[Paper Review] Plutonium-241 as a possible isotope for neutrino mass measurement and capture

N. De Groot|arXiv (Cornell University)|Mar 3, 2022

Neutrino Physics Research4 citations

TL;DR

This paper proposes plutonium-241 ($^{241}$Pu) as a viable alternative to tritium ($^{3}$H) for neutrino mass measurements and cosmic neutrino background (CNB) detection. Due to its low recoil energy (47 meV, 80× smaller than tritium) and similar decay energy (20.78 keV) and half-life (14.4 years), $^{241}$Pu minimizes energy spread issues. The study reports a neutrino capture cross-section of $1.52 \times 10^{-45}$ cm², 40% of tritium’s, making it a promising candidate for future experiments.

ABSTRACT

Tritium has been the isotope of choice for measurements of the neutrino mass and planned detection of the relic neutrino background. The low mass of $^{3}$H leads to large recoil energy of the nucleus. This has emerged as a limiting factor for both measurements. We investigate $^{241}$Pu as an alternative. The recoil is 80x smaller and it has similar decay energy and lifetime as $^{3}$H. We evaluate for the first time its soft-neutrino capture cross-section and find $(σv)_ν = 1.52 imes 10^{-45}$. This is 40% of the capture cross-section for tritium and makes $^{241}$Pu an interesting alternative for $^{3}$H.

Motivation & Objective

• To identify isotopes with reduced recoil energy to improve energy resolution in neutrino mass measurements.
• To evaluate $^{241}$Pu as a viable alternative to tritium ($^{3}$H) for neutrino mass and cosmic neutrino background (CNB) detection.
• To calculate the neutrino capture cross-section on $^{241}$Pu for the first time using measured beta decay spectra.
• To assess the feasibility of $^{241}$Pu in experiments by analyzing background challenges, particularly from $^{237}$U decay.
• To compare $^{241}$Pu's performance with other candidate isotopes like $^{151}$Sm and $^{171}$Tm in terms of energy uncertainty and capture cross-section.

Proposed method

• The neutrino capture cross-section $(\sigma v)_{\nu}$ is calculated using a novel method that extrapolates from the measured beta decay spectrum of $^{241}$Pu.
• The approach relies on the analytical dependence of the beta decay rate on electron and neutrino momenta, using a linear approximation near zero neutrino momentum.
• The method applies the relation $(\sigma v)_{\nu}[1 + \alpha_1 p_\nu / Q + O(p_\nu^2 / Q^2)] = \hbar^3 c^2 \pi^2 / p_\nu^2 \cdot d\Gamma_\beta / dE_e$, derived from the beta decay rate.
• The calculation uses a precise parametrization of the measured $^{241}$Pu beta spectrum and is cross-validated with synthetic spectra generated using the BetaShape software.
• The study evaluates background contributions from $^{237}$U, which decays via $\beta$ decay with a 6.7-day half-life and up to 459 keV energy, and proposes mitigation via recoil separation or $\gamma$-ray veto.
• The results are benchmarked against known values for $^{3}$H, $^{63}$Ni, $^{151}$Sm, and $^{171}$Tm to validate the method and cross-section estimation.

Experimental results

Research questions

• RQ1 Can $^{241}$Pu serve as a low-recoil alternative to tritium for neutrino mass measurements?
• RQ2 What is the neutrino capture cross-section on $^{241}$Pu, and how does it compare to tritium?
• RQ3 Can the energy uncertainty from zero-point motion and recoil be significantly reduced in $^{241}$Pu compared to tritium?
• RQ4 What are the dominant background sources in $^{241}$Pu-based experiments, and can they be mitigated?
• RQ5 How does $^{241}$Pu compare to other candidate isotopes like $^{151}$Sm and $^{171}$Tm in terms of figure of merit and detectability?

Key findings

• The neutrino capture cross-section for $^{241}$Pu is $1.52 \times 10^{-45}$ cm², determined from the measured beta decay spectrum.
• The calculated cross-section from a BetaShape-generated spectrum is $1.34 \times 10^{-45}$ cm², showing 10% agreement and validating the method.
• The recoil energy of $^{241}$Pu is 47 meV, 80 times smaller than tritium’s 3.4 eV, significantly reducing energy spread.
• The $\gamma$ factor for $^{241}$Pu is 25 times smaller than for $^{3}$H, implying a 25-fold reduction in energy uncertainty from zero-point motion.
• The capture cross-section is 40% of tritium’s, making it a viable alternative despite a lower event rate.
• The dominant background comes from $^{237}$U, which has a 6.7-day half-life and decays with up to 459 keV energy, but can be mitigated via recoil separation or $\gamma$-ray veto.