5/2003 Rev 25/2003 Rev 2 II.3.5 – slide II.3.5 – slide 11 of 45 of 45IAEA Post Graduate Educational CourseIAEA Post Graduate Educational Course

Radiation Protection and Safe Use of Radiation SourcesRadiation Protection and Safe Use of Radiation Sources

Session II.3.5Session II.3.5

Part IIPart II Quantities and MeasurementsQuantities and Measurements

Module 3Module 3 Principles of Radiation Principles of Radiation Detection and MeasurementDetection and Measurement

Session 5Session 5 Semiconductor DetectorsSemiconductor Detectors

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Semiconductor DetectorsSemiconductor Detectors

Upon completion of this section the student will be Upon completion of this section the student will be able to explain the process and characteristics of able to explain the process and characteristics of semiconductor detectors including the concepts:semiconductor detectors including the concepts:

N-typeN-type P-typeP-type Intrinsic/Depletion regionIntrinsic/Depletion region ResolutionResolution EfficiencyEfficiency

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Semiconductor DiodesSemiconductor Diodes

Semiconductors are typically made of silicon or Semiconductors are typically made of silicon or germaniumgermanium

For portable detectors, silicon is typically used For portable detectors, silicon is typically used because the band gap is greater which results in because the band gap is greater which results in less thermally generated “noise”less thermally generated “noise”

To reduce this noise in germanium detectors it is To reduce this noise in germanium detectors it is necessary to cool the detectors using liquid necessary to cool the detectors using liquid nitrogennitrogen

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Semiconductor DetectorsSemiconductor Detectors

Silicon forms a crystal that has a diamond Silicon forms a crystal that has a diamond shaped latticeshaped lattice

Each silicon atom has four covalent bondsEach silicon atom has four covalent bonds

In the diagram in the next slide, each In the diagram in the next slide, each covalent bond is represented by a pair of covalent bond is represented by a pair of valence band electronsvalence band electrons

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Semiconductor DetectorsSemiconductor Detectors

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Semiconductor DetectorsSemiconductor Detectors

There are two types of silicon and There are two types of silicon and germanium semiconductor detectors, N-type germanium semiconductor detectors, N-type and P-typeand P-type

N-type detectors have an excess of donor N-type detectors have an excess of donor impurities, usually group V elementsimpurities, usually group V elements

An extra electron is donated at the site of the An extra electron is donated at the site of the impurity resulting in an extra negative impurity resulting in an extra negative chargecharge

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N-Type Si ContainingN-Type Si ContainingGroup V Donor ImpurityGroup V Donor Impurity

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Semiconductor DetectorsSemiconductor Detectors

P-type detectors have an excess of acceptor P-type detectors have an excess of acceptor impurities, usually group III elementsimpurities, usually group III elements

A hole is created at the site of the acceptor A hole is created at the site of the acceptor impurity, this results in a positive charge at impurity, this results in a positive charge at the site of the impuritythe site of the impurity

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P-Type Si ContainingP-Type Si ContainingGroup III Acceptor ImpurityGroup III Acceptor Impurity

PositivePositiveHoleHole

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Semiconductor DetectorsSemiconductor Detectors

The sensitive volume of a diode detector is referred The sensitive volume of a diode detector is referred to as the depletion or intrinsic regionto as the depletion or intrinsic region

This is the region of relative purity at a junction ofThis is the region of relative purity at a junction of

n-type and p-type semiconductor materialn-type and p-type semiconductor material

At this junction, the electrons from the n-type silicon At this junction, the electrons from the n-type silicon migrate across the junction and fill the holes in the migrate across the junction and fill the holes in the p-type silicon to create the p-n junction where there p-type silicon to create the p-n junction where there is no excess of holes or electronsis no excess of holes or electrons

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Semiconductor DetectorsSemiconductor Detectors

When a positive voltage is applied to the n-type When a positive voltage is applied to the n-type material and negative voltage to the p-type material, material and negative voltage to the p-type material, the electrons are pulled further away from this the electrons are pulled further away from this region creating a much thicker depletion regionregion creating a much thicker depletion region

The depletion region acts as the sensitive volume of The depletion region acts as the sensitive volume of the detectorthe detector

Ionizing radiation entering this region will create Ionizing radiation entering this region will create holes and excess electrons which migrate and holes and excess electrons which migrate and cause an electrical pulsecause an electrical pulse

