4. Recent HRIBF Research - Commissioning Experiments at the Low-Energy Radioactive Ion Beam Spectroscopy Station (LeRIBSS)
(K. Rykaczewski for the LeRIBSS collaboration)
The first LeRIBSS experiments were performed in July 2008. The 54-MeV ORIC proton beam impinging on the ~7 grams of the HRIBF 238UCx target typically had an intensity of about 8 μA. Fission products are released as positive ions from the HRIBF ion source. These ions are usually passed through the cesium-loaded charge exchange cell to create the negative ions needed for post-acceleration in the HRIBF Tandem. However, some elements, like zinc and cadmium, do not create negative ions. In the first phase of LeRIBSS experiments, zinc-free copper beams were obtained using the charge exchange cell. Later phases of the experiment used positive ions in order to avoid losses due to the charge exchange process (typically ~ 5% efficiency).
Figure 4-1: The activity collected on tape of the MTC was viewed by two plastic beta energy-loss detectors and four clover gamma detectors during first LeRIBSS experiment.
Figure 4-2: The LeRIBSS digital data acqusition system based on Pixie-16 modules of XIA (lower crate) and the remotely controlled Wiener MPOD-ISEG High Voltage supply for clover and beta detectors (upper crate).
Two LSU plastic beta detectors and four clover gamma-detectors around the thin beam pipe surrounded the collection point, see Fig. 4-1. All detector signals were digitally processed using XIA Pixie-16 modules . This new data acquisition system was developed at the Digital Pulse Processing Laboratory of University of Tennessee Knoxville (UTK) by Robert Grzywacz and his collaborators Sean Liddick and Iain Darby, see, e.g., . The high-voltage for all detectors was remotely controlled using a Wiener MPOD minicrate with ISEG High Voltage cards, see Fig. 4-2.
Figure 4-3: The beta-gated gamma spectrum collected during 3 hours measurement with pure 75Cu negative ion beam.
The decays of copper isotopes in the vicinity of doubly magic 78Ni were studied following the proposals of Jeff Winger and Sergey Ilyushkin of Mississippi State University. Practically pure beams of 75Cu and 77Cu were obtained, at the maximum rates of about 3000 pps and 130 pps, respectively. An example of the low-energy beta-gated gamma spectrum collected during the 3 hour experiment on 75Cu decay is given in Fig. 4-3. The respective time patterns of the 191-keV, 419-keV and 722-keV gamma transitions following 75Cu decay are given in Fig. 4-4. The beam was pulsed with 5-seconds on and 7-seconds off in order to measure the activity grow-in/decay-out profile.
Figure 4-4: The grow-in/decay-out pattern observed for several gamma transitions following beta decay of 75Cu. The total measurement time was about 80 minutes.
The second part of the LeRIBSS experiment was performed without the use of the charge exchange cell. The calibration of the RIB-injector magnet and initial beam tuning were done using positive ions of stable Krypton and Xenon isotopes. The decays of neutron-rich 79Zn, 80Zn and 81Zn isotopes were studied following the HRIBF proposals by Sean Liddick and Steven Padgett of UTK. The resolving power of the HRIBF injector magnet was high enough to separate the neighboring isobars of Z=31, 81Ga and Z=30, 81Zn. The ratio of mass difference between 81Ga and 81Zn to the mass of 81Zn is ΔM:M ~ 1:6400. The positive ions of 81Ga were produced at the rate of about 106 pps, while the rate of 81Zn ions was of the order of 10 pps.
Figure 4-5: The section of beta-gated gamma spectrum measured for 81Zn decay. The 216-keV and 711-keV gamma transitions can only result from the decay of 81Ga produced as a daughter activity of 81Zn. The 81Ga ions, initially produced at a rate about five orders of magnitude higher than that of 81Zn, were practically removed from the separated positive-ion beam.
Despite the large difference in composition of the A=81 beam, practically pure 81Zn samples, at about half its maximum rate, were studied at LeRIBSS after fine tuning of the HRIBF high-resolution beam-injector magnet, see Fig. 4-5. In the preliminary analysis, the half-life of 81Zn was measured to be 315(18) ms, see Fig. 4-6. The gamma transitions feeding the levels up to ~2-MeV excitation energy in the N=50 81Ga daughter nucleus were established using beta-gamma-gamma coincidence information.
Figure 4-6: The grow-in/decay-out pattern observed for the strongest gamma transition at 351 keV following 81Zn beta decay The total measurement time was about 5 hours.