1.5 The e- correlation in the superallowed decays of 32Ar and 33Ar

E.G. Adelberger, M. Beck, A. Garcia,^* N. Koloskamis,^* H.E. Swanson, Z. Zhao and the ISOLDE Collaboration^¤

Precision studies of the e- correlation provide an unbiased probe for rare weak processes involving scalar or tensor interactions.¹ Because of experimental limitations, very little information on this correlation is available for 0-->0 Fermi transitions;^1,2 these are particularly sensitive to scalar decays that could occur, for example, if a massive scalar boson were exchanged instead of the usual W^±. We have recently studied the e- correlation in the superallowed decays of ³²Ar and ³³Ar by measuring the effect of lepton recoil on the line shapes of the narrow (150eV) delayed-proton groups following these decays.

The experiment was performed at the ISOLDE General Purpose On-line Isotope Separator at CERN. Short-lived Ar activities were produced by bombarding CaO targets with 1 GeV protons and ionized in a plasma ion source. The Ar ions were stopped in a 25 µg/cm² carbon foil. Delayed protons were counted using a PIN diode detector setup described elsewhere in this report. Two 8192-channel energy spectra with gains of approximately 0.5 and 1.0 keV/channel were recorded along with the time delay of the event after the proton burst.

Data were taken in a sequence where long ³²Ar runs alternated with shorter ³³Ar runs (the intensity of the ³³Ar beam was 40 times greater than that of the ³²Ar beam). The ³³Ar data provided the energy calibration, as well as being interesting in their own right. A total of 9 × 10⁴ and 1.1 × 10⁶ superallowed decays of ³²Ar and ³³Ar, respectively, were observed. The low-gain spectra are shown in Figs. 1.5-1 and 1.5-2. An expanded high-gain spectrum showing the superallowed group from ³²Ar decay is shown in Fig. 1.5-3. A very preliminary analysis of the ³²Ar data yields an e- correlation coefficient consistent with the standard model value of unity with an uncertainty of 0.026.

Fig. 1.5-1. 32Ar delayed proton spectrum. Fig. 1.5-2. 33Ar delayed proton spectrum.

Fig. 1.5-3. Expanded ³²Ar delayed proton spectrum showing superallowed proton group. The peak in ch 6466 is a pulser.