Thermoluminescent 

Dosimeters

3/1/2011
Contents

Introduction

1. General2. Thermoluminescent Materials

Band Structure and Traps

1. General2. Electron and Hole Traps3. Electron Traps4. Hole Traps5. Energy Absorption and Trapping
Contents

Heating, Recombination and Light Emission

1. General2. Hole Traps as the Recombination Center3. Electron Traps as the Recombination Center4. Effect of Multiple Trap Depths5. Competing Traps

Glow Curves

1. General2. Things that can Affect the Glow Curve Shape3. Handling and Cleaning TL Materials4. UV Sensitivity
Contents

TLD Readers

1. General2. Nitrogen Quenching3. Contact Heating Element4. Heated N2 Gas

5. Infrared lamp Heater

Heating, Annealing and Readout

1. Annealing2. Readout3. Ramp Rate4. Heat Cycle
Contents

TL Material Characteristics

1. Fading2. Sensitivity3. Energy Response4. Supralinearity5. Emission Spectrum

Neutron Dosimetry

1. General2. Harshaw TLDs3. Li-6 and Li-74. Li2B4O7 and CaSO45. LiF Glow Curve6. Albedo Dosimeters 

Appendix: Correction Factors and Algorithms
Introduction
Introduction

1. GeneralWhen scintillators (e.g., NaI, ZnS) absorb energy imionizing radiation, they immediately release some oenergy as light. For each particle of radiation interathe scintillator, a flash of light (scintillation) is produgreater the energy absorbed by the material, the brflash.When thermoluminescent materials (e.g., LiF) absomuch of this energy is trapped rather than released immediately. The material must be heated for this energy to be released as light. 
Introduction

1. GeneralThe total amount of light emitted during the heating reflects the absorbed dose (i.e., the radiation energabsorbed). The emitted light can’t be used to determine the endeposited by individual particles of radiation. It refletotal energy deposited by all the radiation interactioTL material.
Introduction

2. Thermoluminescent Materials

• crystalline solids (chips, rods, pellets, powders)• non-conductors (valence band is full)• thermoluminescence is a fairly common phenom

• most thermoluminescent materials (e.g., NaCl) a

suitable as TL dosimeters

• TL materials suitable for dosimetry: LiF

Li2B4O7CaF2CaSO4Al2O3
Introduction

2. Thermoluminescent Materials

Four LiF chipsandwiched btwo sheets of and mounted aluminum cawith barcodeBulb dosimetmaterials affixa spiral wire theating elemencased in gbulbs. 
Band Structure 

and Trapsand Traps
Band Structure and Traps

1. GeneralAll materials contain impurities and defects. When impurities are added intentionally to a TL malatter is said to be "doped." The added impurities are referred to as activators. identity of the activator might appear in parentheseformula for the TL material, e.g., LiF(Mg), or followincolon, e.g., LiF:Mg. Electrons located at these impurities may possess that electrons in other parts of the solid cannot, i.e.possess energies in the band gap. 
Band Structure and Traps

2. Electron and Hole TrapsTwo types of impurities: electron traps 

hole traps 

Electron traps are impurities with insufficient electrocomplete covalent bonds with the surrounding atomUntil filled by an electron, there is a hole at the elecIn the following figures, the hole is symbolized with TL materials have a specific number of traps, e.g., said to have 11 or 12 traps. The idea is that the mahas a specific number of different types of impuritieElectrons at these impurities possess energies at dlevels (depths) in the band gap. 
CONDUCTION BAND

Sh ll T

Band Structure and Traps3. Electron Traps

VALENCE BAND e

(full)

Shallow Trap

+

e 

e

e

e

e

e

e

e

Middle Trap

Deep Trap +
Band Structure and Traps

3. Electron TrapsThe main electron trap of LiF (the trap that accumumost electrons) is believed to be produced by a matrimer. Magnesium is a natural impurity in LiF and is ordina

is

a natural impurity in LiF and is ordina

found at a concentration of 175 ppm.
Band Structure and Traps

4. Hole TrapsHole traps are impurities with more unpaired electroneeded to form covalent bonds with the surroundinIn the case of LiF, the main hole trap is believed to Ti(OH) complex Like magnesium titanium is an n

n complex. Like magnesium, titanium is an n

impurity in LiF. The titanium concentration is believon the order of 10 ppm.The extra electrons at such impurities are more loobound than those forming covalent bonds. Their enare also higher, just above those in the valence ban
Band Structure and Traps

