PHYSICS CHAPTER 3 THE PRODUCTION OF X-RAYS

Describe elastic collision

- after an elastic collision, the incoming electron comes to rest and the energy from the incoming electron is transferred to the electron that was originally at rest, and assumes the first electrons velocity -this assumes that no energy is lost in collision

Filters for orthovoltage beams are made of

- copper (Cu) - aluminum (Al) - or Thoraeus ( tin copper and aluminum)

The two particles which result from ionization are an electron with a ____ charge and a ____ charged atom.

- e⁻, a negative ion - positively charged atom, lacking one electron with a plus one charge

Describe inelastic collision

-after an inelastic collision only a portion of the energy of the incoming electron (V1) is transferred to the electron at rest. The electron at rest takes off with a velocity of V2 which is always less than the velocity of the incoming electron, v1. The incoming electrons velocity is reduced to "v1'" - aka hitting a care ahead with breaks on, the loss of energy in the second car's breaks prevents it from picking up the kinetic energy and momentum. In this case the kinetic energy is dispersed as heat energy (in break pads)

What are the minimum requirements for x-ray production?

-cathode -source of electron -anode -vacuum -high volatage DC source.

What are the two broad classes of electron interaction?

-collision interaction with subsequent characteristic radiation production -radiative interaction which produce bremsstrahlung radiation

Describe the reflection target of a diagnostic x-ray tube

-electrons hit the target and x-rays are emitted in all directions, but the target absorbs many of them -x-rays are perpendicular to electron beam

What forms of energy are involved in the production of an x-ray beam?

-multiple sources -electric -heat -radiation

How is the design of a diagnostic x-ray tube target different from that for a 4 MV x-ray therapy tube? Why?

-reflective target -therapy target is transmission target, usually thicker, and absorbs electrons, -diagnostic is thinner, and to not necessarily absorb electrons, but produce both brems, and characteristic

Why do we use reflection or reflectance target in diagnostic energy x-ray beams?

-so that x-rays come off the target unimpeded in directions useful for imaging

What are the 6 basic properties of of x-rays?

-unaffected by gravity (no mass) -unaffected by electric field -unaffected by magnetic fields -travel in a straight line -exponentially attenuated by matter -cannot by focused

What are the use of filters in megavoltage beams?

-use transmission target that serves to remove low energy photons. -filters used to adjust intensity distribution of the beam and alter the spectral characteristics of the beam.

What is the maximum energy of a photon in bremsstrahlung production if the incoming electron has a kinetic energy of 4 MeV?

4MeV

Define efficiency regarding diagnostic and therapeutic energies

@diagnostic energies efficiency is 1%, 99% is heat @theraputic energies efficiency is 50%

What is Z and why is it important in x-ray production?

Atomic number; the number of protons in the atoms nucleus. It controls electron shell spacings and therefore electron and photon interactions with the material's electrons as wel as the nuclear charge effecting bremsstrahlung

Describe Bremsstrahlung radiation.

Braking radiation. Radiation produced as a result of a nucleus's electric field exerts force on the electron and causes it to change its direction or velocity.

When an electron interacts not with another electron but with the positive charge of an atomic nucleus, it is called

Bremsstrahlung

What is often combined to transport and dissipate heat in conventional radiation therapy tubes?

Oil and air cooling systems

What are the characteristic x-ray a characteristic of?

The dropping of outer electron into inner electron shells

What causes characteristic x-ray production?

The electron shell energy spacings of the target material

What can be said about bremsstrahlung production and the relationship with the shells on the target atom?

The farther away from the nucleus, the weaker the interaction, and therefore less energy is given to the photon. -more low energy x-rays are produced than high-energy x-rays.

When an outer orbital electron enters a shell to fill a vacancy, if the potential energy is small or from a low Z atom, what is emitted?

The potential energy difference will emit light photon.

Starting from the side facing the target, list in order the materials in the Thoraeus filter.

Tin, highest Z closest copper, absorbs lower energy aluminum -they filter each other

Low kV's are good for soft tissue. True or false?

True

When using a thick transmission target, what is happening to the energy?

When the target is thick enough to stop all of the electrons.

who discovered x-rays

Wilhelm Roentgen in 1895 while experimenting with cathode rays after activation with high voltage. -useful interaction of high energy electrons with matter.

