Calibration of TL phosphor & TLD reader and its use in dose distribution measurements.
Aim: To calibrate TL phosphor and TLD reader and use it in dose distribution measurements.
Requisite: Slab phantom with TLD holder, TLD-TLD 100, TLD Reader-Model 3500, Tweezers
Theory
Many crystalline materials exhibit the phenomenon of thermoluminescence (TL). When
such a crystal is irradiated, a very minute fraction of the absorbed energy is stored in the
crystal lattice. Some of this energy can be recovered later as visible light if the material is
heated. This phenomenon of the release of visible photons by thermal means is known as
TL. The arrangement for measuring the TL output is shown in figure below. The irradiated
material is placed in a heater cup or planchet, where it is heated for a reproducible heating
cycle. The emitted light is measured by a photomultiplier tube (PMT), which converts light
into an electrical current. The current is then amplified and measured by a recorder or a
counter. There are several TL phosphors available, but the most noteworthy are lithium
fluoride (LiF), lithium borate (Li2B4O7), and calcium fluoride (CaF2).
Simplified Diagram of Thermoluminescent Dosimeter Reader
A plot of TL against temperature is called a glow curve . As the temperature of the TL
material exposed to radiation is increased, the probability of releasing trapped electrons
increases. The light emitted (TL) first increases, reaches a maximum value, and falls again to
zero. Because most phosphors contain a number of traps at various energy levels in the
forbidden band, the glow curve may consist of a number of glow peaks as shown in Figure
The different peaks correspond to different “trapped” energy levels.
The purpose for calibrating TLD dosimeters is to ensure that all the dosimeters in a system
will give essentially the same response to a given radiation exposure. Because of natural
variations in TL material responsiveness and in the physical mass of manufactured TL chips
,there is a variation in response from the mean in a population of dosimeters. The
calibration factor for dosimeters is called the Element Correction Coefficient or ECC. The ECC
is used as a multiplier with the reader output (in nanocoulombs) to make the response of
each dosimeter comparable to the average response of a designated group of dosimeters
maintained as calibrated dosimeters.
The purpose for the Reader calibration is to maintain a consistent output from the Reader
the over a period of time based on a convenient local source. By using a set of Calibration
Dosimeters and a consistent local source, the readers performance may be kept at a
constant level in spite of high voltage changes , repairs , dirt accumulation or long term drift.
The calibration factor for readers is known as Reader Calibration Factor or RCF.This factor
converts the raw charge data from the multiplier Tubes (in nanoCoulombs) to dosimetric
units. The two factors are applied according to the following formula
Exposure= (ECCXCharge )/RCF
Procedure
Calibration
1.Anneal the TLD at 4000C for 1hr in oven or as per TTP (Time, Temperature, Profile) defined
for that specific TLD material in softwire.
2.Expose 10 TLD to a known dose : 2 Gy at 5 cm depth, field size 10x10 cm2 ,at 100 SSD.
3.Read the TLD 1 hour after exposure in Gen Cal Dosi mode
4.Choose the TLD whose response is 1U or near to 1U , separate and mark them as
reference dosimeter and others as field dosimeter.
5.Search response database, select all the TLD Generate Calibration Dosimeter, compute
and accept by adjusting the range.
5.Anneal the TLD. Expose it again as in step 2.
6.Read the reference TLD in Dal reader mode. Search response database select the
reference TLD ,from calibration select reader calibration mode include all range,give
irradiation value .Accept and close.
7.Read the field dosimeters in Cal Dosi mode. Search response data from calibration select
Dosimeter calibration ,compute accept and close. Calibration is complete.
7.For dose distribution measurement calculate monitor units for 10x10 cm2 5 cm depth for 2
Gy .Expose it check and read in Read Dosimeter mode .Compare the dose measured and
calculated.
Ressult
TLD Reader and TLD 100 rod (10 numbers) is calibrated.
Measured dose is within 10%.
Observation
Setup
Field Size=10x10 cm2
SSD=100
Depth=5 cm
Dose delivered=2Gy
TLD Id Measured dose % variation
