Physics - Radioactivity
OCR A-Level Physics 2022
Flashcards
Radiation basics and detection
What must you subtract from every Geiger-Muller counter in order to accurately determine the radioactivity of a substance?
The background radiation.
What sort of things does the background radiation come from?
Earth, cosmic rays, food.
What is a cloud chamber used for?
Seeing the path that ionising radiation takes.
How do you determine what type of radiation something is?
Look at how far it travels and what it passes through.
Alpha radiation
What happens to the number of protons and neutrons in the nucleus during alpha radiation?
- Protons: $-2$
- Neutrons: $-2$
Why is alpha radiation strongly ionising?
Because alpha particles have a large mass and charge.
What is the approximate mass of an alpha particle in atomic mass units $u$?
Beta-plus radiation
What happens to the number of protons and neutrons in the nucleus during beta-plus radiation?
- Protons: $-1$
- Neutrons: $+1$
Why is the ionisation of beta-plus radiation typically glossed over?
Because the positron annihilates electrons.
What is the approximate mass of a beta-plus particle (a positron) in atomic mass units $u$?
Why don’t beta-plus particles travel very far?
They very quickly become annihilated.
Beta-minus radiation
What happens to the number of protons and neutrons in the nucleus during beta-minus radiation?
- Protons: $+1$
- Neutrons: $-1$
What is the approximate mass of a beta-minus (an electron) particle in atomic mass units $u$?
Gamma radiation
What happens to the number of protons and neutrons in the nucleus during gamma radiation?
Nothing.
What is gamma radiation typically absorbed by?
A few centimeters of lead or thick concrete.
Orders of magnitude of speed
What order of magnitude is the speed of an alpha particle?
What order of magnitude is the speed of a beta-plus or beta-minus particle?
The radiation-testing experiment
What does the setup for testing what type of radiation a radioactive source is emitting (ideally as a very poorly drawn diagram)?
How can you use this setup to test what type of radiation a source is emitting?
How can you use this setup to test what type of radiation a source is emitting?- Vary the distance of the Geiger-Muller tube.
- Place absorbers between the source and detector.
What distance do you have to place absorbers away when using this setup to ensure you account for all types of radiation?
What distance do you have to place absorbers away when using this setup to ensure you account for all types of radiation? Why is it difficult to use this setup to determine the type of radiation something is by varying the distance between the source and detector?
Why is it difficult to use this setup to determine the type of radiation something is by varying the distance between the source and detector?Because it’s hard to ensure the radiation is travelling directly towards the tube.
Handling sources safely
What four things can you do to ensure safety when handling radioactive sources (MMHD)?
- Minimise exposure time
- Maximise distance to source
- Handle with tongs
- Don’t point at anyone
Deflection in electric and magnetic fields
Why does a beta-plus or a beta-minus get deflected more than an alpha particle in an electric or magnetic field despite having a smaller charge?
Because the mass is much smaller, its much easier for the force to change its motion.
What happens to a gamma ray in a magnetic or electric field?
Nothing; it’s not deflected at all.
Decay equations and conservation
What happens to a “parent nucleus” in nuclear decay?
It decays into a daughter nucleus.
What two quantities are always conserved in nuclear decay and nuclear decay equations?
Nucleon number and proton number.
Here is alpha decay of a mystery element $X$:
\[^A _ Z X \to ^{??} _ {Z-2} Y + ^4 _ 2 \text{He}\]
What is the missing value?
Here is the beta-minus decay of a mystery element $X$:
\[^A _ Z X \to ^A _ {??} Y + ^0 _ {-1}e + ^0 _ 0 \bar{v _ e}\]
What is the missing value?
Here is the beta-plus decay of a mystery element $X$:
\[^A _ Z X \to ^{??} _ {A - 1} Y + ^0 _ {-1}e + ^0 _ 0 \bar{v _ e}\]
What is the missing value?
Here is the gamma decay of a mystery element $X$:
\[^A _ Z X^{*} \to ^{??} _ {Z} X + ^0 _ 0 \gamma\]
What is the missing value?
What does the fun asterisk $X^*$ represent in the gamma decay equation $X$:
\[^A _ Z X^{*} \to ^{??} _ {Z} X + ^0 _ 0 \gamma\]
?
The fact the nucleus is excited.
Choosing the decay mode
What type of radioactive nuclear decay _ doesn’t _ form a new element?
Gamma.
For light nuclei, what is typically true about the ratio of protons to neutrons?
It is roughly the same.
For heavy nuclei, what is typically true about the ratio of protons to neutrons?
There are more neutrons than protons.
What type of radiation is common for very heavy nuclei and why?
Alpha radiation, because the nuclei are trying to quickly reduce their mass.
What type of radiation is common for nuclei with too many protons?
Beta-plus.
What type of radiation is common for nuclei with too many neutrons?
Beta-minus.
What is “greatest number of ion pairs” overly complicated science talk for?
“most ionising”.
Properties of radioactive decay
What does it mean for radioactive decay to be random?
You can’t predict when a specific nucleus will decay.
What does it mean for radioactive decay to be spontaneous?
The presence of other nuclei does not affect the probability of a nucleus decaying (i.e. it’s independent).
In what way is nuclear decay like a dice roll?
- Random; you can assign a probability to a dice roll but not say a specific outcome
- Independent; rolling 1000 dice doesn’t change the odds of what 1 die will be
How can you affect the rate or probability of nuclear decay of a particular isotope?
You can’t.
Half-life and activity
What is the half-life of a radioactive isotope?
The average time it takes for half the active nuclei in the same isotope to decay.
What unit is radioactive activity measured in?
How could you write activity in terms of change in nuclei $\Delta N$ and change in time $\Delta t$?
What proportionality equation links the change in nuclei $\Delta N$ and the change in time $\Delta t$?
The decay equations
What is the formula for activity $A$ after a certain amount of time $t$ has elapsed in nuclear decay?
What is the formula for the nuclei decaying $N$ after a certain amount of time $t$ has elapsed in nuclear decay?
How could you find the value of $\lambda$ given the relationship $A = A _ 0 e^{-\lambda t}$ and a set of data points?
Take the log of both sides and plot $\log A$ against $t$.
What equation links decay constant $\lambda$ and half-life $t _ {1/2}$?
Interpreting the decay constant
How can you interpret the decay constant $\lambda$?
The probability of a nucleus decaying in one second.
Iterative modelling
What is using “iterative modelling” in radioactivity?
Working out the number of undecayed nuclei by Euler’s method.
What is the step for using iterative modelling to find $\Delta N$ given a step size $\Delta t$ and current nuclei $N$?
Carbon dating
Why does Carbon-14 end up in every living organism?
Carbon is produced in the atmosphere, plants eat up the atmosphere and then animals eat plants.
What is the oldest you can date something using carbon dating?
Around 50,000 years.
What assumptions is made when you use carbon dating?
The ratio of carbon-14 to carbon-12 has stayed constant over time
Why can’t you use carbon dating to date rocks on earth?
Because the rocks are old enough that all the carbon-14 has disappeared.
What collides with carbon-12 in the atmosphere to turn it into carbon-14, the element used for radioactive dating?
High-speed protons in cosmic rays.
What is sometimes problematic about carbon dating (and isn’t the fact that you’re assuming the ratio stays the same)?
The count rate is very small compared to the background rate.