Nuclear Physics

Properties of Nuclei
• Every atom contains an extremely dense,
positively charged nucleus
• The nucleus is much smaller than the
overall size of the atom, but contains most
of its total mass
 Properties of Nuclei
• Nucleus as a sphere
– R depends on the total number of nucleons
(neutrons and protons) in the nucleus
– A is the nucleon number
1/3
R= R0A
-15
R0= 1.2x10 m
 Properties of Nuclei

•A is also the mass number

in u
–Proton mass and neutron
mass ~ 1u
27
–1u = 1.6605402 x 10 - kg
 Nuclides and Isotopes

The masses of the building blocks of

atom:
mp= 1.007276u= 1.67263x10-27kg
-27
mn= 1.008665u= 1.674929x10 kg
me= 0.000548580u= 9.10939x10-31kg
 Nuclides and Isotopes

Z is the number N is the

of protons in the number of
nucleus neutrons

A is the sum
Nuclides and Isotopes
A single nuclear
species having
Nuclide specific values of
both Z and N

Same Z
Isotopes but
different N
 Nuclear Binding Energy
MH= mass of protons
• The energy that must be
and
EB added
electrons to separate the
nucleons
mn= mass of neutron
• The magnitude of the energy by
E
M= B mass of neutral atom
which the nucleons are bound
together
ZA
2
c = 931.5MeV/u 2
EB •(ZMH + Nmn - Z M)c A
 Example
• Deuterium has a mass number 2, an
isotope of H. Its nucleus consists of a
proton and a neutron bound together to
form a particle called the deuteron.
Deuterium has an atomic mass of
2.014102u. What is the binding energy of
deuteron?
•
 Nuclear Binding Energy

• From the example, what is the

binding energy per nucleon?
• BE/2 = 2.224 Mev/2
• =1.112MeVper nucleon
 Nuclear Force
• The force that binds protons and
neutron together in the nucleus,
despite the electrical repulsion of the
protons
• An example of strong interaction
 Nuclear Force

1 Does not depend on

2 It has short
charge range (10-15m)

3 Interaction is
within immediate
4 Favors binding
vicinity of pairs
Nuclear Structure
• The Shell Model
• Uses the concept
of filled shells and
subshells and
their relation to
stability
Nuclear Structure
• In atomic
structure, noble
gases are stable
• A comparable
effect occurs in
nuclear structure
Nuclear Structure
• An unusually
stable structure
results when
number of
protons or
number of
neutrons is 2, 8,
20, 28, 50, 82, or
126
• MAGIC NUMBERS
 Nuclear Stability & Radiation

• Radioactivity
– The decay of unstable structures to form
other nuclides by emitting particles and
electromagnetic radiation
– Alpha decay
– Beta decay
– Gamma decay
Nuclear Stability & Radiation

Alpha Emission 4
He
of alpha
decay particle nucleus

N and Z, both Emission occurs

decrease by 2, A with nuclei that are
decreases by 4 too large
Nuclear Stability & Radiation

Beta minus When N/Z ratio

decay occurs is too large

Neutral atomic mass

is larger than that of
the final atom
Nuclear Stability & Radiation

Beta minus Emission involves

is an transformation of
electron a neutron to a:

N decreases and Proton, electron,

Z increases by 1, and an
A doesn’t change antineutrino
Nuclear Stability & Radiation

Beta plus When N/Z ratio

decay occurs is too small

Neutral atomic mass is at least 2

electron masses larger than that
of the final atom
 Nuclear Stability & Radiation

Beta plus is Identical to an

electron but with
a positron opp. charge

A proton is converted
N increases and Z
decreases by 1, A
to a neutron, a
remains the same positron, and an
antineutrino
Nuclear Stability & Radiation

Beta decay Electron combines

with a proton to form
Electron a neutron and an
capture antineutrino

N increases and Z The neutron stays in

the nucleus and the
decreases by 1, A antineutrino is
remains the same emitted
Nuclear Stability & Radiation

Bombardment of One or more

high-energy photons are
particles and or by emitted from the
radioactive nucleus
transformation

The
Gamma Gamma rays or
element
ray gamma ray photons
does not decay (10keV – 5MeV)
change
 Activities and Half-lives

