*1.The Liquid Drop model*Nuclear models occur under a set of hypotheses, each of which explains some aspect of nuclear behavior. The most basic is perhaps the “liquid drop model” which is used to account for nuclear fission, radioactivity. This is illustrated in Fig, 1 below:

*Fig. 1: The Liquid Drop model showing U235 Fission*As the name implies, the nucleus is depicted in terms of a “liquid drop”, which analogous to a liquid macroscopic drop, exhibits surface tension and an excitation energy when disturbed.

This model is premised on the exceedingly short range of nuclear forces which requires that nearest neighbor attractions predominate. This is somewhat similar to the type of attraction between the molecules of a liquid which leads to the property of its surface tension. Thus, in a liquid drop nucleus each nucleon shares its total binding energy with every nucleon.

*a spherical configuration*in the

**minimum energy**or unexcited state. The unstable (compound) nucleus then yields two “daughter nuclei” : La 139 and Mo 95, with two additional neutrons released

Thus,
between the spherical and compound nucleus phase, the “drop” may be said to
undergo oscillations which lead it to overshoot sphericity in two directions –
vertically (converting to an elliptical configuration with semi-major axis in
the vertical sense, and horizontally, with semi-major axis in the horizontal
sense. It finally reaches a point – the compound nucleus phase – at which the
surface tension restoring force is no longer able to contain the long –range
Coulombic repulsion force and the inertia of nuclear matter.

At
this point, the drop reaches a stage of distortion in which the nucleus splits
into two fission fragments, denoted by [A1, Z1] and [A2, Z2]. In order to
further assess the nuclear changes, we use what is called a “semi-empirical
mass formula” which sets out terms as follows:

(1)

**f**_{o}(Z, A) = 1.008142Z + 1.008982 (A – Z)
Where
the Z coefficient is the mass of the hydrogen atom in atomic mass units and the
(A – Z) coefficient is the mass of the neutron in amu. The remaining terms
listed correct for several effects which contribute to the total nuclear
binding energy:

(2)

**f**_{1}(Z, A) = -a_{1}A
This term accounts for the binding energy
and is essentially proportional to the nuclear mass or the volume of the
nucleus.

(3)

**f**_{2}(Z, A) = +a_{2}A^{2/3}
This
term is a positive correction to the surface area of the nucleus, i.e. the effect of surface tension energy.

(4)

**f**_{3}(Z, A) = + a_{3 }(Z^{2}/ A^{1/3 })
This
term accounts for the positive Coulomb energy of the charged nucleus, assumed
to be a sphere of radius proportional to

**A**.^{1/3}
(5)

**f**_{4}(Z, A) = + a_{4 }(Z^{ }- A/2)^{2}/ A
This
term and the last (6) introduce properties specific to the nucleus. This term
is zero for the case of Z = (A – Z) or 2Z = A.

(6)

**f**_{5}(Z, A) = 0, - f(A), +f(A)
Where 0 is the result when ‘Z even’ pairs with (A –
Z) odd, or ‘Z odd’ pairs with (A – Z) even; and –f(A) is the result when Z even
pairs with (A – Z) even, and f(A) is the results when Z odd is paired to (A –
Z) odd. The form of f(A) is determined
by fitting the data. It is found that the best fit for a simple power law
relation is:

**f (A) = a**

_{5}A^{- 1/2}

^{}
Combining
all the terms we get:

**M(Z, A)**=

**1.008142Z + 1.008982 (A – Z) -a**

_{1}A+ a_{2}A^{2/3}**+ a**

_{3 }(Z^{2}/ A^{1/3 }) + a_{4 }(Z^{ }- A/2)^{2}/ A + (-1, 0 , 1) a_{5}A^{– 1/2}
The parameter values designated are:

**a**= 0.01692,_{1 }**a**= 0.01912,_{2 }**a**= 0.000763,_{3}**a**= 0.10178, and_{4 }**a**= 0.012._{5 }**:**

*2. The Nuclear Shell Model*
The shell model of the nucleus treats a
different aspect, namely specific energy levels within the nucleus. To achieve this the nucleons are treated as
independent particles, each of which moves in its own spherically symmetrical
potential well about 50 MeV deep with rounded edges.

*Fig. 2: The potential well for the Shell model*The

*Schrodinger Equation*is then set up for this potential and solved. The solution yields stationary quantum states, somewhat like the states which are associated with electrons in the outer shells of the atom.

Like
the electrons occupying atomic shells, the nucleons are fermions, i.e.
particles each with spin ½ and therefore
obeying the Pauli Exclusion Principle so that no more than two nucleons of the
same type can occupy the same energy level (e.g. one with spin +½, the other
with spin -½.). This leads to particles pairing up in the nucleus: spin up (+
½) protons with spin down (-½) protons, and spin up neutrons with spin down
neutrons.

