Edition 8, No. 3 Summer Quarter 2000 Price: FREE

In order to assist you in preparing proposals involving neutron-rich beams from the HRIBF, a recent test run searching for gamma rays from fusion-evaporation reaction products is reported below. We look forward to receiving your proposals and/or inquiries soon.

Neutron-rich RIBs at the HRIBF

D. C. Radford, C. Baktash, A. Galindo-Uribarri, C. J. Gross, C.-H. Yu, P. E. Mueller and D. W. Stracener
Physics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831

D. J. Hartley and L. L. Riedinger
Department of Physics and Astronomy, University of Tennessee, Knoxville, TN 37996

We have performed a test experiment using neutron-rich radioactive ion beams from the HRIBF facility, together with the CLARION Ge detector array. The RIBs are produced by proton-induced fission of uranium and were accelerated to energies of between 3.0 and 4.0 MeV per nucleon in the Tandem accelerator. The higher energies required the use of the second stripping foil in the high-energy beam tube.

During the main part of the experiment, a beam of 117Ag at 460 MeV was used to bombard a 0.6 mg/cm2 target of 12C. The 117Ag beam current on target was measured to be 3.8 x 105 ions per second. Later, this beam current was optimized by changing stripper foils and by increasing the proton beam current from the ORIC cyclotron from 5 to 9 microamps; this increased the beam to approximately 1.2 x 106 ions per second, which is an order of magnitude higher than was expected for this doubly stripped beam.

With this thin, light target, the count rate in the CLARION array due to scattered radioactive beam was very low. Gamma singles rates increased from about 180/s (background) to about 300/s (with beam on). Gamma-gamma coincidence rates approximately doubled, from about 1/s to about 2/s. Much of this increase in the gamma-gamma coincidence rate could be attributed to the fusion-evaporation events. This result is very promising for gamma-ray spectroscopic studies of fusion-evaporation products with these beams; such studies are the ultimate goal of this test experiment.

A foil-plus-microchannel-plate recoil detector (MCP), placed at the achromat of the RMS, allowed recording of gamma-gamma-recoil coincidence events. Even with the low beam intensity of 3.8 x 105 ions per second and the thin target, we were able to observe several hundred such coincident events per hour, mostly (80%) from the 4n evaporation channel leading to 125I. A sizable fraction of the direct beam also impinged upon the MCP, resulting in an MCP count rate of more than 105/s.

While the A = 117 beam was almost purely silver, beams of other masses from the same source are expected to be strongly mixed isobarically. In the same test run, we also measured the available beams for A = 134, using singly stripped ions. The beam was composed of approximately 4 x 104 134Sb/s, 1 x 106 134Te/s, and 5 x 105 134I/s. These rates were measured at 5 microamps proton current and could therefore be increased significantly.

We greatly appreciate the extraordinary efforts of the HRIBF operations staff in developing and providing these beams. We also thank T. A. Lewis, D. Shapira and J. W. Johnson, who developed the large-area MCP detector.

You may contact us at the addresses below.

Witek Nazarewicz Carl J. Gross
Deputy Director for Science Scientific Liaison
Mail Stop 6368 Mail Stop 6371
+1-865-574-4580 +1-865-576-7698

Holifield Radioactive Ion Beam Facility
Oak Ridge National Laboratory
Oak Ridge, Tennessee 37831 USA
Telephone: +1-865-574-4113
Facsimile: +1-865-574-1268