TLD

Done by: Fatma Al-GafriSr. Medical Physicist

Royal HospitalSultanate of Oman

Outline

o Radiation

o Radiation Detection Devices

o Personal Dosimeters

o What is TLDs

o How does it work

o Types of TLDs

o Advantage & Disadvantage of TLDs

Radiation

o Alpha (α) and Beta (β) particles.

– Do not travel very far.

o Gamma (γ) and X-ray.

– Travel further than Alpha or Beta.

o Radiation that has enough energy to break chemical bonds is referred to as “ionizing radiation”.

o Ionizing radiation is potentially damaging to cells/ tissue.

Radiation Detection Devices

o Instruments that can identify the presence of radiation:

• In the environment

• On the surface of people (external contamination)

• Inside people (internal contamination)

• Received by people as exposure

o There are many types of radiation detection devices. • No single device can detect all kinds of radiation.

• No one device is useful in all situations.

External Contamination

Partial contamination Whole body contamination

Internal contamination

Digestive system Respiratory system

Open wound

Exposure

Types of exposure: Whole body exposure: Person receives penetrating

radiation, ( no portion of the body is shielded). Partial body exposure: Shielding of sufficient thickness

blocks a significant portion of the person from receiving penetrating radiation.

A measure of the amount of ionizations produced in air by photon radiation

What can radiation devices detect and measure?

o Specific types of radiation:

(e.g., alpha, beta, gamma, neutron).

o Specific levels (ranges) of radiation energy (kV).

o "Counts" per unit time (minute or second)

o Accumulated dose (units of gray or rad)

o Current dose rate (units of gray or rad per unit time)

Personal Dosimeters

• What is a personal dosimeter? – A small radiation monitoring device worn

by persons entering environments that may contain radiation .

• Who should wear a personal dosimeter? – Workers in ( emergency / non emergency)

environments that may contain radiation .– Workers in industrial environments where

radiation is used .

Personal monitoring – Why??

• To ensure dose limits are not exceeded 20 mSv /y for whole body.

• To check that doses are ALARA

• Provides documentation in case of ionization incident / emergency.

Types of personal monitor

Chest badge for whole body monitoring:

has two discs of lithium fluoride.

Finger monitor:

Approximately 20mg of lithium fluoride

Where are personal

dosimeters usually worn?

o Flat badges are usually worn on the torso, at the chest level, but can be worn on the forearm .

o Ring shaped badges can be worn on the finger.

TLDThermoluminsecence Dose meter

o The primary form of personal radiation monitoringdosmoter.

o Thermo luminescence is the emission of light byheat.

o TLD measures ionizing radiation exposure bymeasuring the amount of visible light emitted froma crystal in the detector when the crystal is heated.

o The most commonly used TL phosphors are:o lithium fluorideo calcium fluorideo lithium borate o calcium sulphate.

o They are usually manufactured in the form of chips:o (3.2 mm square by 0.9 mm thick)o pelletso small rods o powder

o Lithium fluoride exhibits suitable characteristics and is commonly used in dosimetry .

Why LiF ????

o LiF atomic number 8.1.

o Which is near to the atomic number of soft tissue (7.4), than silver (z=47).

o Can store energy during exposure to ionizing radiation and subsequently release it as light when heated.

Body TLD

Used for X-ray + gamma ray and beta radiation.

Measured does to the

o whole body 𝑯𝒑(10)

o skin 𝑯𝒑(0.07)

Body TLD comprises :

o TLD Card

o Wrapper

o holder

Contains

o 2 pellets( LiF:Mg,Cu,P).

– Thicker elements: strongly penetrating

– Thinner elements: weakly penetrating

– Covered( front + back) with thin retaining layer (PTFE) Polytetrafluoroethylene.

o Cards: bar coded

o Wrapper ( Aluminized polyester) : protect dosimeter from contamination ( chemical+ dirt).

Cont…

o Holder :– Thicker filter (PTFE) : cover thick

element 𝐻𝑝(10)– Circular open window: covered thin

element 𝐻𝑝(0.07)– Rectangular open window: viewing

the wearer information text.

o Printed wearer information includes:– Serial #– Worker #– Change date

o TLD Dosimeters:

– Lithium Fluoride (TLD-100, TLD-600, TLD-700)

– Calcium Fluoride Dysprosium (TLD-200)

– Aluminum Oxide (TLD-500)

– Calcium Sulfate Dysprosium (TLD-900)

– Calcium Fluoride Manganese (TLD-400)

Whole Body TLD

• The TLD gives a measurement of dose absorbed in the TLD in (mGy) with an accuracy of about 10%.

