How can we tell who closed a PO in the system?

Peoplesoft FSCM 9.2, PUM 46. PeopleTools 8.56.20.

We use the PO functionality in Purchasing. When a PO is closed, our users would like to know the userid who took this action (our users use the Buyer's workbench to perform this function). Currently, only the last updated user displays in the PO_HDR record, which is usually our batchuser for the budget check process. What is the best way to capture this information as to who closed the PO? thank you.