4.4 Speed of Response
Th
ere is another advantage to using thoron as a tracer
to locate submarine springs.
Th
at is the almost
instantaneous response of the RAD7 to thoron.
Th
e
fi
rst daughter of thoron, 216-Po, has a half life of
just 150mS.
Th
us in 0.5 seconds the RAD7 window
B, 216-Po, count rate will have nearly reached
equilibrium with the thoron in the chamber. So the
speed of response of the RAD7 to thoron is limited
not be the half life of the polonium daughter but by
the time it takes to get the sample into the
measurement chamber.
We have seen that with the highest sensitivity
con
fi
guration, using a separate pump to circulate air
round the loop, the total time for thoron to go from
the sampling point to the RAD7 is only about 1.5
minutes.
A boat carrying the system, therefore, moving slowly,
will see the thoron count rate increase within a
minute or two of passing over a submarine spring
and drop again shortly therea
ft
er.
Section 4
Thoron In Water
15