The
form or expression for the energy of a nucleus is very closely approximated by the energy associated with a “square well”
potential. The Fermi energy in more explicit form (which we will not elaborate
upon too much beyond this) is:

*E*_{F}= 3^{2/3}

*p*

^{4/3}ħ^{2}

*r*

^{2/3}/ 2M
Where
M is the nucleon mass, ħ = h/2p

**2p) and r is the nucleon density. Performing the operation with known or estimated values, the result is***is the adjusted Planck number (h divided by***If we assume a spherical nucleus then the radius is just:***E*_{F}= 30 MeV.

*R = r*_{o}A^{1/3}

^{}
This
is also the dimension on either side of the central symmetry line of Fig. 2.
The evidence for the shell model includes the following:

i)Particularly
stable nuclei are borne out by “closed shells”, based on our observations.

ii) The model predicts (correctly) that the even
N (or A – Z) nuclei will be most stable and the odd Z, odd N nuclei the least
stable. Experiments bear this out: there are 160 stable nuclides (with even Z,
N) and only 4 with odd Z, N.

iii) The model predicts that for even N, Z
nuclides the total angular momentum J = 0, and that for odd nuclides it is
half-integral. (Borne out by measurements of nuclear magnetic moments.)

*Problem*: Find the diameter of the oxygen (O

^{16}) nucleus.

__Solution:__

We
apply:

*R = r*_{o}A^{1/3}^{}
Where:

**= 1.2 x 10***r*_{o}^{-15}m is the**Bohr radius**, and A = 16. Then:**1.2 x 10**

*R = (*^{-15}m)

**= 3.0238 x 10**

*(16)*^{1/3}^{-15}m

And
the diameter D = 2

**R**= 2(3.0238 x 10^{-15}m)
D
= 6.05 x 10

^{-15}m
The
preceding result can also be generalized to a situation in which the ratio of
radii (between two nuclei) is sought. In this case, we can write:

*R1/ R2 = [r*_{o}A1^{1/3}] / [r_{o}A2^{1/3}]

Or:

*R1/ R2 = (A1/ A2)*^{1/3}

^{}*Example*: How much larger is a copper nucleus than an oxygen nucleus?

Let
R1 denote the radius of the copper
nucleus, and R2, oxygen’s. Then: A1 = 64, and A2 = 16. So:

R1/
R2 = (64/16)

**4/ 2.51 = 1.587**^{1/3 }=

*Thus, the copper nucleus is about 1.587 times the size of the oxygen nucleus.*

*Problems:*

**1)**

**Evaluate the terms of the semi-empirical mass formula for the U 238 nucleus, i.e. if A = 238 and Z = 92. Use this information to find the total mass in atomic mass units (u) and compare it to the standard mass expression. Thence or otherwise, obtain the mass defect and the binding energy as well as the binding energy per nucleon.**

2)The
sketch graph shown below plots the mass number A vs. the binding energy per
nucleon (BE/nucleon) on the vertical axis.

_{}
Using
any models or other explanations, account for the region of greatest stability
indicated.

3)
A quantum square well potential is schematically defined according to:

It is found that the Schrodinger equation to solve becomes:

-ħ

-ħ

^{2}/ 2m ( d**y**

^{2}**/dx**

^{2}) = E**y**

**Thence:**

d

d

**y**

^{2}**/dx**

^{2}+ 2m/ ħ^{2}(E**y**

**) = 0**

And the quantized energy is found to be:

E

_{n}= (h^{2}/ 8m L^{2}) n^{2}
This
can be modified to yield a simplified

**in one dimension by using instead:***Fermi energy*
E

_{n}= (h^{2 }p^{2}/ 8m L^{2}) (N/2)^{2}**Where all the Fermi shell energy levels are presumed occupied up to N/2. Use this to obtain a simplified estimate of the energy associated with the oxygen**

**if we assume its shells filled up to N =4 and use an estimate for L as:**

*nucleus*

*R = r*_{o}A^{1/3}

*(Take m = 1.7 x 10*^{-27}kg)
4)(a)
Compare the value you obtain in (3) to the actual Fermi (shell) energy for
oxygen, if the nuclear density r » 2.3 x 10

^{17}kgm^{-3}.
(b)
Write out at least one technique that might be used to find the density of an
atomic nucleus. State clearly any assumptions made and how you quantify them.

5)(a)
Find the ratio of the helium nucleus radius to that of the uranium 238 nucleus.

(b)
Estimate, using any technique you can think of, the ratio of the nuclear
densities for part (a).

6)
An element has mass number A = 202 and atomic number Z = 80.

a)
Find the diameter of the nucleus and how many times it is greater than that of
hydrogen.

b)
Find the mass defect D
M for this nucleus

c)
From the result in (b) obtain the binding energy and the binding energy per
nucleon, E

_{B}/ A. Using the graph on the opposite page, mark the location of E_{B}/ A.
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