statistical noise on the decay rate for this sample was
about 3 counts/min, or a little less than 0.5%.
Using the first sample as a reference, I can measure
the half-life of I-131. Every few days, I measured the count
rate of this sample over a one hour period. This gave good
statistics. I then plotted up the count rate in counts per
minute, averaged over at least one hour. This is plotted on
a semi-log plot in Figure 16. When plotted on a semi-log
plot, the exponential decay rate is a straight line.
I fitted an exponential decay rate and got a
decay rate of 10.75 days.
The half-life is calculated as the exponential
decay rate x ln( 2) = 7. 45 days. This is pretty
close to the 8.03 day reported half-life of I-131. I
might have gotten a more accurate value if I was
more careful in preparing the sample instead of
just casually throwing it on top of the lid. The
position of the sample and the distance to the
detector is an important factor influencing the
measured count rate.
In addition, I did not do a very good job of
tracking the exact time of day I performed the
measurement. The precise time of each
measurement could be off by as much as 0.25
After the first few days, I’d collected enough
used kitty litter to bag up. Sure enough, it was
hot. I was getting counts of more than 500 counts/min
with the detector close to the bag. I had a five gallon pail
with a sturdy lid in the back yard to store the used kitty
litter until the count rate dropped down. I estimate this
will be in about four half-lives, or about one month. The
count rate will be just above the background by then and
I can send it to the municipal dump.
Maxwell came through his procedure with no
complications. One month after his treatment, his thyroid
and blood work had him almost back to normal. The vet
says we have to wait six months after the procedure for
the final verdict.
In the meantime, I have some other experiments
planned with my Soviet military surplus Geiger counter.
Assuming Homeland Security doesn’t come knocking.
August 2017 39
■ FIGURE 16. Decay rate of the first kitty litter sample averaged
over one hour, measured for three weeks.
■ FIGURE 15 Measuring the first kitty litter sample
with a precision distance from the detector.
■ FIGURE 14. Maxwell at 12 inches, with about an 850
counts-per-minute average reading on day four.