Nuclear fission Nuclear structure and decay Nuclides far from stability Nuclear astrophysics Neutron activation analysis Incorporation monitoring

Set-up of Hahn and Strassmann at Deutsche Museum, Munich

Strassmann, Meitner and Hahn in Mainz for the official inauguration of the Max Planck Institut, 9.7.1956

All my research is more or less related to nuclear fission, which was discovered in close collaboration by Otto Hahn, Lise Meitner and Fritz Strassmann at the Kaiser-Wilhelm Institut in Berlin. After the war, the institute was moved to Mainz and refounded as Max Planck Institut für Chemie (Otto Hahn Institut). Fritz Strassmann was director at the MPI and as professor at the university, he established the Institute for Inorganic Chemistry (later the Institute of Nuclear Chemistry). Zum Vollmond im März 2001 hat die IAU zwei neue Kleinplanetennamen bekanntgegeben [nähere Informationen]: Nr. Name Entdeckungstag, -ort Entdecker 19126 Ottohahn 22.08.1987, Tautenburg F. Börngen 19136 Strassmann 10.01.1989, Tautenburg F. Börngen See Press release and "Mainzer Allgemeine", FAZ and JOGU

Our group studies nuclear structure of very short-lived exotic isotopes by detecting and analysing the radiation emitted following the decay of these highly unstable nuclei. As the production and separation of these rare species demands quite sophisticated instrumentation, such experiments are performed in close collaboration with collegues at several research facilities as:

LOHENGRIN separator at ILL, Grenoble OSTIS separator at ILL, Grenoble (until 1987) SISAK Fast Chemistry at the TRIGA-reactor in Mainz ISOLDE On-line Separator at CERN (general public, users) Geneva IGISOL Separator at Jyväskylä, Finland; LISOL at Leuven/Louvain-la-Neuve LISE at GANIL, Caen Fragment separator FRS at GSI, Darmstadt NSCL at MSU, East Lansing, USA Fragment separator PARRNe at IPNO, Orsay, France

TRIGA MARK II at Mainz The philosophy underlying the TRIGA concept was expressed by Edward Teller in 1956: "What the world needs is a safe reactor! Not only idiot-proof, but Ph.-D. proof." Freeman Dyson demanded: "Inherent safety must be guaranteed by the laws of nature and not merely by the details of its engineering" Ideas underlying also the Generation IV reactor concept, developped without German participation!

Important contributions to the understanding of nuclear structure far from stability are derived from the study of fragments from thermal neutron induced fission of ²³⁵U at high-flux reactors, as the HFR of the ILL, Grenoble. The most neutron-rich isotopes were obtained for the alkaline elements Rb and Cs, as they combine a high thermal ionisation efficiency with short release times from carbon matrices impregnated with uranium carbide. As an example, detailed g-spectroscopy following the b-decay of 59 ms ⁹⁹Rb is shown below:

Decay scheme of 99Sr from b-decay of 99Rb, the most neutron-rich isotope obtained at OSTIS, for which detailed spectroscopy was possible (NSR 1985PfZZ). For extensions of the rotational bands to higher spins from the b-delayed neutron decay of 100Rb observed at ISOLDE, see Ann. Rep. 1998, p. 23) and Phys. Rev. C63 (2001) 054302.

With advanced separation techniques, the range of elements accessible to detailed nuclear spectroscopy has been extended: on the one side, refractory elements are obtained by the chemically non-selective ion-guide technique (IGISOL, Eur. Phys. J. A2 (1998) 17 and in combination with fast chemical separations applying the SISAK technique at the Mainz TRIGA reactor Eur. Phys. J. A1 (1998) 285) and on the other side, highly selective Laser ion sources allow to study nuclei with extremely low production yields in the presence of extremely high contaminations (ISOLDE, recent results on heavy Mn isotopes in Phys. Rev. Lett. 82 (1999) 1391). At LISOL, the combination of both techniques has yielded promising first results on neutron-rich nickel and cobalt isotopes ( Phys. Rev. Lett. 81 (1998) 3100).
For the nuclides with very low production yields, often only "gross" properties as half-lives and the delayed-neutron emission probability P_n can be obtained (i.e. Ann. Rep. 1995, p. 22; Ann. Rep. 1998, p. 24; tables of measured and predicted T_1/2 and P_n-values can be downloaded in PDF). Recently, in collaboration with the Theoretical Division at Los Alamos, existing experimental data for fission products were re-evaluated and compared to theoretical predictions: B. Pfeiffer, K.-L. Kratz, and P. Möller Status of Delayed-Neutron Precursor Data: Half-Lives and Neutron Emission Probabilities; Report: LA-UR-00-5897 and Progr. Nucl. Energ. 41/1-4 (2002) 39-69.

