Californium (symbol Cf, atomic number 98) is a heavy synthetic metal in the actinide series (period 7, f-block). It is a transuranic element (heavier than uranium) with no stable isotopes. At room temperature it is a dense, silvery-gray metal that tarnishes slowly in air. The most common oxidation state is +3 (like many lanthanide analogs), although +2 and +4 compounds are also known. With a melting point around 1170 K and boiling point around 1740 K, californium is a solid under normal conditions. Its isotopes are all radioactive; the longest-lived, ^251Cf (half-life ≈ 898 years), decays by α emission, while ^252Cf (half-life ≈ 2.65 years) is famous for its spontaneous fission. Californium-252 is intensely neutron-emitting – a few micrograms produce “trillions” of neutrons per minute – which makes Cf-252 extremely useful as a portable neutron source for specialized applications. These properties underpin all practical uses of californium, even as its intense radioactivity requires strict shielding and handling precautions.

Atomic Structure and Electron Configuration

Californium
Atomic number	98
Symbol	Cf
Discovery	1950 (Thompson, Street, Ghiorso, Seaborg)
Electron configuration	[Rn] 5f10 7s2
Density	15.1 g/cm^3
Main isotopes	249Cf, 251Cf, 252Cf
Melting point	900 °C
Block	f
Phase STP	Solid
Oxidation states	+2, +3, +4
Wikidata	Q1888

With atomic number 98, californium has 98 electrons. Its ground‐state electron configuration is [Rn] 5f^10 7s^2. In other words, it has a radon-like core plus ten electrons in the 5f shell and two in the 7s shell. These 5f electrons give Cf chemical behavior similar to late lanthanides (dysprosium is immediately above Cf in the periodic table). In periodic trends the actinides exhibit an “actinide contraction” (a shrinking radius across the series); thus californium’s atomic radius is moderately small for an actinide and comparable to the smaller lanthanoids. Californium is quite electropositive: its Pauling electronegativity is only about 1.3 (similar to its neighbors in the actinide/lanthanide series) and its first ionization energy is roughly 6.3 eV (about 607 kJ/mol). These values reflect a metallic, electropositive element that readily loses electrons (typically forming Cf^3+ ions) in chemical reactions. In terms of periodic placement, Cf is a late actinide that behaves chemically much like a trivalent rare-earth metal, although it is heavier and radioactive.

Isotopes and Nuclear Properties

All californium isotopes are radioactive; none occur naturally in any significant quantity. Twenty isotopes (mass numbers 237–256) are known, all with half-lives short on a geological timescale. The longest-lived is ^251Cf (half-life ≈ 898 years); ^249Cf has T½ ≈ 351 years, ^250Cf ≈ 13 years, and ^252Cf ≈ 2.65 years. Most other isotopes decay in minutes or hours. The dominant decay mode of Cf isotopes is alpha emission (turning californium into curium plus an α-particle), but ^252Cf is unique in having a ∼3% branching to spontaneous fission. In ^252Cf spontaneous fission releases energetic neutrons – about 3.7 neutrons per fission on average (roughly 2.3 × 10^6 n/s per microgram of ^252Cf). The fission neutrons have an energy spectrum ranging up to ~13 MeV (most probable around 1 MeV, average ~2–3 MeV). This intense neutron emission makes ^252Cf one of the strongest neutron sources available. Other isotopes mainly emit α particles; for example, ^249Cf and ^251Cf decay almost entirely by α emission.

Because californium isotopes are man-made, they have no abundance in nature except in trace amounts from human activities (e.g. nuclear fallout or spent fuel). ^252Cf and other isotopes are routinely produced in high-flux reactors by successive neutron captures on lighter actinides (plutonium, americium, or curium targets) and then chemical separation. The neutron-capture pathway to ^252Cf from ^238U requires on the order of 15 neutron captures (bypassing fission). In nuclear science, ^249Cf has been used as a target to create new superheavy elements (for example, ^249Cf + ^48Ca produced oganesson, element 118). On the other hand, the short half-lives mean californium cannot be used for radiometric dating in geology or biology – it serves purely as a neutron source or tracer in industrial and scientific contexts. Notably, californium isotopes have nuclear spins and moments characteristic of heavy even-even or odd-A nuclei, but these details matter little for most applications beyond nuclear physics research.

