel plate with Al–2Si alloy matrix to investigate the role of Si addition in the matrix on the radiation stability of the phase(s) in the U–7Mo fuel/matrix interaction layer. A similar interaction layer that forms in irradiated U–7Mo dispersion fuels with pure Al matrix has been found to exhibit poor irradiation stability, likely as a result of poor fission gas retention. The interaction layer for both U–7Mo/Al–2Si and U–7Mo/Al fuels is observed to be amorphous. However, unlike the latter, the amorphous layer for the former was found to effectively retain fission gases in areas with high Si concentration. When the Si concentration becomes relatively low, the fission gas bubbles agglomerate into fewer large pores. Within the U–7Mo fuel particles, a bubble superlattice ordered as fcc structure and oriented parallel to the bcc metal lattice was observed where the average bubble size and the superlattice constant are 3.5 nm and 11.5 nm, respectively. The estimated fission gas inventory in the bubble superlattice correlates well with the fission density in the fuel.