From 1978 to 1980, Robert Janssens was a research associate at the Kernphysisch Versneller Instituut (KVI) at the University of Groningen in the Netherlands. He was a physics professor at the E.R.C. (Military Academy) in Belgium from 1980 to 1981, in fulfillment of his military obligations.
He has worked in the Physics Division of Argonne as an assistant scientist (1981), scientist (1984), and senior scientist (1994). He has been a visiting scientist at the Niels Bohr Institute and at the University of Leuven. He was program head of the Low-Energy Nuclear Physics group from 1994 to 2000.
Janssens has been scientific director of the ATLAS facility since 2000. From this date until 2006, he was also the associate director of the Physics Division, becoming its interim director in 2007. He was appointed division director in 2008. In 2001, he became an adjunct professor at Michigan State University and in 2004 an adjunct professor at the University of Notre Dame.
Janssens studies nuclear structure at the limits of stability, in high-Z nuclei and in neutron-rich nuclei. Some of his recent work relates to changes in shell structure in neutron-rich nuclei just above 48Ca, performing work with ATLAS, the National Superconducting Cyclotron Laboratory at Michigan State University, and the ISAC exotic beam facility at TRIUMF in Vancouver. He has also carried out experiments on long-lived actinide nuclei to determine the degrees of freedom responsible for various excitations as a function of spin. Additionally, he is participating in measurements of the impact of structure and reaction dynamics on nuclear astrophysics processes. He is a co-author of more than 400 publications in refereed scientific journals and he has supervised nine Ph.D. theses.