4. Hole TrapsHole traps are impurities with more unpaired electroneeded to form covalent bonds with the surroundinIn the case of LiF, the main hole trap is believed to Ti(OH) complex Like magnesium titanium is an n

n complex. Like magnesium, titanium is an n

impurity in LiF. The titanium concentration is believon the order of 10 ppm.The extra electrons at such impurities are more loobound than those forming covalent bonds. Their enare also higher, just above those in the valence ban
CONDUCTION BAND

Band Structure and Traps4. Hole Traps

VALENCE BAND e

(full) 

e 

e

e

e

e

e

e

e

Hole Trap e
Band Structure and Traps

5. Energy Absorption and TrappingWhen radiation interacts with a TL material, energy transferred to electrons. Some of these electrons asufficient energy to be promoted from the valence bthe conduction band. Once in the conduction band, the electrons are mosome will move to the electron traps. At an electron trap (now filled), the electrons posseenergy than they did before the radiation interaction
Band Structure and Traps

5. Energy Absorption and TrappingElectrons at the empty hole traps fall to fill the holes in the valence band.In effect, the holes have moved from the valence bathe hole traps the latter are now filled This leaves the hole traps. the latter are now filled. This leaves 

trap filled. Some of the electron traps and hole traps are now This represents trapped energy. 
CONDUCTION BAND

Band Structure and TrapsElectron in the valence band absorbs radiation ene

VALENCE BAND e

(full)

Hole trapElectron trap

e

e 

e

e

e

e

e

e

e
CONDUCTION BAND

Band Structure and TrapsThe electron is promoted to the conduction band. Imoves to a positively charged electron trap.

VALENCE BAND

Hole trap

Filled electron trap

e

e

e 

e

e

+

e

e

e

e
CONDUCTION BAND

Band Structure and TrapsExtra electron at hole trap falls to fill hole in the valeband. 

VALENCE BAND

Hole trap

Filled electron trap

e

e

e 

e

e e

e

e

+ e
CONDUCTION BAND

Band Structure and TrapsBoth the electron trap and the hole trap are now fillerepresents trapped energy. The net effect has been an electron from the hole trap to the electron trap. 

Filled hole trap

Filled electron trap

+

e

e 

e

e

e

e

e

e

e

VALENCE BAND

(full)
Heating, 

Recombination andRecombination and

Light Emission
Heating, Recombination and Light Emiss

1. GeneralUpon heating, many of the trapped electrons and hrecombine at what are known as recombination cenThis recombination results in the emission of light. When heating promotes a trapped electron into the

heating promotes

a trapped electron into the

conduction band, the electron moves to a filled hole this case, the hole trap is the recombination center. The recombination involves the electron falling from conduction band to a lower energy at the filled hole most cases this results in the emission of a photon wavelength is on the order of 300-600 nm.
Heating, Recombination and Light Emiss

2. Hole Trap as the Recombination CenterPost exposure. The electron trap and the hole trap 

CONDUCTION BAND

Filled hole 

trap

Filled 

electron trap

+

e

e

e

e

e

e

e

e

e

VALENCE BAND

(full)
CONDUCTION BAND

Heating, Recombination and Light Emiss2. Hole Trap as the Recombination CenterTL material is heated. Trapped electron promoted tconduction band and moves to the filled hole trap. of light is emitted.

+

e

e 

e

e

e

e

e

e

e

VALENCE BAND

(full)

Filled hole 

trap

Filled 

electron trap Light photon
Heating, Recombination and Light Emiss

2. Hole Trap as the Recombination CenterThe pre-exposure situation: electron and hole traps empty. The electrons are in the lowest possible ene

CONDUCTION BAND

e

e

e

e

e

e

e

e

e

e

VALENCE BAND

(full)

Empty

hole trap

Empty 

electron trap
Heating, Recombination and Light Emiss

3. Electron Trap as the Recombination CenterAnother recombination mechanism: In the following the electron trap serves as the recombination cente

CONDUCTION BAND

Filled hole 

trap

Filled 

electron trap

+

e

e

e

e

e

e

e

e

e

VALENCE BAND

(full)

e
Heating, Recombination and Light Emiss

3. Electron Trap as the Recombination CenterThe TL material is heated. An electron in the valencpromoted to the filled hole trap.

