Exercise: Working With Radioactive Material

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In the first exercise you are going to learn how to use a contamination detector ("monitor") to identify contaminated equipment or areas. The monitor ("Automess") contains a simple gas-filled chamber in which the ionizing radiation is detected (proportional or Geiger-Müller detector). Your supervisor will instruct you on how to use the detector.

In your journal, you are required to write down the important things to remember when using the monitor.

Task 1: Measurements with the external probe
Perform the following measurements with the probe connected to the hand-held monitor, in each case measure five times with ~30 s intervals:

The count rate (counts per second – cps) at the following places: Close to the laboratory bench. Inside the fume hood. Outside of the laboratory.

Was there any significant difference for the three places?

Task 2: Measurements with the internal probe
Remove the probe (unplug the cable from the instrument - be careful so that you destroy the cable¸ ask for help if unsure).

The "Automess" hand-held monitor has a built in probe which is constructed in such a way that it will accurately measure external radiation dose (for γ radiation) for an extensive range of energies and intensities. Dose rate is a measurement of how much “strain” the radiation gives the body. It is measured in Sievert (Sv) per unit time. Sievert is a large unit and dose rate is therefore presented in μSv per hour (the instrument switches to mSv in high radiation field, but this should not happen in low-level training labs!). For comparision, the natural background radiation (in Norway) will give you a yearly dose of about 3-4 mSv (~0.5 μSv per hour).

Measure the dose rate at the same places as you measured the count rate!

Task 3: Measure a strong γ-source
Reconnect the probe. There is a source placed on one of the benches in the laboratory (the place is clearly labeled). The source is well shielded, but there is an opening where you can insert the probe so that it will "see" the source.

Write down the source strength and nucleus. What is the count rate of the source?

Task 4: Calibration of the external probe
In the lab you will find a 90Y/90Sr calibration probe with a disintegration rate of 30 Bq/cm2.

Measure the count rate about 0.5 cm and 5 cm from the surface. What is the relationship between disintegration rate and measured count rate? (I.e. what is the efficiency of the detector).

A surface is regarded as contaminated if the disintegration rate is above 4 Bg/cm2 (for γ and β radiation). For the monitor you are using, will you be able to measure such a disintegration rate? What is the detection limit of your monitor?

'Task 5: Find a contamination on a surface
One of the laboratory benches shall be checked for contamination. For this particular exercise, we have covered and sealed the contamination (not normally the case). Thus, there is no need to worry that you by accident will become contamination. However, for the purpose of training you should observe the same precautions as if it had been a real (unprotected) contamination.   It is very important that you manage these exercises since in exercise Measurement of 99mTc gamma-spectrum and Yield of Tc in Liquid-Liquid Extraction you will work with activety that is not sealed and then you must be able to identify eventual contaminations