Neutron Widths Research Articles

Measurement of theHe3andHe4total neutron cross sections up to 40 MeV

The total neutron cross sections of the helium isotopes were measured in the energy range from 1.5 to 40 MeV. A continuous energy neutron source was used in connection with a 190 m long flight path and high resolution time-of-flight techniques. The energy resolution achieved enabled us to improve considerably the parameters of the narrow ${\frac{3}{2}}^{+}$ resonance in the n+$^{4}\mathrm{He}$ system. The position of the resonance was found to be 22 133\ifmmode\pm\else\textpm\fi{}10 keV, with a total width of 76\ifmmode\pm\else\textpm\fi{}12 keV and a neutron width of 37\ifmmode\pm\else\textpm\fi{}5 keV. This agrees well with previous analyses, which yield ${\ensuremath{\Gamma}}_{\mathrm{n}}\ensuremath{\sim}\frac{1}{2}\ensuremath{\Gamma}$. Absolute values for the cross sections were determined with an overall error smaller than 3%. Comparison with older data shows agreement within the total errors. Our data are 4% to 5% higher than the most recent data set; however, in the case of $^{3}\mathrm{He}$, the data are still below the predictions from an $R$-matrix analysis. It was pointed out recently that a slight disagreement exists between the total neutron cross section of $^{3}\mathrm{He}$ and the sum of the partial n-$^{3}\mathrm{He}$ cross sections. This discrepancy is removed by our new data.NUCLEAR REACTIONS Measured $\ensuremath{\sigma}({E}_{\mathrm{n}})$ for $^{3}\mathrm{He}$, $^{4}\mathrm{He}$, 1.5-40 MeV, high resolution TOF techniques, high pressure gas samples, four-nucleon system, $^{5}\mathrm{He}$ level parameters.

Overlappingβdecay and resonance neutron spectroscopy of levels inKr87

Energy levels in $^{87}\mathrm{Kr}$ have been studied, with special emphasis on the unbound region, using two different methods. The first method comprises neutron capture and transmission measurements on an enriched gas target of $^{86}\mathrm{Kr}$ using neutron time-of-flight techniques. In this way, neutron widths were determined for 39 resonances below 400 keV and capture areas for 14 resonances below 90 keV. The second method is a decay study of 56-s $^{87}\mathrm{Br}$ in which a level scheme for $^{87}\mathrm{Kr}$ has been established that shows 126 levels in the bound and 12 levels in the unbound region. A detailed comparison amongst the neutron resonance, the $\ensuremath{\gamma}$-ray decay, and available delayed neutron results has been made. Almost a one-to-one correspondence exists between the currently observed $p$-wave resonances below 250 keV and levels in $^{87}\mathrm{Kr}$ studied through delayed neutron emission. The overall $\ensuremath{\beta}$-strength distribution derived from the present data shows broad resonancelike structures. However, no marked selectivity is observed in the $\ensuremath{\beta}$ decay to individual levels in the unbound region of $^{87}\mathrm{Kr}$. The neutron capture cross section of $^{86}\mathrm{Kr}$ is found to be about 5 mb for 30-keV neutrons with a Maxwellian energy distribution. The future of delayed neutron spectroscopy as a new tool for obtaining level-density information is discussed.NUCLEAR REACTIONS $^{86}\mathrm{Kr}(n)$, ${E}_{n}=1\ensuremath{-}400$ keV, measured total $\ensuremath{\sigma}(E)$; $^{86}\mathrm{Kr}(n, \ensuremath{\gamma})$,, ${E}_{n}=1\ensuremath{-}90$ keV, measured ${\ensuremath{\sigma}}_{n\ensuremath{\gamma}}$; $^{87}\mathrm{Kr}$ deduced resonances, resonance parameters, neutron strength functions, Maxwellian average cross section; enriched target.RADIOACTIVITY $^{87}\mathrm{Br}$ [from $^{235}\mathrm{U}(n, f)$], measured ${E}_{\ensuremath{\gamma}}$, ${i}_{\ensuremath{\gamma}}$, $\ensuremath{\gamma}\ensuremath{\gamma}$-coinc; deduced $^{87}\mathrm{Kr}$ levels; deduced $^{87}\mathrm{Br}$ ground state $J$, $\ensuremath{\pi}$; Ge(Li) detector, mass separated source.

