Effect of Gaseous Impurities on BF3 Proportional Counters

The effect which ${\mathrm{SiF}}_4$ , ${\mathrm{SO}}_2$ , and ${\mathrm{SF}}_6$ have on the plateau and the pulse size distribution of a proportional counter were investigated. ${\mathrm{SiF}}_4$ was tested at three pressures, 30, 45, and 60 cm, of ${\mathrm{BF}}_3$ and its effect was found to be independent of the counter pressure for the range of value considered. From the variation in the plateau, the attachment probability for ${\mathrm{SiF}}_4$ was calculated to be ${\text{h\hspace{0.17em}=\hspace{0.17em}1.485×10}}^{\text{−5}}$ and its cross section for attachment to be ${\sigma }_a{\text{\hspace{0.17em}=\hspace{0.17em}5.12×10}}^{\text{−20}}\hspace{0.17em}{\mathrm{cm}}^2$ . The amount of these gases permitted, without the counter being affected beyond the limits of tolerance which are set up, were found to be 0.04% for ${\mathrm{SiF}}_4$ , 0.01% for ${\mathrm{SO}}_2$ , and ${\text{2.0×10}}^{\text{−6}}\%$ for ${\mathrm{SF}}_6$ .