Smart Collections for scanned images

I have imported a lot of scanned images into my library, mainly from 35mm negatives but also some 35mm slides and 110 negatives. I would like to create a smart collection for each type but haven't been able to figure out how. 

I have used the "Source Type" field in the "IPTC Extension Administrative" metadata section to identify each type, e.g. "Digitised from a negative on film" and so on. However, I can't find a specific entry for "Source Type" anywhere in the Smart Collection settings but I've tried "Any searchable text", "Any searchable Metadata", "Searchable IPTC" but none of them seem to work. That suggests that the "Source Type" field is not searchable for some reason.

Anybody got any ideas on where I'm going wrong?