Thermoluminescent Dosimeter (TLD)

Dosimetry Methods

2011-06-04T08:44:00.000-07:00

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)

Personal Radiation Dosimeter

2011-06-04T08:39:00.000-07:00

What are the dosimeters?
Personal Radiation Dosimeter is an equipment or device wear by a radiation worker while working in radiation field.
A radiation dosimeter or badge does not provide protection but detects and measures radiation that you have been exposed to. The badge or Personal Radiation Dosimeter will detect high-energy beta, gamma or x-ray radiation. Personal Radiation Dosimeter cannot detect low energy beta radiation from some isotopes, including carbon-14, tritium or sulfur-35.

What types of Personal Radiation Dosimeters are there?
It can be of two badges for most radiation workers, Luxel by Landauer (aluminum oxide dosimeter) and TLDs (thermoluminescent dosimeter). The Luxel badge measures whole body dose from x-radiation, gamma radiation and beta radiation. The TLD measures extremity dose (finger, hands etc.) from x-radiation, gamma radiation and high energy beta radiation. The TLD chip is housed in a plastic ring to be worn on your dominant hand. For more details on the proper procedure for wearing dosimetry. Radiation workers who operate x-ray machines, flouroscopy units, certain unsealed and sealed radioisotopes or are exposed to other sources of gamma or high energy beta radiation are generally required to wear one or more dosimeters.
Luxel Body Badges
The Luxel body badge contains a sheet of radiation-sensitive aluminum oxide sealed in a light and moisture proof packet. When atoms in the aluminum oxide sheet are exposed to radiation, electrons are trapped in an excited state until irradiated with a specific wavelength of laser light. The released energy of excitation, which is given off as visible light, is measured to determine radiation dose.
The packet contains a series of filters designed so that the energy and type of radiation can be determined. In order for the radiation type and energy to be determined, the dosimeter must be worn so that the front of the dosimeter faces towards the source of radiation.
Luxel body dosimeters are among the most sensitive dosimeters available. The minimum detectable dose is 1 millirem for x-rays and gamma rays and 10 millirem for energetic beta radiation.
Ring Badges
The ring dosimeter contains a small radiation-sensitive lithium fluoride crystal. When atoms in the crystal are exposed to radiation, electrons are trapped in an excited state until the crystal is heated to a very high temperature. The released energy of excitation, which is given off as visible light, is measured to determine radiation dose. This phenomenon is called thermo luminescence and dosimeters that use this principle are often referred to as TLDs (thermoluminescent dosimeters). TLD dosimeters are slightly less sensitive than Luxel dosimeters. The minimum detectable dose for TLD ring dosimeters is 30 millirem for x-rays and gamma rays and 40 millirem for energetic beta radiation.
Deep dose is due to penetrating radiations such as x- or gamma radiation. Deep doses are applied against the whole body dose limit. Shallow dose is due to less penetrating radiations such as beta radiation and low energy x-rays. Shallow doses are applied against the skin dose limit. Dose to the lens of the eyes is due to an intermediate range of radiations and energies and is applied against the lens of the eye dose limit. In the case of ring badges, dose is only reported as shallow dose and is applied against the extremities dose limit.
TLD = Thermoluminescence Dosimeter.
LLD = Lowest Limit of Detection.
OSL = Optically Stimulated Luminescence.
Luxel = Registered trademark of Landauer Inc.
badge can’t be used for detecting/monitoring low dose (below one rad) of radiation. Low dose/level of x-ray is monitored with Geiger-Muller type and other counters and dose is monitored by badges, such as silver halide X-ray film, OSL (optically simulated luminescence) and TLD (Thermo Luminescence) dosimeters. Radiation detectors are expensive and bulky. Film, OSL and TLD dosimeters have capability of monitoring one millirad (0.001 rad) dose while lowest detection limit is about a few rads. The film, OSL and TLD dosimeters are not instant and they need to be sent to an analytical laboratory for their analysis. SIRAD is light weight and instant. SIRAD is a casualty dosimeter for monitoring 1-1,000 rads. In an event of dirty bomb detonation and nuclear accident, one needs to monitor this dose range as soon as possible. As can be seen from the table above, SIRAD is ideal for a high range of doses at a very low cost.

Luminescence Phenomena

2011-01-16T00:58:00.000-08:00

Luminescence Phenomena:-

The luminescence is a process of emission of optical radiation from a material from causing other than heating it to incandescence. Luminescent materials for example luminophors or light bearers can absorb energy, store a fraction of it, and convert it into optical radiation which is then emitted. Luminescence embraces many similar but specific effects, each of which can be described by the addition of a prefix to the term luminescence. The prefix generally but not invariably describes the mean by which the material receives its excitation energy. A number of example of this nomenclature are as follows.

1. The Luminescence effect called Photoluminescence is excited by means of photons of ultraviolet, visible and infrared range of light.
2. The Luminescence effect called Triboluminescence is excited by means of rubbing or grinding.
3. The Luminescence effect called Chemiluminescence is excited by means of chemical energy.
4. The Luminescence effect called Electoluminescence is excited by means of Electric field.
5. The Luminescence effect called Radioluminescence is excited by means of Ionising radiations.
6. The Luminescence effect called Cathodoluminescence is excited by means of Cathode rays.
7. The Luminescence effect called Sonoluminescence is excited by means of sound waves.

Fluorescence which are characterised by prompt emission, Phospohorescence which are known by delayed emission, Thermoluminescence are thermally accelerated emission can be excited by various means and any combination of above source of excitation described for other luminescence processes.But it should be noted that the Thermoluminescence, Fluorescence,and Phospohorescence are the particular forms of luminescence and are not related to means of excitation as means of excitation can vary.

Fluorescence:-

The excitation process of Luminescence involves the transfer of energy to electrons and their displacement to a higher energy stat which is called excited state and represented as E. If electrons return promptly to their original energy stat which ground stat and emit optical radiation, then the process is called Fluorescence.

Phospohorescence :-

If there is electron trap found in excited state E, then electron can not return to ground state immediately but their returning will be after some delay, then the process is call Phospohorescence.

Thermoluminescence:-

The transition of electrons directly from a metastable state to ground state is forbidden. The metastable state represents a shallow electron trap and electrons returning from it to the excited state require energy. This energy can be supplied in the form of optical radiation (photo stimulation) or as heat (thermal stimulation). The probability p of escape of an electron from a metastable state to an excited state is governed by the Boltzmann equation.
p = s exp(-delta E/ kT)
where s is a constant, delta E is difference of energy states E and M (trap depth), k is Boltzmann's constant and T is temperature in kelvin.
The effect of temperature on escape of electron is directly proportional as by rising the temperature the escape of electron is increased. Due to this effect the phosphorescence process is effectively accelerated as the progressively deeper metastable states empty with increase in temperature. This process is called Thermoluminescence.

