ne.gif (2791 bytes) NE406 Radiation Protection and Shielding

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 concepts:

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 neutral particles.

Secondary particles: We will use this term for the charged particles that are produced from ionization reactions.

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 air.; 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.