brought it close to the Geiger counter (shown in
Figure 10). To my delight, the buzzer starting
buzzing and the LED light started flashing. When
the source was right on top of the Geiger tube, the
count rate was 950 counts per minute, as
compared to about 45 counts/min as background.
My detector was working! A recording of this
source is shown in Figure 11.
Maxwell is Hot
One week after his injection of I-131, we
picked up Maxwell at the vet and brought him
home. It was a little hard to get him to stay still
long enough to measure his count rate. I had cut
the counting interval down to 10 seconds, but I
still needed the cat to stay still for a few minutes to
get good statistics. Figure 12 shows my first
attempt to get a reading on Maxwell.
I found early on that his count rate was
strongly dependent on how close he was to the
detector. To get stable and reproducible readings
and compare his count rate changes each day, I
decided on measuring him with a fixed distance of
12 inches from his neck. I also waited for him to
settle down in one of his favorite napping
locations. Figure 13 shows the typical setup I used
to get reproducible measurements.
On his fourth day home, his count rate was
about 850 counts per minute at 12 inches. Figure
14 shows these initial stable readings.
On day 11, Maxwell was down to 160 counts
per minute. By day 13, he was down to 92 counts
per minute — only twice above background.
Unfortunately, his poop wasn’t.
What Do You Do with
Part of the reason Maxwell’s counts dropped so fast
was that he was excreting a lot of the radioactive iodine.
This meant his kitty litter was radioactive. I scooped up
the first sample and kept it in a bag to use as a reference.
The decay rate of this clump should reflect the decay rate
of I-131. I kept this around my lab and measured it
periodically, trying to be careful to keep it the same
distance from the detector each time.
I used the lid of the plastic box of the Geiger counter
as the reference position. Figure 15 shows the sample
resting in the detector box while I took readings for an
hour to get good statistics. With a count rate of about 600
counts/min over one hour, I collected about 36,000 total
counts. The standard deviation on this is about
sqrt( 36,000) = 190 counts/per hour = 3 counts/min. My
38 August 2017
■ FIGURE 11. Background count rate and when the smoke
detector source is on top of the GM tube.
■ FIGURE 12. First reading of counts from Maxwell. He was at
more than 6,000 counts per minute — almost at the
■ FIGURE 13. Maxwell napping while I measured his
count rate with the detector 12 inches from his neck.