Physics and Engineering of Radiation Detection

11.2. Quantities Related to Dosimetry 609

The tissue weighting factor for stomach iswT=0.12 as given in Table 11.2.2.

The effective dose is then give by

E = wT·HT,R

=(0.12)(0.3)

=0. 036 Sv =36mSv.

The usual effective dose received during a typical CT scan of abdomen is

around 10mSv, which means that this patient received more than three times

the usual dose.

11.2.BFluxorFluenceRate

We know that the ionizing power of radiation is directly related to the energy it car-
ries. This can be understood by noting that in most gases the number of charge pairs
produced per unit of absorbed energy is almost independent of the type of radiation
(see chapter on gas filled detectors). In case of solids, although this independence
is not guaranteed, still the ionization caused by radiation depends to a large extent
to the energy it delivers. In dosimetry therefore one is interested in determining the
amount of radiation carried by the radiation.Energy fluxis a measure that can be
used to characterize this quantity. Another quantity closely related to energy flux
is theparticle flux.
In order to define particle and energy fluxes, Let us first assume that we have
a mono-energetic beam of particles incident on a material having a cross sectional
areada. IfwecountthenumberofparticlesdNincident on this area in a timedt,
then the particle flux can be calculated from

Φr=

dN

dtda

. (11.2.6)

This shows that the particle flux simply represents the number of particles incident
on a surface per unit area per unit time. In the field of dosimetry this quantity is
also known asparticle fluence rate. Now, since we have a mono-energetic beam of
particles therefore we can use the above relation to compute the energy flux as

Ψr=

EpdN

dtda

, (11.2.7)

whereEpis the energy carried by a single particle. The energy flux is a measure of
the total energy incident on a surface per unit area per unit time. Another term used
for this quantity, specially in dosimetry, is theenergy fluence rate. Energy fluence
rate is actually the preferred terminology adopted widely in the field of dosimetry.
In fact, the usage of flux is discouraged in this field. It is evident that the energy
fluence rate and particle fluence rate are directly related through the relation

Ψr=EpΦ. (11.2.8)

Up until now we have considered the radiation beam to be absolutely mono-
energetic, that is, all the particles had same energy. This is certainly not true since
an absolute mono-energetic beam of particles is impossible to create. A realistic