Download Session II305 Semiconductors Rev050503

Session II.3.5
Part II Quantities and Measurements
Module 3 Principles of Radiation
Detection and Measurement
Session 5 Semiconductor Detectors
5/2003 Rev 2
IAEA Post Graduate Educational Course
Radiation Protection and Safe Use of Radiation Sources
II.3.5 – slide 1 of 45
Semiconductor Detectors
 Upon completion of this section the student will be
able to explain the process and characteristics of
semiconductor detectors including the concepts:
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N-type
P-type
Intrinsic/Depletion region
Resolution
Efficiency
II.3.5 – slide 2 of 45
Semiconductor Diodes
 Semiconductors are typically made of silicon or
germanium
 For portable detectors, silicon is typically used
because the band gap is greater which results in
less thermally generated “noise”
 To reduce this noise in germanium detectors it is
necessary to cool the detectors using liquid nitrogen
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II.3.5 – slide 3 of 45
Semiconductor Detectors
 Silicon forms a crystal that has a diamond
shaped lattice
 Each silicon atom has four covalent bonds
 In the diagram in the next slide, each
covalent bond is represented by a pair of
valence band electrons
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II.3.5 – slide 4 of 45
Semiconductor Detectors
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II.3.5 – slide 5 of 45
Semiconductor Detectors
 There are two types of silicon and
germanium semiconductor detectors, N-type
and P-type
 N-type detectors have an excess of donor
impurities, usually group V elements
 An extra electron is donated at the site of the
impurity resulting in an extra negative
charge
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II.3.5 – slide 6 of 45
N-Type Si Containing
Group V Donor Impurity
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II.3.5 – slide 7 of 45
Semiconductor Detectors
 P-type detectors have an excess of acceptor
impurities, usually group III elements
 A hole is created at the site of the acceptor
impurity, this results in a positive charge at
the site of the impurity
5/2003 Rev 2
II.3.5 – slide 8 of 45
P-Type Si Containing
Group III Acceptor Impurity
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II.3.5 – slide 9 of 45
Semiconductor Detectors
 The sensitive volume of a diode detector is referred
to as the depletion or intrinsic region
 This is the region of relative purity at a junction of
n-type and p-type semiconductor material
 At this junction, the electrons from the n-type silicon
migrate across the junction and fill the holes in the
p-type silicon to create the p-n junction where there
is no excess of holes or electrons
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II.3.5 – slide 10 of 45
Semiconductor Detectors
 When a positive voltage is applied to the n-type
material and negative voltage to the p-type material,
the electrons are pulled further away from this
region creating a much thicker depletion region
 The depletion region acts as the sensitive volume of
the detector
 Ionizing radiation entering this region will create
holes and excess electrons which migrate and
cause an electrical pulse
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II.3.5 – slide 11 of 45
Semiconductor Detectors
Reverse Bias
Anode (+)
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Cathode (-)
Intrinsic/Depletion Region
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II.3.5 – slide 12 of 45
Semiconductor Detectors
 Diode detectors are often referred to as
“PIN” detectors or diodes. “PIN” is from
P-type, Intrinsic region, N-type
 The intrinsic region is several hundred
micrometers thick
 The intrinsic efficiency (ignoring attenuation
from the housing) is 100% for 10 keV
photons
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II.3.5 – slide 13 of 45
Semiconductor Detectors
 The efficiency is reduced to approximately
1% for 150 keV photons and remains more or
less constant above this energy
 Above 60 keV, the interactions involve
Compton scattering almost exclusively
5/2003 Rev 2
II.3.5 – slide 14 of 45
Semiconductor Detectors
Gamma rays transfer
energy to electrons
(principally by
compton scattering)
and these electrons
traverse the intrinsic
region of the detector
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II.3.5 – slide 15 of 45
Semiconductor Detectors
 When a charged particle traverses the
intrinsic (depletion) region, electrons are
promoted from the valence band to the
conduction band
 This results in a hole in the valence band
 Once in the conduction band, the electron is
mobile and it moves to the anode while the
positive hole moves to the cathode (actually
it is displaced by electrons moving to the
anode)
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II.3.5 – slide 16 of 45
Semiconductor Detectors
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II.3.5 – slide 17 of 45
Semiconductor Detectors
 The average energy needed to create an
electron-hole pair in silicon is about 3.6 eV
 The average needed to create an ion pair in
