Radioisotope Safety – Monitoring for Radioactive Contamination

This appendix provides general guidance for monitoring and controlling radioactive contamination, and relating the monitoring results to the contamination limits specified in CNSC licences. The appendix also provides guidance on contamination monitoring instrument selection. Certain CNSC licences authorizing the use of unsealed nuclear substances contain a condition that states the regulatory criteria pertaining to radioactive contamination. The specified contamination criteria should be applied to all areas where unsealed nuclear substances are used/present, or have been used. Notwithstanding these limits, licensees should maintain levels of radioactive contamination as low as reasonably achievable (ALARA).

1.0 Elements of a contamination monitoring program

1.1 Method of measurement

Radioactive contamination may be measured directly or indirectly. Direct measurement generally means the use of portable radiation detection instruments to detect both fixed and removable contamination. Direct measurement may be used when background radiation levels are negligible compared to licence criteria. Indirect measurement only detects removable contamination by means of a sampling program.

1.2 Instrument selection

The ability of various radiation detection instruments to detect radioisotopes will vary by instrument and manufacturer. Guidance on the selection of instruments can be found in the section “Selection of contamination monitoring instruments”. For specific information on a particular model, contact the manufacturer.

2.0 Contamination monitoring

2.1 Purpose

The locations selected for contamination monitoring should be numbered on a plan of the work area. These locations should include working surfaces (such as benches, countertops or fume hoods), storage areas, and non-working surfaces (such as floors, instruments and equipment, door handles, light switches, sink taps and telephones). Several random locations should also be monitored. Too rigid a set of locations may overlook problem areas. The list of locations should be reviewed at a sufficient frequency to determine whether the list is current or new locations should be added.

2.2 Instrument checks

All instruments used for counting samples, such as liquid scintillation counters, well-type crystal gamma counters, gas-flow proportional counters, semiconductor gamma spectrometers and gamma cameras, should be routinely serviced according to the manufacturer’s instructions. Licensees must keep a record of the servicing with the date. Before monitoring for contamination, all instruments should be given operational checks as specified by the manufacturer; i.e., battery check, high-voltage check, response check and the background radiation level should be measured. Licensees must keep a record of the operational checks and background measurement. Similarly, the instruments should be used to measure the radiation from a standard such as a check source for every set of contamination monitoring measurements performed. The results of those measurements allow the determination of the instrument efficiency.

Instruments that are not operating within the parameters of operational checks, or that show anomalous background, blank or standard measurements, should not be used until their proper operation can be verified. These instruments should be tagged indicating that they are out of service and should not be used until their proper operation can be verified.

2.3 Frequency of contamination monitoring

Contamination monitoring frequencies, which should be at least weekly during periods of regular use, should conform to the requirements indicated in the licensee’s radiation protection program. When radioactive substances are not used for a prolonged period of time, contamination monitoring is not required, but such a period should be identified in the records.

2.4 Direct measurement of contamination using a portable meter

Direct measurement instrument readings include both fixed and non-fixed contamination. Subsequently, a direct reading may be used to satisfy licence criteria for non-fixed contamination.

2.5 Indirect measurement of contamination with wipes

Perform the following steps for indirect measurements:

• Wipe each of the locations shown on the plan of the working area with a filter paper, wipe or cottonswab lightly moistened with alcohol or water. Use one numbered wipe per location. If contamination is found, the contaminated area shall be identified and decontaminated.
• Wipe an area of 100 cm². Using uniform and constant pressure, wipe the entire area. In situations where wipes of 100 cm² areas are not feasible such as wipes of some equipment, light switches, etc., take note of the area of the surface wiped and ensure the appropriate conversion factor is applied.
• If necessary, carefully dry the wipe to prevent loss of activity. Since the contamination may be absorbed into the wipe material, the use of a wetting agent may lead to a significant underestimate of alpha and low-energy beta contamination with some counting methods.
• Count the wipes in a low-background area and record all results.
• If the wipes are to be counted on a contamination meter, the wipe should be smaller than or equal to the sensitive area of the detector. Note that the geometry of the wipe material (flat like filter paper or roundlike a swab) may change results.
• Clean any contaminated areas and monitor again. Record results before and after decontamination.

2.6 Decontamination

Any area that is found to have non-fixed contamination exceeding the contamination criteria shall be cleaned and monitored again. If the area cannot be cleaned to meet those criteria, the contaminated area shall be sealed or shielded until the criteria are met. All non-fixed contamination should be removed if possible.

Note: For short-lived radionuclides, the room or area may be posted and secured until the radioisotope decays.

