Slide 1 - Nuclear Instrumentation

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					             Detection of Fission Neutrons
             Using Semi-Insulating Silicon
                  Carbide Detectors*
        Frank H. Ruddy, Robert W. Flammang, John G. Seidel,
              Scott M. Watson**, and James T. Johnson**

        Science & Technology Dept., Westinghouse Electric Co.
                    Pittsburgh, Pennsylvania, USA
         **Idaho National Laboratory, Nuclear Nonproliferation
                 Department, Idaho Falls, Idaho, USA
*Thiswork was supported by the US Department of Homeland Security Domestic Nuclear
Detection Office under Contract HSHQ-07-C-00041




1
      Detection of Fission Neutrons Using Semi-
         Insulating Silicon Carbide Detectors

    ● Silicon Carbide (SiC) semiconductor fission-
      neutron detectors have been developed for US
      Department of Homeland Security applications
    ● SiC detectors have been shown to be capable of
      detecting fission neutrons from concealed Special
      Nuclear Materials in pulsed neutron and photon
      interrogation environments.




2
Advantages of SiC Detectors
   SiC detectors can count fission                        1000
                                                                                                         1000 Hz
    neutrons from SNM                                                                                    2000 Hz
                                                            100
    immediately following intense                                                                        500 Hz
                                                                                                         121-microsecond die-away
    neutron bursts from an                                   10




                                      Counts per Second
    interrogating source                                      1
   Thermal and epithermal
                                                             0.1
    neutron-induced fission
    neutrons will be missed by                              0.01       Neutron Pulse

    alternative detectors, which
                                                           0.001
    require time to recover from
    the neutron burst                                     0.0001
                                                                   0              500            1000             1500          2000
   Highly selective - in the                                                    Time after Start of Neutron Pulse (ms)
    example shown, 2336 fission
    neutrons were detected with a
    235U sample present and zero                             (Results reported at the IEEE2007 NSS)                             3

    counts when the sample was
    removed
Advantages of SiC Detectors
   SiC detectors are
    insensitive to thermal and
    epithermal neutrons and
    gamma rays and provide
    excellent g/n discrimination
   Fast-neutron pulses have
    been observed during
    intense gamma bursts at
    the INL Varitron 10-MV
    electron accelerator
   An unshielded SiC detector
    was located 1 m from the
    converter in a 9000-12,000
    R/hr gamma field               (Results reported at the IEEE2007 NSS)   4
Development of Radiation Detectors Based
on Semi-Insulating Silicon Carbide
   Silicon Carbide (SiC) radiation detectors based on diode
    devices have been developed by many groups for a variety
    of applications
   SiC Schottky diode and p-i-n diode detectors use lightly
    doped epitaxial SiC layers on conducting SiC substrates
   The thickness of the detector active volume corresponds to
    the depletion thickness of the epitaxial layer
   Although SiC epitaxial layers thicker than 200 µm have
    been produced, only layers up to about 120 µm can be
    produced routinely
   Furthermore, to fully deplete the epitaxial layer at modest
    voltages (<1000 V), the unintentional doping concentration
    must be limited to <1014 atoms/cm3 (Nitrogen is typically     5

    used)
                                                                            Gold Wire

    SiC Fast-Neutron Detectors                            Gold Wire
                                                         to Schottky
                                                           Contact
                                                                            to Detector
                                                                              Ground



 A SiC Schottky diode is shown
 The n- epitaxial layer, when depleted
  by a reverse bias, is the active volume
  for the detector
                       Schottky Contact
 Ionization in the
   active volume is             Epitaxial SiC Layer       100µm n- (1014 N/cm3)

   collected under
   the influence of       SiC Conducting Substrate          300 µm
                                  18             3
                          (~10 Nitrogen atoms per cm )     • Reverse Bias Depletes n-
   the applied bias                                          region
                                                           • Ionizing Radiation Produces
 Detector response                                          electron-hole pairs in
                       Back Ohmic Contact                    depleted region
  is due to fast-            4.5 to 6-mm diameter
                                                           • Charge is collected under
                                                             influence of applied voltage
  neutron reactions                                                                   6

– no internal detector die-away time
 SiC Fast-Neutron Detectors
                                                                                                                                         n'
                                The detector response is                                                    n
                                                                                                                                                                       n'
                                 based on ionization produced                                                n
                                                                                                                                        12C
                                                                                                                                                   

