Master White Bkgnd Presentation

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Room Temperature Semiconductor Detectors
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Room Temperature Semiconductor Detectors

• Tutorial Presented at Alabama AM University
• Lodewijk van den Berg
• Constellation Technology Corporation
• With the cooperation of
• Alexsey Bolotnikov
• Brookhaven National Institute Laboratory
• 
  Normal, AL,
  Thursday 13 July, 2006

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Room Temperature Semiconductor Detectors

• What are semiconductor Detectors, and what are
  the Requirements?
• Solid semiconductor material
• Chemically stable and preferably inert to
  atmospheric conditions
• Able to absorb high energy nuclear radiation
  without being destroyed
• Able to convert absorbed radiation photons
  into electronic charges of
• The amount of electronic charges created
  should be a linear function of the
• energy of the radiation
• Should have high quality single crystalline
  structure
• Should have very small levels of impurities (
  lt 10 ppm total)

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Principles of Operation

• A piece of semiconductor material is cut to the
  desired dimensions and
• electrodes are deposited on opposite sides.
• The electrode material is usually a noble
  metal, e.g. Au, Pd, Pt, Ag, but
• sometimes common metals and conductive
  organic polymers are used.
• Contact wires are attached to the electrodes to
  connect the detector to a
• high voltage power supply on one side and a
  signal processing system on
• the other side.
• The charges created by the radiation are
  driven by the bias toward the
• electrodes and are counted and processed by
  the electronic system.
• Charges may not be able to travel the whole
  distance from their point of
• origin to the respective electrodes because
  of trapping at material defects.

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Charge Transport Properties

• The movement of the electronic charges to the
  contacts can be described by
• the mobility and the trapping time (also
  called lifetime).
• The mobility µ with dimensions cm2/Vs (or
  cm/sec per V/cm) determines the
• velocity with which the charge moves in the
  lattice under the force of the
• field E applied to the detector.
• The trapping time ? in seconds represents the
  probability that a charge is
• trapped at a crystalline defect during the
  time that the charge travels through
• the detector.
• The drift length ? (cm) ??E is the average
  distance a charge can travel
• before it is trapped.

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Charge Transport and Collection

• The collection of charges is described in the
  most extensive way by the
• Hecht equation, which considers all the
  possible trapping mechanisms.
• Basically it can be expressed for one type of
  charge (electrons or holes)
• Q Q0 x e d/?
• where Q is the amount of charge collected,
• Q0 is the amount of charge
  generated
• d is the thickness of the
  detector.
• One can see from the equation that in order to
  collect 98 of the charges
• generated, the value of the drift length
  should be ? gt 5d.
• This condition is often difficult to meet in
  semiconductor detectors
• therefore values of ?? gt 2d are often
  accepted.

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Charge Collection and Analysis System
Counts
Multi Channel Analyzer (MCA)
Channels (Energy)
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Charge Collection and Analysis System (continued)

• The schematic on the previous page shows the
  following features
• The high voltage applied to the detector makes
  the charges generated by the
• absorbed radiation move to the respective
  electrodes.
• The side of the detector from which the signal
  is taken is in most cases held at
• a neutral bias.
• Charges which are collected at the opposite
  electrode create an image charge
• on the signal electrode, so that essentially
  all charges are accounted for.
• The charge pulse (current) is processed by the
  preamplifier containing a Field
• Effect Transistor (FET) which removes a large
  part of the continuous current
• flowing through the preamplifier and
  highlights the signal.

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Charge Collection and Analysis System (continued)

• The charges enter the shaping amplifier which
  amplifies the current and also
• determines the time (in microseconds) during
  which charges will be collected.
• The charges from the shaping amplifier are
  processed by the Multi-Channel
• Analyzer. The MCA reads the amount of charge
  in the pulse delivered by the
• shaping amplifier. It sets up a number of bins
  (or channels) over which it
• distributes the number of times a certain
  charge is received.
• This information can be displayed on a PC and
  is called a spectrum.
• Ideally the spectrum of a monochromatic test
  source should be a sharp peak
• distributed over a few channels. The
  broadening of the peak is caused by
• incomplete charge collection, Compton
  scattering and other effects in the
• crystal which cause charges to be lost or not
  generated.

