Regional Basic Professional Training Course in Korea

Regional Basic Professional Training Course (BPTC) on Nuclear Safety
general, for reasonable distances and source sizes, this approximation is adequate. Then a
reference point is picked: one unit distance away. This unit distance can be 1 meter, 1
centimeter‐whatever is the most convenient unit to use. In this text, 1 meter has been
chosen for consistency. As a result, knowing the number of photons emitted per unit time
at a distance of 1 meter (or any other unit distance) from the source, the number of
photons per unit time at any other distance can be obtained, as follows:
Number of photons per
second at 1m
(unit distance)
Example 2.8.1.
⋅ (1m)
2
=
Number of
❙ 106 ❙
per second
elsewhere
photons
⋅
(distance) 2
in meters
from source
A source of photons, as measured at a distance of 1 meter, emits 1000 photons per
second. How many photons per second will be measured at a distance of 5 meters?
Hence:
1000 Photons · 1s ‐1 ·(1m) 2 = x·(5m) 2
Rearranging the above equation, one obtains:
which gives finally
1000 -1
2
Photons
⋅1s
2
25m
Photons
40 =
s
⋅1m
x
= x
Two things are worth noting in the previous example. The first is the tremendous
decrease in the number of photons encountered by the measuring unit, which could be the