Stopping ultrafast particles and observing what has never been seen before
30 May 2008 (Volume 3 Issue 5)
In physics, researchers have been engaged in building larger and larger accelerators, and exploring unknown particles and phenomena resulting from the collision of high-energy particles accelerated to ultrahigh speeds. This methodology, however, is said to be approaching its limit of application, because still larger accelerators would require vast amounts of time and money to build. Reversing this way of thinking, researchers at the Atomic Physics Laboratory in the Advanced Science Institute, formerly the Discovery Research Institute, are working to elucidate the nature of materials at high levels of accuracy that have not been achieved to date, by decelerating the high-speed particles generated by accelerators.
“Dr Shin-ichiro Tomonaga, the 1965 Nobel laureate in physics and an ex-member of RIKEN, is reported to have said, ‘Modern physics is just torturing nature.’ Traditionally, physics has been directed toward bringing hidden truths to light by accelerating particles to render them in high-energy states. We take the reverse approach, based on the idea of stopping fast-moving particles and listening to nature’s whispers,” explains chief scientist Yasunori Yamazaki concerning the laboratory’s research strategy. His major targets are unstable atomic nuclei and antihydrogen.
Stopping ultrafast particles within one second
One of the main areas of research at the laboratory is the study of unstable atomic nuclei, which is conducted under the leadership of Michiharu Wada, senior researcher at the laboratory. An atomic nucleus consists of protons and neutrons. There are a wide variety of atomic nuclei comprised of combinations of different proton and neutron numbers. It should be noted, however, that less than 300 kinds of atomic nuclei are stable in nature. All other atomic nuclei are destined to disintegrate (unstable nuclei).
Currently available models concerning atomic nuclei have been established on the basis of the findings of stable nuclei. “Theoretically, there are more than 10,000 kinds of atomic nuclei, so examining only 300 kinds does not lead to a proper understanding of atomic nuclei,” points out Wada. A method invented in the 1980s made it possible to artificially create a great many unstable nuclei using an intermediate-energy heavy-ion accelerator. Characterizing such nuclei has revealed unique features of atomic nuclei that have systematically refuted the arguments previously held as ‘common sense’ one after the other. For example, some neutron-rich unstable nuclei with many more neutrons than protons were found to have a wide distribution of neutrons on the outer side of the nucleus, in contrast to stable nuclei, in which neutrons and protons are uniformly distributed throughout the nucleus within a common radius. Now the common-sense view of the atomic nucleus is being challenged.
At RIKEN, the Radioactive-Ion-Beam Factory (RIBF)—a state-of-the-art accelerator facility—came into operation in 2007. The RIBF is capable of accelerating heavy ions, such as uranium ions, to levels up to more than 50% the velocity of light, and is expected to produce 4,000 kinds of atomic nuclei, including 1,000 new nuclei. When they are synthesized, the atomic nuclei are moving at ultrahigh speeds, equivalent to temperatures of 10 trillion kelvin. “We are working to accurately determine the true nature of the nuclei by decelerating and stopping them, and cooling them to an ultralow temperature of about 0.1 kelvin within one second, so as to perform ultrahigh precision spectroscopy,” says Wada.
Michiharu Wada and others first proposed the project in 1997. The aim of the project is to develop apparatus for cooling the atomic nuclei of all the elements produced at high energies to ultralow temperatures, and to perform precision measurements of individual nuclei. Wada says, “I don’t like doing research that follows others.” Then how do they achieve instantaneous cooling from 10 trillion Kelvin to 0.1 Kelvin?
First, atomic nuclei moving at ultrahigh speeds are passed through a metal plate to slow them down. The decelerated nuclei are shot into a large two-meter-long vessel filled with gaseous helium, and further decelerated and cooled to room temperature. In this stage, many of the nuclei are in the form of singly charged positive ions. The nuclei are then brought into a vacuum and further cooled to about 0.1 kelvin by a method known as laser cooling. “During this process, it is quite difficult to transport the ions from the gas-vessel to the vacuum. The ions must be drawn out quickly by applying an electric field, but they soon get attached to the vessel walls and electrodes if a simple system is used.” Wada solved this problem using the radiofrequency (RF) carpet electrode —a specially designed electrode that he developed himself (Fig. 1). Using this design, an ion-barrier field is built on the electrode surfaces, and the ions are brought into a vacuum without touching the surface through a small hole at the center of the RF carpet. He adds with a smile, “I tried a variety of electrode shapes and other factors—my efforts and failures yielded piles of rubbish.”
