1. Classroom notes for: Radiation and Life 98.101.201 Professor: Thomas M. Regan Pinanski 207 ext 3283

2. The Four Forces Gravity is an attractive force that acts between all objects with mass. It is a 1/r 2 law; that is, doubling the distance between two objects with mass will serve to decrease the gravitational attraction between them by a factor of four (strictly speaking, this is only true for point masses or spherical bodies). Likewise: tripling the distance will decrease the attraction by a factor of nine; quadrupling the distance will decrease the attraction by a factor of sixteen, and so on… It is (as far as we know) always attractive. There are theories that postulate that antigravity can exist under conditions such as those present at the start of the universe; for the purposes of our class we can safely discuss gravity as being an attractive force only. It is by far the weakest of the forces.There is no danger that the overhead projector will ever fly at me due to gravitational attraction.

3. However, because the earth is so massive, gravity dominates interactions between objects on a macroscopic scale. As a force, it is measured in newtons (kg*m/s 2 ); the rough equivalence is 1 N =.2248 lb. (Physics 3 rd Ed., Tipler, back cover) It plays no role in the study of radiation and its interactions with matter, so will rarely be mentioned again in this class.

4. Electromagnetism The electromagnetic force acts between all objects with charge.  It is also a “1/r 2 “ force (strictly speaking, this is only true for point masses or spherical bodies).  It can be either an attractive or a repulsive force; like charges repel; unlike charges attract.  This attraction binds electrons to the atom; it is only so strong, however.  For progressively larger atoms, the attractive force on the electrons grows weaker for two reasons:  the force diminishes with 1/r 2 ; and  there are other electrons between the outermost shells and the nucleus.

5.  The effective nuclear charge is the positive charge experienced by an electron from the nucleus, equal to the nuclear charge but reduced by any shielding or screening from any intervening electron distribution. (General Chemistry, Ebbing and Wrighton, pp. 292-293)  Thus, under certain circumstances, electrons can be removed from the atom, creating ions.  As a force, it is measured in newtons (kg*m/s 2 ); the rough equivalence is 1 N =.2248 lb. (Physics 3 rd Ed., Tipler, back cover)  It plays an important role in our understanding of radiation and its interactions with matter, so will be mentioned again.

6. You can’t measure the gravitational attraction between the two protons on opposite sides of a medium sized nucleus; however, they are pushing each other apart with an electrostatic force strong enough to tear the nucleus apart. Why doesn’t the electromagnetic force dominate our daily existence? Matter is essentially neutral, and there are very rarely large enough charge buildups for the force to manifest itself on a large scale. And, recalling a question posed when discussing the discovery of the neutron: why doesn’t the nucleus explode apart due to electrostatic repulsion? One of the other forces works to counteract it within the nucleus.

7. The Weak (Nuclear) Force The weak force is responsible for the emission of beta particles from radioactive nuclei. The beta-decay interaction is one of a general class of phenomena known collectively as weak interactions. (Introductory Nuclear Physics, Krane, p. 285) The weak nuclear force can only act over a distance on the order of magnitude of the nuclear size. (http://www.final.gov/pub/electroweak.html)http://www.final.gov/pub/electroweak.html With respect to beta decay, the weak force is actually about 1000 times weaker than the electromagnetic force. (Introductory Nuclear Physics, Krane, p. 285) Weak interactions are responsible for the fact that all the more massive quarks and leptons decay to produce lighter quarks and leptons. When a particle decays, it disappears and in its place two or more particles appear. The sum of the masses of the produced particles is always less than the mass of the original particle. This is why stable matter around us contains only electrons and the lightest two quarks (up and down). (http://www.phy.cuhk.edu.hk/cpep/weak.htmlhttp://www.phy.cuhk.edu.hk/cpep/weak.html

8. The present theory of the weak interaction is based on an exchange-force model. The exchanged particles are known as intermediate vector bosons, represented by W +, W -, and Z 0. (Introductory Nuclear Physics, Krane, p. 703) The existence of the weak bosons was proposed by S. Weinberg and A. Salam who in 1967 separately made the first step toward achieving a unified description of all particle interactions by combining the electromagnetic and weak forces into a single theoretical framework. This electroweak theory postulates that at very high energy the weak and electromagnetic forces become completely equivalent. The pure electroweak force would be mediated by four massless spin-1 particles… At lower energies, the symmetry between weak and electromagnetic forces is broken, and three of the four exchanged particles lose their massless character to become the weak bosons W +, W -, and Z 0. The fourth particle remains massless and is the ordinary photon of electromagnetism. (Introductory Nuclear Physics, Krane, pp. 704-705)

