Total internal conversion coefficient of the 260.9 keV (7+→3−) transition in198mTl

The 1.87 hours 543.7 keV (7+) isomeric state in198Tl is produced via197Au(α, 3n)198mTl (Eα=35 MeV) reaction. The total conversion coefficient of 260.9 keV (7+→3−) is determined for the first time by the intensity balance method. The value of\(\alpha _{\rm T} \left( {260.9 keV} \right)\) is found to be 40.1±8.6 which is in good agreement with the theoretical value of Hager and Seltzer for pureM4 transition. The gamma transition probability of the 260.9 keV (M4) is calculated using the present value of αT and compared with the single-particle estimate.