As part of its portfolio of European Prizes of the European Physical Society, the Nuclear Physics Board of the EPS has created, with sponsorship from the company EURISYS MESURES, a new prize for Nuclear Science in the name of LISE MEITNER.
On behalf of the Chairman of the EPS-Nuclear Physics Board, we are happy to announce that the first recipients are:
Peter Armbruster (GSI, Darmstadt), Gottfried Muenzenberg (GSI, Darmstadt) and Yuri Ts. Oganessian (Flerov Laboratory, Dubna).
The prize is awarded for their unique work over a long period on the synthesis of heavy elements, which has led to the discovery of the new elements in the region of nuclear charges of Z=102 to 105 (Dubnium), as well as Bohrium (Z=107), Hassium (Z=108) and Meitnerium (Z=109). These discoveries involved extensive developments of experimental techniques, and the use of a specific reaction mechanism, the "cold" fusion of two heavy nuclei. Measurements of the properties of these heavy elements provide an important cornerstone of the concept of deformed shells in nuclei, whose existence is responsible for the increased stability of the new nuclei. Because of this work the study of the properties of very heavy elements (Z=108-118) is a very active field in nuclear science.
The physics case:
The new elements (Z=102 109), increased stability for deformed nuclei
The search for missing and new elements has played a key role in chemistry and in nuclear physics in the last 100 years. The discovery of new radio-activities in the first decades of the 20th century was associated with the names of Curie, Joliot, Hahn and Meitner. Mendeleev's original table had missing elements with low mass. These are the radioactive species Technetium (Z=43, T1/2 = 2.6 Million years), and Promethium (Z= 61, T1/2 = 265days), which have rather short life- times compared to the age of our planet (4.5 Billion years). Other short lived elements between Uranium and Lead, namely Polonium (Z=84), Astatinium ( =85), Radium (Z=86), Francium (Z=87), Radon (Z=88) and Protactinuim (Z=89), were discovered and given names which reflected the national origins of the various groups of scientists working in the field. There exists a comparable "stable" island of elements consisting of the isotopes of Thorium 232Th (Z=90) and Uranium (Z=92). Later the hunt for the transuranium elements started with the irradiation of 238U with neutrons. Enrico Fermi received the Nobel prize in 1938 for "demonstrations of the existence of new radioactive elements produced by neutron irradiation". This result was later correctly reinterpreted with the discovery that these new radioactive elements were not transuranic species, but fission fragments. The fission process was discovered in the irradiation of Uranium with neutrons by the chemists Hahn and Strassman in 1938 (the role of Lise Meitner in this work is especially tragic). Already during World War II large amounts of the elements Z=93 (Neptunium) and Z=94 (Plutonium) were produced in nuclear reactors.
Irradiations of 238U with different heavy ion beams (helium, carbon, oxygen isotopes) at Berkeley lead to the discovery of elements with nuclear charges of Z = 95-100. Heavier elements were produced in several laboratories (Sweden, Russia and USA). E.M. McMillan and G. T.Seaborg (from Berkeley) received the chemistry Nobel prize (in 1951) "for the discoveries in the chemistry of the transuranium elements".
Finally, the era of fusion reactions between heavy nuclei started with the availability of beams of nuclei heavier than oxygen or neon in several laboratories in the world. The first heavy transuranium elements were produced with heavy ion beams by the irradiation of heavier targets, like Plutonium (Z = 94) and Americium (Z= 95). In this way several elements in the region of Z = 102-106 were synthesized by the group of G. Flerov, Yu. Lazarev and Yu. Oganessian at the laboratories of the JINR in Dubna, and in LBL in Berkeley by A. Ghiorso's group. Later, physicists (and chemists) turned back to using lighter targets such as lead 208Pb (Z=82) and bismuth 209Bi (Z=83) and the appropriate heavier projectiles. Thus element 102 can be produced with the [(Pu(Z=94)+18O(Z=8)] combination, or with the [208Pb(Z=82) + 48Ca(Z=20)] combination. When the closed shell nuclei Pb and Ca are used the energy balance (the Q-value) of the reaction becomes more negative. The excitation energy Ex of the fused system (Ex=ECM+Q) is minimised and only a very small number of neutrons are emitted. The incident kinetic energy Ecm is chosen to be as low as possible while still being sufficient to overcome the Coulomb repulsion between the colliding nuclei. It is very important to minimise Ex because the decay probability of very heavy elements, in particular the fission decay probability, is thereby reduced. This is the concept of "cold" fusion reactions between two heavy nuclei, which was introduced by Yu. Oganessian in Dubna. Theoretical concepts developed by theorists at Frankfurt had independently also suggested this type of reactions. However, the full impact of this concept of "cold" fusion reactions became clear only in the last 20 years with the synthesis of even heavier elements by the GSI-group lead by Armbruster and Muenzenberg.
