S-wave Resonances Research Articles

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.

C11level structure via theB10(p, γ)reaction

The reaction /sup 10/B(p,..gamma..) /sup 11/C has been investigated in the energy range E/sub p/ = 0.07--2.20 MeV. The broad resonant structure previously observed in the ground state transition near E/sub p/ = 1.2 MeV has been seen also in ..gamma..-ray transitions to excited states. The observed excitation functions as well as the ..gamma..-ray angular distributions can be explained by assuming several broad overlapping resonances. The low-energy data (E/sub p/<0.6 MeV) reveal the existence of two s-wave resonances at E/sub p/ = 0.010 and 0.56 MeV. Spectroscopic factors for several final states have been obtained from observation of direct capture processes to them; they are in fair agreement with results from stripping reaction studies. The present data also provide information on partial and total widths of the states at E/sub x/ = 8--9 MeV. The energy range investigated corresponds to the temperature range of T = (0.01--5) x 10/sup 9/ K. The thermonuclear reaction rates deduced from the present results are compared with previously reported values.

Neutron Capture Resonances in 56Fe and 58Fe in the Energy Range from 10 to 100 keV

The neutron capture cross sections of 56Fe and 58Fe have been measured in the energy range from 10 to 250 keV relative to the gold standard. A pulsed 3-MV Van de Graaff accelerator and the 7Li(p,n) reaction served as a neutron source. Capture gamma rays were detected by two C6D6 detectors, which were operated in coincidence and anticoincidence modes. Two-dimensional data acquisition allowed the offline application of the pulse height weighting technique. The samples were located at a 60-cm flight path. The total time resolution was 1.2 ns allowing an energy resolution of 2 ns/m. The experimental setup was optimized with respect to low background and low neutron sensitivity. The additional 4-cm flight path from the sample to the detector was sufficient to discriminate against the capture of sample scattered neutrons by the additional time of flight. In this way reliable results were obtained even for the strong s-wave resonances of both isotopes. The experimental capture yield was analyzed with the FANAC code. The energy resolution allowed extraction of resonance parameters in the energy range from 10 to 100 keV. Individual systematic uncertainties were found to range between 5 and 10% while the statistical uncertainty is 3 to 5% for most resonances. A comparison to other results exhibits systematic differences of 7 to 11% for 56Fe. The present results for 58Fe differ up to 50% from the only other measurement for this isotope.

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.

Neutron resonance parameters ofZn3068+nand statistical distributions of level spacings and widths

Discrete values of the parameters (E/sub 0/, gGAMMA/sub n/, J/sup ..pi../, GAMMA/sub ..gamma../, etc.) of the resonances in the reaction /sup 68//sub 30/Zn+n have been determined from total cross section measurements from a few keV to 380 keV with a nominal resolution of 0.07 ns/m for the highest energy and from capture cross section measurements up to 130 keV using the pulsed neutron time-of-flight technique with a neutron burst width of 5 ns. The cross section data were analyzed to determine the parameters of the resonances using R-matrix multilevel codes. These results have provided values of average quantities as follows: S/sub 0/ = (2.01 +- 0.34), S/sub 1/ = (0.56 +- 0.05), S/sub 2/ = (0.2 +- 0.1) in units of 10/sup -4/, D/sub 0/ = (5.56 +- 0.43) keV and D/sub 1/ = (1.63 +- 0.14) keV. From these measurements we have also determined the following average radiation widths: (Gamma-bar/sub ..gamma../)/sub l/ = 0 = (302 +- 60) meV and (Gamma-bar/sub ..gamma../)/sub l/ = 1 = (157 +- 7) meV. The investigation of the statistical properties of neutron reduced widths and level spacings showed excellent agreement of the data with the Porter-Thomas distribution for s- and p-wave neutron widthsmore » and with the Dyson-Mehta ..delta../sub 3/ statistic and the Wigner distribution for the s-wave level spacing distribution. In addition, a correlation coefficient of rho = 0.50 +- 0.10 between GAMMA/sup 0//sub n/ and GAMMA/sub ..gamma../ has been observed for s-wave resonances. The value of at (30 +- 10) keV is 19.2 mb.« less

