Source of monoenergetic electrons for the neutrino experiment KATRIN is ready

17. 06. 2021

The strongest calibration source of monoenergetic electrons based on 83Rb was developed and prepared in the NPI. The source was transferred to the Karlsruhe Institute of Technology (KIT). It is currently being employed in the one-month long measurement that allows for detailed investigation of many properties of the complex apparatus of KATRIN.

Unique calibration source of conversion electrons is based on radionuclide generator 83Rb/83mKr. Parent radionuclide 83Rb with a long half-life of 6 days is firmly attached to a solid-phase matrix. The matrix mustn’t release any 83Rb during the measurement on order to avoid highly undesirable contamination of the KATRIN apparatus. In contrast, daughter radionuclide 83mKr with a short half-life of 1.8 hours formed in the decay of 83Rb is an inert gas that continuously leaves the matrix (emanates) almost completely. It then passes through aerosol filters into the KATRIN system and mixes perfectly with tritium. The 83mKr emits during its decay conversion electrons of precisely known energy that serve for calibration. Monitoring of the position and shape of the conversion electron lines in their spectrum can be employed in the investigation of the whole 70 m long KATRIN line, in particular of the gaseous molecular tritium source. Measurement quality depends of the achievable statistic uncertainty that decreases with increasing calibration source activity. That is why we have prepared an 83Rb/83mKr generator within NPI that emanates milliards of 83mKr atoms per second. It corresponds to the parent 83Rb activity of 10 GBq, the highest level that can be used within KATRIN regarding valid permission to handle open radionuclide emitters.

Such a project represented several technological challenges. The first one was just the large-scale production of 83Rb in a nuclear reaction on gaseous krypton target in several-days lasting bombardment with intense proton beam. This aim was achieved thanks to the key instrument of the CANAM infrastructure, Canadian cyclotron TR-24. The irradiation is quite demanding process regarding target technology, beam stability and perfect target tightness – all these parameters were met successfully. The second was large activity handling that required automation of many steps in the 83Rb separation from the target and in the manufacturing of the calibration source itself. We had to combine radiation safety with precise performance of all the operations. In some case, we must replace performance of manual interventions by highly sensitive remote manipulators.

It’s our pleasure to state that the generator seems to meet expectations of the KATRIN team. Last week, the source was attached to KATRIN, and since that time the planned one-month long measurement with use of 83mKr has been ongoing.