Description
The Karlsruhe Tritium Neutrino (KATRIN) Experiment aims to measure the neutrino mass in a model-independent manner with a sensitivity of 0.2 eV/c² (90% C.L.). This is achieved by spectroscopy of the β-decay electrons of molecular tritium close to the kinematic endpoint at 18.6 keV using a high-resolution (~1 eV @ 18.6 keV) integrating spectrometer.
β-decay electrons are provided by a windowless gaseous tritium source (WGTS). It provides a very high count rate as well as a high temporal stability. The WGTS, as part of the closed tritium loop, provides $10^{11}$ β-electrons per second [1] with fluctuations less than 0.1 %/h. The WGTS and the connected tritium processing infrastructure have been operated successfully at the Tritium Laboratory Karlsruhe since 2019 [2]. The first campaign in 2019 set a new upper limit on the neutrino mass of 1.1 eV/c² (90% C.L.) [3]. A second campaign in the same year but with higher source strength (and lower background) allowed a further reduction of the upper limit to 0.8 eV/c² (90% C.L.) [4].
The high-luminosity tritium source is maintained by continuous circulation of high-purity (>98%) molecular tritium gas. Related to “burn-in” effects during the initial campaign, the maximum tritium activity was limited to about a quarter of the nominal column density. This limitation was no longer present in following campaigns. By spring 2022, a total amount of 15.5 kg of T$_2$ was circulated, which has required ≈100 tritium gas transfers from and to the Tritium Laboratory Infrastructure per year for its upkeep.
At the rear end, the WGTS is terminated by a gold-coated stainless steel disc. Its surface has accumulated a small amount of tritiated species which contribute to the measured β-spectrum. This small contribution represents a
systematic effect that needs to be characterized in order to minimize its impact on the neutrino mass. To mitigate this effect, two successful cleaning campaigns based on an in-situ ozone / UV treatment were performed since 2021.
The β-electrons generated in the strong tritium source partly ionize the molecular gas and form a plasma inside the source magnetic field of 2.5 T [1]. Spatial inhomogeneities in this plasma and thus in the starting potential of the β-electrons lead to a distortion of the β-spectrum. By measuring the line-profile of mono-energetic conversion electrons from traces of $^{83m}$Kr [5], used as an atomic reference standard, and co-circulating with the tritium inside of the WGTS, these distortions can be precisely quantified. Due to the low vapor pressure of Kr at the design source temperature of 30 K, the plasma calibration has to be performed at an elevated source temperature (>80 K).
During the second KATRIN campaign at nominal column density it has been shown that the start potential of the β-electrons defined by the plasma was impacted by drifting surface potentials at the boundaries of the source. In 2020, the source setpoint for neutrino mass measurements was changed from T = 30 K to T = 80 K. We will show how this choice has reduced the maximum achievable tritium activity by only 10%, but has allowed to perform several $^{83m}$Kr calibration campaigns under identical conditions as during the long-term neutrino measurements. Moreover, after changing to the new higher temperature setpoint, the observed drift has decreased by about an order of magnitude.
After this change, between early 2020 and late 2021, four measurement phases were successfully conducted, accumulating more than 250 days of β-scanning time. With reduced systematics due to the source potential drift, it will be possible to further improve significantly on the neutrino mass limit.
We acknowledge the support of Helmholtz Association (HGF); Ministry for Education and Research BMBF (05A17PM3, 05A17PX3, 05A17VK2, 05A17PDA, 05A17WO3, 05A20VK3, 05A20PMA and 05A20PX3); Helmholtz Alliance for Astroparticle Physics (HAP); the doctoral school KSETA at KIT; Helmholtz Young Investigator Group (VH-NG-1055); Max Planck Research Group (MaxPlanck@TUM); Deutsche Forschungsgemeinschaft DFG (Research Training Group grant nos. GRK 1694 and GRK 2149); Graduate School grant no. GSC 1085-KSETA, SFB-1258, and Excellence Cluster ORIGINS in Germany; Ministry of Education, Youth and Sport (CANAM-LM2015056, LTT19005) in the Czech Republic; the Department of Energy through grants DE-FG02-97ER41020, DE-FG02-94ER40818, DE-SC0004036, DE-FG02-97ER41033, DE-FG02-97ER41041, DE-SC0011091 and DE-SC0019304; and the Federal Prime Agreement DE-AC02-05CH11231 in the USA. This project has received funding from the European Research Council (ERC) under the European Union Horizon 2020 research and innovation programme (grant agreement no. 852845). We thank the computing cluster support at the Institute for Astroparticle Physics at Karlsruhe Institute of Technology, Max Planck Computing and Data Facility (MPCDF), and National Energy Research Scientific Computing Center (NERSC) at Lawrence Berkeley National Laboratory.
[1] M. Aker, et al., Journal of Instrumentation Volume 16, 2021, T08015.
[2] M. Sturm, et al., Fusion Engineering and Design Volume 170, 2021, 112507.
[3] M. Aker, et al., Physical Review. Letters 123, 2019, 221802.
[4] M. Aker, et al., Nature Physics 18, 2022, 160-166.
[5] K. Altenmüller, et al., Journal of Physics G: Nuclear and Particle Physics Volume 47(6), 2020, 065002.
| Collaboration | The KATRIN Collaboration |
|---|
