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Nuclear Diagnostics & Forensics: Neutron Activation & Radiography

Nuclear Diagnostics & Forensics: Neutron Activation & Radiography. Reactor Neutrons. 1 MeV. The Australian OPAL is an open-pool type research reactor fuelled by low enriched fuel operating at a core thermal power of 20MW. . Reactor Neutron Energy Spectrum.

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Nuclear Diagnostics & Forensics: Neutron Activation & Radiography

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  1. Nuclear Diagnostics & Forensics: Neutron Activation & Radiography W. Udo Schröder, 2012

  2. Reactor Neutrons 1 MeV The Australian OPAL is an open-pool type research reactor fuelled by low enriched fuel operating at a core thermal power of 20MW. Reactor Neutron Energy Spectrum W. Udo Schröder, 2012

  3. Commercial Neutron Generator ING-03 Neutrons can be produced in a variety of reactions, e.g., in nuclear fission reactors or by the D(d,n)3He or T(d,n)4He reactions 1300 mm rear connectors source window Specs: < 3·1010 D(d,n)3He neutrons/s Total yield 2·1016 neutrons Pulse frequency 1-100Hz Pulse width > 0.8 ms, Power 500 W Alternative option: T(d,n)4He, En14.5 MeV Source: All-Russian Research Institute of Automatics VNIIA W. Udo Schröder, 2012

  4. A) Attenuation of incident beam B) Production of sample- characteristic secondary radiation. g-rays (n, g) charged particles (n, a),… neutrons (n, n’) fission fragments (n, f) (5. b± continuous spectrum, not very characteristic) Primary neutrons intensity Transmitted/secondary radiation time Principle of Neutron Imaging/Radiography IncidentTransmitted Neutron interactions with nuclei in sample sample Transmitted or secondary radiation induced by neutrons in the sample appear with the same frequency as the neutron pulses. Special detectors for characteristic secondary radiation/conditions enhance recognition of sample material. W. Udo Schröder, 2012

  5. Fast-Neutron Radiography Example of radiography with fast neutrons Images of electrical switch with color enhancement. (After: Nucl. Eng. UT Austin) • (After: Goldhaber) W. Udo Schröder, 2012

  6. Principle of Thermal-Neutron Activation Prompt Gamma rays,particles Delayed deexcitation Gamma rays Final Nucleus in Ground State Relative to n capture: Prompt g Delayed b-Delayed g Final daughter nucleus in g.s. (N,Z)+n Target in g.s. Energy b- (N+1,Z)+g (N+1,Z)* (N+1,Z+1)+b-+g W. Udo Schröder, 2012

  7. Neutron Capture Cross Sections IAEA Public Data Compilation Resonance Region 105 Thermal Region 10-5 N capture cross section is En dependent. Low-energy neutrons captured easily  large cross section Narrow quantal capture resonances associated with nuclear structure. Gauge magnitude relative to geometrical cross section Resonance Region Thermal Region W. Udo Schröder, 2012

  8. Thermal-Neutron Capture Cross sections Largest cross sections for lowest (=thermal) n energies  used for NAA.Al can used for normalization Gd n capture cross section = 550x geometrical cross section W. Udo Schröder, 2012 http://environmentalchemistry.com/yogi/periodic/crosssection.html

  9. Neutron Spectra Neutron spectra are too hard Not optimal for neutron capture • Moderate n energies to thermal • Use p-rich moderators (water, paraffin, plastics; ~15 cm) PuBe Neutron Source W. Udo Schröder, 2012

  10. Activation and Decay Competition production/decay for a species with N(t) members Irradiation of sample produces unstable nucleus.Constant rate of production P Constant decay rate lActivity A= l·N N P lN Gain- Loss Diff. Equ. Irradiation of Sample Irradiation inefficient for t > 3 t W. Udo Schröder, 2012

  11. Example: 51V Time Dependent NAA Irradiate 51V with thermal neutrons, daughter b-decays to 52Cr* 52Cr* de-excites by g –ray emission Eg= 1.4336MeV A(t)/P vs. t t=0 • Irradiate from t=0 to t. Wait time (t1 – t) • Conduct Ig measurement from t1 to t2 • Convert Ig to A, Extrapolate back to A0 Ig Fit Curve Integrated g intensity activity A, number of active nuclei in sample. W. Udo Schröder, 2012

  12. Produce analog signal  Detector Energy Slow Source Distance r Binary data to computer Amp PreAmp Data Acquisition System Fast EnergyDiscriminator DE-Tag External Time reference signal t0 electron. Clock(TAC) Start Stop Produce timing signal  Time Trigger Activity DA(Dt)/DA(t0) Dt (Channel #) 1 i 0.1 . 0.01 0 100 200 300 400 Dt Measuring “Decay Curves”:Fast-Slow Signal Processing W. Udo Schröder, 2012 Measured: Energy and time of arrival Dt=t-t0 (relative to an external time-zero t0) for radiation (e.g., g-rays), energy discriminator to identify events (DA) in a certain energy interval DE by setting an identifier “tag.” Calibrate Dt axis channel #  time units (s, y,..)Watch that Dt-channel  t.

  13. Observing a Finite Lifetime of the 198Au g.s. E. Norman et al., http://ie.lbl.gov/radioactivedecays/page2 Spectrum of b delayed 198Au g-rays 411.8 keV # 1 Spectrum of b delayed 198Au g-rays • decay of 198Hg exc. state is prompt: tg  tb 11 measurementsEach spectrum ran for 12 hours real time#11 taken 5 days after #1 # 11 411.8 keV W. Udo Schröder, 2012

  14. Thermal Neutron Flux and Saturation Factor Irradiate samplewith thermal neutrons for series of times t, measure sample activity A(t) A(t)/P vs. t A/P (%) W. Udo Schröder, 2012

  15. Fin W. Udo Schröder, 2012

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