Atoms, on a simple level, can be thought of as consisting of negatively charged electrons moving around a nucleus which contains positively charged protons and neutral neutrons (collectively known as nuclides). Since, in an atom, the electrical charges must balance, the atom must have the same number of protons and electrons. How many protons the atom has (its atomic number) defines what element it is (the number of neutrons is irrelevant). An atom with one proton (and one electron to balance the charge) would be an atom of the element hydrogen. Six protons would make it a carbon atom; 92 protons would make it uranium. But there could be two atoms with the same number of protons (they would be the same element) but with different numbers of neutrons.
If you wanted to write the chemical symbol for uranium, you would use the accepted abbreviation, U, which makes sense in English. However, there are some surprises. K stands for potassium which doesn’t seem to make any sense; you have to know that the ‘K’ is from the Latin for potassium which is kalium (besides, P already stands for phosphorus), ‘Na’ for sodium, Latin natrium (S already stands for sulfur), and ‘Fe’ for iron, Latin ferrum. W stands for tungsten; the W comes from the German word for tungsten, wolfram.
The atomic mass (sometimes incorrectly called the atomic weight) of an atom is the sum of the masses of the protons and neutrons it has (the neutron is ever so slightly more massive than the proton). The mass of the electrons is usually ignored because their mass is much, much less than that of protons or neutrons. The simplest possible atom is one with one proton, no neutrons, and one electron which is a ‘kind’ of hydrogen. I say ‘kind of’ because there are other possible ‘kinds’ of hydrogen. You could have an atom with one proton (by definition it would be hydrogen) but it could also have one neutron. That ‘kind’ of hydrogen would have double the atomic mass of the hydrogen without a neutron. A third possibility is a hydrogen with two neutrons or how about one with three neutrons? They would all be hydrogen but hydrogen with different atomic masses.
A better, technical word for different ‘kinds’ of an element (different atomic masses, same element), is ‘isotope.’ ‘Isotope’ would apply, not only to different ‘kinds’ of hydrogen but to different ‘kinds’ of an atom of any element. For example, there are two important isotopes of uranium; one uranium isotope has 146 neutrons and the other, 143 neutrons. Since, by definition, uranium has 92 protons, the atomic mass of the first isotope would be 92 + 146 = 238 amu (atomic mass units) and that of the second isotope, 92 + 143 = 235 amu.
You could play the same trick with T for tritium. Note that the T doesn’t stand for any element; it’s not Tin (Sn because the Latin for tin is stannum). Nor is it element #22 Titanium (Ti), #43 Technetium (Tc), #65 Terbium (Tb), #69 Thulium (Tm), #73 Tantalum (Ta), #81 Thallium (Tl), #90 Thorium (Th), or #117 Tennessine (Ts). Who knew there were so many elements starting with T and every one of them a double letter? Tritiated water (water with at least one tritium substituted for hydrogen) is used in tracing the slow movement of water through aquifers. You spike the groundwater with some tritiated water and later take samples down-gradient, checking for radioactivity from the tritium, the presence of which would indicate that the groundwater had reached that down-gradient location.
The electrons of an atom occupy what a physicist would call energy levels and a chemist would describe as electron shells around the nucleus of the atom. These energy levels/electron shells are only found at certain very definite distances from the nucleus, distances which are unique to a particular element. The electrons cannot be at any other distances which is because the electrons have both particle and wave-like properties (all matter does but it is only at the very small scale of an electron that the wave-like properties of matter normally become important – quantum physics).
The particular energy level/electron shell that an electron occupies corresponds to a certain amount of energy; the farther away from the nucleus that the energy level is, the greater the amount of energy that the electron in that level has. The lowest energy level (closest to the nucleus) can hold two electrons; the next energy level can hold eight, and so on. An atom is at rest when all of its electrons are in the lowest possible energy levels. Depending on how many electrons an atom has, there may be an energy level that is not completely filled with electrons.
An atom is said to be at rest if all of its electrons are at the lowest possible energy levels/electron shells. For example, if a particular atom (of oxygen) has eight electrons, two of them would be at the lowest level and six of them would be in the second level. Since the second level can hold eight electrons, that level would be two electrons short of being fully occupied.
