Thorium-232 in the context of Breeder reactors

Radon is a chemical element; it has symbol Rn and atomic number 86. It is a radioactive noble gas and is colorless and odorless. Of the three naturally occurring radon isotopes, only Rn has a sufficiently long half-life (3.825 days) for it to be released from the soil and rock where it is generated. Radon isotopes are the immediate decay products of radium isotopes.

The instability of Rn, its most stable isotope, makes radon one of the rarest elements. Radon will be present on Earth for several billion more years despite its short half-life, because it is constantly being produced as a step in the decay chains of U and Th, both of which are abundant radioactive nuclides with half-lives of at least several billion years. The decay of radon produces many other short-lived nuclides, known as "radon daughters", ending at stable isotopes of lead. Rn occurs in significant quantities as a step in the normal radioactive decay chain of U, also known as the uranium series, which slowly decays into a variety of radioactive nuclides and eventually decays into stable Pb. Rn occurs in minute quantities as an intermediate step in the decay chain of Th, also known as the thorium series, which eventually decays into stable Pb.

Thorium (₉₀Th) has seven naturally occurring isotopes but none are stable. One isotope, Th, is relatively stable, with a half-life of 1.40×10 years, considerably longer than the age of the Earth, and even slightly longer than the generally accepted age of the universe. This isotope makes up nearly all natural thorium, so thorium was considered to be mononuclidic. However, in 2013, IUPAC reclassified thorium as binuclidic, due to large amounts of Th in deep seawater. Thorium has a characteristic terrestrial isotopic composition and thus a standard atomic weight can be given.

Thirty-one radioisotopes have been characterized, with the most stable being Th, Th with a half-life of 75,400 years, Th with a half-life of 7,916 years, and Th with a half-life of 1.91 years. All of the remaining radioactive isotopes have half-lives that are less than thirty days and the majority of these have half-lives that are less than ten minutes. One isotope, Th, has a nuclear isomer (or metastable state) with a remarkably low excitation energy, recently measured to be 8.355733554021(8) eV It has been proposed to perform laser spectroscopy of the Th nucleus and use the low-energy transition for the development of a nuclear clock of extremely high accuracy.

Uranium-233 (
U or U-233) is a fissile isotope of uranium that is bred from thorium-232 as part of the thorium fuel cycle. Uranium-233 was investigated for use in nuclear weapons and as a reactor fuel. It has been used successfully in experimental nuclear reactors and has been proposed for much wider use as a nuclear fuel. It has a half-life of 159,200 years to alpha decay and is a part of the neptunium decay chain.

Uranium-233 is produced by the neutron irradiation of thorium-232. When thorium-232 absorbs a neutron, it becomes thorium-233, which has a half-life of about 22 minutes. Thorium-233 decays into protactinium-233 through beta decay. Protactinium-233 has a longer half-life of about 27 days to further decay into uranium-233; some proposed molten salt reactor designs attempt to physically isolate the protactinium from further neutron capture before beta decay can occur, to maintain the neutron economy (if it misses the U window, the next fissile target is U, meaning a total of 4 neutrons needed to trigger fission).

Thorium is a chemical element; it has symbol Th and atomic number 90. Thorium is a weakly radioactive light silver metal which tarnishes olive grey when it is exposed to air, forming thorium dioxide; it is moderately soft, malleable, and has a high melting point. Thorium is an electropositive actinide whose chemistry is dominated by the +4 oxidation state; it is quite reactive and can ignite in air when finely divided.

All known thorium isotopes are unstable. The most stable isotope, Th, has a half-life of 14.0 billion years, or about the age of the universe; it decays very slowly via alpha decay, starting a decay chain named the thorium series that ends at stable Pb. On Earth, thorium and uranium are the only elements with no stable or nearly-stable isotopes that still occur naturally in large quantities as primordial elements. Thorium is estimated to be over three times as abundant as uranium in the Earth's crust, and is chiefly refined from monazite sands as a by-product of extracting rare-earth elements.

Lead (₈₂Pb) has four observationally stable isotopes: Pb, Pb, Pb, Pb. Lead-204 is entirely a primordial nuclide and is not a radiogenic nuclide. The three isotopes lead-206, lead-207, and lead-208 represent the ends of three decay chains: the uranium series (or radium series), the actinium series, and the thorium series, respectively; a fourth decay chain, the neptunium series, terminates with the thallium isotope Tl. The three series terminating in lead represent the decay chain products of long-lived primordial U, U, and Th. Each isotope also occurs, to some extent, as primordial isotopes that were made in supernovae, rather than radiogenically as daughter products. The fixed ratio of lead-204 to the primordial amounts of the other lead isotopes may be used as the baseline to estimate the extra amounts of radiogenic lead present in rocks as a result of decay from uranium and thorium. This is the basis for lead–lead dating and uranium–lead dating.

The longest-lived radioisotopes, both decaying by electron capture, are Pb with a half-life of 17.0 million years and Pb with a half-life of 52,500 years. A shorter-lived naturally occurring radioisotope, Pb with a half-life of 22.2 years, is useful for studying the sedimentation chronology of environmental samples on time scales shorter than 100 years.

In nuclear and particle physics, a geoneutrino is a neutrino or antineutrino emitted during the decay of naturally occurring radionuclides in the Earth. Neutrinos, the lightest of the known subatomic particles, lack measurable electromagnetic properties and interact only via the weak nuclear force (when ignoring gravity). Matter is virtually transparent to neutrinos and consequently they travel, unimpeded, at near light speed through the Earth from their point of emission. Collectively, geoneutrinos carry integrated information about the abundances of their radioactive sources inside the Earth. A major objective of the emerging field of neutrino geophysics involves extracting geologically useful information (e.g., abundances of individual geoneutrino-producing elements and their spatial distribution in Earth's interior) from geoneutrino measurements. Analysts from the Borexino collaboration have been able to get to 53 events of neutrinos originating from the interior of the Earth.

Most geoneutrinos are electron antineutrinos originating in β
decay branches of K, Th and U. Together these decay chains account for more than 99% of the present-day radiogenic heat generated inside the Earth. Only geoneutrinos from Th and U decay chains are detectable by the inverse beta-decay mechanism on the free proton because these have energies above the corresponding threshold (1.8 MeV). In neutrino experiments, large underground liquid scintillator detectors record the flashes of light generated from this interaction. As of 2016 geoneutrino measurements at two sites, as reported by the KamLAND and Borexino collaborations, have begun to place constraints on the amount of radiogenic heating in the Earth's interior. A third detector (SNO+) is expected to start collecting data in 2017. JUNO experiment is under construction in Southern China. Another geoneutrino detecting experiment is planned at the China Jinping Underground Laboratory.