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Licensee Outreach Webinar – Contamination Monitoring

Webinar sessions were offered to Directorate of Nuclear Substance Regulation (DNSR) licensees to provide information on the requirements for contamination meter calibration and methods for establishing detection efficiency.

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Factors that may cause the efficiency of an ASP-2 with LEG-1 to decrease year after year while using the same source (Am-241)

It is possible for alpha sources to deteriorate over time because of the very thin plated layer over the radioactive material, which is designed to allow alphas to escape. Dust, debris and/or oxidization can accumulate, add shielding and affect the emission rate.

Beta sources are less susceptible because they are not as easily shielded. It is also possible for the meter to drift, which is why we want to establish or confirm efficiency annually.

Users should consult the manufacturer of these calibration sources to determine appropriate shelf lives.

Content of calibration report or certificate

The CNSC does not mandate or impose requirements on the calibration procedure for calibration service providers. Appendix D.7 of REGDOC-2.7.1, Radiation Protection, provides a list of what should appear in a record of calibration. It is up to the licensee to ensure that its provider includes the appropriate information.

Length of counting time required for an accurate scan when checking hands/feet for contamination

It depends on whether the checking of the hands and feet procedurally translates to “clearing down to X Bq/cm2.” If so, and if counting time can be controlled, it will be dictated by the established efficiency for the target isotope (or worst-case isotope) and the background of the instrument, and by ensuring that the minimum detectable activity (MDA) is about 0.5 times the value of X Bq/cm2.

However, many licensees simply “check” their hands to make sure they are below 2x background using a contamination meter in ratemeter mode. In this case, the counting time is the integration time of the meter (typically 2 to 7 seconds).

Calibrating a large area detector (like the Berthold LB-124B, detector window 150 cm2)

In cases where the instrument’s active detector window size exceeds 100 cm2, when converting counts per minute (CPM) to Bq/cm2, 100 cm2 should be used (unless the actual surface area of the contamination is known, such as in the case of skin contamination).

This is because it is unlikely that any routine surface would be evenly contaminated over an area larger than 100 cm2. Berthold’s own internal conversion factors between counts per second (CPS) and Bq/cm2 assume 100 cm2, not the full active detector area.

Determining the activity on a surface that is larger than the surface area of the detector

The perimeter, and by extension the surface area, of a large spill can be delineated with a contamination meter by finding the transition between an obvious count rate and one that approaches background. Next, multiple locations within the spill area can be counted and converted to Bq/cm2 using a contamination meter with an established efficiency for the target isotope. These results can then be averaged to establish the Bq/cm2 value for the spill, and multiplied by the surface area to estimate total activity spilled.

Frequency for determining MDA

First, the licensee must be able to demonstrate, using a contamination meter, that 2x background is well below (<0.5x) the surface contamination criterion stipulated in the licence. In order to demonstrate this, the detection efficiency for the target isotope (or most difficult to detect isotope per isotope class) must be known.

It is the detection efficiency that must be verified annually. If the detection efficiency has changed more than +/- 20% from the previous year, it should be updated along with the MDA. 

Note that once detection efficiency has been established for the target isotope and a specific detector, that information can be transferred to a long-lived source, such as Cs-137, and then the target isotope detection efficiency can be verified annually with the Cs-137 source using the transfer factor.

Criteria for long-lived sources used to transfer efficiency information (slide 24 of the presentation)

Ideally, a transfer source would have the same type of emissions (alpha, beta, gamma). However, it does not necessarily need to be same, depending on the detector used.

A Cs-137 gamma source can act as a transfer source for Tc-99m using a standard GM pancake, even though Tc-99m emits both gammas and electrons. However, when choosing a suitable surrogate source for the target isotope as a calibration source, the emission type and energies must match as closely as possible.

Survey meters with dual functionality (dose rate and contamination)

Pancake meters are thin-window GM tubes that are used for contamination monitoring. They are sensitive to alpha, beta and gamma radiation. They are not only suitable for I-131 contamination monitoring, but are also an instrument of choice.

Pancake meters should not be used as survey meters (i.e., to measure gamma dose rates) unless they are fitted with a proper H*10 filter supplied by a reputable manufacturer.

