Improved measurement of the ${}^{44}\mathrm{Ti}$ half-life from a 14-year long study

The half-life of ${}^{44}\mathrm{Ti}$ was determined by following the decay of ${}^{44}\mathrm{Ti}$ and ${}^{60}\mathrm{Co}$ for 14 yr. Mixed sources containing ${}^{44}\mathrm{Ti}$ and ${}^{60}\mathrm{Co}$ and pure sources of ${}^{44}\mathrm{Ti}$ and ${}^{60}\mathrm{Co}$ were prepared and their γ-ray spectra were measured with a Ge spectrometer once or twice a year. Counts in the 1157.0-keV γ-ray photopeak of ${}^{44}\mathrm{Ti}$ and the 1173.2- and 1332.5-keV γ-ray photopeaks of ${}^{60}\mathrm{Co}$ were used for this measurement. From the decay of the ${}^{44}\mathrm{Ti}$ and ${}^{60}\mathrm{Co}$ activity ratios, the half-life of ${}^{44}\mathrm{Ti}$ was determined. Using a value of 5.2711±0.0004 yr for ${}^{60}\mathrm{Co}$ half-life, we determined the half-life of ${}^{44}\mathrm{Ti}$ as 58.9±0.3 yr. Special efforts were made to check for systematic errors and these were found to be negligible within the quoted uncertainty.

Figure 1

Partial decay schemes of ${}^{44}\mathrm{Ti}$ and its daughter ${}^{44}\mathrm{Sc}$ from Ref. [1].

Figure 2

γ-ray spectrum of a mixed source of ${}^{44}\mathrm{Ti}$ and ${}^{60}\mathrm{Co}$ measured with a 25% Ge spectrometer. This spectrum was measured 2 years after the start of the experiment. Energies of ${}^{44}\mathrm{Ti}$ and ${}^{60}\mathrm{Co}$ γ rays are given in keV. The source-to-detector distance was 10.2 cm and the counting time was 48 h. B denotes peaks from room background and S denotes sum peaks.

Figure 3

A portion of the γ-ray spectrum displayed in Fig. 2 showing the ${}^{44}\mathrm{Ti}$ and ${}^{60}\mathrm{Co}$ peaks of interest. The lower spectrum represents the room background measured for the same time interval as the source spectrum. The 1155.18-keV peak belongs to ${}^{214}\mathrm{Bi}$, decay product of ${}^{238}\mathrm{U}$, present in concrete. B denotes peaks from room background.

Figure 4

Semilogarithmic plot of the ratios of the areas of the 1157.0-keV peak and the 1173.2-keV peak against the time of measurement for the Argonne data (top figure). The set contains 22 data points. Data points measured very close in time overlap and hence are indistinguishable in the figure. The line represents the best fit to data points and gives a value of 58.8±0.4 yr. The bottom figure shows the differences between the measured data points and the values calculated by the fit.

Figure 5

Semilogarithmic plot of the 1157.0 keV (${}^{44}\mathrm{Ti}$) and 1173.2 keV (${}^{60}\mathrm{Co}$) photopeaks (solid line) collected at Hebrew University for the mixed ${}^{44}\mathrm{Ti}$+${}^{60}\mathrm{Co}$ source and of the fit to the experimental spectrum (dotted line) obtained to extract the ratio of counts between the two peaks (see text for details). The experimental spectrum was collected close to the start of the measurements ($t=0.73$ yr).

Figure 6

A portion of the γ-ray spectrum of a mixed ${}^{60}\mathrm{Co}$ and ${}^{44}\mathrm{Ti}$ source obtained after subtraction of the room background spectrum (bottom panel). Also shown (top panel) are spectra of a pure ${}^{60}\mathrm{Co}$ source (normalized to the counts in the 1173.2-keV peak of the mixed source) and a pure ${}^{44}\mathrm{Ti}$ source (normalized to the counts in the 1157.0-keV peak of the mixed source). All the three spectra were measured at a source-to-detector distance of 10.2 cm and are plotted with the same $x$ and $y$ scales.