Radiation interaction with matter and introduction to radiationdetectors

PAP328 – Lecture part 2 1/ 22

Radiation interaction with matter

• Energy transfer between systems through a medium• Non-ionizing radiation

- Electromagnetic radiation with low energy (radio, radar, infra-red and visiblelight)

• Ionizing radiation- Electromagnetic radiation beyond UV (x-rays, gammas)- Charged particles (alphas, betas, fission fragments, particles in high energyphysics experiments)

• Neutrons, neutrinos etc.– Usually have a reaction which has an end product that can be measured– Not considered in these lectures

PAP328 – Lecture part 2 2/ 22

Photons

• Photons interact withmatter via few processes,each dominating atdifferent energy range

• Photoelectric absorption –up to hundreds of keV

• Compton scattering – up tofew MeV

• Pair production from MeVon (2 times 511 keV).

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Photons

Photoelectric absorption

• Photon is absorbed by an electronin an atom in a discreet processtransferring the photon energy intokinetic energy of the electron

• After absorption the atom may beexcited and will emit an x-ray or anauger electron

• When photoabsorption dominates,the intensity of incoming photonradiation will attenuate but theenergy stays unchanged

• Cross section depends strongly(power of ∼ 4) on the Z of theabsorbing atom and the inverse ofthe energy of the photon

• After few hundred keV the crosssection of photoabsorption getssmall compared to comptonscattering

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

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PhotonsElastic scattering

• In low energy region the radiation can scatter without exiting or ionizing the atomsso 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

Pair production

• Photon turns into electron-positronpair in the Coulomb field of a nucleus

• Excess energy (beyond 1.022MeV) isgiven as kinetic energy of the electronpositron pair

• Positron will normally annihilate fastinto a photon pair

• Scales with Z and and E completelydominating on high energies

PAP328 – Lecture part 2 5/ 22

Charged particles• Bethe-Bloch: e and m are the

charge and mass of electron, Z , Aand ρ the medium atomic number,mass and density, I the meanexcitation potential and N theAvogadro’s number,

• C/Z and δ/2 are correction factorsfor electron screening and densityeffect correction that can be foundtabulated in the literature

• At high energy (GeV+) the energyloss becomes similar for all particles- minimum ionizing particles ormips

d2N

dxdε= K

Z

A

ρ

β21ε2

K =4πNe2

mc2

∆E

∆x= −ρ

2KZ2Aβ2

[ln

2mc2β2

I (1− β2)− β2 −

C

Z−

δ

2

]

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Charged particlesEnergy is lost via ionization and exitation of the electrons in the material

• Particle undergoes a chain of smallenergy losses: dE/dx

• Ionized electrons often haveenough energy to ionize more(delta electrons)

• Ionization clusters by deltaelectrons around the path of theparticle

• Energy loss scales by 1/E - most ofthe energy is lost in the end of thetrack

• Range of alpha particles is cm scalein air, micron scale in solids

• Steady, but energy dependent lossdE/dx in the material

• Normally tracks are straight, dueto the different masses of theparticle and the electrons that actas scatterers

Figure: Image from Fermilab

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Charged particlesElectrons are special

• Mass of electrons is several magnitudes less than anything else: collisionkinematics 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 ofelectrons, they scatter strongly underthe collisions

• Bremstrahlung losses: Very fastelectrons radiate photons whenaccelerating/deaccelerating

• Electron range in a material isdifficult to define due to multiplescattering. Range is still magnitudesmore than for heavier particles

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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

PAP328 – Lecture part 2 9/ 22

Radiation sources in the Detector Laboratory55Fe source• 55Fe decays via electron capture (K-shell) into 55Mn (half-live of 2.75 a).• Electron shell reorganizes and energy is released by Auger electrons(∼ 5− 6.5 keV) or X-rays mostly from K and L shell.• 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.

Figure: Image from AmptekPAP328 – Lecture part 2 10/ 22

Radiation sources in the Detector Laboratory241Am source• 241Am decays via Alpha decay into 237Np.

24195Am 432 a−−−→ 237

93Np + 42α+ γ(59.54 keV)

• Our source is closed so that the alpha particle will not escape the source.• In addition to the alpha, a complicated spectrum of gamma rays is emitted.• Main gamma line is 59.54 keV.

Figure: Image from Amptek

PAP328 – Lecture part 2 11/ 22

Radiation sources in the Detector Laboratory241Am source and smoke detector• In smoke detector 241Am is used to ionize air in two ionization chambers.• One chamber is a closed reference chamber.• A potential difference between the electrodes produces an ion current.• In open chamber smoke particles will bind floating ions causing current

difference.

Figure: Image from WikimediaPAP328 – Lecture part 2 12/ 22

Radiation detectors

• Modes of operation• Getting radiation in• Keeping energy in• Getting signal out• Readout

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Operating modes

Integrating mode

• In an integrating mode the signal iscollected over some time and thesum is then read out• Readout is simple• Time and energy information of the

detected radiation lost

The history of radiation detectors

• Scintillators read via photographicplates 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 energydiscrimination• The time information of each

particle crossing the detector isdetected separately• Optimal for tracking and particle

counting detectors that do not stopthe radiation

Examples

• The Geiger Mueller counter (1928)• Triggers (hodoscope)• Radiation security detectors• Many detectors in HEP experiments(tracking, muon systems)

PAP328 – Lecture part 2 15/ 22

Operating modes

Pulse mode with energydiscrimination• 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 Aluminium 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 theparticles in (MeV scale alpha particles – hundreds of nm to fewmicrons)

• As energy grows the stopping power decreases – GeV scaleprotons go through the detector

• Electrons have longer range but random tracks andbackscattering

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Getting the radiation in the detector

Photons

• In low energy region the intensity of photon radiation scales byI/I0 = exp(−(µ/ρ)x) , where µ/ρ is mass attenuationcoefficient and x the mass thickness of the material (density ×thickness)

• With increasing energy Compton scattering starts to competewith photoabsorption. Energy lost to electrons and intensitylost due to scattering out of the detector

• Compton scattering can also take place in the structure of thedetector (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 volumeof the detector

• If charged particle goes through the detector it will leave partof its kinetic energy to the detector

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

• Due to relatively long range of fast electrons it is possible tolose part of the absorbed energy when the electrons leave theactive volume

• Sometimes the atoms get excited and an characteristic x-ray isemitted, 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 volumeof the detector

• If a photon goes through, it does not leave a trace. If it isabsorbed, it becomes a fast electron and/or photons. Photonspectroscopy is only possible if the full energy of the secondaryelectron is contained within the detector

• When a characteristic x-ray is emitted and leaves the volume anescape peak is created. Energy of the escape peak for photonabsorption 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 createlight

• 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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