From: utzoo!decvax!harpo!uwvax!doug
Newsgroups: net.misc
Title: Re: Spontaneous particles from vacuum?!??
Article-I.D.: uwvax.538
Posted: Sat Aug  7 21:33:52 1982
Received: Sun Aug  8 03:11:26 1982
References: ihnss.128

I should mention that the spontaneous decay of vacuums into non-virtual
particles would only be in the vicinity of an extremely highly charged
nucleus.   In this vicinity, the SA article maintains that it would be
energetically desirable for such decay to occur. 

The nuclei required, if I remember correctly, have proton numbers > 200.
Experiments are being conducted to create (albeit for a short time) such
nuclei.