On the half-life of {sup 44}Ti

One of the few long-lived gamma-ray emitting radioisotopes expected to be produced in substantial quantities during a supernova explosion is {sup 44}Ti. The relevant portions of the decay schemes of {sup 44}Ti and its daughter {sup 44}Sc are shown. {sup 44}Ti decays to {sup 44}Sc emitting {gamma} rays of 68 and 78 keV. {sup 44}Sc subsequently decays with a 3.93-hour half life to {sup 44}Ca emitting an 1,157-keV {gamma}ray. This characteristic 1,157-keV {gamma} ray from the decay of {sup 44}Ti has recently been observed from the supernova remnant Cas A. In order to compare the predicted {gamma}-ray flux to that actually observed from this remnant, one must know the half-life of {sup 44}Ti. However, published values for this quantity range from 46.4 to 66.6 years. Given that the Cas A supernova is believed to have occurred approximately 300 years ago, this translates to an uncertainty by a factor of 4 in the amount of {sup 44}Ti ejected by this supernova. Thus, in order to provide an accurate and reliable value for this important quantity, the authors have performed a new experiment to determine the half-life of {sup 44}Ti. The authors produced {sup 44}Ti via the {sup 45}Sc(p,2n) reaction using 40 MeV protons from the Lawrence Berkeley National Laboratory`s 88-Inch Cyclotron. In the present experiment, the authors attempted to use all three {sup 44}Ti {gamma}-ray lines to determine its half life. However, analysis of the {sup 241}Am and {sup 137}Cs lines produced an incorrect value for the half life of each of these isotopes. On the other hand, the analysis of the {sup 22}Na line produced a result that agreed to within 0.5% of the known value of 2.603 years. Thus, they decided to concentrate their effort on the analysis of the 1,157-keV line. The half life of {sup 44}Ti that they deduce from this experiment is 63 {+-} 3 years