Conducting research on the phenomenon of radioactivity is the only way of gaining insight into the properties of some atomic nuclei. Polish scientists from the Faculty of Physics, University of Warsaw have built a unique detector that has made it possible to observe atypical decays of one of the isotopes of nickel, during which two protons were emitted simultaneously.
An article on the subject will appear in the journal Physical Review C.
Isotopes are varieties of a chemical element that share the same number of protons in the nucleus but differ in the number of neutrons. The mass number is the total number of neutrons and protons. Nickel has 28 protons in the nucleus and at least 30 isotopes, including five stable ones, for example nickel-58. Isotopes of nickel in which the equilibrium between the number of protons and neutrons is disturbed the most are hard to obtain and even harder to study -- they are unstable and decay quickly, transforming into nuclei of other elements. Scientists from the Nuclear Spectroscopy Division at the Institute of Experimental Physics (IFD) of the Faculty of Physics, University of Warsaw (FUW) have undertaken research into nickel-48, a highly peculiar isotope. It has 28 protons and only 20 neutrons in its nucleus. It is the most neutron deficient nucleus ever studied. Such an isotope "lives" only 2 thousandths of a second and then decays. The research by the Polish scientists from FUW has shown that the most frequent decay mode of nickel-48 is two-proton emission.
Protons ejected from a nucleus carry information about its internal structure. In order to gain insight into the structure, the correlations between the emitted particles are studied by observing their tracks. An appropriate device is therefore necessary. "The detectors used earlier recorded electronic signals in which all the information about the correlation between the two protons was lost," says Prof. Marek Pfützner from the Nuclear Spectroscopy Division IFD FUW, head coordinator of the research. In the detection method developed by Polish scientists images are recorded by a camera, making the results easy to interpret -- one can simply see what happened. The cutting-edge detector was built in Warsaw according to the design of Prof. Wojciech Dominik from the Particles and Fundamental Interactions Division IFD FUW. The pioneering device not only makes it possible to gather information about the tracks of charged particles moving inside a chamber but also generates their striking visual image. The experiment using the Polish detector was conducted in the U.S. at the National Superconducting Cyclotron Laboratory in Michigan with the collaboration of the University of Tennessee and the Oak Ridge National Laboratory.
The process of the production of unstable nickel has several stages. Atoms of the stable isotope nickel-58 are accelerated in a cyclotron and subsequently directed towards a revolving nickel target containing a natural mixture of stable isotopes of the element. The collisions cause nuclear reactions and a beam of various isotopes of different elements is created. It falls into a magnetic separator, which makes a selection on the basis of mass number. The selected beam falls into a detector filled with a mixture of gases -- helium, argon and nitrogen. There, as a result of the stopping power of the gaseous medium, the energy is dissipated and the atoms come to a halt. The radioactive decay of their nuclei takes place. The whole event is recorded by a camera. The probability of the formation of a nickel-48 nucleus is very small. Which is why during 156 hours of measurements, when 1017 (ten with seventeen zeros!) projectiles hit the target , only six atoms of this rare isotope were observed. The nuclei of four of them decayed by two-proton emission. The rest underwent a different transformation.
"The simultaneous two-proton emission is a very rare phenomenon -- so far it has only been observed in three other atomic nuclei: magnesium-19, zinc-54 and iron-45," says Zenon Janas, PhD, co-author of the experiment. It was also the physicists from Warsaw that observed the two-proton decay of iron. "The possibility of studying such rare decays, providing rich insight into the internal structure of nuclei, has a great learning value," adds Prof. Pfützner. "It can make it possible to verify hypotheses and models describing this still elusive area of matter that makes up the world around us and ourselves."
"Research in physics of nuclei has a long tradition at the University of Warsaw -- first works in the field date back as early as the 1930s," says Prof. Teresa Rząca-Urban, Dean of FUW, a nuclear physics herself. Before the war, such scientists as Leonard Sosnowski and Andrzej Sołtan, who put the first Polish accelerator into operation in 1934, were at the core of nuclear physics at the University of Warsaw. After the war, Jerzy Pniewski and Marian Danysz gained renown for the discovery of hypernuclei -- atomic nuclei containing unstable particles of matter different from the one that surrounds us. Today, nuclear physicists from Warsaw participate in large international experiments, and the successes of Prof. Pfützner's group show that they can also put forward and carry out their own interesting projects and build the equipment necessary for their realization. The University of Warsaw also has its own large research device -- a heavy ion cyclotron used for research in nuclear and atomic physics and for medical applications.
Nuclear physicists do not limit themselves to fundamental research but also try to meet the current economic needs. Already in November the University of Warsaw introduces new "Nuclear power engineering and nuclear chemistry" interdisciplinary studies, a joint effort of the Faculties of Physics and Chemistry. The introduction of the new interdisciplinary studies focused on nuclear power engineering is related to the expected construction of the first Polish nuclear power plant and the need to train an adequate number of specialists prepared to tackle various aspects of its operation. The curriculum of the studies focuses, among others, on issues related to the production, storage and recycling of reactor fuel. "Students will also gain knowledge of physical phenomena, chemical processes as well as legal and administrative aspects related to the functioning of a nuclear power plant," explains Przemysław Olbratowski, PhD, coordinator of the new interdisciplinary studies.
This year also sees the centenary of the awarding of the Nobel Prize in chemistry to Maria Skłodowska-Curie and the centenary of the discovery of the atomic nucleus. In order to celebrate the anniversaries, Prof. Marek Pfützner organizes "The legacy of Maria Skłodowska-Curie -- 100 years after the discovery of the atomic nucleus" conference. It takes place between 11th and 18th September 2011 in Piaski, Mazury. The great-grandson of our Noble Laureate -- French astrophysicist Yves Langevin has confirmed his presence at the event. The opening lecture will be delivered by Prof. Andrzej Kajetan Wróblewski -- eminent physicist, popularizer and scholar of the history of science.
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