High Resolution Neutron Transmission Research Articles

High Spatial Resolution Neutron Transmission Imaging Using a Superconducting Two-Dimensional Detector

Neutron imaging is one of the most powerful tools for nondestructive inspection owing to the unique characteristics of neutron beams, such as high permeability for many heavy metals, high sensitivity for certain light elements, and isotope selectivity owing to a specific nuclear reaction between an isotope and neutrons. In this study, we employed a superconducting detector, current-biased kinetic-inductance detector (CB-KID) for neutron imaging using a pulsed neutron source. We employed the delay-line method, and high spatial resolution imaging with only four reading channels was achieved. We also performed wavelength-resolved neutron imaging by the time-of-flight method for the pulsed neutron source. We obtained the neutron transmission images of a Gd–Al alloy sample, inside which single crystals of GdAl <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_3$</tex-math></inline-formula> were grown, using the delay-line CB-KID. Single crystals were well imaged, in both shapes and distributions, throughout the Al–Gd alloy. We identified Gd nuclei via neutron transmissions that exhibited characteristic suppression above the neutron wavelength of 0.03 nm. In addition, the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{155}$</tex-math></inline-formula> Gd resonance dip, a dip structure of the transmission caused by the nuclear reaction between an isotope and neutrons, was observed even when the number of events was summed over a limited area of 15 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m × 12 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> m. Gd selective imaging was performed using the resonance dip of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{155}$</tex-math></inline-formula> Gd, and it showed clear Gd distribution even with a limited neutron wavelength range of 1 pm.

Synthetic model for Doppler broadening of neutron absorption resonances in molecular fluids

A general and systematic approach expressed in modern language, accounting for molecular motion effects on Doppler Broadening of Neutron Absorption Resonances (DBNAR) is given the form of a new model. It relies on well validated hypothesis: The separability of atomic from nuclear degrees of freedom, the use of the Van Hove scattering formalism and the fact that a conceptually identical approach produced experimentally proved predictions when applied to DBNAR in solid systems. We treat the molecular internal degrees of freedom approximately as harmonic oscillators. As a second contribution of this work, a synthetic model is presented in order to make the more complete model mentioned above suitable for neutron calculation codes. This second synthetic model reduces to the exact expressions of the complete model in the low and high neutron energy regimes and provides a plausible transition in between. Numerical results are presented for a general hypothetical case to show its strengths and limitations. Also, both models are applied to a real case of the 238U 6.674 eV resonant effective broadened absorption cross-section in UF6 (uranium hexafluoride). A direct experimental validation of our models is still necessary for which a special high resolution neutron transmission experiment ought to be devised at low temperatures and pressures on a gaseous system. It is showed how the synthetic model can be used to make thermometric predictions in an improved fashion in comparison to the effective temperature gas model at low temperatures.

R-Matrix Analysis of 238U High-Resolution Neutron Transmissions and Capture Cross Sections in the Energy Range 0 to 20 keV

The neutron resonance parameters of 238U were obtained from a SAMMY analysis of high-resolution neutron transmission measurements and high-resolution capture cross-section measurements performed at the Oak Ridge Electron Linear Accelerator (ORELA) in the years 1970–1990 and from more recent transmission and capture cross-section measurements performed at the Geel Linear Accelerator. Compared with previous evaluations, the energy range for this resonance analysis was extended from 10 to 20 keV, taking advantage of the high resolution of the most recent ORELA transmission measurements. The experimental database and the method of analysis are described in this paper. The neutron transmissions and the capture cross sections calculated with the resonance parameters are compared with the experimental data. A description is given of the statistical properties of the resonance parameters and of the recommended values of the average parameters. The new evaluation results in a slight decrease of the effective capture resonance integral and improves the prediction of integral thermal benchmarks by 70 to 200 pcm.

R-Matrix Analysis of 232Th Neutron Transmissions and Capture Cross Sections in the Energy Range Thermal to 4 keV

Neutron resonance parameters of 232Th were obtained from the Reich-Moore SAMMY analysis of high-resolution neutron transmission measurements performed at the Oak Ridge Electron Linear Accelerator (ORELA) by Olsen in 1981, along with the high-resolution neutron capture measurements performed in 2005 at the Geel Linear Accelerator (GELINA, Belgium) by Schillebeeckx and at the n-TOF facility (CERN, Switzerland) by Aerts. The ORELA data were analyzed previously by Olsen with the Breit-Wigner multilevel code SIOB, and the results were used in the ENDF/B-VI evaluation. In the new analysis of the Olsen neutron transmissions by the modern computer code SAMMY, better accuracy is obtained for the resonance parameters by including in the experimental database the recent experimental neutron capture data. The experimental database and the method of analysis are described in the report. The neutron transmissions and the capture cross sections calculated with the resonance parameters are compared to the experimental values. A description is given of the statistical properties of the resonance parameters. The new evaluation results in a decrease in the capture resonance integral.

