Transcript Mössbauer spectroscopy

Mössbauer spectroscopy
References:
J.P. Adloff, R. Guillaumont: Fundamentals of
Radiochemistry, CRC Press, Boca Raton, 1993
Mössbauer effect:
recoil free nuclear resonance absorption of g radiation
1,2
Recoil
Recoil
Recoil
ABSORPTION
EMISSION
E
E2
E photon
E1
E photon
E1
E photon=(E2-E1)+R
E photon=(E 2-E1)-R
0
1
-4
Energy
Line width (W) results from: natural width of E2 level + Doppler widening due to temp.
W ~10-6 eV (natural width) + 10-3 eV (temp. effect) → 10-3 eV (overall effect)
R~100 eV in nuclear processes, R’~10-7 eV in optical processes
Realization of nuclear resonance:
Source and absorber contain the same element
(same nuclear energy levels).
Reduction of R by embedding the isotope in a solid
crystal matrix,
cooling the sample (reduced oscillation of the
atoms, reduced R↔reduced W).
The missing part of „2R” energy can be provided
by moving the source due to Doppler effect
57
Co
271 days
10-9 sec
5/2- 137 keV
9%
91%
resonance
absorption
10-7 sec
3/2 14,4 keV
1/2- ground
57
Fe
emitter
57
Fe
absorber
Mössbauer spectrometer:
Source or absorber is moved. (Emission and absorption
spectroscopy, respectively.)
Source or absorber should be in ground state, non-magnetic,
symmetric environment precluding hyperfine splitting of nuclear level.
S source emitting weak g radiation
S
A
g
v
D
A absorber moving with velocity
v (mm/s)
D g radiation detector
The linear motion represents about 10-8 eV.
The resulting Es energy is derived from the Eg source energy:
Es= Eg(1±v/c)
The Mössbauer spectrum
Resonance absorption spectrum :g radiation intensity vs. velocity
(Energy)
0
count rate
-10
0
10
Typical Mössbauer emission spectrum
as the superposition of 2 single lines
according to magnetic splitting of the
nuclear levels in magnetic field:
chemical
shift
5/2-
237
Np
5/2+
-1,2
velocíty (mm/s); energy (10-7 eV)
1,2
count rate
Typical Mössbauer emission spectrum
as the superposition of 5 single lines
according to quadrupole splitting of the
nuclear levels in electric field:
5/2-
237
Np
5/2+
0
-10
0
10
-1,2
velocíty (mm/s); energy (10-7 eV)
20
30
Chemical information in Mössbauer spectra
Spectra reveal splittings of nuclear levels,
determined by the electronic environment.
• Isomer shift: position of the centroid of the line,
oxidation state, covalency of the bondings
• Quadrupole splitting: multiplets
asymetry in the electronic environment, chemical
spin state, intensity of ligand field
• Magnetic splitting: multiplet due to magnetic field
Mössbauer active atoms
• 75 transitions in isotopes of 44 elements
• Radionuclide: MBq activity
alpha, beta, EC or IT
T1/2: hours-hundreds of years
• Conditions to be fulfilled:
- Eg<100 keV,
- emitter should be bound in a lattice
- mean life-time of excited level: 1 ns-100 ns
- solid, cooled absorber (liquid N2), m>100mg
E.g.: 57Co(EC)57Fe: 14,4 keV
241Am(alpha)237Np: 60 keV
Tc, Th, Pa, U, Np, Pu, Am
Application examples
• Analysis of steels:
oxidation state of iron (+2 or +3)
chemical form (oxide, sulfate…)
magnetic properties
• Analysis of iron oxide layers
magnetite, hematite
• Recoil processes in condensed material
• Oxidation states of Np, Am compounds
Other nuclear related methods providing
information on chemical environment
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Positron annihilation spectrometry
Muon spectrometry
Nuclear magnetic resonance
Electron spectroscopies:
photoelectron spectroscopy
conversion electron spectroscopy
Auger electron spectroscopy