426
18.
THE DETECTION ÔF PARTÏCLÉS
"
,
'The circuit
10 of gure 18—9 is capable of uhusually high count—
Ï
ing rates since the total resistance, R = R,; + Rp, through which
the tube disCharges, is from one one-hundredth to one onethousandth of the load resistance used in the preceding circuits.
ln one Case, using a 57 or 6C6 vacuum tube, RP was 2 megohms
and Rg ranged from 1 to 50 megohms, the larger values for smaller
(larger capacity) counter tubes. The grid bias (Eg) (0 to 15 volts
in the case above) must be. adjusted critically for a given value of "
Rg; if too small, the potential drop across RP lowers the voltage
supplied by Ez, to a value below the threshold of the counter tube
'
,
.
.
"
R
-
_
;Rg_
;+E
.
,
FIG. 18—9._ The Neher-Harper circuit.
f;
,
'
'
and, if too large, a blocking ' _ action occurs. When Eg has been
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