Publisher's Note: High resolution neutron transmission and capture measurements of the nucleusPb206[Phys. Rev. C76, 014605 (2007)

Publisher's Note: High resolution neutron transmission and capture measurements of the nucleus<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mmultiscripts><mml:mi mathvariant="normal">Pb</mml:mi><mml:mprescripts /><mml:none /><mml:mrow><mml:mn>206</mml:mn></mml:mrow></mml:mmultiscripts></mml:math>[Phys. Rev. C<b>76</b>, 014605 (2007)

High-resolution neutron transmission and capture measurements of the nucleusPb206

Neutron total and capture reaction cross-section measurements for $^{206}\mathrm{Pb}$ were performed at the GELINA neutron time-of-flight facility in the energy range from 1 to 620 keV. The resonance parameters corresponding to 304 excited nuclear levels in $^{207}\mathrm{Pb}$ were determined from the data using the R-matrix formalism. The results are compared to existing data. From the capture data photon strength functions have been deduced. No evidence has been found for a previously reported enhancement of the $M1$ transition strength, nor for a strong enhancement of the s-wave doorway state in the photon channel. The neutron capture cross section of $^{206}\mathrm{Pb}$ is of importance in stellar evolution calculations of the formation of the elements by neutron capture. Maxwellian-averaged neutron capture cross sections have been calculated at different stellar temperatures ranging up to 100 keV and compared with the results of previous work and evaluated data libraries.

High-resolution neutron capture and transmission measurements for116,120Sn,and their stellar neutron-capture cross sections ats-process temperatures

We have made high-resolution measurements of the neutron capture and total cross sections for ${}^{116}\mathrm{Sn}$ and the neutron capture cross section for ${}^{120}\mathrm{Sn}$ from 50 eV to 500 keV at the Oak Ridge Electron Linear Accelerator (ORELA). There have been no previously reported high-resolution measurements of these cross sections in this energy range. We performed an $\mathcal{R}$-matrix analysis of our new data as well as previous total cross section data for ${}^{120}\mathrm{Sn}$ to extract parameters for 216 resonances in ${}^{117}\mathrm{Sn}$ between 100 eV and 30 keV, and 187 resonances in ${}^{121}\mathrm{Sn}$ between 350 eV and 55 keV. These resonance parameters were used to calculate the average radiation widths, average level spacings, and strength functions for s- and p-wave neutron resonances. The ${}^{116,120}\mathrm{Sn}(n,\ensuremath{\gamma})$ astrophysical reaction rates were determined, to approximately 3% accuracy across the entire range of temperatures needed by the latest stellar models of the s process, from these resonance parameters together with our neutron capture measurements in the unresolved region. At the canonical s-process temperature of $kT=30 \mathrm{keV},$ our ${}^{116}\mathrm{Sn}(n,\ensuremath{\gamma})$ rate agrees to within the experimental uncertainties with the most recent high-precision measurement, and our ${}^{120}\mathrm{Sn}(n,\ensuremath{\gamma})$ rate resolves the discrepancy between two recent high-precision measurements. However, our measurements show that the most recently recommended reaction rates (which are based on extrapolations from previous measurements at higher energies) at $kT=6--8 \mathrm{keV},$ where most of the neutron exposure occurs in stellar s-process models, are too high by approximately 10--20 %. We discuss the astrophysical impact of our new rates.

