What is radioactivity?
Radioactivity can be described as the spontaneous
disintegration (decay) of an unstable nucleus of an atom,
resulting in the emission of radiation from the nucleus.
Instability of an atom's nucleus may result from an excess of
either neutrons or protons.
There can be many forms of emitted radiation, the most
common of which are alpha, beta, and gamma rays.
The Atom
Protons 11p
(1.007276 amu)
Neutrons 10n
Electrons (1.008665 amu)
(0.0005486 amu)
Neon-20 2010Ne
(19.992434 amu)
Alpha Decay
Daughter
Nucleus
Np-237
++
Th-234 Parent Nucleus
Ra-228 Am-241
Rn-222 U-238
Th-232 Alpha Particle
Ra-226 (Helium Nucleus)
(4.00147 amu)
Alpha “rays”
Alpha decay is a radioactive process
in which a particle with two neutrons
and two protons is ejected from the
nucleus of a radioactive atom. The
particle is identical to the nucleus of a
helium atom.
Alpha decay only occurs in very heavy elements such as uranium,
thorium and radium. After an atom ejects an alpha particle, a new
parent atom is formed which has two less neutrons and two less
protons. Because alpha particles contain two protons, they have a
positive charge of two. Alpha particles are very heavy and very
energetic compared to other common types of radiation. These
characteristics allow alpha particles to interact readily with materials
they encounter, including air, causing many ionizations in a very
short distance. Typical alpha particles will travel no more than a few
Daughter
Beta (Negatron) Decay
Nucleus
Osmium-187
Calcium-40
Antineutrino
Parent Nucleus
Rhenium-187
− −
Potassium-40
Beta Particle
(electron)
Beta “rays”
Beta decay is a radioactive process
in which an electron is emitted
from the nucleus of a radioactive
atom, along with an unusual
particle called an antineutrino.
When a nucleus ejects a beta particle, one of the neutrons in the
nucleus is transformed into a proton plus the ejected electron (beta
particle). Since the number of protons in the nucleus has changed, a
new daughter atom is formed which has one less neutron but one
more proton than the parent. Beta particles have a single negative
charge and weigh only a small fraction of a neutron or proton. As a
result, beta particles interact less readily with material than alpha
particles. Depending on the beta particles energy (which depends
on the radioactive atom and how much energy the antineutrino
carries away), beta particles will travel up to several meters in air,
Gamma-Ray Emission
− −
Gamma Ray
Parent Nucleus Daughter Nucleus
Cesium-137 Barium-137m
Molybdenum-99 Technetium-99m
Gamma rays
After a decay reaction, the nucleus is
often in an “excited” state. This means
that the decay has resulted in producing
a nucleus which still has excess energy.
This energy is lost by emitting a pulse
of electromagnetic radiation called a gamma ray, identical in nature
to light or microwaves, but of very high energy.
The gamma ray has no mass and no charge. Gamma rays interact with
material by colliding with the electrons in the shells of atoms. They
lose their energy slowly in material, being able to travel significant
distances before stopping. Depending on their initial energy, gamma
rays can travel from 1 to hundreds of meters in air and can easily go
right through people. It is important to note that most alpha and beta
emitters also emit gamma rays as part of their decay process. However,
there is no such thing as a “pure” gamma emitter. Important gamma
m
Positron Decay
Daughter Neutrino
Nucleus
Boron-11 +
Carbon-13 +
-
Positron Particle
Parent Nucleus
(Positive electron)
Carbon-11
Nitrogen-13
Annihilation
Radiation
Electron Capture and
Characteristic X-Rays
Daughter Nucleus
Iron-57 Neutrino
Parent Atom
Cobalt-57
Characteristic X-Ray
X-Ray Production
(Bremsstrahlung)
Electron
X-Ray
Target Nucleus
Tungsten
Anode (+)
Cathode
(-)
X-Rays
Types of Radiation
++ Paper Plastic Lead Concrete
Alpha
−
− Beta
Gamma and X-rays
0n
1 Neutron
Other types of radiation and summary of radioactive particles
Cosmic radiation: very energetic particles including protons which bombard the
earth from outer space. It is more intense at higher altitudes than at sea level where
the earth's atmosphere is most dense and gives the greatest protection.
Neutrons: uncharged particles which are also very penetrating. On Earth they
mostly come from the splitting, or fissioning, of certain atoms inside a nuclear
reactor. Water and concrete are the most commonly used shields against neutron
radiation from the core of the nuclear reactor.
Radiation Type of Radiation Mass (AMU) Charge Shielding material
Alpha Particle 4 +2 Paper, skin, clothes
Beta Particle 1/1836 ±1 Plastic, glass, light metals
Gamma Electromagnetic Wave 0 0 Dense metal, concrete, Earth
Neutrons Particle 1 0 Water, concrete, polyethylene, oil
Nuclear Equations
Nuclear equations describe events that occur in the nucleus. They are
balanced; i.e., the sum of atomic masses (superscript; number of neutrons
plus protons) on the left must equal the sum of atomic masses on the
right. Similarly, the atomic numbers (subscript; number of protons) must
balance.