Resonance parameters ofNi60+nfrom measurements of transmission and capture yields from 1 to 450 keV

High-resolution transmission and capture measurements of $^{60}\mathrm{Ni}$-enriched targets have been made from a few eV to 1800 keV in transmission and from 2.5 keV to 5 MeV in capture. The transmission data from 1 to 452 keV were analyzed with a multilevel $R$-matrix code which uses Bayes' theorem for the fitting process. This code provides the energies and neutron widths of the resonances inside the 1- to 452-keV region as well as a possible parametrization for outside resonances to describe the smooth cross section in this region. The capture data were analyzed from 2.5 to 452 keV with a least-squares fitting code using the Breit-Wigner formula. Average parameters for the 30 observed $s$-wave resonances were deduced. The average level spacing, ${D}_{0}$, was found to be equal to 15.2\ifmmode\pm\else\textpm\fi{}1.5 keV; the strength function, ${S}_{0}$, equal to (2.2\ifmmode\pm\else\textpm\fi{}0.6)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$; and the average radiation width, ${\overline{\ensuremath{\Gamma}}}_{\ensuremath{\gamma}}$, equal to 1.30\ifmmode\pm\else\textpm\fi{}0.07 eV. The staircase plot of the reduced level widths and the plot of the Lorentz-weighted strength function averaged over various energy intervals show possible evidence for doorway states. The level densities calculated with the Fermi-gas model for $l=0$ and for $l>0$ resonances were compared with the cumulative number of observed resonances, but the analysis is not conclusive. The correlation coefficient $\ensuremath{\rho}$ between ${\ensuremath{\Gamma}}_{n}^{0}$ and ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}}$ is equal to 0.53\ifmmode\pm\else\textpm\fi{}0.18. The average capture cross section as a function of the neutron incident energy is compared to the tail of the giant electric dipole resonance prediction.NUCLEAR REACTIONS $^{60}\mathrm{Ni}(n, n)$, ${E}_{n}=1\ensuremath{-}452$ keV; $^{60}\mathrm{Ni}(n, \ensuremath{\gamma})$, ${E}_{n}=2.5\ensuremath{-}452$ keV; measured ${\ensuremath{\sigma}}_{n,\mathrm{tot}}({E}_{n})$, ${\ensuremath{\sigma}}_{n,\ensuremath{\gamma}}({E}_{n})$; deduced $^{61}\mathrm{Ni}$ resonance parameters ${E}_{0}$, $g{\ensuremath{\Gamma}}_{n}$, ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}}$, $l$, $J$, and average properties ${D}_{0}$, ${S}_{0}$, ${\overline{\ensuremath{\Gamma}}}_{\ensuremath{\gamma}}$; calculated level density for $l=0$ and $l>0$ resonances and correlation $\ensuremath{\rho}({\ensuremath{\Gamma}}_{n}^{0}, {\ensuremath{\Gamma}}_{\ensuremath{\gamma}})$.

Neutron resonance parameters of silver-107 and silver-109.

Neutron transmission measurements were carried out on the separated isotopes of silver using the time-of-flight facility at the Japan Atomic Energy Research Institute electron linear accelerator. Neutrons were detected with the 6Li-glass detectors at 56 and 191 m. The samples used were metallic powder enriched to 98.2% for 107Ag and 99.3% for 109Ag. Transmission data were analyzed with the multi-level Breit-Wigner formula incorporated in a least squares fitting program. Resonance energies and neutron widths were determined for the large number of resolved resonances in the neutron energy region of 400 eV~7 keV. The s-wave strength functions and average level spacings were obtained to be; S0= (0.43±0.05) × 10−4, D0 = 20±2 eV for 107Ag and S0= (0.45 ± 0.05) × 10−4, D0 = 20 ± 2eV for 109Ag.

Resolved Resonance Parameters for 238U from 4 to 6 keV

Neutron widths for 145 resonances from 4 to 6 keV are reported from a least-squares shape analysis of the Oak Ridge Electron Linear Accelerator 150-m flight path, four-sample /sup 238/U transmission data. The resultant s-wave strength function from 4 to 6 keV is found to be substantially smaller than that from 0 to 4 keV.

