BACKGROUND

This invention relates to neutron imaging systems having neutron shields.

Neutron-sensitive microchannel plates (MCP) can be used as effective detectors in neutron imaging applications. A microchannel plate can be formed by bonding a glass plate between an input electrode and an output electrode, and providing a high voltage direct current (DC) field between the electrodes. The glass plate is perforated with a substantially regular, parallel array of microscopic channels, for example, cylindrical and hollow channels. Each channel, which can serve as an independent electron multiplier, has an inner wall surface formed of a semi-conductive and electron emissive layer.

The glass plate can be doped with, for example, boron-10, which can capture neutrons in reactions that generate lithium-7 and alpha particles. As the lithium-7 and alpha particles enter nearby channels and collide against the wall surfaces to produce secondary electrons, a cascade of electrons can be formed as the secondary electrons accelerate along the channels (due to the DC field), and collide against the wall surfaces farther along the channels, thereby increasing the number of secondary electrons. Alternatively, although having a smaller neutron capture cross-section, the glass plate can be doped with lithium-6, resulting in triton and alpha particle reaction products which likewise produce a cascade of electrons. The electron cascades develop along the channels and are amplified into detectable signals that are electronically registered and processed to construct a digital image. The resultant intensity map or image corresponds to the variation in neutron flux striking the microchannel plate surface.

SUMMARY

This disclosure describes a novel microchannel plate-based neutron imaging system that has a high resolution and contrast. Neutrons from a neutron generator pass through a sample object to be imaged and are detected by a microchannel plate neutron detector. The neutrons pass through a flight tube and optionally one or more microchannel plate collimators. A neutron shield wraps around the flight tube to provide a shielded flight tube to prevent neutrons scattered by objects in the environment from entering the flight tube and reaching the neutron detector. Similarly, a neutron shield wraps around the microchannel plate detector to provide a shielded microchannel plate detector to prevent neutrons scattered by objects in the environment from reaching the neutron detector. As a result, most or all of the neutrons that reach the detector are from a collimated neutron beam that passes through the sample object to be imaged without previously being scattered by objects in the environment. This neutron imaging system has a better resolution and contrast compared to a neutron imaging system that does not provide neutron shielding around the flight tube and the microchannel plate detector.

In a general aspect, a neutron imaging system for generating a neutron image of an object is provided. The neutron imaging system includes a neutron generator, a shielded flight tube, a sample chamber, and a shielded neutron detector. The neutron generator is configured to provide neutrons. The shielded flight tube has an input opening, an output opening, a flight tube wall extending from the input opening to the output opening, and a flight tube shield surrounding the flight tube wall. The flight tube is positioned relative to the neutron generator to enable neutrons from the neutron generator to enter the flight tube through the input opening and exit the flight tube through the output opening, and the flight tube shield is configured to substantially block neutrons outside of the flight tube from entering the flight tube through the flight tube wall. The sample chamber is configured to support a sample object at a position to receive neutrons that pass through the output opening of the flight tube. The shielded neutron detector includes a microchannel plate detector and a detector shield. The microchannel plate detector includes an input electrode, an output electrode, and a microchannel plate defining microscopic channels. The input electrode is configured to be connected to a first voltage, and the output electrode is configured to be connected to a second voltage different from the first voltage. The neutron detector is configured to receive at least a portion of the neutrons that pass through the sample object and generate output signals upon detection of the neutrons. The detector shield is configured to substantially block neutrons other than those that have traveled inside the entire length of flight tube and pass through the output opening of the flight tube from reaching the neutron detector.

