Liquid lithium rid the ions in the beam of extra electrons

(ORDO NEWS) — A new method for separating electrons made it possible to increase the power of an ion beam intended for the synthesis of isotopes with unusual ratios of the numbers of protons and neutrons.

In stable isotopes , the number of neutrons usually varies within a few pieces, while in radioactive isotopes it can vary over a much wider range. For example, stable isotopes of nickel have from 30 to 36 neutrons, and radioactive ones have from 20 to 52.

If you place all known isotopes on a map, along the axes of which the numbers of protons and neutrons are plotted, you get a narrow “ridge of stability” and wide “shoals » radioactive isotopes.

The number of possible radioactive isotopes may be about six thousand, and so far scientists have synthesized only about half of them.

The study of unstable isotopes with a large imbalance of protons and neutrons is of great interest – by studying the features of their decay, physicists step by step improve the description of the forces that hold the nuclei together.

It is necessary to study such isotopes literally “on the fly” – their half-life is usually from tenths to thousandths of a second (but sometimes much less). In 2022, the FRIB (Facility for Rare Isotope Beams) laboratory at Michigan State University began this work.

Liquid lithium rid the ions in the beam of extra electrons 1
Map of isotope stability depending on the content of protons (vertically) and neutrons (horizontally). The lines mark the numbers of protons and neutrons that give the nuclei increased stability. Black shows stable isotopes, other colors correspond to the half-life: brown – millions of years, orange – years, green – seconds

The process of obtaining unstable isotopes in the FRIB laboratory is implemented as follows. First, ions are obtained from a heavy element, such as xenon or uranium, which are accelerated and sent to an electron separator (charge stripper).

In it, the ions are deprived of almost all electrons and are sent to the main accelerator, and from there the ion beam hits the target.

Colliding with the target nuclei at a speed of up to half the speed of light, the beam nuclei “scatter” into large fragments, which are sorted by a magnetic field and sent to a trap surrounded by decay detectors. According to the energies and types of emitted particles, scientists restore the structure of the nucleus.

Complexity awaited the developers of the new facility at the stage of electron separation, which is necessary to increase the charge of ions and the efficiency of their acceleration in front of the target.

Collisions produce nuclei with a wide variety of protons and neutrons, but many isotopes are produced too rarely. To increase the rate of their formation, it is necessary to increase the current of the ion beam, and the electron separator cannot withstand this.

Liquid lithium rid the ions in the beam of extra electrons 2
Expected rate of isotope production at FRIB at maximum beam intensity (50 trillion uranium ions per second). Horizontally and vertically – the number of neutrons and protons in the nucleus. The black line shows the boundary of known isotopes. The color coding is logarithmic, with an increase in the value per unit, the rate of formation of isotope nuclei increases 10 times. Zero corresponds to 1 core per second (blue-green), 10 to ten billion per second (red)

In less powerful accelerators, the electron separator consists of graphite foil, but when flying through it, the ions destroy the crystal structure of graphite. It turned out that graphite burns out too quickly in the FRIB beam.

Researchers led by Takuji Kanemura have invented a self-healing electron separator that bypasses the beam power limitation. To do this, they used a powerful stream of molten lithium.

The choice of this particular metal is due to two factors – light lithium atoms are not able to strongly scatter a beam of heavy ions flying through it, and its high boiling point prevents violation of the vacuum that is necessary to maintain the beam.

Liquid lithium rid the ions in the beam of extra electrons 3
Image and diagram of a lithium electron separator. The nozzle is shown in green, the deflector in blue, the ion beam trajectory in red, and the lithium jet in gray, which turns into a film by the deflector and breaks into drops at the bottom. Drops of molten lithium wetting it are visible on the deflector

In a liquid-metal electron separator, a jet of molten lithium exits a nozzle and hits the edge of a deflector, which turns it into a film 10-20 micrometers thick, flying at speeds up to 180 kilometers per hour.

The beam passes through this film, but each volume of the melt is exposed to it only for a very short time, and does not have time to heat up and boil. Using a lithium separator, the researchers were able to raise the beam power to 400 kilowatts.

A similar solution is used in ultra-bright x-ray tubes. Their anode is a jet of metallic gallium capable of withstanding a focused electron beam of such power that it would vaporize even the most refractory materials.

The use of rare radioactive isotopes is not limited to the study of nuclear forces. The FRIB plant produces a huge amount of isotopes – including those used in nuclear medicine and other areas – and scientists plan to collect them for further use.

In addition, this method of obtaining radioactive nuclei can be useful when reaching the ” island of stability ” on the isotope map, which contains superheavy elements with very interesting chemical properties.

So far, scientists have reached only the unstable neutron-deficient edge of this “island”. In its center, isotopes with half-lives of millions of years can be found, but the problem of neutron deficiency does not allow moving deeper.

Usually, superheavy elements are synthesized by bombarding a transuranium target with light nuclei, and all sufficiently stable isotopes from which “projectiles” and targets can be prepared in advance contain too small fractions of neutrons.

Unstable nuclei can contain many more neutrons: if enough of them can be synthesized on the fly and immediately sent to the target, the problem of neutron deficiency can be overcome.

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