There are also Tritium tubes although as you say, they're not too bright.
There is also betavoltatic batteries, basically beta-particle powered solar panels. Depending on the type of isotope, those have a half-life of 50, 100 or 1000 years. They're solid state, very stable and provide a decent voltage for a looooong time. I've seen versions that use aluminium/phosphor bronze terminals because copper corroded too much over the time span!
I have also seen a design for TIMM's that uses a thermal radioisotope to heat the cathode. Basically, a tiny hot spot of Plutonium under the cathode, heating it up locally. I have no idea if it was ever built, but it was intended for very long duration deep space probes back in the 70's.
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There are also Tritium tubes although as you say, they're not too bright.
There is also betavoltatic batteries, basically beta-particle powered solar panels. Depending on the type of isotope, those have a half-life of 50, 100 or 1000 years. They're solid state, very stable and provide a decent voltage for a looooong time. I've seen versions that use aluminium/phosphor bronze terminals because copper corroded too much over the time span!
I have also seen a design for TIMM's that uses a thermal radioisotope to heat the cathode. Basically, a tiny hot spot of Plutonium under the cathode, heating it up locally. I have no idea if it was ever built, but it was intended for very long duration deep space probes back in the 70's.