Quasi-Free Scattering from Relativistic Neutron-Deficient Carbon Isotopes

Abstract

Quasi-free scattering of the neutron-deficient carbon isotopes 10C and 11C has been studied in inverse kinematics at the R3B-LAND setup at the GSI-Helmholtz Centre for Heavy Ion Research. In this experiment, a 40Ar primary beam was incident on a beryllium production target at an energy of 490 AMeV, and the selected reaction residues were then transported through the fragment separator FRS to the experimental area. The incoming beam was identified using the time-of-flight and energy loss, and the incoming angle of the beam was determined from the position on two silicon strip detectors in front of the reaction target. The target area was surrounded by the Crystal Ball NaI array used for gamma- and proton detection and a box consisting of four silicon strip detectors. After passing through the ALADIN magnet, the outgoing fragments were identified and tracked using the position and energy information given by two additional silicon strip detectors, two fiber detectors, and a time-of-flight wall, and protons emitted in-flight were detected by drift chambers. In order to obtain the cross section for quasi-free scattering on hydrogen, a CH2-target and a carbon target were used, and the measurements with the carbon target were used to subtract both the contribution of the carbon and the background in the CH2-measurements. The cross sections obtained with the carbon target for proton and neutron removal from 11C and for neutron removal from 10C are sigma_C=21.6(8) mb, sigma_C=34.8(23) mb, and sigma_C=32.8(39) mb. The work presented here is aimed at a quantitative understanding of spectroscopic factors, which appear to be quenched for deeply bound nuclei. The obtained quasi-free scattering cross sections for the reactions 11C(p,2p)10B, 11C(p,pn)10C, and 10C(p,pn)9C are sigma_H=17.3(8) mb, sigma_H=21.2(20) mb, and sigma_H=19.7(28) mb, respectively. The associated quenching of spectroscopic factors, obtained from comparing the experimental cross sections with theoretical ones from DWIA-calculations is R_s=0.53(2), R_s=0.82(8), and R_s=1,04(15). The momentum distributions for the fragments of all three reactions have been calculated from the measured incoming and outgoing angles and compared to DWIA-calculations. In all three cases there is good agreement between the measured distributions and calculations for a p-shell knockout. The gamma-energy spectra measured in coincidence with 10C and 10B produced in quasi-free scattering reactions indicate a strong population of the low-lying excited states in both cases. The cross section for the reaction 11C(p,p alpha)7Be has been determined to be sigma_H= 5.8(4) mb. Due to the small spatial separation of the decay products of the unbound 9B on the silicon detectors and limited acceptance of the proton drift chambers, no reliable cross section could be determined for the reaction 10C(p,2p)9B. Furthermore, new front-end electronics for double-sided silicon strip detectors have been developed, and initial tests has been completed. The electronics is based on the combination of commercially available readout and triggering ASICs (VA and TA by Gamma-Medica) and ADC (Struck SIS3300) with a customized sequencing module, based on the VUPROM module developed at GSI. The design is flexible, and it is supposed to be used for DSSD prototype detectors built in connection with the ongoing detector development for nuclear structure experiments at the planned FAIR facility.