3B Scientific Teltron Electron Diffraction Tube S - Manual (Page 2)

2
optical system. The resulting tight, monochro-
matic 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
graphitised carbon has been deposited by va-
porisation. 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 re-
sulting from the undeflected electron beam con-
tinues 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 manu-
facturing process or due to later overheating.
3. Technical data
Filament voltage: ≤ 7.0 V AC/DC
Anode voltage: 0 – 5000 V DC
Anode current: typ. 0.15 mA
at 4000 V DC
Lattice constant of graphite:
d
10
= 0.213 nm
d
11
= 0.123 nm
Distance from graphite target
to fluorescent screen:
125 ± 2 mm approx.
Fluorescent screen: 100 mm dia. approx.
Glass bulb: 130 mm dia. approx.
Total length: 260 mm dia. approx.
4. Operation
To perform experiments using the electron dif-
fraction tube, the following equipment is also
required:
1 Tube holder S
1014525
1 High voltage power supply 5 kV (115 V, 50/60 Hz)
1003309
or
1 High voltage power supply 5 kV (230 V, 50/60 Hz)
1003310
1 Analogue multimeter AM51 1003074
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 inserted. Take
note of the unique position of the guide pin.
4.2 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.
4.3 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 destruction.
The anode voltage and the graphite target itself
should be monitored throughout the experiment.
If the graphite target starts to glow or the emis-
sion current rises above 0.2 mA, the anode must
immediately be disconnected from its power
supply
If the diffraction rings are not satisfactorily visi-
ble, the electron beam can be redirected by a
magnet so that it passes through an undamaged
region of the target.
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 diffraction
rings.
Two diffraction rings appear on the fluorescent
screen centred on the undeflected beam in the
middle. The two rings correspond to Bragg re-
flections from atoms in the layers of the graphite
crystal lattice.
Changing the anode voltage causes the rings to
change in diameter. Reducing the voltage
makes the rings wider. This supports de
Broglie's postulate that the wavelength in-
creases as momentum is reduced.
a) Bragg equation: ϑ ⋅ ⋅ = λ sin 2 d
λ = wavelength of the electrones
ϑ = glancing angle of the diffraction ring
d
= lattice plane spacing in graphite
L = distance between sample and screen
D = diameter D of the diffraction ring
R = radius of the diffraction ring
L
D
⋅
= ϑ
2
2 tan
L
R
d
⋅ = λ
b) de-Broglie equation:
p
h
= λ
h = Planck’s constant