Atoms & Nuclear Radiation Flashcards

All 55 cards in this deck

What is radioactive decay?

The random process in which an unstable nucleus gives out radiation as it changes to become more stable.

What is meant by the activity of a radioactive source?

The rate at which a source of unstable nuclei decays. Measured in becquerel (Bq).

What is count-rate?

The number of decays recorded each second by a detector.

Which instrument is used to detect and measure count-rate?

A Geiger-Muller tube.

True or false? It is the electrons of an atom that are unstable in a radioactive substance.

False. It is the nucleus that is unstable, and radiation is emitted from the nucleus.

What does an alpha particle consist of?

Two protons and two neutrons (the same as a helium nucleus).

What is a beta particle?

A high speed electron ejected from the nucleus as a neutron turns into a proton.

What is a gamma ray?

Electromagnetic radiation emitted from the nucleus.

What material stops alpha, beta and gamma radiation?

Alpha: a sheet of paper (or skin). Beta: a few mm of aluminium. Gamma: thick lead or concrete (only reduced).

What is the approximate range in air of alpha, beta and gamma radiation?

Alpha: a few cm. Beta: about a metre. Gamma: very long (many metres or more).

Put alpha, beta and gamma in order of ionising power, strongest first.

Alpha (most ionising), then beta, then gamma (least ionising).

True or false? Gamma radiation is the most ionising type of nuclear radiation.

False. Gamma is the least ionising but the most penetrating; alpha is the most ionising.

Which type of radiation is used in a smoke alarm, and why?

Alpha, because it ionises the air but has a very short range so it cannot escape the alarm.

Which type of radiation is used to monitor the thickness of thin sheets of metal or paper, and why?

Beta, because it partly passes through the sheet so the count-rate changes if the thickness changes.

Which type of radiation is used as a tracer to find leaks in underground pipes, and why?

Gamma, because it is penetrating enough to pass through the soil and be detected at the surface.

True or false? An alpha source would be a good choice for a tracer that must be detected from outside a sealed container.

False. Alpha is stopped by the container walls; a gamma source is needed.

Why is beta, rather than gamma, chosen for a thickness gauge for thin aluminium sheet?

Gamma would pass straight through whatever the thickness, so the count-rate would barely change.

What symbol is used for an alpha particle in a nuclear equation?

$^{4}_{2}\text{He}$

What is the effect of alpha emission on the mass number and atomic number of a nucleus?

Mass number decreases by 4; atomic number (charge) decreases by 2.

What are the steps to complete a nuclear equation for alpha decay?

e.g. $^{226}_{88}\text{Ra} \rightarrow\ ^{?}_{?}\text{Rn} + \alpha$

1. Write alpha as $^{4}_{2}\text{He}$
2. Take 4 off the mass number: $226 - 4 = 222$
3. Take 2 off the atomic number: $88 - 2 = 86$, giving $^{222}_{86}\text{Rn}$

True or false? In alpha decay the charge of the nucleus stays the same.

False. Alpha decay decreases both the mass and the charge of the nucleus.

What symbol is used for a beta particle in a nuclear equation?

$^{0}_{-1}\text{e}$

What is the effect of beta emission on the mass number and atomic number of a nucleus?

Mass number is unchanged; atomic number (charge) increases by 1.

What are the steps to complete a nuclear equation for beta decay?

e.g. $^{14}_{6}\text{C} \rightarrow\ ^{?}_{?}\text{N} + \beta$

1. Write beta as $^{0}_{-1}\text{e}$
2. Keep the mass number the same: $14$
3. Add 1 to the atomic number: $6 + 1 = 7$, giving $^{14}_{7}\text{N}$

True or false? A beta particle is an electron that comes from the outer shells of the atom.

False. It is emitted from the nucleus when a neutron turns into a proton.

What is the effect of gamma emission on the mass number and charge of a nucleus?

Neither changes; the nucleus only loses energy.

True or false? When a nucleus emits a gamma ray it turns into a different element.

False. The atomic number is unchanged, so it is still the same element.

What symbol is used for a neutron in a nuclear equation?

