X-ray radiation sources with low neutron emissions for radiation scanning Number:7,436,932 from the United States Patent and Trademark Office (PTO) owispatent In one example, a radiation source comprises a housing and an acceleration chamber within the housing, with a peak acceleration energy greater than the lowest neutron production threshold of tantalum. A source of charged particles is supported by the housing to emit charged particles into the acceleration chamber. A target is supported by the housing downstream of the acceleration chamber. The target consists essentially of at least one isotope having a neutron production threshold greater than the peak acceleration energy. No neutrons are therefore generated. The source may also comprise a collimator, target shielding, and/or housing shielding comprising at least one isotope having a neutron production threshold greater than the peak acceleration energy, reducing or eliminating neutron generation as compared to the prior art, as well. Systems comprising the source, methods of operation of the source, and methods of manufacture of the source are also disclosed.<H radiation, emissions, X, ray, radiation, 7436932.html, Patent, pto, 7436932

Title: X-ray radiation sources with low neutron emissions for radiation scanning

Abstract: In one example, a radiation source comprises a housing and an acceleration chamber within the housing, with a peak acceleration energy greater than the lowest neutron production threshold of tantalum. A source of charged particles is supported by the housing to emit charged particles into the acceleration chamber. A target is supported by the housing downstream of the acceleration chamber. The target consists essentially of at least one isotope having a neutron production threshold greater than the peak acceleration energy. No neutrons are therefore generated. The source may also comprise a collimator, target shielding, and/or housing shielding comprising at least one isotope having a neutron production threshold greater than the peak acceleration energy, reducing or eliminating neutron generation as compared to the prior art, as well. Systems comprising the source, methods of operation of the source, and methods of manufacture of the source are also disclosed.

Patent Number: 7,436,932 Issued on 10/14/2008 to Clayton

Inventors: Clayton; James E. (Henderson, NV)
Assignee: Varian Medical Systems Technologies, Inc. (Palo Alto, CA)

Appl. No.: 11/165,972

Filed: June 24, 2005

Current U.S. Class: 378/143 ; 378/119

Current International Class: H01J 35/08 (20060101)

Field of Search: 378/119,121,122,142,143,144 313/363.1,359.1,231.01-231.71 250/505.1,515.1

References Cited [Referenced By]

U.S. Patent Documents


4317035	February 1982	Cohen et al.
5115459	May 1992	Bertozzi
5124658	June 1992	Adler
5874811	February 1999	Finlan et al.
6009146	December 1999	Adler et al.
6069936	May 2000	Bjorkholm
6172463	January 2001	Cutler et al.
6445766	September 2002	Whitham
6463123	October 2002	Korenev
6493424	December 2002	Whitham
6628745	September 2003	Annis et al.
6683318	January 2004	Haberer et al.
2005/0077472	April 2005	Korenev
2005/0218348	October 2005	Fehrenbacher et al.
2005/0281383	December 2005	Harding et al.

Other References

"Neutron Contamination From Medical Electron Accelerators", National Council on Radiation Protection and Measurements, NCRP Report No. 79, 1984, pp. i-vi and 1-60. cited by other .
McCall, R. C., et al., "Neutron Sources and Their Characteristics", NBS SP, 1979, vol. 554, pp. 75-86, from Heaton, et al., ed., "Proceedings of a Conference on Neutrons from Electron Medical Accelerators", Proceedings of a Conference held at the National Bureau of Standards, Gaithersburg, MD, Apr. 9-10, 1979. cited by other .
McCall, Richard C., et al., "Improvement of linear accelerator depth-dose curves", Med. Phys., Nov./Dec. 1978, vol. 5, No. 6, pp. 518-524. cited by other .
Gozani, Tsahi, "Active Nondestructive Assay of Nuclear Materials: Principles and Applications", United States Nuclear Regulatory Commission, Jan. 1981, pp. 173-205. cited by other .
MatWeb, Oxygen-free electronic Copper, UNS C10100, http://www.matweb.com/search/SpecificMaterial.asp?bassnum=MC101A, at least as early as Dec. 28, 2004. cited by other .
MatWeb, "GlidCop.RTM. AL-60 Dispersion Strengthened Copper", http://www.matweb.com/search/SpecificMaterial.asp?bassnum=NOMG60, at least as early as Dec. 28, 2004. cited by other.

Primary Examiner: Yun; Jurie
Attorney, Agent or Firm: Sklar; Brandon N. Kaye Scholer LLP

Claims

I claim:

1. A radiation source comprising: a housing; an acceleration chamber within the housing, the acceleration chamber having a peak acceleration energy greater than the neutron production threshold of tantalum, during use; a source of charged particles supported by the housing to emit charged particles into the acceleration chamber; and a target supported by the housing downstream of the acceleration chamber; wherein: impact of the target by the accelerated charged particles generates radiation; and the target consists essentially of at least one isotope having a neutron production threshold greater than the peak acceleration energy.

2. The radiation source of claim 1, wherein: the peak acceleration energy is less than or equal to 8 MeV; and the target is chosen from the group consisting of at least one isotope of carbon, aluminum, scandium, titanium, vanadium, manganese, cobalt, and copper.

3. The radiation source of claim 1, wherein: the peak acceleration energy is greater than 8 MeV and less than or equal to 9 MeV; and the target is chosen from the group consisting of at least one isotope of aluminum, scandium, vanadium, manganese, cobalt, and copper.

4. The radiation source of claim 1, wherein: the peak acceleration energy is greater than 9 MeV and less than or equal to 10 MeV; and the target is chosen from the group consisting of at least one isotope of aluminum, scandium, manganese, and cobalt.

5. The radiation source of claim 1, wherein: the peak acceleration energy is greater than 10 MeV and less than 11 MeV; and the target is chosen from the group consisting of at least one isotope of scandium and aluminum.

6. The radiation source of claim 1, wherein: the peak acceleration energy is greater than 11 and less than about 13.1 MeV; and the target consists essentially of aluminum.

