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Summary of important points on atomic structure

• In the neutral atom, the number of orbiting electrons is equal to the number of protons

in the nucleus. Since the number of electrons determines the chemical behaviour of an

atom, the atomic number (Z) also determines this chemical behaviour. Each element

has different chemical properties and thus each element has a different atomicnumber. These form the basis of the periodic table.

• toms in the ground state are electricall! neutral because the number of positive

charges (protons) is balanced b! the number of negative charges (electrons).

• If an electron is removed, the atom is no longer neutral, but becomes positivel!

charged and is referred to as a positive ion.The process of removing an electron from

an atom is called ioni"ation.

• If an electron is displaced from an inner shell to an outer shell (i.e. to a higher energ!

level), the atom remains neutral but is in an e#cited state. This process is called

e#citation.

• The unit of energ! in the atomic s!stem is the electron volt (e$), % e$& %.'# %% *oules.

Interaction of X-rays with matter

+hen -ra!s strie matter, such as a patient/s tissues, the photons have four possible fates,

sho0n diagrammaticall! in 1igure 2.. The photons ma! be3

• 4ompletel! scattered 0ith no loss of energ!

• bsorbed 0ith total loss of energ!

• Scattered 0ith some absorption and loss of energ!

• Transmitted unchanged.

Definition of terms used in X-ray interactions

• Scattering 5 change in direction of a photon 0ith or 0ithout a loss of energ!

• bsorption 5 deposition of energ!, i.e. removal of energ! from the beam

• ttenuation 5 reduction in the intensit! of the main -ra! beam caused b!

absorption and Scattering.

ttenuation & bsorption 6 Scattering

• loni"ation 5 removal of an electron from a neutral atom producing a negative ion

(the electron) and a positive ion (the remaining atom).

Interaction of X-rays at the atomic levelThere are four main interactions at the atomic level, depending on the energ! of the incoming

 photon, these include3

• 7nmodified or 8a!leigh scattering 5 pure scatter 

• 9hotoelectric effect 5 pure absorption

• 4ompton effect 5 scatter and absorption

• 9air production 5pure absorption.

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:nl! t0o interactions are important in the -ra! energ! range used in dentistr!3

• 9hotoelectric effect

• 4ompton effect.

Photoelectric effect

The photoelectric effect is a pure absorptioninteraction predominating 0ith lo0-energ!

 photons (see 1ig. 2.%).

Summary of the stages in the photoelectric effect

%. The incoming -ra! photon interacts 0ith a bound inner-shell electron of the tissue

atom.

2. The inner-shell electron is e*ected 0ith considerable energ! (no0 called a

 photoelectrori) into the tissues and 0ill undergo further interactions (see belo0).

;. The -ra! photon disappears having deposited all its energ!< the process is therefore

one of pure absorption.

=. The vacanc! 0hich no0 e#ists in the inner electron shell is filled b! outer-shell

electrons dropping from one shell to another.

>. This cascade of electrons to ne0 energ! levels results in the emission of e#cessenerg! in the form of light or heat.

'. tomic stabilit! is finall! achieved b! the capture of a free electron to return the atom

to its neutral state.

?. The high-energ! e*ected photoelectron behaves lie the original high-energ! -ra!

 photon, undergoing man! similar interactions and e*ecting other electrons as it passes

through the tissues. It is these e*ected high-energ! electrons that are responsible for 

the ma*orit! of the ioni"ation interactions 0ithin tissue, and the possible resulting

damage attributable to -ra!s.

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 An individual's average dose from background radiation is estimated at approximately 2

mSv per year in the UK, while in the USA it is estimated at approximately !" tnSv! #hese

 figures are useful to remember when considering the magnitude of the doses associated 

with various diagnostic procedures $see later%!

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Classification of the biological effects

The biologicall!damaging effects of ioni"ing radiation are classified into three main

categories3

• Somatic @ETE8AIBISTI4 effects

•

Somatic ST:4CSTI4 effects.• Denetic ST:4CSTI4 effects.

The somatic effects are further subdivided into3

• cute or immediate effects 5 appearing shortl! after e#posure, e.g. as a result of 

large 0hole bod! doses (Table =.%)

• 4hronic or long-term effects 5 becoming evident after a long period of time, the

socalled latent period (2 !ears or more), e.g. leuaemia.

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Somatic deterministic effects

These are the damaging effects to the bod! of the person e#posed that 0ill definitely result

from a specific high dose of radiation. E#amples include sin reddening and cataract

formation. The severit! of the effect is proportional to the dose received, and in most cases a

threshold dose e#ists belo0 0hich there 0ill be no effect.Somatic stochastic effects

Stochastic effects are those that may develop. Their development is random and depends on

the la0s of chance or probabilit!. E#amples of somatic stochastic effects include leuaemia

and certain tumours.

These damaging effects may be induced 0hen the bod! is e#posed to any dose of radiation.

E#perimentall! it has not been possible to establish a  safe dose  5 i.e. a dose belo0 0hich

stochastic effects do not develop. It is therefore assumed that there is no threshold dose, and

that ever! e#posure to ioni"ing radiation carries 0ith it the possibility of inducing a

stochastic effect.

The lo0er the radiation dose, the lo0er the probabilit! of cell damage. Co0ever, the severit!

of the damage is not related to the si"e of the inducing dose. This is the underl!ing

 philosoph! behind present radiation protection recommendations (see 4h. ').

Genetic stochastic effects

Autations result from an!

sudden change to a gene or 

chromosome. The! can be

caused b! e#ternal factors, such

as radiation or ma! occur 

spontaneousl!.8adiation to the reproductive

organs ma! damage the @B of 

the sperm or egg cells. This ma!

result in a congenital

abnormalit! in the offspring of 

the person irradiated. Co0ever,

there is no certaint! that these

effects 0ill happen, so all

genetic effects are described as

stochastic. cause-and-effect

relationship is difficult, if not

impossible, to prove. lthough

ioni"ing radiation has the

 potential to cause genetic

damage, there are no human data

that sho0 convincing evidence

of a direct lin 0ith radiation.

8is estimates have been based mainl! on e#periments 0ith mice. It is estimated that a dose

to the gonads of .>-%. Sv 0ould double the spontaneous mutation rate. :nce again it isassumed that there is no threshold dose.

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