2b.3 - Half Life

Question 1

What happens every time an alpha, beta or gamma particle is given out?

Answer 1

A radioactive nucleus has disappeared

Question 2

What is the unit for radioactivity?

Answer 2

Becquerel (Bq)

Question 3

What does 1 Bq mean?

Answer 3

One nucleus decaying per second

Question 4

What is the definition of half-life?

Answer 4

The average time it takes for the number of nuclei in a radioactive isotope sample to halve

Question 5

The activity of a radioisotope is 640 cpm (counts per minute). Two hours later it has fallen to 80 cpm. Find the half life of the sample.

Answer 5

INITIAL = 640
ONE = (%2) 320
TWO = (%2) 160
THREE = (%2) 80

Three half lives

Two hours represents three half-lives, so the half life is 120 (mins) % 3 = 40

Question 6

How is a half-life found from the graph?

Answer 6

By finding the time interval on the bottom axis corresponding to a halving of the activity on the vertical axis

Question 7

What type of radiation do smoke detectors use?

Answer 7

Alpha

Question 8

How do smoke detectors work? (3)

Answer 8

• Weak source of alpha is placed in the detector, close to two electrodes
• The source causes ionisation, and a current flows between the electrodes
• If there is a fire, smoke will absorb the radiation so the alarm sounds

Question 9

What types of radiation is used as tracers in medicine?

Answer 9

Beta or gamma

Question 10

How do tracers work? (3)

Answer 10

• Injected or swallowed
• The progress around the body can be followed using an external detector
• A computer converts the reading to a display showing where the strongest reading is coming from

Question 11

What is a well known example of a tracer?

Answer 11

Iodine - 131

Question 12

Why are beta or gamma radiation used as tracers? (2)

Answer 12

• The radiation can pass out of the body

- Short half-life

Question 13

What is radiotherapy?

Answer 13

The treatment of cancer using gamma rays

Question 14

Why is gamma radiation used in radiotherapy?

Answer 14

High doses of gamma will kill living cells

Question 15

How is food and surgical instruments sterilised?

Answer 15

Exposed to high doses of gamma radiation which kill all microbes, keeping food fresher for longer

Question 16

What causes ionisation?

Answer 16

Alpha, beta and gamma enters living cells and collides with molecules

Question 17

What causes cancer? (2)

Answer 17

• Low doses causes minor damage to cells without killing them.
• This causes mutations which divide uncontrollably

Question 18

Which type of radiation is more dangerous outside the body?

Answer 18

Beta and gamma

Question 19

Why are beta and gamma sources more dangerous outside the body?

Answer 19

They can bet inside to delicate organs

Question 20

Why is alpha not dangerous outside the body?

Answer 20

It can’t penetrate the skin

Question 21

Which type of radiation is most dangerous inside the body?

Answer 21

Alpha

Question 22

Why is alpha the most dangerous inside the body?

Answer 22

They do all their damage in a very localised area

Question 23

Why is beta and gamma radiation less dangerous inside the body?

Answer 23

Mostly pass straight out without causing much damage

Question 24

What are the four safety precautions when handling radiation?

Answer 24

• Keep exposure to a minimum
• Always handle with tongs
• Hold the source at arms length
• Keep the source pointing away form the body

Question 25

What types of radiation does lead protect against?

Answer 25

All - it takes a lot to stop gamma