Isomer shift calibration of61Ni by lifetime variation measurements in the electron capture decay of57Ni

In the present work, differential ionization chambers are used in order to measure the difference in the electron capture decay rate of⁵⁷Ni nuclei in several pairs of different chemical compounds. Combination of these values with the corresponding Mössbauer isomer shifts provides a reliable experimental method to determine differences in electron densities at the nucleus. This allows to calibrate the isomer shift scale of the concerned nuclide. By using⁶¹Ni isomer shifts given in the literature, this work leads to the first experimental determination of the⁶¹Ni isomer shift calibration constant α=?(1.8±0.9)·10^?3.a ₀ ³ mm.s^?1. This corresponds to a change in the mean-square charge radius Δ(r ²)=?(7.2±3.4)·10^?4 fm² during the 67.4 keV Mössbauer transition of⁶¹Ni.