Study of Landau-Pomeanchuk-Migdal e ffect

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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.