7. The radiation source of claim 1, further comprising: a collimator coupled to the housing, the collimator comprising at least one isotope having a neutron production threshold greater than the peak acceleration energy.

8. The radiation source of claim 7, further comprising: target shielding surrounding at least a portion of the target, the target shielding comprising at least one isotope having a neutron production threshold greater than the peak acceleration energy.

9. The radiation source of claim 1, further comprising: housing shielding to shield the housing, the housing shielding comprising at least one isotope having a neutron production threshold greater than the peak acceleration energy.

10. The radiation source of claim 1, wherein: the target consists essentially of at least one isotope of copper.

11. The radiation source of claim 1, wherein the peak acceleration energy is at least about 7.7 MeV.

12. A radiation source comprising: a housing; an acceleration chamber within the housing, the acceleration chamber having a peak acceleration energy greater than the lowest neutron production threshold of tungsten, during use; a source of charged particles supported by the housing to emit charged particles into the acceleration chamber; a target supported by the housing downstream of the acceleration chamber; and a collimator coupled to the housing, proximate the target material; wherein: impact of the target by the accelerated charged particles generates radiation; and the collimator comprises at least one isotope having a neutron production threshold greater than the peak acceleration energy.

13. The radiation source of claim 12, wherein the collimator comprises: at least one first section consisting essentially of at least one isotope having a neutron production threshold greater than the peak acceleration energy; and at least one second section comprising at least one isotope having a neutron production threshold less than the peak acceleration energy.

14. The radiation source of claim 13, wherein: the first section is at least partially between the target and the second section.

15. The radiation source of claim 12, further comprising: target shielding to shield the target, the target shielding material comprising at least one isotope having a neutron production threshold greater than the peak acceleration energy.

16. The radiation source of claim 12, further comprising: housing shielding to shield the housing, the housing shielding comprising at least one isotope having a neutron production threshold greater than the peak acceleration energy.

17. The radiation source of claim 12, wherein: the collimator comprises copper.

18. The radiation source of claim 12, wherein: the peak acceleration energy is greater than the lowest neutron production threshold of molybdenum.

19. The radiation source of claim 18, wherein: the peak acceleration energy greater than the neutron production threshold of tantalum.

20. A radiation source comprising: a housing comprising housing material; an acceleration chamber within the housing, the acceleration chamber having a peak acceleration energy during use; a source of charged particles supported by the housing to emit charged particles into the acceleration chamber; a target supported by the housing downstream of the acceleration chamber; and target shielding surrounding at least a portion of the target; wherein: impact of the target material by the accelerated charged particles generates radiation; and the target shielding material comprises: at least one first section consisting essentially of at least one isotope having a neutron production threshold greater than the peak acceleration energy, proximate the target; and at least one second section comprising at least one isotope having a neutron production threshold less than the peak acceleration energy; wherein the at least one first section is at least partially between the target and the at least one second section.

21. The radiation source of claim 20, wherein: the first section of the target shielding comprises copper.

22. The radiation source of claim 20, wherein the peak acceleration energy is greater than the lowest neutron production threshold of molybdenum.

23. The radiation source of claim 22, wherein the peak acceleration energy is greater than the lowest neutron production threshold of tantalum.

24. The radiation source of claim 20, further comprising: housing shielding separate from the target shielding, surrounding, at least in part, the housing.

25. The radiation source of claim 20, wherein the peak acceleration energy is greater than the lowest neutron production threshold of tungsten, during use.

26. A radiation source comprising: a housing; an accelerator chamber within the housing, the acceleration chamber having a peak acceleration energy of less than a lowest neutron production threshold of copper; a source of charged particles to emit charged particles into the accelerator chamber; a target supported by the housing, downstream of the accelerator chamber, wherein impact of the target by the accelerated charged particles generates radiation; a collimator coupled to the housing, proximate the target; target shielding at least partially surrounding the target to shield the target; wherein: the target, the collimator, and the target shielding comprise copper.

27. The radiation source of claim 26, wherein the collimator comprises: a first section consisting essentially of copper, proximate the target; and a second section comprising at least one isotope having a neutron production threshold less than the peak acceleration energy, downstream of the first section.

28. The radiation source of claim 27, wherein the target shielding comprises: a first section consisting essentially of copper, proximate the target; and a second section comprising at least one isotope having a neutron production threshold less than the peak acceleration energy wherein the at least one first section is at least partially between the target and the at least one second section.

29. The radiation source of claim 27, further comprising: housing shielding to shield the housing; wherein the housing shielding comprises copper.

30. The radiation source of claim 27, further comprising: lead shielding surrounding at least a portion of the collimator and the target shielding.

31. The radiation source of claim 27, wherein the peak acceleration energy is less than or equal to about 9 MeV.

32. The radiation source of claim 26, wherein: the acceleration chamber has a peak acceleration energy greater than the lowest neutron production threshold of tungsten.

33. The radiation source of claim 32, wherein the acceleration chamber has a peak acceleration energy greater than the lowest neutron production threshold of molybdenum.

34. The radiation source of claim 33, wherein the acceleration chamber has a peak acceleration energy greater than the neutron production threshold of tantalum.

35. A method of generating radiation, comprising: accelerating charged particles to a peak acceleration energy greater than the neutron production threshold of tantalum; colliding the charged particles with a target consisting essentially of at least one isotope having a neutron production threshold greater than the peak acceleration energy; and generating radiation from the collision of the charged particles with the target, without generating neutrons.

36. The method of claim 35, further comprising: collimating the generated radiation by a collimator comprising at least one isotope having a neutron production threshold greater than the peak acceleration energy.

37. The method of claim 35, further comprising: shielding the target with shielding comprising at least one isotope having a neutron production threshold greater than the peak acceleration energy.

38. The method of claim 35, further comprising: shielding the housing with shielding material comprising at least one isotope having a neutron production threshold greater than the peak acceleration energy.

