IAMP
Nuclear Physics Lab
Institute for Atomic and Molecular Physics
JUSTUS-LIEBIGUNIVERSITÄT
GIESSEN
Leihgesterner Weg 217 (Strahlenzentrum), room 14
Versuch 6 — Mössbauer Effect
The Mössbauer effect is used for the determination of the natural line width of the γ transition from the first excited state to the ground state of 57 Fe. From the natural line width
the lifetime of the first excited state is inferred. Furthermore, the Mössbauer effect is used
for the determination of this state’s magnetic moment and its quadrupole splitting for Fe in
FeSO4 · 7H2 O.
1
References
• K. S. Krane, Introductory Nuclear Physics (John Wiley, New York, 1988) chapter 10.9.
• Manuals of the electronic modules used.
• Web pages of the Nuclear Physics Lab
http://www.strz.uni-giessen.de/amp/NucPhysLab.htm.
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Questions
• What is the Mössbauer effect? How can it be explained?
• How can the Mössbauer effect be studied?
• What is the Debye-Waller factor?
• What kind of nuclei are suitable for the observation of the Mössbauer effect?
• Which conditions must be fulfilled by the γ counter? (57 Co level scheme)
• What is the magnetic hyperfine splitting?
• What is the electric quadrupole splitting?
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Experimental procedure
3.1
•
List of equipment
57
Co single line Mössbauer source with lead shielding and electromagnetic drive (loudspeaker),
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• F. u. H. Mössbauer electronics consisting of oscillator, differential amplifier, and modulator,
• NaJ scintillation counter with thin NaJ crystal,
• preamplifier, amplifier, discriminator,
• VME data acquisition computer, ADC, interface electronics,
• absorber from iron (magnetic),
• absorber from steel (nonmagnetic),
• absorber from FeSO4 · 7H2 O (nonmagnetic).
3.2
•
3.3
List of radioactive samples
57
Co (permanently mounted inside lead shielding).
Adjustment of the electronics
A single line 57 Co γ source is used in the experiment. Set the window of the single channel
analyzer to the 14.4 keV Mössbauer line. Subsequently, connect each output of the electronic
modules with the oscilloscope and observe the pulse shapes. The maximum velocity of the
source can be adjusted with a fine potentiometer within the range (-100)–(+100) mm/s.
The count rate at the single channel analyzer’s output is now measured as function of the
source’s velocity.
3.4
Measurements with various absorbers
The following spectra are to be taken:
1.
57
Co energy spectrum.
2. Absorption spectrum of the steel absorber with ±2 mm/s max. source velocity.
3. Absorption spectrum of the iron absorber with ±7 mm/s max. source velocity.
4. Absorption spectrum of the FeSO4 · 7H2 O absorber with ±4 mm/s max. source velocity..
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Data analysis
Draw a schematic of the experimental setup and describe the purpose of the various electronic
modules. Document the pulse shapes, that were observed at the outputs of the modules, by
giving in each case the pulse height, rise time, and decay time.
4.1
Gamma spectrum
Give an interpretation of the measured
level diagram (Fig. 1).
57
Co γ-spectrum with the aid of the corresponding
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4.2
Natural line width
Determine the natural line width of the transition from the 57 Fe 14.4 keV level to the ground
state from the Mössbauer spectrum of the nonmagnetic steel absorber. Note, that emitter
and absorber both contribute to the observed line width.
4.3
Magnetic moment of the
57
Fe 14.4 keV state
Link the absorption lines that were measured with the magnetic iron absorber to the hyperfine levels of the two lowest 57 Fe states. Determine the magnetic moment µe of the excited
14.4 keV state from the measured hyperfine splitting. The magnetic moment of the 57 Fe
ground state is µg = 0.9µN .
4.4
Quadrupole splitting
Determine the quadrupole splitting of the
corresponding absorption spectrum.
A
57
Fe 14.4 keV state in FeSO4 · 7H2 O from the
Level scheme
Abb. 1: Level scheme for
57
Co.
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