|| RTSM Table Of Contents ||

RADIATION SAFETY TRAINING MANUAL

CHAPTER 7
MEASUREMENTS OF RADIATION EXPOSURE

CHAPTER 7 Table Of Contents

A. FILM BADGE AND FINGER RING DOSIMETERS

Figure 7.1 Dosimetry Request Card
Figure 7.2 Dosimetry Report - Graphic Under Construction

B. BIOASSAYS

1. URINE ASSAY
2. THYROID BIOASSAY

C. SURVEY METERS

Figure 7.3 Common Types of G-M Detectors
Figure 7.4 Common Types of Scintillation Probes

D. WIPE SURVEYS

Figure 7.5 Energy Response for Geiger Counters

Several methods are used to determine radiation exposure. Individual exposure is estimated through personnel monitoring devices such as film badges or finger rings. Internal exposure to radioisotopes from ingestion, inhalation, absorption, or puncture is estimated from bioassay procedures that analyze samples of blood, urine, or tissue or monitor the organ of interest, such as the thyroid gland.

Radiation exposure rates from sources such as stored radioisotopes, radioactive waste, or work areas can be directly measured by using the appropriate survey meter. Finally, radioactivity contamination is measured by analyzing wipe samples in a liquid scintillation counter or well counter.

Each of these methods is considered below.

A. FILM BADGE AND FINGER RING DOSIMETERS

Chapter 3 discussed the occupational dose limits set by regulations and the more stringent limits set by the University of California, San Francisco (UCSF) in adhering to the As Low As is Reasonably Achievable (ALARA) precept. Records from analysis of personal dosimeters are used to verify compliance with these requirements.

Dosimeters are issued by the Radiation Safety Office to those individuals who are authorized to work with sources of ionizing radiation which dosimeters can measure. The most common radiation dosimeters used at UCSF are film badges and finger rings. Film badges have an error range of approximately 20% and finger rings approximately 5%. These dosimeters allow assessment of beta, gamma, and x-ray exposure. Dose assessment for exposure to ³H, ¹⁴C and ³⁵S is performed by urinalysis since the film badges and finger rings are not sufficiently sensitive to these low beta energies.

The following practices pertain to wearing radiation dosimeters. Some are required by regulation and others are common sense.

1. Film badges should be worn at the collar outside of protective clothing, such as a lead apron worn by x-ray personnel. Other acceptable locations are the trunk of the body or shirt pocket. Finger rings should be worn on the same finger when doing procedures and on the hand that is favored. The finger ring should be worn under the glove on the index finger with the detector (located under the label on the top of the ring) pointed towards the palm.

 

2. The radiation dosimeter is to be worn whenever there is a possibility of being exposed to ionizing radiation in the work place. It should never leave the campus or be worn at other institutions. Store the dosimeter in a radiation-free location when it is not being worn. It should not be subjected to high temperatures or high humidity. Radiation dosimeters are not to be worn when receiving medical radiation exposure for diagnosis or therapy.

 

3. Loaning your assigned dosimeter to anyone else to wear is illegal. Never use your assigned dosimeter to monitor an area. Radiation area monitors are available from the Radiation Safety Office on request.

 

4.Your dosimeter is your responsibility. UCSF has over 3,000 to process and exchange monthly, so promptly return your dosimeter each month for processing. Returning dosimeters late or losing dosimeters results in extra work and delays in obtaining reports. You must fill-out a Lost Badge Report for all non-returned dosimeters.

 

5. Do not leave your radiation dosimeter where it can be exposed to radiation.

 

6. The film must be properly placed into the holder every month and stored within the holder. If you are at all uncertain about how to insert the film into the holder, ask your Laboratory Supervisor for assistance. The holder contains special filters that allow for quantitative and qualitative analysis, therefore, for accurate readings the film must be properly inserted. The finger ring is a sealed unit and should be returned intact.

 

7. Loss of a film badge or finger ring must be reported to the Radiation Safety Office as soon as the loss is noted so that a Lost Film Badge Report can be filed. If a dosimeter has been assigned to you, and it is not worn when working with ionizing radiation, this is a violation of the law.

 

8. If there is an incident involving radiation exposure, it should be reported immediately to the Radiation Safety Office. Reporting will allow immediate evaluation of your radiation dosimeter to ascertain whether a significant exposure has taken place. Otherwise there can be a significant delay in knowing the extent of exposure.

