Senin, 14 Februari 2011

Atomic Physics

Modern atomic and nuclear physics is among the most impressive scientific achievements of the last century. Both the theories and techniques of atomic and nuclear physics have played an important role in the life sciences. The theories provided a solid foundation for understanding the structure and interaction of organic molecules, and the techniques provided many tools for both experimental and clinical work. Contributions from this field have been so numerous and influential that it is impossible to do them justice in a single chapter. We will present a brief description of the atom and the nucleus, which will lead into a discussion of the applications of atomic and nuclear physics to the life sciences.

1) Early Models of the Atom
By 1912, through the work of J. J. Thompson, E. Rutherford, and their colleagues, a number of important facts had been discovered about atoms which make up matter:
·   Atoms contain small negatively charged electrons and relatively heavier positively  
   charged protons.
·   The proton is about 2000 times heavier than the electron, but the magnitude of the
   charge on the two is the same.
·   There are as many positively charged protons in an atom as negatively charged
   electrons. The atom as a whole is, therefore, electrically neutral.
·   The identity of an atom is determined by the number of protons it has. For example,
   hydrogen has 1 proton, carbon has 6 protons, and silver has 47 protons.

Through a series of ingenious experiments, Rutherford explained his astonishing results by developing a new atomic model which was presented as follows:  
·   The positive charge in the atom was concentrated in a region that was small relative to
   the size of the atom. He called this concentration of positive charge the nucleus of the
·   Any electrons belonging to the atom were assumed to be in the relative large volume
   outside the nucleus. To explain why these electrons where not pulled into the nucleus
   by the attractive electric force, Rutherford modeled them as moving in orbits around
   the nucleus in the same manner as the planets orbit the Sun. For this reason, this model
   is often referred to as the planetary model of the atom (Figure 1).
·   It was subsequently discovered that the nucleus also contains another particle, the
, which has approximately the same mass as the proton but is electrically
·   Although the nucleus contains most of the atomic mass, it occupies only a small part of
   the total atomic volume. The diameter of the whole atom is on the order of 10−8 cm,
   but the diameter of the nucleus is only about 10−13 cm.

Figure 1: Rutherford ′s planetary model of the atom

Two basic difficulties exist with Rutherford ′s planetary model:
1. The first difficulty is that, an atom emits (and absorbs) certain frequencies of
     electromagnetic radiation and no others, which Rutherford′ s model cannot explain
     this phenomenon.
2. The second difficulty is that Rutherford’s electrons are undergoing a centripetal
     acceleration, but centripetally accelerated charges revolving with frequency ƒ should
     radiate electromagnetic waves of the same frequency ƒ.  

Unfortunately, this classical model leads to a prediction of self- destruction when applied to the atom. As the electron radiates, energy is carried away from the atom, the radius of the electron′ s orbit steadily decrease as this radiation is given off and its frequency of revolution increases. This process would lead to an ever-increasing frequency of emitted radiation and the electron should eventually spiral into the nucleus and finally an ultimate collapse of the atom occurs.

2) Bohr′s Model of the Hydrogen Atom
In 1913, the Danish physicist Niels Bohr proposed a model for the atom that circumvented the difficulties of Rutherford's planetary model, and explained many observations that were puzzling scientists at that time. The most surprising observed property of atoms was the light emitted by them. When an element is put into a flame, it emits light at sharply defined wavelengths, called spectral lines. Each element emits its own characteristic spectrum of light. Prior to Bohr, scientists could not explain why these colors were emitted by atoms. Bohr's model of the atom explained the reason of the sharp spectra. The basic ideas of the Bohr Theory as it applies to the hydrogen atom were expressed by the following assumption:
1. The electron moves in circular orbits around the proton under the influence of the
    electric force of attraction as shown in Figure 2.

Figure 2: Diagram representing Bohr′s model of hydrogen atom. The orbiting electron is allowed to be               
                 only in specific discrete radii.

2. Only certain electron orbits are stable. When in one of these stationary states, as Bohr
    called them, the electron does not emit energy in the form of radiation. Hence the total  
    energy level of the atom remains constant and classical mechanics can be used to         
    describe the electron’s motion. This representation claims that the centripetally
    accelerated electron does not continuously emit radiation, losing energy and
    eventually spiraling into the nucleus, as predicted by Rutherford ′s planetary model.

3. Radiation is emitted by the atom when the electron makes a transition from a more 
    energetic initial orbit to a lower-energy orbit. This transition can not be visualized.
    Bohr was able to calculate the radii of these allowed orbits and shows that, the
    radiation or spectral lines are emitted as a consequence of orbital restrictions. The
    orbital restrictions are most easily illustrated with the simplest atom hydrogen,
    which has a single proton nucleus and one electron orbiting around it. Unless energy
    is added to the atom, the electron is found in the allowed orbit closet to the nucleus. If
    energy is added to the atom, the electron may “jump” to one of the higher allowed
    orbits farther away from the nucleus, but the electron can never occupy the regions
    between the allowed orbits (Figure 3).

Figure 3: Bohr model for the hydrogen atom. The electron orbits about the nucleus and can occupy only    
                 discrete orbits with radii 1, 2, 3, and so on.

4. The Bohr model was very successful in explaining many of the experimental
    observations for the simple hydrogen atom. But to describe the behavior of atoms
    with more than one electron, it was necessary to impose an additional restriction on
    the structure of the atom; the number of electrons in a given orbit cannot be greater
    than 2n2, where n is the order of the orbit from the nucleus, trough which the
    maximum number of electrons in each orbit can be calculated according to the
                                           The first allowed orbit = 2 x (1)2 = 2
                                      The second allowed orbit = 2 x (2)2 = 8
                                          The third allowed orbit = 2 x (3)2 = 18 and so on.

    The atoms are found to be constructed in accordance with these restrictions. Helium
    has two electrons, and, therefore, its first orbit is filled. Lithium has three electrons,
    two of which fill the first orbit; the third electron, therefore, must be in the second
    orbit. This simple sequence is not completely applicable to the very complex atoms,
    but basically this is the way the elements are constructed.

5. A specific amount of energy is associated with each allowed orbital configuration of
    the electron. Therefore, instead of speaking of the electron as being in a certain orbit,
    we can refer to it as having a corresponding amount of energy. Each of these allowed
    values of energy is called an energy level. An energy level diagram for an atom is
    shown in Figure 4. Note that every element has its own characteristic energy level
    structure. The electrons in the atom can occupy only specific energy states; that is, in a
    given atom the electron can have energy E1, E2, E3, and so on, but cannot have energy
    between these two values. This is a direct consequence of the restrictions on the
    allowed electron orbital configurations.

Figure 4: Energy Level for an Atom.

6. The lowest energy level that an electron can occupy is called the ground state. This
    state is associated with the orbital configuration closest to the nucleus. The higher
    allowed energy levels, called excited states, are associated with larger orbits and
    different orbital shapes. Normally the electron occupies the lowest energy level but it
    can be excited into a higher energy state by adding energy to the atom.

