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
What is gamma radiation absorbed completely by?
Several centimetres of lead
What does a Geiger tube consist of?
It is a sealed metal tube that contains argon gas at low pressure, there is a thin mica window at the end of the tube which allows alpha and beta particles to enter the tube (gamma photons can enter the tube through the tube wall as well), a metal rod down the middle of the tube is at a positive potential, the tube wall is connected to the negative terminal of the power supply and is earthed
What happens when a particle of ionising radiation enters the Geiger tube?
The particle ionises the gas atoms along its track, the negative ions are attracted to the rod and the positive ions to the wall, the ions accelerate and collide with other gas atoms producing more ions and these ions produce further ions in the same way so that within a very short time, many ions are created and discharged at the electrons, a pulse of charge passes round the circuit through resistor R causing a voltage pulse across R which is recorded as a single count by the pulse counter
What is the dead time of a Geiger tube and what would happen to a particle that enters the tube at this time?
This is the time taken to regain its non-conducting state after an ionising particle enters it and another particle that enters the tube in this time will not cause a voltage pulse
How can you investigate the range of each type of radiation in air?
Through using the setup of the absorption tests but without the absorbers, the count rate is measured for different distances between the source and the tube starting with the source close to the tube, the background count rate must also be measured in the absence of the source so the corrected count rate can be calculated for each distance
What happens to the count rate when the Geiger tube is beyond the range of alpha particles?
The count rate decreases sharply
What is the range of alpha radiation in air?
A few cm
What is the range of beta radiation in air?
Up to about a metre
What happens to the count rate with beta radiation when the distance increases and why?
The count rate gradually decreases with increasing distance until it is the same as the background count rate at about a distance of 1m, the reason for the gradual decrease of count rate is that beta particles from any given source have a range of initial kinetic energies up to a maximum
Do faster beta particles travel further in air than slower ones and what does this mean?
Yes which means they have a greater initial kinetic energy
What range does gamma radiation have in air?
It has an unlimited range in air
What happens to the count rate with gamma radiation when the distance increases and why?
The count rate gradually decreases with increasing distance because the radiation spreads out in all directions
What can be said about the proportion of gamma photons from the source entering the Geiger tube when the distance increases?
It decreases according to the inverse square law
What does an alpha particle consist of?
It is positively charged and it is made of two protons and two neutrons
What experiment did Rutherford carry out that proved neutralised alpha particles were the same as helium?
Alpha particles were collected as a gas in a glass tube fitted with two electrodes - when a voltage was applied to the electrodes, the gas conducted electricity and emitted light, using a spectrometer Rutherford proved that the spectrum of light from the tube was the same as a tube filled with helium gas
What does beta radiation consist of?
Fast-moving electrons
How was it proved that beta particles were fast-moving electrons?
By measuring the defection of a beam of beta particles using electric and magnetic fields, the measurements were used to work out the specific charge of the particles and this was the same as the specific charge of an electron
What is the formula for specific charge?
charge/mass
Why is an electron created and emitted from a nucleus?
It is emitted from a nucleus with too many neutrons as a result of a neutron suddenly changing into a proton
What happens when a nucleus has too many protons?
It is unstable and emits a positron, the antiparticle of an electron, when a proton changes into a neutron
Where are unstable nuclei found?
They are not present in naturally occurring radioactive substances - they are created when high energy protons collide with nuclei
What does gamma radiation consist of?
Photons with a wavelength of the order of 10^-11m
How were gamma photons discovered?
By using a crystal to diffract a beam of gamma radiation in a similar way to the diffraction of light by a diffraction grating
From where are gamma photons emitted?
From excited nuclei
What is 1MeV equal to?
1.6 x 10^-13J
What is intensity equal to?
The radiation energy per second passing normally through unit area
What is the radiation energy per second from a source that emits n gamma photons per second?
nhf
What is an expression for the intensity and what relationship can be deduced from this?
intensity = nhf/4πr^2 therefore intensity is proportional to 1/r^2
At a distance r from the source, what area do all the photons that are emitted pass through?
4πr^2 as this is the surface area of a sphere
What do radiation workers use to keep as far away as possible from gamma radiation sources to reduce their exposure?
Remote handling devices and lead screens
How can you verify the inverse square law for a gamma source?
Use a Geiger counter to measure the count rate C at different measured distances, d, from the tube and the background count rate, C0, without the source present
What is the corrected count rate proportional to?
