1. Electron Paramagnetic Resonance spectrometer
RPLab Tuesday Seminar
Structure of
Electron Paramagnetic Resonance spectrometer
I’ll talk about the structure of the structure of e.p.r spectrometer.
Kwon Choi

2. 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
First, what is the ep.r
I have presented about this several times, so I’ll briefly talked this.
EP.R is a spectroscopy technique based on the absorption of electromagnteic radiation, (in this technique, mostly microwave radiation)
by a paramagnetic sample.
Then what is the meaning of the term “paramagnetic”?
By a dictionary’s definition, “paramagnetism” is
a property to form a magnetism attracted by an externally applied magnetic field.
The source of paramagnetic properties are due to the presence of some unpaired electrons.
Then what is the “unpaired electron”?
(다음)

3. Thermo Luminescence EPR Here is an orbital of an atom.
There are a pair of electrons in a stable state orbital.
They are spinning in opposite directions
(b) By some external radiation like alpha, beta or gamma rays, one of the paired electrons can be knocked out of the orbital.
And the knocked off electron become an unpaired electron.
© The unpaired electron may be trapped by an impurity, in this illustration by a gangster.
(d) Heating can release the trapped electron.
In this process (d), there is emitting light called Thermo Luminescence.
EP.R observes the trapped unpaired electron directly, which a state illustrated in (b)
The following may be paramagnetic :
Atoms or molecules with an odd number of electrons
(nitrogen, hydrogen atoms)
Free radicals
Ions with partially filled inner electron shells
(ions of transition elements)
Color centers in crystals;
Impurity atoms (donors in semiconductors)
Conduction electrons in metals or semiconductors

4. Origin of an EPR signal Anti-Parallel, E = 1 2 𝑔𝑒𝜇𝐵𝐵0 Microwaveℎν
∆E = ge𝜇BB0 = ℎν
Parallel,
E = 𝑔𝑒𝜇𝐵𝐵0
𝑔𝑒 : g-factor (Zeeman splitting)
𝑔𝑒≈2 for unpaired electrons
𝜇𝐵 : Bohr magneton
𝜇𝐵 = 9.27x10-24 J/T
Previously I said the definition of paramagnetism.
By the definition, when there’s no external magnet field, the sample isn’t magnetic.
Which means that unpaired electrons in sample have no fixed direction.
By the external magnetic field,
the unpaired electrons are aligned in parallel or anti-parallel to the magnetic field.
These two states of electrons (parallel and anti-parallel) have different energy states like these (그림 수식).
The energy states are a function of the strength of magnetic field intensity.
When the microwave with the energy same to the energy gap of these two states,
electrons start to resonant and absorb the energy of the microwave.

5. How energy absorption occur
1. Under the resonance :
E = 1 2 𝑔𝑒𝜇𝐵𝐵0
Energy emission
Energy absorption
E = 𝑔𝑒𝜇𝐵𝐵0
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 <𝟏
Then how the energy absorption occur?
1. I described the electrons of two states resonant,
This means that electrons of each states transit to another states.
Electrons of upper energy state become lower energy state emitting energy,
And those of lower energy state become upper state absorbing energy.
These two transitions have same probability.
2. But electrons of two states have different population, by the Boltzmann’s equation.
The population of the lower energy level is a little greater than that of the upper level.
Consequently energy absorption happens.
But the amount of energy absorption is very little because the small difference in population.
T : temperature
k : Boltzmann constant
The population of the lower level is greater than that
of the upper level.
Net energy absorption

