Physics and Engineering of Radiation Detection

34 Chapter 1. Properties and Sources of Radiation

L K

Κ(α)

M

incident electron

orbital electron

x−rays

Κ(β)

BremsstrahlungFigure 1.6.3: Physical process

of generation of characteristic x-

rays and Bremsstrahlung.

X-ray tube spectra generally have more than one characteristic x-ray peaks since
there are a number of electronic transitions possible following a vacancy created
in one of the inner electronic shells. If an electron from the innermost K-shell is
knocked-off, the vacancy can in principle be filled by any of the electrons in the
outer shells. If an electron from the L-shell jumps in to fill the vacancy, a photon
with an energy ofEγ=EL−EKis emitted. A large number of such photons would
appear as a prominent peak in the spectrum. Such a peak is generally referred to
asKαpeak.Kβpeak is the result of transitions of M-shell electrons to the K-shell
giving off photons with an energy ofEγ=EM−EK(see Fig.1.6.3).
In an x-ray tube the target (anode) is kept very close (typically 1-3cm)tothe
source of electrons (cathode). A high electric potential between cathode and anode
accelerates the electrons to high velocities. The maximum kinetic energy in electron
volts attained by these electrons is equal to the electric potential (in volts) applied
between the two electrodes. For example an x-ray machine working at a potential
of 30kVcan accelerate electrons up to a kinetic energy of 30keV.
X-ray machines are extremely inefficient in the sense that 99% of their energy is
converted into heat and only 1% is used to generate x-rays.

Example:

Calculate the maximum velocity attained by an electron in an electric

potential of 40kV.

Solution:

An electron in an electric field of 40kVcan attain a maximum kinetic energy

of

Tmax =

(

40 × 103

)(

1. 602 × 10 −^19

)

=6. 408 × 10 −^15 J

=40keV.