I've read that nickel-63 is used in surge protectors and voltage regulators.
What is it about nickel-63 that makes it particularly suitable for those applications?
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Nickel-63 is a beta emitter with a half-life of 101 years. That makes it reasonably active, yet long-enough lasting for the application you hint at. Now, what is that application? Gas discharge tubes.
A gas discharge tube (GDT) is used in a variety of applications to divert overvoltage to ground (such as a lightning strike), protecting what is behind the tube. At the simplest, one can consider it two parallel plates with gas in between. One plate is connected to ground, one is on an input line (voltage to a device, from an antenna, etc.). At a high enough voltage, the gas is supposed to break down, usually through an avalanche process, providing a low impedance path to ground and protecting what is downstream of the tube. The problem is, an avalanche process needs a few free electrons around to get it going in a timely fashion. For a high-voltage GDT (1000's of volts or higher), field ionization will likely provide enough electrons quickly enough to have it fire quickly enough to protect the circuit. For lower voltage rated tubes, it is helpful to have a low level of ionization pre-existing in the tube.
A beta emitter inside the tube is one way to consistently have a low concentration of free electrons ready to avalanche in case of enough voltage being present across the device. Nickel is easy to plate on the contacts.
Further, Nickel-63 is readily formed by thermal neutron irradiation of Nickel-62, with a cross-section of nearly 1000 barns (quite large for neutron absorption). So, it is also easy to make by exposing nickel to a reactor environment. (Note one would just expose naturally-occurring nickel, counting on transforming some of the naturally-present Nickel-62 to the 63 isotope).