The Radiation Lab
Unstable atoms fire out radiation at random. Meet alpha, beta and gamma, learn what stops each, balance a decay, and master the exam favourite: half-life.
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The Radiation Lab
Some atoms have unstable nuclei. To become more stable they decay, firing out nuclear radiation, and they do it completely at random. In this lab you will meet the three types of radiation, learn what stops each, balance a decay, and crack the exam favourite: half-life.
The words for it
Five terms carry the whole topic. The last one is the exam favourite:
Random, yet predictable
Nobody can say when any single nucleus will decay, yet a physicist can state a sample's activity in an hour's time with real confidence. How can both be true?
- A sample holds an enormous number of nuclei, and although each one is unpredictable, the proportion decaying in a given time is very consistent across so many
- The nuclei take it in turns, so the order is random but the timing is fixed
- Measuring the sample forces the nuclei to decay at a steady rate
- The physicist controls the temperature, which controls the rate
Alpha, beta and gamma
Three radiations, and every difference between them follows from the first row: a big slow charged lump interacts with everything it meets, and a wave interacts with almost nothing.
Order by penetrating power
Put the three radiations in order of penetrating power, from the LEAST penetrating to the MOST.
- Alpha (α)
- Beta (β)
- Gamma (γ)
Pick the right radiation
- A smoke alarm, where the radiation must not escape the plastic case
- A gauge checking the thickness of aluminium foil as it is rolled
- A medical tracer that has to be detected from outside the patient
- Sterilising surgical instruments already inside sealed packaging
- Alpha, because a few centimetres of air stop it, so it cannot get out to harm anyone
- Beta, because thin metal absorbs some but not all of it, so the count changes as the thickness does
- Gamma, because only the most penetrating radiation gets out through the body to a detector
- Gamma again, because it passes straight through the packaging and kills the bacteria inside
Balancing decay
A decay is written as a nuclear equation, with the mass number on top and the atomic number below. Both must balance across the arrow. Nothing vanishes. Alpha decay throws out 2 protons and 2 neutrons, so mass number −4 and atomic number −2. Beta decay is the stranger one. A neutron turns into a proton, and the electron that is emitted is created in that moment: it does not come from the electron shells. So the atomic number goes up by 1 while the mass number does not change at all, because a neutron and a proton have much the same mass.
Radium decays
Radium-226 has a mass number of 226 and an atomic number of 88. It decays by emitting an ALPHA particle. What is the MASS NUMBER of the nucleus left behind?
And its atomic number
Same decay: radium-226, atomic number 88, emits an alpha particle. What is the ATOMIC NUMBER of the nucleus left behind? (That number identifies it as radon.)
Where does the electron come from?
A nucleus emits a beta particle, which is an electron. Nuclei contain no electrons. So where did it come from?
- A neutron in the nucleus turned into a proton, and the electron was created at that moment and thrown out
- It was pulled in from the innermost electron shell and fired back out
- A proton split into a neutron and an electron, so the atomic number falls by 1
- It was captured from a nearby atom just before the decay
Four halvings
A source has an activity of 6400 Bq and a half-life of 15 minutes. What is its activity ONE HOUR later, in Bq? (Work out how many half-lives fit into an hour first.)
Plot the decay curve
A source has an activity of 8 units at the start and a half-life of 2 hours, so it halves every 2 hours. The points (0,8), (4,2) and (6,1) are shown. Plot the missing point after ONE half-life, at time 2.
Contamination and irradiation
Irradiation is being exposed to radiation from a source outside you. Step away, or put the source back in its lead pot, and it stops. It does not make you radioactive: a hospital radiographer is irradiated at work and goes home no more radioactive than they arrived. Contamination is radioactive material getting onto or into you: dust breathed in, a spill on the skin, a source swallowed. Now it travels home with you and keeps emitting, and you cannot walk away from it. The distinction turns the penetration table upside down. Alpha is the safest to stand near, because your skin stops it. Alpha is the most dangerous to swallow, because inside you there is no skin in the way and it is the most ionising of the three.
The spill
A sealed gamma source is used in a lab, and a separate bottle of radioactive powder is knocked over. Four calls to make.
- A worker stood two metres from the sealed source for a minute before it was locked away. What have they experienced, and what should be done?
- A second worker has powder from the spilled bottle on their hands. Why is this the more serious problem?
- The spilled powder is an alpha emitter. Someone argues it is therefore harmless, since alpha cannot even get through skin. What is wrong with that?
- Which precaution matters most for each hazard?
The lab rules
Alpha radiation is the most ionising but is stopped by _____, while gamma is the most penetrating and needs thick _____. The time for the activity of a source to halve is its _____. Radioactive material getting onto or into an object is called _____, and unlike irradiation it does not stop when you walk away.