How X-Rays Are Absorbed By Matter

An X-ray beam can ionize matter in just a few hundred femtoseconds, researchers say.

AsianScientist (Feb. 9, 2017) – Researchers from Tohoku University now know what happens in the million billionth of a second after x-rays hit matter. Their findings have been published in Nature Communications.

X-rays are one of the most important diagnostic tools in medicine, biology and the material sciences, as they may penetrate deep into material which is opaque to the human eye. Their passage through a sample, however, can have side effects, as the absorption of X-rays deposes energy in deep layers of the sample. In extreme cases, the application of X-rays is limited by these side effects, known as ‘radiation damage.’ Medicine is one area in which the absorbed X-ray dose must be minimized.

Surprisingly, it is unclear what happens when an X-ray is absorbed, for example, in biological tissue consisting of water, biomolecules and metal atoms. One reason for this is that the first few steps of reactions after the absorption of an X-ray, happen extremely fast, within 10-100 femtoseconds.

Within this time, in a complex cascade of events, several electrons are emitted, and positively charged reactive particles (ions) are created. Most experiments done up to now were only able to characterize this final state a long time after the cascading reaction was completed. However, it is the precise understanding of the intermediate steps that is very important for the prediction and design of radiation effects in matter.

A team led by Professor Kiyoshi Ueda has now carried out an experiment that gives an unprecedented detailed view of the first few hundred femtoseconds after absorption of an X-ray by matter.

In a biological system, water molecules are flexibly arranged around the biologically functional molecules, without strongly binding to them. To mimic this layer of water, the researchers used a flexible, weakly bonded aggregate of two different noble gases, neon (Ne) and krypton (Kr), created by cooling them to extremely low temperatures. These Ne-Kr clusters were then exposed to pulsed X-rays of the SPring-8 synchrotron radiation source which, under the conditions chosen for the experiment, preferentially ionized Ne atoms.

By using an advanced experimental set-up, the team was able to record all electrons and ions that were created at every X-ray absorption event. They found that just a few hundred femtoseconds after the initial ionization, the Ne atom that had absorbed the x-ray, as well as two neighboring Kr atoms, were all in an ionized, positively charged state.

The experiment clearly demonstrates that highly localized charge produced by X-rays in matter redistribute over many atomic sites in a surprisingly short time.

“We believe that understanding X-ray initiated processes on a microscopic level will lead to new insights across the disciplines of physics, biology and chemistry,” Ueda said.



The article can be found at: You et al. (2017) Charge Transfer to Ground State Ions Produces Free Electrons.

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Source: Tohoku University.
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