Sec. 10.21] 



GEIGER-MULLER COUNTERS 



327 



With the efficiency of each counter known, the total efficiency of an r-fold 

 coincidence arrangement is simply the product of the separate efficiencies 



or e t = e\t2 



e r . 



c 



-K- 



OUTPUT 



— O 



NEG. 

 PULSE 



10.20. Averaging Effect of Scaling Circuit. The slowest component in a 

 counting circuit is the mechanical register. Even the most efficient registers 

 are slower than the counter tube by several orders of magnitude. In most 

 cases, therefore, scaling circuits are advisable even at moderate counting 

 rates. For a scale factor 5 and a register resolving time T\ no counts are 

 lost or corrections necessary if T\ < st, where r is the counter-tube resolving 

 time, since the counter tube cannot produce two pulses in an interval shorter 

 than the resolving time of the register. On the other hand, it more frequently 

 occurs that T x > st, indicating that pairs of output pulses from the scaling 

 circuit can occur in intervals shorter than the register can function. A 

 scaling circuit, however, tends to average the intervals between output pulses 

 since it passes only one pulse after receiving 5 random input pulses and the 

 intervals tend to be more uniform. For high scale factors, 32 and greater, the 

 probability that pulse intervals for a counting rate n deviate markedly from 

 s/n is small. Thus, for a scale of 64, the standard deviation of the lengths 

 of intervals from the average is only one- 

 eighth the average interval [36]. For 

 most purposes, register losses are negli- 

 gible when Ti < s/n for scale factors of 

 32 and greater. At high counting rates, 

 however, it is usually desirable and often 

 necessary to use scale factors of 128 or 

 decade circuits of 1,000. 



10.21. Quenching Circuits. Quench- 

 ing circuits serve to extinguish the dis- 

 charge in a counter tube by suddenly 

 lowering the tube potential below the 

 voltage at which a discharge can con- 

 tinue. This action should occur in an 

 interval that is short as compared to the 

 drift time of the positive ions, and the 

 operating voltage should be returned to 

 the tube by the time all positive ions have been swept out. The recovery 

 time of the external circuit should be shorter than that of the counting tube 

 but not so short as to allow continuation of the discharge. Generally it is 

 in the order of 10~ 5 sec. 



External quenching circuits are essential to the operation of non-self- 

 quenching counter tubes but usually are not necessary for self-quenching 

 counters. Nevertheless, they are frequently used as the first stage for any 



+ H.V. 

 Fig. 79. Resistance quenching cir- 

 cuit. When the counter tube dis- 

 charges the current that flows through 

 the tube is sufficient to lower the 

 potential across R below the Geiger- 

 Miiller threshold voltage. The time 

 constant of 10~ 2 — 10~ 3 sec is too long 

 for most applications. R = 10 7 — 10 9 

 ohms, C = 2 — 10 yu/xf d. 



