The electrons eventually radiate their energy into the crystal in the form of non-thermal phonons. Most of the 6 keV appears rapidly as non-thermal phonons propagating through the crystal, with energy above the detection threshold of the junctions. Some properties of a superconducting tunnel junction are illustrated in figure 2. Well below TC, most electrons are bound in Cooper pairs. This is the current which is detected.
Cooper pairs can also tunnel through the barrier.
The phonons are absorbed in the aluminum films of the tunnel junctions on one surface of the sapphire crystal, perhaps after several reflections from the crystal faces. The tunnel junctions are current biased, as shown in figure 3. When energy is absorbed, excess current flows. An integrating amplifier produces a signal proportional to the charge. Our tunnel junctions consist of an aluminum electrode, a layer of native oxide, and an aluminum counter-electrode as shown in figure 4.