ALL Experts Redux: The Production Of Heavy Elements

 

Question –

I read in a book that production of elements in the core of stars terminates with   iron.  How are even heavier elements produced?

Thanks!

Answer:

The theoretical answer as to how the heavy elements are produced was first presented by Fowler, Hoyle et al in their paper 'Synthesis of Elements in Stars' in The Review of Modern Physics over four decades ago. Basically, they theorized that in very massive stars (> 3-4 solar masses) the core collapses after the nickel-iron stage is reached and the implosion leads to an enormous explosion called 'supernova'. 

En route to this stage, the gravitational collapse is such that protons and electrons are squeezed together in the core to form countless neutrons which then become available for combination with existing elements formed (up to the Ni-Fe stage). The authors developed their paper to show how two basic neutron capture paths could occur in type I supernovas, leading to elements of very high atomic number. The neutron capture paths were referred to as "the r process" and the "s process". 

The essential feature in each is that a large flux of neutrons becomes available for addition to elements of the iron group. (The "r" in r-process refers to "rapid", e.g. rapid neutron capture path, and the "s" in s-process, refers to "slow", e.g. slow neutron capture path)  Specifically, the r-process features neutron capture on a very short timescale (0.01 sec  <  10 sec) which is able to produce isotopes of elements in the range (70 < A < 270). The latter limit is in the neighborhood of Rutherfordium. The s-process features neutron capture with emission of gamma radiation and occurs over time scales in the range (100 yr. <  10^5 yrs.) and produces isotopes in the range: 23 < A < 46. 

Another process identified in the supernova event is the p-process, which denotes proton capture with emission of gamma radiation - or alternatively, emission of a neutron after gamma ray absorption. Of course, the details are much more complex than presented here - which serves merely to provide a basic answer to your question. 

For example, competing processes can arise - thus the p-process reactions can and do compete with positron emission until proton addition is no longer possible, and positron emission occurs alone. (Bear in mind the positron =e(+) is a positive electron, so that when each such emission occurs the effect is to reduce the atomic number by 1)