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Unformatted text preview: Compton Scattering In this experiment we will scatter a gamma ray in a plastic scintillator detector (target) and detect the outgoing scattered gamma ray in a sodium iodide (NaI) detector. The apparatus allows you to rotate the NaI detector around the target position and measure energy of the scattered gamma ray E γ as a function of the scattering angle θ . You will compare your measured E γ ( θ ) with the distribution expected form Compton scattering theory. This is a double scattering arrangement similar to what shown in Leo, Fig. 2-22, and Fig. 1 below. γ θ γ TARGET NaI DETECTOR Figure 1: Schematic of Compton scattering experiment. Compton scattering occurs when the photon interacts with a (free) electron and scatters through an angle θ defined by the position of the NaI detector. The recoil electron will deposit its energy in the target (here a plastic scintillator) and the recoil photon will be absorbed by the NaI detector. The pulse heights from the NaI detector are recorded in the Multichannel Analyzer (MCA) for each detected scatter. In order to reduce background counts, we use coincident signals from the plastic and NaI detectors. Apparatus In this experiment a plastic scintillating counter is used as a target and an NaI scintillation counter as a detector of the scattered gamma ray. The source is placed in a lead tube that collimates the gamma rays that strike the target. The apparatus is mounted on a table that allows one to set different scattering angles between the incident and scattered gamma ray. A photograph of one of the two set-ups is shown in Fig. 2. The NaI detector is mounted on the arm which rotates while the plastic target is mounted at the pivot point of the NaI detector that is near the exit of the collimator and is stationary. A discriminator and a dual gate generator are used to generate a delay between the pulse that signals a scattering event in the plastic scintillator and the pulse from the NaI detector. The delay is necessary because of the much slower rise and decay time of NaI signal following the scattering event. The procedure for setting up the delay is described below and shown schematically in Fig. 3. The output of the NaI is sent to an amplifier that provides the shaped positive pulse required by 1 Figure 2: Scattering table used for Compton scattering. NaI detector is mounted on a movable arm. The target is a plastic scintillator detector suspended vertically over the table’s center. The source is behind the lead bricks inside a lead-wrapped tube. the multichannel analyzer. For this experiment we use the Norland MCA for which instructions available on the physics 433 web page. The Norland MCA allows one to “gate” whether pulses are measured or not via a TTL-level input on the back of the unit....
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- Fall '11
- NaI, Compton scattering, NaI detector, gate generator