Docstoc

Hamby

Document Sample
Hamby Powered By Docstoc
					 A System for Simultaneous Beta and
     Gamma Spectroscopy and its
Application to Nuclear Non-Proliferation

           D.M. Hamby and A.T. Farsoni

    Nuclear Engineering and Radiation Health Physics
                College of Engineering
                Oregon State University
                   Corvallis, Oregon
Beta Spectroscopy

  • Typically for beta dosimetry
  • Open- and closed-window
        spectral stripping
  • Solid-state, scintillators, thin plastics
  • 20-yr history
Gamma Spectroscopy

•   Very common
•   Simple to complex
•   Solid-state, scintillators
•   High/low resolution
  Scintillator Response

                      The light output from most scintillators can be represented as a simple
                      exponential decay. The current pulse from the PM-tube anode is:

                                                                                        t                         Q       t
                                                                        i(t )  i0e          t
                                                                                                                      e        t
                                                                                                                   t
                                 where t is the scintillator light decay time and Q is the total collected charge


                                   (b) A typical gamma-induced pulse
            260                                                                                              250



            240
                                                                                                             240


            220
                                                                                                             230
Amplitude




                                                                                                 Amplitude
            200

                                                                                                             220

            180
                                                    NaI (Tl)                                                                                 Plastic
                                                                                                             210
            160

                                                 τ = 230 nsec                                                                            τ = 2.4 nsec
                                                                                                             200
            140



                                                                                                             190
            120                                                                                                 0               500   1000          1500   2000   2500
                  0        500            1000                   1500    2000    2500
                                                 Time (2 nsec)                                                                          Time (2 nsec)
Phoswich Detectors

• The combination of dissimilar scintillators optically coupled to
  a single PM tube (phosphor sandwich)




                  Phoswich Detector                                                        Rise-Time Distribution



*Usuda, S. and others, “Phoswich detectors for simultaneous counting of α-, β (gamma )-rays and neutrons” 1997.
 Digital Spectrometers
 Advantages

 Pulse processing algorithm is easy to edit

 The algorithm is stable and reliable, no thermal noise or other fluctuations

 No bulky analog electronics             •Classical Spectrometer

                                                                             Analog
                                               Detector     Preamplifier    Shaping     Multichannel     Histogram
                                                                            Amplifier    Analyzer         Memory


 Post-processing ability
                                          •Digital Spectrometer

                                                                           High-Speed

 More cost-effective
                                                                                         Digital Pulse   Histogram
                                             Detector     Preamplifier                    Processing      Memory
                                                                             ADC




 Effects, such as pile-up, can be corrected or eliminated at the processing level

 Signal capture and processing can be based more easily on coincidence criteria
  between different detectors or different parts of the same detector
Simultaneous Beta/Gamma Spectrometer

     A digital phoswich detection system capable of
   simultaneous spectroscopy with minimal cross-talk.
Triple-Layer Phoswich Design
Aluminized Mylar (6 mm)

          CaF2(Eu) (1.5 mm)       NaI(Tl) (25.4 mm)




                                         76.2 mm      PMT




BC-400 (1.5 mm)      Quartz Optical Window (2.0 mm)
Electron & Photon Energy Deposition

                                                      MCNP modeling for detector optimization.

                       0.6                                                                                                       1
                                                                                     50 keV                                                           BC-400
                       0.5                                                                                                                            CaF2:Eu
                                                                                     100 keV                                    0.1
                                                                                                                                                      NaI(Tl)
 Probability per MeV




                                                                                     500 keV
                       0.4




                                                                                                     Probability per MeV
                                                                                                                               0.01
                                                                                     1.0 MeV

                       0.3                                                           2.0 MeV
                                                                                                                              0.001
                                                                                     3.0 MeV
                       0.2
                                                                                                                             0.0001

