CHAPTER III - PERSONNEL DOSIMETRY AND REGULATORY LIMITS

3.1 Monitoring Radiation Doses from External Sources

3.1.1 TLD Badges

TLD badges are used at ASU to monitor personnel for exposure of the body to penetrating ionizing radiation such as gamma and x-rays, and exposure of the skin to less penetrating radiation such as beta particles. For most individuals results of the TLD badge readings are also used as estimates for the exposure of the lens of the eye. TLD badges must be worn by personnel working with most sources of ionizing radiation. TLD badges may not be required for individuals handling the following radioactive materials, however:

1. ³H, ¹⁴C and/or ³⁵S
2. Micro-curie quantities of material in check sources,
3. Micro-curie quantities of materials in radio-immuno assay kits,
4. Material contained in gas chromatographs or other equipment as a sealed source.

TLD badges must be worn on the trunk of the body at or above the waist. Dosimetry devices must not be taken home or left in laboratory areas where they may be exposed to radiation from radioactive material. The TLD badge is sensitive to heat and humidity. False positive readings may result when badges are left in hot cars, near hot windows, or other sources of heat.

3.1.2 Extremity Dosimetry

Ring dosimetry devices are used at ASU to monitor for radiation exposure to the hands and are issued to personnel handling millicurie quantities of ³²P or other "hard" beta emitters, and to personnel handling large gamma-emitting sources. Ring badges must be worn under gloves with the sensitive portion of the ring toward the source.

3.1.3 Dosimeter Exchange

The TLD badges and the rings are exchanged on a quarterly basis. ORS personnel hand carry replacement TLDs and rings to department offices during the last few days of the quarter. Old TLDs and rings must be hand carried to the ORS. Campus mail must not be used. This policy has been established to avoid exposure of TLDs to sources of radiation, heat, and humidity during transit.

TLD badges should be returned to the ORS during the first 5 working days of the new quarter.

3.1.4 Lost or Damaged Dosimeters

Periodically, dosimetry devices are lost or damaged. This should be reported to ORS staff immediately so that replacement dosimeters can be issued. Lost or damaged dosimeters will cost $10 per badge to replace.

3.2 Monitoring for Exposure from Internal Radioactive Materials

3.2.1 Thyroid Bioassay

All personnel handling more than 100 uCi of ¹²⁵I or ¹³¹I during a calendar month must have a thyroid scan within 6 – 72 hours of use. Thyroid scans are obtained at the ORS on a walk-in basis.

3.2.2 Urine Bioassay and Whole Body Counts

Individuals handling in excess of 10 mCi of ³H in any given month must submit a urine sample for analysis to the Radiation Safety Officer within 6 – 72 hours of use. Personnel handling large unsealed sources of radioactive material other than ³H may be required to submit urine samples or submit to a whole body count.

ORS staff will notify individuals requiring bioassays as needed.

3.3 Regulatory Dose Limits

3.3.1 Limits for Radiation Workers

ARRA has imposed limits on the dose of ionizing radiation which may be received by individuals working with sources of ionizing radiation. These limits are shown in Table 3-1.

3.3.2 Limits to the Embryo/Fetus of Declared Pregnant Workers

Due to concerns about prenatal radiation exposure (Appendix B) ARRA regulations provide separate limits for the embryo/fetus of Declared Pregnant Workers. The limit is 0.5 rem dose equivalent to the fetus during pregnancy.

This limit applies only for workers who have formally declared pregnancy in writing. Declaration of pregnancy should be sent to the Radiation Safety Officer and include the estimated date of conception.

Individuals concerned about radiation and pregnancy should feel free to speak to the Radiation Safety Officer.

3.3.3 Limits for Members of the Public

The regulatory limit for members of the public is 0.1 rem total effective dose equivalent per year. This limit applies to all individuals who are not trained to work with sources of ionizing radiation. At ASU this includes most faculty, staff, and students.

3.4 ALARA

In view of uncertainties that exist concerning the health effects of exposure to low doses of radiation (see Appendix A), it is prudent to keep doses to personnel "as low as is reasonably achievable" (ALARA). Each user of radioactive material at ASU has the responsibility to incorporate shielding and protective devices, and to take any other steps required to keep doses ALARA.

3.4.1 Investigational Levels

In order to maintain doses ALARA, investigational dose levels have been established at ASU. These dose levels are shown in Table 3-2. The limits are evaluated on a quarterly basis.

Personnel exposures equal to or greater than Investigational Level I, will be reviewed by the RSO, who will report the results to the RSC at their next regularly scheduled meeting. The RSC may require corrective actions on the part of the RSO or sub-licensee.

Personnel exposures equal to or exceeding Investigational Level II will be investigated in a timely manner by the RSO who will take immediate action if warranted. A report of the investigation, actions taken, and a copy of the individual's radiation dosimetry history will be presented to the RSC at their regularly scheduled meeting following completion of the investigation. The RSC may impose restrictions on the user, and/or additional conditions on the sub-license under which the exposed individual was working, as warranted.