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Reverse BiasReverse Bias

Intrinsic/Depletion RegionIntrinsic/Depletion Region

Cathode (Cathode (--))Anode (Anode (++))

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Semiconductor DetectorsSemiconductor Detectors

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Semiconductor DetectorsSemiconductor Detectors

Diode detectors are often referred to as Diode detectors are often referred to as “PIN” detectors or diodes. “PIN” is from“PIN” detectors or diodes. “PIN” is fromPP-type, -type, IIntrinsic region, ntrinsic region, NN-type-type

The intrinsic region is several hundred The intrinsic region is several hundred micrometers thickmicrometers thick

The intrinsic efficiency (ignoring attenuation The intrinsic efficiency (ignoring attenuation from the housing) is 100% for 10 keV from the housing) is 100% for 10 keV photonsphotons

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Semiconductor DetectorsSemiconductor Detectors

The efficiency is reduced to approximately The efficiency is reduced to approximately 1% for 150 keV photons and remains more or 1% for 150 keV photons and remains more or less constant above this energyless constant above this energy

Above 60 keV, the interactions involve Above 60 keV, the interactions involve Compton scattering almost exclusivelyCompton scattering almost exclusively

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Semiconductor DetectorsSemiconductor Detectors

Gamma rays transfer Gamma rays transfer energy to electrons energy to electrons (principally by (principally by compton scattering) compton scattering) and these electrons and these electrons traverse the intrinsic traverse the intrinsic region of the detectorregion of the detector e

((++)) ((--))

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Semiconductor DetectorsSemiconductor Detectors

When a charged particle traverses the When a charged particle traverses the intrinsic (depletion) region, electrons are intrinsic (depletion) region, electrons are promoted from the valence band to the promoted from the valence band to the conduction bandconduction band

This results in a hole in the valence bandThis results in a hole in the valence band

Once in the conduction band, the electron is Once in the conduction band, the electron is mobile and it moves to the anode while the mobile and it moves to the anode while the positive hole moves to the cathode (actually positive hole moves to the cathode (actually it is displaced by electrons moving to the it is displaced by electrons moving to the anode)anode)

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Semiconductor DetectorsSemiconductor Detectors

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Semiconductor DetectorsSemiconductor Detectors

The average energy needed to create an The average energy needed to create an electron-hole pair in silicon is about 3.6 eVelectron-hole pair in silicon is about 3.6 eV

The average needed to create an ion pair in The average needed to create an ion pair in gas is about 34 eV, so for the same energy gas is about 34 eV, so for the same energy deposited, we get about 34/3.6 or aboutdeposited, we get about 34/3.6 or about9 times more charged pairs9 times more charged pairs

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Energy ResolutionEnergy Resolution

The energy resolution in a detector is The energy resolution in a detector is E/E, which is E/E, which is proportional to proportional to N where N is the number of charged N where N is the number of charged pairspairs

Using a semiconductor detector, we receive about Using a semiconductor detector, we receive about 9, or 3 times the resolution of a gas ionization 9, or 3 times the resolution of a gas ionization detector systemdetector system

Compared to a scintillation detector which requires Compared to a scintillation detector which requires about 1000 eV to create one photoelectron at the PM about 1000 eV to create one photoelectron at the PM tube, the resolution is about 17 times bettertube, the resolution is about 17 times better

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Germanium vs Silicon DetectorsGermanium vs Silicon Detectors

Germanium (Ge) requires only 2.9 eV to Germanium (Ge) requires only 2.9 eV to create an electron-hole pair vs. 3.6 eV for create an electron-hole pair vs. 3.6 eV for silicon, so the energy resolution issilicon, so the energy resolution is(3.6/2.9) = 1.1 times that of silicon(3.6/2.9) = 1.1 times that of silicon

The problem with Ge is that thermal The problem with Ge is that thermal excitation creates electron-hole pairs. For excitation creates electron-hole pairs. For this reason liquid nitrogen is used to cool this reason liquid nitrogen is used to cool the electronics of germanium systemsthe electronics of germanium systems

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Ge(Li) and Si(Li) DetectorsGe(Li) and Si(Li) Detectors

