Atomic magnetometry for nuclear threat reduction applications

Richards, Abigail (2021). Atomic magnetometry for nuclear threat reduction applications. University of Birmingham. Ph.D.

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The work within this thesis explores the applicability of atomic magnetometry to nuclear threat reduction applications. The scope of nuclear threat reduction is explored in the context of UK Government strategy with research conducted at the Atomic Weapons Establishment. An application space is defined which includes nuclear treaty verification, nuclear forensics and detection science. Within these areas, the requirement to detect shielded nuclear and radiological materials is established and eddy current induction methods are proposed to meet the detection of a sub-set of these materials.
Eddy current induction measurements are conventionally met using coil based technologies, however, the sensitivities of these systems increase as a function frequency. Eddy current induction is a frequency dependent measurement, where penetration through an object is reduced at larger values due to the skin-depth effect. These factors combine to be counterproductive for the measurement of objects in thickly shielded configurations. Atomic magnetometers, which measure magnetic fields through the inference of Zeeman splitting within alkali metal vapours, are proposed as an alternative to coil based sensors. Atomic magnetometer technologies benefit from high sensitivities which can be achieved across broad range of frequencies.
Two systems are explored within this work, the first focusses on the detection of targets in challenging shielding configurations. These include through high conductivity (aluminium), ferrous (steel) and high density (lead) materials which necessitated low frequency measurements and were achieved using a commercially available atomic magnetometer. The second system examines the detection of smaller unshielded objects
with a focus on tuning the excitation frequency to higher values. Tuning the frequency enabled the maximisation of the magnetic field phase response of nuclear materials such as uranium and plutonium. This was achieved by constructing a radio-frequency atomic magnetometer.
In the low frequency regime, an imaging system was constructed that allowed raster scan images of high conductivity materials to be obtained behind aluminium shielding up to 63 mm thick at distances > 200 mm. These same objects were also imaged behind steel plates up to 12 mm thick and lead shielding 100 mm thick, where these values represent the maximum thickness tested. These high conductivity objects are of interest for the confirmation of declarations made within treaty verification or for the detection of illicit items in bag/personnel scanning. Lower conductivity materials such as conductivity surrogates for uranium were imaged behind 35 mm of aluminium and the aforementioned
material thickness values for steel and lead.
In the high frequency regime, a rubidium based radio-frequency magnetometer was constructed, optimised and integrated into an eddy current imaging system. Images of the lower conductivity plutonium surrogate were obtained at frequencies above 20 kHz. Additional work was identified across both the magnetometer and the imaging system to allow the full sensitivity of the system to be exploited. Outside of the imaging function of the system, the frequency of the eddy current excitation field was successfully tuned to enhance the measurement of the uranium and plutonium surrogates over higher conductivity samples such as copper and aluminium.

Type of Work: Thesis (Doctorates > Ph.D.)
Award Type: Doctorates > Ph.D.
Licence: All rights reserved
College/Faculty: Colleges (2008 onwards) > College of Engineering & Physical Sciences
School or Department: School of Physics and Astronomy
Funders: Other
Other Funders: Atomic Weapons Establishment
Subjects: Q Science > QC Physics


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