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How the Glass Works

 

Neutron Sensitive Glass Fibers

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Nucsafe uses a variety of advanced technologies in our products. One of our core technologies is a scintillating glass fiber for neutron detection. It represents a compelling alternative technology for thermal neutron detection versus 3He and BF3 tubes. Its key advantages over gas tubes are its sensitivity, ruggedness, flexible geometry, fast timing, and  dynamic range. More details can be found in a primer on the 6Li neutron sensitive glass fiber technology.

 

The scintillating glass fibers work by incorporating 6Li and Ce3+ into the glass bulk composition. The 6Li has a high cross-section for thermal neutron capture. The capture reaction produces a tritium ion and an alpha particle and kinetic energy. The triton ion will likely interact with a Cerium ion through Coulombic interactions. This interaction results in the excitation of one of the Cerium atoms electrons. The resulting de-excitation of the electron produces a flash of light. This scintillation propagates through the glass fiber which acts as its own wave guide. The fibers are optically coupled to a photo-multiplier tube. At this point, the light is multiplied and converted to a electronic pulse that can be processed and counted.

 

Neutron Detector Technology - The Long and Short of It


Both 3He gas tube and PUMA 6Li scintillating glass fiber optic sensors can be incorporated into a multitude of detector geometries. Each detector technology has benefits and disadvantages. Nucsafe offers both types of neutron detectors in order to use the best technology for each application. In fact, Nucsafe can offer hybrid systems incorporating both types of detector technology in a single system to offer the best of both worlds.


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He gas tubes are the most commonly employed thermal neutron detector. They are a type of proportional detector, the most basic type of radiation detector. Each tube has an active length and diameter and are filled to a certain gas pressure. The length times the diameter yields a volume and the number of atmospheres of pressure defines how much gas is in that volume. The universal gas law can be used to compute the number of moles or atoms of gas that a particular 3He tube contains. Gas pressures range from 2 to 20 atmospheres and typical tubes are 3 to 10 atmospheres. The tubes are typically made of Al or Stainless Steel. The major benefit of 3He detectors is their insensitivity to gamma rays. The major disadvantages is their slow response, sensitivity to vibration and rigid geometry.

 

Thermal neutrons are captured by the 3He atoms that produce charged particles. The charged particles ionize other gas in the tube giving rise to ions and electrons that are accelerated toward their respective contacts and inducing a current in the preamplifier circuitry.

 

The 6Li scintillating glass fiber sensors are a solid state alternative to 3He gas tubes. The glass contains both the neutron sensitive atom 6Li and a scintillator of Ce. The fibers offer greater versatility to match the sample geometry and are flexible enabling wearable devices . These fibers are laid down in ribbons that can vary in length and number, ranging from 1 centimeter to 2 meters in length and 1cm to 16cm in width. Larger widths are made using multiple ribbons. The sensors can consist of a single fiber or tens of thousands of fibers, depending on the neutron flux to be detected. Each sensor is constructed of multiple layers of fiber interleaved with polyethylene, Teflon or other materials specific to the application. Because it is a glass, the distribution of the atoms is uniform throughout the glass fiber providing a uniform efficiency over the entire area of the detector. Another important advantage of the fiber technology is their fast scintillation time. By using high-speed electronics, these detectors have an improved response time. Ionizing radiation interacts with the scintillating fibers and produces light. This light is trapped in the fiber and goes to the fiber end. Here, conversion to an electrical signal takes place. This electrical signal is interpreted as either a neutron or gamma ray interaction, depending on its size.

 

The principal disadvantage of the glass fibers are their higher sensitivity to gamma rays. Using them in high gamma ray fields that are more than 100,000 times higher than the neutron flux does give rise to false neutron counts. Because the neutron count rate in the ambient environment is so low, even a few false counts can give rise to a count rate that is significant with respect to background.

Nucsafe in conjunction with Pacific Northwest National Laboratory has been selected as the developer of a 1999 R&D100 award winning technologies for the PUMA. r&d100.jpg (5227 bytes)
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