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Activity Becquerel
The SI unit of for the activity of a radioactive material is the becquerel Bq, where one Becquerel = 1 disintegration per second.
The traditional unit of activity has been the Curie Ci, where one Curie = 3.7 x 10
10
disintegrations per second.
Absorbed Dose Gray
This is a measurement of the energy imparted to matter by ionising radiation per unit mass of the material. The SI unit of absorbed dose is the gray Gy,
which is equal to an energy absorption of 1 jouleKg. The traditional unit of absorbed dose is the rad, where 1 Gray = 100 rads.
Dose Equivalent Sievert
Equal absorbed doses may not always give rise to equal risks of any biological effect. The relative biological effectiveness of a particular absorbed
dose may be affected by the type of radiation or the radiation conditions. Accordingly the equivalent dose can be expressed as:
Dose equivalent Sievert = Absorbed dose Gray x Modifying Factor. The modifying factor depends on both the quality of the radiation which is
1.0 for the lower energy radiations but rises to 20 for high energy fission fragments and the part of the body affected.
The traditional unit is the rem where 1 sievert = 100 rem.
17.3 External and Internal Radiation
When discussing the health aspects of exposure to ionising radiation and the control of any hazard, it is important to distinguish between external radiation
and internal radiation.
An external radiation hazard is one from radiation sources outside the body
of sufficient energy to penetrate the outer layers of the skin. A summary of the
161 effects of exposure, principles of control and types of monitoring are set out
below: The effects of external exposure can be summarised as:
Minimal hazard Skin and eyes at risk
Whole body at risk penetrating radiation
An internal radiation hazard arises when the body is contaminated with a
radioactive isotope. The presence of radioactive material in the body is often a more serious problem than exposure to external radiation because the
radioactive material: is in intimate contact with the body tissues and organs remember
inverse square law. cannot be removed or shielded irradiates the body 168 hrweek.
Entry to the body can occur through inhalation, ingestion, or skin absorption, In this situation, the effects of exposure are:
Very serious hazard Serious hazard
not normally applicable
17.4 Levels of Radiation
We are all exposed to radiation from natural sources as well as those encountered during work. The Los Alamos National Laboratory in the US
provide an online tool which enables you to calculate your annual radiation dose, see:
http:newnet.lanl.govinfodosecalc.asp accessed February 2010.
This takes into account: Cosmic radiation which increases with height above sea level
The material which your home is made from. Time spent on aircraft
162 Smoking
Medical x–rays Other lifestyle factors.
17.5 Biological Effects of Ionising Radiation
Exposure of living tissue to ionising radiation results in damage to the component cells. Such radiation damage can be useful to mankind as in the
treatment of cancer under carefully controlled conditions, but under most conditions it should be avoided as far as possible. Possible effects are
summarised in the table below.
Acute Effects Chronic Effects
Erythema Blood change
Sterility Death
Cancer Hereditary defects
All forms of ionising radiation produce the same type of injury in irradiated tissues. However, the efficiency with which the tissue reactions are produced
varies with the density of the ionisation in the path of the radiation. Particulate radiations such as alpha particles or neutrons which produce closely packed
tracks of ions are more damaging per unit of energy absorbed than is electromagnetic radiation such as gamma rays or X-rays, which cause more
diffuse ionisation. Since cosmic rays bombard all of the earths surface and naturally occurring
radioactive elements exist everywhere, a certain minimal exposure to so-called background radiation is inevitable. In some regions, radioactive
radon gas occurs naturally in bedrock such as granite. It can exposure miners working underground and can accumulate in the basements of buildings,
which may need special ventilation. Owing to the use of radioactive materials in industry and the use of ionising
radiations in medicine and industry, some groups of people are exposed to increased levels of radiation.
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17.6 Uses of Radiation Industrial
