detection of high energy by noreenmarwat

VIEWS: 13 PAGES: 85

									Noreen Marwat
Senior Scientist
           PAEC
 Includes
    Basic operation principles of different types of
     radiation detectors;
    Physical processes underlying the principles of
     operation of these devices, and
    Comparing and selecting instrumentation best
     suited for different applications.
 Electrical   collection of ions
     Air Ionization
     Gas Ionization
 Scintillation
     Light Production
A  gas-filled detector consists of a volume
  of gas between two electrodes, with an
  electrical potential difference (voltage)
  applied between the electrodes
 Ionizing radiation produces ion pairs in
  the gas
 Positive ions (cations) attracted to
  negative electrode (cathode); electrons
  or anions attracted to positive electrode
  (anode)
 In most detectors, cathode is the wall of
  the container that holds the gas and
  anode is a wire inside the container
 Electrical collection of ions
 No gas amplification (air)
 Used to measure dose rate
 Sensitive to environmental changes
                             +

                             e
                             -   (+) Anode


Resistor
                                     (-) Cathode


               +         -


               Battery or High
               Voltage
 Variable voltage source
 Gas-filled counting chamber
 Two coaxial electrodes well insulated
  from each other
 Electron-pairs
  produced by radiation in fill gas
  move under influence of electric field
  produce measurable current on electrodes, or
  transformed into pulse
          wall

          fill gas
End               Anode (+)            Output
window
Or wall
           Cathode (-)            or    A
                              R
 wall




Incident gamma photon
            wall



beta (β-)                      e   -
                                           e   -
                   e   -                               e   -


                       e   -
                                   e   -
                                               e   -
              e    -
Incident
charged
particle
    +
            e   -       Output

        +
e   -
                    R
                 GAS FILLED DETECTOR
                                       RESPONSE
                           Breakdown Region
Log of Electrical Signal

                           Geiger Mueller
                           Region
                           Proportional Region
                           Ionization Region
                           Recombination




                                Applied Tube Potential (Voltage)
            Recom-
            bination
Number      region      Ionization region
of Ion
Pairs
collected
                       Saturation Voltage


                       100 % of initial
                       ions are collected

                        Voltage
 The  point at which 100% of ions begin to
  be collected
 All ion chambers operate at a voltage that
  produces a saturation current
 The region over which the saturation
  current is produced is called the
  ionization region
 It levels the voltage range because all
  charges are already collected and rate of
  formation is constant
 Ions collected
 Number of ionizations relate to specific
  ionization value of radiation
 Gas filled detectors operate in either
     current mode
         Output is an average value resulting from detection of
          many values
     pulse mode
         One pulse per particle
                 Alpha
                 Particles



Pulse
Height
                Beta
                Particles


             Gamma Photons


         Detector Voltage
 Pulse size depends on # ions produced in
  detector.
 No multiplication of ions due to secondary
  ionization (gas amplification is unity)
 Voltage produced (V) = Q/C
 Where
  Q is total charge collected
  C is capacitance of the ion chanber
 Chamber’s   construction determines is
  operating characteristics
 Physical size, geometry, and materials
  define its ability to maintain a charge
 Operates at a specific voltage
 When operating, the charge collected due
  to ionizing events is
                    Q = CΔV
 The number of ions (N) collected can be
  obtained once the charge is determined:
                   N=Q/k
 Where k is a conversion factor
    (1.6 x 10-19C/e)
   Accuracy of measurement
     Detector Walls composed of air equivalent material or
     tissue equivalent
   Wall thickness
     must allow radiation to enter/ cause interactions
     alpha radiation requires thin wall (allowed to pass)
     gammas require thicker walls (interactions needed)
   Sensitivity
       Air or Fill gas Pressure
                               Air at high pressure
     100



Relative                      Helium at high pressure
       10
Current
(%)                           Air at low pressure

      1.0

                              Helium at low pressure
      0.1
            Applied Voltage (volts)
                       Pulse Height

               Recombination Region


                    Ionization Region




                
                
                
                          Proportional Region




Voltage
          Limited Proportional
                 Region



          Geiger-Mueller Region



            Continuous Discharge Region
 Operates   at higher voltage than ionization
  chamber
 Initial electrons produced by ionization
    are accelerated with enough speed to cause
     additional ionizations
    cause additional free electrons
    produces more electrons than initial event
 Process   is termed: gas amplification
               Recombination Region
                                      Ionization   Proportional Region
                                        Region
Pulse Height




