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Fundamental nuclear properties measured with laser spectroscopy

Periodic Reporting for period 4 - FNPMLS (Fundamental nuclear properties measured with laser spectroscopy)

Okres sprawozdawczy: 2019-10-01 do 2020-03-31

The central research theme of this project has been the study of the fundamental nature of the nuclear force by measuring the properties of exotic nuclei with laser spectroscopy. The programme asked: To what extent do three-nucleon forces modify the location of the neutron drip-line? How does the interplay of central and tensor components of the nucleon-nucleon interaction modify nuclear shell structure in exotic nuclei? How does the shape of the nucleus change as the neutron and proton drip lines are approached?

Laser spectroscopy measures nuclear observables without introducing assumptions from any particular nuclear model. When performed across a chain of isotopes this provide a powerful probe to study the evolution of nuclear structure. The most important cases required experimental and theoretical advances beyond the current state-of-the-art . Measurements were carried out at the ISOLDE facility in CERN, which produced the key radioactive species studied in this project, such as 78Cu. These exotic isotopes were produced at rates down to 10 atoms/second. This required a new level in sensitivity for laser spectroscopy of radioactive isotopes and we developed the collinear resonance ionization spectroscopy (CRIS) to address this. The CRIS method combines the high-resolution of collinear laser spectroscopy with the high sensitivity of resonance ionization spectroscopy. This allowed ground-breaking observations that provide a better understanding of nuclear structure. Measurements were performed around 52Ca, 78Ni, 100Sn and 132Sn and interpreted using with nuclear models that use only the properties of the lightest nuclei as constraints. We were able to perform the first spectroscopic measurements of radium monofluoride (RaF), showing that this system can be laser cooled and trapped for ultra-high precision experiments. The properties of molecular systems offer an opportunity to measure observables relevant for searching for new physics beyond the standard model of particle physics and provide new insight into the atomic nucleus.
Since the start of the project the computational capability of nuclear physics has allowed theoretical tools and models to rapidly evolved beyond the expectations of the community. This has allowed our measurements to address the over-arching questions supported by the rapid deployment and development of ab initio based nuclear models. In order to address the thematic questions within the project extensive effort has been dedicated to improving the experimental techniques. This consolidation process has highlighted where further improvements can be made and the future capability and capacity for the method to deliver cutting edge science has been highlighted. The method developed in this project are now being adopted at international facilities around the world.
We achieved 20 MHz line width while maintaining sensitivity [Groote PRL 2015]. This method provides better than 1 MHz precision on light elements [Koszorús PRC 100 (3), 034304 2019]. We produced high-resolution 3rd harmonic light (240-250 nm and 20 MHz) for spectroscopy of radioactive species [Groote Nat Phys 16 620 (2020)]. A novel laser ablation ion source for was constructed to support experimental preparation and development [Ruiz PRX 8 (4), 041005 2018]. We development of high electric field gradient field ionizer, that will reduce background by more than x100 [Vernon Sci. Rep. 10, 12306 (2020)]. We demonstrated that the CRIS method can be used to study radioactive molecular systems as well as atomic [Ruiz Nature 581 7809 396 (2020)].

We installed and commissioned a high precision laser laboratory in 2015. Existing laser assets in Manchester were provided to support the project providing an operational capacity of 17 lasers. At times all laser systems have been simultaneously used and monitored. A new UHV charge exchange cell was designed and installed. The control of the ion beam was improved with new software, power supplies, irises and Faraday cups to control and monitor the laser and atom beam position and overlap. The upgrades allowed an operational pressure of below 10-10 mbar to be reached helping reduce the observed background by a factor of 1000 compared to the proof of concept CRIS measurement in 2013. We developed a field ionization unit that further reduces the background by more than x100. The commissioning results for this unit have been published in Scientific Reports.

During the project we measured the ground-state spins and magnetic moments of the K-isotopes up to N = 33, crossing for the first time the proposed new N = 32 magic number (paper is in preparation). We have extended the studies on the Z = 29 Cu isotopes up to N = 49 (78Cu). The spins and moments of 76,77,78Cu reproduced well with LSSM calculations, establish clearly the doubly magic nature of 78Ni. The radii have been compared to different ab-initio nuclear theories, that describe binding energies and radii in a quantitative way with nucleon- nucleon interactions derived from chiral EFT, and fitted to observables of isotopes of up to A=4 only. These theories succeed for the first time to reproduce and understand the small odd-even staggering from a microscopy point of view. Laser spectroscopy on tin and indium isotopes just a few nucleons away from the exotic self-conjugate and doubly-magic 100Sn, were studied. The charge radii and moments of isotopes and isomers have been measured across the indium isotope chain from N = 52 to N = 82. The extraction of the moments and charge radii from the laser spectroscopy of indium has been achieved through collaboration with atomic theory. The interpretation of the tin and indium observables will build on the success of the copper work and is currently ongoing. In collaboration with theorists from quantum chemistry and with support from different groups at ISOLDE, the CRIS collaboration measured the low-lying structure of several RaF molecules. To our knowledge, this is the first ever laser spectroscopy measurement of a short-lived radioactive molecule. These achievements constitute a major step towards the use of radioactive molecule for nuclear structure, electroweak physics, and the study of fundamental symmetries.
We have demonstrated high-precision spectroscopy on exotic isotopes produced at rates down to 10 atoms/second extending measurements to the edges of the nuclear chart. The N=32 shell closure in potassium has been studied and the results are ready to be submitted for peer review. The isomer shift of charge radii in neutron-rich indium has been measured and builds on the recent work on neutron rich copper [Groote Nature Physics 2020]. The moments and charge radii of indium and tin are currently being analysed and will be submitted for peer review in 2020. The sensitivity of exotic molecular species for high precision laser spectroscopy is currently being investigated. We have measured the isotopes shifts and hyperfine structure of RaF and this is currently being carefully analysed. Our work has highlighted a new opportunity for understanding the atomic nucleus using molecular systems.