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.

E e

 

E e

 

E

  2

m e 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.

E e

 

E e

  2

E



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

I

o , 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 measure charge produced Microchannel plate Scintillation counter 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.