• The decay rate

Activity • -dN(t)/dt

• Decay constant
λ • Activity is prop. to the number of
radioactive nuclei

-dN(t)/dt •=λN(t)
Activities and Half-lives

• The number of
N(t) remaining nuclei

• Decay constant
λ • Activity is prop. to the number of
radioactive nuclei

-λt
N(t) •=N0e
 Activities and Half-lives

Units for activity

– Ci or curie or Becquerel or Bq
10 10
– 1Ci= 3.70x10 Bq= 3.70x10 decays/s
 Activities and Half-lives
• Time required for the no.
Half life of radioactive nuclei to
decrease to half N0

T1/2 •= 0.693/λ
Tmean • =1/λ=T1/2/ln2=T1/2/0.693
 Radioactive Dating

A small proportion of Plants that obtain

the unstable 14C is their carbon from
present in CO2 in the this source contains
atmosphere the same prop.

C β- decays to 14 N with a
14

half-life of 5730 yrs. The When a plant dies, it

remaining 14C determines stops taking carbon
the age of the organism.
Biological Effects of Radiation
Biological Effects of Radiation

When Break
radiation pass They lose molecular
through energy
matter bonds

Create Ionizing
ions radiation
Biological Effects of Radiation
Charged particles
interact directly with the
electrons in the material

X rays and γ rays Absorption by nuclei

with subsequent
interact by the radioactive decay
photoelectric effect

Neutrons cause Collisions with

ionization by: nuclei
Biological Effects of Radiation

Radiation dosimetry 1rad=0.01J/kg=0.01Gy

The quantitative
description of the effect
of radiation on living SI: J/kg, Gy(gray),
tissue present: rad

Energy delivered to the

Absorbed dose tissue per unit mass
 Biological Effects of Radiation

Radiation RBE ( Sv/Gy or

rem/rad)
X rays and γ rays 1
Electrons 1.0-1.5
Slow neutrons 3-5
Protons 10
α particles 20
Heavy ions 20
Nuclear Reactions
A process that alters
the energy or structure
or composition of
atomic nuclei.
Nuclear Reactions

Nuclear fission
A decay process in
which an unstable
nucleus splits into
two fragments of
comparable mass
Nuclear Reactions
Nuclear fission
• Discovered by the
experiments of
Otto Hahn and
Fritz Strassman
• Bombardment of U
with neutrons
Nuclear Reactions
Nuclear fission
• The resulting
radiation did not
coincide with any
of the know
radioactive nuclide
• Lise Meitner found
out its barium
Nuclear Reactions
Nuclear fission
• With Otto Frisch,
they discovered
that U nuclei was
splitting into 2
massive fragments
called fission
fragments
Nuclear Reactions
Induced fission
• Fission resulting
from neutron
absorption
• Rare Spontaneous
fission occurs
w/out initial
neutron absorption
Nuclear Reactions: Fission
Ba-144

n 3n

U-235 Kr-89
Nuclear Reactions
Nuclear Fusion
• Occurs when 2 or
more small light
nuclei come
together to form a
larger nucleus
 Nuclear Reactions
Nuclear Fusion
1
H1 + 1 H1 2
H1 + β+ + ve

2
H1 + 1 H1 3
He2 + γ

3
He2 + 3He2 4
He2+1H1+1H1

Proton-proton chain
Nuclear Reactions
Nuclear Fusion
• 2 or more nuclei
must be w/in the
range of nuclear
force (approx.
2x10-15m)
Nuclear Reactions
Nuclear Fusion
• Thermonuclear
reactions
• Proton-proton reaction
occurs at “only” 1.5x107K
in the sun
• Cold fusion doesn’t
require high temperatures
 Reaction Energy
When Q is
The difference pos., total
between the mass dec &
masses before and the total
after the reaction KE inc EXOERGIC
Reaction
2
Q= (MA + MB – MC – MD) c
When Q is neg.,
ENDOERGIC total mass inc &
Reaction the total KE dec
 Example
• When lithium (7Li) is bombarded
by a proton, two alpha particles
4
( He) are produced. Find the
reaction energy.
H=1.007825u;
1
H + 7
Li Li=
4 7.016004u;
He + 4
He
1
He= 3
4.002603u 2 2
• Q= 17.35MeV
 Example

• Calculate the reaction energy for

the nuclear reaction represented
below:
H=1.007825u;
4
He + 14
N N= 14.003074u;
17
O + 1
H
2 7 8
He= 4.002603u; O=16.999132u 1
• Q= -1.192MeV
Nuclear Reactions

Nuclear Generator