Gamma-ray spectrometry

• US 20040000645A1
• Filed: 04/10/2003
• Published: 01/01/2004
• Est. Priority Date: 10/11/2000
• Status: Active Grant

• Current Assignee: Symetrica Limited
• Original Assignee: Symetrica Limited
• Inventors: Ramsden, David, Meng, Ling-Jian, Ignatov, Sergei, Ivanov, Oleg
• Granted Patent: US 6,940,071 B2
• Time in Patent Office: Days
• US Class Current: 250/361R
• CPC Class Codes: G01T 1/20 with scintillation detectors

Abstract

Different geometries of scintillation spectrometer are disclosed which provide improved resolution over prior art scintillation spectrometers. By ensuring that photons generated by scintillation events occurring in different locations within the scintillation material generate similar light profiles on the photo-detector, the output signal is made less sensitive to the initial interaction site. This can be achieved in a number of ways, such as: by limiting the exit window of the scintillation crystal to a smaller detector, by introducing an optical spacer (94) between the scintillation crystal and detector (99), and/or by making the crystal longer than necessary to stop the gamma rays. A principal advantage of these new geometries is that deconvolution of the raw-data is more effective, thus improving resolution.

Claims

1. A gamma-ray spectrometer comprising a scintillation body for receiving gamma-rays and creating photons therefrom, and a photon detector having a sensitive area facing the scintillation body so as to receive and detect the photons, wherein the sensitive area of the photon detector presented to receive the photons is no more than 10% of the surface area of the scintillation body.

2. A gamma-ray spectrometer according to claim 1, wherein the sensitive area is between 1% and 10%, more preferably 1% and 5%, of the surface area of the scintillation body.

3. A gamma-ray spectrometer according to claim 1 or 2, wherein the scintillation body has at least a portion of its surface which is curved, and wherein the sensitive area is arranged tangentially to the curved surface portion.

4. A gamma-ray spectrometer according to claim 3, wherein the scintillation body is generally spherical.

5. A gamma-ray spectrometer according to any one of the preceding claims, wherein the photon detector is separated from the scintillation body by a light guiding spacer having a length between 0.3 and 10 times the width of the scintillation body.

10. A gamma-ray spectrometer according to any one of the preceding claims, wherein the photon detector is based on a semiconductor element.

11. A gamma-ray spectrometer according to any one of claims 1 to 9, wherein the photon detector is a PIN diode, a drift diode, a hybrid photodiode (HED) or an avalanche photodiode (APD).

12. A gamma-ray spectrometer according to any one of claims 1 to 9, wherein the photon detector is a photo-multiplier tube (PMT).

13. A gamma-ray spectrometer according to any one of the preceding claims, wherein the scintillation body is packed in a reflective material.

20. A gamma-ray spectrometer according to any one of the preceding claims, further comprising a temperature sensor for measuring temperature of the scintillation body.

26. Use of a gamma-ray spectrometer according to any one of claims 1 to 19:

for determining radioactivity levels in a soil sample,

in an airborne apparatus for mapping radioactivity levels,

for determining radioactivity levels in a liquid sample, such as aqueous effluent,

in a cooling circuit of a nuclear reactor for detecting presence of one or more fission products in the cooling circuit,

for measuring elemental composition of crushed rock by irradiating the crushed rock with neutrons and detecting gamma rays emitted as a consequence,

for verifying the elemental composition of cement before firing by irradiating the cement with neutrons and detecting gamma rays emitted as a consequence,

for measuring the calorific content of coal by irradiating the coal with neutrons and detecting gamma rays emitted as a consequence,

for detecting radioactive material passing through a detection archway or baggage control apparatus, or contained in a shipping container,

for detecting explosives passing through a detection archway or baggage control apparatus, or contained in a shipping container,

for detecting narcotics passing through a detection archway or baggage control apparatus, or contained in a shipping container,

for detecting buried landmines by illuminating the ground with neutrons and detecting gamma rays emitted as a consequence,

for detecting and imaging contraband materials using multi-energy gamma-ray computed tomography (MEGA-CT),

for analysis of the U/Ti ratio in rock chippings generated during drilling of an oil well,

for rock composition analysis by down-the-well neutron analysis,

for rock core analysis,

for radio guided surgery,

in a gamma ray imaging system for imaging radioactive tracers in a patient,

for thickness measurements with a monochromatic beam of gamma rays,

for measuring the cement content of concrete from the natural radioactivity levels in the constituent

for measuring the water content of concrete by illuminating the concrete with neutrons

for measuring the water content of concrete by illuminating the concrete with neutrons.

