Although
it seems like this post should include some commentary on zombies or
video games, we’re going to focus on the term ‘half-life’ as it’s used
in physics, this time. The reason for this is that important research
has been published last month on geothermal heat produced by radioactive
decay.
But before all that complicated stuff let’s start at the
beginning. ‘Half-life’ is actually shortened from ‘half-life period’
which refers to the time in which exactly half of a radioactive
substance decays. This measurement is especially useful because
radioactive materials decay exponentially – meaning that they decay much
more quickly at first than later on, where the decay process drags on
more slowly. This decay rate is directly connected to the rate at which
radioactive materials emit radiation.
Let’s
take iodine as an example. I-131 has a radioactive half-life of just
over 8 days and gives off both alpha and beta radiation (for a
discussion of these radiation types see this post). As I-131 atoms give
off radiation they transform into atoms of Xe-131, a stable (and
non-radioactive) isotope1.
That means if you start with a pure sample of I-131, after 8 days about
half of the sample will be I-131 and half will be Xe-131. If you wait
another 8 days, 1/4 of the sample will be I-131 and 3/4 will be Xe-131,
and so on. As you may expect, the sample of I-131 will emit much more
radiation right at first versus many days later on, when the majority of
the sample is Xe-131.
Not all
materials have a half-life short enough to notice. In fact, the
half-lives of radioactive materials can vary from fractions of a second
to billions of years. These differences lend themselves to varied
applications. Isotopes with short half-lives (such as I-131, Tl-201,
In-111, and Tc-99) are commonly used in medical imaging and therapy
because they show up clearly in the body and become non-effective
quickly so that the patient is not exposed to too much radiation2.
Isotopes with long half-lives (such as U-238, C-14, and K-40) are often
used in radiometric dating, where scientists can measure the abundance
of these isotopes in various materials to determine their age3.
Newly
published research4
from Japanese and Italian scientists also suggests that over half of
the internal heat produced by the earth is caused by long-lasting
radioactive materials such as thorium, uranium, and potassium – a
quantity that adds up to nearly twice as much energy used annually by
everyone on the planet5.
The fact that radioactive materials are responsible for the heat is
important because it helps to explain why our earth is hot enough to
produce volcanoes, mountain ranges, and general plate tectonics while
other planets in our solar system have long since gone cold. The
geothermal heat of our planet isn’t going to cool soon either, thanks to
the fact that the isotopes producing the heat have half-lives of
billions of years.
So
although the adage “all good things must come to an end” (and all bad
ones, too!) may be a great application to radioactive materials,
there’ll be plenty of radiation and geothermal heat for years to come.
5)http://www.scientificamerican.com/blog/post.cfm?id=nuclear-
fission-confirmed-as-source-2011-07-18
D-tect Systems is
supplier of advanced radiation and chemical detection equipment sold
around the world. www.dtectsystems.com.