CONDUCTION BAND

Filled hole 

trap

Filled 

electron trap

+

e

e

e

e

e

e

e VALENCE BAND

e

e

(full)

e
Heating, Recombination and Light Emiss

3. Electron Trap as the Recombination CenterThe hole trap is now empty and there is a hole (vacthe valance band.

CONDUCTION BAND

Empty hole 

trap

Filled 

electron trap

+

e

e

e

e

e

e

e

e

e

VALENCE BAND

e
Heating, Recombination and Light Emiss

3. Electron Trap as the Recombination CenterThe electron in the electron trap falls to fill the hole in the valence band.

CONDUCTION BAND

Empty hole 

trap

Filled 

electron trap

+

e

e

e

e

e

e

e

e

e

VALENCE BAND

e

Light photon
CONDUCTION BAND

Heating, Recombination and Light Emiss3. Electron Trap as the Recombination CenterPre-exposure situation: electron and hole traps are 

e

e

e 

e

e

e

e

e

e

e

VALENCE BAND

(full)

Empty

hole trap

Empty 

electron trap
CONDUCTION 

BAND

Heating, Recombination and Light Emiss4. Effect of Multiple Trap DepthsPost-exposure situation: electron and hole traps arThree types of electron trap (three depths). 

e

e 

e

e

e

e

e

e

e

VALENCE 

BAND

e

e

e

e

e e e

+ + ++++ Full hole traps

Full electron 

traps
CONDUCTION 

BAND

Heating, Recombination and Light Emiss4. Effect of Multiple Trap DepthsTL material is heated: the initial temperature is only enough to free electrons from shallow traps.

e

e 

e

e

e

e

e

e

e

VALENCE 

BAND

e

e

e

Light photons

e

e e e

+ + + + + +
CONDUCTION 

BAND

Heating, Recombination and Light Emiss4. Effect of Multiple Trap DepthsHeating continues: temperature now high enough telectron from middle trap.

e

e 

e

e

e

e

e

e

e

VALENCE 

BAND

e

e

e

e

e e e

Light photon 

+ e + + + e
CONDUCTION 

BAND

Heating, Recombination and Light Emiss4. Effect of Multiple Trap DepthsHeating continues: temperature now high enough telectrons from deep trap.

+ e

e

e 

e

e

e

e

e

e

e

VALENCE 

BAND

e

e

e

e e

e e e

+

+

Light photon 

+
CONDUCTION 

BAND

Heating, Recombination and Light Emiss4. Effect of Multiple Trap DepthsPre-exposure situation: electron and hole traps are 

e

e

e 

e

e

e

e

e

e

e

VALENCE 

BAND

e

e

e

e e

+ + +

+

e e e

Empty hole traps

Empty 

electron traps
CONDUCTION 

Heating, Recombination and Light Emiss5. Competing Traps

+

Competing traps are so deep that the heating process ceases before reaching a temperature high enough to free the electrons.

VALENCE B

+ e + 

These competing traps reduce the sensitivity of the TL material: the larger the fraction of electrons going to competing traps, the lower the light output per unit dose.

+ e 
Heating, Recombination and Light Emiss

5. Competing TrapsThere is an upper limit on the temperature to which materials can be heated for two reasons:

• Over heating can damage the material’s crysta

structure. This will affect the light output and m

material unsuitable for re usematerial unsuitable for re

-use

• If the TL material gets hot enough, it will emit 

incandescent light that is unrelated to the radiaexposure.
CONDUCTION 

Heating, Recombination and Light Emiss5. Competing Traps

+

With some TL materials, sufficiently high exposures can fill the competing traps and thereby increase the sensitivity of the material to subsequent exposures

VALENCE B

e e e 

exposures. In the case of LiF, the competing traps can be filled by exposures on the order of 70,000 R.

e e 
Glow Curves
Glow Curves

1. GeneralA thermoluminescent glow curve is a plot of light oufunction of a TL material’s temperature (or time durheating). As the temperature increases during readout, waveare producedare produced. Each wave of light manifests itself on the glow curvpeak, and each peak represents a trap. By trap, we mean impurities or imperfections of a stype at which electrons will possess a particular eneband gap. The glow curve serves as the record of a TLD read
Glow Curves

1. GeneralSince LiF is said to have 12 traps, we would expect peaks on the LiF glow curve. However, all 12 peakseen because: 

• Some traps are so shallow that electrons escap

before the TL material is analyzed 

• Different traps might be at such similar depths 

peaks overlap and cannot be resolved (distingu

• Other traps are so deep that electrons in them 

freed during the heating process. These are thcompeting traps.
Glow Curves

1. GeneralOnly three or four peaks are discernible in a LiF gloThe temperature at which a peak is produced reflecdepth of the corresponding trap. A peaks height reflects the number of electrons in t

p g

Temperature (EC)

0 100 200 300

Temperature (EC

0 100 200 

Peak 2

Peak 3

Peak 4

Peak 5 LiF
Glow Curves

1. General

CaF2:Dy CaSO4:Dy

The absorbed dose is related to the total amount of (area under glow curve) emitted during the readout calibration. It is less common, but possible, to estimexposure using the height of the main glow peak.

