On the new thermal instruments, the experimental beam shutter is contained within the biological shield of the source, and simply has two positions, open and closed.  Outside the in-pile shutter, all the thermal beam holes are (nominally) identical, and the beam shutter for BT-7 has already been installed.  The beam shutter opening is 6.4 cm wide at the exit, opening to 9 cm on the end toward the source (this may be different on the other instruments).  The vertical opening is 16 cm.  The shutter opens or closes in eleven seconds.

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