Radioactivity

01

Atoms, nuclei and isotopes

An atom has a small positive nucleus, with negatively charged electrons outside it. The nucleus contains protons and usually neutrons.

Proton

A positively charged particle in the nucleus. Its charge is +1 when measured in units of the proton's charge.

Neutron

An uncharged particle in the nucleus. Protons and neutrons are collectively called nucleons.

Electron

A negatively charged particle outside the nucleus. Its charge is -1 in the same units. A neutral atom has equal numbers of protons and electrons.

The nucleus is much smaller than the whole atom. A drawing that makes its particles visible cannot also show their sizes and separations to one realistic scale. Ordinary hydrogen has one proton and no neutron in its nucleus, so a neutron is not present in every atom.

Read a nuclide symbol

A nuclide is a type of nucleus specified by its proton and neutron numbers. Its notation puts the nucleon number A at the upper left and the proton number Z at the lower left of the element symbol.

2311Na means sodium-23, with 23 nucleons and 11 protons.

Proton number, Z

The number of protons in a nucleus. It is also called atomic number and identifies the element.

Nucleon number, A

The total number of protons and neutrons. It is also called mass number. It is a count, not the atom's mass in grams.

Same element, different neutron number

For sodium-23, the neutron count is 23 - 11 = 12. Sodium-24 still has 11 protons, but has 24 - 11 = 13 neutrons. They are isotopes: atoms of the same element with the same proton number but different neutron numbers.

Magnesium-24 has 12 protons and 12 neutrons. It shares nucleon number 24 with sodium-24, but its different proton number makes it a different element. Equal nucleon numbers alone do not identify isotopes.

Removing or adding electrons changes an atom's charge, producing an ion. It does not change its proton or neutron numbers, so it does not create another isotope. Also, the existence of isotopes does not mean that every isotope of an element is radioactive.

02

Radioactive decay and radiation

During radioactive decay, an unstable nucleus loses energy by emitting radiation. The material contains the radioactive nuclei; the radiation is what leaves them.

A nucleus contains protons and usually neutrons. Nuclear decay changes a nucleus, rather than simply removing an electron from an atom's outer region.

Random events, a predictable average pattern

Random

The exact time when a particular unstable nucleus will decay cannot be predicted. Identical nuclei do not all decay after one fixed waiting time.

Spontaneous

The decay does not require an external trigger. An unstable nucleus can undergo the change on its own.

For a large collection of the same radioactive nuclide, the average rate follows a reproducible pattern. This is compatible with uncertainty about individual nuclei. The half-life describes that population pattern.

The nucleus remaining after an emission is called the daughter nucleus. It may itself be radioactive; one emission does not guarantee a stable final nucleus.

Ionisation changes atoms along the radiation's path

Ionisation can occur when radiation removes an electron from an atom or molecule, leaving it charged. This can happen in material that the radiation enters, away from the original radioactive nucleus.

Alpha, beta and gamma can all cause ionisation. Compare what each emission is, how densely it ionises along its path and how readily it penetrates a stated material.

Alpha, α

An alpha particle is a helium nucleus: two protons and two neutrons, with charge +2 in proton-charge units. It is not a whole neutral helium atom.

It typically produces strong, dense ionisation over a short path. Its penetration is low: paper or the outer layer of intact skin can stop typical alpha particles.

Beta-minus, β^-

A beta-minus particle is a fast electron emitted in a nuclear process, with charge -1. It is produced during that change, rather than being an existing electron ejected from an atomic shell.

Compared with typical alpha radiation, it produces less dense ionisation and penetrates farther. A suitable thin metal absorber can substantially reduce it. The energy and absorber determine how much passes through.

Gamma, γ

Gamma radiation is electromagnetic radiation from a nucleus. It has no electric charge and no rest mass. Its nature connects to the electromagnetic spectrum.

It is highly penetrating. Thick lead or concrete can reduce its intensity, but shielding does not necessarily remove every gamma ray. It can cause ionisation even though its ionisation is less dense in the usual comparison.

Passing through paper does not, by itself, identify an emission as gamma: beta can also pass paper. Use the full set of observations under the stated absorber and source conditions.

Penetration and ionisation describe different properties. A short range does not mean harmlessness, especially if radioactive material is close to living cells. The uses and hazards depend on the source's location and the exposure conditions.

03

Background radiation and activity

A detector can register radiation even when the particular source being investigated is absent. Separate this background contribution from the source reading.

Radioactive decay is random, so counts fluctuate. A rate means an amount per unit time: equal raw counts collected over different durations need not mean equal rates.

Background is present without the study source

Natural contributions include cosmic radiation, radioactive materials in rocks and soil, radon from the ground, and radionuclides in food and body materials. Artificial sources can also contribute. Background varies with location and conditions.

Removing the study source therefore does not normally make the detector reading zero. Repeated equal-time measurements also need not give exactly the same count.

Convert counts to matching rates

Five equal one-minute background observations give 18, 21, 19, 22 and 20 counts. Their total is 100 counts in 5 min, so the mean background rate is 100/5 = 20 counts/min.

Activity counts decays, not detector events

The activity A of a radioactive source is the number of nuclear decays per second. Its unit is the becquerel, Bq: 1 Bq means one decay per second.

Here A names a rate. In a nuclide symbol, A instead names nucleon number, which has no unit. Use the quantity and context to distinguish them.

A detector generally registers only a fraction of the source's emissions. Direction, distance, absorbers and detector response matter. A reading of 80 counts/s is therefore not automatically an activity of 80 Bq.

