Why do we use both distance and attenuation to calculate reduction in photon dose rate?

The inverse square dependence of
photon fluence rate or dose rate on distance from a source applies to the
primary photons emitted isotropically from a source whose dimensions are small
compared to the distance from the source to the dose point (referred to as a
point isotropic source). The process of exponential attenuation of photons
applies at any distance in any medium and to any source, but when dealing with
transmission media that present little attenuation we often neglect attenuation
and consider only the reduction in intensity with distance. This is often the
case when dealing with sources in air when the photons are sufficiently
energetic that air attenuation is not a significant factor.

The common approximate exposure rate
equation, X = 6CEn d², where X is the exposure rate (R per hour), C
is the source activity (Ci), E is the photon energy (MeV), n is the photon
yield (number of photons of energy E per disintegration of the radionuclide),
and d is the distance (feet) from a point isotropic source, assumes no
significant attenuation in the medium between the source and dose point. This
is a legitimate assumption when the distance is not excessive and the photon
energy is not very low.

For example, at one meter in air from
a source that emits 1 MeV photons, the reduction in intensity of primary
photons by attenuation in the air is about 1 percent. If the medium were water,
however, the loss of primary photons would have been about 99.9 percent, as
calculated as 1 minus the fractional transmission—i.e., 1 – e^-µd,
where µ is the linear attenuation coefficient for 1 MeV photons in water
(0.0707 cm^-1) and d is 100 cm.

The fact is that the processes of
intensity reduction by distance and by attenuation do not operate exclusively
in many situations involving exposure to photons. It is necessary to consider
both processes in order to account properly for the reduction in primary photon
intensity. Note that I have referred several times to primary photons—i.e.,
those emitted directly by the source. In addition to these, it is often
necessary to account for the effects of secondary photons that arise as a
consequence of interactions of the primary photons in the media through which
the primary photons travel. Compton-scattered photons are among the most
important in this regard. The impact that these secondary photons have on
increasing the dose rate at the point of interest may be substantial and is
often accounted for in calculations by the use of a buildup factor that
represents the factor by which the primary photon dose (rate) is increased by
these secondary radiations. If you are interested in a more detailed discussion
of this process, see the information available at this link on
the HPS website.

In summary, we
do not generally have the option of choosing either distance or attenuation as
an approach to calculating dose rate from a source; we must consider both
reduction of intensity by increasing distance from the source and reduction by
attenuation in any media between the source and dose point. It is only in cases
when one of these reduction mechanisms becomes insignificant compared to the
other that it is appropriate to neglect the lesser effect.

I hope this
clarifies the situation for you.

George
Chabot,
PhD, CHP