Californium
Atomic Data
| Atomic Number | 98 |
| Symbol | Cf |
| Atomic Weight | 251 u |
| Density (STP) | 15.1 g/cm³ |
| Melting Point | 899.85 °C (1173 K) |
| Boiling Point | 1469.85 °C (1743 K) |
| Electronegativity | 1.3 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f14 5s2 5p6 5d10 5f10 6s2 6p6 7s2 |
| Oxidation States | +2, +3, +4 |
| Phase at STP | Solid |
| Category | Actinoid |
| Period / Group | 7 / None |
| CAS Number | 7440-71-3 |
Electron Configuration
[Rn] 5f10 7s2
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 6 | 36 |
| N | 4 | 4d | 10 | 46 |
| N | 4 | 4f | 14 | 60 |
| O | 5 | 5s | 2 | 62 |
| O | 5 | 5p | 6 | 68 |
| O | 5 | 5d | 10 | 78 |
| O | 5 | 5f | 10 | 88 |
| P | 6 | 6s | 2 | 90 |
| P | 6 | 6p | 6 | 96 |
| Q | 7 | 7s | 2 | 98 |
| Total | 98 | 98 | ||
Isotopes of Californium
Californium is monoisotopic: ²⁵²Cf is its only naturally occurring stable isotope, accounting for 100% of all natural Californium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Californium-252 | ²⁵²Cf | 98 | 154 | trace | Stable |
Abundance & Occurrence
Californium is present in Earth's crust at approximately trace amounts by mass and at approximately trace amounts by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Neutron source hazard: Californium-252 (t½ = 2.6 years) undergoes spontaneous fission producing ~2.3 × 1012 neutrons/g/s: one of the most intense portable neutron sources; sources must be stored in hydrogenous shielding (polyethylene) and require personnel dosimetry including neutron dosimetry.
- Alpha radiation: Cf-252 is also an alpha emitter; internal contamination via inhalation or ingestion is a serious internal dose hazard: containment and air monitoring are mandatory.
- Medical source handling: Cf-252 brachytherapy sources require remote afterloading equipment and dedicated radiation-controlled areas; patient and staff exposures must be carefully managed.
- Orphaned source risk: Like other industrial radioisotope sources, Cf-252 sources that are lost, stolen, or abandoned represent a serious radiation hazard; all sources must be tracked under national regulatory authority licences.
Californium in the Real World
Real-World Uses
- Portable neutron source: Californium-252 (t½ = 2.645 yr) undergoes spontaneous fission at a rate of ~2.3×10³ fissions per μg per second, emitting an average of 3.76 neutrons per fission; Cf-252 is the most practical compact neutron source for nuclear reactor start-up, materials neutron activation analysis, and field explosives and drugs detection.
- Cancer brachytherapy: Cf-252 neutron sources implanted in tumour tissue deliver fast neutrons that kill hypoxic cancer cells resistant to X-ray and gamma radiotherapy; used clinically for treatment of locally advanced cervical cancer and melanoma.
- Oil well logging: Cf-252 neutron sources are used in pulsed neutron logging tools that measure formation porosity, water saturation, and lithology in oil and gas exploration wells, replacing chemical neutron sources in some applications due to their higher neutron output per unit mass.
- Nuclear reactor start-up sources: Cf-252 pellets are used as start-up neutron sources in nuclear power reactors, providing the initial neutrons needed to initiate and monitor the chain reaction when fresh, unirradiated fuel is loaded.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is californium used for?
Californium-252 is the most commercially important isotope: it is an intense neutron source, emitting neutrons by spontaneous fission at a rate of about 2.3 million neutrons per second per microgram. These neutrons are used for neutron activation analysis (identifying elements in samples), for start-up neutron sources in nuclear reactors, in nuclear well logging to find oil and gas deposits, and in cancer radiotherapy (californium-252 brachytherapy for cervical cancer).
Why is californium-252 such a useful neutron source?
Californium-252 undergoes spontaneous fission at a high rate: about 3.1% of its decays produce fission, each releasing an average of 3.8 neutrons. This gives Cf-252 the highest known spontaneous fission neutron yield of any practical isotope. A 1-microgram sample emits 2.3 million neutrons per second. This makes it the most intense and compact portable neutron source available, usable without a reactor or accelerator.
How was californium discovered?
Californium was synthesised in February 1950 by Stanley Thompson, Kenneth Street Jr., Albert Ghiorso, and Glenn Seaborg at the University of California, Berkeley. They bombarded curium-242 with helium ions in the 60-inch cyclotron and identified two isotopes. They named it californium after the state of California and the University of California, following the berkelium precedent.
How is californium used to find oil wells?
In neutron well logging, a californium-252 neutron source is lowered into a borehole on a cable. The neutrons it emits are slowed down (moderated) by hydrogen atoms in water and hydrocarbons, and a detector measures how many neutrons return after a set distance. Rock formations saturated with oil or water contain more hydrogen and slow more neutrons than dry rock, creating a measurable signal that allows geologists to map subsurface formations and locate oil and gas deposits.