Temperature (EC)

0 100 200 300

Temperature (EC)

0 100 200 
Glow Curves

2. Things that Can Affect the Glow Curve ShapSome TL materials (e.g. LiF) have glow curves that more susceptible to modification than others. Things that might affect the glow curve shape inclu

• type of activator• type of radiation• rate at which the TL material is heated (ramp ra• thermal and exposure history of the material• handling
Glow Curves

3. Handling and Cleaning of TL MaterialsHandle with vacuum tweezers, not standard tweezecould scratch the material. Do not handle with fingeRinse (not soak) with anhydrous methyl alcohol. Air 4 UV Sensitivity

. UV Sensitivity

The following are very UV sensitive and should be in the absence of UV. Store in opaque containers.

CaF2:Dy (TLD-200)Al2O3 (TLD-500)

CaSO4:Dy (TLD-900)
TLD Readers
TLD Readers

1. GeneralTL materials can be heated using: heating elemen

heated nitrogen infrared light

It is desirable that all parts of the TL material be at same temperature during heating. As such, TL matare usually quite thin (rarely more than 1/8" thick). To permit the temperature to be raised and lowered there should be no large heat sinks in the vicinity of heating chamber or in contact with the TL material. 
TLD Readers

1. GeneralWhen a TL material comes into direct contact with a element or other component of the reader, it might contaminated with any trace chemicals that might bpresent.TLD d th t th TL t i l t h t d TLD readers that expose the TL material to heated 

gas might partially or completely eliminate any direcphysical contact, but any components of the reader exposed to the gas will also be heated. Heating the TL material with infrared light can elimiproblems. 
Heating, Annealing and Readout

2. Nitrogen QuenchingBecause oxidation can lead to a contamination of thmaterial that would affect its response, the heating is usually performed in an atmosphere of pure nitroOxidation can also chan

ge the reflectivit

y in the hea

g y

chamber and affect the amount of light reaching the The presence of oxygen during heating is also undebecause it can enhance the spurious emission of liglight unrelated to a radiation exposure. Because nisuppresses this spurious luminescence, its use is sometimes referred to as nitrogen quenching. 
Digitizer

PMT

0057

Digital R

(arbitra

TLD Readers3. Contact Heating Element

Heating Element

EC

Thermocouple

Te

TLD 
PMT

Filter

Heated TLD

TLD Readers4. Heated N2 Gas

Heated

N2 In 

TLD holder
TLD Readers

5. Infrared Lamp Heater

PMT

TLD Holder 

TLD ElementTLD Element

Infrared 

Heat Lamp
Heating, Annealing 

and Readoutand Readout
Heating, Annealing and Readout

1. AnnealingAnnealing refers to the heating of the TL material foother purpose than measuring the light output.Pre-read Anneal. Performed prior to reading the TLmeasurement of light output). Also known as a pre-post

-irradiation anneal The purpose might be to e

post

-irradiation anneal. The purpose might be to e

light production during readout by unreliable shallow (low temperature peaks)Post-read Anneal. Performed after reading the TLDreferred to as a post-heat or pre-irradiation anneal. purpose might be to ensure that no residual light wemitted the next time the TL material is used. In othwords, ensure that the TL material is a “blank slate.
Heating, Annealing and Readout

1. AnnealingThe following table indicates suggested temperaturdurations of pre-read and post-read anneals. These may have to be much shorter if a large number of Tto be read.

