A new method will help scientists to measure X-rays helicity

Nikolay Kabachnik, the Leading Researcher, DSc.

SINP scientist Nikolay Kabachnik and his colleague from S.-Petersburg University Andrey Kazansky offered a theoretically based method for demonstration and measuring of X-rays helicity (or circular polarization), which provides new opportunities in the fields of biochemistry, physics and applied sciences. Russian theoretical physicists in collaboration with an International research group leaded by the scientists of the European X-ray laser based on free electrons (European XFEL) have evaluated the offered method under conditions of the real experiment. The experiment took place in the International Research Center «Elettra Sincrotrone» (Trieste, Italy) on the FERMI laser based on free electrons. The report on the study was published in Nature Communications.

The Leading Researcher of the SINP Department of physics of the atomic nucleus, DSc. Nikolay Kabachnik told us in layman's terms about the importance of X-rays circular polarity for iologists, physicists, pharmacologists and perfumers and about the method for its demonstration and measuring.

- Nikolay, please, tell us in layman's terms what X-rays circular polarization, or helicity, is.

- X-ray emission is type of electromagnetic radiation, which provide a very important property - polarization. It means that oscillations of the electric field in the electromagnetic wave are spatially oriented. If the electric field oscillates in one plane, it is linear polarization. For instance, light with linear polarization in two mutually perpendicular directions is used in 3D films. In the case when the electric field is curled like a cork-screw, it is circular polarization, it's also called "helicity". There are right circular polarization (if the electric field is curled right, in a clockwise direction) and left one (if it is curled in a counterclockwise direction). This property is typical for all types of electromagnetic radiation, icluding X-rays.

- And what X-rays with circular polarization are applied for?

- Light with circular polarization, including X-rays, is a very important instrument, for instance, in the studies of biological molecules which compose our bodies. The reason is that a variety of biologically important molecules can exist in two enantiomorphous forms: right and left. This property is called chirality. One of the basic natural puzzles is the following: why all flesh at the Earth consist of the molecules composed from amino acids only with left forms of chirality. The right forms can be produced in the laboratory, moreover, if no special measures are taken, an equal quantity of left and right molecules wil be synthesized. While living objects consist of amino acids of only left type, sugar consists of ones of only right type. It is a great puzzle. Why is it so? According to some theories dominating of right (or left) types of living objects' molecules is associated with origin of life at the Earth. In order to study right-left asymmetry of the molecules it is necessary to use radiation with circular polarization, because it is sensitive to the molecules' chirality. By the way, do you wear perfume?

- Yes, I do.

- As it happens, over 800 molecules, which are used for perfumes, provide chirality, i.e. they are right- or left-curled. And the fragrance depends on the type of molecules (right or left), which the perfume consists of. The right molecule can provide plesant aroma, while the left one (with the same composition!) - disgusting smell. So for perfumers it is very important to know how to separate right and left molecules. AN how is it possible to recognize them? By means of radiation with circular polarization.

One more application of radiation helicity is associated with pharmaceutics. Up to 70% of pharmaceuticals distributed through drug-stores contain components with chirality. Moreover, as a rule, right and left molecules of one and the same substance provide different bioactivity, i.e. medical drugs will affect human organism in different ways depending on the type of the substance molecules (right or left). And as you already know, the type of molecules can be recognized by means of X-ray with circular polarization.

And now let me tell you about our purposes. Today we have very powerful X-ray sources with huge intensity, which have been little dreamt for our experimentalists previously. They produce not weak X-rays, as in the clinics (there they are weak in order not to hurt humans). In the field of physical experiments we need more powerful sources. Currently very powerful innovative sources called free electrons lasers (briefly FEL) are developed. They are situated in the USA, Germany, Japan, Italy. In Switzerland it is being manufactured. I work in cooperation with an Institute, which is manufacturing European FEL in Hamburg.It will a giant building, about three and a half kilometers long. In fact it is a huge tube from which powerful pulsed X-ray beams will escape. These X-rays are directed to the studied object, for instance to the viruses or biomolecules. Almost all existing FELs produce X-rays with linear polarization. The only FEL which produce soft X-rays with circular polarization is situated in Italy, Trieste.

- If such FEL already exists, and you have already evaluated your method on it, what are the reasons for manufacturing one more in Hamburg?

- FEL in Trieste is small, and it produces low energy X-rays. And in Hamburg we develop a very big one, which will produce X-rays with energy about hundred times higher, so-called, hard X-rays with very short wavelength. And it is very important for the studies of very small objects, tiny ones, less than micron, with size of one molecule. In order to recognize the details of so small objects, it is necessary to use radiation with very short wavelength. You can't recognize the details using radiation with long wavelength, because in this case the wave will go round the small object without any reactions with it. And the short wave will strike the object, echo from it, and you are able to detect these events. So the smaller object you want to study, the shorter wavelength and harder X-rays you should use. A big laser, being built in Hamburg is intended for the studies of very small objects - biomolecules. Intensity of the beams is also important in order to get more X-ray pulses per second. We'd like to shoot not with a gun, but with a machine-gun with high killing power. Existing lasers are "guns", producing about 100 pulses per second, and the laser in Hamburg, which, I hope, will start its operatin in 2017, is a machine-gun: it will produce 27000 pulses per second. It is clear, that using such "rapid-fire" instrument we'll obtain necessary information more quickly.

- Please, tell us about the method, which you have developed and tested on the FEL in Trieste.

- About three years ago two theoretical physicists - Andrey Kazansky, my friend and colleague from Saint-Petersburg University, and I - offered a method for measuring of circular polarization by measuring of dichroism. What dichroism is? Dichrosim is a property of an object to absorb different quantity of clockwise polarized and counterclockwise polarized light. It is widespread in nature. For instance, fieldspar absorbs light of different types of polarization in different ways. In science dichroism is well-known. We offered to use it in order to measure circular polarization of the soft X-rays. Essentially, the method can be used also for hard X-rays, by it is of higher priority for soft X-rays, because there are no good analyzers of circular polariation, or polarimeters for it. And it is very important, because the results of the experiments with molecules depend on the power of circular polarization and on the direction of electric field curling.

- Could you tell us about the mechanism?

- In order to measure the power of circular polarization of X-ray pulse, it was directed on the target - atom of Helium. Under the influence of X-rays Helium atom emits an electron, and an ion is produced. The ion and the electron "remember" the circular polarization of X-rays, but we can't recognize and see it, as long time ago we couldn't see images on the photographic films till we developed it. In order to "develop" polarization, which was passed by X-ray pulse to the system "electron-ion", we iluminated the target simulataneously by X-rays and IR pulses of a powerful laser with known circular polarization. t is much easier to polarize and to measure polarization of IR-radiation. We can produce IR beam with 100% right or 100% left circular polarization. It is possible to find information wheather the fields in X-rays and IR beams rotate in one or in opposite directions by analysis of electrons escape and their angular distribution. Surely, the experimental results were compared with theory. And we, theoretical physicists, described what should happen. Comparison of theory with experiment allowed to determine the power of circular polarization of X-rays beam.

Using our method the researches can test before the experiments with molecules, what the polarization of their beam - right or left, and what is the power of polarization. And further act in an appropriate way.

- Are you going to continue your studies?

- Our colleagues who work at the accelerator in Trieste, are interested in our method greatly. They need to know polarization of their X-ray beam. They should guarantee to their cutomers that the beam is clockwise or countercloskwise polarized with a cirtain power of polarization. They can do it only by measurements. Physicist do not believe to the words. I suppose, that the eaperiments are to be continued.

- Good luck for you!

- Thank you.