• However what we need to know is not the absorbed dose in the badge, but the deep equivalent dose 𝐻𝑝(10) in (mSv) in tissue.

How does it work

• TLDs work by storing the energy they receive from the ionizing radiation

• until they are heated to a high temperature (around 250°C). ( reader)

• on heating, the absorbed energy is released in the from of visible light.

• A plot of light intensity emitted against temperature is know as a glow curve.

How it worksOperate by

store E receiving from

IR

E released in the form of Photon

by heating 250 C

Light is collected and

measured

Amount of light being

proportional to the

radiation dose (Sv)

TLD reader

• Heat up the TLD using nitrogen gas (250 C).

• Detect the resulting light emission.

• Calculate the radiation exposure.

• Restores the TLD to the original condition.

TLD reader

Thermoluminescence glow peak that results from heating an irradiated TL

o Incident radiation elevates electrons to trap states.o when exposed to heat electrons return to ground state

(Releasing light ). o Light can be measured and used to determine amount of

radiation (Equivalent Dose) .

Dose recording

o Doses usually monitored every month

o Expired TLD’s sent to UK for evaluation

o Records of doses received sent back to Oman

o Should be no greater than 0.5 mSv (Per month).

o Maximum annual dose limit = 20 mSv (WB)

TLDs Results

Detection Limit

• The detection limit of Dosimeter under laboratory conduction is less than 10 microSv

Tissue equivalence

• The detector absorb radiation energy in the same way and to the same extent as human tissue as does the holder material.

SpanLife

• The dosimeter is capable of retaining the stored dose information for extended periods before assessment with no measurable change in response over at least 6 month.

Re-assessment

of TLD

• TL glow curves of all dosemeter reading are kept for at least 5 years.

Extremity TLD

• The dosimeters are designed to measure doses from x ray , beta and gamma radiations to the skin of the extremities in terms of the radiation quantity 𝐻𝑝 0.07 .

• worn on the fingers or taped to the ankles to measure the external equivalent dose to the extremities i.e. hands, forearms, feet and ankles.

• Extremity TLDs are designed to measure beta radiation at a depth equivalent to that of the sensitive basal layer of the skin (about 0.3 mm).

• Small discs of LiF which are sealed in plastic holders.

• The holder shields the TLD from the lower energy betas (70 keV and less) which can not penetrate to the basal layer.

When to Wear Extremity TLDs

whenever you handle unshielded beta sources.

when working with the whole body shielded except for the hands.

when handling small non-shielded gamma sources.

for decontamination jobs when beta contamination is present.

for Iodine source operations.

• The minimum detectable dose for TLD ring dosimeters :– [ 30 millirem for x-rays and gamma

rays] – [40 millirem for energetic beta

radiation] .

• Both the body and ring badges do not detect radiation from beta emitters with energies less than 250 keV.

• To avoid contaminating your ring badge when using open sources, wear your ring under the glove.

Extremity TLD

Advantage (WB TLD)

o Lithium Fluoride is almost tissue equivalent. o No darkroom processing / chemicals required to obtain result. o Small sizeo Reusableo High sensitivity.

– 15% ( low doses)– 3% (high doses)

o They can be used to measure dose over a long period (months or even a year) .

o Excellent resistance to environment (i.e. temperature, humidity)o Can distinguish between types of radiation by using different

lithium isotopes.

Disadvantage ( WB TLD)

• Re-usable: stored signal is cleared .• The process of reading out eliminates the dose

effect, so it can only be done once.• Not permanent record.• Dust on the detector will glow when heated and

will be recorded by the phototube as a false reading.

• The TLDs are sensitive to exposure by ultraviolet light and therefore must be sealed in a light-tight badge.

Neutron Dosimeters

The dosimeter is designed to measure doses fromneutrons to the whole body and to the skin in terms of the radiation quantities Hp(10) and Hp(0.07)

Radon Dosimeters

• These are used in rare situations where, although radon levels are elevated, no cost-effective remediation measures are available.

• This is usually in underground workplaces.

Guidelines for Use

o Never share your badges or wear another person’s badges.

o Do not intentionally expose badges to radiation.

o No matter how curious you are, do not wear your badges when you receive a medical x-ray or other medical radiation treatment. Your badges are intended to document occupational dose, not medical dose.

storage

o Store your badges in a safe place, at work rather than at home.

o Be sure to store badges away from sources of radiation.

o If you store your badges clipped to your lab coat, make sure that your lab coat (or any other lab coat near it) is not contaminated.

o Store your badges away from sources of heat (some badges such as TLDs show some sensitivity to environmental factors like heat). For example, badges left in cars over hot summer weekends may give false exposure readings.

THANK YOU

TERIMA

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