Statistical properties of complex states ofZn3067

High resolution neutron total and capture cross section measurements have been made on the separated isotope /sup 66/Zn in the energy range from a few keV to about 1 MeV using the neutron time-of-flight technique and the pulsed Oak Ridge Electron Linear Accelerator. The nominal resolution of the measurements was 0.07 ns/m. The total cross section data have been analyzed up to 380 keV using a multilevel R-matrix formalism and the capture data up to 130 keV using a least squares fit program. From these analyses we have obtained the values of the resonance parameters E/sub 0/, gGAMMA/sub n/, and GAMMA/sub ..gamma../, and values of average quantities: D/sub l/ = 0 = (4.62 +- 0.55) keV, D/sub l/ = 1 = (1.17 +- 0.14) keV, S/sub 0/ = (2.06 +- 0.36), S/sub 1/ = (0.66 +- 0.06), and S/sub 2/ = (0.50 +- 0.12) in units of 10/sup -4/ eV/sup -1/2/. The values of average radiation widths are (0.39 +- 0.06) eV for s-wave resonances and (0.190 +- 0.013) eV for p-wave resonances. The distributions of neutron reduced widths and of level spacings of s- and p-wave resonances show excellent agreement with the Porter-Thomas and Wigner distributions, respectively. The valuemore » of the ..delta../sub 3/ statistic for the long range correlation of s-wave energy levels is found to be in reasonable agreement with the theoretical prediction of Dyson and Mehta. A value of 0.59 +- 0.10 has been determined for the linear coefficient for the correlation between the neutron reduced widths and the radiation widths of 26 s-wave resonances. This value is highly significant and has a confidence level of 99.7%.« less

Determination of the Capture Width of the 27.7-keV s-Wave Neutron Resonance in Iron-56

The neutron capture width of the 27.7-keV s-wave resonance in 56Fe has been determined using a setup completely different from most of the previous experiments. A pulsed 3-MV Van de Graaff accelerator and the 7Li(p,n) reaction served as a neutron source. Capture gamma rays were observed by a Moxon-Rae detector and gold was used as a standard. The samples were positioned at a flight path of only 7.6 to 8.0 cm. This allowed the use of very thin samples avoiding large multiple scattering corrections. Three metallic disks enriched in 56Fe were used with a thickness between 0.6 and 0.15 mm. Events due to capture of resonance scattered neutrons in the detector or surrounding material were completely eliminated by time-of-flight. The result for the capture width is Γγ = 1.01 eV with a statistical uncertainty of 1.3% and a systematic uncertainty of ∼5%.

Resonance neutron capture in58Fe

The resonance neutron capture cross section of 58Fe has been measured at the Oak Ridge electron linear accelerator with 0.2% energy resolution from 2.5 to 200 keV. Large corrections for scattered neutrons are required for the s-wave resonances which prohibit a study of correlation effects. The s- and p-wave radiative widths are Gamma gamma (s)=0.86+or-0.8 and g Gamma gamma (p)=0.58+or-0.06. Many new l>0 resonances are observed which have not been seen previously in transmission measurements. The Maxwellian averaged capture cross section at kT=30 keV is 15.9+or-1.5 mb.

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.

The Transmission of Neutrons Through Iron-56 at 24.37 keV

A measurement of the 56Fe total neutron cross section at the interference minimum near 24.37 keV shows a value of 7.5 ± 4.2 mb. The data suggest a radiation width of 2 ± 1 eV for the 27.85-keV s-wave resonance of 56Fe. The values Γn, = (1.43 ± 0.06) keV and R′ = (5.67 ± 0.16) × 10−13 cm [(5.67 ± 0.16) fm] for this resonance were deduced from a shape fit.

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.

Autodissociating Rydberg states of positronium hydride

Consequences of the nonrelativistic Coulomb Hamiltonian with a fixed proton are considered for positronium hydride (PsH). An optical-potential method and certain simplifying assumptions are used to compute the lowest s-wave resonance parameters on the basis of the /e(+)H(-)/ configuration. Resonance parameters corresponding to a Ps scattering energy of 4.0190 eV and a width of 0.0303 eV are obtained. These results are shown to be in very close agreement with those of previous studies.

The Total Neutron Cross Section of Boron-10 Between 90 and 420 keV

The neutron total cross section of /sup 10/B was determined between 90 and 420 keV neutron energy by means of a transmission measurement on a boron sample enriched in /sup 10/B. Deviations in shape from other measurements are within the statistical accuracy of the present measurement and are smaller than 1.5%. In the measured energy region, no indication of narrow resonance structure was found, and the deviation of the total cross section from an E/sup -1/2/ energy dependence above 100 keV was confirmed. An analysis using the R-matrix formalism showed that this deviation can be associated with the 7/2/sup +/ s-wave resonance at 370 keV. 2 figures, 3 tables.

Neutron capture cross section of manganese

Total neutron capture cross section measurements have been carried out for /sup 55/Mn (I = 5/2) over the energy interval of 2.5--600 keV. Using time-of-flight techniques and a nonhydrogenous liquid scintillator detector system, resonance energies and capture area were determined for some 143 resonances to 112 keV neutron energy. Values of radiation widths (GAMMA/sub ..gamma../) were evaluated for broad l = 0 resonances using published total cross section data. Although values of GAMMA/sub ..gamma../ vary widely for individual resonances, mean values of /sub l/ = 0 = 750 +- 150 and /sub iltsts1/ = 400 +- 100 MeV were determined. The correlation between GAMMA/sub ..gamma../ and GAMMA/sub n//sup 0/ (for s-wave resonances) was found to give rho = 0.64 +- 0.14 which is statistically significant at the 99.9% confidence level.