Discrete spectrum are shifted to higher energies when ____ is increased

Z

A hypothetical uniform spectrum can be described as

a a theoretical continuous spectrum where the same number of photons are present at every energy.

4. For a hypothetical atom, if an electron vacancy in the K shell (24keV) is filled by an electron transition from the L shell (16keV), a resulting Auger electron emitted from the M shell (7keV) would have a kinetic energy of: a. 1 keV b. 7 keV c. 16 keV d. 24 keV e. 31 keV

a. 1 keV

In superficial x-ray therapy, what filters can be used to increase the average energy of photons?

aluminum and occasionally copper

The production of characteristic x-rays competes directly with the production of : a. bremstrahlung x-rays b. Auger electrons c. gamma rays d. photoluminescence e. fission fragments

b auger electrons ( when the energy of the removal of an electron of a shell)

3. The effective energy on an x-ray beam can be increased by: a. increasing the tube current b. increasing the filtration c. increasing the mAs d. decreasing the tube voltage e. none of the above

b increase filtration

Why is it called a thick target?

because of all the incoming electrons are stopped in the target.

When x-rays are produced, how are they emitted?

both characteristic radiation from target atoms and bremsstrahlung x-rays

_____ is the source of most x-ray production

bremsstrahlung

Continuous spectrum is more intense for higher Z because

bremsstrahlung production is more efficient for higher Z material.

8. A conventional x-ray tube is operated at 100kVp. Which of the following is true? a. The minimum x-ray energy is 100keV b. The average x-ray energy is 100keV c. The maximum x-ray energy is 100keV d. Every x-ray has energy of 100keV

c maximum x-ray i energy is 100keV

1) An atom has electrons with binding energies of 80, 45, 20, and 5 keV. Possible characteristic x-ray energies following the ionization of one of these electrons by an x-ray include: a. 80, 45, 20, and 5 keV b. 75, 60, 40, 35, 25 and 15 keV c. 35, 25, and 15 keV d. 125, 65, and 25 keV

c. 35, 25, and 15 keV

9. Which one of the statements below is true for the characteristic x-rays emitted from a tungsten target when 100 kev electrons are fired at it: a. Have a continuous spectrum of energies up to 100keV b. Are about equal in intensity to the bremsstrahlung c. Have energies equal to differences in the electron binding energies of tungsten d. Do not contribute to the imaging process. e. Have a non-isotropic distribution.

c. have energies equal to differences in the electron binding energies of tungsten

Describe the change in velocity in bremsstrahlung radiation

causes loss in the kinetic energy of the intruding electron and by law of conservation of energy, the energy lost by the electron goes into an x-ray photon.

In megavoltage radiation therapy, ______ photons are relatively low energy and are therefore of no practical importance.

characteristic

Bremsstrahlung radiation produces what kind of spectrum?

continuous spectrum

Describe superficial x-rays

deposit 5mm from the surface -low energy of 5-15 keV -Grenz rays

5. Tungsten has electron shell binding energies of K=69keV, L=12keV, M=2keV. A 60 keV electron striking a tungsten target could produce both bremsstrahlung and characteristic x-rays. Which of the following statements is true regarding x-ray energies from this interaction? a. maximum energy of both brem. and characteristic x-rays is 69 keV b. maximum energy of both brem. and characteristic x-rays is 60 keV c. maximum energy of both brem. and characteristic x-rays is 57 keV d. minimum energy of both brem. and characteristic x-rays is 2 keV e. none of the above

e- none of the above

7. An element contains electrons in the 10, 65, 70, and 72 keV energy levels. Which energy cannot occur in its characteristic x-ray spectrum? a. 5 b. 7 c. 55 d. 60 e. 65

e. 65

The collisions of electrons can be ___, in which momentum and kinetic energy are conserved, or ______, which momentum is conserved but kinetic energy is not conserved.

elastic and inelastic

Energy coming in to the target of an x-ray tube, it is in the form of _________, and the energy coming off is ______ of the newly created photons.

electron kinetic energy; electromagnetic energy

x-rays are produced by the interaction of ____ with ___

electrons with matter

In bremsstrahlung production what is the maximum energy of the x-rays?

equal to the kinetic energy of the incoming electrons. can not be higher.

