Monoenergetic Positron Line and Three New Gamma Transitions inBi206

The monoenergetic positron line (the 1.72-MeV transition) in ${\mathrm{Bi}}^{206}$ was sought using an intense ${\mathrm{Bi}}^{206}$ source in a two-cycle baffled solenoid spectrometer. The source was prepared by bombarding in a deuteron beam a highly purified radiogenic lead target with ${\mathrm{Pb}}^{206}$ isotopic content enriched from 26 to 87.7%. The comparative ${\mathrm{Bi}}^{205}$ contamination is thus reduced to less than 0.05%. The leak through of the opposite sign of particles of the baffle used is better than 1 in ${10}^7$. A small peak of ∼1% above the base line is observed at the predicted momentum value for the monoenergetic positron line (3970 Gcm). The intensity of the positron line is estimated to be 3.2×${10}^{-8\mathrm{}}$ per gamma ray, which is one-sixth of that first reported by Brunner. The retardation factor of this $E1\mathrm{}$ (1.720 MeV) transition is estimated to be around 600. Three new gamma transitions above the highest energy line (1.720 MeV) known previously have been found and identified. These are 1.845, 1.88, and 1.90 MeV. From the ratios of internal conversion to internal pair production, one can assign $E1\mathrm{}$ to both 1.845- and 1.880-MeV transitions. The 1.90-MeV transition is probably also of the $E1\mathrm{}$ type.