Transcript Solid State Detectors

Radiation Detection and Measurement II
IRAD 2731
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What is a semiconductor?
Types of semiconductors
Why is it different than scintillators
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Semiconductor – has electrical
conductivity between metals and
insulators
 Pure- pure Si or Ge crystals are
used to generate signal
Small band gap
Creates hole/electron pair
Numbers of electrons produced is
proportional to energy deposited in
crystal
Conduction band
10eV
Band
Gap
<3eV
Valence band
Scintillator
Solid State
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N-type- material is doped with a “donor
impurity” which has a loosely attached
electron
This generates free electrons easier than pure
Si cause electrons are in different energy
state
Si has 4 electrons ,As or P, are used at doped
material, have 5 electrons
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P-type-material is doped with an “acceptor
impurity” which has a need for an extra
electron
This generates ”holes” easier than pure Si
Si has 4 electrons, AL or B, are used at doped
material, have 3 electrons
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When semiconductors are exposed to
radiation the electrical properties change
Intrinsic- material has been doped with both
n and p type impurities
Doping with both material aligns the holes on
one side and the electrons on the other
Appling reverse bias increases the
hole/electron area
This forms a depletion layer, active volume of
the detector
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Surface barrier detector
PIPS
Silicon detectors
Gemanium detectors
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In pure Si and Ge and natural current exists
that excludes holes/electrons close to the
surface
P-type material is electroplated onto the
surface of a n-type Si surface, usually gold
With reverse bias applied this creates a
depletion layer
Thin dead layer, very little energy loss of
charged particles
Surface Barrier
-Very good resolution , better than p-n
junction detectors
Depletion layer is not as thick (best for low
energy
particles)
-Light sensitive (2-4eV)
-Very low background
-Electronic noise
-Very fragile- can not touch
surface
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Passivated implanted planar silicon
Photo diode
Measures signals as photo current so can be
very sensitive
Low noise
Needs to be shielded from visible light
Alpha/beta detection
More rugged that SSB, lower leakage current,
window material is thinner
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Most common semiconductor
Used to detect heavy charged particles
Alpha spectroscopy
Good energy resolution
SiLi detectors (used for gamma spect) have to
be cold all the time
◦ Prevent the movement of Li inside the Si crystal
◦ BUT not for charged particles
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Designed for highly
penetrating
charged particle Up
to 3 MeV Betas, 30
MeV protons, 140
MeV Alpha
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Used to be doped with Li top get larger
depletion zone
◦ Have to keep cold all the time
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Easier to get high purity Ge than Si cause of
melting temp
GeLi has been replaced with HPGe
HPGe detectors can be warmed to room temp
when not in use
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Planar
◦ Slab of detector
◦ Limited in size
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Coaxial
◦ Can have either n or p type coaxial detectors
◦ Larger active volume of detector
◦ Large dead layer does not affect most gamma rays
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Cryostat- container that holds liquid
Nitrogen (or other cold liquid)
A method of transmitting this to the detector
(usually a copper cold finger)
◦ Can have several orientations
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Detector capsule- consisting of the detector
and electronics housed in protective endcap
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Band gap is only 0.7 ev
Thermal noise will generate tremendous
leakage current leading to noise
Will need to be cold (LN) to operate
◦ Decrease in movement of the atoms in the crystal
will decrease thermal noise
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Have smaller band gap get more pieces of
info from each radiation event
◦ More events better statistics
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Energy resolution depends on
◦ Statistical spread in number of charged carriers
◦ Variations ion charged collect ion efficiencies
◦ Electronic noise
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HPGe have better resolution than
scintillators
◦ which means that you can see gamma peaks
that are closer together than in the scintillation
crystals
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NaI detectors are more efficient than HPGe
HPGe detectors have better resolution than
Na I
BUT have some large HPGe detectors that are
more efficient than their NaI counterparts
More expensive than NaI crystals
◦ NaI gamma spectroscopy system about 10K
◦ Same efficiency HPGe system about $100K
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CZT -cadmium-zinc-telluride crystals
◦ Operates at room temperature
◦ Good energy resolution better than NaI but not as
good as HPGe
◦ Hard to grow
◦ High density
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LaBr
◦ Similar characteristics