K-capture to positon branching ratios in the decay of Ga⁶⁸

Abstract

K-capture to positon branching ratios have been measured in the decay of Ga⁶⁸ using coincidence scintillation spectrometer techniques. The measured values are 1.28±0.12 (to the first excited state) and 0.10±0.02 (to the ground state). Assuming these decays to be pure Gamow-Teller transitions the Fierz interference terms are computed to be b_GT=-0.03±0.02 and +0.03±0.01 respectively (the uncertainty in the latter result arising from uncertainty in N₀, the source strength). The ratio of positons to the first excited state and ground state of Zn⁶⁸ has been found to be (1.76±0.22)×10^-2. A search for a possible low-lying 0⁺ state in Ga⁶⁸ was made through the decay of Ge⁶⁸ using X-ray-X-ray and X-ray-γ-ray coincidences employing NaI(Tl) crystals. Besides the 9 keV K X-ray, annihilation radiation and the known γ-ray in Ga⁶⁸ whose energy was measured to be 1.067±0.035 MeV, no other γ-ray was observed (> 8% of 1.07 γ). The number of positons per 1.07 γ quantum was determined as 19.47±2.10 by comparison with Na²². An upper limit of 0.4% per decay was set on positon emission by Ge⁶⁸.