6. Sweep in order of only a few gauss
Absorption spectrum
EPR spectrum
∆E = ℎν = ge𝜇BB0
Sweep in order of only a few gauss
To know the absorbed energy spectrum, magnetic field sweep about the resonance magnetic field.
The magnetic field sweep is in the range of order of ten gauss, which is very small range of magnetic field strength.
(위 그래프) So the absorbed energy spectrum is the function of magnetic field.
This graph show the energy absorption increases about the resonant magnetic field strength.
(아래 그래프) In practice, we can get the absorption spectrum in the form of first derivative,
I don’t know the detail,
but it is said that due to the modulation of magnet field and a phase-sensitive detector used to enhance the sensitivity.
(다음)
이것이 제가 소개하는 EPR spectrometer의 방식에서 얻게 되는 일반적인 신호형태이다.
 simple absorption spectra will appear similar to the one on the top of Figure 1. However, a phase-sensitive detector is used in EPR spectrometers which converts the normal absorption signal to its first derivative. Then the absorption signal is presented as its first derivative in the spectrum, which is similar to the one on the bottom of Figure 1. Thus, the magnetic field is on the x-axis of EPR spectrum; dχ″/dB, the derivative of the imaginary part of the molecular magnetic susceptibility with respect to the external static magnetic field in arbitrary units is on the y-axis.
The derivative-like line shape is a result of the use of field modulation. In order to get sufficient signal to noise, the B_0 field (large, static field) is modulated (usually at 100kHz) and a lock-in amplifier (or equivalent) is used to reject any frequencies beside 100kHz. The result of this field modulation is that the signal that is obtained is not the value of the absorbance, rather the difference in the absorbances between the ends of the modulation amplitude (more or less) i.e. ~d(absorption)/dB. A slightly more detailed explanation (with a picture, figure 2.7) is here.
There are lots of caveats and such due to the fact that it's not a true derivative, and there is, therefore, some signal distortion.
There is currently (again) development of detection techniques which are not dependent on the modulation of the field for S/N.

7. Difference in measurement method from 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
Relaxation time
Order of sec
Order of 𝛍sec
Too short for time-domain!
As I mentioned, most EPRspectromter measure varying magnetic field.
This is a difference methods from NMR.
Most NMR magnetic field is not varied while radiofrequency vary.
This difference is due to the relaxation time of the observing target.
NMR concerns the nuclear or proton, and this has the relaxation time order of second.
EPR observes the electron, order of micro-second.
This is too short to record in time-domain.
So instead, many EPR are recorded in magnetic field domain.
NMR은 magnetic field의 세기를 constant하게 유지하고 radio frequency를 변화시키면서 측정하고
EPR은 radiofrequency를 constant하게 유지하며 magnetic field의 세기를 변화시키면서 측정한다고 하였다.
이는 측정하는 대상의 relaxation time의 차이에 원인이 있다.
NMR과 EPR은 일반적으로 대상이 되는 nucleus 혹은 electron을 radiofrequency로 excitation 시킨 후 relaxation 되는 동안 시간에 대해 signal의 변화를 측정한다.
NMR의 대상이 되는 nucleus는 relaxation time이 초 단위로, excitation과 relaxation까지 기록하는데 10초 정도 되는 비교적 넉넉한 시간을 확보할 수 있다.
하지만, EPR의 대상인 electron의 relaxation time은 NMR에 비해 굉장히 짧아, 수 microsecond 동안 excitation과 signal sampling 전 과정을 해야 한다.
이를 위해서는 NMR보다 상당히 sophisticate한 hardware가 필요하다.
이 방식을 pulsed EPR이라 하는데, contrast agent를 사용하여 비교적 relaxation time이 긴 경우에 적합하다.
많은 경우, free radiacal의 relaxation time은 time-domain EPR로 측정하기에는 너무 짧아, 다른 방식인 continuous-wave

8. “Maybe” someone think EPR spectrometer is good, and wants to make one ourselves.
So for that case, I’ll describe the components of EPR spectrometer.
Here is some microwave components to compose the EPR spectrometer,
and a photo of that.
D:\Dropbox\RadiationPhysics\Textbook\Electron Paramagnetic Resonance - A Practitioner's Toolkit\03. ch2.pdf

9. Components of EPR spectrometer
Spectrometer detection method
Measure the amount of radiation that is reflected back out of the resonator.
In resonator,
Circulator
Waveguide
Oscillator
Detector
Lock-In Amplifier
Oscillator Power
So this is a spectrometer.
EPR Spectrometer measure how much the microwave radiation is absorbed by the test-sample.
The microwave is produced from the oscillator like klystron,
then the microwave moves along the waveguide to the resonator.
In resonator, the test-sample exist in the magnetic field.
Through the energy abosorption by the test-sample, the microwave is reflected to the waveguide.
This time this microwave is guided only to the detector, and measured how much attenuated.
I’ll describe some components in the following pages.
Power Supply
Multimeter
Gaussmeter
The diode detector is mounted along the microwave and produced a current proportional to the microwave power reflected from the cavity.

