S. Mitsuoka

TRANSPORT EFFICIENCY OF JAERI RECOIL MASS SEPARATOR

Abstract Solid angles and transport efficiencies of the JAERI recoil mass separator were measured by using three procedures, that is, by using (1) α-particles from an 241 Am source, (2) elastic recoils of 197 Au produced in the bombardments of 28 Si beams on 197 Au targets and (3) evaporation residues produced in the fusion reaction of 30 Si+ 141 Pr. The solid angles of 11 and 21 msr were obtained for the focus conditions of the mass dispersion (x | δ m )=−1.1 and 0 cm /% respectively. The measured transport efficiency was compared with the ion-optical calculation of GIOS and a good agreement between the calculation and the present data was obtained. The charge state distributions of low-energy heavy recoils 167 Ta and 197 Au were also measured and compared with the various calculations. It was found that the empirical formula by Shima et al. well reproduced the present data.

The KEK–JAERI joint RNB project

A new ISOL-based radioactive nuclear beam (RNB) facility has been constructed as a joint project of the High Energy Accelerator Research Organization (KEK) and the Japan Atomic Energy Research Institute (JAERI) at the Tandem accelerator of the Tokai site of JAERI. It uses heavy-ion linacs and related equipment of the presently closed KEK-RNB facility. Fission fragments with energy from 0.1 to 8 MeV/u will be available at this facility.

The feature of the JAERI recoil mass separator

Abstract A recoil mass separator has been constructed at the JAERI tandem-booster facility. It has a symmetric configuration of Q1Q2-ED1-MD-ED2-Q3Q4-O, where two electric dipoles ED1 and ED2, and a magnetic dipole MD are used to disperse reaction products by their mass/charge ratio ( m q ) and focus their energies. Two quadrupole doublets (Q1Q2 and Q3Q4) are used for focusing reaction products spread spatially. The octupole magnet O is used to correct a non-linearity of the m q dispersion. The m q and energy acceptances are designed to be ±4% and ±12%, respectively. In order to reduce a beam background scattered from the ED1 anode, the anode is split into two parts, so that the primary beam can pass through without hitting the anode. The performance of the recoil mass separator has been tested by using the 127I and 28Si beams from the JAERI tandem accelerator. The capability of the background suppression at the beam direction is excellent and the obtained mass resolution is A ΔA ≅ 300 . By using the recoil mass separator, a new neutron-deficient isotope 209Th has been produced in the reaction of 32S on 182W at beam energy of 171 MeV. An α-decay energy and a half-life of 209Th have been determined to be 8.080(50) MeV and 3.8−1.5+6.9 ms, respectively.

Effect of nuclear shell structure on fusion reaction

The dependence of the fusion reaction on the nuclear shell structure was investigated for the two reaction systems 82Se + 138Ba and 82Se + 134Ba, where the nucleus 138Ba has a closed neutron shell N=82, while the nucleus 134Ba has a neutron number 78. Evaporation residues for these fusion reactions were measured near the Coulomb barrier region. The measured evaporation residue cross sections for the reaction system 82Se + 138Ba were two orders of magnitude larger than those for the reaction system 82Se + 134Ba in the excitation energy region of 20–30 MeV. The evaporation residue cross sections were compared with those of the other reaction systems that produce the same compound nucleus as the present systems. It was found that the fusion reaction 82Se + 138Ba occurs without hindrance, while that of 82Se + 134Ba is considerably hindered, as commonly observed in the massive reaction system with the charge product ZpZt>1800 of projectile and target. This suggests the importance of the shell closure N=82 in the heavy-ion fusion reaction.

Dependence of heavy-ion fusion reaction on nuclear deformation

In order to study the effect of nuclear deformation on complete fusion between spherical projectiles and well-deformed targets, we have measured the excitation functions at energies around the Coulomb barrier in the same compound nuclear system of 6 0 Ni+ 1 5 4 Sm and 3 2 S+ 1 8 2 W (the compound nucleus 2 1 4 Th), 7 6 Ge+ 1 5 0 Nd and 2 8 Si+ 1 9 8 Pt (the compound nucleus 2 2 6 U), and 6 4 Ni+ 1 5 4 Sm. To get the direct evidence that the heavy projectile really fuses with the deformed target, the fusion evaporation residues emitted to the beam direction were measured by using the JAERI recoil mass separator and identified on the basis of time- and position-correlated α-decays. The angular distribution of fission fragments was also measured to obtain the total fusion cross section. The measured cross sections of the fission and the evaporation residue were compared with the results of a coupled channel calculation and a statistical model calculation, respectively. In 3 2 S+ 1 5 2 W and 2 8 Si+ 1 9 8 Pt reactions having enough smaller charge product Z 1 Z 2 than 1800, good agreements between the data and the calculated results were obtained. In the large Z 1 Z 2 systems of 6 4 , 6 0 Ni+ 1 5 4 Sm and 7 6 Ge+ 1 5 0 Nd, on the other hand, large fusion hindrance was observed up to about three orders of magnitude in the lowest energy region where only collisions around the tip of the deformed target are possible. The fusion hindrance gradually decreased in the high energy region where near side collisions become possible. By assuming of an extra-extra push energy around 20 MeV in the tip collisions and nearly zero in the side collision, the excitation function was reasonably reproduced. This is consistent with the theoretical speculation of hugging fusion or gentle fusion that the compact touching configuration of the side collision is more favorable for complete fusion than the elongated configuration of the tip collision.