Allotropes and Typical Compounds

Metal Allotropes: Californuim metal has multiple solid forms. At ambient pressure it has two allotropes: an α-phase at lower temperatures (a double hexagonal close-packed structure) and a β-phase above about 600–800 °C (face-centered cubic). A third phase appears under very high pressure (~48 GPa). In the α-phase at room temperature, the density of californium metal is about 15.1 g/cm³; in the β-phase it drops to about 8.7 g/cm³. The metal is silvery-white and malleable (easily cut with a knife), but it tarnishes slowly in air, especially if moisture is present (forming surface oxides). Californium also forms alloys with lanthanide metals, though such alloys are not well characterized.

Films and Organo-Compounds: Very thin films of californium metal have been prepared (μg quantities) by reducing Cf2O3 with metals like lanthanum. Some organometallic complexes exist too (for example, Cf metallocene compounds), but they are rare and of academic interest due to intense radioactivity.

Major Inorganic Compounds: As with other actinides, californium most commonly forms compounds with Cf in the +3 oxidation state. Trivalent californium compounds are largely ionic. Representative examples include:

Oxides: Cf2O3 (californium(III) oxide) is a sessquioxide that appears yellow to green. CfO2 (californium(IV) oxide) is also known, a brown-black suboxide.
Halides: Trifluorides, trichlorides, tribromides, and triiodides (CfF3, CfCl3, CfBr3, CfI3) are typically bright-colored solids (for example, CfF3 is a bright green powder, CfCl3 emerald green, CfI3 lemon-yellow). Dihalides such as CfBr2 (yellow) and CfI2 (dark violet) exist as +2 species. Californium(IV) fluoride (CfF4, green solid) is a strong oxidizer; other Cf(IV) halides may be prepared under extreme conditions.
Hydroxides and Sulfides: Cf(OH)3 can be precipitated from solution (gray-green). Cf2S3 (californium sulfide) and other chalcogenides are known from high-temperature reactions.
Complexes: Californium forms coordination compounds and complex salts similarly to the lanthanides. For instance, a polyborate complex Cf[B6O8(OH)5]·(H2O)n has been isolated (californium in +3 state, yielding a pale green pigment). Many anion complexes (like Cf(NO3)3) and organic chelates are possible, though not widely studied.

In general, Cf^3+ salts are stable in air and water (the oxide and fluoride are moisture-resistant). Cf(II) compounds (halides) are powerful reducing agents, while Cf(IV) compounds are strong oxidizers. Bonding is largely ionic with coordination numbers of 8 or 9 being common. No allotropes analogous to carbon (diamond vs graphite) exist beyond the metal structures; all other forms are simply different compounds of californium.

Physical Properties

Californium’s bulk physical properties are dominated by its metallic nature and radioactivity:

Appearance: Metallic californium is silvery-white to gray. It tarnishes in moist air (forming oxides) but is stable in dry air at room temperature.
Density: ~15.1 g/cm³ (15.1×10^3 kg/m³) at 25 °C in the α-phase. (In its high-temperature β-phase the density drops to ~8.7 g/cm³.)
Melting/Boiling Points: Melts around 1173 K (900 °C); boils around 1743 K (1470 °C). In practice, handling is usually far below melting. In vacuum the metal will slowly vaporize above roughly 300 °C.
Crystal Structure: Below ≈ 600–800 °C, α-Cf has a double hexagonal close-packed (dhcp) lattice. Above that temperature it transforms to β-Cf with a face-centered cubic (fcc) structure. Under extreme pressure it becomes an orthorhombic phase.
Mechanical: Californium metal is relatively soft and malleable (it can be cut with a knife in microgram-scale samples).
Magnetism: At very low temperatures californium orders magnetically. Below about 51 K it behaves ferromagnetically or ferrimagnetically; between roughly 51–160 K it is antiferromagnetic. Above ~160 K it is paramagnetic (no spontaneous magnetization). This complex magnetic behavior arises from its unpaired f-electrons.
Conductivity: As a metal, Cf is electrically conductive; however detailed electronic properties are not well quantified.
Thermal Properties: Californium has an (estimated) specific heat similar to other actinides; its bulk modulus (~48 GPa) is comparable to other late actinides and smaller than that of common metals like Al.
Radiation Emission: All californium is intensely radioactive. ^252Cf in particular emits not only α-particles but also copious neutrons and gamma rays. Even microgram quantities produce dose rates that require heavy shielding (lead or concrete for gammas, combined with hydrogenous material for neutrons). The radiation field from californium severely limits direct measurement of some properties. Its spectral lines (X-rays, gamma, and atomic spectra) have been measured in laboratories but are less relevant outside physics research.

