Question

One type of commercial smoke detector contains a minute amount of radioactive americium- \(241\left({ }^{241} \mathrm{Am}\right)\), which decays by \(\alpha\) -particle production. The \(\alpha\) particles ionize molecules in the air, allowing it to conduct an electric current. When smoke particles enter, the conductivity of the air is changed and the alarm buzzes. a. Write the equation for the decay of \({ }_{95}^{241} \mathrm{Am}\) by \(\alpha\) -particle production. b. The complete decay of \({ }^{241}\) Am involves successively \(\alpha, \alpha, \beta\), \(\alpha, \alpha, \beta, \alpha, \alpha, \alpha, \beta, \alpha\), and \(\beta\) production. What is the final stable nucleus produced in this decay series? c. Identify the 11 intermediate nuclides.

Short answer

a. The equation for the decay of Am-241 by α-particle production is: \[^{241}_{95}\text{Am} \rightarrow ^{237}_{93}\text{Np} + ^{4}_{2}\text{He}\] b. The final stable nucleus produced in this decay series is Bismuth-209 (Bi-209). c. The 11 intermediate nuclides in the decay series are: Np-237, Pa-233, U-233, Th-229, Ra-225, Ac-225, Fr-221, At-217, Bi-213, Po-213, and Pb-209.

Step-by-step solution

a. Writing the equation for α-decay of Am-241

For an α-decay, the parent nucleus loses an alpha particle (a helium nucleus) with mass number 4 and atomic number 2. So, the mass number decreases by 4 and the atomic number decreases by 2. Let's write the decay equation for Am-241: \[^{241}_{95}\text{Am} \rightarrow ^{237}_{93}\text{X} + ^{4}_{2}\text{He}\] Here, X represents the element with atomic number 93, which is Neptunium (Np). So, the decay equation becomes: \[^{241}_{95}\text{Am} \rightarrow ^{237}_{93}\text{Np} + ^{4}_{2}\text{He}\]

b. Finding the final stable nucleus in the decay series

The decay series is given as: α, α, β, α, α, β, α, α, α, β, α, β. We will find the final stable nucleus by successively applying each decay process to Am-241. For each α-decay, the mass number decreases by 4 and the atomic number decreases by 2. For each β-decay, the mass number remains the same, but the atomic number increases by 1. Starting with Am-241, let's apply the given decay series: \[^{241}_{95}\text{Am} \xrightarrow[\alpha]{} ^{237}_{93}\text{Np} \xrightarrow[\alpha]{} ^{233}_{91}\text{Pa} \xrightarrow[\beta]{} ^{233}_{92}\text{U} \xrightarrow[\alpha]{} ^{229}_{90}\text{Th}\] \[^{229}_{90}\text{Th} \xrightarrow[\alpha]{} ^{225}_{88}\text{Ra} \xrightarrow[\beta]{} ^{225}_{89}\text{Ac} \xrightarrow[\alpha]{} ^{221}_{87}\text{Fr}\] \[^{221}_{87}\text{Fr} \xrightarrow[\alpha]{} ^{217}_{85}\text{At} \xrightarrow[\alpha]{} ^{213}_{83}\text{Bi} \xrightarrow[\beta]{} ^{213}_{84}\text{Po}\] \[^{213}_{84}\text{Po} \xrightarrow[\alpha]{} ^{209}_{82}\text{Pb} \xrightarrow[\beta]{} ^{209}_{83}\text{Bi}\] The final stable nucleus in the decay series is Bismuth-209 (Bi-209).

c. Identifying the 11 intermediate nuclides

In the above step, we already identified the intermediate nuclides while applying each decay process. The 11 intermediate nuclides in the decay series are: 1. Neptunium-237 (Np-237) 2. Protactinium-233 (Pa-233) 3. Uranium-233 (U-233) 4. Thorium-229 (Th-229) 5. Radium-225 (Ra-225) 6. Actinium-225 (Ac-225) 7. Francium-221 (Fr-221) 8. Astatine-217 (At-217) 9. Bismuth-213 (Bi-213) 10. Polonium-213 (Po-213) 11. Lead-209 (Pb-209)

Key concepts

Alpha Decay

Alpha decay is a common form of radioactive decay where an unstable atomic nucleus releases an alpha particle. An alpha particle consists of two protons and two neutrons, essentially a helium nucleus. When an atom undergoes alpha decay, it loses an alpha particle, causing its mass number to decrease by 4 and its atomic number to decrease by 2. This process transforms the original element into a different element.

For example, in the decay of Americium-241 (Am-241):

• The atomic number changes from 95 (Americium) to 93 (Neptunium).
• The mass number changes from 241 to 237.

This process releases an alpha particle, represented as \[^{4}_{2}\text{He}\]. The equation is:
\[^{241}_{95}\text{Am} \rightarrow ^{237}_{93}\text{Np} + ^{4}_{2}\text{He}\]

Alpha decay is crucial in reducing the radioactivity of heavy elements, turning them into more stable forms over time.

Beta Decay

Beta decay is another type of radioactive decay where a beta particle is emitted from an atomic nucleus. A beta particle can either be an electron or a positron. In beta minus decay, a neutron is transformed into a proton, emitting an electron and an antineutrino. This increases the atomic number by 1, but the mass number remains unchanged.

During the decay series of \(^{241}\text{Am}\), beta decay steps were essential to stabilize the nucleus:

• The atomic number increases by 1, such as from \(^{233}_{91}\text{Pa}\) to \(^{233}_{92}\text{U}\).

The equation looks like:
\[\text{n} \rightarrow \text{p} + \text{e}^{-} + \overline{u}\]

In contrast, beta plus decay occurs when a proton is converted into a neutron, though it's less common in this context. Understanding beta decay helps in comprehending isotopic changes and the stabilization of elements.

Radioisotopes

Radioisotopes are isotopes of an element with an unstable nucleus. These isotopes undergo radioactive decay, releasing particles or electromagnetic waves. The decay process transforms the isotope into a more stable form.

Americium-241, used in smoke detectors, is an example of a radioisotope. It undergoes alpha decay, releasing energy that is used to ionize the air in detectors. This ionization process helps detect smoke by altering the electrical conductivity of the air.

Radioisotopes have various applications beyond smoke detection, including medical imaging, cancer treatment, and carbon dating. Their ability to emit radiation makes them valuable in scientific research and industry, although handling requires strict safety measures due to the potential health risks.

Smoke Detectors

Smoke detectors are essential safety devices in homes and buildings, often utilizing radioisotopes like Americium-241. This radioisotope helps the detector sense smoke through a simple yet effective process.

Here's how it works:

• The decay of \(^{241}\text{Am}\) produces alpha particles.
• The alpha particles ionize the air between two electrodes, allowing a small electric current to flow.
• When smoke enters the detector, it disrupts the ionized air, reducing conductivity and thus altering the current.

This disruption triggers the alarm, alerting occupants of potential fire. Smoke detectors are vital for early fire detection and safety, demonstrating a practical application of nuclear physics in everyday life.