Hi,
if you will read positiv20 data sheet, over that lines is linked, you will find: "its most sensitiv with UV-A", but UV-C`wavelength is not so far, that you can not need it, but the sensitivity is some lower = you must find out what is your correct exposition time with your equipment!
In all day business you will make it some times repeatedly so and so, then the commercial photo plates have not 100% same or stabile sensitivity, than after some standing on your and yours delivery firms stock is the sensitivity more less too>>> you must find another (longer) exposing time too...
What kind of wavelength is better- you can tell it only if you see the sensitivity characteristic of Positiv 20, but as I know this is not public!:-(
You must also try some experimenting...
K.