The basic characteristics of the two commercially available cylindrical proportional counters used in this study are summarized in Table 1. The measurement system consists of a high voltage power supply, a preamplifier suitable for proportional counters (Canberra model 2006), an amplifier (Tennelec model TC243 or Canberra model 2025) and a computer-based multichannel analyzer (Tennelec PCA III or Canberra model Multiport II)