Tatsushi Shima

A Monte Carlo code for multiple neutron scattering events in a thick sample for (n, γ) experiments

Abstract TIME-MULTI, a time-introduced Monte Carlo estimation code for multiple neutron scattering events in a thick sample, was designed to extract accurate neutron-capture cross sections. In order to obtain the energy and time-of-flight spectra of a captured neutron, the neutrons history in the sample was simulated by a Monte Carlo method. The code was applied to the 1H(n, γ)D reaction with different sample thicknesses and was shown to give the correct capture cross section independent of the thickness.

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.

A gas scintillation drift chamber for the 14N(n, p)14C measurement

Abstract A gas scintillation drift chamber was developed to measure the cross section of the 14 N(n, p) 14 C reaction at the keV region. Since it contains N 2 gas in addition to He gas, the reaction events can be detected with an acceptance of 4π. The 3 He in the He gas is enriched to 0.2% in order to normalize the measurement. The chamber uses primary scintillation photons to determine the neutron energies by the time-of-flight method, and uses the trajectories of charged particles to discriminate the true events from events due to both the background and the 3 He(n, p) 3 H reaction. It was shown for the first time that N 2 gas works not only as a sample, as a wavelength shifter, but also as a discharge suppressor for the electron avalanche. The basic properties of the chamber were investigated experimentally: good time- (∼ 1.7 ns), spatial- (∼ 2 mm) and energy- (∼ 16% at 600 keV) resolutions were obtained. From these results, it was estimated that the 14 N(n, p) 14 C cross section can be measured with an error of 10 ∼ 15%.

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.

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.

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.

Nuclear and nuclear astrophysical interest in D(n,γ)3H reaction

Abstract A neutron capture cross section of deuterium was measured for the first time at the neutron energies between 10 and 80 keV by detecting the prompt γ-ray from the capture state to the ground state of triton. The cross section is important to study the nuclear dynamics of the few nucleon systems, and to estimate the elemental abundances in the primordial nucleosynthesis. In order to pick up the small γ-ray signal from huge background we have developed highly sensitive NaI(T1) detector systems with good signal to noise ratio. Use of these detectors together with pulsed neutron beams enabled us to determine the small capture cross section accurately. The cross sections were obtained as 2.12 (35) and 2.04 (3) μbarns at 30.5 and 54.2 keV, respectively, and they are compared with the theoretical ones.

Is 16O a strong neutron poison

The cross section of a 16O(n,γ)17O reaction was measured in the neutron energies of between 10 and 80 keV. The Maxwellian‐averaged cross section at kT=30 keV is 38±4 μb; 200 times larger than the value measured previously. The results may have significant effects for the nucleosynthesis theories of s‐ and p‐processes in metal‐deficient stars due to a strong neutron poison of 16O. The large cross section is attributed to a non‐resonant p‐wave neutron capture.