Personnel Dosimetry: TLDs, OSLs, and Electronic Dosimeters

Objective. Students will describe the operating principles of thermoluminescent dosimeters, optically stimulated luminescence dosimeters, and electronic personal dosimeters, and correctly assign, wear, and exchange dosimetry according to plant access control procedures.

Every worker who enters a Radiologically Controlled Area (RCA) must wear dosimetry that records their radiation exposure. Federal regulations in 10 CFR Part 20 require licensees to monitor occupational dose and to keep exposures As Low As Reasonably Achievable (ALARA). Three dosimeter technologies carry the bulk of this responsibility at US nuclear power plants: thermoluminescent dosimeters (TLDs), optically stimulated luminescence dosimeters (OSLs), and electronic personal dosimeters (EPDs). Understanding how each device works, when to use it, and how to wear it correctly is a foundational skill for every nuclear technician and operator.

Thermoluminescent Dosimeters (TLDs)

A TLD uses crystals—commonly lithium fluoride (LiF)—that have a property called thermoluminescence. When ionizing radiation passes through the crystal, it promotes electrons into higher-energy "trap" states in the crystal lattice, where they remain stored. That stored energy is a direct record of the radiation dose the crystal received.

To read the dose, a laboratory technician heats the crystal using a controlled heating cycle. The trapped electrons release their stored energy as visible light. A photomultiplier tube measures the intensity of that light output, which is proportional to the absorbed dose. After reading, the crystal is re-annealed (thoroughly heated) to clear all remaining traps, resetting it for reuse.

TLD badge assemblies typically contain multiple crystals behind different filtration materials. This filter-crystal arrangement allows the reader to distinguish between beta particles, gamma rays, and neutrons, and to calculate the dose in a way that reflects the energy range of the actual radiation field.

Optically Stimulated Luminescence Dosimeters (OSLs)

OSL dosimeters work on a similar principle to TLDs but are read with light rather than heat. The most common material is aluminum oxide doped with carbon (Al₂O₃:C). Radiation traps electrons in the material just as in a TLD. To read the stored dose, the laboratory stimulates the material with a specific wavelength of laser light. The trapped electrons release a luminescent signal that is measured and converted to a dose value.

OSLs offer several practical advantages: the reading process does not erase all stored information in a single step, so the dosimeter can be re-analyzed multiple times if a dose record needs to be verified. OSL badges are now very common at commercial nuclear plants and are frequently the technology behind the whole-body badge issued at the RCA entrance.

Electronic Personal Dosimeters (EPDs)

Unlike TLDs and OSLs, EPDs give the worker a real-time, continuous readout of accumulated dose and dose rate. Most EPDs use either a silicon diode detector or a small Geiger-Müller tube. The detector generates an electrical pulse each time ionizing radiation interacts with it; the internal electronics count those pulses and convert them to dose units (typically millirem or millisievert) displayed on a digital screen.

Key EPD features for plant operations include:

Preset dose and dose-rate alarms — supervisors or Radiation Protection (RP) technicians set alarm thresholds before a worker enters a work area. When the accumulated dose or instantaneous dose rate reaches the setpoint, the EPD alarms audibly and/or vibrates, directing the worker to immediately stop work and exit the area.
Real-time display — workers can check their accumulated dose at any moment, supporting ALARA decision-making.
Dose logging — many EPDs store a history of dose-rate readings that can be downloaded for review.

The EPD is a direct-reading dosimeter and is used for operational control, but the TLD or OSL badge remains the legal record of dose of record for regulatory reporting purposes.

Wearing Requirements and Dosimeter Assignment

10 CFR Part 20 Subpart C establishes occupational dose limits and monitoring requirements. Plant procedures derived from those regulations specify exactly how dosimetry must be worn.

Whole-body workers wear a primary badge (TLD or OSL) on the front of the torso, between the neck and waist, on the most-exposed side of the body. This badge records deep dose equivalent (DDE), shallow dose equivalent (SDE), and lens-of-eye dose.

Extremity workers performing tasks with significant hand or wrist exposure (such as working directly with radioactive material) are assigned extremity dosimetry—typically a TLD ring badge worn on the dominant hand or the most-exposed extremity.

