Lecture 13: Radiation detectors II

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Lecture 13 Radiation detectors II

• Germanium detectors
• Planar
• Coaxial
• Energy resolution
• Applications in Nuclear Physics
• Imaging in Germanium detectors

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Interaction of gamma-rays with matter

• Gamma radiation is a very penetrating form of
  radiation that does not directly ionise the
  material through with it travels.
• The three main interaction processes responsible
  for gamma-rays interacting with matter are
• Photoelectric absorption Animate!
• Compton Scattering Animate!
• Pair Production Animate!
• Following such an interaction an electron with a
  finite amount of energy will be left in the
  semiconductor material.

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Germanium detectors

• Germanium semiconductor detectors are the
  detector of choice for applications in gamma-ray
  spectroscopy.
• Simple junction detectors find widespread use for
  the detection of alpha particles and other
  short-range radiation but they are not easily
  adaptable for applications with more penetrating
  forms of radiation.
• The major limitation is the maximum depletion
  depth or active volume that can be created 3mm
  for normal semiconductor purity.
• Much thicker detectors are required for gamma-ray
  spectroscopy.

Lower impurity concentration N
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Germanium detectors

• To accomplish this goal there are two general
  approaches
• Further refine the semiconductor reducing the
  impurity concentration to 1010atoms/cm3. Such an
  impurity concentration would give a 1cm depletion
  depth at 1000V reverse bias. Techniques have been
  developed to achieve this goal in Germanium, but
  not in Silicon. Detectors based on this technique
  are called High-Purity Germanium detectors
  (HPGe).
• Create a compensated material where residual
  impurities are balanced by equal concentration of
  dopant atoms. Such lithium ion drifting is added
  to the semiconductor following crystal growth.
  Such detectors are called Ge(Li) detectors.
• HPGe detectors have superseded Ge(Li) detectors
  because HPGes offer much better operational
  convenience.

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Germanium Advantages

• Germanium semiconductor detectors are the
  configuration of choice for gamma-ray
  spectroscopy
• Can be made hyperpure and therefore large
  crystals can be fully depleted.
• High Z (32) compared to Silicon (14)
  (Cross-section for photoelectric absorption ?Z4).
• Excellent energy resolution due to low (3eV)
  ionisation energy much better than scintillator
  technology

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HPGe fabrication

• The starting material is bulk germanium intended
  for use in the semiconductor industry.
• Further processing using the technique of zone
  refining involves impurity levels being
  progressively reduced by locally heating the
  material and slowly passing a melted zone from
  one end of the sample to the other.
• Since impurities tend to be more soluble in the
  molten zone and are swept from the sample.
• Large crystals are grown from the purified melt.
• If the remaining low-level impurities that remain
  are acceptors (donors) the electrical properties
  of the semiconductor will be mildly p-type
  (n-type).

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HPGe detector configurations

• Planar detectors
• The n contact can be formed by lithium
  evaporation and diffusion or ion implantation.
• The detector depletion region is formed by
  reverse biasing the n-p junction.
• The other contact must be non-injecting for a
  majority carrier. It may consist of a p contact
  formed by ion implantation of acceptors (thin) or
  a metal-semiconductor surface barrier (Shottky
  barrier).

Fully depleted detector
Operated at 77K
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Planar Germanium detectors

• Are operated as fully depleted detectors.
• Reverse bias requires that a positive voltage be
  applied to the n contact with respect to the p
  surface.
• The depletion region begins at the n contact and
  extends further into the detector as the voltage
  is raised.
• Further increases in voltage increase the E-field
  everywhere by a uniform amount.
• The detector voltage is increased in order to
  saturate the drift velocity of the charge
  carriers minimising collection time and
  detrimental effects due to carrier recombination
  and trapping.
• Saturation velocity for electrons in germanium at
  77K is reached with a minimum field of 105V/m

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Saturation of drift velocity

• Saturation velocity is reached when the drift
  velocity becomes independent of further increases
  in the E-field.
• The saturation velocity of electrons and holes is
  very similar however the field strength required
  to saturate the hole velocity is two to three
  times larger than electrons.

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Coaxial Germanium detectors

• The maximum depletion depth is limited to 2cm
  for a planar germanium detector, which is a
  serious restriction on the active volume of the
  device.
• To produce a detector with a larger active volume
  which is required for gamma-ray spectroscopy a
  detector is constructed in a cylindrical or
  coaxial geometry.
• One electrode is fabricated on the outer
  cylindrical surface of a long germanium
  cylindrical crystal. A second cylindrical contact
  is provided by removing the core of the crystal
  and placing a contact over the inner cylindrical
  surface.
• A closed-end coaxial configuration is one in
  which only part of the centre core is removed and
  the outer electrode is extended over one flat end
  of the crystal.

