20 CFR §725.231

1. (a)For purposes of this subpart, the term “domicile” means the place of an individual's true, fixed, and permanent home.
2. (b)The domicile of a deceased miner or surviving spouse is determined as of the time of death.
3. (c)If an individual was not domiciled in any State at the pertinent time, the law of the District of Columbia is applied.