7. An atom can be excited from a lower to a higher energy state in a number of different
    ways. The two most common methods of excitation are:
    (a) Electron impact.
    (b) Absorption of electromagnetic radiation.
    (a) Excitation by electron impact occurs most frequently in a gas discharge; if a      
    current is passed through a gas of atoms, the colliding electron is slowed down and
    the electron in the atom is promoted to a higher energy configuration. When the
    excited atoms fall back into the lower energy states, the excess energy is given off
    as electromagnetic radiation. Each atom releases its excess energy in a single
    photon. Therefore, the energy of the photon is simply the difference between the
    energies of the initial state Ei and the final state Ef of the atom. The frequency ƒ of
    the emitted radiation is given by

ƒ= Energy of photon / Planck constant = (E − Ef) / h                (1)

    Where h is a Planck′ s constant with a value = 6.626 x 10-34 J. s

    Transition between each pair of energy levels results, in the emission of light at a
    specific frequency, called transition or resonance frequency. Therefore, a group of
    highly excited atoms of a given element emit light at a number of well-defined
    frequencies which constitute the optical spectrum for that element.

    (b) An atom in a given energy level can also be excited to a higher level by light at a
    specific frequency. The frequency must be such that each photon has just the right
    amount of energy to promote the atom to one of its higher allowed energy states.
    Atoms, therefore, absorb light only at the specific transition frequencies, given by
    Equation 1.  Light at other frequencies is not absorbed. If a beam of white light
    (containing all the frequencies) is passed through a group of atoms of a given species,
    the spectrum of the transmitted light shows gaps corresponding to the absorption of
    the specific frequencies by the atoms. This is called the absorption spectrum of the
    atom. In their undisturbed state, most of the atoms are in the ground state. The
    absorption spectrum, therefore, usually contains only lines associated with transitions
    from the ground state to higher allowed states (Figure 5). Optical spectra are produced
    by the outer electrons of the atom. The inner electrons, those closer to the nucleus, are
    bound more tightly and are consequently more difficult to excite. However, in a
    highly energetic collision with another particle, an inner electron may be excited.
    When in such an excited atom an electron returns to the inner orbit, the excess energy
    is again released as a quantum (photon) of electromagnetic radiation. Because the
    binding energy here is about a thousand times greater than for the outer electrons, the
    frequency of the emitted radiation is correspondingly higher. Electromagnetic
    radiation in this frequency range is called X - rays.

Figure 5: The absorption Spectrum

8. The Bohr model also explained qualitatively the formation of chemical bonds. The
    formation of chemical compounds and matter in bulk is due to the distribution of
    electrons in the atomic orbits. When an orbit is not filled to capacity (which is the
    case for most atoms),  the electrons of one atom can partially occupy the orbit of
    another. This sharing of orbits draws the atoms together and produces bonding
    between atoms. As an example we show in Figure 6, the formation of a hydrogen
    molecule from two hydrogen atoms.

   (a)                                                             (b)     

Figure 6:  A schematic representation for the formation of a hydrogen molecule.
                 (a) Two separate hydrogen atoms.                  
                 (b) When the two atoms are close together, the electrons share each other’s orbit, which results
                       in the binding of the two atoms into a molecule.

      Figure 6 indicate that:
      (a)In the orbit of each of the hydrogen atoms, there is room for another electron.
      (b)A completely filled orbit is the most stable configuration; therefore, when two 
          hydrogen atoms are close together, they share each other’s electrons, in this way
          the orbit of each atom is completely filled part of the time. This shared orbit can
          be pictured as a rubber band pulling the two atoms together.
      Therefore, the sharing of the electrons binds the atoms into a molecule. While the
      sharing of electrons pulls the atoms together, the repulsion force of the nuclei tends
      to keep them apart. The equilibrium separation between atoms in a molecule is
      determined by these two counter forces. In a similar way, more complex molecules,   
      and ultimately bulk matter, are formed.  Atoms with completely filled orbits (these
      are atoms of the so-called noble gases) as helium, neon, argon, krypton, and xenon,
      cannot share electrons with other elements and are, therefore, chemically most inert.

9. Molecules also have characteristic spectra both in emission and in absorption.
     Because molecules are more complicated than atoms, their spectra are
     correspondingly more complex. In addition to the electronic configuration, these
     spectra also depend on the motion of the nuclei. Still the spectra can be interpreted
     and are unique for each type of molecule.

2) Spectroscopy
Spectroscopy is the study of the wavelength distribution of radiation from a sample. The absorption and emission spectra of atoms and molecules are unique for each species. They can serve as fingerprints in identifying atoms and molecules in various substances. Spectroscopic techniques were first used in basic experiments with atoms and molecules, but they were soon adopted in many other areas, including the life sciences as:
·   In biochemistry, spectroscopy is used to identify the products of complex chemical
·   In medicine, spectroscopy is used routinely to determine the concentration of certain   
   atoms and molecules in the body. From a spectroscopic analysis of urine, for example,  
   one can determine the level of mercury in the body. Blood sugar level is measured by
   first producing a chemical reaction in the blood sample which results in a colored
   product. The concentration of this colored product, which is proportional to the blood-
   sugar level, is then measured by absorption spectroscopy.
The basic principles of spectroscopy are simple and characterized by two different forms:
2.1) Emission Spectroscopy
In emission spectroscopy the sample under investigation is excited by an electric current or a flame. The emitted light is then examined and identified. Emission spectra can provide information about the concentration of the various components in the substance. In emission spectroscopy, the intensity of the emitted light in the spectrum is proportional to the number of atoms or molecules of the given species.

2.2) Absorption Spectroscopy
In absorption spectroscopy, the sample is placed in the path of a beam of white light. Examination of the transmitted light reveals the missing wavelengths which identify the
components in the substance. The absorption spectra can also provide information about the concentration of all the components in a substance. In absorption spectroscopy, the amount of absorption can be related to the concentration.

2.3) The Spectrometer
The instrument used to analyze the spectra is called a spectrometer. This device records the intensity of light as a function of wavelength. A spectrometer, in its simplest form, consists of a focusing system, a prism, and a detector for light as shown in Figure 7.

Figure 7: The measurement of spectra.

2.3.1The Measurement of Spectra
The focusing system forms a parallel beam of light which falls on the prism. The prism, which can be rotated, breaks up the beam into its component wavelengths. At this point, the fanned-out spectrum can be photographed and identified as follows:
·   The narrow exit slit intercepts only a portion of the spectrum, through which a small
   section of the spectrum is detected at a time.
·   As the prism is rotated, the whole spectrum is swept sequentially past the slit. The
   position of the prism is calibrated to correspond with the wavelength impinging on the
·   The light that passes through the slit is detected by a photo detector which produces an
   electrical signal proportional to the light intensity. The intensity of the signal as a
   function of wavelength can be displayed on a chart recorder.

Spectrometers used in routine clinical work are automated and can be operated by relatively unskilled personnel. The identification and interpretation of the spectra produced by less well-known molecules, however, require considerable training and skill. In addition to identifying the molecule, such spectra also yield information about the molecular structure.

3) Quantum Mechanics
3.1) Overview
Quantum mechanics is the study of mechanical systems whose dimensions are close to or below the atomic scale, such as molecules, atoms, electrons, protons and other subatomic particles. Quantum mechanics is a fundamental branch of physics with wide applications. Quantum theory generalizes classical mechanics and provides accurate descriptions for many previously unexplained phenomena such as stable electron orbits. The effects of quantum mechanics are typically not observable on macroscopic scales, but become evident at the atomic and subatomic level. The word “quantum” came from the Latin word which means "what quantity". In quantum mechanics, it refers to a discrete unit that quantum theory assigns to certain physical quantities, such as the energy of an atom at rest. The discovery that waves have discrete energy packets (called quanta) that behave in a manner similar to particles led to the branch of physics that deals with atomic and subatomic systems which we today call quantum mechanics.
As mentioned earlier, quantum mechanics is essential to understand the behavior of systems at atomic length scales and smaller. For example, if Newtonian mechanics governed the workings of an atom, electrons would rapidly travel towards and collide with the nucleus, making stable atoms impossible. However, in the natural world the electrons normally remain in an unknown orbital path around the nucleus (Figure 8).

