3.1 USB Gamma Spectrometer

This experiment uses a 1.5” diameter detector and a small electronics box from GammaSpectacular. The detector generates electrical pulses from incoming gamma rays and the instrument box converts them into sound pulses – the volume of the sound corresponds to the magnitude of the electrical pulse. The sound pulses are detected by the PC’s sound card and register as a count in the appropriate bin. Over time, the energy spectrum of the gamma rays are presented.

    The biggest downside to this detector is the incredibly long ‘dead-time’. Because the instrument uses sound pulses and the PC’s soundcard, the sound pulses end up being about 100 s long. No other gamma rays can be detected while this pulse is happening. More expensive instruments have pulses that are just a few microseconds long, leading to much less dead time. If we were doing intensity measurements over time, where we would want to directly compare counts as the intensity decreased, then the dead time of the detector for each measurement would be crucial. Since we aren’t interested in precise comparisons of the amplitude of the peaks in the gamma spectrum, we don’t have to worry about the dead time.

    This instrument comes with a software package that is used to display various interesting data sets. The most interesting for this experiment, obviously, is the histogram of pulse heights (i.e. deposited energy from the gamma ray); that’s our gamma spectrum! This software also comes with a fitting routine that will allow us to quickly fit our data with the Breit-Wigner distribution and find the center of the peaks with relatively decent precision. Instructions for choosing a region of interest are in the detector manual on page 13. If you can press ‘b’ and then ‘e’, then you can fit data!

3.2 Radioactive Sources

You will be using safe, sealed radioactive sources called ‘check sources’. The radioactivity of these sources is incredibly small – less than 10 Ci. For comparison, a banana has a radioactivity of about 0.0005 Ci. So if you feel safe around 20,000 bananas, you should feel safe around these radioactive sources. Even though they are not very radioactive, we will still be using best-practices for handling radioactive material. Specifically, that means using tweezers to handle the sources and placing the thick, steel shield between yourself and the radioactive sample.

    One source of radiation that is ubiquitous is . It has a very long half-life – about a billion years – and makes up a bout 0.012% of the potassium on earth. There is enough in the atmosphere that, if you run the detector for long enough, you should see a peak in the gamma spectrum at 1460.8 keV.