Assessing the Different Types of Thermoluminescence Badges on Harshaw 6600 Plus Reader

Radiation dosimetry is defined as the measurement, usually, of the absorbed dose, or other relevant quantities like KERMA, exposure or equivalent dose, which is produced due to the interaction of the ionizing radiation with a material. That measurement can be achieved using a dosimeter. A dosimeter with its reader is called a dosimetry system. The TLD reader system is used to process a worn TLD dosimeter. The reader automatically heats the TL material. A photo multiplier measures the TL-light released during the heating of the material from room temperature up to a maximum of (250-400 °C) depending on the system. Personal dosimeters should be calibrated in terms of operational quantities for individual monitoring of external exposure (i.e. the personal dose equivalent HP (10) for strongly penetrating radiation and HP (0.07) for weakly penetrating radiation). Personal dosimeters should be irradiated on the ISO water slab phantom 30 cm × 30 cm × 15 cm that provides an approximation of the backscatter conditions of the human body. The purpose for calibrating TLD Cards is to ensure that all cards in a system will give virtually the same response to a given radiation exposure. Because of natural variations in TL material responsiveness and the physical mass of manufactured TL chips, there is a variation in response of as much as 30% (based on 3 sigma) from the mean in a population of dosimeters. The calibration factor for dosimeters is called the Element Correction Coefficient (ECC). The ECC is used as a multiplier with the Reader output (in Nano Coulombs) to make the response of each dosimeter comparable to the average response of a designated group of dosimeters maintained as Calibration Dosimeters.


Introduction
The TLD badge comprises of a lithium fluoride chip/crystal which is doped with titanium and magnesium (LiF: Ti, Mg) and placed in a holder that in-corporates a filter system [1].
A TLD badge does not provide protection but detects and absorbs radiation that you have been exposed to. The badge will detect high energy beta, gamma or x-ray radiation. Dosimeters cannot de-tect low energy beta radiation from some isotopes, including carbon-14, tritium and sulphur-35. The TLD is used to monitor the Personal dose equiva-lent at a depth of 0.07 mm on the body (Skin dose) and the Personal dose equivalent at a depth of 10 mm on the body (Deep dose) received by all occu-pationally exposed workers (OEWs) [1,2] (Figure 1 and Figure 2).
In actual fact, the TLD Badge does not protect the occupational worker from the effects of radiation, neither does it act as a magnet to collect the radia-tion dose, but rather it absorbs the radiation in the field of work [3]. Once you are exposed, the Health Physicist uses your TLD to estimate the amount of radiation you are exposed to in the course of your work. The results obtained after the evaluation of the TLD card with the Harshaw 6600 PLUS TLD Reader will help you to know how much radiation you are exposed to during the course of your work.
This will help estimate your potential risks due to exposure to radiation [4].   x-ray machines, fluoroscopy units, certain sealed and unsealed radioisotopes or who are exposed to other sources of gamma or high energy beta radiation are generally required to wear one or more dosimeters.

Advantage of Harshaw TLD model 6600 plus
New institutions working with radiation sources or radiation emitting devices must apply for TLD badges providing the names and positions of the personnel to be monitored. They also must complete a personnel data form by providing all the required details. Once the Radiation Protection Institute reviews the application, a dosimeter will be issued if needed.
The TLD badge must be worn on the front of the upper torso of the body, between the shoulders and the waist [7,8]. TLD badge should be worn for a period of one (1) month and shall not exceed three (3) months, after which it is to be sent to the service provider, Radiation Protection Institute (RPI), for evaluation and assessment.
The primary occupational whole body dose limit is 20 mSv per year averaged over a five-year period and should not exceed 50 mSv in any single year.
The annual equivalent dose limit for the skin, the hands and feet (extremities) is 500 mSv and for the lens of the eye is 150 mSv.
When an occupationally exposed worker being monitored leaves the facility or changes work locations, please notify the Health Physics and Instrumentation Centre (HPIC) of the Radiation Protection Institute (RPI).

Materials
Harshaw 6600 plus TLD reader: For this project, the Harshaw TLD (Model 6600 PLUS) located at the Radiation Protection Institute is the reader used for estimating whole body and extremity effective doses recorded by the TLDs. A non-contact heating system is used which utilizes a stream of hot nitrogen gas. The Reader is interfaced to a computer where all measured data can also be displayed, stored and printed [2,6]. A hand held scanner is used to identify each TLD card before reading ( Figure 3 and Figure 4).
Thermoluminescent dosimeter: The most widely used material is the LiF with added magnesium and titanium. TLD-100 is LiF: Mg, Ti which due to being near tissue equivalent, unlike Al 2 O 3 or CaSO 4 . For whole body personnel and environmental monitoring, the sensitive elements are securely fixed to sturdy aluminum cards with a barcode to ensure complete chain of custody.

Advantage of instadose
Active Dosimetry Systems, Active Radiation Dosimeters or Electronic Personal Dosimeters have the advantage of providing a direct display of the accumulated radiation dose and dose rate to the wearer as well audible and visual indication of the dose rate level. Smaller than a flash drive, this dosimeter provides an instant read-out. Small Lightweight Design. USB Compatible Detector. It can be used in Dental, Animal Health, Healthcare and Medical Facilities [3].

