Higher-order effects in an allowed positon decay in Co 56

Abstract

The allowed positon decay of Co 56 to the second excited state of Fe 56 has the unusually large log ft value of 8.7. A study of the directional correlation of this positon group with the two cascade gamma rays of energy 0.85- and 1.24-MeV which depopulate this level has been measured as a function of beta energy from 0.7 to 1.2 MeV to yield new information about the decay to this level. A magnetic spectrometer was used as the beta analyzer. An anisotropic correlation was measured, A max = (2.5–3.3)% near 1.2 MeV. This is only the third case where any correlation has been observed in an allowed beta decay and it is much larger than the other two. An explanation may be found in higher order terms predicted by the Gell-Mann theory or in terms of cross terms between allowed and second forbidden matrix elements Unfortunately, better theoretical calculations are needed, especially for large ft decays, before theory and experiment can be compared favourably.