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

2. 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

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
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

4. 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

5. 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

6. 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

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
N Activation RadGraphy
W. Udo Schröder, 2010

8. 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

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
N Activation RadGraphy
W. Udo Schröder, 2010

10. 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

11. 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= MeV 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

12. 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

13. Observing a Finite Lifetime of the 198Au g.s.
E. Norman et al.,
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

14. 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

15. Fin
N Activation RadGraphy
W. Udo Schröder, 2010