The most probable ionisation losses have been measured with a long proportional counter with a 100 cm sensitive length, whose central wire is divided by a small glass ball into two sensitive sections of 10 and 90 cm, respectively. The counter is filled with Ar+10% CH4 at various pressures between 0.1 and 2.0 atm. Measurements of ionisation losses in the sensitive section of 10 cm for 300 MeV/c electrons passing parallel to the central wire are achieved with high accuracy under two different conditions of an incident particle direction. The results show that there exists no transition radiation effect such as Garibyan and Ispiryan (1972) had proposed to explain a discrepancy between the observations and the Landau-Sternheimer theory of ionisation loss.
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