nuclear physics - section 8 - radioactivity Flashcards
The gold foil in an alpha scattering experiment is replaced with another foil of the same size made from a mixture
of isotopes of gold. Nothing else in the experiment is changed.
Explain whether or not the scattering distribution of the monoenergetic alpha
particles remains the same. [1]
the scattering distribution remains the same
because the alpha particles
interact with a nucleus with the same atomic number (charge)
so repulsive force remains the same
describe the alpha scattering experiment [2]
beam of alpha particles fired at a thin sheet of gold foil
in a vacuum so alpha particles don’t collide with air particles
volume of a sphere
4/3 π r^3
In an experiment to investigate the structure of the atom, α particles are directed
normally at a thin metal foil which causes them to be scattered.
In which direction will the number of α particles per second be a maximum? [1]
what does this suggest about the structure of the atom? [1]
straight on
deflection of zero degrees
mostly open space
In an experiment to investigate the structure of the atom, α particles are directed
normally at a thin metal foil which causes them to be scattered.
A small number of α particles are scattered through 180°.
Explain what this suggests about the structure of the atoms in the metal. [2]
most of the mass of an atom is contained in its nucleus (1)
the nucleus contains a positive charge (1)
Why is it necessary to remove the air from the chamber in a Rutherford
scattering experiment? [1]
to prevent interaction
of the α particle with the air (1)
2 conclusions about the nucleus from the Rutherford scattering experiment [2]
nucleus has a positive charge
nucleus contains most mass of the atom
what were the expectations of the alpha scattering experiment?
plum pudding model suggested that a beam of positively charged alpha particles would be slightly scattered by the atoms of the foil if the positive charge was evenly distributed
what were rutherford’s observations in the alpha scattering experiment
some of the particles were bounced back at a much greater force than expected ( small percentage (1 in 10000) were deflected through angles greater than 90 degrees)
most of the alpha particles passed straight through the foil ( about 1 in 2000 were deflected )
most alpha particles passed straight through the foil. some bounced back
what inferences can be made from this
most of the atom’s mass is concentrated in a small region in the atom ( the nucleus ) at the centre
the nucleus is positively charged because it repels alpha particles that come close
nuclear diameter
10^-15 m
rutherford and radiation
he found that radiation ionised the air making it conduct electricity
alpha and beta
order the three types of radiation in order of ionisation power ( lowest to highest )
gamma
beta
alpha
in the alpha scattering experiment give two reasons why the gold foil should be very thin [2]
the α particles must not be absorbed by the foil (1)
the α particles must only be scattered once(1)
order the three types of radiation in order of penetration power ( lowest to highest )
alpha
beta
gamma
what are the three types of radiation made up of?
how was this deduced?
when placed in a magnetic field, both alpha and beta radiation were deflected in opposite directions
gamma wasn’t deflected
this meant alpha was composed of positively charged particles and bet of negatively charged particles
gamma was later discovered to consist of high energy photons
why is ionising radiation seen as dangerous
can kill or mutate cells
this can lead to mutations and lead to things such as cancer
which type of radiation is most dangerous inside the body and why
alpha
Alpha radiation is highly ionising and poorly penetrating
This would mean it would cause localised damage and not be able to leave the body
give an example of real life use of beta decay and explain why beta decay is used for this
used to measure the thickness of aluminium foil or paper
Beta is used because it is moderately penetrating and would be able to pass through to reach the detector on the other side
Gamma radiation is too penetrative and would pass through anything.
which type of radiation follows the inverse square law
gamma radiation
what is the inverse square law
The intensity of radiation is inversely proportional to the square of the distance from the source
what is intensity measured in
watts per square metre
W/m^2
describe an experiment to show the inverse square law
- measure the background radiation without the gamma source present in the room using a geiger muller tube
- Place the source at a set distance from the tube and measure the count rate over a set period of time
- Take an average count rate
- repeat for distances at increasing intervals and subtract the background radiation from each recording
- plot a graph of count rate per minute against 1/distance squared
- should be a straight line through origin to confirm they are directly proportional
what is background radiation
radiation that is constantly present in the surroundings from sources such as rocks and cosmic radiation
what is the decay constant
λ
the probability of a nucleus decaying per second
what are the units for the decay constant
s^-1
what is half life
Time taken for half the unstable nuclei in a substance to decay
half life formula
t½ = ln2 / λ
activity formula
A = λN
activity = decay constant * Number of unstable nuclei in sample
what is activity measured in
Bq
decays per second
unstable nuclei remaining formula
N = N0e^-λt
number of unstable nuclei remaining = original number of unstable nuclei * e^-decay constant * time
what does the unstable nuclei formula show
that radioactive isotopes decay exponentially
which isotope is commonly used in medicine and why
Technetium 99m
- releases gamma radiation
- has a short half-life and doesn’t remain highly radioactive for long
- half life of approximately 6 hours, long enough to be detected
- can be made near to the hospital
what does the NZ graph show
shows relationship between proton number and neutron number
shows a stability curve beginning at N=Z until N=20 and then curves up
where on the NZ curve does beta minus decay occur
above the stability line
where there are excess neutrons
beta minus decay occurs to turn the neutron to proton and become more stable
what type of decay occurs below the stability line and why
beta plus decay
proton rich
beta plus decay occurs to turn protons into neutrons
how does the heavier nuclei often decay
through alpha decay
alpha decay emits a helium nucleus (4,2) making the nuclei less heavy and more stable
sin 45
root2 / 2
