Spectral shapes and a beta-gamma directional correlation in the beta decay of ¹⁷²Tm(J^Π = 2^-)

Abstract

The isotope ¹⁷²Tm populates the 0⁺, 2⁺ and 4⁺ states of the ¹⁷²Yb ground-state rotational band directly in beta decay. The shapes and intensities of the three beta groups have been measured by means of a six-gap magnetic spectrometer operated in coincidence with a NaI(Tl) crystal. The experimental shape factor plots are taken relative to the ⁹⁰Y shape factor measured in the same circumstances, and it is concluded that on this basis the shapes of all three ¹⁷²Tm beta groups agree with the theoretical shape for a first-forbidden unique transition. The directional correlation for the 2^-(β)2⁺(γ)0⁺ cascade has been measured at six energies from 1010 to 1550 keV and these results too are consistent with the assumption that the 2^- →2⁺ beta transition is purely of tensor rank 2. The data for the 2^- →2⁺ transition are analysed on the basis of the modified B_ij approximation, which defines parameters X and Y corresponding to the relative contributions of tensor ranks 0 and 1 respectively. The shape measurement provides the limit X²+Y² ≦ 0.10, whereas the angular correlation measurement gives Y = 1.64X+(0.02±0.03). The reduced transition probabilities (ƒ₁t)^- can therefore be compared with the theoretical intensity rules for λ = 2, which predict the ratios 0.70 : 1 : 0.05 for the branches to the 0⁺, 2⁺ and 4⁺ states. The experimental result is (0.63±0.19) : 1 : (0.046±0.007).