Search

The predominant branch in the $\ensuremath{\beta}$ decay of $^{34}\mathrm{Ar}$ is the superallowed ${0}^{+}\ensuremath{\rightarrow}\phantom{\rule{0.16em}{0ex}}{0}^{+}$ transition to the ground state of $^{34}\mathrm{Cl}$. To determine its important branching ratio one must first establish the ratios for the competing Gamow-Teller branches based on the measured intensities of $\ensuremath{\gamma}$ rays subsequently emitted from the excited states they populate in $^{34}\mathrm{Cl}$. The strongest of these branches populates the ${1}^{+}$ state at 666 keV in $^{34}\mathrm{Cl}$, which has three possible $\ensuremath{\gamma}$-decay paths. We report here a measurement of the decay of this state, which we populated via resonant proton capture in the reaction $^{33}\mathrm{S}$($p,\ensuremath{\gamma}$)$^{34}\mathrm{Cl}$. We find that the intensity of the 519-keV $\ensuremath{\gamma}$-ray path is 1.46(19)% relative to that of the 666-keV path. This result is critical to new precise measurements of the superallowed decay of $^{34}\mathrm{Ar}$.

Superallowed $\ensuremath{\beta}$-decay between nuclear analog states with isospin 1 and spin-parity ${0}^{+}$ offers access to fundamental tests of the properties of the electroweak interaction. With 15 such transitions precisely measured, the data collectively probe the conservation of the weak vector current, set tight limits on the presence of beyond-standard-model scalar currents, and yield the most precise value for the ${V}_{u\phantom{\rule{0}{0ex}}d}$ element of the CKM quark-mixing matrix. ${V}_{u\phantom{\rule{0}{0ex}}d}$ is a critical ingredient required to test the unitarity of that matrix. This work provides an in-depth review of the relevant nuclear physics input and connects these data to the fundamental couplings.

The 30.8 keV ${(1/2)}^{\ensuremath{-}}$ isomeric state in $_{41}^{93}\mathrm{Nb}$ decays by an $M4$ transition to the ${(9/2)}^{+}$ ground state. We have measured the $K$-shell internal conversion coefficient ${\ensuremath{\alpha}}_{K}$ for this transition in order to test the validity of current methods of calculation for a case with relatively low atomic number, low transition energy, and a high value of ${\ensuremath{\alpha}}_{K}$. Taking the fluorescence yield of niobium to be 0.751(4), we obtain ${\ensuremath{\alpha}}_{K}=2.56(9)\ifmmode\times\else\texttimes\fi{}{10}^{4}$, a result that agrees with the Dirac-Fock calculations that take into account the presence of a $K$ vacancy in the final state, and disagrees by 1.7 standard deviations with calculations that ignore the vacancy. We also determine the energy of the isomeric state to be 30.760(5) keV, a fourfold improvement in precision over the currently accepted value.

We have measured the branching ratio for the superallowed ${0}^{+}\ensuremath{\rightarrow}$ ${0}^{+}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$ transition from $^{34}\mathrm{Ar}$ to be 0.9448(8), and determined its $ft$ value to be 3058.1(28) s, a result with $\ifmmode\pm\else\textpm\fi{}0.09$% precision, which is a factor of 3 improvement over the previous result based on current world data. The $ft$-value ratio for the mirror pair of superallowed transitions $^{34}\mathrm{Ar}$ $\ensuremath{\rightarrow}^{34}\mathrm{Cl}$ and $^{34}\mathrm{Cl}$ $\ensuremath{\rightarrow}^{34}\mathrm{S}$ becomes the most precise yet measured and, in a sensitive test of the method used to calculate the isospin-symmetry-breaking correction, ${\ensuremath{\delta}}_{C}$, it agrees well with the ratio as calculated with Woods-Saxon radial wave functions. This confirms the method used in the most recent survey of superallowed decays to extract ${V}_{\text{ud}}$, the up-down quark-mixing element of the Cabibbo-Kobayashi-Maskawa matrix. In addition, our branching-ratio results for the four observed Gamow-Teller branches to ${1}^{+}$ states in $^{34}\mathrm{Cl}$ are shown to agree well with shell-model calculations based on the same effective interactions that were used in the determination of ${\ensuremath{\delta}}_{C}$.

