PC1222 Lab Report: Electron Diffraction||
||Wang Zhikai||
||A0080959N||
||Group B6||
PC1222 Lab Report: Electron Diffraction||
||Wang Zhikai||
||A0080959N||
||Group B6||
1
Objectives
To determine the wave behaviour of electrons.
To determine experimental values for the interatomic spacing of graphite via electron diffraction
patterns.
2
Introduction
All forms of matter have wave as well as particle properties. The wavelength,
, of a particle is related
to its momentum,
p
, by the following equation:
, where
, which is Planck’s
constant. When a beam of electrons strikes a family of parallel crystal atomic planes, each plane will
reflect part of the waves. By Bragg’s Law, constructive interference between the waves occurs when
, where d is the separation between two adjacent planes and
is the angle
the electron beam makes with the plane. When a potential difference of
V
is used to accelerate the beam
of electrons, the momentum of an electron,
p
, is related to
V
by the following equation,
,
where
m
and
e
are the mass and charge of the electron respectively. From the equations above, we
obtain a relationship between
and
V
, given by the equation:
.
In the experiment, a beam of electrons is accelerated through a thin film of graphite target, and a
screen is at a distance of
L
= 0.140m away. The interatomic distance in graphite is 0.142nm and all the
bond angles are 120
°
. From Bragg’s Law and this equation, we obtain the following equation:
, where
D
is the diameter of the diffraction rings produced on the screen. Therefore,
using this equation, we can obtain experimental values for
d
by measuring the different diameters
D
formed on the screen.
3
Methodology
We adjusted the voltage controls on the power supplies to zero and turned them on. We then waited a
few minutes for the filament to warm up, so that it would emit electrons via thermionic emission. We
switched on the filament voltage for one minute to allow the cathode temperature to stabilize before
applying the anode voltage. We then increased the anode voltage gradually while monitoring the anode
current to ensure it remains below 0.2mA so that the graphite target will not be punctured due to
overload of the current.
Next, we measured the diameter of the inner diffraction ring at three different positions along the