Figure 1: Michiharu Wada, Senior Research Scientist, holding a radiofrequency carpet electrode.
The electrode has very thin ring electrodes arranged concentrically at 0.3-mm intervals to form a barrier to keep ions away from the surface of the electrode. Ions are brought into a vacuum through a small hole in the center of each ring the electrode.
Discovering what is beyond common sense
To date, the structures and properties of unstable nuclei have been examined mainly by analyzing the phenomena that occur when they collide with other nuclei at high speeds. However, little is known about the forces that govern the interaction in the collision. Therefore, any data obtained by analyzing such collision-related phenomena unavoidably involve uncertainty. On the other hand, if unstable nuclei are stopped and cooled to ultralow temperatures, it is adequate to make measurements based on electromagnetic forces, such as laser spectroscopy. Because the theory of electromagnetic forces is well established, the results extracted contain no ambiguity. “In addition, laser spectroscopy enables the most accurate measurement of the properties of atoms. The properties of atomic nuclei can be studied with high precision by accurately measuring the properties of atoms,” says Wada. For example, the transition energy of atoms, which will be used as the time standard by the next generation, can be measured with an astonishing level of accuracy, of the order of ten quadrillionths (10 × 10–14) by laser spectroscopy.
Wada and others developed a prototype cooling apparatus into an accelerator facility before RIBF, and have already succeeded in making laser spectroscopy on the unstable nuclei of beryllium isotopes. They are planning to build a new cooling apparatus, SLOWRI (Slow RI-Beam Facility), in the RIBF, and to make accurate measurements of the radii, moments, masses and other parameters of the new atomic nuclei that will be created (Fig. 2). “I will explore atomic nuclei that have not been studied by anyone with unprecedented levels of accuracy,” says Wada. “What I am expecting most is that things beyond common sense—events that have never been imagined before— will be revealed. A great many kinds of atomic nuclei exist, and it is necessary to begin examining each of them one by one.”
Figure 2: Slow RI-Beam Facility (SLOWRI).
An important feature of the apparatus is its capability for precision measurement of the atomic nuclei of all kinds of elements, irrespective of their chemical properties. The construction of SLOWRI remains to be scheduled because of budget issues. In Europe and the USA, projects to construct next-generation accelerator facilities with a performance equivalent to RIBF are ongoing. Following Wada’s research plan, apparatuses like SLOWRI are also being developed.
Challenging the puzzle of antimatter
Another area of research at the laboratory is the study of antihydrogen, which is conducted under the leadership of Yamazaki. Antihydrogen consists of negatively charged antiprotons and positively charged positrons (Fig. 3). Antiprotons and positrons are generically called antiparticles, and they have charges that are opposite to the particles around us, such as protons and electrons, although they share the same mass and lifetime. When a proton collides with an antiproton, or when an electron collides with a positron, pair annihilation occurs to produce light.
A substance consisting of antiparticles, like antihydrogen, is called antimatter. According to observations that have been performed to date, our Universe consists of matter, and there is no other world consisting of antimatter. However, this poses a major riddle. In the Big Bang Theory of the birth of the Universe, it is hypothesized that light existed first, from which particle and antiparticle pairs were produced. Modern physics theorizes that particles and antiparticles were produced in the same number. If this is true, the particles and antiparticles must have annihilated, resulting in the restoration of a Universe in which only light exists.
Figure 3: Hydrogen versus antihydrogen and pair production versus pair annihilation.enlarge image
Were slightly more particles than antiparticles produced for some reason? And was our Universe formed from the substances that escaped pair annihilation? If this is true, there must be some difference in nature between matter and antimatter. However, today’s physics theories are based on the assumption that there is no such difference between matter and antimatter.
Then why does the Universe possess matter substances in abundance? Is the current theory of physics incorrect? To solve this big problem, Yamazaki and his colleagues plan to produce antihydrogen and accurately measure its nature in search of how it may differ from hydrogen.