An atom is excited when one or more of its electrons are at a higher energy level. This is not a stable state and the electron will drop down to a lower energy level, emitting electromagnetic radiation in the process: the greater the drop, the shorter the wavelength (which corresponds to more energy) of the emitted electromagnetic radiation. Electromagnetic radiation, then, has a spectrum or range of wavelengths (energies) which runs from very long wavelength (low energy) radio waves at one end of the spectrum to very short wavelength (very high energy) gamma radiation.
Visible light is part of the spectrum but is restricted to a very narrow range of it. That narrow visible light part of the spectrum can be subdivided into the familiar colors of the rainbow where red light has the longest wavelength of visible light and purple/indigo the shortest.
An important constant is the speed of light, around 186,000 miles/sec or 300,000 km/sec (in a vacuum; it slows down slightly in air, more in water, and even more in glass). More accurately, that is the speed of the entire electromagnetic spectrum; radio waves and gamma rays move at the speed of light. An index of refraction is actually a ratio of the speed of light in a vacuum divided by the speed of light in a transparent medium.
Atoms can become excited by heat. Temperature is a measure of how fast atoms are moving, or vibrating in place if they happen to be part of a solid. That movement is heat which excites the atom. Electrons are continuously being bounced up to higher energy levels as they absorb the heat energy and then the energy is released again as the electrons drop back down to lower energy levels. Your body is warm enough to generate infrared radiation. If something like iron is heated enough to raise electrons to even higher energy levels, the electrons, when they make bigger drops down to lower energy levels, may emit visible light. The iron first becomes red hot, then glows orange, yellow, and finally becomes white hot. The tungsten filament of an incandescent light bulb becomes so hot it glows white hot which is hotter than the surface of the sun which is only yellow hot. Tungsten is used because it has such a high melting point that it can get white hot without melting.
Although electrons only move between specific energy levels, heating something produces a continuous spectrum (all the colors of the rainbow if it is hot enough, which combine to look white). This happens because, although temperature is a measure of how fast everything is moving/vibrating, there is a range of speeds/vibrations; some particles are moving a little slower or faster than others. Movement is kinetic energy which means there is a range of kinetic energies that atoms can have at a given temperature. It is the collision of atoms with other atoms, with a range of kinetic energies, that produces a continuous (thermal) spectrum.
An emission spectrum is produced when the dropping of electrons from higher to lower energy levels produces a very specific set of discrete wavelengths characteristic of a particular element. An emission spectrum can be produced by exciting atoms with high voltage electricity. If the voltage is high enough, you can even produce x rays. Such a spectral ‘fingerprint’ is very useful in identifying elements in stars, and in water. The emission spectrum of hydrogen is relatively simple, consisting of a bright red line, a bright blue line, and two fainter purple lines.
The AAS is a very important instrument in a water lab. If you want to know how much calcium, iron, arsenic, or other elements are dissolved in your drinking water, it is an AAS that will determine that. How it works begins with a tube/lamp with a filament which, when excited with the right voltage of electricity, will produce the emission spectrum of a particular element.
Suppose, for example, you are interested in how much dissolved copper is in your water. The lab would have a ‘copper’ tube which would produce the emission spectrum of copper which would consist of specific wavelengths of visible light characteristic of that element. Unfortunately, if you want to analyze a water sample for another element, you would need to swap out the copper tube for another tube for that other element. There are a few tubes which can simultaneously produce the emission spectra for two or three elements but it still involves the swapping out of a lot of tubes when analyzing for different elements. For this reason, the lab usually analyzes many water samples for one thing, swaps tubes, and then analyzes all of them for something else.
Great. We now have a source of the emission spectrum of copper. At the other end of the machine is a photometer which is a fancy name for a device that measures the intensity of light at a very specific wavelength. If it is copper you are interested in, then you dial the photometer to measure one of the emission wavelengths coming from the copper tube. But it is what is in between the copper lamp and the photometer that is important.
