Transcript Chapter 30: Radioactivity

Chapter 30: Radioactivity
By: PF
Wesley
Herbie
Affel
Summers
History of Radioactivity
• Antoine Henri Becquerel, a French physicist,
discovered radioactivity in 1896.
• Canadians Ernest Rutherford and Frederick
Soddy studied uranium atoms and found that
they changed to other atoms.
• As a result of this observation, polonium and
radium were discovered as new elements by
Marie and Pierre Curie.
• The study of radioactivity allowed for a better
understanding of the nucleus in an atom.
The Nucleus
• Rutherford determined through his
experiments that the number of deflected
α particles is proportional to the square
of the charge.
• Based on this, he further discovered the
numbers of electrons that different types of
atoms contained.
The Nucleus (Cont.)
• Since the atom is neutral, the nucleus must also
posses positive charge to balance the negative
electrons.
• The charge of a proton is equal to that of an
electron (an elementary charge), but it is a
positive charge.
• A proton’s mass is approximately equal to one
atomic mass unit (u). The number of protons is
the atomic number of the atom (Z).
The Nucleus (Cont.)
• Not all of the mass of an atom was compensated
for, so Rutherford determined that there must be
another component.
• Through this idea the neutron was discovered,
which has no charge, but makes up the rest of
the mass.
• The mass number (A), therefore is the sum of
the protons and neutrons in an atom.
• Rutherford understood that the nucleus was very
small. For example, the diameter for a hydrogen
atom is now accepted to be about 2.6 x 10-15 m.
Isotopes
• The puzzle of atomic masses that were not
integral numbers of atomic mass units was
solved with the mass spectrometer.
• Mass spectrometer demonstrated that an
element could have atoms with different masses.
• Example: when analyzing a pure sample of
neon, two spots appeared on the film of the
spectrometer, which were produced by neon
atoms with different masses.
• Different forms of an atom are called
isotopes.
• The nucleus of an isotope is called a
nuclide.
• All isotopes of an atom have the same
number of protons and electrons, but
different numbers of neutrons.
• Many isotopes form naturally
• There is special notation used to describe an
isotope.
• A subscript representing the atomic number, Z,
is written in the lower left of the symbol for the
element.
• A superscript written to the upper left of the
symbol is the mass number, A.
• This notation takes the form Az X
• For example, two isotopes of neon with atomic
22
number 10 are written 10Ne and 20
10Ne
Radioactive Decay
• In 1896, Henri Becquerel discovered that
uranium samples would “fog” photographic
plates.
• This suggested that some type of ray was
passing through the plates. This is called
radiation.
• Several other materials had this same effect;
these materials are called radioactive and
therefore undergo radioactive decay.
Types of Radioactive Decay
• In 1899, Rutherford discovered that
Uranium compounds have three types of
radiation.
• These are separated by their penetrating
ability, α (alpha) being the weakest,
followed by β (beta) and finally γ (gamma),
the strongest.
α Alpha Decay
• An Alpha particle is the nucleus of a Helium
atom, 24He
• This particle contains two protons and two
neutrons, and can be stopped by a simple piece
of paper.
• This particle comes from within the nucleus of
the decaying material
• The mass number of the original nucleus is
therefore decreased by four, while the charge
decreases by two
β Beta Decay
• A beta particle is simply a high speed
ejected electron, -10e
• Beta decay can be stopped with about six
millimeters of aluminum
• The charge of the original atom will
increase by one, since it is losing a
negative charge.
γ Gamma Decay
• A Gamma particle is a high energy photon.
• This particle has negligible mass and no
charge: 00 γ
• It requires several centimeters of lead to
stop gamma radiation.
• The original atom does not change mass
or charge.
Nuclear Reactions and Equations
• A nuclear reaction occurs whenever the
number of neutrons protons in a nucleus
changes.
• Just as in chemical reactions, some
nuclear reactions occur with a release of
energy in the form of the kinetic energy of
the released particles.
• Others occur only when energy is added
to the nucleus.
• Nuclear reactions can be expressed in
words.
• The word equation for the change of
uranium to thorium due to α decay:
• Uranium 238 yields Thorium 234 plus one
α particle.
• Nuclear reactions are more easily
expressed as equations.
• The same reaction expressed is equation
form:
234Th+ 4He
• 238
U
90
2
92
• No nuclear particles are destroyed during
the reaction, thus the sums of the
superscripts are equal on both sides, as
well as the sums of the subscripts.
