How to Measure the Half Life of 234mPa - with MCA system
For this part of the exercise, you will use a NaI detector connected to a Multi-Channel Analyzer (MCA) to determine the disintegration rate of 234mPa. An alternative and more direct, but "old-fashion" method, is to use a GM-tube connected directly to a simple counter. It's described here.
If you look at the radiation from 234mPa (look it up in your nuclear chart!) you will notice that 234mPa only emits very weak gamma-rays. However, due to the high-energy beta-particle we can still measure 234mPa since this high-energy particle will be able to penetrate through the protective shield around the NaI and interact with the NaI crystal. Alternately, we could mount e.g. a plastic detector (NE 102A or similar) on a PM-tube and use this instead. The results will largely be the same (but the NaI is more sensitive to gamma-background, which add uncertainty to the background subtraction).
This description assumes you have the Maestro MCA software from ORTEC. If you are using an alternative system, you will have to consult the manual to figure out how to use it. The procedure should not be very different, though.
We want to make successive 60-s measurements to determine the half-life curve of 234m. This can be done manually by succesive starting-waiting-stopping-saving-clearing operations.
However, with a modern system this tiresome procedure can be automated: In Maestro jargong you do this by preparing a job-description file (it would be called a script file or batch file in most other software). This file contain all the instructions you would have to execute, but can be simplified by using the built-in loop structure. Furthermore, once running, it will execute the correct commands at exactly the right time.
Since the commands execute very rapidly, you will also be able to spend practically all the time during the 234mPa decay actually counting, something witch is not possible if you are doing everything manually.
set_preset_real 60 loop 15 clear start wait save m:\spectra\KJM5911_D130_A???.chn end_loop
set_preset_real 300 clear start wait save m:\spectra\KJM5911_D130_Background.chn
Notice that we have added a 5-min measurement to check for residual activity after the 234mPa has decayed. Any residual activity would be from break-through of 234Th from the column.
Contrary to a simple counting system, the MCA will save spectra containing counts vs. energy. Since we are measuring beta particles, the spectra do not contain specially interesting information and we will simply sum up all the counts in the spectrum and use this number for the decay curve.
The procedure for measuring the 234mPa decay is as follows:
- Measure a background spectrum for as long as possible (e.g. during preparation of the radionuclide generator). Remember to save the spectrum and the filename!
- "Milk" 20 drops into a sample cup from the generator as quickly as possible. Use a stopwatch and start it at the 10 drop.
- Put the sample cup on the detector (you should protect the detector surface with a thin plastic sheet to avoid contamination etc.).
- Start the job-file and note down the time difference between milking and starting. Now, sit back and relax! Alternatively (better), if the job-file is saving spectra to a network disk, you can analyze the spectra as they are produced (using another pc which can read the same disk).
- When the job-file finishes, repeat the measurement. (Remember to renemae or move your spectra, otherwise they will be deleted or the job-file stops.)
From the spectra you should get the following data (by opening each spectrum in Maestro): The measurement start time and the gross count (total number of counts in the spectrum). Use the "sum" command to get the total number of counts (the spectrum must contain no region-of-interest markings).