Erratum: High-resolution neutron capture and transmission measurements, and the stellar neutron-capture cross section of88Sr [Phys. Rev. C62, 055803 (2000)

Erratum: High-resolution neutron capture and transmission measurements, and the stellar neutron-capture cross section of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow /><mml:mrow><mml:mn>88</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>Sr [Phys. Rev. C<b>62</b>, 055803 (2000)

High-resolution neutron capture and transmission measurements, and the stellar neutron-capture cross section of88Sr

We have made new and improved measurements of the neutron capture and total cross sections for ${}^{88}\mathrm{Sr}$ at the Oak Ridge Electron Linear Accelerator (ORELA). Improvements over previous measurements include a wider incident neutron energy range, better resolution, the use of metallic rather than carbonate samples, better background subtraction, reduced sensitivity to sample-dependent backgrounds, and better pulse-height weighting functions. Because of its small cross section, the ${}^{88}\mathrm{Sr}(n,\ensuremath{\gamma})$ reaction is an important bottleneck during s-process nucleosynthesis. Hence, an accurate determination of this rate is needed to better constrain the neutron exposure in s-process models and to better understand the recently discovered isotopic anomalies in certain meteorites. We performed an $\mathcal{R}$-matrix analysis of our capture and transmission data to extract parameters for 101 resonances between 100 eV and 350 keV. In addition, we fitted our transmission data alone to extract parameters for 342 additional resonances between 350 and 950 keV. We used this information to calculate average properties of the ${}^{88}\mathrm{Sr}+n$ system for comparison to previous work. Although previous data and resonance analyses were much less extensive, they are, in general, in good agreement with our results except that the average radiation widths as well as the p-wave correlation coefficients we determined are significantly smaller, and the s-wave correlation coefficient we determined has opposite sign from that reported in previous work. We used these resonance parameters together with a calculation of the small, but significant direct-capture contribution to determine the astrophysical reaction rate for the ${}^{88}\mathrm{Sr}(n,\ensuremath{\gamma})$ reaction to approximately 3% accuracy across the entire range of temperatures needed by s-process models. Our new rate is in good agreement with the results from a high-precision activation measurement at kT=25 keV, but it is approximately 9.5% lower than the rate used in most previous nucleosynthesis calculations in the temperature range (kT=6--8 keV), where most of the neutron exposure occurs in current stellar models of the s process. We discuss the possible astrophysical impact of this new, lower rate.

High resolution neutron capture and transmission measurements on137Baand their impact on the interpretation of meteoric barium anomalies

We have made improved measurements of the neutron capture and total cross sections for ${}^{137}\mathrm{Ba}$ over a sufficiently wide range of energies so that the reaction rate at $s$-process temperatures ($\mathrm{kT}$=6--23 keV) can be determined solely from the data. These rates are crucial for the interpretation of recently discovered anomalies of Ba isotopes in silicon carbide grains from the Murchison meteorite. Recent stellar models of the $s$ process are in agreement with the meteoric anomaly data for Ba only if the ${}^{137}\mathrm{Ba}(n,\ensuremath{\gamma})$ reaction rate is 20% larger than the previously accepted rate. Our reaction rates at $s$-process temperatures are in agreement with the extrapolated reaction rate from the most recent previous measurement. Hence, our results uphold, and place on much firmer footing, the discrepancy between recent stellar models of the $s$-process and the meteoric anomaly data.

New neutron capture and transmission measurements for 134,136Ba at ORELA and their impact on s-process nucleosynthesis calculations

We have made high-resolution neutron capture and transmission measurements on isotopically enriched samples of 134Ba and 136Ba at the Oak Ridge Electron Linear Accelerator (ORELA) in the energy range from 20 eV to 500 keV. Previous measurements had a lower energy limit of 3 – 5 keV, which is too high to determine accurately the Maxwellian-averaged capture cross section at the low temperatures (kT ≈ 6 – 12 keV) favored by the most recent stellar models of the s-process. Our results for the astrophysical reaction rates are in good agreement with the most recent previous measurement at the classical s-process temperature, kT = 30 keV, but show significant differences at lower temperatures. We discuss the astrophysical implications of these differences.

Resonance neutron capture and transmission measurements and the stellar neutron capture cross sections of 134Ba and 136Ba.