Half-Life
The time required for half of the atoms in any given quantity
of a radioactive isotope to decay is the Half-Life of that
element.
Each particular isotope has its own half-life
Half-Life of Selected Isotopes
Activity
The decay rate, or number of decays per second, of a
radioactive substance is called it’s activity. Activity is
proportional to the number of radioactive atoms
present. Therefore, the activity of a particular sample is
also reduced by one half in one half-life.
131
Consider 53 I with a half-life of 8.07 days. If the activity of
a certain sample is 8x10^5 decays per second when
the 131
53 I is produced, 8.07 days later its activity will be
4x10^5 decays per second. After another 8.07 days, its
activity will be 2x10^5 decays per second. The SI unit
for decays per second is a Bequerel, Bq.
Nuclear Bombardment
• Rutherford bombarded many elements
with α particles, using them to cause a
nuclear reaction.
• For example, when nitrogen gas was
bombarded, he noted that high energy
protons were emitted from the gas.
• A proton has a charge of 1, while an α
particle has a charge of 2.
• Rutherford hypothesized that the nitrogen
had been artificially transmuted by the α
particles.
• The unknown results of the transmutation
A
can be written ZX, and the nuclear
reaction can be written….
14
4
A
1
2He + 7N  1H + ZX
• Simple arithmetic shows that the unknown
isotopes number is Z = 2 + 7 - 1= 8 and
the mass number is A = 4 + 14 - 1= 17
• By looking at the appendix D-5 in the
17
book, we find that the isotope must be 8O
• Bombarding Be with α particles produced
a radiation more penetrating that any
previously discovered.
• In 1932, Irene Curie and her husband,
Frederic Joliot, discovered that high speed
protons expelled from paraffin wax that
was exposed to this new radiation from
beryllium.
9
4
• That same year, James Chadwick
showed that the particles emitted
from the beryllium were
uncharged, but had approximately
the same mass as protons.
• In other words, the beryllium
emitted the particle Rutherford had
theorized must be in the nucleus,
the neutron.
• The reaction can be written using
1
the symbol for the neutron, 0n
4
1
12
9
2He + 4Be  6C + 0n
• Since neutrons are uncharged and are not
repelled by the nucleus, neutrons are often
used to bombard nuclei
• Alpha particles from radioactive materials
have fixed energies
• Also, sources that emit large numbers of
particles per second are hard to produce
• Because of this, methods of artificially
accelerating particles to higher energies
are needed…
Linear Accelerators
• A linear
accelerator
consists of a
series of hollow
tubes within a
long evacuated
chamber.
• The tubes are connected to a source of
high frequency alternating voltage.
• There is no electric field within the tube, so a
proton or electron can move at a constant
velocity, inside the tube
• When the first tube has a negative potential, protons
are accelerated into it.
• When the protons is at the end of one tube, the
potential of the second tube is negative with respect
to the first tube.
• This accelerates the proton into the second tube.
• This continues, and the proton keeps accelerating,
gaining 105 eV every time.
• Linear accelerators can be used with both protons
and electrons
• The largest linear accelerator is at Stanford
University in California. It’s 3.3 km long and
accelerates electrons to energies of 20 GeV
(1x1010 eV)
The Synchrotron
• A smaller accelerator
by bending the path
for the particles into a
circle
• The bending
magnets are
separated by
accelerating regions
• In the straight regions, high frequency
alternating voltage accelerates the
particles.
• The strength of the magnetic field and the
length of path are chosen so that the
particles reach the location of the
alternating electric field exactly when the
field’s polarity will accelerate them.
• One of the largest synchrotrons is at the
Fermi National Accelerator lab near
chicago, where protons there can reach
energies of 1 TeV (1x1012eV)
Particle Detectors
Photographic films become “fogged,” or exposed
when α particles, β particles, or y rays strike them.
Thus, photographic film can be used to detect
these particles and rays. Many other devices are
used to detect charged particles and rays. Most of
these devices make use of the fact that a collision
with a high speed particle will remove electrons
from atoms. That is, the high speed particles ionize
the matter that they bombard. In addition, some
substances fluoresce when exposed to certain
types of radiation. Thus, fluorescent substances
can be used to detect radiation.
