STANDARD OPERATING PROCEDURES FOR

RADIATION CONTAMINATION SURVEYS

INTRODUCTION

Whenever unsealed radioactive materials (liquid solutions in vials, ampoules, test tubes) are handled (pipetted, transported, mixed, transferred) it is possible to contaminate laboratory benches, floors, and equipment. Every radiation-handling laboratory must be frequently surveyed to detect the presence of any radioactive contamination.

The purpose of this document is to provide guidelines and techniques for laboratory personnel responsible for conducting contamination surveys.

WHAT CONSTITUTES CONTAMINATION ?

Contamination is defined as the presence of radioactivity in an unwanted area. If the contamination is on a disposable item it may be thrown out into the radioactive waste.

WHAT ARE THE DANGERS FROM CONTAMINATION ?

Contaminated areas in a lab can lead to 3 problems;
  1. external radiation exposure to lab personnel
  2. internal absorption if comes into contact with skin or is inhaled
  3. interference with experiments being conducted in the lab

WHERE DO I LOOK FOR CONTAMINATION ?

Areas that have high potential for contamination; work benches, fume hoods, pipettes, syringes, centrifuges, the floor, inside refrigerators/freezers should be surveyed more frequently. Door handles, telephones, sink faucets should be surveyed once per month.

Persons responsible for conducting surveys should obtain a room diagram and a custom made survey form from the Radiation Safety Office (RSO) and use it to specify areas which will be frequently monitored. The areas should be numbered and identified on the Contamination Survey Report form.

HOW FREQUENTLY MUST I SURVEY ?

Every investigator agrees, as a condition of his/her authorization, that radiation-handling laboratories will be surveyed weekly for contamination. However, it is good laboratory practice to do a contamination survey after each use of radioactive materials.

If no radionuclides are handled in a particular week, no survey is required. However, the survey report form must be filled out and sent to the RSO or a "No Radioactive Material Use" form can be submitted to the RSO.  Since a handling lab may also store radioactive materials, a survey must be performed at least once per month, even if no radioactive materials were handled during that month.

Freezers, cold rooms, counter rooms, and centrifuge rooms must be checked monthly and the Contamination Survey Report form filled out and sent to the RSO.

52 weekly contamination survey reports are expected each year for each radiation handling laboratory.

12 monthly surveys are expected each year for rooms used only to count radioactivity, and those which are used only to store radioactivity such as cold rooms and freezers.

WHAT KINDS OF CONTAMINATION ARE THERE ?

Fixed contamination refers to radioactivity remaining on a surface after repeated decontamination attempts fail to significantly reduce the contamination level.

Removable contamination refers to radioactivity that can be transferred from a surface to an absorbent material, such as filter paper or cotton swabs, by rubbing with moderate pressure and swapping an area of at least 100 centimeters squared.

WHAT IS BACKGROUND RADIATION ?

Every radiation detector indicates counts even without the presence of radiation sources or contamination. This results from electronic "noise" and from the detection of cosmic radiation and other natural radiation sources in the environment. This is called "background" radiation. The background counting rate for most detectors is usually between 10 and 100 counts per minute (cpm).

You must determine the background count rate for your instrument before each contamination survey. Here are some ways to do this;

For a survey meter (GM or NaI), the background count rate is determined by a reading in a location that has no radiation sources within it, such as the hallway or an office.

For a liquid scintillation counter, the background count rate is determined by counting a non-contaminated wipe in the same volume of scintillation cocktail that you use for your wipes to survey for contamination.

For a gamma counter, the background count rate is determined by counting a non-contaminated wipe in the same size container that you will use for your wipes to survey for contamination.

WHAT IS A SURVEY METER ?

A survey meter is a portable device used to measure radiation by counting either the number of air ionizations in a GM tube or light flashes produced in a NaI crystal as a result of radiation's interaction with the detector. The GM detector is very efficient for detecting the presence of high energy beta particle emitters (such as P-32) and a NaI detector may be used to detect gamma and xray emitting radionuclides (such as Cr-51, I-125).