Keep the measurement conditions matched

• Detector and range: use a suitable detector and counter with a usable rate range and response for the emission. At high rates, some detectors miss events arriving close together.
• Background: measure it with the study source absent, keeping the detector settings, location and surrounding conditions comparable.
• Geometry: keep source-detector distance and alignment fixed when comparing a source over time. Changed separation or an added absorber can change counts without any change in source activity.
• Timing: record the actual collection duration. Longer counting or repeated observations can reduce relative random variation in a steady or background rate.
• Calibration: the one-in-four factor above applies only to its stated emission, geometry, shielding and settings. A change can invalidate that estimate.

Longer counting is not always better for following a decaying source: a long interval averages over its change. Use intervals short compared with the decay timescale when assigning a rate to a particular time. Repetition cannot correct an inappropriate calibration or changed geometry.

04

Half-life from numbers and graphs

Half-life is the time taken for half the undecayed radioactive nuclei in a large sample of one nuclide to decay. Over that time, its activity falls to half.

Activity is decays per second. With unchanged detection conditions, the background-corrected source count rate follows the same proportion for the single-nuclide model considered here.

The symbol is t_1/2. It is a time, with SI unit s. A problem may use minutes, hours or days consistently; compare the elapsed time and half-life in matching units.

Halve what remains each time

Half of the radioactive parent nuclei remain after one half-life on average. Half of the whole sample has not vanished: daughter material is still present. After the next half-life, half of the remaining parents decay.

Find half-life from corrected data

This separate supplied model follows one radioactive nuclide. Background has a constant mean of 20 counts/min; daughter contributions are neglected. The rates represent the labelled instants, with detection conditions unchanged.

1. Correct the first pair: 180 - 20 = 160 and 100 - 20 = 80 counts/min.
2. Read the time interval: 160 to 80 takes 4 - 0 = 4 min.
3. Check another pair: the net rate falls from 40 at 8 min to 20 at 12 min, again taking 4 min.

A fall from 160 to 20 counts/min is three halvings: 160, 80, 40, 20. It therefore takes 3 x 4 = 12 min. It is not one half-life simply because the final reading is small.

Halving the uncorrected 180 to 90 counts/min would also halve the background contribution. The background does not follow the source's decay, so that would give the wrong interval.

Read a curve and interpret real measurements

Label elapsed time horizontally and the stated rate vertically. On a corrected curve, choose a source rate, find half that rate, and read the difference between their times. Use another well-separated pair as a check.

The source loses the same fraction over equal half-life intervals, not the same amount. The average curve bends as the rate falls; it does not become zero after two half-lives. Individual measurements scatter around this trend because decay and detection are random.

For a logged count series, retain the timestamps and counting durations. Calculate rates, subtract a comparable background estimate and keep source-detector geometry fixed. A short counting window can give an approximate rate at its labelled time, but an excessively long window blurs a rapid decay. Do not mistake a changed detector position or setting for a new half-life.

05

Uses and hazards

Radioactivity is useful when its emissions can be detected or transfer energy where needed. Those same interactions can damage living tissue.

Alpha is strongly ionising with low penetration; gamma is highly penetrating. Beta lies between them in the usual comparison. Half-life describes how rapidly a source's activity falls, not the range of its radiation.

Follow the source, material and detector

A gamma-emitting medical tracer

Radioactive material inside the body emits radiation that can reach a detector outside. The detected radiation gives information about the tracer's distribution.

The source is inside, and radiation travels outwards towards the detector. This differs from an external X-ray source sending radiation through the body. Suitable penetration is needed for the emitted signal to reach the detector.

A beta thickness gauge

A source and detector sit on opposite sides of a moving sheet. Under matched material and geometry conditions, a thicker sheet absorbs more, giving a lower detected rate. A thinner sheet gives greater transmission.

The chosen radiation must be partly transmitted through the relevant sheet. Alpha may be absorbed too completely, while very penetrating radiation may show too little change. Beta is useful for suitable paper, plastic or light-metal sheets, but is not the correct choice for every material and thickness.

Keep the source output and detector conditions accounted for: a falling count rate caused by source decay or changed alignment would not, by itself, show that the sheet became thicker.

Gamma radiation in cancer treatment

Ionisation can damage or kill tumour cells. It can also damage healthy cells, so the useful effect requires controlled exposure. Gamma rays do not automatically recognise and target cancer cells.

Distinguish irradiation from contamination

Irradiation or exposure

Radiation reaches an object or person. Exposure to an external source does not, by itself, mean radioactive source material has been transferred.

Radioactive contamination

Radioactive material is present where it is not wanted, such as on a surface or inside a body. It can continue emitting from that location.

Ionisation can damage cells or DNA, kill cells or increase cancer risk. The consequences depend on the radiation, amount and duration of exposure, and where its energy is absorbed.

Typical alpha particles have little penetration from outside, but alpha-emitting material inside the body can damage nearby cells. There may be no protective outer layer between that source and the tissue. Gamma can reach tissue from outside. Penetrating power alone does not rank every exposure's hazard.

Activity in Bq gives the source's decay rate. It does not directly give the energy absorbed by a particular person. Equal activities can produce different exposures because the emissions, shielding, separation and source locations can differ.

Match protection to the route of exposure

• Reduce exposure time: less time exposed can reduce the energy received under otherwise unchanged conditions.
• Increase separation: greater distance from an external source can reduce the radiation reaching a person.
• Use suitable shielding: the material and thickness must suit the emission and conditions. Reduced transmission does not mean the source has stopped being radioactive.
• Contain the source material: containment addresses transfer and spread of radioactive material, rather than only shielding radiation arriving from outside.