The neptunium series is a radioactive series beginning with neptunium-237. Its members are produced artificially by nuclear reactions and do not occur naturally because the half-life of the longest-lived isotope in the series is short compared to the age of the earth. This radioactive decay chain consists of unstable heavy atomic nuclei that decay through a sequence of alpha and beta decays until a stable nucleus is achieved. In the case of the neptunium series, the stable nucleus is bismuth-209 (with a half-life of 1.9E19 years) and thallium-205.
Since alpha decay represents the disintegration of a parent nucleus to a daughter through the emission of the nucleus of a helium atom (which contains four nucleons), there are only four decay series. Therefore, the mass number of the members within each series may be expressed as four times an appropriate integer (n) plus the constant for that series. As a result, the neptunium series is known as the 4n+1 series.
You can meet radionuclides from this series in some kinds of smoke detectors. Ionization smoke detectors usually use a radioisotope, typically americium-241, to ionize the air and detect smoke. In this case, americium-241 decays to neptunium-237 and is, in fact, a member of the neptunium series.
Types of Decay in Neptunium Series
Within each radioactive series, there are two main modes of radioactive decay:
- Alpha decay. Alpha decay represents the disintegration of a parent nucleus to a daughter through the emission of the nucleus of a helium atom. Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus. Because of its very large mass (more than 7000 times the mass of the beta particle) and its charge, it heavy ionizes material and has a very short range.
- Beta-decay. Beta-decay or β decay represents the disintegration of a parent nucleus to a daughter through the emission of the beta particle. Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles have a greater range of penetration than alpha particles but still much less than gamma rays. The beta particles emitted are a form of ionizing radiation known as beta rays, and the production of beta particles is termed beta decay.