Structure of Electron Paramagnetic Resonance spectrometer 2014.02.04 RPLab Tuesday Seminar Kwon...
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Transcript of Structure of Electron Paramagnetic Resonance spectrometer 2014.02.04 RPLab Tuesday Seminar Kwon...
Structure of Electron Paramagnetic Reso-
nance spectrometer
2014.02.04 RPLab Tuesday Seminar
Kwon Choi
A spectroscopy technique based on the ab-sorption of electromagnetic radiation by a paramagnetic sample.
– Paramagnetic?– Paramagnetic properties due to the
presence of some unpaired electrons
Electron Paramagnetic Resonance (EPR)
http://encyclopedia2.thefreedictionary.com/Electron+Paramagnetic+Resonance+EPR
EPRThermo Lumi-nescence
Origin of an EPR signal
Parallel, E = -
Anti-Parallel, E =
: g-factor (Zeeman split-ting) for unpaired electrons : Bohr magneton = 9.27x10-24 J/T
E = geBB0 = Mi-crowave
2. Population distribution of electrons between two en-ergy levels :(Boltzmann’s equa-tion)
T : temperaturek : Boltzmann constant
1. Under the resonance :
The population of the lower level is greater than that of the upper level.
Net energy absorp-tion
http://encyclopedia2.thefreedictionary.com/Electron+Paramagnetic+Resonance+EPR
Energy emis-sion
Energy absorp-tion
Both transitions with same probability
=
How energy absorption oc-cur
E = -
E =
http://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Magnetic_Resonance_Spectroscopies/Electron_Paramagnetic_Resonance/EPR%3A_Interpretation
Absorption spec-trum
EPR spectrum
E = = geBB0
Sweep in order of only a few gauss
Difference in measurement method from NMR
http://afni.nimh.nih.gov/sscc/staff/rwcox/ISMRM_2006/ISMRM%20M-F%202006/files/FA_07.pdf
NMR EPR
FieldRadiofrequency varyMagnetic field con-stant
Radiofrequency con-stantMagnetic field vary
Main con-cern
Splitting of nuclear spin states
Splitting of electron spin states
Relax-ation time Order of 0.1-1 sec Order of 0.1-1 sec
Too short for time-do-main!
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,
OscillatorWave-guide
Circula-tor
The diode detector is mounted along the mi-crowave and produced a current proportional to the microwave power reflected from the cavity.
DetectorLock-In
AmplifierOscillator Power
Power Supply
Gaussme-ter
Multime-ter
Components of EPR spectrometer
Oscilla-tor
Wave-guide
Circula-tor
DetectorLock-In
AmplifierOscilla-tor
Power Power Supply
Gaussme-ter
Multime-ter
Oscilla-tor
Generally a klystron tube or a gunn diode
Microwave source
Components of EPR spectrometer
Oscilla-tor
Wave-guide
Circula-tor
DetectorLock-In
AmplifierOscilla-tor
Power Power Supply
Gaussme-ter
Multime-ter
Oscilla-tor
Generally a klystron tube or a gunn diode
Several kind of microwave frequency bands
Penetration depth
longer
shorter
Microwave source
Components of EPR spectrometer
OscillatorWave-guide
Circula-tor
DetectorLock-In
AmplifierOscillator Power
Power Supply
Gaussme-ter
Multime-ter
Res-onator
Res-onator
General optical spectroscopy ; Detect how much light gets through.
Very inefficient in EPR spectroscopy
Need to enhance the sensitivity.
So Microwave Resonator is used.
Why resonator is needed?
Components of EPR spectrometer
OscillatorWave-guide
Circula-tor
DetectorLock-In
AmplifierOscillator Power
Power Supply
Gaussme-ter
Multime-ter
Res-onatorHelps increase sensitivity by “focusing” or “concentrating” the microwave power at the sample.
Res-onator
Various designs
Correspond to a specific wavelength of mi-crowave
OscillatorWave-guide
Circula-tor
DetectorLock-In
AmplifierOscillator Power
Power Supply
Gaussme-ter
Multime-ter
Res-onator
Couple the microwaves into the cavity via an iris.
Iris screw in front of the iris that we can adjust the “matching”.
A Little More Details of Resonator
IrisA connection between the wave guide and the resonator cavity
Various shapes of resonators (here; a metal box)
Magnetic field drives the absorption.
Store MW energy in the form of the standing wave
Specific to the wavelength of MW
OscillatorWave-guide
Circula-tor
DetectorLock-In
AmplifierOscillator Power
Power Supply
Gaussme-ter
Multime-ter
Circula-torTransmit microwave only to a fixed direction - Oscillator -> Resonator - Resonator -> Detector
Circulator & Isola-tor
Isolator
Transmit microwave only in one direction
A kind of circulator
OscillatorWave-guide
Circula-tor
DetectorLock-In
AmplifierOscillator Power
Power Supply
Gaussme-ter
Multime-ter
Circula-torTransmit microwave only to a fixed direction - Oscillator -> Resonator - Resonator -> Detector
Circulator & Isola-tor
IsolatorTransmit microwave only in one direction
Correspond to a specific wavelength of mi-crowave
RF Circula-tor
Yixin Microwave Electronics Co., Ltd.
RF Isolator
OscillatorWave-guide
Circula-tor
DetectorLock-In Ampli-
fierOscillator Power
Power Supply
Gaussme-ter
Multime-ter
Lock-in Amplifier
Detect and measure very small AC signals (~ few nV)Use phase-sensitive technique for noise filter-ing
Experimental example
Simple experimental setup to see EPR phenomena
Helmholtz coils
Oscilloscope
RF Oscillator - 10 MHz ~ 18 MHz (RF range) - 50 Hz current through Helmholtz coils
Signal processing using Lab view
Response curve for dosimetry
EPR spectrum in the first-derivative form
Peak-to-peak amplitude of the most intense EPR peak
Dose resopnsepeak-to-peak intensity vs. dose
Thank you for your at-tention.