Boosting when matching specific field values

[gseoeltru solr]

I am using edismax when executing search against set of news articles. I
would like to also boost the scores of matched documents based on another
field in the documents which I will call "source" which can be set to 3
possible strings.   So if the "source" field has a value "a", then I want
to multiply the score by 1. If the "source" field has a value "b", then I
want to multiple the score by 2 ... and so on. What is the way to go about
doing this ?
Any help here mucho appreciated !