Is the underlined ‘a’ correctly used in the sentence? Can I use ‘not long after’ and ‘soon after’ interchangeably in the sentence?

Is the underlined ‘a’ correctly used in the sentence? Can I use ‘not long after’ and ‘soon after’ interchangeably in the sentence?

At the open forum, Mrs. Sullivan, the guest speaker, answered the audience’s questions. (Not long after, soon after), the master of master of ceremonies awarded a certificate of appreciation and a token to her.