The NuDoubt⁺⁺ Experiment

The NuDoubt⁺⁺ Prototype

Our approach integrates hybrid scintillators, which measure the ratio of Cherenkov and scintillation signals, with opaque scintillators that provide detailed event topology. This combination greatly enhances background discrimination and allows for heavy isotope loading without significantly affecting the scintillation output or energy resolution. Double beta searches using hybrid and opaque scintillators have been suggested in the past, with significant theoretical and experimental advancements over the years. The cylindrical detector prototype, measuring roughly 150 cm in both diameter and height, consists of approximately one metric tonne of hybrid-slow opaque scintillator. It is designed to test the hybrid and opaque particle discrimination capabilities, with optimised wavelength-shifting (WLS) fibres running parallel to the cylinder's symmetry axis. Each fibre connects to silicon photomultipliers (SiPMs) on both ends, enabling precise energy and spatial measurements. This design allows for the deployment of positron, gamma, and electron sources at its center to verify discrimination strategies. Monte Carlo simulations indicate effective containment and energy reconstruction capabilities for electron, positron, and gamma events.

This setup is well-suited for initial measurements of two-neutrino double weak decays such as 2ν2β⁺ and 2νβ⁺EC, and for setting limits on the neutrinoless decays 0νβ⁺EC and 0ν2β⁺. The primary isotopes of interest are ⁷⁸Kr, ¹²⁴Xe, and ¹⁰⁶Cd.

Double Beta Plus Decays

Double beta decay is a rare nuclear process in which the charge of a nucleus changes by two units while the mass number remains unchanged. It can occur when ordinary single beta decay is energetically forbidden or strongly suppressed. Most experimental searches focus on the β−β− channel, where two neutrons are transformed into two protons. NuDoubt++, however, focuses on the complementary charge-decreasing modes, where two protons are converted into two neutrons. These modes include double positron decay (β+β+), positron-emitting electron capture (β+EC), and double electron capture (ECEC). In the Standard Model, they can occur with the emission of two neutrinos. Their neutrinoless versions, if observed, would be a clear sign of physics beyond the Standard Model, including lepton-number violation and the possible Majorana nature of neutrinos.

Positron-emitting modes have distinctive experimental signatures. Each emitted positron eventually annihilates with an electron, producing two 511 keV gamma rays. This creates a combination of local energy deposition from the positron and additional gamma interactions nearby. These multi-site event topologies are especially well suited for a hybrid-opaque scintillator detector, which can use both timing information and spatial light patterns to identify signal-like events and reject backgrounds.

Studying β⁺β⁺, β⁺EC, and ECEC decays provides important tests of nuclear structure calculations and helps improve predictions for nuclear matrix elements. At the same time, searches for neutrinoless versions of these modes offer a complementary path toward discovering lepton-number violation and probing physics beyond the Standard Model.

Hybrid-Opaque Scintillator

A key challenge in rare double-positron decay searches is the suppression of radioactive backgrounds. NuDoubt⁺⁺ addresses this by using a hybrid-opaque scintillator, which combines two powerful event-identification techniques: the separation of Cherenkov and scintillation light, and the reconstruction of local energy-deposition patterns.

The proposed scintillator is based on a slow hybrid mixture of 88% linear alkylbenzene (LAB), 10% diisopropylnaphthalene (DIN), and 2% wax, with 2,5-Diphenyloxazole (PPO) used as a wavelength shifter. The slow scintillation emission delays the scintillation light, allowing the prompt Cherenkov signal to be more clearly identified. This makes it possible to use the Cherenkov-to-scintillation light ratio as a handle for particle identification. At the same time, the addition of wax makes the scintillator opaque. Instead of allowing scintillation light to travel freely through the detector, strong scattering confines the light close to its production point. The light is then collected by wavelength-shifting fibers and read out by SiPMs. This preserves information about the event topology: electrons tend to create compact single light blobs, gamma rays often produce multiple separated blobs through Compton scattering, and positrons can be identified through a combination of their ionization signal and the annihilation gamma signatures.

By combining the hybrid and opaque approaches, NuDoubt⁺⁺ can exploit both timing and topology. The Cherenkov/scintillation ratio helps distinguish different particle types, while the opaque response provides spatial information about where the energy was deposited. This is especially useful for identifying the characteristic signatures of double-positron decay modes and rejecting backgrounds.

Isotope Choice and Scintillator Loading

NuDoubt⁺⁺ focuses on promising isotopes for positron-emitting double beta decay modes, such as ⁷⁸Kr, ¹²⁴Xe, and ¹⁰⁶Cd. These isotopes provide high detectable energies, which helps separate potential signals from many natural radioactive backgrounds.

The hybrid-opaque NoWaSH scintillator also makes high isotope loading more practical. In conventional transparent scintillators, large amounts of isotope can reduce transparency and degrade detector performance. In an opaque scintillator, light is collected locally, so the transparency requirements are much less strict.

Noble gases such as krypton and xenon can be dissolved in the scintillator, with higher loading possible under increased pressure. For cadmium, solid compounds can be dispersed directly in the wax-based scintillator. This flexibility allows NuDoubt⁺⁺ to explore different isotope candidates while maintaining good light collection and energy resolution.

Optimised Wavelength-shifting Light-guides (OWL)

For the NuDoubt⁺⁺ experiment, we plan to utilize Optimised Wavelength-shifting fibres (OWL-fibres) to enhance light collection efficiency. These fibres are designed with the wavelength-shifters located on the outer surface, maximizing the probability of photon capture via total internal reflection. Polystyrene-based OWL-fibres can achieve a theoretical maximum trapping efficiency of up to 38%, capturing nearly four times more photons than conventional doped fibres with central emission points.

Leveraging expertise from IceCube's Wavelength-shifting Optical Modules (WOMs), the OWL-fibres demonstrate superior light collection, making them ideal for high-performance opaque scintillation detectors. Initial tests indicate competitive performance even with reduced attenuation lengths.