2.7 Monitoring records

Contamination monitoring records shall be available for inspection by CNSC staff. These records should include:

• date of measurement
• make and model of the instrument
• monitoring locations
• contamination monitoring results in Bq/cm² before and after decontamination, if applicable
• results of operational checks and background measurements
• standard measurement results
• measured or predicted efficiency
• instrument servicing records should be recorded and updated as necessary
• demonstration that the chosen instrument and counting methods yield a minimum detectable activitybelow the applicable criteria

2.8 Regulatory criteria for radioactive contamination

The licensee shall ensure that for nuclear substances listed in Classes of Nuclear Substances non-fixed contamination in all areas, rooms or enclosures where unsealed nuclear substances are used or stored does not exceed:

• non-fixed contamination in all areas, rooms or enclosures where unsealed nuclear substances are used or stored does not exceed:
  • 3 Bq/cm² for all Class A radionuclides
  • 30 Bq/cm² for all Class B radionuclides
  • 300 Bq/cm² for all Class C radionuclides, averaged over an area not exceeding 100 cm²
• non-fixed contamination in all other areas does not exceed:
  • 0.3 Bq/cm² for all Class A radionuclides
  • 3 Bq/cm² for all Class B radionuclides
  • 30 Bq/cm² for all Class C radionuclides, averaged over an area not exceeding 100 cm²

2.8.1 Relating measurement readings to regulatory criteria

The readings from contamination meters can be related to regulatory criteria if the efficiency of the instrument for a specific nuclear substance is known. Instrument efficiencies for specific nuclear substances can be obtained from the manufacturer or determined using an appropriate standard of known activity. For mixtures of nuclear substances, identify the isotope for which the detector has the lowest response at the applicable contamination limit. Using the following equation, calculate the measurement results in Bq/cm²

$\text{Removable Activity =}\frac{\text{N-NB}}{\text{E}\times \text{60}\times \text{A}\times \text{F}}$

Where:

N = the total count rate in counts per minute (cpm) measured directly or on the wipe.

NB = the normal background count rate (in cpm) from the portable survey instrument or the count rate (in cpm) from a blank sample using a benchtop instrument

E = the instrument efficiency factor (expressed as a decimal, i.e. for 5 percent efficiency, E=0.05) for the radioisotope being measured. Consult the manufacturer or determine using a radioactive source with a known amount of activity in a counting geometry similar to that used when surveying for contamination.

60 = sec/min

A = area wiped (not to exceed 100 cm²) or area of the detector in cm² (for direct measurement)

F = the collection factor for the wipe (used only when calculating indirect wipe monitoring results).

If F is not determined experimentally, a value of F=0.1 (i.e. 10%) shall be used.

2.9 Minimum detectable activity

The minimum detectable activity (MDA) is defined as the minimum amount of activity in a sample that can be detected with a 5-percent probability of erroneously detecting radioactivity when none is present, and a 5-percent probability of not detecting radioactivity when it is present. For any given system designed to count and quantify radioactivity, the MDA should be calculated for the most restrictive scenario (i.e., for the nuclide with the lowest detection efficiency and the most restrictive regulatory criterion). The units of the MDA (Bq, Bq/gr, Bq/cm²) should be the same as those expressed in the licence or regulatory criterion, as applicable. The MDA in Bq/cm², can be calculated as follows:

${\text{MDA (Bq/cm}}^2)=\frac{\text{2}\text{.71 + 4}\text{.66}\sqrt{NB\times \left[\raisebox{1ex}{$T$}\!\left/ \!\raisebox{-1ex}{$60$}\right.\right]}}{E\times T\times A\times F}$

Where the terms NB, E, A and F have the same meanings as the section above and

T = the counting time, in seconds, for indirect wipe monitoring, and is the instrument response time for direct measurements (or the actual time if performing scalar counting). The instrument response time will vary between instruments and is a parameter which can be selected by the user on some devices; e.g., via either software selection of the actual time or “fast/slow” switch set to predefined timesspecified in the user manual. Other instruments may auto-select the response time based on the countrate. Longer response times will improve the MDA, but the instrument shall stay stationary over eacharea for a period that is at least as long as the response time.

Note: the efficiency, and hence the MDA of the instrument is highly dependent on the distance between the source and the detector. The MDA should be calculated for the distance at which the detector will be when monitoring.

2.10 Selection of contamination monitoring instruments

The MDA for a nuclear substance will depend on both the types and energies of radiation emitted by that nuclear substance and on the type of detector used. In general, there are three basic detector design considerations that will impact instrument sensitivity, and each of these parameters will have a different impact, depending upon the type and energy of radiation being detected:

2.10.1 Window thickness and composition

Consideration should be given to whether the window density is small enough to allow the radiation emitted by source to enter the detector. This is critical for low-energy beta radiation and alpha radiation, which can be completely absorbed even by materials as thin as a sheet of paper. Note that some isotopes, such as H-3 or Ni-63, cannot be detected by most instruments, because the beta radiation they emit gets completely absorbed within the window. For such isotopes, indirect monitoring using liquid scintillation is generally the best choice.

