Radiation Protection and Shielding
Lesson 6 - General specifications of radiation sources
Types of sources
In this course, we will be working with four spatial layouts of sources. In this lesson, we want to introduce them and get a little practice in converting from one type to the other. The four are
These are sources that denote particles that are spontaneously emitted from a single point, with no physical source modeled:
This is a mathematical simplification, of course, of a physical source. The book gives two general requirements for assuming this:
and then states that Requirement 2 can be relaxed if source characteristics are modified to account for source self-absorption. This relaxation is generally made by performing a stand-alone calculation of a physical source, carefully collecting information about the number and energy profile of particles (of all types) that escape the source, and then attributing the escaping particles to the mathematical point source in a subsequent calculation.
This allows the analyst to model the effects of the source without having to model the source material in the calculation.
The line source, which has the addition dimension "per unit length," is a similar mathematical simplification of a physical source, but one in which all dimensions but one of the physical source are small in comparison with the dimensions between source and detector. In other words, it is long and skinny ("pencil-like"). We call the dimension we keep the "length", L:
Notice that a differential length of the source emits , therefore acts like a small point source. The total particle emission rate from the line sources is .
Area (or surface) sources,
The analogy continues with the area source. If two dimensions cannot be ignored, we model the source as a flat source and the dimension has a "per unit area" in it. A differential area on the surface, then, acts like a mini-point-source, and the total particle emission rate from the area source becomes .
The final, and most general, form of the source that we will use is a full three dimensional source, where all three spatial dimensions are significant. As an example, consider as a parallelepiped (i.e., block) shaped source:
A volumetric source can also be reduced to the previous, simpler forms: area source, line source, and point source. The exercise at the end of the lesson will give you practice doing this.
Curie vs. Becquerel
The standard, preferred unit of activity is the Becquerel (Bq), which is 1 disintegration/sec. Us old guys still use the Curie (Ci), which is . You are caught in the middle and have to know both.
Activity vs. particle production rate
Be sure to recognize the difference between these two. Sources are often specified in terms of the disintegration rate of decaying isotopes; it is up to the analyst to figure out the resulting particle source rate.
Appendix H in the book gives gamma and x-ray emission information for selected radionuclides. This can be useful.
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