ANNUAL REPORT. PART A: STUDIES IN K-CAPTURE POSITRON BRANCHING RATIOS. PART B: SEARCH FOR A LOW-LYING O$sup +$ STATE IN GALLIUM-68

Abstract

K-capture to positron branching ratios were measured in the decay of Na/ sup 22/, Co/sup 58/, and Ga/sup 68/ -- all pure Gamow-Teller emitters, using coincidence scintillation spectrometer techniques. The measured values are 0.105 plus or minus 0.004 for Na/sup 22/, 5.08 plus or minus 0.17 for Co/sup 58/, and 1.28 plus or minus 0.12 and 0.10 plus or minus 0.02 for Ga/sup 58/. From these the Fierz interference terra is computed to be b =--0.004 plus or minus 0.012, -0.004 plus or minus 0.14, --0-03 plus or minus 0.02, and +0.03 plus or minus 0.01, respectively. These results indicate that the Fierz interference in Gamow-Teller interaction is very small. The decay of 270 day Ge/sup 68/ was investigated in equilibrium with Ga/sup 68/ to look for a possible low-lying 0+ level in Ga/sup 68/ using x-ray--x-ray and x-ray- gamma -ray coincidences. The result was negative. Besides the 9 kev K x ray, the 1.07 Mev gamma ray in the decay of Ga/sup 68/ and annihilation radiation, no other gamma rays were detected (<8% of 1.07 Mev gamma ray). The number of positrons per 1.07 Mev gamma - quantum was determined as 19.47 plus or minus 2.10. The ratio of positrons to the 1.07 Mev level and ground state of Zn/sup 68/ was found to be (1.76 plus or minus 0.22) x 10/sup -2/. Studies with a Ge/sup 68/ source chemically separated from Ga/sup 68/ showed no gamma rays (<1% of total Ge/sup 68/ decays). By following the growth of annihilation radiation, an upper limit of 0.4% per decay cculd be set on the amount of positron emission by Ge/sup 68/. A search for an electric monopole (EO) transition from the 2.3 Mev level to the O/sup +/ ground state of Zn/sup 68/ was made using plastic scintillators and an anti-coincidence arrangement. An upper limit of (5 plus or minus 25) x 10/sup -9/ conversion electrons per decay of Ga/sup 68/ could be set. This is interpreted as evidence against a possible O/sup +/ spin assignment for the 2.3 Mev level, but the interpretation is not conclusive. A brief review is presented regarding the status of Fierz interference in beta-decay as revealed by present studies and other experiments. (auth)