Radiation detectors

A short introduction to radiation detectors

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Disclaimer

• Detector physics is not strictly speaking related to lab instrumentation

• You will be building a proportional counter during this course however

• This is a short walkthrough of things you need to know to write your report

• There are courses for solid state detectors and for gas filled detector (contains scintillators) for more in-depth treatise of detector physics

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Radiation interaction with matter

• Energy transfer between systems through a medium

• Non-ionizing radiation – Electromagnetic radiation with low energy (radio, radar, infra-red and

visible light)

• Ionizing radiation – Electromagnetic radiation beyond UV (x-rays, gammas)

– Charged particles (alphas, betas, fission fragments, particles in high energy physics experiments)

• Neutrons, neutrinos etc. – Usually have a reaction which has an end product that can be

measured

– Not considered in these lectures

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Photons

• Photons interact with matter via few processes, each dominating at different energy range

• Photoelectric absorption – up to hundreds of keV

• Compton scattering – up to few MeV

• Pair production from MeV (2 times 511 keV) on

Image from Gaseous Radiation Detectors, Fundamentals and Applications, Fabio Sauli

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Photons

• Photoelectric absorption

– Photon is absorbed by an electron in an atom in a discreet process transferring the photon energy into kinetic energy of the electron

– After absorption the atom may be excited and will emit an x-ray or an auger electron

– When photoabsorption dominates, the intensity of incoming photon radiation will attenuate but the energy stays unchanged

– Cross section depends strongly (power of ~4) on the Z of the absorbing atom and the inverse of the energy of the photon

– After few hundred keV the cross section of photoabsorption gets small compared to compton scattering

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Image from https://en.wikipedia.org/wiki/Absorption_cross_section

Photons

• Pair production – Photon turns into electron-positron pair in

the Coulomb field of a nucleus – Excess energy (beyond 1.02 MeV) is given as

kinetic energy of the electron positron pair – Positron will normally annihilate fast into a

photon pair – Scales with Z and and E completely

dominating on high energies

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• Elastic scattering – In low energy region the radiation can scatter without exiting or ionizing the atoms so no

energy transfer happens

• Compton scattering – Inelastic scattering: photon gives up a portion of its energy to an electron – Scattering photon is deflected by compton angle which depends on the energy lost to the

electron – Cross section scales with Z and inverse of the photon energy

Charged particles

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• Bethe-Bloch: e and m are the charge and mass of electron, Z, A and ρ the medium atomic number, mass and density and N the Avogadro’s number

• C/Z and δ/2 are correction factors for electron screening and density effect correction that can be found tabulated in the literature

• At high energy (GeV+) the energy loss becomes similar for all particles - minimum ionizing particles or mips

Charged particles

• Energy is lost via ionization and exitation of the electrons in the material – Particle undergoes a chain of small energy losses:

dE/dx – Ionized electrons often have enough energy to ionize

more (delta electrons) – Ionization clusters by delta electrons around the

path of the particle – Energy loss scales by 1/E - most of the energy is lost

in the end of the track – Range of alpha particles is cm scale in air, micron

scale in solids – Steady, but energy dependent loss dE/dx in the

material – Normally tracks are straight, due to the different

masses of the particle and the electrons that act as scatterers

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Charged particles

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• Electrons are special – Mass of electrons is several magnitudes less than anything else: collision

kinematics different

• Electrons are everywhere – Heavy charged particles lose most of their energy via delta electrons – Electron is produced also in photon interactions

• Scattering – Because of the small mass of

electrons, they scatter strongly under the collisions

– Bremstrahlung losses: Very fast electrons radiate photons when accelerating/deaccelerating

– Electron range in a material is difficult to define due to multiple scattering. Range is still magnitudes more than for heavier particles

Non-ionizing electromagnetic radiation

• Low frequencies not relevant for Detector laboratory, except for one thing - noise

• Visible light, even near infrared, is energetic enough to kick electrons free in a semiconductor detector

• Photomultipliers rely on photoelectric effect to get photoelectrons from a metal photocathode