                       0.1
                                                                                                                            0.00001
                        0
                             0   0.05   0.1   0.15   0.2   0.25   0.3   0.35   0.4     0.45    0.5                         0.000001
                                                      Energy (MeV)                                                                    0   0.2   0.4             0.6   0.8   1
                                                                                                                                                       Energy (MeV)




                          Simulated detector response                                                                                 Simulated detector response from
                         from different monoenergetic                                                                                 1.0 MeV gamma-rays in the three
                             electrons in BC400.                                                                                            scintillation layers.
Interaction Scenarios

                                Phoswich Detector
Scenario #


                 BC400
                         CaF2                       NaI:Tl


     1
     2
     3
     4
     5
     6
     7
Radiation Transport Study
Beta and Gamma Discrimination

  Scenario          Scintillation Layers          Total Probability (%)*        Pulse Recorded as:
              BC-400       CaF2       NaI:Tl     Gamma             Beta

     1                                             0.35            12.32               Beta
                ×
     2                                             0.07            81.70               Beta
                ×            ×
     3                                            14.40            4.60              Rejected
                             ×
     4                                             0.06            0.08              Rejected
                ×                          ×
     5                                             2.65            0.03              Rejected
                             ×             ×
     6                                             0.01            0.57              Rejected
                ×            ×             ×
     7                                            12.87            0.00              Gamma
                                           ×
    * Total probabilities are calculated for 1.0 MeV photon/electron events. Events with energies
                          less than 10 keV were excluded as electronic noise.
Data Acquisition System
            Phoswich Detector
                                                                                Gamma & Beta
                                                                               Gamma & Beta
                  BC400                 Scintillation Light Pulses
                Tc = 2.4 ns
                                                                                   Spectra
                                                                                  Spectra
                                                                                  ________                  Host PC
                                                                                 ________
                 CaF2:Eu
                Tc =900 ns                                                        MATLAB
                                                                                 MATLAB


                                                                                   Radiation Pulses are
                 NaI(Tl)
                                                                                 transferred to the PC by
               Tc = 230 ns
                                                                                   fixed-length digital
                                                                                          blocks



                PMT

                              Detector Output




                                                                          12                                  USB
                                                            ADC
              Preamplifier        Nyquist Filter        12 bits/100 MHz
                                                                               FPGA                         Processor

   Signal                                          12
    Out          DAC
                                                                                       Digital Pulse Processor
12-bit, 100 MHz Digital Pulse Processor



                   ADC     Preamplifier
                                           Input signal
USB Port                                    from anode




 FPGA




                                           Output signal
                                DAC       to oscilloscope
                  Data Acquisition System
                  Pulse Processing Algorithm                                       Baseline
                                                                                                                        (a) A typical beta-induced pulse
                                                                                               250


                                                                                               240

                                                                                                                   M2
                                                                                               230


             Four parameters, Baseline, P, M1 and M2                                           220
                                                                                                             M1


             are used for analyzing the anode pulses.                                          210




                                                                                   Amplitude
                                                                                               200
                                                                                                         P

                                                                                               190




                  Fast-Ratio (FR) = (P – M1) / P
                                                                                               180


                                                                                               170

                  Slow-Ratio (SR) = (M1 - M2) / M1                                             160


                                                                                               150
                                                                                                     0            500         1000                   1500   2000   2500
                                                                                                                                     Time (2 nsec)

                      A          B
                                (a) A typical beta-induced pulse
            250


            240


            230


            220


            210
                                                                                  Sums A and B used for calculating the
Amplitude




                                                                                  energy absorption in each layer of the
            200


            190


            180
                                                                                  phoswich detector.
            170


            160


            150
                  0       500         1000                   1500   2000   2500
                                             Time (2 nsec)
 Experimental Results
 Measurements using the Phoswich/DPP
 - Energy Calibration


Beta particles (Emax) from     The Compton edge and               The 662 keV and 1332 kev
14C and 99Tc were used for                                        photopeaks, respectively from
                               photopeak of 137Cs were used
BC400 calibration.                                                137Cs and 60Co, were used for
                               for CaF2(Eu) calibration.
                                                                  NaI(Tl) calibration.



