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

第一年：我们升级了原设计用于观测宇宙初级电子的BETS触发系统；实现了多触发系统，可以同时观测不同触发条件下的伽马射线。第二年：我们选择了东京大学的Norikura山天文台（~ 2800 m）作为观测地点，对大气中微子计算的校准和探测器的测试有意义，我们进行了为期3周的观测。触发率与计算值吻合在2 ~ 3%之间。然而，伽马射线的垂直通量与计算有偏差。现在通过改进数据分析方法很好地解决了这一矛盾。第三年：1)观测是在气球的高度，因此预计会有一些带电粒子从探测器的侧面进入。为了排除这些颗粒，我们在探测器顶部增加了4个尺寸为30cmx 5cmx 1.5cm的塑料闪烁片，用于读出，我们在每个板中嵌入了8根细纤维，并通过小型PMT读取。2)该气球于1999年9月2日在三陆气球中心发射，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很好地吻合，从而证明计算结果和探测器都运行良好。我们在海拔15.3、18.5、21.2和24.7km处实现了准水平飞行（~ 30 min）， 3)得到了这些高度的能谱，并将其转化为垂直通量。光谱很好地近似于3至30 GeV的功率。利用BESS测得的初级强度进行的计算很好地再现了15至21公里处的垂直通量。然而，在25km以上，观测得到的通量比计算得到的要大，这似乎是由于探测器的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间隙进入探测器。利用图像分析可以排除这些质子，初步结果表明我们可以排除这些质子。2000年，我们又增加了反巧合闪烁体，并再次进行了气球观测。利用这些结果和不同高度的μ子谱，我们期望我们可以减少可靠的中微子通量。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