Effects of radiation damage caused by proton irradiation on Multi-Pixel Photon Counters (MPPCs)

We have investigated the effects caused by proton-induced radiation damage on Multi-Pixel Photon Counter (MPPC), a pixelized photon detector developed by Hamamatsu Photonics. The leakage current of irradiated MPPC samples linearly increases with total irradiated doses due to radiation damage, which is not completely recovered even after a year from the irradiation. No significant change has been observed in the gains at least up to 8.0 Gy ($9.1\times10^7$ n/mm$^2$ in 1 MeV neutron equivalent fluence, $\Phi_{\rm eq}$). The device has completely lost its photon-counting capability due to baseline fluctuations and noise pile-up after 21 Gy irradiation ($2.4\times10^8$ n/mm$^2$ in $\Phi_{\rm eq}$), which might be problematic for some applications, such as ring-imaging Cherenkov detectors. We have found that the pulse-height resolution has been slightly deteriorated after 42 Gy irradiation ($4.8\times10^8$ n/mm$^2$ in $\Phi_{\rm eq}$), where the measured sample has been illuminated with a few hundred photons. This effect should be considered in the case of energy-measurement applications.Comment: 18 pages, 10 figure

Improved Measurement of the K+ to pi+ nu nubar Branching Ratio

An additional event near the upper kinematic limit for K+ to pi+ nu nubar has been observed by Experiment E949 at Brookhaven National Laboratory. Combining previously reported and new data, the branching ratio is B(K+ to pi+ nu nubar)= 1.47 (+1.30, - 0.89) x 10-10 based on three events observed in the pion momentum region 211<P<229 MeV/c. At the measured central value of the branching ratio, the additional event had a signal-to-background ratio of 0.9

Further Evidence for the Decay K+ to pi+ neutrino-antineutrino

Additional evidence for the rare kaon decay K+ to pi+ neutrino-antineutrino has been found in a new data set with comparable sensitivity to the previously reported result. One new event was observed in the pion momentum region examined, 211<P<229 MeV/c, bringing the total for the combined data set to two. Including all data taken, the backgrounds were estimated to contribute 0.15 pm 0.05 events. The branching ratio is B=1.57^{+1.75}_{-0.82} 10^{-10}.Comment: 10 pages, 2 figure

Search for the decay KL0→3γK_L^0 \rightarrow 3\gamma

We performed a search for the decay $K_L^0 \rightarrow 3\gamma$ with the E391a detector at KEK. In the data accumulated in 2005, no event was observed in the signal region. Based on the assumption of $K_L^0 \rightarrow 3\gamma$ proceeding via parity-violation, we obtained the single event sensitivity to be $(3.23\pm0.14)\times10^{-8}$, and set an upper limit on the branching ratio to be $7.4\times10^{-8}$ at the 90% confidence level. This is a factor of 3.2 improvement compared to the previous results. The results of $K_L^0 \rightarrow 3\gamma$ proceeding via parity-conservation were also presented in this paper