 

9. Ring badges should be worn whenever the hand may receive significant radiation exposure and must be worn when handling amounts of 5 mCi or more of energetic beta emitters, such as ³²P, and gamma emitters.

The results measured by your radiation dosimeter is UCSF's record of your occupational radiation dose. UCSF has administratively set investigational levels far below State and Federal dose limits. Should your monthly exposure exceed these levels, you will be notified and an inquiry into the causes will be made.

If you are pregnant or considering pregnancy, notify your supervisor so that changes in your working conditions can be considered. The dose to your unborn baby is limited to 500 mrem for the entire gestation period.

Reports of radiation doses are prepared monthly and sent to your dosimeter coordinator. The report is generally available about one month after your badge is exchanged. Check with the coordinator to see where the results can be reviewed. If you have questions that your coordinator can not answer, feel free to call the Radiation Safety Office. See Figure 7.1 for a sample Dosimetry Request Card, and Figure 7.2 for the layout of a sample Dosimetry Report.

Figure 7.1 Dosimetry Request Card

UCSF DOSIMETRY PROGRAM

Name: ______________________________________________________________Sex __________

Last First MI

Box ____________Phone _________ Job Title_______________________________ Dept._______________________

Birthday __________ Social Security # _______________________ Employee ID #(UCSF) ___________________________

M / D / Y

PI ___________________________ RUA # __________________________________ Series Code _______________________

Use Radionuclides: Yes ______________ No _________________ Operate X-ray unit: Yes ____________ No__________

Previous UCSF Dosimetry: Yes _______________ No ___________ If YES, Series Code ________________________

Other Previous Dosimetry: Yes _________________ No _____________ If YES, Please complete and mail

attached form to previous employer

Signature _______________________________________________________________________________________________

For Office Use Only:

Evaluation: Exposed to ionizing radiation > 10% MPD? Yes ______ No __________ Comment ___________________

Film __________________ TLD ___________________________ Series _______________ Part # ____________

G1 / G8 / Other?

Issue Date ____ Previous Hist _______________________ Expos Req __________________ Initial ___________

In order to comply with the State of California requirements, we are requesting a report of the radiation exposure history for the following individual. Please include all available results of bioassays, whole body counts or calculated exposures, where applicable, in the report.

Name ________________________________________________________________________________________________

Social Security Number __________________________________________________________________________________

Department ___________________________________________________________________________________________

Dates of Employment ____________________________________________________________________________________

Signature _____________________________________________________________________________________________

Part # _______________________________

Yours Truly,

Dosimetry Coordinator

Office of Environmental Health and Safety
UCSF
Box 0942
San Francisco, CA 94143

Figure 7.2 Dosimetry Report - Graphic Under Construction

B. BIOASSAYS

Bioassays are performed when persons use unsealed quantities of radioactive materials to determine whether any activity has entered the body. The results are used to estimate internal organ doses, determine the presence of airborne radioactive materials, and to evaluate work habits, experiment design, and facility design. Bioassays are performed by analyzing samples of blood, urine, or tissue, or by monitoring the organ of interest (e.g. thyroid gland) to determine the presence and quantity of radioisotopes.

1. URINE ASSAY

 

UCSF requires that each individual using 100 millicuries or more of ³H, ¹⁴C, ³⁵S, or ³²P per experiment perform a urinalysis after such use and document the results. The results should be kept in the Laboratory Radiation Safety Logbook for review by the Radiation Safety Office and inspection by the State. The counting procedures, maximum values, and action levels are given in Appendix D, Part B of the Radiation Safety Manual.

 

2. THYROID BIOASSAY

 

Persons working with radioiodine may be exposed to airborne concentrations. Thyroid bioassays are performed to verify that exposures are kept to a minimum. The bioassay procedure involves placing a radiation detector, sensitive to emissions to be measured, close to the thyroid gland and obtaining a one minute count. The results are used to estimate the total activity of radioiodine in the thyroid.

The requirements for thyroid bioassays are given in Appendix D, Part C of the Radiation Safety Manual.

C. SURVEY METERS

The most common use of survey meters in UCSF laboratory situations is to detect the presence of contamination during and after experimental work involving radioisotopes.

