Excitation Energy of the Fission Fragments

As noted earlier, the prompt neutrons are emitted from the fully accelerated fragments after scission. The number of these neutrons, Vy, (= 2.4 in the case of 235U) as 

Example Problem How did we get the estimate for v r for the fission of 298114 Assume this nucleus fissions symmetrically. 

Solution The total energy released can be estimated using a modem mass formula (Liran and Zeldes, 1976) as 311 MeV. The total kinetic energy of the fragments can be calculated as 

This leaves a total fragment excitation energy of 311 — 245 = 66 MeV. Since the 7 rays and (3, are emitted only to take away the energy not emitted as neutrons, we shall assume this energy is the same as in 235U(wti /) or 10 MeV. This 

Prompt y-ray emission competes with or follows the last stages of prompt neutron emission. These photons are emitted in times from 10 15-10 7s. Typical y-ray multiplicities of 7-10 photons/fission are observed. These photons, as indicated earlier, cany away 7.5 MeV. This y-ray yield is considerably larger than one would predict if y-ray emission followed neutron emission instead of competing with it. Because of the significant angular momentum of the fission fragments ( 7-10 h) even in spontaneous fission, photon emission can compete with neutron emission. The emitted y rays are mostly dipole radiation with some significant admixture of quadrupole radiation, due to stretched El transitions (J/= 7, — 2). 

---