The Markus Chamber (PTW 23343) Build-up Disk is made of acrylic and available in 2 styles.
Item 664-320 is a Cobalt 60 Disk that is placed over the chamber’s acrylic screw on cap. The disk has a 2.99 cm diameter and is 3.2 mm thick.

Item 664-401 is a Cobalt 60 disk with a shoulder placed over the chamber’s mylar window. It has a 3.2 cm diameter and is 4 mm thick.