Program - Brookhaven National Laboratory

Fission Activities Around the Lohengrin Mass Spectrometer
O. Serot, O. Litaize, CEA, DEN-Cadarache, F-13108 Saint-Paul-lez-Durance, France. C. Amouroux, A.
Letourneau, T. Materna, S. Panebianco, CEA, DSM-Saclay, IRFU/SPHN, F-91191 Gif-sur-Yvette,
France. A. Bidaud, N. Capellan, S. Chabod, G. Kessedjian, F. Martin, C. Sage, LPSC, 53 rue des
Martyrs, F-38026 Grenoble Cedex, France. A. Ebran, CEA, DAM-Ile de France, F-91290 Arpajon,
France. H. Faust, U. Koster, W. Urban, Institut Laue Langevin, 6 rue Jules Horowitz, F-38042,
Grenoble, France. L. Mathieu, CENBG, le Haut Vigneau, BP 120, F-33175 Gradignan, France. J.-M.
Regis, M. Rudigier, IKP, Universität zu Köln, 50937 Köln, Germany.
The “Lohengrin” mass spectrometer is one of the forty instruments built around the reactor of the Institute
Laue-Langevin (France) which delivers a very intense thermal neutron flux. Usually, Lohengrin was
combined with a high-resolution ionization chamber in order to get good nuclear charge discrimination
within a mass line, yielding an accurate isotopic yield determination. Unfortunately, this experimental
procedure can only be applied for fission products with a nuclear charge less than about 42, i.e. in the
light fission fragment region. Since 2008, a huge collaboration has started with the aim of studying various
fission aspects, mainly in the heavy fragment region. For that, a new experimental setup which allows
the isotopic identification by γ-ray spectrometry has been developed and validated. This technique was
applied on the 239 Pu(nth,f) reaction where about 65 fission product yields could be measured with an
uncertainty that has been reduced on average by a factor of 2 compared to that previously available in
the nuclear data libraries. The same γ-ray spectrometric technique is currently applied on the study of
233 U(nth,f) reaction. Our aim is to deduce charge and mass distributions of the fission products and to
complete the experimental data that exist mainly for light fission fragments. Lastly, the measurement of
41 mass yields from the reaction 241 Am(2nth,f) has been also performed. One of the main motivations for
this measurement is to determine whether there is a difference in mass and isotopic yields between the
isomeric state and the ground state of 242 Am. In addition to these activities on fission yield measurements,
various new nanosecond isomers were discovered. Due to the fact that some nanosecond isomers decay by
a highly converted internal transition, their presence can be revealed from a strong deformed ionic charge
distribution compared to a normal Gaussian shape. For some of these isomers, their half-life could be also
estimated. Lastly, a new neutron long-counter detector (called LOENIE for LOng-counter with ENergy
Independent Efficiency), designed to have a detection efficiency independent of the detected neutron energy
has been built. Combining this neutron device with a Germanium detector and a beta-ray detector array
allowed us to measure the probability to emit delayed neutrons (Pn) of some important fission products
for reactor applications.
PD 2 4:00 PM
Conservation of Isospin in n-Rich Fission Fragments
Ashok Kumar Jain, Deepika Choudhury, Bhoomika Maheshwari
Department of Physics, Indian Institute of Technology, Roorkee, India
On the occasion of the 75th anniversary of fission phenomenon, we present a surprisingly simple result
which highlights the important role of isospin and its conservation in neutron rich fission fragments. We
have analysed and investigated the fission fragment mass distribution from two recent measurements using
the heavy-ion reactions 238U(18O,f) [1] and 208Pb(18O,f) [2] along with the isobaric yields from the
thermal neutron fission in the reaction 245Cm(n th.,f) [3]. We find that the conservation of total isospin
explains the overall trend in the observed relative yields of fragment masses in each fission pair partition in
a strikingly precise manner. The effect of shell structure becomes visible only at the magic numbers. The
isospin values involved in the fission of heavy nuclei are very large making the effect dramatic. The fission
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