This emergency smoke detector is installed in a warehouse containing boxes of ethanol solution. Fig. 1 shows its radioactive source, americium-241, written ...
Question 1 Report
This emergency smoke detector is installed in a warehouse containing boxes of ethanol solution. Fig. 1 shows its radioactive source, americium-241, written in nuclear notation. The source emits alpha particles. Alpha radiation ionises air inside the detector, allowing a very small electric current to flow. Smoke particles reduce the current and trigger the alarm. The source is sealed inside a metal holder, so only the nuclear changes need to be considered.
(a) Name the two types of particle found in the nucleus of americium-241. [2]
(b) Use Fig. 1 to calculate the number of neutrons in an atom of americium-241. [2]
(c) Describe the composition of an alpha particle. [2]
(d) Explain why the alpha source is safe to use when it remains inside its sealed holder. [3]
Answer Details
(a) [2] The nucleus contains protons and neutrons.
(b) [2] The mass number is 241 and the atomic number, which is the number of protons, is 95.
\[\text{number of neutrons}=241-95=\mathbf{146}\]
(c) [2] An alpha particle contains two protons and two neutrons.
(d) [3] Alpha particles have low penetration. The sealed metal holder stops or absorbs them, and the radioactive material cannot enter the body while the holder remains sealed. This is why the source can be used safely in this arrangement.
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