Multi-spectral X-ray imaging with a laser-plasma accelerator

Abstract

This master thesis focuses on the creation of an algorithm to perform multispectral
imaging of a metallic target using X-rays. The generation of an ultrashort high-power low-divergent X-ray beam is performed by means of a Ti:Sapphire laser driven wake-field plasma accelerator. The X-rays result from synchrotron radiation, and are therefore broadband. The spectrum will be characterized, along with the transmittance of a variety of different metals, using a technique called "single-photon counting". An X-ray sensitive 4 MP CCD camera is used to perform a statistical analysis of the x-ray beam spectrum, and X-ray absorption spectroscopy (XAS) is used in order to identify a "fingerprint" for each metal. It is demonstrated that the proposed system allows multi-spectral imaging of a Ross filter, being capable of detecting a series of different metals with good spatial
resolution.

Popular Abstract

It is possible to determine an object’s composition by simply shining light through it and observing how the frequency components of the light source (its spectrum) change after transmission. This is because different materials absorb light in unique ways, with some frequencies being absorbed more than others. For example, when looking through a pair of red colored glasses, the world appears red because only the red frequency component of the sunlight (which is white) is transmitted, whereas all other colors are mostly absorbed. This method of studying a material is called "absorption spectroscopy", and it is an extremely powerful tool which can even allow astronomers to determine the species of gases in nebulae that are thousands of light years away from Earth.

In this thesis work, the aim is to be able to determine the composition and location
in space with good resolution of different metals in a 2D image. Visible light cannot be used because metals are opaque to this type of radiation. Instead, X-rays, a more energetic type of light, are produced by means of a technique called laser wakefield acceleration. An intense laser pulse is shot into a gas target, which turns into a plasma, i.e. a state of matter where electrons are separated from their nuclei. The pulse excites a plasma wave, and electrons are trapped behind the laser pulse as it propagates through the target, following in its wake like surfers on a sea wave, accelerating to extreme energies while simultaneously wiggling up and down. As they change direction in this wiggling motion, they lose energy in the form of X-rays. This radiation has many frequencies (it is broadband) and is incredibly short, less than 40 fs. To put this in perspective, light can travel about 300 000km in a single second (more than 7 round trips around the Earth’s equator!), but only travels about 12 microns in 40 fs: less than the width of a human hair.

To measure the spectrum of the X-rays, it is inconvenient to use conventional
methods based on spectrometers which rely on physical dispersive elements such as prisms or crystals, because with these the spectrum can be measured only for single points in the sample, which would be very inconvenient for imaging. Instead, the technique hereby proposed uses nothing more than a CCD camera. When X-rays strike into the pixels of a camera, they generate a number of electrons which is proportional to their energy (and therefore also their frequency). This means that if a few X-ray photons are "sprinkled" here and there on the sensor, it is possible to derive the distribution of the light source frequencies by creating a histogram of these single-photon events, which is nothing but the spectrum! This technique is called single-photon counting, and an algorithm has been developed by the author for this purpose.

After calculating the spectrum before and after the sample in this way, another
algorithm has been developed to automatically detect the presence and type of a metal in an image by comparing the two spectra and using the K-edge of a metal, a dip in the transmission spectrum which occurs at different energies for different metals, that can be used as a sort of fingerprint.

• author: Angella, Andrea
• supervisor: Olle Lundh; Giada Cantono
• organization: Atomic Physics; Department of Physics
• course: PHYM03 20221
• year: 2022
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Multi-spectral imaging, single-photon counting, X-ray absorption spectroscopy, XAS, LWFA, laser-wakefield acceleration, K-edge, imaging, metal-detection algorithm
• publication/series: LRAP
• report number: LRAP584
• language: English
• id: 9090778
• date added to LUP: 2022-06-20 14:37:32
• date last changed: 2022-06-20 14:37:32