Monitoring is an essential component of any radiation safety program. It involves the regular and routine measurement and/or assessment of factors relevant to radiation safety and takes the following forms:
- Monitoring of radiation dose or dose rate.
- Area monitoring, i.e. measurement of radiation dose rate at various points in an area where a radiation-emitting machine or equipment is located, or where radioactive sources are stored, handled or used.
- Technique monitoring, i.e. measurement of the dose or dose rate applicable to specific persons or specific locations, during complex procedures involving radiation sources.
- Personnel monitoring, i.e. measurement of the total dose received by individual radiation workers over a specified period of time.
- Monitoring of radioactivity (count rate):
- measurement of radioactive contamination on surfaces(i.e. benches, floors) and equipment;
- measurement of radioactive contamination on exposed areas of skin and on clothing of radiation workers;
- monitoring of ingestion, inhalation and injection of radioactivity by workers handling unsealed radioisotopes.
- Medical surveillance of radiation workers.
Environmental Health & Safety conducts these radiation surveys. The purpose is to ensure that the room and equipment shielding, and the local practices and procedures are satisfactory. Permit Holders, Heads of Department, Departmental Radiation Safety Officers and individual radiation workers should collaborate with the Environmental Safety Office in these surveys and should report any situation or change in procedure, which may warrant special investigation.
Permit Holders have the responsibility of carrying out recommendations arising from the surveys and of making the results known to their staff. The Permit Holder must retain a copy of each Radiation Survey Report pertaining to his department or laboratory, together with a record of any action taken as a result of such report. These records must be made available on request to authorized persons such as the Radiation Safety Officer and CNSC inspectors. Permit Holders are required to keep radiation survey reports and all related documents up to 3 years in their McGill Radiation Log Book or in other files.
Radiation Users and Nuclear Energy Workers whose main source of exposure is from external beta or X-ray and gamma sources may be subject to routine, continuous monitoring by means of a thermoluminescent dosimeter (TLD) which is worn at all times during working hours. Such monitoring is mandatory for NEWs and recommended for Radiation Users. In addition, NEWs who may be exposed to neutrons must carry a separate neutron badge* . The National Dosimetry Service of the Radiation Protection Bureau, a service of Health Canada in Ottawa, provides the TLDs as whole body and extremity dosimeters. To obtain more information or to subscribe to the service, contact the McGill Radiation Safety Officer. The following paragraphs refer specifically to TLD badges for the monitoring of photons and high-energy electrons. Neutron monitors are considered in Section 5.3.5.
* A person who is likely to be exposed to neutrons to a significant extent must be designated as a NEW, irrespective of the level of and/or or X-ray and radiation to which he/she is also exposed.
The whole body TLDs provided by the National Dosimetry Service comprise two small plaques of lithium fluoride (approx. 3 mm x 3 mm x 1 mm thick) mounted on an aluminum plate, all contained in a plastic holder provided with a pocket clip. The base plate is encoded (by a series of punched holes) so that an individual monitor can be identified.
The National Dosimetry Service provides the whole body TLD badges on a 3-month or 1-month cycle, with distribution being on a department or laboratory basis. The Permit Holder must make the necessary arrangements for personnel monitoring of radiation workers in his/her own department or laboratory and is also responsible for the cost of the service. An outline of the procedure is given below, further details being available from the Environmental Safety Office:
- usually the Permit Holder delegates the organizational aspects of personnel monitoring to another person who is in daily contact with the staff concerned. It is essential for this person to be properly briefed. For convenience, he/she will be referred to in this section as the Monitoring Supervisor;
- the individual radiation worker retains his or her badge or monitor with his/her name attached. The badge is changed every cycle;
- the changeover dates (beginning of a new monitoring period) vary from group to group and the Radiation Protection Bureau (RPB) will notify each monitoring group. Shortly before the changeover date, the Monitoring Supervisor collects the old badges and provides the new badges from everyone in his/her group; and
- the "exposed" plaques are measured automatically by the RPB, and a report of the radiation dose received by each worker is sent to the Radiation Safety Officer (RSO) at Environmental Health & Safety. The RSO will then:
- examine the report and note any unusually high value;
- send a copy to the department or laboratory (minimum once a year or on request);
- retain the original report for record purposes; and
- investigate any dose which is either:
- over 10mSv per 2 consecutive periods of 3 months in the case of an NEW; or
- is significantly greater than the "normal" value (exceeding 0.5 mSv for 2 consecutive periods of 3 months) for the individual concerned or for the group of workers who are engaged in similar activities, as in the case of a Radiation User or the general public. (See section 3.7 for details)
The individual radiation worker must:
- take good care of their assigned monitor at all times;
- wear the monitor at all times during working hours. The badge may be worn either at the wrist as a bracelet or on the finger as a ring and finally at the head/neck area or chest height as a whole body monitor. Where protective clothing such as a lead apron is worn, the badge must be placed under the protective clothing since its function is to record the radiation reaching the body, not the radiation reaching the protective clothing;
- guard the badge as a personal monitor, issued to a named individual. Under no circumstances should the badge be loaned to another person;
- take care that the badge is not dropped or accidentally placed in a position where it could be exposed to a level of radiation higher than the ambient level;
- take care that the badge is not accidentally splashed or otherwise contaminated by a radioactive liquid;
- take care that, outside working hours, the monitor is stored in a safe place which is well away from any radiation source and from any source of intense heat such as a radiator; and
- report any problems with the monitor to the Monitoring Supervisor or to the RSO.
