Measurement of angular correlations in low-energy internally-created e−e+ pairs using Timepix3 pixel detectors

Abstract

We report on measurements of electron–positron pairs internally created in excited ${}^{16}{O}^{\ast }$ nuclei, as detected using Timepix3 pixel detectors. Such experiments, complemented with simulation efforts, form a basis for our long-term programme to remeasure the so-called ATOMKI anomaly, i.e., the discrepancy in the pair-opening-angle distribution between theoretical expectation and experimental findings for ${}^8$Be and ${}^4$He nuclei. Emphasis is placed on studying the ability of the setup, comprising of three Timepix3 detectors surrounding the target, to reconstruct the vertices of e${}^{-}$e${}^{+}$ pairs and the angle between each-pair members. The ${}^{16}{O}^{\ast }$ nucleus, created by transmutation of fluorine upon bombardment with protons tuned to populate the 6.05-MeV (first excited) oxygen state, is a convenient means of producing copious amounts of pairs via the E0 deexcitation. The high yield suits the testing purposes and compensates the disadvantage that thus-created particles are of significantly lower energies than in reactions supposedly displaying the anomaly, to be eventually studied. Items such as multiple scattering in the detector material, gamma-background suppression and geometric limitations are discussed.