There is a tank of oxygen and a tank of acetylene that feed a combined gas to a burner in between the ‘copper’ lamp and the photometer. The burner ignites the gas and produces a flame through which the copper ‘light’ must pass to get to the photometer. below the burner is a beaker of water from which water is fed into an atomizer whose purpose is to turn the water into a mist (just like the head of a perfume bottle) which is sprayed into the flame which turns the water mist into a water vapor cloud. Of course, this is a flame so if you want to maintain that water vapor cloud, you have to continuously feed water into it.
The copper light must now pass through the water vapor cloud on its way to the photometer. If there happens to be some copper in the water, then some of the copper light will be absorbed by the copper in the water which means that the photometer will see less light. The more copper in the water cloud, the less copper light will get through to the photometer. What the photometer sees is the opposite of an emission spectrum; it is a spectrum (actually the photometer only looks at one wavelength) in which the intensity of specific wavelengths are reduced, in this case, those of copper. What it is is an absorption spectrum from which you get part of the name of the AAS, atomic absorption spectro- (spectrum) plus the name of the device that sees it, the photometer.
This procedure is exactly how an astronomer can tell what is in a cold cloud of gas/dust in distant space. Stars produce a spectrum in which it is relatively easy to see the unique emission lines of a particular element and the more intense those lines are, the more of that element is in the star. Cold clouds, however, don’t produce emission spectra but if some starlight behind the cold cloud passes through the cloud, the cloud will absorb the starlight at the specific wavelengths characteristic of particular elements. See the similarity? Other lab instruments look at the entire absorption spectrum, not just a single wavelength, and may include part of the near infrared and/or ultraviolet part of the electromagnetic spectrum. Such instruments can be used to identify compounds, including complex organics. Is that white powder sugar or is it cocaine? Each compound has a unique absorption spectrum which can serve as a kind of fingerprint for that compound.
If you want to quantify exactly how much copper is in the water, you must calibrate the AAS for copper. You start out with a sample of pure water; it doesn’t have any copper in it. You feed it into the flame and then tell the AAS computer that whatever the intensity of the copper wavelength that it is looking at, that intensity represents zero copper in the water. Next you successively feed at least two standard water samples into the flame. These are standards because you know how much copper is in the water because you put it in. Each time, you tell the AAS computer that whatever intensity of light the photometer sees, this represents whatever the copper concentration in the standard is. The machine is now calibrated and ready to analyze unknown water samples for copper.
You should also now recognize why such an analysis takes so long. You have to swap out lamps for each different element and recalibrate the AAS. There are cheaper and quicker methods of measuring specific elements dissolved in water but they are not very accurate; they can be screening tests. If accuracy is important, such as at level three or four testing, you really do need the AAS.
There is an important difference in isotopes. Hydrogen isotopes with one neutron or none at all are quite stable but the hydrogen isotope with two neutrons is not fully stable. Over time, it will literally fall apart, in the process, emitting radiation and turning into something else (we will discuss the details of this later). The hydrogen isotope with three neutrons is completely unstable; if you could somehow make it, it would immediately fall apart and, so, it doesn’t really exist. A particular element, then, may have stable isotopes and some radioactive unstable isotopes. There are even some elements which do not have any stable isotopes at all; uranium is one example of that although it takes a very long time for U-238 to decay (fall apart), emitting radiation in the process.
Sometimes individual isotopes are given their own name. H-1 (hydrogen with no neutrons) is known as protium. H-2 is deuterium and H-3 is tritium. There are no other isotopes of hydrogen; any others are completely unstable. Carbon has several isotopes, the most important being C–12 (98.9% of all carbon), C–13 (1.1%), and C–14 (<< 1%); the other isotopes fall apart very quickly. C–13, like C–12, is completely stable and C–14 is unstable but doesn’t fall apart very quickly. None of the carbon isotopes have their own names and are simply referred to as C–14, etc.
There are three isotopes of radon, none of them stable, including thoron (Rn–220) and actinon (Rn–219). As it turns out, if you have radon gas, it will be almost entirely the Rn–222 isotope which does not have an isotopic name. It is usually the case that if you do not specify a particular isotope of an element, it is assumed that you mean the most abundant isotope of that element which would mean H–1 (protium), C–12, and Rn–222, respectively, for the examples above. Do be careful not to confuse the name of an isotope with the name of an element.