We have made high-resolution neutron capture and transmission measurements on isotopically enriched samples of $^{134}\mathrm{Ba}$ and $^{136}\mathrm{Ba}$ at the Oak Ridge Electron Linear Accelerator (ORELA) in the energy range from 20 eV to 500 keV. Previous measurements had a lower energy limit of 3--5 keV, which is too high to determine accurately the Maxwellian-averaged capture cross section at the low temperatures (kT\ensuremath{\approxeq}8-12 keV) favored by the most recent stellar models of the s process. By fitting the data with a multilevel R-matrix code, we determined parameters for 86 resonances in $^{134}\mathrm{Ba}$ below 11 keV and 92 resonances in $^{136}\mathrm{Ba}$ below 35 keV. Astrophysical reaction rates were calculated using these parameters together with our cross section data for the unresolved resonance region. Our results for the astrophysical reaction rates are in good agreement with the most recent previous measurement at the classical s-process temperature kT=30 keV, but show significant differences at lower temperatures. We determined that these differences were due to the effect of resonances below the energy range of previous experiments and to the use of incorrect neutron widths in a previous resonance analysis. Our data show that the ratio of reaction rates for these two isotopes depends more strongly on temperature than previous measurements indicated. One result of this temperature dependence is that the mean s-process temperature we derived from a classical analysis of the branching at $^{134}\mathrm{Cs}$ is too low to be consistent with the temperature derived from other branching points. This inconsistency is evidence for the need for more sophisticated models of the s process beyond the classical model. We used a reaction network code to explore the changes in the calculated isotopic abundances resulting from our new reaction rates for an s-process scenario based on a stellar model. These calculations indicate that the previously observed 20% discrepancy with respect to the solar barium abundance is reduced but not resolved by our new reaction rates. \textcopyright{} 1996 The American Physical Society.

R-Matrix Analysis of239Pu Neutron Transmissions and Fission Cross Sections in Energy Range from 1.0 keV to 2.5 keV

Abstract The results of the 239Pu high resolution neutron transmission measurements of Harvey et al. and of the 239Pu high resolution fission cross section measurements of Weston & Todd performed at the Oak Ridge Electron Linear Accelerator (ORELA) were analysed in the energy range from 1 keV to 2. 5 keV by the Bayesian code SAMMY using the Reich-Moore approximation of the R-matrix theory. The results obtained in a previous analysis in the energy range from thermal to 1 keV were updated by taking into account the recent renormalization of the experimental fission data by Weston & Todd. The statistical properties of the parameters of the resonances identified in the energy range from thermal to 2. 5 keV were examined and improved values of the average parameters were obtained. The resonance parameters are given in an ENDF-6 format file available from JAERI Nuclear Data Center and from NEA Data Bank (OECD).

R-Matrix Analysis of 239Pu Neutron Transmissions and Fission Cross Sections in Energy Range from 1.0keV to 2.5keV.

Abstract The results of the 239Pu high resolution neutron transmission measurements of Harvey et al. and of the 239Pu high resolution fission cross section measurements of Weston & Todd performed at the Oak Ridge Electron Linear Accelerator (ORELA) were analysed in the energy range from 1 keV to 2. 5 keV by the Bayesian code SAMMY using the Reich-Moore approximation of the R-matrix theory. The results obtained in a previous analysis in the energy range from thermal to 1 keV were updated by taking into account the recent renormalization of the experimental fission data by Weston & Todd. The statistical properties of the parameters of the resonances identified in the energy range from thermal to 2. 5 keV were examined and improved values of the average parameters were obtained. The resonance parameters are given in an ENDF-6 format file available from JAERI Nuclear Data Center and from NEA Data Bank (OECD).

Multilevel resonance analysis of 59Co neutron transmission measurements

Large discrepancies exist between the recent high-resolution ORELA neutron transmission data of 59Co and transmissions computed from the resolved resonance parameters of ENDF/B-VI. In order to provide new resonance parameters consistent with these data, the transmission measurements have been analyzed with the computer code sammy in the energy range 200 eV to 100 keV. The resonance parameters reported in this paper provide an accurate total cross section from 10 −5 eV to 100 keV and correctly reproduce the thermal capture cross section. Thermal cross-section values and related quantities are also reviewed here.

High resolution neutron capture and total cross section measurements of50Cr,52Cr and53Cr

Abstract High resolution neutron capture and transmission measurements were carried out at the 150 MeV Geel linac (GELINA) on the separated isotopes 50Cr, 52Cr and 53Cr. The data sets were atialysed by means of the TACASI, FANAC, FANAL and MULTI computer programs. The analysis of the capture data was completed up to 300, 500 and 200 keV resp. for the three isotopes 50,52,53Cr. The transmission data were analysed resp. up to 800, 500 and 200 keV. The resonance parameters obtained are compared with recent results of other laboratories1.5.

58Ni +ntransmission, capture and differential elastic scattering data analysis in the resonance region

High-resolution neutron transmission, capture and differential elastic scattering measurements have been made for /sup 58/Ni-enriched targets at the Oak Ridge Electron Linear Accelerator (ORELA) from the eV to the MeV region. The three sets of data were analyzed simultaneously from 10 to 450 keV, and the transmission analysis was extended to 650 keV. For the 52 s-wave resonances observed between 10 and 650 keV, the average level spacing, D/sub 0/, and the strength function, S/sub 0/, are 12.2 +- 1.0 keV and (3.1 +- 0.6) x 10/sup -4/, respectively.