The difference between Incandescence and Thermoluminescence:-

Thermoluminescence is quit different from the incandescence as in the process of incandescence the application of heat causes vigorous vibration,collision and excitation of all the atoms of a material, resulting in an emission spectrum approximately to that of a black body radiator at the same temperature. But the form of this emission curve is determined by temperature according to Planck's law. In contrast the Thermoluminescence emission spectrum of a material depends on the species of luminescence atoms present.
Thus Thermoluminescence produced as a result of the absorption of ionising radiation in accordance with common practice and emission of optical rays or photon is caused by heat.

fading rate of CaF2 TLD200

2010-08-18T10:02:00.000-07:00

fading rate of CaF2 TLD200
Becker et al., 1971 conclude that the fading rate increases too rapidly with increasing temperature, thus making it advisable not to use it at least in a hot climate, especially considering that other equally or more sensitive phosphors are now available which are thermally much more stable, including CaSO4 :Dy and Mg2 SiO4 :Tb. Furthermore, the properties of CaF2 :Dy are apparently subject to rather large batch-to-batch fluctuations (Sukis, 1971).
Some other activators for CaF2 have also been tested, among them erbium and terbium. Apparently they offer no substantial advantages. A supposedly "pure" synthetic CaF2 sample exhibited peaks at 60 and 360°C, the ratio between both changing during fading, for example, by a factor of two during five days at 10°C. It is, in principle, possible to use this ratio as an indicator of the time of exposure.

fading of gamma induced TL in CaF2: Dy at varios storage temperature

Due to its high sensitivity, CaF2:Dy has attracted some attention as a possible long-term integrating detector for measurements of the environmental (background) radiation. Its value for this purpose has, however, been subject to some controversy because of the necessity of large fading corrections. Some investigators simply ignore the fading, or wait for at least one day prior to readout to avoid the ~25% fading they find during the first day after exposure (Jones et al., 1971 and Lindeken et al., 1973) or they apply, in addition to the one-day delayed reading, a pre-readout annealing procedure, as discussed above (Denham et al., 1972).
Due to the presence of low-temperature peaks, there is a 12% fading during one day at room temperature in the dark; if the sample is kept in diffuse room light, it increases to 30% during one day and 40% in two days. For example, a post-irradiation annealing for 10 min at 80°C reduces the sensitivity by 20%, but improves the long-term storage stability to 12% fading during one month (Binder et al., 1968). Other authors (McCurdy et al., 1969) report 30% fading during ten days and suggest a post irradiation annealing treatment for 10 min at 115°C. Obviously, such a "stabilizing" pre-readout annealing substantially reduces the sensitivity.

Dysprosium-activated CaF2,TLD-200 from the Harshaw Chemical

2010-08-18T09:54:00.000-07:00

Dysprosium-activated CaF2, commercially available as TLD-200 from the Harshaw Chemical Co. and manufactured similar to CaF2: Mn, is substantially more sensitive to gamma radiation than CaF2: Mn in standard TLD readers (Table 2-5). It is, however, not more sensitive than LiF:Mg,Ti to high energy protons or alpha particles. A response of 3 x 10-7 rad gamma equivalent per 735 MeV proton, and of 2.1 x 10-6 rad per 930 MeV alpha particle has been reported (Jones etal.,1971).

Glow curve of Caf2:Dy immediately after exposure to gamma radiation and after a stabilizing prereading heat treatment for 10 min at 115oC.
The phosphor has a rather complicated glow curve with at least four peaks between 110 and ~270°C (Figure 18), as well as two high temperature peaks at ~340°C and ~400°C. The TL emission spectrum exhibits main peaks at 483.5 and 576.5 nm. The gamma radiation response is rather complex: Supralinearity begins at ~800 R, peaks at ~104 R, and begins to show saturation between 105 and 106 R. The response can,however, be made linear by pre-irradiation and annealing for 2 hr at 600°C. The glow curve undergoes complicated changes as the dose in- creases and/or during fading.

Harshaw TLD Materials Specifications (Thermo)

2010-08-17T09:57:00.000-07:00

Thermoluminescence dosemeters (TLD) are widely used to monitor personal doses due to photon fields in the radiation protection of workers.
The Thermo Scientific:

The Thermo Scientific is Themoluminesent Dosimeter cards and Themoluminesent Dosimeter cards and TLD chip reader manufecturing company. Themoluminesent Dosimeter Cards (TLD cards) manufectured by Themo Scientific have following specifications:

Handling and thermal treatment of Harshaw TLD Materials Specifications (Thermo):
Consistent, well-controlled and repeatable procedures are the key to successful TLD. Variations in annealing temperature will affect dosimeter sensitivity, for example. The following guidelines are advisable to optimize the reproducibility of bare dosimeters.
Handling of Harshaw TLD Materials Specifications (Thermo):
Vacuum tweezers should always be used. (Avoid mechanical tweezers or fingers). Small scratches, loss of mass or foreign deposits affect light emission).
Cleaning Harshaw TLD Materials Specifications (Thermo):
Rinse the dosimeters in analytical grade anhydrous methyl alcohol between normal uses. (Do not soak). Dry by leaving to evaporate for at least one hour. Anneal once before actual use, accurately following the established procedure. The anneal will also assist in removing any residual methylalcohol.
Sensitivity to Ultraviolet Light of Harshaw TLD Materials Specifications (Thermo):
Calcium Fluoride Dysprosium (TLD-200), Aluminum Oxide (TLD-500) and Calcium Sulfate Dysprosium (TLD-900) are extremely sensitive to UV light. These materials should be handled and used in the absence of UV light and stored in opaque containers. Calcium Fluoride Manganese (TLD-400) is moderately UV light sensitive.
Limiting temperatures of Harshaw TLD Materials Specifications (Thermo):
Temperature Significance
240 °C limit for LiF:Mg,Cu,P materials
300 °C limit for PTFE encapsulation
400 °C limit for Kapton encapsulation

Annealing of Harshaw TLD Materials Specifications (Thermo):
For annealing temperatures up to 400 °C, the containers should be made from high temperature stainless steel , oxidized aluminum, preferably thin to assist rapid cooling following annealing. (Do not use non-oxidized aluminum). The use of a dedicated annealing oven reduces the risk of contamination by foreign material. Place the annealing containers on open oven racks with air space all round to avoid inconsistent heat gradients. (Do not stack containers or allow them to touch the oven walls).
EXT-RAD dosimeters

Special features of TLD-100 (LiF:Mg,Ti)

• Nearly tissue-equivalent

• ± 15% sample-to-sample uniformity

• Repeatability to within 2% or better
REFERENCES

1. International Commission on Radiological Protection. General Principles of Monitoring for Radiation Protection of Workers. Publication 35 (Oxford: Pergamon) Ann. ICRP 9(4) (1982).

2. International Commission on Radiological Protection. The 1990–1991 Recommendations of the International Commission on Radiological Protection. Publication 60 (Oxford: Pergamon) Ann. ICRP 21(1–3) (1990).

