636 Chapter 11. Dosimetry and Radiation Protection
After irradiation the material is etched with a suitable chemical, such as hot
sodium hydroxide solution. In earlier days of track etch dosimetry the tracks were
counted by visual inspection, a process that was prone to human errors. Modern
systems are equipped with automated microscope readers and are therefore much
more precise and accurate.
D.1 Advantages and Disadvantages of Track Etch Dosimeters
The advantages of track etch detectors include the following.
Cost Effectiveness:Due to their low cost, these dosimeters have found wide
applications.
Good Sensitivity: Track etch polymers are sensitive to almost all types of
ionizing radiation, with the exception of photons.
Operationally Safe:Since these materials are non-toxic, they can be handled
without any extra precautions. This has made them highly suitable for use in
households by non-specialists for radon dosimetry surveys.
Track etch detectors also suffer from several disadvantages, some of which are
listed below.
Track Fading: Since the tracks left by the radiation fade away with time
therefore the readout can not be delayed.
Insensitivity to Photons:Track etch dosimeters are not sensitive to photons
and can therefore not be used to properly assess the dose in a mixed radiation
environment.
Dependence on Etching Rate:The results depend to some extent on the
etching process. In particular the etching rate has been seen to affect the dose
estimation.
The most commonly used track etch material is the so calledCR39plastic, which
has the composition of polyallyl diglycol carbonate. The material can be bought in
the form of large sheets, which can then be cut into desired sizes.
11.4ActiveDosimetry.............................
Active dosimetry involves use of an electronic detector and is suitable for measuring
both instantaneous and integrated doses. The most commonly employed active
dosimeters are described below.
11.4.AIonChamberDosimetry.....................
Ionization chambers have long been used in all types of dosimetry because of their
simplicity in design and low operating cost. In clinical practice, they have have
become standard dosimetry tools, though the trend is now shifting towards other
types of detectors such as semiconductor devices.
In the following sections we will look at two methods commonly used in ion
chamber dosimetry and discuss their pros and cons.