NEW WORLDS: Colder and colder

How Israeli researchers found a new way to freeze ions.

December 17, 2017 04:26
4 minute read.



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When investigating atoms, scientists face a challenge – at room temperature, individual atoms in a gas have kinetic energy and spin about at high velocities. Temperature is, in essence, the measurement of the relative movement between atoms; thus the goal of getting the atoms to have small relative velocities involves freezing them to extremely cold temperatures.

A group at the Weizmann Institute of Science in Rehovot has now developed a new universal method for cooling ions, and has published its discovery in Physical Review Letters.

Ions – atoms with electric charges – are currently first cooled in traps using electric and magnetic fields and then further cooled with lasers. The new method, which does not require lasers, was developed by scientists Dr. Oded Heber and Dr. Michael Rappaport, and postdoctoral fellows Dr. Reetesh Kumar Gangwar and Dr. Koushik Saha, in the lab of Prof. Daniel Zajfman of the particle physics and astrophysics department – who is also president of the institute.

Heber said that the new method is significant because the cooling process does not depend on either the type or the weight of the ion. Thus it might be used, for example, to investigate the properties of large biological molecules or nanoparticles.

Zajfman and his group had previously created an improved version of an ion trap called an electrostatic ion-beam trap – an apparatus for storing ions that was much smaller than the standard ion storage rings, which tend to be very large and expensive. In an electrostatic trap, ionic molecules oscillate as they fly at speeds up to 10,000 km/hour, and cool down within the trap.

Systems like this can recreate in the lab the sparse matter that exists in interstellar space.

When groups of ions are oscillating in the trap at these high speeds, there is a natural distribution of frequencies.

At this stage, the scientists have a method in which “variable periodic impulse voltage” is applied to separate out and accelerate only the coldest ions in that distribution. By continuing to apply voltages, researchers can eventually end up with the very coldest ions.

“This process,” noted Heber, “is not so much cooling as ‘filtering’ or sorting ions according to the temperatures they have reached.”

In recent experiments, however, the group tuned the trap so the density of the ions in the electrostatic ion-beam trap can be increased 1,000-fold at the edges.

Increasing the density naturally increases the incidence of collisions between the ions in the beam, and the result is that energy gets shared between the ions.

The scientists discovered that there was an enhanced correlation between the position of an ion within the group and its kinetic energy level, the coldest ions being in the center. Indeed, the energy – or temperature – was transferred to the ions at the edges, producing more extremely cold ions in the accelerated bunch. “This surprising process,” said Heber, “already passes the test of genuine cooling.”

In a series of experiments, the ions reached temperatures of about a tenth of a degree above absolute zero. The researchers are currently conducting further experiments to fine-tune the system to lower the ion temperatures even more.

SINGING OF THE MICE Mice, like birds, are natural-born singers. From birth, they emit a wide repertoire of vocalizations especially in the ultrasonic range, which are inaudible to humans.

They emit ultrasonic vocalizations (USVs) to form complex patterns to communicate with each other. The amount of calls and sequences of different types of vocalizations are an important part of the communication process. Mouse vocalizations are also used as a model system for research into neuropsychiatric disorders such as autism.

The mouse “songs” are recorded with ultrasonic microphones and analyzed using manual methods, which are very time-consuming. Some research groups use commercial software, which is not completely accurate.

Researchers at the Konrad Lorenz Institute of Ethology and the Austrian Academy of Sciences recently developed a new method to automatically detect mouse song, which they call the automatic mouse ultrasound detector (A-MUD).

The songs that mice form with the USVs differ not only in the sequence of sounds, but also in their duration and complexity. In this way, the mice can respond specifically to their social environment by addressing possible sexual partners or unfamiliar fellow mice. The scent of a female mouse, for example, can be enough to trigger vocalization in males.

Most studies in the field have concentrated on the vocalizations of domesticated laboratory mice. How wild house mice use their vocalizations, however, remains largely unexplored. “The use of different vocalizations outside of the laboratory could help us to understand when and how the animals communicate with each other in their natural surroundings,” said Dustin Penn, the principal investigator.

“That requires a reliable and efficient method for data processing and analysis,” he added. “With A-MUD, we can make such a method freely available to other research groups. And we are currently working on a second, improved version of our tool.”

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