Thermoluminescent Dosimeter (TLD) - Everything You Need to Know

Reviewed by Mirela Kirr and Joel H. White, ME, Nuclear Engineering

What is a thermoluminescent dosimeter (TLD), and what does it do? In this article, we'll deep dive into everything you need to know about TLDs and their importance in radiation dosimetry.

Key Takeaways

• Purpose of TLDs: Thermoluminescent dosimeters (TLDs) measure and record ionizing radiation exposure, commonly used in workplaces where radiation is present.
• How TLDs Work: TLDs absorb radiation, store energy, and release it as light when heated, with the light emitted proportional to the radiation dose.
• Types of TLDs: TLDs come in different forms, including whole body, ring, wrist, and fetal badges –all designed to monitor specific body parts. Organizations can also use TLD area monitors to measure public radiation dose.
• Importance: TLDs ensure radiation doses remain within safe limits, helping protect workers' health and comply with regulatory standards.

What Is a Thermoluminescent Dosimeter?

A thermoluminescent dosimeter, more commonly called a TLD, is a type of radiation dosimeter. An occupational worker would use a TLD to monitor radiation exposure within the workplace over a period of time using a dosimetry service.

How TLDs Work

As ionizing radiation passes through a TLD, electrons in the material move into dosimetric traps and stay there until the detector heats up. Once the temperature reaches a maximum of approximately 400°C, the electrons in these dosimetric traps begin to move. This movement emits a light pulse, also known as luminescence. A photomultiplier tube (PMT) counts the amount of light emitted – this amount is proportional to the amount of ionizing radiation exposure the dosimeter received.

A TLD is considered a passive radiation detection device and has become the most commonly used solid-state radiation detector for personnel monitoring.

TLDs and the International Atomic Energy Agency

The thermoluminescent dosimeter was invented in 1954 by Professor Farrington Daniels at the University of Wisconsin-Madison.

Since 1969, the International Atomic Energy Agency's dosimetry program has operated a service to validate the calibration of radiation beams. The IAEA mainly focuses on cooperation in the nuclear field and seeks to promote the safe, secure, and peaceful use of nuclear technologies worldwide.

TLDs are a very popular area of study, and researchers often test TLDs' energy response – specifically the difference between standard TLDs and high-sensitivity TLDs – to observe how radiation interacts.

Energy Response: Types of Energy Measured

Thermoluminescent dosimeters measure ionizing radiation, including gamma rays, X-rays, and beta radiation exposures. TLDs measure the amount of visible light emitted from a crystal in the detector.

The most common types of thermoluminescent dosimeters are lithium fluoride, lithium borate, calcium sulfate, and calcium fluoride. Manufacturers couple each type of TLD material with one or more impurities to produce trap states for energetic electrons.

TLDs emit light photons proportional to the irradiation dose when heated. Irradiation, you'll remember, is the process by which an object is exposed to radiation – that's how TLDs measure the energy response.

TLDs can respond to beta, photons (gamma and X-rays), and neutron fields depending on the impurities added to the radiation detector.

Ionizing Radiation Dose and Thermoluminescent Dosimeters

Radiation dose is the measurement of how much energy is deposited into a material from a radiation source. A TLD is capable of measuring the effective dose equivalent at a specific depth and area of the body:

Deep Dose Equivalent = Hp(10)

• The external exposure of the whole body (including the head, upper arms above elbows, torso, and upper legs above the knee)
• Represents the dose equivalent at a tissue depth of 1 cm (1,000 mg/cm²)

Shallow Dose Equivalent = Hp(0.07)

• The external exposure of the skin of the whole body or the skin of extremities
• Represents the dose at a tissue depth of 0.007 cm (7 mg/cm²)

Lens Dose Equivalent = Hp(3)

• The external exposure of the lens of the eye
• Represents the dose at a tissue depth of 0.3 cm (300 mg/cm²)

To learn more about radiation dose, check out our blog – Dosimetry 101: Everything You Need to Know About Radiation Dosimetry

Radiation Exposure and TLDs

Radiation exposure is broken into two types:

• Non-ionizing radiation includes radio waves, cell phones, microwaves, infrared radiation, and visible light.
• Ionizing radiation includes ultraviolet radiation, radon, X-rays, and gamma rays.

