5.1 Nuclear photodisintegration
There are many ways in which high-energy gamma rays can disintegrate nuclei of iron-56, but as an illustration of the physics involved, and the amount of energy that may be absorbed by this photodisintegration process, consider the situation in which a nucleus of iron-56 is broken down into helium nuclei:
The amount of energy absorbed by this process in the forward direction is 
. At the temperature and density of a stellar core, the result is that about three-quarters of the iron nuclei are dissociated.
Activity 7
If each iron nucleus absorbs 124.4 MeV of energy by photodisintegration, and three-quarters of a core of mass 1.4 M☉ is dissociated in this way, calculate the total energy absorbed by this process. (The mass of an iron-56 nucleus is 56u, where 
kg is the atomic mass unit.)
Answer
The number of particles N in a sample of material is given by the total mass of the sample M divided by the individual masses of the particles m, i.e. N=M/m. Three-quarters of the core mass is 
, so the number of iron-56 nuclei is
Each nucleus absorbs 124.4 MeV of energy, which is equivalent to 
. So the core absorbs 
via the photodisintegration of iron-56 nuclei.
At still higher temperatures, helium nuclei will also undergo photodisintegration as follows:
The energy absorbed by this process in the forward direction is 
. At the temperature and density of a stellar core, the result is that about half of the helium-4 nuclei are dissociated.
Activity 8
If each helium-4 nucleus absorbs 28.3 MeV of energy by photodisintegration, and half of the core of mass 1.4 M☉ is dissociated in this way, calculate the total energy absorbed by this process. (The mass of a helium-4 nucleus is 4u, where kg is the atomic mass unit.)
Discussion
The number of particles is N = M/m. Half the core mass is 
, so the number of helium-4 nuclei is
Each nucleus absorbs 28.3 MeV of energy, which is equivalent to 
So the core absorbs 
via the photodisintegration of helium-4 nuclei.
Therefore, the total amount of energy absorbed by nuclear photodisintegration is 
from the dissociation of iron-56 plus 
from the dissociation of helium-4, or about 
in total.
OpenLearn - White dwarfs and neutron stars
Except for third party materials and otherwise, this content is made available under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 Licence, full copyright detail can be found in the acknowledgements section. Please see full copyright statement for details.