KIT – The Research University in the Helmholtz Association h Introduction to the KATRIN Rear Wall (RW) e- Analyzing RW Systematic via Measuring the Rate and Spectrum from RW − Events Non- Mitigated RW Surface(s) How Can We Model the RW Contaminant? Gold RW Surface Conclusions and Further Studies: The spectral shape near the endpoint is analyzed to determine the neutrino mass signature. − Spectrum Endpoint Region KATRIN measures the energy spectrum of the - near the endpoint. Molecular Tritium Decay Products Gaseous tritium undergoes - decay. Molecular Tritium KArlsruhe TRItium Neutrino Experiment (KATRIN) Direct Measurement of the Neutrino Mass KNM1-5 Uncertainties m< 0.8 eV/c2 (90% C.L.) The Rear Wall (RW) Mitigated RW Surface Au Au During KATRIN operatations, electrons bombard the unknown CnHm, breaking the -bonds and the CnHm. Example hydrocarbon chain (CnHm) pre-existing on RW CnHm with -bond opened by electron bombardment Tritium bonds to the newly opened - bonds resulting in amorphous structure. Newly created tritiated amorphous hydrocarbon (aCnTm) on RW − from T decay of aCnTm have a different spectrum than − from source A Possible Origin for the RW Systematic Evolution of the RW rate Evolution of the RW spectrum & Nat. Phys. 18, 160- 166 (2022) ❖We divide our RW data into epochs between surface cleanings with UV and ozone. ❖Integral flow is the total tritium circulated through the source since a given time. RW rate measurements are performed whenever there is no tritium in the gaseous source (e.g., between KATRIN Neutrino Mass Campaign “KNM” etc). For these measurements, we count the rate above a fixed retarding energy threshold. From these measurements, we found the RW rate has a predictable growth during epochs. ❖ ❖ ❖ RW spectrum measurements are performed by varying the retarding energy threshold to scan the RW −spectrum and then fitting a model to that RW −spectrum. Our goal is to find a minimal set of parameters for the RW spectrum model. The RW spectrum parameters are signal, endpoint, and FSD shape. The FSD shape compares the Final State Distribution (FSD) of the RW substance to that of T2. ❖ ❖ ❖ ❖ The RW rate and signal have consistent behavior across epoch 1 and 2. In epoch 3, the RW signal and rate are lower and more stable than in epochs 1 and 2. The RW endpoint and FSD shape have -0.96 correlation coefficient. Further Studies: We plan to next determine the RW uncertainty for KATRIN datasets post KNM1-5. The RW tritium spectrum has a systematic effect on the neutrino mass which can be addressed by including the RW spectrum in neutrino mass fits. This systematic effect has been successfully mitigated. Conclusion: ❖ ❖ ❖ Tritium bonds to the open pi bonds in the CnHm creating aCnTm. UV and ozone cleaning removes the chemisorbed tritium and some of the aCnTm beneath (Fusion Science and Technology 80.3-4 (2024): 303-310.). The aCnTm has been almost removed. We see that there is limited accumulation of tritium activity (T adsorbs poorly on Au). Possible Model: A big, initial CnHm layer that later saturates with tritium. ❖ 1. 2. 3. T T e- e- T + + He Detector Main Spectrometer Gaseous Source Magnetic fields guide - from the source to the detector A bias voltage is applied to the RW to help to control the starting potential of electrons in the gaseous source. CnHm becomes tritiated aCnTm is cleaned from RW Initial CnHm layer on RW Au C H H H H C We acknowledge the support of Helmholtz Association (HGF), Ministry for Education and Research BMBF, the doctoral school KSETA at KIT, Helmholtz Initiative and Networking Fund, Max Planck Research Group, and DFG in Germany; Ministry of Education, Youth and Sport in the Czech Republic; INFN in Italy; the National Science, Research and Innovation Fund via the Program Management Unit for Human Resources & Institutional Development, Research and Innovation in Thailand; and the DOE Office of Science, Nuclear Physics in the United States. This project has received fu