3. Christensen, P., Julius, H. W. and Marshall, T. O. Radiation Protection 73: Technical Recommendations for Monitoring Individuals Occupationally Exposed to External Radiation. (Luxembourg: Commission of the European Communities) CEC Report EUR 14852 EN (1994).
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Rexon UL-320 Series TLD Readers

2010-08-12T23:22:00.000-07:00

Rexon UL-320 Series TLD Readers
Rexon TLD readers are DIRECT CONTACT PLANCHET Heating Processes
ALL TLD Dosimeters And Even Powder
· Superior Design-Lower Cost-User Friendly
· Rexon TLD readers has Bar Code Scanner ,
Rexon TLD readers has Auto N2 shut-off and 10 Nodes for Temperature Profiling
· Rexon TLD readers Displays Digitized Glow-Curve And Temperature File
· Unlimited Configuration And Calibration For Any Unit of Measurement. Simple And Accurate Analysis
· Bar Coded Planchet For Track Of Dosimeters
· Verification For A Built-In Quality Control Check
· Reference Light Source For Real-Time PMT Drift Correction & HV adjust in 1-volt increments
· Rexon TLD readers has Dosimeter Correction Factor Storage And Automatic Call-Up
· Automatically Processes Up To 185 Dosimeters - More with Extended mags. optional.
· Option For Weight Input From Electronic Balance for Powder Samples. PC Programmable Powder
Vibrator.
· Rexon TLD readers has Internal µProcessor-RS 232 Connection to PC or Laptop
UL-320

· Rexon TLD readers are Windows® XP compatible Software
Rexon TLD system uses TLD Material Compatibility:
Chip, Rods, Discs, Pellets, PC Compatible
Rexon TLD system has Teflon-Phosphor( Chips, Rods, Discs).
Capacity: Up to 185 dosimeters (With Disc type Planchets) RS-232 Serial Interface
Computer Requirements: Windows XP, 2000, NT, 98 Larger Capacity Magazine are available. VGA Display Cycle Time: Variable: 0 to 3 min. 8 Mg of RAM (min. req.)
Adjustable in 1 second intervals. Accessories: Bar-coded planchet
Test Light: Stable LED reference light for real time Pmt drift Computer System
compensation. Load Magazine
Rexon TLD system has Environmentally safer than 14CO types Eject Magazine

Rexon TLD system Indicators and controls:
Rexon TLD system Internal regulator eliminates external N2
Rexon TLD system flow control requirement,
N2 solenoid valve
Flow indicator (SCFH)
Low-flow detection sensor
Rexon TLD system On-off switch
User programmable anneal temperature
and duration
Power: 115 VAC/230 VAC @ 50/60H
Dimensions: 50 cm (h) x 28 cm (w) x 51 cm (d)
19.5 in (h) x 11 in (w) x 20 in (d)
Weight: 22 kg, 48.5 lb

Panasonic UD-7900M TLD SYSTEM

2010-08-11T03:46:00.000-07:00

UD-7900M TLD SYSTEM SPECIFICATIONS
Optical Heating System of Panasonic UD-7900M TLD SYSTEM:
( Non-Contact)
Extremely fast non contact heating requiring no gas or annealing ovens. The heat is localized for high
operational reliability.
Simplified Operation Panasonic UD-7900M TLD SYSTEM :
One-press automatic start.Minimal training for operators and technicians.Very easy to service and maintain.
Batch Measurement of Panasonic UD-7900M TLD SYSTEM :
With the Built In Auto-changer batch measurement of 500 badges is possible . This saves time as there is no need to manually feed badges into reader.
Selectable Heating Mode of Panasonic UD-7900M TLD SYSTEM :
Digital heating control of power and time. UD-710A and UD-716A heating emulation. No need to change hardware, connections, or lamp.
• Built in Glow Curve Interface of Panasonic UD-7900M TLD SYSTEM .
• Built in Electronic Real Time Measurement of Panasonic UD-7900M TLD SYSTEM of Heating Temperature Profile.
• Power Supply (factory preset) of Panasonic UD-7900M TLD SYSTEM
100/115/220/230/240V AC/50-60Hz
• Windows based Control Software ; PC and
Monitor are included
• Communications to Host PC or Mainframe is
easy through built in RS232 Interface
• Uses field proven and reliable UD-710/UD-
730 mechanics

System Information of Panasonic UD-7900M TLD SYSTEM
* Next Generation Automated TLD Reader with Built-In Autochanger
* Increased Reliability and Performance
* Patented Panasonic Non-Contact Optical Heating of TLD’s
* Processes all Panasonic UD-800 Series TLD’s
* Pentium Based PC Control and Processing of TLD's
* Uses Field Proven and Reliable UD-710 / UD-730 Mechanics
* SVGA Color Monitor Display of Operational Data, Parameters, and Glow curves
* Standard IBM PC Keyboard and/or Mouse for input of Operational Data
* Microsoft Windows OS and User Friendly Graphical Interface
* High Speed 19.2K+ Baud RS-232C Communications to Host PC or Mainframe
* Built-In Glow Curve Interface
* Selectable Fast (UD-710) or Slow (UD-716) “Soft Cycle” Type Heating
* Patented Electronic Real-Time Measurement of Heating Temperature Profile
* PCI Bus Cards Replace Device Controller, PDP Controller, and Lamp Driver Printed Circuit Boards
Physical
Electrical Characteristics of Panasonic UD-7900M TLD SYSTEM

Input Voltage: 120 / 240 VAC
Main supply voltage not to exceed ±10% nominal voltage
Overvoltage Category II
Input Frequency: 50 / 60 Hz
Max Input Power: 400 W

Measuring Range of Panasonic UD-7900M TLD SYSTEM :
Li2B4O7 10mR ~ 1000R (100μSv ~ 10 Sv)
CaSO4 1mR ~ 50R (10μSv ~ 500mSv)
TL Badge Loading of Panasonic UD-7900M TLD SYSTEM :
Automatic loading with Built In Auto-changer for 10 magazines 50 badges per magazine x10 magazines = 500 badges/loading
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preparing of cement for tld,tld detectors,tld dosimetry disavantages,tld fading dose supra ,tld field reader radiation,tld harshaw heating method,tld materials and systems, panasonic tld dosimetry,advantage and disadvantages of thermoluminescence detection mechanisms,advantages and disadvantages of personal exposure measurement

Direct Read Pocket Dosimeter and Digital Electronic Dosimeter

2010-08-06T03:41:00.000-07:00

A direct reading pocket ionization dosimeter is generally of the size and shape of a fountain pen. The dosimeter contains a small ionization chamber with a volume of approximately two milliliters. Inside the ionization chamber is a central wire anode, and attached to this wire anode is a metal coated quartz fiber. When the anode is charged to a positive potential, the charge is distributed between the wire anode and quartz fiber.
Another type of pocket dosimeter is the Digital Electronic Dosimeter. These dosimeters record dose information and dose rate. These dosimeters most often use Geiger-Müller counters. The output of the radiation detector is collected and, when a predetermined exposure has been reached, the collected charge is discharged to trigger an electronic counter. The counter then displays the accumulated exposure and dose rate in digital form.

