Nuclear Physics

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Nuclear Physics
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Recall how the nucleus was discovered
Scattering of alpha-particles from a thin gold
foil The Rutherford scattering experiment
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nucleus
Electrons
We dont care about the electrons right now
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What particles make up the nucleus?
protons
neutrons
Recall both protons and neutrons are called
NUCLEONS
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Recall some stuff about the nucleus
Mass number
Notation
element
Atomic number
Isotopes nuclei with the same number of protons,
but not necessarily the same number of neutrons
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If we are able to manipulate it, we can change
one element into another example
lead into gold!!
Later we will learn that such things are possible
but not practical
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What is the charge on a proton? The neutron?
What is the nature of the force between the
nucleons?
How does the nucleus hold itself together??!!
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The Four Fundamental Forces 1 Gravity
(weak ,only attractive, long range)
2 Electromagnetic (stronger, both
attractive and repulsive, long range.) 3
Nuclear Strong (Strongest force!
Attractive, short range) 4 Nuclear Weak
(weak and short range)
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The Nucleus is a delicate balance between
1 The attractive nuclear strong force between
all of the nucleons, and 2 The repulsive
electromagnetic force between all of the protons
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What is radioactivity?
Henri Becquerel in 1896 discovered that uranium
caused a photographic plate to turn black.
These things turn black when they are exposed
to light.
What was happening?
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It turns out that our delicate balance of forces
tends to break down when nuclei get large The
electromagnetic forces eventually overcome the
nuclear ones and the nucleus spontaneously breaks
apart
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This instability is called radioactivity
All nuclei having atomic number larger than 83
(bismuth) are radioactive.
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Three types of spontaneous radioactive decay
Alpha Beta Gamma
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Alpha Decay
Parent
Alpha particle
Daughter
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What is an alpha-particle?
It has two protons and two neutrons
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Example Write the nuclear equation for the alpha
decay of uranium 238

Now look at the chart to find this one
THORIUM
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Beta Decay
Electron!
electron (beta)
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Write out the nuclear equations in each case and
identify the daughter nuclei
Alpha decay of 222Rn Beta decay of
3H Beta decay of 90Sr
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Radioactivity and half-life
Henri Becquerel in 1896 discovered that uranium
caused a photographic plate to turn black. This
was a result of alpha- and beta- particles
exposing the film Q.M. can NOT predict exactly
when an individual nucleus will decay. It CAN
predict PROBABILITY that it will decay in a
certain amount of time.
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The most important probability parameter is the
HALF-LIFE
Half-Life the TIME it takes half of a collection
of many nuclei to decay. This depends on the
relative stability of the different nuclei
semi-stable ones last longer than very unstable
ones.
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1 Kg
Example 14C (half-life 6,000 years).
How much is left after 6,000 years?
12,000 years?
18,000 years?
24,000 years?
30,000 years?
36,000 years?
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1 Kg
Example 14C (half-life 6,000 years).
There must be an easier way!
How much is left after 6,000 years?
12,000 years?
18,000 years?
We divided by 2 six times
24,000 years?
30,000 years?
36,000 years?
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(No Transcript)
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How many years would we have to wait for 1 kg of
239Pu (plutonium) with a half-life of 24,000
years to decay to less than 1 of its original
amount?
Now do the same for 238U (uranium) with a
half-life of 4.5 x 109 years.
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We can do the process in reverse to see how much
time it took a substance to decay
Radioactive Dating
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Exposure to ionizing radiation
All of these are IONIZING RADIATION
alpha
beta
gamma
Ionizing radiation causes cell damage that can
lead to DNA damage that can lead to CANCER.
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The unit of radiation exposure is called the
REM (Radiation Equivalent in Man)
This is a direct measure of the number of damaged
cells.
The average dose for one year is about 0.3
Rem Sudden dose of 1000 rems causes death in 30
days!!
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12C and 13C differ in their (a) number of
neutrons. (b) number of protons. (c) number
of electrons in orbit around the nucleus. (d)
chemical behavior. (e) all of the above.
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3890Sr is a radioactive isotope that decays by
beta-decay. Its daughter nucleus is (a) 3686Kr.
(b) 3788Rb. (c) 3789Rb. (d) 3989Y. (e)
3990Y.