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Nature of Penetrating Radiation

The Electromagnetic Spectrum

X-rays and gamma rays differ only in their source of origin. X-rays are producedby an x-ray generator and gamma radiation is the product of radioactive atoms.

They are both part of theelectromagnetic spectrum. Theyare waveforms, as arelight rays, microwaves, and radio waves. X-rays and gamma rays cannot been

seen, felt, or heard. They possess no charge and no mass and, therefore, are notinfluenced by electrical and magnetic fields and will generally travel in straightlines. However, they can be diffracted (bent) in a manner similar to light.

Both X-rays and gamma rays can be characterized by frequency, wavelength, andvelocity. However, they act somewhat like a particle at times in that they occur as

small "packets" of energy and are referred to as "photons." Due to their short

wavelength they have more energy to pass through matter than do the other formsof energy in the electromagnetic spectrum. As they pass through matter, they are

scattered and absorbed and the degree of penetration depends on the kind of matter

and the energy of the rays.

Properties of X-Rays and Gamma Rays

They are not detected by human senses (cannot be seen, heard, felt, etc.). They travel in straight lines at the speed of light. Their paths cannot be changed by electrical or magnetic fields. They can be diffracted to a small degree at interfaces between two different

materials. They pass through matter until they have a chance encounter with an atomic

particle.

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Their degree of penetration depends on their energy and the matter they aretraveling through.

They have enough energy to ionize matter and can damage or destroy livingcells.

X-Radiation

X-rays are just like any other kind of electromagnetic radiation. They can be produced inparcels of energy called photons, just like light. There are two different atomic processes that

can produce X-ray photons. One is called Bremsstrahlung and is a German term meaning

"braking radiation." The other is called K-shell emission. They can both occur in the heavy

atoms of tungsten. Tungsten is often the material chosen for the target or anode of the x-ray

tube.

Both ways of making X-rays involve a change in the state of electrons. However,

Bremsstrahlung is easier to understand using the classical idea that radiation is emitted when

the velocity of the electron shot at the tungsten changes. The negatively charged electron

slows down after swinging around the nucleus of a positively charged tungsten atom. This

energy loss produces X-radiation. Electrons are scattered elastically and inelastically by the

positively charged nucleus. The inelastically scattered electron loses energy, which appears

as Bremsstrahlung. Elastically scattered electrons (which include backscattered electrons) are

generally scattered through larger angles. In the interaction, many photons of different

wavelengths are produced, but none of the photons have more energy than the electron had to

begin with. After emitting the spectrum of X-ray radiation, the original electron is slowed

down or stopped.

Bremsstrahlung RadiationX-ray tubes produce x-ray photons by

accelerating a stream of electrons to energiesof several hundred kilovolts with velocities of

several hundred kilometers per hour and

colliding them into a heavy target material.

The abrupt acceleration of the charged

particles (electrons) produces Bremsstrahlung

photons. X-ray radiation with a continuous

spectrum of energies is produced with a range

from a few keV to a maximum of the energy

of the electron beam. Target materials for

industrial tubes are typically tungsten, which

means that the wave functions of the boundtungsten electrons are required. The inherent

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filtration of an X-ray tube must be computed, which is controlled by the amount that the

electron penetrates into the surface of the target and by the type of vacuum window present.

The bremsstrahlung photons generated within the target material are attenuated as they pass

through typically 50 microns of target material. The beam is further attenuated by the

aluminum or beryllium vacuum window. The results are an elimination of the low energyphotons, 1 keV through l5 keV, and a significant reduction in the portion of the spectrum

from 15 keV through 50 keV. The spectrum from an x-ray tube is further modified by the

filtration caused by the selection of filters used in the setup.

The applet below allows the user to visualize an electron accelerating and interacting with a

heavy target material. The graph keeps a record of the bremsstrahlung photons numbers as a

function of energy. After a few events, the "building up" of the graph may be accomplished

by pressing the "automate" button.

K-shell Emission RadiationRemember that atoms have their electrons arranged in

closed "shells" of different energies. The K-shell is the

lowest energy state of an atom. An incoming electron

can give a K-shell electron enough energy to knock itout of its energy state. About 0.1% of the electrons

produce K-shell vacancies; most produce heat. Then, a

tungsten electron of higher energy (from an outer shell)

can fall into the K-shell. The energy lost by the falling

electron shows up in an emitted x-ray photon.

Meanwhile, higher energy electrons fall into the

vacated energy state in the outer shell, and so on. K-

shell emission produces higher-intensity x-rays than

Bremsstrahlung, and the x-ray photon comes out at a single wavelength.

When outer-shell electrons drop into inner shells, they emit a quantizedphoton

"characteristic" of the element. The energies of the characteristic X-rays produced are only

very weakly dependent on the chemical structure in which the atom is bound, indicating that

the non-bonding shells of atoms are the X-ray source. The resulting characteristic spectrum is

superimposed on the continuum as shown in the graphs below. An atom remains ionized for a

very short time (about 10

-14

second) and thus an atom can be repeatedly ionized by theincident electrons which arrive about every 10-12second.

