J.W. Boldeman

Mass-split dependence of the pre- and post-scission neutron multiplicities for fission of 251Es

Abstract Pre- and post-scission neutron multiplicities have been measured as a function of fission mass-split for the reactions of 105 and 120 MeV 19 F with 232 Th. The post-scission multiplicities show no evidence for persistence of the sawtooth yield observed for spontaneous fission of 252 Cf; the dependence on mass-split is not consistent with the random neck rupture model. The pre-scission multiplicities show no evidence of quasi-fission for asymmetric mass-splits. It is proposed that pre-scission neutron multiplicity measurements should be a useful tool in studying quasi-fission produced by reactions with heavier projectiles.

The neutron capture cross section of natural silicon

Abstract The neutron capture cross section of natural silicon has been measured to 1500 keV using the capture cross section facility at the 40 m station of the Oak Ridge Electron Linear Accelerator. Analysis of the present data, in combination with existing total cross section information, has provided almost complete resonance data for 28Si. On the other hand, the capture kernel only ( gΓ n Γ γ Γ ) has been obtained for neutron capture resonances in 29Si and 30Si. A strong positive correlation has been observed between the radiative width and the corresponding reduced neutron width for p-wave resonances in 28Si, confirming significant valence effects. It is noted that a quantitative valence calculation provides only an approximate estimate of the valence strength for this nucleus. It was not possible to confirm from the present measurements, reported asymmetry observed in two resonances in the 29Si(γ, n) reaction.

Valence component in the neutron capture cross section of 90Zr

Abstract The neutron capture cross section of 90Zr has been measured with high energy resolution between 3 and 200 keV using the capture cross section facility at the 40 m station on the Oak Ridge electron linear accelerator. Through the comparison of the present data with the total cross-section and inverse 91Zt(γ, n) data from Toohey and Jackson, complete resonance parameters have been extracted for 37p 3 2 , 12p 1 2 and 11 s-wave resonances out of a total of 101 observed resonances. The neutron strength functions extracted from the resonance parameters are S 0 = 0.56 × 10 −4 , S 1 = 3.8 × 10 −4 , S 1 ( P 3 2 ) = 4.7 × 10 −4 and S 1 ( 3 2 ) = 1.9 × 10 −4 . It is noteworthy that the S 1 ( 3 2 ) strength function is significantly larger than the S 1 ( P 1 2 ) strength function in agreement with theoretical expectation. A significant correlation (ρ = 0.58) exists between the reduced neutron widths and the radiative widths for the 37(P 3 2 ) resonances. The data give strong confirmation of the valence neutron model. With standard valence calculations all radiative widths can be calculated reasonably from the associated reduced neutron width. However, to explain the measured correlation coefficient, it has been necessary to include with the valence component, singleparticle transitions to the ground and low excited states of 91Zr, which are uncorrelated with the resonance reduced neutron width. The average capture γ-spectrum for neutron capture in 90Zr between 2 and 80 keV has been calculated from the valence model and the present data and is found to be in very close agreement with published experimental data.

Average neutron resonance parameters and radiative capture cross sectins for the isotopes of molybdenum

Abstract The neutron capture cross sections of the stable molybdenum isotopes have been measured with high energy resolution (ΔE/E ≲ 0.2 %), between 3 and 90 keV neutron energy, at the 40 m station of ORELA. Average resonance parameters are extracted for s- and p-wave resonances. The s-wave neutron strength function is close to 0.5 × 10−4 for all isotopes, but the p-wave strength function exhibits a well defined peak near A t~ 95. Both s- and p-wave radiative widths decrease markedly as further neutrons are added to the closed shell. The p-wave radiative widths are generally greater than the s-wave widths showing the presence of non-statistical γ-decay mechanisms. Valence neutron theory fails to explain the magnitude of the p- to s-wave radiative width disparity and doorway state processes are invoked. In particular, the data for 98Mo appear to violate the usual valence theory, since the correlations between radiative and neutron strengths are small. Further, the radiative widths are smaller than can be explained on the valence model. An explanation for the loss of valence strength is advanced. Interpolated resonance parameters allow an estimate for the unknown cross section for 99Mo(n, γ).

