Define radioactive decay .

The spontaneous disintegration of a nucleus to form a more stable nucleus, resulting in the emission of an alpha , beta , or gamma particle.

List the features that show radioactive decay is a random process.

There is an equal probability of any nucleus decaying It cannot be known which particular nucleus will decay next It cannot be known at what time a particular nucleus will decay The rate of decay is unaffected by the surrounding conditions It is only possible to estimate the proportion of nuclei decaying in a given time period

How does the fluctuating count rate of a Geiger-Muller (GM) tube near a radioactive source provide evidence for the randomness of radioactive decay?

The counts are irregular and cannot be predicted, and each count represents the decay of an unstable nucleus, so the fluctuations in count rate provide evidence for the randomness of radioactive decay.

Define the decay constant , λ.

The probability that an individual nucleus will decay per unit of time.

Define the activity , A , of a radioactive sample.

The average number of nuclei that decay per unit of time.

Activity is measured in .......... , where an activity of 1 Bq is equal to one decay per second.

Activity is measured in Becquerels (Bq) , where an activity of 1 Bq is equal to one decay per second.

Define exponential decay .

A model in which the number of undecayed nuclei falls very rapidly, without ever reaching zero.

On a graph of the number of undecayed nuclei against time, the .......... the slope, the larger the decay constant λ.

On a graph of the number of undecayed nuclei against time, the steeper the slope, the larger the decay constant λ.

True or False? A radioactive decay curve eventually reaches zero, showing that all the nuclei in a sample have decayed.

False. Radioactive decay is an exponential decay process, so the number of undecayed nuclei falls very rapidly but never actually reaches zero.

The mass of a substance, in grams, in one mole; its unit is g mol -1 .

Define Avogadro's constant , N A .

The number of atoms in one mole of a substance; equal to 6.02 × 10 23 mol -1 .

The average time taken for a given number of nuclei of a particular isotope to halve.

True or False? A radioactive isotope with a short half-life has a small decay constant.

False. Half-life and the decay constant are inversely proportional , so a shorter half-life means a larger decay constant and a faster rate of decay.

Why does the activity of a sample also halve after one half-life has passed?

Activity A is proportional to the number of undecayed nuclei N , so as N halves, the activity also halves.

How can the half-life of a substance be found from a graph of activity against time?

Draw a line from the curve to the point where the activity has dropped to half its original value, then draw a line down to the time axis; this time value is the half-life.

On a graph of ln N against time t , the gradient of the straight line is equal to .......... .

On a graph of ln N against time t , the gradient of the straight line is equal to −λ .

Why are log graphs preferred over standard decay curves for interpreting radioactive decay data?

Straight-line graphs are more useful than curves for interpreting data, and since nuclei decay exponentially , taking logarithms produces a straight line.

List four applications of radioactivity.

Nuclear power Medicine, e.g. radiotherapy, tracers and sterilising equipment Radiocarbon dating of archaeological artefacts Uranium-lead dating of rock samples Radioisotope power systems

Why does the activity of carbon-14 in an organism begin to fall after it dies?

Living organisms constantly absorb and replace carbon-14 while alive, but after death no more is absorbed, so the activity falls with a half-life of around 5730 years .

Radiocarbon dating is considered a reliable method for samples between .......... and .......... years old.

Radiocarbon dating is considered a reliable method for samples between 500 and 60 000 years old.

Why is carbon dating unreliable for samples younger than 500 years?

The activity of the sample is too high to measure small changes accurately, so the ratio of carbon-14 to carbon-12 is too high to determine an accurate age.

Why is carbon dating unreliable for samples older than 60 000 years?

The activity is too low to distinguish from background radiation, so the ratio of carbon-14 to carbon-12 is too small to determine an accurate age.

True or False? Uranium-238 decays directly into stable lead-206 in a single decay.

False. Uranium-238 decays via a decay chain of several steps, which ends with the stable isotope lead-206 .

Define radioisotope power systems .

Devices that transform the heat released by the decay of a radioisotope into electrical power , used to power space probes and satellites.

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Radioactive Decay (AQA A Level Physics): Flashcards

Exam code: 7408

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FrontRadioactive Decay
Define radioactive decay.
FrontRadioactive Decay
List the features that show radioactive decay is a random process.
BackRadioactive Decay
There is an equal probability of any nucleus decaying
It cannot be known which particular nucleus will decay next
It cannot be known at what time a particular nucleus will decay
The rate of decay is unaffected by the surrounding conditions
It is only possible to estimate the proportion of nuclei decaying in a given time period

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Cards in this collection (32)

