Lesson 14 - Dosimetric quantities
With this lesson, we begin our study of Chapter 5, in which we are switch our point of
view from the SOURCE side of our problem to the DETECTOR side of our problem.
Note: When I say "detector," I am including all possible
effects that we might want as our final answer -- dose to personnel, energy-averaged flux,
response of a "real" detector, reading on a film badge, etc.
In the chapter we will make a (hopefully) smooth transition from physical concepts
(energy deposition of radiation) through biological concepts (biological effects of
radiation) to our final goal of a mathematical concept (a response function to convert
particle fluxes into biological effects).
In this first lesson, we will be concerned with concepts related to physical effects,
which are related to energy deposition in detector material. The reading for
this lesson explains the material rather well, but we will repeat emphasize certain
- Primary particles
- This is the term we will use for the uncharged (neutral) particles, neutrons and gamma
rays. The way we will use it in this class, it includes both source and scattered
- Secondary particles
- We will use this term for the charged particles that are produced from ionization
- Imparted energy
- This is the thermal energy deposited in a material. As stated in the text, this
value is computed by adding the energy of all particles entering the material minus the
energy of all particles leaving the material minus energy that has been
"absorbed" in nuclear changes. What is left is the energy that will be
thermally absorbed by the material.
- Specific energy
- As in other applications, the word "specific" turns a term into a ratio.
In this case the transformation is to a "per unit mass."
- Absorbed dose, D (Gy)
- As for specific energy, this is absorbed energy per unit mass. It is different
from specific energy in that it is the average or expected value, which means that
although specific energy will bounce around with time because of the statistical
variations of actual fluxes, absorbed dose is a deterministic quantity that is most easily
recognized as the average of the statistically varying specific energy.
- The symbol for absorbed dose is D, and the unit is the Gray (Gy), which is equivalent to
joules/kilogram. (An older, but still used, unit is the rad, which
is equivalent to 0.01 Gy.)
- Kerma, K (Gy)
- Why are we still talking? Absorbed dose is really the last word in physical
dosimetric concepts -- energy absorbed in the material per unit mass. What is there
left to say? Well, we have two more practical concepts to learn that are in
use because absorbed dose is hard to calculate and hard to measure.
- Kerma is a concept that is an approximation to absorbed dose that has the advantage that
it is easy to calculate. The reason that absorbed dose is hard to calculate is that
when the uncharged particles (whose transport and interaction rates are relatively easy to
calculate) ionize the material, they produce charged particles (whose transport and
interaction rates are harder to calculate). In reality, these charged
particles carry the energy away from the point where they are created and deposit the
energy somewhere else.
- "Kerma" is an acronym that means (something like) kinetic energy
released in matter. (I am not really sure about the "r".)
It ignores the fact that the charged particles travel and just counts the energy
that is released, rather than deposited, in the material.
- The symbol is K, and the unit is the Gray, like before.
Note: Two assumptions that would make kerma equal absorbed dose are:
- Assuming that the charged particles deposit their energy where they are produced.
- Assuming that the energy contained by charged particles that leave the original material
is exactly balanced by the energy of charged particles that were produced outside the
material, but that enter and deposit their energy in the material.
The second assumption is called "charged particle equilibrium." We will
have more to say about this later.
- Exposure, X (Roentgen)
- The second practical unit is exposure, with symbol X. At first glance, it seems to
be a completely different type of measurement:
- 1. Instead of energy deposition (or energy
released), it is a measure of ionization produced.
- 2. Instead of being defined for all materials, it is defined only for
- 3. Instead of applying to all particles, it applies only to photons.
- The usefulness of the concept is in its ease of measurement. With an instrument
containing a chamber with a known quantity of air, including an electric charge applied
across the chamber, the ionization produced in the air can be collected and measured.
- The unit of exposure is the Roentgen, defined as 2.58e-4 ion Coulombs/kilogram of air.
With this unit, the Roentgen translates approximately to the rad.