Types of detectors
In the Geiger-Mueller tube, particles ionize gas
atoms. The tube contains a gas at low pressure (10
kPa). At one end of the tube is a very thin “window”
through which charged particles or gamma rays
pass.
Modern experiments use spark chambers that
are like giant Geiger-mueller tubes. Plates several
meters in size are separated by a few centimeters.
The gap is filled with a low-pressure gas. A
discharge is produced in the path of a particle
passing through the chamber. A
computer records the discharge
which is later used for analysis.
Fundamental Particles
• The atom was thought to be the smallest
particle into which matter could be divided.
• Rutherford then discovered that the atom
was a nucleus surrounded by electrons.
• Once protons were discovered, they too were
thought to be indivisible.
• Experiments involving the bombardment of
protons by other protons or neutrons
showed that these particles were also made
up of even smaller particles.
Quarks and Leptons
• Protons and neutrons are composed of
Quarks.
• Leptons are particles like electrons and
neutrinos.
• Other particles carry or transmit forces
between particles.
Other Subatomic Particles
• Photons carry electromagnetic forces.
• There are eight types of gluons, which carry
the strong forces that bind quarks into
protons and neutrons and that bind the
nucleus together.
• Three types of weak bosons, which carry a
weak force involved in beta decay.
• The gravitron is the particle responsible for
causing gravity.
Antiparticles (Antimatter)
• For every particle, there is an identical
antiparticle.
• These antiparticles differ from their matching
particles only in charge.
• When a particle and its matching antiparticle
collide, they annihilate each other and are
transformed into photons (particle-antiparticle
pairs) and a massive amount of energy
• Antimatter rockets and bombs are theoretical
designs that would yield incredibly powerful
results (1Kg of matter and 1Kg of antimatter=47
Megatons of TNT)
Particles and Antiparticles
• α particles and γ rays emitted by
radioactive nuclei have single energies
that depend on the decaying nucleus.
• β particles, however, are emitted with a
wide range of energies.
• Niels Bohr suggested that this would mean
that nuclear reactions did not follow the
law of conservation of energy.
• Wolfgang Pauli in 1931 and Enrico Fermi
in 1934 suggested instead that an unseen
particle was emitted along with the β
particle.
• Fermi called it the
neutrino. However, it was
actually the antineutrino
that was emitted. This was
first observed in 1956.
• Neutrons in an unstable nucleus decay by
emitting a β particle and an antineutrino.
• Antineutrinos have no charge and zero
mass, but like a photon carry momentum
and energy.
• A proton in an unstable nucleus decays
into a neutron by emitting a positron and a
neutrino.
• Positrons are positive electrons.
• The energy equivalent of the positron and
electron can be calculated with E=mc2.
E = 2(9.11x10-31 kg)(3.00x108 m/s)2
= (1.64x10-13 J)(1 eV/1.60x10-19 J)
= 1.02x106 eV or 1.02 MeV
• Just as there are positive electrons, there
are also negative protons called
antiprotons. The pair was first created in
the lab by Berkeley in 1955.
• Antiparticles are antimatter. When they
collide with their matter counterparts, they
annihilate each other and release energy
in γ rays.
• Just as matter can be converted to energy,
energy can be converted to matter. For
example, if a γ ray with at least 1.02 MeV
energy passes close to a nucleus, a
positron and electron pair can be
produced. The pair must always be
created together and cannot be created
individually.
The Quark Model of the Nucleons
• The Quark model describes the structure
of the proton and neutron.
• Each nucleon is divided into three quarks.
• There are both up quarks and down
quarks.
• An up quark has a positive 2/3 e value.
• A down quark has a negative 1/3 e value.
The Quark Model of the Nucleons
(Cont.)
• The proton has two up quarks (2/3
e*2=4/3) and one down quark (-1/3 e) for a
balanced charge of +1 e.
• The notation to describe this is p=(uud).
• The neutron has one up quark and two
down quarks.
• The charge therefore is 2/3 – 1/3 – 1/3, or
zero.
• The notation for this is n=(udd).
The Quark Model of the Nucleons
(Cont.)
• In the Quark Model the force that holds the
quarks together is a result of gluons which
are emitted and absorbed.
• The farther away the quarks become the
greater the force, unlike electric force.
• Weak interaction is caused by three
forces: W+, W-, Z0 bosons.
• Weak interaction is witnessed through
beta decay.