T. Kii

Nonresonant direct p - and d -wave neutron capture by 12 C

Discrete gamma-rays from the neutron capture state of 13C to its low-lying bound states have been measured using pulsed neutrons at En = 550 keV. The partial capture cross sections have been determined to be 1.7+/-0.5, 24.2+/-1.0, 2.0+/-0.4 and 1.0+/-0.4 microb for the ground (1/2-), first (1/2+), second (3/2-) and third (5/2+) excited states, respectively. From a comparison with theoretical predictions based on the non-resonant direct radiative capture mechanism, we could determine the spectroscopic factor for the 1/2+ state to be 0.80 +/- 0.04, free from neutron-nucleus interaction ambiguities in the continuum. In addition we have detected the contribution of the non-resonant d-wave capture component in the partial cross sections for transitions leading to the 1/2- and 3/2- states. While the s-wave capture dominates at En < 100 keV, the d-wave component turns out to be very important at higher energies. From the present investigation the 12C(n,gamma)13C reaction rate is obtained for temperatures in the range 10E+7 - 10E+10 K.

Fast neutron capture reactions in nuclear astrophysics

The cross sections of the neutron capture reaction on light nuclei, protons,12C and16O, were measured at astrophysically relevant energies between 10 and 300 keV. They are not only important for estimating yields of elements in the primordial nucleosynthesis, stellar nucleosynthesis of s-, r-, and p-processes, but also to study the role of meson exchange currents, the nuclear structure and the reaction mechanism. In the measurement, we used a prompt γ-ray detection method, combined with a pulsed neutron beam, and a recently developed Monte-Carlo code, TIME-MULTI, to correct for neutron multiple-scattering effects in a sample.

PRECISION TEST OF CHARGE SYMMETRY OF NUCLEAR FORCE VIA PHOTONUCLEAR REACTIONS OF 4HE

Abstract The (γ,p) and (γ,n) reactions of 4 He at the energy of the giant dipole resonance provide a very sensitive tool for investigation of the charge symmetry in nuclear forces. In order to search for a possible breaking of the charge symmetry, we are going to make high-precision measurement of the reaction cross sections by using a Laser-Compton scattered γ-ray beam and a time projection chamber. Since both reaction cross sections are measured simultaneously, the charge symmetry can be tested with high accuracy. In this paper we will show the present status of our project.

Measurement of the 13C(n,γ)14C cross section at stellar energies

Abstract The 13 C(n,γ) 14 C reaction cross section at stellar energy is crucial in studying the roles of the reaction in nucleosynthesis both in the early universe and in stellar helium burning. A new measurement of the cross section was carried out in the energy range of between 10 and 87 keV, and the obtained values are larger by a factor of about 10 compared to the values estimated from the thermal cross section with the 1 v law. The experimental results are shown, and the effects of the 13 C(n,γ) 14 C reaction on nucleosynthesis are discussed.

127I(n, γ)128I reaction in NaI(Tl) due to cosmic-ray muon capture by Pb

Abstract A γ-ray from a neutron-capture reaction on 127 I in an NaI(Tl) detector was observed in background measurement. The neutrons were considered to be produced by cosmic-ray muon captured by Pb, which was used for shielding the spectrometer against γ-ray background. The observed γ-ray yield of the reaction was compared with the value calculated by assuming that the fast neutrons produced in the Pb were thermalized while they passed through a plastic scintillation counter, which surrounded the NaI(Tl) detector, and these thermal neutrons were captured by 127 I in the detector. They agree with each other, supporting the present interpretation. The γ-ray energy from the capture state to the ground state in 128 I was different from the previously measured value using thermal neutrons; the difference could be due to a nonuniformity of the detector. The present result has important implications for developing a low-background γ-ray spectrometer for studying rare physical phenomena.

First direct measurement of a p(n,γ)d reaction cross section at stellar energy

The cross section of the p(n,γ)d reaction was measured for the first time in the neutron energy between 10 and 280 keV. The result is in good agreement with the theoretical values calculated recently by Hale et al. and Ohtsubo et al. The present result, however, is not consistent with the estimated value by using the experimental value of the deuteron photo‐disintegration.

New measurement of the 14N(n,p)14C cross section at stellar energy with a drift chamber

The 14N(n,p)14C reaction is crucial for nucleosynthesis in stars, since it may act as a neutron poison for stellar s‐process. We made a new measurement of its cross section by using a newly developed gas scintillation drift chamber, which contained N2 gas as an active target in addition to the chamber gas. A preliminary value of 1.07±0.31(stat.)±0.19(syst.) mb was obtained at En=49.7 keV.

Pulsed keV neutrons for nuclear astrophysics and recent results of (n,γ) reaction of light nuclei

Pulsed keV neutrons play an important role in studying problems related to the nuclear astrophysics. In the present study we have measured the neutron capture cross sections on D, 7Li and 18O samples in the neutron energies between 10 and 550 keV. They are important not only for the primordial nucleosynthesis theory, but also for the stellar nucleosynthesis theory of less evolved stars and for the solar neutrino problem. While in order to extend the present study of the keV neutron induced reaction of nuclei further we have been working on the improvement of the performance of the NaI(Tl) spectrometer and testing the property of the spallation neutron source at KEK.