Figure 8: The electrons orbiting the atomic nucleus in different orbits, with different energy levels.

This branch of physics was initially developed to provide a better explanation of the atom, especially the spectra of light emitted by different atomic species. The quantum theory of the atom was developed as an explanation for the electron's staying in its orbital, which could not be explained by Newton's laws of motion and by Maxwell's laws of classical electromagnetism. In the formalism of quantum mechanics, the state of a system at a given time is described by a complex wave function (sometimes referred to as orbital in the case of atomic electrons), and more generally, elements of a complex vector space. This abstract mathematical object allows for the calculation of probabilities of outcomes of concrete experiments. For example, it allows one to compute the probability of finding an electron in a particular region around the nucleus at a particular
time. Contrary to classical mechanics, one can never make simultaneous predictions of conjugate variables, such as position and momentum, with arbitrary accuracy. For instance, electrons may be considered to be located somewhere within a region of space, but with their exact positions being unknown. Contours of constant probability, often referred to as “clouds” may be drawn around the nucleus of an atom to conceptualize where the electron might be located with the most probability (Figure 8).
The other exemplar that led to quantum mechanics was the study of electromagnetic waves such as light. When it was found in 1900 by Max Planck that the energy of waves could be described as consisting of small packets or quanta, Albert Einstein exploited this idea to show that an electromagnetic wave such as light could be described by a particle called the photon with a discrete energy dependent on its frequency. This led to a theory of unity between subatomic particles and electromagnetic waves called wave –particle duality in which particles and waves were neither one nor the other, but had certain properties of both...

3.2) The Wave Properties of Particles
In 1924 Louis de Broglie postulated that because photons have both wave and particle characteristics, perhaps all forms of matter have both properties. According to de Broglie, electrons, just like light, have a dual particle - wave nature. Because the photon wavelength can be specified by its momentum: λ = h/p, De Broglie suggested that material particles of momentum p have a characteristic wavelength that is given by the same expression. Thus the de Broglie wavelength of a particle is
λ = h / p = h / m v                                             (2)

From equation 2, the magnitude of the momentum p of a particle of mass m and speed v is p = m v.
Furthermore, in analogy with photons, de Broglie postulated that a particle obey the Einstein relation E = h ƒ (Equation 1), where E is the total energy of the particle. Then the frequency of a particle is
ƒ = E / h

The dual nature of matter is apparent in these last two equations, because each contains both particle concepts (m v and E) and wave concepts (λ and ƒ).

4) The Electron Microscope
It was pointed out earlier, that the size of the smallest object observable by a microscope is about half the wavelength of the illuminating radiation. In light microscopes, this limits the resolution to about 200 nm (2000 A˚). Because of the wave properties of electrons, it is possible to construct microscopes with a resolution nearly 1000 times smaller than this value. It is relatively easy to accelerate electrons in an evacuated chamber to high velocities so that their wavelength is less than 10−10 m (1A˚). Furthermore, the direction of motion of the electrons can be altered by electric and magnetic fields. Thus, suitably shaped fields can act as lenses for the electrons. The short wavelength of electrons coupled with the possibility of focusing them has led to the
development of electron microscopes that can observe objects 1000 times smaller than are visible with light microscopes.
The basic construction of an electron microscope is shown in Figure 9. The similarities between the electron and the light microscope are evident; both have the same basic configuration of two lenses which produce two-stage of magnification.

Figure 9: The electron Microscope

4.1) Measurements
·   Electrons are emitted from a heated filament, and are then accelerated and collimated
   into a beam.
·   The beam passes through the thin sample under examination which diffracts the
   electrons in much the same way as light is diffracted in an optical microscope. But
   because of their short wavelength, the electrons are influenced by much smaller
   structures within the sample.
·   The transmitted electrons are focused into a real image by the objective lens.
·   This image is then further magnified by the projector lens, which projects the final
   image onto film or a fluorescent screen.

Although it is possible to produce electrons with a wavelength much less than 10−10m (1˚A), the theoretical optimum resolution implied by such short wavelengths has not yet been realized. At present, the best resolution of electron microscopes is about 5 x 10−10 m (5A˚).

4.1.1 The Optimum Measuring Conditions
a)Because electrons are scattered by air, the microscope must be contained in an
   evacuated chamber.
b)The samples under examination must be dry and thin.

These conditions, of course, present some limitations in the study of biological materials. The samples have to be specially prepared for electron microscopic examination. They must be dry, thin, and in some cases coated. Nevertheless, electron microscopes have yielded beautiful pictures showing details in cell structure, biological processes, and recently even large molecules such as DNA in the process of replication.

5) X-rays
X-rays are part of the electromagnetic spectrum. X-rays were first observed and documented in 1895 by Wilhelm Conrad Roentgen, a German scientist who found them quite by accident when experimenting with electron beams in a gas discharge tube. Roentgen noticed that, a fluorescent screen in his lab started to glow when the electron beam was turned on. This response in itself was not so surprising. Fluorescent material normally glows in reaction to electromagnetic radiation, but Roentgen's tube was surrounded by heavy black cardboard. Roentgen assumed this has blocked most of the radiation. Roentgen placed various objects between the tube and the screen, and the screen still glowed.  A week later, he took an X-ray photograph of his wife's hand which clearly revealed her wedding ring and her bone as shown in Figure 10. The photograph electrified the general public and aroused great scientific interest in the new form of radiation. Roentgen called it "X" to indicate it was an unknown type of radiation.

Figure 10:  An X-ray picture (radiograph), taken by Wilhelm Röntgen.                                                                                                                                                                                                                                                                

5.1) X-Ray Spectra
X-rays are emitted when high-energy electrons or any other charged particles bombard a metal target. The x-rays spectrum typically consists of a broad continuous band containing a series of sharp lines, as shown in Figure 11.   
5.1.1 Bremsstrahlung X-Rays (Broad band)
As we mentioned earlier, that an accelerated electric charge emits electromagnetic radiation. The x-rays in Figure 11 are the result of the slowing down of high-energy electrons as they strike the target. The electron loses all of its kinetic energy after several interactions with the atoms of the target. The amount of kinetic energy lost in any given interaction varying from zero up to the entire kinetic energy of the electron. Therefore, the wavelength of radiation from these interactions lies in a continuous range from some minimum value up to infinity. It is this general slowing down of the electrons that
provide the continuous curve in Figure 11, which shows the cutoff of x-rays below a minimum wavelength value that depends on the kinetic energy of the incoming electrons. X-ray radiation with its origin in the slowing down of electrons or continuous spectrum shown in the Figure is called Bremsstrahlung, the German word for “braking radiation.”

Figure 11: The x – rays spectrum of a metal target consists of a broad continuous spectrum plus a number
                   of sharp lines, which are due to characteristic x-rays. The data shown were obtained when 37-
                   ke V electrons bombarded a molybdenum target.