We have measured the half-life of $^{34}\mathrm{Ar}$, the parent of a superallowed ${0}^{+}\ensuremath{\rightarrow}{0}^{+}$ $\ensuremath{\beta}$ transition, to be 0.84646(35) s. With a precision of 0.04%, this result is an essential ingredient needed to complete a third pair of mirror superallowed transitions, $^{34}\mathrm{Ar}\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}^{34}\mathrm{Cl}$ and $^{34}\mathrm{Cl}\phantom{\rule{4pt}{0ex}}\ensuremath{\rightarrow}\phantom{\rule{4pt}{0ex}}^{34}\mathrm{S}$, the ratio of whose $ft$ values would provide the most sensitive test yet of the isospin-symmetry-breaking corrections used to obtain ${V}_{ud}$. In a sequence repeated thousands of times, we implanted a very pure $^{34}\mathrm{Ar}$ beam into tape for 0.7 s, and then rapidly transported the collected source to a shielded $4\ensuremath{\pi}$ proportional gas counter, which detected the decay positrons from the combined decays of $^{34}\mathrm{Ar}$ and its daughter $^{34}\mathrm{Cl}$. The data were analyzed as a linked parent-daughter pair. Our new result replaces one we published 13 years ago.

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.

For many fission products, the γ rays emitted following β decay provide an easily-detectable signature that can be used to identify their quantities and distributions in a sample. As a result, γ -ray spectroscopy is often exploited to study fission-product yields, provided sufficiently accurate information on the γ -ray intensity is available. However, in many cases, the uncertainties in the existing nuclear data are large enough that they compromise the precision achievable for modern experiments and applications. To address this need, we have developed a new experimental method that is well suited to precisely measure absolute γ -ray intensities in the β decay of long-lived fission products. The approach involves the production of a radiopure sample by implantation of a mass-separated ion beam from the CAlifornium Rare Isotope Breeder Upgrade (CARIBU) facility on a thin carbon foil. The emitted β -decay radiation is detected with a 4 π gas proportional counter and a meticulously efficiency-calibrated high-purity germanium (HPGe) detector. As a first measurement to demonstrate the approach, we studied the absolute γ -ray intensities of the strongest transitions following the β decay of 95 Zr and its decay-daughter 95 Nb, and determined them to fractional precisions of better than 1–2%. In addition, with a larger sample of activity produced through neutron irradiation of an isotopically-enriched Zr foil, we performed a high-precision measurement of the relative γ -ray intensities following the decay of 95 Zr with just the HPGe detector. The sample-production method at CARIBU and the coincidence detection approach demonstrated here can be applied to study fission products with half-lives longer than a day, which includes isotopes important not only for nuclear-energy and national-security applications, but also for medical-isotope research and environmental monitoring.

We present a method of analysis of nuclear decay spectra affected by extending dead-time, in which a non-trivial, but relatively simple correction for dead-time losses is applied to produce an estimate of the spectrum in the absence of dead-time, with negligible bias and negligible deviation from the original Poissonian statistics. The method requires the measurement of arrival times of all observed decay events, but it does not require any knowledge about the dead-time or the nuclear decay. The parameters of nuclear decay can be determined in a subsequent straight-forward analysis of the estimated dead-time free spectra, individually or in combinations. In a typical realistic situation this approach is expected to contribute 0.001% or less to the systematic error of nuclear half-life or activity. Compared to the exact method of analysis presented in our earlier publication, the present method is much easier to implement and produces the results significantly faster.

