Variable-ratio neutron-gamma ray source

1. A variable-ratio neutron-gamma ray source comprising:

a neutron generator for generating neutrons;

a shield for reducing external radiation exposure;

a collimator for collimating the neutrons into a neutron beam that traverses the shield; and

an external gamma target for generating a dual neutron-gamma ray beam from the neutron beam, wherein the dual neutron-gamma ray beam has a variable neutron-gamma ratio as a function of a thickness of the external gamma target.

2. The source of claim 1 , wherein the external gamma target comprises at least one of graphite or aluminum.

3. The source of claim 1 , wherein the external gamma target comprises at least one of lithium, beryllium or lead.

4. The source of claim 1 , wherein the external gamma target comprises at least one of water or paraffin.

5. The source of claim 1 , wherein the external gamma target comprises a plurality of movable plate targets having different individual thicknesses.

6. The source of claim 1 , wherein the external gamma target comprises a plurality of interchangeable rotary targets having different individual thicknesses.

7. The source of claim 1 , wherein the external gamma target comprises a pivoting half-cylindrical drum having different thicknesses as a function of position.

8. The source of claim 1 , wherein the neutron generator generates neutrons via a fusion process, and wherein the neutrons have a characteristic energy of approximately 14 million electron volts (14 MeV).

9. The source of claim 8 , wherein the external gamma ray target generates gamma rays via inelastic scattering of the neutrons, and wherein the gamma rays have a characteristic energy of approximately four point four million electron volts (4.4 MeV).

10. A variable-ratio neutron-gamma ray detection system comprising:

a source for interrogating a container, the source comprising:

a neutron generator for generating a neutron beam,

an external gamma target for converting the neutron beam into a dual neutron-gamma ray beam having a neutron-gamma ratio, and

an actuator for varying the neutron-gamma ratio by positioning the external gamma target; and

a detector array for detecting an object in the container as a function of neutron and gamma ray attenuation in the beam.

11. The system of claim 10 , wherein the actuator comprises a linear actuator for positioning the external gamma target.

12. The system of claim 10 , wherein the actuator comprises a rotary actuator for rotating the external gamma target.

13. The system of claim 10 , wherein the actuator comprises a pivoting actuator for pivoting the external gamma target.

14. The system of claim 10 , wherein the external gamma target comprises a neutron moderator.

15. The system of claim 10 , wherein the external gamma target comprises a neutron multiplier.

16. The system of claim 10 , wherein the neutron beam has a characteristic neutron energy of at least two point five million electron volts (2.5 MeV).

17. The system of claim 16 , wherein the dual neutron-gamma ray beam has a characteristic gamma ray energy of at least one million electron volts (1 MeV).

18. A method for detecting an object, the method comprising:

illuminating the object with a dual neutron-gamma ray beam;

measuring a neutron-gamma attenuation ratio in the dual neutron-gamma ray beam;

detecting the object as a function of the neutron-gamma attenuation ratio; and

varying a neutron-gamma ratio of the dual neutron-gamma beam by positioning an external gamma target.

19. The method of claim 18 , wherein illuminating the object comprises illuminating a modular cargo container with a dual neutron-gamma ray beam having a characteristic neutron energy of approximately fourteen million electron volts (14 MeV), and a characteristic gamma ray energy of approximately four point four million electron volts (4.4 MeV).

20. The method of claim 18 , wherein detecting the object comprises analyzing the composition of the object as a function of the neutron-gamma attenuation ratio.