Remote detection of fissile material

• US 6,448,562 B1
• Filed: 07/29/1999
• Issued: 09/10/2002
• Est. Priority Date: 07/29/1999
• Status: Expired due to Fees

• Current Assignee: L-3 Communications Corporation (L3Harris Technologies, Inc.)
• Original Assignee: Jaycor, Inc. (Titan Corporation)
• Inventors: Blackwell, William, Zukic, Muamer, Fry, Edward L., Hill, Charles H., Seidler, William A.
• Primary Examiner(s): Hannaher, Constantine
• Assistant Examiner(s): Gabor, Otilia
• Application Number:
  US09/363,955
• Time in Patent Office: 1,139 Days
• Field of Search: 250/372, 250/358.1, 250/363.06
• US Class Current: 250/372
• CPC Class Codes: G01T 3/00 Measuring neutron radiation G01T5/00 takes precedence

Abstract

A remote sensor for detecting a nuclear source, comprises: a Field-of-View (FOV) structure having an aperture therethrough of area A and a Field-of-View (FOV) angle, the FOV angle centered on a source, and subtending a solid angle Ω to the nuclear source; a plurality of optical filters for filtering photons outside a selected Ultraviolet (UV) band and for transmitting in-band photons according to a selected Transfer Function defining an out-of-band rejection ratio and an in-band transmittance ratio for the selected UV band, the Transfer Function supporting a sensor sensitivity S at the selected UV band for the Field-of-View (FOV) structure to support a Signal-to-Noise ratio of greater than one (1) to detect a nuclear source of nuclear material having a brightness of at least about IR, the selected UV band being selected such that naturally occurring in-band photons are at a brightness of about less than 10⁴ R during daylight and are not naturally occurring at night up to about 20 km Earth altitude, and the in-band photons have a mean free path in air large enough and a radiative emission rate short enough to allow the in-band photons to reach the Field-of-View structure; and an optical camera having pixels configured to receive the in-band photons transmitted through the optical filters for measuring the in-band photons, the in-band photons being emitted from airglow caused by ionizing radiation from the nuclear material.

Claims

1. A remote sensor for detecting a nuclear source, the sensor comprising:

a Field-of-View (FOV) structure having an aperture therethrough of area A and a Field-of-View (FOV) angle, the FOV angle centered on a source, and subtending a solid angle Ω

to the nuclear source;

a plurality of optical filters for filtering photons outside a selected Ultraviolet (UV) band and for transmitting in-band photons according to a selected Transfer Function defining an out-of-band rejection ratio and an in-band transmittance ratio for the selected UV band, the Transfer Function supporting a sensor sensitivity S at the selected UV band for the Field-of-View (FOV) structure to support a Signal-to-Noise ratio of greater than one (1) to detect a nuclear source of nuclear material having a brightness of at least about 1R,

the selected UV band being selected such that naturally occurring in-band photons are at a brightness of about less than 10⁴ R during daylight and are not naturally occurring at night up to about 20 km Earth altitude, and the in-band photons have a mean free path in air large enough and a radiative emission rate short enough to allow the in-band photons to reach the Field-of-View structure up to about 20 km Earth altitude; and

an optical camera having a Photomultiplier Tube (PMT) or CCD pixels configured to receive the in-band photons transmitted through the optical filters for measuring the in-band photons, the in-band photons being emitted from airglow caused by ionizing radiation from the nuclear material.

2. The remote sensor of claim 1 wherein the Signal-to-Noise ratio is computed as:

 

(S/n)It/{square root over ((S/n)It+(S/n)(cont)I(cont)t)}

wherein S denotes the sensor sensitivity at the selected (UV) band, n denotes a number of pixels, “

cont”

denotes a contaminant signal, I denotes an intensity of the nuclear source at the selected UV band, and t denotes a pixel integration time.

3. The remote sensor of claim 2 wherein the sensor sensitivity S is computed by:

$S=\frac{{10}^6}{4<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\pi <br><br>}}<br><br>A<br><br>\text{\hspace{1em}<br><br>}<br><br>\mathrm{\Omega <br><br>\varepsilon <br><br>}<br><br>\text{\hspace{1em}<br><br>}<br><br>Q$

wherein;

ε

is the transmittance ratio, and Q denotes a quantum efficiency for the sensor.

