Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
layer(HVL) is used. This concept applies only in the case of photons.
If a beam of photons consists of only a single energy, the beam is referred to as
monoenergetic. A beam of photons may be composed of many energies, generally the
case with X‐ray. As a result, it is common to speak of quality and quality and quantity of
a beam of photons.
Quality refers to the energy of particular photons, and quantity refers to the number of
photons of each specific energy. Quality and quantity are described together in a concept
known as intensity, defined as the total energy contained in the beam quality x quantity
per unit area per unit time. Hence:
I(intensity)
ETotal
=
Area • Time
The area referred to is the cross‐sectional area of the beam at a particular point in space.
Since most beams of photons spread out as they move away from the source, it is
important to specify where the intensity is being calculated.
In the following discussion of HVL, a monoenergetic beam of photons has been assumed.
The application to a non‐monoenergetic (polyenergetic) beam is described afterward.
Consider a beam of photons approaching a barrier, e.g., a simple wall, as shown in Fig.
(2.15). Some of the photons penetrate to the far side of the wall, and some are attenuated
by the wall material. The number of photons attenuated by the wall is a function of both
the energy of the photons and the atomic number and density of the material composing
the wall. In general, two principles apply:
1. The more energetic the photons, up to a certain energy, the less likely they are to be
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