>>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 Amara.org community

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.

Time domain energy nah????

I never took chemistry before ?? but soon I Hav too