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Aperture and Shutter
An important safety feature of the shutter mechanism requires the presence of both air pressure and
electrical power to open. Thus if either air pressure or electrical power is lost while open, the shutter
will close. The shutter opens by a chain of events: electrical, pneumatic, and finally mechanical. First,
the computer closes a switch, sometimes referred to as making a contact output closure. This signal is
sent to the head, through the source backplane and Source 12 radiation interlock board to the shutter
solenoid. The energized solenoid then opens allowing pressurized air into the shutter air cylinder. When
inflated, this air cylinder pulls on the shutter which rotates open. Unless so pressurized the shutter is
closed by the air cylinder's internal spring. Therefore, this spring is an important shutter safety
mechanism, forcing the shutter closed if electrical power or air pressure is lost. Through the source
capsule's rectangular window passes the beam of beta particles. The aperture is a rectangular hole in a
stainless steel cover. This is close to the stationary source capsule and covers the capsule window and
lower portions of two sides. When commanded to open, the shutter rotates away from its normal resting
position between the aperture and the source capsule. This allows the beta particles out through the
open
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