n activation radgraphy w. udo schröder, 2010 1. reactor neutrons n activation radgraphy w. udo...
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1 Nuclear Forensics: Neutron
Activation & Radiography
Reactor Neutrons
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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
1 M
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Commercial Neutron Generator ING-03
Source: All-Russian Research Institute of Automatics VNIIA
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, En14.5 MeV
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
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A) Attenuation of incident beam
B) Production of sample- characteristic secondary radiation.
1. g-rays (n, g)2.charged particles (n, a),…3.neutrons (n, n’)4.fission fragments (n, f)(5. b± continuous spectrum, not very characteristic)
Principle of Neutron Imaging/Radiography
time
inte
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Primary neutrons
Transmitted/secondary radiation
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.
sample
Incident TransmittedNeutron interactions with nuclei in sample
Fast-Neutron Radiography
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Example of radiography with
fast neutronsImages of
electrical switch with color
enhancement. (After: Nucl. Eng. UT Austin)
(After: Goldhaber)
Principle of Thermal-Neutron Activation Analysis
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Delayed deexcitation Gamma ray
(N+1,Z)*
(N,Z)+nTarget in g.s.
(N+1,Z)+g
b-
(N+1,Z+1)+b-
+g
Relative to n capture: Prompt g Delayed b-
Delayed gFinal daughter nucleus in g.s.E
nerg
y
Neutron Capture Cross Sections
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Therm
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Regio
n
Therm
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Regio
n
Resonance Region
Resonance Region
IAEA Public Data Compilation
N capture cross section is En dependent.Low-energy neutrons captured easily large cross sectionNarrow quantal capture resonances associated with nuclear structure.
Gauge magnitude relative to geometrical cross section
2
1 3 2
( ); .
1.2 5 ;1 100
geo R b R nucl radius
R A fm b fm
105
10-5
Thermal-Neutron Capture Cross sections
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http://environmentalchemistry.com/yogi/periodic/crosssection.html
Gd n capture cross section = 550x geometrical cross section
Largest cross sections for lowest (=thermal) n energies used for NAA.Al can used for normalization
Neutron Spectra
Neutron spectra are too hardNot optimal for neutron capture Moderate n energies to thermal Use p-rich moderators
(water, paraffin, plastics; ~15 cm)
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PuBe Neutron Source
0
0
( )
2
NE N E e
E MeV
x-= ×
=
Nuclear Decay
Activation and Decay
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Competition production/decay for a species with N(t) members
t tPN(t ) 1 e A(t ) P 1 e
dN(t )N(t ) P
dt
Irradiation of sample produces unstable nucleus.
Constant rate of production PConstant decay rate lActivity A= l·N
Gain- Loss Differential Equation
t
For t :
A(t ) P 1 e P
Irradiation inefficient for t > 3 t
lN
N
PIrradiation of Sample
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Example: 51V Time Dependent NAA
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Irradiate 51V with thermal neutrons, daughter b-decays to 52Cr*
51 52 5223 23 24V n V Cr e
1 21
t t
det
2det
t tt0det
I t I 0 e I 0 e lifetime 1
IN ; f rel. popul.
f
area 4 r ; r distance source det .
AN e e
Integrated g intensity activity A, number of active nuclei in sample.
A(t)/P vs. t
t=0
Irradiate from t=0 to t. Wait time t-t1
Conduct Ig measurement from t1 to t2
Extrapolate to I g (t) A to A0=A(t)
Fit Curve
52Cr* de-excites by g –ray emission Eg= 1.4336MeV
I g
Pri
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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
EnergyDiscriminator
TimeTrigger
Start
Stop
External Time reference signal t0
Detector
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.
0 100 200 300 400.0.01
0.1
1
i
Dt
Act
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A(D
t)/D
A(t
0)
Dt (Channel #)
t
A( t ) exp
Nuclear Decay
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Observing a Finite Lifetime of the 198Au g.s.
E. Norman et al., http://ie.lbl.gov/radioactivedecays/page2
411.8 keV
411.8 keV
Spectrum of b delayed 198Au g-rays
Spectrum of b delayed 198Au g-raysg decay of 198Hg exc. state is prompt: t g tb
11 measurementsEach spectrum ran for 12 hours real time#11 taken 5 days after #1
# 1
# 11
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Thermal Neutron Flux and Saturation Factor
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Irradiate sample with thermal neutrons for series of times t, measure sample activity A(t)
A(t)/P vs. t
t0 st
t
A (t ) P 1 e P : A
Saturation factor 1 e
sample0abund c n
s molar
ts
sample
abund
molar
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c
n
MA (t )f L
A M
A 1 e
M sample mass
f relative abundance
M molar weight
L 6.023 10 atoms mole
cross section (area)
n flux (n' s sec area )
27
24 2c
Normalization Al n,
0.21b, 1b 10 cm
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Fin