9. The Strong (Nuclear) Force The strong force is an attractive force that acts between a proton and a proton, or a neutron and a proton, or a neutron and a neutron. (Radiation and Health, Luetzelschwab, p. A3 by (Radiation and Health, Luetzelschwab, pp. A3-A4) far the strongest of the forces- about 100 times stronger than the electromagnetic force.(Introductory Nuclear Physics, Krane, p. 285) It has an incredibly short range of about one femtometer (10 -15 m); outside of that range, the force is zero. (Radiation and Health, Luetzelschwab, pp. A2-A3) Thus, unlike gravity and electromagnetism, it suddenly drops to zero at a short distance from the particle, as opposed to gradually fading with distance. Compare the range of the strong force (one femtometer) to the diameter of an average nucleus (ten femtometers). The strong force therefore does not act beyond the bounds of the nucleus; it does not manifest itself on the scale of everyday life.

10. It “battles” the repulsion of the protons to hold the nucleus together. (Radiation and Health, Luetzelschwab, p. A3) Technically, the proper way to describe this situation is to consider the nuclear potential well, the Coulombic barrier, and quantum- mechanical barrier tunneling. Why not just create a nucleus with neutrons only, or just a single proton and varying numbers of neutrons, in which case there would be no “battle”? Recall our earlier example of the isotopes of hydrogen; a maximum of two neutrons are allowed to exist in the nucleus with the single proton. There are no isotopes of hydrogen with three neutrons because nuclei with large imbalances between the number of protons and neutrons will not exist very long; there are no nuclei composed entirely of neutrons or entirely of protons. The strong force involves the exchange of pi mesons between nucleons. This is not a truly fundamental process, because the nucleons and mesons are composite particles. The strong interaction can be treated on a more fundamental basis in terms of the quark model, in which the strong interaction between quarks is mediated by the exchange of field particles called gluons. (Introductory Nuclear Physics, Krane, p. 709)

11. In quantum chromodynamics (QCD), quarks interact by exchanging gluons. The photons are the carriers of the electromagnetic field exactly as the gluons are the carriers of the strong color field. What makes the two theories so different is that the photons themselves carry no electric charge and so are unaffected by electric fields; gluons in contrast carry a net color, and therefore interact directly with the quarks. That is, a quark can emit a gluon and then interact with it and create additional gluons; a photon cannot itself exchange photons with nearby charges. This property of gluons force QCD into a considerable level of mathematical complexity. (Introductory Nuclear Physics, Krane, p. 742) As the distance between two point charges increases, the electric field (corresponding to the density of electric field lines crossing a unit surface area) decreases. The color field remains constant as the distance increases (the gluon-gluon interaction forces the color field lines into a narrow tube). As we try to separate to large distances, the work will eventually exceed the production threshold for creation of a quark-antiquark pair, resulting in the formation of a meson. Thus putting energy into a nucleus in an attempt to liberate a quark is expected to create new mesons, exactly as observed. (Introductory Nuclear Physics, Krane, pp. 743-744)

12. Action at a Distance  All four of the forces operate on the principle of “action at a distance”; that is, the forces can act between objects even if they are separated by vacuum. It’s interesting to contemplate that some of history’s greatest minds, which turn the world upside down, can end up being the biggest defenders of the status quo.  One way to envision this is by thinking of invisible fields, or “lines of force” emanating from bodies.  For instance, all objects with charge can be thought to have an electric field surrounding them. If another charge is brought into the presence of this charge, it will experience a force that pushes it away or pulls it in.  A permanent magnet can be thought to have magnetic lines emanating from it. Observe this yourself by placing a bar magnet directly under a sheet of paper and sprinkling iron filings on the paper.  Another way to envision this by thinking of the forces being transmitted by field particles that are exchanged between objects.  For instance, the electromagnetic force is transmitted by photons that are exchanged between objects with charge; think of two children on roller skates as representing two positively charged protons that will repel each other. They throw a basketball back and forth and are pushed apart, an analogy to the photon, as the carrier of the electromagnetic force, pushing the two like charges apart.

13.  A permanent magnet can be thought to have magnetic lines emanating from it. Observe this yourself by placing a bar magnet directly under a sheet of paper and sprinkling iron filings on the paper.  Another way to envision this by thinking of the forces being transmitted by field particles that are exchanged between objects.  For instance, the electromagnetic force is transmitted by photons that are exchanged between objects with charge; think of two children on roller skates as representing two positively charged protons that will repel each other. They throw a basketball back and forth and are pushed apart, an analogy to the photon, as the carrier of the electromagnetic force, pushing the two like charges apart.

14. The Electromagnetic (EM) Spectrum General Properties  Remember from Bohr that if an electron moves from a higher energy state (shell) to a lower, it gives off a photon.  Conversely, the electron can absorb a quantum and move from a lower to a higher energy state as described by this formula.  Energy (E) = E initial -E final = Planck’s constant (h) x frequency of the radiation (   The photon’s energy dictates into what part of the electromagnetic spectrum it fits.  The electromagnetic spectrum is simply the range of all possible energies for photons.

