Science at Home Videos

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Joanna and Steve: Just science!

Joanna: Hi! I'm Joanna!

Steve: And I'm Steve!

Joanna: Today, you're going to measure the half-life of a radioactive isotope, metastable barium-137! This is an actual experiment that you're going to do using our equipment!

Steve: The half-life of a radioactive substance is the amount of time it takes for half of that substance to undergo decay. Now, half-lives can range from billions of years to a tiny fraction of a second. Happily, the half-life of metastable barium-137 isn't too long and isn't too short. It's perfect for doing a half-life experiment like this within a reasonable amount of time.

Joanna: First off, we need a sample of radioactive barium. We're going to get it from this thing. This device contains a salt made from radioactive cesium-137. As it decays, the cesium turns into barium-137 through beta decay.

Now, barium-137 is a little unusual. Some of the barium-137 atoms that are produced are stable and don't decay any further. Most, however, are what's called metastable. This means that their nuclei contain a little extra energy that isn't released right away. When it is released, it's usually released in the form of a gamma ray. This type of radioactive decay is known as 'isomeric transition.' Unlike alpha and beta decay, it doesn't change the composition of the nucleus. So, you start with metastable barium-137 and you end up with stable barium-137.

So, how do you separate the barium from the cesium? Well, luckily, that's the easy part. Cesium and barium are chemically different from one another. We can easily separate the barium and cesium salts by using a solution of hydrochloric acid and sodium chloride. We simply wash the solution through the generator and the barium is eluted out!

Steve: Now that we have a source, we need a way to measure its activity. We're going to do that using two different detectors. #noname The first one is a Geiger-Müeller tube. It's a cylinder surrounding a wire that's kept at a high voltage. The cylinder is filled with a low pressure gas and when ionizing radiation passes through it, it can knock electrons off of some of the gas particles. Those electrons are then accelerated towards the central wire. As they accelerate towards the central wire, they pick up speed and eventually they're able to knock other electrons off of other gas particles. Those are then accelerated and can knock other electrons off of other gas particles. What you eventually end up with is an avalanche of electrons that strikes the central wire. That creates a current that can be detected and counted with the scaler.

The second one is a scintillator attached to a photomultiplier tube. The scintillator is a crystal of sodium iodide and when ionizing radiation passes through it, it gives off a little flash of light. That light then strikes the front of the photomultiplier tube, and it knocks an electron off the surface of the coating. That electron is then accelerated through a series of little paddles called dynodes. And the trick here is that when an electron strikes a dynode, more than one electron pops off. So, by the time you reach the end of the dynode chain, you again have an avalanche of electrons that can be detected and counted with a scaler.

We're going to place the source between the two detectors and you can choose which one to use. The GM has a lower count rate than the photomultiplier tube, so it'll be easier to read because the numbers won't be flying by really fast. But since it has a lower count rate, it has greater statistical error. You'll get better data using the photomultiplier tube, even if you can't tell what the last couple of digits are.

Joanna: Now we're ready to start! We're going to do a series of trials and place them in their own separate video. Then we're going to do a video to show you how to calculate the half-life using the data that you collected. #noname But, for now, thanks for watching! I hope you'll join us again soon as we continue this experiment!