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Office of Homeland Security & Emergency Coordination
Radiation Safety Division (RSD)
United States Department of Agriculture

Dosimetry

DEFINITION

Dosimetry is the study, measurement, method of measurement, or instrument of measurement of radiation dose. Dosimetry often refers to the status of wearing a personnel badge that measures and monitors dose. It may also refer to dose history and the records where dose history is maintained.

More specifically, radiation dosimetry is the calculation of the absorbed dose in tissue resulting from exposure to ionizing radiation. Dose is reported in units of gray (Gy) for mass, and dose equivalent is reported in units of sieverts (Sv) for biological tissue, where 1 Gy or 1 Sv is equal to 1 joule per kilogram. Traditional units are still prevalent as well, where dose is often reported in rads and dose equivalent in rems. By definition, 1 Gy = 100 rads and 1 Sv = 100 rems. Workers who may be exposed to radiation carry personal dosimeters. These dosimeters measure dose, based on a variety of measuring systems. The average background dose for a human being is about 350 milli-rems per year, resulting mostly from cosmic radiation and natural isotopes in the earth.

Radiation dose refers to the amount of energy deposited in matter and its biological effect on living tissue, and should not be confused with activity, measured in units of curie or becquerel. Exposure to a radioactive source will give a dose which is dependent on the activity, time of exposure, energy of the radiation emitted, distance from the source and shielding. The dose equivalent is then dependent upon the additional assignment of weighting factors describing biological effects for different kinds of radiation on different organs.


DOSIMETRY

Development of Standards
Radiation safety standards and policies are set by a consensus among national and international scientific organizations, such as the Health Physics Society, the National Council on Radiation Protection (NCRP), and the International Commission on Radiological Protection (ICRP). The risks associated with low-level radiation exposure are conservatively calculated to be proportional to those observed with high-level exposure. These calculated risks are compared to other known occupational and environmental hazards, and standards are established to control and limit potential harmful radiation effects. In the United States, the Nuclear Regulatory Commission sets regulatory dose limits for the public and occupationally exposed workers.

Personnel Dosimetry
Radiation monitoring is required when an individual's radiation exposure is likely to exceed 10% of the dose limit, when a new activity is being initiated, or when there is no other method that can adequately determine a dose in an accident situation. Personnel monitoring for radiation exposure is not required as part of the USDA broad scope radioactive materials license, but is a requirement for all irradiator operators. RSD also requires personnel monitoring for nuclear gauge users, and area monitoring for x-ray producing equipment. Through years of monitoring, the USDA has determined that no individual working with unsealed radioactive materials is likely to receive in excess of 10% of the annual dose limits.

Adding or Changing Dosimetry
Complete the “Radiation Safety Program Dosimetry Request RSD-70” and send, fax, or email it to the Radiation Safety Division.

Proper Use of Dosimeters

  • Always wear your dosimeter when working with radioactive material. It should be worn on the shirt front, or on the waste at the pant or skirt belt or loop, with the front or name side of the dosimeter facing outward.
  • Do not take your dosimeter home.
  • Do not store or leave your dosimeter near radioactive material.
  • Do not loan your dosimeter to a coworker or visitor.
  • Do not wear your dosimeter during a medical procedure, where you might have an x-ray taken, for example, at the dentist’s office. (Notify RSD if this happens by mistake).
  • Do not disassemble or otherwise tamper with your dosimeter.
  • Do not continuing wearing the old dosimeter once you receive the new one.


RADIATION DOSE LIMITS

Radiation Workers
The yearly occupational dose limit for a radiation worker is:

5.0 rems for whole body

15.0 rems for lens of the eye

50.0 rems for skin or an extremity

Minor, in an Occupational Setting
The yearly dose limits for a minor working in a restricted area are 10% of the doses for an adult radiation worker. This is 0.5 rems (or 500 milli-rems) for a whole body dose.

Occupational Dose for an Embryo or Fetus
The dose to an embryo or fetus, resulting from the mother’s occupational exposure, shall not exceed 0.5 rems (or 500 milli-rems) during the 9-month period of pregnancy. A woman who works in an area that is restricted for purposes of controlling radiation exposure may declare her pregnancy, in writing, to her supervisor, and request that the dose limits for the embryo or fetus apply to her for the duration of her pregnancy. When this occurs, the supervisor must contact the LRPO and RSD to obtain proper guidance and assistance in implementing this request, and maintain records of actions taken to meet the dose limit of 0.5 rems (or 500 mill-rems).

Internal Radiation Dose Limits
The radiation dose from the inhalation or ingestion of radioactive materials must also be considered in the USDA Radiation Safety Program. The Nuclear Regulatory Commission places limits on the amount of radioactive materials that a radiation worker can take into their body over the course of a working year. This is called the Annual Limit on Intake (ALI). These values have been calculated for several radioisotopes and are listed in 10 CFR 20 Appendix B Tables, Table 1 ALI values for occupational dose …

If a radiation worker receives one ALI, the calculated radiation dose from that intake will be 5 rems. Currently, only the thyroid dose is actively monitored by the Radiation Safety Division. Using standard tables, the whole body effective dose equivalent can be estimated. Regulations require that the total external and internal radiation dose to a worker not exceed 5 rems in one year.

Dose Limits for Members of the Public
NRC or agreement state licensees shall conduct operations so that the dose to individual members of the public from the licensed operation does not exceed 0.1 rem (or 100 milli-rems) in a year.