3B Scientific Electron Diffraction Tube D User Manual
Page 2
2
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 graph-
ite target has slight damage as a result of the
manufacturing process or due to later overheat-
ing.
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 D
1008507
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
2 Pair of Experiment Leads, 75 cm
1002850
1 Experiment Lead, Plug and Socket 1002838
Additionally recommended:
1 Protective Adapter, 3-Pole
1009960
2 Pair of Safety Experiment Leads, 75 cm 1002849
1 Experiment Lead, Safety Plug/Socket 1002839
4.1 Setting up the tube in the tube holder
•
The tube should not be mounted or removed
unless all power supplies are disconnected.
•
Push the jaw clamp sliders on the stanchion
of the tube holder right back so that the jaws
open.
•
Push the bosses of the tube into the jaws.
•
Push the jaw clamps forward on the stan-
chions to secure the tube within the jaws.
•
If necessary plug the protective adapter onto
the connector sockets for the tube.
4.2 Removing tube from the tube holder
•
To remove the tube, push the jaw clamps
right back again and take the tube out of the
jaws.
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 internal resistor is there to prevent damage
to the graphite foil.
The graphite target itself should be monitored
throughout the experiment. If the graphite target
starts to glow, 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. Connect
the negative pole of the anode supply via
the 2-mm socket.
•
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
p
= momentum of the electrones
m
p
U
e
⋅
=
⋅
2
2
U
e
m
h
⋅
⋅
⋅
=
λ
2
m
= electron mas, e = electron charge