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Gamma Radiation

Gamma radiation is one of the three types of natural radioactivity. Gamma rays are

electromagnetic radiation, like X-rays. The other two types of natural radioactivityare alpha and beta radiation, which are in the form of particles. Gamma rays arethe most energetic form of electromagnetic radiation, with a very short wavelength

of less than one-tenth of a nanometer.

Gamma radiation is the product of radioactive atoms. Depending upon the ratio ofneutrons to protons within its nucleus, an isotope of a particular element may be

stable or unstable. When the binding energy is not strong enough to hold the

nucleus of an atom together, the atom is said to be unstable. Atoms with unstablenuclei are constantly changing as a result of the imbalance of energy within the

nucleus. Over time, the nuclei of unstable isotopes spontaneously disintegrate, ortransform, in a process known as radioactive decay. Various types of penetrating

radiation may be emitted from the nucleus and/or its surrounding electrons.

uclides which undergo radioactive decay are called radionuclides. Any material

which contains measurable amounts of one or more radionuclides is a radioactivematerial.

Types Radiation Produced by Radioactive Decay

When an atom undergoes radioactive decay, it emits one or more forms ofradiation with sufficient energy to ionize the atoms with which it interacts. Ionizingradiation can consist of high speed subatomic particles ejected from the nucleus or

electromagnetic radiation (gamma-rays) emitted by either the nucleus or orbital

electrons.

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Alpha Particles

Certain radionuclides of high atomic mass (Ra226, U238, Pu239) decay by theemission of alpha particles. These alpha particles are tightly bound units of two

neutrons and two protons each (He4 nucleus) and have a positive charge. Emission

of an alpha particle from the nucleus results in a decrease of two units of atomicnumber (Z) and four units of mass number (A). Alpha particles are emitted with

discrete energies characteristic of the particular transformation from which theyoriginate. All alpha particles from a particular radionuclide transformation will

have identical energies.

Beta Particles

A nucleus with an unstable ratio of neutrons to protons may decay through the

emission of a high speed electron called a beta particle. This results in a net changeof one unit of atomic number (Z). Beta particles have a negative charge and the

beta particles emitted by a specific radionuclide will range in energy from nearzero up to a maximum value, which is characteristic of the particular

transformation.

Gamma-rays

A nucleus which is in an excited state may emit one or more photons (packets ofelectromagnetic radiation) of discrete energies. The emission of gamma rays does

not alter the number of protons or neutrons in the nucleus but instead has the effectof moving the nucleus from a higher to a lower energy state (unstable to stable).

Gamma ray emission frequently follows beta decay, alpha decay, and other nuclear

decay processes.

Activity (of Radionuclides)

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The quantity which expresses the

degree of radioactivity or the radiationproducing potential of a given amount

of radioactive material is activity. Thecurie was originally defined as thatamount of any radioactive material that

disintegrates at the same rate as onegram of pure radium. The curie has

since been defined more precisely as a

quantity of radioactive material inwhich 3.7 x 1010atoms disintegrate persecond. The International System (SI)

unit for activity is the Becquerel (Bq),

which is that quantity of radioactivematerial in which one atom is

transformed per second. The radioactivity of a given amount of radioactivematerial does not depend upon the mass of material present. For example, two one-

curie sources of Cs-137 might have very different masses depending upon therelative proportion of non-radioactive atoms present in each source. Radioactivity

is expressed as the number of curies or becquerels per unit mass or volume.

The concentration of radioactivity, or the relationship between the mass of

radioactive material and the activity, is called "specific activity." Specific activity

is expressed as the number of curies or becquerels per unit mass or volume. Eachgram of Cobalt-60 will contain approximately 50 curies. Iridium-192 will contain

350 curies for every gram of material. The shorter half-life, the less amount ofmaterial that will be required to produce a given activity or curies. The higher

specific activity of Iridium results in physically smaller sources. This allows

technicians to place the source in closer proximity to the film while maintaininggeometric unsharpness requirements on the radiograph. These unsharpness

requirements may not be met if a source with a low specific activity were used at

similar source to film distances.

Isotope Decay Rate (Half-Life)

Each radionuclide decays at its own

unique rate which cannot be altered byany chemical or physical process. A

useful measure of this rate is the half-

life of the radionuclide. Half-life isdefined as the time required for the

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activity of any particular radionuclide to decrease to one-half of its initial value. In

other words one-half of the atoms have reverted to a more stable state material.

Half-lives of radionuclides range from microseconds to billions of years. Half-lifeof two widely used industrial isotopes are 74 days for Iridium-192, and 5.3 years

for Cobalt-60. More exacting calculations can be made for the half-life of thesematerials, however, these times are commonly used.