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Radiation sources in Detector laboratory

• 55Fe source – 55Fe decays via electron capture into 55Mn which is exited and relaxes by sending

characteristic x-rays – Kα(1,2), Kβ(1,2) have mean energies of 5.89 keV and 6.49 keV respectively but due to

relative intensities the mean energy of them combined is 5.9 keV

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Radiation sources in Detector laboratory

• 241Am source – Alpha source decays via alpha decay into 237Np – Source is closed so the alpha particle will not escape the source. – In addition to the alpha particle, a complicated spectrum of gamma rays is emitted – Main gamma line in 59.54 keV

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• Modes of operation

• Getting radiation in

• Keeping energy in

• Getting signal out

• Readout

Radiation Detectors

Operating modes

• Integrating mode – In an integrating mode the signal is

collected over some time and the sum is then read out

– Readout is simple – Time and energy information of the

detected radiation lost

• The history of radiation detectors – Scintillators read via photographic

plates by C.W. Röntgen (1895) – Proportional counter was read with

current meter (1906) – Radiation security badges – X-ray imaging and tomography

detectors

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Operating modes

• Pulse mode with no energy discrimination – The time information of each particle

crossing the detector is detected separately

– Optimal for tracking and particle counting detectors that do not stop the radiation

• Examples – The Geiger Mueller counter (1928) – Triggers (hodoscope) – Radiation security detectors – Many detectors in HEP experiments

(tracking, muon systems)

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Operating modes

• Pulse mode with energy discrimination – The time and energy

information of each particle is detected

– For spectroscopy – To get the full energy, the

radiation has to be stopped in the detector

– Can measure dE/dx instead

• Examples – Bubble chamber ? – The Aluminun beverage can

detector

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Getting the radiation in the detector

• Charged particles

– Particles are slowed just by air

– The window of the detector has to be thin enough to let the particles in (MeV scale alpha particles – hundreds of nm to few microns)

– As energy grows the stopping power decreases – GeV scale protons go through the detector

– Electrons have longer range but random tracks and backscattering

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Getting the radiation in the detector

• Photons

– In low energy region the intensity of photon radiation scales by I/I0=e-(μ/ρ)x , where μ/ρ is mass attenuation coefficient and x the mass thickness of the material (density*thickness)

– With increasing energy Compton scattering starts to compete with photoabsorption. Energy lost to electrons and intensity lost due to scattering out of the detector

– Compton scattering can also take place in the structure of the detector (scales with Z). Secondary radiation.

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Getting the energy to stay in the detector

• The detector can only see the energy that is collected in the active volume of the detector – If charged particle goes through the detector it will leave

part of its kinetic energy to the detector

– It is possible to get an idea of the energy and/or species of the particle from the amount of its energy loss dE/dx

– Due to relatively long range of fast electrons it is possible to lose part of the absorbed energy when the electrons leave the active volume

– Sometimes the atoms get excited and an characteristic x-ray is emitted, which can leave the active volume.

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Getting the energy to stay in the detector

• The detector can only see the energy that is collected in the active volume of the detector – If a photon goes through, it does not leave a trace. If it is

absorbed, it becomes a fast electron and/or photons. Photon spectroscopy is only possible if the full energy of the secondary electron is contained within the detector

– When a characteristic x-ray is emitted and leaves the volume an escape peak is created. Energy of the escape peak for photon absorption is Eesc = Ephoton – Ex-ray.

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Getting the signal out of the detector

• At this point you have a lot of electrons in the detector volume

– Electrons are collected with electric field

– Their movement induces current in the readout electrode(s)

– The current can be read out directly (ionization chamber)

– Each signal can be collected separately by suitable preamplifier, which amplifies the signal and gives out a voltage pulse

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Getting the signal out of the detector

• Except in scintillator detector the electrons excite the atoms and create light

– Light is collected into a light sensitive detector by a lightguide

– Light sensitive detector (photomultiplier, avalanche diode etc.) generates an electrical pulse which is then amplified

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