  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.

Nuclear Experiment;Instrumentation and Detectors