Lab MP-2 - Ex pe rim e nt MP-2 El e c tro n D i ffrac ti o...

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E x p e r i m e n t M P - 2 Electron Diffraction off Graphite Educational Objective To study wave-like properties of massive particles (i.e. electrons). Experimental Objective To measure the deflection of an electron beam due to diffraction by virtue of its de Broglie wavelength; also, to measure the interplanar spacing of carbon atoms in graphite. Apparatus One Tel-Atomic CRT tube containing: 1. an electron gun, 2. an evacuated clear glass bulb on which is deposited a luminescent screen, and 3. a target of graphitized carbon. Voltage and current meters to: 1. monitor the potential on the reversed-bias field, and 2. monitor the current in the high voltage supply; not to exceed 0.2 mA. Method By varying the potential in the electron gun, one can vary the momentum (i.e. the de Broglie wavelength) of the electrons. Secondly, one can adjust the intensity of the electron beam by varying the reversed-bias potential on the Cathode can (see figure 1). This can be done by varying the potential on the 50- Volt supply. Use this adjustment to keep the HV (high voltage) current less than 0.2 mA. There's no reason to have the HV current exceed 0.1 mA. Two rings are formed on the luminescent screen whose radii are determined by the de Broglie wavelength and the interplanar spacing of the atoms in the target (i.e.graphitized carbon). By plotting the accelerating voltage vs. the diameter of the rings, one can determine the interplanar spacings of the target. Theory Connect the tube as shown in Fig. 1, switch on the heater supply (i.e. the 6-Volt supply) and wait one minute for the cathode heater to stabilize. Slowly adjust the HV supply to about 2000 Volts.
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