Operating Manual

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As there is considered to be no totally safe lower limit below which no damage would be
sustained, the “ALARA” concept is being promoted. ALARA (short for As Low As Reasonably
Achievable), aims to achieve the lowest possible radiation dose whereby economic and soci-
al factors are considered.
The protection from unwanted external irradiation is based on three principles:
• Speed: by working fast, the exposure duration is reduced.
• Distance: the greater the distance, the lower the rate of exposure
(remember the inverse square law).
• Shielding and collimating: materials with high radiation absorbing properties,
such as lead, reduce the exposure rate to a level that can be calculated in advance.
Table 3-19 in section 19.8 shows the half-value thickness of lead for various gamma sources.
19.5 Permissible cumulative dose of radiation
Although the subject of permissible cumulative dose of radiation is complex, the values
given below apply to external irradiation of the whole body.
The values have been established by the ICRP.
• Radiology workers, category A: 20 mSv/year
• Radiology workers, category B: 5 mSv/year
• Public, not being radiology workers: 1 mSv/year
The whole body level of 20 mSv per year is normally interpolated as 0.4 mSv per week and
10 μSv/h at a 40 hr working week.
These levels are acceptable but it is not to be automatically assumed that people working
with radiation actually should receive these doses.
When radiography is carried out in factories and on construction yards etc. special conside-
ration is required for non-radiological workers and demarcation of the area in which radia-
tion is used, and a maximum dose rate of 10 μSv/h applies, is essential.
This is also the maximum value to be measured at the outside surface of a charged isotope
container.
19.3 The effects of exposure to radiation
The understanding of the effect that exposure to radiation has on human beings has grown
over the past 50 years and has led to a substantial reduction of the maximum permissible dose.
There are two categories of biological effects that an overdose of radiation can cause: soma-
tic and genetic effects. Somatic effects are the physical effects.
A reduction in the number of white blood cells is an example of a somatic effect. Much more
is known about the somatic than about the genetic effects of radiation.
Blood cells are very sensitive and the first signs of radiation are found in the blood, which is
why people working in radiology are subjected to periodic blood tests.
The most serious effects of radiation occur when a large dose is received in a short period of
time. Table 2-19 shows doses received over 24 hours and their effects:
The consequences of excess radiation are not necessarily noticeable immediately after the
irradiation. Frequently, they only show up after some time. The time lapse between irradia-
tion and the moment the effects become apparent is called “the latent period”.
Genetic effects can only be assessed over generations.
19.4 Protection against radiation
The International Commission on Radiation Protection (ICRP), a division of the
International Atomic Energy Agency (IAEA), is engaged in providing rules and regulations
for the protection against radiation, as the name suggests. The ICRP has established the
values for radiological and non-radiological workers, as indicated in section 19.5.
Practically all countries have brought their national legislation (laws) on ionising radiation
in line with the ICRP codes. The conditions for registration, transport, storage, protection
and the expertise of preparation and use of radiation sources have been laid down in regu-
lations. The purpose of practical protection against radiation is to prevent any individual
receiving a harmful dose.
Dose received Effects
by the body
0.0 - 0.25 Sv No noticeable effects
0.25 - 0.5 Sv Limited temporary changes in the blood
0.5 - 1.0 Sv Nausea, fatigue
2.0 - 2.5 Sv First lethal cases
5.0 Sv 50 % lethal (MLD = medical lethal dose)
Table 2-19. Effects of radiation doses