First observation of 54Zn and its decay by two-proton emission

Abstract

The nucleus 54 Zn has been observed for the first time in an experiment at the SISSI/LISE3 facility of GANIL in the quasi-fragmentation of a 58 Ni beam at 74.5 MeV/nucleon in a nat Ni target. The fragments were analysed by means of the ALPHA-LISE3 separator and implanted in a silicon-strip detector where correlations in space and time between implantation and subsequent decay events allowed us to generate almost background free decay spectra for about 25 different nuclei at the same time. Eight 54 Zn implantation events were observed. From the correlated decay events, the half-life of 54 Zn is determined to be 3.2 +1.8 −0.8 ms. Seven of the eight implantations are followed by two-proton emission with a decay energy of 1.48(2) MeV. The decay energy and the partial halflife are compared to model predictions and allow for a test of these two-proton decay models.PACS numbers: 23.50.+z, 23.90.+w, 21.10.-k, 27.40.+z Our understanding of nuclear structure is mainly based on results obtained with nuclei close to the line of stability. These studies allowed for understanding of the basic structure of the strong interaction which governs the interplay between neutrons and protons in an atomic nucleus. However, these nuclei close to stability cover only a very small range in isospin, i.e. their proton-toneutron ratio is rather similar. With the advent of machines to produce radioactive nuclei, these basic concepts can now be tested with more and more exotic nuclei having a strong imbalance of neutrons and protons.With these exotic nuclei being much further away from stability, also new phenomena appeared. For nuclei beyond the proton drip line, where the strong force can no longer bind all protons, one-and two-proton (2p) radioactivity was predicted more than 40 years ago by Goldanskii [1]. For odd-Z nuclei, one-proton radioactivity was proposed to occur, whereas for medium-and heavy-mass even-Z nuclei the nuclear pairing energy renders one-proton emission impossible. In this case, twoproton emission is to be expected.One-proton radioactivity was observed for the first time about 20 years ago by Hofmann et al. [2]. Twoproton radioactivity was sought for many years without success. This research field experienced a strong boost with the advent of high-intensity projectilefragmentation facilities. At these facilities, experimentalists could for the first time reach the most promising candidates for two-proton radioactivity. According to recent theoretical predictions, proton drip-line nuclei in the A=40-55 region were identified as the most promising candidates [3][4][5]. The recent observation of two-proton radioactivity of 45 Fe [6,7] confirmed these predictions nicely. In other experiments, less promising candidates like 42 Cr, and 49 Ni could be shown to decay by β-delayed processes [8]. Beyond 45 Fe, 48 Ni and 54 Zn were regarded as possible candidates to exhibit two-proton radioactivity. In the present letter, we report on the first observation of 54 Zn and its decay by two-proton radioactivity. 54 Zn was produced by...

4D-imaging of drip-line radioactivity by detecting proton emission from 54mNi pictured with ACTAR TPC

“…Due to the short half-life of the isomeric state, the ionization signal from the emitted proton is registered together with the huge signal of the ion implantation (three orders of magnitude larger), making the detection impossible with standard techniques such as silicon detectors (e.g. 11 , 15 ). Using ACTAR TPC, the proton signal is clearly visible when the particle track projection on the collection plane creates a signal on different pads of the detector (Fig.…”

“…In this decay, the proton has to carry away impressive 9ħ units of angular momentum, ℓ = 9, to decay into 52 Fe, with only a 1.5% branch to its 0 + ground state known. In the same region of the nuclear chart, two-proton radioactivity was discovered from the ground states of 45 Fe 11 , 12 , 48 Ni 13 , 14 and 54 Zn 15 , 16 .…”

Nuclei near and at the proton dripline

“…In both experiments ions of 45 Fe were implanted into silicon detectors and the evidence of the 2p decay came from the measured total decay energy (Q 2p ≈ 1.2 MeV) and the decay time (T 1/2 = 2.6 ms). The same technique was used later by Blank et al (2005) to discover 2p radioactivity in 54 Zn (T 1/2 = 1.6 ms) and by Goigoux et al (2016) to establish this decay mode in 67 Kr (T 1/2 = 7.4 ms).…”

Mass prediction of proton-rich nuclides with the Coulomb displacement energies in the relativistic point-coupling model

“…This exotic decay mode was first predicted in 1960 [23], and since then lots of attempts have been made in many different systems [24]. Evidence for the ground-state 2p radioactivity for 54 Zn was observed in 2005 [25], and possibly also for 48 Ni [26].…”