(last update: september 2012)
Half-life and branching-ratio for the superallowed 0+ —> 0+ + emitter 42Ti
Date: summer 2008
contact@CENBG: T. Kurtukian-Nieto
The half-life and the branching ratio of the superallowed emitter 42Ti were measured in an experiment performed at the JYFLTRAP facility of the Accelerator Laboratory of the University of Jyväskylä. 42Ti is the heaviest TZ=−1 nucleus for which high-precision measurements of these quantities have been tried. The half-life (T1/2 = 208.14 0.45 ms) is close to the required precision of about 0.1%. The branching ratio for the superallowed decay branch [(BR = 47.7 (12) %], a by-product of the half-life measurement, does not reach the necessary precision yet. Nonetheless, these results allow one to determine the experimental ft value and the corrected Ft value to be 3114 (79) and 3122 (79) s, respectively.
The study of superallowed nuclear decays provides valuable information for testing the standard model of particle physics. The superallowed 0+ —> 0+ decay between T = 1 analog states depends uniquely on the vector part of the weak interaction and, according to the conserved vector current (CVC) hypothesis, its experimental ft value is related to the vector coupling constant gV, a fundamental constant that is the same for all such transitions (see Ref.  and references therein for a review). Consequently, the experimental study of these superallowed transitions serves as a sensitive probe of the conservation of the weak vector current and allows tight limits to be set on the presence of scalar or right-handed currents. Once the CVC hypothesis is verified, these data, together with the decay properties of the muon, allow for the determination of the Vud matrix element of the Cabbibo-Kobayashi-Maskawa quark-mixing matrix.
The aim of the present piece of work is to measure the half-life of 42Ti with a precision close to or better than 0.1 %. In addition, the branching ratio for the superallowed decay is measured with less precision. 42Ti decays by superallowed β+ emission to its isobaric analog state (J = 0+, T = 1), the ground state of 42Sc. Before the measurement reported here, the accepted value for the half-life of 42Ti was 199 6 ms. This value is the weighted average of three measurements, 173 14 ms , 202 5 ms , and 200 20 ms , yielding a precision of the half-life for this nucleus of 3%. In this work, we present a new 42Ti half-life that is 15 times more precise than the previously accepted value for this nucleus, whereas the branching ratio determined in the present work is 10 times more precise than the previously accepted value .
The experiment was performed at the Ion Guide Isotope Separator On-Line (IGISOL)  facility at the Accelerator Laboratory of the University of Jyväskylä. 42Ti was produced by means of a fusion-evaporation reaction of a 3He beam at 17 MeV impinging on a 1.5 mg/cm2 thick natCa target. The reaction products were thermalized in the helium gas of the IGISOL target chamber, extracted by means of a helium gas flow and a sextupole ion guide (SPIG), accelerated to 30 keV, and A/q = 42 selected by the IGISOL magnet. The JYFLTRAP facility—consisting of a radio frequency cooler-buncher (RFQ)  and two Penning traps —was used to prepare pure samples of 42Ti.
The experimental setup used for the half-life measurement was similar to the one used in our recently reported measurement of the half-life of 26Si . Figure 1 shows a picture of the experimental setup used in 2008.
The measurements were structured in cycles of 2600 ms. The ion bunches accumulated in the RFQ for 300 ms were injected into the purification Penning trap and isobarically cleaned for 100 ms. Then, the bunches were ejected from the purification trap through the precision trap to the decay spectroscopy setup, and implanted on a moving Mylar tape, placed at the end of the JYFLTRAP extraction beam line (see Figure 1). The collection spot was located at the center of a 2 mm thick, 4π cylindrical plastic scintillator, which was used to detect the positrons emitted by the implanted nuclei. The plastic scintillator had an entrance hole of 12 mm in diameter and an entrance hole for the tape on the bottom of the cylinder, yielding a beta-particle detection efficiency of 90%. The scintillation light was collected by two photomultiplier tubes (HAMAMATSU H1949-50 and R1828-01), which viewed a special light guide connected to the scintillator. The two photomultipliers were run in coincidence to eliminate most of the noise of the individual tubes. To provide beta-gamma coincidence data for measuring the superallowed branching ratio, and for a further control of the background, three coaxial germanium detectors were placed around the plastic scintillator detector at −90o, 0o, and +90o, with respect to the extraction beam line. These germanium detectors are referred to as Ge1, Ge2, and Ge3, respectively, throughout the text. They were mounted at a distance of about 15 cm from the collection point and had an intrinsic efficiency of 60%.
