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. Alpha particles ( or )- 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

2. 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

3. 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

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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 + 4HeBeta emitter: 14C 14N + e-

Gamma 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