The Beam Counters info page

Upstream

S1

halo veto (hole ~3" x 6")

S2

beam defination

S3

beam (10"O.D. x 3/8" scintilator)

S4

halo veto (12" x 12" x 3/8" scintilator w/ 4" I.D. hole in the center)

Downstream

EV

1994 Elastic Veto

BV

1994 Beam Veto