Module 6 Radioactivity Flashcards
What is radioactivity?
Emission of ionising radiation from an unstable nucleus.
What are the three most common types of radiation?
Alpha, beta minus, and gamma rays.
How can radioactivity be detected?
Using a Geiger muller tube.
What is the usual unit for count rate?
Becquerels.
What are sources of background radiation?
Cosmic radiation, radioactive elements in rocks of Earth’s crust.
What will background radiation result in when measuring activity of samples?
Increased measure of activity by the same amount and therefore a systematic error.
How to account for background radiation when measuring count rate?
Measure count rate with no source present and subtract count rate from future experiments.
What can be changed to make a more suitable container apart from changing the thickness of an absorber?
Use a different type of absorber.
What is the range of alpha particles in air?
A few cm (5-10).
What is the range of beta particles in air?
A few meters.
What is the range of gamma radiation in air?
A few kilometres.
What can alpha particles be absorbed/blocked by?
Paper or skin.
What can beta particles be absorbed/blocked by?
A few mm of aluminium.
What can gamma radiation be absorbed/blocked by?
A few cm of lead or a few m of concrete.
Why do radioactive particles have limited range?
They lose energy by ionising atoms in their path as they pass through a material or medium.
Why do alpha particles have the shortest range?
Large size and charge, therefore cause a lot of ionisation and lose energy quickly.
Why does gamma radiation have a very long range?
No charge, negligible size, high energy, and low ionising power - loses energy very slowly as it passes through matter.
What is the nucleus left after decay has taken place?
Daughter.
Why is gamma radiation emitted?
Emitted along with alpha or beta particles to remove excess energy.
What does spontaneous mean?
Not affected by external factors such as pressure, temperature, or chemical reactions.
What does random mean?
Cannot predict when a certain nucleus will decay or which will decay in a given time.
What does A mean in the decay equation?
Activity. Number of nuclei decaying per unit time.
What is the decay constant?
Probability of a nucleus decaying per unit time.
What is N in the decay equation?
Number of undecayed nuclei.
Why is the decay constant a number between 0 and 1?
It’s a probability.
What does the value of A actually mean?
How much N will decrease by in a second.
What is the equation to find the number of daughter nuclei using the exponential formula?
N= N_0*e^(-λt).
What does the -λN symbol mean in the spreadsheet equation for decay?
-A.
If λ is given in years^-1, what will the value of t be given in the exponential equation?
Years.
How to find N from a mass and molar mass M?
N = (mass / M) * Avogadro’s number.
What value of t must be taken for accurate results in the spreadsheet model?
Much less than half life so N does not vary too much.
What is the approximate half life of carbon 14?
5700 years.
How is carbon-14 made?
In the atmosphere by cosmic rays (cause nitrogen to undergo beta minus decay).
Why does the ratio of carbon-14 to carbon-12 remain the same over time?
Carbon-14 is constantly replenished by cosmic rays and incorporated into living matter by photosynthesis.
Given that the ratio of C-14 to C-12 is 1.3x10^-12, show that the activity of carbon is 0.25Bq per g.
1 gram of C12 has 1/12xNa number of nuclei.
Multiply by ratio to find number of C14 nuclei per g.
Multiply by decay constant (find using half life of 5700 years).
Describe and explain how carbon dating can be used to find the age of a biological sample.
Measure the count rate of the relic to find A.
Measure the count rate of a living sample with a similar mass of carbon, this is A_0.
C-14 decays exponentially so the exponential decay equation can be used.
What are the limitations of carbon dating?
Cannot be used for modern samples as activity will barely change. Cannot be used for samples over 50,000 years old as the activity will be lower than background rate. May not be accurate for older samples as there may have been changes in Earth’s magnetic field which change the ratio of C-14/C-12 by influencing cosmic rays.
Why is uranium used to date old rocks?
Has an extremely long half life.
What is radioactive power?
Activity x energy released per decay.
What device is used to convert radioactive power to electricity?
Using an RTG.
Give examples of where radioactive materials are used as nuclear batteries.
Spacecrafts or pacemakers.