Coming back to tritium, you can see it would be easy to figure out how much tritium was left after one half-life or some multiple of the half-life but how much tritium would be left after ten years or a hundred years? To calculate how much of an isotope is left for any length of time (N), you need this equation:
Since we know that the half–life of tritium is 12.3 years, we can now calculate the decay constant for tritium. Note, however, the units for the decay constant, per time. The units in –λt must cancel out if the first equation is to work. The t for tritium is in years so in order for the units in –λt to cancel out, the unit for the decay constant, per time, must be per year. Using the second equation to calculate m for tritium yields a dimensionless result of about 0.0563 which is a probability. It might be easier to understand if that decimal is turned into a percentage which would be 5.63%. This means that, for tritium, there is a 5.63% chance every year that any given atom of tritium will decay. It should be easy to see now that the larger the decay constant, the faster the unstable isotope will disappear.
When an unstable isotope decays, it spits out radiation; unstable isotopes are radioactive. There are three main types of radiation produced: alpha (α), beta (β–), and gamma (γ) radiation whose names come from the first three letters of the Greek alphabet; it is equivalent to our A, B, and C. There are other possible radiation types; an unstable nucleus could, for example, spit out a neutron, proton, or positron, and there are other modes of decay such as β+ decay, electron capture, and fission, but α, β–, and γ are the most common forms of radiation produced in the decay of an unstable nucleus. Gamma radiation is exactly that, radiation; it is from the extreme end (very high energy, very short wavelength) of the electromagnetic spectrum. The other two, however, are actually particles with a lot of kinetic energy (energy of motion).
Physicists recognize four forces today (is there a fifth force? time will tell): gravity, the electromagnetic force, the strong nuclear force, and the weak nuclear force. Although very important at a human scale and larger, gravity, at the scale of an atomic nucleus, is so weak compared to the other three forces that it can be completely ignored at that scale.
The electromagnetic force which, among many other things, dictates that like charges repel also mandates that the positively charged protons in an atomic nucleus repel one another, introducing an instability in an atomic nucleus with more than one proton. Opposing this electromagnetic repulsion is the strong nuclear force whose attraction between nucleons (protons and neutrons in the nucleus of an atom) overwhelms the repulsion of the electromagnetic force.
The problem is that the strong nuclear force is a very short-range force; its strength drops drastically over short distances, becoming less and less effective as an atomic nucleus becomes larger. The consequence is that there is a limit to how big an atomic nucleus can get and still hold itself together. Another factor is the number of protons and neutrons in a nucleus. For lighter, smaller, nuclei, a stable ratio of protons to neutrons is 1:1 but as nuclei become bigger and heavier, more neutrons, compared to the number of protons, become necessary to form a reasonably stable isotope.
Unlike K-40, Thorium and the two Uranium isotopes do not decay into stable isotopes but create a chain of radioactive daughter isotopes which eventually end up as different stable isotopes of lead. Each intermediate radioactive daughter in the three chains of decay has a different half-life (none of them nearly as long as the original thorium or uranium isotope) and the daughters produce radiation through a series of alpha and beta decays. These three important decay chains include isotopes of radium and radon and will be discussed later – first, what are alpha and beta decay?
Alpha decay is common in large, unstable atomic nuclei (the plural of nucleus) such as the nucleus of a uranium atom. In effect, it is as if a nucleus undergoing alpha decay puts together a bundle of two protons and two neutrons and throws that bundle out of the nucleus with great speed (lots of kinetic energy). You might notice that an alpha particle looks a lot like the nucleus of a helium–4 atom but there are two important differences. The alpha particle is not associated with any electrons (it’s not an atom; it’s a He-4 nucleus) and it has a huge amount of (kinetic) energy.
An isotope that throws out an alpha particle drops its atomic number by two and its atomic mass by four. Both alpha decay and beta decay are examples of the transmutation of elements (one element changes into another element). Unfortunately, there is no natural decay chain that will transmute a lead isotope (element #82) into gold (element # 79). It actually has been done by colliding other elements together but it cost more to do it than the gold was worth.
