Radioactivity, Uses & Dangers

Activity decreases over time

• Every time a nucleus decays, it disappears from the pool of unstable nuclei
• So the number of unstable nuclei left in the sample is always falling
• Fewer unstable nuclei means fewer decays per second, so the activity falls with time as well
• The fall is not linear; it is faster at the start (when many unstable nuclei are present) and slower as time goes on. The shape is an exponential decay curve

Decay is random

• Radioactive decay is a random process: it is impossible to predict which nucleus will decay next, or exactly when any one nucleus will break down
• All you can say is the probability that a given nucleus will decay in the next second
• The randomness can be seen by measuring the count rate of a source using a Geiger–Müller (GM) tube
• The count rate is the number of decays the detector registers per second. When count rate is plotted against time the trace fluctuates up and down rather than following a perfectly smooth curve
• These fluctuations are direct evidence that decay is random. If decay were a steady, clockwork process the count rate would lie exactly on a smooth curve

Activity vs count rate (don't mix them up)

• Activity = rate at which unstable nuclei in the source actually decay (Bq)
• Count rate = rate at which the detector registers radiation
• Count rate is usually smaller than activity, because:
  • The detector does not surround the source completely, so much of the radiation flies off and misses it
  • Some radiation is absorbed by air, the GM tube wall, or any object in between
• Count rate is the practical, measurable quantity; activity is the theoretical quantity

2. Half-life

Definition

• Because individual decays are random, you cannot say "this particular nucleus will decay at 3:42 pm"
• What you can measure precisely is how long it takes for a large sample to halve. This is the half-life:

the half-life of a radioactive isotope is the time taken for the number of unstable nuclei in a sample to fall to half of its original value

• Equivalently, since activity is proportional to the number of unstable nuclei, half-life is the time for the activity of the sample to drop to half its starting value
• Half-life is a property of the isotope itself. Every nucleus of a given isotope has the same probability of decaying per second, so the half-life is constant, and temperature, pressure and chemistry have no effect

Half-lives vary enormously

• Different isotopes have wildly different half-lives:

• Short half-lives mean a very high activity for a short time; long half-lives mean a much lower activity but lasting far longer than any human timescale

Halving step by step

• After each half-life, the number of unstable nuclei (and the activity) is halved:

• The pattern: after n half-lives, the fraction remaining is (1/2)ⁿ

Reading a half-life from a graph

• To find the half-life from an activity-time graph:
  1. Read off the initial activity A₀ from the y-axis at t = 0
  2. Halve it to get A₀/2
  3. Draw a horizontal line from A₀/2 across to the curve, then drop straight down to the time axis
  4. The reading on the time axis is the half-life
• It is a good idea to do this a second time (read at A₀/4) and divide the resulting time by 2 to check; you should get the same answer

3. Uses of radioactivity

The choice of radiation for any application depends on its range (how far it travels and what it can penetrate) and its ionising power.

Smoke detectors: alpha

• A small alpha source (typically americium-241) sits inside the detector
• Inside a small chamber, the alpha particles ionise the air between two electrodes, creating a steady tiny current that flows through a circuit
• When smoke enters the chamber, the smoke particles absorb the alpha radiation before it can ionise the air
• The current drops sharply, and the microchip triggers the alarm

Why alpha is the right choice:

• Alpha is the most strongly ionising radiation, so it creates plenty of ions in the chamber and a strong baseline current
• Alpha is the least penetrating, so it cannot escape the detector, meaning there is no radiation hazard to the person standing under it
• Smoke is dense enough to absorb alpha but not beta or gamma. Beta and gamma radiation would slip through any normal puff of smoke unchanged, so the current would never drop and the alarm would never trip

Measuring the thickness of materials: beta

• A beta source is placed on one side of a moving sheet of paper, cardboard or thin aluminium foil; a GM tube sits on the other side
• Some beta particles are absorbed by the sheet; the rest pass through and are counted by the GM tube
• If the sheet is too thick, more beta particles are absorbed and the count rate falls
• If the sheet is too thin, fewer beta particles are absorbed and the count rate rises
• A control system reads the count rate and adjusts the rollers to keep the thickness constant

Why beta is the right choice:

• Alpha would be stopped completely by even the thinnest paper, so the count rate would be zero no matter what thickness the sheet was
• Gamma would pass through almost unchanged, so changes in thickness would not register
• Beta is partially absorbed, which is exactly what is needed for the count rate to be sensitive to small changes in thickness
• The source must have a long half-life (typically strontium-90, 28 years) so the count rate does not drift during the working life of the gauge

Treating cancer: gamma

• Radiotherapy uses high-energy gamma rays directed at a cancerous tumour
• The gamma beam is aimed from several different directions, all crossing at the tumour. The healthy tissue along any one beam path receives only a small dose, but the tumour at the focus receives the full combined dose from every beam
• The beam is often produced by a rotating cobalt-60 source

Why gamma is the right choice:

• Gamma is highly penetrating, so it can reach a tumour deep inside the body from an external source
• Alpha and beta would be absorbed by the skin and surface tissue before reaching the tumour