5.1.2 Characteristic X- rays (Sharp lines)
The discrete lines shown in Figure 11 called Characteristic x-rays which can be produced according to the following steps:
·   A bombarding electron collides with a target atom.
·   The electron with high energy, removes an electron from an innermost shell (K shell, n
   = 1) of the atom.
·   An electron from a next higher orbit (L shell, n = 2) drops down to fill the vacancy.
   The time interval for this to happen is very short, less than 10-9 s.
·   This transition is accompanied by the emission of a photon whose energy is equal to
   the energy difference between the two levels. Thus the photon emitted has an energy
   corresponding to the K α, the characteristic x-ray line on the curve in Figure 11.
   [In this notation K refers to the final level of the electron and the subscript α, refers to
    the initial level as the first one above the final level. Thus
Kα indicates that, a
    transition occur from energy level
L (n = 2) to energy level K (n = 1)].   
·   If the vacancy in the K (n = 1) shell is filled with electron dropping from the M shell (n
   = 3), the Kβ line in Figure 11 is produced.
   [K β is a transition from level M (n = 3) to level K (n=1)]
Typically, the energy of such transition is greater than 1 000 eV, and the emitted photons have wavelengths in the range of 0.01 nm to 1 nm.

Other characteristic x-rays lines are formed when electrons drop from upper levels to vacancies other than those in the K shell. For example, L lines are produced when vacancies in the L shell are filled by electrons dropping from higher shells. An Lα lines is produced as an electron drops from M shell (n = 3) to the L shell (n = 2), and an Lβ  is produced by a transition from the N shell (n = 4) to the L (n = 2) shell.

5.2) X-Rays Machines
The heart of an X-ray machine is an electrode pair, a cathode and an anode that sits inside a glass vacuum tube. The cathode is a heated filament. The machine passes current through the filament, heating it up. The heat sputters electrons off of the filament surface. The positively-charged anode, a flat disc made of tungsten, draws the electrons across the tube as shown in Figure 12.

Figure 12: The X - Ray Machine

The voltage difference between the cathode and anode is extremely high, so the electrons fly through the tube with a great deal of force. When a speeding electron collides with a tungsten atom, it knocks loose an electron in one of the atom’s lower orbital. An electron in a higher orbital immediately falls to the lower energy level, releasing its extra energy in the form of a photon. It’s a big drop, so the photon has a high energy level; it is an X-ray photon (Figure 13).

Figure 13: The free electron collides with the tungsten atom, knocking an electron out of a lower orbital.                       
                   A higher orbital electron fills the empty position, releasing its excess energy as a photon.

The high-impact collisions involved in x-ray production generate a lot of heat. A motor rotates the anode to keep it from melting (the electron beam isn’t always focused on the same area). A cool oil bath surrounding the envelope also absorbs heat. The entire mechanism is surrounded by a thick lead shield. This keeps the x-rays from escaping in all directions. A small window in the shield lets some of the x-ray photons escape in a narrow beam. The beam passes through a series of filters on its way to the patient. A camera on the other side of the patient records the pattern of x-ray light that passes all the way through the patient’s body (Figure 12).
Generally, doctors keep the film image as a negative. That is, the areas that are exposed to more light appear darker and the areas that are exposed to less light appear lighter. Hard material, such as bone, appears white, and softer material appears black or gray.

Finally, from the preceding discussion, one can show that two different processes give rise to radiation of x-ray frequency:
·   In one process; radiation is emitted by the high - speed electrons themselves as they
   are slowed or even stopped in passing near the positively charged nuclei of the anode
   material. This is the Brehmsstrahlung radiation.
·   In a second process; radiation is emitted by the electrons of the anode atoms when
   incoming electrons from the cathode knock electrons near the nuclei out of orbit and
   they are replaced by other electrons from outer orbits. X - rays produced in this way
   have definite energies just like other line spectra from atomic electrons. They are
   Characteristic x-rays. The spectral lines generated depend on the target (anode)
   element used and thus are called characteristic lines.
In medical applications, the target (anode) is usually tungsten or a more crack resistant alloy of rhenium (5%) and tungsten (95%), but sometimes molybdenum for more specialized applications, such as when soft x-rays are needed as in mammography.
In general, the spectrum of frequencies given off with any particular anode material thus consists of a continuous range of frequencies emitted in the first process, and superimposed on it a number of sharp peaks of intensity corresponding to discrete frequencies at which x- rays are emitted in the second process.

5.3) X-Rays Techniques
5.3.1 Medicine
1. X-ray Picture: Within three weeks of Roentgen’s announcement, two French
    physicians, Oudin and Barthelemy, obtained x-rays of bones in a hand. Since then, x-
    rays have become one of the most important diagnostic tools in medicine. With
    current techniques, it is even possible to view internal body organs that are quite
    transparent to x-rays. This is done by injecting into the organ a fluid opaque to x-
    rays. The walls of the organ then show up clearly by contrast.
2. X-ray Computerized Tomography (CT scan): The usual x-ray picture does not
    provide depth information. The image represents the total attenuation as the x-ray
    beam passes through the object in its path. For example, a conventional x-ray of the
    lung may reveal the existence of a tumor, but it will not show how deep in the lung the
    tumor is located. Several tomographic techniques (CT scans) have been developed to
    produce slice-images within the body which provide depth information. Presently the
    most commonly used of these is x-ray computerized tomography (CT scan) developed
    in the 1960. The basic principles of the technique in its simplest form illustrated in
    Figure 14 are:
·   A thin beam of x-rays passes through the plane we want to visualize and is detected by
   a diametrically opposing detector.
·   For a given angle with respect to the object (in this case the head), the x-ray source-
   detector combination is moved laterally scanning the region of interest as shown by the
   arrow in Figure 14 (a).
·   At each position, the detected signal carries integrated information about x-ray
   transmission properties of the full path in this case A−B.
·   The angle is then changed by a small amount (about 1˚) and the process is repeated full
   circle around the object.

As indicated in Figure 14(b), by rotating the source-detector combination, information can be obtained about the intersection points of the x-ray beams. The Figure shows schematically the scanning beam at two angles with two lateral positions at each angle. While at each position, the detected signal carries integrated information about the full path, two paths that intersect contain common information about the one point of intersection. In the Figure, four such points are shown at the intersection of the beams AB, A′B′, C D, and C′D. The multiple images obtained by translation and rotation contain information about the x-ray transmission properties of each point
within the plane of the object to be studied. These signals are stored and by a rather complex computer analysis a point by point image is constructed of the thin slice scanned within the body.

Figure 14: (a) Basic principle of computed axial tomography.
                   (b)Rotation of the source-detector combination provides information about the x-ray
                        transmission properties of each point within the plane of the object to be studied.

   The visualized slices within the body obtained in this way are typically about 2 mm
   thick. In the more recent versions of the instrument, a fan rather than a beam of x-rays
   scans the object, and an array of multiple detectors is used to record the signal. Data
   acquisition is speeded up in this way yielding an image in a few seconds.

5.3.2 Biology
Crystallography: x-rays have also provided valuable information about the structure of biologically important molecules. The technique used here is called crystallography and can be expressed as follows:
The wavelength of x-rays is on the order of 10−10 m, about the same as the distance between atoms in a molecule or crystal. Therefore, if a beam of x- rays is passed through a crystal, the transmitted rays produce a diffraction pattern that contains information about the structure and composition of the crystal. The diffraction pattern consists of
regions of high and low x-ray intensity which when photographed show spots of varying brightness (Figure 15).

