Calibration and monitoring of the MEG experiment by a proton beam from a Cockcroft-Walton accelerator

The MEG experiment at PSI searches for the decay μ→eγ at a level of ≈10^−13 on the branching ratio BR(μ→eγ/μ→tot), well beyond the present experimental limit (BR≤1.2×10^−11) and is sensitive to the predictions of SUSY-GUT theories. To reach this goal the experiment uses one of the most intense continuous surface muon beams available (≈10^8μ/s) and relies on advanced technology (LXe calorimetry, a gradient-field superconducting spectrometer as well as flexible and powerful trigger and acquisition systems). In order to maintain the highest possible energy, time and spatial resolutions for such detector, frequent calibration and monitoring, using a Cockcroft–Walton proton accelerator, are required. The proton beam is brought to the centre of MEG by a special bellows insertion system and travels in a direction opposite to the one of the normal μ-beam. Protons interact with a lithium tetraborate (Li2B4O7) nuclear target and produce one γ (17.6 MeV) from the reaction 3-7 Li(p, γ)8-4 or two coincident γs (11.67 and 4.4 MeV) from the reaction 11-5 B(p, γ)12-6 C*. The 17.6 MeV γ is used for calibrating and monitoring the LXe calorimeter (σEγ/Eγ=3.85±0.15% at 17.6 MeV) while the coincident 11.67 and 4.4 MeV γs are used to measure the relative timing of the calorimeter and the spectrometer timing counters (σ_delta_t=0.450±0.015ns).