High-precision probing of beta radioactivity at GANIL - 27 Novembre 2008

Beta radioactivity is one of the three most common decay modes of unstable nuclei. It manifests itself as a transformation of a proton into a neutron, or of a neutron into a proton, within the atomic nucleus. In the latter case, the neutron's transformation is accompanied by the emission of an electron and an antineutrino, this being referred to as "beta-minus" radioactivity. However, whereas the electron is easy to observe, it is almost impossible to detect an antineutrino because of its weak interaction with matter. Among the four fundamental forces of nature (gravitation, electromagnetism, weak and strong nuclear forces), it is the weak nuclear force which is responsible for beta radioactivity.

Questions still unsolved

"Certain theoretical fundaments concerning the weak nuclear force still remain mysterious", explains Oscar Naviliat Cuncic, a professor at the LPC laboratory in Caen¹. "The present theory was never challenged by experimentation, but many arguments suggest that other interactions might exist in addition to those already known, and that these might also contribute to the phenomenon of beta radioactivity. Now, the signature of such new interactions could be discovered through high-precision measurements, such as the measurement of the relative emission directions between the electron and the antineutrino during beta decay."

A device specifically designed for the experiment

To carry out this measurement, the team of researchers directed by Oscar Naviliat Cuncic built the LPCTrap² device. This piece of equipment, which is the only one of its kind, allows radioactive ions of a helium isotope (generated using GANIL's SPIRAL facility) to be slowed down and then trapped³ by means of oscillating electric fields, so that they can be maintained in vacuum in the form of a small cloud.

During the beta decay of such a trapped helium nucleus, the emission energies and directions of the nucleus, electron and antineutrino are constrained by the fundamental principles of physics. As for the distribution of the emission patterns, this depends on the type of underlying interaction. By measuring the emission energies and directions of the resulting electron and nucleus, it is thus possible to derive those of the antineutrino. Then, by observing the distribution of the various decay patterns, it is possible to accurately trace backwards to the nature of this interaction.

Millions of observations

The results published in Physical Review Letters were derived from about 100,000 observations of radioactive decays, which was sufficient to demonstrate this technique's efficiency. However, for the measurement itself, it was necessary to collect a larger quantity of data, and additional tests will be required in order to gain better insight into the systematic effects of the conditions under which ions are trapped, and their decays are observed. 
"During a new experiment, carried out in October 2008, we collected more than 4 million observations", said Oscar Naviliat Cuncic, "which would allow us to improve on the present sensitivity. Confirming or infirming this theory is a different story". 


This result was recently published in Physical Review Letters. X. Fléchard et al., Physical Review Letters, Vol. 101, 21 November 2008, p. 212504


¹ Laboratoire de Physique Corpusculaire de Caen (Corpuscular Physics Laboratory of Caen), ENSICAEN, CNRS/IN2P3, University of Basse-
Normandie, Caen, France
² «LPCTrap facility: A novel transparent Paul trap for high-precision experiments»

D. Rodríguez et al., Nuclear Instruments and Methods in Physics Research A 565 (2006) 876-889


³ Techniques for trapping atoms or ions have been used for several years in laboratories for the high-precision measurement of certain properties of the atomic nucleus. These techniques also enable the decay modes of radioactive atoms or ions to be investigated with greater sensitivity than before.