Search

We report for the first time the discrimination of the core fragment knockout and valence nucleon knockout reaction mechanisms at medium energy range, by the use of the recoil proton tagging technique. Intense 8He beams at 82.3 MeV/u were supplied by the RIPS beam line at RIKEN, and impinged on both hydrogen and carbon targets. Recoil protons were detected in coincidence with the forward moving core fragments and neutrons. The core fragment knockout mechanism is identified through the polar angle correlation and checked by various kinematics relations. This mechanism may be used to extract the cluster structure information of unstable nuclei. On the other hand, with the selection of the tagged valence nucleon knockout mechanism, a narrower peak of 7He ground state is obtained. The extracted neutron spectroscopic factor S n = 0.512 ( 18 ) is relatively smaller than the no-tagged one, and is in good agreement with the prediction of ab initio Greenʼs function Monte Carlo calculations.

Knockout reaction experiment for 8He at 82.3 MeV/u on Hydrogen target was carried out at the RIPS beam line in RIKEN. Recoil protons were detected in coincidence with the forward moving core fragments and neutrons. The quasi-free knockout mechanism is identified through the polar angle correlation and checked by various kinematics conditions. The absolute differential cross sections for 6He core cluster are obtained and compared with the simple Glauber model calculations. The extracted spectroscopic factor is close to unity and a shrinking of the cluster size is evidenced.

A knockout reaction experiment was carried out using a 6He beam at 82.3 MeV/u impinged on carbon and hydrogen targets. The recoil protons were detected in coincidence with the forward moving charged fragments and neutrons. It is demonstrated that through the correlation between the recoil protons and forward moving charged fragments, the core fragment knockout and valence nucleon knockout reaction mechanisms can be discriminated at energies around 100 MeV/u. The 5He ground-state resonance was reconstructed based on the selection of the valence nucleon knockout mechanism. The resonance parameters were obtained for both carbon and hydrogen targets, which are consistent with those previously reported in the literature. The importance of selecting appropriate reaction mechanisms is discussed based on reconstructions using events with larger transverse momentum transfer.

The first spectroscopic study for the beta decay of N-21 is carried out based on beta-n, beta-gamma, and beta-n-gamma coincidence measurements. The neutron-rich N-21 nuclei are produced by the fragmentation of the E/A=68.8 MeV Mg-26 primary beam on a thick Be-9 target and are implanted into a thin plastic scintillator that also plays the role of beta detector. The time of flight of the emitted neutrons following the beta decay are measured by the surrounding neutron sphere and neutron wall arrays. In addition, four clover germanium detectors are used to detect the beta-delayed gamma rays. Thirteen new beta-delayed neutron groups are observed with a total branching ratio of 90.5 +/- 4.2%. The half-life for the beta decay of N-21 is determined to be 82.9 +/- 7.5 ms. The level scheme of O-21 is deduced up to about 9 MeV excitation energy. The experimental results for the beta decay of N-21 are compared to the shell-model calculations.

A new transition is reported in the beta-delayed neutron decay of {sup 18}N. The observed neutron energy is 3.78{+-}0.05 MeV with a branching ratio of 0.05{+-}0.03%, which corresponds to an excitation energy of 12.05{+-}0.05 MeV in the daughter nucleus {sup 18}O. The log ft value of this transition is 5.24{+-}0.3, which, together with the previously reported electron scattering data, determines the J{sup {pi}} value of this level to be 1{sup -}. The experimental data are compared to the shell model calculations in different model spaces.

Breakup reactions of the double borromean nucleus 8He were measured at 82.3 MeV/u on CH2 and C targets. The coincident detection of two decaying neutrons and the high performance for neutron cross talk rejection are realized in this experiment. The relative energy spectrum for 8He was reconstructed with good statistics. The spectrum exhibits a structure of two resonant peaks, one at an excitation energy of about 3.0 and the other at about 4.14 MeV, respectively. Substantially larger cross section for the first resonance is observed in comparison to the results reported previously.