Operating Manual

213212
Distance
Since radiation is subjected to the inverse square law, its intensity is reduced with the incre-
ase in distance to the square.
Absorbing barrier and distance
Whenever radiation penetrates a material, the absorption process reduces its intensity.
By placing a high-density material such as lead around the source of radiation, the quantity
of transmitted radiation will decrease. To determine the material thickness required for a
certain reduction in radiation, a factor known as the half-value thickness (HVT) is used.
Table 3-19 shows the HVT-values for lead for various types of gamma sources
Example
To reduce 2.56 mSv/h, measured at 1 meter distance, to 10 μSv/h the required distance
according the inverse square law is
 2560/10 = 16 metres. To achieve the same by placing
a shield, the number of HVTs is calculated as follows:
Required intensity reduction is 2560 / 10 = 256 x
Number of HVTs is, log 256 / log 2 = 8
The example above demonstrates that an intensity of 2.56 mSv/h can be reduced to
10 μSv/h by increasing the distance to 16 metres, or place shielding material of 8 HVTs as
close as possible to the source. If either of these methods cannot be used on its own, a
combination of the two could be considered.
Film dose meter (film badge)
The film badge consists of two pieces of X-ray film con-
tained, with filters, in a special holder. At the end of a
specified period, the films are developed and the densi-
ty measured.
The radiation dose received by the wearer can then be
determined by consulting the density/exposure curves,
and the type of radiation received can be established by
checking the densities behind the filters. Film dose
meters as illustrated in figure 4-19 are a very cheap and reasonably accurate method of
monitoring personnel in selected areas. They measure 25 x 25 x 5 mm, are robust and
convenient to wear.
19.7 Dose registration
Due to legally required monitoring and registration of radiation doses received by radiolo-
gical workers over a specified period of time, dose meters must be worn. Generally, these are
TLD or film badges. The TLD-meter is preferred over the film badge as it is read out electro-
nically and can be linked to a data base. Processing film badges is more complicated. The
films must first be developed before they can be viewed to quantify and register the radia-
tion dose.
Radiation dose monitoring is carried out by a government-authorised organisation which is
responsible for mailing, processing and viewing of the badges. This organisation generates
reports, which contain the individual irradiation doses over a specified period of time, as
well as the accumulated dose.
19.8 Radiation shielding
Protection from radiation (best by distance) can consist of ribbons or ropes and warning
flags to demarcate the area where radiographs are made, or concrete bunkers with doors
which automatically switch off the X-ray equipment as soon as they are opened.
Both methods have the same objective: i.e. to prevent unauthorised people entering the
area of radiation.
An area of radiation can be defined as an area in which the radiation level exceeds the per-
mitted value of 10 μSv/h.
There are three ways to achieve a reduction in intensity:
1. by erecting a demarcation barrier at an appropriate distance,
2. by erecting an absorbing barrier,
3. by a combination of methods 1 and 2.
Fig. 4-19. Filmdosismeter (film badge)
Table 3-19. Half-value thicknesses for lead using different types of gamma sources
Symbol Average energy in Half-value thickness
MeV in mm lead
Cesium137 0.66 8.4
Cobalt60 1.25 13
Iridium192 0.45 2.8
Selenium75 0.32 2.0
Ytterbium169 0.2 1.0
Thulium170 0.072 0.6