Two new isotopes: Boron-20 and Boron-21

I noticed a new paper appear on the arXiv this morning, announcing the discovery of two new isotopes;  boron–20 and boron–21.  Boron has atomic number 5, so each boron nucleus has 5 protons.  There are two stable isotopes; boron-10 with 5 neutron and boron-11 with 6 neutron.  Boron–20 and –21 have 15 and 16 neutrons respectively.  That large asymmetry between the number of neutron and protons makes the isotopes unstable.  They are so unstable that the last neutrons do not even stick on to the nucleus for long and they decay by emitting one or two neutrons.  The live long enough to be identified as a resonance state in experiment, which is exactly what happened in the experiment that led to their discovery. 

Figure from arXiv:1901.00455

The experiment took place at the Radioactive Isotope Beam Facility (RIBF) at RIKEN Nishina Center, in Japan.  A beam of calcium–48 ions was fired at a beryllium target at very high energy, causing some of calcium nuclei to have a number of their protons and neutrons ripped off to give lighter isotopes of various sorts.  From this cocktail of fragments, the experimenters extracted nitrogen–22 and carbon–22 which were then directed onto a second target (of carbon) where some reactions knocked protons out of the nitrogen–22 and carbon–22 nuclei to form the previously-unknown boron isotopes. The snapshot taken from the paper in the arXiv shows the reactions taking place.  The plot is a section of the Segrè chart in which isotopes are shown with increasing proton number in the y-direction and increasing neutron number in the x-direction.  The arrows show the proton knockout reactions leading to the boron isotopes.  The red line marks out the neutron drip-line, separating those nuclei which are stable with respect to losing a neutron, and those which are not.  

Though the paper just appeared on the arXiv today, it was published in Physical Review Letters on 27th December.  Odd to put it on the arXiv only after it has been published elsewhere, but at least it means it reaches a wider audience (including me!), albeit belatedly.  Unfortunately the arXiv is not as widely adopted in nuclear physics as astro or high energy physics.