Chapter 9. Nuclear Analysis Methods

1.Neutron Activation Analysis 2.Accelerator Mass Spectrometry 3.Mössbauer Spectroscopy 4.Ion Beam Analysis

2

The Neutron Activation Analysis Method

Block diagrams of the NAA method

Neutron Source

Material for NAA

radioactive nuclides

Prompt g rays

b rays

Gamma-ray spectrometer

Data analysis and results reporting

Material original

g rays

中子活化分析

3

Chinese NAA Facilities

China Institute of Atomic Energy 中国原子能科学研究院Institute of High Energy Physics, Chinese Academy of Science中国科学院高能物理研究所…

What is NAA?

• Hit source with neutrons

• Sources become radioactive• Then decay in predictable ways

The activity A of the sample increases with bombarding time t

How?• Detect the gamma-rays (prompt and delayed) - with gas

detector, scintillators, semiconductors• Bin number of counts at each energy

An example of gamma-ray spectrum from the activation of a human nail

Neutron sources

• A nuclear reactor• A source that emits

neutrons by fission (e.g. Californium)

• Alpha Source (like Radium) with Beryllium

• D-T fusion in a gas discharge tube

Applications

• Determine the chemical composition of a sample

• Lunar samples, artifacts, forensics• Can identify up to 74 different elements in

gases, liquids, solids, and mixtures• Can also determine the concentration of the

elements of interest:

Advantages

• Small sample sizes (.1mL or .001gm)• Non-destructive• Can analyze multiple element samples• Doesn’t need chemical treatment• High sensitivity, high precision

Sensitivity (picograms) Elements

1 Dy, Eu

1–10 In, Lu, Mn

10–100 Au, Ho, Ir, Re, Sm, W

100–1000 Ag, Ar, As, Br, Cl, Co, Cs, Cu, Er, Ga, Hf, I, La, Sb, Sc, Se, Ta, Tb, Th, Tm, U, V, Yb

1000–104 Al, Ba, Cd, Ce, Cr, Hg, Kr, Gd, Ge, Mo, Na, Nd, Ni, Os, Pd, Rb, Rh, Ru, Sr, Te, Zn, Zr

104–105 Bi, Ca, K, Mg, P, Pt, Si, Sn, Ti, Tl, Xe, Y

105–106 F, Fe, Nb, Ne

107 Pb, S

Limitations

• Interferences can still occur when different component sample elements produce similar gamma rays.

• The detection limit

• bulk matrix

Take sample to the rabbit system apparatus

• The rabbit system works much like the system used by banks at drive-through windows. A canister carries items back and forth between the customer and teller.

• The sample is sent through the wall in a mini canister into the nuclear reactor located behind the wall.

• Once inside the reactor, the sample is irradiated with neutrons.

• After irradiation of the sample in the capsule, and before removing it from the reactor site, it must be determined if the capsule is safe for transfer. A Geiger counter is used to assess whether the radioactive decay has reached low enough levels to be safe.

The prepared sample and standard sample are placed in a “detector” one at a time.

• The detector system counts and records gamma radiation emissions for a period of time.

• Time varies, but is

usually in the range of 5 minutes to an hour.

Specialized software analyzes radiation peaks. Peak data is correlated to specific elements for identification and quantification.

16

Neutron Activation Analysis (NAA)Neutron activation analysis is a multi-, major-, minor-, and trace-element analytical method for the accurate and precise determination of elemental concentrations in materials.

Sensitivity for certain elements are below nanogram level.

The method is based on the detection and measurement of characteristic gamma rays emitted from radioactive isotopes produced in the sample upon irradiation with neutrons.

High resolution germanium semiconductor detector gives specific information about elements.

2. Accelerator Mass Spectrometry

-prior to AMS samples were 14C-dated by counting the number of decays - required large samples and long analysis times

-1977: Nelson et al. and Bennett et al. publish papers in Science demonstrating

the utility of attaching an accelerator to a conventional mass spectrometer

Principle:You cannot quantitatively remove interferring ions to look

for one 14C atom among several quadrillion C atoms.Instead, you a) destroy molecular ions (foil or gas)b) filter by the energy of the ions (detector) to separate

the needle in the haystack.

a) ION SOURCEgenerates negative

carbon ionsby Cs sputtering

b) INJECTOR MAGNETseparates ions by mass,

masses 12, 13, and 14 injected

http://www.physics.arizona.edu/ams/education/ams_principle.htm

c) ACCELERATORgenerates 2.5 million volts,

accelerates C- ions

d) TERMINALC- ions interact with

‘stripper’ gas Ar,become C+ ions,

molecular species CHdestroyed

e) ELECTROSTATIC DEFLECTORspecific charge of ions selected (3+)

f) MAGNETIC SEPARATION13C steered into cup, 14C

passes through to solid detector

g) Si BARRIER DETECTORpulse produced is proportional to the energy of ion, can

differentiate b/t 14C and other ions count rate for modern sample = 100cps

AMS measurement capabilities.

