Radioactivity + Particles

Discuss radioactive decay

Discuss radioactive decay

Radioactive decay is the spontaneous disintegration of a nucleus, which results in the nucleus emitting ionising radiation.

Give sources of background radiation (occurs naturally)

• Cosmic rays from space (mostly from the Sun)

• Radon gas from rock, soil, and building materials

• A little bit in all living things

• Radioactive minerals in the ground

• Food and water

Give sources of radiation de to human activity

• Nuclear accidents e.g. Chernobyl

• Atomic weapons testing

• Emissions from power stations → the nuclear industry is legally allowed to emit tiny amounts of radioactive material into the environment

• Radioisotope tracers used in industry and hospitals

How do you detect radioactivity?

• Using a Geiger-Muller tube (an instrument used for accurate measurements of radioactivity)

• The Geiger-Muller tube gives a count rate - the number of radioactive particles reaching it per second.

• Remove the source to measure/record the background radiation over a time period. Subtract this number from your results.

What is the penetration power of the three different types of radiation?

• Alpha particles: blocked by paper, skin, or a few cm of air

• Beta particles: blocked by thin metal

• Gamma rays: blocked by thick lead or very thick concrete.

Definition for a half-life

The time it takes for half of a radioactive material’s radioactive atoms (now present) to decay.

How is beta or gamma radiation used in medical tracers?

• A source which emits beta or gamma radiation is injected into a patient (or swallowed).

• The radiation penetrates the body tissues and can be detected externally. As the source moves the body, the radiographer uses a detector to monitor its progress on a computer display.

• The radioactive source must have a short half-life so that the initial levels are high enough to be easily detected, but the radioactivity inside the patient quickly disappears.

• Doctors use this method to check whether organs are functioning correctly.

How is radiation used in kidney scans?

A tracer with a radioactive ‘tag’ (so it can be followed into the body with detectors) is injected into the patient’s veins. A gamma camera is used to track the radioactivity.

• Role: Used to diagnose certain kidney diseases as it shows what the kidneys look like and how well they function.

How are gamma rays used to detect leaks in underground pipes?

• Gamma emitting tracers are allowed to flow down the pipes and a detector is used above ground.

• If there’s a crack in the pipe, more radiation will collect outside the pipe, and the detector will show very high radioactivity at that point.

• Gamma is used because it can pass through any rocks or earth surrounding the pipe.

• The radioactive tracer must have a short half-life so as not to cause a long-term hazard if it collects somewhere.

How is beta radiation used in thickness control?

• Beta radiation is directed through the paper being produced, and a detector is put underneath the paper, connected to a control unit.

• When the detected radiation level chances, it means the paper is coming out too thick/thin, so the control unit adjusts the rollers to give the correct thickness.

• Beta radiation must be used, not any others, because then the paper will partly block the radiation. If it goes al though/none does, then the reading won’t chance as the thickness of the paper changes.

How is radiation used in thyroid cancer radiotherapy?

• The radioactive isotope Iodine-131 is used to kill thyroid cancerous cells.

• It circulates the patient’s body in their bloodstream, where cancerous thyroid cells pick up the iodine-131.

• The radiation in the iodine-131 then kills the cancer cells.

What does Giga- mean?

10^9 (or a billion)

How does nuclear radiation cause ionisation?

The radiation travels at a high velocity (being expelled from the radioactive nucleus at high speeds), and it bashes into atoms and knocks off electrons from them, turning them into ions.

Discuss gamma radiation

• Very high frequency EM radiation

• Often accompanies alpha and beta decay. Never emitted on their own.

• Creates when a nucleus emits an alpha or beta particle, leaving the daughter nucleus in an excited state - it can then move to a lower energy state by emitting a gamma ray.

• Mass = 0, Charge = 0

• Travels at the highest speed, but is the lowest penetrating.

• Stopped by a thick layer or lead or very thick concrete. Can pass through many metres of air.

• The emission of a gamma ray does NOT change the composition of the nucleus; it only lowers the energy state associated with the nucleus.