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Closed-end coaxial Germanium

• In the closed end coaxial configuration the
  electric field lines are no longer radial, as
  they would be in the true coaxial case.
• The bulletized closed end geometry rounds the
  corners of the crystal which helps prevent
  localised weak E-fields.

n-type crystal
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Coaxial Germanium detector
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Coaxial Germanium detector

• In order to determine the drift velocity of the
  charge carriers within the detector volume
  Poissons equation is solved.
• Poissons equation in cylindrical coordinates
  becomes
• Treating the case of a true coaxial detector the
  voltage needed to fully deplete the detector can
  be written
• Notice the depletion voltage decreases linearly
  with dopant level.

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Germanium detector spectrum
88Y collected on a High Purity Germanium Detector
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HPGe detector response Dead layer

• To a first approximation the active volume of the
  detector is just the active volume between the n
  and p contacts.
• As already indicated, these contacts may have an
  appreciable thickness and can represent a dead
  layer on the front of the crystal through which
  incident radiation must pass.
• The lithium drifted contact on a germanium
  detector will be a thickness of several hundred
  mm 600mm.
• This will have the affect of attenuated low
  energy gamma-ray and X-ray which enter the
  detector.
• The boron implanted contact however is produced
  by ion implantation and hence can produce a very
  thin contact 0.6mm.

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Germanium Energy resolution

• The overall energy resolution is normally
  determined by a combination of three factors
• WD is the inherent statistical fluctuation in the
  number of charge carriers created.
• This is given by
• Where F is the Fano factor, ? is the ionisation
  energy and E is the gamma-ray energy
• WX is due to incomplete charge collection
  (important in large volume detectors).
• WE is from the broadening effects of all
  electronic components following the detector.

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High Purity Germanium Detectors
The Problem

• Require precise knowledge of the energy, time and
  interaction position of discrete gamma-ray decays
  from excited nuclear states.
• Gamma-radiation is very penetrating.
• High purity germanium (HPGe) detectors can be
  made with large depletion regions.
• HPGe detectors are available with impurity levels
  1012 atoms/cm3.

The Solution
Boron implantation (thin)
Holes
Lithium drifted (thin)
Electrons
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Charge Pulse Response

• Charge pulse response characteristics for a
  closed-end coaxial geometry.
• Multiple interactions are separable from single
  interactions .
• PSA allows interaction position to be determined.

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Ortec 6x4 Segmented Detector

• One 65mm diameter 80mm length crystal
• 24 way segmentation of outer boron implanted
  contact.
• Warm FET configuration.

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Digital Gamma-ray Spectroscopy

• Digital sampling of detector signal provides
  powerful alternative to conventional analogue
  electronics.
• High rate capability
• Improved gain stability
• Ability to extract more than energy and time from
  signal.

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Highly segmented Ge Detectors
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Ortec 6x4 Segmented Detector

• Electrical segmentation of the outer boron
  implanted contact.
• 150?m separation between adjacent electrodes.

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Detector Characterisation
What is characterisation?

• Provides a full description of the detector
  response
• Calculation of reference pulse shapes
• Calculation of E-fields
• Simulation of interaction points
• Full characterisation of germanium crystals
• Detailed source scans
• Define how to characterise detectors
• Define reference points
• Standardise scanning set-ups

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Automated Scanning Tables

• Liverpool System
• Parker linear positioning table
• Pacific scientific stepper motors
• 0.3mCi 137Cs/0.2mCi 57Co
• 1-2mm collimator
• Singles/coincidence system

Precise position calibration
GSI / CSNSM Orsay
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Liverpool Detector Scanning System
Data recorded using 14 bit 80Mhz digital
electronics to disk. Pulse shape analysis of
detector response as a function of position.
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Detector scan results
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Charge pulse response
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Analysis of the pulse shapes

• T30, T60 T90 variation gives the position between
  anode and cathode
• Radial position of interaction in Coaxial
  (Clover)

0.9

• Energy determination
• Moving Window Deconvolution
• Image charges
• Azimuthal position in Clover

0.6
0.3
0
T30
T60
T90
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6x6 Risetime Analysis
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Gamma-ray arrays - Evolution

• Germanium BGO
• Evolution
• Single crystal
• Composite Detectors
• Encapsulated crystals
• Electrically segmented crystals
• Germanium shell

EUROBALL
AGATA
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Lecture 13 Radiation detectors II

• Germanium detectors
• Planar
• Coaxial
• Energy resolution
• Applications in Nuclear Physics
• Imaging in Germanium detectors