The applet below offers an interactive representation of radioactive decay series.

The four series represented are Th232, Ir192, Co60, Ga75, and C14. Use the radio

buttons to select the series that you would like to study. Note that Carbon-14 is notused in radiography, but is one of many useful radioactive isotopes used todetermine the age of fossils. If you are interested in learning more about Carbon-14

Dating, follow this link:Carbon-14 Dating.

The Sequence Info button displays a chart that depicts the path of the series withatomic numbers indicated on the vertical axis on the left, and the number ofneutrons shown along the bottom. Colored arrows represent alpha and beta decays.

To return to the main user interface, click the "Dismiss" button.

Initially, a selected series contains all parent material, and the amount isrepresented by a colored bar on a vertical logarithmic scale. Each line represents a

factor of ten. In order to step forward through the sequence by a specified numberof years, you may type the appropriate number into the "Time Step" field and hit

"Enter." A negative time step will backtrack through the sequence.

You may choose a step interval in years and progress through each step by

pressing the "Enter" key. The "Animate" button will automate the progress throughthe series. You can either choose a time step before you animate or leave it at zero.If the time step is left at zero, the system will choose time steps to optimize

viewing performance.

Ionization

As penetrating radiation moves from point to point in matter, it loses its energy

through various interactions with the atoms it encounters. The rate at which this

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energy loss occurs depends upon the type and energy of the radiation and the

density and atomic composition of the matter through which it is passing.

The various types of penetrating radiation impart their energy to matter primarily

through excitation and ionization of orbital electrons. The term "excitation" is usedto describe an interaction where electrons acquire energy from a passing charged

particle but are not removed completely from their atom. Excited electrons may

subsequently emit energy in the form of x-rays during the process of returning to a

lower energy state. The term "ionization" refers to the complete removal of an

electron from an atom following the transfer of energy from a passing chargedparticle. In describing the intensity of ionization, the term "specific ionization" isoften used. This is defined as the number of ion pairs formed per unit path length

for a given type of radiation.

Because of their double charge andrelatively slow velocity, alpha

particles have a high specific

ionization and a relatively shortrange in matter (a few centimeters in

air and only fractions of a millimeterin tissue). Beta particles have a muchlower specific ionization than alpha

particles and, generally, a greater

range. For example, the relatively

energetic beta particles from P32have a maximum range of 7 metersin air and 8 millimeters in tissue. The low energy betas from H3, on the other hand,

are stopped by only 6 millimeters of air or 6 micrometers of tissue.

Gamma-rays, x-rays, and neutrons are referred to as indirectly ionizing radiation

since, having no charge, they do not directly apply impulses to orbital electrons as

do alpha and beta particles. Electromagnetic radiation proceeds through matter

until there is a chance of interaction with a particle. If the particle is an electron, it

may receive enough energy to be ionized, whereupon it causes further ionizationby direct interactions with other electrons. As a result, indirectly ionizing radiation

(e.g. gamma, x-rays, and neutrons) can cause the liberation of directly ionizingparticles (electrons) deep inside a medium. Because these neutral radiationsundergo only chance encounters with matter, they do not have finite ranges, but

rather are attenuated in an exponential manner. In other words, a given gamma rayhas a definite probability of passing through any medium of any depth.

Neutrons lose energy in matter by collisions which transfer kinetic energy. Thisprocess is called moderation and is most effective if the matter the neutrons collide

with has about the same mass as the neutron. Once slowed down to the sameaverage energy as the matter being interacted with (thermal energies), the neutrons

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have a much greater chance of interacting with a nucleus. Such interactions can

result in material becoming radioactive or can cause radiation to be given off.

Newton's Inverse Square Law

Any point source which spreads itsinfluence equally in all directions

without a limit to its range will obey

the inverse square law. This comes

from strictly geometricalconsiderations. The intensity of the

influence at any given radius (r) is the

source strength divided by the area ofthe sphere. Being strictly geometric inits origin, the inverse square law

applies to diverse phenomena. Pointsources of gravitational force, electric

field, light, sound, and radiation obeythe inverse square law.

As one of the fields which obey the general inverse square law, a point radiationsource can be characterized by the diagram above whether you are talking

aboutRoentgens, rads,orrems.All measures of exposure will drop off by theinverse square law. For example, if the radiation exposure is 100 mR/hr at 1 inchfrom a source, the exposure will be 0.01 mR/hr at 100 inches.

The applet below shows a radioactive source. The distance to the green source is

shown below. You can also drag the little person and his Geiger counter around toa distance of your choice. When the mouse button is released, a point is plotted onthe graph. The dosage the person receives at the particular distance is shown

numerically and graphically. The graph allows you to confirm Newton's InverseSquare Law.

