Classroom notes for: Radiation and Life 98.101.201 Thomas M. Regan Pinanski 207 ext 3283 1
Thus, after the directly ionizing radiation has lost its energy, it is no longer radiation; it simply becomes part of an atom (beta particles and electrons) or becomes a whole atom (alpha particles and protons) no different from other atoms in the target. Bear in mind that we have discussed interactions with the orbital electrons, not the nucleus. Thus, chemical bonds can be broken, and chemical properties altered as a result of exciting the orbital electrons or knocking them from the atom, but nothing is made radioactive. The nucleus is the source of radioactivity, so if it is unaffected by the passage of directly ionizing radiation, then it is not made radioactive, period. 2
Indirectly Ionizing Radiation Indirectly ionizing radiation has no charge. The particles that meet this criterion are grouped as follows: energetic photons such as x- and gamma rays; and neutrons. By virtue of having no charge, indirectly ionizing radiation interacts with matter via different mechanisms than does directly ionizing radiation. The particles have no charge, and thus have no electric fields. Thus, they will not interact with the fields of the orbital e - in the target. In the case of photons, the x- or gamma must make a direct hit on the orbital electron to interact with it (there are exceptions to this rule). Since the probability of this is small, the photons will pass through matter while undergoing only a relatively small number of interactions, and will generally penetrate much more deeply in matter than will directly ionizing radiation. 3
Neutrons Neutrons will not interact with the orbital electrons at all, only the nucleus. In either case, the end result is the production of a secondary charged particle; we ll discuss this further as we consider the different types of indirectly ionizing radiation in greater detail. 4
x-rays and γ-rays In a photoelectric absorption event, the photon is completely absorbed by an orbital e -. The electron gains the photon s energy. If the energy gained is great enough, the electron will be ejected as part of an ionization event (the atom is now one electron short, so it has a positive charge- it is a positive ion).otherwise, the electron is simply elevated to an excited energy state. If the electron is ejected, it now continuously deposits its energy in the target and is known as a secondary charged particle. 5
In a Compton scattering event, the photon simply collides with and gives up some of its energy to an orbital e -. After the collision, the photon continues onward through the material. The electron gains the amount of energy that the photon lost. If the energy gained is great enough, the electron will be ejected as part of an ionization event, otherwise, the electron is simply excited. If the electron is ejected, it now continuously deposits its energy and is known as a secondary charged particle. This effect was first observed by the American physicist Arthur Holly Compton (1892-1962), who won the Nobel Prize for physics in 1927 for his work. (Asimov s Chronology of Science and Discovery, Asimov, p. 551) Incidentally, Compton was one of the first to refer to use the term photon. (Asimov s Chronology of Science and Discovery, Asimov, p. 524) 6
A pair production event is an exception to the rule that the x- and gamma ray photons only interact with the target by hitting the orbital electrons. Recall Einstein s equation: E = m*c 2 This is a statement of the conservation of mass and energy mass and energy are equivalent. That is, mass can disappear, but energy must appear in its place. Conversely, energy can disappear, but mass must appear in its place. During pair production, a photon comes close to a nucleus and disappears (the nucleus is unaffected by this event). It is converted directly into an e - and an e + as allowed by Einstein s formula. 1.022 MeV of the photon s energy is used to create (the mass of) both particles. The energy that is left over is split equally by the electron and the positron, which use it to continuously deposit energy as secondary charged particles. 7
Photonuclear reactions. For all but the most specialized applications (such as therapeutic x-ray machines), photonuclear interactions can be ignored. In a photonuclear interaction an energetic photon (exceeding a few MeV) enters and excites a nucleus, which then emits a proton or neutron. (γ, p) events contribute directly to the kerma, but the relative amount remains less than 5% of that due to pair production. Thus it has been commonly neglected in dosimetry considerations. (Introduction to Radiological Physics and Radiation Dosimetry, Attix, p. 154) Consider a 1-MeV photon traveling through a 10 cm concrete shield. A 1-MeV photon has a mean-free path length of about 7 cm in concrete [1 / (.0637*2.3) = 6.8 cm]. On average, it will travel about 7 cm before interacting. It might travel 7 cm and then undergo a Compton scattering event, after which it leaves the shield without further interactions. Obviously, the photon itself had little effect on the target because it only knocked out a single orbital electron. However, that orbital electron will go on to cause many ionization events itself. 8
Obviously, the photon itself had little effect on the target because it only knocked out a single orbital electron. However, that orbital electron will go on to cause many ionization events itself. Also note: following a photoelectric absorption event or a pair-production event, the gamma-ray photon is gone- it no longer exists; so the fate of the photon is to either disappear or pass right through an object. It does not stay inside the object. As a final thought, bear in mind that each of the first three interactions leaves the nucleus unaffected. Thus, as we discussed previously, nothing is made radioactive. At normal photon energies, photons cannot make nuclei radioactive. 9
Neutrons Neutrons interact only with the nucleus The specific interaction will depend on the target nucleus (what element is it?) and the neutron s energy, as a result, neutrons interact quite readily with some materials, and almost not at all with others. They will either scatter ( bounce ) off of the nucleus, or be absorbed by it. A scattering event can reduce the neutron s energy. After multiple scattering events, the neutron will be at a low energy. Neutrons at the lowest energies (~.025 ev) are known as thermal neutrons; they have energies equivalent to the energies of atoms and molecules at room temperature (the velocity of a thermal neutron is about 2200 m/s). Thermal neutrons are typically more easily absorbed by the nucleus. 10
