Antiscatter grids for low-energy neutron radiography.

Abstract

Background fogging from scattered neutrons is an inherent problem in neutron radiography of hydrogenous substances such as biological tissue (1, 2). This report describes the antiscatter grids that the authors have developed for thermal and epithermal neutron radiography. The design follows the standard technics used in roentgenography except for the choice of materials. Since aluminum is radiolucent to low-energy neutrons, it is employed to separate the neutron opaque planes that define the small acceptance solid angle. Cadmium and indium are the neutron opaque materials used for the thermal and epithermal grids, respectively. Because neutron capture in both cadmium and indium results in prompt gamma cascades, these grids are suggested primarily for neutron radiography tech nics utilizing image transfer foils (3). Atkins (2) has built a similar grid with boron as the opaque material. Even though this grid is significantly less efficient, gamma rays are not produced in boron; thus direct neutron radiography is possible. The indium capture cross section is effective only in the epithermal region where it is characterized by large but isolated resonances. Since the energies of the scattered neutron absorptions must be identical to the image-detection energies if the grid is to be efficient, the indium-aluminum epithermal grid can be used only with an indium transfer foil. On the other hand, the uniformly high cross section of cadmium throughout the thermal region allows any thermal transfer foil to be used with the cadmium-aluminum thermal grid. The dimensions of these linear, parallel (non-focused) grids (4) are 1/8 × 6 × 6 inches. The grid ratio is 15.6∶1 with 77 opaque lines per inch. The grids transmit about 62 per cent of the unscattered neutron flux (less an additional small percentage due to structural irregularities). Both grids are constructed identically. Thin strips of aluminum (0.008 × 0.5 × 6.0 inches) and cadmium or indium (0.005 × 0.5 × 6.0 inches) are bonded together with unfilled epoxy. For ease of subsequent machining, alternate radiolucent and radiopaque strips are stacked in blocks approximately ½ × ½ × 6 inches. Two grid elements per block are then made by sawing the blocks in half, along the long axis, with the cut normal to the plane of the opaque strips. These rough elements are machined with a planing mill to a final height of 1/8 inch to remove the smeared cadmium or indium produced by the sawing. Notches, which are machined into the edges of each element, fit over matching guides on the frame inner edges. To minimize element warping, the elements are mounted in the frame under tension. To eliminate the appearance of grid lines on the radiographs, a mechanical linear motion oscillator was developed to move the grids normal to the neutron beam in a manner analogous to that described by Potter and Bucky (5).