Rad. Physics

What is radioactivity?

The process in which certain unstable atoms or elements undergo spontaneous disintegration while trying to reach a more balanced nuclear state.

What happens during radioactivity?

Unstable atomic nuclei disintegrate and give off energy in the form of particles or rays.

What is radiation?

The emission and propagation of energy through space or a substance in the form of waves or particles.

Are radiation and radioactivity the same thing?

No. Radiation is energy being emitted or transmitted, while radioactivity is the spontaneous disintegration of unstable atoms.

What type of radiation is used in dentistry?

X-radiation.

What is ionization?

The process of converting an atom into ions.

What is an ion?

An atom that gains or loses an electron and becomes electrically unbalanced.

What happens if an atom loses an electron?

It becomes electrically unbalanced and forms an ion.

What particles are involved in ionization?

Electrons.

What must happen for ionization to occur?

enough energy to overcome the electrostatic force binding an electron to the nucleus

How can x-rays cause ionization?

X-ray photons interact with the materials they penetrate and can remove electrons from atoms.

What is the nucleus of an atom?

core of the atom with protons and neutrons

What charge does a proton have?

Positive.

What charge does a neutron have?

No electrical charge.

What charge does an electron have?

Negative.

What determines the atomic number?

number of protons in the nucleus

What holds electrons in their orbits?

Electrostatic attraction, also called binding energy or binding force.

Where is electron binding energy greatest?

In the shell closest to the nucleus.

What are the two types of ionizing radiation?

Particulate radiation and electromagnetic radiation.

What is particulate radiation?

Tiny particles traveling in a straight line at high speeds that transmit kinetic energy.

What is electromagnetic radiation?

Wavelike energy without mass that travels through space or matter and creates electric and magnetic energy.

What are photons?

Bundles or clusters of energy with no mass or weight.

How fast do photons travel?

At the speed of light.

What are the three characteristics of a wave?

Velocity, wavelength, and frequency.

What is velocity?

The speed of a wave.

What is wavelength?

The distance between the crest of one wave and the crest of the next wave.

What is frequency?

The number of wavelengths that pass a certain point in a given amount of time.

What is the relationship between wavelength and frequency?

inversely related

What happens to frequency when wavelength increases?

Frequency decreases.

What type of radiation has a long wavelength?

Low-frequency, low-energy radiation.

What type of radiation has a short wavelength?

High-frequency, high-energy radiation.

What are x-rays?

High-energy, short-wavelength, high-frequency ionizing electromagnetic radiation.

What allows an x-ray to penetrate matter?

Its wavelength and frequency.

What are the three main parts of the dental x-ray machine?

Control panel, extension arm, and tubehead.

What is the tubehead?

metal housing containing the x-ray tube

What does the metal housing of the tubehead do?

Prevents x-rays from escaping and contains the x-ray tube and transformers.

What does insulating oil do?

Surrounds the x-ray tube and prevents overheating.

Where are dental x-rays actually produced?

Inside the x-ray tube.

What do transformers do?

Increase or decrease voltage.

What do aluminum disks do?

Filter out nonpenetrating, longer-wavelength x-rays.

What does the lead collimator do?

Restricts the size of the x-ray beam.

What does the position-indicating device (PID) do?

Aims and shapes the x-ray beam.

What are the two electrodes inside the x-ray tube?

The cathode (-) and anode (+).

What is the cathode?

The negative electrode that supplies the electrons needed to generate x-rays.

What does the tungsten filament do?

Produces electrons when heated.

What does the molybdenum cup do?

Focuses electrons into a narrow beam.

What does amperage represent?

The number of electrons.

What unit measures electron current inside the tubehead?

Milliamperes (mA).

What does mA control?

The number of electrons in the electron stream.

What is the anode?

The positive electrode that converts electrons into x-ray photons.

What is the tungsten target?

The focal spot on the anode that converts electrons into x-ray photons.

What does the copper stem do?

Dissipates heat from the tungsten target.

What is voltage?

The force required to propel electrons from the cathode to the anode.

What unit is used to measure voltage?

Kilovolts (kV).

What does kV control?

The force used to propel electrons from the cathode to the anode.

What are the two electrical circuits in the tubehead?

The low-voltage filament circuit and the high-voltage circuit.

What controls the low-voltage filament circuit?

The mA setting.

What does the high-voltage circuit do?

Provides the power needed to propel electrons to the tungsten target and create x-rays.

What controls the high-voltage circuit?

kV setting

What does the step-down transformer do?

Decreases incoming 110- or 220-line voltage to 3 to 5 volts for the filament circuit.

What does the step-up transformer do?

Increases incoming voltage to 65,000 to 100,000 volts for the high-voltage circuit.

What does the autotransformer do?

Corrects minor fluctuations in current.

What is thermionic emission?

release of electrons from heated tungsten filament

What happens to electrons after thermionic emission?

They have low energy and form a cloud around the filament.

What happens when the exposure button is pushed?

The high-voltage circuit is activated and the electrons are propelled toward the tungsten target.

What happens when the electrons strike the tungsten target?

Their energy is converted into heat and x-rays.

How much of the electron energy is converted into x-rays?

Less than 1%.

How much of the electron energy becomes heat?

About 99%.

How do x-rays leave the tubehead?

They pass through the unleaded glass window, tubehead seal, aluminum disks, lead collimator, and exit through the PID.

What are the possible interactions of x-rays with matter?

No interaction, absorption, or scatter.

What is absorption?

The x-ray photon transfers its energy to matter and is absorbed.

What is the photoelectric effect?

An x-ray photon gives all of its energy to an inner-shell electron, ejecting the electron, and the photon is completely absorbed.

Does the original x-ray photon remain after a photoelectric interaction?

No. It is completely absorbed.

Why is the photoelectric effect important to radiographic images?

It contributes to differences in absorption that help form the image.

What is scatter radiation?

Radiation that changes direction after interacting with matter.

What are the two types of scatter interactions?

Coherent scatter and Compton scatter.

What is coherent scatter?

A low-energy x-ray photon interacts with an atom and changes direction without causing ionization.

Does coherent scatter cause ionization?

No.

What happens to the x-ray photon during coherent scatter?

It changes direction with essentially the same energy.

What is Compton scatter?

An x-ray photon interacts with an outer-shell electron, ejects the electron, and continues in a different direction with less energy.

Does Compton scatter cause ionization?

Yes.

What happens to the photon during Compton scatter?

It loses some energy and changes direction.

Which interaction completely absorbs the x-ray photon?

The photoelectric effect.

Which scatter interaction does NOT cause ionization?

Coherent scatter.

Which scatter interaction ejects an outer-shell electron?

Compton scatter.

What are the two types of x-rays produced at the tungsten target?

Bremsstrahlung radiation and characteristic radiation.

What is Bremsstrahlung radiation?

X-rays produced when high-speed electrons are slowed or stopped by the nucleus of a tungsten atom.

What does Bremsstrahlung mean?

Braking radiation.

What happens to an electron during Bremsstrahlung x-ray production?

It is slowed or deflected and loses energy that is emitted as an x-ray photon.

What is characteristic radiation?

X-rays produced when a high-speed electron ejects an inner-shell electron from a tungsten atom and an outer-shell electron drops down to fill the vacancy.

Why is it called characteristic radiation?

energy of the resulting x-ray for the tungsten atom

What type of energy is used to produce x-rays?

Kinetic energy from rapidly moving electrons is converted into x-ray energy and heat when the electrons strike the tungsten target.

Why is dental anatomy important when interpreting radiographs?

You must recognize normal oral structures and anatomy so they can be distinguished from abnormalities on the radiographic image.