Radioactivity Notes
Isotopes:
- Typically, each element contains only half stable isotopes; the remaining isotopes tend to be radioactive due to their unstable nuclei, which decay and emit radiation.
- e.g., carbon-14
Radioactive Decay:
- The nuclei of unstable isotopes break down at random.
→ You have no control over the process of decay.
- When it decays, it emits various types of radiation, including alpha, beta, gamma, or neutrons.
- In this process, the nucleus often becomes a new element.
Background Radiation:
- There's low-level background nuclear radiation everywhere. It comes from:
→ Cosmic rays (mainly from the sun)
→ Living things
→ Substances on Earth: Soil, rocks, food, and building materials.
→ Human activity causes radiation due to nuclear waste and fallout from nuclear explosions.
Ionising Radiation:
- Nuclear radiation causes ionisation by smashing into atoms and knocking electrons off of them.
→ Atoms are turned into ions.
- The less ionising and less damage the radiation causes, the further it can penetrate before hitting an atom and stopping.
- A G-M tube or photographic film can detect ionising radiation.
Alpha Particles: ⁴₂He / α
- It consists of two protons and two neutrons.
- Big, heavy, and slow-moving → ∴ not very penetrating.
- ∴ of their size, they are concising → bash into lots of atoms and electrons off them before they slow down → creates lots of ions.
- Magnetic and electric fields deflect these positively charged alpha particles.
- Paper, skin, or a few centimetres of air can block low-penetration-power particles.
²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂He
→ The mass number decreases by 4, and the atomic number decreases by 2.
Beta Particles: ⁰₋₁e / β⁻
- When a neutron transforms into a proton and an electron, the nucleus of an atom emits an electron.
- Move quickly and are relatively small in size.
- They penetrate moderately before colliding and also exhibit slight ionisation.
- Electric and magnetic fields deflect them due to their negative charge.
- Thin metal (aluminium) blocks beta particles.
¹⁸⁷₇₅Re → ¹⁸⁷₇₆Os + ⁰₋₁e
→ The number of atoms increases by one, indicating a proton increase.
Gamma Rays: γ
- It has no mass, just energy in an electromagnetic wave.
- It has the ability to penetrate deep into various materials.
→ Either thick lead or thick concrete stopped the process.
- They are weakly ionising, meaning they tend to pass through rather than collide with atoms, eventually hitting something and causing damage.
- Since there is no charge, it is not deflected by electric or magnetic fields.
- Gamma emission always occurs after alpha or beta decay, not independently.
- If a nucleus has excess energy, it uses it by emitting a gamma ray.
⁹⁹₄₃Tc → ⁹⁹₄₃Tc + ⁰₀γ
→ No change
Neutron Emission:
¹³₄Be → ¹²₄Be + ¹₀n
→ The mass number decreases by 1.
Investigating the Penetration of Radiation:
- A Geiger-Müller detector, which provides a count rate, can detect ionising radiation. The count rate indicates the quantity of radioactive particles that reach it in a second.
- Remove the source in order to measure the background count over a specific time period, such as 30 seconds.
- Divide your count by time period to get a background count rate.
- Repeat the process two more times, then find the mean and remove it from all results.
- Replace the source and measure count rate with no material present. After measuring the count rate three times and taking a mean, insert additional materials between the source and the detector.
→ Insert the read count rate for each material to allow the radiation to penetrate it. If the count rate significantly decreases, the material is either absorbing or blocking the radiation.
- If it drops to 0 after background count is subtracted, then radiation is completely absorbed.
Half-life:
- As radioactivity decreases over time, the number of decays per second (Bq) also decreases.
- Radioactive decay is a random process, and you can't predict when one unstable nucleus is going to decay.
- Alternatively, you can make predictions about decay when you have a large number of unstable nuclei.
The rate of decay of a radioactive material depends on:
- The type of material.
- The number of undecayed nuclei present. The greater the number of nuclei present, the greater the rate of decay.
- The half-life of a radioactive material is the time it takes for half of the radioactive atoms to decay.
Over here, the half-life is two days:
- 80 → 40: 2 days
- 40 → 20: 2 days
- 20 → 10: 2 days
Make sure to check at least 3 times.
Examples:
1) A radioactive element has an activity of 400 Bq. 3 hours later it has an activity of 50 Bq. What's the half-life?
0 half-life 400 : 3h = 3 half-lives
1 half-life 200 :. 1h = 1 half-life
2 half-lives 100
3 half-lives 50 half-life = 1h
2) A radioactive element with an activity of 1200 Bq is detected. 4 hours later, the count is 300 Bq. What's the half-life?
0 1200 4h = 2 half-lives
1 600 :. 1 half-life = 2h
2 300
3) A radioactive element has an activity of 1000 Bq. 12 hours later the count is 125 Bq. What's the half-life?
0 1000 12h = 3 half-lives
1 500 :. 4h = 1 half-life
2 250
3 125
4) A radioactive sample has a half-life of 8h. Is the activity 800 Bq now? After how long will it be 100 Bq?
0 800 3 x 8 = 24h
1 400
2 200
3 100
Nuclear Fusion and Fission:
- Nuclear fission is the splitting of an atom (e.g., Uranium-235), which releases energy.
- A U-235 can split if it absorbs a slow-moving neutron.
- Each time this occurs, it releases a small number of neutrons. These neutrons could potentially impact other uranium-235 nuclei, leading to their splitting, and so on. This is a chain reaction.
- These new nuclei typically pose disposal and waste issues due to their radioactivity.
- Each nucleus splitting gives out a lot of energy; this is in the kinetic energy stores of the fission products, the daughter nuclei, and the neutrons.
- Thermal energy stores in a reactor use this energy to produce steam, which powers a turbine.
Nuclear Reactors:
- Nuclear reactors possess a significant amount of energy. Uranium nuclei must absorb them to sustain the chain reaction.
- The moderator, usually water or graphite, slows down neutrons.
- Control rods, often made of boron, limit the rate of fission by absorbing extra neutrons.
- The high-energy neutrons and gamma rays released in fission are highly penetrating, ionising radiation.
→ A thick concrete structure requires shielding to absorb it.
- Pumped around the reactor, a substance transfers energy by heating the water in a heat exchanger. The water turns to steam, which drives a turbine that turns a generator and generates electricity.
Nuclear Fusion:
- Two light nuclei collide at high speed and fuse to create a larger, heavier nucleus.
- The mass of the heavier nucleus is significantly less than that of the two separate, light nuclei. Radiation releases the converted energy from mass.
- Fusion releases much more energy than fission. Still, it only happens at high temperatures and pressures. The positively charged nuclei must approach the fuse very closely and move very quickly to overcome the strong force caused by electrostatic repulsion.
→ We haven't found a way to use fusion to generate energy; the conditions needed mean fusion reactors are really hard and expensive to build.