to ion chasbers, scintillation ¢stectors, or photoconductive crystals.
See Figure 2.1, In operation, the charge on C, holds tube 7, well
beyond cutoff. The output current ef the sensing elenent discharges C,
at 2 rate dependent won the redistion level. when the voltage at the
grid of T, reaches the grid bees, T, conducts, feeds 8 negetive signal
the grid of T., and initiates a regenerative action which rapidly cuts
T,+ Then C, chazyes to # potential equal to B-plus less the cathode
voltage and the grid-to-cathode drop through the diode action of the g@
of Ty»
When C, is completely charged, the circuit returns to its nome
condition of T, conducting end T, cutoff.
The circuit will seman in %
condition until C, 4s once more discharged by the output of the senein:
elenent. The output of this clrowlt consists of pulses that have a’Se;
rete proportional to the cutpyt current of the sensing element.
general, decay of the gamme-exposure rate from fallout contamination i:
Given byt
where:
I= 1,0
(2.1)
I = the gamme-expoeure rete at tine +
1, = the gume-eposure zete at unit time
x = the decay constant (given as 1.2 for groes fission producti
Measurements of the decay constant require good (short) time
resolution at early times (< emell, I Large) when the changes in game:
exposure rate are most rapid. At later times (t large, I small), the
rate of change of the gamma-exposure rate of wreseme radiation is
much emaller, and the instrument systen need nothavesuch goodtime
resolution.
The instrusent for the measurement of residuel-gasue