Natural Radioactivity
• Some isotopes of elements have unstable
nuclei---these are radioactive
• All isotopes of elements with Z > 83 are
radioactive
• By releasing energy (radiation) the nuclei
can become stable
• We are exposed to radiation from many
sources, including sunlight, x-rays and
building materials
Some Common Forms of
Radiation
Three Main Types of Radiation Emitted During Decay
1.
2.
3.
4
Alpha particles ( or 2 He )
- Have two protons + two neutons
- Have greatest mass of the three
- Can travel 2-4 cm in air and 0.05 mm in tissue
- Protection: lab coat and gloves, distance
Beta particles ( or e-)
- High energy electrons
- Can travel 2-3 m in air and 4-8 cm in tissue
- Protection: lab coat, gloves, plexiglass and distance
Gamma rays ()
- High energy rays (like x-rays)
- Have no measurable mass
- Can travel 500 m in air and > 50 cm in tissue
- Protection: lead or thick concrete, distance
Alpha, Beta and Gamma Emission
Radiation and Safety
• Alpha, beta and gamma are forms of ionizing radiation
• Ionizing radiation causes electrons to be knocked out of
atoms or molecules, creating unstable ions (will discuss
ions, which are charged particles, later)
• Radioactive elements are used frequently in medical
and research applications
• Exposure to ionizing radiation should be minimized,
especially with repeated exposure
• Intensity of radiation drops off as 1/D2, where D = ratio
of new distance from source to old distance
• So, if you move twice as far from the source, you
receive 1/4 the exposure
Nuclear Equations for Radioactive Decay
• Radioactive decay occurs by the following
process:
Radioactive nucleus  New nucleus (more
stable) + Radiation
• Balance nuclear equations so that the atomic
numbers and the mass numbers add up the
same on both sides
• Examples:
Alpha emitter: 238U  234Th + 4He
Beta emitter: 14C  14N + eGamma emitter: 99mTc  99Tc + 
Producing Radioactive Isotopes
• Stable isotopes can be converted to radioactive
ones by bombardment with neutrons, protons or
alpha particles
• This process is called transmutation (changing
one element to another element)
• When the stable nucleus absorbs a high energy
particle, it becomes unstable (or radioactive)
• Example:
66Zn
+ H 
67Ga
Radiation Detection and Measurement
• Geiger counter is used to measure beta or gamma radiation
• Radiation is measured in units of activity
• One Curie (Ci) = 3.7 x 1010 disintegrations per second (often
use micro or millicuries)
• One rad (radiation absorbed dose) = amount of radiation
absorbed per 1 gram of tissue
• Rem = rad x damage factor (measure of biological damage
from radiation)
• The average exposure to radiation in the US is 0.17 rem/year
(a small, but significant, amount)
• Larger doses can cause radiation sickness
• The LD50 = 500 rem for humans (means half of those exposed
to that amount will die)
• Maximum permissible dose = 5 rem/year
Half-life of a Radioisotope
• Half-life = time needed for 1/2 of sample to
decay
• Some radioisotopes have very short half-lives
(very unstable, such as 15O = 2 min.)
• Some have very long half-lives (very stable,
238U = 4.5 x 109 years)
• A decay curve is a plot of the amount of
radioactive isotope (activity) vs. time
Medical Applications of Radioactive Isotopes
• Gamma rays are best for medical detection since they can
travel far enough through tissue to be detected
• Since they are damaging to tissues, the lowest possible dose
is used
• PET scans use positron emitters (C-11)
• Positrons are particles with the same mass electrons, but
with a positive charge (when they collide with electrons,
the mass is annihilated and gamma rays are produced)
• Ionizing radiation is most damaging to rapidly dividing
cells (bone marrow, skin)
• Since cancer cells are rapidly dividing, radiation can be
used to treat tumors, without damaging surrounding tissue
with less rapidly diving cells (adult bone, nerves, muscle)
• Beta emitters are often used in cancer treatment because of
their limited range of activity in tissue
Nuclear Fission
• Fission = splitting of nucleus into smaller nuclei
• Example:
235U
+ n  91Kr + 142Ba + 3n + 
• A lot of energy is released during fission
• A very small amount of mass is lost upon splitting, and
according to Einstein’s equation E = mc2, where c is
the speed of light (3 x 108 m/s), the energy produced
by this loss of mass is very large
• This is how we get nuclear power
• Also, since 3 neutrons are produced upon splitting, a
chain reaction occurs, accelerating the reaction
• What happens if you do this in a closed container?
Nuclear Fusion
• Fusion = combining two smaller nuclei to form one
larger one
• Example: 3H + 2H  4He + n
• Again, mass is lost and a large amount of energy is
produced (more than in fission)
• Very high temperature is required due to strong
repulsion between H nuclei, so the method is currently
impractical
• Cold fusion is the holy grail of nuclear chemistry
• Fusion occurs in the sun, providing heat and light