Study of Landau-Pomeanchuk-Migdal e ffect

当高能电子进入非晶介质时，由于邻近原子的多重库仑散射，伽马射线的产生明显受到抑制。多重散射对这种电子轫致的影响被称为Landau-Pomeanchuk-Migdal （LPM）效应。淋浴起始点对电子轫致截面非常敏感，在LPM效应的实验检验中是最有效的。为了研究高能电子的阵花起始点，于2007年7月在欧洲核子研究中心的H4束流线上，将9个辐射长度的乳剂腔暴露在20GeV、50GeV、100GeV和250GeV的电子束下。乳化液室（ECC）由核乳化液板和铅板交替堆叠而成。在ECC中，可以以一微米或更小的精度测量每个乳液板中淋浴轨迹的位置。由于这种高分辨率，我们可以详细地检查阵雨的起始点，并明确区分由电子、伽马射线和强子事件引起的阵雨。暴露后，ECC被空运到日本，开始分析电子阵雨。然而，通过不完整的暴露条件设置来检查淋浴点分布所必需的数量的电子没有暴露。因此，2008年7月在欧洲核子研究中心的H4束流线上进行了一次重复暴露实验。波束时间被接受。

When the electron of high energy enters into the amorphous medium, the generation of gamma rays is remarkably suppressed because of the multiple Coulomb scattering by neighboring atoms. The influence of the multiple scattering on this electron bremsstrahlung is known as the Landau-Pomeanchuk-Migdal (LPM) effect. The shower starting points, which is very sensitive to the cross section of electron bremsstrahlung, is the most effective in an experimental examination of the LPM effect. To study the shower starting point of high-energy electrons, emulsion chamber of 9 radiation length was exposed to 20GeV, 50GeV, 100GeV, and 250GeV electron beams in the H4 beam line of CERN in July 2007. Emulsion chamber (ECC) consist of nuclear emulsion plates and lead plates, which are stacked alternately. In ECC, it is possible to measure the location of shower tracks in each emulsion plate with a precision of one micron or less. Because of this high resolution, we can inspect the shower starting points in detail and unambiguously distinguish shower due to electrons, gamma rays, and hadronic events. After exposed, ECC was airlifted to Japan and started the analysis of electron showers. However, electrons of a number necessary for examining the shower starting point distribution by an incomplete setting of exposure conditions are not exposed. Therefore, a repeated experiment of the exposure is conducted in the H4 beam line of CERN in July 2008. The beam time was accepted.