50 research outputs found
pairs from a nuclear transition signaling an elusive light neutral boson
Electron-positron pairs have been observed in the 10.95-MeV decay
in O. The branching ratio of the ee pairs compared to the
3.84-MeV decay of the level is deduced to be
. This magnetic monopole (M0) transition cannot proceed by
-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 or boson. Indeed, we do observe a sharp peak in the
angular correlation with all the characteristics belonging to the
intermediate emission of such a boson with an invariant mass of 8.5(5)
MeV/c. It may play a role in the current quest for light dark matter in the
universe.Comment: 6 page
A Re-evaluation of Evidence for Light Neutral Bosons in Nuclear Emulsions
Electron-positron pair-production data obtained by bombardment of emulsion
detectors with either cosmic rays or projectiles with mass between one and 207
and kinetic energies between 18 GeV and 32 TeV have been re-analysed using a
consistent and conservative model of the background from electromagnetic pair
conversion. The combined data yield a spectrum of putative neutral bosons
decaying to e+e- pairs, with masses between 3 and 20 MeV/c^2 and femtosecond
lifetimes. The statistical significance against background for these "X-bosons"
varies between 2 and 8 sigma. The cross-section for direct production of
X-bosons increases slowly with projectile energy, remaining over 1,000 times
smaller the the pion production cross-section.Comment: major revision with improved figures; accepted by Int J Mod Phys
Measuring the ß-decay Properties of Neutron-rich Exotic Pm, Sm, Eu, and Gd Isotopes to Constrain the Nucleosynthesis Yields in the Rare-earth Region
The ß-delayed neutron-emission probabilities of 28 exotic neutron-rich isotopes of Pm, Sm, Eu, and Gd were measured for the first time at RIKEN Nishina Center using the Advanced Implantation Detector Array (AIDA) and the BRIKEN neutron detector array. The existing ß-decay half-life (T1/2) database was significantly increased toward more neutron-rich isotopes, and uncertainties for previously measured values were decreased. The new data not only constrain the theoretical predictions of half-lives and ß-delayed neutron-emission probabilities, but also allow for probing the mechanisms of formation of the high-mass wing of the rare-earth peak located at A ˜ 160 in the r-process abundance distribution through astrophysical reaction network calculations. An uncertainty quantification of the calculated abundance patterns with the new data shows a reduction of the uncertainty in the rare-earth peak region. The newly introduced variance-based sensitivity analysis method offers valuable insight into the influence of important nuclear physics inputs on the calculated abundance patterns. The analysis has identified the half-lives of 168Sm and of several gadolinium isotopes as some of the key variables among the current experimental data to understand the remaining abundance uncertainty at A = 167–172.Peer ReviewedPostprint (published version
A pair spectrometer for measuring multipolarities of energetic nuclear transitions
A multi-detector array has been designed and constructed for the simultaneous measurement of energy- and angular correlations of electron-positron pairs. Experimental results are obtained over a wide angular range for high-energy transitions in 16O, 12C and 8Be. A comparison with GEANT simulations demonstrates that angular correlations between 50 and 180 degrees of the electron-positron pairs in the energy range between 6 and 18 MeV can be determined with sufficient resolution and efficiency.An electron–positron pair spectrometer has been designed and constructed for the simultaneous measurement of energy- and angular correlations of e+e- pairs. Experimental results are obtained over a wide angular range for high-energy transitions in 16 O, 12 C and 8 Be. The results showed that the angular correlations between 50 and 180° of the e+e- pairs in the energy range between 6 and 18 MeV can be determined with sufficient resolution and efficiency in good agreement with the GEANT simulations.A multi-detector array has been designed and constructed for the simultaneous measurement of energy- and angular correlations of electron-positron pairs. Experimental results are obtained over a wide angular range for high-energy transitions in 16O, 12C and 8Be. A comparison with GEANT simulations demonstrates that angular correlations between 50 and 180 degrees of the electron-positron pairs in the energy range between 6 and 18 MeV can be determined with sufficient resolution and efficiency