Enhanced material defect imaging with a radio-frequency atomic magnetometer

Abstract

Imaging of structural defects in a material can be realized with a radio-frequency atomic magnetometer by monitoring the material's response to a radio-frequency excitation field. We demonstrate two measurement configurations that enable the increase of the amplitude and phase contrast in images that represent a structural defect in electrically conductive and magnetically permeable samples. Both concepts involve the elimination of the excitation field component, orthogonal to the sample surface, from the atomic magnetometer signal. The first method relies on the implementation of a set of coils that directly compensates the excitation field component in the magnetometer signal. The second takes advantage of the fact that the radio-frequency magnetometer is not sensitive to the magnetic field oscillating along one of its axes. Results from simple modelling confirm the experimental observation and are discussed in detail. Published under license by AIP Publishing. https://doi.Published under license by AIP Publishing.FIG. 3. The modelled change in the signal phase (dotted black line) and the amplitude (solid red line) of the magnetic resonance signal over a recess recorded by a magnetometer for various amplitudes of the primary field components. The vertical axis of the image array represents changes in the vertical component, while the horizontal axis represents changes in the horizontal component of the primary field. Amplitude is expressed in units of b y,max .Journal of Applied Physics ARTICLE scitation.org/journal/jap

Inductive Imaging of the Concealed Defects with Radio-Frequency Atomic Magnetometers

“…Our previous studies have been focused on measuring object thinning in the form of a recess [22][23][24][25][26]. Analysis of the amplitude and phase images validated this method as a tool for monitoring defect (recess) depth.…”

“…In particular, it enables operation at low frequencies, where the change in the penetration depth of the rf field is most significant. This category of sensors includes giant magnetoresistance (GMR) magnetometers [12][13][14], superconducting quantum interference devices (SQUIDs) [15,16] and atomic magnetometers [17][18][19][20][21][22][23][24][25][26].…”

“…Implementation of an rf atomic magnetometer as a magnetic field sensor brings a superior field sensitivity (∼ fT/ √ Hz) [11] and several novel functionalities to the NDT measurement process. In particular, the semi-vector mapping of the secondary field components generated by a defect was recently demonstrated in NDT measurements with an rf atomic magnetometer, although this sensor is typically regarded as scalar in nature [23,24]. The operation of a magnetometer in the so-called spin maser mode (self-generated drive for rf magnetic field) addresses the issue of the sensor's narrow bandwidth [25,26].…”

“…For the uncovered defect, the amplitude of the defect signature increases with frequency, which is a simple consequence of the growth of induction with rf field frequency. Most of the observations reported in [22][23][24][25][26] have been performed with the rf primary field frequency in a range of tens of kHz. One of the main challenges for existing NDT methods is the detection of concealed defects; e.g., defects covered by an insulating material, buried within, or located on an inner surface of the object.…”

Generation of atomic spin orientation with a linearly polarized beam in room-temperature alkali-metal vapor

“…5 of a radio-frequency atomic magnetometer. The rf frequency range of the presented technique (1 kHz-30 kHz) is interesting in the context of magnetic induction based non-destructive testing, where a low operating frequency translates into a deeper penetration depth of the (socalled primary) magnetic field [25]. The measurement configuration discussed here combines the efficient generation of the F=4 atomic orientation and off-resonant probing usually achieved with two/ three independent lasers.…”