Target Interactions

3 things needed to produce x-rays

source of electrons, acceleration of those electrons, stopping of electrons

xray photons are produced when

the high speed electrons from the cathode strike an anode target

the electrons that form the thermionic cloud around the filament strike the anode target traveling at

nearly half the speed of light

when incident electrons strike the target they

transfer their tremendous kinetic energy to the atoms of the target material

the greater the number/speed of the incident electrons, the greater the

quality and quantity of photons produced

target interactions all occur within

0.25-0.5 mm of the surface of the target

after 1000+ interactions

when will the incident electron have finally slowed down enough to be conducted through the anode and remainder of high volt circuit

what percent of KE is converted to heat during xray production

99%

how much of target interactions result in xray photon being produced

1%

tube loading

refers to the heat created within the xray tube, limits the heat that can be put into the xray tube before damaging it and thus limiting the power in each acquisition

why has so much gone into the development of the thermal aspects of the tube

tube loading

incident electrons transfer just enough energy to excite the outer shell electrons of the target atoms to where they will emit

infrared radiation (heat)

two types of interactions in diagnostic range

bremsstrahlung and characteristic

the type of interaction that occurs depends on the

kinetic energy of the incident electron and the binding energy of the electron shells of the target

Brehsstrahlung interactions

german word for braking, most of the xray beam produced, no real interaction just the nucleus slowing the electron

a bremms incident electron interacts with the

force field of the nucleus

during brems the energy lost during the braking is

emitted as the xray photon

brems photons energies are the

difference between the entering and exiting kinetic energy of the electron

in brems the amount of kinetic energy lost is determined by the

distance the electron is from the nucleus

in Brems the closer the electron can get

he more energy it will lose and the more energy the Brem photon will have

brem photon energies are

individually unpredictable and poly-energetic

during Brems if an incident electron comes in and gets very close to the nucleus, how much energy will the photon have

max number/almost all

energy of Brem photon is dependent on

energy of incident electron and proximity of the incident electron to the nucleus of the atom

the maximum amount of photon energy (keV) is equal or less than the energy of the

tube potential (kVp)

during brems only when the incident electron loses all of its excess kinetic energy would the electron

drift away and join the current flow

characteristic interactions occur only when

the incident electron ejects an inner shell electron from orbit causing a vacany/hole

K shell binding energy of tungsten

69.5 keV

due to 69.5 keV of tungsten K shell, characterstic interactions only occur when kVp is

70 and above

characteristic interactions

inner shell electron ejected, atom ionized, incident electron continues in different path, inner shell has vacancy

characteristic xray photons are produced from the

electrons dropping down into a new shell

A characteristic interaction X-ray photon produced has an energy that is the

difference between the binding energy of the outer and inner shells between which the electron dropped

characteristic x-rays have very predictable energies since they can be

calculated from the binding energies

tungsten, with a high atomic number (74) has sufficiency electron shells to

produce relatively high energy characteristic photons

tungsten K shell binding energy

69.5 keV

tungsten L shell binding energy

12.1 keV

characteristics photon energy for tungsten

K - L → 57.4 keV

only electron drops into the K-shell will produce

characteristic photons within diagnostic x-ray range

the further out the dropping electron’s original position the

greater the energy imparted to the characteristic photon

as a result of characteristics and bremsstrahlung radiation generationa

a spectrum of x-ray energy is produced w/i the xray beam

between 80-100 kVp how much of the primary beam is produced by Brem IX

80-90%

between 80-100 kVp how much of the primary beam is produced by characteristic IX

10-20%

Characteristic photons will not comprise any of the useful beam until the kVp is above 70 because removal of a K-shell electron from tungsten
requires

69.5 keV

combine to form the complete primary beam spectrum

Brems and characteristic

these photons do not have suffiecient energy and are absorbed by filtration

L shell characteristic emissions

these emissions form a characteristic peak

K shell emissions

the spectrum can be manipulated by changing the

tube current (mAs) or voltage settings (kVp)

how can radiographers apply different spectra of xray beam to different body parts

changing mAs and kVp

The average primary beam photon has a keV energy of only about

30-40% of the kVp

changing mAs changes

amplitude (quantity)

change in kVp changes

amplitude and average energy (quantity and quality)

Which of the following statements is true regarding x-ray production?

Almost all of the kinetic energy of the incident electrons is converted to heat.

Which of the following occurs when the kinetic energy of the incident electrons is transferred to the outer-shell electrons during x-ray production?

The outer-shell electrons vibrate, resulting in the emission of infrared radiation.

Which factor determines the types of target interactions that occur in the production of x-ray photons in the diagnostic range?

The energy of the incident electrons

Which of the following best describes the energy of a
bremsstrahlung photon?

The energy of the bremsstrahlung photon is the difference between the entering and exiting kinetic energies of the incoming electron.

Which of the following occurs at the end of the characteristic cascade?

The outermost shell is missing an electron.

Which interaction produces x-ray photons only if the kVp is 70 or above?

Characteristic