Greetings!
This course is intended to provide you with an understanding of what happens when ionizing radiation interacts with biological tissue. Many of you have learned much of this material either in other academic settings or as part of your on-the-job training, particularly the parts of the course that relate to ionizing radiation itself: how it is produced, what it is used for, how to deliver it, how to quantitatite it, and how to minimize exposure of people and things to it. You have also gotten some information about the biological effects of radiation from various sources. In this course the emphasis is on the biological effects, both harmful and beneficial, of radiation. But to put those biological issues in context we will spend some time discussing radiation physics and radiation chemistry. I am hoping we won't have to put in a lot of time on those subjects, since most of you will have treated those subjects elsewhere.
Who is your instructor?
I am in the biology faculty within the Biological, Chemical, and
Physical Sciences Department at Illinois Institute of Technology.
But my graduate degree is in physics, and I have a secondary appointment
at IIT in physics, so I'm reasonably familiar with physics and chemistry
as well as biology.
My graduate work and my professional concentration have been in macromolecular
crystallography, which is the study of the three-dimensional structures
of large biomolecules by means of X-ray diffraction. So although I am
not a health physicist by specialization, I do use ionizing radiation
(in the form of X-rays, both from laboratory sources and electron
storage rings) in my professional life, and my research is often
affected by concerns for the radiation safety of my experiments.
In a sense I'm a consumer of knowledge about radiation biophysics.
I also did a one-year postdoc (1983-84) at a toxicology laboratory
in Albuquerque where much of the research is on the biological effects
of ionizing radiation, and some of that understanding has (I hope) rubbed off.
How is this course going to work?
This course is an experiment, and we will be together from Monday
4 August through Thursday 7 August for most of the day.
I'm not unrealistic enough to think that you can absorb all the
material of a one-semester, three-semester-hour radiation biophysics
course in that brief interval, but I'll strive to introduce you to all
the topics that I do cover in my one-semester course, which your colleague,
Tim Wright, is taking this summer.
You can compare notes with him to see whether what you'll get is
comparable to what he got.
The course is primarily lecture-based, although we'll certainly try
to get some discussions going as well.
We'll provide you with a few computer-lab experiences,
including a chance to try out a Health Physics game that my colleague,
Dr. Laurence Friedman, has developed as part of Illinios
will be taught on Tuesday and Thursday evenings from
the first full week of June through the last week in July.
You're being provided with HTML and PowerPoint notes for the course,
all of them derived from the corresponding notes that I use in
the one-semester course.
I'm striving to edit out the parts of the lectures that depend
heavily on the textbook we use in that course,
which is Edward Alpen, Radiation Biophysics, 2nd Ed.
(1998: Academic Press, 483 pp., cloth; ISBN 0120530856).
I encourage you to get a copy of this book: it's pretty well balanced,
has useful problems at the ends of the chapters, and is clearly written.
Alpen provides an entertaining historical perspective on the early days of radiation research. Much of the early work on ionizing radiation occurred over a short interval, between 1895 (when Roentgen characterized X-rays) and about 1905, by which time Becquerel had discovered natural radioactivity and Rutherford and Thomson reported on the ionizing properties of X-rays. X-rays began to be used in medicine only a year after Roentgen's discovery, and the first medically observable damage to tissue from X-rays occurred in 1896 as well. Alpen includes in his textbook a quote from Thomas Edison's autobiography, in which Edison's invention of the fluoroscope leads to hair loss in one of Edison's assistants. Edison says, "I then concluded it would not do, and that it would not be a very popular kind of light; so I dropped it." So the history of radiation biophysics goes back almost as far as the history of X-rays.
Not all radiation that is of interest to health physicists is ionizing. Many of the interactions between photons and biological tissue involve changes to rotational or vibrational states of the atoms in the biological tissue, rather than production of actual ions. Non-ionizing radiation sources are of intense interest to many people, including environmental health researchers who want to know how exposure to ultraviolet radiation at 270 nm causes skin erythemas, and safety officers who want to know if the microwave oven in the break room is going to cause medical problems. But in this course we will focus on interactions that do result in ionization of the target materials, i.e., we will spend almost all our time talking about ionizing radiation.