Chemical Reactivity and Trends

Californium behaves chemically much like other heavy actinides and analogous lanthanides (rare-earth metals). Its low electronegativity and large radius make it quite electropositive and reactive toward many non-metals, especially in elevated temperatures or aggressive conditions:

Corrosion and Air/Oxygen: Californium metal tarnishes slowly in moist air (forming a dull oxide layer). Dry air causes less rapid oxidation. On heating, Cf metal will react with oxygen to form oxides (mainly Cf2O3) and with chalcogens (S, Se) to give CfSx compounds. It also slowly reacts with water vapor or liquid water to form Cf(OH)3 and hydrogen gas. In practice, Cf metal is often stored under inert atmosphere or in vacuum to prevent corrosion.
Acids and Bases: Californium dissolves readily in acids. For example, dry HCl gas reacts quickly with Cf to yield CfCl3 and hydrogen. In aqueous strong acids, Cf^3+ goes into solution (as CfCl3, Cf(NO3)3, etc). Trivalent Cf salts behave like those of other actinides: they hydrolyze only weakly, precipitating Cf(OH)3 when the pH is raised. Californium(III) oxide Cf2O3 is basic to amphoteric (dissolves in acids, reacts with strong alkali to some extent).
Halogens: Californium metal reacts with fluorine or chlorine at high temperatures to form CfF3 or CfCl3, respectively. The +3 halides are stable, and with excess halogen or under rigorous conditions, CfF4 can form (a strong fluorinating agent).
Hydrogen and Nitrogen: On heating (several hundred °C), Cf metal will absorb hydrogen to form hydrides (nominally CfH2). It also reacts with nitrogen at ∼1200 °C to form CfN. These binary compounds are studied mainly to probe bonding; CfN is a refractory solid.
Complexation: In solution, Cf^3+ behaves like the heavy rare-earth ions. It complexes with ligands such as EDTA, carbonates, or citrate, but data are scarce due to radioactivity. In many solvents Cf^3+ remains as the nine-coordinate aqua ion [Cf(H2O)9]^3+, similar to the analogous dysprosium(III) ion.
Oxidation States: The +3 state is by far the most stable. Cf(II) is a strong reducing agent (Cf^2+ salts easily oxidize to Cf^3+). Cf(IV) compounds (like CfO2 or CfF4) are strong oxidizers akin to Ce(IV) salts. No higher (e.g. +5) are known.
Reactivity Series Context: Californium is more noble than the alkali/alkaline-earth metals but still a very active metal. It lies near the bottom of the electrochemical series among actinides, readily losing electrons to semi-metals. In practical terms, Cf metal must be handled in controlled lab conditions; diluted acids and oxidizers can corrode it quickly.
Passivation/Corrosion: Unlike aluminum or chromium, californium does not form a self-protecting oxide layer; it corrodes deeper into the metal. In air or acid, any exposed Cf will continue to form oxide or hydrate with time.

Overall, californium’s chemical trends follow those of late actinides and of its lanthanide homologs (dysprosium, terbium): predominantly +3 ionic chemistry, basic oxide/hydroxide, idiosyncratic +2/+4 extremes, and electropositive metal behavior.

Occurrence and Production

Californium is essentially exclusively synthetic. Its natural (terrestrial) abundance is negligible – estimated at less than 10^-16 relative to common elements. Any californium found today comes from human activity. Tiny traces of Cf isotopes were briefly present in fallout from nuclear weapons tests, but all such atoms have long since decayed. Small quantities of californium (mostly ^252Cf) are created as byproducts of plutonium- and uranium-fueled research reactors (for example, in spent nuclear fuel or specialized targets). However, even spent fuel contains only micrograms of Cf at most, and recovering it is not practical.

