Pakistan Institute of Engineering & Applied Sciences (PIEAS)Lectures on Radiation Detection Delivered at Workshop in PNRA (April, 2008)Dr. Nasir M MirzaDeputy Chief Scientist, Department of Physics & Applied mathematics, PIEAS, P.O. Nilore, 45650, Islamabad.

Email: nmm@pieas.edu.pk Ph: +92 51 9290273 (ext: 3059)

Semiconductor DetectorsRecommended Text Books Glenn F Knoll s Radiation Detection & Measurement (recent edition).

Lecture 4:

Semiconductor Detectors

Using Solid as Detection MediumIn many radiation detection applications, the use of solid medium is of great advantage For high energy electrons and gammas, solid state detectors are much smaller than gas filled detectorsScintillation detectors good detection efficiency but poor resolution ( W value = 100 eV or more)Energy resolution can be improved by increasing number of charge carriers possible in semiconductors (much lower W value)

Semiconductor Diode DetectorsDesirable features of (semiconductor diode detectors) or solid state detectorsSuperior Energy Resolution Compact Size Fast Timing Characteristics Effective Thickness Can be varied according to the requirement Semiconductor Materials Silicon Used for charged particle spectroscopy Germanium - Used for gamma ray spectroscopy

Band Structure in SolidsValence Band (V.B)Corresponds to those outer shell electrons that are bound to specific lattice sites within the crystal Conduction Band (C.B)Represents the electrons that are free to migrate through the crystal Forbidden Band (F.B) or Band gapThe size of width determines whether the material is classified as a semiconductor or insulator

Compensated MaterialsIf donor and acceptor impurities are present in a semiconductor in equal concentration, the material is said to be compensatedBehaves like intrinsic material, therefore designated with IAlthough number densities of charge carrier nearly that of intrinsic material, but due to impurities present may behave differently

Heavily Doped MaterialThin layers of semiconductor material that have high concentration of impuritiesHas very high conductivity valuesUsed in making electrical contactsDenoted by: n+ Heavely doped n-type material p+ Heavely doped p-type material

Radiation InteractionPassage of charged particle through semiconductor production of many e-h pairs along the track Average energy expended by charged particle called ionization energy (W) to produce an ion pair in Si or Ge is about 3 eV as compared to 30 eV in gas detectors A large # of charge carriers is produced The increased # of charge carriers has two beneficial effects on energy resolution Statistical fluctuation in # of carriers per pulse becomes a small fraction of the total Signal to noise ratio of preamplifier is better even at low energies

Charge CollectionIn Si or Ge semiconductor detectors, the electron mobility is within a factor of ~ 2-3 of hole mobility, so the collection times are much close to being equivalent Both carrier types must therefore be completely collected for the resulting pulse for faithful energy measurement

The Semiconductor Junction (Contd.)Depletion RegionThe region over which the charge balance exists is called the depletion region and extends into both the p and n sides of the junction Potential Difference The buildup of net charge within the region of the junction leads to the establishment of an electric potential difference across the junctionResistivityThe depleted region is compensated of charges, therefore has high value of resistivity

The Semiconductor Junction (Contd.)Radiation InteractionInteraction of radiation within depletion region create e-h pairs in the regionElectric FieldThe existence of electric field across junctionSweeping the electrons back to n-type materialSweeping the holes back to p-type materialElectrical SignalThe collection of e-h pairs constitute an electrical signal

The Semiconductor Junction (Contd.)The depletion region acts as a detector but with a poor performance due to small value of potential differenceSignal is lost due to recombination and trapping of e-h pairs Signal to Noise RatioThickness of depletion region is small and the capacitance is large so signal to noise ratio is decreased

The Semiconductor Junction (Contd.)Reverse Biasing of the Junctionp-side of the junction is made negative w. r. t. n-side i.e. connecting p-side of junction with negative terminal of the battery and n-side of the junction with positive terminal of the battery Potential difference is enhanced Minority carriers are attracted across the junctionThickness of the depletion regionThickness of the depletion region is increased which is given by

Where is dielectric constant, V is applied voltage, N dopant concentration, and e is charge on electron.

Semiconductor Detector ConfigurationsDiffused Junction DetectorsSurface Barrier DetectorsIon Implanted LayersFully Depleted DetectorsPassive Planer Detectors

Surface Barrier DetectorsJunction Formationn-type Si crystal Etching its surfaceOxidation of surface (+ve layer formation)Electrical ContactsEvaporation of thin gold layer for electrical contacts Oxide layer is intermediate between gold layer and electrical contactDetectors PropertiesVery thin dead layer Sensitive to light, operated in darkness Kept in a coverThin entrance window should be handled carefully (avoid exposure to vapors and direct handling of surface)

Fully Depleted DetectorsDepletion layer width is given by So depletion region increases with increasing VIf voltage is increased to such an extent that depletion region extends fully across silicon wafer then such detectors refer to fully depleted detectorsHave several advantages over partially depleted detectors so preferred

Leakage CurrentWhen reverse bias is applied across the detector, a small current is observed without any radiation which is the leakage currentIt is Related to detector: Bulk volumeSurface area