During the review of a laboratory's application to obtain a Radiation Use Authorization permit, the Radiation Safety Office will verify that appropriate survey meters are available to detect the radioactive material to be used in the laboratory. All UCSF survey meters must be calibrated at least annually. Calibrations are performed by the Radiation Safety Office and a calibration sticker is attached to the meter noting the next calibration due date. Before using a survey meter, be sure that it has been calibrated within the past year and check the battery. If you are suspicious about the operation of a survey instrument, notify your Laboratory Supervisor.

The most common radioisotopes used in research at UCSF are ³H, ¹⁴C, ³²P, ³⁵S, and ¹²⁵I. ³H cannot be detected with a survey meter. ³²P is quite energetic and easily detected; ¹⁴C and ³⁵S are low energy beta-emitters and are detected with much lower efficiency than ³²P. The Radiation Safety Office requires that a meter having a scintillation probe be used for ¹²⁵I detection.

The two most common types of survey meters used in laboratories at UCSF are the Geiger-Mueller (G-M) and scintillation counter. UCSF G-M survey meters will be equipped with a thin end-window to permit detection of low energy beta radiation and a speaker that produces an audible noise relative to the radioactivity being detected.

G-M detectors are filled with a gas such as argon. The meter must be carefully handled to prevent damage by rupture of the thin end-window. The G-M tube consists of a center wire surrounded by a concentric cylinder. A high voltage is applied between the wire and the cylinder. Each radioactive event causes electrical discharge in the G-M detector tube. The electrons are collected on the central wire to produce a signal. G-M tubes may be located inside the meter or contained in probes having several shapes, such as a "pancake" probe or cylindrical probe. See Figure 7.3 for sample G-M detectors.

Figure 7.3 Common Types of G-M Detectors

END WINDOW G-M DETECTOR	PANCAKE G-M DETECTOR

The scintillation meter operates under a different principal than the G-M meter. When radiation interacts with the scintillation detector (often a crystal made of NaI activated with Thallium), a light flash is produced that is "seen" by a photomultiplier (PM) tube. The initial stream of electrons produced by light falling on an electron-emitting surface is multiplied to produce a current that makes the meter register. These units are required when working with ¹²⁵I. See Figure 7.4 for sample Scintillation Probes.

Figure 7.4 Common Types of Scintillation Probes

Become familiar with the survey instruments in the laboratory. Ask your Laboratory Supervisor to demonstrate the proper use of each instrument, review the manufacturer's manual for each instrument, and learn the important characteristics of the instrument such as energy dependence. An example of an energy dependence curve (measured dose rate vs energy) is given in Figure 7.5.

Some practical steps in use of survey meters:

1. Use the instrument carefully; most instruments have a slow response when surveying for low level contamination.

 

2. Avoid contaminating the instrument or breaking the thin window of the detector.

 

3. Do not cover the probe of the instrument with Parafilm or other plastic wraps. This will reduce the instrument's detection efficiency.

 

4. Check to battery to make sure it is in operable range.

 

5. Hold the instrument as close to the surface as you can, without touching the surface.

D. WIPE SURVEYS

Wipe surveys must be routinely performed as part of the laboratory contamination control program. Direct monitoring with a survey meter is a means to detect the presence of contamination and should be the first step in conducting a wipe test survey. Wipe tests are an effective method to measure the extent of contamination. In addition, wipe surveys are the only effective way to detect contamination from low-energy beta emitters such as ³H.

Upon completion of a thorough meter survey, identify the areas to be tested and mark them on a laboratory diagram. Wearing disposable gloves, use filter paper, or another appropriate media such as cotton swabs, to wipe each test area. The wipe area should be at least 100 cm² or approximately 4" x 4." Label each wipe and include a control wipe to obtain a background count. Count the samples in a liquid scintillation counter (a well counter can be used for gamma emitters).

Review the results and note those samples that exceed 2 x background in uncontrolled areas and 25 x background in controlled areas. These areas must be decontaminated and re-wiped to document that decontamination has been performed. Monitor your hands and clothing after you have completed the survey.

Figure 7.5 Energy Response for Geiger Counters

.|| Top of page || || Chapter 6 || Chapter 8 || Table Of Contents ||