Some adventures in research

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It is a joyful experience for a scientist when he gains a deaper insight into the nature of the subject of his research. Often, such discoveries are the result of a careful planning, but in many cases important discoveries happen by chance, fully unexpectedly. It is not an exception that first indications are overlooked or mis-interpreted, and the true implication becomes evident only little by little. On this page I have recorded a few discoveries in which I was personally involved. The pleasure in these findings was a kind of personal compensation for my research work. Others may judge on the importance.

Obstructions on the fission path (new!)

Sometimes, it happens that pertinant signatures in experimental data that carry valuable information about burning questionf of research are overlooked for decades, until the importance of these signatures is recognized. One of these signatures is the impact of the obstruction on the fission path - the shape evolution of a fissioning nucleus towards scission - by a third barrier in the fission of light actinides. Already in 1973, the systematics of the fission-fragment mass distributions from the fission of heavier actinites was found to be violated in thermal-neutron-induced fission of 229Th (Unik et al.,  Proc. Symp. Phys. Chem. Fission, Rochester 1973, IAEA Vienna (1974), vol. 2, p. 19): In particular, the asymmetric component of the mass distribution was found to be driven to higher mass-asymmetry. Only recently, this observation was interpreted and attributed to the obstruction on the fission path by the third barrier, which is particulary high in the light actinides (Schmidt et al., The abnormal mass distributions and other anomalies, found in the fission of the light actinides are attributed to a suppression of trajectories passing by compact configurations, which cannot travers the third barrier without tunneling. Thus, the memory on the mass-asymmetric distortions, induced by the second barrier is wiped out. In contrast, the memory on the mass-asymmtric distortions behind the second barrier is fully preserved at higher excitation energies that exceed the third barrier due to the influence of inertia. Our interpretation of these observations is in severe conflict with the wide-spread claim of theoretical models, which allege that the role of collective inertia in fission dynamics is irrelevant. They also falsify the validity of largely used approaches that neglect the influence of inertia on nuclear dynamics, like statistical  scission-point models and the use of the Smoluchowsky equation in stochastic approaches to fission.    

Maxwell's demon on the nuclear level

   It has been overlooked so far that two moderately excited nuclei show a very bizarre behaviour when they are brought in thermal contact: All the excitation energy accumulates in one of the nuclei, while the other one looses all its energy. This behaviour is a consequence of the melting of the nuclear superfluid phase due to which the number of degrees of freedom grows in proportion to the excitation energy. This leads to a constant temperature, independent of excitation energy but with a specific value which depends on the nature of the nucleus. In an attempt to establish thermal equilibrium, thermal energy is moved from the hotter nucleus (the one with the higher temperature) to the colder nucleus (the one with the lower temperature), just like a hot cup of coffee cools down to room temperature. However, the temperatures of the two nuclei stay the same, and, therefore, this process only ends when the hotter one of the two nuclei has lost all its excitation energy. This situation is realized in the scission configuration of nuclear fission at moderate excitation energies. This effect explains why an increase of excitation energy is translated into an increase of the number of emitted neutrons from the heavy fragment, only. [K.-H. Schmidt, B. Jurado, Phys. Rev. Lett. 104 (2010) 212501]. This observation has been made in the 1960s already, but remained unexplained up to now.
   The process of energy sorting was first described by Maxwell in 1871. Since he wanted to illustrate the statistical character of thermodynamics, Maxwell pointed out that a demon could violate the second law of thermodynamics by opening and closing a trap door between two compartments of a chamber containing gas. By only opening the door when fast molecules
approach it from the right, or slow ones from the left, the thermal energy would accumulate in the left compartment. This would allow operating a thermal engine and converting pure thermal energy into mechanical energy, a process which is not allowed by the second law of thermodynamics. Many attempts were made to prove the impossibility of such perpetuum mobile of the second kind, and to exorcise Maywell's demon (see J. Earman, J. D. Norton, Stud. Hist. Phil. Mod. Phys. 29, 435 (1998)).
   Maxwell's demon on the nuclear level sorts the excitation energy, but not the temperature. Since the energy sorting is driven by entropy, there is no reason to be apprehensive of a violation of the second law.

