Techniques for detecting X-rays and gamma-rays
Pair production
Creation of elementary particle and its antiparticle from a photon.
Occurs only if enough energy present to create the pair – at least the total
rest mass energy of the two particles.
Ee  Ee  E  2me c 2
 Z
2
Techniques for detecting X-rays and gamma-rays
Pair production
The positron and electron after creation produce trails of ionisation until
eventually they have expended all their energy. If the positron comes to rest
near an electron it will annihilate to create a pair of 0.511 keV gamma rays.
This is a good calibration source for any detector.
Ee  Ee  2E
Techniques for detecting X-rays and gamma-rays
Bringing this all together
For a fixed Z the photoelectric effect is dominant at low photon energies.
Pair production is dominant at high energies.
Mid energy interactions favour Compton scattering.
Mistakes
Techniques for detecting X-rays and gamma-rays
Mass absorption coefficient
The mass absorption coefficient is a measurement of how strongly a
substance absorbs photons at a given energy.
Monoenergetic photons with an incident intensity Io, penetrating a layer of

material with mass thickness x, mass absorption coefficient   , and



density ρ, emerges with intensity I given by the exponential attenuation law:
 
I ( x)  I o exp 
 
 
  x
 

 

Techniques for detecting X-rays and gamma-rays
Total mass attenuation coefficient
This graph shows both absorption and scattering processes.
Scattering is process in which a portion of the photons coming from a source
scatter (bounce) off molecules and other small particles in the atmosphere or
target in the case of a detector.
The line in the graph
indicating the total absorption
coefficient only receives a
contribution from absorption
processes and disregards any
contribution due to scattering.
The total attenuation coefficient
receives contributions from both
scattering and absorption
processes.
Techniques for detecting X-rays and gamma-rays
Rayleigh scattering
Rayleigh scattering is the scattering of photons by tiny particles. It can occur
when photons pass through solids or liquids, but most often in gases. (In the
case of our blue sky it is these scattered photons that give the sky its
brightness and colour.
Techniques for detecting X-rays and gamma-rays
Line for Compton scattering and one for Compton absorption – why ?
In Compton effect the interacting photon passes on in a new direction
(Compton scatters) having given up part of its energy to smack into an
electron (Compton absorption).
This means that the photon will have deviated from the direct line of sight
between source and detector.
 
I ( x)  I o exp 
 
 
  x
 

 

Techniques for detecting X-rays and gamma-rays
So do I use the total attenuation or absorption coefficient ?
If you have a detector with which you want to measure the photon rate from a
distant source then you would want to know the TOTAL attenuation
coefficient as you are interested in the absolute intensity arriving at your
target.
If you would just like to know the intensity as a consequence only of
absorption and wish to ignore completely the effect of scattering, then you
would want to know the total absorption coefficient at the energy of interest.
http://physics.nist.gov/PhysRefData/XrayMassCoef/tab3.html
http://physics.nist.gov/PhysRefData/XrayMassCoef/tab4.html
On the NIST website:
The total mass attenuation coefficient is defined as
The total mass absorption coefficient is  en 
  

 

Techniques for detecting X-rays and gamma-rays
Detectors types used to detect x-rays and gamma rays
Detectors are designed to allow photons to interact with some sort of target
material causing them to interact via the photoelectric effect or Compton
effect or pair production.
Photoelectric effect = photon is absorbed by a target atom and photoelectron
is created. This shoots off in a specific direction and bounces into other
atoms creating a trail of ionisation along its path until it loses all its energy.
Compton effect = the photon gradually loses its energy by momentum
transfer to orbital electrons until all original photon energy is lost.
Pair production = the photon is absorbed and all its energy (minus the
energy required to create them i.e. the rest mass) is given in the form of
kinetic energy to an electron and a positron which then create ionisation
trails.
Techniques for detecting X-rays and gamma-rays
Detectors types used to detect x-rays and gamma rays
Detectors then either measure this charge directly or measure it indirectly
by for example recording the light produced as the ionised atoms and
electrons recombine. We will look at the following types…
Some detectors can only measure the total number of photons, others can
record number as a function of energy (creating a spectrum) and some can
record the path of the photon as it interacts.
Proportional counter
Semiconductor detector
MPPC
Microchannel plate
Scintillation counter
measure charge produced
measure light from scintillation
Techniques for detecting X-rays and gamma-rays
Detectors types used to detect x-rays and gamma rays
Proportional counter
Semiconductor detector
MPPC
Microchannel plate
Scintillator
Main detector types
Proportional counters
Gas (e.g. argon + 10%
methane) filled container
with a central electrode to
attract the charge
(ionisation) created by the
photon.
Primary electrons "see" an
increasing electrical field on
route to the central anode
wire.
Electrons speed up and create additional ionisation colliding with atoms in
their path. These ‘new’ ( secondary) electrons collide with other atoms
creating a cascade amplifying the original signal.
The electron cloud reaches the central wire where a current pulse is
recorded by basic and rugged electronics.
If all photon energy is given up inside the chamber then the size of the
pulse is proportional to the energy of the original photon.
Main detector types
Proportional counters
http://www.youtube.com/watch?v=cAIKp0cu7UM
Some are now capable of discerning the path of ionisation through the gas.
Rectangular boxes containing grids of orthogonal wires (Multi-wire
Proportional Chambers).
Ionisation track is drifted
.
within an electric field toward
grids.
Upon arrival it creates a
signal on both sets of wires
and triangulation provides x
and y coordinates.
z coordinate is determined
by measuring the drift time
from the ionization event to
the wires.
Path of ionisation can be
reconstructed.
Main detector types
Proportional counters
The ionisation trail has
been determined using
x ,y ,z, coordinates.
Main detector types
Scintillation Detectors
Scintillators produce light when either ionising particles pass through
them or photons interact with target atoms.
They can be gases or liquids or solids (but they must always be
transparent to the light they produce!)
An incident photon
interacts to create high
speed electrons which
go on to create a path of
ionisation through the
target.
Main detector types
Scintillation Detectors
Look at the table below and appreciate that the original X-ray photon
energy must be suitably high to enable a respectable output signal from
the PMT.
Putting these values together sodium iodide scintillation detectors need
around 230 eV of incident photon energy to create an electron which will
then pass to the 1st dynode.