The alpha particle inevitably collides with something else, transfers some of its momentum to whatever it hit, and slows down a bit. If whatever it hit is a compound, the transferred energy may break bonds which, in a living cell, could promote cancer. The alpha particle continues to collide with other atoms/molecules, transferring some of its momentum to whatever it hits and slowing down (it loses kinetic energy). Eventually, the alpha particle slows down enough (loses enough kinetic energy) that it can acquire two electrons at which point it becomes an ordinary helium atom.
There are two interesting results from all of this (aside from any bond-breaking). You might wonder where the helium comes from that fills balloons and dirigibles. Let helium gas loose in the atmosphere and it will escape into space. The alpha decay of elements like uranium and thorium deep within the earth produces helium gas which will attempt to rise up to the surface of the Earth where it can escape. Sometimes, however, the rock through which the gas is trying to rise is impermeable enough that the rock traps the helium underground. Such rock might also trap natural gas (methane) underground. If there is enough trapped natural gas, someone may drill a well into the rock to extract it. Along with the natural gas comes a small amount of helium which can be separated from the methane by cooling and compressing the methane until it liquifies, leaving the helium as a gas which can now be easily separated from the liquified methane.
Another interesting consequence of alpha decay is that it produces heat. Temperature can be thought of as a measure of how fast atoms, ions, or molecules are moving or vibrating in place; the higher the temperature, the faster everything is moving/vibrating. As the alpha particle transfers momentum to whatever it hits and slows down, whatever it hits speeds up a little and passes on a little of its momentum to whatever it hits. Everybody now moves a little faster (has a little bit more kinetic energy). The temperature goes up a tiny fraction of a degree. This is how radioactive decay can heat the Earth. Without it, the Earth would be a very different place, probably no plate tectonics or volcanism which, by the way, would not be a good thing.
Unlike protons, free neutrons (unassociated with protons) have a short half–life of about 10.5 minutes. It is only within the nucleus of an atom that a neutron is ~ stable. In a large nucleus with an excess of neutrons, however, the excess neutrons tend to form a skin around the outer surface of the nucleus. These excess neutrons are farther from the stabilizing influence of the protons. It may happen that the weak nuclear force, which holds a particular neutron in the outer skin together, weakens enough to allow the neutron to decay.
Beta decay is the decay of a neutron. What happens is that the neutron splits into what looks a lot like an electron (with one important difference), a proton, and an anti-neutrino. The ‘electron’ and the anti-neutrino are thrown out of the nucleus at great speed, in the process releasing a lot of energy in the form of kinetic energy (the great speed) and gamma radiation (beta radiation is associated with gamma radiation). It is that huge kinetic energy that distinguishes a beta particle from an ordinary electron. The proton left behind increases the atomic number of the daughter isotope by one (transmutation of an element).
That the daughter isotope has one more proton than the parent isotope means that the newly-formed daughter isotope is short one electron. When another electron is acquired, it must drop down many energy levels to get as close as possible to the nucleus of the isotope. In doing so, there is a release of electromagnetic energy and since the drop of the electron is quite large, the emitted electromagnetic energy is gamma radiation which is why beta decay is associated with the emission of gamma radiation.
The story of the beta particle is similar to that of an alpha particle. It hits other particles (atoms, ions molecules), transfers some of its momentum to whatever it hits, possibly breaks bonds (ionizing radiation), creates heat, and eventually loses enough kinetic energy to become an ordinary electron.
Some very large unstable isotopes, such as U–235 and Pu–239, instead of spitting out alpha or beta particles, may simply split (fission) into several larger radioactive chunks along with a shower of smaller particles and a lot of energy. Unaided (spontaneous) fission is usually quite rare, for example, in U–238 there is only one spontaneous fission for every 2,230,000 α decays. Another way of looking at it is that the half–life of U–238 due to spontaneous fission is a whopping 350 quadrillion years as compared with 4.5 billion years for alpha decay.
