Speaker
Description
The $\beta$ decay of neutron-rich nuclei produced via fission processes from nuclear reactors have played a crucial role in developing our understanding of neutrinos within the standard model of particle physics. Reactor antineutrino experiments are unique in providing intense fluxes of with pure electron flavour ($\bar{\nu_e}$) within an MeV-scale energy range which are exploited to perform three-neutrino-flavour oscillation experiments [1] and the search for a fourth-flavour sterile neutrino leading to new physics beyond the standard model. The Reactor Antineutrino Anomaly (RAA) refers to an ~6% deficit in antineutrino measured detection rates [2] and an excess of antineutrinos at 5-7 MeV known as the ‘shoulder’ when compared to state-of-the-art Huber-Muller model predictions [3,4]. This anomaly has prompted a flurry of activity from both theory and experiment over the past 15 years to resolve this disagreement and significant progress has been achieved. These antineutrinos are produced via the $\beta$ decay of fission fragments and therefore, the origin of the RAA lies in the details of the $\beta$-decay processes. Despite their importance, much of the existing $\beta$-decay data is unsatisfactory, and improvements are essential to the future of reactor antineutrino experiments.
The $\beta$ decay of $^{92}$Rb is one of the main contributors to the reactor high-energy antineutrino spectrum and consequently an important contributor to the RAA. Recent studies of this decay using Total Absorption Spectroscopy (TAS) [5,6] reveal significant discrepancies with significant additional feeding to high-lying levels when compared with previous work utilising High-Resolution Spectroscopy (HRS) performed in the 1970s. This discrepancy can be attributed to the Pandemonium effect leading to incorrect $\beta$ feeding measurements in the HRS data which in turn lead to incorrect predictions of the antineutrino flux produced. While the TAS method is excellent at obtaining reliable $\beta$ feeding measurements, it is a limited probe of nuclear structure and exploiting both methods is essential to obtain a comprehensive understanding of this decay.
We have thus revisited the $\beta$ decay of $^{92}$Rb ($J^{\pi}=0^-, Q_{\beta}=8.1$ MeV) with the GRIFFIN spectrometer at TRIUMF that consists of up to 16 Compton-supressed HPGe clover detectors. Due to the high intensity of radioactive beam of $^{92}$Rb and the high efficiency of GRIFFIN for detecting $\gamma$ rays, we have obtained an unparalleled picture of $^{92}$Sr with over 180 levels populated and over 850 -ray transitions placed within the level scheme up to and beyond the neutron separation energy of $^{92}$Sr. The $\beta$ feeding of $^{92}$Sr measured in this work compare very well with the most recent TAS study demonstrating a significant suppression of this Pandemonium effect.
This work reveals the $\beta$ decay of $^{92}$Rb populated numerous high-lying levels in $^{92}$Sr. These levels are situated in the energy region of the Pygmy Dipole Resonance (PDR) that manifests as an enhancement of electric dipole strength at the low-energy tail of the Giant Dipole Resonance (GDR) near the neutron separation energy of $^{92}$Sr. The PDR is interpreted as an out-of-phase oscillation between the neutron skin and an isospin saturation core; however, this remains a matter of debate. The new information of nuclear levels in $^{92}$Sr from our study demonstrate the possibility in exploiting $\beta$ decay to investigate the PDR in nuclei and provide a complementary approach to existing techniques.
[1] T. Araki et al. (KamLAND Collaboration), Phys. Rev. Lett. 94, 081801 (2005)
[2] F. P. An et al. (Daya Bay Collaboration), Phys. Rev. Lett. 116, 061801 (2016)
[3] P. Huber, Phys. Rev. C 84, 024617 (2011)
[4] T. A. Mueller et al. Phys. Rev. C 83, 054615 (2011)
[5] B. C. Rasco et al. Phys. Rev. Lett. 117, 092501 (2016)