11.4. Active Dosimetry 647
11.4.DQuartzFiberElectroscope....................
Quartz fiber electroscope is a pocket dosimeter that has the capability of instanta-
neous dose readout. It is therefore generally called a Self-Reading Pocket-Dosimeter
or SRPD. It consists of the following main components.
Ionization Chamber:This consists of a very small volume gas filled chamber
where the incident radiation produces electron ion pairs.Quartz Fiber Electrometer:This is used to measure the charge produced
by the radiation in the ionization chamber. It consists of a movable quartz
fiber and a metal frame forming its two electrodes. The separation of these
electrodes depends on the amount of charge on the metal electrode.Microscope:The purpose of the microscope is to facilitate the reading of the
fiber image.The working principle of the electroscope is quite simple. The incident radiation
produces charge pairs in the chamber volume, which creates a current flow. Con-
sequently the charge on the electrode decreases. Due to this decrease, the quartz
fiber moves and its deflection can be projected by a light source to a calibrated scale
through an objective lens. To do this, the user points the dosimeter to any external
light source while looking into the microscope eyepiece. Since the deflection of the
fiber is proportional to the current flowing through the chamber, therefore it is a
measure of the dose delivered by the radiation. The electroscope is pre-calibrated
and therefore no calibration is required by the user. Upon full discharge, the elec-
troscope can no longer be used before being charged again. This is done by applying
high voltage through a charging pin.
Fig.11.4.5 shows the working principle of a typical quartz fiber electroscope. The
electrometer of an electroscope that has not seen any radiation is fully charged and
the reading pointer is at 0 position. As the incident radiation produces electron
ion pairs in the ionization chamber, the effective voltage on the fixed electrode
decreases. Consequently the quartz fiber moves towards the fixed electrode and the
reading pointer move forward.
D.1 Advantages and Disadvantages of Quartz Fiber Electroscope
The advantages of this instrument include the following.
Visual Readout: Even though the quartz fiber electroscope is an active
dosimeter, it does not require any electronic circuitry to read the dose. The
readout is done visually at any time.Good Accuracy:Although the accuracy of the electrometer can not be com-
pared with that of an electronic dosimeter, it gives a modestly accurate mea-
sure of the integrated dose. In most organizations the electroscope is used in
conjunction with some other dosimeter, such as a TLD.Repeated Usage:The electroscope can be used repeatedly after being recharged,
which is fairly simple and is done through a special unit.