Determination of the Atmospheric Neutrino Flux by the Observation of Atmospheric Gamma

第一年：我们对原来为观测初级宇宙电子而设计的博彩触发系统进行了升级；实现了同时观测不同触发条件下的伽马射线的多触发系统。第二年：我们选择了Mt.东京大学Norikura天文台(~2800米)作为一个对大气中微子计算的定标和探测器测试有意义的观测地点，我们进行了3周的观测。触发率与计算值在2~3%范围内一致。然而，伽马射线的垂直通量与计算结果有偏差。现在，通过改进数据分析方法，这一矛盾得到了很好的解决。第三年：1)观测是在气球高度进行的，因此预计会有一些带电粒子从探测器一侧进入。为了阻止这种颗粒，我们在DET…的顶部周围添加了4块大小为30cmx5cmx1.5 cm的塑料闪烁体更多的是，为了读出，我们在每个板上嵌入了8根细纤维，它们由一个小型光电倍增管读出。2)该气球于1999年9月2日在三陆气球中心发射升空，飞行约10小时后回收。从海平面到25公里的触发率与计算结果吻合较好，从而证明了计算和探测器的良好工作。在15.3、18.5、21.2和24.7公里高度实现了准水平飞行(~30min)。3)得到了这些高度的能谱，并将其转换为垂直通量。在3GeV到30GeV之间的功率很好地近似了光谱。利用BESS测量的初级强度进行计算，很好地再现了15~21公里的垂直通量。然而，观测给出的通量比25公里以上的计算得出的通量大，这似乎是由于探测器某些部分缺乏反符合闪烁体；在更高的高度，预计会有更多的初级质子，它们可以像中性粒子一样从没有闪烁体的缝隙进入探测器。用图像分析的方法可以对这些质子进行拒绝，初步结果表明我们可以拒绝这些质子。2000年，我们正在增加反符合闪烁体，并再次进行气球观测。利用这些结果和不同高度的µ子谱，我们预计可以减少可靠的中微子流量。较少

First year: We have upgraded the trigger system of the BETS originally designed for the observation of primary cosmic electrons; a multi trigger system was realized by which we can observe gamma rays with different trigger conditions simultaneously.Second year: We selected Mt. Norikura Observatory of Tokyo University (〜2800 m) as a site where we could do an observation meaningful for the calibration of atmospheric neutrino calculations as well as the test of the detector, We made 3 week observation. The trigger rate was coincident with the calculation within 2〜3%. However, the vertical flux of the gamma rays was deviated from the calculation. This contradiction was now well resolved by improving the data analysis method.Third year: 1) The observation is at the balloon altitudes so that a number of charged particles are expected to enter from the side of the detector. To reject such particles, we added 4 plastic scintillator plates of size 30cmx 5cmx 1.5cm around the top part of the det … More ector, For read-out, we embedded 8 thin fibers in each plate and they are read by a small PMT. 2) The balloon was launched at Sanriku Balloon Center on Sep., 2, 1999 and recovered after 〜10 hour flight. The trigger rates from the sea level to 25 km were nicely coincident with a calculation and thus proved that both of the calculation and detector were well operating. We realized quasi level flight(〜30 min) at 15.3, 18.5, 21.2, and 24.7km above sea level, 3) The energy spectrum was obtained at these altitudes, and they are converted into the vertical flux. The spectrum was well approximated by a power at 3 to 30 GeV. The vertical fluxes at 15 to 21 km were well reproduced by the calculations using the BESS measured primary intensity. However, the observation gave a larger flux than the calculation above 25 km, This seems to be due to the lack of anti-coincidence scintillator at some part of the detector; at higher altitudes, more primary protons are expected and they can enter the detector as if neutral particles from the gap where there is no scintillator. These protons could be rejected by employing image analysis and preliminary result showed we could to it. In year 2000, we are adding anti-coincidence scintillator and making a balloon observation again. Using these results and the muons spectrum at various altitudes, we expect we can reduce a reliable neutrino flux. Less