N Activation RadGraphy W. Udo Schröder, 2010 1. Reactor Neutrons N Activation RadGraphy W. Udo Schröder, 2010 2 The Australian OPAL is an open-pool type

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W. Udo Schröder, 2010

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

eV


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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

nsi

ty

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

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Neutron Capture Cross Sections

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Therm

al

Regio

n

Therm

al

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

http://environmentalchemistry.com/yogi/periodic/crosssection.html

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


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

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s M

eas


1

2

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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y D

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


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

23

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

Documents

N Activation RadGraphy W. Udo Schröder, 2010 1. Reactor Neutrons N Activation RadGraphy W. Udo Schröder, 2010 2 The Australian OPAL is an open-pool type