Investigational limits exceeding those listed in Table 3-2 may be established by the RSC for a worker or group of workers when the higher investigational levels are consistent with good ALARA practice for the work being conducted by the individual or group.

3.5 Reports to Workers on Radiation Dosimetry

The ORS keeps careful records on results of radiation dosimetry and bioassay for personnel on the ASU dosimetry program. These records are available for review by these personnel.

3.5.1 Review of Records in the ORS

Personnel issued dosimeter devices are welcome to review dosimetry results on file in the ORS. This review should be arranged with the RSO. The RSO requires positive identification of individuals seeking access to dosimetry since these records are covered by state and federal privacy laws.

3.5.2 Annual Dosimetry Report

During the spring of each year, a report on dosimetry results for the previous calendar year is sent to each individual issued dosimeters at ASU. A summary of individual dosimetry results are also sent to the sub-licensee responsible for supervising the work requiring dosimetry.

3.5.3 Notification of Results Exceeding Investigation Limits

Personnel will be notified quickly of results which exceed Investigation Levels listed in Table 3-4. Personnel are not notified quarterly of routine dosimetry results which do not exceed the Investigation Levels.

3.6 Radiation Dosimetry Units

The following paragraphs explain the dosimetry units used in this chapter.

3.6.1 Absorbed Dose

The amount of energy absorbed by irradiated tissue is an important variable in the assessment of radiation risk and damage. The absorbed dose is defined as the energy absorbed per unit mass of tissue. The traditional unit for absorbed dose is the rad.

1 rad = 100 erg / gram

The rad is being replaced by a new unit based on the International System of Units (SI). The new unit is the gray.

1 gray = 1 joule / kg

Spending a little time with the units will reveal that

1 gray = 100 rad

The SI units have not found widespread use in radiation protection in this country. It is the system of units which is used internationally, however, and will eventually replace the older units here.

3.6.2 Dose Equivalent

Alpha, beta, gamma/x-radiation, and neutrons differ in the tissue damage produced for a given absorbed dose. Special units of dose equivalent are used to adjust the absorbed dose for this difference. The traditional unit of dose equivalent is the rem.

1 rem = 1 rad x Q

Q is called the quality factor and is assigned to radiation based on the relative risk for a given dose. Currently a quality factor of 1 is used for photons, electrons, and positrons. A quality factor of from 2.3 to 10 is used for neutrons, depending on their energy, and a quality factor of 20 is used for alpha particles. The SI unit for dose equivalent is the sievert.

1 sievert = 1 gray x Q

3.6.3 Exposure

The energy absorbed by irradiated tissue is rarely measured directly. Most radiation detection instrumentation used in radiation protection measures the number of ion pairs produced in a volume of gas. The traditional unit used to measure ionization in air is the roentgen:

1 roentgen = 2.58 x 10-4 coulombs / kg air

The roentgen is defined only for x-rays and gamma rays. It is not used for beta, alpha, or neutron radiation.

Exposure of 1 roentgen of radiation results in an absorbed dose to tissue of 0.97 rad. For purposes of radiation protection and dosimetry, it is usually assumed that the roentgen, rad, and rem are numerically equivalent for gamma-rays and x-rays.

3.6.4 Effective Dose Equivalent

The various organs and tissues in the body differ in their sensitivity to radiation. For instance, the bone marrow and other blood forming tissues of the body are much more sensitive to radiation than the skin of the body. In order to quantify the risk from radiation exposure when the body is not irradiated uniformly (different doses are delivered to different organs or tissues of the body) a unit called the effective dose equivalent has been developed. The effective dose equivalent is given the symbol H and is defined as:

where D_i is the dose equivalent received by the ith tissue or organ, and W_i is a weighting factor which is assigned to the ith tissue or organ depending on its sensitivity to radiation. Weighting factors currently in use are listed in the accompanying table. The units of the effective dose equivalent are the rem and the sievert depending on which is used for the individual tissue or organ dose equivalent.

3.6.5 Committed Effective Dose Equivalent

When radioactive materials are inhaled, ingested, or otherwise internalized, they may be retained in some tissues for a long period of time. In some cases a fraction of the material may remain in the body for years. The committed dose equivalent is the dose equivalent that will be received by tissues or organs from an intake of radioactive material during the 50 year period following the intake. The committed effective dose equivalent is the effective dose equivalent that will be received from an intake of radioactive material by an individual during the 50 year period following the intake.

3.6.6 Deep Dose Equivalent

The deep dose equivalent is the dose equivalent at a tissue depth of 1 cm.

3.6.7 Shallow Dose Equivalent

The shallow dose equivalent is the dose equivalent at a tissue depth of 0.0007 cm averaged over an area of 1 square centimeter.

3.6.8 Eye Dose Equivalent

The eye dose equivalent is the dose equivalent to the lens of the eye.

3.6.9 Total Effective Dose Equivalent

The total effective dose equivalent is the sum of the committed effective dose equivalent for all intakes of radioactive material and the deep dose equivalent to the whole body resulting from exposure to external sources of radiation.