But fission can be induced in some isotopes by hitting their nuclei with neutrons of the right speed (kinetic energy). It wouldn’t work for U–238 which would just absorb the neutron and turn into U-239 which would quickly (half–life 23.5 minutes) spit out a beta particle and turn into Np-239 which would quickly (half-life 02.35 days) spit out another beta particle and turn into Pu-239 (half-life 24,400 yrs) which is fissionable. Normal nuclear reactors are not optimized to produce fissionable plutonium; some do accumulate in used fuel rods. Breeder reactors are designed to much more efficiently convert the much more abundant but non-fissionable U-238 into fissionable Pu-239. The two most important fissionable (you can induce it to fission) isotopes are U-235 and Pu-239. The fissionable U-235 is much less common than U-238, making up only 0.3% of uranium. U-238 makes up 99.7% of uranium and separating the two isotopes is very very difficult and expensive which is maybe not such a bad thing.
The larger radioactive ‘chunks’ produced in fission include some important radioactive isotopes found in the environment as a result of atomic bomb testing or nuclear reactor failures: tritium, cobalt-60, strontium-90, iodine-131, cesium-137, and americium-241. Plutonium-239 from plutonium processing and storage facilities can also escape confinement and end up in groundwater as is known to have happened near Hanford, Washington, the site of a breeder reactor and the source of the plutonium in the atomic bomb dropped on Nagasaki, Japan. Incidentally, plutonium is not only dangerously radioactive (alpha radiation) but is very chemically toxic too.
Granite is the common rock with the greatest concentration of trace uranium, typically on the order of several ppm which is not normally enough to pose any significant health threat. However, through weathering and erosion, granite can become a source of uranium which can be naturally concentrated in deposits in other areas. Mining of that uranium can produce tailings with elevated levels of uranium which can enter the aquatic ecosystem; some 21 states have or have had uranium mines. Uranium has an affinity for phosphate and can be found associated with phosphate deposits such as in central Florida. The uranium concentrations are not high enough to be economic but uranium daughters such as radium and radon can be a problem in that area.
Uranium is also associated with black shales (reducing environments) and can sometimes be present in high enough trace concentrations to pose a radon risk to structures built on that kind of rock. Granite, or rather the radon from the uranium in it, can be a significant problem as is the case in the granite state (New Hampshire), Galway, Ireland, and many more areas of granite. Radon is usually considered to be an air problem but there are towns in New England that are supplied with well water from wells drilled into granite whose water needs to be treated for radon. The immediate parent of radon is radium and radium is present in significant concentrations in the artesian springs of Saratoga Springs, New York.
There is no natural source of plutonium; every bit of plutonium on the Earth today was made by humans and, except a small amount of tritium produced by the interaction of cosmic radiation with air molecules, the same can be said of the fission isotopes described above. The Earth may have had some of these radioactive isotopes when it first formed but with such relatively short half-lives, they all disappeared shortly thereafter and there are no natural processes to generate more of them.
Although the table above doesn’t specify how a particular isotope decays or what kind of radiation it emits, this can easily be figured out by comparing the parent isotope with its daughter isotope. If it is an alpha decay, the atomic number (lower left) will be two less in the daughter and the atomic mass (upper right) will be four less. If it is a beta decay, the atomic number will be one more in the daughter but the atomic mass will remain the same. Beta decay is associated with gamma radiation and the wavelengths of the emitted gamma radiation is characteristic of the decay of a specific isotope. By measuring the energy/wavelengths of the gamma radiation, you can identify what isotope is emitting it.
You should also note that there is considerable variation in the half-lives and that, although there is not a strict alternation between alpha and beta decay, there are plenty of both in the decay chains to a stable isotope of lead. Some configurations of protons and neutrons within the nucleus are more stable than other configurations which is why there is a variation in the half-lives. As to whether a given isotope emits an alpha or a beta particle depends on the stability of the neutrons near the surface of the atomic nucleus.
