Principles of radiation detection

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Principles of radiation detection
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Overview

• Principles of radiation detection (gamma and
  neutrons)
• Fundamentals of spectroscopy
• Gamma spectrometers
• Neutron detectors

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Principles of radiation detection

• The basic process used for radiation detection
  is
• Radiation ionises the material
• gt production of free charges
• (electron/ions in gases or electron/holes in
  semiconductors)
• 2) The charges move through the electric field
  applied to the detector material and are
  collected at the electrodes
• A current (voltage) peak is measured to flow
  through the detector revealing the detection of
  radiation (counting)
• When the charge production is proportional to the
  energy of the ionising radiation we can measure
  it (spectroscopy)

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Fundamentals of spectroscopy

• Each isotope decays by emitting photons (gamma
  rays) of well-defined energy
• Ex. Cs/Ba-137 662 keV
• Co-60 1173, 1332 keV
• U-235 186, 144, 163, 205, keV
• Am-241 59, keV
• Pu-239 129, 375, 414, 99, keV

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Fundamentals of spectroscopy

• If a gamma detector can measure the energy of
  photons (spectrometer) we can identify the
  material
• For instance, in a gas the energy necessary to
  ionise the atom (generating an ion/electron pair)
  is fixed.
• gt the charge generated is proportional to the
  energy of the photon
• By applying an electric field the current flow
  is proportional to the charge and therefore to
  the energy of the photon (provided the full
  energy of the photon is deposited within the
  detector)

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Fundamentals of spectroscopy

• Spectrum energy distribution of detected photons

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Fundamentals of spectroscopy

• Efficiency
• photons detected / photons emitted
• The efficiency increases with the detector volume
• Generally the intrinsic efficiency is expressed
  as relative to a reference detector (3x3 NaI)

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Fundamentals of spectroscopy

• Resolution
• ideal detector real detector
• FWHM Full Width at Half Maximum - f(E)

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Gamma spectrometers

• Processes of photon interaction with matter

Ionisation (gas)
Charge collection
Electron/hole production (semiconductor crystals)
Light emission (solid/liquid scintillators)
Photocatode (light-gtelectrons) PM tube
(electron multiplication)
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Gamma spectrometers
Scintillation detector
PHOTON
LIGHT
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Gamma spectrometers

• Comparison of spectra from different
  spectrometers

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Gamma spectrometers
Current technology in radiation detection
(efficiency) Detector family Surface Volume
Liquid scintillators Any Any Plastic
scintillators (PVT) 104 cm2 101 l Inorganic
scintillators (NaI, LaBr3,) 102 cm2 100
l Semiconductors (HPGe,) 101 cm2 10-1 l
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Gamma spectrometers
Current technology in radiation detection
(resolution) Detector family Resolution Capabi
lity Liquid scintillators No No
discrim. Plastic scintillators (PVT) Poor Categor
isation Inorganic scintillators
(NaI,) Good Simple spectra Semiconductors
(HPGe,) Excellent Complex spectra
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Gamma spectrometers
Liquid scintillators Plastic scintillators
(PVT) Inorganic scintillators (NaI,
LaBr3,) Semiconductors (HPGe,)
Resolution
Efficiency
No perfect solution !
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Gamma spectrometers
As a consequence a two-step procedure is
applied I Alarm triggered by a high efficiency
fixed portal monitor (RPM) II Secondary
inspection done by the front-line officer using
portable equipment
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Gamma spectrometers
High-resolution spectroscopy can improve the
capability to detect illegal material inside
legal shipments of radioactive sources
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Principles of neutron detection

• Neutron no Electrical Charge
• No Ionisation
• No Direct Detection
• Indirect
• Conversion
• Neutron ? Charged Particle
• Charged Particle Detection

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Neutron detectors
He-3 proportional counters
ANODE WIRE
n 3He ? p 3H 765 keV
The proton and the triton ionise the He
gas electrons are driven towards the anode
wire, ions flow to the cathode wall
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Neutron detectors

• We loose the information on the neutron energy
• Neutron counting (no spectroscopy)
• We cannot identify what has produced the neutrons
• Neutron background in environment nearly zero
• Detection of neutrons is always suspicious
  (ALARM!)