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It is always a surprise to a student when first turning a counter on, to see the build up of counts for no apparent reason. It is therefore a shock when we realise that ionising radiation has always existed and will always exist and cannot be avoided. Its existence is thought to provide a mechanism for natural evolution.
 

The source of the radiation is quite varied but includes;

• "cosmic rays" - a mixture of particles that have their origin as emissions largely from outerspace - literally outside our Solar System. Usually we receive, at ground level, "secondary" rays - debris formed when the "primary" rays strike the atoms of the upper atmosphere.
• Emissions from the rocks either directly or through gaseous radon emissions from the rocks.
• All material is to some slight extent radioactive (including ourselves through C-14.  Note however, that fluorescent lights are no more active than ourselves !)

We also receive dosages from artificial sources most notably;

• X-rays from medical sources or from older TV or computer screens
• medical treatment using radiation sources
• Nuclear Bomb testing!

DOSAGES
Every emission stopping in a medium such as a human body leaves its energy as, finally, heat. This accounts for the radiation burns which are similar to ordinary heat burns but may be deeper and, more importantly, create much more serious chemical changes at the molecular level.
Each emission deposits about 1MeV of energy, give or take.  In ordinary terms of energy this seems tiny, about 2 x 10^-13 joules.  A joule is the energy that a litre of milk dropped through 10cm has on hitting the floor.  For this reason, individual emissions do not do much damage.
Suppose, however that you are exposed to a high activity source at close range, something of the order of 10¹² Bq.  In a short time your body will receive energy of the order of a joule. This would be very serious and probably fatal, in the long term, if not in the short, as major damage will have been done to the dividing cells. Cancer would be likely.

The unit to describe absorbed energy is the gray, Gy.

Because radiations penetrate or pass through materials to some extent, we only include the absorbed or stopped part of the radiation.

Example, A 300gram tumor is given 2J of radiation absorbed.

Absorbed Dose is then 2 / 0.3 Gy = 6.7 Gy

We can protect ourselves from high dosages by distance or shielding, eg concrete or lead.

Unfortunately, the gray is an absolute measure and doesn't differentiate between the damage caused by 1Gy of α or 1Gy of γ. They certainly have a different effect because of their different ionising power.

A new term is defined to take this into account - the sievert, Sv. This term is used when talking about radiation dosage in medicine or in dosages from a nuclear  bomb or nuclear accident.
( You will still hear of two older terms, rem = 10^-2 Sv and rad = 10^-2 Gy .)

Quality Factor is a number given to the type of dose that, loosely expressed, gives its relative danger.

= 1     for  X,  γ, β radiations
= 10   for neutrons
= 20   for α radiation,  if ingested.

Thus α radiation , if in the body, is roughly 20 times more dangerous than the same energy deposited in γ-rays. An α radiation emitter, Po-210, was used to murder a Russian, Alexander Litvinenko, in London in 2006.

In practice, the absorbed radiation is measured fairly crudely for a medical worker; a photographic film badge with filters (shielding) measures the different types of radiation. The film is developed regularly and checked for fogging.  This is considered good enough!
 

The above tumour, if the radiation is β, receives 6.7 x 1 Sv = 6.7 Sv. Had the radiation been α radiation, the dosage would have been 6.7 x 20 = 134 Sv.
 
 
 

The effects of radiation dosages depends on the amount of dose and the exposed region of the body. The radiation creates chemically very powerful ions in the body which alter the chemicals of the body. This in turn affects the ability of the cell to divide, the basis of cellular life. The cell may cease dividing or it may go berserk, cancerous.

Generally,

After a single dose of as little as 10 mSv, the chance of a later cancer (5 - 30 years) is significantly higher.

It is not possible to predict from individual to individual what will happen when subjected to a given dosage.  One can only give a probability of likely outcome.  The information has been gained from studies of the victims of Hiroshima and Nagasaki as well as studies on animals and now studies on the people of the Ukraine affected by Chernobyl.

The information on small doses, mSv, is not well understood though it is clear that humans and all animals have some tolerance to these radiation and can fully recover.

Where the limits stand is very much a matter of debate.  The recommended maximum dosage of the members of the public over and above natural radiation is 1 mSv per year.  For radiation workers, 50 mSv per year.

These are additional to the natural dosage of about 2.3 mSv per year.
 
 

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