TLD cards measure ionizing radiation. Proper handling and storage of TLD elements ensures accurate radiation exposure measurement and occupational dose reporting.

Thermoluminescent Dosimeter Types

Radiation Detection Company offers a wide range of affordable and comprehensive radiation protection solutions to accommodate the needs of any organization, big or small. We've outlined our personal thermoluminescent dosimeter types in detail below.

The best dosimeter is the one that works for your radiation-emitting equipment and your team. If you need help understanding what dosimeters your organization needs, please contact our world-class customer service team, and they will be happy to provide guidance.

Whole Body TLD

Our whole body TLD badge is a four-element radiation TLD badge with a minimal reportable dose of 10 mrem (0.10 mSv) and a lower limit of detection of 1 mrem (0.01 mSv). These dosimeter badges are used to monitor exposure to a single occupational worker who is potentially interacting with radioactive materials or works around radiation-producing equipment.

Ring Badge TLD

Our XTLD-XBG extremity is a single-element ring dosimeter for extremities. The ring badge is optimal for radiation workers whose job functions potentially require their arms or legs to receive a higher exposure. 

Wrist Badge TLD

Our TLD-XBGN is a four-element radiation TLD badge with a minimal reportable dose of 10 mrem (0.10 mSv) and a lower limit of detection of 1 mrem (0.01 mSv). The badge is fitted with a Velcro strap around the wrist. The wrist badge is ideal for radiation workers whose arms may receive a higher potential exposure.

Fetal Monitoring TLD

Our TLD-XBGN fetal monitor is a four-element radiation TLD badge with a minimal reportable dose of 10 mrem (0.10 mSv) and a lower limit of detection of 1 mrem (0.01 mSv).

Fetal Monitors are available for radiation workers who are pregnant to ensure that the fetus does not exceed a significant radiation dose of 500 mrem for the entire gestational period or 50 mrem per month. Pregnant employees should wear the badge on the abdomen to provide the best estimate of the dose to the embryo/fetus.

Area TLD

Organizations can use area TLD cards to ensure that the public is not exposed to more than 100 mrem/year (above background) from outside a room/building. In addition, they can monitor radiation doses for employees at risk of potential exposure in the workplace.

These TLD cards can be placed in a room where the radiation source is located or near a source that radiation could potentially reach.

Reliable Radiation Monitoring From RDC

TLDs are one of many dosimetry options available from Radiation Detection Company. Want to learn more about RDC’s TLD badge solutions? Please visit our Solutions page to view our full suite of offerings.

Do you have a question that we did not address in this article? Please reach out to our Customer Care team, and one of our specialists will be happy to assist you.

Frequently Asked Questions

What is a thermoluminescent dosimeter used for?

A thermoluminescent dosimeter (TLD) measures and records ionizing radiation exposure by absorbing radiation, storing the energy, and releasing it as light when heated. The emitted light indicates the radiation dose. TLDs are used for occupational monitoring in healthcare, nuclear power, veterinary clinics, dental offices, research labs, environmental monitoring, medical applications, and industrial processes.

What is the purpose of the TLD badge?

The purpose of a TLD badge is to measure and record ionizing radiation exposure for individuals working in environments where radiation is present. TLD badges help ensure that radiation doses remain within safe limits, protecting workers' health and safety by providing accurate monitoring and data for regulatory compliance.

What is the principle of TLD?

The principles of Thermoluminescent Dosimeters (TLD) involve:

1. Radiation Exposure: TLD materials absorb and store energy from ionizing radiation.
2. Energy Storage: This energy excites electrons, which get trapped in the material.
3. Heat Stimulation: When heated, trapped electrons are released.
4. Light Emission: Released electrons emit light (thermoluminescence) as they return to their original state.
5. Measurement: The emitted light is measured to determine the radiation dose.

The amount of light emitted is proportional to the radiation exposure, enabling accurate dose measurement for radiation monitoring and safety.

How does TLDs work?

Thermoluminescent dosimeters (TLDs) measure radiation exposure through a process involving absorption, storage, and release of energy.

What is the most commonly used personal dosimeter?

The most commonly used personal dosimeter is the thermoluminescent dosimeter (TLD). TLDs are popular due to their accuracy, sensitivity, and ability to measure a wide range of radiation doses. They are widely used in various settings, including healthcare, nuclear power plants, research laboratories, and other industries where radiation monitoring is essential.

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