The element with 159 neutrons is Californium, specifically the isotope Californium-257 (\(^{257}\text{Cf}\)). Every atom is defined by its nucleus, which contains protons and neutrons, collectively known as nucleons. The number of protons, or the atomic number (\(Z\)), determines the element’s chemical identity, while the number of neutrons (\(N\)) can vary, creating different isotopes. Californium has an atomic number of 98. When an atom has 98 protons and 159 neutrons, the mass number is 257 (98 + 159 = 257), resulting in Californium-257. This isotope is highly radioactive and unstable, which is typical for elements this heavy.
Identifying Elements Based on Neutron Count
Determining the element from a fixed neutron count requires knowing the mass number (\(A\)) to calculate the atomic number (\(Z\)). The fundamental relationship \(A = Z + N\) governs the composition of the atomic nucleus. If only the neutron count (\(N=159\)) is known, the element’s identity (\(Z\)) must be deduced by finding a viable isotope that exists with a known mass number (\(A\)).
A neutron count of 159 is exceptionally high, suggesting the element must be very heavy and possess a large number of protons. Atoms with a low atomic number, such as oxygen or iron, cannot physically bind 159 neutrons into a stable or even short-lived nucleus. For heavy elements, stability requires a neutron-to-proton ratio significantly greater than 1:1. For the Actinide series (\(Z=90\) to \(100\)), this ratio is typically around 1.5:1.
The only element in this heavy range that forms a known, highly unstable isotope with \(N=159\) is Californium (\(Z=98\)). This results in the isotope Californium-257 (\(^{257}\text{Cf}\)). By seeking the heaviest known isotope with this neutron count, the calculation \(A – N = Z\) confirms the atomic number 98 (257 – 159 = 98).
Characteristics of Californium-257
Californium (Cf) is a synthetic element with the atomic number 98, making it the sixth transuranic element in the Actinide series. It was first synthesized in 1950 by researchers at the University of California Radiation Laboratory in Berkeley, including Glenn T. Seaborg and Albert Ghiorso. The initial discovery involved bombarding Curium-242 with alpha particles using a cyclotron. The element was named after the university and the state of California.
Californium-257 is one of the twenty known isotopes of the element. The most stable isotope is Californium-251, which has a half-life of 898 years. In contrast, Californium-257 is significantly less stable than Cf-251.
The production of Californium isotopes typically occurs within specialized high-flux nuclear reactors, such as those at Oak Ridge National Laboratory. This is achieved through the prolonged neutron bombardment of lighter elements like plutonium or americium.
Why 159 Neutrons Means Extreme Instability
The extreme neutron count of 159 relative to 98 protons causes the nucleus’s inherent instability, a concept related to the “Valley of Stability.” As the atomic number increases, the repulsive force between protons (Coulomb force) grows stronger. This requires a disproportionately larger number of neutrons to provide the strong nuclear force needed to hold the nucleus together.
For Californium-257, the neutron-to-proton (\(N/Z\)) ratio is approximately 1.62 (159/98). This ratio is higher than that of the most stable isotopes in this region, placing it on the neutron-rich side of the stability curve. Highly unstable, neutron-rich nuclei like Californium-257 primarily undergo spontaneous fission and alpha decay.
Spontaneous Fission
Spontaneous fission is a decay mode where the nucleus splits into two or more smaller nuclei, releasing energy and several neutrons. This process is a dominant decay pathway for elements beyond Uranium (\(Z=92\)). The electrostatic repulsion between the numerous protons begins to overcome the strong nuclear force that binds the nucleus, causing the split.
Alpha Decay
Alpha decay involves the emission of an alpha particle, which is a helium nucleus (two protons and two neutrons). This process effectively reduces both the atomic number and the mass number. Alpha decay is a step toward achieving increased nuclear stability.
Research and Industrial Uses of Heavy Isotopes
While Californium-257 is primarily a research isotope, the practical applications of the element are largely centered on Californium-252 (\(^{252}\text{Cf}\)), which has 154 neutrons. Californium-252 is highly valued as an intense, portable neutron source due to its high rate of spontaneous fission. A tiny amount of this isotope can produce a vast stream of neutrons, making it a reliable tool for various industrial and scientific fields.
Californium-252 is used in several applications, including:
- Well logging in the oil industry, where emitted neutrons help determine the porosity and composition of geological formations.
- Prompt-Gamma Neutron Activation Analysis (PGNAA) for scanning materials like coal and cement to determine their elemental composition.
- Neutron radiography, a non-destructive testing method used to inspect components such as aircraft parts and nuclear fuel rods for internal defects.
- Cancer therapy, where neutron emissions are used to target and destroy malignant cells.
- Synthesis of even heavier, short-lived elements, using Californium-249 as a target material in particle accelerators.