Since the first cyclotron beam fifty years ago, the Texas A&M Cyclotron Institute has functioned as a university-based laboratory. Over the past two decades it has been funded by the U.S. Department of Energy, the State of Texas, the Welch Foundation, and commercial sale of beam-time. The first accelerator is a conventional isochronous cyclotron modeled after the 88″ cyclotron at the Lawrence Berkeley Laboratory. It arose from a proposal funded by the U.S. Atomic Energy Commission, the R. A. Welch Foundation (a private foundation headquartered in Houston), and the State of Texas and continued to operate from 1967 to 1987. After recommissioning in 2007 as a higher-field K150 machine, it is still in operation. The K500 cyclotron, funded in 1980 by the Welch Foundation and Texas A&M University and modeled after the Michigan State NSCL K500 cyclotron, is a superconducting isochronous cyclotron whose first beam was extracted in 1988.

Superallowed $\ensuremath{\beta}$ decay is an important tool in characterizing the vector component of the weak interaction. Precise $\ensuremath{\beta}$-decay results presently give the best measurement of the up-down quark weak-mixing element, ${V}_{u\phantom{\rule{0}{0ex}}d}$. This new result completes the measurement of a second set of superallowed mirror transitions (${}^{26}$Si-${}^{26m}$Al and ${}^{26m}$Al-${}^{26}$Mg in this case) and has the precision to constrain the theoretical isospin-symmetry-breaking corrections needed in the extraction of ${V}_{u\phantom{\rule{0}{0ex}}d}$.

The ${\ensuremath{\beta}}^{+}$ decay of $^{21}\mathrm{Na}$ to $^{21}\mathrm{Ne}$ is one of the four cases currently being used to determine the value of ${V}_{\mathrm{ud}}$ from nuclear mirror ($T=1/2$) transitions. To date, published values for the half-life of $^{21}\mathrm{Na}$ have not been consistent with one another. We report here a new measurement of the half-life: $22.4615\ifmmode\pm\else\textpm\fi{}{0.0039}_{(\mathrm{stat})}\ifmmode\pm\else\textpm\fi{}{{}_{0.0015}^{0.0001}}_{(\mathrm{syst})}s$. This result increases the world average of the $^{21}\mathrm{Na}$ half-life by 0.0048 s and reduces the uncertainty by a factor of 1.5.

We have measured the half-life of 30S, the parent of a superallowed 0+-to-0+ beta transition, to high precision using very pure sources and a 4pi proportional gas counter to detect the decay positrons. Our result for the half-life is 1.17992(34) s. As a byproduct of this measurement, we determined the half-life of its daughter, 30P, to be 2.501(2) min., Comment: 9 pages, 7 figures

We have measured the $K$-shell and total internal conversion coefficients (ICCs), ${\ensuremath{\alpha}}_{K}$ and ${\ensuremath{\alpha}}_{T}$, for the 109.3-keV $M4$ transition in $^{125}\mathrm{Te}$ to be 185.0(40) and 350.0(38), respectively. Previously this transition's ICCs were considered to be anomalous, with measured values lying below calculated ones. When compared with Dirac-Fock calculations, our new results show good agreement. The ${\ensuremath{\alpha}}_{K}$ result agrees well with the version of the theory that takes account of the $K$-shell atomic vacancy and disagrees with the one that does not. This is consistent with our conclusion drawn from a series of measurements on high multipolarity transitions.

We have measured the $K$-shell internal conversion coefficient, ${\ensuremath{\alpha}}_{K}$, for the 88.2-keV $M4$ transition from the 106-day isomer to the ground state in $^{127}\mathrm{Te}$ to be 484(6). When compared with Dirac-Fock calculations of ${\ensuremath{\alpha}}_{K}$, this result agrees well with the version of the theory that incorporates the effect of the $K$-shell atomic vacancy and disagrees with the one that does not. As a byproduct of this measurement, we have determined the $\ensuremath{\beta}$ branching from the isomer to be 2.14(3)%.

Precise measurements of superallowed ${0}^{+}\ensuremath{\rightarrow}{0}^{+}$ $\ensuremath{\beta}$ decay presently provide the most precise value for the weak mixing amplitude ${V}_{u\phantom{\rule{0}{0ex}}d}$. As the largest element of the CKM matrix, ${V}_{u\phantom{\rule{0}{0ex}}d}$ is a critical piece of the Standard Model of the electroweak interaction. The new, precise Penning-trap mass measurement of the decay energy for the superallowed transition in ${}^{42}$Sc opens the door for a much more precise $f\phantom{\rule{0}{0ex}}t$ value determination if its half-life can be measured more precisely as well.