Figure 15:  Arrangement for detecting diffraction of X-rays by a crystal.

Diffraction studies are most successfully done with molecules that can be formed into a regular periodic crystalline array. Many biological molecules can in fact be crystallized under the proper conditions. It should be noted, however, that the diffraction pattern is not a unique, unambiguous picture of the molecules in the crystal. The pattern is a mapping of the collective effect of the arrayed molecules on the x-rays that pass through the crystal. The structure of the individual molecule must be deduced from the indirect evidence provided by the diffraction pattern.
If the crystal has a simple structure such as sodium chloride, the x-ray diffraction pattern is also simple and relatively easy to interpret. Complicated crystals, however, such as those synthesized from organic molecules, produce very complex diffraction patterns. But, even in this case, it is possible to obtain some information about the structure of the molecules forming the crystal. To resolve the three-dimensional features of the molecules, diffraction patterns   must be formed from thousands of different angles. The patterns are then analyzed, with the aid of a computer. These types of studies provided critical information for the determination of the structure for penicillin, vitamin B12, DNA, and many other biologically important molecules.

5.4) Medical Applications of X-Ray
X- Ray radiation, used in medicine into two different ways:
1.  Diagnosis (imaging)
2. Therapy [The treatment of neoplastic disease (abnormal growth of cells) by using x-
     rays to prevent or to slow the proliferation of malignant cells by decreasing the rate of
     mitosis or impairing DNA synthesis].

5.4.1 Diagnostic Imaging
X-ray radiation is used for imaging internal body structures, hard and soft tissue, for diagnosis; x-rays hard tissues such as bones and teeth absorb more x-rays and show as lighter areas on x-ray film; A contrast medium can be used to highlight soft tissues in
still x-ray pictures, or can be followed on x-ray motion-picture films as it moves through the body or part of the body to record body processes.
Contrast Medium; in a normal x-ray picture, most soft tissue doesn't show up clearly. To focus in on organs, or to examine the blood vessels that make up the circulatory system, doctors must introduce contrast media into the body. Contrast media are liquids that absorb x-rays more effectively than the surrounding tissue. To bring organs in the digestive and endocrine systems into focus, a patient will swallow a contrast media mixture, typically a barium compound. If the doctors want to examine blood vessels or other elements in the circulatory system, they will inject contrast media into the patient's bloodstream.

Arthrography (Joint x-ray): Arthrography is the x-ray examination of joint that uses a special form of x-ray called fluoroscopy and a contrast material containing iodine. Fluoroscopy makes it possible to see internal organs in motion. When iodine is injected into the joint space, it coats the inner lining of the joint structures and appears bright white on an arthrogram, allowing the radiologist to assess the anatomy and function of the joint (areas where the dye leaks out indicate a tear in the tendons). Arthrographic images help physicians evaluate alterations in structure and function of a joint and help to determine the possible need for treatment, including surgery or joint replacement. The procedure is most often used to identify abnormalities within the shoulder, wrist, hip, knee (Figure 16), and ankle.

Figure 16: C T Arthrogram of the knee.

Catheter Angiography: In catheter angiography, a thin plastic tube, called a catheter, is inserted into an artery through a small incision in the skin. Once the catheter is guided to the area being examined, a contrast material is injected through the tube and images are captured using a small dose of ionizing radiation (x-rays). Catheter angiography is used to examine blood vessels in key areas of the body including the brain, kidneys, pelvis, legs, lungs, heart, and neck.

Mammography: Mammography is a specific type of imaging that uses a low dose x-ray system to examine breasts. A mammography exam, called a mammogram, is used to aid in the diagnosis of breast diseases.

Bone X-ray: A bone x-ray shows images of bones in the body, including the hand (Figure 17), wrist, arm, foot, ankle, knee and leg. A bone x-ray is used to determine whether a bone has been fractured or if a joint is dislocated. It also can ensure that a fracture has been properly aligned and stabilized for healing following treatment. The image can show whether there is a build up of fluid in the joint or around a bone. It can also evaluate injury or damage from conditions such as infection, arthritis, abnormal bone growths or other bone diseases, such as osteoporosis. This technique is also used to assist in the detection and diagnosis of cancer.

 Figure 17: X-ray showing frontal view of both hands.

Lower Gastrointestinal Tract X-ray: Lower gastrointestinal tract radiography, also called a lower GI, is an x-ray examination of the large intestine (colon) as shown in Figure 18. This includes the examination of the right or ascending colon, the transverse colon, the left or descending colon and the rectum. The appendix and a portion of the small intestine may also be included. The lower GI use x-ray (fluoroscopy) and a contrast material (barium). A physician may order a lower GI examination to detect; ulcers, benign tumors, cancer, and signs of other intestinal diseases.

Figure 18: This image shows the right side of the large intestine. Air (dark) distends the bowel and barium
                   (white) coats the inner lining.

Renal Examination: X-raying the kidneys, are usually done after injecting a contrast medium into a vein. A series of x-rays is then taken to show the renal outline and collecting system and structures of the kidneys, as well as ureters and the bladder.

Hysterosalpingography: Hysterosalpingography is primarily used to examine women,  who have difficulty to become  pregnant  by  allowing  the  radiologist to  evaluate the shape and structure of the uterus, the openness of the fallopian tubes. The procedure can be used to investigate repeated miscarriages that result from congenital abnormalities of the uterus, and to determine the presence and severity of these abnormalities, including: tumor masses or adhesions uterine fibroids (The word hystero means uterus, while salpingo means tubes in latin).                    

Dental X-rays: Structures that are dense (such as silver fillings or metal restoration) will block most of the photons and  will  appear  white  on developed   film. Air beneath the teeth will  be  black  on film,  teeth and tissue will appear as shades of gray. The test is performed in the dentist's clinics. There are four types of x-rays:
1. Bite-wing: The bite-wing is when the patient bites on a paper tab and shows the crown
    portions of the upper and lower teeth together (Figure19).

Figure 19: Bite-Wings X-rays are some of the most typical x-rays a Dentist will take.  These types of x-
                   rays are very effective at discovering tooth decay. It is called Bitewing because the x-ray
                   film holder provides a surface to bite down on and hold the x-ray securely in place.

2. Periapical X-rays: The periapical x-rays shows one or two complete teeth from crown
     to root (Figure 20).

Figure 20: Periapical X-rays are taken to get a more effective examination of the entire tooth area from
                   crown to root.  These types of x-rays provide a complete side view and typically a complete
                   set consists of 14 films with each tooth appearing in two different films from two different

3. Occlusal X-rays: Occlusal x-ray captures the upper or the lower teeth in one shot,
    while the film rests on the biting surface of the teeth (Figure 21).

Figure 21: Occlusal X-rays are less common than either Bite-Wing or Periapical X-rays. These types of
                    x-rays are taken to show the lower or the upper jaw.

4. Panoramic: Panoramic x-ray requires a special machine that rotates around the head.
    The x-ray captures the entire jaws and teeth in one shot. It's used to plan treatment for
    dental implants, check for impacted wisdom teeth, and detect jaw problems. A
    panoramic x-ray is not good for detecting cavities, unless the decay is very advanced
    and deep( Figure 22).