Advantages and Disadvantages of TLD

2010-08-06T03:34:00.000-07:00

Advantages of Thermoluminescent Dosimeter TLDs:
Thermoluminescent dosimeters (TLD) are often used instead of the film badge. Like a film badge, it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received, if any. Thermoluminescent dosimeters can measure doses as low as 1 millirem, but under routine conditions their low-dose capability is approximately the same as for film badges. TLDs have a precision of approximately 15% for low doses. This precision improves to approximately 3% for high doses. The advantages of a TLD over other personnel monitors is its linearity of response to dose, its relative energy independence, and its sensitivity to low doses. It is also reusable, which is an advantage over film badges. However, no permanent record or re-readability is provided and an immediate, on the job readout is not possible.
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 radio frequency 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)
Instead of reading the optical density (blackness) of a film, as is done with film badges, the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured. The "glow curve" produced by this process is then related to the radiation exposure. The process can be repeated many times
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Annealing Procedures of Lif TLD chips and LIF cards

2010-08-06T03:14:00.000-07:00

Thermoluminescence Dosimeter (TLD) distributions are usually assumed to be normally distributed. Analysis of large TLD data sets shows this assumption to be false, with distributions that exhibit significant low-side tailing. The arithmetic mean therefore may not be the best estimator for the determination of absorbed dose from these measurements. This work suggests that the use of simple robust estimators may substantially reduce measurement uncertainties.
Which annealing cycle for LiF TLD-100 is best: (1) anneal for 400 degrees, one hour, followed by 80 degrees, 24 hours, or (2) anneal for 400 degrees, one hour, followed by 100 degrees, two hours? Which cycle is best for daily use of TLDs and gives the more accurate signal? Why do these different annealing procedures exist?

In order to maintain the same sensitivity for dosimetry purposes, it is important to be consistent in the annealing program used. The general consensus arrived at quite some time ago among various individuals studying thermal effects on LiF, as to the optimum annealing cycle to maintain good sensitivity and consistency in behavior, is the first one you mentioned—that is, 400 degrees C for one hour followed by 24 hours at 80 degrees C.

While the first cycle you note is probably still considered the gold standard, many people use the shorter cycle in routine work, and I see no problem with using that if you are concerned about the rather long time required for the first method. At any rate, I would recommend that you use only one method for annealing. Do not switch among different annealing cycles.

The properties of BeO thermoluminescent (TL) dosimeters have been investigated in the 1.0-6.0 Gy range. Annealing procedure, fading, calibration and TL dependence on energy and dose were studied along with other relevant physical features. Supralinearity phenomena have been found above 0.5 Gy, but repeated measurements showed a marked stability and reproducibility of the TL response curve vs. dose. The BeO TLDs near tissue equivalence, good sensitivity and integrity to high radiation doses, make them especially suitable for a variety of dosimetric purposes in the radiotherapy field.
LiF:Mg, Ti-TLD weist eine erhebliche Strahlenschädigung auf, wenn es zu 106 Gy einer 60Co--Strahlung ausgesetzt wird. Nach einer Nachbestrahlungstemperung bei 400°C von 1 h und dann bei 100°C von 2 h beträgt die Thermolumineszenzempfindlichkeit eines geschädigten Dosimeters etwa ein Viertel seines ursprünglichen Wertes. Es wird versucht, die Strahlenschädigung mit einer Hoch-temperaturtemperung auszuheilen. Durch Probentemperung bei 750°C und 3 h können die Empfindlichkeiten wiederhergestellt werden. Das Verfahren der thermischen Behandlung wird ausführlich beschrieben.
The effects of high absorbed dose on the reusability of CaF2:Mn thermoluminescence

dosimeters (TLDs) are investigated to determine a recommended upper limit on absorbed dose for TLDs that are to be reused. This investigation examines degradation in the uniformity of response and changes in
sensitivity of a batch of TLDs when exposed to gamma-radiation doses ranging from 1 Gy to 1000 Gy, and
confirms earlier work suggesting that CaF2:Mn TLDs should not be reused in applications where cumulative
absorbed doses are likely to exceed 100 Gy. CaF2 TLD were annealed for 1 hour at 4000C and
allowed to ecel at room temperature. This is the annealing procedure normally followed by the RML during
A TLD preparation, and is in accordance with manufacture's recommendations and ASTM standards.

The Harshaw 3500 TLD Reader

2010-08-06T02:57:00.000-07:00

The Harshaw 3500 TLD Reader:
The Harshaw 3500 TLD Reader provides cost-effective measurements of the radiation dose absorbed by individual TLD elements: ribbons (chips), rods, micro-cubes or powders.
The Harshaw 3500 includes a sample drawer for a single element TLD dosimeter, a linear, programmable
heating system and a cooled photomultiplier tube with associated electronics to measure the TL light
output. The manually-operated Harshaw 3500 is used in medical physics, health physics, materials
research, food irradiation and industrial applications.

Key Features of The Harshaw 3500 TLD Reader:
- Planchet heating incorporates welded thermocouple for best temperature reproducibility
- Heating profile includes pre-heat,acquire and anneal cycles
- Heating temperature capability up to 600 °C (1112 °F)
- 7 decade glowcurve acquisition range
- Optional neutral density filter to extend the high measurement range
Advantages of a separate computer with The Harshaw 3500 TLD Reader:

- Minimum initial investment with The Harshaw 3500 TLD Reader
- Extremely flexible parametric adjustments, implemented in software
- The computer can be used for other purposes when not required for TLD
- Use of commercial software for data manipulation, report generation and storage
Applications of The Harshaw 3500 TLD Reader:
- Radiotherapy planning verification
- Total body irradiation dose verification
- Skin irradiation dose verification
- Stereotactic beam output factor measurement
- Critical organ dose verification
- Diagnostic dose studies
- CT dose measurement for quality assurance
- Environmental dosimetry with The Harshaw 3500 TLD Reader
- Testing for irradiated food with The Harshaw 3500 TLD Reader
- Radioactive dating with The Harshaw 3500 TLD Reader
- High dose verification for electronic components Dosimetry performance using LiF;Mg,Ti chips. (TLD-100) Radiations measured: Photon, energies >5 keV; Neutron, thermal to 100 MeV; Electron/beta,
energies >70 keV
Range of The Harshaw 3500 TLD Reader: 10 μGy to 1 Gy (1 mrad to 100 rad) linear; 1 Gy to 20 Gy (100rad to 2000 rad) supralinear with The Harshaw 3500 TLD Reader
Tissue equivalence of The Harshaw 3500 TLD Reader: nearly tissue equivalent
Fading of The Harshaw 3500 TLD Reader: <20% in 3 months without

Harshaw 5500 Automatic Dosimetry Reader

2010-08-06T02:47:00.000-07:00

Harshaw 5500 Automatic Dosimetry Reader
The Harshaw 5500 TLD Reader provides cost-effective measurements of the radiation dose absorbed by
individual TLD elements. The instrument includes an automatic sample changer and carrier disk for
automatic processing of up to 50 TLD dosimeter elements in a single loading.
Product Specifications of Harshaw 5500 Automatic Dosimetry Reader:
Automatic background subtraction capability
Easy to operate, service and maintain
Compact and attractive
Linear contactless hot gas heating
Optional glow curve
deconvolution software
Optional neutral density filters

The Harshaw 5500 has a linear, programmable heating system and a cooled photomultiplier tube with associated electronics to measure the TL light output. The WinREMS Software, which runs on a separate
computer, provides the user interface, the reader control and the applications software.
Harshaw 5500 Automatic Dosimetry Reader Key Features

