Radioactivity and Nuclear Energy Flashcards
Rutherford Alpha Scattering Experiment
Professor Rutherford wanted to see what happened to alpha particles when they
collided with atoms
A few alpha particles were scattered by angles less than 90º, also as expected
As expected, most of the alpha particles
went straight through
Under protest, Geiger and Marsden placed the microscope behind the gold leaf.
Much to their surprise, a very small number of alpha particles (about 1 in 8000) bounced off the gold atoms!
Rutherford’s experiment proved that most of the mass of an atom was concentrated in an incredibly tiny central nucleus – it was the beginning of the nuclear age
Isotope
Isotopes have the same number of protons but a different number of neutrons
Alpha Particles
2 Protons and 2 Neutrons (a helium nucleus.)
Big, heavy and slow moving.
It doesn’t penetrate very far, (paper stops it).
Strongly ionizing.
Alpha - Leaves the nucleus with 2 less protons and two less neutrons
Beta Particles
An electron.
very fast (but not as fast as light).
Moderately penetrating, (thin aluminium stops it).
Moderately ionizing.
Beta –When an electron is emitted, the decay changes a neutron into a proton
Positron (Beta+) – When a positron is emitted the decay changes a proton into a neutron
Gamma Wave
An electromagnetic wave.
No mass, no charge.
Pass through everything, (thick lead stops it).
Weakly ionizing.
Gamma –No physical effect on the nucleus. A photon is given off by an overexcited nucleus, so the nucleus just loses some energy
Deflection by an electric field
Alpha particles are positively charged so are attracted to the negative terminal.
Beta radiation is an electron which is negatively charged so is attracted to the positive terminal. Beta particles have less mass than alpha, so are deflected more
Gamma radiation has no charge so it is not deflected in an electric field
Ionisation
Ionisation is any process that removes an (negatively charged) electron from a neutral atom leaving the atom with a net positive charge
The Geiger Muller counter
The GM tube is a hollow cylinder filled with a gas at low pressure.
The tube has a thin window made of mica at one end.
There is a central electrode inside the GM tube.
A high voltage supply is connected across the
casing of the tube and the central electrode
When ionising radiation enters the tube it produces ions in the gas.
The ions created in the gas enable the tube to conduct.
A current is produced in the tube for a short time.
The current produces a voltage pulse. Each voltage pulse corresponds to one ionising radiation entering the GM tube. The voltage pulse is amplified and counted.
Nuclear Activity
The activity of a radioactive source is measured in Becquerel (Bq), which has units of s^-1
1 Bq = 1 nucleus decay per second
𝐴𝑐𝑡𝑖𝑣𝑖𝑡𝑦 = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓𝑑𝑖𝑠𝑖𝑛𝑡𝑒𝑔𝑟𝑎𝑡𝑖𝑜𝑛𝑠 / 𝑡𝑖𝑚𝑒 𝑡𝑎𝑘𝑒𝑛 = 𝑁 / t
Strong Force
The Strong force binds fundamental particles together into nuclei. It works on the scale of atomic nuclei, so we never have a macroscopic experience of it
The Strong force is the strongest fundamental force by a lot. Therefore the energies needed to study it are huge, and we are still designing ways to investigate it.
Decay Chain
A series of radioactive decays of different radioactive decay products as a sequential series of transformations. Most radioisotopes do not decay directly to a stable state, but rather undergo a series of decays until eventually a stable isotope is reached
N vs Z plot
Nuclear stability depends on the ‘balance’ between the strong & electrostatic force.
As Z increased beyond 20, stable nuclei have more neutrons than protons. The neutrons help bind the nuclei together without increasing the repulsive electrostatic force
For light isotopes (0 < Z < 20) stable nuclei
follow N=Z i.e. equal number of protons &
neutrons
Alpha emitters occur when Z > 60 (approx) with
nuclei above the line of stability. The nuclei
have more neutrons than protons but the
nuclei are too large so the electrostatic
repulsion is greater than the strong force.
Beta plus emitters are proton rich (neutron poor)
so convert a proton into a neutron (emitting
Beta + and an electron neutrino)
Radioactive decay chains
A series of radioactive decays of different radioactive decay products as a sequential series of transformations. Most radioisotopes do not decay directly to a stable state, but rather undergo a series of decays until eventually a stable isotope is reached.
Electron Capture
If a nucleus has too many protons sometimes a
proton turns into a neutron. This is due to an electron (close to the nucleus) exchanging a W+ boson particle with the proton. This causes the proton to turn into a neutron and the electron into a neutrino.
Positron emission versus electron
capture
The emission of a positron and the capture of an electron are twin reactions which both result in the reduction of the number of protons by 1 (from Z to Z1) and the production of a neutrino.
The positron observed in the final stage of the beta decay is a new particle requiring the 0.511 MeV of its rest mass energy to be created.
Electron capture does not have an energy threshold..
In both cases, practically all the energy released is carried by the light particles.
Ionisation damage
destroy cell membranes causing cells to die
damage vital molecules such as DNA. Cell division can be affected causing cancer (uncontrolled growth & cell division). Damaged DNA in a sex cell (sperm or egg) can cause a mutation that’s passed on to future generations
How to reduce exposure to ionisation
Reduce exposure time
Make use of shielding (eg. Lead/concrete)
Increase the distance between you and the source
Intensity and Range
Intensity is the amount of energy per second passing normally through unit area.
We know E = hf for a photon. Therefore, for a gamma radiation source that emits ‘n’ photons per second the energy emitted per second = nhf.
From a uniformly radiating point source will spread out over the surface of a sphere. So at a distance r from the point source they will be spread out over 4pir^22
Therefore, the intensity, I, at a distance r from a point source will be = nhf / 4pir^2
So, I is directly proportional to k/r2 where k = nhf / 4pi
Inverse Square law
𝑰 = 𝒌 / 𝒙^𝟐
𝒘𝒉𝒆𝒓𝒆,
𝑰 =Intensity (Counts per second)
𝒌 =constant
𝒙 = 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑓𝑟𝑜𝑚 𝑠𝑜𝑢𝑟𝑐𝑒 (m)
Background radiation
Background radiation is a measure of the
level of ionizing radiation present in the
environment at a particular location which
is not due to deliberate introduction
of radiation sources.
Background radiation originates from a
variety of sources, both natural and
artificial.
Modelling Decay
The average number of decays per second is
called the activity and can be written as A or
𝐴 = Δ𝑁/Δ𝑡
The total number of nuclei left after a certain time is represented by N
The decay constant
dN /dt = -lambda N
lambda is the decay constant.
It is the probability that any particular nuclei will decay in the next second. If you then multiply that by the number of particles, you
will get the number of particles that are likely to decay in the next second (aka, the activity)
The Negative sign is included because N is a decreasing quantity and therefore dN/dt is negative, while N itself is positive.
Half life
The rate at which a radioactive isotope decays is measured in half-life.
The term half-life is defined as the time it takes for one-half of the atoms of a radioactive material to disintegrate
Carbon Dating
Some of the carbon found in nature consists of the radioactive isotope carbon-14. Carbon-14 nuclei are continuously being produced by the atmosphere.
