V. E. Mayes

Precise efficiency calibration of an HPGe detector: source measurements and Monte Carlo calculations with sub-percent precision

With the goal of measuring precise gamma-ray intensities for short-lived (< 5 s) accelerator-produced activities, we have calibrated the efficiency of an HPGe detector between 53 and 1836keV to sub-percent precision with a combination of source measurements and Monte Carlo calculations. Using known or independently measured detector dimensions, we have achieved both relative and absolute agreement (the latter, to 0.1%) between the calculated and measured efficiencies with only two adjustable detector parameters, the thicknesses of the contact dead layers.

High precision measurement of the superallowed 0+ --> 0+ beta decay of 22Mg.

The half-life, 3.8755(12) s, and superallowed branching ratio, 0.5315(12), for 22Mg beta decay have been measured with high precision. The latter depended on gamma-ray intensities being measured with an HPGe detector calibrated for relative efficiencies to an unprecedented 0.15%. Previous precise measurements of 0+ --> 0+ transitions have been restricted to the nine that populate stable daughter nuclei. No more such cases exist, and any improvement in a critical Cabibbo-Kobayashi-Maskawa unitarity test must depend on precise measurements of more exotic nuclei. With this branching-ratio measurement, we show those to be possible for T(z)=-1 parents. We obtain a corrected Ft value of 3071(9) s, in good agreement with expectations.

Precise half-life measurements for the superallowed {beta}{sup +} emitters {sup 34}Ar and {sup 34}Cl

To contribute meaningfully to any test of the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, the measured ft value of a superallowed 0{sup +}{yields}0{sup +} {beta}{sup +} transition must be obtained to a precision of 0.1% or better. We have determined the half-life of the superallowed emitter {sup 34}Ar to be 843.8(4)ms; the quoted precision, 0.05%, is a factor of five improvement on the best previous measurement and meets this demanding requirement. Our measurement employed a high-efficiency gas counter, which was sensitive to positrons from both {sup 34}Ar and its daughter {sup 34}Cl. We achieved the required precision on {sup 34}Ar by analyzing the parent-daughter composite decay with a new fitting technique. We also obtained an improved half-life for {sup 34}Cl of 1.5268(5) s, which has 0.03% precision and is a factor of two improvement on previous results. As a by-product of these measurements, we determined the half-life of {sup 35}Ar to be 1.7754(11) s.

Precise ft‐value Measurement for the Superallowed 0+ → 0+ β Decay of 22Mg

Very accurate measurements of the half‐life, 3.8755(12) s, and the branching ratio, 0.5315(12), are reported for the superallowed 0+ → 0+ β‐decay of 22Mg. The precision in the branching ratio was achieved by the efficiency calibration of an HPGe detector to 0.15% precision for γ‐ray energies between 50 and 1400 keV. The ft‐value extracted from the measurement, corrected for small calculated radiative effects is Ft=3071(9) s. With this measurement, we open up a new series of superallowed emitters with Tz = −1 to precision measurements. This series will permit direct tests of the calculated nuclear‐structure‐dependent correction terms essential to the determination of the vector coupling constant, GV, and of Vud, the up‐down quark‐mixing element of the Cabibbo‐Kobayashi‐Moskawa (CKM) matrix. As the top row of the CKM matrix currently fails the unitarity test by more than two standard deviations, measurements to improve the precision with which Vud is known are of paramount importance.