Radioactivity and Half-Life Demonstration Background

This demonstration simulates radioactive decay. A sample of a radioactive element consists of a large number of radioactive nuclei that do not all decay at the same time. As unstable atoms are transformed, the radioactivity of a substance decreases. The time required for the initial number of radioactive nuclei to decrease by half is called half-life. Half-life is a characteristic of each radioactive isotope. Depending on the isotope, it may range from a few fractions of a second to several billion years, as shown in the following chart:

Isotope	Half-life
Polonium-214	0.164 second
Oxygen-15	2 minutes
Bismuth-212	60.5 minutes
Sodium-24	15 hours
Iodine-131	8 days
Phosphorus-32	14.3 days
Cobalt-60	5.3 years
Carbon-14	5,730 years
Plutonium-239	24,110 years
Uranium-238	4.5 billion years

As radioactive elements decay, they change into other elements. Decay continues until a nonradioactive nucleus is formed. This process is called the decay series. There are 3 types of radioactive decay

• Alpha decay: The nucleus releases an alpha particle. This decreases the mass number by 4 and the atomic number by 2. Uranium-238 undergoes alpha decay.

• Beta decay: The nucleus releases a beta particle. This does not decrease the mass number, but does decrease the atomic number by one. Carbon-14 undergoes beta decay.

• Gamma decay: The nucleus releases a gamma ray. Gamma decay almost always accompanies alpha and beta decay. The nucleus does not change. It makes a transition to a lower energy state.

The radioactivity of a substance is measured by the number of decays that occur during a given period of time. Here are 2 common units of measurement for radioactivity

• The curie (Ci) - 1 Ci = 3.7 x 10¹⁰ decays/sec

• The becquerel (Bq), an SI unit 1 Bq = 1 decay/sec

These units do not accurately measure how dangerous a given amount of radiation might be for humans. For medical purposes, other units of measurement reflecting this aspect are more appropriate.

The exercise --

Materials needed

•

Shoebox (or similar size box) with lid

•

200 pennies (or any coins of the same denomination)

•

Graph paper

•

Notebook paper

•

Pen or pencil

Procedure

1.

Put the 200 coins in the shoebox tails up. Do not overlap them.

2.

Place the lid on the box.

3.

Hold the box with your thumbs placed on top of the lid.

4.

Shake the box with quick up-and-down motions.

5.

Set the box down. Take off the lid and remove all the coins that face heads up.

6.

Record the number of coins remaining in the box in the chart below.

7.

Repeat steps 2 through 6 until no coins remain in the box.

Trial	Coins remaining
0
1
2
3
4
5
6
7
8
9
10
11
12

8.

Prepare a graph of your data. Let the x-axis represent the number of trials. Let the y-axis represent the number of coins remaining in the box. Be prepared to interpret the graph and discuss your results.