                                            



                                            




                                                      Voltage
 Proportional     counters
    can distinguish between different radiation
     types
    specifically alpha and beta-gamma
 Differential    detection capability
    due to size of pulses produced by initial
     ionizing events
    requires voltage setting in range of 900 to
     1,300 volts
      alpha pulses above discriminator
      beta/gamma pulses too small
Ionization
Current                 Beta-Gamma Plateau


             Alpha Plateau




                    Detector Voltage
 Common   type of proportional counter
 Fixed radiation detection instrument used
  in counting rooms
 Windowed or windowless
 Both employ 2 geometry
    essentially all radiation emitted from the
     surface of the source enters active volume of
     detector
 Windowless
    used for alpha detection
Fill gas
outlet                              Fill gas
                                     inlet

                         anode                 (window-
             Detector                          optional)


                     sample      O-ring




   Sample planchet
 Fill   gas
     selected to enhance gas multiplication
     no appreciable electron attachment
     most common is P-10 (90% Argon and 10%
      methane)
 Operate at voltages above proportional
  detectors
 Each primary ionization
  produces a complete avalanche of ions
   throughout the detector volume
  called a Townsend Avalanche
  continues until maximum number of ion pairs
   are produced
  avalanche may be propagated by
   photoelectrons
  quenching is used to prevent process
 No  proportional relationship between energy
  of incident radiation and number of
  ionizations detected
 A level pulse height occurs throughout the
  entire voltage range
 IonChamber: simple, accurate, wide
 range, sensitivity is function of chamber
 size, no dead time

 ProportionalCounter: discriminate hi/lo
 LET, higher sensitivity than ion chamber

 GM Tube: cheap, little/no amplification,
 thin window for low energy; limited life
 Know   operating principles of your detector
    Contamination only?
    High range?
    Alpha / beta detection?
    Dose rate?
    Alpha/beta shield?
 Power   supply requirements
    Stable?
    Batteries ok?
 Temperature,       pressure correction
  requirements
 Calibration
    Frequency
    Nuclides
 Minimum  time at which detector recovers
  enough to start another avalanche (pulse)
 The dead time may be set by:
  limiting processes in the detector, or
  associated electronics
 “Dead   time losses”
    can become severe in high counting rates
    corrections must be made to measurements
 Term   is used loosely - beware!
 Time interval between dead time and full
  recovery
 Recovery Time = Resolving time- dead time
 Minimum time interval that must elapse after
 detection of an ionizing particle before a
 second particle can be detected.
 For some systems (GMs) dead time may be
  large.
 A correction to the observed count rate can
  be calculated as:
                         Ro
                  Rc 
 Where                1 R0 T
  T is the resolving time
  R0 is the observed count rate and
  RC is the corrected count rate
 “Exposure”  is the parameter measuring the
  ionization of air.
 Geiger tube measures ionization pulses per
  second - a “count rate”.
 The number of ionizations in the Geiger tube
  is a constant for a particular energy but is
  energy dependent.
 GMs have a high sensitivity but are very
 dependent upon the energy of photon
 radiations.
  R

20




1.2
1.0
0.8



      10   100   1000   E, keV
           poor energy response may be
 Detector’s
 corrected by adding a compensation
 sheath
  Thin layers of metal are constructed around
   the GM to attenuate the lower photon
   energies, where the fluence per unit dose rate
   is high, to a higher degree than the higher
   energies.
  The modified or compensated response, shown
   as a dashed line on the next graph, may be
   independent of energy within ± 20% over the
   range 50 keV to 1.25 MeV.
  Compensation sheaths also influence an
   instrument’s directional (polar) response and
   prevent beta and very low energy photon
   radiations from reaching the Geiger tube.
  R

20




1.2
1.0
0.8



      10   100   1000   E, keV
 Emit     light when irradiated
    promptly (<10-8s)
        fluorescence
    delayed (>10-8s)
        phosphorescence
 Can     be
    liquid
    solid
    gas
    organic
    inorganic
Energy


