ELECTROMAGNETIC RADIATION

NOVEMBER 8, 1895 ROENTGEN DISCOVERED X-RAYS IN HIS

LAB IN WURZBURG GERMANY

ROENTGEN EXPERIMENTED WITH CATHODE RAYS USING THE

CROOKE’S TUBE

PHOTONS ARE ENERGY DISTURBANCES MOVING THROUGH

THE SPACE WITH THE SPEED OF LIGHT (c) in VACUUM

PHOTONS HAVE NO MASS AND NO CHARGE. THEY HAVE MAGNETIC AND

ELECTRIC FIELDS CHANGING IN SINUSOIDAL FASHION

SPEED OF LIGHT =

• 3 x 10 8 m/s

•3 x 10 5 km/s

• 300,000 km/s

• 186,400 miles/s

GAMMA vs X-RAYS

X-RAY PRODUCTION

ELECTRON CLOUD IN THE TARGET

GAMMA EMISSION

THREE WAVE PARAMETERS ARE NEEDED TO DESCRIBE

ELECTROMAGNETIC RADIATION:

VELOCITY, WAVELENGTH, AND FREQUENCY

Frequency and WavelengthThe sine wave model of electromagnetic energy describes variations in the electric and magnetic fields as the photon travels with velocity c. The important properties of this model are frequency, represented by f, and wavelength, represented by the Greek letter lambda (λ).

ELECTROMAGNETIC WAVE EQUATION

c=f x Λ

ELECTROMAGNETIC SPECTRUM

Short wavelength

High frequency High energy

Long wavelength Low frequency

Low energy

THE ENERGY OF PHOTON IS DIRECTLY PROPORTIONAL

TO ITS FREQUENCY

THE ONLY DIFFERENCE BETWEEN X-RAYS AND GAMMA RAYS IS THEIR

ORIGIN

VISIBLE LIGHT IS IDENTIFIED BY:

• WAVELENGTH

RF IS IDENTIFIED BY:

• FREQUENCY

X-RAYS ARE IDENTIFIED BY:

• ENERGY

X-RAYS BEHAVE AS THEY ARE PARTICLES

PROPERTIES OF X-RAYS• HIGHLY PENETRATING, INVISIBLE RAYS

• ELECTRICALLY NEUTRAL

• POLYENERGETIC

• LIBERATE MINUTE AMOUNTS OF HEAT ON PASSING THROUGH MATTER

• TRAVEL ORDINARILY IN STRAIGHT LINES

• TRAVEL WITH THE SPEED OF LIGHT IN VACUUM

• IONIZE GASES INDIRECTLY

• CAUSE FLUORESCENSE OF CERTAIN CRYSTALS

• CANNOT BE FOCUSED BY LENS

• AFFECT PHOTOGRAPHIC FILM

• PRODUCE CHEMICAL AND BIOLOGICAL CHANGES• PRODUCE SECONDARY AND SCATTER RADIATION

RADIATION ATTENUATION IS THE REDUCTION IN

INTENSITY RESULTING FROM SCATTERING AND

ABSORPTION

STRUCTURES THAT ABSORB X-RAYS ARE CALLED

• RADIOPAQUE ????

STRUCTURES THAT ATTENUATE X-RAYS ARE

CALLEDRADIOLUCENT ??

In physics, an inverse-square law is any physical law stating that a specified physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity.

•The intensity of ELECTROMAGNETIC RADIATION from a point source (energy per unit of area perpendicular to the source) is inversely proportional to the square of the distance from the source; so an object (of the same size) twice as far away, receives only one-quarter the energy (in the same time period).

The picture above demonstrates the typical x-ray tube used to produce a point source of x-rays. Then as radiation exits the tube it diverges to cover an increasingly larger area as the distance from the source increases. Notice that area "A" is smaller and the radiation is more concentrated than in an equal area "A1" which is some distance from "A." Each square A1 is the same size as "A" but only 1/4 the number of photons occupies it because of the divergence of the radiation with increasing distance.

In spite of the advances in radiation protection, such as collimators, cones, and positive beam limiting devices, distance is still the best tool for radiation protection and remains the most common method of protecting personnel, visitors, and adjacent patients from ionizing radiation use. But few persons in the health care environment understand why distance effectively protects them and therefore continuously question, “At what distance am I considered safe? The answer lies in understanding the relationship of ones distance from a source to exposure intensity. The type(s) of radiation one is exposed to as well as its energy content are also factors that affect personal dose. A safe distance can be accurately estimated from the vector of radiation exposure and its initial intensity using the inverse square law. The radiographer should note that this law applies only to a point source of radiation such as the primary beam. Additionally, the inverse square law applies only to electromagnetic radiation (x-rays and gamma rays), and does not apply to particulate ionizing radiation, or scatter radiation which is the major type of occupational radiation exposure personnel should encounter.

Animationshttp://highered.mcgraw-hill.com/sites/dl/free/007299181x/59233/6_2b.htm

Virtual Lab Link

http://astro.unl.edu/classaction/animations/stellarprops/lightdetector.html

Demonstrates the inverse square law of light with a lightbulb and detector. The lightbulb's intensity and the detector's distance can be adjusted to see how they affect the reading. There are two bulbs and detectors to allow side-by-side comparisons