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Materials

• Many materials have been investigated in the
  past. In the following table a
• selection is made using criteria which will
  help to focus this discussion.
• High density of a material improves the
  absorption of the radiation.
• The type of interaction needs to be considered.
  Materials containing high Z
• element(s) have a higher full-energy peak
  efficiency. This means that the
• energy of the radiation is more efficiently
  converted into the maximum
• number of counts possible. This relationship
  is related to Z3.
• Higher Z materials also have lower Compton
  Scattering, which is a loss of
• energy of the absorbed radiation photon by
  means of elastic scattering.
• Materials with a small electronic band-gap have
  low resistivity. This causes
• a high leakage current and a noisy spectrum
  when bias is applied

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Materials (contd)
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Comments on Materials Table

• The resistivities for the different materials
  given in the previous table have
• been calculated on the basis of their
  measured band gap.
• In reality, the actual crystals grown contain
  many defects which usually
• lowers the resistivity by as much as two or
  three orders of magnitude.
• The standard for all nuclear research and
  measurements is still LN2
• cooled high purity Ge (HPGE), but cooling is
  not readily available
• for field applications.
• The only two solid state detector materials
  presently available for ambient
• temperature applications are CdZnTe and HgI2
  .
• This discussion will continue with the
  description of some properties and
• applications of CZT. A separate presentation
  will discuss HgI2.

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Room Temperature DetectorsCZT

• Review of CZT Properties
• Resistivity after doping
  1010 Ohm.cm
• Electron Mobility
  1350 cm2/Vsec
• Hole Mobility
  120 cm2/Vsec
• Electron Mu-Tau
  1x10-3 cm2/V
• Hole Mu-Tau
  6x10-6 cm2/V
• Crystal can be grown in large sizes (2 inch
  diameter and 10 inches long or
• larger), but contain inclusions and
  segregations.
• The low values of the mu-tau for holes makes it
  impossible to make planar
• detectors with large thicknesses.

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Room Temperature DetectorsCZT Bulk Leakage
Currents(Brookhaven National Laboratory)
measurements of bulk resistivity values 1010
Ohm.cm
V1/2
Fitting results 3x109 Ohm-cm Imarad 5x1010
Ohm-cm eV-Products 3x1010 Ohm-cm Yinnel Tech
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Room Temperature DetectorsCZT

• The low mu-tau product of the holes in CZT
  combined with the relatively low
• resistivity makes it impossible to make
  planar detectors. Thicker detectors can
• be used by using single charge collection of
  the electrons only. Three detector
• configurations have been developed to make
  this possible.
• Coplanar Grid detectors, where the anode is
  formed by two interwoven anode
• grids. One grid serves as the signal anode,
  and the other is a steering grid used
• to drive the electrons to the anode grid. (P.
  Luke at U.C. Berkeley)
• Pixellated anodes where each pixel is connected
  to analysis channel of a
• CMOS based ASIC. (Zhong He and coworkers at
  Univ. of Michigan)
• Frish Grid detectors where the long narrow body
  of the detector is wrapped
• in a teflon/conductor as the cathode. (
  McGregor, KSU, and Bolotnikov, BNL)

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Room Temperature DetectorsCZTCo-planar Grid
Detectors
The bottom of the detector has a solid contact
and is the cathode. The top has two parallel
grids one serves the anode and the other as a
steering grid. Dimensions of the detector 1 cm
x 1 cm x 1 cm.
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Room Temperature DetectorsCZTPixellated Detector
The back of the detector is a solid cathode
contact. The front has an array of small pixel
contacts as sketched in the lower figure. The
anode pixels are connected to an ASIC as
shown. Dimensions of the detector 1 cm x 1 cm x
7 mm.
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Room Temperature DetectorsCZTFrisch Grid
Detectors
The top figure shows the bare detector. The
bottom figure shows the teflon wrap and the
conductive shield which acts as the cathode. The
top surface has the anode contact. Dimensions of
the detectors 6 mm x 6 mm x 3 mm up to 15 mm x
10 mm x 10 mm.
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Room Temperature DetectorsCZTCs-137 Spectrum
(BNL)
Spectrum of Cs-137 with detector 5 mm
thick. Resolution 1.1 FWHM. Single charge
collection.
Energy resolution 1.1 (Frisch-ring) vs. 0.9
(3D device) Resolution is limited by material
non-uniformities!
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Room Temperature Semiconductor DetectorsCZT and
HgI2

• Summary
• Room temperature semiconductor detectors are
  able to provide nuclear
• spectra with a resolution adequate for many
  applications.
• Since they are solid state devices, they are
  very rugged and are suitable for
• devices used in the field (no glass
  components).
• The signal output is stable with temperature
  and does not drift.
• Improvements in the material are needed,
  especially in the crystal growth.
• Specifically, the single crystal material needs
  to be more homogeneous with
• respect to its electronic properties, and
  segregation and inclusions need to be
• minimized. In this way, detector bodies with
  larger volumes will become
• available, so that efficiency can be maximized.