Precise test of internal-conversion theory: αK measurements for transitions in nine nuclei spanning 45 ≤ Z ≤ 78.

We have been testing the theory used to calculate internal-conversion coefficients (ICCs) by making a series of measurements of αK values with precision better than ±2%. So far we have measured E3 transitions in three nuclei, 103Rh, 111Cd and 134Cs; and M4 transitions in six nuclei, 119Sn, 125Te, 127Te, 137Ba, 193Ir and 197Pt. Together, these span a wide range of A and Z values. In all cases, the results strongly favor Dirac-Fock calculations in which the final-state electron wave function has been computed in an atomic field that includes the vacancy created by the internal-conversion process.

ß decay of (38)Ca: sensitive test of isospin symmetry-breaking corrections from mirror superallowed 0+ → 0+ transitions.

We report the first branching-ratio measurement of the superallowed 0+→0+ß transition from Ca38. The result, 0.7728(16), leads to an ft value of 3062.3(68) s with a relative precision of ±0.2%. This makes possible a high-precision comparison of the ft values for the mirror superallowed transitions, Ca38→38mK and K38m→Ar38, which sensitively tests the isospin symmetry-breaking corrections required to extract Vud, the up-down quark-mixing element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, from superallowed ß decay. The result supports the corrections currently used and points the way to even tighter constraints on CKM unitarity.

Precise test of internal-conversion theory: transitions measured in five nuclei spanning 50≤Z≤78.

In a research program aimed at testing calculated internal-conversion coefficients (ICCs), we have made precise measurements of αK values for transitions in five nuclei, (197)Pt, (193)Ir, (137)Ba, (134)Cs and (119)Sn, which span a wide range of A and Z values. In all cases, the results strongly favor calculations in which the final-state electron wave function has been computed using a potential that includes the atomic vacancy created by the internal-conversion process.

FT values measured to ±0.1% for superallowed beta transitions: metrology at sub-second time scales.

Because of angular-momentum conservation, superallowed ß decay between 0(+) analog states involves only the vector part of the weak interaction, so its measured ft value can be used to determine the vector coupling constant, G(V). If many such transitions are measured, then the constancy of G(V) can be established and several important tests made on fundamentals of the electroweak Standard Model. We have developed apparatus that allows us to measure half-lives to ±0.03% and branching ratios to ±0.1% or better, for cyclotron-produced activities with half-lives as short as 100 ms. We present an overview of the equipment and a summary of more than 10 years of results.

Do radioactive half-lives vary with the Earth-to-Sun distance?

Recently, Jenkins, Fischbach and collaborators have claimed evidence that radionuclide half-lives vary systematically over a ±0.1% range as a function of the oscillating distance between the Earth and the Sun, based on multi-year activity measurements. We have avoided the time-dependent instabilities to which such measurements are susceptible by directly measuring the half-life of (198)Au (t(1/2)=2.695 d) on seven occasions spread out in time to cover the complete range of Earth-Sun distances. We observe no systematic oscillations in half-life and can set an upper limit on their amplitude of ±0.02%.

Experimental validation of the largest calculated isospin-symmetry-breaking effect in a superallowed Fermi decay.

A precision measurement of the γ yields following the ß decay of (32)Cl has determined its isobaric-analogue branch to be (22.47(-0.18)(+0.21))%. Since it is an almost pure-Fermi decay, we can also determine the amount of isospin-symmetry breaking in this superallowed transition. We find a very large value, δ(C) = 5.3(9)%, in agreement with a shell-model calculation. This result sets a benchmark for isospin-symmetry-breaking calculations and lends support for similarly calculated, yet smaller, corrections that are currently applied to 0+ → 0 + transitions for tests of the standard model.

Tests of nuclear half-lives as a function of the host medium and temperature: refutation of recent claims.

