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 mu -> e gamma at a level of approximate to 10(-13) on the branching ratio BR(mu -> e gamma/mu -> tot), well beyond the present experimental limit (BR <= 1.2 x 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 (approximate to 10(8) mu/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 mu-beam. Protons interact with a lithium tetraborate (Li(2)B(4)O(7)) nuclear target and produce one gamma (17.6 MeV) from the reaction (7)(3)Li(p,gamma)(4)(8)Be or two coincident gamma s (11.67 and 4.4 MeV) from the reaction (11)(5)B(P,gamma(1))(6)(12)C*. The 17.6 MeV gamma is used for calibrating and monitoring the LXe calorimeter (sigma(E gamma)/E(gamma) = 3.85 +/- 0.15% at 17.6 MeV) while the coincident 11.67 and 4.4 MeV gamma s are used to measure the relative timing of the calorimeter and the spectrometer timing counters (sigma(Delta t) = 0.450 +/- 0.015 ns). (C) 2011 Elsevier B.V. All rights reserved.