Intensity
What is the formula for the corrected count rate?
C - C0 = k/(d + d0)^2 where d0 is the distance from the source to the edge of container it is kept in
Describe the graph that is drawn from the corrected count rate formula
A graph of d (on the y axis) against 1/(C - C0)^1/2 should give a straight line with a negative intercept -d0
Is the energy of a gamma photon constant from a given source?
Yes
What is the relative mass also known as?
The mass in atomic units
What is 1 atomic mass unit equal to?
1/12 x mass of a (12)(6)Carbon atom
What happens in beta - emission?
A neutron in a neutron-rich nucleus changes into a proton and an electron antineutrino is emitted with a beta - particle
What happens in beta + emission?
A proton in a proton rich nucleus changes into a neutron and an electron neutrino is emitted with a beta + particle
What happens in electron capture?
Some proton-rich nuclides can capture an inner-shell electron, this causes a proton in the nucleus to change into a neutron with the emission of an electron neutrino ve at the same time, the inner-shell vacancy is filled by an outer-shell electron and as a result, an X-ray photon is emitted by the atom
Does the emission of gamma photons change the number of protons or neutrons of a nucleus?
No
When is a gamma photon emitted?
It is emitted by a nucleus that has excess energy after it has emitted an alpha or beta - particle
Why is ionising radiation hazardous?
It damages living cells
What does ionising radiation include?
X-rays, protons and neutrons as well as alpha, beta and gamma radiation
Why does ionising radiation affect living cells?
It can destroy cell membranes which causes cells to die and it can damage vital molecules such as DNA directly or indirectly by creating ‘free radical’ ions which react with vital molecules, normal cell division is affected and nuclei become damaged, damaged DNA may cause cells to divide and grow uncontrollably causing a tumour which may be cancerous, damaged DNA in a sex cell may cause a mutation which may be passed on to future generations
What are somatic effects?
Effects affecting the health of the affected person
What are genetic effects?
Effects affecting the health of future generations
At what kind of doses does cell mutation and cancerous growth occur at?
Low doses as well as high doses
What must a person using equipment that produces ionising radiation wear?
A film badge to monitor his or her exposure to ionising radiation
What is a film badge composed of?
It contains a strip of photographic film in a light-proof wrapper, different areas of the film are covered by absorbers of different materials and different thicknesses
What occurs when the film of a film badge is developed?
The amount of exposure to each form of ionising radiation can be estimated from the blackening of the film
What happens if a film badge is overexposed?
The wearer is not allowed to continue working with the equipment
What does the biological effect of ionising radiation depend on?
The dose received and the type of radiation
How is the dose of radiation measured?
In terms of energy absorbed per unit mass of matter from the radiation
Why is alpha radiation more damaging than gamma radiation?
It produces far more ions per millimetre than gamma radiation in the same substance so it is far more damaging
Why is alpha radiation from a source outside the body less damaging than from a source within the body/
Alpha radiation from a source outside the body cannot penetrate the skin’s outer layer of dead cells
What is the radiation dose measured in?
In sieverts (Sv)
What is background radiation due to?
Cosmic radiation and from radioactive materials in rocks, soil and in the air
Why does background radiation vary with location?
Due to local geological features
In what should radioactive material be stored?
In lead-lined containers and the lead lining must be thick enough to reduce the gamma radiation from the source in the container to background radiation
Where can radon gas accumulate?
In poorly ventilated areas of buildings in certain locations
What do regulations require about the stored radioactive material?
The containers are kept under ‘lock and key’ and a record of the sources are kept
What should no source be allowed to come into contact with?
Skin
How should solid sources be transferred and why?
Using handling tools such as tongs to ensure the material is further away from the user and thus the intensity of the radiation is reduced and beyond the range of other types of radiation
What should happen to liquid and gas sources and solids in powder form?
They should be sealed in containers to ensure the radioactive gas cannot be breathed in and radioactive liquid cannot be splashed on the skin or drunk
What happens the longer a person is exposed to ionising radiation?
The greater the dose of radiation the person receives
What is the relationship between the intensity of a gamma beam and an absorber that is passes through?
The intensity of the gamma beam decreases exponentially with the thickness of the absorber, if a certain thickness of a material cuts the intensity of a gamma beam to half, twice the thickness will cut the intensity to a quarter
What does ALARA stand for?
As low as reasonably achievable
How are risks from radioactive sources reduced?
By increasing the distance from sources and shortening the time exposure