When radioactive isotopes that decay by alpha, beta(-), beta(+) and electron-capture transitions are placed in a conducting host material, it has recently been claimed that their half-lives show a significant dependence on temperature-up to a 6% difference between room temperature and 12K. We have refuted two of these claims by measuring: (1) the beta(-) decay of (198)Au in gold, for which we find the half life to be the same within 0.04% between room temperature and 19K; and (2) the electron-capture decay of (97)Ru in ruthenium, for which we find any half-life difference to be <0.1%.

QEC values of the superallowed beta emitters 34Cl and 38Km.

The superallowed beta-decay Q(EC) values of (34)Cl and (38)K(m) have been measured with an online Penning trap to be 5491.662(47) keV and 6044.223(41) keV, respectively. The new values are more precise than the previous high-precision reaction-based values but are consistent with them and establish that there are no significant systematic differences between the two types of measurements.

QEC values of the superallowed beta emitters 50Mn and 54Co.

Using a new fast cleaning procedure to prepare isomerically pure ion samples, we have measured the beta-decay Q(EC) values of the superallowed beta emitters 5(0)Mn and (54)Co to be 7634.48(7) and 8244.54(10) keV, respectively, results which differ significantly from the previously accepted values. The corrected Ft values derived from our results strongly support new isospin-symmetry-breaking corrections that lead to a higher value of the up-down quark mixing element V(ud) and improved confirmation of the unitarity of the Cabibbo-Kobayashi-Maskawa matrix.

Precise tests of internal-conversion theory.

Using the 165.9-keV M1 transition in (139)La as a calibrant, we have determined the K-shell internal conversion coefficients (ICCs) for the 127.5-keV E3 transition in (134)Cs and the 661.7-keV M4 transition in (137)Ba to 0.6% precision. Unlike previous less-precise measurements of these ICCs, which differed significantly from calculated values, our results agree well with calculations that include the atomic K-shell vacancy and disagree with those that exclude it. These results, which are consistent with what we also found for (193)Ir, confirm our earlier conclusions but for different energies, multipolarities and atomic numbers.

Test of internal-conversion theory with precise gamma- and X-ray spectroscopy.

Precise measurements of K-shell internal conversion coefficients (ICCs) are being used to determine the best method for calculating these coefficients. A recent result for the M4 transition from 193Irm has been refined, demonstrating conclusively that the atomic vacancy created by the ejected electron must be properly accounted for in the calculation of ICCs. Measurements of additional cases are discussed.

Q values of the superallowed beta emitters 26Alm, 42Sc, and 46V and their impact on Vud and the unitarity of the Cabibbo-Kobayashi-Maskawa matrix.

The beta-decay Q(EC) values of the superallowed beta emitters 26Alm, 42Sc, and 46V have been measured with a Penning trap to a relative precision of better than 8 x 10(-9). Our result for 46V, 7052.72(31) keV, confirms a recent measurement that differed from the previously accepted reaction-based Q(EC) value. However, our results for 26Alm and 42Sc, 4232.83(13) keV and 6426.13(21) keV, are consistent with previous reaction-based values. By eliminating the possibility of a systematic difference between the two techniques, this result demonstrates that no significant shift in the deduced value of Vud should be anticipated.

Q value of the superallowed decay of 46V and its influence on Vud and the unitarity of the Cabibbo-Kobayashi-Maskawa matrix.

The masses of the radioactive nuclei (46)V and its decay daughter (46)Ti have been measured with the Canadian Penning Trap on-line Penning trap mass spectrometer to a precision of 1 x 10(-8). A Q(EC) value of 7052.90(40) keV for the superallowed beta decay of (46)V is obtained from the difference of these two masses. With this precise Q value, the Ft value for this decay is determined with improved precision. An investigation of an earlier Q-value measurement for (46)V uncovers a set of 7 measurements that cannot be reconciled with modern data and affects previous evaluations of V(ud) from superallowed Fermi decays. A new evaluation, adding our new data and removing the discredited subset, yields new values for G(V) and V(ud). When combined with recent results for V(us), this yields modified constraints for the unitarity of the Cabibbo-Kobayashi-Maskawa matrix and other extensions of the standard model.