If the distance is too small, the dosage will be too high and our brave technician

will face severe medical effects. To clear the graph, select a new material, or thesame one again. Moving the mouse from the white area to the gray will turn off the

sound!

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What dosage in mR/hr is considered safe? Better find out!

The red dosage lines represent 2, 5, and 100 mR/hr levels.

Exercise: Assume you are standing three feet from a a 15 Curie Cobalt-60 source.How many mR/hr dosages are you getting?

Carbon-14 Dating

Radio-carbon dating is a method of obtaining age estimates on organic materials.The word "estimates" is used because there is a significant amount of uncertainty

in these measurements. Each sample type has specific problems associated with its

use for dating purposes, including contamination and special environmental

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effects. More information on the sources of error in carbon dating are presented at

the bottom of this page.

The method was developed immediately following World War II by Willard F.

Libby and coworkers and has provided age determinations in archeology, geology,geophysics, and other branches of science. Radiocarbon dating estimates can beobtained on wood, charcoal, marine and freshwater shells, bone and antler, and

peat and organic-bearing sediments. They can also be obtained from carbonate

deposits such as tufa, calcite, marl, dissolved carbon dioxide, and carbonates in

ocean, lake and groundwater sources.

Carbon dioxide is distributed on a worldwide basis into various atmospheric,

biospheric, and hydrospheric reservoirs on a time scale much shorter than its half-life. Measurements have shown that in recent history, radiocarbon levels have

remained relatively constant in most of the biosphere due to the metabolicprocesses in living organisms and the relatively rapid turnover of carbonates insurface ocean waters.However, changes in the atmosphere over the ages are a source ofuncertainty in the measurements.

Carbon (C) has three naturally occurring isotopes. Both C-12 and C-13 are stable, but C-14

decays by very weak beta decay to nitrogen-14 with a half-life of approximately 5,730

years. Naturally occurring radiocarbon is produced as a secondary effect of cosmic-ray

bombardment of the upper atmosphere. Plants transpire to take in atmospheric carbon, which

is the beginning of absorption of carbon into the food chain. Animals eat the plants and this

action introduces carbon into their bodies.

After the organism dies, carbon-14 continues to decay without being replaced. Tomeasure the amount of radiocarbon left in a artifact, scientists burn a small piece to

convert it into carbon dioxide gas. Radiation counters are used to detect the

electrons given off by decaying C-14 as it turns into nitrogen. The amount of C-14

is compared to the amount of C-12, the stable form of carbon, to determine howmuch radiocarbon has decayed, thereby dating the artifact.

Exponential Decay Formula: A = A0* 2^(-t/k)

Where "A" is the present amount of the radioactive isotope, "A0" is the original

amount of the radioactive isotope that is measured in the same units as "A." The

value "t" is the time it takes to reduce the original amount of the isotope to thepresent amount, and "k" is the half-life of the isotope, measured in the same units

as "t."

The applet allows you to choose the C-14 to C-12 ratio, then calculates the age of

our skull from the formula above.

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Uncertainty in Carbon DatingAs mentioned above, there is significant uncertainty in carbon dating. There are several

variables that contribute to this uncertainty. First, as mentioned previously, the proportions of

C-14 in the atmosphere in historic times is unknown. The C-14:C-12 atmospheric ratio

is known to vary over time and it is not at all certain that the curve is well behaved.

Complicating things further, various plants have differing abilities to exclude significant

proportions of the C-14 in their intake. This varies with environmental conditions as

well. The varying rates at which C-14 is excluded in plantsalso means that the apparent age

of a living animal may be affected by an animals diet. An animal that ingested plantswithrelatively low C-14 proportions would be dated older than their true age.

Attempts are often made to index C-14 proportions using samples of know age. While this

may be useful to eliminate the uncertainty of atmospheric proportions of C-14, it

does not compensate for local conditions such as which plant species are in the diet. The

uncertainty in the measurement leads some to conclude that the method is far less predictive

of age than is commonly supposed, especially for older samples.

Isotope Decay Rate (Half-

Life)

Each radionuclide decays at its own

unique rate which cannot be altered byany chemical or physical process. A

useful measure of this rate is the half-life of the radionuclide. Half-life is

defined as the time required for theactivity of any particular radionuclideto decrease to one-half of its initial

value. In other words one-half of theatoms have reverted to a more stable state material. Half-lives of radionuclidesrange from microseconds to billions of years. Half-life of two widely used

industrial isotopes are 74 days for Iridium-192, and 5.3 years for Cobalt-60. More

exacting calculations can be made for the half-life of these materials, however,these times are commonly used.

The applet below offers an interactive representation of radioactive decay series.

The four series represented are Th232, Ir192, Co60, Ga75, and C14. Use the radiobuttons to select the series that you would like to study. Note that Carbon-14 is not

used in radiography, but is one of many useful radioactive isotopes used to

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determine the age of fossils. If you are interested in learning more about Carbon-14

Dating, follow this link:Carbon-14 Dating.

