First observation of a reactor-status effect on the beta+ decay rate of 22Na

In the search for an electron antineutrino detection method with sensitivity below the 1.8 MeV threshold for the inverse beta decay reaction, beta decay counting experiments with ca. 3 kBq 22Na and 60Co sources were conducted at unit #1 (2.775 GW_th) of the Koeberg Nuclear Power Station in South Africa. The setup consisted of one NaI crystal to measure de-excitation and annihilation photons associated with beta decay. Its volume and well shape were chosen to use coincidence summing in order to differentiate between electron capture and beta+ emission in 22Na. The setup was shielded from the reactor core by 8 m of uninterrupted concrete. Background radiation, responsible for ca. 1% of the total countrate with either source, increased by merely 3% when the reactor status changed from OFF to ON. Normalized countrates of three energy regions-of-interest (TOT, MED, HI) were parameterized to jointly describe the time dependence of two instrumental effects and a reactor-status step function in a least-squares regression analysis. With the 22Na source, the fractional countrate changes in the step from reactor OFF to ON were: (delA/A)_TOT = [-3.02 +- 0.14(stat) +- 0.07(syst)] x 10^-4, (delA/A)_MED = [+1.44 +- 0.42(stat) +- 0.07(syst)] x 10^-4, and (delA/A)_HI = [-2.70 +- 0.26(stat) +- 0.04(syst)] x 10^-4. The uncertainty budget is incomplete because it does not contain the possible influence from environmental factors and the finite stability of the MCA clock-oscillator. No reactor-status dependence was observed with the 60Co source. The corresponding cross sections are [1.55 +- 0.07(stat)] x 10^-25 cm^2 for EC + beta+ decay in 22Na and [0.5 +- 1.5(stat)] x 10^-26 cm^2 for beta- decay of 60Co. The negative sign for TOT and HI activity changes in 22Na points to an antineutrino related interference effect on the beta+ decay of 22Na and rules out reactor neutron induced reactions.