An electron in a lower shell of an atom can be _____ to a higher shell level in an atom, one that is more loosely bound to the nucleus and has a higher potential energy. This process is called

excitation.

Which equation gives an estimate of the fraction of the electron energy converted to photons?

f= 3.5×10⁻⁴ZE where f is the fraction of the incident electron energy converted to photons, Z is the atomic number of the target. E is the maximum energy of the electron in MeV.

Define added filtration

filter deliberately placed in beam to reduced patient dose during diagnostic radiographs

Besides the very low-energy x-rays being absorbed by the glass envelope and target, what else can be used?

filters.

Where is most of the very low-energy x-rays absorbed?

glass envelope and in the target itself.

Targets of x-rays are usually made of ___

high atomic number materials

As the z value of the target material goes up, the efficiency ____

increases

Filtration reduces _____ and _____ but usually raises ___ by preferentially removing low-energy x-rays.

intensity and dose; average energy

An x-ray beam can be characterized by its ____ and _____

intensity and energy distribution

Sometimes enough energy is given to the electron to remove it totally from the atom. This is called ____.

ionization

Describe tube voltage

kVp -effects both the number of x-ray photons emerging (stronger than mA) and it affects the energy distribution. -the higher the average energy o the beam

Describe the difference between high energy and low energy beams interacting with target and the direction of their photons

low energies- x-rays are produced and emitted equally in all directions higher energies (diagnostic x-ray tubes)- most will go at right angles with some foward megavoltage energies (linac machines) - more forward with less angular spread ( divergence cone)

Describe tube current

mA the number of x-ray photons emitted is directly proportional to the tube current. -tube current has no effect on the energy distribution of the x-rays, only intensity.

If tumors are deep by a few cenitmeters, higher energies in the orthovoltage range ( 150-400 kVp) have been used, and what else is done to the beam?

more filtration is used to selectively remove low energy components of the beam

Electrons may interact with many different types of particles but electrons interactions are usually with ____.

other electrons

In a transmission target, what is the direction of the the energy emitted?

photon energy comes off the target in the same direction as the electrons enter

A theoretical line spectrum graph can be described as

produced by characteristic photon - characteristic radiation from exited or ionized atoms produces a series of photons with discrete energies, thus producing multiple lines in the spectrum.

After the energy produces energy when hitting a target, where is the remainder energy ?

producing heat in anode.

superficial and orthovoltage tubes use what kind of target?

reflection

For diagnostic energies, what is usually used as a target?

reflection or reflectance target

The exact energy of the emitted photon from orbital electron filling vacant shells is dependant on _____. This process is called

the characteristic of the atoms that initially lost the electron. Characteristic radiation, and the x-rays produced are called characteristic x-rays.

The direction of x-ray production depends on what?

the energy of the incoming electrons

The _____ is the best way to describe the various energies of the photons produced.

the energy spectrum

When an outer orbital electron enter a shell to fill a vacancy, if the potential energy is large, or high Z , what is emitted?

the potential energy difference is emitting as a photon.

The closer the electron comes to the nucleus in bremsstrahlung production, describe the effects of the production.

the stronger the electrostatic attraction and therefore the more the change of the electrons velocity (breaking). the magnitude of this attraction yields the energy of the x-ray photon.

Why are filters used in medical x-ray beams?

to change their spectral characteristics -or to deposit maximum energy in the diseased tissues.

Describe Thoraeus filter

uses tin copper and aluminum -TIn with the highest Z is nearest to the x-ray tube but transparent to photons below 29 keV, and produces characteristic x-rays -Copper absorbs energies produced or missed by the tin. also transparent to photons below 9 keV and produce characteristic x-rays -aluminum , closest to patient, will absorb the characteristic x-rays from copper.

Describe energy spectrum

various energies of the photons produced -graph displaying number of photons in small energy intervals vs. photon energy.

Describe collision interaction

when electrons interact with electrons in the target and transfer momentum and energy like balls on a pool table

Define hardening the beam

when filters are used on softer less penetrating x-rays are selectively removed to increase the average energy of photons.

Define self filtration

x-rays absorbed in escaping from the target material and x-ray tube.