GERMANIUM GAMMA -RAY DETECTORS BY BAYAN YOUSEF JARADAT Phys.641 Nuclear Physics 1 First Semester 2010/2011 PROF. NIDAL ERSHAIDAT

TABLE OF CONTENT Introduction Introduction General description of Germanium detectors General description of Germanium detectors Types of germanium detectors Types of germanium detectors Configurations of (HPGe) detector Configurations of (HPGe) detector Operational characteristic Operational characteristic Application Application

INTRODUCTION The considered detectors consist essentially of a piece of solid material. The germanium is used because it has a high density and atomic number, in which electrons and holes are produced when a gamma ray is absorbed. The gamma ray is a photon of electromagnetic radiation which emitted from unstable atomic nuclei with very short wavelength and high energy and penetration power.

General description of 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.

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 LN 2 Dewar. The sensitive detector surfaces are thus protected from moisture and condensible contaminants.

Types of germanium detectors  Hyper pure germanium (HPGe) detector.  Germanium Lithium- drifted (GeLi) detector.

Germanium lithium detector When one of donor material (practically Li) is introduced into Ge, the donor atoms are easily ionized and are mobile enough to drift at elevated temperature under the influence of a strong field. The lithium mobility is much greater in germanium and remains high enough at room temperature, which leads to an undesirable redistribution of the lithium. Therefore, the lithium profile of Ge (Li) detectors must always be preserved at liquid N 2 temperature while Si (Li) detectors can be stored at room temperature for a short period due to low mobility of the Li ion in silicon.

Hyper pure germanium detector  High- purity germanium with an impurity concentration of less than atoms/m 3.  Either p-type or n-type.  Volume up to 200cm 3.

Configurations of (HPGe) detector  Planar configuration An example of a planar HPGe detector using a p-type crystal. In this configuration, the electric contacts are provided on the two flat surfaces of a germanium crystal. An example of a planar HPGe detector using a p-type crystal. In this configuration, the electric contacts are provided on the two flat surfaces of a germanium crystal. The n + contact can be formed by direct implantation of donor atoms using an accelerator. The n + contact can be formed by direct implantation of donor atoms using an accelerator.

The detector depletion region is formed by reverse biasing this n + junction. The detector depletion region is formed by reverse biasing this n + -p junction. The contact at the opposite face of the crystal must be a noninjecting contact for a majority carrier. It may consist of a p + contact produced by ion implantation of acceptor atoms The contact at the opposite face of the crystal must be a noninjecting contact for a majority carrier. It may consist of a p + contact produced by ion implantation of acceptor atoms

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

OPERATIONAL CHARACTERISTIC Energy Resolution Energy Resolution The most important advantage of the germanium detector, compared to other types of radiation counters, is energy resolution: the ability to The most important advantage of the germanium detector, compared to other types of radiation counters, is 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:

Bayan jaradat20

Bayan jaradat21 Efficiency Calibration Efficiencies for the germanium detectors are usually specified as a percentage of the 3" x 3" NaI detector photopeak efficiency at a specified energy, most often the higher photon energy of cobalt-60 energy (1.33 MeV).

Efficiency Calibration The relative efficiency is conveniently used for quoting the peak efficiency of a HPGe detector. The relative efficiency is defined as: The relative efficiency is conveniently used for quoting the peak efficiency of a HPGe detector. The relative efficiency is defined as: Rel. eff. = HPGe peak effi/NaI (Tl) (3″×3″) peak effi Rel. eff. = HPGe peak effi/NaI (Tl) (3″×3″) peak effi at 1332 keV from 60 Co. The source-detector distance of 25 cm is uniquely used for this definition. The absolute peak efficiency of a 3″×3″ NaI (Tl) is 1.2×10 -3 in this geometry. at 1332 keV from 60 Co. The source-detector distance of 25 cm is uniquely used for this definition. The absolute peak efficiency of a 3″×3″ NaI (Tl) is 1.2×10 -3 in this geometry.

Bayan jaradat23 The reason for presenting germanium efficiencies relative to NaI is that germanium detectors are available in different geometries, such as planar detectors, coaxial detectors, and others, all of which have different efficiencies even when their volumes are the same. Using the efficiencies relative to NaI may reduce some uncertainties The reason for presenting germanium efficiencies relative to NaI is that germanium detectors are available in different geometries, such as planar detectors, coaxial detectors, and others, all of which have different efficiencies even when their volumes are the same. Using the efficiencies relative to NaI may reduce some uncertainties

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

THANK YOU