The Sequence Info button displays a chart that depicts the path of the series with

atomic numbers indicated on the vertical axis on the left, and the number ofneutrons shown along the bottom. Colored arrows represent alpha and beta decays.To return to the main user interface, click the "Dismiss" button.

Initially, a selected series contains all parent material, and the amount isrepresented by a colored bar on a vertical logarithmic scale. Each line represents afactor of ten. In order to step forward through the sequence by a specified number

of years, you may type the appropriate number into the "Time Step" field and hit

"Enter." A negative time step will backtrack through the sequence.

You may choose a step interval in years and progress through each step bypressing the "Enter" key. The "Animate" button will automate the progress through

the series. You can either choose a time step before you animate or leave it at zero.If the time step is left at zero, the system will choose time steps to optimize

viewing performance.

Interaction Between Penetrating Radiationand Matter

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When x-rays or gamma rays are directed into an

object, some of the photons interact with the

particles of the matter and their energy can be

absorbed or scattered. This absorption and

scattering is called attenuation. Other photonstravel completely through the object without

interacting with any of the material's particles.

The number of photons transmitted through a

material depends on the thickness, density and

atomic number of the material, and the energy

of the individual photons.

Even when they have the same energy, photons

travel different distances within a material

simply based on the probability of their

encounter with one or more of the particles ofthe matter and the type of encounter that

occurs. Since the probability of an encounter

increases with the distance traveled, the number

of photons reaching a specific point within the

matter decreases exponentially with distance

traveled. As shown in the graphic to the right,

if 1000 photons are aimed at ten 1 cm layers of

a material and there is a 10% chance of a

photon being attenuated in this layer, then there

will be 100 photons attenuated. This leave 900

photos to travel into the next layer where 10%of these photos will be attenuated. By

continuing this progression, the exponential

shape of the curve becomes apparent.

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The formula that describes this curve is:

The factor that indicates how much attenuation will take place per cm (10% in thisexample) is known as the linear attenuation coefficient, m. The above equation

and the linear attenuation coefficient will be discussed in more detail on thefollowing page.

Transmitted Intensity and

Linear Attenuation Coefficient

For a narrow beam of mono-energetic photons, the change in x-ray beam intensityat some distance in a material can be expressed in the form of an equation as:

Where: dI = the change in intensity

I = the initial intensity

n = the number of atoms/cm

s =a proportionality constant that reflects the total probability of a

photon being scattered or absorbed

dx = the incremental thickness of material traversed

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When this equation is integrated, it becomes:

The number of atoms/cm3 (n) and the proportionality constant (s) are usuallycombined to yield the linear attenuation coefficient (m). Therefore the equation

becomes:

Where: I = the intensity of photons transmitted across some distance x

I0 = the initial intensity of photons

s =a proportionality constant that reflects the total probability of a

photon being scattered or absorbed

m = the linear attenuation coefficient

x = distance traveled

The Linear Attenuation Coefficient (m)

The linear attenuation coefficient (m) describes the fraction of a beam of x-rays orgamma rays that is absorbed or scattered per unit thickness of the absorber. This

value basically accounts for the number of atoms in a cubic cm volume of material

and the probability of a photon being scattered or absorbed from the nucleus or anelectron of one of these atoms. The linear attenuation coefficients for a variety ofmaterials and x-ray energies are available in various reference books.

Using the transmitted intensity equation above, linear attenuation coefficients can

be used to make a number of calculations. These include:

the intensity of the energy transmitted through a material when the incidentx-ray intensity, the material and the material thickness are known.

the intensity of the incident x-ray energy when the transmitted x-rayintensity, material, and material thickness are known.

the thickness of the material when the incident and transmitted intensity, andthe material are known.

the material can be determined from the value of m when the incident andtransmitted intensity, and the material thickness are known.

Half-Value Layer

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The thickness of any given materialwhere 50% of the incident energy

has been attenuated is know as thehalf-value layer (HVL). The HVL is

expressed in units of distance (mmor cm). Like the attenuationcoefficient, it is photon energy

dependant. Increasing thepenetrating energy of a stream of

photons will result in an increase in

a material's HVL. The HVL is inversely proportional

to the attenuation coefficient. If an

incident energy of 1 and a

transmitted energy is 0.5 is pluggedinto the equation introduced on the preceding page, it can be seen that the

HVL multiplied by m must equal 0.693.

