Epithermal Index Research Articles

Bias Effects on g- and s-Factors in Westcott Convention

For accuracy improvement of neutron activation analysis and neutron capture cross sections, bias effects are investigated on g- and s-factors in the Westcott convention. As origins of biases, a joining function shape, neutron temperature, and sample temperature have been investigated. Biases are quantitatively deduced for two 1/v isotopes (197Au, 59Co) and six non-1/v isotopes (241Am, 151Eu, 103Rh, 115In, 177Hf, 226Ra). The s-factor calculated with a joining function deduced recently by a detailed Monte Carlo simulation is compared to s-factors calculated with traditional joining functions by Westcott. The results show the bias induced by the sample temperature is small, in the order of 0.1% for the g-factor and in the order of 1% for the s-factor. On the other hand, the bias size induced by a joining function shape for the s-factor depends significantly on both isotopes and neutron temperature. As a result, the reaction rates are also affected significantly. The bias size for the reaction rate is given in the case of an epithermal neutron index r = 0.1, for the eight isotopes.

Calculations of the thermal and fast neutron fluxes in the Syrian miniature neutron source reactor using the MCNP-4C code

The MCNP-4C code, based on the probabilistic approach, was used to model the 3D configuration of the core of the Syrian miniature neutron source reactor (MNSR). The continuous energy neutron cross sections from the ENDF/B-VI library were used to calculate the thermal and fast neutron fluxes in the inner and outer irradiation sites of MNSR. The thermal fluxes in the MNSR inner irradiation sites were also measured experimentally by the multiple foil activation method ( 197Au (n, γ) 198Au and 59Co (n, γ) 60Co). The foils were irradiated simultaneously in each of the five MNSR inner irradiation sites to measure the thermal neutron flux and the epithermal index in each site. The calculated and measured results agree well.

Measurement of Effective Capture Cross Section of 238Np for Thermal Neutrons

The effective capture cross section (σ) of 238Np for thermal neutrons has been measured as data for the burn-up analysis of irradiated fuels and for the study of nuclear transmutation of minor actinides. A sample of 237Np was irradiated for 10 hours at the Kyoto University Reactor. Neptunium-238 and -239 were produced simultaneously via the double neutron capture reactions 237Np(n, γ)238Np(n, γ)239Np. The neutron flux at the irradiation position was monitored using Au and Co wires. The epithermal index r√T/T 0 in Westcott's convention was deduced as 0.03. To measure the activities of 238Np and 239Np, a high-purity Ge detector with a BGO (Bi4Ge3O12) anti-coincidence shield has been developed. The σ of 238Np was deduced from the ratio of these activities. The result obtained is 479± 24 b for the σ of 238Np. The result is compared with data in the evaluated nuclear data libraries, ENDL-86, ENDF/B-VI, and JENDL-3.3.

Measurement of Effective Capture Cross Section of 238Np for Thermal Neutrons

The effective capture cross section (σ) of 238Np for thermal neutrons has been measured as data for the burn-up analysis of irradiated fuels and for the study of nuclear transmutation of minor actinides. A sample of 237Np was irradiated for 10 hours at the Kyoto University Reactor. Neptunium-238 and -239 were produced simultaneously via the double neutron capture reactions 237Np(n, γ)238Np(n, γ)239Np. The neutron flux at the irradiation position was monitored using Au and Co wires. The epithermal index r√T/T 0 in Westcott's convention was deduced as 0.03. To measure the activities of 238Np and 239Np, a high-purity Ge detector with a BGO (Bi4Ge3O12) anti-coincidence shield has been developed. The σ of 238Np was deduced from the ratio of these activities. The result obtained is 479± 24 b for the σ of 238Np. The result is compared with data in the evaluated nuclear data libraries, ENDL-86, ENDF/B-VI, and JENDL-3.3.

Measurement of Thermal Neutron Capture Cross Section and Resonance Integral of the 129(n,.GAMMA.)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 format...