39. A system for examining a cargo conveyance, comprising: a radiation source positioned to irradiate an object; and a detector positioned to receive radiation after interacting with the object; wherein the radiation source comprises: a housing; an acceleration chamber supported by the housing, the acceleration chamber having a peak acceleration energy of less than the lowest neutron production threshold of copper and greater than the lowest neutron production threshold of tungsten; a source of charged particles supported by the housing to emit charged particles into the acceleration chamber; a target supported by the housing downstream of the acceleration chamber; wherein: the target consists essentially of at least one isotope of copper; and impact of the target material by the accelerated charged particles generates radiation, without producing neutrons.

40. The system of claim 39, further comprising: a collimator coupled to the housing; and target shielding supported by the housing, partially around the target; and wherein: at least one of the collimator and the target shielding comprises copper.

41. The system of claim 39, further comprising: shielding over at least a portion of the housing; wherein the shielding comprises copper.

42. The system of claim 39, further comprising: a conveyor to support the object for scanning and to convey the object through the system, the conveyor being configured to support and convey a cargo conveyance.

43. The system of claim 42, wherein the cargo conveyance has a thickness of at least 5 feet (1.5 meters).

44. The system of claim 43, wherein: the cargo conveyance is a standard cargo conveyance.

45. The system of claim 39, wherein: the peak acceleration energy is greater than the lowest neutron production threshold of molybdenum.

46. The system of claim 45, wherein: the peak acceleration energy is greater than the lowest neutron production threshold of tantalum.

47. The system of claim 39, wherein: the peak acceleration energy is less than about 9.9 MeV and greater than about 6.1 MeV.

48. A radiation source, comprising: a housing; an acceleration chamber within the housing, the acceleration chamber having a peak acceleration energy greater than the neutron production threshold of tantalum, during use; a source of charged particles supported by the housing to emit charged particles into the acceleration chamber; and a target supported by the housing downstream of the acceleration chamber; wherein: impact of the target by the accelerated charged particles generates radiation; and the target consists essentially of a low atomic number material comprising at least one isotope having a neutron production threshold greater than the peak acceleration energy.

49. The radiation source of claim 48, wherein the low atomic number material consists essentially of copper.

50. A radiation source comprising: a housing; an acceleration chamber within the housing, the acceleration chamber having a peak acceleration energy greater then the neutron production threshold of tantalum and less than about 13.1 MeV, during use; a source of charged particles supported by the housing to emit charged particles into the acceleration chamber; and a target supported by the housing downstream of the acceleration chamber, the target consisting essentially of aluminum; wherein impact of the target by the accelerated charged particles generates radiation.

51. A radiation source comprising: a housing; an acceleration chamber within the housing, the acceleration chamber having a peak acceleration energy of at least 6.1 MeV and less than 9.9 MeV, during use; a source of charged particles supported by the housing to emit charged particles into the acceleration chamber; a target supported by the housing downstream of the acceleration chamber, wherein impact of the target by the accelerated charged particles generates radiation; and a collimator coupled to the housing, proximate the target material, the collimator comprising copper.

52. The radiation source of claim 51 wherein the collimator comprises: at least one first section consisting essentially of copper; and at least one second section comprising at least one isotope having a neutron production threshold less than the peak acceleration energy.

53. A radiation source comprising: a housing comprising housing material; an acceleration chamber within the housing, the acceleration chamber having a peak acceleration energy of at least 6.1 MeV and less than 9.9 Mev; a source of charged particles supported by the housing to emit charged particles into the acceleration chamber; a target supported by the housing downstream of the acceleration chamber, wherein impact of the target material by the accelerated charged particles generates radiation; and target shielding surrounding at least a portion of the target, the target shielding comprising copper.

54. The radiation source of claim 53, wherein the acceleration chamber has a peak acceleration energy greater than the lowest neutron production threshold of tungsten, during use.

55. The radiation source of claim 53, wherein the target shielding comprises: at least one first section consisting essentially of copper; and at least one second section comprising at least one isotope having a neutron production threshold less than the peak acceleration energy; wherein the at least one first section is at least partially between the target and the at least one second section.

56. A method of generating radiation, comprising: accelerating charged particles to a peak acceleration energy greater than the neutron production threshold of tungsten; colliding the charged particles with a target; generating radiation from the collision of the charged particles with the target; and collimating the generated radiation by a collimator comprising at least one isotope having a neutron production threshold greater than the peak acceleration energy.

57. The method of claim 56, further comprising: absorbing radiation generated by the target with target shielding comprising at least one isotope having a neutron production threshold greater than the peak acceleration energy.

58. The method of claim 57, further comprising: absorbing radiation generated by the target by housing shielding comprising at least one isotope having a neutron production threshold greater than the peak acceleration energy.

59. The method of claim 56, further comprising: absorbing radiation generated by the target by housing shielding comprising at least one isotope having a neutron production threshold greater than the peak acceleration energy.

60. A method of generating radiation, comprising: accelerating charged particles to a peak acceleration energy; colliding the charged particles with a target; generating radiation from the collision of the charged particles with the target, without generating neutrons; and absorbing a portion of the generated radiation by housing shielding comprising: at least one first section of material consisting essentially of at least one isotope having a neutron production threshold greater than the peak acceleration energy, proximate the target; and at least one second section of material comprising at least one isotope having a neutron production threshold less than the peak acceleration energy; wherein the at least one first section is at least partially between the target and the at least one second section.

61. The method of claim 60, comprising: accelerating the charged particles to a peak acceleration energy greater than the neutron production threshold of tungsten.

62. The method of claim 60, comprising: absorbing at least a portion of the generated radiation by at least one first section comprising copper.

63. The method of claim 60,further comprising: absorbing at least a portion of the generated radiation by target shielding comprising at least one isotope having a neutron production threshold greater then the peak acceleration energy, the target shielding being proximate to the target.

64. A method of examining contents of an object, comprising: accelerating charged particles to a peak acceleration energy greater than the neutron production threshold of tantalum; colliding the charged particles with a target consisting essentially of at least one isotope having a neutron production threshold greater than the peak acceleration energy; generating radiation from the collision of the charged particles with the target, without generating neutrons; scanning an object by the generated radiation; and detecting radiation inteacting with the object.