More sophisticated spectroscopy is possible for higher yields (as g-g-time coincidences to determine level lifetimes; see, i.e. Ann. Rep. 1995, p. 24), allowing to construct detailed excitation schemes from which spins, parities, nuclear deformations (for a visualisation of shapes click here or see P. Möller et al. Nature 409 (2001) 785) can be extracted. Then, applying b-g coincidence techniques, Q_b-values can be measured and by this the important nuclear masses. Comparing these results to theoretical mass models (a recent publication: At. Data Nucl. Data Tables 66 (1997) 131), hints for further improvements are obtained, which are essential for applications in fields as nuclear astrophysics dealing with far unstable nuclides not accessible for measurements with nowadays techniques (On-line access to calculated Nuclear Properties for Astrophysics by P. Möller).

Fit to the delayed coincidences between the 140 keV line feeding the isomeric level at 1838 keV and the deexciting transition of 1693 keV (filled red squares). For comparison, the coincidences between the 145 keV and the 1693 keV lines with the known t1/2=2.8 ns of the 2+ state are shown (open blue squares). Partial level scheme of 98Sr showing the transitions relevant for the discussion of the isomeric level at 1838 keV. For further details, see Ann. Rep. 1995, p. 24.

Further analysis revealed a deformed K=3 band with probable even parity built on the isomer at 1838 keV. It is interpreted as a two-quasi-neutron excitation in accordance with a quantum Monte Carlo pairing calculation in an article in Phys. Rev. C65, 024318 (2002).

The nuclear physics data are then used as input to nuclear astrophysics calculations of the interconnected items of energy production and nucleosynthesis in stars (see, e.g. Phys. Rep. 294 (1998) 167-263, Il Nuovo Cimento 111A (1998) 1043 (PDF-file) or Nuclear Structure Studies at ISOLDE and their Impact on the Astrophysical r-Process in Hyperfine Interactions 129 (2000) 185). We are concentrating on the reproduction of the solar-system abundance pattern of the heaviest elements synthesized in the rapid neutron capture process (or r-process), which takes place in type II supernovae explosions representing the final stages in the life of massive stars (For a review of our method see: K.-L. Kratz et al., Ap. J. 403, 216(1993)). Comparing our calculated with observed patterns, we can derive information on astrophysical as well as nuclear structure parameters (Ann. Rep. 1996 and B. Pfeiffer et al. Z. Phys. A357, 235(1997) ).

W.D. Harkins THE EVOLUTION OF THE ELEMENTS AND THE STABILITY OF COMPLEX ATOMS. I. A NEW PERIODIC SYSTEM WHICH SHOWS A RELATION BETWEEN THE ABUNDANCE OF THE ELEMENTS AND THE STRUCTURE OF THE NUCLEI OF ATOMS. J. Am. Chem. Soc. 39 (1917) 856 - 879

Remark: Our procedure to derive nuclear structure information from observed isotopic and elemental abundances in meteorites and stars is regarded by certain collegues as absolutely meaningless. (As if we were in the footsteps of Ptolemaios (main) work "Apotelesmatika" [better known as "Tetrabiblos"], in which he laid the basis of astrology under the assumption, that our lifes are intimately interrelated to the stars.) Very recently, the above mentioned article of W.D. Harkins was brought to our attention, in which the author "shows a relation between the abundance of the elements (in meteorites) and the structure of the nuclei of atoms" already in the year 1917. As an example, the Fig. 1 displayed above shows that "even-numbered elements are more abundant than odd-numbered ones". That this odd-even effect is valid up to the heaviest elements can be seen in the figure below displaying r-process abundances in an old Halo star.

