Alpha particles

Alpha particles

Helium nuclei, which are abundant throughout the universe both as radioactive-decay products and as key participants in stellar fusion reactions. Alpha particles can also be generated in the laboratory, either by ionizing helium or from nuclear reactions. They expend their energy rapidly as they pass through matter, primarily by taking part in ionization processes, and consequently have short penetration ranges. Numerous technological applications of alpha particles can be found in fields as diverse as medicine, space exploration, and geology. Alpha particles are also major factors in the health concerns associated with nuclear waste and other radiation hazards.

The helium nucleus, or alpha particle (α), with mass 4.00150 atomic mass units (u) and charge +2, is a strongly bound cluster of two protons (p) and two neutrons (n). Its stability is evident from mass-energy conservation in the hypothetical fusion reaction 2p + 2n → α. The product mass (= 4.00150 u) is less than the reactant mass (= 2 × 1.00728 u + 2 × 1.00866 u) by 0.03038 u. By using Einstein's relation E = mc² (where c is the speed of light), this decrease in mass m (the alpha-particle binding energy) is equivalent to 28.3 MeV of energy E. The enormous magnitude of this energy is reflected in the fact that the fusion transformation of hydrogen into helium is the main process responsible for the Sun's energy. See Conservation of energy, Energy, Nuclear binding energy

Alpha radioactivity

Coulombic repulsion between the protons within a nucleus leads to increasingly larger ratios of neutron number N to proton number Z for stable nuclei, as the mass numbers increase. Neutron-deficient nuclei can improve their N/Z ratios by means of alpha decay. The decay occurs because the parent nucleus has a total mass greater than the sum of the masses of the daughter nucleus and the alpha particle. The energy converted from mass energy to kinetic energy, called the Q value, is shared between the daughter nucleus and the alpha particle in accordance with the conservation of momentum. Thus, each radioactive alpha-emitting nuclide emits the alpha with a characteristic kinetic energy, which is one fingerprint in identification of the emitter. See Nuclear reaction, Radioactivity

There are three major natural series, or chains, through which isotopes of heavy elements decay by successions of alpha decays. Within these series, and with all reaction-produced alpha emitters as well, each isotope decays with a characteristic half-life and emits alpha particles of particular energies and intensities. The presence of these radioactive nuclides in nature depends upon either a continuous production mechanism, for example the interaction of cosmic rays with the atmosphere, or extremely long half-lives of heavy radioactive nuclides produced in past cataclysmic astrophysical events, which accounts for uranium and thorium ores in the Earth. The relative abundances of uranium-238, uranium-235, and their stable final decay products in ores of heavy elements can be used to calculate the age of the ore, and presumably the age of the Earth.

In addition to the study of alpha-particle emitters that appear in nature, alpha decay has provided a useful tool to study artificial nuclei, which do not exist in nature due to their short half-lives. Alpha decay is a very important decay mode for nuclei far from stability with a ratio of protons to neutrons that is too large to be stable, especially for nuclei with atomic mass greater than 150 u. Because of the ease of detecting and interpreting decay alpha particles, their observation has aided tremendously in studying these nuclei far from stability, extending the study of nuclei to the very edge of nuclear existence. Nuclear structure information for more than 400 nuclides has been obtained in this way. In addition, fine structure peaks appear in the alpha-particle spectra for many of these nuclides; each such fine structure peak gives similar information about an excited state in the daughter nucleus.

Interactions with matter

By virtue of their kinetic energy, double positive charge, and large mass, alpha particles follow fairly straight paths in matter, interacting strongly with atomic electrons as they slow down and stop. These electrons may be excited to higher energy states in their host atoms, or they may be ejected, forming ion pairs in which the initial host atom becomes positively charged and the electron leaves. The more energetic ejected electrons, known as delta electrons, cause considerable secondary ionization, which accounts for 60–80% of the total ionization. A cascade of processes occurs along the alpha particle's track, leading to tens of thousands of disruptive events per alpha particle.

The amount of energy expended by an alpha particle to form a single ion pair in passing through a medium is nearly independent of the alpha particle's energy, but it depends strongly on the absorbing medium. While it takes about 35 eV in air and 43 eV in helium to form an ion pair, an energy of only 2.9 eV is required in germanium and 3.6 eV in silicon. The energies expended in gases are roughly correlated to their ionization potentials. For germanium, silicon, and other semiconductors, the lower ion pair energy is, effectively, the amount required to raise an electron to the conduction band. See Ionization potential, Semiconductor

The distance (or range) that an alpha particle travels before it stops depends both on the energy of the particle and on the absorbing medium. The passage of alpha particles through silicon is a particularly important example. The semiconductor industry now produces chips so small that alpha particles from contaminants in the packaging materials can disrupt the memory-array areas of the chips, a serious problem which has been researched in considerable detail.

In biological systems, the ionization and excitation produced by alpha particles can damage or kill cells. By rupturing chemical bonds and forming highly reactive free radicals, alpha particles can be far more destructive than other forms of radiation which interact less strongly with matter. See Charged particle beams

Applications

In the promising medical field of charged-particle radiotherapy, alpha particles are useful in the treatment of inaccessible tumors and vascular disorders. The ionizing power of alpha particles is concentrated near the ends of their paths. Thus they can deliver destructive energy to a tumor while doing little damage to nearby healthy tissue. With proper acceleration, positioning, and dosage, the energy can be delivered so precisely that alpha-particle radiotherapy is uniquely suited for treating highly localized tumors near sensitive normal tissue (for example, the spinal cord).

The element-specific energies of backscattered (Rutherford-scattered) alpha particles are used in remote probes to analyze the mineral composition of geological formations. In particular, alpha particles scattered by light elements transfer more energy than those scattered by heavy elements. In another alpha-particle device, the energy from ²³⁸Pu alpha decay is reliably harnessed in batteries based on the Brayton cycle, and used to power scientific equipment left on the Moon. Large power systems of this type are contemplated for use in space stations.