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Electron Diffraction from Graphite: Apparatus, Exams of Physics

Physics

The electron diffraction tube consists of an electron "gun" which emits a narrow converging beam of electrons within an evacuated clear glass bulb.

Typology: Exams

2021/2022

Uploaded on 09/27/2022

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Physics'320'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''Fall'(12)'2017'
!
Electron!Diffraction!from!Graphite:!Apparatus!
'
The' electron' diffraction' tube' consists' of' an' electron' "gun"' which' emits' a' narrow'
converging' beam' of' electrons' within' an' evacuated' clear' glass' bulb.' ' Across' the' exit'
aperture'of'the'gun'lies'a'micromesh'nickel'grid'onto'which'has'been'vaporized'a'thin'layer'
of' graphitized' carbon.' ' The' beam' penetrates' through' this' carbon' target' and' is' diffracted'
into' two' rings.' ' These' rings' are' observed' on' the' inner' front' surface' of' the' bulb' which'is'
coated' with' a' luminescent' screen.' ' Graphite' consists' of' atomic' layers' with' a' honeycomb'
structure,' where' the' spacing' between' the' layers' is' several' times' larger' that' the' in-layer'
atom-atom'spacing.''Thus,'we'can'treat'each'graphite'layer'as'a'diffraction'grating'with'line'
spacing'corresponding' to'the'separation'between'"lines"'of'carbon' atoms.' 'Two'distinct'
"line"'spacings,'denoted'd10'and'd11'in'the'figure'below,'are' responsible'for'the'diffraction.''
The'source'of'the'electron' beam'is' a'heated'oxide-coated'cathode.'' The'focus' and'intensity'
of'the'beam'can'be'controlled'by'adjusting'a'bias'voltage'(0-50'V)'on'the'cathode'can.''''
'
'
'
!
from'geometry:'
d10'='31/2'd11'
Graphite'layer'
(honeycomb'lattice)'
pf3

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Physics 320 Fall (12) 201 7

Electron Diffraction from Graphite: Apparatus

The electron diffraction tube consists of an electron "gun" which emits a narrow converging beam of electrons within an evacuated clear glass bulb. Across the exit aperture of the gun lies a micromesh nickel grid onto which has been vaporized a thin layer of graphitized carbon. The beam penetrates through this carbon target and is diffracted into two rings. These rings are observed on the inner front surface of the bulb which is coated with a luminescent screen. Graphite consists of atomic layers with a honeycomb structure, where the spacing between the layers is several times larger that the in-layer atom-atom spacing. Thus, we can treat each graphite layer as a diffraction grating with line spacing corresponding to the separation between "lines" of carbon atoms. Two distinct "line" spacings, denoted d 10 and d 11 in the figure below, are responsible for the diffraction. The source of the electron beam is a heated oxide-coated cathode. The focus and intensity of the beam can be controlled by adjusting a bias voltage (0- 50 V) on the cathode can. from geometry: d 10 = 31/2^ d 11 Graphite layer (honeycomb lattice)

Electron Diffraction from Graphite: Experimental Details

The electron diffraction tube should be connected to two separate power supplies and an ammeter as shown in figure on the preceding page. The Pasco power supply is used for both the filament voltage VF and the anode (accelerating) voltage VA. The Elenco power supply is used for the external bias voltage. The digital multimeter is used to measure the electron beam current. Turn-On Procedure: Be sure that the voltage dials of both the Elenco and Pasco power supplies are set at zero. Turn on both power supplies. Wait for at least one minute for the filament of the electron tube to heat up (you should see a red glow in the back of the tube). The multimeter, which measures the electron beam current, should be set to the 200 μA scale. Turn up the voltage on the Elenco power supply to about 10 volts. Turn up the voltage on the Pasco power supply to about 2,000 V (i.e. 2.0 kV). You should see the electron beam spot and two diffraction rings on the phosphorescent screen of the tube. Be sure the electron beam current does NOT EXCEED 150 μA. Adjust the bias voltage to "focus" the diffraction rings. Increasing the bias voltage reduces the beam intensity. If the bias voltage is made too large, the diffraction rings will be distorted or the beam will shut down. Try to keep the beam current in the range 50- 100 μA. The bias voltage should be kept below 50 volts. Data Taking: You will need to determine the radius of each of the two rings seen on the front of the tube. (Think in terms of the ring being formed on the outer surface of the tube.) A good way to get measurements for the radii is either to use a flexible ruler, or better yet, mark the ring locations on a piece of paper held against the tube. Measuring the inner and outer radii of each ring will give you an uncertainty estimate on this measurement. This, of course, does not give the true radius of the ring. The equation for getting the radius is given below. In your report, you will need to derive that equation. (It is easy to do from the diagram shown.) Increase the bias voltage (Elenco power supply) by about 5 volts and then increase the voltage of the Pasco power supply about 500 volts (i.e., up to 2.5 kV). CAUTION: Be sure to increase the Elenco power supply voltage first. Adjust the bias voltage so that the digital multimeter reads between 50 and 100 μA. Again make measurements of the ring radii. Repeat this procedure of increasing the accelerating voltage in steps of 500 V (being sure to increase the bias voltage first) and making measurements at each setting, up to 5.0 kV. Turn off procedure: When turning down the voltages, first turn down the Pasco power supply voltage and then the Elenco power supply voltage.