Unit 5: Radioactivity Flashcards
What was Thomson’s model of the atom?
He thought it would be like a ‘currant bun’ with electrons dotted in the atom, the positive charge was supposedly spread throughout the atom
What do unstable atoms emit?
Radiation such as alpha, beta and gamma
What does alpha radiation consist of?
Fast-moving positively charged particles
What were the results that Rutherford was expecting from his experiment due to Thomson’s model?
The alpha particles directed at the thin metal foil might be scattered slightly by the atoms of the foil if the positive charge was spread out
What type of beam of alpha particles was used and where in Rutherford’s experiment?
A narrow beam of alpha particles was used (that all had the same kinetic energy) in an evacuated container to probe the atom, it was directed at thin metal foil
What were the alpha particles detected by in Rutherford’s experiment?
A detector which could be moved round at a constant distance from the point of impact of the beam on the metal foil
Why was a microscope used in Rutherford’s experiment?
To observe the pinpoints of light emitted by the alpha particles hitting a fluorescent screen
What did Rutherford measure in his experiment?
The number of alpha particles reaching the detector per minute for different angles of deflection - from 0 to approximately 180 degrees
What did Rutherford observe in his experiment?
Most alpha particles passed straight through with little/no defection (1 in 2000 deflected) and a small percentage were deflected through angles of more than 90° (1 in 10000)
Why must the alpha particles used in Rutherford’s experiment have the same speed?
Slow particles would be deflected more
Why must the container in Rutherford’s experiment be evacuated?
Alpha particles would be stopped by air molecules
Why must the source of alpha particles in Rutherford’s experiment have a long half-life?
Later readings would be lower than earlier readings due to radioactive decay of the source nuclei
What were the conclusions drawn from Rutherford’s experiment?
Most of the atom’s mass is concentrated in a small region i.e. the nucleus which is at the centre of the atom and the nucleus is positively charged as it repels alpha particles
What happens if an alpha particle collides head on with a nucleus in Rutherford’s experiment?
The angle of deflection is 180°
What are the consequences of the initial direction of an alpha particle being closer to the head-on direction in Rutherford’s experiment?
The greater the deflection as the electrostatic force of repulsion between an alpha particle and a nucleus increases with decreased separation and the smaller the least distance of approach of the alpha particles to the nucleus
What is the magnitude of the charge of a nucleus?
+Ze where e is the charge of the electron and Z is the atomic number of the element
Why must the foil used in Rutherford’s experiment be very thin?
Otherwise the alpha particles are scattered more than once
What is the probability of an alpha particle being deflected by a given atom when the foil has n layers of atoms and what does this probability depend on?
1 in 10000n and this probability depends on the effective area of cross-section of the nucleus to that of the atom
What is the formula that involves the diameter of the nucleus and the diameter of the atom?
d^2 = D^2/10000n where d = diameter of nucleus, D = diameter of atom and n = number of layers
How can you estimate the diameter of a nucleus with alpha particles?
From the least distance of approach in a head-o4 collision between an alpha particle of known kinetic energy and a nucleus
What happens at the least distance of approach between an alpha particle and a nucleus?
The alpha particle stops momentarily and the potential energy of the alpha particle in the electric field of the nucleus is equal to the initial kinetic energy of the particle
What is the formula for estimating the diameter of a nucleus with alpha particles?
E = Q1Q2/4πε0d where Q1 is the charge of the alpha particle, Q2 is the charge of the nucleus and d is the least distance of approach
What does radiation do to air?
It ionises it and makes it conduct electricity
List the types of radiation in order of how penetrating they are starting with the most penetrating
Gamma, beta then alpha
What does gamma radiation consist of?
High energy photons
How can an ionisation chamber and a picoammeter be used to investigate the effect of each type of radiation?
Ions created in the chamber are attracted to the oppositely charged electrodes where they are discharged, electrons pass through the picoammeter as a result of ionisation in the chamber, the current is proportional to the number of ions per second created in the chamber
How strong is the ionisation that alpha produces and what range does it have?
Alpha radiation causes strong ionisation and it has a range in air of no more than a few centimetres
How strong is the ionisation that beta produces and what range does it have?
Beta radiation has a much weaker ionising effect than alpha radiation, its range varies up to a metre or more in air
Does a beta particle produce fewer ions per millimetre along its path than an alpha particle does?
Yes
How strong is the ionisation that gamma produces and why?
It has a much weaker ionising effect than either alpha or beta radiation as the photons carry no charge
Which type of particles have the same range in air as each other from a given source?
Alpha particles and not beta
Why are alpha particles from a given isotope always emitted with the same kinetic energy?
Each alpha particle and the nucleus that emits it move apart with equal and opposite amounts of momentum
Why are beta particles from a given isotope not emitted with the same kinetic energy?
In beta emission, the nucleus, the beta particle and the neutrino/antineutrino share the energy released in variable proportions
What does a cloud chamber contain?
Air saturated with a vapour at a very low temperature
What happens when an alpha or beta particle passes through the cloud chamber?
Due to ionisation of the air, the particles leave a visible track of minute condensed vapour droplets as the air space is supersaturated
What happens when an ionising particle passes through the supersaturated vapour in a cloud chamber?
The ions produced trigger the formation of droplets
What can be said about the tracks that the alpha particles produce in a cloud chamber?
They produce straight tracks that radiate from the source and are easily visible - the tracks from a given isotope are all of the same length indicating that the alpha particles have the same range
What can be said about the tracks that the beta particles produce in a cloud chamber?
The beta particles produce wispy tracks that are easily deflected as a result of collisions with air molecules - the tracks are not as easy to see since beta isn’t as ionising as alpha
What equipment is required to carry out an absorption test?
A geiger tube, a counter, the source in a sealed container and the absorber
What is registered as a single count by the counter used in the absorption test?
Each particle that enters the tube
What is the equation for the count rate?
count rate = number of counts/time taken
What happens before the source is tested in the absorption test?
The count rate due to background radioactivity must be measured i.e. count rate before the source is present
How is the absorption test carried out?
The count rate is measured with the source at a fixed distance from the table without any absorber present, the background count rate is then subtracted from the count rate with the source present to give the corrected (i.e. true) count rate from the source, the count rate is then measured with the absorber in a fixed position between the source and the tube, the corrected count rates with and without the absorber can then be compared
How can the effect of absorber thickness be investigated in the absorption tests?
By using absorbers of different thickness of the same material
What is alpha radiation absorbed completely by?
Paper and thin metal foil
What is beta radiation absorbed completely by?
5mm of metal