Californium (Cf) is a synthetic, highly radioactive metal belonging to the actinide series. Researchers at the University of California, Berkeley, first synthesized this element in 1950. Californium is not found naturally on Earth and must be produced in specialized high-flux nuclear reactors. This complex production results in the element being one of the most expensive materials in the world. Due to its scarcity and high cost, applications are limited to highly specific, high-value uses in industry and medicine, primarily focusing on the isotope Californium-252 (Cf-252).
The Unique Properties as a Neutron Source
The utility of Californium-252 stems from its unique nuclear property of undergoing spontaneous fission. In this decay process, the Cf-252 nucleus splits into smaller nuclei, releasing a large number of neutrons without requiring an external accelerator. Approximately 3% of Cf-252 decay events result in this spontaneous fission, with each event yielding an average of 3.7 neutrons. This mechanism makes Cf-252 an extremely reliable and self-contained source of intense neutron radiation.
One microgram of Cf-252 can emit up to 170 million neutrons every minute, establishing it as a highly portable neutron source. This high neutron flux density is a significant advantage over other neutron-emitting materials, which typically require large, complex machinery. The isotope’s half-life of about 2.645 years allows it to provide a consistent and predictable neutron stream for a practical period of time.
Industrial Uses and Non-Destructive Testing
The ability of Cf-252 to act as a portable neutron source is utilized extensively in various industrial sectors, often for non-destructive examination (NDE). Neutron radiography is a primary application, where the neutrons are used to inspect the internal structure of materials. Unlike X-rays, neutrons can easily pass through dense metal components to reveal hydrogen-containing materials like plastics, insulation, or explosives hidden within. This technique is valuable for inspecting aircraft components or nuclear fuel rods to verify their integrity and composition.
In the energy sector, Cf-252 is incorporated into specialized tools for oil well logging and moisture gauging. These tools are lowered into boreholes where the neutrons interact with the surrounding geological formations via neutron activation analysis. The resulting data helps geologists determine the porosity and permeability of the material, which is necessary for identifying oil-bearing layers and water content deep underground.
Furthermore, the element is widely used in security applications, such as the detection of explosives and landmines. Prompt Gamma Neutron Activation Analysis (PGNAA) uses Cf-252 to analyze the elemental composition of materials. This analysis specifically looks for high concentrations of elements like nitrogen that are characteristic of many explosive compounds.
Targeted Medical Treatments
Californium-252 has a specialized role in oncology, particularly in the treatment of cancers that show resistance to conventional photon (X-ray) radiation therapy. This application is known as Neutron Brachytherapy (NBT), where a sealed Cf-252 source is temporarily placed inside the body, directly within or immediately adjacent to the tumor. The neutrons emitted by the source are a form of high linear energy transfer (LET) radiation.
High-LET neutrons are highly effective because they cause dense, single-hit, non-repairable damage to the DNA of tumor cells. NBT has been effectively studied for treating advanced and bulky tumors, such as certain types of cervical, rectal, and prostate cancers. The localized placement of the Cf-252 source ensures that the high dose of destructive radiation is concentrated on the tumor while minimizing exposure to surrounding healthy tissues.
Nuclear Reactor Ignition and Scientific Research
Beyond industrial and medical applications, Cf-252 is indispensable in the operation of large-scale nuclear infrastructure. The isotope is incorporated into reactor startup sources, which are inserted into the core of nuclear reactors to initiate the fission chain reaction. When a reactor core is loaded or restarted, the background neutron flux is often insufficient to achieve a controlled chain reaction. The steady and predictable stream of neutrons from Cf-252 provides the necessary trigger to safely bring the reactor to its operational state of criticality.
Synthesis of Heavier Elements
In the field of fundamental science, Californium plays a role in the synthesis of even heavier elements. Historically, Cf-252 has been used as a target material in particle accelerators to bombard it with ions, leading to the creation of transuranic elements with higher atomic numbers.
Materials Testing
Furthermore, Cf-252 sources are used in high-flux research facilities to conduct materials science experiments. These experiments involve testing the resilience of materials and solid-state detectors to neutron damage, which is a necessary step for developing components used in advanced nuclear systems.