Californium is a synthetic, radioactive metallic element not found in significant natural quantities on Earth. This transuranic element, with atomic number 98, must be artificially produced in specialized environments. Its extreme rarity and potent radioactivity make it one of the most expensive elements, with Californium-252 (Cf-252) prices reaching substantial amounts per gram.
The Starting Materials
Californium production begins with heavy, transuranic elements chosen for their high atomic mass, suitable for successive neutron capture and decay processes. Primary target materials include Curium-244 (Cm-244), Plutonium-239 (Pu-239), or Berkelium-249 (Bk-249). These isotopes absorb multiple neutrons, gradually increasing their atomic mass. The target material is prepared for efficient bombardment within a nuclear reactor.
Nuclear Reactor Synthesis
Californium production occurs within a high-flux nuclear reactor, such as the High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory (ORNL) in the United States. HFIR is the western world’s sole supplier of Californium-252.
Within the reactor, target materials like Curium or Berkelium undergo prolonged neutron bombardment. This involves successive neutron captures, increasing their mass number. For example, bombarding Berkelium-249 with neutrons forms Berkelium-250, which then undergoes beta decay to produce Californium-250. Further bombardment can lead to Californium-252.
Following neutron capture, some of these unstable, heavier isotopes undergo beta decay, where a neutron transforms into a proton, emitting an electron (beta particle). This transformation changes the element’s atomic number, moving it up the periodic table and eventually forming isotopes of Californium. The high neutron flux and extended irradiation periods, sometimes lasting nearly two years, enable this multi-step transmutation.
Separation and Handling
Once the irradiation process is complete, the irradiated target material, now containing Californium along with unreacted target material, fission products, and other transuranic elements, must undergo complex chemical separation. Techniques like ion-exchange chromatography or solvent extraction are employed to isolate the Californium. This purification is challenging due to the presence of many chemically similar elements, especially other actinides and lanthanides.
Given Californium’s intense radioactivity, specialized facilities and stringent safety protocols are essential for its manipulation and storage. Operations take place in heavily shielded “hot cells,” which are containment chambers designed to protect personnel from high radiation levels. These hot cells feature thick concrete or lead walls, lead-glass viewing windows, and remote manipulators that allow technicians to handle the radioactive materials without direct contact.
The Radiochemical Engineering Development Center (REDC) at ORNL, for instance, operates such hot cells for processing irradiated targets and purifying Californium. These facilities ensure that the highly radioactive Californium-252, which spontaneously emits neutrons, is managed safely throughout its separation, purification, and encapsulation for various applications. Proper containment and shielding are maintained to minimize radiation exposure.
Key Uses
Californium-252 is highly valued for its ability to spontaneously emit neutrons, making it a powerful and compact neutron source. This property leads to its diverse applications across various fields.
One significant use is in neutron radiography, a non-destructive testing method that utilizes neutrons to inspect the internal structures of materials, detecting flaws or hidden features that X-rays might miss. In the oil and gas industry, Californium-252 sources are used in well logging to determine the porosity and water saturation of geological formations. The neutrons emitted help analyze rock composition and identify hydrocarbon reserves.
Additionally, Californium-252 serves as a startup neutron source for nuclear reactors, providing the initial neutrons needed to begin a controlled chain reaction. Californium has also been explored for medical applications, particularly in neutron capture therapy for certain cancers, though its use in this area has become less common. Its strong neutron emission also makes it useful in research for studying other transuranic elements and investigating nuclear reactions. The unique characteristics of Californium-252 justify the significant effort and resources dedicated to its production.