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Contamination Monitoring
Contamination
control is a primary emphasis of the radiation safety program
at the NIH. Prudent contamination control involves careful planning,
process containment, protective coverings, personal protective
equipment, and avoidance of spills. However, contamination monitoring
is the most important element of a contamination control program.
Routine contamination monitoring at NIH is accomplished through
a variety of contamination surveys.
Daily Monitoring
Monthly Surveys
Survey Service
Comprehensive Surveys
Surveying for Removable Contamination
Counting Methods
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Daily
Monitoring |
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A daily
contamination survey is required each day after any work with
unsealed forms of radioactive material.
A daily
survey should cover the hands, lab coat, work area, floor
adjacent to the work area, shoes (top and bottom), and potentially
contaminated equipment.
- Any
area found to be contaminated must be decontaminated immediately
and re-surveyed. For decontamination
procedures click here.
- Contaminated
lab equipment that is dedicated for radioactive material
work (and likely to be re-contaminated) should be appropriately
labeled.
A portable
survey meter is acceptable as long as the sensitivity of the
instrument allows for efficient detection of the radionuclides
used.
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Recommend
survey meters for specific isotopes
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Isotope
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Meter
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P-32
and other high-energy beta emitters |
Geiger-Mueller
(G-M) detector |
I-125
and low-energy gamma emitters |
Sodium
iodide (NaI) detector |
Alpha
emitters |
Zinc
sulfide (ZnS) detector |
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When
using a portable survey meter, a count rate of more than two
times the background typically indicates the presence of contamination.
For low-energy
beta emitters, a smear survey is required (with an appropriate
counting technique). Portable survey meters are not appropriate,
due to their extremely low efficiency.
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Monthly
Surveys |
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A thorough
contamination survey of the entire lab module is required
to be performed once each calendar month. How lab modules
are determined can be found in the supplement for the survey
form.
A minimum
of ten locations must be surveyed, including at least two
smears on the floor. Smear locations should include areas
and items that have a higher potential for contamination.
Locations should be varied from month to month. Some suggested
areas to swipe for contamination are:
- Work
areas, lab benches, and desks
- Sinks
- Lab
equipment used with radioactive materials
- Refrigerators
and freezers
- Incubators
- Floor
adjacent to work areas and near hallway entrances
- Door
handles
Smears
should be counted by an appropriate counting technique (LSC
or gamma counter).
Any contamination
identified on a monthly survey must be decontaminated and
re-surveyed to satisfy the ALARA policy and demonstrate compliance
with the removable contamination limits for restricted areas.
Monthly
surveys must be documented on a Form NIH
88-12 , with:
- A diagram
of the laboratory
- Locations
of smears
- Description
of contamination identified during the survey
- Description
of re-survey results
The original
copy of the completed survey form must be submitted to DRS,
along with the counter printout. A copy of the survey must
also be kept on file in the lab's Radiation Safety Records,
and retained for a minimum of three years (for inspection
by DRS personnel or NRC inspectors).
If radioactive
materials are not used for the month a survey from must still
be completed and submitted to DRS, however smears are not
required.
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Monthly
Survey Service |
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Monthly surveys may be contracted out to Clym Environmental Services (Clym), DRS’s regulatory compliance support contractor.
The cost per lab module is $40 per month and may be paid by CAN, credit card, or purchase order.
To make arrangements for this service or for more information, contact Clym at 301-435-7953, 888-289-2324 or beth@clymenvironmental.com.
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Comprehensive
(Contractor) Surveys |
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In
addition to daily and monthly surveys, DRS maintains a staff
of contract technicians who perform routine comprehensive surveys
of all posted laboratories. The frequency is determined by DRS,
and varies for each lab (semiannual, quarterly, bimonthly, monthly,
or weekly).
Like daily
and monthly surveys, comprehensive surveys are designed to
monitor for contamination. However, these surveys also include
the measurement of radiation levels, inspection of lab records,
and evaluation of general compliance with the NIH Radiation
Safety Program.
Results
of a comprehensive survey are reported to DRS on a Radiation
Survey Report [contractor survey form], including any items
of noncompliance. The Area Health Physicist reviews each comprehensive
survey and determined an appropriate course of action.
A copy
of each Radiation Survey Report is also furnished to the Authorized
User of the lab, along with a set of Radiation
Safety Survey Information Sheets to describe the survey
results and suggest corrective actions.
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Surveying for Removable Contamination |
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Removable
contamination is defined as the radioactivity that can be transferred
from a surface by rubbing with moderate pressure. The level
of removable contamination is a primary focus of contamination
control at NIH.
The smear
(or wipe/swipe test) is the universal method of assessing
removable contamination, and the required method for monitoring
certain radionuclides.
A small
cloth, filter paper, or fiberglass disk is used to "wipe"
a sample of removable contamination. Fiberglass disks are
preferable for low-energy beta emitters, since they dissolve
or become transparent in liquid scintillation fluid.
A smear
should cover a minimum surface area of 100 cm² (approximately
equal to a square measuring 4" by 4"). Smears should
be dry, and taken using moderate pressure.
Smears
must be separated to avoid cross contamination. Smear packets
make this easy, and are commonly available at the NIH Self
Service Stores.
If counting
results are reported in counts per minute (cpm), they must
be converted to decays per minute (dpm) using the counting
efficiency:
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dpm
= cpm / counting efficiency
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The
maximum allowable limits for removable contamination are as
follows:
Restricted
areas |
Unrestricted
areas |
2,200
dpm/100 cm² beta/gamma and 220 dpm/100 cm² alpha |
220
dpm/100 cm² beta/gamma and 22 dpm/100 cm² alpha |
Data should
be maintained regarding the date and location of each smear.
For monthly surveys, the smears should be documented on the
survey form (NIH 88-12).
Smears
demonstrate that there is not more removable contamination,
but survey meter indicates a count rate of greater than 1000
cpm. Contact your Area Health Physicist for further instructions.
Don't waste your time scrubbing when you can't remove anymore
contamination. The Area Health Physicist can apply regulatory
limits for fixed contamination and advise you accordingly.
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Counting
Methods |
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Smears
to assess beta contamination must be analyzed with a liquid
scintillation counter (LSC). Smears to assess gamma contamination
should be analyzed with a gamma counter.
An LSC
may be used to assess gamma contamination, but close attention
must be given to the efficiency of the counter for the radionuclide
in question.
The manufacturer
or qualified service representative should calibrate counters
on an annual basis. This calibration determines the counting
efficiency for common radionuclides used at NIH, which must
be used to convert between cpm and dpm.
When possible,
liquid scintillation samples should be dark adapted before
analysis to reduce the effect of chemiluminescence. This can
be accomplished by simply leaving the rack in the counter
(with the cover closed) for 30 minutes or more.
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