Xray Di®raction and Absorption
Experiment XDA
University of Florida | Department of Physics
PHY4803L | Advanced Physics Laboratory
Objective
You will investigate the di®raction of xrays
from crystalline samples and the absorption
of xrays passing through metal foils. Di®rac-
tion patterns show sharp maxima (peaks) at
characteristic angles that depend on the wave-
length of the xrays and the structure and
orientation of the crystal. Absorption edges
appear at xray energies that depend on the
atomic number of the foil material. A Geiger-
M?uller tube is used to detect the xrays as the
scattering angle is varied.
References
Generally look in the QC481 and QD945 sec-
tions.
A. H. Compton and S. K. Allison, Xrays in
Theory and Experiment
C. Kittel, Introduction to Solid State Physics
B. D. Cullity, Elements of Xray Di®raction
Teltron manual, The Production, Properties,
and Uses of Xrays
Xray Emission and Absorption
When an electron beam of energy around
20 keV strikes a metal target, two di®erent
processes produce xrays. In one process, the
deceleration of beam electrons from collisions
with the target produces a broad continuum
of radiation called bremsstrahlung (braking ra-
diation) having a short wavelength limit that
arises because the energy of the photon hc=¸
can be no larger than the kinetic energy of the
electron. In the other process, beam electrons
knock atomic electrons in the target out of in-
ner shells. When electrons from higher shells
fall into the vacant inner shells, a series of dis-
crete xrays lines characteristic of the target
material are emitted.
In our machine, which has a copper target,
only two emission lines are of appreciable in-
tensity. Copper K® xrays (¸ = 0:1542 nm)
are produced when an n = 2 electron makes
a transition to a vacancy in the n = 1 shell.
A weaker K¯ xray with a shorter wavelength
(¸ = 0:1392 nm) occurs when the vacancy is
¯lled by an n = 3 electron.
The reverse process of xray absorption by
an atom also occurs if the xray has either an
energy exactly equal to the energy di®erence
between an energy level occupied by an atomic
electron and a vacant upper energy level, or an
energy su±cient to eject the atomic electron
(ionization). For the xray energies and metals
considered in this experiment, the ionization
of a K-shell electron is the dominant mech-
anism when the xray energy is high enough,
which leads to the absorption of the initial
xray photon and the ejection of an electron|a
XDA 1