A free neutron (one not in an atomic nucleus) is not stable; it has a half-life of about 10.3 minutes. It is, however, stable in an atomic nucleus – well, sort of. It is the weak nuclear force that holds a neutron together but it is not strong enough (after all, it is the weak nuclear force) unless it is associated with protons; this is probably an interaction between the three quarks that make up protons and neutrons. The strong nuclear force is 100xs the strength of the electromagnetic force near the nucleus and 1 million times the weak nuclear force. Within a nucleus, then, the weak nuclear force holds the individual neutrons together and the strong nuclear force (probably another interaction between quarks) binds the protons together against the repulsion of the electromagnetic force.
In a large, heavy nucleus, extra neutrons are required to spread out the strong nuclear force to hold a larger nucleus together. The protons and neutrons form shells or layers sort of analogous to the energy or shell layers of electrons outside of the nucleus. Excess neutrons tend to form a ‘neutron skin’ around the rest of the nucleus.
If there aren’t too many excess neutrons in a given nucleus, the ‘neutron skin’ will be thin and clusters of alpha particles may be found in it. These surface alpha clusters are not as tightly bound to the nucleus because they are at the nuclear surface, farther from the stabilizing influence of the other nucleons, a stabilizing influence which is not equal in all directions. It may also happen that the nucleus is not spherical; some nuclei can be ellipsoidal which would make alpha clusters at an end of the ellipsoid even less stable, more weakly bound to the nucleus. Such conditions would promote alpha decay.
Should there be more excess neutrons, the neutron ‘skin’ would be thicker and surface alpha clusters would tend not to form. The stabilizing influence of the protons would be spread over more surface neutrons, lessening the effect for each neutron, which would make the neutrons less stable. Result, beta decay.
There are a wide variety of methods for measuring radiation and, depending on the instrumentation, important differences in what is actually measured. Some instruments measure Activity (from which, radioactivity) which is how many disintegrations per second (dps) or minute (dpm) occur in a given quantity of a radioactive isotope. Other instruments measure the energy of the emitted energy. But how much energy is emitted is not the same as the amount of energy absorbed by something such as a particular material or a human being; how much energy is absorbed requires yet another set of units. How much damage occurs as a result of the absorption of a given amount of energy depends on the type of energy, whether it is alpha, beta, or gamma radiation, yielding even more units. Finally, there are units which are used to describe the effect of the radiation on an entire population.
Further complicating the many different units of radiation are three different sets of units from: the English system, the standard (mks) metric system, and the non-standard (cgs) system. The differences in these three measurement systems can be demonstrated in describing the density of water which would be in units of mass per volume.
Granite is the common rock which has the most amount of trace uranium in it, usually several ppm. Buildings, such as Grand Central Station in NYC and some buildings in Washington, D.C. which are constructed of granite, can have several times higher background radiation levels because of gamma radiation coming from the granite which might merit some mild concern after many years of exposure. Radiation from a kitchen granite countertop is probably insignificant.
Groundwater from granite bedrock, such as is found in the New England states, is not rendered radioactive because of exposure to the granite. The water, however, may pick up significant amounts of radon gas from the granite which can be released into the air in a home (see the section on radon gas).
Not all of the radiation to which you might be exposed will be absorbed by your body; just like light, some will be reflected. How much radiation is absorbed depends on what the material is. The rad (you should recognize that it is an acronym) is an older cgs unit (100 erg/gram) most often applied to how much radiation is absorbed by human tissue. Since soft body tissue will absorb 97 ergs per gram, the rad, for body tissue, is roughly equivalent to the roentgen. Its mks equivalent is the Gray (J/kg). 1 Gy = 100 rads. A typical hospital CT scan results in about 7 mGy.
Rads and Grays still say nothing about the type of radiation which is important in how much damage the radiation can do to human tissue. As it happens, alpha particles will do twenty times the damage to tissue than beta or gamma radiation will do, even when the amount of radiation absorbed is the same. The RBE is a factor which allows comparison of the relative effectiveness of different types of radiation to damage tissue.
Rem is another acronym, Roentgen Equivalent Man, and is Rads x RBE. Since the Rad is a cgs unit, so too is the Rem. Note that for beta and gamma radiation, 1 Rad = 1 Rem but for alpha radiation, 1 Rad = 20 Rem. The mks equivalent of the Rem is the Sievert which is Grays x RBE. 1 Sv = 100 Rems. A typical background radiation exposure results in a few mSv. A CT scan can add up to another 10 mSv. Nuclear employees in the U.S. are limited to 20 mSv/yr.
