Patent Number: 
Section: claims

1. A locking mechanism for a radiographic projector,the radiographic projector being adapted to hold a radioactive source in a source holder, and comprising a housing defining a channel along which the source holder is adapted to be moved between a storage position inside the housing and an exposed position outside of the housing,wherein the locking mechanism is adapted to lock the source holder in the and comprises:a first locking member moveable between a locked state preventing movement of the source holder out of the storage position and an unlocked state allowing movement of the source holder out of the storage position;wherein the first locking member is biased towards the locked state;a second locking member being adapted to prevent movement of the first locking member to the locked state when the source holder is not in the storage position, and to allow movement of the first locking member to the locked state when the source holder is in the storage position; anda third locking member, adapted to engage the first locking member, as a result of movement of the first locking member from the locked state to the unlocked state, to retain the first locking member in the unlocked state, and adapted to disengage said first locking member when said second locking member prevents movement of the first locking member to a locked state. 2. A locking mechanism according to claim 1, wherein the third locking member is decoupled from the source holder, allowing the third locking member to latch the first locking member in the unlocked state without movement of the source holder along the channel. 3. A locking mechanism according to claim 1, wherein one of the third locking member and the first locking member comprises at least one detent, and the other of the third locking member and the first locking member comprises at least one recess for engaging with the or each respective detent, and wherein the third locking member is biased towards the first locking member. 4. A locking mechanism according to claim 3, wherein release of the third locking member from the first locking member, by engagement of the first locking member with the first locking member, results in misalignment of the or each detent relative to the or each respective recess. 5. A locking mechanism according to claim 1, wherein the second locking member is biased towards engagement with the first locking member. 6. A locking mechanism according to claim 5, wherein the second locking member is movable in a direction parallel to the axis of the channel, and is biased along the axis of the channel in a forward sense. 7. A locking mechanism according to claim 5, wherein the presence of a source holder in the storage position in the radiographic projector blocks the second locking member from engaging the first locking member. 8. A locking mechanism according to claim 1, wherein the second locking member is adapted to be coupled to the source holder by a tube through which the source holder passes, wherein an end portion of the source holder has a diameter larger than the internal diameter of the tube, such that retraction of the source holder to its storage position causes the end portion of the source holder to exert a force on said tube and said second locking member to move the second locking member in a rearward sense. 9. A locking mechanism according to claim 8, wherein the second locking member is integrally formed with the tube. 10. A locking mechanism according to claim 1, wherein the first locking member is slidable in a direction transverse to the axis of the channel. 11. A locking mechanism according to claim 1, further comprising first and second locking pins engageable with the first locking member in the locked state, wherein displacement of the first locking pin is actuated by coupling a guide apparatus to the projector, and displacement of the second locking pin is actuated by coupling a remote wind-out apparatus to the projector, and wherein the displacement of the first and second locking pins frees the first locking member for movement to the unlocked state.