The advantage of a survey meter is that it can be used to quickly survey a large area. After determining the background count rate you can usually detect the presence of contamination by an increase in the rate of "clicking" of the audible signal given off by the detectors and/or by a swing of the meter needle from left to right.

The RSO recommends that all laboratories regularly using P-32 purchase a GM detector. These detectors may be purchased for about $600. Call the RSO for purchasing information or visit Ludlum Measurements, Inc. for various survey meters and their prices.

Laboratories using gamma emitters (Cr-51,and I-125) should purchase a NaI solid scintillation detector instead of a GM detector because of the higher detection efficiency. These detectors may be purchased for $600 - $800.

Labs using both P-32 and gamma emitters can buy a detector which has an internal GM detector and an external NaI solid scintillation detector.  An example of such a detector is Ludlum's Model 3-98.

WHAT IS WIPE TESTING ?

Wipe testing involves the swabbing of areas of a laboratory to determine if removable contamination exists. Swabbing may be done with Q-tips, filter paper, or any other dry absorbent material. Use normal pressure (equivalent to amount of pressure exerted in writing with a number 3 pencil). On a 90 cm. x 60 cm. (3' X 2') work bench we recommend that you obtain five wipes. Each wipe must cover at least 100 cm2 of area.

The swabs, or "wipes" are then counted in either a liquid scintillation detector or a gamma counter.

We recommend that you add 7 milliliters of glacial acetic acid to every liter of toluene or xylene based scintillation cocktail to minimize the chance of false high readings due to chemiluminescence.

HOW DO I KNOW WHEN I HAVE FOUND CONTAMINATION ?

To determine if an area has contamination, compare the background counts/minute to the counts/minute of the wipe or survey meter. If the ratio of counts/minute exceeds 3:1 the area contains significant contamination and should be decontaminated and resurveyed until the ratio is less than 3:1. For example, if your background is 25 cpm, you must decontaminate the area if the survey exceeds 75 cpm.

WHAT INSTRUMENTS DO I USE TO DETECT CONTAMINATION ?

Contamination can only be detected with either a wipe test using a liquid scintillation counter or a gamma counter, or an area survey using a survey meter, depending on which radionuclides are present and if it is fixed or removable contamination.

Survey meters cannot be used to survey for removable contamination. This can only be done with a wipe test. However, a wipe test can not be used to evaluate an area of fixed contamination. This can only be done with a survey meter.

HOW RELIABLE ARE RADIATION DETECTORS ?

All survey meters must be tested before each use with a radiation check source to see if the instrument responds. If the instrument fails to respond to within 20% of the normal reading it must be repaired and calibrated.

Contact the RSO for recommendations on the purchase of a check source. Costs range from $50 to $100.  Ludlum Measurements, Inc. has a Cs-137 check source with a holder available.

You should also check the background count rate every time you use the detector to ensure that it is not contaminated.

All survey meters used to survey for contamination must be calibrated each year.

Liquid scintillation and gamma counters must also be periodically checked to make sure that they are not contaminated and working properly. You should also perform a Chi-square test to determine counting reliability. The RSO can help you evaluate your counters.

WHAT IS DETECTOR EFFICIENCY ?

You must determine the efficiency of the detector which is used for contamination surveys as this information is requested by the NRC and is needed for the Contamination Survey Report form.

Detector efficiency is the detected fraction (usually counts per minute) of all radiation emanating from a "standard" radiation source. A standard is a radiation source with a known number of disintegrations per minute (dpm).

% Detector efficiency = (observed cpm of standard / dpm of standard) x 100

LIQUID SCINTILLATION DETECTOR EFFICIENCY

The efficiency of your liquid scintillation detector can be easily checked by inserting a "standard" vial into the counting rack. Apply the above formula.

A "standard" is a radiation source with a known quantity of dpm. Unquenched H-3 and C-14 "standards" are commercially available in the price range of $250 - $300. The RSO can provide purchasing information.

To estimate the efficiency of other radionuclides such as I-125 and P-32, prepare a "standard" by removing between 0.1 microcuries and 0.5 microcuries from your stock vial. One microcurie is equal to 2.22 * 10E6 dpm. Place it in a vial with the same amount of scintillation cocktail that you use with your wipe test. Take a one minute count and use the above formula.