Measurement and Resonance Analysis of the 232Th Total Neutron Cross Section

Using the Oak Ridge Electron Linear Accelerator pulsed neutron source and a 1-mm-thick lithium glass detector, neutron transmission spectra through /sup 232/Th have been measured at 22- and 40-m flight paths. At 22 m, transmission spectra through samples of five thicknesses were measured from 7 mev to 15 ev. At 40 m, spectra through samples of eight thicknesses were measured from 15 ev to 4 kev. The resulting total cross section from 0.1 to 20.0 ev is smaller than that given by the ENDF/B-V evaluation. Least-squares shape analysis of the transmissions up to 2.0 kev gives larger neutron widths above 0.5 kev than those previously reported. An average radiation width of 25.2 mev is obtained for 19 low-energy s-wave resonances.

Resonance structure of 32S + n from transmission and differential elastic scattering experiments

Neutron transmission experiments were performed on 32S between 0.18 MeV and 19 MeV and differential elastic scattering experiments up to 2 MeV. The resonance parameters, including spin and parity, were determined up to 1.7 MeV. The distribution of neutron strength was determined for s 1 2 , P 1 2 , P 3 2 , d 3 2 , d 5 2 and f-wave levels, and is compared to recent model calculations. The isotopic purity of some possible T = 3 2 isobaric analog states was deduced from their neutron widths.

Electric dipole transitions from neutron capture inYb173resonances

Primary neutron capture $\ensuremath{\gamma}$ rays have been studied from 49 neutron resonances in $^{173}\mathrm{Yb}$ in the 10-530 eV energy range. These resonances are assigned $J=2$ and $J=3$ on the basis of the intensity ratios of suitable pairs of low-energy $\ensuremath{\gamma}$ rays. The measured intensities of the high-energy primary $\ensuremath{\gamma}$ rays have been converted to partial radiation widths. These widths have been subjected to a correlation analysis against reduced neutron widths, with the result that convincing evidence was not found for previously reported nonstatistical behavior in this nucleus. The ratio of the average population of a particular low-lying level from $J=3$ resonances to its population from $J=2$ resonances was found to be in satisfactory agreement with a cascade model. Resonances in the 10-200 eV range were noted to exhibit an enhancement of $E1$ transition probabilities to $K=2$ final states as compared to $K=0$ states. A level scheme for $^{174}\mathrm{Yb}$ was constructed, and the neutron separation energy for this nucleus was deduced as 7464.5\ifmmode\pm\else\textpm\fi{}1.0 keV.NUCLEAR REACTIONS $^{173}\mathrm{Yb}$($n$,$\ensuremath{\gamma}$), $E=0.01\ensuremath{-}0.86$ keV; measured ${E}_{\ensuremath{\gamma}}$, ${I}_{\ensuremath{\gamma}}$. $^{174}\mathrm{Yb}$ deduced resonances, $J$, deduced levels, partial radiation widths, correlations, neutron separation energy. Enriched target, Ge(Li) detector.

Baryon- and lepton-number violation by electroweak instantons

We make a quantitative study of instanton-induced baryon- and lepton-number-violating processes in an SU(2) x U(1) electroweak gauge theory at zero and finite temperatures (in the ''dilute-instanton-gas'' approximation). As an example we consider a simplified model involving only the proton, neutron, electron, and electron neutrino. At zero temperature the total cross sections for p+n..-->.. e-bar+nu-bar and eleven other similar reactions are of order s x 10/sup -195/ cm/sup 2/, where s is the total center-of-momentum energy squared in GeV/sup 2/. The neutron decays via n..-->..p-bar+e-bar+nu-bar with a lifetime of the order 10/sup 146/ years. The cross sections and neutron decay width decrease with temperature because color-electric-charge screening reduces the self-dual-instanton density at finite temperature. At high temperature the cross sections (for a given s) and neutron decay width fall off as T/sup -47/3/ in this simplified model. It is suggested that correctly treating the instanton gas as very dense (as discussed by Berg, Luscher, and Stehr) and including finite-energy tunneling solutions could increase the predicted reaction rates.

