32 August 2017
Aweb search revealed there is another treatment for hyperthyroidism with a 95% success rate in cats, based on radioactive Iodine-131. The idea
is to kill off the overactive regions of the thyroid with
radiation. Because the thyroid selectively absorbs iodine, a
radioactive form of iodine is injected into the cat. It
concentrates in the thyroid and over a few weeks, kills off
some of the thyroid. When done correctly, this is a
permanent cure. However, it means making him
radioactive, but only for a few weeks.
Radioactivity and Iodine
Every element on the periodic table has just one
unique atomic number. This is the number of protons in its
nucleus; the same as the number of electrons in the atom.
The atomic number of iodine is 53. Iodine can have a
wide range of neutrons in its nucleus. We call these
different forms of iodine (with different numbers of
There are 37 known isotopes of iodine — all with 53
protons in the nucleus, but with 55 to 93 neutrons. We
track the total number of particles in the nucleus as the
atomic weight. For iodine, it varies from I-108 to I-144.
All of these isotopes are radioactive except I-127,
which is stable. Iodine-131 has special use in medical
treatments. This is partly because iodine is biologically
active and is selectively taken up by the thyroid. It is a
beta and gamma emitter with a half-life of eight days. This
is perfect for targeted radiation attacks on thyroid cells.
There are three types of radioactive particles emitted
in radioactive decay: alpha particles, which are two
protons and two neutrons; beta particles, which are high
energy electrons; and gamma particles, which are high
energy light photons.
Alpha particles have the potential to produce the
most biological damage, but they don’t penetrate much
more than the thickness of a piece of paper. However, if
the radioactive source is ingested or inhaled, it can do a
lot of local cell damage.
Radon is a noble gas which decays and emits alpha
particles. Breathe it in and it decays in your lungs, and you
have a higher chance of contracting lung cancer. This is
why we test for radon in basements where it is a decay
product of uranium into radium, and then into radon,
naturally occurring in bedrock.
Beta particles don’t penetrate more than a quarter of
an inch in tissue. This means that if they can be placed
strategically in tissue you want to kill off, they will deposit
all of their lethal effects very localized. Beta emitters make
the perfect surgical strike killers.
Gamma particles are just a form of light but of very
short wavelength, or high energy per photon. They travel
much farther through the body and through air than beta
particles. These will make it out through the body and are
used to image deep tissue. This is the basis of PET
(Positron Emission Tomography), for example. You’re
injected with a biologically active material that emits
positrons — an anti-electron. As soon as it is emitted, it
immediately annihilates with the first electron it
encounters, and the pair gives off two gamma particles
which make their way out of the body and are detected.
Iodine-131 decays into Xenon-131 by emitting a beta
particle with energies ranging from about 190 keV to 606
keV. This is what does all the medical work. This is a lot of
energy to be deposited locally into the targeted tissue. The
new Xenon nucleus is created in an excited nuclear state.
Immediately after the neutron emits the beta particle and
decays into a proton, the new proton jiggles around, drops
down to a ground state, and emits a gamma particle with
an energy of 364 keV.
By comparison, a dental x-ray may be as high as 70
keV. This 364 keV gamma ray particle makes its way
outside the body and we see it as “radiation.” With I-131,
we get a 2-for-1. The beta particle does the therapeutic
work and the gamma particle allows us to see it in action.
Every gamma particle we detect means there was a beta
particle killing off thyroid tissue.
The half-life of I-131 is eight days. This means every
eight days, the number of I-131 atoms left is reduced in
half, and the rate of decay has decreased by half. After 16
days, there are about 1/2 x 1/2 or 25% of the starting
number of I-131 atoms left, which means the radiation
emitted is reduced to 25%. After 60 days (or about eight
half-lives), there is only 1/ 2^ 8 = 0.4% of the radiation
levels as initially.
I expected Maxwell’s radiation levels to drop off faster
than an eight day half-life because he was constantly
losing radioactive iodine from his body as it cycled
through. It was flushed out of his system and ended up in
his kitty litter, which would also be radioactive.
The vets at the clinic warned us not to spend much
contact time with Maxwell until his radiation levels
dropped and to not throw his litter out in the trash. They
said the radiation levels in his used cat litter might set off
radiation detectors at municipal dumps.
I wanted to build a radiation detector to see how hot
Maxwell was initially, and how quickly his radiation levels
(and his used cat litter) dropped off.
Since I was only going to detect gamma particles, a
Geiger counter was the obvious detector choice. The
original name for the detector is really a Geiger Muller
tube, named after Hans Geiger and Walther Muller, who
developed the detector in 1928. It’s sometimes referred to
as a GM tube, including both inventors.
A GM tube is basically a vacuum tube with about
1/10 an atmosphere of air. A central wire (the anode) is at
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