Transcript Part E - User Web Areas at the

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 ?
http://www.youtube.com/watch?v=qkjCe0r5-cw&feature=related
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.
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
Doped semiconductors
n-type semiconductor = dopant e.g.
phosphorus added which contributes extra
electrons, dramatically increasing the
conductivity.
p-type semiconductor = dopant e.g. boron
produces extra vacancies or holes, which
likewise increase the conductivity.
http://www.youtube.com/watch?v=AgkQrCeJF1Y
http://www.youtube.com/watch?v=U8daujO20nM&feature=related
Main detector types
Doped semiconductors
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".
Main detector types
Doped semiconductors
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.
http://www.youtube.com/watch?v=DbjR-2knrpo&feature=related
http://www.youtube.com/watch?v=kaSXVfWUqEw&feature=related
Main detector types
Semiconductor detectors
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).
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.
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.
Main detector types
Semiconductor detectors
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.
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
Microchannel plate detector
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.
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.