The Stuff that Nuclei Are Made Of
Water, like all of the other matter surrounding us, is either solid, liquid, or gaseous depending on its temperature and pressure. In the same way, nuclear matter - the charged protons and electrically neutral neutrons forming the nucleus of an atom - can assume various states. Indeed, GSI researchers were recently able to provide convincing evidence for the long sought-after phase transition from liquid to gaseous nuclear matter. At even higher temperatures and pressures, the basic building-blocks of the nucleus are expected to break down into a new form of matter - the quark- gluon plasma.Almost all of an atom's matter is located in the nucleus. Atomic nuclei are thus unimaginably dense compared to chemical elements or chemical compounds. The density of a nucleus is more than 14 orders of magnitude (1 x 1014) greater than that of water. But, on the other hand, the forces between the individual components of the nucleus - the nucleons - vary according to distance in a manner remarkably similar to those between molecules in a liquid. At very short distances, the binding forces repel; at medium nucleon distances, they attract. In fact, in many ways, atomic nuclei behave very much like drops of liquid.
If we pursue this line of investigation, it isn't long before we are confronted by an interesting question. Can nuclear matter - like water - change state and become, for example, gaseous. The physicist's response to such a question is to try to solve a more general question - that of the equation of state of nuclear matter, which links the thermodynamic variables of pressure, density and temperature.
Information about this equation can be obtained from the collision of heavy atomic nuclei at relativistic velocities - i.e. speeds approaching that of light - such as those provided by the Heavy Ion Synchrotron (SIS) in Darmstadt. Here, the density and temperature of the nuclear matter can be varied over wide ranges and under controlled conditions. On the one hand, some of the kinetic energy of the colliding atomic nuclei results in the internal excitation of the nuclei: they are heated. On the other hand, wherever the atomic nuclei collide head-on - in the small volume of the collision zone - compressed nuclear matter is created for an extremely short period of time. Careful and precise study of the resulting processes occurring provides insight into the properties of such hot and compressed nuclear matter.
Nuclear matter boils just like water
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| A view into the detector chamber of the opened-up Time Projection Chamber (TPC), which is being used with the ALADIN spectrometer to study the break-up of hot nuclei. The spectrometer magnet can be seen in the background (on the right, in blue). |
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| The caloric curve of nuclear matter (left diagram) exhibits behavior analogous to the temperature curve of boiling water (right diagram). The nuclear matter temperature remains constant for a time despite an increase in excitation energy before eventually beginning to climb again. The data measured by GSI are shown in red (compared to data from other labs). |
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| The FOPI detector covers the entire solid angle around the point of collision. A large magnet (red) encloses various detector components. The accelerated heavy ion beam strikes the target foil from the left. The particle tracks (nucleons green, pions red) recorded in an event have been reconstructed by a computer. |
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| The central detector chamber of the FOPI detector has been removed for installation work. Many young scientists and Ph.D. students participated in preparing, carrying out, and evaluating these major experiments. |
For more than 20 years, theory has predicted that nuclear matter undergoes a liquid-gas phase transition upon reaching a certain critical temperature. However, it was only recently that convincing experimental evidence for this phase transition was finally delivered by a GSI research group. This was accomplished in a series of experiments in which heavy projectile nuclei, such as the nuclei of gold atoms, were fired at light target-nuclei, such as carbon. Because of their low mass, the light nuclei were not able to compress the projected heavy- nuclei to a noticeable extent at the moment of impact. The projectiles were, however, heated so much by the collisions that they disintegrated into numerous fragments. By precisely identifying and measuring these fragments, it was possible to derive the excitation energy and nuclear temperature of the heavy projectiles at the time of break-up.
Plotting these values on a temperature-energy diagram produced phase curves like those typical for the vaporization of water, namely a plateau followed by another rise.
This behavior can be interpreted as follows: upon reaching the plateau, the nucleus begins to boil. Adding more energy does not result in a further increase in temperature. Instead, the bonds between the nucleons are broken in the liquid nuclear matter which then transforms into a gas with a lower density. Only after this transformation is complete does adding more energy result in a renewed increase in temperature. This further increase in temperature was observed for the first time in the ALADIN experiments and is a clear indication of the phase transition.
