CORDIS - Forschungsergebnisse der EU

Exploring the shell structure of exotic Sn isotopes with an Active Target

Periodic Reporting for period 1 - MagicTin (Exploring the shell structure of exotic Sn isotopes with an Active Target)

Berichtszeitraum: 2016-01-01 bis 2017-12-31

The Nuclear scale is a bridge between the world of elementary particles and the Matter at the Atomic level. Nuclei are composed by protons and neutrons: to different proton numbers correspond different atomic species, while a different number of neutrons corresponds to different isotopes of the same Element. Protons and neutrons are called nucleons. Understanding the force that binds nucleons in the nucleus is still one of the major challenges of Nuclear Physics. This force is understood as the residual interaction between the quarks constituting the nucleons and can’t be mathematically treated in a simple way. Moreover, nuclei can be composed by many nucleons, making the treatment of the residual Nuclear Force a very complex many-body quantum mechanical problem. Up to now, effective forces have been employed and very successful theory has been the Nuclear Shell Model. Only recently, “ab-initio” methods started providing good results. In the search for a global theoretical approach capable of explaining the Nuclear Force, new experimental data are still crucial.
The relative number of protons and neutrons in the nucleus rules its stability and abundance. When building the Nuclear Chart, both the number of protons and neutrons are considered. It is observed that stable matter tends to accumulate along the bisector of this Chart, where the Nuclear binding and the Coulomb repulsion (pushing protons apart) find an equilibrium. Larger number of protons require an increasingly large number of neutrons to provide stability to the Nucleus, that’s why this so-called Valley of Nuclear Stability bends towards more neutron-rich nuclei at high proton number. Unstable (radioactive) nuclei also exist. Those can be artificially produced in Nuclear reactions or can be found in Nature if their lifetime is very long. Nuclei far from the Valley of Stability are also called Exotic Nuclei.
The Nuclear binding has a limit: beyond a certain amount of neutrons and protons the system becomes unbound and brakes into smaller its constituents. Where are the limits of the Nuclear Chart? Which are the properties of multi-nucleon systems under the most extreme conditions? Can we build a unified description of the nuclear properties? How do these affect astrophysical processes, state-of-the-art technologies and medical applications?
These are some of the most relevant issues faced by today’s Nuclear Physicists. In this context, worldwide efforts to tackle the nature of exotic nuclei comprise the construction of new-generation Radioactive Ion Beam facilities and new Detectors capable of exploiting at best the produced ions beams. The MagicTin project joined this endeavor with the specific aim of studying the properties of the most neutron rich Tin isotopes. Those isotopes (50 protons, N neutrons) are quite special in their structure since one of the recurring features along the Nuclear Chart, that is the closure of the Nuclear Shells, plays a very specific role.
Closed shells imply a lower attitude of the Nucleus against excitation, together with larger excitation energies. The proton shell is closed in the Tin isotopes and the neutron shells are closed at least in two Tin isotopes at N=50 and N=82, namely for 100Sn and 132Sn. Which are the spectroscopic properties of those Nuclei and of the surrounding ones? Do the most exotic Tin isotopes follow the same trends? Today we know that, on the neutron rich side of the chain, nuclei beyond N=90 are bound but very little is known about their structure properties. Is there a new shell closure at N=90 as some theories predict? To give an answer to these questions, new experimental tools need to be developed and used. With the MagicTin project I have joined the efforts of two ERC projects (ATCAR TPC and SpecMAT) that aim at the construction of a new generation of detectors capable of exploiting the artificially produced ion beams of exotic nuclei at the most extreme borders of the Nuclear Chart. Using standard experimental approaches, indeed, requires beam intensities of the order of 10^6 pps or more. These new tools aim at exploiting beams whose intensities can be as low as 10^2-10^3 pps. This is done by using a gas target that is, at the same time, a time projection chamber (a sort of 3D camera). Having a nuclear reaction occurring already inside the detector allows for unprecedented detection efficiencies and very low detection thresholds. Features like the particle detection dynamic range, the maximum event rate acceptable and the energy and particle identification resolution need to be characterized and optimized before actual experiments can be run.
The knowledge of Nuclear Data finds application in several field of Technology, from Homeland security to Energy production, from monitoring environmental radioactivity to nuclear medicine. Having a full and clear picture of the Nuclear landscape at a fundamental level is of paramount importance both for providing reliable input to the several applications in different fields, as well as for improving the basic knowledge of the laws governing Nature, that is at the basis of the Human being progress.
The Experienced Researcher (E.R.) worked in the framework of two main research projects: ACTAR TPC [ERC by G.F. Grynier, n. 335593] and SpecMAT [ERC by R. Raabe, n. 617156].
The ACTAR TPC collaboration has been developing a new generation Active Target to be used for several applications. Among the others, the measurement of direct reactions with the most exotic, low-intensity, radioactive beams will represent a real breakthrough in Nuclear Spectroscopy. The ACTAR TPC collaboration produced a prototype detector (ACTAR Demonstrator) and run the first test experiments in 2015. In 2016, the E.R. participated in the analysis of the data collected and co-supervised a Master Thesis on that topic. Some technical limitations of the device emerged during the analysis and were fixed by upgrading the Demonstrator. Two more in-beam tests were carried on in 2016 and proved the suitability of the device’ performances. In the same period, the E.R. submitted one Letter of Intent to the SPES Scientific Advisory Committee for the use of exotic Tin radioactive ions beams.
Once the final ACTAR TPC detector started being built, the ACTAR Demonstrator was moved to INFN-LNL for further R&D, in view of its use with Tin isotopes.
The activities related to the SpecMAT project concerned the development of gamma-ray auxiliary detectors for active targets. On this subject the E.R. contributed to the laboratory tests done to characterizing prototype detectors and defining the final specifications. The E.R. also contributed to the development of a GEANT4 simulation of the scintillators.
MagicTin contributed to developing new generation Active Target detectors. The ACTAR TPC is now running experiments at GANIL laboratory while the ACTAR Demonstrator was moved to INFN-LNL where it will be used for further R&D in view of its use with the Radioactive Ion Beams at the SPES facility. At this purpose, test measurements will also be performed at INFN-LNS during 2018.
In terms of training, the MagicTin project allowed the E.R. to join the two main European collaborations working in the field of Active Targets and to extend the physics scopes of those activities to the heavier mass region (neutron rich Tin nuclei). The training received allowed the E.R. to obtain a permanent position in an Italian research institution.