Production: The only practical source of californium is purposeful production in high-flux nuclear reactors. The main method is to irradiate actinide targets with neutrons. For example, neutron irradiation of plutonium or americium samples can lead stepwise to curium, berkelium, and finally californium isotopes. Oak Ridge National Laboratory (ORNL) in the USA and the Research Institute of Atomic Reactors (RIAR) in Russia are the world’s primary producers of ^252Cf. Oak Ridge’s High Flux Isotope Reactor (HFIR) has been dedicated for decades to isotope production, where targets (often curium oxide) are irradiated for times on the order of 1–2 years. After irradiation, the mixture of actinides is processed in radiochemical “hot cells” to separate and purify ^252Cf (and other heavy actinides). This multi-step process yields californium in the form of salts or metal, often as milligram quantities per year. (In 1995, ORNL was producing on the order of 0.5 grams of Cf per year.) A recent news report notes that only two facilities in the world – ORNL HFIR and Russia’s reactor – can produce ^252Cf in useful quantities.

Because californium is so expensive to make, production is governed by supply contracts. The U.S. Department of Energy certifies sales of ^252Cf to industry and research (for example, under a multiyear contract with a consortium of users). Fees reflect its rarity: in the 1970s Cf-252 went for about $10 per microgram, rising to ~$60/μg by the late 1990s (not counting customization or shipping). In practice, laboratories borrow or rent tiny encapsulated sources on loan from suppliers due to the high cost.

Cosmic Occurrence: In astrophysics, californium (like all transuranics) would be created by rapid neutron-capture processes (r-process) such as in supernovae or neutron-star mergers. However, even cosmic californium decays quickly, so no primordial Cf remains today. Thus, all californium used is human-made.

Applications and Technology

Despite its scarcity and radioactivity, californium (especially ^252Cf) has several specialized applications that exploit its intense neutron emission. Key uses include:

Neutron Activation Analysis (Assay): ^252Cf is widely used as a portable neutron source for prompt gamma neutron activation analysis (PGNAA). In this technique, materials such as coal, cement, minerals, or waste are irradiated with neutrons, causing their atoms to emit characteristic gamma rays. By analyzing these gamma rays, the elemental composition can be determined rapidly and nondestructively. Coal and cement plants use Cf-252 sources to check for impurities (e.g. measuring chlorine, sulfur or moisture content). The oil and gas industry employs Cf-252 in well logging: a slim source is sent down boreholes to irradiate rock, and neutron moderation/gamma detection reveals porosity, water/oil content or shale layers. In essence, ^252Cf “assays” geological and industrial supplies on the spot.

Neutron Radiography and Imaging: Californium sources are used to produce neutron beams for imaging (neutron radiography). Because neutrons penetrate many metals and respond differently to hydrogenous materials, neutron imaging can see through lead or steel where X-rays cannot. Portable ^252Cf-based neutron radiography units can inspect welds, rocket motors, engine parts, or luggage in security. For example, the U.S. military uses Cf-252 in the PINS (Portable Isotopic Neutron Spectroscopy) system to detect and identify explosives or chemical agents in munitions by shooting neutrons into the object and analyzing the resulting prompt gamma rays. Similar systems guard ports and borders against hidden nuclear materials or contraband.

Nuclear Reactor Startup: A prime use of ^252Cf is to initiate the chain reaction in a fresh or long-dormant nuclear reactor. A small Cf-252 neutron source (often sealed in a stainless-steel “probe” or wire) is placed in the reactor core or near it before fuel loading. The steady flux of neutrons from the source “pre-neutronates” the core so that when the reactor approaches criticality, it can more easily attain a controlled chain reaction. Cf-252 sources have been used in naval reactors and commercial power reactors alike. For example, in recent years ORNL produced Cf-252 specifically for starting up new reactors at Plant Vogtle (Georgia, USA) and other projects.

Nuclear Waste Assay: Similar to PGNAA, Cf-252 can assay nuclear waste containers by detecting neutron-induced fission or activation in the waste, allowing determination of fissile content or hazardous isotopes without opening the canister. Its penetrating neutrons make it useful for validating waste forms.

Calibration and Instrumentation: ^252Cf is a standard neutron calibration source. It is used to test and calibrate neutron detectors (He-3 tubes, scintillators, dosimeters) and even gamma counters (via associated gamma emissions). Its availability of a known, steady neutron output (that decreases predictably with its half-life) makes it ideal for standardized radiation instrumentation checks.

