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 0 Microwave ℎν Parallel, 1 E = - 0 2 ∆E = geBB0 = ℎν : g-factor (Zeeman splitting) ≈ 2 for unpaired electrons : Bohr magneton = 9.27x10-24 J/T How energy absorption occur 1. Under the resonance : 1 E = 2 0 Energy emission 1 2 E = - 0 Energy absorption Both transitions with same probability 2. Population distribution of electrons between two energy levels : (Boltzmann’s equation) = # # The population of the lower level is greater than that of the upper level. < T : temperature k : Boltzmann constant Net energy absorption http://encyclopedia2.thefreedictionary.com/Electron+Paramagnetic+Resonance+EPR Absorption spectrum EPR spectrum ∆E = ℎν = geBB0 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 sec 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.