Hurdles in mass spectrometry

1) Abundance sensitivity - ratio of signal at massm to signal at m+1

- better with better vacuum- acceptable values: 1-3ppm at 1amu

2) Mass discrimination

- heavier atoms not ionized as efficiently as light atoms

- can contribute 1% errors to isotope values

- can correct with known (natural) isotope ratios within run, or with known standards between runs

3) Dark Noise - detector will register signal even without an ion beam

- must measure prior to run to get “instrument blank” if needed

4) Detector “gain” - what is the relationship between the electronic signal recorded

by the detector and the number of ions that it has counted?

- usually close to 1 after factory calibration- changes as detector “ages”- must quantify with standards

Cardinal rule of mass spectrometry:Your measurements are only as good as your STANDARDS!

Hurdles in mass spectrometry (cont.)

Chapter 9. Nuclear Analysis Methods

1.Neutron Activation Analysis 2.Accelerator Mass Spectrometry 3.Mössbauer Spectroscopy 4.Ion Beam Analysis 5.Synchrotron Radiation Facility

Em ission Absorption

R ecoil

Free emitting and absorbing atoms

mcE=E 2

2

R2

gEnergy of recoil

γ-ray energy

Mass of atom

Em ission Absorption

N o recoil

McE=E 2

2

R2

g

Emitting and absorbing atoms fixed in a lattice

Mass of particle

Mössbauer spectroscopy is the recoil-free emission and absorption of gamma rays

Appearance of Mössbauer spectraDepending on the local environments of the Fe atoms and the magnetic properties, Mössbauer spectra of iron oxides can consist of a singlet, a doublet, or a sextet.

Symmetric chargeNo magnetic field

Asymmetric chargeNo magnetic field

Symmetric or asymmetric charge Magnetic field (internal or external)

Δ Bhfδ Isom

er s

hift

Qua

drup

ole

split

ting

Mag

netic

hyp

erfin

e fie

ld

Use of Mössbauer spectroscopy as a “fingerprinting” technique

1.0 1.5

1

-0.5 0.5Isomer shift (mm/s)

0

2

3

4

0.0

[6]Fe(II)

[6]Fe(III)[6]Fe3+

[4]Fe3+

[6]Fe2+

[4]Fe2+

[sq]Fe2+

[8]Fe2+

[5]Fe3+

[5]Fe2+

Isomer shifts and quadrupole splittings of Fe-bearing phases vary systematically as a function of Fe oxidation, Fe spin states, and Fe coordination.Knowledge of the Mössbauer parameters can therefore be used to “fingerprint” an unknown phase.

Elements of the periodic table which have known Mössbauer isotopes (shown in red font). Those which are used the most are shaded with black

Strengths and weaknesses of 57Fe Mössbauer spectroscopy

• Sensitive only to 57Fe(no matrix effects)

• Sensitive to oxidation state• Allows distinction of

magnetic phases• Very sensitive towards

magnetic phases• Non-destructive• Resolution limited by

uncertainty principle

• Sensitive only to 57Fe(“sees” only 57Fe)

• Coordination ? to ±• Paramagnetic phase data

often ambiguous• Diamagnetic element

substitution & relaxation• Slow• If possible, use other

techniques as well

Strengths Weaknesses

Often a combination of Mössbauer spectroscopy with other techniques can help solve problems that cannot be resolved using Mössbauer spectroscopy alone.

Chapter 9. Nuclear Analysis Methods

1.Neutron Activation Analysis 2.Accelerator Mass Spectrometry 3.Mössbauer Spectroscopy 4.Ion Beam Analysis RBS, PIXE5.Synchrotron Radiation Facility

Rutherford Backscattering (RBS) is• Elastic scattering of protons, 4He, 6,7Li, ...≠ Nuclear Reaction Analysis (NRA): Inelastic scattering, nuclear reactions≠ Detection of recoils: Elastic Recoil Detection Analysis (ERD)≠ Particle Induced X-ray Emission (PIXE)≠ Particle Induced γ-ray Emission (PIGE)• RBS is a badly selected name, as it includes:- Scattering with non-Rutherford cross sections- Back- and forward scattering• Sometimes called Particle Elastic scattering Spectrometry (PES)

