A smoke detector contains a small sealed source of the isotope americium-241, represented in nuclide notation as ^24195Am. This isotope decays by emitting an alpha particle to form a new nuclide, G. The nucleus of G is produced in an excited (higher-energy) state, and almost imm…

Answer: ((a)) ^24195Am → ^23793G + ^42He ((b)) G still has nucleon number 237 and proton number 93 after emitting the gamma ray, because a gamma ray has no mass and no charge, so its emission changes only the energy of the nucleus, not its nucleon number or proton number. ((c)) Alpha particles are strongly ionising over a very short range, so within the small air gap they reliably ionise the air between the closely spaced electrodes to produce a steady, measurable current, while their short range means they cannot escape the sealed unit and pose little…. Part (a): Balancing the alpha-decay equation An alpha particle is a helium nucleus, ^42He, carrying away 4 from the nucleon number and 2 from the proton number. Starting nucleon and proton numbers of americium-241: A = 241, Z = 95 Subtract…

IGCSE Physics 0625: Alpha and Gamma Decay of Americium-241 in a Smoke Detector

A smoke detector contains a small sealed source of the isotope americium-241, represented in nuclide notation as ${}^{241}_{95}\text{Am}$ . This isotope decays by emitting an alpha particle to form a new nuclide, G. The nucleus of G is produced in an excited (higher-energy) state, and almost immediately loses its extra energy by emitting a gamma ray to reach its stable ground state.

Inside the detector, alpha particles travel across a small air gap between two electrodes, ionising the air and allowing a tiny, continuous electric current to flow. If smoke enters the gap, some alpha particles are absorbed by the smoke particles, the current falls, and the alarm is triggered.

(a) Write a balanced nuclide equation for the alpha decay of ${}^{241}_{95}\text{Am}$ to G, including the nucleon number and proton number of G and the symbol for the alpha particle emitted. [2]

(b) State the nucleon number and proton number of G after it has also emitted the gamma ray, and explain your reasoning. [2]

(c) Suggest why alpha particles, rather than beta particles or gamma rays, are suitable for use inside this small, sealed detector unit. [2]

Part (a): Balancing the alpha-decay equation

An alpha particle is a helium nucleus, ${}^{4}_{2}\text{He}$ , carrying away 4 from the nucleon number and 2 from the proton number.

Starting nucleon and proton numbers of americium-241: $A = 241, \quad Z = 95$

Subtract the alpha particle’s numbers to find G: $A_G = 241 - 4 = 237 \qquad Z_G = 95 - 2 = 93$

So the balanced equation is: ${}^{241}_{95}\text{Am} \rightarrow {}^{237}_{93}\text{G} + {}^{4}_{2}\text{He}$

Check: nucleon numbers: $237 + 4 = 241$ ✓. Proton numbers: $93 + 2 = 95$ ✓.

Part (b): The effect of the gamma emission

A gamma ray is electromagnetic radiation with no mass and no charge. It has a nucleon number of $0$ and a proton number of $0$ . Emitting a gamma ray therefore removes energy from the nucleus but does not change how many protons or neutrons it contains.

So after emitting the gamma ray, G still has: $A = 237 \qquad Z = 93$

The nucleon number and proton number of G are unchanged by the gamma emission, only the alpha decay in part (a) changed them.

Part (c): Why alpha particles suit this sealed detector

Inside the smoke detector, the alpha particles only need to cross a very small air gap between two closely spaced electrodes:

Alpha particles are the most strongly ionising of the three types of radiation, so even over the short distance of the air gap, they reliably ionise enough air molecules to sustain a small, steady, measurable current.
Alpha particles also have the shortest range in air (a few centimetres at most), so they are almost entirely absorbed within the sealed unit itself, they cannot travel far enough to escape the casing and reach a person outside, keeping the device safe to have in a room.
Beta or gamma radiation would ionise the air in such a short gap far less effectively (they are less ionising), giving a weaker or less reliable current, and gamma radiation in particular would pass straight through the detector’s casing and escape, becoming an unnecessary radiation hazard to anyone nearby.

Final answers

(a) ${}^{241}_{95}\text{Am} \rightarrow {}^{237}_{93}\text{G} + {}^{4}_{2}\text{He}$
(b) After the gamma emission, G still has nucleon number 237 and proton number 93. Gamma radiation has no mass or charge.
(c) Alpha particles are strongly ionising over a very short range, so they work well and stay contained within the small, sealed detector unit.