Individual Dosimetric Monitoring (IDM) is necessary because it ensures continuous control of the individual absorbed dose received by persons working in an environment with ionizing radiation. This allows for the timely detection of exceedance of dose limit values and the implementation of appropriate protective measures.

IDM is mandatory and must be carried out in accordance with Article 5 of Ordinance No. 32 on the Conditions and Procedure for Performing Individual Dosimetry Monitoring of persons working with sources of ionizing radiation and pursuant to Article 65, Paragraph 1, Item 3 of the Health Act of the Republic of Bulgaria.

For IDM, Individual Thermoluminescent Dosimeters (TLDs) manufactured by Thermo Scientific (Germany) are used. They are worn on the chest during work in an ionizing radiation environment. Each dosimeter consists of:

• A TLD card with a unique barcode, containing two passive detectors for the determination of the personal dose equivalent Hp(10) and the personal skin dose equivalent Hp(0.07);
• A plastic protective holder with a fastening clip in which the TLD card is placed.

To measure the accumulated dose in the TLDs, an automated thermoluminescent dosimetry system is used — Thermo Harshaw 6600 PLUS, equipped with specialized software and capable of reading up to 200 TLD cards without operator intervention.

The measured doses from the individual TLDs are corrected using the doses measured by control TLDs, thereby eliminating the influence of natural background radiation.

The calibration of the system is traceable to a Secondary Standard Dosimetry Laboratory (SSDL) at the National Institute of Nuclear Sciences “Vinča”, Serbia. Initial and periodic verification of the system is performed by the Bulgarian Institute of Metrology (BIM) – General Directorate “Measuring Instruments”, Lovech sector.

• Supply of TLDs and service for a defined period, regulated by contract or purchase order;
• Staff training;
• Dose result processing and reporting;
• Issuance of an IDM Certificate containing the results of the dose exposure of the monitored persons and an assessment of their compliance with the applicable regulatory requirements, in accordance with the Radiation Protection Ordinance.