Radiation

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Dangers of radiation

Ionising radiation includes X-rays, protons, neutrons, alpha, beta and gamma radiation.

Such radiation damages molecules such as DNA directly, or by producing atoms with free radical electrons (known as free radical ions) which react with the DNA and other vital molecules.

Damaged DNA can lead to cells splitting uncontrollably or mutating cells such that they are a danger to the body.

Damaged gametes can lead to damaged offspring.

There is no evidence of a threshold of safe ionising radiation

In order to monitor the amount of radiation in an area, workers wear a badge like the one shown below

In the badge there is a photographic film which will blacken as radiation hits and thus develops it.

Parts 2 and 3 show different thicknesses of plastic, and hence how developed the film beneath is, is an indicator of the strength of radiation.

Parts 4, 5, 6 are samples of tin, lead and cadmium respectively. These indicate the type of radiation due to different types being absorbed by different substances.

This allows the wearer to determine the strength and type of radiation hitting them and so lets them judge whether it is safe or not.

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Dosage and background radiation

Radiation dosage can be measured in sieverts (Sv)

One sievert is the equivalent of 250kV of X-ray radiation that would have the same effect.

Maximum exposure limits are based on this and the recommended limit is 15 mSv per year, however the workplace limit is 2 mSv per year.

This is based off of the existing data from Nagasaki which estimated 3 deaths per 100 000 survivors for each millisievert.

Background radiation can come from many sources, as the pie chart below shows.

Radioactive materials are stored in containers lined with a sufficient thickness of lead as to bring the radiation dose down to the background level.

When using radioactive materials,

Sources should be transferred using handling tools (tongs or glove box or robots).

Fluid sources should be in sealed containers.

Handlers should not ingest the sources.

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Radioactive decay

An isotope emitting alpha or beta radiation changes its nucleon distribution (an alpha reduces it by two protons and two neutrons, beta reduces it by a neutron and gains a proton).

Over time, the unstable nucleus' decay follows an exponential curve (in much the same capacitor discharge does).

The time taken for the mass of an unstable isotope to decrease to half, is called the Half-Life.

The number of nuclei of an isotope that disintegrate per second is the Activity, A of said isotope. This is measured in becquerels (Bq).

The activity of an isotope is proportional to the its mass.

Energy transfer per second from a source (power) = AE (number of particles per second x energy on each particle) (power = energy/time)

Theory of radioactive decay

Rate of decay of nuclei with time is dN/dt

since dN is proportional to N and dt,

dN = -λNdt

Where λ is the decay constant.

- means dN is a decrease.

So dN/dt = -λN

Activity, A = dN/dt so A = λN

- no longer present due to A being a scalar quantity.

rearrange for N (number at given time),

N =N0e^-(λ)t

Hence since N, mass, count rate and A are all affected by the exponential,

m=m0e^-λt A=A0e^-λt C=C0e^-λt

The decay constant λ, is the probability of an individual nucleus decaying per second.

Half-life = ln(2)/λ

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Radioactive isotopes in use

Carbon dating:

Carbon-14 is produced in the atmosphere from cosmic rays causing neutrons to decay in carbon atoms which then collide with nitrogen nuclei to form a carbon-14.

These carbon-14 are taken in by plants as a result of photosynthesis.

Once a given plant has died, a small percentage of the carbon isotopes are carbon-14 and so are partially radioactive. This number is proportional to the length of time the plant has been dead for due to no photosynthesis resulting in fresh carbon-14 being taken in when a plant is dead.

This allows scientists to check the age of dead plants by measuring the radioactivity and compare it to the activity of a similar living plant.

Argon dating:

Potassium gets trapped by rocks which then decays through electron capture into argon gas, releasing a proton.

40K19 + 0e-1 > 40Ar18 + νe

The potassium can also decay into calcium (this is eight times more likely than the previous decay)

40K19 > 0β-1 + 40Ca20 + ̅νe

The age of a given rock therefore can be calculated by measuring the proportion of argon to potassium.

For every N potassium atoms are present, if there is 1 argon, then there must have been N+9 potassium atoms since the proportion of calcium to argon is 1:8

Original potassium = Number present + 9x argon.

Then substitute the potassium into the equation from before where N = number now and N0 = original number.

Radioactive tracers:

Radioactive tracers are used to follow the path of a substance through a system

They should have a stable enough half-life for necessary measurements to be made and short enough for quick decay after use.

Beta or Gamma emission is required so it can be detected from outside the system.

Engine wear:

Rate of wear in an engine can be determined by fitting radioactive rings around a piston such that the greater the rate of wear, the greater the reactivity of the engine oil due to radioactive atoms transferring to the engine oil as a result of wear.

Such rings are made by exposing them to neutron radiation in a nuclear reactor. Each nucleus that absorbs a neutron, becomes unstable and decays through beta emission.

Thickness monitoring:

Metal foil being produced needs to be of a certain thickness, this thickness can be measured by passing radiation through the foil as it is being rolled out.

If the detector reading drops, then the foil is too thick and so the gap between the rollers is deceased and vice-versa.

A beta emitter with a long half life is used since it can be detected through a reasonable range and will not need replacing often. Alpha radiation would not pass through and gamma would not be hindered significantly enough.

power sources for remote devices

A radiation source can be stored in a thermally-insulated container which absorbs all radiation emitted. A thermocouple is attached to the container which produces electricity from the heat generated by the container absorbing radiation.

The isotope needs to be long lasting enough so it doesn't need frequent replacement and yet with a short enough half life that the energy output is substantial enough.

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Decay modes and the N/Z graph