Precise Information from Cold Ions
Highly-charged and cooled ion beams orbiting in storage rings have opened up new experimental fields and previously unimaginable perspectives in atomic physics. Hitherto inaccessible objects of study such as hydrogen-like uranium are enabling physicists to develop precise experimental methods for testing fundamental theories. In addition, the ion beams are also providing astrophysicists with important spectroscopic data.At the beginning of the century, physics took a quantum leap with the experimental study and subsequent theoretical explanation of the spectral lines produced by simple atoms with only one or two electrons. And here we are not only referring to the quantum leap postulated by Niels Bohr when he established the first model of the atom. We are also talking about a major advance towards the development of our scientific picture of the universe. As the principles underlying the hydrogen atom spectrum became clearer and our understanding of them deeper, a new branch of physics began to take shape. It came to be known as quantum mechanics, and without it many fundamental technical advances of our time, such as microelectronics and the laser, would scarcely have been conceivable.
Experiment and theory have continually challenged and driven each other forward. For example, at one point, Paul Dirac's theoretical work on the hydrogen atom seemed to have finally unified quantum mechanics, on one hand, and the theory of special relativity, on the other. However, increasingly more precise experimental techniques were able to detect a slight shift - subsequently known as the 'Lamb Shift,' in honor of its discoverer, Willis Lamb - in the hydrogen spectrum not accounted for in Dirac's model.
Quantum electrodynamics, which is today considered to embody the central theory of atomic physics, developed from that experimental observation. Moreover, quantum electrodynamics is also regarded as the most accurately tested and best confirmed of all physics theories. So far, though, it has only been possible to test it on hydrogen-like atoms possessing relatively low nuclear charge (Z) of less than 15.
Simple atoms test complex theories
Given the assumption that the effects of quantum electrodynamics increase very sharply with increasing Z - this increase is proportional to the fourth power of the nuclear charge - physicists have always been extremely interested in developing precise experiments using atoms with the highest possible Z and the fewest possible electrons. The object of such studies is to exactly determine the binding energies of an electron in an atom carrying a charge Z. In this sense, the aim is essentially to repeat the early pioneering experiments on the hydrogen atom, but now using heavier hydrogen-like ions. The results of such experiments can then be compared with the predictions given in quantum electrodynamics in order to confirm or refute the theory, or alternatively show it to be incomplete.
But beyond this fundamental dimension, there are also strong astrophysical incentives for studying highly-charged heavy atoms. For, along with electrons and neutrons, highly ionized atoms constitute the matter of hot stellar plasmas. All the elements heavier than iron that exist in the universe originated - and are still originating today - in such stellar matter at a temperature of several hundred million degrees. The rate and speed of their synthesis is closely related to the number of bound electrons, and, thus, to the charge state of the atom. For this reason, astrophysicists have always longed for the opportunity to be able to recreate and investigate stellar matter under laboratory conditions.
However, it was only with the advent of great advances in accelerator technology and instrumentation, that steps could be taken towards this goal. For example, at GSI, it has recently become possible to produce so-called "naked" ions - atomic nuclei which have been completely stripped of their electron shells - for all the elements up to uranium. The only other facility to have been successful in this area is the Livermore Laboratory in the USA. There, however, the highly-charged ions are produced by another process which employs an electron beam ion trap. Such experimental possibilities have existed for four years now in Darmstadt. In this short time, results have already greatly exceeded original expectations, mostly thanks to the installation of a beam cooler, which gives the ions a nearly uniform orbiting velocity.
Electrons as "coolants" for heavy ions
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| The electron cooler at the experimental storage ring (ESR). The electrons come from above on the right and are deflected into the horizontal plane. They then accompany the ion beam, coming from the right, in the horizontal tube section which is about two meters long (the smooth yellow tube in the center of the picture). The electrons cool the ion beam through energy exchange before being deflected upwards out of the ion path (rear left of picture). |
During this time, each ion orbiting inside the storage ring exchanges energy with the electron beam more than a million times per second and thereby attains the exact speed of the electron beam within a fraction of a second. At the same time the diameter of the ion beam decreases to a few millimeters and its divergence is drastically reduced. In the reference frame of the ion beam, this process corresponds to a reduction of the disorderly motions of the individual particles of the beam. It can be described formally as a reduction in beam temperature. This is why we say the ions have been cooled by the electron beam. As a result of this cooling, an intense monoenergetic ion beam can orbit inside the highly-evacuated storage ring for up to several hours.
These highly charged ions can now capture electrons, for example, from the electron beam, thereby causing formerly free states in the ion to be occupied. X-rays are emitted during the transition of such a trapped electron to lower-level states - all the way down to the ground state. Accurate measurement of the energies of these X-ray quanta gives precise information about the binding energies of an electron in the atom concerned. In this way, the earlier spectroscopic studies of hydrogen can now be repeated on heavy hydrogen-like atoms. However, this time, quantum electrodynamics aspects are particularly pronounced because of the higher nuclear charges. For example, the Lamb Shift of the ground state for hydrogen-like uranium is about 450 eV, compared with only 0.000032 eV for the hydrogen atom.
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| X-ray transitions in the ground state of hydrogen-like uranium. Because of the fast motion of the uranium ions orbiting inside the ESR, the observed X-ray lines are shifted to higher energies. |
For instance, the binding energy of the electron in the ground state of hydrogen-like uranium can be determined with a previously unattained accuracy of 10-4. Although amounting to a very creditable 16 eV in 132,000 eV, this level of accuracy is still not adequate for a critical test of the current theoretical predictions of quantum electrodynamics, which are about an order of magnitude more exact. However, physicists are optimistic that they will soon be able to attain the precision required for a critical test of the theory using better detectors and more sophisticated detection methods.
