a. Chemical Change. Radiologic specialists are familiar with one of the
benefits of chemical change. The ionization of silver bromide crystals in a film emulsion
causes a chemical change such that those crystals will reduce to black metallic silver
when exposed to developing chemicals. The un-ionized crystals remain chemically
inert to the developer and are removed during the fixing and clearing' process. Thus,
the degree of darkening on a film badge, when processed under controlled conditions,
is used to determine the quantity or total dose of radiation received by the wearer.
b. Electrical Change. The electrical changes brought about by the ionization
process are also useful in detecting the presence of x-radiation. It is possible to detect
and measure radiation from the number of freed electrons since and x-ray beam will
provide a large number of such electrons where there was essentially none before.
4-11. TOTAL DOSE AND DOSE RATE
Total dose and dose rate are two terms that must be clearly understood before
further discussion of the detection and measurement of x-radiation. Total dose is the
total amount of radiation received, such as 50 rem, with no indication of the time during
which it was received. It may have been 1 minute or 1 year. As the dose gets larger,
this time element becomes extremely critical, and in the case of a careless x-ray
specialist, perhaps even deadly. Therefore, it is necessary to be able to discuss
radiation doses in terms of the rate at which they are being received. Dose rate is the
amount of radiation received per unit time such as 50 rem per hour (50 rem/hr). A dose
rate of 100 rem/min for 4 minutes would result in a total dose of 400 rem. Additional
discussion of dosage and dose rates will be found in Section VI.
4-12. ION CHAMBER
An ion chamber (figure 4-7) is an example of an instrument that measures
radiation dose or exposure rates. A difference in potential across the chamber causes
movement of the free electrons. The rate of current flow then reflects the amount of
radiation striking the chamber. The instrument can be calibrated by using a known
quantity of radiation. Figure 4-7A shows no reading on the meter because there is no
ionization (no electrons are being freed); consequently, there is no electron flow. In
figure 4-7B, ionization is taking place (electrons are being freed) and the application of a
potential difference is causing electron flow that results in a reading on the meter. The
meter, of course, would be calibrated in roentgens, or some other units. Two examples
of instruments used to measure radiation in this manner are the Geiger-Mueller Counter
and the Jordan Ion Chamber.
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