Standard Physics Terms
Nuclide: An isotope or isomer. Nuclide is the primary key that links the data across the various tables in the IFFY database.
Isotope: A specific combination of protons and neutrons in the nucleus of an atom. The sum of the number of protons and neutrons is the isotope’s atomic weight. Isotopes of the same element have the same number of protons, but differ in the number of neutrons. IFFY designates an isotope by its atomic weight, followed by a dash, followed by one or two letters symbolizing the element. (14-C won’t be found here. It’s outside the range of fission fragments.)
: A nucleus in an excited energy state. Designated by -1 or -2 after the isotope name. Think of them as football-shaped, which will soon relax to the ground state.
Internal Transform (IT): The decay mode of an isomer to the ground state of the isotope.
Primordial: An isotope (or the longest-lived isomer, 180-Ta-1) with a half-life long enough to survive since the formation of Earth. Primordial isotopes are considered stable here.
Fission: The process of splitting an atom. Unless otherwise stated, the fuel atom involved will be 235-U.
Fission fragment/product: One of the two nuclei created in a fission event. A few neutrons are also freed, which keep the chain reaction going.
Fissile Fuel: An atom that will fission when it absorbs a Slow neutron, such as 235-U. (A fast neutron will split just about anything.)
Fertile fuel (or fuelstock): An atom that will become fissile after it captures one neutron. 232-Th and 238-U are the primary sources of atoms that will someday become fuel.
Neutron Poison: An isotope that likes to absorb the free neutrons needed to keep the chain reaction going. The premier poison is 135-Xe.
Barn: A unit of area about the size of a 235-U nucleus in cross-section. Apparently, a neutron doesn’t have to hit the nucleus head-on to cause fission. The cross-section for neutron absorption for 135-Xe is over 2.6 million barns, so it will reach into the next county to snatch a neutron.
Radioactive Decay Modes
There are a number of ways an unstable nuclide can change to a different nuclide. The Internal Transform mode is mentioned above.
Beta Minus (B-): This is easily the most popular decay mode for fission fragments. Technically, it uses the weak nuclear force to switch a quark inside a neutron from down to up. More practically, it turns a neutron into a proton. This is the old alchemists’ dream, changing one element into another. This transmutation moves the nucleus one cell to the right on the periodic table.
An isomer can skip over its ground state cousin by decaying via the B- mode. This happens for 85-Kr-1, which reduces the population of 85-Kr by ~78%. 85-Kr is one of the bothersome waste products with a half-life in the range of ten to thirty years. (This is practically the only reason we need the extra complexity that comes with isomers!)
B-m: This differs from B- because the result is an isomer. 85-Br becomes 85-Kr-1, so the nucleus does not drop all the way to the next rung (see Ladder analogy below). The “m” means meta-stable, which describes the isomer’s energy state.
Beta Plus (B+) and Electron Capture (EC): These are symmetric with B-, and relevant to the shadowed decay chains. The nucleus becomes the next lighter element. Some nuclides decay by EC, some B+, and some both. In either case a proton changes to a neutron.
EC-m/B+m: This is the shadowed side’s equivalent to B-m. This is the only way to get from a Shadowed chain to a main chain! Due to this, the shadowed side is processed first, so the evolution of fission fragments can be done in one pass.
B-N: This is the most common form of neutron (N) emission. 136-Sb, 100-Rb, and 98-Rb are so unstable they eject two neutrons in B-2N decay.
B-mn: A neutron is emitted and the result is an isomer.
Stable valley: A “proper” combination of neutrons and protons is stable. Fission usually results in nuclei that have too many neutrons. These are located on the neutron-heavy side of the stable valley, energetically “uphill” of the stable isotopes. They roll downhill, radiating energy via radioactive decay as they spontaneously seek to become stable. The NuDat website has a wonderful interactive graphic of the valley.
Decay chain: The evolutionary path of an unstable nucleus generally stays at the same atomic weight. For example, the lightest IFFY decay chain starts at 65-Fe and transmutes through 65-Co and 65-Ni, and stops at stable 65-Cu.
A highly unstable nucleus can eject a neutron, which pushes the nucleus down to the next decay chain. In fact, the lightest decay chain in the data is for atomic weight 66. 11% of the time 66‑Mn emits a neutron, which changes it to atomic weight 65. (We had to add this chain by hand.) The heaviest decay chains in the source data are of atomic weight 172.
Shadowed chains: The main decay chains cover neutron-heavy nuclei plus the stable isotope at the end of the chain. The shadowed chains involve the rare neutron-poor isotopes. The analogy is that of a mountain and the rain shadow it causes, which is exactly opposite of the valley analogy! The terminating stable isotope does not have to be the same as the isotope at the end of the main chain at that atomic weight, because there are sometimes more than one stable isotope at a given atomic weight.
Sink: Unstable nuclei “drain” to a stable terminator. Again, this is energetically downhill, which is the only possible direction for a spontaneous physical process such as radioactive decay. There are three sinks for atomic weight 138 and 96, and two for many others, especially when the atomic weight is an even number..
Unstable islands: Nature likes even numbers, apparently. If there are two sinks at a given atomic weight, they usually have an even number of protons. The “odd” isotope between them might decay either way. These are the islands in the stream of the stable valley.
Decay Ladder: This analogy emphasizes the discrete, as opposed to continuous, nature of radioactive decay, and its downward direction with respect to energy. The sink at the bottom of each major decay chain is assigned a Rung value of zero. Isotopes with too many neutrons are given higher rung values, always integers. The data for one chain has nine rungs.
Shadowed chains and unstable islands are given larger rung values. When decay is processed, these come first.
Isomers are given an extra 0.1 or 0.2 in their Rung value (and the analogy suffers a bit).
Family relationships: The decaying fission fragment is called the parent. It decays to a daughter, which becomes a parent when/if it decays. Isomers are called cousins.
New IFFY Terms
Rung: In a simple case, the rung is equal to the number of destabilizing neutrons above the stable number in the Sink. Rungs provides the (descending) order in which the decay chains can be processed. A more complex treatment adds 15 or 20 for shadow-side nuclides, or 100 for unstable islands, in order to process all decaying nuclides in one pass.
Time_Bin: A method of aggregating nuclides by how long they live. Most half-life references are xx sec, xx min, xx years. We keep the unit, but not the xx.
Half-Life Equivalent (HLE): A measure of how long a nucleus lives when subjected to a steady neutron flux. The neutrons can cause fission or plumping. The neutrons come from outside the nucleus affected, whereas the changes due to natural radioactivity are due to internal instability.
HLE_Bin: Similar to Time_Bin but only covers neutron absorbing isotopes, including fuels and failed fuels. Time frames are slightly different, using weeks, months and decades but not seconds or minutes.
(Absorption) Class: A roman numeral I, X, C, or M denoting the range of neutron absorption cross-section. Z is for zero, but actually means less than one. I = 1…9, X=10…99, C=100…999, M means >= 1000 barns.
Plump: A nucleus will get heavier by one atomic number when it absorbs a neutron, and this often triggers transmutation to another element. Some stable isotopes will plump up to another stable isotope of the same element, which is still referred to as transmutation. We don’t like to use the verb transmute in this case, so we found a one syllable substitute.
Failed Fuel: A fissile fuel atom that fails to fission will plump. It may also transmute. Either way, it will no longer be fuel. It may become fertile fuel or sludge.
Sludge: An heavy atom that requires more than one neutron to evolve into a fuel atom. 236-U needs three more before becoming fissile 239-Pu. This is wasteful of neutrons, which are really expensive. Many sludge isotopes have a low cross-section for capturing neutrons, so they sit in the reactor for a long time.