Developments

• Scintillators based on polyvinyltoluene and silicone rubbers
• Scintillators loaded with Gd, B, group 3, lanthanides, actinides, and select other elements
• UV-, blue-, blue-green-, and green-emitting scintillators (patents applied for)
• Patented method to add neutron sensitivity to gamma ray spectrometers (US Patent 6011266)

Description Gadolinium has the highest cross section for the capture of thermal neutrons among the naturally occurring elements. However, there have been few reports of the fabrication of Gd-loaded solid organic scintillators to date. Such scintillators would offer the advantages of ease of manufacture, forming, custom emission spectra, and, with only 1% Gd by weight, 100% absorption efficiency in a 1-cm thickness. Under the sponsorship of the Department of Energy, workers at Oak Ridge have discovered a method to make these scintillators a reality.

The photograph above shows samples of Gd-loaded plastic; the disk at the left is 6 cm in diameter (click on image for expanded view)

(click on image for expanded view)

In addition to plastics, silicone rubbers have been evaluated as carriers for scintillating materials. Silicone rubber has the advantages of flexibility, low shrinkage on curing, room temperature curing, curing in the presence of oxygen, and ability to m aintain its mechanical properties to over 100ºC. The material may also be fabricated into light pipes having an essentially undetectable optical joint to rubber scintillator. Methods have been found to dissolve Gd and other lanthanides, actinides, B and other group 13 elements, and a variety of fluors. The photograph above shows two samples of blue-emitting rubber scintillator and a vial containing four separate pieces of green-emitting material, which were polymerized sequentially in place. Optical interfaces (except for the uppermost one) are nearly i nvisible. Individual disks are approximately 2.5 cm in diameter and 1 cm thick.

The photograph at the right shows silicone rubber light pipe polymerized in flexible plastic tubing and bent at 90º. Sunlight is incident from the upper left and is clearly seen exiting the light pipe toward the viewer. Scintillator or light pipe polyme rized in flexible tubing can be pushed into plumbing or be positioned to follow the contour of convoluted piping to detect radioactive material hidden or trapped in obscure places. Oak Ridge researchers have obtained a patent covering the application of boron or boron-rich materials to gamma ray spectrometers to confer neutron sensitivity without compromising gamma ray sensitivity. By coating the spectrometer with 1 to 2 mm of boro n, the single gamma ray emitted from the 10B(n,a) 7Li reaction produces a photopeak at 478 keV in a spectrometer made from a high-Z, high density detecting medium. Several methods of fabricating self-su pporting plates and cylinders of elemental boron, boron carbide, and boron-loaded epoxy have been developed.

(click on image for expanded view)

(click on image for expanded view)

The photograph at the right shows, from left to right, a cadmium tungstate crystal, a miniature photomultiplier module, a cesium iodide crystal, and a semicircular sample of solid boron carbide. The scale is in inches. A palm-sized neutron/gamma ray det ector is readily made by covering the cadmium tungstate crystal with 1-mm thick slices of boron carbide and mounting it on the photomultiplier module. Since the boron is so close to the crystal, gamma rays subsequent to neutron capture have a high probab ility of entering it and generating a photopeak. The mean free path of 478 keV gamma rays in cadmium tungstate is approximately 1 cm, making the photopeak efficiency for even small crystals useful. In addition, since boron (or boron carbide) has such lo w Z, 1-2 mm of this material has essentially no effect on gamma rays above 40 keV. These materials and components are configurable to satisfy many radiation detection requirements.

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