If x is the HVL then m times HVL must equal 0.693 (since the number

0.693 is the exponent value that gives a value of 0.5). Therefore, the HVL and m are related as follows:

The HVL is often used in

radiography simply because it iseasier to remember values and

perform simple calculations. In a

shielding calculation, such asillustrated to the right, it can be seen

that if the thickness of one HVL is

known, it is possible to quickly

determine how much material isneeded to reduce the intensity to less

than 1%. Approximate HVL for Various

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Materials when Radiation is from a Gamma Source

Half-Value Layer, mm (inch)

Source Concrete Steel Lead Tungsten Uranium

Iridium-192 44.5 (1.75) 12.7 (0.5) 4.8 (0.19) 3.3 (0.13) 2.8 (0.11)

Cobalt-60 60.5 (2.38) 21.6 (0.85) 12.5 (0.49) 7.9 (0.31) 6.9 (0.27)

Approximate Half-Value Layer for Various Materials when Radiation isfrom an X-ray Source

Half-Value Layer, mm (inch)

Peak Voltage (kVp) Lead Concrete

50 0.06 (0.002) 4.32 (0.170)

100 0.27 (0.010) 15.10 (0.595)

150 0.30 (0.012) 22.32 (0.879)

200 0.52 (0.021) 25.0 (0.984)

250 0.88 (0.035) 28.0 (1.102)

300 1.47 (0.055) 31.21 (1.229)

400 2.5 (0.098) 33.0 (1.299)

1000 7.9 (0.311) 44.45 (1.75)

Note: The values presented on this page are intended for educationalpurposes. Other sources of information should be consulted when designing

shielding for radiation sources.

Sources of Attenuation The attenuation that results due to the interaction between penetrating

radiation and matter is not a simple process. A single interaction eventbetween a primary x-ray photon and a particle of matter does not usuallyresult in the photon changing to some other form of energy and effectively

disappearing. Several interaction events are usually involved and the totalattenuation is the sum of the attenuation due to different types ofinteractions. These interactions include the photoelectric effect, scattering,

and pair production. The figure below shows an approximation of the total

absorption coefficient,(), in red, for iron plotted as a function of radiationenergy. The four radiation-matter interactions that contribute to the total

absorption are shown in black. The four types of interactionsare: photoelectric (PE), Compton scattering (C), pair production (PP),

andThomson or Rayleigh scattering (R). Since most industrial radiographyis done in the 0.1 to 1.5 MeV range, it can be seen from the plot that

photoelectric and Compton scattering account for the majority of attenuationencountered.

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Summary of different mechanisms that cause attenuation of an incident

x-ray beam Photoelectric (PE)absorption of x-rays

occurs when the x-ray photon is absorbed,

resulting in the ejection of electrons from

the outer shell of the atom, and hence theionization of the atom. Subsequently, the

ionized atom returns to the neutral state withthe emission of an x-ray characteristic of the

atom. This subsequent emission of lower energy photons is generallyabsorbed and does not contribute to (or hinder) the image making process.Photoelectron absorption is the dominant process for x-ray absorption up to

energies of about 500 KeV. Photoelectron absorption is also dominant for

atoms of high atomic numbers. Compton scattering (C)occurs when the

incident x-ray photon is deflected from itsoriginal path by an interaction with an

electron. The electron gains energy and is

ejected from its orbital position. The x-rayphoton loses energy due to the interaction

but continues to travel through the materialalong an altered path. Since the scattered x-ray photon has less energy, it,

therefore, has a longer wavelength than the incident photon. The event is

also known as incoherent scattering because the photon energy changeresulting from an interaction is not always orderly and consistent. The

energy shift depends on the angle of scattering and not on the nature of the

scattering medium. Click here for more information on Compton scatteringand the relationship between the scatter angle and photon energy.

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Pair production (PP)can occurwhen the x-ray photon energy is

greater than 1.02 MeV, but reallyonly becomes significant at energies

around 10 MeV. Pair productionoccurs when an electron and positronare created with the annihilation of

the x-ray photon. Positrons are very short lived and disappear (positronannihilation) with the formation of two photons of 0.51 MeV energy. Pair

production is of particular importance when high-energy photons pass

through materials of a high atomic number. Below are other interaction phenomenon that can occur. Under special

circumstances these may need to be

considered, but are generally

negligible. Thomson scattering (R), also known

as Rayleigh, coherent, or classicalscattering, occurs when the x-ray

photon interacts with the whole atomso that the photon is scattered with no change in internal energy to the

scattering atom, nor to the x-ray photon. Thomson scattering is never more

than a minor contributor to the absorption coefficient. The scattering occurswithout the loss of energy. Scattering is mainly in the forward direction.

Photodisintegration (PD)is theprocess by which the x-ray

photon is captured by the nucleusof the atom with the ejection of a

particle from the nucleus when

all the energy of the x-ray is

given to the nucleus. Because of the enormously high energies involved, thisprocess may be neglected for the energies of x-rays used in radiography.

