Detection of energetic particles and gamma rays
Semiconductor detectors
Peter Dendooven
Basic Detection Techniques 2009-2010http://www.astro.rug.nl/~peletier/DetectionTechniques.html
2
Contents
• Interaction of radiation with matter– high-energy photons
– charged particles• heavy charged particles
• electrons
– neutral particles• neutrons
• neutrinos
• General radiation detection concepts– pulse mode operation
– energy spectrum
– detector efficiency
– timing
• Radiation detectors– semiconductor detectors
• operation principle
• examples (silicon, germanium)
• other materials
– scintillation detectors• principle
• organic scintillators
• inorganic scintillators
• photosensors
– gas detectors• ionisation
• proportional
• Geiger
Note: also known as solid-state detectors
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Semiconductor band structure
ni: intrinsic density of electrons in conduction band
pi: intrinsic density of hole in valence band
Nc,v: number of states in conduction, valence band
Eg: band gap at 0 K
A: temperature-independent constant
!
ni = pi = NcN v e"
Eg
2kT = AT3/ 2 e"
Eg
2kT
metalinsulator
>6 eV
semiconductor
~1 eV
electrons
holes
energ
y
occupied
empty
conduction band
valence band
band gap at T=0 K
at T>0 K
kT at 300 K = 0.025 eV
4
Relevant properties of intrinsic Si and Ge
(*) due to the small band gap, Ge needs to be cooled to reduce the leakage
current to an acceptable level (usually LN2 cooling, 77 K is used)
(*)
2.96
3.62
3.76
ionisation energy (eV) 300 K
77 K
3.6 x 104
4.2 x 104
2.1 x 104
1.1 x 104
mobility (cm2/V/s) at 77 K: electrons
holes
3900
1900
1350
480
mobility (cm2/V/s) at 300 K: electrons
holes
2.4 x 10131.5 x 1010intrinsic carrier density at 300 K (/cm3)
0.665
0.746
1.115
1.165
band gap (eV) 300 K
0 K
1612dielectric constant (relative to vacuum)
4.41 x 10224.96 x 1022atomic density (atoms/cm3)
5.322.33density (g/cm3)
3214atomic number
GeSi
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How to get a signal ?
• mimimum ionizing particle deposits ~400 eV/µm
creates ~3.3 x 104 e-h pairs
• free charge carriers in the same volume
4.5 x 109
! particle signal is drowned
solution: reduce number of free charge carriers,
i.e. deplete the semiconductor !! doping
! blocking contact
300 µm
1 cm
1 cm
Si wafer
!
r E he I
!
I = Ie + Ih = A ni e (ve + vh )
= A ni e E (µe + µh )
A: surface area
v: velocityµ: mobility
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Doped semiconductors: n-type and p-type
• pentavalent elements (group V/15, e.g. P, As, Sb)
have one electron too much to fit in: “donor
impurities”
• extra electrons are lightly bound
- energy level close to the conduction band
- thermally excited into the conduction band
- recombination with holes: ne >> nh
! n-type semiconductors
- electrons are the majority charge carriers
- holes are the minority charge carriers
donor level
acceptor level
• trivalent elements (group III/13, e.g. Ga, B, In) have
one electron too little to fit in: “acceptor impurities”
• electrons in missing bond slightly less bound
- energy level close to the valence band
- thermally excited electrons fill the acceptor level,
creating holes
- holes recombine with conduction band
electrons: nh >> ne
! p-type semiconductors
- holes are the majority charge carriers
- electrons are the minority charge carriers
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Conductivity of doped semiconductors
• typical doping levels for detector silicon: 1012 atoms/cm3
• heavily doped semiconductors (n+, p+): 1020 atoms/cm3
results in very high conductivity (good for contacts)
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Action of radiation
• e-h pairs are produced along track (Ne=Nh)
• energy needed per e-h pair is largely independent on
– energy of radiation
– type of radiation
– temperature
! average energy needed = ionisation energy (")
• " is small: Si, 300 K: 3.62 eV
Ge, 77 K: 2.96 eV
! good energy resolution
semiconductor
~1 eV
electrons
holes
advantages of semiconductor detectors:
• good energy resolution
• high density, relatively high atomic number (Ge):
- good stopping power
- good foton interaction probability
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The Fano factor
statistical variation in the number of e-h pairs
– # of e-h pairs
– if excitations are independent: Poisson statistics
!
N =E
"
": ionisation energy
E: energy deposited by radiation quantum
!
"N
= N =E
#variancestandard deviation
!
F "observed statistical variance
E #
!
"N
2=
E
#
Fano factor:
for Si, Ge: F ~ 0.1-0.15 ~ 3 times better than Poisson statistics
!
"EFWHM
stat
not fully understood phenomenon
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Blocking contact: the p-n junction
n-typemobile electrons
p-typemobile holes
depletion region
diffusion: holes to n-region, electrons to p-region
uncompensated fixed charges build up
emerging “contact” potential stops diffusion
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The depletion region detects ionising radiation
• thermally generated charge carriers are quickly swept away
due to the contact potential
!highly suppressed charge carrier density
! relatively small amount of charge carriers created by an
ionising particle is easily detected
• poor performance because:
- small contact potential (~1 V): slow-moving charges can be
trapped, resulting in incomplete charge collection
- depletion layer is thin:
- high capacitance ! large electronic noise
- small sensitive volume cannot detect high-energy radiation
depletion region
13
increased
depletion region
+ –
Reverse biasing
• bias: 100 - 1000 V/cm
• V >> contact potential
• depletion region thickness increases
– smaller capacitance, smaller electronic noise
– quick and complete charge collection
• very large electric field: multiplication ! silicon avalanche detector
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Maximizing the depletion region
• normal semiconductor purity: depletion 2-3 mm maximum
• problem for “long-range” radiation (e.g. high-energy #-rays)
• increasing d only by decreasing N
– further refining techniques
• Si: not (yet) possible
• Ge: high-purity germanium (HPGe)
– N ~1010 atoms/cm3 (relative impurity 10-12 !)
– depletion up to a few cm
– compensated material by lithium ion drifting
• Si(Li), Ge(Li): up to ~2 cm (so-called p-i-n structure)
!
d =2 " V
e N
#
$ %
&
' (
1/ 2
d: depletion region thickness
V: reverse bias voltage
": dielectric constant
e: electronic charge
N: net impurity concentration (atoms/cm3)
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Germanium detector configurations
$, %: ultrapure n, p type
planar coaxial
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Some examples of Si detectors
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Some examples of Si detectors
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Anatomy of a Ge detector
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Some examples of Ge detectors
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Some examples of Ge detectors
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Other semiconductor materials
• Why ?
– higher atomic number for higher #-ray interaction probability
– room-temperature operation (&Ge)
• Commercially available detectors
– CdTe
– Cd1-xZnxTe (CZT)
– HgI2
• Large crystal growth problems cause slow development
– impurities
– defects
! small volumes only
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Properties high-Z semiconductor detectors
4.32.136.480/53HgI2
(300 K)
5.01.64648/30/52Cd0.8Zn0.2Te
(300 K)
4.431.526.0648/52CdTe(300 K)
2.980.725.3232Ge
(77 K)
3.611.122.3314Si
(300 K)
ionisation
energy
(eV)
bandgap
(eV)
density
(g/cm3)
atomic
numbermaterial