Ultra-intense 83Rb/83mKr emanation generator for the source plasma calibration at the KATRIN neutrino mass experiment

Isomer 83mKr of stable 83Kr emits monoenergetic conversion electrons that represent a unique calibration tool in the KATRIN experiment. In contrast to other monoenergetic electron sources, like electron gun, 83mKr as a gas may be homogeneously mixed with tritium. The conversion electrons then undergo similar effects as the β-electrons from tritium decay. It allows for quantification of the electron energy losses in the tritium gas and of the tritium cold plasma impact that originates in its small temporal instabilities and spatial inhomogeneity. Electron conversion lines spectra measurement reflects these distortions that will also affect the β-spectrum of tritium. Besides that, 83mKr doesn’t bring any contamination risk to the KATRIN system due to its reasonably short half-life (1.8620 h). On the other hand, 83mKr is to be continuously supplied to the system in order to achieve required counting statistics. It is possible thanks to the fact that 83mKr may be provided in the long-term from the decay of much longer-lived 83Rb parent isotope (86.2 d).

We have, therefore, developed emanation generator 83Rb/83mKr at the Nuclear Physics Institute, Czech Academy of Sciences. This 83mKr source makes use of the parent 83Rb deposition into a few tens of zeolite spherules. While 83Rb is fixed in the aluminosilicate matrix of zeolite, ca 80 % of 83mKr atoms born from its decay leave its surface [1]. The emanation 83Rb/83mKr source is a core of a generator system based on Swagelok components that injects 83mKr into the KATRIN tritium source [2]. Optimal production route for 83Rb is proton activation of natural krypton. The proton-induced nuclear reactions natKr(p,xn) have reasonably high yield of 83Rb at moderate proton energies available at cyclotrons. We have, therefore, designed, constructed and tested gaseous target systems compatible with the U-120M and the TR-24 cyclotrons operated at the Nuclear Physics Institute within the Centre of Accelerators and Nuclear Analytical Methods (CANAM) infrastructure. Increasing activity demands resulted in subsequent development of several krypton pressurized target generations that slowly enhanced the 83Rb production rate from 14 MBq/h up to ca 130MBq/h.

This production capabilities allowed us to supply several 1–2 GBq 83Rb/83mKr sources for calibration measurements in KATRIN. Initially, 83mKr emanated spontaneously from the zeolite to the tritium source [3,4]. Soon afterwards, a dedicated Gaseous Krypton Source (GKrS) was designed and implemented [5]. It comprises the 83Rb/83mKr source, tritium source tube and the injection loops. The GKrS enabled the KATRIN to employ three injection combinations: 83mKr, T2 + 83mKr and D2 + 83mKr. In 2020, it was revealed that originally planned neglecting plasma effects [6] is impossible, if we want to achieve the desirable sensitivity to neutrino mass. In order to comply with the aim, an ultra-high intensity 83mKr source reveals to be inevitable [7]. The activity of the source should be as high as possible, but fulfilling the legal limit of Tritium Laboratory Karlsruhe handling license for 83Rb, i.e. 10 GBq. Production of such amount of 83Rb and manufacturing of 83Rb/83mKr source from this activity is a technical challenge. It namely represents several days long irradiation of the gaseous Kr target with a high proton beam current, responsible handling and processing of the massively activated target holder and production of high-quality 83Rb/83mKr source at extreme dose-rates.

These conditions required implementation of an appropriate technology retaining favourable source properties, while minimizing contamination risk and the personnel radiation burden. The experience and efforts invested in long-lasting development of gaseous target systems resulted in a robust natKr target. The tests demonstrated that is able to withstand 5 days long activation with 24 MeV protons at 45 μA beam current. The target was processed remotely in a dedicated hot cell. The designed technology allowed for recovery of more than 90 % of the formed 83Rb from the target chamber. Production of the 83Rb/83mKr source itself was performed in semi-automated mode as a continuous process taking several days. Deposition efficiency of 83Rb in zeolite carrier ranged between 92 and 96 %. All the taken measures avoided unacceptable personnel radiation exposure without compromising the 83Rb/83mKr quality, in particular unnoticeable release of 83Rb from the zeolite matrix and high emanation efficiency of 83mKr from the matrix.

The first ultra-high intensity 83mKr calibration source was introduced in KATRIN measurement in summer 2021, the second one has been installed in March 2022. We refer to the follow-up contribution of our colleagues focused on the investigation of the electrical potential of the KATRIN tritium source using the source [8].

Acknowledgements
We acknowledge the support of Helmholtz Association (HGF), Ministry for Education and Research BMBF (05A20PMA, 05A20PX3, 05A20VK3), Helmholtz Alliance for Astroparticle Physics (HAP), the doctoral school KSETA at KIT, and Helmholtz Young Investigator Group (VHNG-1055), Max Planck Research Group (MaxPlanck@TUM), and Deutsche Forschungsgemeinschaft DFG (Research Training Groups Grants No., GRK 1694 and GRK 2149, Graduate School Grant No. GSC 1085-KSETA, and SFB-1258) in Germany; Ministry of Education, Youth and Sport (CANAMLM2015056, LTT19005) in the Czech Republic.

References
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