First limits on double beta decays in $$^\mathbf{232}$$232 Th
- PDF / 798,715 Bytes
- 6 Pages / 595.276 x 790.866 pts Page_size
- 102 Downloads / 157 Views
Regular Article - Experimental Physics
First limits on double beta decays in 232 Th M. Laubenstein1,a , B. Lehnert2,b , S. S. Nagorny3,c 1
INFN-Laboratori Nazionali del Gran Sasso, 67100 Assergi, AQ, Italy Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA 3 Physics Department, Queen’s University, Kingston, ON K7L 3N6, Canada
2
Received: 9 April 2020 / Accepted: 23 July 2020 © The Author(s) 2020
Abstract As one of the primordial radioactive isotopes, 232 Th mainly undergoes α-decay with a half-life of 1.402 · 1010 years. However, it is also one of 35 double beta decay candidates in which the single β-decay is forbidden or strongly suppressed. 181 mg of thorium contained in a gas mantle were measured in a HPGe well-detector at the Gran Sasso Underground Laboratory with a total exposure of 3.25 g×d. We obtain half-life limits on all double beta decay modes of 232 Th to excited states of 232 U on the order of 1011−15 years. For the most likely transition into the 0+ 1 state we find a lower half-life limit of 6.7 · 1014 years (90% C.I.). These are the first constraints on double beta decay excited state transition in 232 Th.
1 Introduction Double beta decay (DBD) is a second order weak nuclear decay and subject to intense study. While the Standard Model process of two neutrino double beta (2νββ) decay is experimentally observed in 11 out of 35 possible DBD nuclides [1,2], the lepton number violating process of zero neutrino double beta (0νββ) decay remains elusive to date. The latter would have profound implications for particle physics and cosmology, implying the Majorana nature of the neutrino and allowing to understand the matter-antimatter asymmetry in the Universe via Leptogenesis [3]. Even though the 2νββ and 0νββ modes require fundamentally different physics, they are connected through the same experimental techniques and share common challenges for nuclear theory. In order to interpret experimentally measured decay rates as a new lepton number violating process, nuclear matrix elements (NME) are required which are notoriously a e-mail:
difficult to calculate. These calculations can be improved and tested by any additional experimental information of observable 2νββ decays [4]. The most likely transition for DBD is into the ground state of the daughter nucleus which is typically a 0+ − 0+ transition. However, if the Q-value of the isotope is large enough, also transitions into excited state can occur. The measurement of the ground and excited state decay rates in the same nucleus are especially useful for testing nuclear models. Comparing both rates cancels many poorly constraint model parameters and allows for a more direct test of nuclear theory [5]. The end of 20th century and the first quarter of the 21st century could be considered as a “golden age” for direct counting experiments looking for DBD. Many experiments exploiting various detector techniques were proposed and realized within this time period. The highest sensitivities were achieved with the “source = d
Data Loading...
