date: January 2001
The study of superallowed Fermi 0+g.s. -> 0+g.s. beta transitions in Z=N nuclei is a test of the properties of the electroweak interaction. The ft value can be calculated from the characteristics of the transition: the branching ratio, the Q value and the half-life. This value, with some correction factors, can be expressed in terms of the weak interaction by the weak vector coupling constant Gv :
where f is the phase space factor, t is the partial half-life of an allowed Fermi beta-transition, dr and dc are the radiative and the Coulomb corrections [Tow92], [Wil93], [Orm95], [Sag96], respectively, K is a constant [Wil93] |Mv|2=T(T+1)-T3(T3+1) is the Fermi matrix element. In the case of odd-odd nuclei with T=1, its value is equal to 2.
The value Gv must be constant according to the conserved vector current (CVC) hypothesis. From Ref. [CKM63] the Cabibbo-Kobayashi-Maskawa (CKM) matrix must verify the unitarity condition
The up-down quark matrix element Vud of the CKM matrix can be calculated from the ratio :
where Gu is the corresponding coupling constant of the muon decay [Mar86], [Sir86].
It was found, with good precision, that for nine nuclei with T=1 (from 10C to 54Co) the Ft value is constant and equal to (3072.3 +/- 2.0) s [Har98]. This result agrees with the CVC hypothesis. However, the unitariry condition is not fullfilled with a deviation of 2.2 sigma.
Our project is to extend the measurements of superallowed Fermi decays to more exotic nuclei (from 62Ga to 86Tc), where the uncertainty in the Ft value is still big. A precision of 0.1 ms in the half-life of 62Ga is needed to get an accurate value of the Ft constant. Also a very weak branching ratio of a decay from the 62Ga ground state to the first excited 0+ level of the 62Zn nucleus is expected to allow the extraction of a value of the dc correction.
Decay schemes and more precise branching ratios of non-analog Fermi decays and of Gamow-Teller decays are basically unknown between 62Ga and 86Tc.
62Ga is a good candidate for this kind of studies as it is expected to have a decay scheme (cf Figure 1) similar to 54Co with a predominant Fermi transition to the isobaric analog state (IAS) and a secondary transition to the 0+2 state at 2.330(10)MeV [Fir96]. Non-analog decay branches of about 0.04% have been found for 46V and 54Co [Hag94].
In 62Ga this branching ratio is also expected. However, no 1+ state is presently known for a Gamow-Teller transition. The QEC value is 9171±26 MeV [Dav79] and many experiments are actually trying to measure its half-life, branching ratios and QEC value with a precision of 10-4.
Decay scheme of 62Ga :
Figure 1: Decay of 62Ga
62Ga ions are produced at IGISOL by the 64Zn(p,3n)62Ga fusion-evaporation reaction at 48 MeV by a H- beam with an intensity of 20 microA. Residues are thermalized in a He chamber before being extracted, selected in a magnetic dipole and stopped in the detection device.
The ions of interest are implanted on a tape transport device surrounded by a 2mm-cylindrical plastic scintillator (for beta detection) and 3 high-efficiency Germanium detectors (for gamma detection) in close geometry around the implantation point as schematized in Figure 2:
Figure 2: Detection set-up composed of an implantation moving tape surrounded by a beta detector (cylindrical scintillator), and 3 Germanium detectors in a close geometry.
To avoid long-lived contaminants produced with higher rates than 62Ga, ions are implanted during cycles of 200 ms of collection (equivalent to twice the 62Ga half-life) and 200 ms decaying periods before moving the tape. The absolute branching ratio will be determined by integrating the number of beta particles under the decay curve.
954.0(4) keV transition:
In the experiment performed in January 2001 with the same set-up, a 215 pps beam of 62Ga ions was obtained (with a 48 MeV proton beam), leading to the determination of the 2+ -> 0+ branching ratio of 0.116(33) % observed in coincidence with betas (660 counts in gamma peak (Figure 3) and 14.7 106 betas (Figure 4) integrated during 65.8 h experiment with 50 % beam ON/OFF). Total efficiency of the 3 Germanium detectors at 954 keV was of 3.8(2) %. This gives an upper limit of the superallowed Fermi transition from 62Ga 0+g.s. -> 0+g.s. of 99.881(33)% considering that all beta-transitions populate states decaying via the 2+ state at 954 keV.