We have measured the K -shell and total internal conversion coefficients for the 150.8-keV E 3 transition in 111 Cd: αK = 1.449(18) and αT = 2.217(26) respectively. The αK value agrees well with Dirac-Fock calculations, in which the effect of the K -shell atomic vacancy is included. It is consistent with our previous precise measurements of αK values, which cover a range of atomic numbers, and extends that range down to Z = 48. The αT measurement, however, disagrees by about two standard deviations from the calculated αT value, whether the atomic vacancy is included or not.

Because of angular-momentum conservation, superallowed β 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, GV. If many such transitions are measured, then the constancy of GV 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.

A digital β - counting method has been developed for high-precision nuclear β - decay half-life experiments that use a gas proportional counter. An 8-bit, 1-GS/s sampling-rate digitizer was used to record the waveforms from the detector and a software filter was designed, tested and applied successfully to discriminate genuine β - decay events from spurious signals by pulse-shape analysis. The method of using a high-speed digitizer for precision β counting is described in detail. We have extensively tested the digitizer and the off-line filter by analyzing saved waveforms from the decay of Al m 26 acquired at rates up to 10,000 per second. The half-life we obtain for Al m 26 is 6345.30±0.90 ms, which agrees well with previous published measurements and is as precise as the best of them. This work demonstrates the feasibility of applying a high-speed digitizer and off-line digital signal processing techniques for high-precision nuclear β - decay half-life measurements.

The Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing matrix is a central pillar of the Electroweak Standard Model. The elements of the matrix must be determined from experiment, but the Model requires the matrix itself to be unitary. Any deviation from unitarity would signal the presence of “new physics” beyond the Standard Model, so tests of CKM unitarity have attracted considerable attention. Currently the most precise test is of the normalization of the top row, which has now reached a precision of 0.06% based on measurements of superallowed 0+ nuclear β decay and of kaon semileptonic and leptonic decays. This work overviews the status of the normalization tests and speculates on likely future improvements.

A time-interval method of beta-decay half-life analysis is described. The method is expected to produce results that are highly accurate (for the given number of events analyzed), regardless of the event rate, nature of the detection-system dead time, and/or extent of the dead time. This was verified by applying the method in a systematic way to an array of simulated data sets characterized by typical as well as extreme combinations of simulation parameters. The results of the analysis are presented and discussed.

We have measured the $K$-shell and total internal-conversion coefficients, ${\ensuremath{\alpha}}_{K}$ and ${\ensuremath{\alpha}}_{T}$, for the 150.8-keV $E3$ transition in $^{111}\text{Cd}$ to be 1.449(18) and 2.217(26), respectively. The ${\ensuremath{\alpha}}_{K}$ result agrees well with Dirac-Fock calculations in which the effect of the $K$-shell atomic vacancy is accounted for; it extends our precision tests of ${\ensuremath{\alpha}}_{K}$ calculations to $Z=48$, the lowest $Z$ yet measured. However, the result for ${\ensuremath{\alpha}}_{T}$ disagrees by about two standard deviations from the calculated ${\ensuremath{\alpha}}_{T}$ value, whether or not the atomic vacancy is included.

Currently, the most restrictive test of the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix is anchored by nuclear beta decay. Precise measurements of the ft -values for superallowed beta transitions between analog 0 + states are used to determine G V , the vector coupling constant; this, in turn, yields V u d , the up-down element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. When the value for V u d obtained from the latest survey of world data is combined with V u s and V u b , the other top-row elements, it leads to the most demanding test available of the unitarity of that matrix. Important ingredients required in the extraction of V u d from experiment are calculated radiative corrections as well as corrections to account for the isospin symmetry breaking that occurs between the analog parent and daughter states of each superallowed transition. The latter are well supported by comparison with experiment. The current value for the unitarity sum is 0.99995(61), a result that is in remarkable agreement with Standard Model expectations.

