Dosimetry Methods

Saturday, June 4, 2011

RADIATION DOSIMETERS
The radiation dosimeters are used to record the radiation doses of raidation workers. There are different types of radiation dosimeters used for this purpose so accordingly there are different techniques used for dosimetry. Some of the methods or techniques of radiation dosimetry are being described here.

A. ACTIVE MONITORING OF DOSE LEVELS OR DOSE RECIEVED BY ANY RADIATION WORKER:
A.1 Electronic Dosimeters EPDS:
These EPDS are to be wear by radiation worker before entering to the radiation field or starting its job.
Examples: Geiger-Muller or semiconductor detector
Detection: Detect x-rays and gamma radiation
Properties: Relatively expensive, usually quite rugged, reusable
A.2 Quartz Fiber Electroscope (QFE):

Consist of a small ionization chamber and quartz fiber
Radiation change the deflects the quartz fiber
Deflection is displayed in the eyepiece lens
Major disadvantage: Easily damaged if dropped or roughly handled.
B. PASSIVE MONITORING
B.1 Film Dosimeters/Badges

Used as a personal dosimeter
Use highly sensitivity silver halide film
Fitted with a range of filters
Distinguish beta, x-ray, gamma and thermal neutrons
Dose is determined by degree of blackening (optical density) and comparing it with calibrated films
Provides permanent record of an individual’s dose
Adverse effects of light and heat
Relatively short shelf life (months)
Require dark room facilities (development chemicals)
Significant manual handling during assessment
B.2 Thermoluminescent Dosimeters (TLDs)

Used as personal and environmental dosimeter
Use Thermo-Luminescent (TL) materials
Electrons are raised/trapped at higher energy levels
The energy is released as light when heated
Light emitted is converted into an electrical signal
Light emitted is proportional to incident radiation
Lithium (LiF:Mn) based TLDs for personal dosimetry: Because they are tissue-equivalent
Calcium (CaF2:Dy, CaSO4:Dy) based TLDs for environmental monitoring: due to their high sensitivity
Lithium borate (Li2B4O7:Mn) TLDs for high dose range dosimetry
TL materials are available in many different forms: e.g. powder, hot pressed chips, pellets, impregnated Teflon disks
Read-out instrument (reader):are required
Method to heat the TLD material: Electrical, hot gas or a radiofrequency heater, Heated in an inert gas during read-out
Device to convert the light output to an electrical pulse
Light signal is amplified using a photomultiplier (PM)
Small size (only milligram quantities of TL material is needed)
TLDs can be reused
Disadvantages of TLDs

Only one time reading during heating, cannot be repeated
Subject to fading (due to temperature or light effects)
B.3 Nuclear emulsion or track etch dosimeters

Neutron dosimetry and alpha particles (e.g., from radon)