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Extremely rare observations of “tennis-like” vibrations of lead

The tennis ball deforms when it bounces off the strings of a racket. It flattens for a moment and elongates along the direction of motion. Traces of similar oscillations occurring in the 208Pb lead nuclei have recently been registered by scientists from the PAS Institute of Nuclear Physics. The only previous observation of a similar phenomenon is more than thirty years old.

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Interior of the vacuum chamber for observations of the lead nuclei oscillations. A target made of lead 208Pb is placed in the middle between two cylindrical gamma radiation detectors. A proton beam from the PROTEUS cyclotron, coming from the lower left corner, hits the target. Scattered proton detectors are visible in the upper right corner.

The unique experiment, described in the journal Physical Review C, was carried out at the Bronowice Cyclotron Center, part of the Institute of Nuclear Physicsc of the Polish Academy of Sciences in Cracow. The 208Pb nuclei of lead atoms were excited by collisions with protons. Physicists performed a series of sophisticated measurements of gamma quanta, and the collected data confirmed that there are "tennis" vibrations in the nuclei. 

We are able to observe subtle oscillations of 208Pb lead nuclei using our facility due to the specific features of the Proteus C-235 accelerator we have here. The main purpose of the device is to irra-diate tumours, including eye tumours. However, one of the four lines of the accelerator was designed with physics research in mind. The uniqueness of the accelerator is due to the energy range of the protons it delivers. Throughout the world, almost all accelerators available to physicists give protons energies either significantly lower or significantly higher than ours”, explains Prof. Adam Maj.

Gamma quanta

In the Proteus cyclotron protons can reach energies of 70 to 230 megaelectronvolts (for comparison: the energy of protons in the LHC accelerator can be even hundreds of thousands of times greater). When excited by a collision with a proton, the lead nucleus can decay into secondary particles or move to a lower energy state, which is combined with the emission of a gamma-ray quantum. The two cases are fundamentally different: the energies of secondary particles can be virtually arbitrary, while the energies of the gamma quanta must correspond to the differences between the specific energy states of the nucleus. This means that it is the gamma quanta that carry the most valuable information about the structure of the atomic nucleus.

Decays of gamma resonances in "cold" nuclei

“Our international team specializes in observations of decays with emission of gamma quanta of particular excitations of nuclei, known as giant resonances”, says Dr. Maria Kmiecik (IFJ PAN), and then specifies: “So far we have studied decays of such resonances in 'hot' nuclei, i.e. those excited to high energies. However, now, thanks to an appropriate choice of experimental conditions and measurement devices, the HECTOR system of high-energy gamma-ray detectors from the University of Milan and the KRATTA matrix detector for scattered protons built in Cracow, we have managed to see decays of gamma resonances in 'cold' nuclei, i.e. those excited on ground states. What is particularly important we were also able to observe oscillations of the nucleus as a whole, being the effect of a giant quadrupole resonance.”

Giant resonances

When a single proton with appropriately selected energy collides with the spherical nucleus of lead 208Pb, it can stimulate them to various oscillations, especially those associated with giant resonances. Physicists use the adjective "giant" to emphasize that resonances of this type occur much more frequently than others. 

Giant resonances come in two basic varieties. 

  1. Giant dipole resonance (GDR) – protons and neutrons in the nucleus oscillate with respect to each other. As a whole, the surface of the nucleus does not change shape then, it only vibrates.  
  1. Giant quadrupole resonance (GQR) – manifests itself in the form of a deformation of the entire surface of the nucleus, which begins to alternately flatten and elongate along a certain direction. The phenomenon resembles a deformation of a tennis ball after bouncing off a racket, momentarily it compresses and extends along the direction of the bounce.

The scale of the difficulty 

The detection of gamma quanta emitted by excited 208Pb lead nuclei is not a simple task. In the case of a giant dipole resonance, which is much easier to induce, decay with gamma emission occurs about a hundred times less frequently than the decay by particles standardly observed. In the case of quadrupole resonance, the probability of emission of a gamma quantum decreases another hundred times, and observations are made more difficult by the fact that the phenomenon occurs on a background of its simpler cousin”, explains Dr. Barbara Wasilewska from the Institute of Nuclear Physics.

Thirty years after the first observation 

The results obtained by physicists in Crocow perfectly harmonize with the results of the experiment held over 30 years ago. At the same time, they carry new, qualitatively important information. Scientists who previously recorded the excitation and gamma decay of the giant quadrupole resonance conducted their measurements by bombarding lead targets with heavy ions. Meanwhile, the present result clearly indicates that even much lighter protons can be used to make the surface of heavy atomic nuclei vibrate.

Warm-up before the next experiments 

These measurements are an introduction to a series of more sophisticated experiments on similar phenomena in other atomic nuclei. The team working at the Bronowice Cyclotron Center has already started new activities, with even more improved equipment. The gamma quantum measurement system was replaced with a new generation PARIS detector system. Scientists are particularly interested in pygmy resonances and oscillations of non-spherical nuclei, which still elude theorists' predictions.

The paper γ decay to the ground state from the excitations above the neutron threshold in the 208Pb(p,pγ) reaction at 85 MeV” has been recently published in Physical Review C.

The research was carried out with funding from European Horizon 2020 grants (IDEAAL and ENSAR2) with the support of a grant from the Polish National Science Centre.

Source of information and image: PAS Institute of Nuclear Physics