date: March 2007
Participants: I. Matea, B. Blank, J. Giovinazzo, J. Huikari, J.-L. Pedroza, J. Souin
In collaboration with: JYFL, ISOLDE CERN, CSIC Madrid
The beta decay half-life of 26Si was measured with a relative precision of 1.4*10-3. The measurement yields a value of 2.2283(27)s which is in good agreement with previous measurements but has a precision that is better by a factor of 4. In the same experiment, we have also measured the non-analogue branching ratios and could determine the super-allowed one with a precision of 3%. The experiment was done at the Accelerator Laboratory of the University of Jyvaskyla where we used the IGISOL technique with the JYFLTRAP facility to separate pure samples of 26Si.
Super-allowed 0+ -> 0+ beta decay allows to test the weak-interaction standard model  with its CVC hypothesis and to determine the Vud matrix element of the Cabibbo-Kobayashi-Maskawa (CKM) quark mixing matrix. This matrix has to be unitary:
The best determination of the Vud matrix element is achieved with beta decay, where it can be determined from the corrected Ft value. The Ft value itself is a function of the decay Q value, the half-life and the branching ratio of the super-allowed decay. Vud is determined as follows [2,3] :
K is a constant and G’F is the coupling constant for pure leptonic decays determined from the weak decay of the muon.
26si is one of the Tz nuclei for which we contributed to a precise determination of the three quantities:
Half-life of the emitter (T1/2),
Masse difference between emitter and daughter nucleus (QEC)
Super-allowed branching ration for the 0+ -> 0+ decay branch (B.R.)
After our efforts on 62Ga, we performed a new precision experiment at the University of Jyväskylä (Finland) in March 2007 on 26Si .
26Si was produced by the fusion-evaporation reaction 27Al(p,2n)26Si. A primary proton beam at 35 MeV and an intensity of 45A impinged on a 2.3mg/cm2 natAl target at the entrance of IGISOL. The reaction products were selected by the analyzing magnet of IGISOL and the Penning trap JYFLTRAP, which allowed to have a pure 26Si beam.
Figure 1: Schematic drawing of the experimental setup showing the plastic scintillator to detect the beta particles as well as three Germanium clover detectors for the gamma rays.
The 26Si was then directed onto a tape situated in the center of a cylindrical plastic scintillator to detect the beta particles, which was itself surrounded by three Germanium detectors to detect the gamma rays.
A possible source of contamination with 26Alm could be an insufficient resolving power of the trap system. The 20Hz step frequency scan presented in Figure 2 was done in order to have a rough estimate of such a possible overlap.
Figure 2: Isobaric scan around 26Si and 26Alm. The background was measured by inserting a beam stopper in the line.
Different runs were performed by varying the trap or the electronics parameters without any experimental biais to be detected. Figure 3 shows the decay time spoectrum from one run, where the rate is decomposed in the different contributions.
Figure 3: The time distribution obtained for a single run (full circles). The full line is the result of the fit and the contributions from 26Si, 26Alm and the background are represented separately. A half-life of 2.229(11)s was obtained for this run.
The final result for the half-life of the 26Si ground state is 2.2283(27)s. Previous measurements of the ground state half-life were reported by Hardy et al.  (2.210(21) s) and Wilson  (2.240(10) s).
The BR for the super-allowed decay was already measured with a precision of about 1% . The main purpose of the present experiment being the half-life measurement, we were not aiming to achieve the required precision (10-3) on the BR. Nevertheless, we have analyzed the gamma spectra of the three germanium detectors to determine the BR.
The spectrum is presented in Figure 4. The only gamma rays that are present come either from gamma or positron scattering in the lead bricks surrounding the germanium detectors, from the positron-electron annihilation, or from the beta decay of 26Si.
Figure 4: Gamma spectrum in coincidence with a $\beta$ ray detected by the plastic scintillator. The X-rays are coming from the low-radioactivity lead bricks used to reduce the $\gamma$ background.
We deduced an absolute beta-decay branch for non-analogue transitions of 24.31(58)% resulting in a absolute $\beta$-decay branch for the super-allowed transition of 75.69(58)\%. For the transitions that were not observed in our experiment we have used the relative intensities from .
After averaging our values with literature data, we can calculate the ft value and obtain ft = 3024(26)s and Ft = 3047(26)s.
 R. P. Feynman and M. Gell-Mann, Phys. Rev. 109, 193 (1958).
 I. S. Towner and J. C. Hardy, Phys. Rev. C 77, 025501 (2008).
 I. S. Towner and J. C. Hardy, Phys. Rev. C 66, 035501 (2002).
 I. Matea et al., Eur. Phys. J. A37, 151 (2008)
 J.C. Hardy et al., Nucl. Phys. A246, 61 (1975).
 H.S. Wilson et al., Phys. Rev. C22, 1696 (1980).
 J.C. Hardy and I.S. Towner, Phys. Rev. C71, 055501 (2005).
In the same section :