Description

FIELD OF THE INVENTION

X-ray radiation sources and, more particularly, X-ray radiation sources with low neutron emissions for radiation scanning of objects.

BACKGROUND OF THE INVENTION

X-ray radiation sources are commonly used in radiation inspection systems for non-destructive inspection of objects. X-ray radiation may be generated in such sources by the impact of a beam of accelerated electrons on a high atomic number ("Z") target material, such as tungsten or tantalum. The electrons are accelerated by a potential difference established across a chamber, referred to as an acceleration energy. The deceleration of the incident electrons by the nuclei of the atoms of the target material generates radiation, referred to as Bremsstrahlung. A collimator is provided to direct some of the generated radiation onto an object to be inspected and to form the generated radiation into a beam of a desired size and shape. One or more radiation detectors are provided to measure radiation transmitted through and/or scattered from the object. The body of the radiation source may also be shielded. In order to prevent radiation from escaping the radiation inspection system, shielding is also provided around the system as a whole.

Small objects, such as luggage and carry-on bags, are typically examined by radiation in the kilovolt range. However, radiation in the kilovolt range may not penetrate objects thicker than about 5 feet (1.52 meters), particularly if the object is filled with dense material. Standard cargo containers are typically 20-50 feet long (6.1-15.2 meters), 8 feet high (2.4 meters) and 6-9 feet wide (1.8-2.7 meters). Air cargo containers, which are used to contain a plurality of pieces of luggage or other cargo to be stored in the body of an airplane, may range in size (length, height, width (thickness)) from about 35.times.21.times.21 inches (0.89.times.0.53.times.0.53 meters) up to about 240.times.118.times.96 inches (6.1.times.3.0.times.2.4 meters). Large collections of objects, such as many pieces of luggage, may also be supported on a pallet. Pallets, which may have supporting sidewalls, may be of comparable sizes as cargo containers, at least when supporting objects. The term "cargo conveyance" is used to refer to all types of cargo containers and comparably sized pallets (and other such platforms) supporting objects.

Higher energy radiation beams are required to penetrate through denser materials than less dense materials and through thicker materials than less thick materials. The low energies used in typical X-ray luggage and bag scanners, described above, are generally too low to penetrate through the much larger cargo containers, particularly those with widths or thicknesses of 5 feet (1.5 meters) or more. While the required energy level depends on the contents of the container and the width of the container, radiation in the megavolt range is typically required. 6 MeV to 10 MeV may be used, for example. 9 MeV is commonly used because it will penetrate through most cargo containers, regardless of the contents. However, high Z and medium Z metals commonly used in X-ray radiation sources, such as tungsten, tantalum, and molybdenum comprise stable isotopes having neutron production thresholds (the energy required to remove a neutron from a nucleus of an atom of the isotope) in a range of about 6 MeV to about 10 MeV. For example, the calculated neutron production thresholds for the stable isotopes of tungsten range from 6.191 MeV to 8.415 MeV. The calculated neutron production threshold for the stable isotope of tantalum is 7.651 MeV. The calculated neutron production thresholds for the stable isotopes of molybdenum range from 7.369 MeV to 12.667 MeV. Since 6 MeV to 10 MeV is a common range to examine cargo conveyances, neutrons are typically produced.

Because of their ability to absorb larger amounts of photons than lower atomic number metals per unit volume, high Z metals, such as tungsten and lead, are also typically used to shield the target and collimate the radiation beam. The stable isotopes of lead have calculated neutron production thresholds of from 6.737 MeV to 8.394 MeV. If the generated X-ray radiation used to examine the objects has an energy above the neutron production threshold of the shielding material and collimator, neutrons will also be produced.

Since neutrons may be harmful to people proximate the scanning system, thicker shielding may be required to prevent the escape of neutrons from the scanning system or the room containing the scanning system. This may increase the size and cost of the system. Concrete walls are commonly used to shield a room containing a scanning system, preventing or decreasing the amount of neutrons and X-rays that may escape the room. If space or other requirements prevent the use of concrete walls, then a multi-layer wall may be used. For example, a thick wall of polyethylene or borated polyethylene may be used as an inner layer to shield neutrons and lead or steel may be used as an outer layer to shield X-rays. The outer layer also shields gamma rays emitted by the polyethylene.

Varian Medical Systems, Inc., ("Varian") Palo Alto, Calif., sells X-ray radiation sources for medical therapy supported by a rotatable gantry that also supports a detector array. The gantry, the source, and the detector comprise an integrated unit, which is sold under the trade name CLINAC.RTM.. The radiation source comprises a copper target and tungsten shielding. Copper generates sufficient X-ray radiation for therapeutic purposes, and is less expensive than tungsten. CLINACs.RTM. are available at 4 MeV, 6 MeV, 10 MeV and above. Copper has two stable isotopes, copper-65, with a calculated neutron production threshold of 9.910 MeV, and copper-63, with a calculated neutron production threshold of 10.852 MeV. Since in the 10 MeV and above models of the CLINAC.RTM. the acceleration energy is above the neutron production threshold of copper-65 and tungsten, neutrons are produced. The collimator comprises a combination of tungsten and lead, which will also generate neutrons. The tungsten shielding will generate neutrons, as well.

Efforts have been made to reduce neutron emission from X-ray sources used in medical therapy, such as radiation treatment. See, for example, Neutron Contamination from Medical Electron Accelerators, NRCP Report No. 79, National Council on Radiation Protection and Measurements, Bethesda, Md., pp. 59-60 (1995). It is said to be difficult to reduce the number of neutrons produced per useful photon rad of radiation, in the space available in existing sources. (Id.) It is noted that neutron emission may be reduced by absorbing unwanted neutrons in a medium Z material, such as iron, instead of tungsten or lead; but it is also noted that much more iron is required than tungsten or lead and there is insufficient space to take advantage of this reduction completely. (Id.)

