The Pulse Shape of a Proportional Counter

The shape of the pulse of a proportional counter is understood as one follows the events that lead to the formation of the pulse. A cylindrical counter will be considered, such as that shown in Fig. 5.14. 

The pulse V t) is shown by the solid line of Fig. 5.17. The pulse rises quickly and reaches half of its maximum in time of the order of microseconds. Then it bends and rises at a much slower rate, until about a millisecond later it reaches its final value, Q/C. 

The pulse of Fig. 5.17 was derived under the assumption that all the ions were produced at the same point. In reality, the ions are produced along the track of the incident particle. This modifies the shape of the pulse during its initial rise but it leaves it virtually unaffected during the later period. 

The pulse of Fig. 5.17 is unacceptably long, even for a modest counting rate. As in the case of the ionization chamber, the pulse is chopped off at some convenient time with the help of a differentiating circuit (Chap. 10). The result will be a pulse shown by the dashed line in Fig. 5.17. 

3 The Change of Counting Rate with High Voltage— 

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