A novel source of X-rays

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novel source of X-rays
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Optical method generated for the production of high precision bright beam X-Ray radiation.

For over a century, medical imaging has made use of X-rays produced in a specialised type of vacuum tube. The major disadvantage of this method lies in the poor quality of the emitted radiation. The source emits radiation from a large spot into all directions and over a broad energy range. These features are responsible for the relatively modest resolution attainable with this mode of imaging.

X-rays generated in synchrotrons provide much higher resolution, but their dimensions and cost preclude their routine use in clinical settings.

Imaging of microscopic structures in any sample of matter requires the use of a very brilliant beam of light with a very short wavelength. Brilliant radiation is able to concentrate a maximum amount of light quanta or photons of a single defined wavelength within the smallest area and shortest duration. Hard X-radiation is therefore ideal for this purpose, because it penetrates matter and exhibits wavelengths of a few hundredths of a nanometre. Unfortunately, the only sources of high-intensity beams of hard X-rays so far available are particle accelerators, which are typically huge and highly expensive. But there is, in principle, a far more economical and compact way of doing the job – with optical light.

A team at the Laboratory for Attosecond Physics, which is run jointly by LMU and the MPQ, has now taken an important step towards realising this goal by generating bright beams of hard X-radiation by purely optical means. Moreover, the wavelength of the emitted radiation can be readily adjusted to cater for different applications. With the aid of two laser pulses, the researchers have generated ultrashort bursts of X-rays with defined wavelengths tailored for different applications. The new source can image structures of varying composition with a resolution of less than 10 microns. This breakthrough opens up a range of promising perspectives in materials science, biology and – in particular – medicine.

The physicists focused a laser pulse, lasting 25 femtoseconds and packing 60TW of power, onto a fine jet of hydrogen gas. Note that the output of a nuclear power station is very modest by comparison, but here each pulse only lasts for 25 millionths of a billionth of a second.

One of the great advantages of the new system in comparison with conventional X-ray sources is that the wavelength of the emitted light can be tuned over a wide range. This ability to alter the wavelength allows radiologists to analyse different types of tissue, for instance.

Not only is the laser-driven radiation tuneable and extremely bright, it is produced in pulsed form. Each 25fs laser pulse gives rise to X-ray flashes of a few fs duration. This makes it useful for applications such as time-resolved spectroscopy, which is used to investigate ultrafast processes at the atomic level. The intensity of the pulses is not yet high enough for this task, but the researchers hope to overcome this obstacle with the aid of the facilities at the new Centre for Advanced Laser Applications (CALA), now being built on the Garching Campus.

The new optically generated radiation can also be combined with phase-contrast X-ray tomography, an imaging procedure that extracts information from the light that is scattered (rather than that absorbed) by an object. “Using this method, we can already image structures as small as 10 microns in diameter in opaque materials,” Stefan Karsch explains. “With our new X-ray source, we will be able to obtain even more detailed information from living tissues and other materials,” he adds.

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