But what happens if the atomic nuclei are not only heated, but also compressed? This situation can be brought about in head-on collisions at relativistic energies between heavy projectiles and heavy target nuclei. At SIS energies, the nucleons in the overlap zone of the colliding nuclei are compressed to two or three times their normal density for a very brief time-span of 10-22 seconds.
Despite this extremely short time-span, information can be obtained about this high density phase - primarily using two methods of analysis. The compression phase, in which the nuclear matter is compressed like a spring, is followed by an explosion-like expansion phase, in which the nuclei break up into numerous pieces known as nuclear fragments. The mass and velocity distribution of the released nuclear fragments can be used to reconstruct the nucleus-nucleus collision and derive a value for the initial compression.
The essential instrument for these studies is the 4Π or FOPI (from Four Pi) detector, with which all of the charged particles emitted during the expansion phase are registered according to size, direction, and velocity.
The mathematical symbol 4Π represents the entire solid angle and is meant to indicate that each emitted particle really is detected and measured. The explosive expansion observed after the collision of two gold nuclei is shown as an example of how the data collected with this detector look.
The analysis of these data reveals that more than half of the energy involved in the nucleus-nucleus collision turns up again in the collective expansion following the compression. Consequently, only the difference could have gone into exciting the nucleons' internal degrees of freedom, or into a disorderly movement of nucleons, i.e., a temperature increase.
Recourse to theoretical model calculations is necessary if we are to gain precise insight into the nucleus-nucleus collisions and the resulting properties of the nuclear matter. An example of one such calculation - and at the same time a good illustration of a high energy nucleus- nucleus collision - is shown in the next figure. The scenario observed in these nucleus-nucleus collisions has many similarities to a supernova explosion. In such an explosion, the interior of a star is compressed by a gravitational collapse to many times the normal nuclear density, and then explosively propels its outer shells of mass into space, leaving behind a neutron star.
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| An explosion-like expansion of nuclear fragments from a central collision between gold nuclei traveling at 50% the speed of light as observed with the FOPI detector. The practically identical mean speed of the larger fragments contradicts the predictions of the thermal expansion model and suggests a collective expansion of the collision zone, during which all fragments move away from the center of the reaction with essentially the same speed. |
Another option for studying compressed nuclear matter during the collision of two atomic nuclei is to observe the radiation they emit. Individual nucleons in the collision zone can be excited to higher-energy states - to so-called nucleon resonances.
In other words, nucleons have an internal structure. They are composed of quarks and gluons and can be excited into higher states by means of interaction with their environment, for example, by means of collisions with other nucleons in a hot, compressed, reaction zone. The nucleon resonances decay within a very short time - during the nucleus-nucleus collision itself - emitting mesons and other particles such as gamma quanta or electron-positron pairs. These can be measured and thus act as messengers from the hot collision-zone, which is commonly referred to as the "fireball" in research circles.
At SIS energies, it is chiefly the so-called delta resonance - the lowest level of nucleon excitation - which is produced. This then decays via the emission of π-mesons. Measurements with the FOPI detector, TAPS (Two Arm Photon Spectrometer) and the KaoS magnet spectrometer (Kaon or K-meson Spectrometer) have shown that up to one third of the nucleons in the compression zone exist for a short time as delta resonances. In other words, a significant portion of the nucleus-nucleus collision energy is used to excite the nucleons' internal degrees of freedom.
This has important consequences for the excitation of even higher nucleon resonances or for the emission of mesons heavier than the π-meson, such as the η-meson or the K-meson. Experiments on the KaoS spectrometer reveal that the production of K-mesons results primarily from collisions in the compression zone between a delta resonance and a nucleon or even between two delta resonances. In fact, the considerable concentration of delta resonances even allows anti-protons to be created during high-energy delta-delta collisions, as has already been demonstrated by experiments at the FRS fragment separator.