Measuring the efficiency of all isotopes used in the department / using the isotope of the lowest efficiency if within the same class to determine the MDA

The MDA and efficiency should be established for the most difficult to detect isotope per isotope class. However, common sense must also prevail. If, for example, 99% of the time Tc‑99m is used versus 1% for Ga-67, then focus on Tc-99m.

Or, for example, if a specific room uses only I-131, but other Class B isotopes are used in the department that are more difficult to detect, I-131 efficiency and MDA must be known for measurements in that one room.

Bottle sources and sheet (flood) sources

There are a few issues with using these sources to determine detection efficiency for contamination meters. One is that only gamma counting efficiency can be established because the electron emissions are blocked. The resulting efficiency may be lower than the true efficiency. A specific problem with a sheet source is that, because of its large size, it does not represent typical contamination on a surface.

In addition, since gamma counting is being performed, the efficiency may be overestimated because of all the gamma emissions from parts of the sheet source that are further away from the detector.

It is best to use 100 cm2 (or smaller) calibration sources. A specific issue with a quality control (QC) source for a dose calibrator is that the activity may be too high. It may be better to use theoretical efficiencies for detectors rather than sheet or QC sources.

Use of a decommissioning tool for contamination monitoring

The DNSR decommissioning tool can be used to establish a CPM trigger level that corresponds to a Bq/cm2 value below the regulatory criterion for the target isotope.

The tool will do the calculation for you. You can also copy/paste the “Data – open criteria” sheet and use it for weekly swipes. You could save one file per week or a sheet per week in the file.

Frequency for efficiency determinations - CNSC expectations

CNSC inspectors will review the annual efficiency determination like they do detector calibration reports. Please consider that efficiency must be determined or confirmed to be within +/-20% of the original value annually. This could be as simple as an accuracy or constancy check, with a long-lived check source used as a transfer source. The verification must be documented.

List of surrogate sources for isotopes used in nuclear medicine

A list of suggested surrogate sources for isotopes used in nuclear medicine is available upon request by contacting your licensing officer at licence-permis@cnsc-ccsn.gc.ca.

MDA calculation requirements for multiple isotopes used in a department

MDAs do not need to be calculated for each isotope used in a department. The MDA and efficiency should be established for the most difficult to detect isotope per isotope class. However, common sense must also prevail. If, for example, 99% of the time Tc-99m is used versus 1% for Ga-67, then focus on Tc-99m. Or, for example, if a specific room uses only I-131, but other Class B isotopes are used in the department that are more difficult to detect, I-131 efficiency and MDA must be known for measurements in that one room.

Service providers who assist with detector efficiency measurements

The CNSC does not endorse any service providers. However, there is a list of service providers on the website.

Use of National Institute of Standards and Technology (NIST) sources in bottle geometry

One of the issues with using this type of source is that the bottle will block electron emissions. This may or may not be a problem, depending on what detector you use for contamination measurements. If the detector is sensitive to betas/electrons, you are only establishing gamma detection efficiency and missing part of the picture.

The other issue is that your dose calibrator source may be too high in terms of activity. It is best to use a calibration source whose activity is in the range of the regulatory criterion (tens of Bq/cm2, not MBq). That said, it can still be done, it is just not ideal.

Responsibility for monitoring a service provider’s procedures when a licensed provider is used to perform detector calibrations and determine efficiencies

Radiation safety officers are responsible for ensuring that the service provider is using a calibration source that is the same as the target isotope, or an appropriate surrogate, and that the calibration source used is designed to allow all emissions to escape that are being detected by the licensee’s detector. For example, many third-party providers will use Co-57 as a surrogate for Tc-99m for thin-window GM, but most Co-57 sources do not allow the electrons to escape.

The information on Co-57 gamma efficiency is still useful but does not provide the entire picture. If it is determined that the efficiency established by the third-party provider is not complete and underestimates efficiency, the information may still be used because, although it is less accurate, it is conservative.

Difference between 4pi (4π) and 2pi (2π) efficiencies

4pi (4π) refers to 360 degrees, and 2pi (2π) to 180 degrees. So if a calibration certificate refers to 4pi (4π) emissions, it is all of the emissions from the source, in all directions. If it refers to 2pi (2π) emissions, it would only be the emissions travelling in one direction towards the detector. Since we are concerned with absolute efficiency (CPS/source activity), we want to use 4pi (4π) emissions – in other words, the true activity of the source as a reference when establishing efficiency.

