Performance in the Presence of Gamma Flux’

Tests have been performed in the GETR to determine the effect of gamma flux on the counter’s sensitivity. A bundle of three detectors was lowered into the Z trail cable with the reactor at full power. The detectors in the bundle were Counter No. 1, Campbellbr No. 3, and an unlined 1/4-inch — diameter ion chamber. The reactor was then scrammed and the outputs of the three detectors re­corded as a function of time.

The gamma flux at any time was determined by the reading of the unlined ion chamber. The neutron flux was determined by the reading of the Campbell channel using a neutron sensitivity of

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1. 89.x 10 nv/div, after correcting the reading for gamma contribution using a gamma sensitivity of 3.23 x 10 R hr /div. The counting rate was determined by the reading of Counter No. 1 after correcting for counting losses by using the data of Figure 6-13. ■

The results are shown in the following graphs. Figure 8-1 shows the gamma flux, Figure 8-2 shows the total reading of the Campbell (MSV) channel and those parts of this reading that are caused by gammas and by neutrons, Figure 8-3 shows the observed and corrected counting rates, and Figure 8-4 shows the calculated counting sensitivity. There is’ considerable spread in the count­ing sensitivity before 19 minutes, when the correction for counting loss is large, but after that time the correction is smaller and the spread in the counting sensitivity is quite small.

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The average counting sensitivity, after 18 minutes from scram, is 0. 67 x 10 sec nv in an

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average gamma flux of 2.2 x 10 R/h. This is to be compared with a counting sensitivity of 1. 52 x 10’^ sec”^ nvat gamma flux of about 10^ R/h

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♦Assuming sources provide 10 nv to the detector at startup. ,

4 x 107