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When the dosimeter is fully charged and the hairline image is at zero on the
graduated scale, the surfaces of the frame/fiber and the ion chamber will carry charges of
opposite polarity.
When an ionizing ray penetrates the dosimeter and its ion
chamber, electrons are ejected from the chamber wall into the air volume of the
electrometer. These electrons collide with the air molecules causing the air to ionize.
The ionized air molecules within the chamber form positively and negatively charged
particles. These oppositely charged particles will be attracted to either the frame/fiber
or the ion chamber wall, depending on their polarity, because opposite charges attract.
As the ionized particles land on these charged surfaces they
become neutralized. This results in a small current flow which will decrease the original
voltage charge on the electrometer which causes the fiber to be less repelled by the
frame. This causes a change in the hairline image observed on the scale toward the upscale
direction indicating an exposure. The voltage decrease is directly proportional to the
quantity of ionizing radiation which penetrated the ion chamber. The relationship between
the fiber and the graduated scale is such that, as the fiber moves closer to the frame,
the hairline image of the fiber on the scale moves to higher values.
The movement of the fiber is a function of the total amount
of radiation to which the ion chamber has been exposed, irrespective of the rate of
radiation exposure. When the dosimeter has been exposed to a quantity of radiation equal
to the full scale reading of the instrument, the hairline image of the fiber will appear
at the highest value graduation on the scale, and the electrometer will have been
discharged to approximately 100 volts.
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