Use of cobalt-57 sources for detection equipment validation

One issue with this is that only gamma counting efficiency can be established because the electron emissions are blocked. Another issue is that if a large sheet source is used, its large size does not represent typical contamination on a surface. Since gamma counting is being performed, the efficiency may be overestimated because of all the gamma emissions detected from parts of the sheet source that are further away from the detector.

It is best to use 100 cm2 (or smaller) calibration sources. If the Co-57 source is small, it can be used to establish gamma-only counting efficiency as a surrogate for Tc-99m. Electron counting efficiency can then be added to the gamma efficiency by using a beta emitting surrogate or using a theoretical value.

Use of non-NIST sources

NIST-traceable sources are the most common traceable sources available here in North America. However, NIST is a U.S. institution, and there are other equivalent institutions in other countries that deal with standards.

Non-traceable sources can be used but are less desirable, as the tolerance on the reference activity may either be unknown or high. For some common untraceable Cs-137 sources, the uncertainty may be as high as 50%.

In these cases, it may be better to use theoretical efficiency values. Other sources may not be traceable to a recognized standard but may still have a tolerance listed on the source or source documentation. If this uncertainty is within +/-20%, it can be used.

Safety precautions for source preparation

Standard lab personal protective equipment will suffice, such as a lab coat, latex/nitrile gloves and safety glasses.

If working with volatile isotopes, such as those of radioiodine, respiratory protection and/or the use of fume hoods may be required.

Third-party contamination meter calibration

The CNSC’s expectation for the calibration of gamma survey meters is that they measure within +/- 20% of the true dose of radiation throughout their dose rate operating range.

The CNSC’s expectation for counting instruments used to quantify contamination levels sent for calibration was not clearly defined prior to the establishment of section 25 of the Radiation Protection Regulations (RPR 25) and REGDOC 2.7.1. As a result, some third-party calibrators may only check the electronics using a waveform generator to send pulses into the circuit to verify the response rate. The actual radiation detector might not be checked, and the certificate received would serve as a counting verification but it would not fit with what we currently define as a contamination meter calibration. Other calibrators may provide detection efficiencies for a select few standard isotopes, which may or may not be suitable surrogates for a licensee’s target isotopes.

In the case of contamination meters, what we now mean by calibration is the determination of detection efficiency (as outlined in REGDOC-2.7.1) for the target isotope(s).

Legal requirement for annual efficiency determination

When a requirement is set out in the regulations, and no specific time frame is stipulated (as in the case of RPR 25), the default frequency is annually. If the annual accuracy test is within +/- 20% of the original accuracy test, performed after the efficiency was originally determined, there is no need to update the efficiency.

The annual calibration (i.e., establishing or confirming the efficiency of contamination meters) would be considered done by virtue of the accuracy test. 

In addition, establishing the efficiency of every isotope is not necessary. Generally speaking, detection efficiency should be established for the most difficult to detect isotope per isotope class.

Use of surrogate sources for 1 cm2 check sources

Standard sources should not exceed 100 cm2.

Document retention requirements for efficiencies carried out by a service provider

Licensees are responsible for retaining records on calibration and on the determination of instrument efficiency for a period of one year following the licence expiry date, regardless of who has carried it out.

Percentage of error when using a dose calibrator

Any measurement will include various sources of uncertainty. For example, the activity uncertainty associated with a traceable standard source may be +/-5% to 10%. For a non‑traceable source, it may be +/-20% to 30%. Other sources of uncertainty include the distance between the source and the detector. At a minimum, the CNSC recommends that the counting uncertainty be calculated and presented with the results, and would require it for a permanent decommissioning.

Ideally, the standard source with the lowest level of uncertainty should be used.

Efficiency values to be used for contamination from multiple isotopes (nuclear substances)

Generally, detection efficiency should be established for the most difficult to detect isotope per isotope class.

Suggested surrogate source for F-18

Cs-137 is suggested for beta and gamma counting. A list of surrogate sources is available upon request.

Use of Tc-99m for efficiency determination for both Ga-67 and Tc-99m for decommissioning

Even though the detection efficiency is lower for Ga-67, if the Tc/Ga quantity ratio in the department is 10 or more, for example, it would be reasonable to decommission based on Tc‑99m.

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