- Harshaw 5500 Automatic Dosimetry Reader has Thermoelectric PMT cooler for maximum gain stability
- Harshaw 5500 Automatic Dosimetry Reader has Measurement quality assurance
- Harshaw 5500 Automatic Dosimetry Reader has Unattended automatic background subtraction capability
- Harshaw 5500 Automatic Dosimetry Reader is Easy to operate, service and maintain
-Harshaw 5500 Automatic Dosimetry Reader is Compact and attractive
- Harshaw 5500 Automatic Dosimetry Reader has Optional calibration software
- Harshaw 5500 Automatic Dosimetry Reader has Unattended automatic operation for up to 50 dosimeters
- Harshaw 5500 Automatic Dosimetry Reader has Multiple, programmable, linear time-temperature profiles
- Harshaw 5500 Automatic Dosimetry Reader has Heating profile includes pre-heat, acquire and anneal cycles
- Heating by hot gas, temperature capability up to 600 °C (1112 °F)
- 7 decade dynamic acquisition ranges
Light detection system of Harshaw 5500 Automatic Dosimetry Reader

Dynamic range: 7 decades.
Warmup time: 30 min
Linearity of Harshaw 5500 Automatic Dosimetry Reader: <1% deviation
Stability: <1.0 mGy STD DEV of 10 consecutive readings
Dark Current: <50 mGy 137Cs equivalent
Test light: temperature-stabilized LED
Stability: <0.5% STD DEV of 10 consecutive readings
Color: blue (wavelength 470 nm)
Dosimeter heating system of Harshaw 5500 Automatic Dosimetry Reader
Method: linear hot nitrogen gas Time/Temperature Profile (TTP) Temp. reproducibility: (±1 °C)
Environmental requirements of Harshaw 5500 Automatic Dosimetry Reader
Electrical: 100/120/220/240 Vac,50/60 Hz.
Gas: N2, pressure 2 bar (30 psi)±20%, 5.6 l/min (12 scfh)
Operating temperature range of Harshaw 5500 Automatic Dosimetry Reader:0 to 40 °C (32 to 104 ° F)
Storage temperature range of Harshaw 5500 Automatic Dosimetry Reader:-10 to 60 °C (14 to 140 °F)
Shock: withstands 20 mm drop on to concrete surface
Humidity: functions within specifications after 24 hr exposure to 95% RH and subsequent 6 hr recovery
Light intensity of Harshaw 5500 Automatic Dosimetry Reader: maintains specifications while exposed to
light up to 1000 lux with cover in place.
Applications of Harshaw 5500 Automatic Dosimetry Reader
Radiotherapy planning verification
Radiation hardness verification
Total body irradiation dose verification
Harshaw 5500 Automatic Dosimetry Reader

Harshaw Automated TLD Reader 8800 High Capacity

2010-07-24T04:44:00.000-07:00

Harshaw Automated TLD Reader 8800 High Capacity
The 8800 TLD reader is automatic system manufectured by HARSHAW, its important features are as follows:

•The Harshaw 8800 TLD reader Measures beta, gamma, X-ray and neutron doses, singly and mixed with each ohther, means mixed field doses cal also be calculated speratly, bu using this system

•The Harshaw 8800 TLD reader Automatically reads a carousel containing up to 1400 four-element cards at 140 cards per hour , Its speed is quit reasonable.

•The Harshaw 8800 TLD reader have Dosimeters and algorithms meet international accreditation requirements including all DOELAP and NVLAP

•The Harshaw 8800 TLD reader have a unique health physics record system tracks and maintains records

•The Harshaw 8800 TLD reader is Precise, highly controlled, highly reliable linear heating by gas: -Produces uniform heating -Provides better reproducibility -Extends dosimeter life Host computer with VGA color monitor

•The Harshaw 8800 TLD reader uses a hot gas heating method; its capicity is 1400 card at a time.

•The Harshaw 8800 TLD reader have Menu-driven control of "expose", "transport cards", and "diagnostics" Explicit on-screen text -no cryptics

•The Harshaw 8800 TLD reader have Optional 90Sr internal irradiator

•The Harshaw 8800 TLD reader has Optional UPS prevents loss of data during power failures

•The Harshaw 8800 TLD reader have Field-proven reliability

The TLD Model 8800 (HARSHAW) is controlled through screen dialogues, using the mouse to make selections from the drop-down menus. It can also be networked with harshaw TLD Model 4500 and harshaw TLD Model 6600 in discrete or shared dosimetry applications.
In Harshaw TLD 8800 system The PMT Clean-Out Drawer:-
•The photomultiplier tube assembly In Harshaw TLD 8800 system is accessed for inspection or cleaning, as required, via a small clean-out drawer.

•In Harshaw TLD 8800 system Hot gas flow control:-

•The precise flow of hot gas to the four heater tubes for the TLD dosimeter elements is controlled by four precision flowmeters adjusted and balanced during factory setup.

•High Voltage Potentiometers are mounted within the Reader, allowing precise adjustment
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Detectors used for Radiation detection and measurement

2010-07-22T23:12:00.000-07:00

Radiation Detection Devices
The instruments or radiation detectors / monitors used to detect radiation or measure the radiation field are referred to as radiation detection devices. But the Instruments used to measure radiation in terms of doses either for occupational or environmental are called radiation dosimeters. It means that radiation dosimeter is the device which measure the dose / dose rate from the radiation field.

Methods of Detection for the radiation.

There are several methods of detecting radiation because there are different types of radiation are, so to detect them we will use different methods. The method to detect the radiations are based on physical and chemical effects produced by radiation exposure. These methods to detect the radiations are :-

1. Ionization chamber (IONs chamber) to measure or detect the dose rate, active detector

2. Photographic effect , photo graphic films, passive detector

3. Luminescence TLDs, Thermoluminescence passive detector

4. Scintillation Plastic scintillators, organic scintillator, non organic scintillator

Ionization :
The ability of radiation to produce ionization in air is the basis for radiation detection by the ionization chamber. It consists of an electrode positioned in the middle of a cylinder that contains gas. When x-rays enter the chamber, they ionize the gas to form negative ions (electrons) and positive ions (positrons). The electrons are collected by the positively charged rod, while the positive ions are attracted to the negatively charged wall of the cylinder. The resulting small current from the chamber is subsequently amplified and measured. The strength of the current is proportional to the radiation intensity.

Photographic effect :
The photographic effect, which refers to the ability of radiation to blacken photographic films, is the basis of detectors that use film.

Luminescence :
Luminescence describes the property by which certain materials emit light when stimulated by a physiological process, a chemical or electrical action, or by heat. When radiation strikes these materials, the electrons are raised to higher orbital levels. When they fall back to their original orbital level, light is emitted. The amount of light emitted is proportional to the radiation intensity. Lithium fluoride, for example, will emit light when stimulated by heat. This is the fundamental basis of thermoluminescence dosimetry (TLD), a method used to measure exposure to patients and personnel.