Measurement of Thermal Neutron Cross Section and Resonance Integral of the Reaction99Tc(n, γ)100Tc

To obtain fundamental data for research on the transmutation of nuclear waste, the thermal neutron cross section and the resonance integral of the reaction 99Tc(n, γ)100Tc have been measured using an activation method. Four ammonium pertechnetate targets containing 371–375 kBq of 99Tc were irradiated for 2 min with reactor neutrons. Activation detectors of Co/Al and Au/Al alloy wires were irradiated for 10 min to monitor the neutron flux and the fraction of the epithermal part (Westcott's epithermal index). The Tc samples and flux monitors were irradiated with and without a Cd capsule. The γ-ray spectra from the irradiated samples were measured using a high purity Ge detector. The thermal neutron cross section (2,200 m/s neutron cross section) and the resonance integral of the 99Tc(n, γ)100Tc reaction were found to be 22.9±1.3 b and 398±38b, respectively. The thermal neutron cross section obtained agrees with the previously reported values (20±2b by Lucas, 24.8 by Pattenden, 24±4 by Ovechkin) within the limits of error. On the other hand, the resonance integral is twice the value reported by Lucas (186±16b).

Measurement of Thermal Neutron Cross Section and Resonance Integral of the Reaction 99Tc(n,.GAMMA.)100Tc.

To obtain fundamental data for research on the transmutation of nuclear waste, the thermal neutron cross section and the resonance integral of the reaction 99 Tc(n,γ) 100 Tc have been measured using an activation method. Four ammonium pertechnetate targets containing 371∼375 kBq of 99 Tc were irradiated for 2 min with reactor neutrons. Activation detectors of Co/Al and Au/Al alloy wires were irradiated for 10 min to monitor the neutron flux and the fraction of the epithermal part (Westcott's epithermal index). The Tc samples and flux monitors were irradiated with and without a Cd capsule. The γ-ray spectra from the irradiated samples were measured using a high purity Ge detector. The thermal neutron cross section (2,200m/s neutron cross section) and the resonance integral of the 99 Tc(n,η) 100 Tc reaction were found to be 22.9±1.3b and 398±38b, respectively. The thermal neutron cross section obtained agrees with the previously reported values (20±2b by Lucas, 24.8 by Pattenden, 24±4 by Ovechkin) within the limits of error. On the other hand, the resonance integral is twice the value reported by Lucas (186±16b).

Measurement of the Thermal Neutron Cross Section of the 90Sr(n,.GAMMA.)91Sr Reaction.

To obtain fundamental data for the research of the transmutation of nuclear waste, the thermal neutron cross section of the reaction 90Sr(n, γ)91Sr has been measured by means of a radiochemical method. Strontium chloride targets containing about 2 MBq of 90Sr were irradiated for 10 min with reactor neutrons. Activation detectors of Co/Al and Au/Al alloy wires were irradiated simultaneously to monitor the neutron flux and Westcott's epithermal index r (T/T 0)1/2. The irradiated Sr samples were purified chemically and their r-ray spectra were measured with a high purity Ge detector. Since the targets included 85Sr, a γ-ray emitter and a tracer for 90Sr, and a ratio of the amount of 85Sr to that of 90Sr was measured in advance, an effective cross section of the 90Sr(n, γ)91Sr reaction for a reactor neutron spectrum was determined from the ratio of radioactivities of 91Sr and 85Sr, and neutron flux data. The resonance integral was also measured by a modified Cd-ratio measurement. Considering an upper limit of...

Measurement of energy spectrum parameters in MNSR irradiation site by solid track method-dual foil method

The solid track method-dual foil method i.e. 239Pu–235U pair and 115In—55Mn pair,which have been used to measure the neutron temperature and epithermal index in the irradiation site on the prototype MNSR, are described in this article. The experimental principle and method, measuring technique of the solid track method, the experimental results and error analysis are explained in details. The results are compared with those obtained by the dual foil mehtod-176 Lu—164Dy and 115In—55Mn pair, and they are in good coincidence within t̀he error scope.

Reactor-neutron-capture cross sections of 165Dy isomers

The yield of 166Dy produced via the double neutron capture of 164Dy was determined by the activation method. The γ-emission probabilities in the decay of 166Dy and its daughter 166Ho were measured and used for the determination of the yield. The reactor-neutron-capture cross sections of 165Dy isomers were deduced from the dependence of the yield on the length of the irradiation time. Obtained cross sections were 2000 ± 600 b for the metastable state and 3530 ± 330 b for the ground state, respectively, with the choice of the Westcott epithermal index r · √( T/ T 0) = 0.017. The ratio of the cross sections between the pair of isomers was discussed together with the cross section ratios of other isomer pairs on the basis of the compound formation model.

