57 Radioactive Materials Safety Manual 131I IODINE-131 Half - Life Physical Biological Effective 8.0 days 138 days 7.6 days Radiation Emitted Type Energy (max) Beta 606 keV (89.3%) Gamma 364 keV (81.2%) 637 keV (7.3%) External Hazard 1 meter 1 cm Dose rate from a 1 mCi (37 MBq) point source: 0.28 mrad/hr (2.8 µGy/hr) 2.8 rad/hr (28 mGy/hr) Internal Hazard Effective Committed Dose Ingestion Inhalation Equivalent per Unit Intake: 53.3 mrem/ µCi (1.44 x 10-8 Sv/Bq) 32.9 mrem/ µCi (8.89 x 10-9 Sv/Bq) Annual Limit on Intake: (ALI)* 30 µCi 50 µCi *ALIs expressed are from Appendix B, Table 1 – Iowa Administrative Code 641.40 (136C). Special Considerations • Volatilization of iodine from NaI solutions and from solutions of certain labeled compounds is a major concern. • 131I 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 131I solutions. • Because 131I is concentrated in the thyroid, it has a low 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 incidents where significant uptake is suspected. • Shielding – 1.3 cm lead provides 95% attenuation; lead HVL is 0.3 cm. • Surveys – GM meter surveys, wipes analyzed in an LSC or gamma counter. • General – Bare source vials of 131I should be handled with leaded rubber gloves, shielded containers, tongs, or vial/tube racks.
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