Test of Fermi gas model predictions of level density in 137Xe.

We have studied the unbound levels of $^{137}\mathrm{Xe}$ via neutron resonance reactions and \ensuremath{\beta} decay of $^{137}\mathrm{I}$. High-resolution neutron transmission data revealed a total of 35 resonances below a neutron energy of 500 keV. At least 27 of the resonances are p-wave resonances. The near absence of s-wave resonances in this energy region resulted in an extremely low value for the s-wave neutron strength function. The current \ensuremath{\gamma}-ray data complement earlier studies of \ensuremath{\gamma} rays and delayed neutrons following the \ensuremath{\beta} decay of $^{137}\mathrm{I}$. By combining all data, we have obtained a detailed picture of the level density in $^{137}\mathrm{Xe}$ for a wide range of angular momenta. With the exception of ${(1/2)}^{+}$ and ${(1/2)}^{\mathrm{\ensuremath{-}}}$ levels, the overall agreement is good between the current data and predictions of the Fermi gas model.

Measurements of the neutron transmission and capture cross sections inPb204

High resolution neutron transmission measurements have been performed on $^{204}\mathrm{Pb}$ in the energy interval $E=0.4\ensuremath{-}105$ keV. The transmission data were analyzed using a multilevel $R$-matrix code to deduce resonance parameters. Previously obtained neutron capture data were reanalyzed in the interval 2.6-86 keV. Values of $\frac{G{\ensuremath{\Gamma}}_{\mathrm{n}}{\ensuremath{\Gamma}}_{\ensuremath{\gamma}}}{\ensuremath{\Gamma}}$ were determined from the capture data. For those resonances where ${\ensuremath{\Gamma}}_{\mathrm{n}}$ could be determined from the transmission data, the capture data were analyzed to extract ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}}$. Our results yield an average capture for a stellar temperature $kT=30$ keV of 89.5\ifmmode\pm\else\textpm\fi{}4.5 mb. The $s$-wave level density for $^{205}\mathrm{Pb}$ corresponding to the neutron energy range investigated (i.e., $E\ensuremath{\sim}105$ keV) relative to that for $^{207}\mathrm{Pb}$ (which has about the same neutron separation energy) is greater by about a factor of 10. The average $s$-wave strength function in this energy region is determined as ${S}_{0}=0.93\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$. This is an order of magnitude greater than that for a similar energy region in $^{206}\mathrm{Pb}$ + n where a doorway is observed at $E\ensuremath{\sim}500$ keV. However, the strength function in the initial $E=0\ensuremath{-}100$ keV in $^{204}\mathrm{Pb}$ + n is almost identical to the average value of that for $^{206}\mathrm{Pb}$ + n when the averaging interval for the latter is taken as ${E}_{\mathrm{n}}\ensuremath{\approx}0\ensuremath{-}1000$ keV (i.e., over the doorway state). This suggests that the $s$-wave doorway state observed in the higher mass lead isotopes is completely mixed with "background" states in $^{205}\mathrm{Pb}$, and most likely no intermediate structure will be observed in the $s$-wave strength function for the $^{204}\mathrm{Pb}$ + n reaction.

Neutron Resonance Parameters of Rubidium-85 and Rubidium-87

High resolution neutron transmission and low background capture measurements were carried out on the separated rubidium isotopes, using the time-of-flight facility of the linear accelerator of Japan Atomic Energy Research Institute. Resonance parameters and associated quantities were deduced as follows: For 85Rb, gΓ n values were determined for 138 resonance levels in the energy region below 18.5 keV. s-wave strength function was obtained to be S0=(0.94±0.11)×10−4, average level spacing ≤D>=133±11 eV and average radiative width ≤Γ r >=328±18 me V. For 87Rb, gΓ n values were determined for 30 resonance levels in the energy region below 48.6 keV and the following quantities were deduced: S0=(1.15±0.3)×10−4, D=1,380±250 eV and ≤Γγ>=166±30 meV. For 85Rb average properties of resonances are in good agreement with the prediction of the statistical model. On the other hand, for 87R9b the average properties of resonances deviate from the prediction of the statistical model; four strong s-wave resonances cluster within an energy interval of 5 keV, and they carry about 37% of s-wave strength below 48.6 keV.