Germanium with lithium ions used to create Germanium with lithium ions used to create the depletion zone form what is known as a the depletion zone form what is known as a Ge(Li) “jelly” detectorGe(Li) “jelly” detector

Silicon with lithium ions used to create the Silicon with lithium ions used to create the depletion zone comprise what is known as a depletion zone comprise what is known as a Si(Li) “silly” detectorSi(Li) “silly” detector

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Ge(Li) and Si(Li) DetectorsGe(Li) and Si(Li) Detectors

For gamma ray detection, the detector For gamma ray detection, the detector efficiency for the photoelectric effect is efficiency for the photoelectric effect is proportional to Zproportional to Z55, where Z is the atomic , where Z is the atomic number of the detector materialnumber of the detector material

Since for Ge, Z=32, and the Z of Si is 14, Ge Since for Ge, Z=32, and the Z of Si is 14, Ge detectors are about 62 times more efficient detectors are about 62 times more efficient than Si detectorsthan Si detectors

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Germanium DetectorsGermanium Detectors

Germanium detectors are semiconductor Germanium detectors are semiconductor diodes having a p-i-n structure in which diodes having a p-i-n structure in which the intrinsic (I) region is sensitive to the intrinsic (I) region is sensitive to ionizing radiation, particularly x rays and ionizing radiation, particularly x rays and gamma rays. Under reverse bias, an gamma rays. Under reverse bias, an electric field extends across the intrinsic electric field extends across the intrinsic or depleted region. or depleted region.

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Germanium DetectorsGermanium Detectors

When photons interact with the material When photons interact with the material within the depleted volume of a detector, within the depleted volume of a detector, charge carriers (holes and electrons) are charge carriers (holes and electrons) are produced and are swept by the electric produced and are swept by the electric field to the P and N electrodes. This field to the P and N electrodes. This charge, which is in proportion to the charge, which is in proportion to the energy deposited in the detector by the energy deposited in the detector by the incoming photon, is converted into a incoming photon, is converted into a voltage pulse by an integral charge voltage pulse by an integral charge sensitive preamplifier. sensitive preamplifier.

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Germanium DetectorsGermanium Detectors

Because germanium has relatively low Because germanium has relatively low band gap, these detectors must be cooled band gap, these detectors must be cooled in order to reduce the thermal generation in order to reduce the thermal generation of charge carriers (thus reverse leakage of charge carriers (thus reverse leakage current) to an acceptable level. Otherwise, current) to an acceptable level. Otherwise, leakage current induced noise destroys leakage current induced noise destroys the energy resolution of the detector. the energy resolution of the detector.

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Germanium DetectorsGermanium Detectors

Liquid nitrogen, which has a temperature Liquid nitrogen, which has a temperature of 77 of 77 K is the common cooling medium K is the common cooling medium for such detectors. The detector is for such detectors. The detector is mounted in a vacuum chamber which is mounted in a vacuum chamber which is attached to or inserted into an LNattached to or inserted into an LN22 dewar dewar called a cryostat. The sensitive detector called a cryostat. The sensitive detector surfaces are thus protected from moisture surfaces are thus protected from moisture and condensible contaminants. and condensible contaminants. Electrically cooled cryostats are also Electrically cooled cryostats are also available.available.

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Germanium DetectorsGermanium Detectors

There are two types of germanium detectors, p-There are two types of germanium detectors, p-type and n-type.type and n-type.

The detectors are connected to a preamplifier. The detectors are connected to a preamplifier. There are only two basic types of preamplifiers in There are only two basic types of preamplifiers in use on Ge detectors. These are charge sensitive use on Ge detectors. These are charge sensitive preamplifiers, which employ either dynamic preamplifiers, which employ either dynamic charge restoration (RC feedback), or pulsed charge restoration (RC feedback), or pulsed charge restoration (Pulsed optical or Transistor charge restoration (Pulsed optical or Transistor reset) methods to discharge the integrator. reset) methods to discharge the integrator.

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Broad Energy Ge (BEGe) Detector covers the Broad Energy Ge (BEGe) Detector covers the energy range of 3 keV to 3 MeV. The resolution at energy range of 3 keV to 3 MeV. The resolution at low energies is equivalent to that of low energy low energies is equivalent to that of low energy Ge detectors and the resolution at high energy is Ge detectors and the resolution at high energy is comparable to that of good quality coaxial comparable to that of good quality coaxial detectors.detectors.