                        99Tc
                                                                     137Cs   + 60Co
                                  137Cs      Compton Edge at
          Emax = 292 keV                                            Photopeak at           Photopeak at
                                                477 keV
                                                                      662 keV               1332 keV
                                                                                   Photopeak at
                                                   Photopeak at                     1172 keV
                        14C
                                                     662 keV
       Emax = 156 keV
Experimental Results
Measurements using the Phoswich/DPP
- Anode Signal Pulses


  Signal Pulses from 90Sr/90Y         Signal Pulses from 137Cs
Experimental Results
Measurements using the Phoswich/DPP
- Real-Time and Simultaneous Beta/Gamma Spectroscopy

                     Pure Beta Source: 99Tc
Experimental Results
Measurements using the Phoswich/DPP
- Real-Time and Simultaneous Beta/Gamma Spectroscopy

              (Beta-shielded) Gamma Source: 137Cs
Experimental Results
Measurements using the Phoswich/DPP
- Real-Time and Simultaneous Beta/Gamma Spectroscopy

             Mixed Beta/Gamma Field: 99Tc + 137Cs
Experimental Results
Measurements using the Phoswich/DPP
- Response to Gamma Sources (beta-shielded)

                         Percentage of events and simultaneous beta/gamma
                             energy spectra from (a) 137Cs and (b) 60Co


(a)                                                              (b)                                     18.2%

         91.6%                             6.6%
                                         1.8%                                                             2.1%
                                                                           79.7%




      Beta Recorded   Gamma Recorded   Pulse Rejected                  Beta Recorded   Gamma Recorded   Pulse Rejected




                       137Cs                                                                60Co



                                                FWHM (662 keV)
                                                   ~ 6.7%
Experimental Results
Measurements using the Phoswich/DPP
- Response to Pure Beta Sources

                                  Percentage of events and simultaneous beta/gamma
                                  energy spectra from (a)14C, (b) 99Tc and (c) 90Sr/ 90Y

                                                                                                                                      1.9%
(a)                                                (b)                                    0.6%         (c)
                                                                                          0.6%
    98.8%
                                                         98.8%                                                                               28.0%
                                                                                                             70.1%
                                      0.8%
                                      0.4%




                                                     Beta Recorded   Gamma Recorded   Pulse Rejected     Beta Recorded   Gamma Recorded   Pulse Rejected
 Beta Recorded   Gamma Recorded   Pulse Rejected




                    14C                                               99Tc                                                 90Sr/ 90Y
Radioxenon Decay Products
energy in units of keV


    Isotope        bmax              g            c.e.    X ray

     131m                          164            129      30
      133          346              81             45      31
     133m                          233            199      30
      135          901             250            214      31



                       g) ce, X) Triple Coincidence
                   (b, (b,or (ce, X) Double Coincidence
 Dual Phoswich for Coincidence Counting
 Utilizing both light coincidence and signal coincidence

                                             Sample
                         CaF2
                                   Out


Sig                                                           Sig


        PMT 1        NaI                              PMT 2
HV                                                            HV




                Quartz             In    Coupling
                           BC400          Sleve
133m
       Xe Coincidence
Probability: 0.115

          199 keV c.e.   30 keV Ka X-ray
 133
    Xe Triple Coincidence
 Probability: 0.017         45 keV c.e.



346 keV bmax                              31 keV Ka X-ray
Conclusions on Simultaneous Spectroscopy

    Prototype digital simultaneous beta/gamma spectrometer

    Customized digital pulse processor (12-bit, 100 MHz)

    Optimization through particle transport simulation (MCNP)

    Gamma resolution as low as 6.7% (@ 662 keV)

    < 2% false positives

    Better discrimination at lower energies

    Double/Triple coincidence detection

    High selectivity

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:3
posted:11/17/2012
language:English
pages:27