A novel neutron lens based on reflection has recently been developed that could greatly benefit neutron imaging and other neutron scattering techniques.
A practical lens helps solve two of the challenges in developing neutron instrumentation: neutron sources are weak and it is inefficient to focus neutrons due to the small neutron refractive index. For instance, to obtain high resolution neutron images, one must strongly collimate the neutron beam, which sacrifices even more neutron flux. With a lens, collimation is no longer needed to achieve spatial resolution so that one can increase the usable flux by a least a factor of 100! The novel lens is a Wolter Optic similar in design to the telescope of the CHANDRA x-ray observatory. NASA has developed a new method to create a lens using nested, thin foils of nickel, so that the lens will have high neutron or x-ray throughput and focal lengths on the order of a few meters. Also, the lens can also be made to have a magnification of 10, which will improve the achievable spatial resolution of neutron imaging to 1 µm.
MIT, NASA and NIST are collaboratively developing the world’s first practical white beam neutron microscope. A proof-of-principle experiment was conducted with a prototype lens that had a magnification of 4. As shown in Figure 1, the lens performed well, creating images from a divergent source just like a microscope with magnification 4. The spatial resolution of this prototype lens was about 70 µm and can be improved upon with a technique called differential deposition. Demonstration of higher resolution optics is expected in 2016.
Wolter Optics Principle and Challenges
Wolter optics are composed of two conic sections, i.e. paraboloids, hyperboloids, and ellipsoids. Shown in Figure 2 is a sketch of a configuration utilizing an ellipsoid and hyperboloid for neutron image magnification. One designs the optic by aligning the foci of the “virtual hyperbolic” and elliptical sections. The object is placed at the focus of the hyperbolic section of the mirror. Neutrons that pass through the object and reflect for the first time from the hyperbolic mirror section also appear to originate from the focus of the ellipse on the right hand side. When the neutron makes the second reflection from the elliptical section of the mirror, it is directed to the focus of the elliptical section on the left, where on places the detector. Since the mirrors are conic sections, the focal plane is curved, and the curvature increases with length and decreases as the square root of the radius.
Figure 2. Top, the basic principle of how Wolter optics form neutron images, including the primary aberration of field curvature. In addition to reducing field curvature, another design goal is to create a lens with a large enough depth of focus to look at real objects.
On the bottom of the figure is a plot showing the fraction of neutrons from the neutron guide that are focused onto the detector. The black line compares the fluence rate at BT2 for high resolution imaging to that for the cold neutron imaging instrument at NG-6 with a single focusing mirror. The focusing mirror is composed of two parabolic sections that are 10 cm in length and have a total focal length of 7 m and form a neutron image with magnification of one. By increasing the divergence of the guide by using a supermirror coating with twice the reflectivity of pure nickel, more neutrons are focused by the mirrors and allow the use larger diameter mirrors.
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