Effect of Photon Energy on AttenuationAbsorption characteristics will increase or decrease as the energy of the x-

ray is increased or decreased. Since attenuation characteristics of materialsare important in the development of contrast in a radiograph, an

understanding of the relationship between material thickness, absorptionproperties, and photon energy is fundamental to producing a quality

radiograph. A radiograph with higher contrast will provide greater

probability of detection of a given discontinuity. An understanding ofabsorption is also necessary when designing x-ray and gamma ray shielding,

cabinets, or exposure vaults. The applet below can be used to investigate the effect that photon energy

has on the type of interaction that the photon is likely to have with a particle

of the material (shown in gray). Various materials and material thicknesses

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scattered x-ray photon has less energy, it has a longer wavelength and less

penetrating than the incident photon.

Compton effect was first observed by Arthur Compton in 1923 and thisdiscovery led to his award of the 1927 Nobel Prize in Physics. The

discovery is important because it demonstrates that light cannot beexplained purely as a wave phenomenon. Compton's work convinced thescientific community that light can behave as a stream of particles (photons)

whose energy is proportional to the frequency. The change in wavelength of the scattered photon is given by:

Where: l = wavelength of incident x-ray photon

l' = wavelength of scattered x-ray photon

h =Planck's Constant: The fundamental constant equal to the ratio of

the energy E of a quantum of energy to its frequency v: E=hv.

me = the mass of an electron at rest

c = the speed of light

q = The scattering angle of the scattered photon

The applet below demonstrates Compton scattering as calculated with theKlein-Nishina formula, which provides an accurate prediction of the angulardistribution of x-rays and gamma-rays that are incident upon a singleelectron. Before this formula was derived, the electron cross section had

been classically derived by the British physicist and discoverer of theelectron, J.J. Thomson. However, scattering experiments showed significant

deviations from the results predicted by Thomson's model. The Klein-Nishina formula incorporates the Breit-Dirac recoil factor, R, also known as

radiation pressure. The formula also corrects for relativistic quantum

mechanics and takes into account the interaction of the spin and magnetic

moment of the electron with electromagnetic radiation. Quantum mechanicsis

a system of mechanics based on quantum theory to provide a consistentexplanation of both electromagnetic wave and atomic structure.

The applet shows that when a photon of a given energy hits an atom, it issometimes reflected in a different direction. At the same time, it losesenergy to an electron that is ejected from the atom. Theta is the angle

between the scattered photon direction and the path of the incident photon.Phi is the angle between the scattered electron direction and the path of the

incident photon.

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Geometric Unsharpness

Geometric unsharpness refers to the loss of definition that is the result of geometric factors of

the radiographic equipment and setup. It occurs because the radiation does not originate from

a single point but rather over an area. Consider the images below which show two sources of

different sizes, the paths of the radiation from each edge of the source to each edge of the

feature of the sample, the locations where this radiation will expose the film and the density

profile across the film. In the first image, the radiation originates at a very small source. Since

all of the radiation originates from basically the same point, very little geometric unsharpness

is produced in the image. In the second image, the source size is larger and the different paths

that the rays of radiation can take from their point of origin in the source causes the edges of

the notch to be less defined.

The three factors controlling unsharpness are source size, source to object distance, and

object to detector distance. The source size is obtained by referencing manufacturers

specifications for a given X-ray or gamma ray source. Industrial x-ray tubes often have focal

spot sizes of 1.5 mm squared but microfocus systems have spot sizes in the 30 micron range.

As the source size decreases, the geometric unsharpness also decreases. For a given size

source, the unsharpness can also be decreased by increasing the source to object distance, but

this comes with a reduction in radiation intensity.

The object to detector distance is usually kept as small as possible to help minimize

unsharpness. However, there are situations, such as when using geometric enlargement, when

the object is separated from the detector, which will reduce the definition. The applet belowallow the geometric unsharpness to be visualized as the source size, source to object distance,

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and source to detector distance are varied. The area of varying density at the edge of a feature

that results due to geometric factors is called the penumbra. The penumbra is the gray area

seen in the applet.

Codes and standards used in industrial radiography

require that geometric unsharpness be limited. In general,

the allowable amount is 1/100 of the material

thickness up to a maximum of 0.040 inch. These values

refer to the degree of penumbra shadow in a radiographic

image. Since the penumbra is not nearly as well defined as

shown in the image to the right, it is difficult to measure it

in a radiograph. Therefore it is typically

calculated. The source size must be obtained from the

equipment manufacturer or measured. Then theunsharpness can be calculated using measurements made

of the setup.

For the case, such as that shown to the right, where a

sample of significant thickness is placed adjacent to the

detector, the

following

formula is used to calculate the maximum

amount of unsharpness due to specimen

thickness:

Ug = f * b/a

f = source focal-spot size

a = distance from the source to front surface of

the object

b = the thickness of the object

For the case when the detector is not placed

next to the sample, such as when geometric

magnification is being used, the calculation

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becomes:

Ug = f* b/a

f = source focal-spot size.

a = distance from x-ray source to front surface of material/objectb = distance from the front surface of the object to the detector

Filters in Radiography

At x-ray energies, filters consist of material placed in the useful beam to absorb,

preferentially, radiation based on energy level or to modify the spatial distributionof the beam. Filtration is required to absorb the lower-energy x-ray photonsemitted by the tube before they reach the target. The use of filters produce a

cleaner image by absorbing the lower energy x-ray photons that tend to scatter

more.