A New Measurement of the 8Li(α,n)11B Reaction for Astrophysical Interest

The 8Li(α,n)11B reaction has been measured directly and exclusively in the energy region of Ecm=0.45–1.75 MeV by using highly efficient detector system covering Ecm= 0.56 MeV, which corresponds to the Gamow window at T9=1. This experiment has been performed in the condition of inverse kinematics by using low‐energy radioactive 8Li beam at the Tandem accelerator facility of Japan Atomic Energy Research Institute. The reaction cross section obtained in the present measurement is consistent with that of the previous exclusive measurements within the errors in an overlapping energy region, but is less than half of that of the inclusive measurements, in particular for lower energy region.

Effect of Closed Shell Structure on Heavy-Ion Fusion Reactions

The effect of the nuclear shell structure on the heavy-ion fusion reaction was investigated for the reaction systems 8 2 Se + 1 3 8 Ba, 8 2 Se + 1 3 4 Ba, 1 6 O + 2 0 4 Pb, 8 6 Kr + 1 3 4 Ba, 8 6 Kr + 1 3 8 Ba and 8 2 Se + n a t . Ce. Evaporation residues for these fusion reactions were measured using a recoil mass separator (JAERI-RMS) near Coulomb barrier region. The measured evaporation residue cross sections for the reactions 8 2 Se + 1 3 8 Ba and 8 6 Kr + 1 3 8 Ba were two orders of magnitude and one order of magnitude larger than those for the reactions 8 2 Se + 1 3 4 Ba and 8 6 Kr + 1 3 4 Ba, respectively, at the excitation energy region of 10 ∼ 30 MeV. The evaporation residue cross sections were compared with those of the other reaction systems that make the same or similar compound nuclei as the present reaction systems. It was found that the evaporation residue cross sections correlate strongly with the sum of the shell energies for both projectile and target nuclei, i.e. the evaporation residue cross sections increase as the sum of the shell energy decreases.

Dependence of Heavy-ion Fusion Reaction on Nuclear Deformation and Nuclear Shell Structure

The dependence of the fusion probability on the orientation of deformed nucleus was investigated for the reactions 60, Ni + Sm and Ge + Nd. Evaporation residues were measured for these reaction systems by the JAERI recoil mass separator in the vicinity of the Coulomb barrier and the fusion probability was extracted as a function of bombarding energy. It was found that the fusion probability depends strongly on the orientation of the nuclear deformation. The fusion probability is considerably reduced when the projectiles collide at the tip of the deformed nuclei. On the other hand, when the projectiles collide at the side of the deformed nuclei, the fusion occurs without hindrance. This phenomenon is understood qualitatively by comparing the distance between the mass centers of two colliding nuclei at touching with the position of the saddle point of the compound nucleus. The dependence of the fusion probability on the nuclear shell closure was also investigated for the reactions Se + Ba, where the nucleus Ba has a closed neutron shell of N = 82 and the nucleus Ba has the neutron number N = 78, four neutrons less than the closed shell. The measured evaporation residue cross section for the reaction Se + Ba was well reproduced by statistical model calculations taking into account a subbarrier fusion enhancement, while the evaporation residue cross section for the reaction Se + Ba was about 100 times smaller than that for the fusion reaction Se + Ba. This suggests that the shell closure plays an important role in the fusion process.

β-delayed neutron andγ-ray spectroscopy of17C utilizing spin-polarized17B

Excited states in 17C were investigated through the measurement of beta?-delayed neutrons and gamma rays emitted in the ? decay of 17B. In the measurement, three negative-parity states and two inconclusive states, were identified in 17C above the neutron threshold energy, and seven gamma-lines were identified in a beta?-delayed multiple neutron emission of the 17B ? decay. From these transitions, the beta?-decay scheme of 17B was determined. In the present work, the fibeta-NMR technique is combined with the ?-delayed particle measurements using a fragmentation-induced spin-polarized 17B beam. This new scheme allows us to determine the spin parity of beta?-decay feeding excited states based on the difference in the discrete fibeta-decay asymmetry parameters, provided the states are connected through the Gamow-Teller transition. In this work, 1/2-, 3/2-, and (5/2-) are assigned to the observed states at Ex = 2.71(2), 3.93(2), and 4.05(2) MeV in 17C, respectively.

Beta-delayed neutron and gamma-ray spectroscopy of 17 C utilizing spin-polarized 17 B

Excited states in 17C were investigated through the measurement of beta?-delayed neutrons and gamma rays emitted in the ? decay of 17B. In the measurement, three negative-parity states and two inconclusive states, were identified in 17C above the neutron threshold energy, and seven gamma-lines were identified in a beta?-delayed multiple neutron emission of the 17B ? decay. From these transitions, the beta?-decay scheme of 17B was determined. In the present work, the fibeta-NMR technique is combined with the ?-delayed particle measurements using a fragmentation-induced spin-polarized 17B beam. This new scheme allows us to determine the spin parity of beta?-decay feeding excited states based on the difference in the discrete fibeta-decay asymmetry parameters, provided the states are connected through the Gamow-Teller transition. In this work, 1/2-, 3/2-, and (5/2-) are assigned to the observed states at Ex = 2.71(2), 3.93(2), and 4.05(2) MeV in 17C, respectively.