15.  Photon energy is typically measured in electron volts (eV), keV, or MeV.  1 eV = the kinetic energy gained by a single electron accelerated through a potential difference of 1 volt. (Radiation Safety and Control, Volume 1, French and Skrable, p. 18)  Remember Roentgen and Thomson’s cathode tubes; the energy gained by the electron as it speeds from the cathode (negative terminal) to the anode will be one electron volt if the terminals have a one-volt difference between them.  1 eV = 1.602 x 10 -19 joules.  This is a tiny amount of energy; a 100-watt light bulb emanates 100 joules of heat and light energy each second.

16. Components of the Spectrum The full range of the spectrum, from least energetic to most, is: radio waves; The sending of signals by radio waves reached a climax on December 12, 1901. Marconi broadcast radio waves from the southeastern tip of England, using balloons to lift his antenna as high as possible. The signals were received in Newfoundland. This day is usually considered the one on which radio was invented, and Marconi is given credit as the inventor. (Asimov’s Chronology of Science and Discovery, Asimov, p. 480)

18. www.users.mis.net/~pthrush/ lighting/incgraph.gif

20. microwaves; Molecules can be made to vibrate and rotate; again, the energy associated with either motion is quantized, and molecules possess rotation and vibrational energy levels in addition to those due to their electrons. Only polar molecules can absorb a photon and make a rotational transition to an excited state. For instance, water molecules are polar, and if exposed to an electromagnetic wave, they will swing around, trying to stay lined up with the alternating electric field. This will occur with particular vigor at any one of its rotational resonances. Consequently, water molecules efficiently and dissipatively absorb microwave radiation at or near such a frequency. The microwave oven (12.2 GHz) is an obvious application. On the other hand, nonpolar molecules, such as carbon dioxide, hydrogen, nitrogen, oxygen, and methane, cannot make rotational transitions by way of the absorption of photons. (Optics, Hecht, pp. 74-75)

21. infrared waves; Infrared is copiously emitted in a continuous spectrum from hot bodies, such as electric heaters, glowing coals, and ordinary house radiators. Roughly half the electromagnetic energy from the sun is infrared, and the common lightbulb actually radiates far more infrared than light. Like all warm- blooded creatures, we too are infrared emitters. (Optics, Hecht, p. 76) Many molecules have both vibrational and rotational resonances in the infrared and are good absorbers, which is on reason infrared is often misleadingly called “heat waves”- just put your face in the sunshine and feel the resulting buildup of thermal energy. (Optics, Hecht, p. 76) As previously mentioned: heat is an energy flow to or from a body by virtue of a temperature difference.

22. visible light; This is essentially the only part of the electromagnetic spectrum that humans can see. ultraviolet ; Humans cannot see UV very well, because the cornea absorbs it, particularly at the shorter wavelengths, while the eye lens absorbs most strongly beyond 300 nm. A person who has had a lens removed because of cataracts can see UV ( > 300 nm). In addition to insects, such as honeybees, a fair number of other creatures can visually respond to UV. Pigeons, for example, are capable of recognizing patterns illuminated by UV and probably employ that ability to navigate by the sun even on overcast days. (Optics, Hecht, p. 78) xrays ; and gamma (  ) rays X- and gamma rays will be discussed in greater depth in the coming lectures. All of these travel at the speed of light (c), 3x10 8 m/s (“c” is from the Latin word celer, meaning fast). (Optics, Hecht, pp. 44-45)

23. Wave-Particle Duality Is electromagnetic energy best described as either a particle or a wave? It depends upon the experiment used to examine it. Consider its wave-like properties. In Energy (E) = h x  “ ” is the frequency of the electromagnetic energy. Frequency (measured in cycles/sec) is a property of waves. Planck’s formula can also be written as: In Energy (E) = h x c /  ” ” is the wavelength of the electromagnetic energy. The wavelength is simply the distance between successive wave crests. As shown by Einstein’s description of the photoelectric effect, electromagnetic energy can also certainly be viewed as being bundled in photons. For this class, we’ll almost always speak of photons when we consider how radiation interacts with matter.

24. Understanding X-Rays Now we can understand the puzzling properties of x- rays. –Invisible- essentially the only part of the electromagnetic spectrum that humans can see is visible light; all other forms of electromagnetic energy are invisible to the human eye. –Didn’t reflect/refract like light waves- x-rays have a different frequency, so they behave differently. –Couldn’t deflect them with a magnet- they are electromagnetic energy, and as such carry no charge. Only objects with charge are affected by magnets. –They seemed to come from the wall of the cathode tube- this is yet to be answered.