Personal Dosimetry
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Direct answer
Personal dosimetry measures the dose a worker actually receives, using thermoluminescent dosimeters (TLD), optically stimulated luminescence (OSL) badges or, historically, film badges worn on the trunk. In India the national personnel monitoring service run through Bhabha Atomic Research Centre supplies CaSO4:Dy TLD badges to radiology staff, exchanged quarterly, and the record is kept for the worker's lifetime. Interventional staff wear two badges: one under the apron at the waist or chest to estimate effective dose, and one at the collar above the apron to estimate dose to the thyroid and lens of the eye. Pocket and electronic dosimeters give immediate readings for high-dose procedures.
What you must remember
- TLD principle: trapped electrons in the phosphor are released as light on heating (the glow curve), with light output proportional to dose; the badge is reusable after readout.
- Indian practice: CaSO4:Dy (dysprosium-doped calcium sulphate) TLDs through BARC's personnel monitoring, issued and read quarterly; LiF:Mg,Ti (TLD-100) is tissue-equivalent and the international reference phosphor.
- OSL badges use aluminium-oxide (Al2O3:C) stimulable by laser light — highly sensitive, stable, and read optically without heating; they dominate commercial services worldwide.
- Film badge: photographic film behind filters (open window, plastic, aluminium, copper, cadmium) allowing radiation energy and type to be estimated; it fades with heat and humidity.
- Whole-body badge position: on the trunk at chest or waist, under the lead apron; collar badge: at the neck, outside the apron, estimating unshielded eye and thyroid dose.
- Ring or wrist badges serve nuclear medicine and interventional staff whose hands approach the beam; the 500 mSv annual extremity limit applies.
- Pocket (quartz fibre) dosimeters are direct-reading instruments for daily checks; electronic personal dosimeters give real-time readings and alarms.
- Dose records must be retained lifelong and transferred between employers; doses exceeding investigation levels (commonly around 1-1.5 mSv in a monitoring period in Indian practice) trigger a review of work practices.
Two badges, one interventional radiologist
Follow the monthly numbers of an interventional radiologist to see why double monitoring exists. Her collar badge, worn at the neck outside a 0.5 mm apron, reads 0.6 mSv for the month — the lens, thyroid and unshielded head receive exposure of this order, and the reduced lens limit of 20 mSv per year is exactly what this badge polices. Her under-apron waist badge reads 0.03 mSv, because the apron absorbs well over 90 per cent of scatter at fluoroscopic energies. Effective dose lies far closer to the under-apron figure than the collar figure, and one accepted combination weights them — roughly E = 0.5 × (under-apron dose) + 0.025 × (collar dose) — for staff who work regularly inside aprons near scatter.
The numbers also teach the corrective actions. If the collar badge climbs, length of beam-on time, position relative to the patient, and use of ceiling-suspended screens come under scrutiny. If the under-apron badge climbs, the apron itself is suspect — lead aprons should be fluoroscoped or radiographed annually for cracks, especially at folds and side seams. And if a badge records a dose on a day she was on leave, the badge was left inside the room during someone else's screening: badges record the radiation where they are, not the work a person did — which is why badges live on the worker or in a shielded rack.
Where students slip
Placement is the classic error: a badge worn outside the apron overestimates effective dose (and one worn inside for a collar purpose underestimates eye dose), so the exam answer must specify both position and purpose. The second slip is physical: leaving the badge on the patient table, inside the fluoroscopy room, or worse, in the primary beam "to test it" — this records a spurious high dose that enters the legal record. Third, understanding the physics: candidates describe TLD readout as "photographic" — there is no film; heat releases trapped electrons as light measured by a photomultiplier, and the phosphor can be reused after annealing. A final viva favourite: why LiF is preferred internationally — tissue equivalence (effective atomic number near that of muscle), unlike CaSO4:Dy, whose higher sensitivity suits the lower-dose quarterly Indian badge.
Frequently asked questions
How does a thermoluminescent dosimeter work?
Radiation excites electrons into traps in the crystal lattice; controlled heating releases them, emitting light proportional to absorbed dose, measured as a glow curve by a photomultiplier tube.
Which TLD phosphor does the Indian personnel monitoring service use?
CaSO4:Dy (dysprosium-activated calcium sulphate), issued and read quarterly through the Bhabha Atomic Research Centre monitoring service.
Why do interventional staff wear two badges?
The under-apron badge estimates shielded effective dose, while the collar badge outside the apron estimates unshielded thyroid and lens dose — each policing a different dose limit.
When is a ring or wrist dosimeter required?
For hands likely to approach the beam or handle radioactive material — nuclear medicine preparation and some interventional work — against the 500 mSv annual extremity limit.