M. Kabasawa

Status of the cyric neutron TOF facilities upgrade

Abstract We describe progress in the Tohoku neutron time-of-flight (TOF) facilities improved to facilitate high resolution measurements for neutrons in the range 10 ≤ E n ≲ 60 MeV. Efforts have been concentrated on completing the neutron detection system consisting of twelve neutron detectors, in which 23 1 of NE213 liquid scintillator are encapsulated, and a CAMAC-based data acquisition system. A systematic study of the neutron detection efficiency has been performed by Monte Carlo calculation for monochromatic neutrons with E n ≲ 34 MeV. Results have been tested by counting neutrons from the 7 Li(p, n) 7 Be reaction and comparing the yield with the absolute neutron fluence determined by activation.

Facilities for neutron induced experiments at the Tohoku University cyclotron

Abstract A facility for neutron induced experiments has been developed using the neutron time-of-flight spectrometer with a beam swinger system at the Tohoku University cyclotron. Quasi-monoenergetic neutrons have been obtained as a secondary beam from the 7 Li(p, n) 7 Be reaction at an incident energy of 37 MeV. Neutrons are available up to 40 MeV with an energy resolution of 650 keV (FWHM). Differential cross sections for scattered neutrons from 12 C were measured over an angular range from 10° to 145° with a 44 m long flight path.

Spectroscopy of 24Al through the 24Mg(p, n)24Al reaction at Ep = 35 MeV

Abstract An experimental study of the 24 Mg(p, n) 24 Al reaction at E p = 35 MeV was carried out. High-resolution neutron time-of-flight spectra were measured at 17 angles in a range of 0°–110°. Neutron peaks corresponding to seventeen states in 24 A1 have been resolved. A number of spin-parity assignments have been made based on distorted-wave Born-approximation (DWBA) analyses for the measured angular distributions, using microscopic transition amplitudes obtained from the 1s0d shell model for positive-parity states and those from the 0s0p-1s0d-1p0f open-shell randomphase approximation (OSRPA) for negative-parity states. Reasonable correlation between the low-energy (p, n) cross section and the β-decay strength has been found for the observed 0 + →1 + Gamow-Teller transitions leading to the 0.439, 1.116 and 3.023 MeV states in 24 Al. The 1.56 MeV 5 + state has been identified as a 0 h ω stretched state, in addition to the 5.54 MeV, 6 state of 1 h ω character. Candidates for the 1 h ω negative-parity transitions leading to the 2, 3, 4 states are also presented.

12C(p, n)12N and 16O(p, n)16F reactions at Ep = 35 and 40 MeV: Reliability of the information obtained from DWBA analysis of low-energy (p, n) data

Abstract A high-resolution study of the 12C(p, n)12N and 16O(p, n)16F reactions was made at Ep = 35 and 40 MeV. The low-lying states in 12N(1+, 2+ and 2−) and in 16F(0−, 1−, 2− and 3−) were clearly resolved, and their angular distributions were measured. Extensive DWBA analysis was made and compared with the data. The calculated angular distribution shapes are found to be in agreement with the data and insensitive to the choice of the parameters involved. On the other hand, the magnitudes of the DWBA cross sections depend strongly on the bound state parameters in the case of a transition from a tightly bound state to a loosley bound state. In the other cases the overall uncertainty of the DWBA cross section magnitudes was estimated to be about ±30%. Within this uncertainty the experimental cross sections for the 12C(p, n) reaction were explained by the calculation, but those for the 16O(p, n) reaction were not: the observed strengths were about a half of the calculated values. Since these results agree with those at intermediate energies, the origin of the discrepancy is considered to be in the structure of the mass 16 nuclei rather than in reaction dynamics. In general, the present results compare well with those at intermediate energies, indicating that the structure information extracted from low-energy, high-resolution (p, n) data is basically sound if careful analysis of the data is made.

A comparative study of the 13C(p, p′)13C and 13C(p, n)13N reactions at Ep = 35 MeV☆

Abstract Differential cross sections were measured at Ep = 35 MeV for the 13C(p, n) and 13C(p, p′) reactions leading to the four low-lying states in the mirror nuclei 13N and 13C. In addition, the analyzing powers were measured for the 13C(p, p′) reaction. The data are generally well accounted for by DWBA calculations except for the 13 C(p, p′) 13 C (3.09 MeV , 1 2 + ) reaction, for which the calculations cannot even reproduce the qualitative features of the data. A comparison of the (p, n) and the (p, p′) results suggests that the isoscalar part of the 13 C ( g.s. , 1 2 − ) → 13 C (3.09 MeV , 1 2 + ) transition is not correctly described by currently available shell-model wave functions.

Excitation of ΔL = 1 resonances in the (p,n) reaction on 90Zr, 120Sn, 140Ce and 208Pb at Ep = 41 MeV

Abstract A broad peak was systematically observed at about 10 MeV above the Gamow-Teller resonance in the 90 Zr, 120 Sn, 140 Ce, 208 Pb (p,n) reactions at E p = 41 MeV. The angular distribution shapes of these peaks support the interpretation that they they correspond to ΔL = 1 state. The data are compared with TDA-DWBA calculations.

Isovector ΔJπ=0− transitions observed in the (p,n) reactions on 13C, 14,15N and the tensor part of the effective N-N interaction

Abstract The angular distributions for the (p,n) reactions on 13C and 14,15N were measured at Ep=35 MeV. A shoulder was systematically observed around 50° in the angular distributions for the 1 2 − → 1 2 + and 1+ → 1− transitions. This shoulder was interpreted as being due to the tensor interaction in the ΔJ(ΔL,ΔS)=0(1,1) channel.

The 31P(d, n)32S reaction at 25 MeV

Abstract Differential cross sections for the 31P(d, n)32S reaction have been measured at Ed = 25 MeV using the time-of-flight technique. Levels more than thirty-five up to an excitation energy 13.4 MeV were observed. The experimental angular distributions for bound states were analysed with the distorted wave Born approximation theory. Those for the proton unbound states were analysed by the method of Vincent and Fortune. Spectroscopic factors obtained are compared with the results of the other experiments, the shell model calculations and sum rule limit. Especially, spectroscopic factors for unbound states are successfully compared with the results obtained from the other one-proton stripping reaction and those from the proton resonance scattering on 31P.

sup 14 C( p , n ) sup 14 N reaction at E sub p =35 MeV

Differential cross sections for the {sup 14}C({ital p},{ital n}){sup 14}N reaction were measured at {ital E}{sub {ital p}}=35 MeV. A number of spin-isospin excitations have been observed including Gamow-Teller-type 0{sup +}{r arrow}1{sup +} and 0{h bar}{omega} and 1{h bar}{omega} jump stretched transitions. Distorted-wave Born-approximation calculations using shell-model wave functions have successfully reproduced the experimental results. Renormalization factors of about 0.5 were required for the spin-flip transitions leading to the 3.947-MeV 1{sup +} state and to the 7.026-MeV 2{sup +} state, where {Delta}{ital J}({Delta}{ital L},{Delta}{ital S})=1(0,1) and 2(2,1), respectively. Proton and neutron optical-potential parameters were derived in the course of the present work.