There are several kinds of ionizing radiation. Alpha particles, which are 4He nuclei, can certainly dislodge electrons from atoms with which they interact, so they are ionizing radiation. Beams of atoms or neutrons can collide with atomic nuclei in ways that give rise to freed charges, so they can be ionizing radiation. Energetic electrons, whether derived from beta decays or from accelerators, can collide with atoms and knock electrons out of those atoms; they thereby leave behind negative ions (the electrons they dislodge) and positive ions (the atoms that have been collided with, which now carry net positive charges). Finally, photons (whether derived from a gamma-ray radioactive events, from Bremsstrahlung effects with electrons, with characteristic emissions from atomic targets, or from cosmic rays) can dislodge electrons from atoms to produce ionizations. The photon has to be sufficiently energetic to overcome the stabilizing energy associated with keeping the electron attached to its nucleus. The amount of photon energy required to strip an electron out of an atom depends on the atomic number, as you might expect; all other things being equal, the lower the atomic number, the easier it is to ionize an atom. A 13.6 eV photon is energetic enough to ionize a hydrogen atom; this energy corresponds to a photon wavelength of 911 Ångstrom, which falls in ultraviolet range. Other atoms hold onto their electrons more diligently than that: ionizing a K-shell electron from selenium requires absorption of a 12684 eV photon, and ionizing a K-shell electron from lead requires an 88005 eV photon. So apart from interactions with hydrogen, we generally describe photons as ionizing only if they carry at least 1 keV of energy each.
A note about energy in photon beams is in order here. The fact that a single photon carries a high energy does not mean that a beam composed of those photons will carry huge amounts of power: the power in a photon beam is the product of the energy in each individual photon multiplied by the flux, defined as the number of photons that pass a point per second. So a beam may be weak in the sense that the power in it is small, but if the photon energies in it are high, it will function as ionizing radiation. Conversely, we may construct a device that delivers a high power to a sample by offering a very high flux at relatively low photon energy. This device will not be able to ionize a target, although it may be able to do a lot of other things to it by exciting rotational and vibrational activity in the target.
The world of radiation research has gone through a major change in the units that it uses to express quantities. As recently as the 1970's when I was learning radiation quantitation, the traditional units for activity, dose, energy imparted, and equivalent dose were still in common use. In this course we will use the more modern units except in dealing with older research papers. Thus the units of interest are:
| Quantity | Definition | SI Unit | Definition | Old Unit | Definition | Conversion |
| Exposure | ΔQ / Δm | C kg-1 | Roentgen | 1 esu cm-3 | 1 R = 2.58*10-4 C kg-1 | |
| Dose | ΔED / Δm | Gray | Joule/kg | rad | 100 erg/g | 100 rad = 1 Gy |
| Equivalent Dose | ΔEDWT,R / Δm | Sievert | Joule/kg | Rem | 100 erg/g | 100 rem = 1 Sv |
| Energy Imparted | ΔED | Joule | kg-m2/s2 | erg | g-cm2/s2 | 107 erg = 1 J |
A few comments on this table are in order. Exposure is only defined for photons, and it is measured in terms of the amount of charge produced per unit mass in air. What we measure is the amount of charge engendered per unit mass of air. It is a non sequitur to talk about an exposure of 1 Roentgen to alpha particles, because they aren't photons. It is also a non sequitur to talk bout a Roentgen of radiation in tissue, because tissue isn't air (except lung tissue, which is a special case). The conversion factor from Roentgen to the more fundamental units (electrons per kg, or Coulombs/kg) is given above; it is based on the rather quaint "esu", or "electostatic charge unit", which is a unit of charge created to make the constant in Coulomb's equation (the k in F = kq1q2r-2) come out as 1.
Equivalent Dose is an expression of the fact that certain types of radiation have a greater affect on absorbing materials than others. Specifically, forms of radiation that dump most of their energy over a small linear range (so-called "high-LET" radiation) generally cause more damage per gray of basic dose than forms of radiation that spread their effects over a longer path-legnth. We describe the significance of these special characteristics of high-LET radiation in terms of a weight factor, WT,R, which is a unitless value that depends on the type of radiation R and might also depend on the type of medium T in which the interaction occurs. By convention WT,R ≥ 1, and we expect that photons and most electrons have WT,R = 1, independent of the target type T; values of WT,R as high as 20 occur for alpha particles passing through soft matter like tissue. It is, in my opinion, a bit bizarre that the equivalent dose value we obtain when we multiply the raw dose by this weight factor is actually given a different unit, namely, the Sievert in SI units and the Rem in older units, from the unit of dose itself. But in fact that is the way it works; we say that a gray of alpha particles produces 20 Sv of equivalent dose in the target. At heart the Sievert is really the same unit as a Gray in that it is a Joule of energy imparted per kg of absorbing mass; it's simply applied in a different way. The Gray, which is the unit of dose, and the Sievert, which is the unit of equivalent dose, have names that commemorate pioneering radiation biologists: Louis Gray (1905-65) and Rolf M. Sievert.