Download Topic 7: Atomic and nuclear physics 7.1 The atom

Topic 7: Atomic and nuclear physics
7.2 Radioactive Decay
3 hours
Radioactivity
• At the end of the 19th century and in the early
part of the 20th, it was discovered, mainly due to
the work of Henri Becquerel and Marie and Pierre
Curie, that some nuclei are unstable. That is to
say, nuclei spontaneously emit a particle or
particles, they decay, and become different
nuclui. This phenomenon is called redioactivity.
• There are three distinct emissions that take place,
called alpha (a), beta (b), and gamma (g)
radiations.
1903 Nobel Prize in Physics
Awarded to Becquerel "in
recognition of the
extraordinary services he has
rendered by his discovery of
spontaneous radioactivity“
and to the Curie’s "in
recognition of the
extraordinary services they
have rendered by their joint
researches on the radiation
phenomena discovered by
Professor Henri Becquerel"
Antoine Henri
Becquerel
Pierre Curie
Marie Curie, née
Sklodowska
1/2 of the prize
1/4 of the prize
1/4 of the prize
France
France
France
École
Polytechnique
Paris, France
École municipale
de physique et de
chimie industrielles
(Municipal School
of Industrial
Physics and
Chemistry)
Paris, France
b. 1852
d. 1908
b. 1859
d. 1906
b. 1867
(in Warsaw,
Poland, then
Russian Empire)
d. 1934
Ionization
• a, b, and g radiations ionize air as they pass
through it; this means they knock electrons
out of the atoms of the gases in the air.
• An a-particle of energy 2 MeV will produce
about 10 000 ion pairs per mm along its air
path. A b-particle of the same energy will
produce about 100 ion pairs per mm and a gray about 1 ion pair per mm.
Three Distinct Emissions
• The existence of three distinct emissions is
confirmed by letting these pass through a
magnetic or electric field and observing the
three separate beams.
Properties of a, b, and g Radiations
Characteristic
Alpha Particle (a)
Beta Particle (b+, b-)
Gamma Ray (g)
Nature
Helium-4 Nucleus
Fast Electron (or
Positron)
Photon (high
frequency EM
radiation)
Charge
+2e
-e (or +e)
0
Mass
6.64 x 10-27 kg
9.1 x 10-31 kg
0
Maximum Speed
~0.06c
~0.98c
c
Penetration Power
A few cm of air or a
thin sheet of paper
A few m of air or a
few mm of metal
A few cm of lead
Detection
Causes strong
fluorescence
Causes fluorescence
Causes weak
fluorescence
Affects photographic
film
Affects photographic
film
Affects photographic
film
Is affected by electric Is affected by electric In not affected by
and magnetic fields
and magnetic fields
electric and
magnetic fields
Detecting Radiation:
The Geiger-Müller Tube
• A Geiger–Müller tube consists of a tube filled with a low-pressure
(~0.1 Atm) inert gas such as helium, neon or argon and an organic
vapor or a halogen gas and contains electrodes, between which
there is a potential difference of several hundred volts, but no
current flowing. The walls of the tube are either metal or the inside
coated with metal or graphite to form the cathode while the anode
is a wire passing up the center of the tube.
• When ionizing radiation passes through the tube, some of the gas
molecules are ionized, creating positively charged ions, and
electrons. The strong electric field created by the tube's electrodes
accelerates the ions towards the cathode and the electrons towards
the anode. The ion pairs gain sufficient energy to ionize further gas
molecules through collisions on the way, creating an avalanche of
charged particles.This results in a short, intense pulse of current
which passes (or cascades) from the negative electrode to the
positive electrode and is measured or counted.
• Most detectors include an audio amplifier that produce an audible
click on discharge. The number of pulses per second measures the
intensity of the radiation field.
The Geiger-Müller Tube
The Geiger Counter
A.K.A.
The Geiger
-Müller
Detector
Detecting Radiation:
The Ionization Chamber
• The ionization chamber is the simplest of all
gas-filled radiation detectors, and is used for
the detection or measurement of ionizing
radiation. Conventionally, the term "ionization
chamber" is used exclusively to describe those
detectors which collect ion pairs from gases.
