Dissertation: Development of measurement devices led to a breakthrough in the study of exotic nuclei (Virtanen)
High-precision atomic mass measurements of exotic radioactive nuclei are vital for understanding the interactions of the nucleons of the atomic nucleus. Mass measurements offer also information on the astrophysical origin of the elements as well as on the decay paths of certain long-lived radioactive isotopes towards stable matter. At the same time, these measurements produce valuable information, based on which the models of the residual strong interaction at the core of nuclear theory can be developed and tested against experimental results.
- Atomic mass measurements face two challenges: the short half-lives of the exotic radioactive nuclides and the composition of the produced ion beams. These beams are composed of a broad range of ions, most of which hinder measurements of the ions of interest. The devices developed in this thesis work can tackle these challenges, explains Doctoral Researcher Ville Virtanen from the University of Jyväskylä.
Development of measurement devices enables the research of short-lived nuclides
In Ville Virtanen’s dissertation research, a Multi-Reflection Time-of-Flight Mass Spectrometer was integrated and commissioned within the Accelerator Laboratory’s Ion-Guide Isotope Separator On-Line (IGISOL) facility. The MR-ToF-MS can be used to separate and measure the atomic masses of exotic radioactive nuclei. To use this mass-spectrometer, the bunching section of the Radio-Frequency Quadrupole cooler-buncher (RFQ cooler-buncher) of IGISOL was reconstructed.
- The mass spectrometer is especially useful to measure the atomic masses of the short-lived radioactive isotopes that decay in less than a tenth of a second. It can be utilized as a fast mass separator for several laser- and decay spectroscopy measurements as well as other experiments, specifies Virtanen.
A mystery was solved after 19 years
The equipment developed in this thesis work was used to perform measurements to solve the problem of two-proton decay of silver-94 that has puzzled the nuclear physics community since 2006. Decay by the emission of two protons is a rare mode of radioactive decay in which, as the name suggests, two protons tunnel out of the atomic nucleus.
- This particular decay mode has so far been found to occur in a small group of atomic nuclei, all of which have an even number of protons, except for the odd proton number silver isotope 94Ag, clarifies Virtanen.
An odd number of protons has so far been expected to favor the tunneling of a single proton. The time-of-flight mass measurements performed in this work show that this silver isotope is significantly lighter than previously thought, revealing two-proton emission from 94Ag to be practically impossible. The research shows that evidence of two-proton emission is found only for nuclei with an even proton number.
- Silver-94 is a special nucleus as it consists of an equal number of protons and neutrons; a situation where especially the effects of interactions between protons and neutrons are expected to surface more strongly. The nuclear binding energy of this nucleus therefore informs us on the strength of the proton-neutron interaction, points out Virtanen.
M.Sc Ville Virtanen defends his doctoral dissertation ”Mass separation and atomic mass measurements of exotic radioactive nuclides with a multi-reflection time-of-flight mass spectrometer at the ion-guide isotope separator on-line facility” on 15.10.2025 starting 12:00 in Ylistönrinne auditorium FYS1. The opponent is Dr. Vladimir Manea (Laboratoire de Physique des 2 Infinis Irène Joliot-Curie) and the custos is senior lecturer Tommi Eronen (University of Jyväskylä). The defence will be held in english
The dissertation “Mass separation and atomic mass measurements of exotic radioactive nuclides with a multi-reflection time-of-flight mass spectrometer at the ion-guide isotope separator on-line facility” is available in the JYX publication archive: https://jyx.jyu.fi/jyx/Record/jyx_123456789_106005?sid=209819217
