planch·et 
(plănchĭt)
(plănchĭt)n.
1. A flat disk of metal ready for stamping as a coin; a coin blank.
2. A small shallow metal container in which a radioactive substance is deposited for measurement of its activity.
[ Diminutive of planch, flat plate, slab, from Middle English plaunche, plank, from Old French planche, from Late Latin planca, from feminine of Latin plancus, flat; see plāk-1 in the Appendix of Indo-European roots.]