3. Recent HRIBF Research - Measurement of Nuclear Structure Properties
( E. Padilla-Rodal & A. Galindo-Uribarri, Spokespersons)
As part of a research program devoted to the measurement of nuclear structure properties of neutron-rich nuclei around the mass A~80 region, during the months of September and October 2007 our collaboration performed three experiments using Radioactive Ion Beams (RIBs) produced at the Holifield Radioactive Ion Beam Facility (HRIBF):
RIB-144 Coulomb excitation of n-rich nuclei.
RIB-135 Static quadrupole moment of the first 2+ in 78Ge.
RIB-151 g-factor measurements of n-rich isotopes near N=50.
Our aim has been to obtain a comprehensive picture of the shell structure in this region through the study of a series of properties: E (2+), B (E2), g-factors and electric quadrupole moment Q. The beams, instrumentation and techniques developed at HRIBF specifically for this purpose have allowed us to systematically study the behavior of these observables along isotopic and isotonic chains using both stable and radioactive nuclei under almost identical experimental conditions.
We have done pioneering studies of the evolution of collectivity in the neutron-rich radioactive isotopes 78,80,82Ge using the B(E2) as an indicator of the nuclear structure . For this work we also did a systematic measurement of the stable germanium and selenium isotopes. Recently a collaboration using REX-ISOLDE at CERN reported on the first observation of the 2+ state in 80Zn by Coulomb excitation . This together with our measurements on other N=50 nuclei gives a more complete picture of the nuclear structure for this isotones.
High quality radioactive ion beams were delivered to the RMS experimental station for the three new experiments. In Fig. 3-1 we show a scheme of the experimental setup. Around the target position we used the combination of the CLARION array, consisting of 11 HP-Ge clover detectors formed by four crystals, each segmented longitudinally in half, and the new particle array BAREBALL, with 54 CsI(Tl) detectors with minimum absorbers arranged in five rings. The de-excitation gamma-rays emitted by the Coulomb-excited projectile were detected in coincidence with the light target recoil nuclei. The beam composition was continuously monitored using a Bragg curve detector placed at zero degrees with respect to the incoming beam.
Figure 3-1: Experimental setup for the three RIB experiments performed in the mass A~ 80 region.
The aim of the first experiment was to measure the B(E2) values of the N=50 84Se. The conditions were very favorable in terms of beam purity and intensity. We bombarded a natural Al target [~1.5 mg/cm2 ] with an A=84 RIB at a beam energy of 193.2 MeV (2.3A MeV). The measured average beam intensity was 4X104pps, and the beam composition was 45% 84Se, 54% 84Br, and 1% 84Rb.
Figure 3-2: Left panel shows a typical two dimensional spectrum from the Bragg Curve Detector placed at zero degrees for an A=84 RIB. In the right panel one can see the projection of the charge distribution along a tilted axis, showing the two main components of the beam.
84Se is the N=50 isotone that lies right in the middle between Z= 28 and Z= 40 and therefore is expected to have maximum sensitivity to constrain the shell model effective interaction. In Fig. 3-3 we show a particle-gamma coincidence spectrum Doppler corrected event-by-event using both the clover segmentation and the recoil information obtained with BAREBALL. New transitions in the odd-odd 84Br isobar and a very clean peak for the 2+1 to 0+g transition in 84Se are observed.
Figure 3-3: Gamma-ray energy spectrum for an A=84 RIB.
The aim of the second experiment was to measure the electrical quadrupole moment of 78Ge. Using a sulfur purification technique , a pure beam of 78Ge was produced with typical intensity of 2X106 pps. We bombarded two targets of 12C and 24Mg alternately to minimize systematic effects such as beam intensity, beam composition and target thickness variations.
In the last experiment we intended to measure the gyromagnetic ratio in the mass A~80 region (78Ge and 80Ge) using the recoil-in-vacuum technique. Fig. 3-4 shows the angular correlation for 78Ge and 76Ge. The beam conditions for the data of 80Ge were less favorable as the overall source intensity had started to decline. The analysis of these data is in progress to determine if more data will be required for 80Ge.
Figure 3-4: Angular correlation for the Stable Ion Beam (SIB) 76Ge and the RIB 78Ge.
Data sets using stable beams under nearly equal conditions to the RIB experiments were obtained for reference, normalization or calibration. Tests have been performed with the Bragg Detector to measure target thickness homogeneity and energy loss variations due to intense beam bombardment.
 E. Padilla-Rodal et al. Phys. Rev. Lett. 94 (2005) 122501.
 J. Van de Walle et al., Phys. Rev. Lett. 99 (2007) 142501.
 D.W. Stracener, HRIBF NewsLetter Supplement, Edition 9, No.3, Spring Quarter 2001.