Study of the 49Ti energy levels using the (γ, γ′) and the (γ, n) reactions

An unbound level in 49Ti at 8884 keV was photoexcited using a γ-beam obtained from the Cr(n, γ) reaction. The γ-decay and n-decay properties of this level were studied using angular distributions and polarization measurements. Thus spin and parity assignments of some levels in 49Ti were made. The neutron width and the total radiative widths of the 8884 keV level were determined to be: Γ n = 0.25 ± 0.05 eV, Γ γ = 2.55 ± 0.80 eV.

Neutron resonance parameters and radiative capture cross sections of 147Sm and 149Sm

Neutron capture and transmission measurements have been carried out on the separated isotopes of 147Sm (98.34 %) and 149Sm (97.72 %) at the 55 m time-of-flight station of the Japan Atomic Energy Research Institute electron linear accelerator. Resonance energies and neutron widths for a large number of resolved resonances were determined up to 2 keV for 147Sm and 520 eV for 149Sm. Radiation widths for 5 resonances in 147Sm + n and 7 resonances in 149Sm + n were derived. The s-wave strength functions, average level spacings and average radiation widths were obtained to be: 10 4S 0 = 4.8 ± 0.5, D = 5.7 ± 0.5 eV and Γ γ = 69 ± 2 meV for 147 Sm ; a 10 4S 0 = 4.6 ± 0.6, D = 2.2 ± 0.2 eV and Γ γ = 62 ± 2 meV for 149 Sm . The average capture cr sections were deduced from 3.3 to 300 keV with an estimated accuracy of 5 to 15 %. The measured capture cross sections for 149Sm are largely different from the evaluated data, which are obtained based on the statistical model calculation. Possible reasons for this disagreement are discussed.

Neutron widths and level spacings ofZn64+n

High resolution neutron total cross-section measurements on the separated isotope $^{64}\mathrm{Zn}$ have been made in energy range of few eV to about 370 keV using a pulsed neutron time-of-flight technique with a nominal resolution of 0.07 ns/m. We have analyzed the cross section data up to 370 keV using a multilevel multichannel computer code and obtained values of the resonance parameters (${E}_{0},{J}^{\ensuremath{\pi}},g{\ensuremath{\Gamma}}_{n}$). From these results we obtain the following values for the average properties of the resonance parameters: ${〈D〉}_{l=0}=(3.59\ifmmode\pm\else\textpm\fi{}0.16)$ keV, ${S}_{0}(\ifmmode\times\else\texttimes\fi{}{10}^{4})=1.89\ifmmode\pm\else\textpm\fi{}0.26$, and ${S}_{1}(\ifmmode\times\else\texttimes\fi{}{10}^{4})=0.59\ifmmode\pm\else\textpm\fi{}0.06$. We have also investigated the neutron width and nearest neighbor spacing distributions for 103 $s$-wave resonances up to 370 keV. The neutron width distribution follows a chi-squared distribution with $\ensuremath{\nu}=1.15\ifmmode\pm\else\textpm\fi{}0.30$. We have determined values for Dyson-Mehta's ${\ensuremath{\Delta}}_{3}$ statistic for $s$-wave levels up to 300 keV as a function of energy. These are found to be in excellent agreement with the predicted values as explained in Table V. Similarly the linear coefficient of correlation between nearest neighbor spacings gives a value of $\ensuremath{\rho}=\ensuremath{-}0.35$ in agreement with the predicted value of the random matrix model. The value for the mean $p$-wave level spacing is larger than expected from the ($2J+1$) density law, indicative of missed $p$-wave levels.NUCLEAR REACTIONS $^{64}\mathrm{Zn}(n,n)$, $E=\mathrm{few}$ eV to \ensuremath{\sim}370 keV. Measured ${\ensuremath{\sigma}}_{t}$ vs ${E}_{n}$, deduced resonance parameters. Resonance energies ${E}_{\ensuremath{\lambda}}$, ${J}^{\ensuremath{\pi}}$, ${\ensuremath{\Gamma}}_{n}^{0}$, ${\ensuremath{\Gamma}}_{n}^{1}$, ${S}_{0}$, ${S}_{1}$, ${〈D〉}_{l=0}$, ${〈D〉}_{l=1}$.