                                 by neutron-induced reaction                                                                                   28Si
                                                                                                             n
                                 products in the detector active                                                                     9Be                  n'

                                 volume                                                                      n
                                                                                                                                    


                                Carbon and Silicon neutron-                                                                              


                                 reaction response is                                                    1-mm gold over             120-mm n-                  300-mm SiC    Back ohmic

                                 proportional to active volume                                           Schottky contact           active layer                substrate      contact




                           120

                                                                                                                                               p
                                                 Maximum Depletion
                                                                                                                                - + -+ -
                           100                     Depth 100 mm
                                                                                                                                  + -+
                                                                                                         n
Depletion Thickness (mm)




                            80



                            60
                                                                                                                                               n’

                                                                       N = 1.2 x 1014 cm-3 nitrogen
                            40



                            20                                                                            Polyethylene                       120-mm n-          300-mm SiC
                                                                                                             (CH2)n         1-mm gold over active layer          substrate      7
                                                                                                                                                                             Back ohmic
                             0                                                                           Converter Layer    Schottky contact                                   contact
                                 0   200   400      600      800      1000     1200     1400      1600
                                                     Reverse Bias (volts)
Development of Radiation Detectors Based
on Semi-Insulating Silicon Carbide
 An alternative is to use semi-insulating SiC
 Semi-insulating SiC detectors have been
  demonstrated
   – M. Rogalla, et al., Nuclear Physics B 78, 516-
     520 (1999)
   – W. Cunningham, et al., Nuclear Instruments &
     Methods A 509, 127-131 (2003)
 However, the semi-insulating SiC used was heavily
  doped with Vanadium, which produces deep-level
  charge traps and causes low charge-collection   8

  efficiency
Development of Radiation Detectors Based
on Semi-Insulating Silicon Carbide
 Recently, high-quality un-doped semi-insulating
  SiC wafer material has become available from
  Cree, Inc.
 These wafers contain low residual impurities
  corresponding to background net doping
  concentrations <105 cm-3
 360-µm thick wafers are readily available, and
  thicknesses up to 2-mm are potentially available
 SiC fast-neutron detectors using these semi-
  insulating materials have been manufactured and    9


  tested
Diode Detectors vs. Semi-Insulating SiC
    Schottky Contact                                                     Schottky Contact

            Epitaxial SiC Layer         100µm n- (1014 N/cm3)


                                                                                Semi-Insulating SiC
       SiC Conducting Substrate          300 µm                                       (360 µm)
       (~1018 Nitrogen atoms per cm3)   • Reverse Bias Depletes n-
                                          region
                                        • Ionizing Radiation Produces
                                          electron-hole pairs in
    Back Ohmic Contact                    depleted region                Back Ohmic Contact
                                        • Charge is collected under
          4.5 to 6-mm diameter
                                          influence of applied voltage     1-mm, 3-mm or 6-mm diameter

           SiC Schottky Diode                  Semi-Insulating SiC
   SiC diode detectors – thickness of epitaxial layer determines
    active volume for the detector (limited to ~120 µm)
   Semi-insulating (S-I) detectors – entire wafer thickness
    comprises the active volume
                                                                  10
   150-µm, 160- µm, and 360- µm S-I SiC detectors have been
    tested and up to 2000-µm wafers are available
SiC Detector Fabrication
   0.78-mm2, 7.07-mm2, and
    28.3-mm2 detectors were
    made on 150-µm, 160-µm
    and 360-µm SiC wafers by
    GeneSiC Semiconductor,
    Inc. of Dulles, VA USA
Device reticule is repeated over         53
                                         6 mm Devices
  entire 7.62-cm wafer area              Per wafer

                            6 mm
                            Device
                                         55
                                         3 mm Devices
                                         Per wafer
                           Alignments/
                           TLMs etc.
                                         57
                                         1 mm Devices
                                         Per wafer
                           3mm with
                           GND band

                           3mm without
                           GND band          11

                            1 mm
                            Device
Present Work

 Fission-neutron response characteristics were
  compared for a 28.3-mm2 x 100-µm Schottky diode
  and a 28.3-mm2 x 160- µm S-I SiC detector
 Both detectors were equipped with 100- µm
  polyethylene proton recoil converter foils
 Detection of thermal-neutron induced fission
  neutrons was demonstrated in a pulsed neutron
  interrogation environment

                                                12
Detection of 252Cf Fission Neutrons
                                                                  Detectors were placed within an
                                                                   RF shielding box at a distance
                                                                   of 10 mm from a 2.11 x 107 s-1
                                                                   252Cf neutron source

