Investigating Isomeric States of No-254 and No-255

Abstract

The study of the isomeric states of No-254 and No-255 was performed with the SuperHeavy element RECoil detector (SHREC) at the focal plane of the Berkeley Gas-filled Separator (BGS) at Lawrence Berkeley National Laboratory (LBNL), Berkeley, USA. The Pb-208(Ca-48,xn)No-(256-x) fusion-evaporation reaction was utilised to produce both nobelium isotopes with beam energies of 230 MeV and 225 MeV, respectively. The experiment was part of the commissioning of the BGS and SHREC in the context of the ongoing Element 120 campaign at LBNL. Through in-beam studies various isomeric states were identified and their half-lives were measured. The study involved an independent analysis of raw traces from SHREC detector elements using a trace analysis routine for the Lund Nuclear Structure Group. Through recoil-e-α correlations, the half-lives of the 16+ and 8- isomeric states of No-254 were measured to be t_1/2=214(18) μs and t_1/2=267(4) ms, respectively. For No-255, recoil-e-α-e correlations were used to measure a half-life of t_1/2=126(95) μs, attributed to the unresolved contributions from the 11/2- and the 27/2+ isomeric states.

Popular Abstract

The discovery of new elements has always been an exciting topic in the world of science. Over the last 100 years, many elements were discovered and thoroughly studied using both simple and complex methods, marking the near end of discovering new elements. Physicists refer to the proton and neutron composition of elements as nuclei. Currently, physicists have pushed forward with attempts to find new exotic nuclei. These can be rare isotopes of rigorously studied nuclei, or entirely new, heavier nuclei. Heavy nuclei are so named due to the large number of protons in them. These nuclei have a number of protons (Z) between 92 and 103. Superheavy nuclei have a proton number Z ≥ 104.

To study (super)heavy nuclei, physicists need to synthesise them, as they are not available in the environment naturally. Thus, a technique called fusion-evaporation reaction is used. This technique is when a beam of accelerated medium to light Z ions hit a target foil made of a medium to heavy Z material and fuse together. This fusion creates a nucleus with the number of protons equal to the sum of the number of protons of the beam and target. This synthetic nucleus holds a large amount of energy, resulting in the evaporation of the a few number of neutrons (N) to release some of that energy. This nucleus is then selected and guided using a magnetic separator towards a detector, where it is completely stopped inside the detector, or as it is commonly called, implanted. Only a handful of facilitates in the world are able to utilise this technique and guide the nucleus created to study its properties. The nucleus then decays to release energy through emitting gamma rays and electrons at various energies. This decay happens through various steps that typically occur almost instantaneously one after the other. In spite of that, some steps take a considerably longer time to occur. Each step in the decay is called a state, and the term used to describe the time each state has, is called a half-life. States with exceptionally long half-lives are called isomeric states.

For this thesis, an experiment was conducted at Lawrence Berkeley National Laboratory, Berkeley, USA to study the isomeric states of No-254 and No-255. This was done using the fusion-evaporation reaction Pb-208(Ca-48,xn)No-(256-x). Pb-208 was the material of the target foil and Ca-48 was the ion-beam that was used. They can create a highly excited state of 256No* followed by the evaporation of either x=1 or x=2 neutrons. The ion-beam was accelerated to 230 MeV and 225 MeV to favour either the evaporation of 2 or 1 neutrons, respectively. The nuclei of interest were then guided and selected using the Berkeley Gas-filled Separator which had the SuperHeavy element RECoil detector (SHREC) at its focal plane to detect them and their radioactive decay properties.

The signals produced from the implantation and subsequent decay of both isotopes in SHREC were analysed using a proprietary trace analysis routine for the Lund Nuclear Structure Group, which was rewritten as part of this work. Through recoil-e-α correlations the half-lives of the 16+ and 8- isomeric states of No-254 were measured to be t_1/2=214(18) μs and t_1/2=267(4) ms, respectively. For No-255, recoil-e-α-e correlations were used to measure a half-life of t_1/2=126(95) μs, attributed to the unresolved contributions from the 11/2- and the 27/2+ isomeric states.

• author: Mohamed, Marwan
• supervisor: Luis Sarmiento; Dirk Rudolph
• organization: Department of Physics; Particle and nuclear physics
• course: FYSM64 20242
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: SHREC, SuperHeavy Elements, Heavy Elements, Nuclear Structure, Nuclei, nobelium, isomeric states, K-isomers, 254No, 255No, Moving Window Deconvolution, BGS, half-life
• language: English
• id: 9203426
• date added to LUP: 2025-06-23 08:53:01
• date last changed: 2025-06-23 08:53:01