Detectors

					Lesson 17

 Detectors
            Introduction
• When radiation interacts with matter,
  result is the production of energetic
  electrons. (Neutrons lead to secondary
  processes that involve charged species)
• Want to collect these electrons to
  determine the occurrence of radiation
  striking the detector, the energy of the
  radiation, and the time of arrival of the
  radiation.
  Detector characteristics
• Sensitivity of the detector
• Energy Resolution of the detector
• Time resolution of the detector or
  itgs pulse resolving time
• Detector efficiency
Summary of detector types
• Gas Ionization
• Ionization in a Solid (Semiconductor
  detectors)
• Solid Scintillators
• Liquid Scintillators
• Nuclear Emulsions
 Detectors based on gas ionization
• Ion chambers




      35 eV/ion pair>105 ion pairs created.

     Collect this charge using a capacitor, V=Q/C

NO AMPLIFICATION OF THE PRIMARY IONIZATION
   Uses of Ion Chambers
• High radiation fields (reactors)
  measuring output currents.
• Need for exact measurement of
  ionization (health physics)
• Tracking devices
         Gas amplification
• If the electric fields are strong enough, the
  ions can be accelerated and when they strike
  the gas molecules, they can cause further
  ionization.
The Result
      Proportional counters
• Gas amplification creates output pulse whose
  magnitude is linearly proportional to energy
  deposit in the gas.
• Gas amplification factors are 103-104.
• Will distinguish between alpha and beta
  radiation
       Practical aspects
                                gas flow

                                typical gas: P10,
                                 90% Ar,
                                 10% methane




Sensitive to ,, X-rays, charged particles
Fast response, dead time ~ s
   Geiger- Müller Counters

• When the gas amplification factor reaches 108, the size
  of the output pulse is a constant, independent of the
  initial energy deposit.
• In this region, the Geiger- Müller region, the detector
  behaves like a spark plug with a single large discharge.
• Large dead times, 100-300µs, result
• No information about the energy of the radiation is
  obtained or its time characteristics.
• Need for quencher in counter gas, finite lifetime of
  detectors which are sealed tubes.
• Simple cheap electronics
  Semiconductor Radiation
        Detectors
• “Solid state ionization chambers”
• Most common semiconductor used is
  Si. One also uses Ge for detection
  of photons.
• Need very pure materials--use
  tricks to achieve this
Semiconductor physics
          p-n junction




Create a region around the p-n junction
where there is no excess of either n or p
carriers. This region is called the “depletion
region”.
Advantages of Si detectors
• Compact, ranges of charged
  particles are µ
• Energy needed to create +- pair is
  3.6 eV instead of 35eV. Superior
  resolution.
• Pulse timing ~ 100ns.
             Ge detectors
• Ge is used in place of Si for detecting gamma
  rays.
• Energy to create +- pair = 2.9 eV instead of
  3.6 eV
• Z=32 vs Z=14
• Downside, forbidden gap is 0.66eV, thermal
  excitation is possible, solve by cooling detector
  to LN2 temperatures.
• Historical oddity: Ge(Li) vs Ge
   Types of Si detectors
• Surface barrier, PIN diodes, Si(Li)
• Surface barrier construction
  Details of SB detectors
• Superior resolution
• Can be made “ruggedized” or for
  low backgrounds
• Used in particle telescopes, dE/dx,
  E stacks
• Delicate and expensive
             PIN diodes
• Cheap
• p-I-n sandwich
• strip detectors
       QuickTime™ and a
        decompressor
are neede d to see this picture.
            Si(Li) detectors
• Ultra-pure region created by chemical compensation, i.e.,
  drifting a Li layer into p type material.
• Advantage= large depleted region (mm)
• Used for -detection.
• Advantages, compact, large stopping power (solid),
  superior resolution (1-2 keV)
• Expensive
• Cooled to reduce noise
               Ge detectors
• Detectors of choice for detecting -rays
• Superior resolution
      Scintillation detectors
• Energy depositlightsignal
• Mechanism (organic scintillators)




Note that absorption and re-emission have different spectra
       Organic scintillators
• Types: solid, liquid (organic scintillator in
  organic liquid), solid solution(organic scintillator
  in plastic)
• fast response (~ ns)
• sensitive (used for) heavy charged particles and
  electrons.
• made into various shapes and sizes
      Liquid Scintillators
• Dissolve radioactive material in the
  scintillator
• Have primary fluor (PPO) and wave
  length shifter (POPOP)>
• Used to count low energy 
• Quenching
Inorganic scintillators (NaI
           (Tl))




  Emission of light by activator center
              NaI(Tl)
• Workhorse gamma ray detector
• Usual size 3” x 3”
• 230 ns decay time for light output
• Other common inorganic scintillators
  are BaF2, BGO
NaI detector operation
Nuclear electronics
Nuclear statistics

       Table 18-2 Typical Sequence of
       Counts of a long-Lived Sample
                  (170Tm)*

    Measurement               cp0.1m                    xi-xm    (xi-xm)2
       Number
          1                    1880                      -18      324
          2                    1887                      -11      121
          3                    1915                       17      289
          4                    1851                      -47     2209
          5                    1874                      -24      576
          6                    1853                      -45     2025
          7                    1931                       33     10899
          8                    1886                      -32     1024
          9                    1980                       82     6724
          10                   1893                       -5      25
          11                   1976                       78     6084
          12                   1876                      -22      484
          13                   1901                       3        9
          14                   1979                       81     6561
          15                   1836                      -62     3844
          16                   1832                      -66     4536
          17                   1930                       32     1024
          18                   1917                       19      361
          19                   1899                       1        1
          20                   1890                       -8      64
                      f.
*We are indebted to Pro R.A. Schmitt for providing these data.
       Distribution functions
Most general distribution describing radioactive decay
is called the Binomial Distribution

                                     x            n-x
  P(x)=(n!/((n-x)!x!)p (1-p)
        n=# trials, p is probability of success
      Poisson distribution
• If p small ( p <<1), approximate binomial
  distribution by Poisson distribution
            P(x) = (xm)x exp(-xm)/x!
    where
                   xm = pn

• Note that the Poisson distribution is
  asymmetric
      Example of use of statistics

• Consider data of Table 18.2
• mean = 1898
• standard deviation, , = 44.2 where

                      N

                      x  x 
                             i      m
                                        2


               2    i 1
                             N 1

  For Poisson distribution
     
 Gaussian (normal) distribution

               1       x  x 2 
     P(x)         exp
                      
                               m
                                   
              2xm        2xm  




     Interval distribution

            1
      I(t)  exp( t / tm )dt
            tm

                 Counts occur in “bunches”!!



        Operation     Answer        Uncertainty
      Addition       A+B          (A2+B2)1/2
      Subtraction    A-B          (A2+B2)1/2
Multiplication A*B           A*B((A/A)2+(B/B)2)1/2
Division         A/B         A/B((A/A)2+(B/B)2)1/2
Simple statistics
Uncertainties for some common operations
          Operation   Answer   Uncertainty

   Addition           A+B       (σA2+σB2)1/2
   Subtraction        A-B       (σA2+σB2)1/2
   Multiplication     A*B      A*B((σA/A)2+(σB/B)2)1/2
   Division           A/B         A/B((σA/A)2+(σB/B)2)1/2

				
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