Diagnosing cancer: tracers and PET scans

• A radioactive tracer is a small amount of a short-half-life radioisotope (often technetium-99m, half-life 6 hours) that is swallowed, inhaled or injected into the patient
• The tracer is chemically attached to a substance the body takes up, glucose for example. The substance carries the tracer to the organ being studied
• A detector (PET scanner) outside the body picks up the gamma emissions and builds up a map showing where the tracer has gone
• Bright spots show areas of high activity in the body. For example, fast-growing tumour cells take up far more glucose than healthy ones, so a tumour shows up as a bright spot

What makes a good tracer:

• Gamma emitter: gamma is the only radiation that can escape the body to reach the external detector
• Short half-life: the tracer should decay quickly so the patient is not radioactive for very long, but not so quickly that it decays before the scan is finished. A few hours is ideal

Sterilising medical equipment: gamma

• Surgical instruments are sealed inside their plastic packaging and exposed to a strong gamma source
• The gamma radiation kills any bacteria, viruses or spores on the instruments
• Because gamma is highly penetrating, it passes straight through the packaging. The instrument is sterilised without opening the seal, so it remains sterile until the surgeon opens the packet
• This method does not need high temperatures, so heat-sensitive instruments (plastics, electronics) can be sterilised this way

Irradiating food: gamma

• Food is exposed to gamma rays before being sold. The radiation kills bacteria, moulds and insects that would spoil the food or cause illness
• The food does not become radioactive (this is irradiation, not contamination; see Section 4)
• The food lasts longer on the shelf and is safer to eat
• Irradiated food is labelled with the Radura symbol, a stylised flower inside a broken circle

Carbon dating: beta

• Living things absorb carbon-14 continuously while they are alive, so the carbon-14 fraction in their tissues stays at the atmospheric level
• When the organism dies, no new carbon-14 is taken in. The carbon-14 already present decays (by beta emission) with a half-life of 5700 years
• The age of an ancient object (wood, bone, charcoal) can be worked out by measuring the remaining carbon-14 activity and counting how many half-lives have passed
• Useful for objects up to about 50 000 years old; for older rocks, uranium dating is used instead

4. Contamination and irradiation

These two words are commonly mixed up in exams. They describe completely different events.

Contamination

Contamination is the accidental transfer of a radioactive substance onto, or into, another object

• The contaminated object now physically contains radioactive atoms, usually in tiny amounts, on the surface or inside the body
• Because the radioactive atoms are part of the object, the object itself is now radioactive and will keep emitting radiation until those atoms decay away
• Typical examples:
  • A radiation leak at a nuclear plant deposits radioactive dust on the ground and on workers' clothes
  • Inhaling radioactive radon gas in a poorly ventilated cellar, where radon atoms enter the lungs and decay there
  • Eating food grown on contaminated soil, so radioactive atoms enter the body

Irradiation

Irradiation is the process of exposing an object to ionising radiation from an external source

• The object is in the path of the radiation while the source is nearby, but no radioactive material is transferred onto or into it
• As soon as the source is removed or shielded, the irradiation stops, and the object is not radioactive afterwards
• Typical examples:
  • A patient receiving a dental X-ray is irradiated while the X-ray machine is on, but does not become radioactive
  • A worker in a hospital who walks past a shielded gamma source receives a small dose of irradiation
  • Food being passed through a gamma sterilisation chamber

Side-by-side comparison

Half-life and which is more dangerous

• A short half-life means a high activity, with many decays per second per unit mass. Most of the radiation is emitted quickly
  • This makes irradiation worse: while the source is active, anyone nearby is hit with a heavy stream of radiation
  • For external doses (X-ray rooms, gamma sterilisers), short-half-life sources are the bigger danger
• A long half-life means a low activity but lasting for a very long time
  • This makes contamination worse: a contaminated object stays radioactive for years or centuries and keeps irradiating its surroundings the whole time
  • For environmental clean-up (nuclear leaks, buried waste), long-half-life isotopes are the bigger danger

5. Dangers of ionising radiation

How radiation harms cells

• All three types of nuclear radiation (and X-rays) are ionising: they knock electrons off atoms inside the body
• This can damage the DNA inside a cell in two main ways:
  1. Low doses: the DNA is damaged but not destroyed. The cell may repair the damage incorrectly, producing a mutation. A mutation in a gene that controls cell division can lead to cancer
  2. High doses: large numbers of cells are killed outright. Tissues stop working. Symptoms include radiation sickness (nausea, hair loss, burns) and, at very high doses, death
• Cells that divide quickly (bone marrow, gut lining, hair follicles, embryos) are most sensitive
• This is also why radiotherapy works: cancer cells divide faster than most healthy cells, so they are killed preferentially by a carefully aimed dose

Inside the body vs outside the body

• The dangers of alpha, beta and gamma flip around depending on whether the source is outside or inside the body:

• This is why alpha-emitting dust or gas is especially feared. Outside the body the alpha would barely reach the skin; inhaled or swallowed it irradiates the lungs or gut at point-blank range
• Polonium-210 (an alpha emitter) is a famous example: harmless if held in a sealed lump, lethal in microgram quantities if ingested