6. A gamma-ray spectrometer comprising a scintillation body for absorbing gamma rays at locations within the scintillation body and creating photons therefrom, and a photon detector arranged to receive and detect the photons, wherein the photon detector is separated from the scintillation body by a light guiding spacer having a length between 0.3 and 10 times the width of the scintillation body so as to spread the photons so that their intensity profile across the photon detector is relatively invariant to the locations where the gamma rays are absorbed in the scintillation body.

7. A gamma-ray spectrometer according to claim 6, wherein the length of the light guiding spacer is at least 0.4 or 0.5 times the width of the scintillation body.

8. A gamma-ray spectrometer according to claim 6 or 7, wherein the length of the light guiding spacer is no more than 1, 2, 4, 6 and 8 times the width of the scintillation body.

9. A gamma-ray spectrometer according to claim 6, 7 or 8, wherein the light-guiding spacer is packed in a reflective material.

14. A gamma-ray spectrometer comprising a scintillation body for absorbing gamma-rays of at least a first energy at locations within the scintillation body and creating photons therefrom, and a photon detector arranged to receive and detect the photons, wherein the scintillation body is dimensioned to have a length of at least twice the attenuation length of gamma rays of the at least first energy in the scintillation body, so as to spread the photons so that their intensity profile across the photon detector is relatively invariant to the locations where the gamma rays are absorbed in the scintillation body, wherein the scintillation body is made of CrI and has a length of at least 0.56, 5.6 or 48.8 mm, the scintillation body is made of NaI and has a length of at least 0.86, 8.2 or 62.4 mm, the-scintillation body is made of LSO and has a length of at least 1.2, 2,2 or 24.4 mm, or the scintillation body is made of YAP and has a length of at least 1.96, 14.0 or 44.8 mm.

15. A gamma-ray spectrometer according to claim 14, wherein the scintillation body comprises an array of pillars.

16. A gamma-ray spectrometer according to claim 15, wherein the pillars are lately isolated from each other with reflective material.

17. A gamma-ray spectrometer according to any one of claims 14 to 16, wherein the photon detector comprises an array of detection elements.

18. A gamma-ray spectrometer according to claim 17, wherein the array of detection elements is made of an array of discrete photodiodes, a monolithic array of photodiodes, a multi-pixel hybrid photodiode, or an electron-bombarded charged coupled detector (CCD).

19. A gamma-ray spectrometer according to claim 17 or 18, fur comprising a coded-aperture mask, collimator or pin hole arranged before the scintillation body to allow for imaging across the array of detection elements.

21. A method of gamma-ray spectroscopy comprising:

providing an object to be analysed based on gamma rays, and

collecting energy-loss data for the object with a gamma-ray spectrometer comprising a scintillation body for receiving gamma-rays and creating photons therefrom, and a photon detector having a sensitive area facing the scintillation body so as to receive and detect the photons, wherein the sensitive area of the photon detector presented to receive the photons is no more than 10% of the surface area of the scintillation body.

24. A method according to claim 21, 22 or 23, further comprising:

processing the energy-loss data by deconvolution using a response function computed for the gamma-ray spectrometer.

25. A method according to claim 24, wherein the energy loss-data is collected with temperature data indicating the temperature of the scintillation body, and the processing compensates the energy-loss data in respect of temperature using the temperature data.

22. A method of gamma-ray spectroscopy comprising:

providing an object to be analysed based on gamma rays, and

collecting energy-loss data for the object with a gamma-ray spectrometer comprising a scintillation body for absorbing gamma-rays and creating photons therefrom, and a photon detector arranged to receive and detect the photons, wherein the photon detector is separated from the scintillation body by a light guiding spacer having a length between 0.3 and 10 times the width of the scintillation body so as to spread the photons so that they more uniformly illuminate the photon detector.

23. A method of gamma-ray spectroscopy comprising:

providing an object to be analysed based on gamma rays of at least a first energy, and

collecting energy-loss data for the object with a gamma-ray spectrometer comprising a scintillation body for absorbing the gamma-rays and creating photons therefrom, and a photon detector arranged to receive and detect the photons, wherein the scintillation body is dimensioned to have a length of at least twice the attenuation length of gamma rays of the at least first energy in the scintillation body, so as to spread the photons so that they more uniformly illuminate the photon detector.