                                                                  A fast Ortec VT-120 amplifier
                                                                   was used with an Acqiris
                                                                   digitizer to record response
                                                                   pulses
                                                                             Pulse amplitude
                                                                               spectra were
                                                       Semi-Insulating SiC Detector

252Cf   Source
                 SiC Schottky Diode

                                      252Cf
                                                                               recorded for the
                                              Source
                                                                               Schottky diode and
                                                                               for the S-I SiC
                                                                               detector
                                                                                              13
               SiC Schottky Diode 252Cf Fission-
               Neutron Results
                         400                                                                           200
                                                                                                                                                              Pulse amplitude
counts per 300 seconds




                                                                                                                                           300 V
                                                                                                                                                          




                                                                              counts per 30 seconds
                         350                                   30 V
                                                 (a)           50 V                                                          (a)           400 V
                         300
                         250
                                                               100 V
                                                               200 V
                                                               800 V
                                                                                                       150                                 500 V
                                                                                                                                           600 V              spectra were
                                                                                                                                           700 V
                         200
                         150
                                                               900 V                                   100                                                    recorded as a
                         100                                                                            50                                                    function of
                          50
                           0                                                                             0
                                                                                                                                                              voltage at two
                               0       0.02    0.04   0.06
                                           Pulse Height (volts)
                                                               0.08     0.1                                  0    0.02    0.04   0.06
                                                                                                                      Pulse Height (volts)
                                                                                                                                          0.08      0.1       different
                                                                                                                                                              triggering levels
                         1000                                                                          1000                                                  Pulse amplitude
counts per 300 seconds




                                                                              counts per 300 seconds




                                                                30 V                                                                        300 V
                                                 (b)            50 V
                                                                100 V
                                                                                                                              (b)           400 V
                                                                                                                                            500 V             increases with
                          100                                   200 V
                                                                800 V
                                                                900 V
                                                                                                        100                                 600 V
                                                                                                                                            700 V             voltage
                           10                                                                            10
                                                                                                                                                             Count rate
                                                                                                                                                              increases with
                               1                                                                          1                                                   voltage
                                   0   0.02   0.04   0.06    0.08       0.1                                   0   0.02   0.04   0.06    0.08        0.1
                                          Pulse Height (volts)                                                       Pulse Height (volts)
                                                                                                                                                                            14

                               Triggering Level 1                                                            Triggering Level 2
SiC Schottky Diode 252Cf Fission-
Neutron Results
 The count rate increases are due to increasing
  depletion depth with voltage
 The increases in pulse amplitude are a result of
  decreasing capacitance with voltage
  (C1/thickness)
 The maximum pulse height is Vmax=Q/C , where Q
  is the charge collected and C is the capacitance
 Since the neutron-induced reaction products have
  ranges that are much less than the active volume
  dimensions, Q is nearly constant. Therefore the
  increase in pulse amplitude with voltage is a result
  of increasing depletion depth and decreasing C.        15
                        SiC Schottky Diode 252Cf Fission-
                        Neutron Results
                        30000
                                    Triggering Level 1
                                    Triggering Level 2
                        25000
Counts per 30 seconds




                        20000                                                                       The integral counts for each
                        15000
                                                                                                     spectrum are shown as a
                        10000
                                                                                                     function of voltage
                         5000

                                                                                                    The depletion depth for
                            0
                                0        200             400
                                                               Bias (-V )
                                                                            600   800     1000
                                                                                                     each voltage can be
                        35000
                                    Triggering Level 1
                                                                                                     calculated
                                    Triggering Level 2
                        30000
                                    Linear (Triggering Level 1)
                                    Linear (Triggering Level 2)
                                                                                                    The number of counts
                                                                                                     recorded is directly
Counts per 30 seconds




                        25000


                        20000


                        15000
                                                                                                     proportional to depletion
                        10000
                                                                                                     depth or active volume – as
                         5000
                                                                                                     expected
                                                                                                                                16
                            0
                                0         20             40                 60     80      100
                                    Depletion Depth - Average Doping Concentration (mm)
               S-I SiC Detector 252Cf Fission-Neutron
               Results
                                                                                                    400                                                       Pulse amplitude
counts per 600 seconds