         Excited state

         Ground state, last filled
         (outer) orbital
A  large number of different scintillation
  crystals exist for a variety of applications.
 Some important characteristics of
  scintillators are:
    Density and atomic number (Z)
    Light output (wavelength + intensity)
    Decay time (duration of the scintillation light
     pulse)
    Mechanical and optical properties
    Cost
   Standard laboratory method for measuring radiation from
    beta-emitting nuclides.
   Samples are dissolved or suspended in a "cocktail"
    containing an aromatic solvent (historically benzene or
    toluene, and small amounts of other additives known as
    fluors.
     Beta particles transfer energy to the solvent molecules, which
      in turn transfer their energy to the fluors;
     Excited fluor molecules dissipate the energy by emitting light.
     Each beta emission (ideally) results in a pulse of light.
     Scintillation cocktails may contain additives to shift the
      wavelength of the emitted light to make it more easily
      detected.
   Samples are placed in small transparent or translucent
    (often glass) vials that are loaded into an instrument
    known as a liquid scintillation counter.
 Most   are crystals of alkali metals (iodides)
    NaI(Tl)
    CsI(Tl)
    CaI(Na)
 Impurity     in trace amounts
    “activator” causes luminescence
 Inorganic   scintillators have greater:
    light output
    longer delayed light emission
    higher atomic numbers
    than organic scintillators
 Inorganic   scintillators also
    linear energy response (light output is  energy
     absorbed)
 Solids   have
     Lattice structure (molecular level)
     Quantized energy levels
     Valence bands
     Conduction bands
               e-




Ge


     As+

              Shared
           electron pair
 Radiation  interaction in scintillator produces
  light (may be in visible range)
 Quantification of output requires light
  amplification and detection device(s)
 This is accomplished with the:
    Photocathode
    Photomultiplier tube
 Both   components are
    placed together as one unit
    optically coupled to the scintillator
                                 Photocathode
             Scintillation
             event                 Photomultiplier tube
Gamma ray
                                Dynodes