2.10.2 Detector density

Every radiation detector functions by detecting interactions between the radiation and a material within the detector. There are two broad classes of detectors: gas-filled detectors, and solid or liquid scintillators. Gas-filled detectors, such as Geiger detectors and proportional counters, will generally work well for detecting alpha or beta radiation, since these types of radiation will cause interactions even in low-density materials. Conversely, gamma rays may readily pass through a low-density gas without interaction, especially at high energies. Solid scintillators, such as NaI detectors, are generally much better suited to detecting gamma radiation. Thin crystal detectors are suitable for low-energy gamma emitters such as Tc-99m, while thicker detectors will enhance sensitivity for high-energy gammas such as those from Cs-137 or Co-60.

2.10.3 Detector output

Every time radiation interacts with a detector, a tiny amount of energy is released within the detector. This energy is then converted into an electronic signal that can be measured. Some detectors, such as Geiger counters, produce uniform pulses which can be counted. Other systems, such as scintillators or proportional counters, may produce a signal that is proportional to the amount of energy released in the initial radiation interaction. This can be used to distinguish between different types of radiation or different energies of radiation of the same type. Such detectors are useful in applications where distinguishing between multiple different isotopes may be necessary.

Hand-held contamination monitoring instrument*	Recommended applications**
Thin-window G-M detector	Beta emitters, alpha emitters
Gas-filled proportional detector	Variable, refer to manufacturers specifications
Thin-crystal sodium iodide scintillation detector	Low-energy gamma emitters (<200 keV)
Thick-crystal sodium iodide scintillation detector	High-energy gamma emitters (>200 keV)
Organic/plastic scintillation detector	Generally specifically designed for alpha and beta detection with low background. Gamma detection is variable; refer to manufacturers specifications.
Zinc sulphide scintillation detector	Alpha emitters
Thick zinc sulphide scintillator with proprietary discrimination	Beta emitters, alpha emitters, gamma emitters

Non-portable monitoring instruments (wipe counters)	Recommended applications**
Liquid scintillation counter	Alpha and beta wipe samples, especially for very low-energy beta emitters such as H-3, Ni-63, and C-14
Sodium iodide well counter	Gamma wipe samples, allows for spectroscopic analysis of different isotopes if multiple isotopes are being used
Gas-flow proportional counter	Gamma wipe samples, allows for high-resolution spectroscopic analysis of different isotopes if multiple isotopes are being used
Semiconductor gamma spectrometer (High Purity Germanium)	Gamma wipe samples, allows for high-resolution spectroscopic analysis of different isotopes if multiple isotopes are being used

* Ion chambers are another major type of portable detector. These devices are intended for measurement of radiation dose rates rather than contamination. In general, they are poorly suited to contamination monitoring and should not be used for this purpose.

** Nuclear substances that decay via emission of alpha or beta particles often also emit gamma rays. Many isotopes, especially high atomic number materials such as uranium and radium, may exist in equilibrium with the other isotopes in their “decay chain”, which in turn emit many different types and energies of radiation. When choosing a contamination monitor, it is important to consider exactly what types of radiation will be present. For example, positron emission tomography (PET) isotopes decay by the emission of a positron (beta+) which in turn produces two high-energy (511 keV) gamma rays. It is the gamma rays that are of primary importance in the use of these isotopes, and a thick crystal NaI scintillator will detect these gammas very efficiently. However, a thin window Geiger detector will detect the beta+ emissions even more efficiently, and will generally have a much lower background (NB) count-rate.

Note: For further information on nuclide specific instrument selection, please see the CNSC’s Radionuclide Information Booklet

2.11 Detector efficiency

The detector efficiency depends upon:

• the type of detector (ex: Geiger-Müller, NaI scintillation, plastic/organic scintillation, proportional counter…)
• the detector size and shape
• the distance from the detector to the radioactive material
• the nuclear substance and the type of radiation measured (alpha, beta and gamma radiations and their energies)
• the backscatter of radiation toward the detector
• the absorption of the radiation before it reaches the detector (by air, by the material itself and by the detector covering)

The detector efficiency can be determined by:

1. Counting an appropriate standard source of known activity with your detector, in counts per second (cps).
   $\text{Efficiency = }\frac{\text{(detector count rate - background count rate)}}{\text{known activity of standard source}}$
2. Referring to the documentation supplied by the vendor for your specific nuclear substance(s). If not specified, by contacting the vendor for the required information.