H. Kitazawa

Systematics of the pygmy resonance in keV neutron capture γ-ray spectra of nuclei with N ≈ 82–126

Abstract We have measured capture gamma-ray spectra of Pr, Tb, Ho, Lu, Ta and Au at neutron energies of 10 to 800 keV with an anti-Compton NaI(Tl) detector, employing a time-of-flight technique. An anomalous bump, so-called the pygmy resonance, was observed in all these spectra. Remarkable features of the pygmy resonance were found to be that the resonance energy and the electric-dipole strength exhausted in the resonance increase with neutron number but these quantities decrease precipitously around the neutron magic number of N = 82. Comparison with theoretical calculations suggests that the pygmy resonance is mainly made up of neutron particle-hole states decoupled from the giant electric-dipole resonance and is excited in a collective mode.

A heavy shield for the gamma-ray detector used in fast neutron experiments

Abstract The 80-20 wt.% mixture of paraffin and boric acid was proposed as a shielding material for the gamma-ray detector used in fast neutron experiments. Its neutron shielding capability was evaluated by neutron and gamma-ray transport calculations and compared with that of other materials. As a result, it was found that the borated paraffin will be an effective and economical shielding material against fast neutrons if it is used with a lead shield. Moreover, a heavy shield for the gamma-ray detector has been constructed, and the typical shield performance was investigated for keV-neutron capture experiments.

Neutron capture cross sections of light nuclei in primordial nucleosynthesis

Abstract Neutron radiative capture cross sections of 7 Li and 12 C were measured at a neutron energy of 30 keV to study the production rate of intermediate-mass nuclei in primordial nucleosynthesis. The reaction cross sections were obtained by using pulsed neutrons and by observing prompt γ-rays from the captured state. The present results favor the nucleosynthesis of intermediate-mass nuclei in the early universe.

( n , α) Reaction on 12 C Induced by 14.1 MeV Neutrons

The ( n , α) reaction on 12 C has been investigated by bombarding a thin polyethylene film with 14.1 MeV neutrons. The emitted α-particles were detected by a CsI(T1) scintillation detector and distinguished from other particles by applying the pulse-shape discrimination method. The angular distribution of the α-particles leaving the residual nucleus 9 Be in the ground state was strongly peaked at about 40° and at the backward hemisphere in the center-of-mass angles, predicting the presence of direct interaction processes. The ground state transition cross section of 76±11 mb was obtained. The forward peaking was compared with the plane wave theory, for both the pick-up and knock-on processes. The reduced width amplitude of the α-cluster at the nuclear surface of 12 C was determined by attempting the theoretical fit to the backward peak of the angular distribution in terms of the heavy-particle stripping model. Comparing this value with those obtained from other reactions on light nuclei, it seems that the...

Anomaly of the alpha-particle optical potential in the 27Al(n, α)24Na reaction

Abstract We determined an alpha-particle optical potential whose real Woods-Saxon potential exhibits a fast increase of its strength in the vicinity of the Coulomb barrier, using the dispersion theory and taking account of available data on the nuclear rainbow scattering at intermediate alpha-particle energies. This potential was applied with considerable success to the Hauser-Feshbach model calculation of the 27 Al(n, α) 24 Na reaction cross sections below 20 MeV neutron energy.

γ-ray transitions following p-wave neutron resonance capture and off-resonance capture by 28Si

Abstract We have measured γ-ray spectra from the 565 keV and 806 keV p 3 2 wave neutron resonance capture by 28 Si and from the off-resonance capture at 485 keV with an anti-Compton NaI(Tl) detector, employing a time-of-flight technique. Particularly the latter resonance transitions have been skillfully isolated from the 802 keV narrow resonance for the first time. Partial radiative widths of both p-wave resonances were obtained for the transitions to the ground ( 1 2 + ), 1273 keV ( 3 2 + ), 2028 keV ( 5 2 + ), 2425 keV ( 3 2 + ), and 3067 keV ( 5 2 + ) states in 29 Si. For both resonances, the Lane-Mughabghab optical-model formula of the valence model reproduces reasonably the observed partial radiative widths for the transitions to the ground state and second excited state while other widths are not well predicted by this formula, even if the uncertainty of the optical potential parameters and spectroscopic factors which were used in the calculation is taken into account. Moreover, the results show that off-resonance capture is successfully explained by the direct capture model in the distorted-wave Born approximation.

(n, α) reactions on heavy nuclei induced by 15.1 MeV neutrons

Abstract The energy spectra of α-particles from the (n, α) reactions on 141 Pr, 165 Ho, 175 Lu, 181 Ta and 197 Au induced by 14 MeV neutrons at 0° have been measured with a n-type silicon surface barrier semiconductor. They cannot be accounted for by the statistical model, with the inclusion of the shell effect for the level density, which is derived from the free gas model for a two-fermion system. The angular distribution of α-particles from 141 Pr is strongly forward-peaked. It seems that there is an appreciable contribution of direct processes to these reactions.

keV-neutron capture cross sections of light nuclei and nucleosynthesis

We measured the keV-neutron capture cross sections of 6Li, 16O, 20Ne, and 22Ne which are important parameters for studies on the primordial and/or stellar nucleosyntheses. The present measurement of the 6Li(n,γ)7Li reaction was the first one in the keV region. In the measurement of the 16O(n,γ)17O reaction, we succeeded in observing the interference between the 434-keV p-wave neutron resonance capture and the p-wave neutron potential capture. For the measurements of 20Ne and 22Ne, liquid samples were prepared and the p-wave neutron capture was observed.

Valence-neutron capture in the 434 keV p32-wave resonance of 16O

Abstract We have measured γ-ray spectra from the 434 keV p 3 2 - wave neutron resonance capture and from the off-resonance capture at 280 keV with an anti-Compton NaI(Tl) detector, employing a time-of-flight technique. Partial radiative widths and partial off-resonance capture cross sections were obtained for the transitions to the ground ( 5 2 + ) and 871 keV ( 1 2 + ) states in 17 O. The radiative widths are well predicted by the valence capture formula of Lane and Mughabghab. Moreover, we found that the potential-capture theory substantially reproduces the partial off-resonance capture cross sections.

Direct and collective captures of energetic neutrons by ellipsoidally deformed nuclei

Abstract The direct and collective theories for nucleon radiative capture are applied to the ellipsoidally deformed nucleus within the framework of an adiabatic approximation. A volume-type form factor rf ( r ) is used for the particle-vibration coupling Hamiltonian between a projectile and the E1 vibrational mode of a target nucleus. Numerical application is made for the 238 U(n, γ) 239 U reaction with 5 to 20 MeV neutrons. Contributions to the capture cross section from the rotational bands for all Nilsson levels in 239 U are calculated to produce the excitation curve and capture γ-ray spectra for this reaction. The coupling strength is taken to be 110 MeV and the deformation parameter to be 0.268, with only the Y 2 deformation of the target nucleus being taken into consideration. Good agreement is obtained between the calculated cross sections and the observed data. Discussions are given for some ambiguities in the calculation, i.e. the depth of the central part of an optical potential, the coupling strength, the giant resonance parameters, and the number of final states of a captured nucleon in the residual nucleus. The number of Nilsson levels and the members belonging to each rotational band chosen, strongly influence the results of the calculations.