Section of Radiochemistry
Institute of Chemistry
Faculty of Mathematics and Natural sciences
University of Oslo
The task in this Laboratory Exercise is to record a disintegration curve of 234mPa and from this curve determine the half-life of the nuclide. The234mPa radionuclide is obtained from a generator system consisting of an ion exchanger column with fixed 234Th where the daughter is milked by a liquid elution process. The α particles from the produced 234mPa-source is recorded by a GM-detector.
Principle Behind Mother-Daughter Relationship (read)
How to Measure the Half Life
For this part of the exercise, you will use a GM-probe connected to a simple counter to determine the disintegration rate of 234mPa.
Before you get your sample, make sure you know exactly what to do. Test the counting procedure without a sample to ensure that this is the case.
1. Make a good background measurement, i.e. use a long counting time (at least 30 min). It would be smart to start the background measurement before you prepare the radionuclide generator, as this will take at least one hour. Then you can start directly with the 234mPa measurements once the generator is ready.
Fig 3: Insertion of aluminum counting-vessel into a detector.
2. Select a preset counting time of 60 s. Make a table in which you can write down your results.
3. Get a stopwatch and learn how to use it.
4. You will count your sample repeatedly in 60 s intervals in order to get the disintegration curve for 234mPa. Between each interval a break of 30 s is recommended for writing down the result, clearing the spectrum/counter and prepare for the next measurement. Make sure you write down the exact time you start each counting - do not cheat to fit your planned schedule, write down the actual time!
5. Write down the total number of counts in the spectrum for each measurement.
6. Repeat the 60 s counting until there is no more 234mPa left, then do a 600 s background measurement (remember to change the preset counting time).
7. Repeat the measurement so you get two complete disintegration curves for 234mPa.
How to Measure the Half Life with a 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.
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 234mPa. 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 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 which is not possible if you are doing everything manually.
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:
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).
Determining the Half-Life
Use a high-quality data plotting and fitting program (e.g. Origin) to analyse the data. The fitting must take the uncertainity into account (do not use Excel), otherwise you will get the wrong result.
Notice that you always shall use the 1/3 of the time into each measuremnent as the "middle time point". This is due to decay - after 1/3 of the time you will have equally many counts before and after the 1/3 point (i.e. it is the "middle point".
Alternativ/Extra: Plot the gross counts instead of the net counts and ask Origin to fit both the background and the decay.
The equipment needed should be ready. The solutions needed should also be prepared. It is usually a good idea to not use to large columns as the time needed for the solution to pas trough the dowex will increase quite a lot with increased volume.
Please leave suggestions and feedback in the comments.