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Extending the north-east limit of the chart of nuclides

The existence of nuclei with exotic combinations of protons and neutrons provides fundamental information on the forces acting between nucleons. The maximum number of neutrons a given number of protons can bind, neutron drip line1, is only known for the lightest chemical elements, up to oxygen. For heavier elements, the larger its atomic number, the farther from this limit is the most neutron-rich known isotope. The properties of heavy neutron-rich nuclei also have a direct impact on understanding the observed abundances of chemical elements heavier than iron in our Universe. Above half of the abundances of these elements are thought to be produced in rapid-neutron capture reactions, r-process, taking place in violent stellar scenarios2 where heavy neutron-rich nuclei, far beyond the ones known up today, are produced. Here we present a major step forward in the production of heavy neutron-rich nuclei: the discovery of 73 new neutron-rich isotopes of chemical elements between tantalum (Z=72) and actinium (Z=89). This result proves that cold-fragmentation reactions3 at relativistic energies are governed by large fluctuations in isospin and energy dissipation making possible the massive production of heavy neutron-rich nuclei, paving then the way for the full understanding of the origin of the heavier elements in our Universe. It is expected that further studies providing ground and structural properties of the nuclei presented here will reveal further details on the nuclear shell evolution along Z=82 and N=126, but also on the understanding of the stellar nucleosyntheis r-process around the waiting point at A~190 defining the speed of the matter flow towards heavier fissioning nuclei.

The access to unexplored regions of the chart of nuclides far from stability offers unique opportunities in modern nuclear physics but also in nuclear astrophysics. Studying ground and structural properties of nuclei close to the drip lines, crossing shell gaps or in particular regions of deformation represents the most important testing ground for improving the description of the nuclear force acting in the nuclear many body system. Some examples of new phenomena constraining the nuclear interaction, that appear far from stability, are new shell gaps 4 , evidencing the role of the tensor term in the nuclear force 5 , or abnormal matter distributions such as haloes or neutron skins 6 .
Many of the stellar nucleosynthesis processes also involve nuclei far from stability, in particular, those taking place in violent scenarios like novae or super-novae, leading to the production of the heaviest nuclei in the Universe 7 . Clear examples are the rapid-proton capture, rp-process, or the rapid neutron capture, r-process, where nuclei approaching the proton and neutron drip lines, respectively, are produced. The full 3 understanding of these nucleosynthesis processes requires then, experimental information on the bulk and structural properties of many of those nuclei.
The number of nuclear species bound by the nuclear force can be estimated using theoretical descriptions of the nuclear binding energy. According to the finite range droplet model 8 , around 6000 nuclear systems could be bound by the nuclear force. The last edition of the chart of nuclides 9 includes up to 2962 nuclei observed so far. The known nuclei reach the proton drip line up to mercury (Z=80), while the exact location of the neutron drip line is only known up to oxygen (Z=8). Then, the heavier the element the further is the last known neutron-rich isotope from the estimated position of the drip line. This situation becomes dramatic for elements above dysprosium (Z=66), where the present limit of the chart of nuclides is just few neutrons away from stability.
The presence in this area of the chart of nuclides of regions of deformation, the last known nuclear shells (Z=82, N=126) and the last waiting point of the stellar nucleosynthesis r-process, provides strong motivation for the present efforts for extending the north-east limit of the nuclear chart.
The production of nuclei far from stability requires two conditions: an optimum reaction mechanism populating the nuclear species of interest with the largest possible rate and an efficient experimental technique to identify and separate those nuclei. The choice of the reaction mechanism is limited by the possible ions that can be accelerated, their intensities and the maximum useful target thickness according to the projectile energy. A large fraction of the proton drip line is reachable using spallation, fragmentation and fusion-evaporation reactions 10 . Light nuclei at or close to the neutron drip line are accessible by fragmentation and spallation reactions 11,12 . Fission is successfully used for producing medium-mass neutron-rich nuclei 13 , although the fragmentation of 136 Xe is also competitive in some particular cases 14 , and the combination of both reaction mechanisms has been proposed as a future option 15 . Some 4 years ago it was proposed to use fragmentation reactions at high energy to overcome the present limitation for producing heavy neutron-rich nuclei 16 . The main argument was that this process is governed by large fluctuations in the neutron excess of the final reaction products but at the same time profit from relatively large useful target thicknesses. The present work aimed at the verification of these arguments. The experiment we present here aimed at investigating the production of heavy neutron-rich nuclei taking advantage of the relativistic heavy-ion beams produced with the SIS synchrotron at GSI, Darmstadt. Beams of 238 U and 208 Pb ions were accelerated up to 1 A GeV with a typical intensity of 10 8 ions/s, that was continuously monitored using a secondary electron monitor. The beams extracted from the synchrotron were then directed to a beryllium production target with a thickness of 1600 mg/cm 2 .
Projectile fragments, flying forward, were then separated and unambiguously identified 5 using the magnetic spectrometer FRagment Separator (FRS) 19 . The FRS is a zerodegree, magnetic spectrometer with a resolving power B/B~1500, a momentum acceptance p/p~ 1.5% and an angular acceptance of 15  hydrogen-like ions, having almost the same magnetic rigidity but a much larger production cross section. Two conditions seem then mandatory for the unambiguous identification of heavy neutron-rich nuclei: minimize the fraction of none fully stripped ions and force and identify changes in the atomic charge state of the transmitted nuclei. 6 In the present experiment both requirements were fulfil by using beams at relativistic energies but also niobium stripper foils at the target, at the intermediate image plane and in between the two MUSIC chambers. By measuring the difference in magnetic rigidity induced by the atomic charge changing at the intermediate image plane of the spectrometer and the different energy losses in both MUSIC chambers, also due to atomic charge changes at the stripper between both chambers, we could guarantee a ratio of contaminants below 30%.
Using this method, and scanning with the FRS magnets the region of magnetic rigidities larger than 12.91 Tm, we were able to produce and identify more than 130 heavy neutron-rich isotopes of elements between tantalum (Z=72) and actinium (Z=89), 73 of them for the first time, using roughly five measurement days per beam. In figure 2 we show a single identification matrix produced combining the more than 20 different This particular reaction channel demonstrates the large fluctuations governing the fragmentation process both in isospin but also in excitation energy, being the key issue in the production of the most neutron-rich nuclei.
The dotted and thick-solid lines in figure 3, showing the most neutron-rich nuclei known in 1958 and before the present experiments, clearly indicate how the use of coldfragmentation reactions at relativistic energies represents a major step forward in the production of heavy neutron-rich nuclei since this reaction mechanism made possible to produce and identify more nuclei in this region of the chart of nuclides that during the past 50 years of research. Indeed, after these measurements 22, 23,24 , several experiments could already investigate ground and structural properties of some of these nuclei 25,26,27 , taking advantage of the cold-fragmentation mechanism for their production.
In summary, our work shows that projectile-fragmentation reactions at relativistic energies investigated in-flight constitute an optimum tool for exploring the north-east region of the chart of nuclides. In the present case we could synthesize for the first time