First Direct Mass Measurements of Nuclides around Z=100 with a Multireflection Time-of-Flight Mass Spectrograph

Abstract

The masses of 246 Es, 251 Fm, and the transfermium nuclei 249−252 Md and 254 No, produced by hot-and cold-fusion reactions, in the vicinity of the deformed N ¼ 152 neutron shell closure, have been directly measured using a multireflection time-of-flight mass spectrograph. Mt, were determined. These new masses were compared with theoretical global mass models and demonstrated to be in good agreement with macroscopic-microscopic models in this region. The empirical shell gap parameter δ 2n derived from three isotopic masses was updated with the new masses and corroborates the existence of the deformed N ¼ 152 neutron shell closure for Md and Lr. DOI: 10.1103/PhysRevLett.120.152501 Precision mass measurements of unstable nuclei, providing a direct measure of the nuclear binding energy, are invaluable for the study of nuclear shell evolution and collective effects, such as deformations, far from stability [1,2]. For transfermium nuclei and the yet poorly investigated region towards the superheavy nuclei (SHN), where proton repulsion becomes a generally dominant feature, the description of nuclear lifetimes depends crucially on shell stabilization effects mainly driven by deformed shells [3][4][5]. Theoretical studies with increasing particle numbers investigate the so-called "island of stability" [6], where features like the continuing decrease of energy gaps [7] and the emergence of shape coexistence [8] have a crucial impact on the predicted position and localization of stability regions and the corresponding lifetimes of the nuclei. Although the first experimental evidence for SHN has reached the region of the predicted subshell closure at N ¼ 162 [9-11], the deformed shell closure at N ¼ 152 for transfermium nuclei (see, e.g., [12]) and, as recently pointed out, weaker shell effects in the vicinity [13], still represents the cutting edge for thorough experimental investigations. The transfermium nuclei, however, can be produced only online, in heavy-ion fusion and nucleon transfer reactions, and consequently only low yields are available for study, necessitating highly efficient techniques. Direct mass measurements of transfermium nuclei have so far been performed for only six nuclei-four isotopes of nobelium and two isotopes of lawrencium-with the Penning trap mass spectrometer SHIPTRAP [14,15].In this Letter, we report the first implementation of a multireflection time-of-flight mass spectrograph (MRTOF MS) for transfermium nuclei as shown in Fig. 1, including new mass measurements of 246 Es, 251 Fm, [249][250][251][252]

Quantified limits of the nuclear landscape

“…Our dataset combines all experimental masses from AME2003 [42] and AME2016 [43] augmented by the recently measured masses from Refs. [44][45][46][47][48][49][50]. For nuclei where experiments have been repeated, we take the most recent value.…”

Colloquium : Superheavy elements: Oganesson and beyond

“…Direct mass measurements of transfermium nuclei have been carried out with the Penning trap mass spectrometer for[252][253][254][255]256 Lr(Block et al, 2010;Ramirez et al, 2012). Recently mass excesses of several transfermium nuclei were obtained by means of time-of-flight mass spectrometry(Ito et al, 2018), which enabled mass determination of heavier nuclei, up to 261 Bh and 266 Mt. The mass of an atom M A can be decomposed intoM A = Z(m p + m e ) + N m n − (B nuc + B el )/c 2 ,where m p , m e and m n are, respectively, proton, electron, and neutron masses known to high precision, and B nuc and B el are the nuclear and electronic binding energies.…”

New mass anchor points for neutron-deficient heavy nuclei from direct mass measurements of radium and actinium isotopes

“…The time-of-flight data has been analyzed using the binned maximum-likelihood estimator provided by the ROOT package [33] developed at CERN. As ion peaks from MR-TOF spectra often have non-Gaussian shapes, meanwhile asymmetric fit functions are preferred to provide higher accuracies as, e.g., reported in a recent analysis of transfermium nuclei [22]. The chosen probability density function (PDF) in the present work is a Gaussian hybrid with two exponential tails previously applied for γ-spectroscopy [34].…”