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| The theoretically simulated spatial and temporal progression of a nuclear collision
viewed at 3 10-23 s intervals (viewed in the center-of-mass system): the approach phase; due
to relativistic effects, the nuclei are shortened in the direction of motion (left); the high-density
phase (middle); and the expansion phase (right). It can be seen that nucleon resonances (green)
are excited in the high-density phase, before decaying via the emission of mesons (red). In the
final phase of the collision, all of the particles fly out in all directions like an explosion.
Animation as Mpeg-File(2682810 Bytes) Further videos: Der theoretisch simulierte räuliche und zeitliche Ablauf des Kernzusammenpralls, bei verschiedenen Energien (E) und Abständen (b). Nukleonen sind blau, Nukleonen-Resonanzen grün und Mesonen rot dargestellt. Diese Filmsequenzen beruhen auf Rechnungen von Steffen A. Bass mit dem QuantenMolekularDynamik Model mit Isospinfreiheitsgraden (IQMD). Gold on Gold E = 1 GeV pro Nukleon, b = 12 Fermi (Mpeg-File with 1121715 Bytes) E = 1 GeV pro Nukleon, b = 6 Fermi (Mpeg-File with 2195804 Bytes) E = 1 GeV pro Nukleon, b = 3 Fermi Mpeg-File with 2269136 Bytes) E = 1 GeV pro Nukleon, b = 0 Fermi (Mpeg-File with 2526016 Bytes) |
On the trail of the "primordial soup"
While nuclear collisions at SIS energies are still governed by the properties of the nucleons, at even higher energies it is the properties of quarks that play the decisive role. A transition from hadronic matter to a new phase of matter - the quark-gluon plasma - is expected in reactions between heavy nuclei. At a sufficient energy density, the nucleons should break down into quasi-free quarks and gluons. Speculation of this nature is also of tremendous cosmological interest, since it is suspected that such a phase transition occurred in the opposite direction approximately 10-5 seconds after the big bang. In other words, many physicists believe that the original "primordial soup" consisting of quark-gluon matter transformed into nucleons and mesons.
In order to investigate this process - itself so important for an understanding of the evolution of the early universe - GSI, in cooperation with other European institutes and the European nuclear research laboratory (CERN), developed a lead-injector for the large SPS synchrotron at CERN. In continuation of a long-running research program, many studies of reactions between very heavy lead nuclei with impact velocities of more than 99% of the speed of light have been carried out since November 1994.
Theory predicts various signatures for the phase transition - for example, an excessive production of s-quarks (s for strange) and the emission of high-energy, electromagnetic radiation. All such signals or signatures should become more distinctive, the greater the highly- excited reaction volume. This is actually the advantage offered by using lead beams in experiments instead of the beams of sulfur used previously.
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| A TPC detector (Time Projection Chamber) records the particle tracks resulting from a reaction between lead nuclei possessing energies of 160 GeV per nucleon. The image is a computer reconstruction of the particle paths registered in a large-volume track chamber. The beam direction is toward the observer. |
These experiments would seem to have a fascinating future now that the CERN member-states have decided to go ahead with the construction of the LHC (Large Hadron Collider). Current plans envisage a key role for GSI in the construction of a heavy ion detector for lead-lead collisions at this European super-accelerator. As a consequence, it will be possible to conduct experiments at energy levels 300 times greater than has been possible to date.
Summarizing, it would thus seem that nuclear matter manifests itself in a myriad of forms, each of which can be realized by means of the appropriate input of energy and compression. Even at relatively slight increases in energy, nuclear matter undergoes a phase transition from the normal, liquid-like state to a nucleon gas. At higher beam energies, such as those provided by the SIS at GSI, nuclear matter can be compressed to roughly three times its normal density. At the same time, nucleon resonances are created by the excitation of the nucleon substructure. But even this nucleon substructure disintegrates at still higher energies, where we can assume that a plasma consisting of quarks and gluons comes into existence. However, there is one thing we can say for certain - the "stuff" from which atomic nuclei are ultimately made is indeed a manifold wonderland.