I.
Introduction/Background:
A.
Electron
paramagnetic resonance (EPR) spectroscopy
1.
EPR
is a technique for studying paramagnetic materials
a)
Paramagnetic
materials are species that contain unpaired electrons
2.
Paramagnetic materials absorb microwave radiation when subjected to a magnetic field
3. The frequency of microwave radiation generally used in EPR is in the range of 1-10 GHz
a)
Microwaves
(300MHz-300GHz) represent a subset of electromagnetic radiation that are generally
referred to as radio waves (3kHz-300GHz)
B.
Low
frequency electron paramagnetic resonance (LFEPR) spectroscopy and EPR MOUSE
1.
LFEPR
is a variation of EPR that utilizes a lower frequency radiation (around
300 MHz-1 GHz) than in conventional EPR which utilizes higher
frequencies (1-10 GHz)
2.
EPR
MOUSE is a further variation of EPR
3. LFEPR, and EPR MOUSE devices have 2 main components
a) The magnet which creates a magnetic field
b) The radio frequency (RF) probe
4. The equation, hν = gβB , represents the relationship between the applied magnetic field and the frequency of the absorbed radiation
a) h = Planck’s constant, ν=frequency, g=g-factor
(constant), β= Bohr magneton (constant), B=
magnetic field
3. LFEPR, and EPR MOUSE devices have 2 main components
a) The magnet which creates a magnetic field
b) The radio frequency (RF) probe
4. The equation, hν = gβB , represents the relationship between the applied magnetic field and the frequency of the absorbed radiation
a) h = Planck’s constant, ν=frequency, g=g-factor
(constant), β= Bohr magneton (constant), B=
magnetic field
II.
Rationale
& Purpose:
A.
The
goal of the first half of this project was to characterize the magnetic field
of various magnet designs and measure the sensitivity of the RF probe to aid in
the construction of the EPR MOUSE.
B.
The
goal of the second half of this project is to demonstrate the capabilities of
the current EPR MOUSE design and establish the feasibility of detecting
paramagnetic species on a variety of surfaces of practical relevance.
C.
Advantages
of LFEPR as opposed to EPR
1.
Can
study larger objects non-invasively
a)
Conventional
(higher frequency) EPR can only study samples smaller than 125mm3 in
volume
b)
LFEPR
can be used to study important cultural artifacts that are multiple liters in volume
2.
Can
study larger objects as opposed to EPR
D.
Advantages
of EPR MOUSE
1.
Can
study surfaces (the sample does not need to be placed inside a spectrometer)
a)
No
sample size limitation
b)
Only
limitation is the depth of penetration of the magnetic field and the radiation
into the sample
III.
Methods:
A.
Characterization
of the magnetic field of different iron yoke designs
1.
Goal
was to optimize the homogeneity of the magnetic field because the RF absorption
is dependent on it.
2.
Used
the Finite Element Method Magnetics (FEMM) software to generate the theoretical
magnetic field distribution around the yoke
3.
Compared
the theoretical prediction (using FEMM) to experimental measurements to ensure
accuracy
B.
Determination
of the sensitivity of the RF probe
1.
Used
an EPR machine with a known homogenous magnetic field
2.
Measured
using DPPH (2,2-diphenyl-1-picrylhydrazyl), a known paramagnetic species that
is commonly used for EPR measurements
3.
Measured
the sensitivity of the probe at numerous locations in the 3D space (in the
XZ,YZ, and YX)
4.
Analyzed
this data to determine the sensitivity of the RF probe
C.
Demonstration
of the capabilities of the EPR MOUSE
1.
Currency
a)
Used
the MOUSE to scan the surface of a US dollar bill and recorded the spatial
distribution of the ferromagnetic signal, and created a 2D image.
2.
Paint/pigments
a)
Used
the MOUSE to identify paramagnetic pigments used in various paints based off
the signal produced
3.
Barcode
a)
Used
the MOUSE to scan a barcode and record data based off its ferromagnetic signal
(from toner ink) and created a 1D image of the barcode
IV.
Results
& Conclusion (not final)
A.
Based
off the projects conducted to demonstrate the capabilities of the MOUSE, the
EPR MOUSE has a number of potential applications including:
1.
Analysis
of paintings and historical artifacts
2.
Authentication
of currency and other paper documents
a)
Paramagnetic
or ferromagnetic watermarks
3.
An
alternative to optical barcode readers
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