Chapter 25 - Radioactivity Flashcards
Define ionising power
The ability of radiation to be able to cause damage to other particles, and become ions.
Alpha particles definition/ description
Positively charged particles.
A Helium atom- 2 particles and 2 protons.
Beta particles definition/ description
FAST moving electrons (B-)
Deflected by E & B fields.
Gamma particles definition/ description
High energy photons with no charge and they travel at the speed of light.
Absorption of alpha particles
They have a large mass and charge so they can interact with many things and hence can ionise easily. So they have a short range.
How can alpha particles be stopped?
A few cm of air or a thin sheet of paper.
They are deflected by E & B fields.
Absorption of beta particles
They have a smaller mass and charge, so a larger range in air. Fast travelling (due to smaller mass)
How can beta particles be stopped?
Stopped by 1-3mm of aluminium to stop.
Deflected by E & B Fields.
Absorption of gamma particles
No charge, and so even less ionising.
They become more ionising the further they travel.
How can gamma particles be stopped?
With thick lead.
Not deflected by E & B Fields.
What is the nucleus before the decay called?
Parent nucleus.
What is an alpha particle?
4 2 He.
What is a beta minus
0 -1 e.
What is a beta plus
0 +1 e.
What is a gamma?
Not a particle.
What causes beta decay
The weak nuclear force.
They generally have too many neutrons for stability (beta- minus).
So a neutron decays into a particle, an electron and n anti electron neutrino.
When does gamma decay happen
Emitted when a nucleus has surplus energy after an alpha/ beta emission.
Gamma decay equation
It stays the same, but it just releases the gamma wave.
What does “natural radioactivity” mean
Spontaneous and random.
Define the half life
Half life of an isotope is the average time it takes for half the number of an active nuclei in the sample to decay.
Define activity
The rate at which the nuclei decay.
Activity equation
A = λ N
A- activity
N- Number of undecayed nuclei N left in the source.
λ- Decay constant.
Define the decay constant
The probability of decay of an individual nucleus per unit time.
Radioactive decay calculations
for the number of undecayed nuclei
N =N0 e ^(-λt)
N = number of undecayed nuclei.
Radioactive decay calculations for the activity
A = A0 e ^(-λt)
Decay constant and half life equation
λ t1/2 = ln(2)
What is the half life of C-14
5730 years.
The ratio of C-14/ C-12 is atmospheric carbon= and it is almost constant at 1.3 x 10^-12.
Limitations of Carbon-dating
- You are assuming that the ration of C-14/ C-12 has remained constant over the years.
But, it is often affected by volcanic eruptions, burning fossil fuels. - and affected by solar flares and nuclear bombs.
Note about conventional current
In magnetic fields, the conventional current flow ISNT the flow of electrons. The electrons travel in the opposite way to the conventional current.
Dangers of radiation
- Ionising radiation is dangerous because it damages living cells.
- Cell DNA is damaged either directly or by creating free radicals that react with DNA.
- Damaged DNA may cause cells to divide and grow uncontrollably, causing a tumour, which can be cancerous.
- High doses of radiation can kill living cells.
Precautions of radiation
- No source should come into contact with skin.
- Solid sources must be transferred using tongs, robots or glove boxes.
- Liquid, gas and solids in powder form must be in a sealed container so they aren’t inhaled.
- Only use when necessary.
What are the 2 methods of detection ?
Gas chamber and a geiger tube
Gas Chamber
It contains air saturated with water at a low temperature. The alpha and beta particles will ionise the air and the water molecules will be attracted to the ions, creating a track of minute, condensed water tracks.
What do alpha particle tracks in gas chambers look like?
Straight, radiating from the source.
The same isotope gives the same length.
The same range or ionising power.
What do beta particles look like in gas chambers?
Wispy tracks and so B-particles are easily deflected by air molecules.
Less easy to easy because they are less ionising.
How does a geiger-muller tube work?
- It contains of a sealed tube with argon gas at a low pressure.
- The thin mica window allows the alpha and beta particles inside. The radiation ionises the gas particles in its track. The negative ions are attracted to the rod, and the positive ions are attracted to the walls.
- The ions collide with each other more often.
- As the particles touch the rod or the walls, they discharge, creating a pulse of charge to travel around the circuit and a voltage pulse across R, which is recorded as a click.
Absorption- the Geiger tube
Before the count rate is examined, you must calculate the background radiation first. Then, subtract the background radiation from the count rate examined.