Varian also sells a Linatron.RTM. series of X-ray sources that generate X-ray radiation in the range of 1-10 MeV. In these sources, the target is tungsten, typically in the form of a disk. A disk of copper is attached to the downstream side of the tungsten, of the electron beam, to dissipate heat and to act as a final electron stop for electrons passing through the tungsten target. The tungsten target is the primary source of X-ray radiation. It is believed that a small amount of X-ray radiation may be generated by the copper disk as well, by the electrons passing through the tungsten target. If the acceleration energy of the source is greater than the neutron production threshold of the tungsten, neutrons may be produced.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the invention, a radiation source is disclosed comprising a housing and an acceleration chamber within the housing. The acceleration chamber has a peak acceleration energy greater than the neutron production threshold of tantalum, during use. A source of charged particles is supported by the housing to emit charged particles into the acceleration chamber. A target is supported by the housing downstream of the acceleration chamber. Impact of the target by the accelerated charged particles generates radiation. The target consists essentially of at least one isotope having a neutron production threshold greater than the peak acceleration energy. The peak acceleration energy may be greater than about 7.7 MeV, for example. In a source with a peak acceleration energy greater than the peak acceleration energy of tantalum, targets comprising tungsten, tantalum, or molybdenum, which include isotopes with neutron production thresholds less than the peak acceleration energy, neutrons are generated.

In one example, if the peak acceleration energy is less than or equal to 8 MeV, the target is chosen from the group consisting of at least one isotope of carbon, aluminum, scandium, titanium, vanadium, manganese, cobalt, and copper. In another example, if the peak acceleration energy is greater than 8 MeV and less than or equal to 9 MeV, the target is chosen from the group consisting of at least one isotope of aluminum, scandium, vanadium, manganese, cobalt, and copper. In another example, if the peak acceleration energy is greater than 9 MeV and less than or equal to 10 MeV, the target is chosen from the group consisting of at least one isotope of aluminum, scandium, manganese, and cobalt. In another example, if the peak acceleration energy is greater than 10 MeV and less than 11 MeV, the target is chosen from the group consisting of at least one isotope of scandium and aluminum. In another example, if the peak acceleration energy is greater than 11 and less than about 13.1 MeV, the target is aluminum. Copper is a preferred material if the peak acceleration energy is less than about 9.9 MeV.

The radiation source may further comprise a collimator, target shielding, and housing shielding. Any or all of these components may also comprise at least one isotope having a neutron production threshold greater than the peak acceleration energy.

In accordance with other embodiments of the invention, a radiation source is disclosed comprising a housing and an acceleration chamber within the housing. The acceleration chamber has a peak acceleration energy greater than the lowest neutron production threshold of tungsten, during use. A source of charged particles is supported by the housing to emit charged particles into the acceleration chamber. A target is supported by the housing downstream of the acceleration chamber. The peak acceleration energy may be greater than the lowest neutron production threshold of molybdenum or tantalum, as well. The peak acceleration energy may be greater than about 6.2 MeV, 6.8 MeV, or 7.7 MeV. In a collimator embodiment, a collimator is coupled to the housing, proximate the target material, and the collimator comprises at least one isotope having a neutron production threshold greater than the peak acceleration energy. In a target shielding embodiment, target shielding surrounds at least a portion of the target, and the target shielding comprises at least one isotope having a neutron production threshold greater than the peak acceleration energy.

It is noted that some of the advantages of the present invention may be obtained if only certain components of the source consist essentially of at least one isotope having a neutron production threshold greater than the peak acceleration energy of the source. Therefore, in the collimator embodiment, the collimator may comprise at least one first section consisting essentially of at least one isotope having a neutron production threshold greater than the peak acceleration energy and at least one second section comprising at least one isotope having a neutron production threshold less than the peak acceleration energy. In the target shielding embodiment, the target shielding may comprise at least one first section consisting essentially of at least one isotope having a neutron production threshold greater than the peak acceleration energy and at least one second section comprising at least one isotope having a neutron production threshold less than the peak acceleration energy. In either case, neutron production is reduced as compared to the use of prior art collimators and target shielding in radiation sources. The collimator and/or the target shielding may comprise copper.

In accordance with another embodiment, a radiation source is disclosed comprising a housing and an accelerator chamber within the housing. In this embodiment, the acceleration chamber has a peak acceleration energy of less than the lowest neutron production threshold of copper. A source of charged particles is supported by the housing to emit charged particles into the accelerator chamber and a target is supported by the housing, downstream of the accelerator chamber. A collimator is coupled to the housing proximate the target. Target shielding at least partially surrounds the target to shield the target. The target, the collimator, and the target shielding comprise copper. The peak acceleration energy may be less than about 9.9 MeV, for example. The peak acceleration energy may be less than or equal to about 9 MeV. The peak acceleration energy may be greater than the lowest neutron production threshold of tungsten, molybdenum, or tantalum, as well. The peak acceleration energy may be greater than about 6.1 MeV, 6.8 MeV, or 7.7 MeV.

The collimator may comprise a first section consisting essentially of copper, proximate the target, and a second section comprising at least one isotope having a neutron production threshold less than the peak acceleration energy, downstream of the first section. The target shielding may comprise a first section consisting essentially of copper, proximate the target and a second section comprising at least one isotope having a neutron production threshold less than the peak acceleration energy, upstream of the first section. The source may further comprise housing shielding to shield the housing, wherein the housing shielding comprises copper. Lead shielding may surround at least a portion of the collimator and the target shielding.

In accordance with another embodiment of the invention, a radiation source is disclosed, as above, wherein the target consists essentially of a low atomic number material comprising at least one isotope having a neutron production threshold greater than the peak acceleration energy of the source.