Combined with recent determinations of the abundances of rare earth elements and the cosmochronometer thorium in very old halo stars by optical spectroscopy with the Hubble Space Telescope, decisive contributions to open questions on the Galactic nucleosynthesis and the age of the Galaxy will result (Ann. Rep. 1996 and Nucl. Phys. A630 (1998) 352c ).

Elemental r-abundances calculated with ETFSI-Q masses (red line) [J.M. Pearson et al., Phys. Lett. B387 (1996) 455] are compared to the respective Nr,solar values (green filled circles). Superimposed are measured abundances (blue filled squares) from the metal-poor halo star CS22892-052 [C. Sneden et al., Ap. J. 591 (2003) 936], which were normalized to the solar rare-earth values (see J.J. Cowan et al., Ap.J. 521 (1999) 194-205).

The observation of "solar" neutron-capture element abundance distributions in four metal-poor halo stars indicates to a unique r-process site in the Galaxy (at least for Z > 55). The estimated decay-age of T = (15.6 ± 4) billion years for the thorium in these stars represent a lower limit for the age of the Galaxy and (evidently) the Universe (see, B. Pfeiffer et al. in IX. Workshop on Nuclear Astrophysics and J.J. Cowan et al., Ap.J. 521 (1999) 194-205).
Recently, new perspectives for radioactive dating of old stars opened with the first simultaneous detection of thorium and uranium in the metal-poor star CS31082-001 by R. Cayrel et al. in Nature 409 (2001) 691. Combined with the Basel-Mainz initial r-process production values (see J.J. Cowan et al., Ap.J. 521 (1999) 194-205) an age of (12.5±3) Gyr was obtained (for more details see R-Process Cosmo-Chronometers).
(These age determinations are cited in Ned Wright's Frequently Asked Questions in Cosmology: How old is the Universe?.)
First calculations on the proposed two r-process components can be found in my contribution to the conference Nuclei in the Cosmos 2000.

Further information on the origin of the elements in our Galaxy may be obtained from the analysis of the composition of Galactic cosmic rays. The ultraheavy cosmic rays (heavier than neodynium with Z=60) appear to come from a pure r-process (explosive) source (see Trek and ECCO-detectors on the space stations Mir and ACCESS in preparation for ISS in 2005, respectively). If confirmed by the next generation of experiments (as the ECCO-detector or advanced versions of TIGER, the Trans-Iron Galactic Element Recorder as first payload for the Ultra-Long Duration Balloon Project), all the heavier neutron-capture elements originate in a unique explosive scenario, which remained unchanged over the history of the Galaxy from the onset of r-process nucleosynthesis in the first halo stars until present times (as evidenced by the Galactic cosmic rays with an estimated mean lifetime in the order of 10 million years). [See, e.g. R.E. Lingenfelter et al., Ap.J. 591 (2003) 228 and talk at Seeon Workshop]

Some articles of our astrophysical collaborations have already been listed as part of the Nuclear Astrophysics Bibliography.

In perhaps even not so far future, nuclear fission and space exploration will be intimately linked. NASA has started projects to power spacecraft by fission reactors: Project Prometheus. [Reactors to be started in orbit, not to lift-off from the surface by nuclear explosions as in the (in)famous "Project Orion" of the 50/60'ies. A project of General Atomic developped in parallel to the TRIGA reactor by partly the same persons.] As a first application,Project JIMO (Jupiter Icy Moons Orbiter) is foreseen. The high electrical power output of the fission reactor will be used for electrical propulsion and will also power for a first time energy consuming scientific payloads as radar emitters to search for the presumed oceans beneath the ice crusts of the 3 outer Galilean satellites of Jupiter. The electrical power will also enable the use of high-gain antennas to transmit a high data rate to Earth. For the design of the reactors, nuclear structure data are needed, data which also our group has measured since a long time. And one should not forget to mention that neutron detectors similar in design to the ones applied in our research are on board the "Mars Odyssey" orbiting Mars and searching for water beneath the surface.

Bernd Pfeiffer's links I: Bernd Pfeiffer's links II: A list of my publications: A look at my private homepage:

Dr. B. Pfeiffer GSI Helmholtzzentrum für Schwerionenforschung Planckstr. 1 D-64291 Darmstadt

Professional: bpfeiffe@uni-mainz.de ; B.Pfeiffer@gsi.de Private: Dr._Bernd-Pfeiffer@t-online.de

Last updated March 2012.