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.