25.1. The Nucleus http://www.ck12.org
through a process calledtransmutation. Uranium, for example, can turn into lead through a sequence of multiple
transmutations.
The transmutation process occurs when a particle is captured by the nucleus and through different nuclear decay or
disintegration processes. Nuclei which spontaneously decay are unstable and are called radioactive.
The original nucleus of the radioactive atom is called theparentand the nuclei formed by the decay are called the
daughters.
Radioactivity was first discovered in 1896 by the French physicist Henri Becquerel (1852-1908). Becquerel was
conducting research on phosphorescent substances (substances that remained luminous after exposure to light) when
he discovered that rocks containing uranium darkened a photographic plate in the absence of light. He surmised that
perhaps yet another new form of radiation (x-rays had been discovered the previous year) must be the reason. The
new radiation occurred spontaneously, unlike x-rays which were produced when high-energy electrons impacted
glass or metal.
Shortly after Becquerel’s discovery, Marie Curie (1867-1934) and her husband Pierre Curie (1859-1906) discovered
two new highly radioactive elements: polonium and radium.
In contrast to a stable nucleus, the nucleus of a radioactive atom has a greater mass than the sum of the masses
of its fission products. A greater mass in the nucleus means a smaller amount of binding energy available to hold
the nucleus together (seeFigure25.2). The extra mass in the nucleus is converted into the kinetic energy of the
daughters or into electromagnetic energy.
Three types of radioactive decay
There are three spontaneous decay processes: alpha(α)decay, beta(β)decay, and gamma(γ)decay. Two of the
processes,αdecay andβdecay, result in transmutations. Some transmutations require only one step to produce
a stable atom. Other transmutations involve multiple decay processes of different kinds, until a stable nucleus is
eventually formed.
We begin our discussion withαdecay.
Alpha decay
Alpha decay, as the name implies, occurs when anαparticle (a helium nucleus^42 He) is ejected from the nucleus of
a parent atom.
For example, polonium changes into lead by alpha decay:^21084 Po→^20682 Pb+^42 H
Illustrative Example 26.1.1
a. What is the energy of theαparticle produced in the decay process:^21084 Po→^20682 Pb+^42 He?
We will ignore the kinetic energy of the recoil of the daughter (lead) atom since it is very small compared to the
kinetic energy of the alpha particle. (Conservation of momentum shows that theαparticle’s velocity is more than
51 times greater than the lead nuclei.) Useful information:
mPo= 209. 982848 u
mPb= 205. 974440 u
mα=He= 4. 002602 uSolution:
We use a strategy similar to the one used in computing the binding energy.