Half Life

Half-life is the time it takes for half of the atoms of a sample to decay.

For example:• A student was testing a sample of 8

grams of radioactive protactinium. Protactinium has a a half life of 1 minute and decays into actinium.

• After 1 minute there would be 4 g of protactinium (and 4 g of actinium).

• After 2 minutes there would be 2 g of protactinium remaining (and now 6g of actinium).

• After 3 minutes there would be 1 g of protactinium remaining (and now 7g of actinium)

Half Life

Dating materials using half-livesQuestion: Uranium decays into lead. The half life of uranium is 4,000,000 years. A sample of radioactive rock contains 7 times as much lead as it does uranium. Calculate the age of the sample.

8

8

Answer: The sample was originally completely uranium…

…of the sample

was uranium

4

8

2

8

1

8Now only 4/8 of

the uranium remains – the

other 4/8 is lead

Now only 2/8 of uranium

remains – the other 6/8 is

lead

Now only 1/8 of uranium

remains – the other 7/8 is

leadSo it must have taken 3 half lives for the sample to decay until only 1/8 remained (which means that there is 7 times as much lead). Each half life is 4,000,000 years so the sample is 12,000,000 years old.

1 half life later…

1 half life later…

1 half life later…

Potassium decays into argon. The half life of potassium is 1.3 billion years. A sample of rock from Mars is found to contain three argon atoms for every atom of potassium. How old is the rock?

The rock must be 2 half lives old – 2.6 billion years

Radioactive substances emit radiation from the nuclei of their atoms all the time.

The half-life of a radioactive isotope is Either the time it takes for the number of

nuclei of the isotope in a sample to halveor the time it takes for the count rate from a

sample containing the isotope to fall to half its initial level.

Types of Radiation

What is radioactive decay?

How do materials affect radiation?

How do magnetic fields effect radiation?

Description

What is alpha (α) radiation?

Electric charge +2

Relative atomic mass4

Penetrating power Stopped by paper or a few centimetres of air

Ionizing effect Strongly ionizing

Effect of magnetic/ electric field

Weakly deflected

2 neutrons, 2 protons

Note:– An alpha particle is the same as a helium nucleus

What is beta (β) radiation?

Description

Electric charge

Relative atomic mass

Penetrating power

Ionizing effect

Effect of magnetic/ electric field

-1

1/1860

Stopped by a few millimetres of aluminium

Weakly ionizing

Strongly deflected

High energy electron

Gamma () radiation

Description

Electric charge

Relative atomic mass

Penetrating power

Ionizing effect

Effect of magnetic/ electric field

0

0

Stopped by several centimetres of lead or several metres of concrete

Very weakly ionizingNot deflected

High energy electromagnetic radiation

Types of radiation and penetrating power

Types of radiation and range in air

Uses of Radiation

Smoke alarms contain a weak source of alpha radiation.

How can radiation detect a fire?

The alpha particles ionize the air.

If there is smoke present, it interacts with the ions produced by the alpha particles and ionization is reduced.

This means that less current is flowing through the air, which causes the alarm to sound.

α

α

smoke particle

How is radiation used in making paper?

How can radiation find leaks in pipes?

How can radiation detect cracks?Gamma rays can also be used to detect cracks after an object has been welded.

If a gamma source is placed on one side of the welded metal, and a photographic film on the other side, any flaws will show up on the film like an X-ray.

Gamma rays are like X-rays.

welded metal pipe

photographic film

welding flaws

HighLevelnuclearwaste

Microbes can be killed using gammaradiation

Increasingdose tumor

view throughthe head

skull

healthybraintissue

Gamma rays can be used to treat brain tumors

Uses of radiation

Dangers of ionizing radiations

As well as the normal laboratory safety rules you follow, are there any extra rules concerning radioactivity?

Radiation safetyThe three types of radiation differ in their effects and physical nature.

All radioactive sources must be handled safely.

The hazard symbol for radiation is shown below:

Radioactive materials could be very dangerous to handle if no safety precautions were taken.

Radiation safety measures

use tongs or a robotic arm to handle radioactive materials.

keep exposure times as short as possible monitor exposure with a film dose badge

label radioactive sources clearly store radioactive sources in

shielded containers wear protective clothing

The safety precautions are:

This is because people and their clothing could become contaminated.

Write down on first side of the paper

Background radiation is the radiation all around us.

Background radiation

How many different sources of background radiation can you think of?

Most of the radioactivity you are exposed to is from natural sources.

Sources of Radiation in Modern Society

Calculating background radiation