GERMANIUM GAMMA -RAY GERMANIUM GAMMA -RAY DETECTORSDETECTORS

BY

BAYAN YOUSEF JARADATPhys.641 Nuclear Physics 1First Semester 2010/2011

PROF. NIDAL ERSHAIDATPROF. NIDAL ERSHAIDAT

TABLE OF CONTENTTABLE OF CONTENT

IntroductionIntroduction General description of Germanium detectorsGeneral description of Germanium detectors Types of germanium detectorsTypes of germanium detectors Configurations of (HPGe) detectorConfigurations of (HPGe) detector Operational characteristicOperational characteristic ApplicationApplication

INTRODUCTIONINTRODUCTION

The considered detectors consist essentially The considered detectors consist essentially of a piece of solid material. The of a piece of solid material. The germanium is used because it has a high germanium is used because it has a high density and atomic number, in which density and atomic number, in which electrons and holes are produced when a electrons and holes are produced when a gamma ray is absorbedgamma ray is absorbed..

The gamma ray is a photon of The gamma ray is a photon of electromagnetic radiation which emitted electromagnetic radiation which emitted from unstable atomic nuclei with very from unstable atomic nuclei with very short wavelength and high energy and short wavelength and high energy and penetration powerpenetration power..

General description of Germanium General description of Germanium detectorsdetectors

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.

Because germanium has relatively low band gap, these detectors must be cooled in order to reduce the thermal generation of charge carriers to an acceptable level. Otherwise, leakage current induced noise destroys the energy resolution of the detector. 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. The sensitive detector surfaces are thus protected from moisture and condensible contaminants.

Types of germanium Types of germanium detectorsdetectors

Hyper pure germanium (HPGe) Hyper pure germanium (HPGe) detector.detector.

Germanium Lithium- drifted (GeLi) Germanium Lithium- drifted (GeLi) detector. detector.

Hyper pure germanium Hyper pure germanium detectordetector

High- purity germanium with an impurity High- purity germanium with an impurity concentration of less than 10concentration of less than 101616 atoms/m atoms/m33..

Either p-type or n-type.Either p-type or n-type.

Volume up to 200cmVolume up to 200cm33 . .

Configurations of (HPGe) Configurations of (HPGe) detectordetector

Planar configurationPlanar configuration

An example of a planar HPGe detector using a An example of a planar HPGe detector using a p-type crystal. In this configuration, the electric p-type crystal. In this configuration, the electric contacts are provided on the two flat surfaces of contacts are provided on the two flat surfaces of a germanium crystal.a germanium crystal.

The nThe n++ contact can be formed by direct contact can be formed by direct implantation of donor atoms using an implantation of donor atoms using an accelerator.accelerator.

Coaxial ConfigurationCoaxial Configuration

The detector is basically a cylinder of The detector is basically a cylinder of germanium with an n- type contact on the germanium with an n- type contact on the outer surface, and a p-type contact on the outer surface, and a p-type contact on the surface of an axial well.surface of an axial well.

OPERATIONAL OPERATIONAL CHARACTERISTICCHARACTERISTIC

Energy ResolutionEnergy Resolution

The most important advantage of the The most important advantage of the germanium detector, compared to germanium detector, compared to other types of radiation counters, is other types of radiation counters, is energy resolution: the ability to energy resolution: the ability to resolve two peaks that are close together in energy.

The parameter used to specify the detector resolution is the full width of the (full-energy) photopeak at half its maximum height (FWHM). If a standard Gaussian shape is assumed for the photopeak the FWHM is given by:

2ln2FWHM

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Efficiency CalibrationEfficiency Calibration

The relative efficiency is conveniently used The relative efficiency is conveniently used for quoting the peak efficiency of a HPGe for quoting the peak efficiency of a HPGe detector. The relative efficiency is defined as:detector. The relative efficiency is defined as:

Rel. eff. = HPGe peak effi/NaI (Tl) (3″×3″) peak effiRel. eff. = HPGe peak effi/NaI (Tl) (3″×3″) peak effi

at 1332 keV from at 1332 keV from 6060Co. The source-detector Co. The source-detector distance of 25 cm is uniquely used for this distance of 25 cm is uniquely used for this definition. The absolute peak efficiency of a definition. The absolute peak efficiency of a 3″×3″ NaI (Tl) is 1.2×103″×3″ NaI (Tl) is 1.2×10-3-3 in this geometry. in this geometry.

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The reason for presenting The reason for presenting germanium efficiencies relative germanium efficiencies relative to NaI is that germanium to NaI is that germanium detectors are available in detectors are available in different geometries, such as different geometries, such as planar detectors, coaxial planar detectors, coaxial detectors, and others, all of detectors, and others, all of which have different efficiencies which have different efficiencies even when their volumes are the even when their volumes are the samesame. . Using the efficiencies Using the efficiencies relative to NaI may reduce some relative to NaI may reduce some uncertaintiesuncertainties   

APPLICATIONAPPLICATION Beta decay of Beta decay of 9090Sr and the mass of neutrinoSr and the mass of neutrino In nuclear beta decays, the weak interaction In nuclear beta decays, the weak interaction transforms either an up quark into a down quark transforms either an up quark into a down quark with the emission of a positron and an electron anti-with the emission of a positron and an electron anti-neutrino, or a down quark into an up quark with the neutrino, or a down quark into an up quark with the emission of an electron and an electron neutrino. emission of an electron and an electron neutrino. Since the quarks are inside nucleons, this either Since the quarks are inside nucleons, this either transforms a proton into a neutron or vice versa. In transforms a proton into a neutron or vice versa. In this experiment the goal is to measure and this experiment the goal is to measure and understand the beta spectrum of Strontium-90, and understand the beta spectrum of Strontium-90, and to use this spectrum to set an upper limit on the to use this spectrum to set an upper limit on the mass of the electron neutrino. Current evidence mass of the electron neutrino. Current evidence suggests that at least some neutrinos have non-suggests that at least some neutrinos have non-zero masszero mass . .

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