Compton Scattering Lab Guide

Experimental goals

In this experiment, you will measure:

1. the energies of Compton-scattered gamma-ray photons and recoil electrons,

2. the frequency of scattering as a function of angle, θ, and

3. the total cross section of electrons for Compton scattering.

Experimental arrangement

The experimental arrangement for the Compton experiment is shown schematically in Figure 1. The 2”x2” cylindrical “recoil electron” or “target” scintillator detector (Canberra model 802-3/2007), is irradiated by a beam of 661.6 keV photons emitted by about 100 µCi (≈ 1 microgram!) of 137Cs located at the end of a hole in a large lead brick which acts as a gamma-ray “howitzer”. If a photon entering the target scintillator scatters from a loosely bound, effectively free, electron, the resulting recoil electron may lose all of its energy in the target, causing a scintillation pulse with an amplitude proportional to the energy of the recoil electron. If the scattered photon emerges from the target scintillator without further interaction, and if its trajectory passes through the NaI crystal of the “scattered photon” detector, it will have a substantial probability of depositing all of its energy by a single photoelectric interaction or by a sequence of Compton scatterings and photoelectric interactions. This will produce a scintillation pulse that contributes to the “photopeak” of the pulse height spectrum. The median channel of the photopeak in the multichannel analyzer (MCA) display is a good measure of the median energy of the detected photons. If the scattered photon undergoes a Compton scattering in the scatter scintillator and then escapes from the scintillator, the resulting pulse, with a size proportional to the energy of the Compton recoil electron, will be registered in the Compton recoil continuum of the spectrum.

Procedure

Keep the lead door of the gamma-ray howitzer closed when not in use. The purpose of the procedures described below is to acquaint you with the operation of the equipment, particularly the coincidence scheme which permits the experiment to achieve a good signal/background ratio with a minimum amount of shielding. The steps below suggest a sequence of tests and adjustments to achieve that purpose, but you should feel free to devise your own.