Precise beta-branching-ratio measurements are required in order to determine ft-values as a part of our program to test the Electroweak Standard Model via unitarity of the Cabibbo-Kobayashi-Moskawa matrix. For the measurements to be useful in this test, their precision must be close to 0.1 %. In a branching-ratio measurement, we position the radioactive sample between a thin plastic scintillator used to detect beta-particles, and a HPGe detector for gamma-rays. Both beta singles and beta-gamma coincidences are recorded. Although the branching ratio depends most strongly on the HPGe detector efficiency, it has some sensitivity to the energy dependence of the beta-detector efficiency. We report here on a study of our beta-detector response function, which used Monte Carlo calculations performed by the Geant4 toolkit. Results of the simulations are compared to measured beta-spectra from several standard beta-sources., 9 pages, 8 figures; preprint submited to Nucl. Instr. and Meth. A, May 2008

Using the 165.9-keV M 1 transition in 139 La as a calibrant, we have determined the K -shell internal conversion coefficients (ICCs) for the 127.5-keV E 3 transition in 134 Cs and the 661.7-keV M 4 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.

Prompted by recent measurements, we surveyed world data and calculated radiative and isospin-symmetry-breaking corrections for the superallowed ${0}^{+}\ensuremath{\rightarrow}{0}^{+}$ Fermi transitions from $^{42}\mathrm{Ti},^{46}\mathrm{Cr},^{50}\mathrm{Fe}$, and $^{54}\mathrm{Ni}$. This increases the number of such transition with a complete set of calculated corrections from 20 to 23. The results are compared with their equivalents for the mirror superallowed transitions from $^{42}\mathrm{Sc},^{46}\mathrm{V},^{50}\mathrm{Mn}$, and $^{54}\mathrm{Co}$. The predicted $ft$-value asymmetries of these mirror pairs are sensitive to the correction terms and provide motivation for improving measurement precision so as to be able to test the corrections. To aid in that endeavor, we present a parametrization for calculating the $f$ values for the new transitions to $\ifmmode\pm\else\textpm\fi{}0.01%$.

We present the full description of a measurement of the branching ratios for the $\ensuremath{\beta}$ decay of $^{38}\mathrm{Ca}$. This decay includes five allowed ${0}^{+}\ensuremath{\rightarrow}{1}^{+}$ branches and a superallowed ${0}^{+}\ensuremath{\rightarrow}{0}^{+}$ one. With our new result for the latter, we determine its $ft$ value to be $3062.3\ifmmode\pm\else\textpm\fi{}6.8$ s, a result whose precision (0.2%) is comparable to the precision of the 13 well-known ${0}^{+}\ensuremath{\rightarrow}{0}^{+}$ transitions used up until now for the determination of ${V}_{\text{ud}}$, the up-down quark-mixing element of the Cabibbo-Kobayashi-Maskawa matrix. The $^{38}\mathrm{Ca}$ superallowed transition thus becomes the first addition to this set of transitions in nearly a decade and the first for which a precise mirror comparison is possible, thus enabling an improved test of the isospin-symmetry-breaking corrections required for the extraction of ${V}_{\text{ud}}$.

Precise measurements of the $\ensuremath{\beta}$ decay between nuclear states of spin-parity ${0}^{+}$ and isospin 1 provide fundamental tests of the properties of the electroweak interaction. Collectively, these transitions sensitively probe the conservation of the vector weak current, set tight limits on the presence of scalar currents, and provide the most precise value for ${V}_{u\phantom{\rule{0}{0ex}}d}$. The latter result has become a linchpin in the most demanding test of the unitarity of the CKM matrix.

