Study On Magnetic Moment Imaging Base On Hanle Effect

Magnetic field measurement technology has always been a hot spot in scientific research.It has a very wide range of applications in such fields as mineral resources exploration,for example,geological hazard warning,biomedical imaging and basic physics research.In recent years with the continuous development of magnetic field measurement technology some very high sensitivity magnetic field measurement devices have emerged,such as superconducting quantum interference device(SQUID),optical atomic magnetometers and medical diagnostic apparatuses based on magnetic resonance imaging(MRI).Although the optical atomic magnetometers are developed relatively late,they have had a unique potential due to high sensitivity of magnetic measurement and become a center of attention nowadays.In this paper we use the circularly polarized light to pump the cesium atom at the ground state and polarize them,and then the optical absorption coefficient of the cesium is changed.According to the Hanle effect,when the polarized atoms are exposed to a magnetic field,it is possible that the state of polarization will be destroyed and radiate fluorescence outward.Fluorescence intensity is dependent on the magnetic strength,showing as a narrow resonance structure in the center of zero field strength.Depending on the corresponding relation between the fluorescence and the magnetic strength,we can measure the magnetic field and image it.In this experiment a cesium cell with buffer gas mixture(8 mbar Ar and 45 mbar Ne)serves as a sensor of the magnetic image because the gas can limit effectively the cesium movement in the cell and cause the polarized layer to be divided into a polarized atom array.A uniform scanning magnetic field is used to selectively destroy the polarization state of the atoms in the magnetic field under test.The magnetic field to be measured is generated by an energized coil and can be equivalent to a magnetic dipole to some extent.A charge coupled device(CCD)receives the fluorescence radiated by the array of the polarized atoms layer,the magnetic field distribution in the plane is recorded and constitute the measured image.As we all known,effective positioning magnetic source has been difficult to acquire in practice.Magnetic field measurement imaging with high sensitivity and high spatial resolution is expected to bring strong help to solve the inverse problem of the source.In my thesis,based on the image the position parameter of the source is deduced by means of Green function,thedeviation of which is about 0.64 mm by analyzing a series of the experimentally recorded images.We also estimate the spatial resolution of the magnetic distribution image recorded by the magnetometer array.The result has showed that if the image plane is located near the source the resolution of 0.96 mm may be realized experimentally because it depends on magnetic gradient,nevertheless on the view of mean free path the polarized atoms may only have 9.5 μm distance without colliding with buffer gas and other Cs,so the system has a limit of spatial resolution,and these show a very excellent spatial resolution capability indicating that it has very large research value and application prospects.