Medical (Neutron Therapy): Californium-252 has been used in cancer therapy. As a neutron emitter, it has very high linear energy-transfer (LET) radiation which can be effective against certain tumors. Cf-252 brachytherapy (placing a source in or near a tumor) has been tried for cervical, endometrial, and brain cancers. In practice, its use is limited and experimental, because neutrons require extensive shielding and the sources are very expensive. Nonetheless, some specialized cancer centers have used ^252Cf for high-grade tumors where other radiation failed.

Scientific Research: Californium is used in basic research. ^252Cf provides neutrons for neutron diffraction and neutron spectroscopy studies of materials (for example, to probe crystal structures of metal hydrides or biological samples). It is also used in nuclear physics experiments to study fission processes or as a target for synthesizing new elements (as mentioned, ^249Cf has been used to make element 118). Its high neutron yield makes it an excellent source for fundamental neutron-related experiments where reactor or accelerator sources are impractical.

Other Industrial Uses: Smaller niche applications include moisture/density gauges and pipeline inspection (in oilfields), where a neutron source like Cf-252 can penetrate soil or thick materials. It has also been used in research on neutron radiography of cultural artifacts (e.g. examining meteorites for water content) due to its portability compared to reactor sources.

In each of these applications, the Cf-252 source is typically a tiny sealed capsule or agragation (often a flat wire or rod coated with ^252Cf salt) that can be handled remotely. Users must account for the source’s finite half-life: a Cf-252 source loses about half its intensity every 2.65 years, so it is often replaced or “recharged” periodically. In many cases, compact neutron generators (accelerator tubes) can be an alternative, but ^252Cf remains unmatched in portability and high neutron output without need for external power.

Biology, Environment, and Safety

Californium has no known biological role and is considered hazardous solely due to its radioactivity. It is extremely toxic if incorporated into the body; chemically it is a heavy metal, but the dominant danger is radiological. Californium-252 (and other isotopes) emit a mix of high-energy radiations (α-particles, γ-rays and neutrons), so exposure must be taken very seriously:

Health Effects: If californium is ingested or inhaled, only a tiny fraction (≈0.05%) enters the blood. Of that, most (≈65%) accumulates in the bone (especially bone surfaces), with the remainder ~25% in the liver (and the rest excreted). In bone and liver, the radioactive decay of Cf irradiates nearby tissues. Bone marrow in particular is extremely sensitive; Cf accumulation disrupts red blood cell formation and can lead to anemia, leukemia, and other cancers. Even external exposure can be hazardous: neutron radiation penetrates deeply and can damage internal organs. Skin contact with strong sources can cause burns and localized tissue damage from neutrons and gamma-rays. In short, any significant intake of Cf into the body would be fatal if not treated immediately; even small internal deposits carry high cancer and organ-damage risk over time.

Environmental Fate: Californium released to the environment (for example, by an accident) would largely stay put; its heavy ions bind strongly to soil and sediments. It has very low mobility in water and does not bioaccumulate in food webs (the total released Cfalls from nuclear testing was negligible). Over time, radioactive decay reduces any environmental Cf. There is no natural cycling of Cf in the biosphere like stable elements. Regulations treat it as a high-level radioactive contaminant.

Safety and Handling: Due to its intense radioactivity, californium must be handled with stringent precautions. All work is done in gloveboxes or hot cells with lead/steel shielding and neutron-absorbing materials (like water or polyethylene) surrounding the source. Distance and time are strictly controlled. A modest 1-μg Cf-252 source can emit ~10^8 neutrons per second and significant gamma radiation; even brief unshielded exposure at a meter’s distance could exceed legal dose limits. Californium sources are shipped and stored in heavy containers, often with lead and wax or borated water to filter gammas and neutrons. Transport of Cf-252 follows Category I (“special form”) regulations due to its high activity.

 Laboratory standards often set exposure limits in terms of microsieverts per hour, and protective gear (lab coats, gloves) are used to avoid skin contamination. Continuous monitoring for contamination is routine. Because californium can adhere to surfaces or resuspend as dust, any spills demand immediate radiological decontamination procedures. As with other alpha-emitters, inhalation is the greatest danger. In the event of uptake, chelation therapy (e.g. diethylenetriaminepentaacetic acid) has been used to help remove heavy actinides like curium/californium from the body.

