Speaker
Description
Anaïs Lépine, LP2iB, on behalf of the WISArD collaboration
The Standard Model (SM) of particle physics has proven to be a successful
theory describing three of the four fundamental interactions. However, some
open questions remain and motivate precise measurements to search for evidence
of physics beyond the SM predictions. Nuclear β-decay provides a sensitive
probe of the weak interaction sector, for instance, the measurements of the shape of the β-energy spectrum can be used to search for exotic currents by extracting the Fierz term bF.
To constrain possible extensions of the SM at the same level as direct searches
at high energy, the required precision on bF must be at the level of a few 10−3.
This raises several experimental challenges, in particular partial energy de-
position caused by electron backscattering.
The SPELS (Spectrum of 32P ELectron Shape) project addresses this issue
by placing two detectors, facing each other, inside the magnetic field of WIS-
ArD’s superconducting magnet. This setup ensures a 4π solid angle and guides
the backscattered particles from one detector to the opposite one, thereby min-
imizing the energy loss of β particles from backscattering.
Following the first β-spectrum shape measurement at WISArD in 2020, using
scintillators and silicon photomultipliers, significant upgrades have been made.
A key improvement is the use of lithium-drifted silicon (Si(Li)) detectors, which
provide a higher energy resolution and a lower energy threshold. To cool these
detectors down, a new cooling system, based on active glycol circulation and
Peltier elements has been developed in Leuven. The setup has been optimized
at LP2iB in Bordeaux and a first successful commissioning run was conducted in
February 2026 at ISOLDE with an 114In source. The final data-taking campaign
is scheduled in July, with measurements of the β-shapes of 32P and 114In, two
isotopes with long lifetimes and endpoint energies in the 1-3 MeV range, where
sensitivity to the Fierz term is optimal.