                                                                           counts per 600 seconds
                                                                                                                                          -800 V
                         600                               +100 V                                                              (a)        -600 V
                         500                   (a)
                                                           +200 V
                                                           +300 V                                   300
                                                                                                                                          -400 V
                                                                                                                                          -300 V
                                                                                                                                                               spectra were
                                                           +400 V
                         400                               +500 V
                                                           +600 V
                                                                                                                                          -200 V
                                                                                                                                                               recorded as a
                                                                                                    200
                         300                               +700 V
                                                           +800 V                                                                                              function of voltage
                         200
                         100
                                                                                                    100                                                        for both positive
                           0                                                                          0
                                                                                                                                                               and negative
                               0    0.01    0.02   0.03     0.04    0.05                                  0        0.01    0.02   0.03     0.04     0.05       biases
                                        Pulse Height (volts)                                                           Pulse Height (volts)
                                                                                                                                                              Pulse heights
                         1000                                                                       1000
                                                                                                                                                               increase with
                                                                           counts per 600 seconds
counts per 600 seconds




                                                           +100 V                                                               (b)        -800 V
                                                           +200 V                                                                          -600 V

                          100
                                               (b)
                                                           +300 V
                                                           +400 V
                                                                                                     100
                                                                                                                                           -400 V
                                                                                                                                           -300 V
                                                                                                                                                               increasing voltage
                                                           +500 V
                                                           +600 V
                                                           +700 V
                                                                                                                                           -200 V
                                                                                                                                                              Since C is
                                                           +800 V
                           10                                                                         10                                                       constant, increase
                                                                                                                                                               must be due to Q
                            1
                                0   0.01   0.02   0.03    0.04      0.05
                                                                                                          1
                                                                                                              0    0.01   0.02   0.03    0.04       0.05
                                                                                                                                                               – more charge
                                       Pulse Height (volts)                                                           Pulse Height (volts)                     collected at higher
                                    Positive Bias                                                                 Negative Bias                                voltages        17
         S-I SiC Detector 252Cf Fission-Neutron
         Results
                          1000
                                                                                      Pulse height spectra
 counts per 600 seconds




                                                                     -800 V
                                                                     +800 V
                           100                                                         are similar at the same
                            10
                                                                                       absolute voltage
                                                                                      More counts are
                             1
                                 0     0.01   0.02   0.03    0.04          0.05
                                                                                       observed when bias is
                                          Pulse Height (volts)                         negative
                      25000          Negative Bias                                    Both show more counts
                                                                                       at higher voltages –
                                     Positive Bias
                                     Linear (Positive Bias)
                                     Linear (Negative Bias)
                      20000

                                                                                       positive-bias case is
Counts per 60 sec




                      15000
                                                                                       nearly linear
                      10000
                                                                                      Many fewer counts
                          5000                                                         observed for S-I SiC
                             0
                                                                                       than for Schottky     18

                                 0      200        400        600    800      1000
                                                 Absolute Bias (V)
        S-I SiC Detector 252Cf Fission-Neutron
        Results
                     1000
                                         Semi-Insulating SiC -400 V
                                         Semi-Insulating SiC -600 V         Pulse amplitudes are also much
counts per channel




                                         Semi-Insulating SiC -800 V
                      100
                                         Schottky Diode -200 V               less for S-I SiC than for the
                                         Schottky Diode -900 V
                                                                             Schottky diode
                       10
                                                                            S-I result at 800 V is similar to
                                                                             Schottky result at 200 V – the
                                                                             200-V Schottky depletion depth
                           1
                               0   0.02     0.04      0.06        0.08
                                                                             is only 39 µm
                                     Pulse Height (volts)                   Both reduced pulse heights and
                     400
                                                                             fewer counts in S-I SiC are
                     350
                                      Semi-Insulating SiC -800 V             indicative of incomplete charge
counts per channel




                     300
                                      Schottky Diode -200 V                  collection
                     250
                                                                            Although increasing counts were
                     200
                     150
                                                                             observed with increasing
                     100                                                     voltages for S-I SiC, high-voltage
                      50                                                     testing was limited by detector
                       0                                                     packaging issues                   19
                           0        0.02         0.04             0.06
                                   Pulse Height (volts)
 S-I SiC Detector Packaging Issues
                        1.2E-09
                                                                                         On-chip I-V
                                                150-µm
                                                160-µm
                                                                                          measurements yielded
                        1.0E-09
                                                Linear (150-µm)
                                                Linear (160-µm)
                                                                                          excellent results
                        8.0E-10
Leakage Current (amp)