Define radioactive decay.
The spontaneous disintegration of a nucleus to form a more stable nucleus, resulting in the emission of an alpha, beta, or gamma particle.
List the features that show radioactive decay is a random process.
- There is an equal probability of any nucleus decaying
- It cannot be known which particular nucleus will decay next
- It cannot be known at what time a particular nucleus will decay
- The rate of decay is unaffected by the surrounding conditions
- It is only possible to estimate the proportion of nuclei decaying in a given time period
How does the fluctuating count rate of a Geiger-Muller (GM) tube near a radioactive source provide evidence for the randomness of radioactive decay?
The counts are irregular and cannot be predicted, and each count represents the decay of an unstable nucleus, so the fluctuations in count rate provide evidence for the randomness of radioactive decay.
Define the decay constant, λ.
The probability that an individual nucleus will decay per unit of time.
Define the activity, A, of a radioactive sample.
The average number of nuclei that decay per unit of time.
Activity is measured in .........., where an activity of 1 Bq is equal to one decay per second.
Activity is measured in Becquerels (Bq), where an activity of 1 Bq is equal to one decay per second.
State the equation linking activity A, decay constant λ and number of undecayed nuclei N.
$A = \frac{Δ N}{Δ t} = - λ N$
True or False?
The activity of a radioactive sample depends only on its decay constant, not on the number of undecayed nuclei remaining.
False.
Activity is calculated using $A = λ N$ , so it depends on both the decay constant and the number of undecayed nuclei remaining.
Define exponential decay.
A model in which the number of undecayed nuclei falls very rapidly, without ever reaching zero.
State the equation for the number of undecayed nuclei N remaining after time t.
$N = N_{0} e^{- λ t}$ where N₀ is the initial number of undecayed nuclei and λ is the decay constant.
State the equations for activity A and count rate C in exponential form.
- $A = A_{0} e^{- λ t}$
- $C = C_{0} e^{- λ t}$
On a graph of the number of undecayed nuclei against time, the .......... the slope, the larger the decay constant λ.
On a graph of the number of undecayed nuclei against time, the steeper the slope, the larger the decay constant λ.
True or False?
A radioactive decay curve eventually reaches zero, showing that all the nuclei in a sample have decayed.
False.
Radioactive decay is an exponential decay process, so the number of undecayed nuclei falls very rapidly but never actually reaches zero.
Define molar mass.
The mass of a substance, in grams, in one mole; its unit is g mol^-1.
Define Avogadro's constant, N_A.
The number of atoms in one mole of a substance; equal to 6.02 × 10²³ mol^-1.
State the equation used to calculate the number of nuclei in a sample from its mass and molar mass.
$number of nuclei = \frac{mass \times N_{A}}{molar mass}$
Define half-life.
The average time taken for a given number of nuclei of a particular isotope to halve.
State the equation for half-life t_1/2 in terms of the decay constant λ.
$t_{1 / 2} = \frac{\ln 2}{λ} ≃ \frac{0.693}{λ}$
True or False?
A radioactive isotope with a short half-life has a small decay constant.
False.
Half-life and the decay constant are inversely proportional, so a shorter half-life means a larger decay constant and a faster rate of decay.
Why does the activity of a sample also halve after one half-life has passed?
Activity A is proportional to the number of undecayed nuclei N, so as N halves, the activity also halves.
How can the half-life of a substance be found from a graph of activity against time?
Draw a line from the curve to the point where the activity has dropped to half its original value, then draw a line down to the time axis; this time value is the half-life.
On a graph of ln N against time t, the gradient of the straight line is equal to ...........
On a graph of ln N against time t, the gradient of the straight line is equal to −λ.
When $N = N_{0} e^{- λ t}$ is written in straight-line form y = mx + c, what do y, x and c represent?
- y = ln N
- x = t
- c = ln(N₀)
Why are log graphs preferred over standard decay curves for interpreting radioactive decay data?
Straight-line graphs are more useful than curves for interpreting data, and since nuclei decay exponentially, taking logarithms produces a straight line.
List four applications of radioactivity.
- Nuclear power
- Medicine, e.g. radiotherapy, tracers and sterilising equipment
- Radiocarbon dating of archaeological artefacts
- Uranium-lead dating of rock samples
- Radioisotope power systems
How is carbon-14 formed in the atmosphere?
Cosmic rays knock neutrons out of nuclei; these neutrons then collide with nitrogen nuclei in the air: $_{}^{1}n +_{}^{14}N \to_{}^{14}C +_{}^{1}p$
Why does the activity of carbon-14 in an organism begin to fall after it dies?
Living organisms constantly absorb and replace carbon-14 while alive, but after death no more is absorbed, so the activity falls with a half-life of around 5730 years.
Radiocarbon dating is considered a reliable method for samples between .......... and .......... years old.
Radiocarbon dating is considered a reliable method for samples between 500 and 60 000 years old.
Why is carbon dating unreliable for samples younger than 500 years?
The activity of the sample is too high to measure small changes accurately, so the ratio of carbon-14 to carbon-12 is too high to determine an accurate age.
Why is carbon dating unreliable for samples older than 60 000 years?
The activity is too low to distinguish from background radiation, so the ratio of carbon-14 to carbon-12 is too small to determine an accurate age.
True or False?
Uranium-238 decays directly into stable lead-206 in a single decay.
False.
Uranium-238 decays via a decay chain of several steps, which ends with the stable isotope lead-206.
Define radioisotope power systems.
Devices that transform the heat released by the decay of a radioisotope into electrical power, used to power space probes and satellites.

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