Smoke Detectors
• The ionization chamber has found wide and
beneficial use in smoke detectors. In a smoke
detector, the gap between the plates is exposed to
the open air. The chamber contains a small amount
of americium-241, which is an emitter of alpha
particles. These alpha particles carry a substantial
amount of energy, and when they collide with gas in
the ionization chamber (mostly nitrogen and oxygen)
the uncharged gas molecules will lose one or more
electrons and become charged ions. Since the plates
are at different voltages the ions and electrons will
be attracted to the plates. The flow of ions between
the plates represents a measurable current. If smoke
enters the detector, it disrupts this current because
ions strike smoke particles and are neutralized. This
drop in current triggers the alarm.
Serge Plots
• There are about 2500 nuclides (nuclei
with a specific number of protons and
neutrons) but only about 300 of
them are stable, the rest are unstable.
• As the number of protons in the
nucleus increases the electrostatic
repulsion between them grows, but
the strong nuclear force does not
grow proportionally since it is a short
range force. Therefore extra neutrons
must be added to increase the
nuclear forces without participating in
the repulsive electric force.
• However, too many neutrons will
make the nucleus unstable and it will
emit a-particles to try and reach a
more stable state.
Alpha Decay
• Alpha decay is a type of radioactive decay in
which an atomic nucleus emits an alpha
particle, and thereby transforms (or 'decays')
into an atom with a mass number 4 less and
atomic number 2 less. We may express such a
decay by the nuclear reaction equation:
Example of Alpha Decay
Beta Decay (A.K.A. Beta Minus Decay)
• In β− decay, the weak interaction converts a neutron (n)
into a proton (p) while emitting an electron (e−) and an
antineutrino (νe):
n → p + e− + νe
• The antineutreno is the antiparticle of a neutreno and
is emitted in order to conserve momentum and energy.
Neutrinos (meaning "small neutral ones") are
elementary particles that often travel close to the
speed of light, are electrically neutral, are able to pass
through ordinary matter almost undisturbed and are
thus extremely difficult to detect. Neutrinos have a
minuscule, but nonzero mass. They are denoted by the
Greek letter ν (nu).
Beta Decay (A.K.A. Beta Minus Decay)
• At the fundamental level
(as depicted in the
Feynman diagram to the
right), this is due to the
conversion of a down
quark to an up quark by
emission of a W− boson
(an elementary particles
that mediate the weak
nuclear force); the W−
boson subsequently
decays into an electron
and an antineutrino.
Beta Plus Decay (HL Only)
• In β+ decay, energy is used to convert a proton
into a neutron, a positron (e+) and a neutrino (νe):
energy + p → n + e+ + νe
• So, unlike β−, β+ decay cannot occur in isolation,
because it requires energy, the mass of the
neutron being greater than the mass of the
proton.
The Nuclear Reaction for b--Decay
• For example:
Gamma Decay
• Gamma rays are often produced alongside
other forms of radiation such as alpha or beta.
When a nucleus emits an α or β particle, the
daughter nucleus is sometimes left in an
excited state. It can then jump down to a
lower level by emitting a gamma ray in much
the same way that an atomic electron can
jump to a lower level by emitting visible light
or ultraviolet radiation.
Gamma Decay
60
Co
27
→
60
Ni*
28
+
e−
60
Ni*
28
→
60
Ni
28
+
γ
+
νe
+
γ
+
1.17 MeV
+
1.33 MeV
Gamma Ray Energies
• Typical gamma ray energies are in the mega
electron volt (MeV) range and have a
wavelength of
l = hc / DE
where DE is the photon’s energy, h is Planck’s
constant (6.63 x 10-34 J s) and c is the speed of
light.
The Law of Radioactive Decay
• Radioactive decay is a random and spontaneous
process and the rate of decay decreases
exponentially with time. The law of radioactive
decay states that the number of nuclei that will
decay per second (i.e. the rate of decay) is
proportional to the number of atoms present
that have not yet decayed.
• There exists a certain interval of time, called the
half-life, such that after each half-life the number
of nuclei that have not yet decayed is reduced by
a factor of 2.
Homework:
• Tsokos, Page 378, Questions 1 to 18