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William P. Gammel added For_this_lab_the_only__.tex
over 8 years ago
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For this lab, the only peak we are interested in from Figure (), is the photopeak, which is produced by the photoeletric effect. Consider a gamma ray striking an ion in a scintillator crystal. The gamma ray should be absorbed by the ion, and all of it's energy is transferred to a bound electron. The bound electron is thus freed, and begins to move rapidly through the crystal. The energy of the gamma ray () is far greater than the energy of the electron bound to the ion, therefore the energy of the freed electron assumes the value of the energy of the gamma ray. The photoelectric effect will then cause a voltage peak in the spectrum seen by the photomultiplier, known as the photopeak. The height of the voltage peak should correspond to the energy of the initial gamma ray.
Though not used in this lab, it is also important at least mention the two other processes by which (), in order to explain the presence of other structure seen in the spectral distributions for various isotopes. The Compton Edge, seen in Figure (), occurs via a process known as Compton scattering. In this scenario, energy is not absorbed by an ion in the scintillator crystal, and is instead scattered through an angle () by an electron. The gamma ray then moves off with a reduced energy and a change in momentum. The electron will carry away some of the gamma rays energy. The energy of the electron, which is the energy lost by the gamma ray will vary depending on the angle (). The photomultiplier will produce a spectrum (Compton spectrum) which corresponds to the angle dependent energies. However, the energy produced by the Compton scattered electrons is essentially a constant, thus, the Compton spectrum will appear as a flat plateau up to a feature known as the Compton Edge, which corresponds to the largest possible scattering angle (). When (), the energy of the electron will be at a maximum. After the Compton Edge, the spectrum will drop off quite sharply.
However, the compton scattering previously explained only applies to gamma rays that were scattered by electrons in the scintillator. Gamma rays can also be scattered into the scintillator from outside interactions, which will result in a peak in the photomultiplier spectrum known as the backscatter peak.
