Tuesday, 28 April 2015

A lot of beta decay

Working where I do -- in a nuclear physics research group that is one of the largest in the UK -- we get a pretty good programme of visitor seminars.  As a rule, it's one per week, with sundry other visitors speaking as and when they happen to be here, or passing.  In fact, quite a few visitors make use of the fact that we are near London and pop down to catch up and give a talk when in transit.

Today we had a talk from Giuseppe Lorusso.  He works at NPL, which is almost certainly our closest neighbour in the world of nuclear physics, up the road in Teddington.  His appointment there comes in conjunction with a closer collaboration between our two institutions, which includes a couple of our academics now having joint Surrey–NPL appointments.  

Giuseppe talked mainly about his work at RIKEN, where I visited last week.  In particular, it was concerned with an impressive set of new measurements of beta–decay lifetimes of neutron–rich isotopes, many of which had not been measured before, with some which had, but with poorer accuracy.  He gave a nice motivation in terms of the generation of elements heavier than iron.  While all elements heavier than lithium (element #3) are made in stars, elements heavier than iron (e.g. gold, lead, uranium, tin, xenon, and a whole host of others) are made only in particular stellar processes.  They have to involve nuclear reactions involving neutron capture.  It is clear from the current abundances of elements that at least some of these processes have to be really fast -- with lots of neutron captures happening in the order of seconds or minutes with subsequent beta decay leading to the isotopes we see now.  It used to be a matter of faith that core–collapse supernovae were the events that gave rise to this rapid neutron–capture process (the "r–process"), but that no longer seems certain, since the models of such events don't seem to have the right neutron fluxes or temperatures.   Current best guesses include high–magnetic–field spinning supernovae, or merging neutron stars, though they each have their problems.

In any case, to understand what goes on in these processes, we need to know the decay lifetimes of nuclei all over this region.  The difficulty is that these very neutron–rich isotopes only exist in nature during these stellar events.  We can make them artificially, but only now with the facilities on offer today have we been able to get close.  In particular, at RIKEN, where a beam of uranium–238 nuclei is collided with a beryllium target, almost anything lighter than uranium can be generated.  One of the things that makes RIKEN special is its ability to separate out all the results of the reactions in a clean way.  Giuseppe's talk showed a wonderful particle identification plot showing the large number of isotopes generated in the experiments;  tens of which had never had their beta–decay half lives measured.  What did the new data tell us about the stellar processes?  The main conclusion seemed to be that (unlike in very light nuclei) the "magic numbers" representing very stable nuclei seem to be pretty robust, lasting well in to the neutron rich region.  This is in contrast to some theoretical ideas suggesting that the nuclear spin–orbit interaction would become weaker here.  

Giuseppe is pictured above, just as he was talking about neutron star mergers.