Mutual support of magicities

   Shell effects are an important feature of nuclear structure. Similar to atoms with closed electron shells, which have the exceptional chemical properties of rare gases, nuclei with "magic" numbers of their constituents, protons and neutrons,  also possess peculiar characteristics. Nuclei with 2, 8, 20, 28, 50, 82, and 126 neutrons as well as with 2, 8, 20, 28, and 82 protons are particularly stable and extremely symmetric. The nuclear shell model, developed in 1949 by E. P. Wigner, M. Goeppert-Mayer and J. H. D. Jensen (see "Elementary Theory of Nuclear Shell Structure", John Wiley a. Sons, Inc., New York; Chapman a. Hall, Ltd., London 1955) explains these structural effects by quantenmechanical regularities.
   According to the basic idea of the nuclear shell model, neutrons and protons move independently from each other in the common potential well, which is formed by the attractive interaction between neighboured nucleons (a general term for neutrons and protons). From measured alpha-decay energies of nuclei around the 126-neutron-shell closure we deduced that the shell effects of neutrons and protons support each other mutually (K.-H. Schmidt et al., "Alpha-decay properties of new protactinium isotopes", Nucl. Phys. A 318 (1979) 253, K.-H. Schmidt, D. Vermeulen, "Mutual Support of Magicities", Contribution to the conference AMCO 6, 1979). This was an evidence for an unexpected interaction between neutrons and protons, which was not explained by the original shell model (see N. Zeldes et al., "Mutual support of magicities and residual effective interactions near 208Pb", Nucl. Phys. A 399 (1983) 11). This kind of interaction naturally explained a hitherto unexplained inconsistency between the apparent strengths of the shells in the single-particle energies and the shell effect obtained by the Strutinsky procedure (see M. Brack et al., "Funny hills: the shell-correction approach to nuclear shell effects and its application to the fission process", Rev. Mod. Phys. 44 (1972) 320). Later, it was proposed that this coupling between neutrons and protons is induced by the shape of the common nuclear potential (M. Bender et al., "The Z=82 shell closure in neutron-deficient Pb isotopes", Eur. Phys. J. A 14 (2002) 23). However, the model calculations presented in that work do not reproduce the pecularities of the nuclear structure in the direct vicinity of the doubly magic 208Pb very well. Therefore, the question on the nature of the residual interaction, which is responsible for this coupling between the neutron and the proton system is not finally settled. 
   The variation of the shell strength in nuclei with a large neutron deficiency or excess with respect to stable nuclei is presently one of the most important research subjects in the physics of nuclear structure. This phenomenon strongly influences the processes in supernovae or other violent cosmic scenarios, which lead to the synthesis of the heavier elements in the universe. Our finding represents one of the early studies in this field.

There is no perfect scientific fraude

   An analysis by means of statistical mathematics [1] revealed that the first claim on the discovery of element 118 [2] was doubtful. It was shown that the reported decay times are concentrated in a time range, which is too narrow to be compatible with the expected feature of true nuclear decays. Since a scientific fraud appeared to be inconceivable at that time, a technical failure was suspected to be the cause of these signals. 
   Only two years later the same authors admitted [3] that the observations described in ref. [2] could not be reproduced. Finally it turned out that the data, which were the basis of ref. [2] were unretrievable in the recorded data of the experiment.
   If the publication [2] was based on manipulated data, the individual decay times were generated without considering the laws of statistical mathematics. Thus, the data themselves carried the indication for their falseness, which could be unrevealed by the appropriate analysis.
[1] "A new test for random events of an exponential distribution", K.-H. Schmidt, Eur. Phys. J. A 8 (2000) 141
[2] "Observation of Superheavy Nuclei Produced in the Reaction of 86Kr with 208Pb"
V. Ninov, K. E. Gregorich, W. Loveland, A. Ghiorso, D. C. Hoffman, D. M. Lee, H. Nitsche, W. J. Swiatecki, U. W. Kirbach, C. A. Laue, J. L. Adams, J. B. Patin, D. A. Shaughnessy, D. A. Strellis, P. A. Wilk, Phys. Rev. Lett. 83 (1999) 1104
[3] "Editorial Note: Observation of Superheavy Nuclei Produced in the Reaction of 86Kr with 208Pb [Phys. Rev. Lett. 83, 1104 (1999)]"
V. Ninov, K. E. Gregorich, W. Loveland, A. Ghiorso, D. C. Hoffman, D. M. Lee, H. Nitsche, W. J. Swiatecki, U. W. Kirbach, C. A. Laue, J. L. Adams, J. B. Patin, D. A. Shaughnessy, D. A. Strellis, P. A. Wilk, Phys. Rev. Lett. 89 (2002) 039901