Advantage of optically stimulated luminescence (OSL)
The combination of OSL and Al 2 O 3 : has many advantages: high sensitivity, improving low dose precision and enabling thin layer dosimeters; possibility to re-analysis the dosimeters as very little of the signal is lost in each read. In physics, optically stimulated luminescence (OSL) is a method for measuring doses from ionizing radiation [6,7]. Radiation Dosimetry, which is the measurement of accumulated radiation dose in the tissues of health care, nuclear, research and other workers, as well as in building materials in regions of nuclear disaster. Optically stimulated luminescence (OSL) is a process in which a pre-irradiated (exposed to ionizing radiation) material when subjected to an appropriate optical stimulation, emits a light signal proportional to the absorbed dose. The wavelength of the emitted light is the characteristic of the OSL material.

Types of TLD Bagdes
1. Chest badge: It is used for estimation of the whole-body dose, worn at the chest level. 2. Wrist badge: It has a strap to be worn around wrist. It is used to estimate the dose to hands and to the forearms when they are likely to be selectively exposed. consists of 92.5% of Li-7 and 7.5% of Li-6. Each card consists of four/two chips. In this project, five hundred (500) new two elements TLD 100 were used. The TLD cards are fixed in their holders which consist of two aluminum filters of different thicknesses; 0.07 mm (for skin dose measurements) and 10 mm (for deep dose measurements). One corner of the TLD card is notched to make sure it is placed correctly in the Reader holder system. The Reader holder system protects the cards from the environment and keeps the filtration media which attenuate different types of radiation in order to ensure selective en-trapments of the TLD-100 [3,9] (Figure 5, Figure  6 and Figure 7).
Apart from the chips, each card has a unique barcode.

Cs-137 calibration source:
The Air kerma rate (k air ) of the Cs-137 source used in the Secondary Standards Dosimetry Laboratory for the calibration was 1. mSv/hr [9,10].    c. The dosimeters were then exposed to a known radiation source (normally Cs 137) within two hours after annealing.
d. The dosimeters were again stored in a shielded enclosure for one hour at a temperature below 30 °C.
e. The dosimeters were read for calibration purposes.

Generation of Calibration Dosimeters
This procedure established the Element Correction Co-efficient (ECC) of these cards without reference to the Reader [12][13][14]. These cards were retained for subsequent re-calibration of the Reader. Below were the procedures in generating Calibration Dosimeters: 1. The cards were loaded into the reader and set the Reader function on the Result Screen to a "Remote Operation".

2.
From the WIN REMS main menu on PC, "Data Acquisition" was selected. From the Data Acquisition Menu, "Time Temperature Profile (TTP)" was also selected.
3. The TTP was set to the required condition and return to the Data Acquisition Menu.

4.
From the Data Acquisition Menu, "Acquisition setup" was selected. The Acquisition setup was set to the required conditions. 5. "Read Cards" was selected and then "Enter", this brought up the Read Cards Screen. A unique group Number and the appropriate TTP number was entered.
6. "Function Field" was selected. From the pop-up menu "Generate Cal. Dosimeters" was selected.
7. "Start Reading" was selected and the comment prompt was responded, then "Start". The Reader began to read the Cards and display the data in accordance with both the setting in the TTP and Acquisition setup screen.

8.
After completing the Reading process, "QA Procedure" was selected from the Main Menu; then "Generate Calibration Cards" was selected. Then the same Group number assigned earlier on and an acceptable ECC range for each position (ii and iii for a two-element position cards) were entered and the ECC calculations was initiated.
9. When the calculation process was completed,

Methodology
Calibration of Harshaw TLD 6600 plus reader: Full calibration of the Harshaw Model TLD 6600 PLUS required calibrating both the Reader and all the TLD cards used in the system [5]. Before setting up the system it was important that the following directions and steps were carefully followed; (i) Dosimeter Preparation.
(iii) Calibration of Reader.
(iv) Calibration of new TLD Field cards.

Dosimeter Preparation
Before beginning any calibration procedures, the new dosimeters acquired were sensitized. The dosimeters were annealed and irradiated for a minimum of three cycles to sensitize the TLD centers [11]. The next step was to set a fixed fade time between irradiation and readout for all dosimeters. Any length of time that suits the operation was acceptable, but it had to be consistent from one time to the next. The Procedure was as follows: a. The Reader annealed the Dosimeters to clear them of all residual exposure.
b. After annealing, the dosimeters were stored in a shield enclosure for one hour at a temperature below 30 °C before irradiation.

4.
A unique Group Number was entered and the number of the TTP being calibrated which has been set to the required conditions and the cards were read.

5.
On the Reader Calibration Screen, the Nominal Irradiation Value (1000 uSv) to which the cards were exposed was set. Also enter the Four (4) TLDs were used as golden cards to establish the Reader Calibration Factor (RCF). The ECC range for calibrating the reader was between 0.9 and 1.1 allowing for only ± 10% deviation.
The RCFs for elements ii and iii were established as 0.059079 and 0.047272 (Figure 8, Figure 9 and Figure 10). a multiplier with the reader output (charge in Nano coulombs) to make the response of each dosimeter comparable to the average response of a group of calibration dosimeters [15,16]