In accordance with another embodiment of the invention, a method of generating radiation is disclosed comprising accelerating charged particles to a peak acceleration energy greater than the lowest neutron production threshold of tantalum and colliding the charged particles with a target consisting essentially of at least one isotope having a neutron production threshold less than the peak acceleration energy. The method further comprises generating radiation from the collision of the charged particles with the target, without generating neutrons. The method may further comprise collimating the generated radiation by a collimator comprising at least one isotope having a neutron production threshold greater than the peak acceleration energy. The method may further comprise shielding the target with shielding material comprising at least one isotope having a neutron production threshold greater than the peak acceleration energy. The method may further comprise shielding the housing with shielding material comprising at least one isotope having a neutron production threshold greater than the peak acceleration energy. The peak acceleration energy may be greater than about 7.7 MeV, for example.

In accordance with another embodiment of the invention, a system for examining a cargo conveyance is disclosed comprising a radiation source positioned to irradiate an object and a detector to receive radiation after interacting with the object. The radiation source comprises a housing and an acceleration chamber supported by the housing. The acceleration chamber has a peak acceleration energy less than the lowest neutron production threshold of copper and greater than the lowest neutron production threshold of tungsten. The peak acceleration energy may be less than about 9.9 MeV, for example. The peak acceleration energy may be greater than 6.1 MeV, for example. A source of charged particles is supported by the housing to emit charged particles into the acceleration chamber and a target is supported by the housing downstream of the acceleration chamber. The target consists essentially of at least one isotope of copper. Other isotopes of other materials that will not generate neutrons at the peak acceleration energy of the source, may be included, without materially affecting the basic characteristics of the target with respect to neutron production. A collimator is coupled to the housing and target shielding is supported by the housing, partially around the target. Either or both of the collimator and the target shielding also comprises copper. The system may further comprise shielding material over at least a portion of the housing, which also comprises at least one isotope having a peak production threshold less than the peak acceleration energy. A conveyor adapted to support and convey a cargo conveyance may also be provided. The cargo conveyance may be greater than or equal to 5 feet thick. The cargo conveyance may be a standard cargo conveyance. The peak acceleration energy may be at least about 6.2 MeV. The peak acceleration energy may be at least about 7.7 MeV.

In accordance with another embodiment of the invention, a method of manufacturing a radiation source is disclosed comprising selecting for at least one of a target, a collimator, and target shielding (in other words, the target, the collimator, and/or the target shielding) at least one isotope having a neutron production threshold less than a peak acceleration energy of the source and assembling the source including the selected material. The method may further comprise selecting housing shielding material consisting essentially of at least one isotope having a neutron production threshold less than the peak acceleration energy and assembling the source with the selected housing shielding material.

The method may further comprise preparing a preliminary design for a radiation source to meet, at least, neutron production requirements, inputting the preliminary design into a simulation to predict neutron production, and receiving an output of the simulation. If the output does not meet requirements, the method may further comprise adjusting the design and inputting the adjusted design into the simulation to predict neutron production. The method may further comprise preparing the preliminary design based on size requirements and/or X-ray radiation generation requirements.

As used herein, the term "about" refers to differences due to round off errors and typical measurement capabilities; the term "at least one of" means "any one or more of the following"; the term "consisting essentially of" means that isotopes of the same and/or other materials that will not generate neutrons at the peak acceleration energy of the source, may be included; and the term "peak acceleration energy" means "maximum" acceleration energy.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an example of a radiation source in accordance with an embodiment of the invention;

FIG. 2 is a graph showing the neutron production thresholds for isotopes with the lowest threshold, for a number of materials;

FIG. 3 is an example of a method of manufacturing an X-ray radiation source with no neutron production, in accordance with an embodiment of the invention;

FIG. 4 is an example of a cylindrical X-ray head for use in an X-ray source, in accordance with an embodiment of the invention;

FIG. 5 is a cross-sectional, perspective view of the X-ray head of FIG. 4;

FIG. 6 is an enlarged cross-sectional view of the target assembly of FIG. 5;

FIG. 7 is an example of an iterative process to design a source in accordance with another embodiment of the invention; and

FIG. 8 is a front view of a cargo scanning system, in accordance with an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with embodiments of the invention, radiation sources, such as X-ray radiation sources, provide no neutron production or reduced neutron production as compared to radiation sources comprising typical materials for the source, collimator, and/or shielding by using materials with neutron production thresholds above the peak acceleration energy of the source. (As mentioned above, here, the term "peak acceleration energy" means "maximum" acceleration energy.) For example, an X-ray source with a peak acceleration energy of less than about 9.9 MeV, comprising a copper target, a copper collimator, and copper shielding of the target and housing, will generate no neutrons. Where it is not feasible to use only materials with neutron production thresholds below the peak acceleration energy of the source due to size, weight, and/or cost constraints, neutron production may be reduced by using such materials for all or part of certain components. By proper selection of materials, no or reduced neutron production may be provided in radiation sources with peak acceleration energies, and hence peak radiation energies, in the range of from about 6.2 MeV to less than about 13.1 MeV.

One of the advantages of reduced or no neutron production is that the physical size of the shielding of the source and the scanning system as a whole, including the room containing the scanning system, may be reduced. Also, the risk of activation of certain materials of or within the object under test and the scanning room by neutron capture reactions, may also be reduced.

The class of materials that may be used is referred to herein as "low atomic number materials" or "low Z materials" because those materials have atomic numbers ("Z") significantly less than that of tungsten (Z=74), tantalum (Z=73), lead (Z=82), and molybdenum (42), the common materials used in the prior art. Copper, for example, has an atomic number of 29. In one example, the low Z material may have an atomic number Z of 30 or less. Appropriate low atomic number materials for a particular application have their lowest neutron production threshold greater than the peak acceleration energy of the source, so that no neutrons are generated by that material.

It has been found that due to their size differences, more neutrons may be generated by the shielding and collimator materials than by the target. It is also noted that not all low Z materials are appropriate in the preferred peak acceleration range for examining cargo conveyances of from about 6 MeV to 10 MeV, which is a useful range for examining cargo conveyances, which can be greater than 5 feet (1.524 meters) thick. Beryllium, for example, with an atomic number of 4, has the lowest neutron production threshold of all elements at 1.665 MeV.