Sources of background radiation

• Everyone is exposed to background radiation all the time. It comes from:
  • Radon gas seeping up from rocks (the single biggest contribution in the UK, especially in granite areas)
  • Cosmic rays from the Sun and outer space
  • Rocks and soil containing tiny amounts of uranium, thorium and potassium
  • Food and drink containing trace amounts of naturally radioactive isotopes (potassium-40, carbon-14)
  • Medical sources such as X-rays, CT scans and radiotherapy
  • Nuclear power and weapons testing (a small but measurable contribution)
• Background radiation levels are low and most people receive a very small dose each year. The risk of cancer at these levels is tiny, but it is not zero, which is why exposure should be kept as low as reasonably achievable

6. Safe handling of radioactive sources

There are three guiding principles for reducing the dose anyone receives from a source: time, distance and shielding.

Time

• The shorter the time you spend near a source, the smaller the total dose you receive
• Workers handling sources keep their tasks brief and the source out of its container only as long as needed

Distance

• Radiation spreads out from a source, so the further away you are, the lower the intensity at your position
• For a point source, the intensity follows an inverse-square law: doubling the distance from the source divides the dose rate by 4
• That is why sources are handled with tongs rather than fingers; the extra distance dramatically reduces the dose to the hand

Shielding

• A barrier between the source and the worker absorbs much of the radiation
• The right shielding depends on the radiation:
  • Alpha: a sheet of paper or even a few cm of air is enough
  • Beta: a few mm of aluminium
  • Gamma: several cm of lead, or thick concrete; never fully stopped, only reduced
• X-ray rooms have lead-lined walls and lead aprons for the patient; nuclear waste is stored in thick concrete-and-steel casks

Practical safety rules in a school or hospital

• Keep sources in a lead-lined box when not in use
• Use tongs to lift and move sources; never touch with bare hands
• Point the source away from people
• Wear gloves, lab coats and (where appropriate) lead aprons or full lead-lined suits
• Where the source is a powder, dust or gas, wear an airtight suit to prevent contamination, keeping the radioactive material out of the eyes, nose and mouth
• Record how long the source is out, and how it has been used
• Wear a film badge (dosemeter), see below, so that any over-exposure is detected

The film badge / dosemeter

• A dosemeter is a small badge clipped to the clothing of anyone working with radiation
• The badge contains photographic film (or a modern equivalent), partly covered by strips of different absorber materials, typically paper, aluminium and lead, each of varying thickness
• Over a week or month the badge is removed and processed. The amount of darkening behind each absorber shows the dose of each type of radiation the wearer has received:
  • Darkening behind no absorber but not behind the paper window ⇒ alpha (or low-energy radiation)
  • Darkening behind paper but not aluminium ⇒ beta
  • Darkening even behind lead ⇒ gamma or X-rays
• If the total dose exceeds the safe limit, the worker is taken off radiation work until the dose drops

7. Disposal of nuclear waste

Radioactive waste from power stations, hospitals and research labs has to be stored safely for as long as it remains active. How it is stored depends on the type of radiation and the half-life.

Low-level waste

• Materials with only alpha or weak beta activity (used gloves, paper, syringes from hospitals)
• Sealed inside plastic bags or metal canisters and buried in shallow, monitored landfill sites
• The packaging easily absorbs the alpha and beta emissions

Intermediate-level waste

• More strongly active beta and gamma waste (reactor parts, contaminated tools)
• Sealed inside steel drums, often surrounded by concrete to form a heavy block
• Stored in concrete-lined silos above ground or in shallow burial sites

High-level waste

• Spent nuclear fuel rods and the products of fission
• Strongly gamma-emitting, with very long half-lives (thousands to millions of years)
• Storage strategy:
  • First cooled in water-filled ponds for several years next to the reactor; water shields the gamma and removes the heat of decay
  • Then encased in vitrified glass blocks (mixing the waste into molten glass that solidifies into a solid, leach-resistant form)
  • Then sealed inside lead-lined, thick concrete casks
  • Finally buried deep underground in geologically stable sites such as old salt mines or stable rock formations where there is no groundwater movement

Why it is so hard

• Storage must last as long as the waste is active. For some isotopes this is tens of thousands of years, far longer than any civilisation has lasted
• Containers must:
  • Resist corrosion for centuries (stainless steel with copper outer layers; expensive to manufacture)
  • Survive earthquakes, floods and accidental impact
  • Stop water reaching the waste (water carries dissolved radioactive atoms into the environment)
• The chosen burial site must have:
  • Low risk of earthquakes and other natural disasters
  • No groundwater flow that could transport radioactivity to lakes, rivers or aquifers
  • High security so the waste cannot be stolen or accidentally exposed
• For very low-activity liquid waste, dilution in large volumes of seawater is sometimes used. This works only because the radioactivity is spread so thinly that any single point in the sea is at or near background level; it relies on the radioactivity being too diffuse to harm anything, not on it disappearing