10. Components of EPR spectrometer
Circulator
Waveguide
Oscillator
Detector
Lock-In Amplifier
Oscillator Power
Power Supply
Multimeter
Gaussmeter
Oscillator
Microwave source
Generally klystron or gunn-diode is used for oscillator.
So, oscillator is a microwave source that send microwave to the resonator through the waveguide.
I used the term “Microwave”, but there is some separate frequency bands in this.
(다음 페이지)
이 oscillator의 주된 역할은 microwave source로서 waveguide를 통해 resonator로 microwave를 보낸다.
한 마디로 microwave라고 해도 주파수에 따라 여러 대역이 있는데,
EPR spectrometer에서도 용도에 따라 몇 가지 주파수대를 달리하여 사용한다.
(표) EPR에서 주로 사용하는 microwave frequency band는 L-band, X-band, Q-band 이며,
주파수에 따라 penetration할 수 있는 depth가 다르기 때문에
in vivo EPR 연구에서는 주파수가 작고, penetration depth가 비교적 긴 L-band를 사용하는 경우가 많다.
*gunn diode : a kind of a semiconductor electronic component, used in high-frequency electronics (고주파 발진기)
Generally a klystron tube or a gunn diode

11. Components of EPR spectrometer
Circulator
Waveguide
Oscillator
Detector
Lock-In Amplifier
Oscillator Power
Power Supply
Multimeter
longer
Gaussmeter
Oscillator
Microwave source
Penetration depth
Each band has different wavelength, so different penetration depth.
So, In EPR spectrometer, different microwave band is used by the main purpose.
L-band has relatively long penetration depth, so In vivo EPR studies are mainly performed in this band.
X-band is mostly used in commercially available EPR spectrometer.
And some studies use Q-band for more sensitive analysis.
*gunn diode : a kind of a semiconductor electronic component, used in high-frequency electronics (고주파 발진기)
Generally a klystron tube or a gunn diode
Several kind of microwave frequency bands
shorter

12. Components of EPR spectrometer
Circulator
Waveguide
Oscillator
Detector
Lock-In Amplifier
Resonator
Oscillator Power
Power Supply
Multimeter
Gaussmeter
Resonator
Now through the waveguide and circulator, the microwave goes to the resonator.
I’ll describe about circulator later.

13. Why resonator is needed?
General optical spectroscopy ; Detect how much light gets through.
Very inefficient in EPR spectroscopy
In optical spectroscopy, spectra are usually acquired by light through the sample and detecting how much light gets through.
But this technique is not very effective for EPR spectroscopy.
As I mentioned earlier, the amount of absorption in EPR sample is by very little number of difference in electrons of two states.
So We need to increase sensitivity to detect this small absorption signal well.
And the resonator comes forward.
Need to enhance the sensitivity.
So Microwave Resonator is used.

14. Components of EPR spectrometer
Circulator
Waveguide
Oscillator
Detector
Lock-In Amplifier
Resonator
Oscillator Power
Power Supply
Multimeter
Gaussmeter
Resonator
Helps increase sensitivity by “focusing” or “concentrating” the microwave power at the sample.
So the resonator helps to increase the sensitivity by focusing the microwave power at the sample.
Resonators have various shapes and dimension.
And the dimension and shape is depend on the wavelength of microwave.
So the design of the resonator should be considered if someone wants to make one.
Various designs
Correspond to a specific wavelength of microwave

15. A Little More Details of Resonator
Circulator
Waveguide
Oscillator
Detector
Lock-In Amplifier
Resonator
Oscillator Power
Power Supply
Multimeter
Gaussmeter
Iris
A connection between the wave guide and the resonator cavity
So not all the resonator is a cavity, but here I describe it as a metal box shape cavity.
When the microwave is guided through waveguide, there is a construction named Iris exists as the part of the resonator.
This is a hole connecting the waveguide and cavity.
And have a role to couple the microwave and cavity to make it resonance.
The iris is matched using the screw named iris-screw, and this matching transform the impedances of the cavity and the waveguide.
When the sample absorbs the microwave energy, the increased microwave energy absorption changes the impedance of the cavity. So the cavity is no longer coupled and microwave will be reflected back resulting an EPR signal.
Couple the microwaves into the cavity via an iris.
Iris screw in front of the iris that we can adjust the “matching”.

