Half-Life Definition•Half-life: the TIME it takes for half of a

radioisotope sample to ‘decay’ and become another element.

• What are the three ways this can happen?–Alpha ()–Beta ()–Gamma ()

Notice: it takes a very long time to completely decay!

Left with at least a tiny amount for > 70 half lives!

Example:

Thorium-234 half-life is 24 days.• Begin with a 1 kg of Th-234, after 24 days,

you will have ½ kg of Th-234 (other ½ kg is something else).

• After 48 days?– ¼ kg (0.25 kg = ½ of ½ )

• After 72 days?– 1/8 kg (0.125 kg = ½ of ½ of ½ )

• After 96 days?• 1/16 kg (0.0625 kg = ½ of ½ of ½ of ½ )

Now you try!Now you try!Start with 400 grams of Iodine- 131

Iodine- 131 has a half life of 8 days

In 8 days you will have?

In 16 days you will have?

In 24 days?

How long to get down to 25 grams?

The half-life for the beta decay of iodine-131 is 8 days.

Half-life Graph

Starting Sample of Iodine-131

Half-life GraphThe half-life for the beta decay of iodine-131 is 8 days.

Sample after 1 cycle

Half-life GraphThe half-life for the beta decay of iodine-131 is 8 days.

Sample after 2 cycles

Half-life GraphThe half-life for the beta decay of iodine-131 is 8 days.

Sample after 3 cycles

Iridium-182 undergoes beta decay to form osmium-182. The half-life of iridium-182 is 15 minutes. After 45 minutes, how much iridium-182 will remain of an original 1-gram sample?

• Calculate how many half-lives will elapse during the total time of decay.

Half-life Calculations

• After three half-lives, the amount of iridium-182 has been reduced by half three times.

Half-life Calculations

• After 45 minutes, 0.125 gram of iridium-182 remains. (1g x 1/8 = 0.125g)

• 0.875 gram of the sample has decayed into osmium-182.

• What if we wanted to know the amount of iridium after 150 minutes? (10 Half-Life cycles)

• What are you going to do?• After 45 minutes, the amount of iridium-182 was

been reduced by half three times.

Half-life Calculations

# of Half-Life Cycles

X Y-grams= Z-grams

Half-Life

Starting Amount of Isotope Amount of Isotope Left

Half-life Calculations

Half-life Calculations

2.23 x 10 -7g2.32 x 10 -7g