No peak was distinguished from background for the expected 1376(10)keV transition between the excited 0+ state and 2+ state, either in the singles spectrum nor in coincidence with the 954.0(4) keV. An upper limit of 0.019(2) % for this transition has been determined. We should be able to observe this transition in a future experiment, when a factor of 10 more statistics is reached.
From the present upper limit, we deduce an upper limit of the isospin mixing matrix element deltaIM of 0.093% using a two-level mixing model. This value can be compared to theoretical predictions from Ormand and Brown [Orm95] of 0.169% and 0.079% depending on the effective interaction used in the shell model.
No clear beta-gamma-gamma coincidence transition was established due to the low statistics and the limited efficency of Germanium detectors. We can only mention that a small peak at 2226.6 keV can be distinguished from background, certainly corresponding to the 2225 keV transition mentionned by J. Döring et al. [Dör00]. However, this peak does not correspond to the 0+2 -> 0+g.s transition (2330(10) keV previously tabulated in [Fir96]. The branching ratio for this gamma line is obtained from beta-gamma (954 keV) -gamma coincidences (shown on Figure 5) and is 0.057(29) %.
Figure 3: Experimental gamma spectrum obtained with the Germanium detectors. The insert shows a zoom on the 954 keV peak, obtained in coincidence with a beta particle.
Figure 4: Experimental beta spectrum obtained in the plastic scintillator. The threshold used to analyse the data is also indicated.
Figure 5: Experimental beta-gamma(954 keV)-gamma spectrum with a zoom on the 2226 keV peak.
A new experiment will be proposed in the next Program Advisory Committee in Jyväskylä (September 2001) [Pro01] containing two parts:
to improve the measurement of branching ratios in the beta decay of 62Ga
(14 days beam time requested)
to measure half-life of 62Ga with a precision of 0.1 ms (14 days beam time requested)
Besides the longer beam times, the essential improvements will be:
- higher intensity primary beam due to upgrades of the IGISOL beam line
- higher gamma efficiency with high-efficiency clover detectors
- higher beta efficiency due to a new design of the beta detector (cf. Figure 6)
- higher collection rate with a new tape drive
Figure 6: Optimized plastic scintillator for beta detection with 87 % efficiency under development at CENBG. The tape entrance is represented as a rectangular hole "RECT" at the bottom and the beam hole as a circular hole noticed "BEAM".
[Tow92]: I. S. Towner, Nucl. Phys. A540, 478 (1992)
[Wil93]: D. H. Wilkinson, Nucl.Instrum. Methods Phys. Res., Sect. A 335, 172 (1993)
[Orm95]: W. E. Ormand and B. A. Brown, Phys. Rev. C 52, 2455 (1995)
[Sag96]: H. Sagawa et al., Phys. Rev. C 53, 2163 (1996)
[CKM63]: N. Cabibbo, Phys. Rev. Lett. 10, 531 (1963), M. Kobayashi and T. Maskawa, Prog. Theor. Phys. 49, 652 (1973)
[Mar86]: W. A. Marciano and A. Sirlin, Phys. Rev. Lett. 56, 22 (1986)
[Sir86]: A. Sirlin and R. Zucchini, Phys. Rev. Lett. 57, 1994 (1986)
[Har98]: J. C. Hardy and I. S. Towner, nucl-th/9812036 14 Dec 1998
[Fir96]: Firestone et al, Table of Isotopes, 8th edition (1996)
[Hag94]: E. Hagberg et al. Phys. Rev. Lett.73 (1994)396
[Dav79]: C. N. Davids et al. Phys. Rev. C19 (1979)1463
[Orm85]: W. E. Ormand, B.A. Brown, Phys. Rev. C52, 2455 (1995)
[Dör00]: J. Döring et al. Contribution to the International Workshop PINGST 2000, Lund University
[Pro01]: B. Blank et al. "Detail studies of the decay of 62Ga" proposed at Jyvaskyla PAC (september 2001)
From CENBG Bordeaux-Gradignan (left to right):
Maria-Jose Lopez Jimenez
In the same section :