Radioactivity

It is found that nuclei with mass numbers greater than about 100 spontaneously decay into other types of nuclei. Such nuclei are said to be radioactive, and there are three main types of such decays.

 
• Alpha ($\alpha$) decay occurs by emission of an alpha particle (⁴²He nucleus). An example of this is the decay of uranium:

  238 *1.5mm92U	$$\rightarrow$$	234 *1.5mm90Th	+	42He.
  tex2html_deferred(parent)		tex2html_deferred(daughter)		tex2html_deferred($&alpha#alpha;$-particle)

  Notice that in this $\alpha$-decay process the ^{238 *1.5mm92}U nucleus (called the parent nucleus) loses two protons and two neutrons (this can be seen by the atomic number (Z) and mass number (A) of the daughter nucleus, ^{234 *1.5mm90}Th). The total number of protons and total number of nucleons is, however, the same on both sides of the reaction.

  Alpha particles are, in general, the least penetrating of the particles produced in a radioactive decay due to their relatively large size and their charge (+2e). Alpha particles are usually stopped or absorbed by a few centimeters of air or a sheet of paper.  

• Beta ($\beta$) decay occurs by emission of a beta particle (either an electron or a positron). A positron is identical to an electron, except it has charge of + e. An example of $\beta$-decay is the decay of carbon to nitrogen:

  14 *1.5mm6C	$$\rightarrow$$	14 *1.5mm7N	+	*2.25mm0 - 1e	+	$$\nu$$
  tex2html_deferred(parent)		tex2html_deferred(daughter)		tex2html_deferred(electron)		tex2html_deferred(neutrino)

  The notation _{$\mp$ 1 *2.25mm0}e denotes an electron (-) or positron (+). $\nu$ is a neutrino, which has little or no rest mass and is electrically neutral. The neutrino was proposed in 1930 by Pauli to account for the apparent loss of energy and momentum during a $\beta$-decay event. Neutrinos interact very weakly with matter, making them very hard to detect. A large project, SNO, is underway in Sudbury, Ontario, looking at detecting neutrinos using detectors placed at the bottom of old mine shafts.

  Beta particles are more penetrating than alpha particles and can usually travel a few meters in air, or a few millimeters through aluminum, before being absorbed or stopped.  

• Gamma ($\gamma$) decay occurs by emission of gamma particles (photons, or quanta of light). Gamma decay usually results when a nucleus is left in an excited state, with excess energy, after an alpha or beta decay. An example of this is the decay of cesium to barium in an excited state (Ba^*) by $\beta$-decay, and then the excited barium (Ba^*) decays by $\gamma$-decay:
  $$\mbox{$\beta$-decay}$$$$\left\{\vphantom{ \mbox{\hspace*{5mm}} \begin{array}{ccccccc} {}^... ...{*} & + & {}^{\hspace*{2.25mm} 0}_{-1}{\rm e} & + & \nu \\ \end{array}}\right.$$ $$\begin{array}{ccccccc} {}^{137}_{\hspace*{1.5mm} 55}{\rm Cs} & \r... ...\rm Ba}^{*} & + & {}^{\hspace*{2.25mm} 0}_{-1}{\rm e} & + & \nu \\ \end{array}$$ $$\left.\vphantom{ \mbox{\hspace*{5mm}} \begin{array}{ccccccc} {}^{... ...{*} & + & {}^{\hspace*{2.25mm} 0}_{-1}{\rm e} & + & \nu \\ \end{array}}\right.$$
  then
  $$\mbox{$\gamma$-decay}$$$$\left\{\vphantom{ \mbox{\hspace*{5mm}} \begin{array}{ccccccc} {}^... ... & \mbox{\small (barium)} & & \mbox{\small (gamma ray)} \\ \end{array}}\right.$$ $$\begin{array}{ccccccc} {}^{137}_{\hspace*{1.5mm} 56}{\rm Ba}^{*} ... ...ium*)} & & \mbox{\small (barium)} & & \mbox{\small (gamma ray)} \\ \end{array}$$ $$\left.\vphantom{ \mbox{\hspace*{5mm}} \begin{array}{ccccccc} {}^{... ... & \mbox{\small (barium)} & & \mbox{\small (gamma ray)} \\ \end{array}}\right.$$
  Notice that the gamma decay does not change Z or A.

  Gamma particles are the most penetrating of the decay products, being capable of penetrating a few centimeters of lead if they have enough energy.

As with the previous discussion of binding energy, it is found that the mass of the decaying particle on the left-hand-side of these equations is greater than the sum of the masses of the decay products on the right-hand-sides. This ``loss'' in mass is interpreted, through Equation (13.2), as a conversion into other forms of energy, mainly kinetic energy of the products. This will be illustrated in some examples later. Although certainly dangerous to life, low-level radioactivity has some important applications in our everyday life, including carbon dating and smoke detectors.