While you probably do not have a scanning tunneling microscope installed in your kitchen, chances are you do have another device that is equally dependent on the effect of quantum tunneling. At the heart of your average smoke detector is an ionization source used to detect the presence of smoke particles. The source is typically a small quantity of americium-241, a heavy radioactive isotope that decays through the emission of an alpha particle. (Alpha particles are composed of two protons plus two neutrons.) And alpha decay, it turns out, is yet another tunneling phenomenon.
The Russian physicist George Gamow discovered this fact in 1928, about 30 years after alpha particles were first observed. He reasoned that an alpha particle could be considered to move freely within a larger atomic nucleus (the “parent” nucleus). Immediately outside the nucleus, there would be a potential barrier due to the electromagnetic force of the transformed nucleus (the “daughter”) acting on the alpha particle.
We saw that the potential energy of one charged particle due to the presence of a second is inversely proportional to the distance between the two. (In that case, we were considering the attraction of an electron to a proton; in this case we are talking about the repulsion between the alpha particle and the daughter nucleus.) What this means is that the potential “barrier” felt by the alpha particle gets smaller with greater distance from the parent nucleus. This means that alpha particles with sufficient energy can tunnel through the barrier and escape. The result is alpha decay.
The alpha particles emitted from the americium-241 source collide with air molecules, sometimes leading to their ionization. This leads to a current produced by negatively charged electrons and positively charged, ionized molecules. The amount of current drops when smoke particles start to get in the way, and this telltale change is used as a detector of smoke.
Due to concerns with disposing of radioactive americium, smoke detectors based on optical detectors (light sensors) are increasingly common. These devices combine either an incandescent light bulb or an infrared light-emitting diode (LED, itself a quantum device) with a light. In the absence of smoke, the light source points slightly away from the sensor. However, when smoke particles are present, they can scatter some of the light into the sensor, which will trigger the alarm.
An alpha particle can escape from its parent nucleus by tunneling through the potential barrier formed by electromagnetic repulsion. Here, r is the distance from the center of the parent nucleus, U is the potential energy function, and Ψ is the alpha particle’s wave function.
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