3.6 - Nuclear Decay
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54 Terms
What are unstable elements?
elements that if left long enough will eventually decay to form a more stable element
what is nuclear decay?
the process of elements decaying and becoming more stable
why is the process called nuclear decay?
the nucleus of the decaying atom changes
nuclear decay has a ... nature
probabilistic
what is does probabilistic nature mean?
it is impossible to predict exactly when a single nucleus will decay
why is it impossible to predict exactly when a single nucleus will decay?
radioactive substances tend to contain large numbers of individual atoms
what do we predict instead about nuclear decay?
the proportion of the atoms which decay after a certain time
what are the three ways a nucleus can decay?
alpha, beta and gamma
what does the unstable nucleus emit during alpha decay?
an alpha particle
what is an alpha particle identical to?
a helium nucleus containing two protons and two neutrons
what happens to the nucleus after alpha decay?
it becomes lighter and more stable
alpha decay generally happens to ... why?
larger elements, they are unstable due to proton-proton repulsion within the nucleus since these elements have more protons
what happens to the nucleus during beta decay?
a neutron decays into a proton, ejecting an electron (beta particle) in the process
when will an elements undergo beta decay?
when it has too many neutrons and needs to become more stable
in beta decay, the ... is unchanged but the ... increases by one
mass number, atomic number
what is conserved in order to preserve the lepton number?
an anti-electron neutrino
why are beta particles emitted at a very high speed?
they have a small mass but a relatively large amount of kinetic energy during the decay
what happens during gamma decay?
a single gamma photon is emitted, with no change to the composition of the nucleus
how can protons and neutrons exist in the nucleus?
in their own excited states
when is a gamma photon emitted?
when the nucleons transition to a lower energy level
what is alpha's range in air?
2-10 cm
what is beta's range in air?
around 1m
what is gamma's range in air?
infinite range - follows inverse square law
how ionising is alpha?
highly
how ionising is beta?
weakly
how ionising is gamma?
very weakly
is alpha deflected by electric and magnetic fields?
yes
is beta deflected by electric and magnetic fields?
yes
is gamma deflected by electric and magnetic fields?
no
what is alpha absorbed by?
paper
what is beta absorbed by?
aluminium foil (around 3mm)
what is gamma absorbed by?
several metres of concrete or several inches of lead
why do alpha particles move slowly in comparison to the other forms of radiation?
they contain 4 nucleons which compared to particles makes them very heavy particles
why do alpha particles lose their energy very quickly?
slower particles are more likely to collide with particles in air
why do gamma particles move at the speed of light and effectively never stopped by air?
it is essentially electromagnetic radiation
what does the ionisation of a particle measure?
its ability to remove electrons from other atoms
why are alpha particles highly ionising?
they carry a strong positive charge, giving them the ability to 'rip' electrons away from atoms they come into contact with
what happens when alpha ionises particles? what does this result in?
energy is transferred from the alpha particle to electron in order to ionise it, this results in them loosing their energy and being stopped rapidly
why do both alpha and beta interact with electric and magnetic fields?
they are charged particles
how do you analyse the penetrating power of each type of radiation?
use a geiger-muller tube and counter, find the background count when the source is not present
place the source of radiation close to the GM tube and measure the count rate
place a sheet of paper between the source and GM tube and measure count rate again, if the count rate decreases significantly, the source is emitting alpha radiation
repeat the above step using aluminium foil and several inches of lead. if there is a significant decrease in count rate for aluminium foil, then beta radiation is emitted, if there is a significant decrease in count rate for the lead block, then gamma is being emitted
why does the amount of radiation being emitted over time decrease?
the number of decays per second is proportional to the number of undecayed atoms left in the sample
what is the activity of a substance?
the number of decays detected each second
what does N represent?
the number of undecayed atoms in the sample
what is activity measured in?
Bq
what is 1Bq equal to?
the activity of a sample in which one atom decays every second
what can activity also be described as?
the rate of N decreasing
the rate that N decreases is proportional to ..., what is this an example of?
N, exponential decay
what does it mean if an atom has a large decay constant?
the atoms are more unstable and decay much faster
Describe this exponential decay graph
both start with the same number of undecayed atoms
initially both decay rapidly, then slow down as time increases until N tends to 0
what is the half-life of a radioactive nucleus?
the average amount of time it takes for half of a radioactive sample to decay