Transcript ELECTROMAGNETIC RADIATION

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 Wavelength
The 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
CALLED
RADIOLUCENT ??
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 onequarter 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.
Animations
http://highered.mcgrawhill.com/sites/dl/free/007299181x/59233/6_2b.htm
Virtual Lab Link
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
http://astro.unl.edu/classaction/animations/stellarprops/l
ightdetector.html