Scintillation :
Scintillation refers to a flash of light. It is a property of certain crystals such as sodium iodide and cesium iodide to absorb radiation and convert it to light. This light is then directed to a photomultiplier tube, which then converts the light into an electrical pulse. The size of the pulse is proportional to the light intensity, which is in turn proportional to the energy of the radiation.
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Specification for TLD Model 4500 Manual Readers

2010-07-22T08:35:00.000-07:00

Specification for TLD Model 4500 Manual Readers

1. Processing Time of TLD Model 4500 Manual Readers:
For chipstrates or two-element cards (The TLD card having two chips) it takes only < 35 sec.;
For DXT-RAD or four-element cards (The TLD card having Four chips) it takes only <1 min.
2. Dynamic Range of TLD Model 4500 Manual Readers:
Seven decades
3. Linearity of TLD Model 4500 Manual Readers:
TLD Model 4500 Manual Readers has <1% deviation, above 2 x dark current
4. Test Light Stabilty of TLD Model 4500 Manual Readers:
TLD Model 4500 Manual Readers has <0.5% short-term variabilility based on one standard deviation of 10 consecutive measurements
5. High Voltage of TLD Model 4500 Manual Readers:
TLD Model 4500 Manual Readers has <0.005% short-term variation
6. Radiation Types and Energies of TLD Model 4500 Manual Readers:
Photons>1 keV, neutrons, thermal energy to 100MeV, betas>70 keV
7. Operating Temperature of TLD Model 4500 Manual Readers:
0° to 40°C (32 to 104°F)
8. Storage Temperature of TLD Model 4500 Manual Readers:
-10° to +60°C (14° to 140°F)
9. Humidity of TLD Model 4500 Manual Readers:
After 24 hr. exposure to 95% RH, within specifications after 6 hr.
10. Light Intensity of TLD Model 4500 Manual Readers:
Withstands 1000 lux with cover fitted
11. Dimensions of TLD Model 4500 Manual Readers:
14.75H x 18.25W x 20.5 in.D (370 x 460 x 500mm)
12. Weight of TLD Model 4500 Manual Readers:
77.5 lb. (35kg)
13. Stablity of TLD Model 4500 Manual Readers:
TLD Model 4500 Manual Readers has <1µGy standard deviation for 10 readings of background
Dose-ralated figures apply to TLD-100 (LiF:Mg, Ti) material.
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Harshaw 4500 Manual TLD Reader

2010-07-22T08:23:00.000-07:00

Harshaw 4500 Manual TLD Reader:-
The Model 4500 connects via a serial interface to an external PC through RS-232 serial interface cable, which provides control over the setup, time-temperature profiles (TTPs), analysis and data recording. It needs a seperate computer for data acquisition and control. On the TLD reader 4500 there is a start button and four indicator lights control and monitor the operation. The start button on the TLD 4500 manual reader is used to start the read operation of TLD card or TLD chip within the system. The whole process sequance is indicated by four LEDs on the reader.

Manual and system operations: The harshaw manual TLD reader 4500 (load TLD dosimeter, select group file for setup operations including the Time-temperature profile, define and modify the setup, define and edit the acquisition parameters used during the read operation, enter dosimeter identity and information of dosimeter, and the last step is to start readout of TLD),In the Harshaw TLD system 4500 (provision to apply TTP, associate light output with dosimeter identity and calibration factors, store result for history record, index to second element-pair and heating parameters if necessary)

Harshaw 4500 Manual TLD Reader has Dual photomultiplier tubes and associated electronics enable it to read cards in two positions simultaneously, which saves 50% time for readout if the TLD card has more than two TLD chips.

Harshaw 4500 Manual TLD Reader has provision for Card identification by entering it through keyboard of the computer and there is option to use an external bar code scanner with the TLD system.

Reads 2-, 3-, or 4-element cards and extremity carrier cards using hot gas heating, N2 gas can be used during the heating cycle. This manual TLD read can read the all types of Harshaw TLD cards, those TLD cards having two TLD chips within the TLD card, three TLD chips, and four TLD chip in a TLD card.

The Harshaw 4500 Manual TLD Reader Reads single TLD chips, rods or powders using contact planchet heating. This is the beauty of manual TLD system.

Harshaw 4500 Manual TLD Reader Reads card elements in pairs; the pairs of a 4-element card are sequenced automatically

Harshaw 4500 Manual TLD Reader Interfaces with WinREMS and software options such as Dose Algorithms, Glow Curve Analyzer, Chain-of-Custody and Health Physics Records System

IT is Easy to integrate into existing TLD systems with minimum additional training which is quit optional in some cases.

System hardware and software expansions can be transparent to the user, and maximize the utility of purchased items.

External PC can be used for other applications also. IT can also be used for record keeping.

Capacity to improve system performance as PC upgrades become available

All PC functions are external to the reader.

Minimum initial reader investment.

Energy dependence and Directional dependence of radiation monitors

2010-07-22T03:55:00.000-07:00

Energy dependence
Here we will learn in general the Influence of energies on dosimeters and especially on thermoluminescent dosimeters. The response of a dosimetry system having any type of detector or dosimeter M/Q is generally a function of radiation beam quality or energy of the radiation to which the detector is exposed. Since the dosimetry systems are calibrated at a specified radiation beam quality with a known radiation source, type of radiation source can vary with different types of detector being calibrated and used over a much wider energy range, the variation of the response of a dosimetry system with radiation quality is called energy dependence and this factor later need to be corrected.
Ideally, what we demand is the energy response should be flat. It means that the system calibration
should be independent of energy over a certain range of radiation qualities. But In reality, the energy correction has to be included in the determination of the quantity Q for most measurement situations. Forexample Ιn radiotherapy, the quantity of interest is the dose to water or the dose to tissue infact. As no dosimeter is water or tissue equivalent for all radiation beam qualities. Thus the energy dependence is an important characteristic of a dosimetry system and it should be corrected as much as possible.
So this topic is valid in these situations:
Energy dependence of radiation monitors, Energy dependence of TLD, Energy dependence of GMT, Energy dependence of Ion chamber, Energy dependence of film badges, Energy dependence of pen type dosimeters, even energy dependence and correction in installed area radiation monitors and all detectors.
Directional dependence
As above we study about energy correction, here we will discuss the directional dependence in the response of any type of detector. So first of all we will learn what is the directional dependence is? How the directional dependence changes the results of radiation detectors.
The variation in response of a dosimeter with the angle of incidence of radiation is known as the directional, or angular, dependence of the dosimeter. Dosimeters usually exhibit directional dependence, due to their constructional details, physical size and the energy of the incident radiation. Directional dependence is important in certain applications, for example in in vivo dosimetry while using semiconductor dosimeters. Therapy dosimeters are generally used in the same geometry as that in which they are calibrated. here we learn a basic point, care should be taken in the calibration of radiation detector, calibrate the detectors in the same geometry and direction as they will be used in the field or actual situations of insalling the detectors.
Thus again this information is valid for a wide range of radiation detectors, from ionization chamber to thermoluminescent dosimeters. i-e
Directional dependence of radiation monitors, Directional dependence of TLD, Directional dependenceof GMT, Directional dependence of Ion chamber, Directional dependence of film badges, Directional dependence of pen type dosimeters, Directional dependence CAF2 TLDs, Directional dependence CaSO4 Thermoluminescent dosimeters,
Spatial resolution and physical size
Since the dose is a point quantity, the dosimeter should allow the determination of the dose from a very small volume (i.e. one needs a ‘point dosimeter’ to characterize the dose at a point). Τhe position of the point where the dose is determined (i.e. its spatial location) should be well defined in a reference coordinate system.
Thermoluminescent dosimeters (TLDs) come in very small dimensions and their use, to a great extent, approximates a point measurement. Film dosimeters have excellent 2-D and gels 3-D resolution, where the point measurement is limited only by the resolution of the evaluation system.
Ionization chamber type dosimeters, however, are of finite size to give the required sensitivity, although the new type of pinpoint microchambers partially overcomes the problem.
Spatial resolution and physical size of radiation monitors, Spatial resolution and physical size of TLD, Spatial resolution and physical size of GMT, Spatial resolution and physical size of Ion chamber, Spatial resolution and physical size of film badges, Spatial resolution and physical size of pen type dosimeters, Spatial resolution and physical size CAF2 TLDs, Spatial resolution and physical size CaSO4 Thermoluminescent dosimeters,The Harshaw 8841 TLD comprises three TLD-700H chips (99.7% 7LiF and 0.03% 6LiF by weight) and one TLD-600H chip (4.4% 7LiF and 95.6% 6LiF by weight).