                                          Photoelectrons
       Fluor crystal NaI (Tl)
Reflector housing
 Photocathode     material
 Dynodes
    electrodes which eject additional electrons
     after being struck by an electron
    Multiple dynodes result in 106 or more signal
     enhancement
 Collector
    accumulates all electrons produced from final
     dynode
 Resistor
    collected current passed through resistor to
     generate voltage pulse
 Usedto detect low energy (ie., low range)
 radiations
    beta
    alpha
 Sample  is immersed in scintillant
 Provides 4  geometry
 Quenching can limit output
  chemical
  color quenching
  optical quenching
 Conventional  neutron meters surround a
  thermal neutron detector with a large and
  heavy (20 lb) polyethylene neutron
  moderator.
 Other meters utilizes multiple windows
  formed of a fast neutron scintillator (ZnS in
  an epoxy matrix), with both a thermal
  neutron detector and a photomultiplier tube.
 Radiation   damage results inchange in
  scintillation characteristics caused by
  prolonged exposure to intense radiation.
 Manifests as decrease of optical
  transmission of a crystal
    decreased pulse height
    deterioration of the energy resolution
 Radiationdamage other than activation
 may be partially reversible; i.e. the
 absorption bands disappear slowly in
 time.
 Doped  alkali halide scintillators such as
  NaI(Tl) and CsI(Tl) are rather susceptible
  to radiation damage.
 All known scintillation materials show
  more or less damage when exposed to
  large radiation doses.
 Effects usually observed in thick (> 5 cm)
  crystals.
 A material is usually called radiation hard
  if no measurable effects occur at a dose
  of 10,000 Gray. Examples of radiation
  hard materials are CdWO4 and GSO.
 Each  scintillation material has characteristic
  emission spectrum.
 Spectrum shape is sometimes dependent on
  the type of excitation (photons / particles).
 Emission spectrum is important when
  choosing the optimum readout device (PMT
  /photodiode) and the required window
  material.
 Emission spectrum of some common
  scintillation materials shown in next two
  slides.
      output (photons per MeV gamma) of
 Light
 most scintillators is a function of
 temperature.
  Radiative transitions, responsible for the
   production of scintillation light compete with
   non-radiative transitions (no light production).
  In most light output is quenched (decreased) at
   higher temperatures.
  An exception is the fast component of BaF2
   where intensity is essentially temperature
   independent.
http://www.scionixusa.com/pages/navbar/scin_crystals.html
 Following table lists characteristics such as high
  density, fast decay etc.
 Choice of a certain scintillation crystal in a
  radiation detector depends strongly on the
  application.
 Questions such as :
       What is the energy of the radiation to measure ?
       What is the expected count rate ?
       What are the experimental conditions (temperature,
        shock)?
Material   Important Properties           Major Applications
                                          General scintillation counting, health
           Very high light output, good
NaI(Tl)                                     physics, environmental
              energy resolution
                                            monitoring, high temperature use
           Noon-hygroscopic, rugged,      Particle and high energy physics,
CsI(Tl)      long wavelength                 general radiation detection,
             emission                        photodiode readout, phoswiches
                                          Geophysical, general radiation
CsI(Na)    High light output, rugged
                                            detection
           Fast, non-hygroscopic,
CsI
              radiation hard, low light   Physics (calorimetry)
undoped
              output
CaF2(Eu) Low Z, high light outut           detection, ,  phoswiches
                                          DC measurement of X-rays (high
           Very high density, low           intensity), readout with
CdWO4
              afterglow, radiation hard     photodiodes, Computerized
                                            Tomography (CT)
           Fast, low density and Z,
Plastics                                  Particle detection, beta detection
              high light output
Material Important Properties                Major Applications
6LiI(Eu)    High neutron cross-section,      Thermal neutron detection and
               high light output                spectroscopy
6Li   -     High neutron cross-section,
                                             Thermal neutron detection
      glass    non-hygroscopic
                                             Positron life time studies, physics
BaF2        Ultra-fast sub-ns UV emission
                                                research, fast timing
YAP(Ce                                       MHz X-ray spectroscopy, synchrotron
            High light output, low Z, fast
  )                                            physics
GSO(Ce High density and Z, fast,
                                             Physics research
  )       radiation hard
                                             Particle physics, geophysical
BGO         High density and Z                  research, PET, anti-Compton
                                                spectrometers
                                             DC measurement of X-rays (high
            Very high density, low             intensity), readout with
CdWO4
               afterglow, radiation hard       photodiodes, Computerized
                                               Tomography (CT)
            Fast, low density and Z, high
Plastics                                     Particle detection, beta detection
               light output
 Highlysensitive surface contamination
  probes incorporate a range phosphors
 Examples include:
    zinc sulphide (ZnS(Ag)) powder coatings (5–10
     mg·cm–2) on glass or plastic substrates or
     coated directly onto the photomultiplier
     window for detecting alpha and other heavy
     particles;
    cesium iodide (CsI(Tl)) that is thinly machined
     (0.25 mm) and that may be bent into various
     shapes;
    and plastic phosphors in thin sheets or powders
     fixed to a glass base for beta detection.
 Most common type of detector
 Gas amplification
 Multiplication factor 108-1010
 Long dead time
                                                +
 Energy dependence
 Variety of uses                               e
                                                -    (+) Anode
    Count rate
    Dose/Dose rate     Resistor
                                                        (-) Cathode
    Surface activity
                                   +        -


                                   Battery or High
                                   Voltage
 Alpha-Beta  discrimination
 Gas multiplication 106
 Low dead time
 Used in labs and neutron detectors



                              Alpha+Beta
                                           Alpha+
                    Beta                   Beta
  Background                               Beta
      Photo Cathode         Dynodes

                                           -
                                      HV

                                           +
Scintillator
                 Focusing
                   cup
                                      Resistor
      Photo Cathode            Dynodes

               e-                             -
                                         HV
               e-
                                              +
Scintillator
                    Focusing
                      cup
                                         Resistor
                                         Photo Peak
Number of Counts




                           Compton
                            Edge




                   Channel Number (Energy)
Multi-Channel Analysis




     identiFINDER
 Stores absorbed energy
 Crystalline material
 Read by heating and detecting emitted light
 Primary occupational dosimeter
Thermoluminescent Dosimeter




                               Whole Body and Wrist TLD Holders




Panasonic UD-802AS Dosimeter



                                 Harshaw DXT-RAD Ring TLD
 High energy resolution
 Low efficiency for gamma (Diode Detector)
 High efficiency for gamma (Ge Detector)
 Must be liquid nitrogen cooled (PINS)




                http://www.missouri.edu/~glascock/naafig5.gif
Semi Conductor Detectors




   High-purity Germanium
 Principles       of radiation detection
     Electrical collection of ions
         Gas ionization
         Air ionization
     Scintillation - Light production
 Characteristics          of sensitive detectors
     Dense detecting medium
     High efficiency
     Low background

								
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