The decay window was triggered by the extraction signal and the decay measurements lasted 2 s (≈10 half-lives). During the measuring time, the cyclotron beam was turned off, which yielded a well-defined constant background on the decay spectrum. After the decay measurement, the tape was moved for a period of 200 ms (about 10 cm) and a new cycle started. These cycles were repeated until the desired statistics were achieved. Cycles with different experimental conditions like trigger thresholds or detector high voltages formed different runs.
To test systematic effects linked to the data acquisition system (DAQ), three independent systems were used, each one of them running with different parameters and data recording modes: DAQ1 and DAQ2 were running in a cycle-by-cycle mode and had a predefined non-extendible and fixed dead time per event of 2 and 8 μs, respectively, and DAQ3 was running in an event-by-event mode, with a dead time of 100 μs. The dead times were chosen to be much longer than the series of dead times in the event treatment by the electronics or the data processing. DAQ1 and DAQ2 only registered the time difference between the extraction signal from the trap and the beta trigger, being simple but fast DAQ systems. DAQ3 registered the time differences described above but also the energy signals from the germanium detectors in coincidence with the master trigger signal, given by the coincident beta signal from the two photomultipliers. Even though the three DAQs were fed by the same data, the dead-time distortion of the underlying Poisson distributed data were different in the three cases, enabling us to test the influence of the actual dead time of the DAQs on the final dead-time-corrected half-life result.
A total of 184 689 cycles divided into 80 experimental runs were collected during the experiment for each DAQ. In total, about 1.6 × 106 42Ti decays have been detected. Figure 2 shows a typical dead-time corrected decay curve from a single run with the decomposed individual contributions from 42Ti, 42Sc, and the background. The half-life determined for this run is 208.79 3.22 ms.
The half-lives obtained from the fits and the normalized 2 for each run and DAQ are shown in Figure 3. As can be seen, the reduced 2 for the fit of each run is always below 1.1 for the three data sets. We obtain an error-weighted average half-life over all runs of 208.14 0.45 ms for DAQ1, 208.12 0.45 ms for DAQ2, and 208.15 0.45 ms for DAQ3. Because the data sets are not independent measurements, the final value of the half-life is given by the mean of the three data sets. The resulting experimental half-life value for the 42Ti ground state is T1/2 = 208.14 0.45 ms.
For details on the analysis and systematic errors see Ref. .
The branching ratio for the 0+ —> 0+ transition was already measured in the past with a precision of about 30% . Our experimental setup and in particular the cycle times were not optimized for the gamma-ray measurement. Nevertheless, we are able to improve by a factor of 10 the precision of the branching ratio for the superallowed decay. Due to the gamma-ray statistics we obtained, only the most intense gamma ray at 611 keV was observable (see Figure 4). The absolute branching ratio for the 0+ —> 0+ transition is then obtained by means of the relative intensities of the other γ transitions compared with the 611 keV transition taken from Ref. .
With the 611 keV efficiency determined with the corrected efficiency curve, the corrected number of counts for the 611 keV γ ray in the 42Ti spectra, and the number of 42Ti implanted in the tape as obtained from the fit of the decay time curve, we can calculate the 611 keV branching ratio. We obtained values of 51.2 (15) % and 51.0 (17) % for detectors 1 and 2. We noticed that the branching ratio of the third detector was not consistent with the branching ratios of the other two detectors. For a reason unknown at present, the efficiency of this detector decreased by about 20% during the course of the experiment. This was evident when plotting, for all runs, the ratio of the number of counts in the 511 keV peak for different pairs of detectors. As a consequence, the third detector was not taken into account for the present results.
Using the relative intensity of gamma rays in the beta-decay daughter, total/611 = 1.023 (12) from Ref. , we can determine the branching ratio, BR, for the 0+ —> 0+ transition. For detector Ge1, we obtained BR = 47.6(16) %, whereas the value for Ge2 is BR = 47.8 (19)%. This yields a final average value of BR = 47.7 (12) % for the superallowed decay branch. This value compares well with the average value from the literature of BR = 43 (14) % .
We have obtained the most precise value yet for the 42Ti half-life of 208.14 0.45 ms. This result, with a 0.2% precision, is 15 times more precise that any previous result. This value is precise enough to test the correction factors calculated to determine the nucleus-independent Ft value and to contribute to a stringent test of CVC, once the superallowed branching ratio, also determined in the present work [BR = 47.7 (12) %] is determined with the necessary precision. 42Ti is thus the heaviest TZ = −1 nucleus, for which high-precision measurements of these quantities have been performed.
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