5.1 Explore Operating Voltage

1. Turn on the ST360 to activate the high voltage supply, timers and counter. The power switch is the red pushbutton at the right rear of the box. The ST360 is connected by a coaxial cable to the Geiger tube behind the curved steel shield. The box automatically discriminates and counts pulses from the Geiger tube and may be manually or programmatically set up. Place the check source in the plastic square on the top or second shelf of the Geiger tube mount.
2. Record the count rate as a function of voltage from 400 to 1050 volts in appropriate voltage steps. You could use a stopwatch and a spreadsheet program for this part. (Hint: the ST360 software may have features to automate this process.) DO NOT EXCEED 1050 VOLTS!! Great accuracy is not necessary for this measurement. Only five to ten per cent error is required. Test the count rate from the source and choose a time interval that achieves this accuracy for measurements in the higher voltage range.
3. For the remainder of your experiments, use an operating voltage in the plateau region of your plot.

5.2 Poisson Distribution

1. Use the program in the multi-sample decay or half-life mode and measure the number of decays of the check source repeatedly for 10-second intervals. You will therefore have 6 samples per minute for approximately 30 minutes or ≈180 samples. The samples will have some average value and a statistical fluctuation corresponding to a Poisson distribution. The frequency of occurrence of any particular measurement should follow the Poisson distribution, as described in Section 3.2.

5.3 Detector Resolving Time

1. A way of determining the resolving time is outlined in the ST360 Manual online and in the lab. It involves the use of two half disks containing nearly equal amounts of , a beta emitter with ≈4 yr halflife, and a single dummy disk.
2. Perform this experiment with the plastic tray that holds the samples about halfway between the detector and the detector and the table.
3. Use the tweezers provided and place the two active semi-disks in the plastic holder with the yellow labels down. The geometry of their placement is crucial; they should be positioned as if a single disk.
4. Measure the activity of the disks for about 30-60 seconds. You need to measure them for long enough that the uncertainty in the rate due to Poisson statistics is small (5-10 % or better). Determine the rate and its uncertainty for the ‘full’ disk.
5. Now replace each semi-disk in turn with the dummy piece, being careful to reproduce the same geometry and return the plastic holder to the top shelf. Measure the rates and their uncertainty for each geometry.
6. Use the results to estimate  and its uncertainty.
   One would expect that

   (12)  

5.4 Halflife Determination

1. With the check source removed, start collecting data before heading to the Nuclear Research Building for the activation of your sample. Remember that you are looking for a short-lived component; a sample interval of the order of minutes would not be appropriate. A sample interval of 5 to 10 seconds might be more appropriate. Enough data should be taken to characterize the long-lived component over several lifetimes. 20-ish hours may be appropriate for this purpose.
2. In collaboration with a supervisor from the NAS staff and the 2P03 staff, the prepared capsule of one of our samples is irradiated in the reactor for a few seconds using a pneumatic ’rabbit’ system.
3. The irradiated specimen is transferred to the laboratory as quickly as feasible and placed in the apparatus. The sample must be placed approximately where the disks were for the resolving time measurements so that the geometry is the same.
4. Refer to the McMaster Nuclear Reactor web site (https://mnr.mcmaster.ca/index.php/about/ overview) for details about the reactor.
5. You will need to come in a day or so later to retrieve your data, or send a Teams message to Dr. FitzGreen and he’ll message you back the data.