How to Measure the Decay of n-activated Silver
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 the n-activated silver.
Pre-lab task: Determine what kind of gamma radiation to expect from the silver isotopes produced in the irradiation and their associated realtive intensity (e.g. from [Berkeley/Lund database]).
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 20-s measurements followed by 120-s ones to determine the half-life curve of the silver isotopes. 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 actually counting, something witch is not possible if you are doing everything manually.
set_preset_real 20 loop 7 clear start wait save m:\spectra\KJM5911_D130_A???.chn end_loop
set_preset_real 120 loop 7 clear start wait save m:\spectra\KJM5911_D130_B???.chn end_loop
set_preset_real 300 clear start wait save m:\spectra\KJM5911_D130_Background.chn
This job-file will perform 8 20-s measurment, then 8 120-s measurements and finally a 5-min background measurement.
The MCA will save spectra containing counts vs. energy. The two interesting gamma-rays from the n-activated silver will overlap and you will not be able to differentiate between them in the NaI spectra. Thus, we will simply use the gross counts and subtract the background as if we had used a simple counter.
The procedure for measuring each irradiated silver disk is as follows:
- Measure a background spectrum for as long as possible if you have not already done this.
- Get the irradiated silver disk and put it as quickly as possible on top of the detector.
- Start the job-file and note down the time between end-of-irradiation and starting the job-file.
- 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 for the different irradiation times (irradiation times = 12, 24, 48, 72, and 144 s). (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).
Alternative procedure: Select the relevant spectrum region with the photo-peaks and only use the integrals under this (double-)peak for analysing the data.