The last two labs of Physics 1140 involve gamma ray spectroscopy, the measurement of the energies and intensities of gamma rays emitted by radioactive nuclei. Today you will gain experience with some of the nuclear instrumentation and data acquisition software used in gamma ray spectroscopy. You will then perform a relevant experiment of your choice.

On your bench, you will find a sodium iodide (NaI) crystal detector, a cylindrical object strapped to a wooden cradle. When a gamma ray passes through the crystal it deposits some or all of its energy. The energy deposited into the detector results in the generation of a voltage pulse with a height proportional to the energy. The multichannel analyzer (MCA) in the UCS-30 – the box on the bench – measures the voltage of the pulses. This information is then sent to the computer, where it is plotted by the USX software to make an energy spectrum. The horizontal axis is related to the energy, and the vertical axis is the number of counts in a particular energy “bin”. When a gamma ray deposits all of its energy (as it could through the photoelectric effect), this will contribute to a relatively narrow peak in the spectrum, but when only some of the energy is deposited (as in Compton scattering, in which the scattered photon escapes with the rest of the energy) the result will be a broad plateau since the fraction of energy deposited varies.

Equipment Data Collection and Analysis Summary Questions