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Introduction

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.

Principle

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 successive starting-waiting-stopping-saving-clearing operations.

However, with a modern system this tiresome procedure can be automated: In Maestro jargon 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.

Job-description file

stop

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 measurement, then 8 120-s measurements and finally a 5-min background measurement.

Procedure

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:

1. Measure a background spectrum for as long as possible if you have not already done this.
2. 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).

1. When the job-file finishes, repeat the measurement for the different irradiation times (irradiation times = 12, 24, 48, 72, and 144 s). (Remember to rename 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 analyzing the data.