Dosimetry Devices: OSL, TLD, Film Badge & Pocket Ionization Chambers
Introduction to Personnel Dosimetry
Personnel dosimetry devices measure and record the radiation dose received by radiation workers. Every radiographer who works in an area where radiation exposure is possible is required to wear a dosimeter.
Definition: A personnel dosimeter is a passive or active device worn on the body that quantifies occupational radiation exposure over a defined monitoring period. Results are used to ensure doses remain within regulatory limits and to comply with ALARA principles.
For the ARRT exam, you must know:
- The type of detector material used in each device
- The dose range, advantages, and limitations of each device
- Correct placement and program requirements
- The role of the control dosimeter
The four primary device types tested are: film badge, TLD, OSL, and pocket ionization chamber. The electronic personal dosimeter (EPD) is a secondary device you should recognize.
Device Types, Detectors & Dose Ranges
Definition: The film badge dosimeter uses radiation-sensitive film enclosed in a light-tight plastic holder fitted with metal filters (aluminum, copper, tin, and lead). Comparing optical density under each filter identifies the type and energy of radiation received.
Film is chemically developed and read optically — the degree of darkening (optical density) is proportional to dose received.
- Dose range: approximately 0.1 mSv – 10 Sv
- Advantages: permanent record (film is stored); energy discrimination via filters; inexpensive
- Disadvantages: film can be damaged by heat, humidity, and light; not reusable; typically read only monthly or bi-weekly
Exam Trap: Film badges provide a permanent record because the developed film is retained — but the TLD does not provide a permanent record because heating destroys the stored signal. Know this distinction cold.
Definition: The TLD uses crystals — typically lithium fluoride (LiF) — that trap excited electrons when exposed to radiation. When heated to approximately 220°C, the crystals release trapped energy as visible light (thermoluminescence). Light output is proportional to dose.
- Dose range: approximately 0.1 mSv – 10 Sv
- Advantages: can be reused after reading; accurate over a wide dose range; small and versatile
- Disadvantages: reading destroys the stored dose information (not a permanent record); requires specialized heating/reading equipment
Key Distinction: TLDs are reusable (the crystal can be re-used after reading), but the dose record itself is destroyed during the heating process. This is a classic ARRT distractor — reusable device ≠ permanent record.
Definition: The OSL dosimeter is the current standard in most U.S. facilities. It uses aluminum oxide (Al₂O₃:C) crystals that store energy when irradiated. During reading, a laser light stimulates the crystals to release light proportional to dose.
- Dose range: approximately 0.01 mSv – 15 Sv (sensitive down to 10 µSv)
- Advantages: extremely sensitive; can be re-read multiple times without destroying stored information; provides energy and dose data; typically worn for 3-month periods
- Disadvantages: more expensive than film badge
OSL vs. TLD — Critical Difference: The OSL can be re-read multiple times because laser stimulation does not destroy the stored signal. The TLD cannot be re-read because heating permanently erases the signal. This is one of the most frequently tested distinctions.
Definition: The pocket ionization chamber (pocket dosimeter) is a small gas-filled ionization chamber providing immediate, real-time dose readout. Ionization of the gas moves a charged quartz fiber on an internal electrometer scale, read through an eyepiece.
- Dose range: typically 0–200 mR or 0–600 mR depending on model
- Advantages: immediate readout without processing; can be read throughout the workday
- Disadvantages: can be discharged by physical shock; no permanent record; must be read and reset before each use; less accurate than TLD or OSL
The Electronic Personal Dosimeter (EPD) is a solid-state detector with a digital readout providing real-time dose and dose-rate data with an audible alarm. It is used in high-dose environments and is typically paired with an OSL or TLD for legal dosimetry records — the EPD alone does not satisfy legal record requirements.
Exam Reminder: The pocket dosimeter provides real-time readings but is not used as the official legal dose record. It is a supplemental, field-use device.
