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This paper discusses the effects of radiation damage to SiPMs on the performances of plastic scintillator counters with series-connected SiPM readout, focusing on timing measurements. The performances of a counter composed of a $120 \times 40 \times5~\mathrm{mm}^3$ scintillator tile read out by two sets of six SiPMs from AdvanSiD connected in series attached on the short sides are presented, for different combinations of SiPMs at various levels of irradiation. Firstly, six SiPMs were equally irradiated with electrons from $^{90}$Sr sources up to a fluence of $\Phi_\mathrm{e^-}\approx 3 \times 10^{12}~\mathrm{cm}^{-2}$. The timing resolution of the counter gradually deteriorated by the increase in dark current. The dark current and the deterioration were reduced when the counter was cooled from 30$^\circ$C to 10$^\circ$C. Secondly, 33 SiPMs were irradiated with reactor neutrons. The characteristics of counters read out by series-connected SiPMs with non-uniform damage levels, were investigated. The signal pulse height, the time response, and the timing resolution depend on the hit position in the counter, when SiPMs' irradiation is not uniform., Comment: 13 pages, 27 figures, Major revision based on the reviewers' comments

This study aimed to make a timing counter of the best time resolution with an about 10 cm square plastic scintillator read out by multiple SiPMs. To achieve this, SiPM connection scheme, counter geometry and difference of plastic scintillators were investigated. As a result, the best time resolution was achieved with a counter of 80 × 175 × 5 mm 3 EJ-230 read out on each side by four readout channels of four SiPMs connected in series, marking σ t = 28 ps with position dependence less than 5 ps.

We studied the impact of radiation damage on scintillation counters read out by six SiPMs connected in series focusing on their time resolution. Six SiPMs from AdvanSiD were irradiated step by step with a 37 MBq 90Sr source. At the integrated dose expected at the end of the MEG II experiment, the time resolution at 30 °C was measured to be worse by ∼ 29%. However, the deterioration is found to be reduced to ∼ 6% at 10 °C since thermal dark noise is suppressed. We also tested the effect of connecting in series differently damaged SiPMs. We observed a position dependence of the time center in a counter, which worsened the time resolution.

The liquid xenon (LXe) γ-ray detector for the MEG II experiment is based on a highly granular scintillation readout with Multi-Pixel Photon Counters (MPPCs). Here we report on the development of a large-area MPPC sensitive to LXe scintillation light in deep-UV range for the MEG II LXe detector. A prototype model of the deep-UV MPPC with an active area of 12 × 12 mm 2 was successfully tested in LXe, showing an excellent performance such as a high photon detection efficiency for the LXe scintillation light (about 20% at Δ V = 2.5 V ), a high internal gain (about 10 6 at Δ V = 2.5 V ) and an excellent single photoelectron resolution. The sensor chip of the MPPC is segmented into four sectors, which are then connected in series in order to reduce the overall sensor capacitance. The signal fall time of about 135 ns for the non-segmented sensor was significantly reduced down to 25 ns with the series-connected sensor segments. The preliminary results on the performance of the deep-UV MPPC are presented.

The MEG experiment at PSI searches for the decay μ → e γ at a level of ≈ 10 − 13 on the branching ratio BR ( μ → e γ / 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 8 μ / 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 Li ( p , γ ) 3 7 Be 4 8 or two coincident γ s (11.67 and 4.4 MeV) from the reaction B ( p , γ 1 ) 5 11 C ⁎ 6 12 . The 17.6 MeV γ is used for calibrating and monitoring the LXe calorimeter ( σ E γ / E γ = 3.85 ± 0.15 % at 17.6 MeV) while the coincident 11.67 and 4.4 MeV γ s are used to measure the relative timing of the calorimeter and the spectrometer timing counters ( σ Δ t = 0.450 ± 0.015 ns ).

An innovative gamma-ray detector with liquid xenon was constructed for the MEG experiment to search for the lepton-flavor violating muon decay, μ + → e + γ . Excellent properties of liquid xenon enable us to measure the energy, timing and position of incident gamma ray with high resolutions. We have developed dedicated reconstruction algorithms for the new detector. Using calibration data with π - charge exchange reaction taken before and after the physics data taking, performance of the detector during the run in 2008 was studied in detail and good resolutions were demonstrated.