Neutron capture cross section inZn64

Total neutron capture cross section measurements of the separated isotope of /sup 64//sub 30/Zn have been made in the energy interval of 2.5--900 keV. These were made using time-of-flight techniques and a total energy ..gamma..-ray detector system with a nominal resolution of about 0.12 ns/m. Resonance energies and capture areas were determined for 191 resonances up to 131 keV. Values of the radiation widths and neutron widths for many resonances have been determined making use of the results available from transmission measurements. From these results mean values of 726=(plus-or-minus60) meV for s-wave and 272=(plus-or-minus30) meV for p-wave resonances have been determined. A value for the p-wave strength function S/sub 1/0.75=(plus-or-minus0.08) x 10/sup -4/ eV/sup -1/2/ was obtained for neutron energy up to 130 keV. Bayes' theorem for conditional probability was used to identify d-wave levels and from these resonances a lower limiting value for d-wave strength function S/sub 2/0.78=(plus-or-minus0.12) x 10/sup -4/ eV/sup -1/2/ was obtained. The values for the p-wave mean level spacing for all resonances observed up to 130 keV, and after elimination of d-wave resonances, were found to be (0.68 +- 0.04) and (1.10 +- 0.07) keV, respectively. An analysis of the linear correlation coefficient (rho) betweenmore » GAMMA/sup 0//sub n/ and GAMMA/sub ..gamma../ gave a value of (0.47 +- 0.08) at a confidence level of 99.7%.« less

Radiative widths of neutron scattering resonances

The radiative widths of resonances with large neutron widths in 19F, 24Mg, 27Al, 28Si, 56Fe and 207Pb have been measured relative to gold in the 20-80 keV neutron energy range. Moxon-Rae gamma -ray detectors were used with time-of-flight discrimination of scattered neutron events. These results help resolve existing discrepancies in the data by elucidating the contribution of resonance neutron scattering to capture gamma -ray yields.

Resonance structure ofS32+nfrom measurements of neutron total and capture cross sections

Neutron total and capture cross sections of /sup 32/S have been measured up to 1100 keV neutron energy (E/sub exc/(/sup 33/S) =9700 keV). Spin and parity assignments have been made for 28 of the 64 resonances found in this region. Values of total radiation widths, reduced neutron widths, level spacings, and neutron strength functions have been evaluated for s/sub 1/2/, p/sub 1/2/, p/sub 3/2/, and d/sub 5/2/ levels. Single particle contributions using the valency model account for a significant portion of the total radiation width only for the p/sub 1/2/-wave resonances. A significant number of resonances can be identified with reported levels excited in /sup 32/S(d,p) and /sup 29/Si(..cap alpha..,n) reactions. A calculation of the Maxwellian average cross section appropriate to stellar interiors indicates an average capture cross section at 30 keV, sigma-bar approx. = 4.2(2) mb, a result that is relatively insensitive to the assumed stellar temperature. Direct (potential) capture and the s-wave resonance capture contributions to the thermal capture cross section do not fully account for the reported thermal cross section (530 +- 40 mb) and a bound state is invoked to account for the discrepancy.

Neutron resonance parameters of 238U

Neutron transmission measurements on natural uranium samples were performed in the energy region from 20 eV up to 4.7 keV on a 190 m flight path of the JAERI 120 MeV linac neutron time-of-flight spectrometer. Samples were all metallic slabs with three thicknesses of 0.00725, 0.0144 and 0.0236 atoms/barn, respectively. One of them was cooled down to 77 K to reduce Doppler broadening effect. The best nominal resolution of the measurements was 0.3 ns/m. Special attention has been paid to background determination, because its shape was found to depend on the thickness of the sample in the beam. Resonance parameters Γ ∘ n are obtained for 180 resonances in the energy region up to 4.7 keV with the Atta-Harvey area-analysis programme. Excluding p-wave resonances assigned by Corvi et al., the average level spacing, the average reduced neutron width and the strength function were determined to be D ̄ = 21.9 eV , Γ ∘ n = 2.47 ± 0.33 meV and S 0 = (1.13 ± 0.13) × 10 −4, respectively. The statistics of the level spacings are in agreement with those predicted by the theory of Dyson and Mehta and are inconsistent with an uncorrelated Wigner distribution. Results are compared with currently available experimental data.