History

History

1970’s: RBS becomes a popular method due to invention of silicon solid state detectors• 1977: H.H. Andersen and J.F. ZieglerStopping Powers of H, He in All Elements• 1977: J.W. Mayer and E. RiminiIon Beam Handbook for Materials Analysis• 1979: R.A. JarjisNuclear Cross Section Data for Surface Analysis• 1985: M. ThompsonComputer code RUMP for analysis of RBS spectra• 1995: J.R. Tesmer and M. NastasiHandbook of Modern Ion Beam Materials Analysis• 1997: M. MayerComputer code SIMNRA for analysis of RBS, NRA spectra

i, Kinematic factor:

2. RBS

0

1E

E

)cos1]()(2[1 22121 cMMMM

2

2

1

2

121

22

2

1

)(1

cos)(]sin)(1[

MM

MM

MM

)1()1(

])1()1([

2

1

2

1

2

2

1

2

1

MM

MM

MM

MM o180

o90

ii,

?21 MM

Rutherford Cross Section

Mass Resolution

X-ray production cross-section su

su is a product of three factors:

su = sS • wS • ru

where sS = S-shell ionization cross section (S = K, L, M, …),wS = fluorescence yield,ru = transition probabiliy.

4.2 PROTON INDUCED X-RAY EMISSION (PIXE)

A PIXE spectrum consists of two components:

peaks due to characteristic X-rays, &a background continuum,

PIXE spectrum of the above tungsten rich area

X-ray energy (keV)

2 4 6 8 10 12 14

X-r

ay c

ount

s

1

10

100

1000PS

Cl

K

CaK

CaKb

FeK

FeKb

WL

WLb

WLg

WLb

WLl

TiK

TiKb

X-ray energy (kev)

X-ra

y co

unts

as can been seen from the spectrum, which is obtained from a lung tissue taken from a patient suffered from hard metal lung disease:

protons

Si(Li)

X-ray

X-ray

X-ray

X-rayTarget

Eo

Where = solid angle subtended by the detector at the target, Np = number of incident protons that hit the specimen, nz = number of sample atoms per unit area of the specimen, z is the atomic number of the element, su = production cross-section for u - line x-rays, eu = detection efficiency for u - line x-rays.

The X-ray yieldThe number of counts under the X-ray peak corresponding to the principal characteristic X-ray line of an element is called the yield (Yu ) for the u - line. It is a product of 5 quantities:

Yu = • Np• nz• su• eu 4

YiYiYi NN /YY

YiYiYi NN /YQY

The knowledge on the number of protons that hit the specimen in a PIXE measurement is required for quantitative PIXE analysis. Np can be measured directly or indirectly.

As protons are positively charged, Np is often measured by charge integration and quoted in units of micro-Coulombs (or mC). The charge carried by a proton is 1.60210 x 10-19 Coulombs. The charge carried by Np protons is therefore:

DETERMINATION OF Np

Qp = 1.60210 x 10-13 x Np mC

DETERMINATION OF Np

Use of a Faraday cup coupled to a charge integrator.Use of rotating vane or chopper which periodically intercepts and

samples the proton beam.

Measuring the back-scattered protons from the specimen

The detection efficiency of a Si(Li) X-ray detector is dependent on the X-ray energy. It is usually determined theoretically using the parameters (i.e. thicknesses of Si diode, Be window, gold contact and Si dead layer) provided by the detector manufacturer. However, calibration standards (targets containing one or more elements of known concentrations) are often used to determined e experimentally.

Detection efficiency e

X-ray Energy (keV)0 2 4 6 8 10 12

dsdE

1000

100

10

1

SEB

p-bremsstrahlung

Total

SOURCES OF BACKGROUND

Ee=4meE/M

1. bremsstrahlung

（ P， γ）Cosmic raysInsulating samplesSubstrate

SOURCES OF BACKGROUND

DETECTION LIMITS

The detection limits for the various elements in PIXE are determined by the sensitivity factors on one hand, and on the other hand by the spectral background intensity where the element signal is expected. It is now a general practice to define the detection limit, DLz for an element Z as the amount of the element that gives rise to a net peak intensity equal to 3 times the standard deviation of the background intensity, NB, in the spectral interval of the principal X-ray line, i.e.

DLz = 3 std. dev. (NB) 3 (NB)½

Absorption filter

multi-elemental？

brain tissue PIXE spectrum

La

multi-elemental？

Urine PIXE spectrUM from people exposed to Cd.

Cd K Cd Kα: 23.17 keV detection efficiency is low

L α: 3.13 keV

K Kα: 3.31 keV

Cd peak is hard to identify by PIXE.Detected by GFAS as ~10 ppm/CR

(PIXE)

• Physics/cross sections• Experimental• Software developm.• Complementary/ competing methods• Bio-PIXE

To preserve the health of human, animal and plant, how do we apply PIXE

Ion beam analysis

卢瑟福背散射离子激发 X射线分析

核反应分析

离子发光扫描透射显微术

离子束感生电荷弹性反冲分析

二次电子