Section 16.7
Lasers
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FIGURE 16.10 Argon-ion laser. From www.nationallaser.com produce short-duration highly intense light pulses, others operate in a continuous mode. Lasers are now widely used in science, technology and increasingly also in medicine. Figure 16.10 shows an Argon-ion laser that emitsgreen or blue light (depending on settings) and is one of the lasers frequentlyused in medical applications.
16.7.1 Lasers 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 lighton various types of tissue. Further, technology had to be developed for precisecontrol of light intensity and duration and for accurate positioning of the focalpoint. While the surgical use of lasers is growing in many areas of medicineand dentistry, the positional accuracy of laser tissue-removal is particularlyimportant in neurosurgery and eye surgery where a fraction of a millimeteroffset can make the difference between success and failure.
Ophthalmologists were among the first to use lasers for a wide range of procedures. The repair of retinal detachments and retinal tears is one suchapplication. As a result of trauma or disease, the retina may detach from theback of the eye or may develop tears. Left untreated this condition leads to aloss of vision. Laser procedures have been very successful in arresting the retinal degeneration and restoring normal vision. Laser light is focused through
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Chapter 16
Atomic Physics
FIGURE 16.11 Physician performing laser eye surgery. From www.trustyguides.com the iris onto the boundary of the detached or torn region of the retina. Thetissue is burnt and the subsequent scarring “welds” the retina to the underlying tissue.
In another ophthalmological application, lasers are used to treat diabetic retinopathy. Diabetes often causes disorders in blood circulation includingleaks in the retinal blood vessels. Such a condition can cause serious damageto the retina and the optic nerve. Laser light focused on the damaged bloodvessel seals the leak and halts further retinal deterioration. Unfortunately, thecourse of the disease is not halted and new leaks develop that require repeatedtreatments. Figure 16.11 shows a typical eye surgery setup.
A relatively recent but now widely used application of lasers in ophthal mology is the LASIK technique (Laser-Assisted in Situ Keratomileusis). Thisis a laser surgical procedure that reshapes the cornea with the aim of correcting focusing problems associated with myopia, hyperopia and astigmatism. Inthis procedure, the computer that controls the laser is first programmed for theamount and location of the corneal tissue to be removed. Then using a cuttinginstrument called a microkeratome a flap is cut in the front part of the cornea Exercises
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and the flap is folded back. The mid-part of the cornea is reshaped by thecomputer controlled laser pulses that deliver the correct amount of energy toevaporate the corneal tissue at the set locations. As a result of this procedure
the need for eyeglasses is often eliminated.
EXERCISES
16-1. Explain the operation of a spectrometer and describe two possible uses for this device.
16-2. Describe the process of X-ray computerized tomography. What infor mation does this process provide that ordinary X-ray images do not?
16-3. Describe the operation of a helium–neon laser. Include a description of the method for obtaining the inverted population distribution.
16-4. Two laser commonly used in laser surgery are the CO2 laser and the argon-ion laser. Describe the method for obtaining the invertedpopulation distribution in these two lasers.
Nuclear Physics
17.1
The Nucleus isotopes. All the nucleiof the oxygen atom, for example, contain 8 protons but the number of neutronsin the nucleus may be 8, 9, or 10. These are the isotopes of oxygen. They aredesignated as 16O, 17O, and 18O. This is a general type of nuclear symbolism
8
8
8
in which the subscript to the chemical symbol of the element is the number ofprotons in the nucleus and the superscript is the sum of the number of protonsand neutrons. The number of neutrons often determines the stability of thenucleus.
The nuclei of most naturally occurring atoms are stable. They do not change when left alone. There are, however, many unstable nuclei whichundergo transformations accompanied by the emission of energetic radiation.
It has been found that the emanations from these radioactive nuclei fall intothree categories: (1) alpha (α) particles, which are high-speed helium nucleiconsisting of two protons and two neutrons; (2) beta (β) particles, which arevery high-speed electrons; and (3) gamma (γ) rays, which are highly energeticphotons.
The radioactive nucleus of a given element does not emit all three radiations simultaneously. Some nuclei emit alpha particles, others emit beta particles,and the emission of gamma rays may accompany either event.
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Radioactivity is associated with the transmutation of the nucleus from one element to another. Thus, for example, when radium emits an alpha particle,the nucleus is transformed into the element radon. The details of the processare discussed in most physics texts (see [16-2]).
The decay or transmutation of a given radioactive nucleus is a random event. Some nuclei decay sooner; others decay later. If, however, we dealwith a large number of radioactive nuclei, it is possible, by using the laws ofprobability, to predict accurately the decay rate for the aggregate. This decayrate is characterized by the half-life, which is the time interval for half theoriginal nuclei to undergo transmutation.
There is a great variation in the half-life of radioactive elements. Some decay very quickly and have a half-life of only a few microseconds or less.
Others decay slowly with a half-life of many thousands of years. Only thevery long-lived radioactive elements occur naturally in the Earth’s crust. One
of these, for example, is the uranium isotope 238U, which has a half-life of
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4.51 × 109 years. The short-lived radioactive isotopes can be produced inaccelerators by bombarding certain stable elements with high-energy particles.
Naturally occurring phosphorus, for example, has 15 protons and 16 neutronsin its nucleus (31P). The radioactive phosphorus isotope 32P with 17 neutrons
15
15
can be produced by bombarding sulfur with neutrons. The reaction is 32S + neutron → 32P + proton
16
15
This radioactive phosphorus has a half-life of 14.3 days. Radioactive isotopesof other elements can be produced in a similar way. Many of these isotopeshave been very useful in biological and clinical work.
17.2 Magnetic Resonance Imaging (MRI), introduced in the early 1980s, is the most recentaddition to medical imaging techniques. This technique utilizes the magneticproperties of the nucleus to provide images of internal body organs with detailedinformation about soft-tissue structure.
The imaging techniques we have discussed so far (X-ray and ultrasound) are in principle relatively simple. They utilize reflected or transmitted energyto visualize internal structures. Magnetic resonance imaging is more complex.
It utilizes the principles of nuclear magnetic resonance (NMR) developed in