Friday, 17 May 2019

Well hello neptunium-220

The discovery has been announced, in Physical Review Letters (here, but paywalled) of a new isotope of element number 93, Neptunium (Np).  It's Np–220, with 93 protons and 127 neutrons.  

This isotope is a long way from the nearest stable isotope, as can be seen on this section of the nuclear chart, with proton number Z increasing along the vertical axis and neutron number N along the horizontal axis:

Np–220 is at the top left of the chart.  The stable isotopes are the black ones, with the nearest either involving losing many protons to go towards bismuth and lead, or to gain many neutrons to get towards the stable uranium isotopes. 

It's possible to make these very far-from-stability isotopes by reacting together two lighter nuclei, in which the stable isotopes tend to have a N:Z ratio which is close to that of Np–220.  The actual reaction used was to fuse argon–40 with rhenium–185, making a very excited Np–225 nucleus, and then looking for decays in which 5 neutrons are emitted. 

The experiments were performed in Lanzhou, China, and the resulting observation of alpha decay of Np–220 led to the conclusion that the extra stability conferred by the N=126 magic number survives into this far-from-stability region.

Thanks, as ever, to Ed Simpson, for providing the #1 online chart of the isotopes at

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