Transcript Part 1 - Lnk2Lrn

IB Physics: Atomic, Nuclear,
and Particle Physics
• Nucleus
– Nucleons (A) = Protons (Z) + Neutrons (N)
– Mass and Atomic Numbers
– Number of protons & neutrons in nucleus is limited.
• Radioactivity is the decay of nuclei to more stable element via emission of
“radiation” (α or β particles,  rays, etc.).
• Half-Life (2n exponential decay)
• Isotopes - 3000 known nuclei, but only 266 stable ones!
• Radioactive Processes
– α, β, and γ-rays
– Natural radioactivity > At. No. 83
– Fusion (Joining) v. Fission (Splitting) of Atoms – both release energy.
Page 1
Nucleus: Particle Composition
• Z protons + N neutrons = A nucleons (1 – 10 fm 10-15diam.).
• 1920: Ernest Rutherford
– Bombarded Au foil with Alpha particles
– Most of atom is empty space with massive + charged nucleus.
• 1932: James Chadwick discovered neutron (bombarded Be with α).
• Isotope: same Z (# protons), different N (# neutrons).
– 15O and 16O … or … 12C and 14C … or … 238U and 235U …
Page 2
The Atom: Particle Properties
Particle
Charge
Proton
+1.6x10-19 1.007276
Neutron
Electron
0
amu
1.008665
–1.6x10-19 5.4858×10-4
kg
1.67x10-27
~1.67x10-27
9.11x10-31
Recall the 4 Models:
1. Single Indivisible Particle
2. Plum-Pudding
3. Planetary
4. Planetary-Quantum (Bohr Model)
Page 3
Nucleus: Binding Energy
• Nuclear mass is slightly less than mass of constituent protons and
neutrons due to nuclear binding energy.
• Binding energy per nucleon
peaks at A = 56
(~8 MeV/nucleon) and
slowly decreases.
• Energy is released when a
heavy nucleus (A~200)
fissions into lighter nuclei
near A~60.
Binding Energy / Nucleon ( MeV)
• Mass is converted to energy when a nucleus is formed, E = mc2.
Peaks at Fe
(A = 56)
Fission
(A ~ 200)
Nucleon Number A Page 4
Radioactivity: Historical Overview
• 1896: Becquerel accidentally discovered that a mysterious rock
emitted invisible radiation onto a photographic plate.
• 1898: Marie and Pierre Curie discovered polonium (Z=84) and radium
(Z = 88), two new radioactive elements.
• 1903: Becquerel and the Curie’s received the Nobel prize in physics
for radioactive studies.
• 1911: Marie Curie received a 2nd Nobel prize (in chemistry) for
discovery of polonium and radium.
• 1938: Hahn (1944 Nobel prize) and Strassmann discovered nuclear
fission - Lisa Meitner played a key role!
• 1938: Enrico Fermi received the Nobel prize in physics for producing
new radioactive elements via neutron irradiation, and work with
nuclear reactions.
Page 5
Contributors to the Study of Nuclear Physics
The Nobel Prize in Physics 1903
"in recognition of the extraordinary
services they have rendered by their
joint researches on the radiation
phenomena discovered by Professor
Henri Becquerel"
Pierre Curie
France
Marie Curie
France
1/4 of the prize
1/4 of the prize
"in recognition of the
extraordinary services he
has rendered by his
discovery of spontaneous
radioactivity"
Antoine Henri Becquerel
1/2 of the prize (France)
Page 6
Recall: Nuclear Physics.
Page 7
Radioactive Decay Examples
=
+
Page 8
Types of Nuclear Radiation
ALPHA (a) PARTICLE is identical to helium
nucleus. It has 2 protons and 2 neutrons, mass
number of 4 and atomic number of 2.
BETA (b) PARTICLE is a high-energy
Electron. It has a negative charge and
mass number of 0.
GAMMA () RAYS are high-energy radiation,
like X-rays. They contain no mass or charge,
only energy. λ = 10-10 to 10-15 m
Page 9
Alpha Emitters
Note: An Alpha particle has the same structure as a Helium nucleus.
Page 10
Alpha Emitters:
226
88 Ra  ?
+ 42He
First, let’s figure out the identity of the new
nucleus. How? Determine the atomic number:
88 – 2 = 86. The new element is Rn.
Next, figure out the mass number of the new
nucleus: 226 – 4 = 222.
22688Ra  22286Rn + 42He
Page 11
Beta Emitters
Note: During Beta Decay, a Neutron spontaneously changes to aPage
Proton.
12
Beta Emitters
•
6C  X
14
+ 0-1e
• To find the new mass # we take 14-0 = 14
• To find the new atomic # we take 6+1 = 7
• The element with atomic number 7 is Nitrogen
•
14 N +
C

6
7
14
0
-1e
Page 13
Producing Radioactive Isotopes:
TRANSMUTATION is the process of changing
one element into another (can be via
bombardment OR emission of radiation).
This can be natural or artificial.
A stable atom can be bombarded with fastmoving a particles, protons, or neutrons.
A radioactive isotope is called a
RADIOISOTOPE.
Page 14
A stable atom can be bombarded with
fast-moving a particles.
Page 15
Radiation Exposure:
BACKGROUND RADIATION is the
radiation that is in the environment.
Background radiation can come from food,
building materials, cosmic rays, etc.
The air molecules in the atmosphere block out
some cosmic rays.
Page 16
Page 17
Radiation Exposure:
Radiation sickness from too much radiation.
Nausea, vertigo, and fatigue (side effects of
chemotherapy). More exposure can lead to
death.
Exposure is measured by LD50 or lethal dose
that is expected to cause death in 50% of the
people receiving that dose.
To minimize problems, workers often wear
badges to monitor the maximum permissible
dose.
Page 18
Page 19
a
basements of buildings
Note: Chart displays average values of doses for common isotopes.
Page 20
Radioactivity
End of Part 1.
Page 21