Download General description of Germanium detectors

GERMANIUM GAMMA -RAY
DETECTORS
BY
BAYAN YOUSEF JARADAT
Phys.641 Nuclear Physics 1
First Semester 2010/2011
PROF. NIDAL ERSHAIDAT
TABLE OF CONTENT
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Introduction
General description of Germanium detectors
Types of germanium detectors
Configurations of (HPGe) detector
Operational characteristic
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 LN2
Dewar. The sensitive detector surfaces are thus
protected from moisture and condensible
contaminants.
Types of germanium
detectors
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Hyper pure germanium (HPGe) detector.
Germanium Lithium- drifted (GeLi)
detector.
Hyper pure germanium detector
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High- purity germanium with an impurity concentration
of less than 1016 atoms/m3.
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Either p-type or n-type.
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Volume up to 200cm3 .
Configurations of (HPGe)
detector
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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.
The n+ contact can be formed by direct implantation
of donor atoms using an accelerator.
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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.
OPERATIONAL
CHARACTERISTIC
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Energy Resolution
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:
FWHM  2 ln 2
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Efficiency Calibration
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
at 1332 keV from 60Co. 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.
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
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APPLICATION
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Beta decay of 90Sr 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.
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