Fermi Gas Model - Valparaiso University
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Transcript Fermi Gas Model - Valparaiso University
Fermi Gas Model
Heisenberg Uncertainty Principle
h
dpx dx h dpx
dx
Particle in dx will have a minimum uncertainty in px of dpx
px
dx
Next particle in dx will have a momentum px
h
px px dpx px
dx
Particles with px in dpx have minimum x-separation dx
dx dpx h
Heisenberg Uncertainty Principle
Identical conditions apply for the y, py, and z, pz -Therefore, in a fully degenerate system of fermions,
(i.e., all fermions in their lowest energy state),
we have 1 particle in each 6-dimensionl volume --
dV ps dx dp x dy dp y dz dpz h 3
dV ps dV dp 3
Phase space
=
volume
Spatial
Momentum
volume
volume
Heisenberg Uncertainty Principle
In some dVps the maximum number dN of unique quantum
states (fermions) is
pz
dN
dV ps
h
dN
p
3
py
dV 4 p 2 dp
2
3
px
Number of states in a shell in p-space
between p and p + dp
Only Heisenberg uncertainty principle; completely general
FGM for the nucleus
Treat protons & neutrons separately
Consider a simple model for nucleus-V (x) 0 ; 0 x L
V (x) ; x L
V (y) 0 ; 0 y L
V (y) ; y L
V (z) 0 ;
V (z) ; z L
0zL
V V (x) V (y) V (z)
2
2M
2 V E
x (x) y (y) z (z)
E E x E y Ez
n x
2
x (x)
sin
L L
2
2
n
Ex
2
2ML
x
FGM for the nucleus
E E x E y Ez
2
E
Total energy eigenvalue
degenerate
ni 1,2,3,
eigenvalues
x (x) y (y) z (z)
unique states
2
2
2
n
n
n
y
z
2 x
2ML
n x
2
x (x)
sin
L L
Ex
2
2
n
2 x
2ML
n x n y n z x, y,z
2
p
x
Ex
px
nx
2M
L
FGM for the nucleus
2
px
n x p n x n 2y nz2 p n x n y nz
L
L
unique states
quantized momentum states
p x , p x , p x
n x n y n z x, y,z
h
dp
L L
3
from Heisenberg
uncertainty relation
3
3
px
pz
n x ,n y ,nz
L
p
dp 3
p x , p x , p x
py
FGM for the nucleus
Assume extreme degeneracy all low levels filled up to a
maximum -- called the Fermi level (EF)
All momentum states up to pF are filled (occupied)
We want to estimate EF and pF for nuclei -The number N of momentum states within
the momentum-sphere up to pF is -1 4 pF3
N
8 3 dp 3
pz
p
one p-state per dp3
1/8 of sphere because nx, ny, nz > 0
px
dp 3
p x , p x , p x
py
FGM for the nucleus
1 4 pF3
1 4 pF3
N
pF 2ME F
N
3
3
8 3 dp
8 3 2
L
2 spin states
3/2
V
2ME
1 2 4
F
3
L3 V
N
N
p
V
2 2
F
3
3
8 2 3
EF
pF
2
3N 2 / 3
2M
V
1/ 3
1/
3
N
2
3
Fermi energy
nucleon(s)
(most energetic
Fermi momentum
V
(most energetic nucleon(s)
protons
N=Z
neutrons N
= (A-Z)
FGM for the nucleus
Protons
pF
Neutrons
1/ 3
1/
3
Z
2
3
V
pF
1/ 3
1/
3
A
Z
2
3
V
Assume Z = N
pF
V
1/ 3
1/
3
A
/2
2
3
4
R3
3
R Ro A1/ 3
V
pF
4
V Ro3A 4.18 Ro3A
3
1/ 3
1/
3
A
/2
2
3
V
3 2 1/ 3
pF
Ro 2 4.18
FGM for the nucleus
Protons
pF
Neutrons
1/ 3
1/
3
Z
2
3
V
pF
1/ 3
1/
3
A
Z
2
3
V
Assume Z = N
3
2 1/
197Mev 3
300
pF
MeV /c
Roc 2 4.18
Ro
pF 231 MeV /c
(Ro 1.3F)
EF
2
pF
28 MeV
2M
FGM potential
Test of
FGM
not FGM