F. W. N. de Boer

Excess in nuclear pairs near 9 MeV/ invariant mass

Data on internal pair conversion in widely different nuclear transitions with energies above 12 MeV have an apparent excess in pairs around 9 MeV/ invariant mass. While the deviations individually may be circumstantial, in combination they appear to point to an overall anomaly.

E+e- pairs from a nuclear transition signaling an elusive light neutral boson

Electron‐positron pairs have been observed in the 10.95‐MeV 0− → 0+ decay in 16O. The branching ratio of the e+e− pairs compared to the 3.84‐MeV 0− → 2+ γ decay of the level is deduced to be 20(5) × 10−5. This magnetic monopole (M0) transition cannot proceed by γ‐ray decay and is, to first order, forbidden for internal pair creation. However, the transition may also proceed by the emission of a light neutral 0− or 1+ boson. Indeed, we do observe a sharp peak in the e+e− angular correlation with all the characteristics belonging to the intermediate emission of such a boson with an invariant mass of 8.5(5) MeV/c2. It may play a role in the current quest for light dark matter in the universe.

A multi-detector array for high energy nuclear e+e? pair spectroscopy

A multi-detector array has been constructed for the simultaneous measurement of energy and angular correlation of electron–positron pairs produced in internal pair conversion (IPC) of nuclear transitions up to 18 MeV. The response functions of the individual detectors have been measured with mono-energetic beams of electrons. Experimental results obtained with 1.6 MeV protons on targets containing 11B and 19F show clear IPC over a wide angular range. A comparison with GEANT simulations demonstrates that angular correlations of e+e− pairs of transitions in the energy range between 6 and 18 MeV can be determined with sufficient resolution and efficiency to search for deviations from IPC due to the creation and subsequent decay into e+e− of a hypothetical short-lived neutral boson.

On multi-pion multi-nucleon states

Commets are presented concerning hadronically stable clusters of negatively charged pions and neutrons (‘pineuts’) with reference to recent theoretical and experimental studies on the subject.

Comment on a search for a short-lived isoscalar axion with mass between 4 and 15 MeV/

Institut de Physique Nucleaire, Universite Catholique de Louvain, B1348 Louvain-la-Neuve, Belgium Received 15 January 1997, in final form 11 March 1997 Abstract: Triggered by a recently observed deviation in internal pair conversion at invariant mass, indicating the possible existence of a new pseudoscalar boson, a previous constraint is reconsidered.