The isomeric cross-section ratio in double neutron capture of 93Nb

The yields of 95Nb isomers were investigated by means of the double neutron capture of 93Nb target. The effective cross section of 94Nb(n,γ)95mNb and 94Nb(n,γ)95gNb reactions were found, by the activation method, to be 0.61 ± 0.03 barn and 14.6 ± 0.3 barn, respectively, with the choice of Wescott's epithermal index r√(T/T0) = 0.022. The observed isomeric cross section ratio of the reaction was compared with the theoretical prediction on the basis of the statistical model. As a result, the spin of the compound nucleus was assigned and the contribution of the quadrupole transition was deduced in the gamma cascade process.

Behavior of the rare earth elements in the Oklo natural reactor

A systematic analysis of the abundance and isotopic composition of ten REE (and Ba) in a sample from the reactor zone at the Oklo (Gabon) ore deposit is presented. From this analysis some constraints on the behavior of the REE group during and after the reaction time are determined. Taking into account the complex state of this sample, the main characteristics of the reactor are deduced. From the abundances of isotopes with large neutron capture cross sections the amount of REE (supposed to be natural) added after the reaction time is calculated first. The comparison of the fluence values obtained respectively from the analysis of the Nd and Dy isotopic spectrum indicates a rather good stability of the REE group during the reaction time. The total number ( 235U) f of 235U nuclei that fissioned during the reaction calculated from different elements of the REE group provides an indication on the relative migration of these elements since the reaction time. Results of these computations suggest a migration of the heavy REE relative to the light REE to outside the reaction zone. As a consequence of these deductions on the behavior of the REE in the Oklo ore, the light REE appear the most appropriate to study the characteristics of the reactor. The reactor characteristics we deduced from this sample (fluence, epithermal index, proportion of nuclides that fissioned, duration of the reactor) are within the range of the ones determined in other studies dealing with REE analysis.

Effects of Neutron Capture Transformations on the Decay Power of Fission Products

Neutron capture effects on the decay power of fission products have been examined by varying the fissile nuclide, neutron spectrum, neutron flux, and irradiation and cooling times. Neutron capture transformations of fission products usually increase the decay power. However, at short cooling times, i.e., <C104 s, the capture effects are small, especially in a thermal reactor, where the negative contribution of 135 Xe offsets the positive contributions of other nuclides. The capture effect exhibits peaks at cooling times of 106 and 108 s and becomes negligible at 109 s. The former peak results mainly from the increases in the activities of103Ru, 134Cs, 136 Cs, 148Pm, 148Pm, and 156Eu, and the latter by activities of 134 Cs and 154Eu. The capture effect increases with increase of the flux level or irradiation time, and it is approximately proportional to the integrated flux at long cooling times. There is only a slight difference between the capture effect of two thermal reactors with epithermal indices of 0.1 and 0.2. In fast reactors, the effect is smaller than in thermal reactors at cooling times over 105 s, and depends only a little on the fissile nuclide. The decay power in fast reactors depends on the cross-section library selected to less than ∼ 1%.

Activation measurements of the cross section of the reaction 147Pm( n, γ) 148 gPm for reactor neutrons

The effective cross section of the reaction 147Pm( n, γ) 148 g Pm has been measured for reactor neutrons. 197Au and 59Co monitors have been used to determine the 2200 m/sec neutron fluxes in the spectra employed, and cadmium ratio measurements for gold and the silver-cobalt activity ratio method have been used to determine the epithermal indices. The amounts of 148 g Pm formed, and the amounts of 147Pm from which they were formed, were both determined after purification of the irradiated material. γ-Spectrometry was used for the former purpose, and the 4πβ-γ tracer method, with 46Sc as the tracer, was used for the latter. The cross section of the reaction for Maxwellian neutrons ( σ 0 g) and its reduced resonance integral (Σ′) were found to be 96·0 ± 1·8 barns ( T = 60°C) and 1274 ± 66 barns respectively.

Gamma-ray spectrometric measurements of the capture cross section of gold-198 for reactor neutrons

By using a calibrated gamma-ray spectrometer to measure the absolute disintegration rates of 198Au and 199Au in mixtures of the two, the capture cross section of 198Au for reactor neutrons, σ ∧ 8 , has been measured as a function of Westcott's epithermal index parameter, r√ T T 0 , over the range encountered in the high-flux irradiation positions of the PLUTO reactor. Assuming the 2200 m/sec capture cross section of 59Co to be 37·4 barns it was found that σ ̂ 8( barns)=25102−46535r T T 0 The capture cross section for Maxwellian neutrons ( T = 70°C) was found to be 25102±371 barns.

