Download Electron diffraction tube and mounting 06721

Electron diffraction tube and mounting
06721-00
PHYWE Systeme GmbH & Co. KG
Robert-Bosch-Breite 10
D-37079 Göttingen
Phone
Fax
E-mail
+49 (0) 551 604-0
+49 (0) 551 604-107
[email protected]
Operating instructions
The unit complies with
the corresponding EC
guidelines.
Fig 1: 06721-00 Electron diffraction tube and mounting
CONTENTS
1
SAEFTY PRECAUTIONS
2
PURPOSE AND CHARACTERISTICS
3
FUNCTIONAL- AND OPERATING ELEMENTS
4
NOTES ON OPERATION
5 DIFFRACTION OF ELECTRONS THROUGH
GRAPHITE (WAVE CHARACTERISTICS OF
ELECTRONS)
6 ELECTRON OPTICAL IMAGE OF THE SUPPORT
MESH NET (PARTICLE CHARACTERISTICS OF
ELECTRONS)
7
PROTECTION AGAINST RADIATION
8
TECHNICAL SPECIFICATIONS
9
LITERATURE REFERENCES
10 ACCESSORIES
11 NOTES ON THE GUARANTEE
12 WASTE DISPOSAL
1
SAEFTY PRECAUTIONS
•
Carefully read these operating instructions completely
before operating this instruments. This is necessary to
avoid damage to it, as well as for user-safety.
Only use the instruments for the purpose for which it was
designed.
Only use the instruments in dry rooms in which there is
no risk of explosion.
Do not start up this instruments in case of visible signs of
damage to it.
Do not exert mechanical force on the tube.
The Power supply, 0...600 VDC, supplies voltages
that are dangerous to contact and is therefore only
to be operated under expert supervision. The regulations that are valid for working with dangerous electrical voltages are to be strictly followed to exclude
the possibility of harm to life or health. In particular,
the circuitry (experimental set-up) is first to be completely set up and be re-checked in currentless condition (absolute disconnection from the mains,
mains plug unplugged!) before the instruments are
connected to the mains and switched on.
Interventions in, and changes to, the circuit are
therefore also only to be performed in currentless
condition. Further connection is only to be made
with shockproof safety leads.
•
•
•
•
•
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2
PURPOSE AND CHARACTERISTICS
The electron diffraction tube is used to investigate and demonstrate the particle and the wave nature of electrons.
In an evacuated spherical glass flask, a focused electron
beam of uniform kinetic energy goes through a thin polycrystalline graphite film. The electrons diffracted by the graphite
crystals randomly distributed in space generate an interference pattern on the interior face of a fluorescent layer placed
on the inside of the glass tube (Debye-Scherrer rings), the
evaluation of which allows to verify de Broglie’s relation.
The particle properties of electrons can be verified by means
of an electron optical image of the copper wire mesh carrying
the graphite sample.
3
+ 300 V
FUNCTIONAL- AND OPERATING ELEMENTS
The evacuated electron diffraction tube contains an electron
beam generating system, of which a schematic representation is given in figure 2. Electrons coming out of an incandescent cathode K (cathode heating H) are accelerated through
the electric field generated by the electrode system G1...G4
and focused. The Wehnelt cylinder G1 sieves out a thin electron beam which is submitted to slight preliminary acceleration through the grid G2. The electrons are then strongly accelerated through the highly positive potential of G3. The
electron beam is focused by means of G3 and G4, which together constitute an electron optical lens system.
The accelerated electrons impinge on a thin polycrystalline
graphite film which is supported by a copper mesh net. The
electrons are diffracted with different intensities at the graphite film and enter the spherical part of the tube, where they
impinge on a fluorescent layer on the interior side of the glass
tube. The fluorescence which is generated at the points
where the electrons impinge, allows to observe an interference pattern consisting of concentric circles.
0… + 300 V
Fig. 3: Schematic of the wiring for operating the electron diffraction tube
Too great an anode current would reduce the service life of
the tube and could destroy the graphite sample. A not too
high brightness should therefore be selected for the image on
the tube.
The following settings should be selected:
a) When used to present the electron diffraction pattern:
-50 V
Wehnelt G1:
Electrode G2:
300 V
Electrode G3:
2…10 kV
0V
Electrode G4:
b) When used to illustrate the supporting grid of the graphite
sample as electron microscope:
Wehnelt G1:
0…-50 V
Electrode G2:
300 V
Electrode G3:
0…3.5 kV
Electrode G4:
0…300 V
Caution! When used with a high voltage > 3.5 kV, the
Wehnelt voltage (focussing) which limits the current is not to
be reduced below -50 V otherwise a too high anode current
would flow!
The tube should not be separated from the base socket. If
this occurs, make sure when replacing the tube the correct
contact allocation is respected. It is imperative to avoid bending the thin connector pins.
Fig. 2: The function elements of the beam generating system
of the electron diffraction tube.
4
The function elements of the beam generating system of the
electron diffraction tube are directly connected to the correspondingly marked connectors of the mounting
(fig .3). An alternating voltage is sufficient for the cathode
heating. The other functions basically require well smoothed
continuous voltages.
NOTES ON OPERATION
This instrument is only to be put into operation under specialist supervision in a controlled electromagnetic environment in
research, educational and training facilities (schools, universities, institutes and laboratories).
This means that in such an environment, no mobile phones
etc. are to be used in the immediate vicinity. The individual
connecting leads are each not to be longer than 2 m.