Discuss Beta particles

• High KE electrons

• Expelled when a neutron changes to a proton in the nucleus. To maintain the conservation of charge, the nucleus, in the process, also emits an electron and an antineutrino.

• Mass = 1/1850 (very small), Charge = -1

• Moderate penetrating → stopped by a few mm of aluminium and travels a few metres in air.

Discuss alpha particles

• High velocity Helium nucleus, consisting of 2 protons and 2 electrons. It is expelled from the nucleus at high speeds (though still travels the slowest relatively)

• Mass = 4, Charge = 2+

• Low penetrating → stopped by a few cm of air or a thin sheet of paper.

How is low-level (slightly radioactive) nuclear waste disposed of?

It is buried in secure landfills

How is high-level nuclear waste disposed of?

Because it stays radioactive for tens of thousands of years, it is often sealed into glass blocks, which are then sealed in metal canisters. These could then be buried deep underground.

• The site of burial has to be geologically stable (e.g. not suffer from earthquakes), since large rock movements could disturb the canisters and allow radioactive material to leak out.

• If this material gets into groundwater, it could contaminate the soil, plants, rivers etc… and get into our drinking water.

What is nuclear fission?

The splitting of an unstable nucleus into two lighter ‘daughter’ nuclei, this process also releases more neutrons.

• Occurs when a slow-moving neutron is absorbed by a large nucleus.

• The mass loss of the large nucleus is converted into energy, mainly heat energy

• It can be spontaneous, but in a nuclear reactor it’s made to happen by a uranium-235 nucleus.

• The production of more neutrons (2-3 per uranium atom), go on to split more uranium atoms, cuasing a chain reaction.

• Energy is transferred into the KE stores of the fission products (daughter nuclei and the neutrons)

Issues with nuclear fission?

• The daughter nuclei produced from the split are usually radioactive which means there is a lot of nuclear waste which is difficult, risky, and expensive to safely dispose of.

• 10-20 year construction time

• Huge capital cost to build + maintain

• Radioactive leakages pose great danger to the environment and ourselves.

What does a moderator do + material it is made out of?

A moderator, slows down neutrons so they can be absorbed more easily by the uranium-235 nuclei. This speeds up the fission rate.

They are usually made of graphite or water.

What are the control rods + material they’re made of?

Control rods absorb excess neutrons, limiting the rate of fission.

They are often made of boron, because it is a non-fissile material.

The control rods can be raised out of the reactor to allow more fission to take place.

How is a nuclear reactor shielded?

Due to the highly penetrating ionising radiation (high KE neutrons + gamma rays), thick concrete structures - which may also contain lead - is put around the reactor to absorb the ionising radiation.

How is energy transferred from a nuclear to kinetic store in a nuclear reactor?

• A substance (e.g. CO2) is pumped around the reactor , which transfers the energy (by heating) to the water in the heat exchanger.

• The water turns to steam

• The steam turns a turbine

• The turbine turns a generator and generates electrical energy.

Pros of nuclear fission?

• No emission of CO2 from fossil fuels → contributes little to global warming

• Provides a reliable, large-scale source of energy

• Provides lots of jobs in construction

How does nuclear fusion work?

Two light nuclei collide at high speed and fuse to create a larger, heavier nucleus.

• E.g. hydrogen nuclei fuse to make a helium nucleus in the Sun.

• This process also releases huge amounts of energy → as some of the mass of the lighter nuclei is converted to energy then released as radiation.

• Requirements: Very high temperatures (10 million ‘C) + pressures. This is to give the lighter nuclei sufficiently high KE to overcome the electrostatic repulsion from the two positive nuclei involved.

Pros + Cons of nuclear fusion?

Pros:

• Clean → doesn’t produce CO2

• Produces plentiful fuel

• If we manage to create nuclear fusion, it could become a cheap fuel option

Cons:

• It requires extremely high temperatures and pressures, making it very expensive and not yet viable.