Nuclear Physics
Phys 435
1437/1436
.
Nuclear Physics (PHYS 435)
9.15-8
9.15-8
Ga39
Ga39
10-11
10-11
zz.alfull@gmail.com
)
(
3
:
1
-
1
.
3
-
1
:
2
.
3
1
6
2
9
3
6
2
:
-
:
-
:
:
3
4
5
-
-
6
.
:
6
7
-Q
2
.
6
2
42
14
–
:
/
8
References
1. ATOMIC AND NUCLEAR PHYSICS An Introduction, 3rd edition by T. A.
LITTLEFIELD, and N.THORLEY, 1912, ISBN 978-1-4684-1470-7 (eBook)
2. Introductory Nuclear Physics ,K. S. Krane, John Wiley&sons; 3rd edition (1988)
ISBN-0-471-80553-X
3. Basic Ideas and Concepts in Nuclear Physics: An Introductory Approach,Third
Edition (Fundamental & Applied Nuclear Physics), K. Heyde ,Taylor & Francis; 3rd
edition (July 1, 2004)
4.Elements of nuclear physics , Walter. E. Meyerhof ,NEW YORK , McGraw-Hill
BOOK COMPANY .1967
)2(
40
)1(
20
2/4
20
12/29
10
10
My website
www.zainab-alfull.com 
My email 
zz.alfull@gmail.com 
‫هللا‬
‫ل‬
‫أ هللا‬
‫ج له‬
‫ج ه‬
Atom
historical introduction
First lecture
Atoms in philosophy

The idea that matter is made up of discrete units is a very
old one, appearing in many ancient cultures such as
Greece and India. The word "atom", in fact, was coined
by ancient Greek philosophers. However, these ideas
were founded in philosophical and theological reasoning
rather than evidence and experimentation. As a result,
their views on what atoms look like and how they behave
were incorrect. They also could not convince everybody, so
atomism was but one of a number of competing theories
on the nature of matter. It was not until the 19th century
that the idea was embraced and refined by scientists,
when the blossoming science of chemistry produced
discoveries that only the concept of atoms could explain.
First evidence-based theory
Brownian motion

In 1827, Robert Brown used a microscope to look at
dust grains floating in water and discovered that they
moved about erratically, a phenomenon that
became known as "Brownian motion". This was
thought to be caused by water molecules knocking the
grains about. In 1905 Albert Einstein produced the
first mathematical analysis of the motion.
French physicist Jean Perrin used Einstein's work
to experimentally determine the mass and
dimensions of atoms, thereby conclusively verifying
Dalton's atomic theory.
Discovery of the electron

The physicist J. J. Thomson measured the mass of cathode rays,
showing they were made of particles, but were around 1800
times lighter than the lightest atom, hydrogen. Therefore,
they were not atoms, but a new particle, the first subatomic
particle to be discovered, which he originally called
"corpuscle" but was later named electron, after particles
postulated by George Johnstone Stoney in 1874. He also
showed they were identical to particles given off by
photoelectric and radioactive materials. It was quickly
recognized that they are the particles that carry electric currents in
metal wires, and carry the negative electric charge within atoms.
Thomson was given the 1906 Nobel Prize in Physics for this work.
Thus he overturned the belief that atoms are the indivisible,
ultimate particles of matter. Thomson also incorrectly
postulated that the low mass, negatively charged electrons
were distributed throughout the atom in a uniform sea of
positive charge. This became known as the plum pudding
model.
1. Thomson knew That cathode rays emitted by a
hot filament could be deflected by a magnet. This
suggested that they carried electric charge;
 2. in fact, the direction of the curvature required
that the charge be negative, It seemed, therefore,
that these were not rays at all, but rather
streams of particles.
 3. By passing the beam through electric and
magnetic fields, and adjusting the field strength
until the net deflection was zero, Thomson was
able to determine the velocity of the particles as
well as their charge-to mass ratio. This ratio
turned out to be enormously greater than for
any known ion, indicating that either the charge
was extremely large or the mass was very small.
Indirect evidence pointed to the second
conclusion.
 4. Thomson called the particles corpuscles, and
their charge the electron. Later the word
electron was applied to the particles themselves.

5. Thomson correctly surmised that these
electrons were essential constituents of
atoms; however, since atoms as a whole are
electrically neutral and very much heavier
than electrons, there immediately arose the
problem of how the compensating plus
charge and the bulk of the mass-is
distributed within an atom.
 6. Thomson himself imagined that the
electrons were suspended in a heavy,
positively charged paste,.
 But Thomson’s model was decisively repudiated by
Rutherford’s famous scattering experiment, which
showed that the positive charge, and most of the
mass, was concentrated in a tiny core, or nucleus, at
the center of the atom.

Discovery of the nucleus

In 1909, Hans Geiger and Ernest Marsden, under the
direction of Ernest Rutherford, bombarded a metal
foil with alpha particles to observe how they
scattered. They expected all the alpha particles to
pass straight through with little deflection, because
Thomson's model said that the charges in the atom
are so diffuse that their electric fields could not affect
the alpha particles much. However, Geiger and
Marsden spotted alpha particles being deflected by
angles greater than 90°, which was supposed to be
impossible according to Thomson's model. To explain
this, Rutherford proposed that the positive charge of
the atom is concentrated in a tiny nucleus at the
center of the atom





Rutherford demonstrated this by firing a beam of a particles
(ionized helium atoms) into a thin sheet of gold foil had the
gold atoms consisted of rather diffuse spheres, as Thomson
supposed, then all of the a-particles should have been
deflected a bit, but none would have been deflected much any
more than a bullet is deflected much when it passes, say,
through a bag of sawdust. What in fact occurred was that most
of the Alfa particles passed through the gold completely
undisturbed, but a few of them bounced off at wild angles.
Rutherford’s conclusion was that the
1. Alfa particles had encountered something very small, very
hard, and very heavy.
2. Evidently the positive charge,
3. and virtually all of the mass, was concentrated at the center,
occupying only a tiny fraction of the volume of the atom
4. (the electrons are too light to play any role in the
scattering; they are knocked right out of the way by the much
heavier a-particles).
5. The nucleus of the lightest atom
(hydrogen) was given the name proton by
Rutherford. In 1914 Niels Bohr
 1. proposed
a model for hydrogen
consisting of a single electron circling the
proton, held in orbit by the mutual
attraction of opposite charges.
 2. Using a primitive version of the quantum
theory, Bohr was able to calculate the
spectrum of hydrogen, and the agreement
with experiment was nothing short of
spectacular. It was natural then to suppose
that the nuclei of heavier atoms were
composed of two or more protons bound
together, supporting a like number of
orbiting electrons.


Unfortunately, the next heavier atom (helium),
although it does indeed carry two electrons,
weighs four times as much as hydrogen, and
lithium (three electrons) is seven times the
weight of hydrogen, and so it goes. This dilemma
was finally resolved in 1932 with Chadwick‘s
discovery of the neutron-an electrically neutral
twin to the proton. The helium nucleus, it turns
out, contains two neutrons in addition to the two
protons; lithium evidently includes four; and in
general the heavier nuclei carry very roughly the
same number of neutrons as protons. (The
number of neutrons is in fact somewhat flexible:
the same atom, chemically speaking, may come
in several different isotopes, all with the
same number of protons, but with varying
numbers of neutrons.)

The discovery of the neutron put the
final touch on what we might call the
classical period in elementary particle
physics. Never before has physics
offered so simple and satisfying an
answer to the question, “What is
matter made of?” In 1932 it
was all just protons, neutrons,
and electrons.