Regulations: Cf-252 (and other Cf isotopes) are classified as highly dangerous radioactive sources by agencies like the IAEA and NRC. Personnel working with Cf must have special licenses and training. Because it is a potential “dirty bomb” material (neutrons make other materials radioactive), security is tight; only approved institutions may obtain it. Fortunately, its short half-lives mean that nearly all Cf in existence decays away with time if kept contained. Any used or decayed sources become radioactive waste requiring long-term management (since daughter isotopes are also radioactive).

In summary, while californium’s radioactivity makes it a powerful tool, it also makes it extremely hazardous. All uses are restricted to specialized facilities. There is no threshold for “safe” ingestion; even tiny microgram amounts internally carried can cause harm. Thus californium compounds and sources are handled with the utmost care, under heavy shielding, and with institutional controls on storage and disposal.

History and Etymology

Californium was first synthesized in early 1950 at the Lawrence Berkeley National Laboratory (then UC Radiation Laboratory). A team led by Stanley G. Thompson, Kenneth Street Jr., Albert Ghiorso, and Glenn T. Seaborg bombarded curium-242 targets with helium ions (alpha particles) in Berkeley’s 60-inch cyclotron. On February 9, 1950, they produced a few thousand atoms of element 98 (then called “element 98”) which they identified by its radioactive decay properties. The team announced the discovery in March 1950. The name californium was chosen to honor the state of California and the University of California (a break from the usual naming conventions for transuranics, which often used names of scientists or mythical concepts). It was the sixth transuranium element to be discovered (after neptunium, plutonium, americium, curium, and berkelium).

Initially only trace amounts (atoms) of californium were produced. In 1954, scientists at the Idaho National Reactor Testing Station managed to create “weighable” microgram quantities by intense neutron irradiation of plutonium targets. Around that time, they also noticed element 98’s exceptionally high rate of spontaneous fission, later understood to be due to ^252Cf. In 1960, Berkeley chemists generated the first californium compounds (CfCl3, CfO, etc.) by isolating and treating Cf with acids.

The High Flux Isotope Reactor (HFIR) at Oak Ridge began routine production of Cf in the 1960s. By the 1970s, hundreds of milligrams of ^252Cf were being produced annually at ORNL and shipped to users. In 1974–1975, researchers prepared the first bulk metal forms: reducings as little as microgram quantities of Cf2O3 with lanthanum metal yielded thin films of metallic californium (crystalline Cf metal was confirmed in 1975).

Historically, californium had a significant price tag: the U.S. government sold Cf-252 (encapsulated sources) to industry at about $10 per microgram in the early 1970s. By the 1990s that price had risen (e.g. ~$60/μg by 1999). The high cost reflected the difficulty of production.

The element’s name “californium” comes from California, the first element named after a U.S. state. (It also implicitly honors the University of California.) This commemorates the place of its discovery. In cultural or industrial terms, californium’s milestones are tied to nuclear technology and research: its unique neutron source applications and role in synthesizing new elements (e.g. Californium’s handgun FUSION experiment yielded element 118, oganesson, in 2002) are its lasting legacies. Apart from science, californium has little presence, but its discovery marked a substantial advance in transuranic chemistry and nuclear instrumentation.

Data Table

Property	Value
Symbol	Cf
Atomic number (Z)	98
Classification	Transuranic actinide (f-block, synthetic)
Electron configuration	[Rn] 5f^10 7s^2
Common oxidation states	+3 (principally); also +2 and +4 known
Phase at STP	Solid metallic
Density	15.1 g/cm³ (15.1×10^3 kg/m³) (α-Cf at 25 °C)
Melting point	≈ 1173 K (900 °C)
Boiling point	≈ 1743 K (1470 °C)
Electronegativity (Pauling)	1.3
First ionization energy	6.3 eV (≈ 607 kJ/mol)
Isotopes	No stable isotopes. Longest-lived is ^251Cf (T½ ≈900 y); common source ^252Cf (T½ 2.645 y).
Discovered	1950, Lawrence Berkeley Lab (Thompson, Street Jr., Ghiorso, Seaborg)
Named for	California (U.S. state and University of California)