gas is about 34 eV, so for the same energy
deposited, we get about 34/3.6 or about
9 times more charged pairs
5/2003 Rev 2
II.3.5 – slide 18 of 45
Energy Resolution
 The energy resolution in a detector is E/E, which is
proportional to N where N is the number of charged
pairs
 Using a semiconductor detector, we receive about
9, or 3 times the resolution of a gas ionization
detector system
 Compared to a scintillation detector which requires
about 1000 eV to create one photoelectron at the PM
tube, the resolution is about 17 times better
5/2003 Rev 2
II.3.5 – slide 19 of 45
Germanium vs Silicon Detectors
 Germanium (Ge) requires only 2.9 eV to
create an electron-hole pair vs. 3.6 eV for
silicon, so the energy resolution is
(3.6/2.9) = 1.1 times that of silicon
 The problem with Ge is that thermal
excitation creates electron-hole pairs. For
this reason liquid nitrogen is used to cool
the electronics of germanium systems
5/2003 Rev 2
II.3.5 – slide 20 of 45
Ge(Li) and Si(Li) Detectors
 Germanium with lithium ions used to create
the depletion zone form what is known as a
Ge(Li) “jelly” detector
 Silicon with lithium ions used to create the
depletion zone comprise what is known as a
Si(Li) “silly” detector
5/2003 Rev 2
II.3.5 – slide 21 of 45
Ge(Li) and Si(Li) Detectors
 For gamma ray detection, the detector
efficiency for the photoelectric effect is
proportional to Z5, where Z is the atomic
number of the detector material
 Since for Ge, Z=32, and the Z of Si is 14, Ge
detectors are about 62 times more efficient
than Si detectors
5/2003 Rev 2
II.3.5 – slide 22 of 45
Germanium Detectors
 Germanium detectors are semiconductor
diodes having a p-i-n structure in which
the intrinsic (I) region is sensitive to
ionizing radiation, particularly x rays and
gamma rays. Under reverse bias, an
electric field extends across the intrinsic
or depleted region.
5/2003 Rev 2
II.3.5 – slide 23 of 45
Germanium Detectors
 When photons interact with the material
within the depleted volume of a detector,
charge carriers (holes and electrons) are
produced and are swept by the electric
field to the P and N electrodes. This
charge, which is in proportion to the
energy deposited in the detector by the
incoming photon, is converted into a
voltage pulse by an integral charge
sensitive preamplifier.
5/2003 Rev 2
II.3.5 – slide 24 of 45
Germanium Detectors
 Because germanium has relatively low
band gap, these detectors must be cooled
in order to reduce the thermal generation
of charge carriers (thus reverse leakage
current) to an acceptable level. Otherwise,
leakage current induced noise destroys
the energy resolution of the detector.
5/2003 Rev 2
II.3.5 – slide 25 of 45
Germanium Detectors
 Liquid nitrogen, which has a temperature
of 77 °K is the common cooling medium
for such detectors. The detector is
mounted in a vacuum chamber which is
attached to or inserted into an LN2 dewar
called a cryostat. The sensitive detector
surfaces are thus protected from moisture
and condensible contaminants.
Electrically cooled cryostats are also
available.
5/2003 Rev 2
II.3.5 – slide 26 of 45
Germanium Detectors
 There are two types of germanium detectors, ptype and n-type.
 The detectors are connected to a preamplifier.
There are only two basic types of preamplifiers in
use on Ge detectors. These are charge sensitive
preamplifiers, which employ either dynamic
charge restoration (RC feedback), or pulsed
charge restoration (Pulsed optical or Transistor
reset) methods to discharge the integrator.
5/2003 Rev 2
II.3.5 – slide 27 of 45
Broad Energy Detectors
 Broad Energy Ge (BEGe) Detector covers the
energy range of 3 keV to 3 MeV. The resolution at
low energies is equivalent to that of low energy
Ge detectors and the resolution at high energy is
comparable to that of good quality coaxial
detectors.
 Most importantly the BEGe has a short, fat shape
which greatly enhances the efficiency below 1
MeV for typical sample geometries. This shape is
chosen for optimum efficiency for real samples in
the energy range that is most important for
routine gamma analysis.
5/2003 Rev 2
II.3.5 – slide 28 of 45
Broad Energy Detectors
 In addition to higher efficiency for typical
samples, the BEGe exhibits lower background
than typical coaxial detectors because it is more
transparent to high energy cosmogenic
background radiation that permeates above
ground laboratories and to high energy gammas
from naturally occurring radioisotopes such as
40K and 208Tl (Thorium). This aspect of thin
detector performance has long been recognized
in applications such as actinide lung burden
analysis.
5/2003 Rev 2
II.3.5 – slide 29 of 45
Broad Energy Detectors
 The BEGe is designed with an electrode structure
that enhances low energy resolution and is
fabricated from select germanium having an
impurity profile that improves charge collection
(thus resolution and peak shape) at high
energies. Indeed, this ensures good resolution
and peak shape over the entire mid-range which
is particularly important in analysis of the
complex spectra from uranium and plutonium.