FIG. 1 is a schematic cross-sectional view of an example of a radiation source 100, such as an X-ray linear accelerator, in accordance with an embodiment of the present invention. The linear accelerator 100 comprises a housing 110 defining an acceleration chamber 120. The housing defines an input 125 and an output 130. A target 140 is supported in or near the output 130 of the housing 110. The target 140 is a material that generates Bremsstrahlung radiation in response to impact by accelerated charged particles. In one example, the target 140 preferably consists essentially of isotopes of a low Z material having a neutron production threshold less than the acceleration potential of the acceleration chamber 120. Copper may be used, for example. An electron gun 150 extends through the input 125. The electron gun comprises a filament 160, which is suspended in the acceleration chamber 120. A magnetron 165 is coupled to the acceleration chamber 120 to create an electromagnetic field within the acceleration chamber. The electromagnetic field accelerates electrons generated by the filament 160 to a desired energy level within the acceleration chamber 120. The accelerated electrons form an electron beam 170 that strikes the target 140 in the output 130, causing the emission of photons in the form of a beam of X-ray radiation 175. The housing 110 may comprise a thin copper wall, for example.

In this example, the target shielding material 180 circumferentially surrounds the target 140, to prevent the escape of X-ray radiation in a direction perpendicular to the electron beam. Housing shielding 182 may be provided around the housing 110, if needed. The need for housing shielding 182 may depend on the accelerator design, the target shielding 180, and the required level of attenuation. If the accelerator uses a solenoid or otherwise narrowly focuses the electron beam on the target 140, and/or required attenuation is low, housing shielding may not be needed to shield stray electrons. If the target shielding 180 extends sufficiently behind the target, then the housing shielding 182 may not be needed to shield radiation emitted behind the target. The embodiment of FIGS. 4-6 shows such a design. In accordance with this embodiment of the invention, all or a portion of the target shielding 180 and the housing shielding 182, if present, also comprise isotopes of appropriate low Z materials having a neutron production threshold less than the peak acceleration energy of the source, to avoid the generation of neutrons by the shielding, although that is not required. Copper may be used, for example. The target shielding 180 and the housing shielding 182 may be a single piece or one or more separate pieces of material. The shielding materials 180 and 182 may be the same or different.

A collimator 190 is coupled to a distal end of the housing 110. It may be connected to the target shielding 182 and to the housing shielding 182, for example. The material of the collimator 190 defines a passage 195 to allow passage of the radiation 175. The passage is shaped to define the radiation beam 175. In accordance with this embodiment, all or a portion of the collimator 190 is composed of isotopes of a low Z material to avoid the generation of neutrons, although that is also not required. Copper may be used, for example. The collimator 190 may also comprise a single piece of material or multiple pieces.

The thicknesses of the shielding material 180, 182 and the collimator 190 may vary in different locations on the linear accelerator 100. At peak energies above 1 MeV, the intensity of the radiation emitted from the target is greatest in the forward direction, along the axis of the path of the electron beam 170 through the target 140. The intensity decreases as the angle from the axis increases. The collimator 190 may therefore be thicker than the shielding material 180, 182. The shielding material 180, 182 may also be thicker at angles closer to the axis, as is known in the art. When the target 140 is made of an appropriate low Z material, with a neutron production threshold above the peak acceleration energy of the source, the shielding 180, 182 and the collimator 190 need not be as thick as when the target is a high Z material because neutrons need not be shielded.

The different components of the source 100 may comprise different materials. For example, a natural copper target 140, a natural copper shielding 180a, 180b, and a natural iron collimator 190 may be used in an X-ray source 100 operating at a peak energy less than the neutron production threshold of iron-57 of 7.646 MeV.

A low Z material may have different stable isotopes with different neutron production thresholds. The isotopic composition of the low Z material therefore has to be considered when selecting appropriate materials for the target 140, shielding 180, 182, and collimator 190 to achieve a desired reduction or elimination of neutrons. To avoid all neutron production, the target 140, shielding 180, 182 and collimating materials 190 are selected such that the neutron production threshold for all isotopes exceeds the peak energy of the particular radiation source. In one example, naturally occurring copper comprises 69.17% of copper-63, which has a calculated neutron production threshold of 10.852 MeV, and 30.83% of copper-65, which has a calculated neutron production threshold of 9.910 MeV. Naturally occurring copper may therefore be used as a target 140, shielding 180, 182, and collimator 190 material for sources operating at a peak acceleration energy below 9.910 MeV, for zero neutron production. Isotopically pure copper-63 can be used as a target 140, shielding 180, 182, and collimator 190 material for sources operating at a peak acceleration energy below 10.852 MeV.

In another example, naturally occurring iron comprises 91.75% iron-56, having a calculated neutron production threshold of 11.197 MeV, 5.85% iron-54, having a calculated neutron production threshold of 13.37 MeV, 2.12% iron-57, having a calculated neutron production threshold of 7.646 MeV, and 0.28% iron-58, having a calculated neutron production threshold of 10.044 MeV. Hence, if naturally-occurring iron is used as a target 140, shielding 180, 182, and collimator 190, an X-ray source operating at a peak acceleration energy of less than 7.646 MeV will not generate neutrons. Isotopically pure iron shielding consisting of only iron-56 could be used as a target 140, shielding 180a, 180b, and collimator 190 in X-ray sources 100 operating at peak acceleration energies up to 11.197 MeV without generating neutrons.