See Appendix G for more details.
Personnel monitoring, of the type described in the preceding paragraphs is a satisfactory general indicator of the whole-body dose arising from external sources of penetrating radiation such as X- and gamma-rays. However, the system has some important limitations:
- the badge reading can be interpreted in terms of a whole-body dose only if the ambient radiation is penetrating, i.e., photons in the MeV energy range, or at least several hundred keV; and isotropic i.e., either the radiation comes from several directions or the radiation worker changes his orientation frequently with respect to the source of radiation. If these conditions are not met, then the badge reading represents only the dose to superficial tissues and/or to part of the body such as the front of the trunk;
- the badge does not record any additional dose received by the extremities and/or face and neck in some procedures;
- the badge does not record doses due to low-energy-particles such as those from tritium(H-3), carbon-14(C-14) and sulphur-35(S-35);
- the 3-monthly cycle may be too long for individuals whose work carries a higher-than-average risk of radiation exposure; and
- the badge does not record internal exposure arising from ingestion, inhalation or injection of radioactive materials.
The first limitation (1) cannot be overcome. Each situation must be assessed to determine what the badge reading represents. In most cases, the badge reading is so low that it is of no importance whether it represents a whole-body or a partial-body dose.
Limitations 2 and 4 may be overcome by the use of monthly monitors or monitors such as TLD "finger badges" or pen-type pocket dosimeters. Arrangements for the issue of these monitors may be made through the Radiation Safety Officer. Monitoring of this type is usually considered as "Technique Monitoring" i.e., it is carried out as a special investigation to determine the relationship between the badge reading and the dose received by other parts of the body.
Limitation 3 can only be resolved by the purchase of special monitors, which are sensitive to low-energies; this comes under the heading of "Area Monitoring". Limitation 5 is overcome to some extent by the use of bioassay procedures as discussed in Section 5.5.
Radiation workers who may handle more than 50 MBq of phosphorus-32 (P-32), strontium-89 (Sr-89), strontium-90 (Sr-90) or yttrium-90 (Y-90) are required to wear an extremity TLD as a ring badge in addition to the whole body TLD. The National Dosimetry Service (NDS) also supplies the extremity TLDs and for additional information on extremity TLD personnel monitoring contact the RSO.
Radiation workers who may be exposed externally to neutrons (i.e. for unshielded neutron sources in excess of 20 GBq) are required to wear a special "neutron" badge in addition to the ordinary TLD badge. The National Dosimetry Service (NDS) also supplies the neutron monitor. However, users should be aware that the present neutron badge is somewhat unsatisfactory and the safety of the staff will depend on a thorough area survey (using a calibrated neutron survey meter) combined with good working practices. Further details on neutron personnel monitoring may be obtained from the RSO.
Portable monitors, suitable for measuring contamination arising from the type of radionuclide stored or used in that laboratory must be available. This should be used regularly to monitor accessible surfaces on benches, walls, floors and equipment, whenever there is a possibility that a liquid radioisotope has been splashed or spilled. The monitor should also be used on exposed skin surfaces and clothes of radiation workers when procedures involving the manipulation of significant activities of radionuclides are completed.
Any laboratory where unsealed radioisotopes are stored and/or used must undergo regular surface contamination checks called "wipe tests". Suspected surfaces are wiped with a moist swab of raw cotton or filter paper in order to remove contamination and the swab is then offered to a sensitive detector, such as a liquid scintillation counter or gamma well counter. Allowing for statistical uncertainties in low-level counting, any count rate which appears to be above the 0.5 Becquerels per centimeter squared (Bq/cm2) is regarded as evidence of contamination. Decontamination procedures are required and discussed in Section 5.8. Wipe tests are done at least once a week for frequent users (i.e. daily or weekly) or after each radioisotope procedure for occasional users (i.e. monthly or bimonthly). The results must be placed in the McGill Radiation Log Book and kept for a maximum of three years. For more complete details on wipe tests consult Section 5.7.
