EMR is an acronym for Electron Magnetic Resonance, other common names are EPR (Electron Paramagnetic Resonance) and ESR (Electron Spin Resonance).
When a material contains “unpaired” electrons and is exposed to an external magnetic field B0, the two energy levels of the two eigenstates “spin up” and “spin down” split as a result of the quantization of the angular momentum (Zeeman splitting). By adding a precise amount of polarized microwave energy, that meets the resonance condition: ΔE = g |mB| B0, the system absorbs energy and undergoes transitions between the electron’s spin up and spin down states, showing up as an energy absorption spectrum. The EMR-signal is usually recorded as the first derivative of the absorption spectrum with on the horizontal axis B0 (or g) and at the vertical axis the signal intensity expressed in arbitrary units (au). The EMR spectrum of isolated unpaired electrons would show a singlet line. The amount of energy absorbed is a measure of the concentration of unpaired electrons present in the material and is expressed as the “peak-to-peak” height.
An isolated electron spin system is hypothetic. When interactions occur between the electron’s spin angular momenta and the momenta present in the surrounding lattice, further splitting of the absorption lines is observed (hyperfine splitting). The position of the lines in the spectrum and the shape of the lines provide qualitative information of the surroundings (molecule structure). The signal intensity provides information on the population density of the distinct energy levels.
Quantitative EMR spectroscopy is the basis for dosimetry. The relative concentration of unpaired electrons can be expressed as the peak-to-peak height of absorption line(s) and hence, after proper calibration, to the absorbed dose. The amino acid L-α-alanine is the most studied radiosensitive material for more than 50 years and shows good dosimetrical properties. When ionizing radiation interacts with the alanine, unpaired electrons are created which show a high stability in time.
What can we do?
Dose measurements in radiation facilities
ransfer dosimetry programs for quality assurance
Where can we be active?
Hospitals (radiotherapy departments)
Facilities for the radiation of food and materials
Regulatory authorities (quality assurance)