                                                                                         Detectors showed
                        6.0E-10
                                                                                          resistive behavior with
                        4.0E-10                                                           leakage currents
                        2.0E-10
                                                                                          generally less than 1
                        0.0E+00
                                                                                          nanoampere
                        -2.0E-10
                                                                                         Packaged devices had
                                   0      100           200         300   400   500       high leakage currents
                                                          Bias (volts)
                                                                                          and breakdown
                                         (Measurements made by                            voltages ≤ 800 V
                                       GeneSiC Semiconductor, Inc.)                                            20
           S-I SiC Neutron Efficiencies
                                                                          6


                                                                          5
                                                                                                                                            100 mm                S-I SiC results are both
                                                                                                                                                                   disappointing and
                                                    Relative Efficiency




                                                                          4
                                                                                                          50 mm


                                                                          3
                                                                                                                                                                   encouraging
                                                                          2
                                                                                  -20 Volts
                                                                                                                                                                    – Fast-Neutron detection
                                                                          1
                                                                                                                                                                      efficiencies are lower than
                                                                          0
                                                                              0          20         40            60
                                                                                                         Depletion Depth (mm)
                                                                                                                                80           100      120             expected
                                            0.009
                                                                                                                                                                    – Results indicate that higher
                                                                                    Schottky Measurements
                                                                                    Linear - Schottky                                                                 efficiencies can be
Intrinsic Efficiency (counts per neutron)




                                            0.008                                   S-I SiC Measurements

                                            0.007
                                                                                    Series4
                                                                                    Series5                                                           360 mm          obtained than for thickest
                                                                                                      160 mm
                                            0.006                                                                                                                     Schottky diodes available
                                            0.005

                                            0.004
                                                                                                    150 mm
                                                                                                                                           D-D Measurement
                                                                                                                                                800 V
                                                                                                                                                                    – Higher efficiencies can be
                                            0.003                                       120 mm
                                                                                                                                                                      expected with better
                                            0.002
                                                                                                                  252
                                                                                                                                                                      packaging and higher
                                            0.001
                                                                                                                       Cf Measurement
                                                                                                                           400 V                                      voltages                   21
                                               0
                                                         0                         50         100         150           200          250       300     350
                                                                                                         Depletion Depth (mm)
    Improvement of Fast-Neutron Efficiency
       for Semi-Insulating SiC Detectors
   In-addition to higher
    voltage packaging, 3D
    electrode designs can
    be explored
   Proposed by Parker, et
    al. (NIMA 395, 328-343
    [1997]), 3D
    configurations are
    being explored for SiC
    as well as Si and other
    semiconductor
    detectors for high-
    energy physics
    experiments                          22
Summary and Conclusions
 Semi-insulating SiC neutron detectors are a
  promising alternative to diode detectors
 Semi-insulating SiC detectors are less expensive
  than epitaxial SiC diode detectors
 Detectors with 360-µm thick active layers are
  readily available, and wafers up to 2 mm thick can
  be obtained
 However, charge collection problems with semi-
  insulating detectors must be overcome
                                                       23
24
Advantages of SiC Detectors
   Temperature insensitivity              (Results reported at the IEEE2006 NSS)
     – No drift in response
                                                                    Gold Wire
                                                  Gold Wire         to Detector
       observed in the range from                to Schottky
                                                   Contact
                                                                      Ground

       from 18 0C to 304 0C
   Resistance to Radiation
    Effects
     – The response of SiC                                Unirradiated
       detectors is unaffected by
       large radiation exposures
                                                Following 22.7MGy 137Cs
                                                    Gamma Exposure
Therefore, SiC detectors can
  operate in extreme ambient
  temperature and radiation
  environments for extended
  time periods                                                                    25


                               Detector still works after
                             massive gamma-ray exposure
Advantages of SiC Detectors
   Unprecedented signal-to-
    background has been
    achieved in high-energy        Timing       With HEU    Without HEU           Net HEU
                                    Group
    neutron interrogation             G1
                                  (0-110 µs)
                                                 37634          29294             8340+259

    environments                      G2
                                 (130-400 µs)
                                                  1857            0                1857+43
                                      G3          394             0                394+20
   2336 fission neutrons were   (400-670 µs)
                                      G4           85             0                  85+9
                                 (670-940 µs)
    observed between 100-ms                                Net delayed counts = 2336+48 (+2.1%)

    source bursts with zero
                                    (Results reported at the IEEE2007 NSS)
    background when the SNM
    was removed

                                                                                            26

				
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