2014.02.04 RPLab Tuesday Seminar
Structure of
Electron Paramagnetic Resonance
spectrometer
Kwon Choi
Electron Paramagnetic Resonance (EPR)
A spectroscopy technique based on the
absorption of electromagnetic radiation by a
paramagnetic sample.
– Paramagnetic?
– Paramagnetic properties due to the
presence of some unpaired electrons
http://encyclopedia2.thefreedictionary.com/Electron+Paramagnetic+Resonance+EPR
EPR
Thermo
Luminescence
Origin of an EPR signal
Anti-Parallel,
1
E = 2 𝑔𝑒𝜇𝐵𝐵0
Microwave
ℎν
Parallel,
1
E = - 𝑔𝑒𝜇𝐵𝐵0
2
∆E = ge𝜇BB0 = ℎν
𝑔𝑒 : g-factor (Zeeman splitting)
𝑔𝑒 ≈ 2 for unpaired electrons
𝜇𝐵 : Bohr magneton
𝜇𝐵 = 9.27x10-24 J/T
How energy absorption occur
1. Under the resonance :
1
E = 2 𝑔𝑒𝜇𝐵𝐵0
Energy
emission
1
2
E = - 𝑔𝑒𝜇𝐵𝐵0
Energy
absorption
Both transitions with
same probability
2. Population distribution of electrons between two energy levels :
(Boltzmann’s equation)
=
# 𝒐𝒇 𝒖𝒑𝒑𝒆𝒓 𝒍𝒆𝒗𝒆𝒍 𝒆𝒍𝒆𝒄𝒕𝒓𝒐𝒏𝒔
# 𝒐𝒇 𝒍𝒐𝒘𝒆𝒓 𝒍𝒆𝒗𝒆𝒍 𝒆𝒍𝒆𝒄𝒕𝒓𝒐𝒏𝒔
The population of the lower level is greater than that
of the upper level.
<𝟏
T : temperature
k : Boltzmann constant
Net energy absorption
http://encyclopedia2.thefreedictionary.com/Electron+Paramagnetic+Resonance+EPR
Absorption spectrum
EPR spectrum
∆E = ℎν = ge𝜇BB0
Sweep in order of only a few gauss
http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Magnetic_Resonance_Spectroscopies/Elect
ron_Paramagnetic_Resonance/EPR%3A_Interpretation
Difference in measurement method from NMR
NMR
EPR
Field
Radiofrequency vary
Magnetic field constant
Radiofrequency constant
Magnetic field vary
Main
concern
Splitting of nuclear spin
states
Splitting of electron spin
states
Order of 0.1-1 sec
Order of 0.1-1 𝛍sec
Relaxation
time
Too short for time-domain!
http://afni.nimh.nih.gov/sscc/staff/rwcox/ISMRM_2006/ISMRM%20M-F%202006/files/FA_07.pdf
D:\Dropbox\RadiationPhysics\Textbook\Electron Paramagnetic Resonance - A Practitioner's Toolkit\03. ch2.pdf
Components of EPR spectrometer
Spectrometer detection method
Measure the amount of radiation that is reflected back out of the
resonator.
In resonator,
Circulator
Oscillator
Oscillator
Power
Waveguide
Detector
Lock-In
Amplifier
Power
Supply
Gaussmeter
The diode detector is mounted along the microwave
and produced a current proportional to the
microwave power reflected from the cavity.
Multimeter
Components of EPR spectrometer
Circulator
Oscillator
Oscillator
Power
Oscillator
Waveguide
Detector
Lock-In
Amplifier
Power
Supply
Gaussmeter
Microwave source
Generally a klystron tube or a gunn diode
Multimeter
Components of EPR spectrometer
Circulator
Oscillator
Oscillator
Power
Waveguide
Detector
Lock-In
Amplifier
Power
Supply
Multimeter
longer
Oscillator
Gaussmeter
Microwave source
Penetration depth
Generally a klystron tube or a gunn diode
Several kind of microwave frequency bands
shorter
Components of EPR spectrometer
Circulator
Oscillator
Oscillator
Power
Resonator
Waveguide
Detector
Lock-In
Amplifier
Resonator
Power
Supply
Gaussmeter
Multimeter
Why resonator is needed?
General optical spectroscopy ; Detect how much light gets through.
Very inefficient in EPR spectroscopy
Need to enhance the sensitivity.
So Microwave Resonator is used.
Components of EPR spectrometer
Circulator
Oscillator
Oscillator
Power
Resonator
Waveguide
Detector
Lock-In
Amplifier
Resonator
Power
Supply
Multimeter
Gaussmeter
Helps increase sensitivity by “focusing” or “concentrating” the microwave power
at the sample.
Various designs
Correspond to a specific wavelength of microwave
A Little More Details of Resonator
Circulator
Oscillator
Oscillator
Power
Waveguide
Detector
Lock-In
Amplifier
Resonator
Power
Supply
Multimeter
Gaussmeter
Iris
A connection between the wave guide and the resonator cavity
Couple the microwaves into the cavity via an iris.
Iris screw in front of the iris that we can adjust the
“matching”.
Various shapes of resonators
(here; a metal box)
Store MW energy in the form of the
standing wave
Magnetic field drives the absorption.
Specific to the wavelength of MW
Circulator & Isolator
Oscillator
Waveguide
Circulator
Detector
Lock-In
Amplifier
Oscillator
Power
Circulator
Power
Supply
Gaussmeter
Transmit microwave only to a fixed direction
- Oscillator -> Resonator
- Resonator -> Detector
Isolator
A kind of circulator
Transmit microwave only in one direction
Multimeter
Circulator & Isolator
Oscillator
Waveguide
Circulator
Detector
Lock-In
Amplifier
Oscillator
Power
Circulator
Power
Supply
Multimeter
Gaussmeter
Transmit microwave only to a fixed direction
- Oscillator -> Resonator
- Resonator -> Detector
Isolator
RF Circulator
RF Isolator
Transmit microwave only in one direction
Yixin Microwave Electronics Co., Ltd.
Correspond to a specific
wavelength of microwave
Oscillator
Waveguide
Oscillator
Power
Lock-in Amplifier
Circulator
Detector
Lock-In
Amplifier
Power
Supply
Gaussmeter
Detect and measure very small AC signals (~ few nV)
Use phase-sensitive technique for noise filtering
Multimeter
Experimental example
Simple experimental setup to see EPR phenomena
Helmholtz coils
RF Oscillator
- 10 MHz ~ 18 MHz (RF range)
- 50 Hz current through
Helmholtz coils
Oscilloscope
Signal processing using Lab view
Response curve for dosimetry
Peak-to-peak amplitude of the
most intense EPR peak
Dose resopnse
peak-to-peak intensity vs. dose
EPR spectrum in the first-derivative form
Thank you for your attention.