It was 1955 when antiprotons were generated for the first time using an accelerator. In 2002 it became possible to decelerate antiprotons and mix them with positrons to produce antihydrogen experimentally. Synthesis of antihydrogen was achieved using antiprotons supplied by Conseil Européen pour la Recherche Nucléaire (CERN) in Switzerland.
An experimental study for the synthesis and accumulation of antihydrogen
First, Yamazaki and his colleagues developed a technology for accumulating large amounts of positrons and antiprotons for use as the ingredients of antihydrogen. “We had been the world record holders for the number of antiprotons accumulated for several years, and we achieved an accumulation of the order of ten million in 2007.” They also developed a new technique for mixing positrons and antiprotons in a magnetic field trap in a special configuration to synthesize and accumulate antihydrogen. Yamazaki continues, “Actually, the development was achieved by taking advantage of the discovery of an unexpected computational result by a graduate student in our laboratory.” It was found that the trap is capable of accumulating antihydrogen produced at relatively high temperatures, and that the accumulated antihydrogen undergoes spontaneous cooling while moving in the trap. Hence, the new technique allowed simultaneous synthesis, accumulation, and cooling to be achieved.
Yamazaki and his colleagues developed an apparatus for antihydrogen synthesis (Fig. 4) based on the new technique, and began an experimental study to synthesize and accumulate antihydrogen at CERN in 2007. “Our first goal was to introduce antiprotons into the apparatus and accumulate them. This was quite easily achieved with a very high efficiency of more than 70% in November.” He continues to describe how they successfully accumulated three million antiprotons, several hundred times as many as the maximum number that had been attained using conventional apparatus. “Generally, in the world of science, a change in the amount handled by one order of magnitude will qualitatively change the world. Based on this achievement, we are going to produce and accumulate a large amount of antihydrogen for the first time in the world.”
Figure 4: Antihydrogen synthesizer.
The Atomic Physics Laboratory is also planning to perform experiments in which antiprotons and unstable atomic nuclei are simultaneously trapped to synthesize antiprotonic unstable nuclear atoms, and the annihilation signals are detected to investigate the distribution of protons and neutrons on the surfaces of the unstable atomic nuclei.
After succeeding in large-scale synthesis and accumulation, what properties of antihydrogen are they planning to examine? “Hydrogen and antihydrogen behave as small magnets. First, I would like to determine the strength of the magnet of antihydrogen to a precision of a tenth of a millionth (10–7) using microwaves, and compare it to hydrogen.
The currently prevailing theory says that hydrogen and antihydrogen share exactly the same value. If they were found to have different values, what would happen? It would be an astounding event,” replies Yamazaki. It is generally believed that they probably share the same value. However, he explains that in fact, antiparticles were a ‘nuisance’ that emerged when the British physicist Paul Dirac synthesized a unified theory of quantum mechanics and special relativity in 1929. It seems Dirac himself did not believe in the existence of antiparticles initially. Two years later in 1931, however, the genius began suggesting the existence of antielectrons (positrons) and antiprotons, a prediction which profoundly impressed Yamazaki. Additionally, as if synchronized with this, positrons were actually detected in cosmic rays in 1932. “The beyond-common-sense thing that had been predicted, but not believed, by Dirac proved to be true, and this is the basis for the view of the world in modern physics.”
Yamazaki and his colleagues are also planning to make experiments to examine the gravitational forces between antihydrogen and matter. “To date no experiments have been made to determine how gravity is exerted between matter and antimatter. There is even a theory available that gravity is exerted on antimatter in the reverse direction, that is, not as a pulling force, but as a repelling force.” If this is true, antihydrogen will go upward in response to the Earth’s gravity, rather than dropping toward the ground. From the common-sense view, this is very unlikely, but it is of paramount importance to perform experiments since natural science is essentially a discipline of experimentation.
Curiosity about unknown things
“I have one more investigation I want to describe now,” continues Yamazaki. Particular structures in biological cells are destroyed with a precision of several hundred nanometers (1 nm is one billionth of a meter, 10–9 m) by means of the beam technology established in his laboratory, and their functions are examined.
“I have begun joint research with RIKEN biologists using the new technology. I am finding cells quite interesting. Our new technology will allow us to elucidate intracellular functions that have not been clear. This research is not relevant to investigations of atomic nuclei and antimatter, but in my mind there is no major difference. Everything interesting or everything unknown is the subject of my research.”