Procedure:
In order to controllably place the radioactive source with respect to the detector we placed a Lego baseplate in the light-tight box containing the detector and source. This allows the students to be able to place the source with reproducible precision, and to have discrete steps to move the sample away without having to use a micrometer.
The energy of the alpha particle is related to the current pulse, and thus the amplified voltage and channel number, however this calibration must be performed. For alpha radiation, this calibration must be done in vacuum to avoid the energy loss of intervening particles. Similarly, the radioactive source used for calibration must be thin such that there isn't much self-absorption of alphas in the source itself. We have tested our detectors in vacuum with a thin film 241Am source using both our amplifier/MCA apparatus as well as a commercial amplifier/MCA. We find a conversion of \(G_1=0.264\ \pm0.001\ \frac{e}{eV}=0.264\ \pm0.001\ \frac{C}{J}\) consistent over several photodiodes. The gain of the preamplifier/MCA can be calibrated using pulses, though we have found that using the nominal gain of the simple op-amp feedback circuits gives accuracy of a few percent. This amplifier calibration was done by the students during the first iteration of this experiment, however we intend to simplify the procedure for the upcoming fall term, since the experiment was too long for the time allotted. Using a well-calibrated detector to measure the spectra of other sources allows the propagation of this calibration to all the sources and detectors. For purposes of this exercise, a calibration of 0.4 keV/channel (for 16384 channels) can be assumed.
Two experiments are described here:
- Measuring the energy loss as a function of distance in air, and
- Measuring the energy loss as a function of mylar thickness.
Both cases start with a manual measurement of alpha energies using the oscilloscope function of the STEMlab and a lab-book histogram, followed by an introduction to the MCA software and measurement of a spectrum with the sample close to the detector.
Alpha spectrum measurement:
- Connect the photodiode to the amplifier input to input 1 of the STEMlab through BNC cables. Connect +/- 15V and ground banana cables to the preamplifier, however do not power on at this time.
- Being careful to not touch either the photodiode active surface or the americium source, open the detector box and place the source close (< 5mm away) from the photodiode. We use Lego to allow for a reproducible separation. Alignment of the diode with the source is critical.
- Close the box containing the source.
- Power on the amplifier.
- Connect the USB power supply to the STEMlab and look for indicator LEDs to light up.
- Connect to the STEMlab using your laptop, tablet or phone wifi. Each STEMlab has a label taped to it. Connect to the Wi-Fi spot titled "rpXXtoday", where XX is the number on the instrument and then enter the password by the same name, and then open the web page at 192.168.128.1. There are other ways to connect to these instruments that we use in class (using ethernet and the university's LAN) however this works well for the demonstration.
- The STEMlabs can be reprogrammed in various ways, however for this experiment we will use the oscilloscope and multi-channel analyzer. Open the oscilloscope.
- Using the oscilloscope we have the students measure the heights of 10-20 peaks from the alpha particles detected in the oscilloscope. The STEMlab oscilloscope works similarly to many digital storage oscilloscopes. Here we zoom in, set a positive trigger voltage, turn on peak-to-peak measuring and use single triggering to help the measurements.
- When the measurements are done well enough, we ask the students to switch back to the STEMlab home screen, and open up the MCA application.
- Set up the MCA to take positive pulses, sampling every 64 ns, with auto baseline and a threshold of 0 channels (to begin with). Take data for a short time and adjust the threshold to remove the spurious peaks at low energy. Take data until there are several hundred counts per channel at the peak. Export your data to CSV.
- The calibration for the detector/amplifier/MCA system is approximately 0.4 keV/channel for the full 214=16384 channel mode, or 6.4 keV/channel for 210=1024 channels.