Figure 22: Panoramic or Panorex (a type of film) X-ray is also very commonly done on an "initial" visit
                   of a dentist.  As the name suggests, a Panoramic X-ray makes a complete half circle from ear
                   to ear to produce a complete two dimensional representation of all teeth.  Panoramic X-rays
                   give the dentist an overall picture of all your teeth and jaw bones.

In General, dental x-rays may be used to identify the number, size, and position of teeth, unemerged or impacted teeth, the presence and extent of dental cavities, bone damage (such as from periodontitis), abscessed teeth, fractured jaw.

5.4.2 X-ray Therapy Radiation Treatment
This kind of treatment is based on the uses of high energy, penetrating waves or particles such as x-rays to destroy cancer cells or keep them from reproducing. One of the
characteristics of cancer cells is that they grow and divide faster than normal cells. Radiation also damages normal cells, but because normal cells are growing more slowly, they are better able to repair radiation damage than are cancer cells. In order to give normal cells time to heal and reduce side effects, radiation treatments are often given in small doses over a six or seven week period. It is used in more than half of all cancer cases. Radiation therapy can be used in different ways:
·   Alone to kill cancer.
·   Before surgery to shrink a tumor and make it easier to remove.
·   During surgery to kill cancer cells that may remain in surrounding tissue after the surgery.
·   After surgery to kill cancer cells remaining in the body.
·   To shrink an inoperable tumor in order to reduce pain and improve quality of life in combination with chemotherapy.

6) Lasers
In this section, we will explore the nature of laser light and apply our knowledge of atomic structure to describe the mechanism involved in the operation of laser. This will be followed by describing a variety of applications of lasers in our technological society.

6.1) The Nature of Laser Light. 
6.1.1 Properties of Laser Light
The primary properties of laser that make it useful in these technological applications are:
·   Laser light is coherent; the individual rays in a laser beam maintain a fixed phase
   relationship with each other.
·   Laser light is monochromatic; light in a laser beam has a very narrow range of
·   Laser light has a small angle of divergence: the beam spreads out very little, even over
   long distances.
6.1.2 Lasers – Operation
In order to understand the origin of these properties, let us combine our knowledge of atomic energy levels from this chapter with some special requirements for the atoms that emit laser light.
We have described, how an incident photon can cause atomic energy level transitions either upward (stimulated absorption) or downward (stimulated emission). The two situations are equally probable. When light is incident on a collection of atoms, a net absorption of energy usually occurs because when the system is in thermal equilibrium, many more atoms are in the ground state than in the excited states. The situation can be inverted so that more atoms are in excited states than in the ground state, a net emission of photons can result. Such a condition is called population inversion.
Population inversion is, in fact, the fundamental principle involved in the operation of a laser (an acronym for light amplification by stimulated emission of radiation). The full name indicates one of the requirements for laser light: this means that, to achieve laser action, the process of stimulated emission must occur.
Suppose an atom is in the excited state E2, as in Figure 23, when a photon with energy hƒ= E2 – E1 is incident on it, we can show that:

·   The incoming photon can stimulate the excited atom to return to the ground state and thereby emit a second photon having the same energy and traveling in the same direction. The incident photon is not absorbed, so after the stimulated emission, there are two identical photons: the incident photon and the emitted photon. The emitted photon is in phase with the incident photon.
·   These photons can stimulate other atoms to emit photons in a chain of similar processes. The many photons produced in this fashion are the source of the intense, coherent light in a laser. 

Figure 23: Stimulated emission of a photon by an incoming photon of energy = E2 – E1.      
 Initially, the atom is in the excited state. The incoming photon stimulates the atom to emit a
                      second photon of energy given by = E2 – E1.

6.1.3 Conditions for Build-Up of Photons
For the stimulated emission to result in laser light there must be a buildup of photons in the system. The following three conditions must be satisfied in order to achieve this buildup:
1. The system must be in a state of population inversion; there must be more atoms in an
    excited state than in the ground state. That must be true because the number of
    photons emitted must be greater than the number absorbed.
2. The excited state of the system must be a metastable state; which means that its
    lifetime must be long compared with the usually short lifetimes of excited state, which
    are typically 10-8s. In this case, the population inversion can be established and
    stimulated emission is likely to occur before spontaneous emission as shown in
    Figure 24.
    [Spontaneous emission; Once an atom is in an excited state, the excited atom can
    make a transition to a lower energy level, because this process happens naturally, it is
    known as spontaneous emission].

Figure 24: Spontaneous emission of a photon by an atom that is initially in the excited state E2.            
                     When the atom falls to the ground state, it emits a photon of energy = E2 – E1.

3. The emitted photons must be confined in the system long enough to enable them to
    stimulate further emission from other excited atoms. That is achieved by using
    reflecting mirrors at the ends of the system. One end is made totally reflecting, and the
    other is partially reflecting. A fraction of the light intensity passes through the
    partially reflecting end, forming the beam of laser light (Figure 25).

Figure 25: Schematic diagram of a laser design.
                  The tube contains the atoms that are the active medium. An external source of energy (for
                  example, an optical or electrical device) pumps the atoms to the excited states. The parallel end
                  mirrors confine the photons to the tube, but mirror 2 is only partially reflective.

6.2) Laser Construction
A laser is effectively a machine that makes billions of atoms pump out trillions of photons all at once so they line up to form a really concentrated light beam.
A red laser contains a long crystal made of ruby as shown in Figure 26 as a red bar with a flash tube (yellow zig-zag lines) wrapped around it. The flash tube looks a bit like a
fluorescent strip light, only it's coiled around the ruby crystal and it flashes every so often like a camera's flash gun.

Figure 26: A construction for laser light by using a crystal and a flash tube.

The following represents how the flash tube and the crystal make laser light:
1. A high-voltage electric supply makes the tube flash on and off.                                 
2. Every time the tube flashes, it "pumps" energy into the ruby crystal. The flashes it
    makes inject energy into the crystal in the form of photons.
3. Atoms in the ruby crystal (large green blobs) soak up this energy in an absorption
    process. When an atom absorbs a photon of energy, one of its electrons jumps
    from a low energy level to a higher one. This puts the atom into an excited state, but
    makes it unstable. Because the excited atom is unstable, the electron can stay in the
    higher energy level only for a few milliseconds. It falls back to its original level,
    living off the energy it absorbed as a new photon of light radiation (small blue blob).
    This is the spontaneous emission process.
4. The photons that atoms give off zoom up and down inside the ruby crystal, traveling
    at the speed of light.
5. Every so often, one of these photons hits an already excited atom. When this happens,
    the excited atom gives off two photons of light instead of one. This is the stimulated
emission described earlier
. Now one photon of light has produced two and the light
    has been amplified (increased in strength). In other words, "light amplification" (an
    increase in the amount of light) has been caused by "stimulated emission of radiation"
    (hence the name "laser", because that's exactly how a laser works!)
6. A mirror at one end of the laser tube keeps the photons bouncing back and forth inside
    the crystal.
7. A partial mirror at the other end of the tube bounces some photons back into the
     crystal but lets some escape.
8. The escaping photons form a very concentrated beam of powerful laser light.

