Speaker
Description
Isomer $^{83m}$Kr of stable $^{83}$Kr emits monoenergetic conversion electrons that represent a unique calibration tool in the KATRIN experiment. In contrast to other monoenergetic electron sources, like electron gun, $^{83m}$Kr 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, $^{83m}$Kr doesn’t bring any contamination risk to the KATRIN system due to its reasonably short half-life (1.8620 h). On the other hand, $^{83m}$Kr is to be continuously supplied to the system in order to achieve required counting statistics. It is possible thanks to the fact that $^{83m}$Kr may be provided in the long-term from the decay of much longer-lived $^{83}$Rb parent isotope (86.2 d).
We have, therefore, developed emanation generator $^{83}$Rb/$^{83m}$Kr at the Nuclear Physics Institute, Czech Academy of Sciences. This $^{83m}$Kr source makes use of the parent $^{83}$Rb deposition into a few tens of zeolite spherules. While $^{83}$Rb is fixed in the aluminosilicate matrix of zeolite, ca 80 % of $^{83m}$Kr atoms born from its decay leave its surface [1]. The emanation $^{83}$Rb/$^{83m}$Kr source is a core of a generator system based on Swagelok components that injects $^{83m}$Kr into the KATRIN tritium source [2].
Optimal production route for $^{83}$Rb is proton activation of natural krypton. The proton-induced nuclear reactions $^{nat}$Kr(p,xn) have reasonably high yield of $^{83}$Rb 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 $^{83}$Rb production rate from 14 MBq/h up to ca 130MBq/h.
This production capabilities allowed us to supply several 1–2 GBq $^{83}$Rb/$^{83m}$Kr sources for calibration measurements in KATRIN. Initially, $^{83m}$Kr 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 $^{83}$Rb/$^{83m}$Kr source, tritium source tube and the injection loops. The GKrS enabled the KATRIN to employ three injection combinations: $^{83m}$Kr, T$_{2}$+$^{83m}$Kr and D$_{2}$+$^{83m}$Kr. 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 $^{83m}$Kr 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 $^{83}$Rb, i.e. 10 GBq.
Production of such amount of $^{83}$Rb and manufacturing of $^{83}$Rb/$^{83m}$Kr 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 $^{83}$Rb/$^{83m}$Kr 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 $^{nat}$Kr 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 $^{83}$Rb from the target chamber. Production of the $^{83}$Rb/$^{83m}$Kr source itself was performed in semi-automated mode as a continuous process taking several days. Deposition efficiency of $^{83}$Rb in zeolite carrier ranged between 92 and 96 %. All the taken measures avoided unacceptable personnel radiation exposure without compromising the $^{83}$Rb/$^{83m}$Kr quality, in particular unnoticeable release of $^{83}$Rb from the zeolite matrix and high emanation efficiency of $^{83m}$Kr from the matrix.
The first ultra-high intensity $^{83m}$Kr 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
[1] D. Vénos et al., Gaseous source of $^{83m}$Kr conversion electrons for the neutrino experiment KATRIN. JINST 9(12), P12010 (2014). https://doi.org/10.1088/1748-0221/9/12/P12010
[2] J. Sentkerestiová et al., Gaseous $^{83m}$Kr generator for KATRIN. JINST 13(04), P04018 (2018). https://doi.org/10.1088/1748-0221/13/04/P04018
[3] M. Arenz et al. (KATRIN), First transmission of electrons and ions through the KATRIN beamline. JINST 13(04), P04020 (2018). https://doi.org/10.1088/1748-0221/13/04/P04020
[4] K. Altenmüller et al. (KATRIN), High-resolution spectroscopy of gaseous $^{83m}$Kr conversion electrons with the KATRIN experiment. J. Phys. G 47, 065002 (2020). https://doi.org/10.1088/1361-6471/ab8480
[5] M. Aker et al. (KATRIN), The design, construction and commissioning of the KATRIN experiment. JINST 16(16), T08015 (2021). https://doi.org/10.1088/1748-0221/16/08/T08015
[6] M. Aker et al. (KATRIN), Improved Upper Limit on the Neutrino Mass from a Direct Kinematic Method by KATRIN. Phys. Rev. Lett. 123, 221802 (2019). https://doi.org/10.1103/PhysRevLett.123.221802
[7] M. Aker et al. (KATRIN), Direct neutrino-mass measurement with subelectronvolt sensitivity. Nature Phys. 18(2), 160-166 (2022). https://doi.org/10.1038/s41567-021-01463-1
[8] Machatschek et al., Observables of the electrical potential of the KATRIN tritium source from calibration with a high-intensity $^{83m}$Kr source, contribution to the Neutrino 2022 conference
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