HalfHalf--LifeLife

••

HALFHALF--LIFELIFE

is the time that it takes for is the time that it takes for 1/2 a sample to decompose.1/2 a sample to decompose.

Determining Age using Radioactive Dating

• Half-life Ex: Carbon-14 = 5730 years»Uranium-235 = 704 million years

Common Radioactive IsotopesIsotope Half-Life Radiation

Emitted

Carbon-14 5,730 years β, γ

Radon-222 3.8 days α

Uranium-235 7.0 x 108 years α, γ

Uranium-238 4.46 x 109 years α

Radioactive Decay Graph

Imagine a batch of 36 parent atoms. These spontaneously decay to daughter atoms (in green).

This graph shows the number of parent atoms vs. time for a generic radioactive isotope.

HalfHalf--LifeLife

Decay of 20.0 mg of Decay of 20.0 mg of 1515O. What remains after 3 halfO. What remains after 3 half--lives? lives? After 5 halfAfter 5 half--lives?lives?

Sample Problem

• The half-life of K-42 is 12.4 hours. How much of a 750g sample is left after 62 hours? Solution:

62 hours/ 12.4 hour = 5 half-lives have gone by.

750g x .5 x.5 x .5 x .5 x .5 (each .5 is a ½

life)

Answer:

23.4 g

Radiation is a natural phenomenon

We are exposed frequently to sources of radiation (most naturally)

In fact, you emit radiation from K-40 inside you.

Learning Check!

The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 39 hours?

Effects of RadiationEffects of Radiation

Geiger Counter

• Used to detect radioactive substances

Radiocarbon DatingRadiocarbon DatingRadioactive CRadioactive C--14 is formed in the upper atmosphere 14 is formed in the upper atmosphere

by nuclear reactions initiated by neutrons in by nuclear reactions initiated by neutrons in cosmic radiationcosmic radiation

1414N + N + 11 oo n n ------> > 1414C + C + 11HH

The CThe C--14 is oxidized to CO14 is oxidized to CO22 , which circulates , which circulates through the biosphere.through the biosphere.

When a plant dies, the CWhen a plant dies, the C--14 is not replenished.14 is not replenished.But the CBut the C--14 continues to decay with t14 continues to decay with t1/21/2 = 5730 = 5730

years.years.Activity of a sample can be used to date the sample.Activity of a sample can be used to date the sample.

Important Uses of Radioactive Isotopes- Bone Scans

Patient is injected with a radioactive isotope (Tc-99) that is attached to another molecule (phosphate).

This molecule with a radioactive tag travels through the body and accumulates in areas that bone growth is high (injuries).

A special scanner picks up on the gamma rays being emitted by Tc-99

Nuclear Medicine: ImagingNuclear Medicine: Imaging

Thyroid imaging using TcThyroid imaging using Tc--99m99m

Food IrradiationFood Irradiation

••Food can be irradiated with Food can be irradiated with γγ

rays from rays from 6060Co or Co or 137137Cs.Cs.••Irradiated milk has a shelf life of 3 mo. Irradiated milk has a shelf life of 3 mo. without refrigeration.without refrigeration.••USDA has approved irradiation of meats USDA has approved irradiation of meats and eggs.and eggs.

Uses of Gamma Radiation• Because of it’s high

frequency and penetrating power, gamma is useful in:– sterilization of medical

equipment by killing bacteria

– used to kill bacteria and insects in foodstuffs, particularly meat, marshmallows, pies, eggs, and vegetables, to maintain freshness

“Gamma Knife”-

Brain tumors are hit with gamma rays in this device.