We present a parameterization of the statistical rate function, f, for 20 superallowed 0+-to-0+ nuclear beta transitions between T=1 analog states, and for 18 superallowed "mirror" transitions between analog T=1/2 states. All these transitions are of interest in the determination of V_{ud}. Although most of the transition Q_{EC} values have been measured, their precision will undoubtedly be improved in future. Our parameterization allows a user to easily calculate the corresponding new f value to high precision (+/-0.01%) without complicated computing., Comment: 5 pages

Currently, the most restrictive test of the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) ma- trix is anchored by nuclear beta decay. Precise measurements of the ft -values for superallowed beta transitions between analog 0 + states are used to determine GV, the vector coupling constant; this, in turn, yields Vud ,t he up-down quark-mixing element of the CKM matrix. The determination of a transition's ft -value requires the measurement of three quantities: its Q value, branching ratio and parent half-life. To achieve 0.1% precision on the final result, each of these quantities must be measured to substantially better precision, for which spe- cial techniques have had to be developed. A new survey and analysis of world data reveals that there are now fourteen such transitions with ft -values known to ∼ 0.1% precision or better, and that they span a wide range of nuclear masses, from 10 C, the lightest parent, to 74 Rb, the heaviest. Of particular interest is the recent com- pletion of the first mirror pair of 0 + → 0 + transitions, 38 Ca → 38m Ka nd 38m K → 38 Ar, which provides a valuable constraint on the calculated isospin-symmetry-breaking corrections needed to derive GV from the experimental data. As anticipated by the Conserved Vector Current hypothesis, CVC, all fourteen transitions yield consistent values for GV .T he value ofVud derived from their average makes it by far the most precisely known element of the CKM matrix, which, when combined with the other top-row elements, Vus and Vub, leads to the most demanding test available of the unitarity of that matrix. Since CKM unitarity is a key pillar of the Electroweak Standard Model, this test is of fundamental significance.

Central to the Electroweak Standard Model is the Cabibbo-Kobayashi-Maskawa (CKM) matrix, a unitary 3 × 3 matrix that relates the weak- to the mass-eigenstates of the three generations of quarks. The Standard Model itself does not prescribe the values of the individual matrix elements—they must each be determined experimentally—but it does insist that the full matrix must satisfy all conditions of unitarity. Thus, any conclusive experimental demonstration that the unitarity conditions are not met would prove the need for “new physics” extending beyond the current Standard Model. Conversely, if the experiment were instead to confirm unitarity, this result would circumscribe the scope of any possible new physics to an extent limited only by the quoted experimental uncertainties on the unitarity sum. Either way, there is compelling motivation here for precision measurements of the CKM matrix elements aimed at the most demanding tests possible of the matrix's unitarity.

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.

A recent Penning-trap measurement of the masses of V 46 and Ti 46 leads to a Q EC value that disagrees significantly with the previously accepted value, and destroys overall consistency among the nine most precisely characterized T = 1 superallowed β emitters. This raises the possibility of a systematic discrepancy between Penning-trap measurements and the reaction-based measurements upon which the Q EC values depended in the past. We carefully re-analyze ( n , γ ) and ( p , γ ) reaction measurements in the 24 ≤ A ≤ 28 mass region, and compare the results to very precise Penning-trap measurements of the stable nuclei Mg 24 , Mg 26 and Si 28 . We thus determine upper limits to possible systematic effects in the reaction results, and go on to establish limits for the mass of radioactive Al 26 , to which future on-line Penning-trap measurements can be compared. We stress the urgency of identifying or ruling out possible systematic effects.

The ft values for superallowed 0+ → 0+ nuclear β transitions are nearly independent of nuclear-structure ambiguities and depend uniquely on the vector part of the weak interaction. They thus provide access to clean tests of some of the fundamental precepts of weak-interaction theory and can even probe physics beyond the standard model. We have just completed a new survey and overview of world data on such transitions, including not just the nine cases considered in the past, but also eleven more from T = 1 parents: even-even Tz = - 1 nuclei from 18Ne to 42Ti; and odd-odd nuclei from 62Ga to 74Rb. These new cases all involve more exotic nuclei and, to yield comparable precision, present real experimental challenges. Nevertheless, three of these new cases are already known well enough to contribute - together with the nine well known transitions - to the setting of limits on fundamental weak-interaction parameters. The remaining eight cases show promise for making important contributions in future.

The study of superallowed nuclear β -decay currently provides the most precise and convincing confirmation of the conservation of the vector current (CVC) and is a key component of the most demanding available test of the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, a basic pillar of the Electroweak Standard Model. The status of these fundamental studies is summarized and recent work described.