Use this table to compare all four primary dosimetry devices for rapid ARRT review:
| Device | Detector Material | Dose Range | Permanent Record? | Reusable? | Real-Time Readout? |
|---|---|---|---|---|---|
| Film Badge | Radiation-sensitive film + metal filters | 0.1 mSv – 10 Sv | ✅ Yes | ❌ No | ❌ No |
| TLD | Lithium fluoride (LiF) crystals | 0.1 mSv – 10 Sv | ❌ No (signal destroyed on read) | ✅ Yes (crystal reused) | ❌ No |
| OSL | Aluminum oxide (Al₂O₃:C) crystals | 0.01 mSv – 15 Sv | ✅ Yes (re-readable) | ✅ Yes | ❌ No |
| Pocket Dosimeter | Gas-filled ionization chamber | 0–200 or 0–600 mR | ❌ No | ✅ Yes (reset) | ✅ Yes |
Placement, Wear Protocols & Special Situations
Standard Placement: The whole-body dosimeter is worn at collar level (outside the lead apron) or at waist level outside the apron, depending on facility protocol.
Placement rules you must know for the ARRT exam:
- Fluoroscopy with lead apron: The dosimeter is worn at collar level, outside the apron — this position records dose to the head, neck, and lens of the eye, which are not protected by the apron.
- Second dosimeter under the apron: A second badge worn at waist level under the apron estimates the protected trunk dose.
- Declared pregnant worker: A second dosimeter is worn at waist level (abdomen) to estimate fetal dose.
- Extremity dosimeter (ring badge): Worn on the finger for workers who handle radioactive materials where hand dose may be elevated.
Positioning Trap: During fluoroscopy, placing the dosimeter under the lead apron at the waist will significantly underestimate the dose to the unshielded head and neck. The collar-level position outside the apron is the correct standard placement for fluoroscopy workers.
Dosimetry programs are governed by regulatory and institutional requirements. The ARRT exam tests knowledge of proper use and program administration.
Exchange & Record Requirements
- Dosimeters must be exchanged regularly — monthly or quarterly depending on device and facility
- The Radiation Safety Officer (RSO) reviews dose results
- Records must be maintained for the duration of employment plus 30 years (varies by state)
- Workers must be notified of their dose results
- NVLAP (National Voluntary Laboratory Accreditation Program) accredits dosimetry processing laboratories
Proper Use Rules
- Never wear another worker's dosimeter — this invalidates both workers' records
- Store dosimeters at the exchange station (low-radiation area) when not in use
- Do not expose the dosimeter to radiation when not being worn
- Return dosimeters on time for processing
Control Dosimeter
Definition: A control dosimeter is a badge kept in the storage area (never worn) that measures background radiation. Its reading is subtracted from worn dosimeters to ensure records reflect occupational exposure only, not environmental background.
Exam Scenario: A radiographer's monthly film badge shows an unexpectedly elevated reading. The RSO checks the control badge — if the control badge also shows an elevated reading, the cause is likely environmental/background rather than occupational exposure. This is why the control dosimeter is essential to program integrity.
Key Concepts Summary
⚠ KEY CONCEPTS — Test-Ready Recall
- Film badge: uses radiation-sensitive film + metal filters; provides a permanent record; damaged by heat, humidity, light; dose range 0.1 mSv – 10 Sv
- TLD: uses lithium fluoride (LiF) crystals; heated to ~220°C to read; reusable crystal but reading destroys the stored signal — no permanent record
- OSL: uses aluminum oxide (Al₂O₃:C); read by laser; can be re-read multiple times; most sensitive (down to 10 µSv); current U.S. standard; worn 3 months
- Pocket dosimeter: gas-filled ionization chamber; real-time readout; no permanent record; can be discharged by physical shock; must be reset before each use
- Fluoroscopy placement: dosimeter at collar level, outside the lead apron — records unshielded head/neck/lens dose
- Declared pregnant worker: second dosimeter at waist/abdomen to estimate fetal dose
- Control dosimeter: stored in low-radiation area; measures background; subtracted from worker readings to isolate occupational dose
- Never wear another worker's dosimeter; never expose a dosimeter to radiation when not worn
- Records kept for duration of employment + 30 years; processing labs accredited by NVLAP
Sign up to track progress
Create a free account to mark lessons complete, track your learning progress, and access study tools