6.3) Classifications of Lasers
Lasers are classified into four broad areas depending on the potential for causing biological damage. Safety thresholds for lasers are expressed in terms of Maximum
Permissible Exposure (MPE). When you use a laser, it should be labeled with one of these four classes designations:
Class 1
These lasers cannot emit laser radiation at known hazard levels under normal operating conditions, because they are completely enclosed (prohibit or limits access to the laser radiation). Examples of this class are: laser printers and laser disc players.
Class 1a
This is a special designation that applies only to lasers that are "not intended for viewing," such as a supermarket laser scanner. The upper power limit of class1a  is 4.0 mW.
Class 2
These are low-power visible lasers that emit above class 1 levels but at a radiant power not above 1 mW. The concept is that the human aversion reaction to bright light will protect a person and are capable of creating eye damage through chronic exposure. The human eye blink reflex, which occurs within 0.25 seconds of exposure to Class 2 laser beam. Examples are: Helium-Neon lasers and some laser pointers.
Class 3a
These are intermediate - power lasers from 1 to 5 mW, which are hazardous only for intra-beam viewing. A warning label shall be placed on or near the laser in a conspicuous location of caution users, to avoid staring into the beam or directing the beam toward the eye of individuals. Example of this type of laser: Helium-Neon lasers, some solid state laser pointers and most pen-like pointing lasers are in this class.
Class 3b
These are moderate-power lasers from 5 mW to 500 mW . These lasers will produce an eye hazard if viewed directly.   
Class 4
These are high-power lasers from 500 mW if continuous wave (cw), or 10 J/cm2 if pulsed, which are hazardous to view under any condition.

6.4) Types of Lasers 
There are many different types of lasers. The laser medium can be a solid, gas, liquid or semiconductor. A laser concentrates high energies into an intense narrow beam of no divergent monochromatic electromagnetic radiation; Lasers using various substances (ruby, argon, krypton, neodymium, helium-neon, carbon dioxide) are available. Lasers are commonly designated by the type of lasing material employed.
Solid-State Lasers: is a high-power laser operating in the infrared, used for cutting, welding and marking of metals and other materials, have lasing material distributed in a solid matrix (such as the ruby or neodymium: yttrium-aluminum garnet "YAG" lasers).
Gas Lasers: helium and helium-neon, He-Ne are the most common gas lasers, have a primary output of visible red light. CO2 lasers emit energy in the far-infrared, 10.6 micrometer, and are used for cutting hard materials.
Excimer Lasers: Use reactive gases, such as chlorine and fluorine, mixed with inert gases such as argon, krypton or xenon. When electrically stimulated, a pseudomolecule or dimmer is produced. When lased, the dimmer produces light in the ultraviolet range (the name excimer is derived from the terms excited and dimmers).

Dye Lasers: use complex organic dyes, such as rhodamine 6G, in liquid solution or suspension as lasing media. They are tunable over a broad range of wavelengths
Semiconductor Lasers: sometimes called diode lasers, are not solid- state lasers. These electronic devices are generally very small and use low power. They may be built into larger arrays, such as the writing source in some laser printers , laser pointers or CD players.

6.5) Applications
There are many scientific, military, medical and commercial laser applications which have been developed since the invention of the laser in the 1958. The coherency, high monochromatic, and ability to reach extremely high powers, are all properties which allow for these specialized applications of laser.
Commercial: The most widespread use of lasers is in optical storage devices  
such as compact disc and DVD players, in which the laser (a few millimeters in size) scans the surface of the disc. Other common applications of lasers are bar code readers, laser printers and laser pointers.
Defense: Marking targets, guiding munitions, missile defense, electro-optical countermeasures (EOCM) and alternative to radar.
Industry: Lasers are used for cutting and welding the steel and other metals and for inscribing patterns (such as the letters on computer Key boards).
6.5.1 Medical Applications
Because various laser wavelengths can be absorbed in specific biological tissues, lasers have a number of medical applications:
·   Certain laser procedures have greatly reduced blindness in patients with glaucoma and
diabetes. Glaucoma is a widespread eye condition characterized by a high fluid
pressure in the eye, a condition that can lead to destruction of the optic nerve. A simple
laser operation, (iridectomy) can “burnˮ open a tiny hole in a clogged membrane relieving the destructive pressure. A serious side effect of diabetes is neovascularization, the proliferation of weak blood vessels, which often leak blood. When neovascularization occurs in the retina, vision deteriorates (diabetic retinopathy) and finally is destroyed. Today, it is possible to direct the green light from an argon ion laser through the clear eye lens and eye fluid, focus on the retina edges, and photocoagulate the leaky vessels. Even people who have only minor vision defects such as nearsightedness are benefiting from the use of lasers to reshape the cornea,  
changing its focal length and reducing the need for eyeglasses. 
·   Laser surgery is now an everyday occurrence at hospitals and medical clinics around the word. Infrared light at 10 μ m from a carbon dioxide laser can cut through muscle tissue, primarily by vaporizing the water contained in cellular material. Laser power of approximately 100 W is required in this technique. The advantage of the “laserˮ knife over conventional methods is that laser radiation cut tissue and coagulates blood at the same time, leading to a substantial reduction in blood loss. In addition, the technique virtually eliminates cell migration, an important consideration when tumors are being removed.
·   A laser beam can be trapped in fine optical fiber light guides (endoscopes) by means of total internal reflection. An endoscope can be introduced through natural orifices, conducted around internal organs, and directed to specific interior body locations, eliminating the need for invasive surgery. For example, bleeding in the gastrointestinal tract can be optically cauterized by endoscopes inserted through the patient's mouth.    
·   In medical and biological research, it is often important to isolate and collect unusual cells for study and growth. A laser cell separator exploits the tagging of specific cells with fluorescent dyes. All cells are then dropped from a tiny charged nozzle and laser- scanned for the dye tag. If triggered by the correct light-emitting tag, a small voltage applied to parallel plates deflects the falling electrically charged cell into a collection beaker.
6.5.2 Laser surgery
Before lasers could be successfully used in surgical procedures, a wide range of studies had to be conducted to understand the effect of intense light on various types of tissues. Further, technology had to be developed for precise control of light intensity and duration and for accurate positioning of the focal point. While the surgical use of lasers is growing in many areas of medicine and dentistry, the positional accuracy of laser tissue-removal is particularly important in neurosurgery and eye surgery where a fraction of a millimeter offset can make the difference between success and failure.
In laser skin resurfacing, a laser is used to remove areas of damaged or wrinkled skin, layer by layer. The procedure is most commonly used to minimize the appearance of fine lines, especially around the mouth and the eyes. However, it is also effective in treating facial scars or areas of uneven pigmentation, sun damaged skin, acne scars, broken blood vessels, wrinkles, (Figure 27 (a), (b) and (c)) and facial redness. Laser resurfacing may be performed on the whole face or in specific regions.



Figure 27: (a) Acne Scars, (b) Broken Blood Vessels, and (c) Wrinkles.

Ophthalmologists were among the first to use lasers for a wide range of procedures. Figure 28 describes the treatment of the disorder of the eye to restore normal vision (lasik eye surgery).
(a) The cornea is the transparent part of the eye that covers the iris. It is also the main      
      light bending part of the eye.                      

(b) Anesthetic eye drops are given to numb the eye, and the surgeon marks the cornea
      with water-soluble ink to guide replacement of the corneal flap.