Negative friction in relativistic heavy-ion collisions

   A surprising discovery has been made at the Heavy-ion Synchrotron SIS18 of the Gesellschaft für Schwerionenforschung (GSI): Two atomic nuclei, which hit each-other at high velocities in a mid-peripheral collision, are not slowed down, but the pieces  outside the immediate collision zone leave the reaction with even higher relative velocity (M. V. Ricciardi et al., Experimental indications for the response of the spectator to the participant blast, Phys. Rev. Lett. 90 (2003) 212302). As an illustration of this finding one may imagine that someone shoots with a gun on a bag of potatoes, and the bag moves towards the rifle-man after the shot! The kind of unusual property of nuclear matter, which is responsible for this "negative friction", is still being investigated.
   One knows already since long that two atomic nuclei that collide with very high velocity shear off each-other: Only the parts that do immediately hit each-other form a fireball, which heats up very much and which moves on with an intermediate velocity and explodes. The parts outside this overlap zone move on with about the same velocity as before the collision. If the overlap zone and thus the fireball is small, the surviving parts of the two nuclei are slowed down a bit. This is what one intuitively expects as a consequence of friction between the two objects. In our experiment we investigated what happens if the overlap region increases. We have found that in this case the surviving parts of the two nuclei move on with a higher relative velocity than before the collision.
   If one wants to learn about the properties of nuclear matter, one cannot simply take a piece of nuclear matter, weigh it, put it under pressure or heat it. The practically only possibility to investigate nuclear matter is shooting an atomic nucleus with high kinetic energy on another atomic nucleus and detect the residual fragments. Heavy-ion accelerators serve to provide a beam of high-energetic atoms. More accurately, these are ions, because the negative electric charge of the electron shell does not balance the positive charge of the protons in the nucleus, and therefore the atom has a residual electrical charge. These high-energetic heavy ions impinge on a target foil. Although the nuclei occupy only a very small fraction of the volume, the nucleus of the heavy ion may hit the nucleus of a target atom. The fragments emerging from the reaction are detected or measured with dedicated detectors and elaborate spectrometers. Our finding has been made with the FRS, which is the largest high-resolution magnetic spectrometer of GSI.
   In a theoretical work by Shi, Danielewicz und Lacey (L. Shi, P. Danielewicz, R. Lacey, Phys. Rev. C 64 (2001) 034601), the effect of negative friction in high-energetic nucleus-nucleus collisions has been attributed to the influence of the momentum-dependent nuclear force. It is supposed that the nuclear force - similar to the electromagnetic force -  consists of a static component (similar to the electrostatic Coulomb force) and a dynamic component (similar to the magnetic force). Our experiment should be capable to measure the strength of the dynamic, momentum-dependent part of the nuclear force. This information is important to determine the individual strenghts of the two components of the nuclear force, since previous experiments usually measured the total force, resulting from the two components. The static component alone is decisive for the incompressibility of nuclear matter, which is still insufficiently known.
   Recent experiments, in which the sizes of the interacting nuclei and the projectile energy was varied (V. Henzl, thesis, Techn. Univ. Prague, 2006), are not directly compatible with the theoretical estimations of Shi, Danielewicz und Lacey. Therefore, the reason for the "negative friction" is not yet finally understood.