Lesson 12 Fission

Importance of Fission Technological importance (reactors, bombs) Socio-political importance Role of chemists Very difficult problem

Overview of fission

Probability of Fission Divide study of fission into two parts, the gs  saddle point (probability of fission) and the saddle  scission point (distribution of fission products) Use liquid drop model to study gs  saddle point

Liquid drop model

Limits on the Periodic Table Notice that x  (a c /2a s ) Z limit =2(a s /a c )A limit Z limit ~ 125 For all stable nuclei, x < 1 As nucleus deforms, pot. energy increases by Eventually Coulomb energy will cause deformation energy to decrease, ie, get fission barrier.

Fission Barriers

Shell Effects

Consequences of Double Humped Fission Barriers Spontaneously fissioning isomers Superdeformed nuclei Subthreshold resonances

Spontaneous fission

Understanding spontaneous fission lifetimes

Spontaneously fissioning isomers

Spntaneously fissioning isomers are nuclei caught in states in the second minimum of the fission potential energy surface. Their sf decay is enhanced relative to gs sf. Lifetimes are s Typically c/a =2:1

Sub-threshold fission resonances

“Normal fission”--the fission transition state nucleus Has the same role as the transition state in chemical reactions. prob. of fission =Aexp(-B f /T) B n > B f ( 235 U); B n < B f ( 238 U) Big Three ( 233 U, 235 U, 239 Pu)

Fission probability Fission probablity  (N f /(N f +N n +N gamma +N ch.p. ))

Multiple chance fission

n/fn/f

n/fn/f

Fission Product Distributions TKE Distribution

Fission Mass Distributions

Fission Product Charge Distributions

Energetics of Fission Q value ~ 200 MeV TKE ~172 MeV Neutrons ~18 MeV Gammas ~ 7.5 MeV , etc ~2.5 MeV

Prompt Neutrons

Prompt Neutron Spectra Average neutron energy ~ 2 MeV Spectrum: frame of moving fragment; Maxwellian P (E)=E n exp (-E n /T) lab frame; Watt spectrum

Fission Fragment Angular Distributions