The effective dose links the equivalent dose with the risk of developing long-term health effects, some type of cancer. A dose of 1 Sv corresponds to a 5.5% chance of developing a cancer.
The amount of radiation exposure of a population from a radiation source is expressed in man-rems or, simply, the exposure in rems x population. 1000 people exposed to 1 mrem or 100 people exposed to 10 mrem, etc., would be equivalent to one man-rem. It has been estimated that for every ten thousand man-rems, one latent cancer will appear in the next 30 years and there will be one genetic defect in later generations in the population. 100 man-rems = 1 person-sievert.
What tissues are affected by radiation depends on the type of radiation. While gamma radiation can penetrate the entire body and beta less so, alpha radiation cannot even penetrate the outermost dead skin cells of the body. Alpha radiation does damage when it is in the body so that the first thing it hits is living cells. Radon gas, an alpha emitter, does its damage when it is inhaled into the lungs which is why it is associated with lung cancer. Radon in water, when consumed, does not appear to be associated with things like stomach cancer; it is only associated with lung cancer.
Other radioactive isotopes may be preferentially concentrated in certain body organs, one of the most specific of which is Iodine-131, a beta and gamma emitter, which is concentrated in the thyroid. Doctors take advantage of this specificity by treating hyperthyroidism and carcinoma of the thyroid with calibrated doses of I-131 which will will, at a low dose, kill some of the hyperactive thyroid cells, reducing their activity, or, at a higher dose, kill all of the cancerous thyroid cells. The I-131 which doesn’t decay in the body is naturally excreted from the body which leads to an interesting story.
A nuclear power plant was under construction in our area and as part of the process, the local area was tested for sources of radioactivity. Power plants (and not just nuclear power plants) need water for their cooling towers, water which is usually drawn from a river. Such was the case with this power plant which, when it tested the river water coming into the plant, discovered that there were detectable levels of I-131 in the river water which was strange because, although the fission of uranium does produce I-131, no nuclear fuel had yet entered the plant. Not wanting to be later accused of being the source of this I-131 when operations did begin, the power plant asked us at Wilkes University to track the source of this radioactive isotope.
It should be stated that detectable does not necessarily mean dangerous. The amount detected in the river water was very low and no threat whatsoever but the power plant did not want to be held responsible for even that much. The level was so low that we found it easier to track the I-131 upriver, not by sampling the water itself, but by taking samples of river diatoms which concentrated iodine in their shells. We followed the I-131 up to the treated effluent discharge of the local sewage treatment plant; there was no I-131 in the river water upriver of the effluent discharge. Some of our physics majors had the joy of sampling the sewage as it traveled through the sewage treatment process and we confirmed that it was in the sewage.
It then occurred to us that the source of the I-131 might be some of the local hospitals. We asked the hospitals to tell us when they used I-131 for treatment of hyperthyroidism or carcinoma of the thyroid. Sure enough, every time there was a treatment, we would see I-131 within a day or two in the sewage.
Nuclear power plants often have large supplies of non-radioactive iodine pills. Should there be a release of fission products from the nuclear plant, which could include I-131, the iodine pills would be distributed to the local population, the purpose of which would be to saturate the thyroid with non-radioactive iodine so that it would not concentrate any I-131 that might get into the body.
Major nuclear plant accidents (Chernobyl, Fukushima Daiichi) and nuclear bombs can release enough radiation to generate acute radiation effects, including death. The likelihood of death as a result of exposure to toxic chemicals as well as radiation can be described in an LD50 which is the exposure at which half of a population can be expected to die within a short time. For radiation exposure, the LD50 (lethal dose-50% of population) is about 500 rems. But effects can be long-term too. The classic long-term effect for radiation exposure is some form of cancer which might not develop for decades. Madame Marie Curie died of aplastic anemia (too few bone marrow cells meaning too few red blood cells produced) induced by exposure to radiation.