TL Material Pre read Anneal Post read AnnealTL Material Pre

-read Anneal Post

-read Anneal

LiF 10 min at 100 

EC 2 hr at 100 

E

C

CaF

2 (Dy) 20 min at 100 

EC 10 min at 320 

E

C

CaF

2 (Mn) none 1 min at 320 

E

C

Li

2

B

4

O

7 (Mn) 10 min at 100 

E

C none

CaSO

4 (Dy) 10 min at 100 

EC 1 hr at 600 

E

C
Heating, Annealing and Readout

2. ReadoutIn general, temperatures between 100 and 300

oC a

employed to produce the peaks of interest on the gcurve. In the case of LiF, the material is usually heated to 

Excessive temperatures are avoided because they damage the TL material and/or result in the emissioincandescent light, i.e. light unrelated to the radiatioexposure history of the material.
Heating, Annealing and Readout

3. Ramp RateThe ramp rate is the rate at which the temperature TL material is raised. A typical ramp rate is 10

oC/s 

rates can vary from 0.5 to 30

oC/s. 

The ramp rate can affect the relative heights of vari

peaks and it can also affect the temperature at whicpeaks and it can also affect the temperature at whic

peaks are produced: the higher the heating rate ththe temperature at which the peak occurs. Increasramp rate also decreases the dose at which supralicommences. Although the exact ramp rate being employed is noextremely important, it is important to choose a speramp rate and stick to it.
Heating, Annealing and Readout

4. Heat CycleThe following figures show typical heating cycles fogeneral, the post-read anneal is done immediately readout (figure on left). Sometimes the TLDs are cimmediately after readout (figure on right) and annelater as a batch.

Temp

(

EC)

Time (s)

0 20 40 60 80

Temp

(

EC)

Time 

0 20 40 

100300200

100300200

Ramp

Post-read

Anneal

Pre-read

Anneal

Read Read Pre-read Anneal
TL Material 

Characteristics
TL Material Characteristics

1. FadingFading is an undesirable decrease in light output thoccurs between the irradiation and readout of a TLDEven at room temperature, some of the electrons in material might have enough thermal energy to climth tthe 

traps.

Fading is most likely to involve shallow traps where 

energy required to free electrons is least. As such, peaks on the glow curve produced at low temperatupeaks associated with shallow traps) can be expecshow the most fading. 
TL Material Characteristics

1. FadingA glow peak half-life is the time it takes, at ambient temperature, for the area under the peak to decreaone half. For LiF:

Peak 1 associated with the shallowest trap, has a 

of approximately 10 minutesof approximately 10 minutes

Peak 2 associated with the next shallowest trap, 

half life of approximately 10 hours

Peak 3 has a half life of 0.5 yearsPeak 4 has a half life of approximately 7 yearsPeak 5 has a half life of 80 years 

The intense fading of peaks 1 and 2 make them unroutine dosimetry.
TL Material Characteristics

1. FadingThe figure on the left shows a LiF glow curve produimmediately after the exposure. The curve on the riproduced two days after the exposure - note the cdisappearance of peak 2. 

Temperature (

EC)

0 100 200 300

LiF

Temperature (

EC

0 100 200 

LiF
TL Material Characteristics

1. FadingWith some TL materials, light (especially UV) can ethe rate of fading. Such materials are said to be ligsensitive. The following materials are particularly lisensitive: Al

2

O

3, CaF

2:Mn, CaSO

4:Mn

It i i tk

t li ht h t 

It is wise 

to 

keep any exposure 

to li

ght as s

hor

t as p

Indoors, low UV (gold) fluorescsent lights could be employed. Light induced fading can be a concern when TLDs in environmental dosimetry because of the possible exposure to intense sunlight and the long exposure 
TL Material Characteristics

1. FadingLiF with its high temperature main glow peak at 200

less than 10% fading per year.CaSO

4(Mn), due to a low temperature main glow pe

110

oC, exhibits 40% fading the first year! 

LiF

(Mg, Ti)

CaF

2

(Mn)

CaF

2

(Dy)

CaSO

4

(Dy)

CaSO

4

(Tm)

Ca

(M

TLD-100 TLD-400 TLD-200 TLD-900 

Main Glow Peak Temp.

200 EC 300 EC 180 EC 220 EC 220 EC 11

Fading <10%/yr 7% 1st 

24 hrs10% 1st month

10% 1st 24 hrs.16% in 2 wks

little little 40% 

1st 
TL Material Characteristics

2. SensitivitySensitivity is the intensity of the emitted light per unThe greater the sensitivity, the lower the absorbed dcan be measured.All TL materials have the same sensitivity for beta pThe key characteristic that determines a TL materiasensitivity to photons (gamma and x-rays) is its effeatomic number. The higher the effective atomic numthe greater the sensitivity. Nevertheless, sensitivity depends on a number of ofactors, e.g., the number of competing traps, the wavelength of the emitted light.
TL Material Characteristics

2. SensitivityIt is common to report the sensitivity of a given TL mrelative to that of LiF. Sensitivity depends on the enthe radiation, and it is standard practice to report thsensitivity for Co-60.

