Method and apparatus for reducing afterglow noise in an X-ray inspection system

• US 5,666,393 A
• Filed: 06/04/1996
• Issued: 09/09/1997
• Est. Priority Date: 02/17/1994
• Status: Expired due to Term

• Current Assignee: American Science and Engineering Incorporated (OSI Systems Incorporated)
• Original Assignee: Martin Annis
• Inventors: Annis, Martin
• Primary Examiner(s): Church, Craig E.
• Application Number:
  US08/658,012
• Time in Patent Office: 462 Days
• Field of Search: 378/57, 378/98.2, 378/98.8, 378/146
• US Class Current: 378/57
• CPC Class Codes: G01N 23/04 and forming images of the material

Abstract

An improved X-ray inspection system comprises a pulsed X-ray source configured to emit short, X-ray pulses for reducing afterglow noise. A rotating cylindrical collimator limits the X rays to a pencil-beam, which is directed across and through an object prior to interception by a detector. The detector comprises a plurality of scintillating screens optically coupled to photoemissive detecting elements. When the short X-ray pulses generated by the source are intercepted by the detector, the outputs of the detecting elements are sampled for only a short period of time, which sampling period is immediately followed by a substantially longer quiescent period of time during which the elements are not sampled. Therefore, only a slight portion of the typical afterglow noise occurs during measurement of a useful X-ray signal.

Claims

1. An improved X-ray inspection system for reducing afterglow noise during the inspection of an object, said improved system comprising:

a pulsed X-ray source for radiating a cone of X ray pulses that penetrate the object at a rate of at least about 2,000 pulses per second, said pulses being asserted for a duration period of time and adjacent of said pulses being separated by a separation period of time, said separation period being substantially longer than said duration period; and

a detector for intercepting said X-ray pulses penetrating the object and for transforming said X-ray pulses into image data, said detector comprising at least one scintillating screen optically coupled to a plurality of photoemissive detecting elements such that when said X-ray pulses are intercepted by said detector, outputs of said photoemissive detecting elements are sampled for a sampling period of time immediately followed by a quiescent period of time during which said photoemissive detecting elements are not sampled, said quiescent period being substantially longer than said sampling period so as to significantly reduce afterglow noise during inspection of the object.

2. The improved X-ray inspection system of claim 1 wherein said separation period is approximately equal to said quiescent period and wherein said duration period is approximately equal to said sampling period.

3. The improved X-ray inspection system of claim 1 further comprising a rotating cylindrical collimator for converting said X-ray pulses into a pencil-beam of X-ray pulses.

4. The improved X-ray inspection system of claim 3 further comprising an enclosure for containing said source, said enclosure having an integrally-formed precollimator that limits said radiated cone to a fan-beam of X ray pulses.

5. The improved X-ray inspection system of claim 4 wherein said precollimator comprises a shield and a slit.

6. The improved X-ray inspection system of claim 5 further comprising a fixed-slit collimator arranged between said rotating cylindrical collimator and the object, said fixed-slit collimator further limiting said pencil-beam of X-ray pulses prior to penetrating the object.

7. The improved X-ray inspection system of claim 1 wherein said source is a high-energy pulsed linear accelerator having a repetition rate of about 2,000 to 10,000 pulses per second.

8. The improved X-ray inspection system of claim 1 wherein said source is a low-energy X-ray tube having a peak energy of 200 to 800 KeV.

9. The improved X-ray inspection system of claim 1 wherein said detector comprises a plurality of scintillating screens arranged in a back-to-back configuration to increase the efficiency of said detector.

10. The improved X-ray inspection system of claim 9 wherein said photoemissive detecting elements comprise photomultiplier tubes.

11. A method of reducing afterglow noise during inspection of an object by an X-ray inspection system, said method comprising the steps of:

generating X-ray pulses at a rate of least 2,000 pulses per second, wherein each of said pulses has a time duration that is substantially less than a separation of time between adjacent of said pulses;

intercepting said X-ray pulses using a scintillating screen optically coupled to a plurality of photoemissive detecting elements of a detector, each of said photoemissive detecting elements generating an electrical output signal in response to visible light produced by said scintillating screen; and

sampling said outputs of said plurality of photoemissive detecting elements during said duration of time of said X-ray pulses to reduce the afterglow noise of said system.

12. Apparatus for reducing afterglow noise during inspection of an object by an X-ray inspection system, said apparatus comprising:

means for generating X-ray pulses at rate of at least about 2,000 pulses per second, wherein each of said pulses has a time duration that is substantially less than a separation of time between adjacent of said pulses;

means for intercepting said X-ray pulses using a scintillating screen optically coupled to a plurality of photoemissive detecting elements of a detector, said photoemissive detecting elements generating current at outputs thereof in response to visible light produced by said scintillating screen; and

means for sampling said outputs of said plurality of photoemissive detecting elements during said duration of time of said X-ray pulses to thereby reduce the afterglow noise of said system.