Instructional Guide for MCG Research Personnel- Laboratory Surveys

Introduction

Principal Authorized Users (PAU) are required, as a condition of their authorization, to perform weekly surveys. Surveys are used to identify and quantify radioactive contamination and radiation levels. This guide is provided to assist you in performing surveys and documenting routine weekly surveys.

Surveys are required to be performed by the Principal Authorized User in all labs under a permit that allows use of radioactive material. This includes common equipment areas where several labs are using radioactive material. One Principal Authorized User is generally responsible for ensuring that the area is surveyed on a weekly basis.

Types of Surveys

There are three categories of survey that are performed in the lab: fixed contamination, removable contamination, and ambient dose rate surveys.

Fixed contamination surveys are used to locate radioactive material that can not be removed by applying moderate pressure. Fixed contamination is quantified by :

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  • A Surface Emission Rate (SER), in gross counts per minute (cpm).
  • Good radiological practice is to make sure that the SER is kept below twice the ambient background.

Removable contamination are used to locate radioactive material than can be removed by applying moderate pressure. Removable contamination presents a personnel hazard and also an internal hazard due to the possibility of ingestion. Removable contamination is defined in units of:

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  • Disintegrations per minute, divided by a surface area of one hundred centimeters squared (dpm/100cm²).
  • The MCG criteria for removable contamination is 200 dpm/100cm².
  • Contaminated areas generally have removable contamination, but may have a combination of the two types.

Dose Rate Surveys measure the amount of ionization in air, taken 30 cm from the source of radiation.

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  • The MCG action level is that ambient area dose rates must be kept below 2 mrem/hr. Possible sources of readings above this level are unshielded source vials and waste containers.

Survey Instrumentation

One key to an effective contamination survey program is the selection of the proper instrument. For detecting a Surface Emission or Exposure Rate the two most common portable survey meters found in the MCG Research facilities are Geiger Mueller (GM) survey meters and NaI scintillation survey meters.

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  • The GM meter is best used for P-32, a high energy beta emitter, but can also identify areas heavily contaminated with lower energy betas such S-35 or C-14, for which the GM has a relatively low efficiency.
  • Note that exposure readings displayed in mR/hr using a GM are a conservative over response.
  • Note also that regulatory limits are expressed in units of mrem/hr. For purposes of laboratory surveys the quality factor converting exposure rate to dose rate is one, so that displayed units of mR/hr are equivalent to mrem/hr.
  • Thin crystal NaI scintillation survey meters are used to identify I-125 contamination. GM survey meters are poor detectors of I-125. No common portable survey meter can detect H-3.

Wipe tests must be used to survey an area for H-3 contamination. Due to their high efficiency, wipe test counters, such as liquid scintillation counters and gamma counters are very effective tools for identifying removable contamination. Gamma counters are used for counting wipe tests for photon emitters, such as: I-125, I-131, Tc-99m, or Cr-51.

The liquid scintillation counter is the best instrument for detecting removable beta contamination. Its efficiency is high for a wide range of b -emitting nuclides. It is the best choice for removable contamination surveys in radioactive material laboratories not working with pure photon emitters such as those mentioned above.

When one is unsure of the correct counting instrument to utilize, the Radiation Safety Office should be contacted at extension 1-9826.

Survey Frequency

Routine contamination surveys are required to be performed weekly when radioactive material is used. A good working practice is to perform a survey following each experiment involving radioactive material. If your laboratory uses radioactive material infrequently, you may suspend routine contamination surveys as long as you document the non-use. A record of the survey performed after the last use must be on file, however. It is a requirement of the MCG Radiation Safety Guide that non-use of isotopes be documented if no survey was performed for the week. For your convenience, there is a column on the survey form to easily document non-use of isotopes for the week.

Portable Instrument Check

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  • Check Radiation Safety Office instrument calibration label. Contact the Radiation Safety Office for calibration if calibration is greater than one year old.
  • Check batteries.
  • Turn on audio circuit.
  • Perform operational check. Expose meter to attached check source to verify that the instrument is operating.
  • Determine background reading by holding the meter over an area that is not contaminated.
  • Record on the survey form the date of the survey and indicate the instrument used and the required instrument information.

Survey Locations

Survey locations should be chosen to reflect both areas where there is a likelihood of detecting contamination and also for which contamination might be accidentally spread.

Some suggested locations:

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  • Countertops, including the edges
  • Fume hoods (aprons, sashes, sash handles)
  • Refrigerator and freezer door handles
  • Areas surrounding sinks designated for radioactive material
  • Floor, around working areas and lab entrances, waste containers, and fume hoods
  • Equipment used with radioactive material, especially common equipment
  • Doorknobs and the floor at entrances to the lab

Utilizing the provided laboratory survey form, indicate the survey locations with numbers corresponding to the locations.

Fixed Contamination Survey

Slowly move the probe over designated areas in a zigzag pattern, listening for an increase in the pulse rate as an indication of increased activity.

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  • If an area with a count rate more than two times background is identified, determine the extent of the contaminated area and decontaminate.
  • After decontaminating any area, resurvey to verify that the area is clean.
  • If the count rate is greater than two times background, repeat the decontamination and resurvey until levels are less than two times background.
  • Fixed contamination surveys do not have to be documented, but are an extremely good radiological practice. These surveys will help you locate possible contamination that can be quantified with a removable contamination survey.

Removable Contamination Survey

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  • Identify sampling points/locations.
  • Using filter paper, with a diameter less than two inches, wipe an area approximately 100 cm². This can be done by making an S-shaped curve about 12 inches long to increase the chance of picking up contamination.
  • Count the wipes on a liquid scintillator or gamma counter.
  • Divide the cpm given by the counter by the instrument efficiency to obtain dpm/100cm² and document on the survey form under the "R" on the left side of the survey location column corresponding to the location where the wipe was taken.
  • If an area of removable contamination is identified as being greater than 200 dpm/100cm², decontaminate and resurvey the area.
  • Repeat the cleaning and resurvey until the area has removable contamination less than two times background.
  • Record counting results, identified contamination, subsequent actions, and resurvey results.

Decontamination Guidelines

Any location in which the count rate is more than 200 dpm/100cm² should be considered contaminated.

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  • Clean the area with soap and water, a commercial cleaning solution, or a commercial decontamination solution specifically designed for radioactive contamination.
  • Resurvey the area to verify that the contamination has been removed. If the contamination appears to be fixed, mark the area and notify the radiation safety office for assistance. If high level contamination is discovered and assistance in decontaminating the area is required, contact the Radiation Safety Office.

Dose Rate Surveys

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  • Dose Rate surveys are performed with the detector 30 cm from the source.
  • Record the exposure readings under the "E" of the column corresponding to the location
  • The MCG action level for exposure rates at 30 cm is 2 mrem /hr.
  • If decontamination measures can not bring the exposure rate below 2 mrem/hr, contact MCG Radiation Safety.
  • Typically, only waste containers containing P-32 will exceed these levels and should be shielded to bring the exposure rate to less than 2 mrem/hr and the container should be disposed of as soon as possible.

Please feel free to contact the Radiation Safety office at extension 1-9826 with any questions regarding laboratory surveys or other radioactive material use issues.

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