Quick reference
| Property | Value |
|---|---|
| Symbol | Cf |
| Atomic number | 98 |
| Atomic weight | [251] (most stable isotope) |
| Group | Actinide |
| Discovered by | Stanley G. Thompson, Kenneth Street Jr., Albert Ghiorso and Glenn T. Seaborg |
| Discovery date | 1950 |
| Name origin | Named after the state of California and the University of California |
| Density | About 15.1 g/cm³ |
| Melting point | About 900 °C |
| Appearance | Silvery-white radioactive metal |
A made-in-the-lab element
Californium is a synthetic element: it isn’t found in nature in useful amounts and must be made in laboratories or nuclear reactors. It’s a transuranium element, meaning its atomic number is greater than uranium’s (92).
Discovery
On February 9, 1950, a team at the University of California, Berkeley, bombarded a tiny sample of curium-242 with alpha particles (helium nuclei) using a cyclotron, a type of particle accelerator. They created californium-245, with a half-life of about 45 minutes.
The team had produced only about 5000 atoms. They named the element after their state and university. Glenn Seaborg, a member of the team, helped discover ten elements and later had element 106, seaborgium, named after him.
How it’s made today
Californium is produced in tiny amounts in special high-flux nuclear reactors, mainly at Oak Ridge National Laboratory in Tennessee and in Russia. Heavy elements like curium are bombarded with neutrons for months until some atoms capture enough neutrons to become californium.
Only small amounts, often less than a gram a year, are produced worldwide. This makes californium one of the most expensive substances on Earth: californium-252 has been priced at tens of millions of dollars per gram.
Isotopes
Californium has about 20 known isotopes. The most useful is californium-252.
| Isotope | Half-life | Notes |
|---|---|---|
| Cf-251 | About 900 years | Most stable isotope |
| Cf-249 | About 351 years | Used to make heavier elements, including oganesson |
| Cf-252 | About 2.6 years | Powerful neutron source |
A powerful neutron source
Californium-252 is unusual: about 3% of its atoms decay by spontaneous fission, splitting on their own and releasing neutrons. Just one microgram (a millionth of a gram) gives off about 2.3 million neutrons per second. This makes it an extremely strong neutron source.
Uses of californium-252
| Use | How it helps |
|---|---|
| Starting nuclear reactors | Provides neutrons to begin the chain reaction |
| Oil exploration | Neutron probes lowered into wells identify rock layers holding oil and water |
| Mining | Detecting gold and silver ores |
| Security | Detecting explosives and hidden nuclear materials in luggage and cargo |
| Medicine | Treating certain cancers (neutron brachytherapy), especially some cervical and brain tumours |
| Industry | Checking aircraft parts and welds for flaws |
| Research | Making heavier elements |
Californium-249 was used as the target to create element 118, oganesson, in 2002.
Safety
Californium is highly radioactive and dangerous. Its neutrons can damage living tissue, and if it enters the body, it builds up in bones, where it can damage bone marrow. Scientists handle it only in tiny amounts, with heavy shielding and remote-controlled tools.
Californium in the periodic table
Californium belongs to the actinides, the row of radioactive elements at the bottom of the periodic table. All actinides are radioactive. Only actinium, thorium, protactinium and uranium (and tiny traces of neptunium and plutonium) occur naturally in significant amounts; the rest, including californium, are made by scientists.
Common mistakes
- Thinking californium is found in nature. It’s synthetic.
- Thinking all radioactive elements are equally dangerous. It depends on the type of radiation, amount and half-life.
Try it yourself
- Find californium on the periodic table. Name its neighbours and their origins (many are named after scientists or places).
- Make a list of all the elements named after places. Which ones are named after U.S. states or cities?
- Calculate: if you start with 8 µg of californium-252, how much remains after 5.2 years? (Hint: 2 half-lives.)
(Answer to 3: 2 µg)
Quick recap
- Californium (Cf, atomic number 98) is a synthetic, radioactive actinide.
- It was first made at Berkeley in 1950 and named after California.
- It’s produced in tiny amounts in special reactors and is extremely expensive.
- Californium-252 is a powerful neutron source used in reactors, oil exploration, security and medicine.
- It must be handled with heavy shielding.