Experimental Studies of Proton-Unbound Nuclei via In-Flight Decay Spectroscopy
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II. Physikalisches Institut
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Several exotic nuclei, located beyond the proton dripline, have been studied in the present thesis. Four new isotopes were discovered, and a variety of nuclear structure findings could be accomplished. In particular, this thesis reports the first observation and spectroscopy of the proton-unbound isotopes in the vicinity of very neutron-deficient isotopes of argon (28Cl, 30Cl, 29Ar, 31Ar and 31K). The corresponding experiments have been performed with the fragment separator FRS of GSI using a start version of the EXPERT setup, which is under construction and designed for experiments with even more exotic nuclei at the Super-FRS of FAIR.
A secondary beam of 31Ar ions has been created via projectile fragmentation of 885 MeV/u 36Ar ions with an intensity of 10^9 ions/s impinging on the production target at the entrance of the FRS. The spatial separation of 31Ar ions has been performed with the FRS operated in a special ion-optical mode applying a degrader at F1 to achieve an achromatic focus at the central focal plane F2, where the detector setup of the EXPERT pilot experiment has been installed. The setup consisted of the 4.8 g/cm^2 Be secondary target and an array of silicon microstrip detectors. The proton-unbound isotopes of interest have been studied by the well-established in-flight tracking technique, where the trajectories of the decay products are measured with the array of silicon microstrip detectors. This allows to measure particularly short-lived nuclei, which exhibit half-lives of the range from ns to ps or even below. Using the in-flight decay spectroscopy, the 1p and 2p emission processes have been reconstructed from the measured angular correlation in double "heavy ion + proton" and triple "heavy ion + two protons" coincidences, respectively.
In the course of the analysis, the two previously unknown isotopes 28,29Cl, which are unbound with respect to 1p emission, have been observed for the first time. To their ground states, the 1p-separation energies S_p = -1.60(8) and S_p = -0.48(2) MeV, respectively, could be assigned. Excited states of the 2p emitter 31Ar have also been identified for the first time. The high level of isobaric symmetry observed in the 31Ar - 31Al mirror pair allows to assign a 2p-separation energy S_2p = 6(34) keV to the ground state of 31Ar. The obtained ground state energy of 31Ar is the most accurate evaluation available so far. It improves the results of previous estimates by a factor of three. Also, the 2p emitter 29Ar has been discovered. The first excited state of this nucleus with S_2p of -5.50(18) MeV has been identified. One more highlight of this thesis is the discovery of 31K: its observation marks the nuclide, that is hitherto found to be farthest away (four mass units) beyond the proton dripline. It is a 3p emitter and has been studied by means of the angular correlation of its decay products, 28S and 3p. The 3p-separation energy of the 31K ground state has been assigned to S_3p = -4.6(2) MeV. An upper half-life limit of 10 ps of 31K has been derived from the measured decay-vertex distribution.
Overall, the performed studies and obtained results are an essential first step towards wider nuclear-structure studies far beyond the proton dripline, where basic nuclear properties, theoretical concepts (like mean-field theory) and the strong nuclear force in general may be examined. They may open a transition from ordered nucleons in nuclei to amorphous nucleon matter.
Further investigations exploiting the in-flight decay technique shall be performed within the EXPERT project at the future super-conducting fragment separator Super-FRS. Leading to this direction, this thesis presents recent developments of new EXPERT detectors, which aim at the tracking of neutrons originating from neutron-decay reactions (NeuRad detector) and Time-of-Flight measurements for improved secondary-beam identification (ToF detector) at the Super-FRS. Moreover, the further development and refinement of the in-flight decay technique by using in addition precise longitudinal momentum measurements of the heavy ion from the decay performed by the subsequent spectrometer stages of the separator, has been discussed. A novel, complementary method has been proposed and proven to provide a model-independent evaluation of the spectroscopic information. Finally, prospects to forthcoming physics cases that can be studied, dedicated experiment proposals, intended developments of the EXPERT detectors, and ideas for further investigations of proton-unbound nuclear systems using the in-flight decay technique have been discussed.
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