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Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).
1
Neutron Resonance Reactions
Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).
2
Neutron Activation Analysis
(Z,A) + n (Z, A+1)-
(Z+1, A+1)
(-delayed -ray)
Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).
3
Neutron Attenuation
Neutrons
ndxI
dIt
TargetThickness “x”
nxo
teII
Similar to -attenuation. Why?
Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).
4
Neutron Moderation
Show that, after elastic scattering the ratio between the final neutron energy E\ and its initial energy E is given by:
For a head-on collision:
After n s-wave collisions:
where
HW 44HW 44
2
2\
)1(
cos21
A
AA
E
E CM
2
min
\
1
1
A
A
E
E
nEEn lnln \
1
1ln
2
)1(1ln
2
\
A
A
A
A
E
E
av
Lethargy?
Lethargy?
Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).
5
HW 44 HW 44 (continued)(continued)
How many collisions are needed to thermalize a 2 MeV neutron if the moderator was: 1H 2H 4He 12C 238U
Discuss the effect of the thermal motion of the moderator atoms.
Neutron Moderation
Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).
6
Nuclear Fission
~200 MeV
Fission
Fusi
on
Coulomb effectSurface effect
Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).
7
Nuclear Fission• B.E. per nucleon for 238U (BEU) and 119Pd (BEPd) ?• 2x119xBEPd – 238xBEU = ?? K.E. of the fragments 1011 J/g• Burning coal 105 J/g• Why not spontaneous?• Two 119Pd fragments just touching The Coulomb barrier is:
• Crude …! What if 79Zn and 159Sm? Large neutron excess, released neutrons, sharp potential edge…!
MeVMeVfm
fmMeVV 2142502.12
)46(.44.1
2
Crude!Crude!
Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).
8
Nuclear Fission
• 238U (t½ = 4.5x109 y) for -decay.• 238U (t½ 1016 y) for fission.• Heavier nuclei??• Energy absorption from a neutron (for example) could form an intermediate state probably above barrier induced fission.• Height of barrier is called activation energy.
Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).
9
Nuclear Fission
Liquid Drop
Shell
Act
iva
tion
Ene
rgy
(MeV
)
Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).
10
Nuclear Fission
Surface Term Bs = - as A⅔
Coulomb Term BC = - aC Z(Z-1) / A⅓
3
3
4R
2
3
4ab=
1
)1(
Rb
Ra23 abR
...)1( 252
...)1( 251
Volume Term (the same)
32
31
52
51 )1( AaAZZa SC fission
47~2
A
Z
Crude: QM and original shape could be different from spherical.
Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).
11
Nuclear Fission
48300
)120( 2
Extrapolation to 47 10-20 s.
Consistent with activation energy curve for A = 300.
Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).
12
Nuclear Fission
235U + n93Rb + 141Cs + 2nNot unique.
Low-energy fission processes.
Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).
13
Nuclear Fission
Z1 + Z2 = 92Z1 37, Z2 55A1 95, A2 140Large neutron excess
Most stable:Z=45 Z=58Prompt neutronsPrompt neutrons within 10-16 s.Number depends on nature of fragments and on incident particle energy.The average number is characteristic of the process.
Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).
14
Nuclear Fission
The average number of neutrons is different, but the distribution is Gaussian.
Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).
15
Delayed neutronsDelayed neutrons
Higher than Sn?
~ 1 delayed neutron per 100 fissions, but essential for control of the reactor.
Follow -decay and find the most
long-lived isotope (waste) in this
case.
Nuclear and Radiation Physics, BAU, First Semester, 2007-2008 (Saed Dababneh).
16
Nuclear Fission