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Radioactive Decay

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Nuclear Decay Processes

You did what to my hamburger?

Food irradiation is a sensitive subject for many people. The practice involves exposing the food to ionizing radiation in order to kill harmful bacteria (such as salmonella) that cause sickness. The food is essentially unchanged and does not lose any nutritive value. Parasites and insect pests are easily destroyed by this process, while bacteria take longer to kill. Viruses are not affected by the radiation treatment. But don’t worry – the food is not radioactive and you will not glow in the dark if you eat it.

Nuclear Decay Processes

Radioactive decay involves the emission of a particle and/or energy as one atom changes into another. In most instances, the atom changes its identity to become a new element. There are four different types of emissions that occur.

Alpha Emission

Alpha (α) decay involves the release of helium ions from the nucleus of an atom. This ion consists of two protons and two neutrons and has a 2+ charge. Release of an α-particle produces a new atom that has an atomic number two less than the original atom and an atomic weight that is four less. A typical alpha decay reaction is the conversion of uranium-238 to thorium:

{^{238}_{\phantom{0}92}\text{U}} \rightarrow {^{234}_{\phantom{0}90}\text{Th}} + {^4_2 \alpha^+}

We see a decrease of two in the atomic number (uranium to thorium) and a decrease of four in the atomic weight (238 to 234). Usually the emission is not written with atomic number and weight indicated since it is a common particle whose properties should be memorized. Quite often the alpha emission is accompanied by gamma (γ) radiation, a form of energy release. Many of the largest elements in the periodic table are alpha-emitters.

Emission of an alpha particle from the nucleus.

Beta Emission

Beta (β) decay is a more complicated process. Unlike the α-emission, which simply expels a particle, the β-emission involves the transformation of a neutron in the nucleus to a proton and an electron. The electron is then ejected from the nucleus. In the process, the atomic number increases by one while the atomic weight stays the same. As is the case with α-emissions, β-emissions are often accompanied by γ-radiation.

Beta emission.

A typical beta decay process involves carbon-14, often used in radioactive dating techniques. The reaction forms nitrogen-14 and an electron:

{^{14}_{\phantom{0}6}\text{C}} \rightarrow {^{14}_{\phantom{0}7}\text{N}} + {^{\phantom{-}0}_{-1}e}

Again, the beta emission is usually simply indicated by the Greek letter β; memorization of the process is necessary in order to follow nuclear calculations in which the Greek letter β without further notation.

Gamma Emission

Gamma (γ) radiation is simply energy. It may be released by itself or more commonly in association with other radiation events. There is no change of atomic number or atomic weight in a simple γ-emission. Often, an isotope may produce γ-radiation as a result of a transition in a metastable isotope. This type of isotope may just “settle,” with a shifting of particles in the nucleus. The composition of the atom is not altered, but the nucleus could be considered more “comfortable” after the shift. This shift increases the stability of the isotope from the energetically unstable (or “metastable”) isotope to a more stable form of the nucleus.

Positron Emission

A positron is a positive electron (a form of antimatter). This rare type of emission occurs when a proton is converted to a neutron and a positron in the nucleus, with ejection of the positron. The atomic number will decrease by one while the atomic weight does not change. A positron is often designated by β + .

Carbon-11 emits a positron to become boron-11:

{^{11}_{\phantom{0}6}\text{C}} \rightarrow {^{11}_{\phantom{0}5}\text{B}} + {^{\phantom{+}0}_{+1} \beta}

Summary

  • Radioactive decay processes are described.

Practice

Read the material at the link below and answer the questions at Practice Problem 3.

Review

  1. What is gamma emission?
  2. What isotope is formed when U-238 emits an alpha particle?
  3. What isotope produces boron-11 when it emits a positron?

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