LiF CaF CaF CaSO CaSO CaLiF

(Mg, Ti)

CaF

2

(Mn)

CaF

2

(Dy)

CaSO

4

(Dy)

CaSO

4

(Tm)

Ca

(M

TLD-100 TLD-400 TLD-200 TLD-900 

Effective Atomic Number

8.2 16.3 16.3 15.3 15.3 15

Sensitivity Relative to LiF for Co-60

1.0 10 30 20 70 7
TL Material Characteristics

2. SensitivityThe sensitivity of a TL material depends in part on hare doped.For example high sensitivity LiF materials are availa(e.g., TLD-100H) that are doped with magnesium, c

d h h LiF M C P Th h 15 ti 

an

d p

hosp

horous: LiF:Mg, 

Cu, 

P. They 

have 15 tim

sensitivity of standard LiF.
TL Material Characteristics

3. Energy ResponseThe accuracy of a TLD's measurement of dose shoindependent of the radiation energy, i.e. the same aof light would be emitted for a dose of 1 rad from 10gamma rays as for 1 rad from 1000 keV gamma rayTh f ll TL t i l i ti llThe response o

f all TL ma

terials is essenti

ally ener

independent for photon energies above 300 keV. ATLDs calibrated with Cs-137 (662 keV) can make aassessments of an absorbed dose due to Co-60 (11332 keV).
TL Material Characteristics

3. Energy ResponseHowever, at energies below 300 keV or so, many Trespond because the light output per unit dose incrthe energy of the radiation decreases. If calibrated with Cs-137 (662 keV), these TLDs wo" t" hi h b b d d th d if th "report" a hi

g

her a

bsor

b

e

d

dose than occurre

d if the 

exposure was due to Am-241 (60 keV).To be energy independent below 300 keV, a TL mamust be tissue equivalent. That is, it must have the effective atomic number as human tissue (ca. 7.42)various TL materials available, LiF and Li

2

B

4

O

7 com

closest to being tissue equivalent.
TL Material Characteristics

3. Energy Response

Relative R

10

CaSO

4

esponse

01

Photon Energy (keV)

10 100 1000

LiF

Li

2

B

4

O

7
TL Material Characteristics

3. Energy ResponseThe following table indicates a TL material’s energy independence (or dependence) by giving its responkeV (where the maximum over-response occurs) rethat at 1173 and 1332 keV (i.e. Co-60). The ideal rresponse is1.0 which indicates that the material is eid d tin

depen

den

t.

LiF

(Mg, Ti)

CaF

2

(Mn)

CaF

2

(Dy)

CaSO

4

(Dy)

CaSO

4

(Tm)

Ca

(

TLD-100 TLD-400 TLD-200 TLD-900 

Effective Atomic Number

8.2 16.3 16.3 15.3 15.3 1

Energy Response (30 keV/Co-60)

1.35 13 12.5 10 10 
TL Material Characteristics

3. Energy ResponseIdeally, TLDs that significantly over-respond at low (e.g. CaSO

4) would be calibrated at the photon ene

they will be used to measure the dose. Alternatively, a filter of the appropriate material (e.g

d thi k i ht b l d th TLD t d 

an

d thi

c

kness might

be place

d over the TLD

to re

du

over-response at low energies. Another option is to estimate the photon energy and apply a correction factor. The energy of the radiatioestimated by comparing the response of a TL mateunder different types of filtration.
TL Material Characteristics

4. SupralinearityThe calibration factor employed to calculate the dosnot vary with the magnitude of the dose.Expressed another way, we want the intensity of themitted by a TL material to be linearly related to dod bli th d d bl th li ht t tdoubling the 

dose 

doubles the li

ght ou

t pu

t.