Leakage Current (Contd.)Bulk volume leakage current Minority carriers are conducted across the junction, a steady current is generated that is prop. to the area of the junctionThermal generation of e-h pairs within depletion junctionThe rate increases with volume of depletion region. Can only be reduced by cooling the material Si detectors can be operated at room temGe detectors can not operated at room temp

Leakage Current (Contd.)Surface Leakage Takes place at the edges of the detector due large P.D across the edges Depends onDetector encapsulation Humidity Contamination of surface by finger prints Condensable vapors (vacuum pump oil etc)Guard ring can reduce the surface leakage Leakage current changes Affect resolution of detector Indicate abnormal behavior of detector Useful to monitor degree of radiation damage

Dead LayerEnergy loss before reaching the active volume of Heavely charged particles Weakly penetrating radiations Dead layer is due toThe metal electrode The intermediate thickness of silicon beneath the electrode The change of electric field Can be measured experimentallyEnergy Calibration Dead Layer Issues

Application of Silicon Diode Detectors

General Charged Particle SpectroscopyAlpha Particle SpectroscopyFission Fragment SpectroscopyParticle Identification

Fission Fragment SpectroscopyPerformance TestDetector performance can be tested with 252Cf fission fragment spectroscopy (spectrum).The spectrum provides a check on detector properties such asResolution Low energy tailing Internal multiplication in detector

Germanium Gamma-Ray Detectors

General ConsiderationsSurface barrier and other junction detectors are suitable for -particles and short range radiation. Thickness of depletion layer is small (2-3 mm) even with very high voltages (near breakdown voltage), therefore cant be used for -ray measurements

General Considerations (Contd.)Thickness of depletion layer is given by

Where V is the reverse bias voltage, is the dielectric constant, e is the electronic charge, and N is the net impurity concentration of the bulk semiconductor material Reverse bias cant increase up to break down voltage, the only choice is to reduce the impurity level to increase d The detectors that are manufactured from ultra pure germanium are called HPGe (high purity germanium) detectorsThe pure crystal impurity level ~1010 atom/cm3 which at 1000 Volts corresponds to a d of 10 mm

General Considerations (Contd.)Such a low impurity concentration corresponds to levels that are less than ppb level Techniques for Germanium has been developed to achieve the goal of high purity, but not in siliconDetectors made of such high purity Germanium are called HPGe detectorsDepletion region width of several centimeters attained

General Considerations (Contd.)Second approach is to create compensated materialResidual impurities balanced by equal concentration of dopant atoms of the opposite typeProcess of lithium ion drifting used for perfect compensation in grown crystals

Configurations of Germanium DetectorsPlanar configuration of p-type HPGe detector n+ contact is made of Li p+ contact is made of B

Configurations of Germanium Detectors (Contd.)The detector depletion region is formed by reverse biasing n+-p junction HPGe detectors are generally operated as fully depleted detectors Reverse biasing: Depletion region begins at n+ edge and extends deep into p-region, until detector is fully depleted Detector are applied some over voltage ----- electric field becomes highImpart saturated drift velocities to charge carriers

Configurations of Germanium Detectors (Contd.)Minimize the collection time Reduce carrier recombination & trapping HPGe detectors are operated at 77 K temperature to overcome the bulk leakage current. Maximum applied voltage 3-5 kV, limited by break down and surface leakage. Detectors can also be fabricated of high purity n-type material.n+ and p+ contacts are provided at each surface of the wafer

Configurations of Germanium Detectors (Contd.)Reverse biasing: Roles of the two contacts is reversed p+-layer: serves as the rectifying contactDepletion region extends from p+ contact to n-type crystal n+-contact: Functions as a blocking contact When fully depleted and operated with a large over voltage, the electric field in the planar detectors is uniform from one contact to the other The maximum depth of a planar detector is 1-2 cm Total active volume does not exceed 10-30 cm3. High capacitance

Configurations of Germanium Detectors (Contd.)Coaxial ConfigurationHPGe is made in a cylindrical geometryCore of the cylinder is removedElectrical contacts are provided on outer cylindrical surface and inner cylindrical surfaceMuch larger active volumes (~750 cm3) can be produced by larger dimensions in axial direction Detector capacitance is small by keeping small inner diameter

Configurations of Germanium Detectors (Contd.)ClosedEnded Coaxial Reduce leakage current Provide a planar thin window for weakly penetrating radiation Ends are rounded (bulletized) to keep electric field almost uniform Welltype Configuration Sources can be put inside the detectorDetection efficiency is increased

Germanium Detector Operational CharacteristicsAn insulated container used especially to store liquefied gases, having a double wall with a vacuum between the walls and silvered surfaces facing the vacuum is called a Dewar FlaskDetector coolingBand gap of Ge ~ 0.7 eVTherefore thermally induced leakage current at room temperatureOperation of HPGe at Low temperatureCooled with Liquid nitrogen (77k) in a dewarMechanical closed-cycle refrigerationFor Ge(Li) detectors, temperatures must be kept at 77k as room temperature will cause a catastrophic redistribution of the drifted lithium

Germanium Detector Operational Characteristics (Contd.)

Germanium Detector Operational Characteristics (Contd.)Energy ResolutionBest detectors for gamma ray spectroscopyAll -ray spectroscopy of complex energy spectra is carried out with germanium detectors

Full = Statistical + Charge collection + Electronics