Studies in nuclear spectroscopy using the (γ, γ') and the (γ, n) reactions

The technique of using elastic and inelastic resonance scattering of monochromatic protons for studying the spectroscopic properties of energy levels in various nuclei is discussed. The photons are produced by thermal neutron capture in some metallic elements. The resonance levels are photoexcited by a random overlap between one of the incident γ-lines and one nuclear level of the scattering isotope. It is shown that such (γ, γ') reactions may be used for measuring the excitation energies, the spins, parities and radiative widths of the resonance levels. Also the excitation energies and spins of the low lying levels, the absolute strengths of E1, M1, E2, M2 transitions from the resonance level can be obtained. For unbound levels, the study of the (γ, n) reaction can provide information on the spin, parity, neutron width of the resonance level, interference between various multipolarities, and spectroscopic information on the energy level of the A − 1 isotope. The advantages of using stronger γ-sources from hugh flux reactors is discussed.

TheO16(d,p)O17reaction to the unbound 5.09-MeV state ofO17

New measurements of the $^{16}\mathrm{O}$($d$,$p$) reaction cross sections to the ground-, 0.87-, and 5.09-MeV states in $^{17}\mathrm{O}$ have been performed for eleven angles from 17.5\ifmmode^\circ\else\textdegree\fi{} to 150\ifmmode^\circ\else\textdegree\fi{} at a bombarding energy of 12 MeV. The results are compared with earlier measurements at the same energy. The new measurements are shown to resolve a discrepancy observed for the comparison of the neutron strength of the unbound 5.09-MeV state extracted from earlier ($d$,$p$) studies with that from neutron scattering experiments. These results indicate that the method of Vincent and Fortune for distorted-wave analysis of stripping to unbound states yields a reliable value for the neutron width for this important test case. Measurements of $^{16}\mathrm{O}$($d$,$d$) elastic scattering cross sections performed simultaneously are reported.NUCLEAR REACTIONS $^{16}\mathrm{O}$($d$,$d$) elastic scattering and $^{16}\mathrm{O}$($d$,$p$)$^{17}\mathrm{O}$ (0.0, 0.87, 5.09 MeV), $E=12$ MeV; measured $\ensuremath{\sigma}(\ensuremath{\theta})$; $\ensuremath{\theta}=17.5\ifmmode^\circ\else\textdegree\fi{}\ensuremath{-}150\ifmmode^\circ\else\textdegree\fi{}$, eleven angles; deduced neutron width from the ($d$,$p$) reaction to 5.09-MeV state.

Neutron Resonance Parameters for Neptunium-237

Capture, elastic scattering, and total cross-section measurements were performed on 237Np between 8 and 204 eV. The neutron widths, Γn, were obtained for 200 resonances and the radioactive widths, Γγ, for 25 resonances. The mean capture width 〈Γγ〉 = 41.2 ± 2.9 meV, and the mean s-wave resonance spacing D0 = 0.740 ± 0.061 eV. The s-wave strength function was found to be S0 = (1.02 ± 0.14) × 10−4.

Low-energy neutron resonances inPb206

Neutron resonance reactions on an enriched /sup 206/Pb target have been studied below 50 KeV using neutron time-of-flight techniques. The neutron and radiation widths of seventeen resonances were obtained from transmission and capture cross section measurements. Spin and parity assignments for eight resonances were made on the basis of the transmission measurements and the decay patterns of ..gamma.. rays measured with a Ge(Li)detector. The ..gamma..-ray spectra are strikingly simple, being composed of less than five primary ..gamma.. rays. The neutron separation energy for /sup 207/Pb was determined to be 6738.2 +- 0.7 KeV. The magnetic dipole transitions from several p-wave resonances to the first three excited states in /sup 207/Pb show some enhancement. A possible doorway state in the p/sub 1/2/ neutron channel has been observed in the /sup 206/Pb + n reaction at E/sub n/ approx. = 36 keV.