At low doses, this is always the case. Unfortunatelyoutput per unit dose can increase when doses abovcertain magnitude are exceeded.Supralinearity refers to this increase in light output dose. 
TL Material Characteristics

4. SupralinearityIf the dose to a TL material exceeds that at which supralinearity occurs, the dose will be overestimate(assuming the calibration factor was determined at dose in the linear region).Th hi h th d t hi h li it b i The hi

g

her the 

dose a

t whi

c

h supralinearity 

begins, 

better.LiF is the worst TL material in terms of supralinearitonset of supralinearity with LiF is just above 1 Gy (For this reason, LiF is sometimes paired wit h Li

2

B

4

which supralinearity doesn’t begin until 300 rads or 
TL Material Characteristics

4. Supralinearity

Light Output

( bit it )

LiF

(arbitrary unit

s

)

1

Dose (Gy)

0 1 2 

Onset of supra
TL Material Characteristics

5. Emission SpectrumThe TL emission spectrum of a material should ovemuch as possible with the absorption spectrum of tMost PMTs peak absorption efficiencies are in the 4nm range.

LiF CaF CaF CaSO CaSO CaLiF

(Mg, Ti)

CaF

2

(Mn)

CaF

2

(Dy)

CaSO

4

(Dy)

CaSO

4

(Tm)

Ca

(M

TLD-100 TLD-400 TLD-200 TLD-900 

Emission 

Spectrum 

(nm)

350-600 440-650 350-590 460-590 350-530 450

Peak 

Emission (nm)

400 500 480-575 480-575 450-520 5
Neutron Dosimetry
Neutron Dosimetry

1. GeneralThe errors associated with neutrons dosimetry tend large. Accuracy, even in the best of circumstance, is expebe on the order of ±50% and precision ±30%. The major source of error is our inability to determinneutron energy spectrum associated with the exposRelatively little progress has been made in this area the last 20 years.
Neutron Dosimetry

1. GeneralNatural lithium is a mix of Li-6 and Li-7.Li-6 has a high cross section for neutrons. Li-7 doeTLDs using natural lithium (a mix of Li-6 and Li-7) a

iti t t b f th i Li 6 tt

sensitive to neutrons because of their Li-6 content.

The Harshaw TLD-600 consists almost exclusively The Harshaw TLD-700 consists almost exclusively 
Neutron Dosimetry

2. Harshaw TLDs

TLD-100 LiF (Li-6 & Li-7) neutron sensitive

TLD-200 CaF: Dy neutron insensitivTLD-400 CaF: Mn neutron insensitiv

TLD 600 LiF (Li 6) t iti

-600 LiF (Li-6) neutron sensitive

TLD-700 LiF (Li-7) neutron insensitivTLD-800 Li2B4O7 (Li-6 & Li-7) neutron sensitive

TLD-900 CaSO4: Dy neutron insensitiv
Neutron Dosimetry

3. Li-6 and Li-7Some TLD systems used to estimate the neutron dcontain both TLD-600 and TLD-700 elements.The difference between the readings of the two elerepresents the response to neutrons (assuming tha

i di t b t ti l )

one is responding to beta particles).

The response of the TLD-700 element represents tresponse to photons.
Neutron Dosimetry

4. Li2B4O7 and CaSO4Some TLD systems (e.g., Panasonic) that determinneutron dose contain both Li2B4O7 and CaSO4 elem

The difference between the readings of the two elerepresents the response to neutrons.

The Panasonic TLD determines the neutron dose aThe Panasonic TLD determines the neutron dose a

H = NF (E2 - E4)

The response E2 (Li2B4O7) is to neutrons and gamm

response E4 (CaSO4) is due to gammas only. The 

calibration factor (NF) converts this to a dose equivNot as sensitive as using TLD-600 and TLD-700. 
Neutron Dosimetry

5. LiF Glow CurveIt is possible to evaluate neutron exposures by anadifferent peaks on the LiF glow curve:A TLD is read at two temperatures: first 250oC, then 

The response at 250oC is due to neutrons and gam

Neutron, but not gamma, exposures produce a wepeak(s) around 300oC . The neutron dose is determ

directly from the TL response at 325oC. 

The latter is used to estimate the neutron contributiresponse at 250oC. The response at 250o is correc

neutron contribution and then used to calculate the exposure. 
Neutron Dosimetry

6. Albedo DosimetersMost exposures involve neutrons from 1 eV to 1 MeUnfortunately lithium-containing materials are almounresponsive to anything but thermal neutrons. Any high energy neutrons must be moderated (slowif th t b dif they are to be measured. 