In another general aspect, a neutron imaging system is provided. The neutron imaging system includes a neutron generator, an elongated flight tube, a sample chamber, a neutron detector, and a neutron shield that substantially surrounds the flight tube and the neutron detector. The neutron generator is configured to generate a neutron beam. The elongated flight tube has an input opening, an output opening, and a flight tube wall extending from the input opening to the output opening. The flight tube is positioned relative to the neutron generator to enable neutrons from the neutron generator to enter the flight tube through the input opening and exit the flight tube through the output opening, and the flight tube has a length to diameter (L/D) ratio of at least 10. The sample chamber is configured to support a sample at a position to receive neutrons that pass through the output opening of the flight tube. The neutron detector includes a microchannel plate detector having an input electrode, an output electrode, and a glass plate including microscopic channels. The input electrode is configured to be connected to a first voltage, the output electrode is configured to be connected to a second voltage different from the first voltage, and the neutron detector is configured receive at least a portion of the neutrons that pass through the sample and generate output signals upon detection of the neutrons. The neutron imaging system includes circuitry configured to process the output signals of the neutron detector to generate an image or a video of the sample. The neutron shield substantially surrounds the flight tube and the neutron detector, in which the neutron shield has an opening around the input opening of the flight tube to allow neutrons to enter the flight tube through the input opening. The neutron shield includes a material that is configured to substantially block neutrons outside of the flight tube from entering the flight tube through the flight tube wall, the neutron shield is configured to allow neutrons exiting the output opening of the flight tube to reach the neutron detector and substantially block neutrons other than those that have traveled inside the entire length of flight tube and exit the output opening of the flight tube from reaching the neutron detector.

In another general aspect, a neutron imaging system includes a neutron generator, a flight tube, a stage, a neutron imaging module, and a neutron shield. The neutron generator is configured to provide neutrons. The flight tube has an input opening, an output opening, and a flight tube wall extending from the input opening to the output opening. The flight tube is positioned relative to the neutron generator to enable neutrons from the neutron generator to enter the flight tube through the input opening and exit the flight tube through the output opening. The stage is configured to support a sample object at a position to receive neutrons that pass through the entire length of flight tube and exit through the output opening of the flight tube. The neutron imaging module has a neutron-sensitive component that is sensitive to neutrons and configured to receive neutrons that pass through the sample object and generate neutron detection signals that can be used to generate an image or video of the sample object. The neutron shield surrounds at least a portion of the flight tube and at least a portion of the neutron imaging module to block at least a portion of stray neutrons that travel toward the neutron-sensitive component of the neutron imaging module, in which the stray neutrons do not enter the flight tube through the input opening of the flight tube.

Implementations of the neutron imaging system can include one or more of the following features. The neutron shield can include at least one of borated-polyethylene, lithiated-polyethylene, boron carbide (B4C), borated carbide aluminum (Boral®, B4CAl), Boraflex (silicone polymer and boron carbide powder), borated silicone sheet (Flexi-Boron), gadolinium, cadmium, boron-loaded bricks, or lithium-loaded bricks. The neutron shield can surround the flight tube, the stage, and the neutron imaging module, and can have an input opening that aligns with an output opening of the neutron generator, and the input opening of the neutron shield can allow neutrons emitted from the output opening of the neutron generator to reach the flight tube. In some examples, the neutron-sensitive component can include a microchannel plate that has a material that is sensitive to neutrons. In some examples, the neutron imaging module can include an anode that receives charge pulses output from microchannels of the microchannel plate, and a delay line readout that is capacitively coupled to the anode. The neutron imaging system can include a constant fraction discriminator (CFD) and an amplifier to process signals provided by the delay line readout. In some examples, the neutron imaging module can include a phosphor or phosphorous screen that receives charge pulses output from microchannels of the microchannel plate and generates light signals based on the charge pulses. The imaging module can include a camera configured to detect the light signals generated by the phosphor screen. In some examples, the microchannel plate can be disposed in an evacuated hermetically sealed image tube. In some examples, the microchannel plate can be disposed in an actively pumped vacuum enclosure. In some examples, the neutron-sensitive component can include a neutron-sensitive scintillator. In some examples, the neutron imaging module can include a component to convert light from the scintillator to electron charge signals, and a microchannel plate to amplify the electron charge signals. The component for converting the light from the scintillator to electron charge signals can include a photo-cathode. In some examples, the neutron imaging module can include a camera configured to detect the light signals generated by the scintillator. The neutron generator can include a compact linear accelerator, and the neutron generator is configured to produce neutrons by fusing isotopes of hydrogen.

Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. In case of conflict with patent applications or patents incorporated herein by reference, the present specification, including definitions, will control.