When a neutron is absorbed by the nucleus, the new nucleus that results may be induced to emit a charged particle directly, or it may become radioactive (this is called an activation). For instance, if 1 H-2 absorbs a neutron, it becomes radioactive 1 H-3. It will be the secondary charged particle that is emitted that will deposit energy in the target. For example: 8 O-16 + 0 n-1 -> 7 N-16 + 1 p-1 + Q In this reaction, the oxygen nucleus absorbs a neutron and is induced to emit a proton. The neutron is the indirectly ionizing radiation and the proton is the secondary charged particle that continuously deposits energy in the target. Thus, this interaction mechanism is also a source of ionizing radiation. This is an important reaction in nuclear reactors. As an aside, this can be considered an activation reaction because N-16 is radioactive (t-½=7.13 s, Eγ=6 or 7 MeV) 11
Summary Radiation- particles or electromagnetic energy. Ionizing radiation- can knock orbital electrons out of atoms. In broad terms, there are six different sources of ionizing radiation (radioactive nuclei are only one of the six sources). In broad terms, ionizing radiation can also be characterized by the manner in which it interacts with matter; either directly or indirectly. In either case, once the ionizing radiation interacts with matter, it is no longer still inside the target, and only neutrons have an appreciable ability to make other nuclei radioactive. 12
In terms of its ability to penetrate matter, we can establish a rough rule-of-thumb (ROT) regarding the various types of ionizing radiation. A ROT is simply a generalization that doesn t necessarily hold true under all circumstances. Directly ionizing radiation. Heavy charged particles (α, p +, and recoil daughter nuclei) will typically be stopped by very thin shields such as a piece of paper or thin plastic. Light charged particles (β -, β +, e -, and e + ) will typically be stopped by a moderate layer of plastic or thick clothing. Neither type tends to penetrate very deeply in matter; for instance, heavy charged particles such as alphas will only travel cm in air and less than mm in water or tissue (Radiation Safety and Control HW #2, French) 13
Indirectly ionizing radiation. X- and γ-ray photons will typically be stopped by a moderate layer of lead or a relatively thick layer of concrete. There is no ROT for neutrons because they have highly variable penetrability in matter depending on their energies and the type of material. 14
Basic Radiological Science Radioactivity Defined Radioactivity is the spontaneous emission of ionizing radiation (directly or indirectly ionizing) by an energetically unstable nucleus. The atom s nucleus transforms into another one; this is known as radioactive decay. The word disintegration is sometimes used in place of decay ; but this is an unfortunate misnomer that often causes confusion because the atom doesn t literally disappear 15
Radioactivity as a Random Process Radioactivity is a completely random process. Consider alpha decay. 92 U-238 ---> 90 Th-234 + 2 α-4 + Q We can identify the end products by balancing the reaction, and then we can calculate the particle energies using the Q- value. We cannot predict exactly the decay will occur; it is impossible to predict when a particular atom will decay. However, if we have a huge number of atoms, we can predict, on average, how will decay during a given time interval. Radioactive decay is completely random- you can never pick when an individual atom will emit radiation and transform into something else. It's impossible. However, if you have a "large" number of atoms (even a single gram of H 2 0 will have on the order of a billiontrillion atoms- that's "large"!), you can use statistics to estimate how many, on average, will decay over a given time interval. 16
Let's think of this another way. Consider 10 coins. If I asked you to predict which ones would come up "heads" and which "tails" before I flipped them, you'd tell me I'm crazy. The odds of correctly picking, for all 10, which will be heads and which would be tails are very, very, tiny. This is equivalent to trying to predict when the individual atoms will decay. If on the other hand, I asked you to predict, on average, how many of the group of 10 will come up heads, you'd tell me that's a "no-brainer". You'd say "5", and even though it sometimes might be 4, or 6, or 7, etc If I flipped the 10 of them many times and took the average number of heads, the number would be darn close to 5. This is equivalent to predicting how many atoms, on average, will decay over a given time interval. 17
Half-life Now, suppose I arbitrarily pick the time interval to be the interval over which one half of the atoms will decay. There's no magic here, the choice of 1/2 was completely arbitrary. I could just as easily have chosen 1/10, or 1/3, or 13/127, for that matter. We call that time interval the "half-life" (often abbreviated t-½ ), and we can predict, using statistics (as I mentioned above), how many of the radioactive atoms will decay over that interval. For each type of radioactive isotope, the half-life will be different, because the structure of the nucleus is different for each. Half-lives can range from less than nanoseconds (billionths of seconds) or less, all the way to billions of years or longer. 18
Consider 92 U-238. It has a half-life of 4.5 billion years. If 1000 atoms of this uranium were initially present, 4.5 billion years later, 500 would have undergone radioactive decay, leaving behind 500. 4.5 billion years after that, one half of the remaining 500 would have undergone decay, leaving behind 250. That's an important point- the sample is always losing half of what is remaining, not half of the original amount. Otherwise, after one halflife, you'd have 500 atoms left, and after the second halflife you'd lose the remaining 500 (1/2 of 1000). Bear in mind that the decays occur over the course of the half-life; 500 atoms don t suddenly transform when the clock strikes 4.5 billion years. This can be summarized in a table and graphically. 19
fraction of original amount remaining as dependent upon the elapsed time time (= number of fraction of original elapsed half-lives) amount remaining 1 ½ = 1/2 2 ½ x ½ = 1/4 3 ½ x ½ x ½ = 1/8 4 ½ x ½ x ½ x ½ =1/16 5 ½ x ½ x ½ x ½ x ½ =1/32 Then 1/64, 1/128, 1/256, etc The actual time elapsed depends on the half-life of the radioactive isotope. It s important to remember that the whole time we have 1000 atoms; nothing has disappeared; the lost uranium has transformed into something else (in this case, thorium, which also happens to be radioactive). 20