You can't force anyone to do publicity. You wouldn't want to. And you don't need to.
Every day of their lives, journalists face people who don't want to talk. If the story is good, they find a way to get it anyway. Your job as a PR professional is to help the journalists get their story. If your main character is AWOL, you need to find another "human face" for the project, and maybe even use the absence of your star scientist to create a bit of intrigue.
I can tell you from experience that scientists do not usually make for great interview subjects. They are sources for the basic information you need, and, very often, little else. Journalists accept this because science is not a field people enter seeking stardom (well, at least not stardom outside of the scientific community). They do their talking at a laboratory bench, not at a microphone.
Still, it helps if you can find someone involved in the research who is gregarious and outgoing. Start there, and give the journalists a good launching point. But if this breakthrough is as important as you say it is, you should also be helping the reporters to focus on its impact. Who is this going to effect? What is it going to change? Find the people whose lives will be improved by this research and make them the main characters of the story. Do this work yourself and when it comes time to pitch an editor, you have a packaged story with all the elements for a feature, instead of a brief on page B17.
Forcing your mad scientist to a microphone can backfire. If he's dry or confrontational, you're in trouble. But you can certainly sit down with him yourself and get all of the relevant material you may need. As an employee of your company, he is obligated to at least do this. And he should also know that you may be contacting him again with any follow up questions. It's OK if he wants to stay out of the limelight, but he can't expect his breakthrough to do the same.
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