Does anyone know where I could get access to the Camp Douglas, Ill (Union POW camp) Roll of Prisoners of War?

My gggrandfather, a private in the 46th TN Inf named John J. Simmons, was captured on Island No. 10 and sent to Camp Douglas. I have information that he took the oath to the Union and was released. I want to find out if this information is correct from official documents.

I would greatly appreciate any help.

Thanks,

Clay