2
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5
DIFFRACTION OF ELECTRONS THROUGH
GRAPHITE
(WAVE CHARACTERISTICS OF ELECTRONS)
The crystallites of the graphite film, which are randomly distributed in space, have the hexagonal crystal structure typical
for graphite. The two largest lattice plane distances are:
d1 = 213 pm und d2 = 123 pm.
The diffracted electrons have the following de Broglie wavelength, which depends on anode voltage Ua (as related to
earth):
λ=
1500 kV
pm
Ua
In the experiment, the electrode voltages recommended
above are set to begin with. The negative Wehnelt voltage is
reduced until the most intense possible refraction rings have
formed. Fig. 4 shows the result of an experimental example.
The two rings which can be recognised comply with Bragg’s
condition
2 d1,2 sin ϑ 1,2 = λ =
Fig. 4: Example of an experimental result
1500 kV
pm
Ua
where ϑ1,2 are the diffraction angles of the electron beam
which depend on the anode voltage Ua (cf. fig. 5). The diffraction angles can be determined by means of the corresponding diameters r of the diffraction rings by means of the equation
ϑ=
1
2r
arcsin
4
D
The best way to measure the diameters r is with a plastic
vernier caliper. The caliper should as far as possible be applied to the intensity maximums of the rings.
Next to the two intense diffraction rings, further rings appear
with increasing anode voltage, which are clearly seen if the
room is completely darkened.
The unit constant D = 127 mm corresponds to the largest distance between the graphite layer and the interior wall of the
glass tube. Due to manufacturing techniques, it is impossible
to indicate an exact value of D. According to the manufacturing tolerance for D, one must reckon with an error of 2.5%.
The velocity of the electrons is about 22% of the velocity of
light for the maximum value of the anode voltage of 12 kV, so
that a non relativistic consideration is sufficient. This approximation of the de Broglie wavelength is affected by an
error of about 2.5%.
Fig. 5: The diffraction angle of the electron beam
6
ELECTRON OPTICAL IMAGE OF THE SUPPORT MESH NET
(PARTICLE CHARACTERISTICS OF ELECTRONS)
If the electron beam which impinges on the sample is broadened sufficiently, an image of the copper support mesh grid
can be represented on the luminous screen. The voltages of
the Wehnelt cylinder G1 and the anode voltage G3 are decreased until the diameter of the luminous spot is large
enough to recognise the projection shadow of the support
mesh grid.
The voltage at the Wehnelt cylinder G1 is in the -40 V... 0 V
range in this experiment.
Whereas in the diffraction experiment, the anode current
strength can only be max. 0.1 mA, it can increase here up to
1 mA according to the settings. The resistor is more strongly
loaded that the 1.5 W nominal rating.
The experiment for electron-optical imaging is only to be
carried out for a brief time! The 10 MW resistor heats up
(so only max. 3 to 5 min duration of experiment).
3
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7
PROTECTION AGAINST RADIATION
When operating the electron diffraction tube, X-rays are generated. The electron diffraction tube is a perturbing radiation
emitter in the sense of the Regulations Concerning Protection
against X-Ray Induced Damages of 8 January 1987 (X-Ray
Regulation - RöV). The use of such an object is settled in §
5(2)-RöV. As the local dose rate for the maximum anode
voltage Ua = 12 kV is far below the defined limit value, the
electron diffraction tube may be used in classes without taking any particular precautions. The use of the tube mustnot
be reported and no special authorisation must be required.
11 NOTES ON THE GUARANTEE
We guarantee the instrument supplied by us for a period of
24 months within the EU, or for 12 months outside of the EU.
Excepted from the guarantee are damages that result from
disregarding the Operating Instructions, from improper handling of the instrument or from natural wear.
The manufacturer can only be held responsible for the function and technical safety characteristics of the instrument,
when maintenance, repairs and alterations to the instrument
are only carried out by the manufacturer or by personnel who
have been explicitly authorized by him to do so.
12 WASTE DISPOSAL
The packaging consists predominately of environmentally
compatible materials that can be passed on for disposal by
the local recycling service.
Attention!
In order to avoid damaging the electron diffraction tube,
it is not allowed to apply higher voltages than the maximum anode voltage of 12 kV.
8
TECHNICAL SPECIFICATIONS
Diffraction preparation
Support mesh grid
Nominal distance between
preparation and interior wall
of the tube
Luminous screen diameter
Cathode heating
Wehnelt voltage (smoothed
)
Preliminary acceleration voltage
(smoothed)
Anode voltage (smoothed)
Anode current
Focusing voltage (smoothed)
9
Should you no longer require this product, do not dispose of it with the household refuse.
Please return it to the address below for
proper waste disposal.
graphite
copper
127 mm ±3 mm
100 mm
U~ = 6.3 V/300 mA
0... -50 V
+250 V
+2...+12 kV
< 1 mA
0...+250 V
PHYWE Systeme GmbH & Co. KG
Abteilung Kundendienst (Customer Service)
Robert-Bosch-Breite 10
D-37079 Göttingen
Phone
Fax
+49 (0) 551 604-274
+49 (0) 551 604-246
LITERATURE REFERENCES
Handbook Laboratory Experiments Physics 16502-32
10 ACCESSORIES
Power supply, 0...600 VDC
High voltage supply unit, 0...10 kV
oder
High voltage supply unit, 0...25 kV
High value resistor, 10 MOhm
13672-93
13673-93
13671-93
07160-00
4
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06721-00 / 4513