Session II.3.5 Part II Quantities and Measurements Module 3 Principles of Radiation Detection and Measurement Session 5 Semiconductor Detectors IAEA Post Graduate Educational Course Radiation Protection and Safe Use of Radiation Sources

Semiconductor Detectors Upon completion of this section the student will be able to explain the process and characteristics of semiconductor detectors including the concepts: N-type P-type Intrinsic/Depletion region Resolution Efficiency

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

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

Semiconductor Detectors

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

N-Type Si Containing Group V Donor Impurity Extra Electron

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

Group III Acceptor Impurity P-Type Si Containing Group III Acceptor Impurity e e e e e e e e Positive Hole +

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

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

Semiconductor Detectors Reverse Bias Intrinsic/Depletion Region Cathode (-) Anode (+) + + - -

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

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

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

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)

Semiconductor Detectors + The “hole,” that is, the + charge, appears to move from the left to right in this slide as the electron moves to fill in the “hole.”

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

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 A photocathode requires about 1000 eV to liberate one photoelectron. The semiconductor detector is then about 1000/3.6 = 17 times better that a PM (photo multiplier) detection, or scintillation system.

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

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

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 Efficiency ratio - Ge: Z5 = 325 ; Si: Z5 = 145; 325/145 = 62.

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.

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.

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.

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.

Germanium Detectors There are two types of germanium detectors, p-type 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.

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.

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.

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.

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.

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.

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.

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.

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.

Germanium Detectors

Comparison of Broad Energy and Coax Detectors

Broad Energy Detectors

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.

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.

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.

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

Extended Range Germanium Detectors The response curves illustrate the efficiency of the XtRa detector compared to a conventional Ge detector.

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

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)