Study Of ~9Li Cluster States

Nuclear clustering is an essential concept in nuclear multi-body dynamics.Nucleon clustering inside a nucleus is an intriguing phenomenon which has given rise to alternative view of the nuclear basic structure property of the coexistence of mean-field dynamics and clustering dynamics.Nuclear structure and reaction mechanism have attracted continuous attention over the past decades.The interest in nuclear clustering has been pushed strongly due to the study of neutron-rich and of exotic weakly bound nuclei.This is a field which has attracted worldwide attention,because the weakly-bound nuclear systems exhibit unique features related to the quantal properties of the many body systems,like halos and clustering.6He+t cluster states in the breakup reaction of 9Li on a Pb target have been measured at 32.7 MeV/nucleon for the first time,which is very meaningful for studying the cluster structure properties in unstable nuclei.A peak at 9.8 MeV with a wide decay width was clearly observed.The relative orbital angular momentum L(L=1)of 6He+t coincident fragments was identified for this peak through angular correlation analysis and generator coordinate method(GCM)calculation.A continuum discretized coupled channel(CDCC)method calculation based on 6He+t cluster configuration was performed to verify the cluster resonance states,and the spin-parity of 6He+t cluster resonance state,3/2—,was determined.Based on multipole-decomposition(MD)analysis,a strong monopole strength comparable to the typical single-particle strength was extracted.These results support the picture of strong 6He+t clustering in 9Li.The experiment was carried out at the Heavy Ion Research Facility in Lanzhou,Radioactive Ion Beam Line in Lanzhou(HIRFL-RIBLL).A 32.7 MeV/nucleon secondary beam 9Li was produced on primary Be target bombarded by a 12 C primary beam at 53.7 MeV/u delivered by the HIRFL.The beam particle identification was realized event by event using the time-of-flight(TOF),energy loss(?E)and B? method.Three position-sensitive parallel-plate avalanche counters(PPACs)monitored the secondary beam to a self-supported Pb target with a thickness of 526.9 mg/cm2.The position and incident angle of the beam particles at the target were determined by extrapolating the position information provided by PPACs event by event.A zero-degree ?E-E telescope,covering the ? angles from 0o to 10 o,was used to record the positions and energies of the charged fragments produced in the 9Li+Pb reactions.The energies and tracks of the two coincident charged particles 6He+t are measured by the ?E-E telescope array and the excitation energies of these events are reconstructed by the invariant mass method.The excitation energy spectrum decaying into 6He+t is above the decay threshold energy of 9Li.Two peaks at 9.8 MeV and 12.5 MeV were observed through double-Gauss fitting.We adopted a Breit-Wigner(BW)shaped resonance function to fit the first peak in order to evaluate their formation and decay natures.The extracted width of BW function for the first peak is ?= 1.4±0.5 MeV(the error is statistical only)through a least square fitting.An model-independent angular correlation method was applied to examine the relative orbital angular momentum of the first resonance peak.The obtained relative orbital angular momentum is L=1.Taking into account of the intrinsic spin of 6He(0+)and t(1/2+),the total angular momentum of 6He+t resonance states should be obtained through the coupling of the spins of the clusters and the orbital angular momentum of the intercluster motion.Thus,the total angular momentum of 6He+t resonance state of the first peak could be 1/2— or 3/2—.From the GCM calculation,the third 1/2— and the second 3/2— states with L=1 might be the candidates of the first peak(9.8 MeV);the second 5/2— and the second 7/2— states with L=3 might be the candidates of the second peak(12.5 MeV).It can be concluded that the calculated values reproduce the data fairly well.From the CDCC calculation,the distribution of the 3/2— partial wave presents a pronounced resonance peak around the relative energy of 2.0 MeV while comparing the distribution of the other partial waves,which agrees with the present experimental result and the GCM calculation.The angular distribution of the differential cross section for 3/2— partial wave calculated from CDCC method,well reproduce the distribution of experimental points.The monopole excitation from the ground state to the excited states can be a good probe to experimentally observe cluster resonances of light nuclei which appear in the energy region a few MeV above the threshold energies.In present work,The extracted isospin scalar monopole transition matrix element(M(IS0))=4.5±0.4 fm2 for the 9.8 MeV excited state of 9Li is comparable to those typical cluster states in 12 C,16O and unstable nucleus 12 Be.From the GCM calculation for 9Li with 6He+t configuration,a value of ~6 fm2 for M(IS0)at excitation energy of 9.8 MeV was obtained.These theoratical results agree well with our present observation.In summary,based on present experimental studies,the picture of strong clustering of 6He+t at 9.8 MeV in unstable nucleus 9Li is supported.