Abstract: A defeatable safety interlock device for blocking radiation from an instrument when a portion of the protective housing therefor is removed or displaced. When defeated, it is not possible to remove or displace the interlocked portion of the protective housing, nor is it possible to replace that interlocked portion during such defeat. The interlock device includes a shutter, a locking pin and a latch. The shutter blocks the radiation upon activation of the interlock device and controls the position of the locking pin. The latch is secured to the displaceable portion and includes a slotted portion movable in a restricted path, the passage being controlled by the position of the locking pin. The locking pin is seated in a slotted ramp movable with the shutter to raise and lower the pin relative to the path followed by the slotted portion of the latch.