2b.3 - Half Life Flashcards
What happens every time an alpha, beta or gamma particle is given out?
A radioactive nucleus has disappeared
What is the unit for radioactivity?
Becquerel (Bq)
What does 1 Bq mean?
One nucleus decaying per second
What is the definition of half-life?
The average time it takes for the number of nuclei in a radioactive isotope sample to halve
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.
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
How is a half-life found from the graph?
By finding the time interval on the bottom axis corresponding to a halving of the activity on the vertical axis
What type of radiation do smoke detectors use?
Alpha
How do smoke detectors work? (3)
- 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
What types of radiation is used as tracers in medicine?
Beta or gamma
How do tracers work? (3)
- 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
What is a well known example of a tracer?
Iodine - 131
Why are beta or gamma radiation used as tracers? (2)
- The radiation can pass out of the body
- Short half-life
What is radiotherapy?
The treatment of cancer using gamma rays
Why is gamma radiation used in radiotherapy?
High doses of gamma will kill living cells
How is food and surgical instruments sterilised?
Exposed to high doses of gamma radiation which kill all microbes, keeping food fresher for longer
What causes ionisation?
Alpha, beta and gamma enters living cells and collides with molecules
What causes cancer? (2)
- Low doses causes minor damage to cells without killing them.
- This causes mutations which divide uncontrollably
Which type of radiation is more dangerous outside the body?
Beta and gamma
Why are beta and gamma sources more dangerous outside the body?
They can bet inside to delicate organs
Why is alpha not dangerous outside the body?
It can’t penetrate the skin
Which type of radiation is most dangerous inside the body?
Alpha
Why is alpha the most dangerous inside the body?
They do all their damage in a very localised area
Why is beta and gamma radiation less dangerous inside the body?
Mostly pass straight out without causing much damage
What are the four safety precautions when handling radiation?
- Keep exposure to a minimum
- Always handle with tongs
- Hold the source at arms length
- Keep the source pointing away form the body
What types of radiation does lead protect against?
All - it takes a lot to stop gamma