Most importantly the BEGe has a short, fat shape Most importantly the BEGe has a short, fat shape which greatly enhances the efficiency below 1 which greatly enhances the efficiency below 1 MeV for typical sample geometries. This shape is MeV for typical sample geometries. This shape is chosen for optimum efficiency for real samples in chosen for optimum efficiency for real samples in the energy range that is most important for the energy range that is most important for routine gamma analysis. routine gamma analysis.

Broad Energy DetectorsBroad Energy Detectors

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In addition to higher efficiency for typical In addition to higher efficiency for typical samples, the BEGe exhibits lower background samples, the BEGe exhibits lower background than typical coaxial detectors because it is more than typical coaxial detectors because it is more transparent to high energy cosmogenic transparent to high energy cosmogenic background radiation that permeates above background radiation that permeates above ground laboratories and to high energy gammas ground laboratories and to high energy gammas from naturally occurring radioisotopes such as from naturally occurring radioisotopes such as 4040K and K and 208208Tl (Thorium). This aspect of thin Tl (Thorium). This aspect of thin detector performance has long been recognized detector performance has long been recognized in applications such as actinide lung burden in applications such as actinide lung burden analysis. analysis.

Broad Energy DetectorsBroad Energy Detectors

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The BEGe is designed with an electrode structure The BEGe is designed with an electrode structure that enhances low energy resolution and is that enhances low energy resolution and is fabricated from select germanium having an fabricated from select germanium having an impurity profile that improves charge collection impurity profile that improves charge collection (thus resolution and peak shape) at high (thus resolution and peak shape) at high energies. Indeed, this ensures good resolution energies. Indeed, this ensures good resolution and peak shape over the entire mid-range which and peak shape over the entire mid-range which is particularly important in analysis of the is particularly important in analysis of the complex spectra from uranium and plutonium.complex spectra from uranium and plutonium.

Broad Energy DetectorsBroad Energy Detectors

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In addition to routine sample counting, there are In addition to routine sample counting, there are many applications in which the BEGe Detector many applications in which the BEGe Detector really excels. In internal dosimetry the BEGe really excels. In internal dosimetry the BEGe gives the high resolution and low background gives the high resolution and low background need for actinide lung burden analysis and the need for actinide lung burden analysis and the efficiency and resolution at high energy for whole efficiency and resolution at high energy for whole body counting. The same is true of certain waste body counting. The same is true of certain waste assay systems particularly those involving assay systems particularly those involving special nuclear materials.special nuclear materials.

Broad Energy DetectorsBroad Energy Detectors

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The BEGe detector and associated The BEGe detector and associated preamplifier are normally optimized for preamplifier are normally optimized for energy rates of less than 40 000 MeV/sec. energy rates of less than 40 000 MeV/sec. Charge collection times prohibit the use of Charge collection times prohibit the use of short amplifier shaping time constants. short amplifier shaping time constants. Resolution is specified with shaping time Resolution is specified with shaping time constants of 4-6 microseconds typically.constants of 4-6 microseconds typically.

Broad Energy DetectorsBroad Energy Detectors

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Another big advantage of the BEGe is that Another big advantage of the BEGe is that the detector dimensions  are virtually the the detector dimensions  are virtually the same on a model by model basis. This same on a model by model basis. This means that like units can be substituted in means that like units can be substituted in an application without complete an application without complete recalibration and that computer modeling recalibration and that computer modeling can be done once for each detector size can be done once for each detector size and used for all detectors of that model. and used for all detectors of that model.

Broad Energy DetectorsBroad Energy Detectors

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Absolute Efficiency of the Canberra Absolute Efficiency of the Canberra Industries BE5030 compared to a Coaxial Industries BE5030 compared to a Coaxial Detector of 60 mm diameter by 80 mm Detector of 60 mm diameter by 80 mm length for a source measuring 74 mm length for a source measuring 74 mm diameter by 21 mm thick located on the diameter by 21 mm thick located on the detector end cap. Both detectors have detector end cap. Both detectors have approximately 50% Relative Efficiency for approximately 50% Relative Efficiency for a a 6060Co point source at 25 cm. Co point source at 25 cm.

