3.4 NEUTRON-INDUCED ACTIVITY
The MG-3 detector head buried at 30 yards from ground zero operated satisfactorily.
rate versus time was obtained for 12 minutes after time zero.
Dose
The dose rate beyond this time
was below the sensible threshold of the instrument (0.1 r/hr).
Figure 3.11 shows that the gamma radiation decayed according to the equation y = at > after
0.4 minute, where y was the dose rate in roentgens per hour, and t was the time in minutes.
The constants in this equation were a = 2.89 and b= 1.21. These were determined from a
least-squares fit of the data points. The decay exponent of -1.21 suggests that the detector was
measuring fission-product decay, rather than neutron-induced activity in the soil. Thus, it is
believed that close to ground zero the radiation level from induced activity of the soil was insignificant when compared to that from fission products.
As mentioned in Section 2.2.5, the detector head was buried below ground in boric acid and
paraffin to minimize neutron-induced activity in the head itself. From all indications, it is
certain that there was no activation of the detector head.
Calibration of the chart recorder indicated that the chart read within +1 percent of the detector
reading and that the lag time of the chart needle was less than 1 second. Thus the decay recorded
was the actual decay of the radioactivity above ground.
3.5 FIELD TEST OF MG-3
The fallout-radiation dose rate at the far-out MG-3 station (650 yards, 85-degree axis) was
never high enough to be recorded by the chart recorder (0.1 r/hr). However, the close-in MG-3
did operate satisfactorily (Section 3.3). On this basis, it is believed that the field testing of the
MG-3 fallout detector was a success.
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