Several gamma background situations are now considered and plotted:

Case І: Ф = 10′ R/h.

= 0.110+ (3.0X10"9) (107 R/h) = 0. 140

and it can be seen from the graph that the count rate is less than 0. 01 sec* at a discriminator setting of 2. 5 volts,

Case II: At a count rate of 1 count/sec due to gamma, with discriminator set at

2. 5 volts,

c _ -5 •

4. 75

A plot of count-rate indication versus discriminator setting, at 1.45 x Ю4 nv

. ‘• П •

neutron flux and with both zero gamma and 2.5×10 R/h gamma, is shown in Figure 6-9 in Section VI. .

Case III: Conversely, if the area of the counter electrodes were larger, the count

rate due to gamma would increase. For a counter with 10 times the present

• • 7 •

electrode area the variance is found to be at 10 R/h, ‘a 2. (107 R/h) = 0.30,

thus
or
S

The large increase in pile-up count rate with increased electrode area shows that a larger counter would not be desirable.

The second possible effect that could limit the low end of the range is the electric potential distribution change in the gas volume of the chamber (space charge effects). When the flux levels are high enough, the electric field will go to zero in the vicinity of the positive electrode, and recombination will commence. As a result both the pulse height and the average current will reduce in magnitude. This effect, however, does not occur at flux levels that will be encountered by the in-core counter; for example, it would commence for Counter No. 1 at a gamma level of 4 x 108 R/h.