11. Monitoring
Contents
11.1.1 Personal Whole Body Dosimeter
11.1.2 Personal Finger Dosimeter
11.2 Monitoring Surface Contamination
11. Monitoring
11.1.1 Personal Whole Body Dosimeter
Personal dosimeters are issued on a 2-monthly basis. The whole badge is now designed as a single integrated unit, so please do not attempt to disassemble it.
Your badge should be worn at all times when exposure to ionising radiations is likely. Personal Dosimeters should be worn on lapel or breast pocket of your laboratory coat.
Please assist your DRPS by ensuring that your badge is available for changing at the end of the wear period. Prompt changing of badges helps all concerned and makes the dose results available sooner.
11.1.2 Personal Finger Dosimeter
These ring badges are used to monitor the hands of personnel who come into close contact with radioactive material or are engaged in other activities where their extremities may be exposed to an X-ray beam. Typically they are issued to people working with significant amounts of P-32 or other 'hard' beta-gamma emitters.
They are issued on a 2-monthly basis and are available in 3 sizes - small, medium and large. Unless otherwise instructed the standard medium size will be issued.
The personal dosimeter is encapsulated inside the identification cover. The whole ring is returned at the end of the wear period, please do not disassemble.
Note: Personal dosimeters are issued for use by the named person at the University of St-Andrews only. They are not to be used by any other person or to be taken to be used at any other establishment. If monitoring is required at another establishment it should be provided at that establishment.
11.2 Monitoring Surface Contamination
The most common types of instrument used for monitoring contamination is a count-rate meter that measures counts per second (cps). Many different types of probes can be used with such a meter. The main types of probes used are:
- Scintillation Probes - which are best for detecting x-rays and gamma rays in the range of a few KeV to several MeV;
- Geiger-Muller Tubes (G-M tubes) - Glass walled G-M tubes are best for detecting high energy beta and gamma-emitters. Thin-end G-M tubes can detect low energy beta particles such as those emitted from 35S and 14C.
If such contamination count rate meters cannot be used e.g. for very low energy beta particles (e.g. particles from 3H) or for a particles, then a 'Wipe Test' should be performed. The procedure for such a test includes:
- Mark out a defined area where the contamination is suspected;
- Moisten filter paper with 70% ethanol;
- Wipe the defined surface with the filter paper;
- Repeat the procedure over different areas of the potentially contaminated surface;
- Put the filter paper in an appropriate scintillant vial with scintillant and count the number of dissociations per minute.
To determine the amount of contamination present, it should be assumed that one tenth of the contamination is taken up by the filter paper.
11.3 Air Monitoring
Airborne contamination can be one of the main risks in a radiochemical laboratory. If any dust or spray producing operation is carried out then some form of airborne contamination monitoring should be carried out. The most common form of such monitoring is to collect a known volume of air and pass it over a filter which is then counted in a scintillation counter.
11.4 Thyroid Monitoring
Measurement of the amount of radioactive iodine in the thyroid can be performed with a contamination monitor with a type 5-42 scintillation probe for detection X-rays and gamma-emissions. This probe will detect small amounts of iodine in the thyroid (e.g. 3.7 KBq of 125I will give a measurement of 25-35 cps on such a system).
All persons using radioactive iodine should regularly monitor their thyroid and record their results. It is very important to carry out such monitoring if workers are performing iodination type reactions. Written records of all such monitoring should be kept.
11.5 Dose Rate Monitoring
Regular 'Dose-Rate' monitoring of 'Controlled' and 'Supervised' areas should be performed and written records of such monitoring kept.
The instrument for such monitoring is a 'Dose-Rate' meter which is made of a compensated Geiger-Muller tube or an Ionisation Chamber detector. It should be noted that using ionisation chambers can significantly underestimate the dose-rate if the beam does not fully traverse the chamber.
An estimation of the 'Dose-Rate' can be obtained using a contamination 'Count-Rate' monitor. This procedure should always be carried out some distance away from known radioactive sources. For beta particles, a dose of approximately 1 microSv hr-1 is the equivalent of 2-4 counts per second on a Count-Rate meter. For gamma emissions of > 0.15 MeV, 2 to 4 counts per second is the equivalent of 1 microSv hr-1. For gamma emissions of < 0.15 MeV, the sensitivity of the machine will depend on the peak energy of the gamma emissions e.g. 125I which has a photon energy of 0.03 KeV, approximately 6 cps will be equivalent to a dose of 1 microSv hr -1 .
Radiation surveys for pencil beam X-ray equipment should be performed with a very small sized Geiger-Muller tube with a thin window end capable of detecting low energy beams.
11.6 Monitor Testing / Repair
The University Radiation Protection Service (URPS) will test contamination monitors for Schools/Units at regular intervals and arrange for repair work