$^{1}_{0}\text{n}$

What is the effect of emitting a neutron on the mass number and charge of a nucleus?

Mass number decreases by 1; the charge (atomic number) is unchanged.

What is meant by the 'random nature of radioactive decay'?

You cannot predict which nucleus will decay next, or when a particular nucleus will decay.

True or false? Radioactive decay can be triggered or speeded up by heating the sample.

False. Decay is a random nuclear process, unaffected by temperature or chemical conditions.

True or false? Although individual decays are random, the half-life of an isotope is predictable.

True. With very large numbers of nuclei, the overall rate of decay follows a reliable pattern.

Why does a measured count-rate vary slightly from second to second even for a steady source?

Because decay is random, so the number of decays in each second is not exactly the same.

What is the half-life of a radioactive isotope?

The time it takes for the number of nuclei of the isotope in a sample to halve, or for the count-rate (activity) to fall to half its initial level.

What happens to the activity of a sample after one half-life has passed?

It falls to half its previous value.

What are the steps to find the half-life from a graph of count-rate against time?

e.g. the graph starts at 80 counts per second

1. Read the starting count-rate: 80 counts/s
2. Halve it: 40 counts/s
3. Read off the time when the curve reaches 40 counts/s — that time is the half-life.

True or false? After two half-lives a radioactive sample has completely decayed.

False. A quarter of the original nuclei remain undecayed.

True or false? The half-life of an isotope stays the same as the sample decays.

True. Half-life is a fixed property of the isotope.

What fraction of the original nuclei remains after $n$ half-lives?

$\left(\frac{1}{2}\right)^{n}$

e.g. after 3 half-lives, $\frac{1}{8}$ remains.

What are the steps to find the net decline in activity as a ratio after a given number of half-lives?

e.g. after 3 half-lives

1. Halve once per half-life: $1 \rightarrow \frac{1}{2} \rightarrow \frac{1}{4} \rightarrow \frac{1}{8}$
2. Fraction remaining $= \frac{1}{8}$
3. Net decline as a ratio $= 1:8$

What are the steps to find how long it takes for activity to fall to a given fraction?

e.g. fall to one quarter, half-life 60 s

1. Count the halvings: $1 \rightarrow \frac{1}{2} \rightarrow \frac{1}{4}$, so 2 half-lives
2. Multiply by the half-life: $2 \times 60 = 120$ s

True or false? A net decline of 1:16 means four half-lives have passed.

True. $\frac{1}{2}$ four times gives $\frac{1}{16}$ remaining.

What is radioactive contamination?

The unwanted presence of materials containing radioactive atoms on other materials.

What is irradiation?

The process of exposing an object to nuclear radiation.

True or false? An object that has been irradiated becomes radioactive.

False. The irradiated object does not become radioactive.

What causes the hazard from radioactive contamination?

The decay of the contaminating atoms; the type of radiation emitted affects the level of hazard.

How does the hazard stop in each case: irradiation and contamination?

Irradiation stops as soon as the source is removed or shielded; contamination continues until the radioactive atoms are removed or have decayed.

Which type of radiation is most hazardous inside the body (from contamination), and why?

Alpha, because it is the most ionising and is all absorbed by nearby cells.

Which types of radiation are most hazardous outside the body (from irradiation), and why?

Gamma and beta, because they are penetrating enough to pass through skin into the body.

State two precautions used to protect against a radioactive source during irradiation.

Handle the source with long tongs / keep your distance, and use shielding such as lead or a lead-lined box; also limit exposure time.

State two precautions used to avoid becoming contaminated by a radioactive source.

Wear gloves and protective clothing, and never handle sources directly or eat near them.

True or false? Alpha sources are relatively safe outside the body because alpha is stopped by skin.

True. The danger comes if an alpha source is swallowed or inhaled, causing contamination.

Why is it important that findings of studies into the effects of radiation on humans are published and shared with other scientists?

So that the findings can be checked by peer review.

What is peer review?

The checking of a scientist's findings and methods by other scientists working in the same field.

True or false? The conclusions of a single unpublished study into the effects of radiation should be accepted as reliable.

False. Findings must be published and peer reviewed before they can be trusted.