When a laser beam is reflected onto the ion beam and superimposed upon it, fascinating possibilities for precision spectroscopy begin to open up. Whenever an ion "discovers" a light quantum of matching energy - the amount of energy corresponding exactly to one of its possible excitation energies - it can absorb that quantum and "jump" into an excited state. A fundamental point here is that the energy of the laser photons in the system of fast-moving ions, measured in the laboratory, appears to be shifted according to the transformation required by the theory of special relativity. As a result, the ion beam can be "tuned" to the required light quanta energy by simply varying the beam velocity. In other words, we have a versatile and extremely precise method for determining resonance energies.
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| A mirror deflects the laser beam from the "laser booth" into the experimental storage ring, where it is made to overlap with the heavy ion beam. (The course of the beam has been sketched in the picture.) |
Previously, it had only been possible to measure hyperfine splitting in hydrogen and helium single-electron systems. Consequently, the successful bismuth experiment really did represent a "quantum jump" in experimental technique. Once it became possible to measure the fluorescent light signal, which is a function of the laser energy (or of the beam velocity), to an accuracy of 10-4, new experimental challenges quickly appeared on the horizon. Not only has it been possible to investigate nuclear physics parameters such as the distribution of charge and currents in the bismuth nucleus, quantum electrodynamic effects have also been researched. In order to clearly distinguish between the two components, further measurements of hyperfine splitting of nuclides adjacent to bismuth are planned. As described in the section on the "Structure of the Atomic Nucleus," these nuclides can be generated as exotic beams from the fragment separator. Thus it will be possible to measure the role played by the magnetic interaction in quantum electrodynamics for the first time.
Naked ions sometimes have shorter lives
Now that highly-charged ions can be circulated for many hours in heavy ion storage rings without significant energy losses, several other processes occurring naturally in hot stellar plasmas can be studied in a terrestrial laboratory. For instance, it has proved possible for the first time to observe and measure bound beta decay - a special form of radioactive beta decay predicted more than 40 years ago. In this process the electron produced in the beta decay is not emitted as a free particle as in normal beta decay. Instead, it is immediately trapped again in an unoccupied state of the same atom. This bound beta decay has only minimal importance for the neutral atoms that make up the normal state of the matter on Earth since the sites in the discrete energy shells are all occupied in neutral atoms. In stellar material, by contrast, when atoms have completely or partially lost their shell electrons, bound beta decay is an important, and sometimes even the sole, decay process.
One particularly interesting case occurs when an electron is no longer able to leave the atom because of the inadequate decay energy available to it. In this case, beta decay is not possible for the neutral atom: it remains stable. For the ionized or even naked atom, on the other hand, bound beta decay is possible and decay occurs. In this way, naked atoms can have a significantly shorter life than atoms "covered" with electrons. And this is precisely what happens in the case of the dysprosium nuclide with mass number A = 163 and atomic number Z = 66.
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| A neutral dysprosium atom with fully occupied electron shells (left half of the figure) is stable. On the other hand, a highly ionized atom with unoccupied electron states (right half) can decay. The letters K, L, and M designate the electron shells. |
Mysterious lines in the heaviest collision systems
Naturally, work in heavy ion atomic physics at Darmstadt is not restricted to experiments with the new ESR storage ring. In pursuance of a rich tradition, experiments are still being carried out at the "old" UNILAC. For instance, in two independent experiments performed a decade ago, narrow electron-positron lines with sum energies in the range of 500 to 800 keV were observed in collisions involving extremely heavy participants. Such collisions included uranium on tantalum and uranium on thorium - with injection energies close to the Coulomb barrier. For a long time, these lines could not be related to any known processes. Ambitious attempts to explain them as decay signals of still unknown particles proved equally difficult to substantiate. Now, after having made substantial improvements to the equipment, physicists plan to carry out highly complex, beam time-intensive experiments with a view to constructing a systematic database with which to explain the origin of the earlier observed structures.
Other UNILAC experiments have provided information about static properties of the atoms, such as their binding energies, or about dynamic properties such as ionization and electron- electron correlations. Most recently, recoil ion momentum spectroscopy has proved to be a new and particularly promising experimental method. The ions generated in a collision and the electrons released by ionization are extracted from the interaction zone by electrical and magnetic fields and led to counters sensitive to position and energy. Thus the kinematics and energy transfer in an atomic reaction can be determined completely and with high resolution. Now this method is also to be applied to the storage ring, where it will distinctly enhance the spectroscopic potential.
Another long-known spectroscopic method has also produced impressive results with the ESR: a free electron can be "resonantly" captured in a highly charged atom if the energy released on capture exactly matches the excitation energy for a bound electron. This dielectronic resonance recombination can be realized in the storage ring by quickly and accurately "tuning" the energy of the cooling electrons to that of the ions. Thus certain excited states can be deliberately produced in a highly charged atom. The binding energies of lithium-like gold atoms - gold nuclei with just three bound electrons - have been determined in the ESR to an accuracy of about 1 eV. Should physicists succeed in further improving the quality of the cooled ion beam, it ought to be possible to improve upon this degree of accuracy even more.
Highly charged ions in storage-cooler rings and ion traps have opened up new experimental fields in atomic physics. New types of experiments can be carried out with unique precision. But so far, only a small proportion of the enormous potential of this type of research has been exploited. In order to tap this potential further, still better experimental methods and techniques will have to be developed. Should this goal be achieved, there will be every justification for saying that the storage rings used in heavy ion research - not least of all, that at Darmstadt - really have promoted a quantum leap in atomic physics.