Nuclear Physics
Tiny central group of particles in an atom.
A hydrogen atom is the smallest atom and is on the
order of about 10-10 m across.
A nucleus is only about 10-14 m across.
A nucleus is made up of the protons and neutrons.
Nucleus has a net positive charge.
If the protons are positively charged and the neutrons
are neutral, how does the nucleus stay together?
Protons repel each other via the electric force.
Because the protons are so close together and
there mass is so small, the gravitational force is
not strong enough to keep them together.
A new force called the strong nuclear force keep
them together.
There is another force called the weak nuclear
force which is involved in radioactive beta decay.
These 4 forces are the fundamental forces that
determine how the universe behaves.
Three types of “glue” in the universe keeps
particles together.
1) Gravitational force holds objects together at
the astronomical level.
2) Electric force holds solids and liquids together
at the atomic level.
3) The nuclear force holds the nucleus together.
The nuclear force is very strong but acts only up
to a distance of about 10-15 m.
(As the size scale goes down, the strength of the forces
go up.)
Nucleus has protons and neutrons.
Each of these are made up of quarks.
There are 6 types of quarks. Protons and neutrons
are made up of Up and Down quarks.
Up quarks have charge of +(2/3)e
Down quarks have charge of –(1/3)e
Protons have 2 up quarks and 1 down quarks.
Neutrons have 2 down and 1 up quarks.
Masses:
Mass of proton is 1.67265 x 10-27 kg
Mass of neutron is 1.67495 x 10-27 kg
Neutron is a tiny bit larger.
We will see later that a neutron can decay into a
proton with some other ‘junk’ left over.
Elements are described as having atoms with the
same number of protons. Example: all oxygen
atoms have 8 protons.
The number of protons is called the atomic number.
Atoms with the same number of protons but
different number of neutrons are isotopes.
Example: ordinary hydrogen has 1 P and 0 N.
Deuterium has 1 P and 1 N.
Tritium has 1 P and 2 N.
All have the same atomic number = 1.
Atomic weight or atomic mass or mass number is
the number of protons and the number neutrons
added up.
Different isotopes of the same element have the
same atomic number but different atomic masses.
Another example is carbon. 12 13 14
6
C, 6 C, 6 C
Typically elements with small atomic numbers
have only a few neutrons. (#p about equal to #n)
For large atoms, such as uranium ( 92 P), there
are a lot more neutrons (for uranium there are
about 140 N). The large number of protons need
more neutral neutrons to be in between them.
Radioactivity
First observed by accident when a
photographic plate was stored in a drawer
with an uranium compound.
The plate had been exposed even though it
has kept in a dark drawer.
Energy from the uranium exposed the plate.
Radioactivity – results when an unstable nucleus
emits energy in the form of particles or
electromagnetic waves.
The original material decays, or turns into two
entities which when added up have the same
energy as the original material.
Radioactive isotopes – isotopes that are radioactive.
All isotopes with an atomic number > 83 is
radioactive. Some smaller isotopes are also
radioactive. Example: 14C.
Stable nuclei don’t naturally radioactively decay.
They need some outside influence.
Unstable nuclei decay with no outside disturbance.
3 types of radioactive decay:
alpha – radioactive nucleus spits out an alpha
particle. Alpha particles ( ) are identical to a
helium nucleus. 2 p and 2 n.
Air can absorb alpha particles. They will collide
with the air and pick up electrons and become
normal helium atoms.
beta decay – ( ) radioactive nucleus spits out an
electron. Amazingly, a nucleus can make an
electron.
Most radioactive isotopes can only give off either
alpha or beta particles. (one or the other)
However these two are usually accompanied by a
gamma-ray photon. This is the 3rd type of decay.
Gamma rays are a form of electromagnetic radiation
(light with wavelength smaller than 10-12m)
Parent nucleus is what is present before the
decay (nuclear reaction).
Daughter nucleus is present after the nuclear
reaction.
238
234
92 U
90Th
uranium is the parent
thorium is the daughter
Half life – determines the rate of how fast the
nuclear material decays.
half life – the time that is needed for half of the
material to decay.
see tables 15.1 for some examples
and the graph below.
The half life for each radioactive isotope is
constant. Using this information it is possible to
date old objects depending on the amount of
radioactive material that is present.
Carbon dating using the 14C isotope is common.