In addition to regular monitoring carried out by the radiation workers in a laboratory, every laboratory handling unsealed radioisotopes is subject to regular inspection and survey by Environmental Health & Safety. Such inspections are carried out annually, but more frequent surveys may be needed in some cases. The Permit Holder should request a special survey whenever an accident has occurred which might have resulted in contamination, or whenever a complex new procedure has been carried out for the first time. The rules listed in Section 5.2 for "Area and Technique" surveys carried out by Environmental Health & Safety also apply to the Contamination Surveys discussed here.
The CNSC has established effective dose limits for persons during a specified period. For the purpose of calculating the effective dose, one of the parameters used is the annual limit on intake (ALI). The ALI is defined as the activity, in Becquerels, of a radionuclide that will deliver an effective dose of 20 mSv during the 50 year period after the radionuclide is taken into the body of a person 18 years or older or during the period beginning at intake and ending at age 70 after it is taken into the body of a person less than 18 years old. See Appendix C, for a list of ALI according to radioisotope.
Radioactive materials may be ingested, inhaled or injected through the skin. If the absorbed radionuclide is a gamma-emitter, it can be detected and measured by a sensitive external counter. This is the procedure used for measuring the ingestion of radionuclides such as I-125, which concentrates mainly in the thyroid gland. A thyroid bioassay service is operated by Environmental Health & Safety and is available to all workers who handle iodine radioisotopes. Radiation workers in this category should be monitored regularly, at a frequency which depends on the particular isotope handled and, in the case of short-lived isotopes, on the individual workload.
The most commonly used radioisotope of iodine is I-125 (half-life 60 days) and in this case the monitoring frequency is one month, irrespective of the workload. The CNSC Regulatory Document R-58 requires persons handling I-125 or I-131 at above specified activity levels to submit to thyroid monitoring. These tests must be done within 5 days of the iodine manipulation. For details and arrangements for thyroid scans contact Environmental Health & Safety. The minimum activities requiring evaluation are listed below:
|Types of operations||Activity handled in unsealed form|
|Processes carried out in open room||2 MBq (54 µCi)|
|Processes carried out in fume hood||200 MBq (5.4 mCi)|
|Processes carried out within closed glove boxes||20 GBq (540 mCi)|
Bioassay procedures for other radionuclides have been developed and made available to all laboratory personnel. Body burden assessments can be performed using two methods:
- Excreta analysis (urine and faeces); and
- Whole-body counting (approximate for gamma emitting radionuclides)
In accident or emergency situations Environmental Health & Safety is able to carry out, or to make arrangements for any type of bioassay which may be required.
The radiation doses received by the great majority of radiation workers are so low that no correlation can be demonstrated between dose and any known physiological effect, and no deleterious effect can be unequivocally linked to a radiation exposure. Consequently, medical surveillance has no role in assessing the effectiveness of a program of radiation protection. Surveillance is therefore limited to the minority of radiation workers who, because of the nature of their work and/or the type of radiation sources they handle, are classified as Nuclear Energy Workers (NEWs). These workers are more likely than non-NEWs to receive a significant dose in the event of an accident.
Any person whose position is classified as a NEW should undergo a complete pre-employment medical examination. A NEW whose whole-body dose in the preceding 12-month period, as evidenced by personnel exposure records, exceeds 50 mSv (5 rem) must undergo further medical examination. This is also required in cases of accidental over-exposure (real or suspected). Where the over-exposure is severe (200 mSv (20 rem) or more), a cytogenetic examination is required.
The most effective method for measuring radioactive surface contamination is the wipe test technique. This procedure will indicate only the levels of removable contamination. No removable contamination should be tolerated. Begin the wipe test with a sketch of the laboratory that includes marked and numbered locations to be examined. Usually, 10 to 20 locations are adequate for most laboratories. The wipe test method includes the following steps:
- Moisten a filter paper (2 cm) with alcohol or water, and wipe over an area approximately 100 cm2 (10 cm x 10cm). Please note dry wipes can also be used.
- Place the filter paper in a vial and count in a gamma well counter (for gamma and/or X-ray contamination) or in a vial containing scintillation fluid, shake and count in a liquid scintillation counter for alpha and/or beta contamination. If a single radioisotope is being used in the laboratory, then appropriate window settings and a quench curve (only for liquid scintillation counting) are recommended. However, if several radioisotopes are being handled or contamination is unknown, then operate at full spectrum with no quench correction. It is suggested that before counting, the vials set aside for liquid scintillation counting be stored for 24 hours to reduce chemiluminescence.