4. The remote sensor of claim 3 wherein the selected UV is selected from a group of bands consisting of 391.4 nm, 247 nm, 280 nm, 308 nm, 320 nm, 337.1 nm, 346.6 nm, and 427.8 nm.

5. The remote sensor of claim 1 wherein the pixels are photo multiplier tube (PMT) pixels.

6. The remote sensor of claim 4 wherein the transfer function defines an out-of-band rejection ratio for ultraviolet as 10⁻

6, an out-of-band rejection ratio for visible light as 10<sup>−

11</sup>, an out-of-band rejection ratio for infrared as 10<sup>−

8</sup>, and the transmittance ratio for in-band photons as 90%.

7. The remote sensor of claim 1 wherein the selected UV band is less than 200 nm.

8. The sensor of claim 7 wherein the optical filters comprise π

multilayers of a LaF₃ layer and a MgF₂ layer.

9. The remote sensor of claim 1 wherein the selected UV band within 200 nm to 300 nm.

10. The remote sensor of claim 9 wherein the optical filters comprise alternating π

multilayers of a Sc₂O₃ layer and a SiO₂ layer.

11. The remote sensor of claim 1 wherein the selected UV band is 300 nm to 350 nm.

12. The remote sensor of claim 11 wherein the optical filters comprise alternating π

multilayers of a HfO₂ layer and a SiO₂ layer.

13. The remote sensor of claim 1 wherein the selected UV band is above 350 nm.

14. The sensor of claim 13 wherein the optical filters comprise alternating non-repeating π

multilayers of a HfO₂ layer and a SiO₂ layer, the π

multilayers tuned for a maximum throughput at the selected UV band.

15. The remote sensor of claim 1 further comprising:

an optical platform containing the plurality of optical filters and the optical camera.

16. The remote sensor of claim 15 wherein the selected UV band is 391.4 nm.

17. The remote sensor of claim 16 wherein the selected UV band is 391.4 nm and further wherein the Field-of-View structure is configured to achieve an f number (f#) supporting a required sensor sensitivity at the selected band.

18. The remote sensor of claim 17 further comprising:

an image processor coupled to the optical camera, for mapping radiation corresponding to the solid angle Ω

to at least 1000 pixels.

19. The remote sensor of claim 18 wherein optical information contained by each one of the at least 1000 pixels is further mapped to another set of pixels to further enhance resolution of an optical image produced by the image processor.

23. The remote sensor of claim 1 wherein the pixels are charged coupled device (CCD) pixels.

20. A method of remote sensing of nuclear materials, the method comprising the steps of:

selecting an ultraviolet band such that naturally occurring in-band photons are at a brightness of about less than 10⁴ R up to 20 km Earth'"'"'s altitude are not naturally reflected by Earth'"'"'s surface at night, and the in-band photons have a mean free path in air large enough and a radiative emission rate short enough to allow the in-photons to reach a Field-of-View structure coupled to the sensor;

defining a Transfer Function defining an out-of-band rejection ratio and an in-band transmittance ratio for the selected UV band, the Transfer Function supporting a high enough sensor sensitivity S at the selected UV band and with the Field-of-View (FOV) structure to support a Signal-to-Noise ratio of greater than one (1) for a source of the nuclear material, the source having a brightness of at least 1R at the selected UV band;

filtering out-of-band photons received through the Field-of-View structure with the optical filters having reflective properties according to the selected Transfer Function;

transmitting in-band photons received through the Field-of-View structure with the optical filters having reflective properties set according to the selected Transfer Function; and

counting transmitted in-band photons from the optical filters at a photon imaging device.

21. The method of claim 20 further including the step of:

mapping counted transmitted in-band photons to at least 1000 pixels.

22. The method of claim 21 further including the step of:

mapping each one of the at least 1000 pixel counts to another set of pixels to increase image resolution.