Main detector types Scintillation Detector Spectrum

Main detector types Scintillation Detector Spectrum

Compton scattered photon energy as a function of scattering angle for different incident photon energies.

Note how irrespective of the incident energy, a 180 degree reflection of the photon reduces its energy to between 170 and 220 keV.

Main detector types Scintillation Detector Spectrum

A = the photopeak produced by photoelectric absorption. B = the Compton background continuum. C = the Compton edge: the maximum energy that can be given to the recoiling electron in the first Compton scattering.D = a couple of unrelated Compton scattering events occur at the same time and the analysis software integrates them as one higher energy signal. It is also possible that a photon may undergo Compton scattering twice and then leave the target without depositing all its energy.E = the backscatter peak formed when the source photon does not interact at all with the target but instead Compton scatters with general stuff in the lab via the Compton effect, its recoiling photon making an angle of pretty much 120 to 240 degrees to its original travel. F = contribution from background photons coming from the rest of the universe.G = the electrical noise remembering that the PMT amplifies greatly any stray charge landing on any dynode stage.

Main detector types Influence of detector size and material on energy spectrum produced

The size of the detector and target material really influences the energy spectrum.

Remember different materials are more or less likely to interact with photons via the photoelectric effect, Compton scattering and pair production and the size of the target determines whether all the energy is deposited within the target.

Main detector types Influence of detector size and material on energy spectrum produced

Main detector types Influence of detector size and material on energy spectrum produced

Main detector types Influence of detector size and material on energy spectrum produced

Main detector types What is a semiconductor ?

Valence band contains bound electrons which form bonds between atoms.

Conduction band contains electrons which are free to move between atoms.

Conductors: valence and conduction bands overlap = always available electrons to move freely between atoms conducting electricity.

Insulator: band gap at room temperature > 6 eV thermal energy isn’t enough to promote an electron from valence band to conduction band.

Semiconductor: band gap is ~ 1 eV allowing promotion of electrons to the conduction band enabling them to be used for conduction.

http://www.youtube.com/watch?v=qkjCe0r5-cw&feature=related

Main detector types How can we turn this into a detector ?

Cool semiconductor down to liquid nitrogen temperature to empty conduction band.

Photons interacting in semiconductor promote electrons into conduction band. If a voltage is applied this charge will be removed from the detector.

But we can do better …

Main detector types

p-type semiconductor = dopant e.g. boron produces extra vacancies or holes, which likewise increase the conductivity.

Doped semiconductors

http://www.youtube.com/watch?v=AgkQrCeJF1Y

http://www.youtube.com/watch?v=U8daujO20nM&feature=related

n-type semiconductor = dopant e.g. phosphorus added which contributes extra electrons, dramatically increasing the conductivity.

Main detector types

When p-type and n-type materials are placed in contact with each other a diode is produced.

Some free electrons in the n-region are free to diffuse across the junction and combine with holes leaving behind positive ions at the donor impurity sites. This forms what is called a "depletion region".

Doped semiconductors

Main detector types

Space charge builds up across depletion region inhibiting further transfer.

A reverse voltage on the pn junction will pull electrons (black dots) towards the anode and holes (white dots) towards the cathode widening the depletion layer.

Doped semiconductors

http://www.youtube.com/watch?v=kaSXVfWUqEw&feature=related

http://www.youtube.com/watch?v=DbjR-2knrpo&feature=related

Main detector types

Photon strikes semiconductor promoting electron from valence to conduction band creating electron(-) - hole(+) pairs. (This is analogous to electron-ion pairs generated in proportional gas detectors).

Semiconductor detectors

Integral of current equals the total charge generated by the incident particle.Amount of energy required to create an electron-hole pair is known, so energy of the incident radiation can be found.

If this happens in the depletion region, the strong internal field will rapidly separate the pairs before they recombine, electrons drifting towards the anode, and holes to the cathode, resulting in a net current across the diode.

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Ionization energy of 3.6 eV is needed to produce electron-hole pair in silicon; low compared to approx 300 eV for sodium iodide crystal scintillator to produce an electron on the PMT 1st stage dynode.

Semiconductor detectors

This means better resolution as more electrons are produced for same gamma.

Difference in energy resolution between scintillator detector (a) and semiconductor detector (b) is shown in the figure.However there is no charge multiplication in the semiconductor and so the signal-to-noise ratio is a critical issue requiring low-noise electronics.

Main detector types

2mm slab of highly resistive material with tiny tubes or slots (microchannels). Microchannels 10 μm in diameter apart by approximately 15 μm parallel and entering the plate at ~8° from normal. High voltage is applied through the length of the channels.

Microchannel plate detector

Main detector types Microchannel plate detector

Incident photon enters a channel and frees (via photoelectric emission) an electron from channel wall. Under influence of field this electron strikes the adjacent wall, freeing several electrons (via "secondary emission"). These electrons give rise to more electrons.

Main detector types Microchannel plate detector

A cloud of several thousand electrons emerge from the rear of the plate where they are detected often by a single metal anode measuring total current. In some applications each channel is monitored independently to produce an image.

Main detector types

Multi Pixel Photon Counter (MPPC) and Charge Coupled Devices (CCDs)

The MPPC consists of an array of APD (Avalanche Photo-Diode) pixels arranged on a substrate of area around 1cm by 1cm.

They have superb photon detection ability, excellent cost versus performance and are very compact. They will most likely supersede expensive large PMTs in near future.