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How Neutron Bells Toll
December 1, 2014   
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An international team of physicists has observed—for the first time with such precision—vibrations of the surface of a heavy nucleus, lead 208Pb. Their measurements have made it possible to unravel the secrets of neutron oscillations in the atomic nucleus and determine how many neutrons on the surface of the nucleus participate in unique vibrations known as pygmy resonances. These findings give scientists a new insight into the mysteries of the universe. They may also have far-reaching implications for both theoretical and applied physics.

If an accelerated ion of high energy impacts the nucleus of a heavy element, it makes the nucleus vibrate in a specific manner: all of its neutrons begin to oscillate collectively with respect to all of its protons. However, close to the point of impact, these constituents of the atomic nucleus may display additional vibrations on the surface of the nucleus, called pygmy resonances. Up to now, the number of particles on the surface of the nucleus involved in a pygmy resonance—or the number of neutrons particularly affected by the ion impact—was unknown. This mystery could only be resolved by performing extremely precise measurements. Such measurements were possible at a nuclear research facility in Legnaro, Italy, where an international team of nuclear physicists used for this purpose the recently assembled AGATA gamma-spectrometer of the latest generation.

“You can figuratively compare resonances within the nucleus to what happens during an earthquake, where all buildings vibrate in a more or less consistent rhythm, just like neutrons in a giant resonance. But close to the earthquake’s epicenter, bells on church towers may toll with their own rhythm – in an analogy to a pygmy resonance”, says Prof. Adam Maj of the Polish Academy of Sciences’ Institute of Nuclear Physics in Cracow.

So far in studies of pigmy resonances, heavy nuclei were bombarded by light particles. This could excite the nucleus in many different ways, so it was difficult to unravel the vibration phenomena. But this time, much heavier ions of oxygen 17O were used to bombard lead 208Pb target nuclei. If such ions hit such atomic nuclei, vibrations of the target nucleus occur almost exclusively on its surface.

The accuracy of the measurements carried out using the AGATA spectrometer was so high that for the first time the researchers were able to “see” what was happening at surface of the nucleus. As a result, they were able to reliably assess how neutrons actually vibrate and how many of them take part in the pygmy resonance caused by the collision.

The AGATA (Advanced Gamma Tracking Array) is an ultra-modern instrument for recording gamma radiation that recently began operating at the Legnaro National Laboratories, part of the Italian Institute of Nuclear Physics. Studies using the AGATA spectrometer are being carried out by a team of physicists from several nuclear research laboratories in Poland, Italy, Germany, France, Spain, Sweden, Norway and Britain. The Polish group in Legnaro is a team of physicists from the Cracow-based Institute of Nuclear Physics who have studied nuclear resonances for many years.

The basic building blocks of atomic nuclei are positively charged protons whose electric charge must be equal to the total charge of negative electrons in that atom. Thus, the number of protons in the nucleus determines the number of electrons required to balance their charge. In turn, electrons are responsible for the chemical properties of the elements, hence the number of protons in the nucleus determines what the chemical element is. The atomic nuclei also contain neutrons that are proton-like particles but without any electric charge. Atomic nuclei that have the same number of protons but a different number of neutrons are called isotopes of that element.

In many elements the number of neutrons in the nucleus is equal to or close to the number of protons. However, in the nuclei of heavy elements the number of neutrons may considerably exceed the number of protons. In the experiments performed at Legnaro the target nucleus of lead 208Pb contained 82 protons and 126 neutrons.

”These excess neutrons tend to place themselves at the surface of the nucleus, forming a ‘neutron skin’ that surrounds the protons and the remaining neutrons of the nucleus,” explains Mateusz Krzysiek, a Ph.D. student at the Institute of Nuclear Physics.

It has been known for a few decades that if the nucleus of a heavy element is hit by another particle, such as an electron or a helium nucleus (which consists of two protons and two neutrons bound together), the neutrons in the target nucleus will vibrate together with respect to protons in that nucleus. These mutual collective oscillations of protons and neutrons occur at a high frequency, therefore with a lot of energy. Physicists call such oscillations the giant dipole resonance. But the nature of oscillations of the neutron skin of the atomic nucleus—whether it oscillates by rocking sideways or whether it ”breathes” back and forth with respect to the center of the nucleus—remained an open question.

In atomic nuclei that have a neutron skin there is yet another mode of oscillation: the skin neutrons located close to the point of impact will not only take part in their collective motion against protons, but may also vibrate on their own. The energy of this specific vibration is so high that to release it, the nucleus will emit high-energy gamma-ray quanta. Far fewer neutrons are involved in such resonances than in a giant resonance, therefore their gamma-ray signal is much weaker and more difficult to detect. For these reasons such localized vibrations have been termed pygmy resonances.

Around the world, pygmy resonance studies are becoming increasingly popular among nuclear physicists. This is because such resonances played a significant role in the development of neutron stars and in the synthesis of elements during the early evolution stages of the universe after the Big Bang.
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