It is noted that if an iron target is used in an X-ray source operating up to (but less than) 10.044 MeV, only about 2.12% of the target 140, shielding 180, 182, and collimator 190 would generate neutrons. Therefore, at a peak energy of less than 10.044 MeV, a naturally occurring iron target 140, shielding 180, 182, and collimator 190 would provide a significantly reduced amount of neutron production, as compared to the use of tungsten, but would not eliminate it. If some amount of neutron generation may be tolerated, but less neutron generation is desired than if tungsten, tantalum, molybdenum, or lead are used, metals may be chosen that provide certain isotopes having neutron production thresholds below the peak energy of the source and certain isotopes having neutron production thresholds above the peak energy of the source. For example, in a source having a peak acceleration energy of 8.5 MeV, the following materials may be used for the target 140, the shielding 180, 182, and the collimator 190: magnesium (10% of which consists of magnesium-24 having a calculated neutron production threshold of 7.331 MeV), iron (2.12% of which consists of iron-57 having a calculated neutron production threshold of 7.646 MeV), nickel (3.63% of which consists of nickel-61 having a calculated neutron production threshold of 7.82 MeV), and zinc (4.10% of which consists of zinc-67 having a calculated neutron production threshold of 7.051 MeV). Different materials from this group may be used for the target 140, the shielding 180, 182, and the collimator 190. Such reductions could also be advantageous in particular applications. Shielding requirements may be reduced, for example.

In another example of a configuration that will reduce but not necessarily completely eliminate neutron production, certain components, such as the target 140, for example, may be an appropriate low Z material, while one or more other components, such as the collimator 190, the target shielding 180, and/or the housing shielding 182 may comprise high Z materials that may generate neutrons, such as tungsten, tantalum or lead, for example. Use of tungsten, tantalum, lead, or other such materials may provide better X-ray radiation generation or shielding, and may therefore be needed for certain components, or part of certain components, to meet performance and size requirements for the source, for example, as discussed below. Therefore, in accordance with another embodiment of the invention, neutron production is reduced by use of an appropriate low Z material for at least one but not necessarily all components, as compared to the use of tungsten, tantalum, molybdenum, or lead for that component.

Use of isotopically purified high Z atom material as a target 140, shielding 180, 182, and/or collimator 190 may also decrease or eliminate neutron generation, and provide some or all of the benefits of the use of a high Z material. For example, isotopically pure tungsten-182, which has a calculated neutron production threshold of 8.064 MeV, can be used in a source having a peak acceleration energy less than 8.064 MeV, without producing neutrons. Isotopically pure molybdenum-94, which has a peak acceleration energy of 9.677 MeV, can be used in a source having a peak acceleration energy less than 9.677 MeV, without producing neutrons. Mixtures of appropriate isotopes that are above the peak acceleration energy can be used, as well. For example, a mixture of molybdenum-96 and molybdenum-94 can be used in a source having a peak acceleration energy less than 8.064 MeV without producing neutrons.

While typically a metal, the target 140, shielding 180, 182, and/or the collimator 190 may be a non-metal, such as carbon. One stable isotope of carbon, carbon-13, which has an abundance of only 1.11%, has a calculated neutron production threshold of 8.071 MeV. The other stable isotope of carbon, carbon-12, which has an abundance of 98.89%, has a calculated neutron production threshold of 18.721 MeV. A source with a peak acceleration energy of less than 8.071 MeV would generate no neutrons, while in a source operating at above 8.071 and less than 18.721, only 1.11% of the carbon (the carbon-13) would generate neutrons. Preferably, a stable form of carbon is used, such as graphite or diamond, for example.

As is known in the art, the neutron production threshold for every isotope of a metal, including low atomic number metals, can be calculated according to the following equation: Threshold (MeV)=(Mass Excess (Z, A-1)+Neutron Mass Excess)-Mass Excess (Z, A).

In the equation above, Z is an atomic number of an atom of an element; A is a mass number (sum of the number of protons and neutrons) of an atom of the element; (Z, A) is an original isotope of an atom of the element (before it loses a neutron as a result of interacting with an X-ray radiation); and (Z, A-1) is a mass of an atom of a resulting isotope of the element after loss of one neutron. Mass Excess (Z, A-1) is the equivalent energy of the mass of the resulting isotope, as compared to carbon-12, which has a mass excess of 0 MeV. The Neutron Mass Excess is a constant equal to 8.071 MeV, which is the energy of the difference between the neutron mass of a particular isotope and the neutron mass of carbon-12, which is set at 0 MeV. The Mass Excess (Z, A) is the equivalent energy of the mass excess of the initial isotope. Mass instead of Mass Excess may be used to determine the threshold as well, as is known in the art.

For example, the neutron production threshold for magnesium-24 is calculated as follows. Magnesium-24 is an original isotope which, after interaction with a beam of X-ray photons, emits one neutron and becomes magnesium-23, a resulting isotope. The mass excess (Z, A-1) of magnesium-23 (-5.473 MeV) is added to neutron mass excess (8.071 MeV). The mass excess (Z, A) of magnesium-24 is then subtracted (-13.933 MeV), yielding a neutron production threshold of 16.531 MeV, as shown below: (-5.473 MeV+8.071 MeV)-(-13.933 MeV)=16.531 MeV.

Large negative mass excesses (Z, A) show that the protons and neutrons in a nucleus of an atom are tightly bound together. An amount of external energy exceeding the absolute value of the negative mass excess is necessary to remove one neutron from the nucleus. In other words, the neutron production threshold of a particular isotope is a minimum energy a photon needs to have to emit one neutron from a nucleus of that isotope.

Calculated neutron production thresholds for each isotope of several materials are summarized in Table I, below. In Table I, some abundances may not add to 100%, due to round off errors. Abundance information is not provided for unstable isotopes. Neutron production thresholds for isotopes of elements ranging from beryllium to uranium may also be found in Neutron Contamination from Medical Electron Accelerators, National Council on Radiation Protection and Measurements, Bethesda, Md., pp. 18-23 (1995), where they are referred to as "separation energies."

TABLE-US-00001 TABLE I Mass Atomic Excess Neutron Production Abundance Element/Isotope Number (MeV) Threshold (MeV) (%) Beryllium-8 4 4.942 Beryllium-9 4 11.348 1.665 100.00% Carbon-11 6 10.650 Carbon-12 6 0 18.721 98.89% Carbon-13 6 3.125 8.071 1.11% 100.00% Magnesium-23 12