Broad Energy DetectorsBroad Energy Detectors

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With cross-sectional areas of 20 to 50 cmWith cross-sectional areas of 20 to 50 cm22 and thickness’ of 20 to 30 mm, the and thickness’ of 20 to 30 mm, the nominal relative efficiency is given below nominal relative efficiency is given below along with the specifications for the entire along with the specifications for the entire range of models. BEGe detectors are range of models. BEGe detectors are normally equipped with our low normally equipped with our low background composite carbon windows. background composite carbon windows. Beryllium or aluminum windows are also Beryllium or aluminum windows are also available. available.

Broad Energy DetectorsBroad Energy Detectors

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Germanium DetectorsGermanium Detectors

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Comparison of Broad Energy Comparison of Broad Energy and Coax Detectorsand Coax Detectors

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Broad Energy DetectorsBroad Energy Detectors

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The extended range coaxial germanium The extended range coaxial germanium detector haves a unique thin-window detector haves a unique thin-window contact on the front surface which contact on the front surface which extends the useful energy range down to 3 extends the useful energy range down to 3 keV. Conventional coaxial detectors have keV. Conventional coaxial detectors have a lithium-diffused contact typically a lithium-diffused contact typically between 0.5 and 1.5 mm thick. This dead between 0.5 and 1.5 mm thick. This dead layer stops most photons below 40 keV or layer stops most photons below 40 keV or so rendering the detector virtually so rendering the detector virtually worthless at low energies. worthless at low energies.

Extended RangeExtended RangeGermanium Detectors Germanium Detectors

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The extended range detector, with its The extended range detector, with its exclusive thin entrance window and with a exclusive thin entrance window and with a Beryllium cryostat window, offers all the Beryllium cryostat window, offers all the advantages of conventional standard advantages of conventional standard coaxial detectors such as high efficiency, coaxial detectors such as high efficiency, good resolution, and moderate cost along good resolution, and moderate cost along with the energy response of the more with the energy response of the more expensive Reverse Electrode Ge (REGe) expensive Reverse Electrode Ge (REGe) detector. detector.

Extended RangeExtended RangeGermanium Detectors Germanium Detectors

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The effective window thickness can be The effective window thickness can be determined experimentally by comparing determined experimentally by comparing the intensities of the 22 keV and 88 keV the intensities of the 22 keV and 88 keV peaks from peaks from 109109Cd. With the standard 0.5 Cd. With the standard 0.5 mm Be window, the XtRa detector is mm Be window, the XtRa detector is guaranteed to give a 22 to 88 keV intensity guaranteed to give a 22 to 88 keV intensity ratio of greater than 20:1. Aluminum ratio of greater than 20:1. Aluminum windows are also available. windows are also available.

Extended RangeExtended RangeGermanium Detectors Germanium Detectors

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Extended Range DetectorsExtended Range Detectors

The response curves The response curves illustrate the illustrate the efficiency of the XtRa efficiency of the XtRa detector compared to detector compared to a conventional Ge a conventional Ge detector.detector.

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Extended Range Germanium Extended Range Germanium Detectors Detectors

The response curves The response curves illustrate the illustrate the efficiency of the XtRa efficiency of the XtRa detector compared to detector compared to a conventional Ge a conventional Ge detector.detector.

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Spectroscopy from 3 keV up Spectroscopy from 3 keV up Wide range of efficiencies Wide range of efficiencies High resolution – good peak shape High resolution – good peak shape Excellent timing resolution Excellent timing resolution High energy rate capability High energy rate capability Diode FET protection Diode FET protection Warm-up/HV shutdown Warm-up/HV shutdown High rate indicator High rate indicator

Extended Range Germanium Extended Range Germanium Detectors Detectors

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Where to Get More InformationWhere to Get More Information

Cember, H., Introduction to Health Physics, 3Cember, H., Introduction to Health Physics, 3rdrd Edition, McGraw-Hill, New York (2000)Edition, McGraw-Hill, New York (2000)

Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds., Table of Isotopes (8Table of Isotopes (8thth Edition, 1999 update), Wiley, Edition, 1999 update), Wiley, New York (1999)New York (1999)

International Atomic Energy Agency, The Safe Use International Atomic Energy Agency, The Safe Use of Radiation Sources, Training Course Series No. 6, of Radiation Sources, Training Course Series No. 6, IAEA, Vienna (1995)IAEA, Vienna (1995)