Valence neutron capture in 54Fe

Abstract The neutron capture cross section of 54 Fe has been measured with 0.2% energy resolution from 2.5 to 500 keV. A large and significant correlation is observed between the s-wave reduced neutron widths and the corresponding total radiative widths. The valence model readily accounts for this correlations as well as over half the average value of the s-wave radiative widths.

Resonance neutron capture in 56Fe

Abstract The neutron capture cross section of 56 Fe has been measured with 0.2–0.3% energy resolution from 2.5 keV up to the inelastic neutron scattering threshold. Results are compared with recent total cross-section data and average parameters are derived for s-, p- and d-wave resonances. The low correlation coefficient observed between the s-wave reduced neutron and radiative widths is consistent with the minor contribution of the valence capture mechanism as calculated in the framework of the optical model. Broad E1 and M1 doorway states for s-, p- and d-wave resonances are postulated to explain the cross-section data and γ-ray spectra up to 1 MeV.

Odd-even effects in radiative neutron capture by 42Ca, 43Ca and 44Ca

Abstract The neutron capture cross sections of −44 Ca have been measured with high resolution ( ΔE E ∼ 0.2 % ) at the 40 m station of the Oak Ridge Electron Linear Accelerator. Resonance parameters have been extracted for levels below 230 keV in 42 Ca, below 40 keV in 43 Ca, and below 165 keV in 44 Ca. The average s-wave resonance parameters obtained from our data are as follows: 〈 D 〉 = 8.6± 1.0 keV, 1.5± 0.2 keV and 16 ± 2 keV for 42, 43, 44 Ca respectively. The corresponding s-wave neutron strength functions are respectively 10 4 S 0 = 2.7 ± 0.8, 3.1 ± 0.8 and 2.0 ± 0.8. The s-wave radiative widths for the even- A isotopes are considerably greater than for 43 Ca. In addition, p-wave radiative widths in the even- A isotopes are anomalously small.

Fission fragment angular distributions for neutron fission of 232Th and their interpretation with a triple-humped fission barrier

Abstract The fission fragment angular distributions have been measured for the neutron fission of 232 Th at a number of energies near the neutron threshold. An exhaustive search has been made for a set of transition states and barrier parameters that would simultaneously fit the angular distributions and reproduce quantitatively the structure seen in the neutron fission cross section. No satisfactory fit to both types of data could be obtained with a double-humped fission barrier. However, use of a triple-humped fission barrier does provide a reasonable fit to all the experimental data.

The neutron total and capture cross sections of 92, 94Zr☆

Abstract The neutron total and capture cross sections of92,94Zr have been measured at the 80 and 40 m flight stations, respectively, on the Oak Ridge Electron Linear Accelerator (ORELA). Resonance analysis of the total cross sections gave values of s, p 3 2 and p 1 2 strength functions. Strong intermediate structure was observed in the p 3 2 cross section of 92Zr. Significant single particle transitions identified in the p-wave capture cross section indicate the importance of the valence process for both nuclei. It is noted that the intermediate structure in the p 3 2 total cross section leads to enhanced valence effects in the capture cross section. The effects of non statistical E1 transitions following particle-hole annihilation in the compound states were not observed in either the 92Zr or 94Zr capture cross sections as they had been in that for 90Zr.

Valence neutron capture in 88Sr

Abstract The neutron capture cross section of 88Sr has been measured with high energy resolution between 2.5 and 400 keV using the capture cross-section facility at the 40 m station on the Oak Ridge Electron Linear Accelerator. Strong positive correlations have been observed between the reduced neutron and radiative widths for both p 3 2 and p 1 2 resonances. The valence process dominates the p-wave neutron capture mechanism. Comparison of calculated valence effects, using the optical model formulation of the valence theory, with the experimental data, indicates a reliability in the calculation of approximately 25%.