3B Scientific Teltron Electron Diffraction Tube S - Manual (Page 6)

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2. Description
The electron diffraction tube illustrates the
wave nature of electrons by allowing obser-
vation of interference caused by a beam of
electrons passing through a polycrystalline
graphite target on a fluorescent screen (De-
bye-Scherrer diffraction). The wavelength of
the electrons can be calculated for various
anode voltages from the radius of the dif-
fracted rings and the distance between the
crystal layers in the graphite. The tube also
confirms the de Broglie hypothesis.
The electron diffraction tube is a highly
evacuated tube with an electron gun consist-
ing of a pure tungsten heater filament (4)
and a cylindrical anode (5) all contained in a
clear glass bulb. The electrons emitted by the
heated cathode are constrained to a narrow
beam by an aperture and are then focussed
by means of an electron-optical system. The
resulting tight, monochromatic beam then
passes through a micro-mesh nickel grating
(7) situated at the aperture of the gun. Onto
this grid, a thin layer of polycrystalline graph-
itised carbon has been deposited by vaporisa-
tion. This layer affects the electrons in the
beam much like a diffraction grating. The
result of this diffraction is seen in the form of
an image comprising two concentric rings
that become visible on the fluorescent screen
(8) A spot resulting from the undeflected
electron beam continues to be visible at the
centre of the rings.
A magnet is also supplied with the tube. This
allows the direction of the electron beam to
be changed, which may be necessary if the
graphite target has slight damage as a result
of the manufacturing process or due to later
overheating.
3. Technical data
Filament voltage: ≤ 7,5 V AC/DC
Anode voltage: 2000 V – 5000 V DC
Anode current: typ. 0,15 mA bei
4000 V DC
Lattice constant of graphite:
d
10
= 0.213 nm
d
11
= 0.123 nm
Dimensions:
Distance from graphite target
to fluorescent screen: 135 mm approx.
Fluorescent screen:
100 mm Ø approx.
Glass bulb: 130 mm Ø approx.
Total length: 260 mm Ø approx.
4. Operation
To perform experiments using the electron
diffraction tube, the following equipment is
also required:
1 Tube holder U18500 (TEL 2501) to hold
the tube
1 6 kV power supply U21060 to supply the
required power to the filament, anode
and cathode. Max. voltage 6000 V
1 Meter for measuring anode current, e.g.
multimeter U17451
4.1 Setting up the electron diffraction
tube in the tube holder
• Press tube gently into the stock of the
holder and push until the pins are fully in-
serted. Take note of the unique position
of the guide pin.
4.1 Removing the electron diffraction
tube from the tube holder
• To remove the tube, apply pressure with
the middle finger on the guide pin and the
thumb on the tail-stock until the pins
loosen, then pull out the tube.
4.2 General instructions
The graphite foil on the diffraction grating is
only a few layers of molecules thick and any
current greater 0.2 mA can cause its destruc-
tion.
The anode voltage and the graphite target
itself should be monitored throughout the
experiment. If the graphite target starts to
glow or the emission current rises above 0.2
mA, the anode must immediately be discon-
nected from its power supply
If the diffraction rings are not satisfactorily
visible, the electron beam can be redirected
by a magnet so that it passes through an
undamaged region of the target.
The electron beam can be focussed by apply-
ing a focussing voltage of 0 – 50 V DC (con-
nected as in Fig. 2). This can be used to
make the rings clearer and easier to observe
at lower anode voltages.
5. Example experiment
• Set u the experiment as in Fig. 2.
• Apply the heater voltage and wait about 1
minute for the heater temperature to
achieve thermal stability
• Apply an anode voltage of 4 kV.
• Determine the diameter D of the diffrac-
tion rings.