The total filtration of the beam includes the inherent filtration (composed of part of

the x-ray tube and tube housing) and the added filtration (thin sheets of a metalinserted in the x-ray beam). Filters are typically placed at or near the x-ray port inthe direct path of the x-ray beam. Placing a thin sheet of copper between the part

and the film cassette has also proven an effective method of filtration.

For industrial radiography, the filters added to the x-ray beam are most oftenconstructed of high atomic number materials such as lead, copper, or brass. Filters

for medical radiography are usually made of aluminum (Al). The amount of both

the inherent and the added filtration are stated in mm of Al or mm of Al equivalent.The amount of filtration of the x-ray beam is specified by and based on the voltage

potential (keV) used to produce the beam. The thickness of filter materials is

dependent on atomic numbers, kilovoltage settings, and the desired filtrationfactor.

Gamma radiography produces relatively high energy levels at essentially

monochromatic radiation, therefore filtration is not a useful technique and is

seldom used.

Secondary (Scatter) Radiation

Secondary or scatter radiation must often be taken into consideration whenproducing a radiograph. The scattered photons create a loss of contrast and

definition. Often secondary radiation is thought of as radiation striking the film

reflected from an object in the immediate area, such as a wall, or from the table orfloor where the part is resting. Side scatter originates from walls, or objects on the

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source side of the film. Control of side scatter can be achieved by moving objects

in the room away from the film, moving the x-ray tube to the center of the vault, or

placing a collimator at the exit port, thus reducing the diverging radiationsurrounding the central beam.

It is often called backscatter when it comes from objects behind the film. Industrycodes and standards often require that a lead letter "B" be placed on the back of the

cassette to verify the control of backscatter. If

the letter "B" shows as a "ghost" image on the

film, a significant amount of backscatterradiation is reaching the film. The image of the"B" is often very nondistinct as shown in the

image to the right. The arrow points to the area

of backscatter radiation from the lead "B"

located on the back side of the film. Thecontrol of backscatter radiation is achieved by

backing the film in the cassette with a sheet of lead that is at least 0.010 inch thick.It is a common practice in industry to place a 0.005" lead screen in front and a

0.010" screen behind the film.

Undercut

Another condition that must often be

controlled when producing a radiograph

is called undercut. Parts with holes,

hollow areas, or abrupt thicknesschanges are likely to suffer fromundercut if controls are not put in place.

Undercut appears as a darkening of the

radiograph in the area of the thicknesstransition. This results in a loss of

resolution or blurring at the transition

area. Undercut occurs due to scattering within the film. At the edges of a part orareas where the part transitions from thick to thin, the intensity of the radiation

reaching the film is much greater than in the thicker areas of the part. The highlevel of radiation intensity reaching the film results in a high level of scattering

within the film. It should also be noted that the faster the film speed, the more

undercut that is likely to occur. Scattering from within the walls of the part alsocontributes to undercut, but research has shown that scattering within the film is

the primary cause. Masks are used to control undercut. Sheets of lead cut to fill

holes or surround the part and metallic shot and liquid absorbers are often used asmasks.

Radiation Safety

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Ionizing radiation is an extremely important NDT tool but it can

pose a hazard to human health. For this reason, special

precautions must be observed when using and working aroundionizing radiation. The possession of radioactive materials and

use of radiation producing devices in the United States isgoverned by strict regulatory controls. The primary regulatoryauthority for most types and uses of radioactive materials is the

federal Nuclear Regulatory Commission (NRC). However, morethan half of the states in the US have entered into "agreement" with the NRC to

assume regulatory control of radioactive material use within their borders. As part

of the agreement process, the states must adopt and enforce regulations comparableto those found in Title 10 of the Code of Federal Regulations. Regulations forcontrol of radioactive material used in Iowa are found in Chapter 136C of the Iowa

Code.

For most situations, the types and maximum quantities of radioactive materials

possessed, the manner in which they may be used, and the individuals authorizedto use radioactive materials are stipulated in the form of a "specific" license from

the appropriate regulatory authority. In Iowa, this authority is the Iowa Departmentof Public Health. However, for certain institutions which routinely use large

quantities of numerous types of radioactive materials, the exact quantities ofmaterials and details of use may not be specified in the license. Instead, the licensegrants the institution the authority and responsibility for setting the specific

requirements for radioactive material use within its facilities. These licensees are

termed "broadscope" and require a Radiation Safety Committee and usually a full-time Radiation Safety Officer.