Radiation Dosimeter and properties of dosimeters

2010-07-22T03:46:00.000-07:00

A radiation dosimeter is a device, instrument or system that measures or evaluates, either directly or indirectly, the quantities exposure, kerma, absorbed dose or equivalent dose, or their time derivatives (rates), or related quantities of ionizing radiation. A dosimeter along with its reader is referred to as a dosimetry system. Measurement of a dosimetric quantity is the process of finding the value of the quantity experimentally using dosimetry systems. The result of a measurement is the value of a dosimetric quantity expressed as the product of a numerical value and an appropriate unit.
The progress in passive thermoluminescence dosimetry systems is mainly related to the implementation of tissue equivalent high sensitivity materials such as LiF:Mg,Cu,P. However, the impact of replacing a previous TL system normally based on LiF:Mg,Ti needs to be considered. As an alternative to TLDs, optically stimulated luminescence (OSL) detectors based on Al2O3:C are also becoming important in environmental monitoring.
To function as a radiation dosimeter, the dosimeter must possess at least one physical property that is a function of the measured dosimetric quantity and that can be used for radiation dosimetry with proper calibration. In order to be useful, radiation dosimeters must exhibit several desirable characteristics.

For example, in radiotherapy exact knowledge of both the absorbed dose to

water at a specified point and its spatial distribution are of importance, as well as the possibility of deriving the dose to an organ of interest in the patient. In this context, the desirable dosimeter properties will be characterized by accuracy and precision, linearity, dose or dose rate dependence, energy

response, directional dependence and spatial resolution.
Accuracy and precision
In radiotherapy dosimetry the uncertainty associated with the
measurement is often expressed in terms of accuracy and precision. The
precision of dosimetry measurements specifies the reproducibility of the
measurements under similar conditions and can be estimated from the data
obtained in repeated measurements. High precision is associated with a small standard deviation of the distribution of the measurement results. The accuracy of dosimetry measurements is the proximity of their expectation value to the ‘true value’ of the measured quantity. Results of measurements cannot be absolutely accurate and the inaccuracy of a measurement result is characterized
as ‘uncertainty’.
The error of measurement is the difference between the measured value
of a quantity and the true value of that quantity.
● An error has both a numerical value and a sign.
● Typically, the measurement errors are not known exactly, but they are
estimated in the best possible way, and, where possible, compensating
corrections are introduced.
● After application of all known corrections, the expectation value for
errors should be zero and the only quantities of concern are the uncertainties.
Linearity of dosimeters:
Ideally, the dosimeter reading M should be linearly proportional to the
dosimetric quantity Q. However, beyond a certain dose range a non-linearity
sets in. The linearity range and the non-linearity behaviour depend on the type
of dosimeter and its physical characteristics.
Two typical examples of response characteristics of dosimetry systems are
first exhibits linearity with dose, then a supralinear behaviour, and finally saturation. exhibits linearity and then saturation at high doses.
In general, a non-linear behaviour should be corrected for. A dosimeter
and its reader may both exhibit non-linear characteristics, but their combined effect could produce linearity over a wider range.
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high sensitivity, CaF2:Dy

2010-05-15T14:20:00.000-07:00

high sensitivity, CaF2:Dy

Due to its high sensitivity, CaF2:Dy has attracted some attention as a possible long-term integrating detector for measurements of the environmental (background) radiation. Its value for this purpose has, however, been subject to some controversy because of the necessity of large fading corrections. Some investigators simply ignore the fading, or wait for at least one day prior to readout to avoid the ~25% fading they find during the first day after exposure or they apply, in addition to the one-day delayed reading, a pre-readout annealing procedure, as discussed above (Denham et al., 1972).

Others (Becker et al., 1971) conclude that the fading rate increases too rapidly with increasing temperature (Figure 19), thus making it advisable not to use it at least in a hot climate, especially considering that other equally or more sensitive phosphors are now available which are thermally much more stable, including CaSO4 :Dy and Mg2 SiO4 :Tb. Furthermore, the properties of CaF2 :Dy are apparently subject to rather large batch-to-batch fluctuations (Sukis, 1971).

Some other activators for CaF2 have also been tested, among them erbium and terbium. Apparently they offer no substantial advantages. A supposedly "pure" synthetic CaF2 sample exhibited peaks at 60 and 360°C, the ratio between both changing during fading, for example, by a factor of two during five days at 10°C. It is, in principle, possible to use this ratio as an indicator of the time of exposure.
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advantage of CaF2

2010-05-15T14:15:00.000-07:00

A significant advantage of CaF2 is that it is naturally occurring (as the mineral fluorite) and can thus be cheaply obtained in developing countries. A standard natural fluorite used to be marketed by MBLE, Belgium, although this is now discontinued. One major problem of using natural fluorite is the complexity of its glow-curve. The natural mineral contains many activators, which appear to be predominantly rare earths. Binder & Cameron (1969) introduced Synthetic rare-earth-doped calcium fluoride detectors (CaF2: Dy) and by Lucas & Kaspar (1977; CaF2: Tm). Ginther & Kirk (1957) introduced CaF2:Mn. The glow-curves for MBLE fluorite, CaF2:Dy and CaF2:Mn are shown in figure 16. The fading of the glow peaks in CaF2 is suprisingly high considering their estimated half-lives from E and s calculations and several fading rates have been reported. Considering the processes occurring during isothermal annealing in alkali halides (LiF) it is relevant to recall that impurity clustering has also been observed to influence thermoluminescence in CaF2.
A key problem in the use of CaF2 dosimeters for the measurement of low doses has been the "self-dosing," namely, a build-up of a TL signal without external radiation exposure either in the case of natural fluorites because of the inherent radioactivity of the phosphor (8 to 12 mrad/year self-dosing occur in the M.B.L.E. phosphor) or in the case of glass-encapsulated CaF2: Mn due to potassium content of the glass envelope and/or cement.

