Nuclear Forensics: Neutron Activation & Radiography N Activation RadGraphy W. Udo Schröder, 2010

Reactor Neutrons N Activation RadGraphy W. Udo Schröder, 2010 The Australian OPAL is an open-pool type research reactor fuelled by low enriched fuel operating at a core thermal power of 20MW. 1 MeV Reactor Neutron Energy Spectrum N Activation RadGraphy W. Udo Schröder, 2010

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 N Activation RadGraphy Alternative option: T(d,n)4He, En14.5 MeV Source: All-Russian Research Institute of Automatics VNIIA W. Udo Schröder, 2010

Principle of Neutron Imaging/Radiography Neutron interactions with nuclei in sample sample Incident Transmitted 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) time intensity Primary neutrons Transmitted/secondary radiation Transmitted or secondary radiation induced by neutrons in the sample appear with the same frequency as the neutron pulses. N Activation RadGraphy Special detectors for characteristic secondary radiation/conditions enhance recognition of sample material. W. Udo Schröder, 2010

Fast-Neutron Radiography Example of radiography with fast neutrons Images of electrical switch with color enhancement. (After: Nucl. Eng. UT Austin) N Activation RadGraphy (After: Goldhaber) W. Udo Schröder, 2010

Principle of Thermal-Neutron Activation Analysis Delayed deexcitation Gamma ray (N+1,Z)* (N,Z)+n Target in g.s. (N+1,Z)+g b- (N+1,Z+1)+b-+g Relative to n capture: Prompt g Delayed b- Delayed g Final daughter nucleus in g.s. Energy N Activation RadGraphy W. Udo Schröder, 2010

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 N Activation RadGraphy W. Udo Schröder, 2010

Thermal-Neutron Capture Cross sections Largest cross sections for lowest (=thermal) n energies  used for NAA. Al can used for normalization N Activation RadGraphy Gd n capture cross section = 550x geometrical cross section W. Udo Schröder, 2010 http://environmentalchemistry.com/yogi/periodic/crosssection.html

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 N Activation RadGraphy W. Udo Schröder, 2010

Activation and Decay N P lN Competition production/decay for a species with N(t) members Irradiation of sample produces unstable nucleus. Constant rate of production P Constant decay rate l Activity A= l·N Gain- Loss Differential Equation lN N P Irradiation of Sample W. Udo Schröder, 2007 Irradiation inefficient for t > 3 t Nuclear Decay

Example: 51V Time Dependent NAA Irradiate 51V with thermal neutrons, daughter b-decays to 52Cr* A(t)/P vs. t t=0 Fit Curve 52Cr* de-excites by g –ray emission Eg= 1.4336MeV Ig Irradiate from t=0 to t. Wait time t-t1 Conduct Ig measurement from t1 to t2 Extrapolate to Ig (t) A to A0=A(t) N Activation RadGraphy Integrated g intensity  activity A, number of active nuclei in sample. W. Udo Schröder, 2010

Measuring “Decay Curves”: Fast-Slow Signal Processing Source Distance r Slow Fast PreAmp Amp Produce timing signal  electron. Clock (TAC) Data Acquisition System Energy DE-Tag Produce analog signal  Binary data to computer Energy Discriminator Time Trigger Start Stop External Time reference signal t0 Detector Principles Meas 100 200 300 400 . 0.01 0.1 1 i Dt Activity DA(Dt)/DA(t0) Dt (Channel #) 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. W. Udo Schröder, 2004

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 decay of 198Hg exc. state is prompt: tg  tb 11 measurements Each spectrum ran for 12 hours real time #11 taken 5 days after #1 Spectrum of b delayed 198Au g-rays W. Udo Schröder, 2007 # 11 411.8 keV Nuclear Decay

Thermal Neutron Flux and Saturation Factor Irradiate sample with thermal neutrons for series of times t, measure sample activity A(t) A(t)/P vs. t N Activation RadGraphy W. Udo Schröder, 2010

Fin N Activation RadGraphy W. Udo Schröder, 2010