5/2003 Rev 2
II.3.5 – slide 30 of 45
Broad Energy Detectors
 In addition to routine sample counting, there are
many applications in which the BEGe Detector
really excels. In internal dosimetry the BEGe
gives the high resolution and low background
need for actinide lung burden analysis and the
efficiency and resolution at high energy for whole
body counting. The same is true of certain waste
assay systems particularly those involving
special nuclear materials.
5/2003 Rev 2
II.3.5 – slide 31 of 45
Broad Energy Detectors
 The BEGe detector and associated
preamplifier are normally optimized for
energy rates of less than 40 000 MeV/sec.
Charge collection times prohibit the use of
short amplifier shaping time constants.
Resolution is specified with shaping time
constants of 4-6 microseconds typically.
5/2003 Rev 2
II.3.5 – slide 32 of 45
Broad Energy Detectors
 Another big advantage of the BEGe is that
the detector dimensions are virtually the
same on a model by model basis. This
means that like units can be substituted in
an application without complete
recalibration and that computer modeling
can be done once for each detector size
and used for all detectors of that model.
5/2003 Rev 2
II.3.5 – slide 33 of 45
Broad Energy Detectors
 Absolute Efficiency of the Canberra
Industries BE5030 compared to a Coaxial
Detector of 60 mm diameter by 80 mm
length for a source measuring 74 mm
diameter by 21 mm thick located on the
detector end cap. Both detectors have
approximately 50% Relative Efficiency for
a 60Co point source at 25 cm.
5/2003 Rev 2
II.3.5 – slide 34 of 45
Broad Energy Detectors
 With cross-sectional areas of 20 to 50 cm2
and thickness’ of 20 to 30 mm, the nominal
relative efficiency is given below along
with the specifications for the entire range
of models. BEGe detectors are normally
equipped with our low background
composite carbon windows. Beryllium or
aluminum windows are also available.
5/2003 Rev 2
II.3.5 – slide 35 of 45
Germanium Detectors
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II.3.5 – slide 36 of 45
Comparison of Broad Energy
and Coax Detectors
5/2003 Rev 2
II.3.5 – slide 37 of 45
Broad Energy Detectors
5/2003 Rev 2
II.3.5 – slide 38 of 45
Extended Range
Germanium Detectors
 The extended range coaxial germanium
detector haves a unique thin-window
contact on the front surface which
extends the useful energy range down to 3
keV. Conventional coaxial detectors have
a lithium-diffused contact typically
between 0.5 and 1.5 mm thick. This dead
layer stops most photons below 40 keV or
so rendering the detector virtually
worthless at low energies.
5/2003 Rev 2
II.3.5 – slide 39 of 45
Extended Range
Germanium Detectors
 The extended range detector, with its
exclusive thin entrance window and with a
Beryllium cryostat window, offers all the
advantages of conventional standard
coaxial detectors such as high efficiency,
good resolution, and moderate cost along
with the energy response of the more
expensive Reverse Electrode Ge (REGe)
detector.
5/2003 Rev 2
II.3.5 – slide 40 of 45
Extended Range
Germanium Detectors
 The effective window thickness can be
determined experimentally by comparing
the intensities of the 22 keV and 88 keV
peaks from 109Cd. With the standard 0.5
mm Be window, the XtRa detector is
guaranteed to give a 22 to 88 keV intensity
ratio of greater than 20:1. Aluminum
windows are also available.
5/2003 Rev 2
II.3.5 – slide 41 of 45
Extended Range Detectors
 The response curves
illustrate the
efficiency of the XtRa
detector compared to
a conventional Ge
detector.
5/2003 Rev 2
II.3.5 – slide 42 of 45
Extended Range Germanium
Detectors
 The response curves
illustrate the
efficiency of the XtRa
detector compared to
a conventional Ge
detector.
5/2003 Rev 2
II.3.5 – slide 43 of 45
Extended Range Germanium
Detectors
 Spectroscopy from 3 keV up
 Wide range of efficiencies
 High resolution – good peak shape
 Excellent timing resolution
 High energy rate capability
 Diode FET protection
 Warm-up/HV shutdown
 High rate indicator
5/2003 Rev 2
II.3.5 – slide 44 of 45
Where to Get More Information
 Cember, H., Introduction to Health Physics, 3rd
Edition, McGraw-Hill, New York (2000)
 Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds.,
Table of Isotopes (8th Edition, 1999 update), Wiley,
New York (1999)
 International Atomic Energy Agency, The Safe Use
of Radiation Sources, Training Course Series No. 6,
IAEA, Vienna (1995)
5/2003 Rev 2
II.3.5 – slide 45 of 45