The processes responsible for the creation of elements more massive than iron are not well understood. Possible production mechanisms involve the rapid capture of protons (rp-process) or the rapid capture of neutrons (r-process), which are thought to occur in explosive astrophysical events such as novae, x-ray bursts, and supernovae. Mass measurements of the nuclides involved with uncertainties on the order of 100 keV or better are critical to determine the process ‘paths’, the energy output of the events, and the resulting nuclide abundances. Particularly important are the masses of ‘waiting-point’ nuclides along the rp-process path where the process stalls until the subsequent β decay of the nuclides. This paper will discuss the precise mass measurements made of isotopes along the rp-process and r-process paths using the Canadian Penning Trap mass spectrometer, including the mass of the critical waiting-point nuclide 68 Se.

In the Standard Model of elementary particles, quark-mixing is expressed in terms of a 3 x 3 unitary matrix V, the so called Cabibbo-Kobayashi-Maskawa (CKM) matrix. Significant unitarity checks are so far possible for the first row of this matrix. This article reviews the experimental and theoretical information on these matrix elements. On the experimental side, we find a 2.2 $\sigma$ to 2.7 $\sigma$ deviation from unitarity, which conflicts with the Standard Model.

Precise measured data and an extensive set of Monte Carlo calculations have been combined to optimize the parameters for a 280 cm3 n-type Ge semiconductor photon detector, with the outcome that the Monte Carlo calculations now provide a very precise detector efficiency curve. The detector position and its length were determined from a set of measurements that included an axial scan. Measurements of the relative efficiencies based on radionuclides with accurately known relative photon emission rates were combined with efficiencies calculated by a Monte Carlo photon and electron transport code to adjust the detector diameter, internal deadlayer thickness, and the effective external deadlayer thickness. Our results show that, in a well-studied situation, it is possible to use Monte Carlo calculations to aid in the determination of a Ge detector's efficiency with an accuracy of about 0.2% from 50 to 1400 keV.

The Canadian Penning Trap (CPT) mass spectrometer is designed to make precise mass measurements on a variety of stable and short-lived isotopes. Modifications to the injection system of the CPT have been implemented in recent months, the purpose being to more efficiently collect and transfer weakly-produced reaction products from the target to the Penning trap. These include a magnetic triplet situated after the target chamber to increase the acceptance of the Enge spectrograph, a velocity filter to more effectively separate the beam from the reaction products and the replacement of the Paul trap with a linear trap resulting in more efficient capture and accumulation of ions from the ion cooler. This paper will discuss these recent modifications and how they have increased our ability in making mass measurements on isotopes of low abundance, including those from a 252 Cf fission source.

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 198Au (t1/2=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%.

Precise measurements of the intensities of superallowed Fermi 0+ → 0+β-decays have provided a powerful test of the CVC hypothesis at the level of 3×10-4 and also led to a result in disagreement with unitarity for the CKM matrix at the 98% confidence level. It is essential to address possible trivial explanations for the apparent non-unitarity such as uncertainties in the isospin symmetry-breaking corrections. We have carefully studied the 74Rb →74Kr β-decay in order to measure the non-analog β-decay branching to the 0+ state at 508 keV in 74Kr. We have determined that this branching is < 3×10-4, far smaller than any published theoretical estimate. We also show that high-precision, complete spectroscopy, measuring the major β-branches to excited 0+ and 1+ states, must be performed if one is to obtain a meaningful branching ratio to the excited 0+ state and concomitantly deal, in a substantial way, with the possibility of β feeding to an array of 1+ states.

The combination of the K500 superconducting cyclotron and the recoil spectrometer MARS have been used to generate secondary radioactive ion beams ranging from 7Be at 12 MeV/u to 62Ga at 37 MeV/u. Secondary beams are produced via transfer reactions in inverse kinematics using stable beams from the cyclotron on gas targets. The radioactive beams have been used for reaction measurements and β-decay studies.

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.

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.

Potassium-37 is one of several isotopes that could be used to directly test the standard model weak interaction.