Particle Detectors: (4.3) Cat's-Whisker Detector: (2) Description: (2. 1) Crystal: A natural mineral crystal forms the semiconductor side of the junction. The most common crystal used was galena (PbS, lead sulfide), a naturally occurring ore of lead, although many other minerals were also used including silicon, iron pyrite, molybdenum and carborundum. Galena is a semiconductor with a small bandgap of about 0. 4 eV and is used without treatment directly as it is mined. However, not all galena crystals would function in a detector; galena with good detecting properties was rare and had no reliable visual characteristics distinguishing it from galena samples with poor detecting properties. A rough pebble of detecting mineral about the size of a pea was mounted in a metal cup, which formed one side of the circuit. The electrical contact between the cup and the crystal had to be good, because this contact must not act as a second rectifying junction, which would prevent the device from functioning. To make good contact with the crystal, it was either clamped with setscrews or mounted in solder. Because the relatively high melting temperature of tin-lead solder can damage many crystals, a low-melting-point (well under 200^oF) alloy such as Wood's metal was used. One surface was left exposed to allow contact with the cat's-whisker wire