The main experimental difficulty in identifying the new heavy elements is the low probability of their formation, which calls for unusual high intensities of ion beam beams to be used on fragile targets, and the separation of the surviving compound nucleus from the very high flux of incident projectile nuclei, typically at 0 degrees with respect to the beam direction. For this purpose the GSI-Giessen group built a "Wien"-filter (consisting of crossed magnetic and electric fields), which is a velocity filter. This instrument has been in operation at the UNILAC heavy ion accelerator of the GSI since 1976. The universal heavy ion accelerator "UNILAC", designed by Ch. Schmelzer, went into operation at GSI in 1974. The second step in the identification of very short lived isotopes produced with cross sections of nano-barns and below is a measurement of the nuclear charge and mass. This is achieved by a measurement of the decay energies and half-lives of a complete a -particle decay chain down to elements with known properties. The chemistry of these elements has recently been carried out with only several atoms per experiment!
In the middle of the sixties several groups of theoretical physicists proposed, that a new "island of stability" of very heavy elements (called "Super heavy elements") would exist in the region of nuclear charges of Z=114-124, similar to the very stable "magic" spherical nuclei like the Pb-isotopes, and some isotopes of Z=114 were predicted to be stable enough to be detected in nature. Later their life times were found to be much shorter than the 5 Billion years of the planet earth. Just as interesting, these studies showed that deformed nuclei with charge number between Z = 104 to 112 and away from closed shells may also have increased stability. Generally the Coulomb repulsion between the protons becomes so strong that the nuclear forces could stabilise only nuclei up to a maximum charge of Z~100. However, detailed theoretical studies in the last 20 years of several groups in Europe, in particular in Poland, Sweden and the Ukraine have shown, that because of certain quantal effects associated with nuclear deformation, nuclei are stabilised to the extend, that they can be observed in the laboratory. The study of the properties of the elements Z = 102 109 by the 3 laureates over the last 30 years and the observation of their increased stability has beautifully confirmed this concept of "deformed magic" nuclei.
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Life of
Lise Meitner |
In 1907 she moved to Berlin in order to "increase her true understanding of physics". She was "permitted" to hear lectures by Max. Planck, with whom she kept contact throughout her life. Soon after coming to Berlin, she became "collaborator" of the chemist Otto Hahn in the Kaiser Wilhelm Institute. She was, however, not supposed to use the main entry of the Institute, and Otto Hahn installed a laboratory in a shack ("Holzwerkstatt"). Photographs of this site are well known.
The collaboration was extremely fruitful. As the physicist in the partnership Lise Meitner was the driving force. Many important publications emerged from this period of her life. In 1913 she became a Scientific Member of the Kaiser-Wilhelm-Gesellschaft (a permanent position). From 1918 she was responsible of her own group in the radio-physical laboratory of the KW-Institut in Berlin-Dahlem (the building still exists). In 1918 Lise Meitner and Otto Hahn discovered element Z=91, which they named "Protactinium", because it was the missing link between the elements Uranium (Z=92) and Actinium (Z=89).
She looked set for a brilliant future in science. In famous photographs taken at the SOLVAY congresses in Brussels we see her in the first row among the first rank of scientists of that time (90% of them got the Nobel prize). She was awarded the title of Professor in 1928, but because of her Jewish origin, this was withdrawn from her in 1933/34.
As an Austrian citizen, she stayed in Germany until 1938 under the Nazi regime. When Austria was occupied she escaped to Sweden (without an exit visa) with the help of Dutch colleagues. She missed participation in the discovery of fission by only 6 months. Nuclear fission was discovered by Otto Hahn and Strassman by very careful chemical separation techniques. The culmination of the collaboration of Hahn and Meitner was published without her name, possibly also for political reasons. In 1939 she published the first interpretation of nuclear fission with her nephew Otto Frisch who coined the term "fission".
Her life in Sweden was difficult. In the second half of the last century, women were seldom seen in leading positions in science. She became Professor of Physics in Stockholm (Sweden) at the age of 67, and acquired Swedish citizenship in 1948. In 1960 she moved to Cambridge (England) in order to live close to her nephew Otto Frisch , she died there in 1968 and is buried in England.
Only in the late stages of her life was she honoured by various national and international institutions and received high-ranking prizes. To mention only the last one, the Enrico Fermi Award of the USA in 1966, which was given to the team Hahn, Meitner and Strassmann. In 1945 the Nobel Prize for Chemistry (not Physics) for the discovery of fission was given to Otto Hahn alone. Her life and work has received more attention recently. Articles about her life have appeared in Physics Today (Sept. 1997, p. 26) and Scientific American (Jan. 1998, p. 58). In Germany, schools (and gymnasia) in more than 10 cities bear her name. In the state of Hessen, a Lise Meitner prize, which provides support for women in the natural sciences, has been awarded for 7 years. The Technische Universit't Wien has a literature prize (Lise Meitner Literaturpreis) for texts written by female authors with emphasis on technical issues and science. In 1998 the Institute of Physics of the Humboldt University of Berlin created a Lise Meitner prize for outstanding PhD Thesis work. This was recently awarded for the second time.