- 3. Contamination is present if wipes exceed 0.5 Becquerels per centimetre squared (Bq/cm2). The contaminated area must be cleaned with water and detergent or with a commercial decontamination solution such as Decon75, Count-Off, Rad-Con or Contrad 70. Begin with the perimeter of the spill area and work towards the centre, being careful not to spread the contamination during cleaning. Repeat the wipe testing until the measurements are at or below 0.5 Bq/cm2.
The formula used to calculate "Bq/cm2" is:
|Bq/cm2||=||CPM net / [ C.E. x 60 x 100 x Weff ]|
|Bq/cm2||=||Becquerel per centimetre squared|
(use the counter C.E. for that radioisotope, or for simplicity use a C.E. of 50% for all radioisotopes)
|60||=||is for 60 seconds and related to counts per second (cps)|
|100||=||for a maximum surface area of 100 cm2 (10 cm x 10 cm)|
|Weff||=||Wipe Efficiency (use 10% or 0.1 for wet wipes, and 1% or 0.01 for dry wipes)|
In radiation safety, "decontamination" refers to the removal of loose or fixed surface radioactivity, and is required whenever wipe tests reveal contamination. The Radiation Safety Officer should be consulted for any decontamination effort.
Selection of a cleanser or decontaminant depends on factors such as the nature of the item to be decontaminated and the amount of dirt trapping the contamination. Either a commercial detergent or soap may be used. The key to effective decontamination is to use plenty of cleanser, a good brush or scouring pad, lots of water rinses, and absorbent paper to dry the area. Inadequate rinsing and drying may yield falsely elevated counts in post-cleaning wipe tests due to chemiluminescence, as a result of cleanser residue.
As a rule of thumb, if the contamination is dry (such as powder), keep it dry. Remove the powder by scraping away the contamination and pick up the small particles with adhesive tape. If contamination is wet, use absorbent materials to pick up the moisture.
Porous surfaces (such as wood and unpainted concrete) difficult to decontaminate and may require disposal. Some isotopes (such as tritium) may become chemically bonded to the surface and are extremely difficult to decontaminate.
Metals may be decontaminated with dilute mineral acids (nitric) a 10% solution of sodium citrate, or with ammonium bifluoride. When all other procedures fail for stainless steel, use hydrochloric acid - this process is effective but unfortunately it will etch the surface. As for bases, commercial polish cleaners will work well. Plastics may be cleaned with ammonium citrate, dilute acids or organic solvents.
For decontamination of both wet and dry surfaces, a final wiping with water or alcohol may be necessary. Decontamination should always be followed by wipe tests to confirm that the remaining radiation activity has been reduced to acceptable levels (less than or equal to 0.5 Bq/cm2). Finally, all used decontamination materials must be discarded as radioactive waste.
The Permit Holder shall ensure that all areas (i.e. laboratories, storage and waste facilities) identified on his/her Internal Permit are decommissioned or free from radioactivity upon the expiry or termination of the permit. Decommissioning shall include:
(1) Transfer or removal of all radioactive materials or devices to an approved site.
(2) Appropriate disposal of all radioactive waste.
(3) Removal of all radioactive warning signs and labels.
(4) Monitor all areas, decontaminate and remove surface contamination (i.e. loose and fixed) to meet the McGill and ultimately the CNSC prescribed limits. (See table below)
(5) Prepare a Decommissioning Report Form [.pdf] describing how the decommissioning requirements have been satisfied and forward it to Radiation Safety Officer.
(6) Update all records.
(7) Records must be retained by the Permit Holder for the period ending 3 years after expiry date of the last Internal Permit issued.
(8) Decommissioning Criteria: By Radionuclide Classification1. See table below.
CNSC AND MCGILL DECOMMISSIONING CRITERIA
|Radionuclide classif.||Fixed and non-fixed CNSC decommissioning limit (avg over area not to exceed 100 cm2)||Fixed and non-fixed McGill decommissioning limit (avg over area not to exceed 100 cm2)|
|Class A||0.3 Bq/cm2||0.05 Bq/cm2|
|Class B||3.0 Bq/cm2||0.5 Bq/cm2|
|Class C||30 Bq/cm2||0.5 Bq/cm2|
Note: The McGill radioactive surface contamination & decommissioning standard is more restrictive compared to the CNSC contamination and decommissioning standards.
1 For details on the radionuclide classifications, consult Section 6.2.