Chem143 Measuring Radiation

>>In this video, we’re
going to talk a little bit about measuring radiation, the
units that we use to measure it and a little bit about how we
measure it, just a little bit. So the tool that we
most commonly use to measure radiation is
called a Geiger counter. And the basic Geiger counter
works something like this. It has a tube filled with
a gas, maybe argon gas, and it has an opening,
well a covered opening so the gas can escape
or something that radiation can
pass through easily. And when that radiation passes
through and hits these atoms of argon gas, or whatever
the gas is, it ionizes. That means it knocks
an electron off and that creates a
positively charged atom, like an argon plus
charge, and an electron. Now in this tube, there’s a
positive end and a negative end. And anything with a negative
charge will be pulled towards the positive end, anything with a positive towards
the negative end. And when you create these
things with charges, the Geiger counter itself sends
a, there’s a signal that’s sent up this wire to the
Geiger counter and that causes the Geiger
counter to make a sound. The more radiation it hits, the more charged particles
there are, the more clicks that you hear on
a Geiger counter. That’s basically, you know, roughly how a Geiger
counter works. Now, the different types of
radiation that we’ve seen, basically the alpha
particles, beta particles and the gamma rays, what this
table does is it shows you about how far they can
travel through the air. Alpha particle not very
far, 2 to 4 centimeters. The beta particle
will go 2 to 3 meters, and a gamma ray can
go half a kilometer. That’s pretty far. How far do they penetrate
into biological tissue? Well, alpha particle not very
far, like a half of a hundredth of a millimeter, not very
far, just the surface, whereas a beta particle can
go 4 to 5 millimeters in. That’s enough to do some damage. And whereas the gamma ray
can basically go straight through a human being,
you know, half of a meter. And the way you shield against
these different particles, paper or clothing,
just something thin like that will shield
an alpha particle. For a beta particle, you need,
you know, really heavy clothing, you know, lab coats,
gloves, things like that. Whereas to shield a gamma ray,
you need lead or thick concrete. So the units that we
use when we’re talking about radiation depend
upon what we’re talking about that radiation doing. The common units we use
for what’s called activity, activity is just how
much of an isotope, of a sample of an isotope,
is decaying per second, and the common unit which we’ll
talk about is called the curie. The symbol is capital C,
lowercase i. So 1 curie is equal to 3.7 times 10 to the tenth
disintegrations per second, or decays per second. In other words, that many
isotopes decaying per second is 1 curie and that
number corresponds to the disintegration
rate of 1 gram of radium, a radioactive isotope of radium. And so if something had
an activity of 2 curies, it would be twice this, or we could have the SI
prefixes like micro or milli. If it’s 1 millicurie,
it’s 1/1000 of this or 10 to the seventh, 3.7
times 10 to the seventh. Alright, so that’s activity. That’s the unit we’ll use. Now how much they’ll
absorb the dose, radiation absorbed
dose, the rad. This measures how much energy
is hitting whatever is being irradiated per gram of
the irradiated material. So it’s joules, which
is energy per gram. And this gram is not grams of
the isotope but rather grams of whatever the radiation
is hitting, your body or maybe a detector or
something like that. And that’s about 1 times 10 to
the minus fifth joules per gram. So that’s different
from activity. Activity is how many of
these isotopes are decaying per second. Absorbed dose, or rad, that’s how much energy
is hitting 1 gram of the irradiated material. Now we also have
biological damage or the rem. And this is a measure of the
damage that radiation can do and it’s how much energy hits
per gram or rad times the RBE, radiation biological equivalent. And the RBE is just a factor
that you multiply the number of rads by and it depends upon
what kind of radiation there is. If it’s an x-ray, a beta ray,
or gamma ray, then RBE is just 1 and so the amount of damage, the
rem that it can do is the same as the number of rads. If it’s a high-energy proton or
a neutron, then the RBE is 10. So you multiply the
number of rads times 10. So it can do more damage. If it’s an alpha particle,
then the RBE is 20. And, you know, multiply
the number of rads times 20 to get how much damage or
the REMs that it can do. So we have activity, absorbed
dose, and biological damage. These three units
measure different things. So because activity
depends upon how much of an isotope is present,
we can treat it just like we can amount
of the isotope. So when we were doing
half-life calculations, we talked about if we
initially had this many grams of an isotope, how many grams
would be left, for example. We can talk about it the same
way but in terms of activities. So if we have this much
activity to begin with, after this much time,
what’s the activity is going to depend upon the half-life,
just like the amount did. So let’s do an example. So given that the half-life
for cerium-141 is 32.5 days, and if you have a
sample of cerium-141 that now currently has an
activity of 8.0 microcuries, remember this micro means
times 10 to the minus sixth, so it’s a, you know,
small amount. So anyway, if the activity
after 130 days is 8 microcuries, what’s the initial
activity, in microcuries? Well, we set it up
just like we did with the other half-life
problems. And in this one, there’s
two ways we can do it, just like before. We’ll talk about both of those. But first, no matter
what, we have to find out how many half-lives
have gone by. Well, 130 days have
passed and we know that there’re 32.5 days
per every one half-life. So if you just divide
the two numbers, it tells us that 4
half-lives have gone by. So one way to do it would be to
kind of work our way backwards. Okay? So if we now
have 8 microcuries, so that would be
after 4 half-lives. So if we double that
times 2, 8 times 2 is 16, that means after 3 half-lives,
there are 16 microcuries. Double the 16 is 32,
so after 2 half-lives, there is 32 microcuries
of activity. And then double that, that’s 64. That means after 1 half-life, there were 64 microcuries
of activity. So if we double that one more
time, 128 or we get 2 sig figs so round it to 130 microcuries
was the initial activity or we can use a formula just
we did on the other problem. That is if we take
the initial amount, or in this case the initial
activity, times 1/2 raised to the power of however many
half-lives have gone by, that’s equal to how much we
have left, the current activity. In this case, we’re solving
for the initial activity. In the problem that we
did before, we solved for, I believe, how much was left. So if we just divide both sides
by 1/2 to the fourth power, we can take 8 divided by
1/2 to the fourth power and you guys should put
this in your calculator and make sure you
get the same number. And this gives us
128 microcuries, which again two sig figs
because of this 8.0, rounds to 130 microcuries
of initial activity. Subtitles by the community

3 thoughts on “Chem143 Measuring Radiation

  1. You made this so easy for me to understand a million thank you. My Bio Chem professor goes way to fast for me to understand him.

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