A frequently noted problem with CaF2 is the buildup of a significant response within the phosphor during storage periods. This is possibly due to 'self-dose'.The ultra-violet response of CaF2 can be enhanced several fold by annealing at 900oC in air, while the same treatment significantly reduces the sensitivity to ionizing radiation. Pradhan & Bhatt (1983) examine this further and relate the increased ultra-violet sensitivity to surface oxidation and speculate that the reduction in the response may be related to a reduction of the charge state of the rare earth impurities before irradiation.
Synthetic CaF2 :Mn (Mn contentration usually 3 mol %) is generally prepared by coprecipitation of CaF2 and MnF2 from a chloride solution with ammonium fluoride, followed by drying and firing at temperatures up to 1,200°C in an inert atmosphere, grinding of the sintered mass, and sieving to a convenient particle size of ~100 m.
The phosphor has a rather complicated glow curve with at least four peaks between 110 and ~270°C (Figure 18), as well as two high temperature peaks at ~340°C and ~400°C. The TL emission spectrum exhibits main peaks at 483.5 and 576.5 nm. The gamma radiation response is rather complex: Supralinearity begins at ~800 R, peaks at ~104 R, and begins to show saturation between 105 and 106 R.
The photon energy dependence of CaF2:Dy was measured to amount to a factor of about 12 to 17 between 30 keVand 1 MeV. The peak ratio in the glow curve changes, as in most other phosphors, as a function of LET. Due to the presence of low-temperature peaks, there is a 12% fading during one day at room temperature in the dark; if the sample is kept in diffuse room light, it increases to 30% during one day and 40% in two days. For example, a post-irradiation annealing for 10 min at 80°C reduces the sensitivity by 20%, but improves the long-term storage stability to 12% fading during one month (Binder et al., 1968). Other authors (McCurdy et al., 1969) report 30% fading during ten days and suggest a post irradiation annealing treatment for 10 min at 115°C (Figure 18). Obviously, such a "stabilizing" pre-readout annealing substantially reduces the sensitivity.

annealing of Natural CaF2

2010-05-15T14:07:00.000-07:00

A pressure annealing for 5 to 15 miD at 400 to 450°C is suggested, which leaves some of the initially filled very deep traps unannealed. Natural CaF2 is quite sensitive to visible ultra- violet light. As in most other TL phosphors,

1. In pre-irradiated samples, light-induced bleaching (optical stimulation) occurs which is particularly pronounced in the low-temperature peaks; during 1 hr of exposure to 500 lux daylight there is ~10% fading, increasing to ~25% after 4 hr.

2. In unirradiated samples, ultraviolet light induces a TL signal; as the deep traps are usually not completely annealed at a high temperature prior to the first use, there is also some UV- induced trapped electron transfer from deep to shallow traps.

3. In pre-irradiated samples that have been conventionally evaluated by heating briefly to a temperature somewhat above the main TL peak, ultraviolet light exposure causes the acceleration of the normally very slow transfer of electrons from deep, unannealed traps into the shallower traps which are annealed during standard readout.

Thus, a repeated rereading of a previously "read" detector at a reduced sensitivity level becomes possible by the proper use of ultraviolet exposure previous integrated gamma exposures above ~5 rad can be repeatedly reread. In Figure 13, the competing effects of UV- induced TL build'-up, fading, and transfer are illustrated; Figure 14 shows the loss of stored TL information during such repeated transfer and readout cycles, The strong effect of ultraviolet light on natural CaF2 has also been studied as a possible method for the integrating measurement of ultraviolet light, Saturation, followed by a decrease of the TL signal, occurs in a Brazilian fluorite during exposure to 365 nm light. The M.B.L.E. fluorite was found to be most"sensitive at 300 nm, but it exhibits a similar nonlinearity due to the competing reactions. Although the response of fluorite is usually quite linear, a slight supralinearity has been observed in some samples at high dose-levels (in an Indian fluorite, for example, between 1 and 4 x 104 rad). Pre-irradiation to ~6 x 104 rad, followed by annealing for 1 hr at 450°C, increased the sensitivity by a factor of ~4.

The photon energy dependence of CaF2, which can be calculated to amount to an over sensitivity by about a factor of 12 at ~25 keV , is found in experiments to be slightly less (~9) and can be compensated with a good metal filter down to ~35 keV within approximately +15%. The effect of grain size on the calculated energy response becomes rather pronounced for very small grains and may amount to a deviation by a factor of six from the value which has been calculated on the basis of the mass energy absorption coefficients

Calcium fluoride

2010-05-15T13:51:00.000-07:00

Calcium fluoride

It is high Z thermoluminescent phosphors with their less desirable photon energy response. The TL of natural fluorite, a rather widespread mineral, has been the subject of investigations as far back as 1903. During the early years of modern solid-state dosimetry, fluorite was studied under the dosimetric point of view in Germany and Switzerland (Luchner, 1957 and tI Grogler et al., 1958). A complex glow curve with rapidly fading peaks around 80 to 100°C, reasonably stable peaks at 150 to 200°C, and very stable peaks at ~250 to 300°C was reported. In the early 1960's a major effort was made in Belgium by M.B.L.E., a Brussels affiliate of Philips, Eindhoven, to introduce a commercial system based on a fluorite of undisclosed origin. This material has been the subject of intense research efforts not only in the manufacturer's laboratories (Brooke and Schayes, 1967; van Espen, 1968; and I Brooke, 1969), but also by various independent researchers (Harvey, 1967; Aitken, 1968; Fleming, 1968; Adam and Katriel, 1971; and Burgkhardt i and Piesch, 1972). Since the commercial production of this system was discontinued in 1970. More recently, the declining interest in natural fluorite was somewhat revived in developing countries, where a local supply of large quantities of an inexpensive TL phosphor has obvious advantages even if its dosimetric properties are not ideal. As can be seen in Figure, fluorites from different sources may exhibit rather different glow curves. The response of all peaks is fairly linear in all fluorites, with saturation of the peaks occurring at slightly different dose levels in the 104 to 105 rad gamma radiation range. Attempts to correlate the various peaks with certain impurities and to understand the mechanism of TL have been partially successful.

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Aluminum oxide, Al2O3

2010-05-15T13:43:00.000-07:00

Aluminum oxide, Al2O3

The dosimetric properties of this material were first described by Rieke & Daniels (1957) with a later investigation of its TLD behavior by McDougall & Rudin (1970). The emission from Cr3 + free Al2O3 is predominantly at, 410nm, which is a much more desirable wavelength for TLD. The main dosimetric peak in Al2O3 appears at ~ 250oC but the glow curve it self is quit complex figure. Flame treatment (to 200oC) is seen to produce large increases in sensitivity and to introduce new, high temperature peaks into the glow-curve. In particular, a peak at 625°C is produced which appears to have reasonable dosimetric properties.

TLD material Beryllium oxide, BeO

2010-05-15T13:33:00.000-07:00

Beryllium oxide, BeO

Tochilin, Goldstein & Miller (1969) and later Scarpa (1970) looked at the potential usefulness of BeO as a TLD material. The material is seen to have some interesting dosimetric properties including an LET response, which increases with increasing LET (as opposed to the usual decrease), a low thermal neutron response (but a higher fast neutron sensitivity) and good tissue equivalence (Zeff = 7.13). To offset these features, the material also exhibits strong light sensitivity, small linear range, a complex glow-curve structure and (in powder form) high toxicity. The glow-curve shape depends critically upon the dopants (e.g., Al3 +, F-) and the preparation conditions. Glow-curves from hot pressed and sintered BeO are shown in figure.