Precise half-life measurements for 38Ca and 39Ca

Precise half-life measurement of the superallowedβ+emitter38Ca

“…These were each obtained by observation and analysis of the decay of β-delayed γ-rays from the daughter, 38 K. The first result lies about one-and-a-half of its error bar away from our quoted half-life; the other three agree well within their error bars. There is a much more recent measured value, 443.8(19) ms [8], which was obtained in a similar manner to ours, from a measurement of the decay positrons in a gas counter; in their case, though, sample purity was achieved by collecting the 38 Ca first in a Penning trap. Our half-life result agrees well with theirs but with an uncertainty five times smaller.…”

“…The value of Q EC is used to determine the statistical rate function, f , while t 1/2 and R are combined to obtain the partial half-life, t. Before our half-life measurement of 38 Ca, which is reported here, only its Q EC value was known to the desired precision: it had been measured with Penning-traps to ±0.006% [5][6][7], which corresponds to an f -value precision of ±0.03% [2]. The most precise half-life result was only known to ±0.4% [8] and the superallowed branching ratio has not yet been measured at all.…”

Precise measurement of branching ratios in theβdecay ofCa38

“…V B convincingly rule out the possibility of large numbers of unobserved weak transitions to higher excited states that sum to appreciable strength -the Pandemonium effect [24,25] -so we can safely conclude that the Gamow-Teller sum we have obtained accounts for all the non-superallowed strength in the decay of 38 Ca. The branching ratio for the superallowed 0 + → 0 + transition to the 130-keV analog state is thus 0.7728 (16), a result we obtain simply by subtracting the total GamowTeller branching ratio from unity. Note that, as we do so, we convert the relative precision of our measurement, which is 0.70% (= 0.0016/0.2272), to a relative precision of 0.21% (= 0.0016/0.7728) for the quantity we have sought to obtain: the superallowed branching ratio.…”

“…The total branching ratio for all five Gamow-Teller transitions -to the 1 + states at 459, 1698, 3341, 3856 and 3977 keV -is 0.2272 (16). This is simply the sum of the corresponding branching ratios in column six of Table V.…”

“…This nuclide is not present in the implanted beam, but it naturally grows in the collected sample as the 38 Ca decays. Since the half-lives of 38 Ca and 38m K are well known [1,16,17], the ratio of their activities can easily be calculated if the 38 Ca implantation rate is known as a function of time during the collection period [17]. To enable this calculation, we recorded the time profile of ions detected by the scintillator at the exit of MARS for each individual cycle.…”