New limits on fundamental weak-interaction parameters from superallowed beta decay.

A new survey of all world data on superallowed beta decays provides demanding tests of, and tight constraints on, the weak interaction. In confirmation of the conserved vector current hypothesis, the vector coupling constant G(V) is demonstrated to be constant to better than three parts in 10(4), and any induced scalar current is limited to f(S)

Precise efficiency calibration of an HPGe detector up to 3.5 MeV, with measurements and Monte Carlo calculations.

Previously we used relative and absolute efficiency measurements combined with Monte Carlo calculations to define the efficiency of an HPGe gamma-ray detector with 0.2% accuracy from 50 to 1400 keV. This work has been extended to 4.8 MeV with measurements of relative efficiencies from 24Na, 56Co, and 66Ga sources. The combined results of experiment and calculation yield an efficiency curve up to 3.5 MeV with 0.4% accuracy. Single- and double-escape peak contributions also agree with calculation if positron annihilation-in-flight is incorporated.

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.

Superallowed beta decay of nuclei with a >or= 62: the limiting effect of weak Gamow-Teller branches.

The most precise value of V(ud), which is obtained from superallowed nuclear beta decay, leads to a violation of Cabibbo-Kobayashi-Maskawa unitarity by 2.2sigma. Experiments are underway on two continents to test and improve this result through decay studies of odd-odd N = Z nuclei with A>or=62. We show, in a series of illustrative shell-model calculations, that numerous weak Gamow-Teller branches are expected to compete with the superallowed branch in each of these nuclei. Though the total Gamow-Teller strength is significant, many of the individual branches will be unobservably weak. Thus, new techniques must be developed if reliable ft values are to be obtained with 0.1% precision for the superallowed branches.

Production of Co-60 sources for high-accuracy efficiency calibrations of gamma-ray spectrometers.

Activity determination by gamma-ray spectrometry is a valuable aid which is widely used where radioactive sources are applied. For the construction of an efficiency curve vs. energy in a standard geometry, the accuracy is limited not only by the uncertainty of peak area determination and the knowledge of the relevant emission probabilities but also by the uncertainties of the activity values applied to efficiency calibration with standard sources. Recently, a method to determine a relative efficiency function using selected radionuclides with gamma cascades of well known emission probabilities has been developed which yields very accurate efficiency values. Such a curve has to be converted into an absolute efficiency curve by calibration of the detector system with at least one source of very well known activity. Especially for this purpose, a set of Co-60 gamma-ray spectrometer sources was produced at the PTB. The activity values (10-45 kBq) were measured by the 4pi beta-gamma coincidence technique with a standard uncertainty of less than 0.1%. Production and measurement of these sources are described in the present paper. Special care was taken to include all contributions to the standard uncertainty of the activity value when establishing the uncertainty budget for the activity of each of these sources. The activity ratio of two of these sources, as obtained by the absolute measurements, was checked by independent gamma-ray counting at two other laboratories, where the precision achieved was less than 0.2%. The procedure described is considered to be a step towards more accurately determined efficiency functions in gamma-ray spectrometry.

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.

Precise half-life measurement for the superallowed 0(+)-->0(+) beta emitter (74)Rb: first results from the new radioactive beam facility (ISAC) at TRIUMF.

Presently, the world data for superallowed beta decay leads to a result in disagreement (at the 98% confidence level) with the predictions of the minimal standard model for the unitarity of the Cabibbo-Kobayashi-Maskawa matrix. Precise data for the superallowed 0(+)-->0(+) beta decay of (74)Rb would provide a critical test of the nucleus-dependent isospin symmetry-breaking corrections that must be calculated for these superallowed Fermi beta decays. The present work reports the first precise measurement of the half-life for (74)Rb ( t(1/2) = 64.761+/-0.031 ms). The data were obtained at the radioactive beam facility (ISAC) at TRIUMF using a beam of approximately 4000 (74)Rb ions s(-1).