Studies of positron electron pair production in {sup 238}U + {sup 181}Ta

Technical Report · Tue Aug 01 04:00:00 EDT 1995

DOI:https://doi.org/10.2172/166356· OSTI ID:166356

Ahmad, I.; Back, B. B.; Betts, R. R. ^[1]

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1. and others

Following the completion of APEX in late 1993, a two-week run on the {sup 238}U + {sup 181}Ta system at 6.1 and 6.3 MeV/u with 1 mg/cm{sup 2} targets provided, for the first time, data in which the expected sharp sum-energy lines should appear. Data from previous experiments show evidence for sharp sum-energy lines at 625, 748 and 805 keV, observed at bombarding energies from 5.9 to 6.3 MeV/u. The 625- and 809-keV lines display the characteristics of equal-energy back-to-back emission whereas the 748-keV line shows a rather different behavior. In our measurements, average beam currents of 2-3 pnA from the ATLAS accelerator were used to bombard 1-mg/cm{sup 2} rolled {sup 181}Ta targets, the energy loss in which corresponds to the ranges of bombarding energies over which the sharp sum-energy lines were previously reported. A run at 5.95 MeV/u for {sup 238}U + {sup 181}Ta followed in May 1994. These data were analyzed extensively. Sum-energy spectra measured in coincidence with scattered ions in the range 20{degrees} < {theta} < 68{degrees}. No evidence is found for the sharp sum-energy lines reported previously and, depending on the scenario assumed for the production mechanism and kinematics of the pairs, upper limits on cross sections at the 90% confidence limit range from 10-100 times smaller than the values that can be deduced from the earlier reports. We are in the process of refining the data analysis and simulations of the apparatus in order to finalize these numbers for publication.

• Research Organization: Argonne National Lab., IL (United States)
• OSTI ID: 166356
• Report Number(s): ANL--95/14; ON: DE96000985
• Country of Publication: United States
• Language: English

Related Subjects

66 PHYSICS
ELECTRON PAIRS
PAIR PRODUCTION
PROGRESS REPORT
TANTALUM 181 TARGET
URANIUM 238 REACTIONS