Albedo dosimeters rely on the body to slow down thneutrons and scatter them towards the dosimeter. For albedo dosimeters to work properly, they must close to the body. Changes in the dosimeter positioaffect its response and the accuracy of the measure
Neutron Dosimetry

6. Albedo DosimetersAlbedo dosimeters can be used for area and enviromonitoring if they are mounted next to hydrogenous e.g., a 4” thick piece of Lucite.
Appendix:

Correction Factors andCorrection Factors and

Algorithms
Correction Factors and Algorithms

1. Correction FactorsNo two TLD elements (individual chips) have exactsame light output per unit dose. As such, it is necessary to determine a “rank correcfactor” for each element. When an element's response is multiplied by its rancorrection factor, the corrected response will be the for a given radiation dose as the corrected responsother element.In some cases, it might be acceptable to use an avcalibration factor for a group of batched elements , elements with similar responses.
Correction Factors and Algorithms

2. Desired TLD OutputA TLD dosimeter must assess the dose equivalent (equivalent dose) to the:

• skin/shallow dose (at a depth of 7 mg/cm

2) 

• lens of the eye (at a depth of 300 mg/cm

2) 

• deep dose (at a depth of 1000 mg/cm

2

)

deep dose (at

a depth of 1000 mg/cm

)

If the TL material over-responds to low energy photdosimeter must be able to estimate the photon enecorrect for any over-response. The following section describes the algorithms used Panasonic TLD system to accomplish the above as estimate any contribution from betas.
Correction Factors and Algorithms

3. Typical Panasonic TLDThe typical Panasonic TLD employs four TL elemen

Element 

Response 

Designation

Element Type Filter Thickness Filter Material 

E1 Li

2

B

4

O

7 (Cu) 14 m

g/cm

2

plastic be

1 2

4

7 ( )

g

p

E

2 Li

2

B

4

O

7 (Cu) 300 mg/cm

2 plastic be

E

3 CaSO

4 (Tm) 300 mg/cm

2 plastic be

E

4 CaSO

4 (Tm) 1000 mg/cm

2 plastic and 

lead
Correction Factors and Algorithms

4. Determining the Type and Energy of Radiatioa. The exposure is considered to be only from pho

(gamma rays and/or x-rays) when:

E1/E

2 < 8 and E

3/E

4 > 1.5.

I thi th f th h t i ti t In this case, the energy o

f the p

h

o

tons is estima

t

e

curve on the following slide
Correction Factors and Algorithms

4. Determining the Type and Energy of Radiatioa. 
Correction Factors and Algorithms

4. Determining the Type and Energy of Radiatiob. High energy photons are indicated when E

3/E

4 < 

In this case, the shallow dose, dose to the lens oand the deep dose are equal. 

Th d i d t th th d th i th The 

dose assigne

d a

t these three 

depths is the r

element E

2. 

Element E

2 provides the most accurate estimate 

dose because Li

2

B

4

O

7 is tissue equivalent and E

E1, is covered with enough plastic for electronic e

to exist. 
Correction Factors and Algorithms

4. Determining the Type and Energy of Radiatioc. Low energy photons are indicated when E

3/E

4 > 

In this case, shallow dose is greatest, the dose tof the eye is next, and the deep dose is the loweThe e

quations that calculate these doses em

plo

q p

response E

2. They also take into account the ra

The smaller this ratio, the higher the gamma enethe more similar the three doses. In the extreme case where E

3/E

4 > 24 (very low 

rays), the shallow dose is estimated to be 1.85 Eto the lens of the eye is 1.43 E

2, and the deep do

E

2.
Correction Factors and Algorithms

4. Determining the Type and Energy of Radiatiod. A beta contribution to the dose is indicated whe

Betas are also indicated if E

3/E

2 is less than 1.5 

is greater than 1.1. When the dose results from both beta and gamm

t ib ti i d t i d b E

gamma con

t

rib

ution is 

d

e

termine

d

by 

E

4. 

The filtration over this element is sufficiently thicprevent a response to the betas. However, sincelement is CaSO

4, it may over-respond if the ga

component is low energy. Fortunately, the lead E

4 flattens the response at low energies.
Correction Factors and Algorithms

4. Determining the Type and Energy of Radiatiod. The shallow beta dose is based on E1 (beta+ga

minus E

4 (gamma only). The equation includes 

factor (BF) that takes into account the beta ene

Shallow beta dose = BF (E1 – E

4

)
Correction Factors and Algorithms

4. Determining the Type and Energy of Radiatioe. The dose to the lens of the eye is only calculate

energy betas are present. The latter is indicated when the ratio (E

2 - E

4)/(E

greater than 0.25. In this case:

beta dose to the lens of the eye = 0.12 (E1