SURVEY METER EFFICIENCY

The efficiency of most survey meter depends on the type of detector and window thickness. Place the radiation standard as close as possible to the detector surface.

To determine the efficiency for radionuclides such as C-14, S-35, P-32, Cr-51, and I-125, you can prepare a "standard" by removing between 0.1 microcuries and 0.5 microcuries from your stock vial. Place it in a planchet or on filter paper. Record the number of counts per minute and apply the above formula.

GAMMA COUNTER EFFICIENCY

To determine the efficiency of your gamma counter for gamma ray emitters such as I-125 and Cr-51, prepare a "standard" by removing between 0.1 and 0.5 microcuries from your stock vial. Place it in the same size vial that you use with your wipe test. Take a one minute count and use the above formula.

HOW DO I DECONTAMINATE ?

Decontamination is the removal of radioactive contamination and may be done using a commercial detergent such as "Count-Off"(New England Nuclear) or "Isoclean"(ISOLAB). When decontaminating use a paper towel or other absorbent material after allowing the detergent to settle on the contaminated area for several minutes. If the contamination is successfully removed, the absorbent material is contaminated and must be disposed as radioactive waste. After decontaminating the surface, repeat the survey procedure. Continue decontaminating and resurveying until the survey indicates the contamination is less than 3 times the background counts/minute.

HOW DO I RECORD RESULTS ?

The weekly and monthly reports must be filed with the RSO using a personalized Contamination Survey Report form, which can be custom made for each lab.

Most of the form is self-explanatory.

Give a brief description of the areas surveyed such as; fume hood, bench #1, floor by fume hood, etc.

If you are doing a wipe test with a 2- or 3-channel scintillation detector, record the results for each channel or set one of the channels for an "open window" (the entire spectrum).

If you are doing a area survey with a survey meter, record your results under the column labelled "GM".

If you are trying to detect only one contaminant, you can set the detector window for that radionuclide, or you can use a wide open window. Be consistent ! If you have calculated the efficiency of your detector for a particular radionuclide with a wide open window, then your wipe test results should be counted with a wide open window.

RSO CUSTOMIZED SURVEY REPORT FORMS

In order to reduce some of the paperwork for the labs, the RSO can make personalized survey report forms. Much of the information requested on the Contamination Survey Report Form will be repeated from week to week. Record the new information onto your personal survey form. Make copies of the customized report form.

INSPECTIONS BY RSO

The RSO is required by our NRC license to conduct a minimum of one annual inspection of all radiation-handling and storage rooms at UVM. The RSO has the authority to inspect as often as necessary. Part of the inspection involves a review of the recent history of surveys conducted by laboratory personnel, a spot check for contamination, evidence of smoking, eating, or drinking, and the presence of food or drinks in a refrigerator.

ASSISTANCE

RSO personnel are available to help with any of the above areas. Call x62570

CONTAMINATION SURVEY REPORT TO THE RADIATION SAFETY OFFICE

Working in a contamination-free laboratory builds confidence in conducting experiments. Surveying the laboratory for radiation contamination is essential to prevent cross-contamination of equipment, counting samples, and personnel.

REMINDERS

  1. Please use one form for each room.
  2. Send a weekly report for each radiation handling laboratory during active or inactive periods.
  3. Send a monthly report for each laboratory not used for radiation handling but used for waste storage, counting, or radionuclide storage in freezers and cold rooms.
  4. Use an open "window" on your scintillation counter if you are checking for more than one contaminant.
  5. Survey the areas most likely to become contaminated; e.g. work benches, floors, sinks, fume hoods, instruments, and waste storage areas.
  6. If the count rate of the surveyed area exceeds three times the background counting rate, it must be decontaminated and resurveyed until the count rate is less than three times background. Record the results of resurveys as indicated below.
  7. Call x62570 to obtain a brochure on methods of conducting surveys and decontamination techniques.
  8. No smoking, eating, or drinking is permitted in a radiation handling laboratory.
  9. If contamination is found, the investigator and other laboratory personnel must be informed.
  10. Send the contamination report to the Radiation Safety Office, Room 004 Rowell Building.