Chapter 9. Nuclear Analysis Methods
1.Neutron Activation Analysis
2.Accelerator Mass Spectrometry
3.Mössbauer Spectroscopy
4.Ion Beam Analysis
The Neutron Activation Analysis Method
中子活化分析
Neutron
Source
Material
for NAA
Prompt g rays
b rays
radioactive
nuclides
Material
original
g rays
Gamma-ray
spectrometer
Data analysis and
results reporting
Block diagrams of the NAA method
2
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
…
3
What is NAA?
• Hit source with neutrons
The activity A of the sample
increases with bombarding time t
• Sources become radioactive
• Then decay in predictable ways
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.
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.
16
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.
c) ACCELERATOR
generates 2.5 million volts,
accelerates C- ions
b) INJECTOR MAGNET
separates ions by mass,
masses 12, 13, and 14 injected
a) ION SOURCE
generates negative
carbon ions
by Cs sputtering
d) TERMINAL
C- ions interact with
‘stripper’ gas Ar,
become C+ ions,
molecular species CH
destroyed
e) ELECTROSTATIC DEFLECTOR
specific charge of ions selected (3+)
f) MAGNETIC SEPARATION
13C steered into cup, 14C
passes through to solid detector
g) Si BARRIER DETECTOR
pulse produced is proportional to the energy of ion, can
differentiate b/t 14C and other ions count rate for modern
sample = 100cps
http://www.physics.arizona.edu/ams/education/ams_principle.htm
AMS measurement capabilities.
Hurdles in mass spectrometry
1) Abundance sensitivity - ratio of signal at mass
m 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
Hurdles in mass spectrometry (cont.)
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!
Chapter 9. Nuclear Analysis Methods
1.Neutron Activation Analysis
2.Accelerator Mass Spectrometry
3.Mössbauer Spectroscopy
4.Ion Beam Analysis
5.Synchrotron Radiation Facility
Free emitting and absorbing atoms
Emission
Absorption
Recoil
Energy of recoil
2
g
E
ER =
2
2mc
γ-ray energy
Mass of atom
Emitting and absorbing atoms fixed in a lattice
Emission
Absorption
No recoil
2
Mössbauer spectroscopy is the grecoil-free emission
R
and absorption of gamma rays 2
E
E =
2Mc
Mass of particle
Appearance of Mössbauer spectra
Symmetric charge
No magnetic field
Asymmetric charge
No magnetic field
Bhf
Magnetic hyperfine field
Δ
Quadrupole splitting
δ
Isomer shift
Depending 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 or asymmetric charge
Magnetic field (internal or external)
Use of Mössbauer spectroscopy as a
“fingerprinting” technique
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.
4
Fe2+
[6]
3
[5]
Fe3+
[4]
2
[6]
Fe3+
Fe(III)
[4]
Fe2+
[8]
Fe2+
Fe3+
[6]
[5]
Fe2+
1
[6]
Fe(II)
0
-0.5
[sq]
0.0
Fe2+
0.5
Isomer shift (mm/s)
1.0
1.5
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
Strengths
• 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
Weaknesses
• 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
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, PIXE
5.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. Ziegler
Stopping Powers of H, He in All Elements
• 1977: J.W. Mayer and E. Rimini
Ion Beam Handbook for Materials Analysis
• 1979: R.A. Jarjis
Nuclear Cross Section Data for Surface Analysis
• 1985: M. Thompson
Computer code RUMP for analysis of RBS spectra
• 1995: J.R. Tesmer and M. Nastasi
Handbook of Modern Ion Beam Materials Analysis
• 1997: M. Mayer
Computer code SIMNRA for analysis of RBS, NRA spectra
2. RBS
i, Kinematic factor:
  E1 E
0
  1  [2 M 1M 2 ( M 1  M 2 ) 2 ](1  cos c )


M
M
2
2
1
1
[1  ( M ) sin  ]  ( M ) cos 
2
2


M
1 ( 1
)


M
2


1
2
ii,
[(1  M 1
) (1  M 1
)]2
M2
M2


(1  M 1
) (1  M 1
)
M
M
2
2

2
  180o
  90o
M1  M 2 ?
Rutherford Cross Section
Mass Resolution
4.2 PROTON INDUCED X-RAY EMISSION (PIXE)
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.
A PIXE spectrum consists of two components:
peaks due to characteristic X-rays, &
a background continuum,
as can been seen from the
spectrum, which is obtained
from a lung tissue taken
from a patient suffered from
hard metal lung disease:
CaK
Cl
1000
PS
K
WL
FeK
CaKb
TiK
100
WLb
FeKb
WLg
TiKb
WL
X-ray counts
X-ray counts
WLb
l
10
Si(Li)
X-rayTarget X-ray
1
2
4
6
8
10
12
14
X-ray energy (keV)
PIXE spectrum of the above tungsten rich area
X-ray energy (kev)
Eo
protons
X-ray
X-ray
The X-ray yield
The 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:
Yi  iY YY Ni / NY

Yu = 4 • Np• nz• su• eu
Yi  QiY YY N i / NY
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.
DETERMINATION OF Np
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:
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
Detection efficiency e
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.
SOURCES OF BACKGROUND
1. bremsstrahlung
1000
Total
ds 100
dE
SEB
Ee=4meE/M
10
p-bremsstrahlung
1
0
2
4
6
8
X-ray Energy (keV)
10
12
SOURCES OF BACKGROUND
（P，γ）
Cosmic rays
Insulating samples
Substrate
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？
La
brain tissue PIXE spectrum
multi-elemental？
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.
Urine PIXE spectrUM from people exposed to Cd. 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
二次电子
离子发光
扫描透射显微术
弹性反冲分析
离子束感生电荷
卢瑟福背散射
核反应分析
离子激发X射线分析