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The Warsaw Voice » Other » June 17, 2009
Physics
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In Search of Heavy Nuclei
June 17, 2009   
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Software developed by Polish physicists was used in a breakthrough experiment that created the nuclei of a new mercury isotope, 208 Hg. The experiment was conducted at the GSI Helmholtzzentrum fr Schwerionenforschung research center in Darmstadt, Germany, and described in Physical Review Letters, a journal published by the American Physical Society.

The software was created by Zygmunt Patyk, Ph.D., from the Andrzej So速an Institute for Nuclear Studies in 安ierk near Warsaw. The nuclei of the 208Hg mercury isotope obtained in the experiment consist of 80 protons and 128 neutrons. According to Marek Paw這wski, Ph.D., a spokesman for the institute, the experiment enabled physicists to "explore a new region on the map of heavy nuclei."

The atomic nucleus is built of nucleons, the common name for protons and neutrons, which account for almost the entire mass of an atom. The number of nucleons in the nucleus is what differentiates elements from one another. Nuclei with the highest numbers of protons and neutrons are called heavy nuclei.

Atoms of the same element have the same number of protons in the nucleus (atomic number), but in nature they can sometimes differ in the number of neutrons and thus the total number of nucleons (mass number). Such variations are called isotopes. Isotopes of the same element usually have similar physical and chemical properties, but as the difference in the mass number increases, differences in the properties-such as density, the melting and boiling points, and the temperature of sublimation-become greater as well. Different atomic masses in isotopes also result in different degrees of reactivity.

Magic numbers
Paw這wski says there is another important term in nuclear physics, that of "magic" numbers. These are numbers of neutrons and protons in a nucleus that make the nucleus differ significantly from those with similar mass numbers. The main difference is that the nucleus is more stable and less reactive. Scientists who deal with the structure of atoms have known these magic numbers for long, identifying them as 2, 8, 20, 28, 50 and 82. Neutrons have three other magic numbers, 126, 162 and, possibly, 184, while for protons additional magic numbers are 108 and, possibly, 114. Atomic nuclei with a specific number of protons and neutrons are called magic nuclei.

When the number of electrons in a shell reaches the maximum value (2, 10, 18, 36, 54 and 86 for consecutive shells), the shell closes and the element becomes a noble gas that can hardly react with a different element. Similar phenomena occur in the nucleus independently for protons and neutrons.

Relying on current knowledge, physicists have designed a map of heavy atomic nuclei, marking all the magic nuclei on it. The map has the form of a coordinate system whose horizontal axis marks the number of neutrons in the nucleus and the vertical axis indicates the number of protons. Each heavy element has its firm position on the map, depending on the atomic mass. One of the nuclei on the map is the 208Pb of lead with 82 protons and 126 neutrons. The heaviest magic nucleus known to date, it is doubly magic due to the number of both protons and neutrons. It has another characteristic feature in that it divides the heavy isotope map into four quadrants.

"As it turns out, it is relatively easy to create nuclei in three of the four quadrants, whereas nuclei in the fourth quadrant, ones with a surplus of neutrons and a shortage of protons, are a long shot," says Paw這wski. The 208Hg mercury isotope created in Darmstadt is located in the fourth quadrant and so its formation is an enormous achievement for the nuclear physics community, according to Paw這wski.

Software with a difference
The experiment which produced the isotope would not have succeeded without Patyk's software, which was used to measure the new isotope's mass. The software took 12 years to develop. It makes it possible to simultaneously analyze tens of thousands of spectrums obtained in experiments.

"When you use the E=mc2 formula to convert the mass of the 208Hg nucleus into energy, the result is over 190 million kiloelectronvolts (keV)," says Patyk. "The software made it possible to measure the energy with a very high level of accuracy."

The research project in which Patyk took part was designed to help explain the origin of elements whose atomic masses are larger than that of iron. Researchers suppose that elements heavier than iron are only created when stars explode. "A supernova blast releases enormous amounts of neutrons which are later captured by nuclei," says Patyk.

The significance of measuring the masses and life spans of atomic nuclei extends beyond understanding the origin of heavy elements, experts say. Such measurements also provide data on newly discovered phenomena that occur in nuclei. They help check the accuracy of physics theories and determine the values of fundamental physical constants, or quantities that are generally believed to be universal in nature and constant in time.

Julia Paw這wska
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