New search for correlated e+e- pairs in the α decay of 241Am (full text)

2013 - A new search for production of correlated e+e- pairs in the α decay of 241Am has been carried out to investigate internal pair production process (IPP).

In the seventies experimental and theoretical investigations of weak interactions in nuclei suggested the presence of various electromagnetic phenomena, which modify the basic decay processes such as IPP in β and α decay. In this latter case two modes have been suggested: i) the α particles are accelerated in the Coulomb field of the daughter nucleus and emit bremsstrahlung γs, with energy above 2mec2, giving rise to e+e- pairs; ii) the daughter nucleus originates in an excited state, and the resulting γ quantum (real or virtual) is converted to a e+e- pair. Both processes are possible but the dominant contribution to the IPP in the α decay of 241Am arises from the mode i).

The results have been obtained through dedicated measurements with some pairs of the NaI(Tl) detectors of the DAMA/LIBRA set-up. In the measurements an 241Am source of about 35 kBq activity was placed in the middle of each used NaI(Tl) pair for a total of 6 sources used. The experimental procedure is based on the detection of the two γs from the positron annihilation through the study of double coincidences in pairs of faced highly radiopure NaI(Tl) detectors.

The experimental data show an excess of double coincidences in the 511511 keV energy region which are not explained by known analysed background processes. The obtained relative activity for the IPP with respect to α decay of 241Am is

λ = (4.7 ± 0.63) · 10-9
this value is of the same order of magnitude as previous determinations.