Declared pregnant workers receive a second, separate dosimeter worn at the waist to monitor fetal dose in compliance with the lower dose limit that applies to the embryo/fetus under 10 CFR Part 20.

High-radiation area (HRA) workers must wear their EPD with alarm setpoints set to values approved by the RP supervisor before entering the HRA. The EPD alarm protects the worker from unexpected dose-rate increases.

Visitors are assigned dosimetry consistent with plant access-control procedures; typically a whole-body badge and an EPD if they enter posted areas.

Exchange and Control of Dosimetry

Dosimeters must never be left in radiation areas when not being worn—stored dose would appear as worker dose on the record. Workers exchange (turn in and receive fresh) TLD/OSL badges on a schedule set by the plant RP program, commonly monthly or quarterly. Badge exchange records are maintained to ensure dose-of-record continuity. Lost or damaged dosimeters must be reported immediately to the RP department; a dose assessment will be performed using area monitoring records and work history.

Safety Notes

Never wear another worker's dosimeter. Dose records are personal legal records.
Never attempt to shield your dosimeter from radiation; doing so falsifies a dose record and is a regulatory violation.
Always verify your EPD alarm setpoints are set before entering any posted area. An EPD with no alarm setpoint provides no dose-rate protection.
Follow LOTO (lockout/tagout) and all applicable PPE requirements for the task; dosimetry monitors exposure but does not reduce it.
Contamination of a dosimeter should be reported to RP staff immediately; do not attempt to decontaminate it yourself.

Key terms10

Thermoluminescent Dosimeter (TLD)A passive dosimeter that records absorbed radiation dose by trapping electrons in a crystal lattice; dose is read by heating the crystal and measuring the emitted light.

Optically Stimulated Luminescence (OSL) DosimeterA passive dosimeter in which radiation-trapped electrons are released by laser light stimulation; the resulting luminescent signal is proportional to absorbed dose and the material can be re-read multiple times.

Electronic Personal Dosimeter (EPD)A real-time, direct-reading dosimeter using a semiconductor or gas-filled detector that displays accumulated dose and dose rate and can alarm at preset thresholds.

Deep Dose Equivalent (DDE)The dose equivalent at a tissue depth of 1 cm (10 mm), used to represent whole-body dose from penetrating radiation; reported on the primary badge.

Shallow Dose Equivalent (SDE)The dose equivalent at a tissue depth of 0.007 cm (7 mg/cm²), used to represent skin and extremity dose from weakly penetrating radiation such as beta particles.

ALARAAs Low As Reasonably Achievable; the regulatory principle requiring radiation exposures to be kept as low as practicable below dose limits, considering economic and practical factors.

Dose of RecordThe legally required official dose measurement for a radiation worker, typically provided by the TLD or OSL badge; used for regulatory reporting under 10 CFR Part 20.

Extremity DosimeterA small TLD badge (commonly a ring badge) worn on a finger or wrist to measure dose to the hands or other extremities when they are more exposed than the whole body.

Alarm SetpointA pre-programmed dose or dose-rate threshold entered into an EPD by an RP technician or supervisor; when reached, the device alarms to warn the worker to exit the area.

Radiologically Controlled Area (RCA)Any area in a nuclear facility where access is controlled to protect individuals from radiation exposure or radioactive contamination; entry requires dosimetry and authorization.

In class: Dosimeter teardown and anatomy review: instructor shows cross-section diagrams and, where available, sample non-activated TLD/OSL badge components; students label components and write a brief explanation of how energy is stored and read. · Scenario-based dosimeter assignment drill: students receive five worker profiles (whole-body worker, extremity work, declared pregnant worker, visitor, high-radiation area worker) and select the correct dosimeter type(s) and wearing location for each, citing the applicable regulatory or plant-procedure basis. · Electronic dosimeter (EPD) reading exercise: using training EPDs or simulators, students enter a simulated RCA, note the preset alarm setpoint, read the accumulated dose display at intervals, and record entries and exits in a simulated access-control log.
Materials: Sample TLD and OSL badge assemblies (non-activated or training units), electronic personal dosimeters or training simulators, 10 CFR Part 20 subpart C reference, sample plant access-control procedure, dosimetry assignment log forms
Check: Written 15-question quiz on dosimeter principles, wearing requirements, and regulatory basis for assignment decisions; 70% passing score required.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.