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by
Dr. Adriana Banu
JMU Department of Physics and Astronomy
January 28, Meet the Scientist: Saturday Morning Physics at JMU’12
A few fundamentals…
What chemists like…
92 building blocks
(chemical elements)
Periodic Table of Elements
1896 Mendeleyev
o all matter (solids, liquids and gases) is composed of atoms
o chemical element = matter that is composed of only one type of atom
o atom = the smallest particle of any element that still retains the characteristics of the element
not to scale
Atom = nucleus + electrons
-e
(10-10 m)
+Ze
Nucleus = protons + neutrons
(10-14 m)
Electron (discovered in 1897 by J. J. Thomson)
Neutron (discovered in 1932 by J. Chadwick)
1906
1935
Stable and Unstable Atoms
How do atomic particles interact inside the atom?
o there are forces within the atom that account for protons
(positive charges) and electrons (negative charges):
like charges repel and unlike charges attract each other
q1
F21
F12
q2
Attractive force between q1 and q2.
r
F21
q1
q2
Repulsive force between q1 and q2.
r
F12
Stable and Unstable Atoms
What makes the protons stay together in an atom?
o the strong nuclear force which opposes and overcomes the electrical
repulsion force between the protons
The energy associated with the strong force is called the binding energy.
Does the nucleus of an atom ever lose particles?
o stable(unstable) nucleus:
the binding energy is(not) great enough to hold the nucleus together.
Unstable atoms will lose neutrons or protons as they attempt
to become stable. They are called radioactive atoms.
What is a radioactive decay?
o spontaneous breakdown of an atomic nucleus resulting in the
release of nuclear radiation
What is nuclear radiation?
o energy and matter released during radioactive decay
 3 types observed: ,  and 
What nuclear physicists like…
~ 3000 currently known nuclides
~ 270 stables only !
~ 7000 expected to exist
Color Key:
Stable
+ emission
- emission
 particle emission
Spontaneous fission
Z
N
A
X
Z
N
A chemical element (X) is uniquely identified by
the atomic number Z !
Nuclides that have the same Z but different N are called isotopes !
Mass number: A = N + Z
What is radioactivity?
o emission of nuclear radiation due to a change in the nucleus
Transmutation occurs when a radioactive element attempts to
become stabilized and transforms into a new element.
He-4
(2 protons + 2 neutrons)

Radium-226
(88 protons + 138 neutrons)
Radon-222
(86 protons + 136 neutrons)
The Beginnings…
It was 1895 in Europe…
the discovery of X-rays
1869 – first observation
of “cathode rays”
Wilhelm Conrad Röntgen
1901 – Nobel Prize for Physics
(first ever)
Modern “Alchemy”: radioactivity
1896 Becquerel discovered radioactivity
The Nobel Prize in Physics 1903
A. H. Becquerel
Pierre Curie
Uranium mineral
Marie Curie
1898 – Marie Curie discovered two new
radioactive elements:
Polonium & Radium
The Nobel Prize in Chemistry 1911
Curies’ work on radioactivity…
Marie Curie studied the “uranium rays” through their electrical effects
using the quartz piezoelectroscope, an unusually sensitive electroscope
invented by Pierre Curie himself (see picture below)
Classical electroscope used
to detect electrical effects
in the air
(briefly)Marie Curie’s legacy
o coined the word “radioactivity” to describe each substance’s
power to give off invisible ionizing rays/radiations
o radioactivity is an atomic property
o discovery of new radioactive elements- Polonium and Radium
o isolated Radium and determined its atomic weight
o suggested that Becquerel rays might be pieces of matter
o the first scientist honored with 2 Nobel prizes (physics & chemistry)
o first female professor at the prestigious University of Paris (Sorbonne)
o founded a private Radium Institute (1914) to conduct research in
chemistry, physics and medicine (the beginning of radiotherapy)
Rutherford joins the quest
Meet the “uranium rays”:
Ernest Rutherford
The Nobel Prize in Chemistry 1908
“for his investigations into the disintegration
of the elements and the chemistry of
radioactive substances”
Rutherford – father of nuclear physics
Discovery of the nucleus!
fluorescent
ffl
screen
u
non-deflected
particles
gold foil

deflected particles
radioactive source
“Father of nuclear physics” in his own words:
“It was as though you had fired a fifteen inch shell at a piece
of tissue paper and it had bounced back and hit you.”
Where does the energy come from?
He-4
(2 protons + 2 neutrons)

Radium-226
(88 protons + 138 neutrons)
Radon-222
(86 protons + 136 neutrons)
1 kg of radium would be converted into 0.999977 kg of radon and alpha particles.
The loss in mass is only 0.000023 kg = 23 mg!
Energy = mc2 = mass x (speed of light)2
= 0.000023 x (3 x 108)2 = 2.07 x 1012 joules.
Equivalent to the energy from over 400 tonnes of TNT!!!
1 kg Ra (nuclear)  4*105 kg TNT (chemical)
238Pu
Vanishing radioactivity
o no matter how much radioactive material is present initially, half of it
will disappear after a time interval known as the “half-life”, T1/2
o after two half-lives, only a quarter of the original material remains…
o radioactivity is a random process that follows the laws of probability
Exponential decay law:
N  N 0e
0.693

T1/ 2
(decay probability)
 t
Radioactive half-life
Half-lives for various radioisotopes can range from a few
microseconds (10-6 s) to billons of years:
Radioisotope
Half-life
Polonium-215
0.0018 seconds
Radon-222
38 seconds
Sodium-24
15 hours
Iodine-131
8.07 days
Cobalt-60
5.26 years
Caesium-137
30.17 years
Radium-226
1600 years
Carbon-14
5730 years
Uranium-234
0.25 million years
Potassium-40
1.3 billion years
Uranium-238
4.5 billion years
~ 99%
~ 0.7%
~ 0.006%
How to measure the decay of radioactive isotopes?
Basic unit of measure the
radioactivity: curie (C)
1 C = 37 x 109 decays/second
Exposure
Amount of radiation striking an
object: Röntgen (R)
Dose
Amount of energy absorbed by an
object exposed to radiation: gray (Gy)
(in USA: 1 rad = 0.01 Gy)
Dose equivalent
Amount of biological damage caused
by radiation: Sievert (Sv)
(in USA: 1 rem = 0.01 Sv = 10 mSv)
Human body is naturally radioactive
This is the story of a mysterious science which
entered history at the onset of major transitions
in science, and itself triggered these changes…
From its modest beginnings as a minor phenomenon,
radioactivity quickly developed into a major
research field…
The radioactive decays of naturally occurring minerals
containing uranium and thorium are in large part
responsible for the birth of the study of nuclear physics…
A remarkable episode in modern science,
radioactivity has the dual attractions of a fascinating
history and dramatic consequences for humanity…
What did we learn?
Got questions?