505. 円筒燃料内照射によるラジオアイソトープの製造

The neutron flux and energy distribution inside the cylindrical fuel of JRR-2 used for irradiation have been measured by activation method, and the results compared with those obtained by irradiation at the same position without the fuel.Threshold detectors such as nickel and sulphur were used to measure the fast neutron flux. Gold and cobalt metal foils were used as thermal neutron monitor. In order to measure the contribution of resonance neutrons to the effective activation cross section, elements possessing different resonance neutron absorption energies were irradiated along with the flux monitors.It was estimated that approximately 30% of the thermal neutrons were converted to fast neutrons inside the cylindrical fuel and that the fast neutron flux was 6 times as high as obtained in the absence of fuel material, but that the epithermal index was found unchanged.

Experimental Analysis of Energy Spectrum Parameters in Slightly Enriched UO2-H2O Lattices

The relative neutron temperature represented by the activation ratio (AR) of Lu-Cu, and the modified epithermal index in the form were measured by foil activation for a wide range of water-to-fuel volume ratios, in slightly enriched UO2-H2O lattices. With tight lattices, the values of the activation ratio thus experimentally determined agree better with the two dimensional collision probability calculation than with the one dimensional cylindricalized cell calculation. The values of measured with the use of Au-Mn-Ni alloy foil without Cdfilter are in good agreement with the results calculated by the extended TEMPEST code. The neutron temperature Tn and the epithermal index r in a unit cell, derived from the quantities AR and , can be expressed respectively by from which, it is found that a linear relationship between is obtained in the form

Different activation techniques for the study of epithermal spectra, applied to heavy-water lattices of varying fuel-to-moderator ratio

Spectral indices at the cell boundary have been studied as functions of lattice pitch in the reference core of the Swedish RO reactor. Epithermal indices were determined by activation of 115In, employing three different techniques: the two-foil, the cadmium ratio and the sandwich foil methods. The latter of these has the advantage of being independent of assumptions about foil cross sections of spectral functions, and it gives a spectrum index that lends itself readily to comparisons with theoretical multigroup calculations. Alternatively the results can be expressed in terms of the Westcott parameters r and T n when this is justified by the spectral conditions. The agreement between the three methods investigated is generally good. Good agreement is also found with multigroup collision probability calculations of different degrees of complexity.

Neutron flux measurements with cobalt

A method for accurately measuring the neutron flux in reactor experiments on a routine basis is summarized. Cobalt or cobalt-aluminum alloy wire monitors are prepared with a wire cutting device. After irradiation, the Co 60 activity in each is measured with an ionization chamber or well-type NaI crystal scintillation counter. Correction factors for neutron self-shielding, γ-ray self-absorption in the Co 60 measurement, and for neutron leakage through cadmium in cadmium ratio measurements are presented graphically. The results, with appropriate corrections, are interpreted using the Westcott convention to give the thermal neutron flux and the “epithermal index”, a measure of epithermal flux related to the cadmium ratio. Sources of experimental error and uncertainties in the input data are reviewed and it is concluded that the method is accurate to a few percent. The accuracy will improve as more accurate input data become available.

Effective Cadmium Cutoff Energies for Non-1/VDetectors

Effective cadmium cutoff energies were calculated for 239Pu, 235U, 115In, Au, 119Ir and 176Lu as a function of the cadmium filter thickness, the mast probable energy of the Maxwellian flux and the epi-thermal index. An infinite slab geometry and isotropic flux were assumed in the calculations. The calculated values of the cutoff energies are compared with those for 1/V detector, and are related to the resonance energies near the cutoff energy. If the resonance energy is lower than that of the 1/V detector, and the neutron spectrum and filter are same, the cutoff energy is lower than that of the 1/V detector. Such cases include 235U and 176Lu with thin filter, 191Ir with thick filter, and 239Pu with either thin or thick filter. If the resonance energy is higher than the E c of the 1/V detector, the cutoff energy is higher, such cases being 191Ir with thin filter, 176Lu with thick filter and 115In with either thin or thick filter. And if the resonance energy is very much higher than the E c of the 1/V detec...