226
Alpha rays: 88 Ra → 2He + 86 Rn
4 222
This describes decay of Radium, found in many rocks and minerals. Radium in basement
materials decays to the gas Radon, which itself is radioactive. Radon, if breathed into the
lungs, emits radioactive particles and is very likely to cause cancer.
14
Beta rays: 6 C(s) → 14
7 N+ 0 e
-1
This describes decay of carbon-14. It is used in “carbon dating” of fossilized materials, and
is also commonly used as a tracer for carbon in biochemical experiments.
131
I→
131 0 0
Gamma rays: 53 54 Ne + -1
e + 0
This describes decay of iodine-131. It is commonly used as a tracer for iodine in
biochemical and physiological (thyroid) experiments.
Half-Life
The time required for the
1200 amount of radioactive material
1000 to decrease by one-half
800
Activity600
400
200
0
New 1 Half- 2 Half- 3 Half- 4 Half-
Life Lives Lives Lives
Half-life
Half-life is the time required for the
quantity of a radioactive material to
be reduced to one-half its original
value.
Decay is a random process which follows an exponential curve.
The number of radioactive nuclei remaining after time (t) is given
by:
N(t) = N(0) * e(- t)
where
N(0) = original number of atoms
N(t) = number remaining at time t
= decay constant (unique for each isotope; related to
probability of decay)
Derivation of half-life equation
# of atoms decaying at any time is
proportional to # of atoms present. -dN/dt = N
Separate differentials dN/N = -dt
Integrate ʃdN/N = -ʃdt
Evaluate between limits lnN = -t
N: N(0), N(t)
t: 0, t ln N(t) – lnN(0) = -t
Rearrange and take antilog ln(N(t)/N(0)) = -t
Let N(t) = ½ N(0); i.e., t = t ½ N(t)/N(0) = e(-t)
Solve for
= ln(2)/t ½
Substitute ; take ln of both sides; rearrange;
N(t)/N(0) = e (-ln(2)t/t ½)
take antilog of both sides; solve for N(t)
N(t) = N(0)2-(t/t ½)
Commonly used isotopes
Units of radioactivity
When given a certain amount of
radioactive material, it is customary to
refer to the quantity based on its activity
rather than its mass. The activity is
simply the number of disintegrations
or transformations the quantity of material undergoes in a given period
of time.
The two most common units of activity are the Curie (Ci) and the
Becquerel (Bq). One Curie is equal to 3.7x1010 disintegrations per
second. One Becquerel is equal to one disintegration per second.
Because the Ci is so large and the Bq is so small, we often use
prefixes to define levels of activity. Examples of these prefixes
follow:
(m) milli (10 ) –3 () micro (10 ) –6 (n) nano (10 ) –9 (p) pico (10 ) -
Units of exposure
and dose
The SI unit of exposure (X) is the
coulomb/kilogram (C/kg). The
traditional unit is the roentgen (R). 1 R = 2.58 x 10-4 C/kg. R only
applies to absorption of gamma rays and x-rays in air.
The SI unit of absorbed dose (D, energy absorbed by an object per
unit mass) is the Gray (Gy). The traditional unit is the rad.
100 rad = 1 Gy = 1 J/kg. D applies to all radiations at all energies in
all absorbers.
The SI unit of dose equivalent (H, the absorbed dose multiplied by a
“quality factor” that accounts for the different biological effectiveness
of different types of radiation) is the Sievert (Sv). The traditional unit
is the rem. 100 rem = 1 Sv.
Quality factor
A quality factor (Q) is used to compare the biological damage
producing potential of various types of radiation, given equal
absorbed doses. This factor is used to convert units of
absorbed dose (Grays) to units of absorbed dose equivalents
(Sieverts). The quality factors for frequently encountered
types of radiation are:
Radiation Q
Gammas and x-rays 1
Beta particles & electrons 1
Alpha particles & fission fragments 20
Neutrons 10
Measures of Radioactivity
• Activity: The quantity of radioactive
material present at a given time:
– Curie (Ci) : 3.7x1010
disintegration per
second (dps)
– milliCurie (mCi): 3.7x107 dps
– microCurie (mCi): 3.7x104 dps
– picoCuries (pCi): .037 dps
– Becquerel (Bq): 1 dps
– megaBecquerel (MBq): 1x106
dps
Radiation Units
• Roentgen: A unit for measuring the
amount of gamma or X rays in air
• Rad: A unit for measuring absorbed
energy from radiation
• Rem: A unit for measuring biological
damage from radiation
Contamination vs Radiation
Radiation and Contamination are often
confused. Radiation is energy, while
contamination is the physical presence of
a radioactive material on something.
So, you may have contamination on your
shoe, but not radiation.
Radiation Versus Radioactive Contamination
• Radiation is particles or waves of energy emitted
from unstable atoms.
• Radioactive Contamination is radioactive material
usually in any location you do not want it.
Ionizing Electromagnetic
Radiation
Ionizing Electromagnetic Radiations do
have enough energy to remove electrons
from atoms, such as:
• X-rays
• Gamma rays
Units of Contamination
Contamination, or the presence of
radioactive material on something is
measured as count on a detector per some
time like a minute (cpm), or by the actual
decay rate (dps).
