235U+0 92 n elastic scatter 235U+on+Y inelastic scatter bn+232U→232U*→ 236 92 U+y radiative capture YH+Y+y1+y2+.. fission
Neutron Scattering X+ elastic scattering n+X一 X*+n inelastic scattering A+121 Ecos+VE(4-1+o)+A(4+1 E E nin aI max a=(A-1)2/(A+1)2 The elastic scattering is the main mechanism in moderating neutrons in thermal nuclear reactors
Neutron Scattering The elastic scattering is the main mechanism in moderating neutrons in thermal nuclear reactors
The corresponding neutron energy loss (△B)=E-Ev=E-1 2 (E+aE)=(1-a)E Average Logarithmic Energy Loss on a logarithmic energy scale a neutron loses the same amount of E =1+ Ina =s logarithmic energy per elastic scatter, regardless of its initial energy average number of scatters required to bring a neutron of initial energy E,to a lower energy E2 E sEs
The corresponding neutron energy loss Average Logarithmic Energy Loss on a logarithmic energy scale a neutron loses the same amount of logarithmic energy per elastic scatter, regardless of its initial energy average number of scatters required to bring a neutron of initial energy E1 to a lower energy E2
Slowing of neutron(moderation)by various materials Here n is the number of elastic scatters to slow on the average, a neutron from 2 Mev to 0.025 eV Material H 0 182 H2o 1&16 0.920 19.8 0.111 0.725 25.1 2&16 0.509 35.7 He 0.3600.425 42.8 Be 06400.207 88.1 12 0.7160.158 115 238 U 238 0.9830.00842172 lsH2 O a good moderator(慢化剂)
Slowing of neutron (moderation) by various materials. Here n is the number of elastic scatters to slow, on the average, a neutron from 2 MeV to 0.025 eV Is H2O a good moderator (慢化剂)
Thermal Neutrons Cross sections Thermal neutron capture cross sections(oc) Thermal neutron cross section for fission(o() C 011cd G。/b0.330000520.00341.82 0000219.820 Moderators: H2O VS D2O VS C Fermi's used Cd for emergency Nuclear fission
Nuclear Fission 15 Thermal Neutrons Cross Sections Thermal neutron capture cross sections (c ) Thermal neutron cross section for fission (f ) 1H 2H 12C 14N 16O 113Cd c /b 0.33 0.00052 0.0034 1.82 0.0002 19,820 Moderators: H2O vs. D2O vs. C Fermi’s used Cd for emergency