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Radioactivity

Radioactivity

Atomic Physics – the

study of the clouds of

electrons that make up the

atom.

But now we need to delve

deeper…

… into the atomic nucleus.

Radioactivity

Wilhelm Roentgen

1895

Radioactivity

Radioactivity

Antoine Henri Becquerel

1896

Radioactivity

Antoine Henri Becquerel

Radioactivity

◼ Radioactivity is the process of the atomic

nucleus emitting energetic subatomic

particles.

◼ This process is nothing new – it has been

around far longer than the human race!

Radioactivity

Three Distinct Types of Radiation

◼ Alpha - α

◼ Beta - β

◼ Gamma - Ɣ

Radioactivity

Environmental Radiation

Units of Radiation

◼ Radiation dosage is commonly measured

in rads.

◼ 1 rad = 0.01 joule of radiant energy absorbed

per kg of tissue.

◼ The measure for radiation dosage based

on potential damage is the rem (roentgen

equivalent man)

Units of Radiation

◼ A lethal dose of radiation for a human is

about 500 rems. You have about a 50%

chance of surviving a dose of this

magnitude delivered over a short time.

◼ Average person in U.S. is exposed to 360

millirem each year (a millirem is 1/1000th

of a rem)

The Strong Force

The Strong Force

The nucleus is made up of nucleons, the

collective name for protons and neutrons.

Quarks

Each nucleon is made up of three fundamental

particles called, quarks.

The Strong Force

Just like there are energy levels for the orbital

electrons, there are energy levels within the

nucleus. Changes of energy states in radioactive

nuclei result in the emission of gamma-ray photons.

Question: If like charges

repel, what holds all those

positively charged protons

close together in the

nucleus?!

The Strong Force

Comparison of the strengths of the

STRONG force and the ELECTRIC force

over distance.

The Strong Force

Since the strong force is only “strong” over extremely short

distances, a large nucleus is not as stable as a small one.

The Strong Force

In the Helium-4 nucleus, all four nucleons attract by the

strong force. Only two (the protons) feel the repulsive

electric force.

The Strong Force

Neutrons act as

“Nuclear cement”,

adding to the attractive

force ONLY.

Elements toward the

end of the periodic

table have many more

neutrons per proton

than the lighter ones!

The Strong Force

Lithium-6

Uranium-238

U-238 has about 1.5 times as many neutrons per proton

as Li-6. Neutrons add to nucleus stability!

The Strong Force

A neutron all by itself, however, is radioactive! It

spontaneously transforms to a proton and an electron!

The Strong Force

Summing Up So Far

1.Which two fundamental forces have visible

effects within the nucleus of an atom?

2.Which one acts over greater distance?

3.Which force acts on ALL nucleons?

4.Which force acts only on protons?

Summing Up So Far

1. In the nucleus of ordinary hydrogen,

does the strong force play a role?

2. How about the electric force?

Radioactive Half-Life

◼ The radioactive decay rate of an element

is measured in terms of a characteristic

time called the half-life.





◼ This is the time it takes for half of an original

quantity of a radioactive isotope to decay.





◼ This is the time it takes for half of an original

quantity of a radioactive isotope to decay.

The half-life of Uranium-238 is 4.5 billion years!


Radium-226 has a half-life of 1620 years.


Check Question:

If a sample of a radioactive isotope has a half-

life of 1 day, how much of the original sample

will be left at the end of the second day?

Radiation Detectors

A Geiger counter detects

incoming radiation by a

short pulse of current

triggered when radiation

ionizes a gas in the tube.

A scintillation counter

detects incoming radiation

by flashes of light

produced by particles or

gamma rays passing

through the counter.

Review of Elements

Transmutation

Transmutation

The changing of one chemical element into another is

called atomic transmutation.

Alpha Decay

Beta Decay

Gamma Decay

Electron Capture (EC)

Positron Emission

Five types of Decay

◼ Alpha Decay

◼ Beta Decay

◼ Gamma Decay

◼ Electron Capture

◼ Positron Emission

Transmutation

Radioactive elements can decay backward or

forward in the periodic table.

Transmutation

What nucleus results when Krypton-81

undergoes electron capture?

What nucleus results when Silicon-27

undergoes positron emission?

Carbon Dating

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