Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
unconcerned [ʌnkənˈsəːnd] adj (person) беззабо́тный*беззабо́тен
unconcerned about равноду́шный*равноду́шен к +dat
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.