(c) The surgeon performs a keratectomy which creates a corneal flap. The flap is lifted
      and reflected exposing the cornea beneath (A keratectomy is a procedure that uses a
      small instrument that makes a cut in the cornea as it moves across

(d) A computer-controlled laser reshapes the cornea to the prescribed shape for clear


(e) The corneal flap is repositioned and bonds to the cut edge of the cornea quickly.


Figure 28: Lasik Eye Surgery.

Laser procedure promises to eliminate the need for radiation, preserve speech, shorten treatment time and significantly improve care in other ways for many patients whose cancer is diagnosed early.  The therapy uses heat from laser to destroy the tumor’s blood supply and cancer cells, which damages surrounding tissues far less than radiation and different types of lasers.

Otolaryngology is the use of laser systems for excising skin tumors, cancers, birthmarks and other disorders of the head and neck; problems of the voice box, throat, mouth, nose and ear may be amenable to different laser treatments. Nodules or polyps on the larynx and blood vessel defects in the upper airway are disorders that could be treated with laser. In another instance, laser surgery might be performed to remove the stapes from the middle ear for treatment of otosclerosis. A new outpatient could eliminate the need for radiation in treating early cancer of the larynx, or voice box. Pulses of green laser light selectively destroy blood vessels feeding the tumor without burning the vocal cords as shown in Figure 29.

Figure 29: The use of laser in treating tumor in the voice box.

Finally, laser Surgery is often referred to as "bloodless surgery," laser procedures usually involve less bleeding than conventional surgery. The heat generated by the laser keeps the surgical site free of germs and reduces the risk of infection. Because a smaller incision is required, laser procedures often take less time (and cost less money) than traditional surgery. Sealing off blood vessels and nerves reduces bleeding, swelling, scarring, pain, and the length of the recovery period.

Dental Laser
A dental laser (Figure 30) is a type of laser designed specifically for use in oral surgery or dentistry.

Figure 30: Dental Laser

Lasers are so precise in what they can do; there are several unique types of lasers used in dentistry for varied purposes in hard or soft tissue. They are designed with different wavelengths that have different absorption capabilities that recognize and interact with different types of human tissue. For example,longer wave lasers such as the Er: YAG (erbium) lasers, are used for teeth and bone because they have an affinity for water and can recognize the decayed part of the tooth because of the differences in water content between solid and decayed tooth structure.
The laser emits light energy either in continuous or pulsed states. Most dental lasers involve pulsed energy release. The pulse, or burst of light, delivers the laser’s heat energy quickly (each pulse lasts only a few ten thousandths of a second) and then allows the tissue a time to cool before initiating another blast. This heat energy pulse works by either boiling away water within and between the cells, or by causing micro expansion of hard tissue, resulting in a tiny explosion that removes the unwanted tissue. The procedure consists of a series of “pops” that work to remove decay or cut away tissue or tooth structure.
When laser used for surgical procedures, it acts as a cutting instrument or a vaporizer of tissue. When used for cutting, a-filling, the laser helps to strengthen the bond between the filling and the tooth. When used in teeth whitening procedures, the laser acts as a heat source.
Several variants of dental laser are in use, with the most common being diode lasers, carbon dioxide lasers and yttrium aluminum garnet laser. Different lasers use different wavelengths and these mean they are better suited for different applications. For example, diode in the 810–900 nm range are well absorbed by red colored tissues such as the gingival, increasingly being used in place of electro surgery and standard surgery for soft tissue applications such as tissue contouring and gingivectomy.
There are many different types of dental lasers, but all can be classified as one of two kinds of hard or soft tissue dental lasers. Laser dentistry is a new technique that can
improve the precision of your treatment while minimizing pain and recovery time (Figure 31).

Figure 31: The use of dental laser in oral surgery.  

The application of lasers in dentistry opens the door for dentists to perform a wide variety of dental procedures; they otherwise may not be capable of performing as:
·   Benign Tumors: Dental lasers may be used for the painless and suture-free removal of
   benign tumors from the gums, palate, sides of cheeks, and lips.
·   Cavity Detector: Low intensity soft tissue dental lasers may be used for the early
   detection of cavities by providing a reading of the by-products produced by decay.
·   Tooth and Gum Tissues: Optical Coherence Tomography (CT scan) is a safer way to
   see inside tooth and gums in real time.
·   Cold Sores: Low intensity dental lasers reduce pain associated with cold sores and
   minimize healing time.
·   Crown Lengthening: Dental lasers can reshape gum tissue and bone to expose
   healthier tooth structure. Such reshaping called crown lengthening; provides a stronger
   foundation for a restoration.
·   Dental Fillings: Hard tissue dental lasers may eliminate the need for a local anesthetic
   injection and the traditional turbine drill. Lasers used in dental filling procedures are
   capable of killing bacteria located in a cavity and this may lead to better long term
   tooth restorations.
·   Soft Tissue Folds (Epulis): Dental lasers may be used for the painless and suture-free
   removal of soft tissue folds often caused by ill-fitting dentures.
·   Teeth Whitening: Low intensity soft tissue dental lasers may be used to speed up the
   bleaching process associated with teeth whitening and with the use of a whitening gel. 
   The translucent bleaching gel is applied to the teeth and a laser light is used to activate
   the crystals to absorb the energy from the light and penetrate the teeth enamel to
   increase the lightening effect on the teeth. The length of time in the cosmetic dentist's
   chair depends on the degree of discoloration you have (Figure 32).

Figure 32:  Before and after using laser for whitening.

·   Temporomandibular Joint Treatment: Dental lasers may be used to quickly reduce
   pain and inflammation of the temporomandibular jaw joint.
·   Gummy Smile: Dental lasers can reshape gum tissue to expose healthy tooth structure
   and improve the appearance of a gummy smile.
·   Tooth Sensitivity: Dental lasers may be used to seal tubules (located on the root of the
   tooth) that are responsible for hot and cold tooth sensitivity.

أعلى النموذجأسفل النموذج
1)Serway, R.  and Jewett, J., Physics for Scientists and Engineers with  ModernPhysics,
    7 th  Edition, Thomson , Brooks/COLE 2008.
2)Davidovits, P. Physics in Biology and Medicine, 3rd Edition, Elsevier 2007.
     2007.ppt#281,3, How X-rays interact with the tissues in your body

114)http://geocities. com/muldoon432/Laser_Types_and_Classifications.htm.

Assignment 5

1-Explain the operation of a spectrometer and describe two possible uses for this device.
2-How are X ray produced?
3-What are the applications of X rays in medicine?
4-Describe the 4 types of X ray images used in dentistry.
5-Describe the process of X-ray computerized tomography. What information does this
   process provide that ordinary X-ray images do not?
6-Calculate the frequency of a photon of energy 4.8 x 10-19 J;
                                                                                                              [ƒ= 7.24 X 1014 Hz]
7-Calculate the de Broglie wavelength for a proton moving with a speed of 106m/s.
   [mass of proton = 1.67 X 10-27 kg]
                                                                                                [λ = 3.97 X 10-13m]
8-Describe the operation of a laser. Include a description of the method for obtaining the
   inverted population distribution.
9-What are the applications of laser in medicine in general and in dentistry in particular?

1)Serway, R. and Jewett, J., Physics for Scientists and Engineers with Modern Physics,    
   7 th 
Edition, Thomson , Brooks/COLE 2008.
2)Davidovits, P. Physics in Biology and Medicine, 3rd Edition, Elsevier 2007.
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