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 a series of 30-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. 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.