Tamio Yamagata

Photodisintegration cross section measurements on {sup 186}W, {sup 187}Re, and {sup 188}Os: Implications for the Re-Os cosmochronology

Cross sections of the {sup 186}W, {sup 187}Re, {sup 188}Os({gamma},n) reactions were measured using quasimonochromatic photon beams from laser Compton scattering with average energies from 7.3 to 10.9 MeV. The results are compared with the predictions of Hauser-Feshbach statistical calculations using four different sets of input parameters. In addition, the inverse neutron capture cross sections were evaluated by constraining the model parameters, especially the E1 strength function, on the basis of the experimental data. The present experiment helps to further constrain the correction factor F{sub {sigma}} for the neutron capture on the 9.75 keV state in {sup 187}Os. Implications of F{sub {sigma}} for the Re-Os cosmochronology are discussed with a focus on the uncertainty in the estimate of the age of the galaxy.

Studies of (α,d) Reactions on N=28 Nuclei. I. 51V(α,d)53Cr

Angular distributions of deuterons to 13 levels in 53 Cr were measured up to 3.72 MeV of excitation energy using 22 MeV alpha particles. The angular distributions are compared with the zero-range DWBA calculations employing the explicit the explicit wave functions for the 53 Cr states given by a shell model calculation. The 53 Cr states above E x =1.28 MeV which are only weakly excited by one-nucleon transfer reactions are found to be rather strongly excited. They are assigned as the states of which main configuration corresponds to the coupling of one neutron outside the N =28 core with the J π ≠0 + protons. An attempt is made to determine the absolute normalization constant for the (α,d) cross sections and the value D 0 2 ∼10 6 MeV 2 fm 3 is obtained.

Photoneutron Cross Sections for Au Revisited: Measurements with Laser Compton Scattering γ-Rays and Data Reduction by a Least-Squares Method

Photoneutron cross section measurements were made for Au in the entire energy range of the (γ,n) channel based on a direct neutron counting with quasi-monochromatic γ-rays produced in inverse Compton scattering of laser photons with relativistic electrons. The data were analyzed by a least-squares method to deduce photoneutron cross sections. The analysis significantly reduced experimental uncertainties compared with those resulting from the photon difference method. The result is compared with the previous data by direct neutron counting with γ-rays produced in positron annihilation in flight and by photoactivation with bremsstrahlung. The present data are in good agreement with the previous data near the neutron threshold, while there remain some discrepancies between the present and the previous data above 10 MeV.

Studies of (α, d) Reactions on N=28 Nuclei. II. 52Cr(α, d)54Mn

Angular distributions of deuterons from the 52 Cr(α, d) 54 Mn reaction at E α =24 MeV have been measured and are analyzed in terms of the zero-range DWBA theory. Transferred angular momenta are assigned for 20 transitions up to 4.72 MeV excitation in 54 Mn. Strong transitions to the 0.363 and 4.715 MeV states in 54 Mn are interpreted as due to high angular momentum transfers, and these levels are assigned J π =5 + and 8 - , respectively. Seven L =0 transitions leading to E x =1.449, 1.623, 1.777, 1.913, 2.126, 2.873 and 3.835 MeV states in 54 Mn have been observed, and these levels are assigned as J π =1 + . The transition strengths for the low-lying states are discussed on the basis of a simple shell model and are found to be in good agreement with the shell model predictions.

Cluster-excitations in 6Li and 7Li

Some dipole resonances in 6 Li and 7 Li nuclei were investigated via the ( 7 Li, 7 Be) reaction with an incident energy of 65A MeV. Based on the observed excitation energy, width, and cross section of each resonance, the relevant resonances are inferred to be due to excitations of the α-clusters in 6 Li and 7 Li and the deuteron-cluster in 6 Li.

Striking behavior of photoneutron cross sections for 90Zr near threshold

Photoneutron cross sections for 90Zr near neutron threshold were examined with a photon difference technique using laser Compton scattering γ‐ray beams. The cross section decreasing in the vicinity of neutron threshold exhibited a steep rise within 150 keV above the threshold. We draw attention to a possible relation of this striking behavior to the 3P valence neutron resonance known in neutron capture in the mass region A≈90.

THE γSF method

The γ‐ray strength function (γSF) interconnects radiative neutron capture and photoneutron emission as a common ingredient in the statistical model. Outlined here is an indirect method of determining radiative neutron‐capture cross sections for unstable nuclei based on the γ‐ray strength function. Application examples of the γSF method are demonstrated.

APPLICATION OF THE γSF METHOD TO PALLADIUM

Photoneutron cross sections were measured for 108Pd, 106Pd, and 105Pd with laser‐Compton scattering γ‐ray beams in an application of the γSF method to a radioactive nucleus 107Pd. We present radiative neutron cross sections for 107Pd[6.5×106 y] obtained with the γSF method.

Extra γ-ray strength for 116,117Sn arising from pygmy dipole resonance

Photoneutron cross sections were measured for 117Sn and 116Sn near neutron thresholds with quasi‐monochromatic laser Compton scattering γ‐rays. The measured cross sections for 117Sn and 116Sn are strongly enhanced from the threshold behavior expected for L = 1 neutron emissions after E1 photoexcitation. This suggests the presence of extra γ‐ray strength in the low‐energy tail of the giant dipole resonance. The present cross sections were analyzed together with radiative neutron capture cross sections for 116Sn within the framework of the statistical model calculation. It is shown that the extra γ‐ray strength can be interpreted as pygmy E1 resonance which was previously reported in the nuclear resonance fluorescence experiment for 116Sn and 124Sn.

The γ‐ray Strength Function in Nuclear Astrophysics

The γ‐ray strength function is a key statistical nuclear ingredient to the s‐, r‐ and p‐process nucleosynthesis of heavy elements. It has recently become apparent that extra γ‐ray strengths that appear near neutron threshold on top of the low‐energy tail of GDR play an important role in neutron capture and photodisintegration. We discuss extra γ‐ray strengths of pigmy E1 and giant M1 nature based on the latest experimental photoneutron cross sections measured with laser Compton scattering γ‐rays.