210 Po

Polonium

210

Alexander

Litvinenko

Nuclear Radiation

We will look at three types of nuclear radiation.

Radiation Symbol Range

alpha

beta

gamma

α

β

γ

few centimetres through air

several centimetres through air

several metres through air

Absorption

Alpha

Paper

Beta

A few mm of

Aluminium

Gamma

Several cm of Lead

Safety Precautions

1. Always use forceps to lift a source (never bare hands).

2. Ensure source is pointed away from the body.

3. Never bring towards eye to examine source.

4. When in use, always attended by an authorised person. Returned to a locked and labelled store which is shielded after use.

5. After using radioactive sources, wash hands thoroughly before eating.

6. In the UK, students under the age of 16 may not handle radiation sources.

Nuclear radiation can kill or damage living cells.

The extent of damage depends on:

• type of tissue

• type of radiation

• the total amount of energy absorbed.

Effect on Living Cells

Physicists measure the damage by calculating the equivalent dose.

Equivalent dose is measured in units called Sieverts ( S ).

Radiotherapy

Radiotherapy uses nuclear radiation to kill cancer cells.

A beam of nuclear radiation is fired at the cancer cells from different directions to minimise the damage to the surrounding healthy cells.

The cancer cells are damaged which stops them reproducing.

The tumour then shrinks.

Brain Tumour

A beam of invisible nuclear radiation is fired at the tumour.

The direction is changed at the next dose to protect the healthy cells around the tumour.

The Gamma Camera

The gamma camera is a detector of gamma radiation.

It can be used to produce an image of inside the body.

A radioactive chemical called a tracer ( which emits gamma radiation ) is usually injected into the patient’s bloodstream.

It gives off gamma radiation as it travels around the body. This gamma radiation is detected by the gamma camera.

Image of a patient’s kidneys using a gamma

camera.

+

+

--

Nucleus +

-

Proton

Neutron

Electron

The Atom

The mass of the atom is concentrated in the centre called the nucleus.

The nucleus contains positive particles called protons and neutral particles called neutrons.

Most of the atom is empty space.

Orbiting the nucleus are negative particles called electrons.

In an atom there are always the same number of protons and electrons.

The positive and negative charges cancel out each other so the atom has an overall neutral charge.

Nuclear radiation can change neutral atoms into charged ions.

Nuclear Radiatio

n+

+-

Nucleus

-

Ionisation

If nuclear radiation passes through or close to an atom it can remove one or more of the orbiting electrons.

This upsets the balance between positive and negative.

It is no longer neutral.

The neutral atom has been changed into a charged ion.

This is called IONISATION.

ALPHA causes more ionisation than BETA or GAMMA.

If an atom loses an electron it becomes a positive ion.

If another atom gains this electron it will become a negative ion.

This is a detector of nuclear radiation.

Counter

Nuclear Radiation

Tube of gas atoms

Central Electrode ( high

voltage )

mica window

The Geiger-Muller Tube

If nuclear radiation enters the G-M tube through the thin mica window it will IONISE some gas atoms in the tube.

These charged IONS are attracted to the high voltage central electrode.

A pulse of electricity is produced which is recorded by the counter.

Activity & Dose Equivalent

Activity

The activity of a radioactive source is measured in becquerels ( Bq ) or kilobecquerels ( kBq ).

20 Bq means 20 atoms disintegrating in one second.

Dose Equivalent

The dose equivalent is a measure of the biological effect of radiation.

The dose equivalent is measured in Sieverts ( Sv ) or millisieverts ( mSv ).

The amount of radiation emitted from a radioactive source is called its activity.

Activity is measured in units called Becquerels ( Bq ).

The activity of a radioactive source decreases with time.

Activity ( kBq )

time ( hours )

1000

3 6 9

500

Half-Life = 3 hours

Radioactive Decay

The half-life of a radioactive source is the time it takes for the activity to half.

Counter

G-M Tube Radioactive Liquid

Measuring Half-Life

Stopwatch

1. Measure the background count rate.

2. Place the radioactive source next to the GM tube.

3. Measure the count rate of the substance every minute.

4. Subtract the background count rate from every reading.

Time(min)

Activity(kBq)

Corrected Activity(kBq)

0 100 985 64 6210 43 4115 29 2720 21 1925 18 1630 17 15

Background radiation = 2 kBq

To Do…1. Use the table of results to plot a graph of corrected activity

and time.

2. Predict from your graph, the half life of the radioactive source.

0

10

20

30

40

50

60

70

80

90

100

110

0 5 10 15 20 25 30 35

Time (min)

Cor

rect

ed A

ctiv

ity

(kBq)

Half-life = 7 years

Half-Life Calculations

Example 1

The Half-Life of a radioactive source is 3 days.

(a) How long will it take the Activity to fall from 2000 kBq to 250 kBq?

2000 1000 500 250

It will take 9 days.

3 days 3 days 3 days

(b) What will the activity be after a further 6-days?

250 125 62.53 days 3 days

Activity after a further six days is

62.5 kBq.

Example 2

The activity of a radioactive source falls from 200 kBq to 25 kBq in a time of 24-hours.

What is the half life of the radioactive source?

200 100 50 251 half-life 1 half-life 1 half-life

3 half lives = 24-hours

1 half-life = 24-hours

3

1 half-life = 8-hours

Attempt the following questions showing ALL your working.

Q1. A radioisotope has a half life of 5-hours. If its initial count rate is 320 cps, what is the count rate after 15-hours?

Q2. Uranium has a half-life of 4,000 years. If the activity of a sample is 48 kBq in the year 2000 AD, what will its activity

be in the year 14000 AD?

Q3. The half-life of a radioactive substance is 5,600 years. How long will it take for the activity to fall to one eighth (1/8) of what it was?

Q4. The activity of a radionuclide in 1985 was 1,200 Bq. In what year will the activity be 75 Bq if its half-life is 12 years?

Q5. Calculate the half-life of a radioactive sample whose activity falls from 128 kBq to 4 kBq in a time of 40-hours.

Q6. The activity of a radioisotope is measured to be 20 MBq. Twelve days later the activity has fallen to 5 MBq. What is

the half-life?

Q7. A Geiger counter measures the corrected count rate of a radioactive gas to be 80 cpm (counts per minute). One

minute later the count rate has decreased to 10 cpm. What is the half- life of this radioactive gas?

Q8. In 1970 the activity of a radioisotope is found to be 100 kBq. In 1982 this activity had fallen to 25 kBq. What is the half-life of the radioisotope?