54 Radioactive Materials Safety Manual Environmental Health and Safety 99mTc TECHNETIUM-99m Half - Life Physical Biological Effective 0.25 days (6 hours) 1 day 0.2 day (4.8 hours) Radiation Emitted Type Energy (max) Electron 119.5 keV (8.8 %) 137.5 keV (1.9 %) Gamma 140.4 keV (89.1 %) External Hazard 1 meter 1 cm Dose rate from a 1 mCi (37 MBq) point source: 0.72 mrad/hr (7.2 µGy/hr) 7.2 rad/hr (72 mGy/hr) Internal Hazard Effective Committed Dose Ingestion Inhalation Equivalent per Unit Intake: 0.06 mrem/µCi (1.68 x 10-11 Sv/Bq) 0.03 mrem/µCi (8.8 x 10-12 Sv/Bq) Annual Limit on Intake: (ALI)* 80 mCi 200 mCi *ALIs expressed are from Appendix B, Table 1 – Iowa Administrative Code 641.40 (136C). Special Considerations • 99mTc emits relatively energetic gamma rays which can pose a significant external hazard. • High external doses to the fingers can result from direct handling of unshielded vials of millicurie quantity 99mTc solutions. • Because of its relatively short effective half-life, 99mTc has a relatively large annual limit on intake. Radiation Safety Practices • Dosimetry – Whole body badge required when working with quantities of 100 µCi (3.7 MBq) or more; ring badge required when working with quantities of 1 mCi (37 MBq) or more. • Bioassay – Required for individuals involved in spills or other incidents where significant uptake is suspected. • Shielding – 2.1 mm lead provides 95% attenuation; lead HVL is 0.5 mm. • Surveys – NaI meter surveys, wipes analyzed in an LSC or gamma counter. • General – Unshielded source vials of 99mTc should be handled with leaded rubber gloves; shielded containers, tongs, or vial/tube racks.
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