Wednesday, 6 March 2019

The zirconium-88 neutron caputre cross-section mystery

Figure: The neutron capture cross section of
measured nuclei.  The new measurement of
zirconium-88 is the second-highest point. 
From Shusterman et al., Nature 565, 328 (2019)
doi: 10.1038/s41586-018-0838-z
Imagine you are a neutron,  newly–released from a fissioning nucleus in a reactor.  Your new–found freedom involves flying around in an environment full of other nuclei.  If you crash into one, it might grab you and keep you, converting the nucleus to the next-heaviest isotope.

Fortunately for you, you can see all these nuclei and they all look very small, and easy to avoid crashing into.  Except there are a few sneaky ones which can grab you as you fly past. According to a new paper published in Nature by Shusterman et al., zirconium–88 is one such nucleus, for reasons that are not at all clear.

The protons and neutrons in 88Zr define a nuclear density which extends up to a radius of a little over 5 fm (= 5 x 10–15m) which amounts to a cross–sectional area of ≈ π x (5 x 10–15)2 = 2 x 10–29 m2.  According to Shusterman et al., a 88Zr nucleus presents an effective cross–sectional area of  9 x 10-23 m2. That's around a million times larger than the extent of the matter that makes up the nucleus.  The effective radius of 88Zr for neutron capture is around 5 x 10–12m.  This is around the same radius as the innermost electrons circling round a zirconium atom.

So a neutron only needs to go in the vicinity of zirconium–88 for it to get captured.  This result is a surprise.  The predictions previous to the experimental work gave a predicted cross section around 4 orders of magnitude smaller. 

The reason why 88Zr appears so large to neutrons is a mystery, then.  Presumably there is a strong resonance which makes the nucleus really likely to accept a neutron coming into it with just the right energy (the measurement is integrated over a range of energies).  But then, all nuclei have resonances like this.  Why is it so strong in 88Zr?  We don't know.

This result has consequences in modelling of stellar nucleosynthesis, where the absorption of neutrons is one of the key processes happening in novae and supernovae.  It also means that significant quantities of 88Zr in nuclear reactors — created during the fission process, for example — would be a reactor "poison", sucking up the slow neutrons which are the basis of the nuclear chain reaction.  

Plenty to ponder about this result.  

A plot from the paper is shown here.  The large value for zirconium–88 is the second-largest measured ever.  The largest cross-section, in xenon–135 was measured in 1948. 

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