A comparison between exact and DWBA stripping to a resonance

Exact cross sections for deuteron stripping to a resonance from a heavy target were obtained by solving the Faddeev-Lovelace equations for separable s-wave potentials. The s-wave resonance in the neutron-target subsystem is described by an energy-dependent term in the potential, of the type arising from a BSEC. The DWBA is derived without recourse to an adjustable optical potential. The DWBA and exact cross section, without the background contribution, are evaluated and compared. Interference effects between the resonance and background amplitudes turned out to be important and of the kind observed experimentally.

The generalised scattering length and resonances in e+-H S-wave scattering

Using as a definition of resonance, a discrete complex eigenvalue of the rotated Hamiltonian Halpha obtained from H by the transformation r to rei alpha , it is shown that an infinite sequence of S-wave resonances (disregarding relativistic effects), approaching the H(n=2) threshold along a line, will exist. The location of these resonances depends asymptotically on the generalised scattering length b, which is related to the form of the solution of Schrodinger's equation at the threshold energy. With the help of the Kohn variational principle, b is calculated using Pekeris basis functions (1958), which give rise to a sparse matrix. It is found that the resonances have a small width.

Study of Kπ scattering using the reactions [formula omitted

An elastic Kπ partial-wave analysis is presented. It is based on high statistics data for the reactions K ± p → K ±π + n and K ± p → K ±π −Δ ++ at 13 GeV obtained in a spectrometer experiment performed at SLAC. For each reaction, a t-dependent parametrization of the production amplitudes provides information on both the Kπ mass dependence of the production mechanisms and on Kπ scattering. Knowledge of the t-dependence then allows a calculation of the Kπ partial-wave amplitudes for Kπ masses from 0.7 to 1.9 GeV. The results of such analyses using data for (i) the neutral recoil reactions, (ii) the Δ ++ recoil reactions, and (iii) both neutron and Δ ++ recoil reactions simultaneously, are presented. Besides the leading J P = 1 −, 2 +, and 3 − resonances at M K π = 0.896, 1.434, and 1.78 GeV, there is evidence in two of the four possible partial-wave solutions for a broad P-wave resonant-like structure in the region of 1700 MeV. The I = 1 2 S-wave magnitude rises slowly and smoothly to a maximum near 1400 MeV, but then decreases rapidly between 1400 and 1600 MeV. This structure is strongly indicative of an S-wave resonance near 1450 MeV. The charge-two Kπ reaction is dominated by S-wave scattering with a total cross section decreasing from 4 mb at 0.9 GeV to 2 mb at 1.5 GeV. Both the I = 1 2 S-wave below 1400 MeV and the I = 3 2 S-wave are well described by an effective range parametrization.

Resonance and background interference in 54Fe neutron capture

Asymmetric s-wave resonances observed in the neutron capture cross section of 54Fe can be explained by interference between resonance valence components. An upper limit is obtained for the background capture cross section which is much smaller than that expected from the large correlation observed between s-wave reduced neutron widths and radiative widths.

Neutron Resonance Spectroscopy on 209Bi

The neutron transmission through 209Bi was measured with high resolution at both the 80 and 200 m stations of the Oak Ridge Electron Linear Accelerator. Resonance parameters for s-wave levels below 270 keY and for p-wave levels below 100 keY bombarding energy were obtained. Approximately 25 % of the expected neutron strength of a particle-vibrator doorway was identified in the energy range analysed for s-wave resonances. The average s-wave level spacing is s = 4�5 � 0�6 ke V after correcting for missed levels. The s- and p-wave neutron strength functions are respectively: 104 So = 0�65 � 0�15 (E &lt; 170 keY); 104 Sl = o� 25 � 0�07 (E &lt; 60 keY). Both strength functions show evidence of energy-dependent structure.

Measurement of polarized target asymmetry in γp → π+n below 1.02 GeV

The polarized target asymmetry for the process γp → π +n has been measured for incident photon energies below 1.02 GeV over a range of c.m. angles from 40° to 160°. π + mesons from a polarized butanol target were detected by a magnetic spectrometer. The results are compared with predictions given by existing analyses. A tentative interpretation of the data is performed, and a larger contribution of S-wave resonances is suggested. The photocouplings of dominant resonances were hardly changed by the inclusion of new data and they seem to be almost uniquely determined.