5.2 Radioactivity (Cambridge (CIE) IGCSE Physics)

False.

Nuclear decay is spontaneous and random.

False.

It is not possible to predict when a nucleus will decay or which nucleus will decay. But it is possible to estimate how many nuclei will decay, or the probability that a nucleus will decay in a given time frame.

An alpha particle is composed of two protons and two neutrons. It is essentially a helium nucleus.

A beta particle is composed of a fast-moving electron.

False.

Alpha particles have a charge of +2 due to having two positively charged protons. It is beta particles that have a charge of -1 due to the negatively charged electron.

Alpha particles are the most ionising form of nuclear radiation. Beta particles are next, and gamma radiation is the least ionising.

The types of nuclear radiation in order of increasing penetrating power are:

• alpha (least penetrating, can be stopped by paper)

• beta (can stopped by a few mm of aluminium)

• gamma (most penetrating, can be reduced by a few mm of lead)

Alpha particles have the greatest charge of all the nuclear decay radiation at +2. Therefore, they attract electrons from other atoms very strongly. Alpha particles also have the greatest mass, and therefore a high kinetic energy. These factors combined make alpha particles highly ionising.

Gamma radiation has no charge so it is less ionising than alpha and beta particles. However, gamma radiation is still capable of ionising atoms as it is the highest energy electromagnetic wave. (Remember that UV, X-rays and gamma rays are ionising.)

Beta particles have a negligible mass, but they travel at very high speeds. This means that they have a high kinetic energy. Beta (minus) particles also have a charge of -1 so they repel electrons, knocking them out of atoms. These factors combined make them ionising.

Nuclear radiation can be deflected in an electric field if it has charge. Therefore, alpha and beta particles can be deflected in an electric field.

False.

Alpha particles are positively charged and opposite charges attract, therefore alpha particles will deflect toward the negatively charged plate in an electric field.

True.

Gamma radiation has no charge therefore it cannot be deflected by electric or magnetic fields.

Beta particles are deflected the most when travelling through an electric field. Alpha particles have a greater mass so they get deflected the least.

True.

The most stable nuclei have approximately equal numbers of protons and neutrons.

True.

A nucleus becomes stable by reducing its overall nuclear energy. It does this by emitting high-energy alpha and beta particles and gamma radiation.

The two factors that can cause an isotope to be radioactive are:

• an excess of neutrons in the nucleus

• the nucleus being too heavy

An isotope will decay into a new element if the number of protons changes. Therefore, alpha and beta decay will produce a new element.

Alpha decay results in the mass number decreasing by 4, because 4 nucleons are emitted.

True.

A neutron changes into a proton and an electron during beta decay. The high-energy electron is emitted from the nucleus.

Alpha decay results in the atomic number decreasing by 2, because 2 protons are emitted.

Gamma decay has no effect on the mass number or the atomic number. Gamma radiation reduces the energy of the nucleus but has no effect on the number of particles.

The atomic number increases by 1 as a result of beta decay. A neutron changes into a proton and an electron. The extra proton causes the atomic number to increase by 1, so a new element is formed.

The charge on a nucleus will decrease by 2 after it emits an alpha particle because 2 protons are emitted.

The mass number remains unchanged by beta decay. A neutron changes into a proton which has the same mass. A beta particle is emitted but this change in mass is negligible.

Half-life is the time taken for half the nuclei of a particular isotope in any sample to decay.

True.

Half-life is constant for any particular isotope.

The activity of a radioactive sample will have halved after one half-life

True.

Half-life is different for different isotopes.

The symbol for activity on a half-life graph is .

The value of the activity of a radioactive sample after two half-lives is of its original value.

The symbol for original activity on a half-life graph is .

To determine the half-life of an isotope on a graph of activity over time:

• identify the point at which the activity has halved and draw a line to the curve

• track that point down to the time axis

After three half-lives, the proportion of original nuclei remaining in the sample will be .

When determining the activity of a radioactive sample, the count rate of the background radiation must be subtracted from the activity of the sample.

Alpha radiation is used in smoke detectors.

Beta radiation is used in measuring and controlling the thickness of materials.

Gamma radiation is used for irradiating food to kill bacteria.

Gamma radiation is used for sterilising equipment.

A smoke alarm works as follows:

• alpha particles ionise the air inside the detector creating a current

• the alpha emitter is blocked by the presence of smoke particles

• the alarm is triggered when the sensor no longer detects alpha particles

Alpha particles are stopped by paper, so they would not penetrate the material to reach the detector.

Gamma radiation can penetrate packaging and objects so all surfaces of the food or equipment can be irradiated. Gamma radiation is ionising enough to kill any microorganism, therefore it is used for sterilisation.

Beta particles can penetrate materials such as paper and aluminium foil, so the number of beta particles detected by a sensor can be used to measure and control the thickness of the substance.

Gamma radiation is used in the treatment of cancer in the process of radiotherapy. Gamma radiation can damage cells and tissues so this can be used to target cancer cells. Gamma rays can penetrate the body to reach the cancer cells, but the beams must be emitted from different angles to reduce the damage to healthy tissues.

The three effects of ionising nuclear radiation that you need to know are:

• cell death (and therefore tissue damage)

• mutations

• cancer

Ionising radiation can remove electrons from the atoms that make up DNA and therefore change the structure of the molecule. A structural change in DNA can lead to mutated cells. Mutated cells usually either die or get destroyed by the immune system, but some mutations can cause cells to replicate uncontrollably and lead to cancer.

Radioactive waste is disposed of by:

• containing it in marked containers

• burying it underground

• leaving it undisturbed for thousands of years

Radioactive samples should be handled by:

• storing the sample encased in lead

• using gloves and tongues to move the sample

• wearing protective clothing

• keeping the sample at a distance from the body

• minimising the exposure time

• washing hands after to avoid contamination

A dose is the amount of radiation received by a person. It is measured in sieverts (Sv).

Over-exposure to nuclear radiation could lead to radiation poisoning.

A dosemeter is a device worn by people who work with radiation that measures the amount of radiation received.