16. Various shapes of resonators (here; a metal box)
Store MW energy in the form of the standing wave
Magnetic field drives the absorption.
As I mentioned before, the resonator have various design.
In the resonator cavity, microwave energy is stored in the form of the standing wave.
This is the reason that each resonator has a design specific to the wavelength.
Standing wave makes the magnetic field and the electric field (like the figure 그림).
Magnetic field is driving the microwave absorption,
Sample is placed in the place with maximum magnetic field, so that the signal and sensitivity can be maximized.
Specific to the wavelength of MW

17. Circulator Isolator Circulator & Isolator Circulator Oscillator
Waveguide
Oscillator
Detector
Lock-In Amplifier
Oscillator Power
Power Supply
Multimeter
Gaussmeter
Circulator
Transmit microwave only to a fixed direction
- Oscillator -> Resonator
- Resonator -> Detector
After the energy absorption in the resonator, microwave is reflected through the waveguide.
There is a RF component named circulator.
Through this microwave can proceed only to the fixed direction.
So the reflected microwave can go only to the detector.
And microwave from oscillator only to the resonator.
During these, the isolator, a kind of circulator, helps this process.
Resonator에서 reflect된 microwave는 다시 waveguide를 통해서 나오게 된다.
이때 waveguide 내에서 microwave에 방향성을 정해주어 reclect된 wave만이 detector로 갈 수 있어야 한다.
이를 돕는 것이 circulator의 역할이다.
Circulator는 microwave의 방향성을 control하여,
Microwave source인 Oscillator에서 waveguide를 통해 온 microwave는 resonator 방향으로만,
Resonator에서 반사된 microwave는 detector 방향으로만 갈 수 있도록 한다.
이때 Isolator가 waveguide의 중간에서 마찬 가지로 microwave가 한 방향으로만 가도록 돕는다.
Isolator
A kind of circulator
Transmit microwave only in one direction

18. Circulator Isolator Circulator & Isolator Circulator Oscillator
Waveguide
Oscillator
Detector
Lock-In Amplifier
Oscillator Power
Power Supply
Multimeter
Gaussmeter
Circulator
Transmit microwave only to a fixed direction
- Oscillator -> Resonator
- Resonator -> Detector
Also, these components are specific to the microwave wavelength.
So if someone think about making one, also determined in advance.
또한 이 component들도 microwave의 wavelength에 특화되어 있기 때문에 제작하는데 있어 사전에 정해 둘 필요가 있다.
Isolator
Correspond to a specific wavelength of microwave
RF Circulator
RF Isolator
Transmit microwave only in one direction
Yixin Microwave Electronics Co., Ltd.

19. Lock-in Amplifier Circulator Oscillator Detector Lock-In Amplifier
Waveguide
Oscillator
Detector
Lock-In Amplifier
Oscillator Power
Power Supply
Multimeter
Gaussmeter
Lock-in Amplifier
Detect and measure very small AC signals (~ few nV)
The signal sensitivity has been enhanced.
But still, the strength of signal is small and noise is relatively big.
To compensate this, lock-in amplifier is used.
Lock-in amplifier can detect very small signal to the order of nano-voltage.
Two input signal is required for lock-in amplifier; signal from the detector and reference signal.
The two signals have common phase and frequency.
Signal with different from these phase and frequency is rejected as noise
so that only intended signals can be processed.
이를 위해 phase-sensitive technique을 사용하며,
여기에는 두 개의 input signal이 필요하며, 이는 detector에서 받는 signal과 reference signal이다.
Reference signal는 detector에서 받는 signal과 phase와 frequency를 공유하여,
이것과 frequency와 phase가 다른 신호는 noise로서 reject되어 본래 measure하고자 하는 signal만을 catch할 수 있다.
Use phase-sensitive technique for noise filtering

20. 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

21. 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

22. Thank you for your attention.