IEEE Nuclear Science Symposium and Medical Imaging Conference, Portland, Oregon, October 20031 Abstract-- In most neutron scattering experiments and in boronneutron capture therapy, the angular spread of the neutronbeam is defined by the quality of the neutron collimator. Atypical collimator consists of a large number of parallel platescoated with neutron absorbing material, and at present theseplates are at least few centimeters in length. In order to obtaincollimation in both vertical and horizontal planes, twoorthogonally aligned collimators are installed in the neutronbeam. We present a new type of high performance neutroncollimator made with Gd-doped microchannel plates (MCPs).Such collimators are only few millimeters thick and the rockingcurve is expected to be even sharper than that of conventional0.5o collimators. While collimation is performed in twoperpendicular planes simultaneously, the geometry of these newcollimators can be changed so that the degree of collimation ineach direction is controlled independently. The modeling of theproposed collimator indicates that for the existing MCPtechnology the rocking curve can be made as sharp as 0.2oFWHM, which can be further improved by currentdevelopments in the MCP technology. The preliminaryexperimental evaluation of our first very thin (only 0.6 mm)MCP collimators confirms the accuracy of our numerical model.Index Terms—Neutron optics, collimators.I. INTRODUCTIONhermal neutron beams are usually collimated with aSoller slit collimator, comprising an array of absorbingfilms separated by neutron-transparent spacers. Currentlyproduced collimators use stacked arrays of stretchedpolyethylene films or thin single crystal silicon wafers withabsorbing coatings [1]-[3]. The thickness of the spacers andthe absorbing coatings are optimized in order to achieve highdegree collimation. In addition, the shorter the collimator, thenarrower the transmitting channel must be in order to obtainthe required collimation. Recent stacking of 160 µm thicksilicon wafers coated with 4 µm of gadolinium metal, forinstance, allows the construction of a 2.75 cm collimator with Manuscript received October 29, 2003.A. S. Tremsin is with the Space Sciences Laboratory, UC Berkeley,Berkeley, CA 94720 USA (e-mail: [email protected]).D. F. R. Mildner is with the National Institute of Standards &Technology, Gaithersburg, MD 20899 USA (e-mail: [email protected]).W. B. Feller is with NOVA Scientific, Inc.660 Main St. P.O. Box 928Sturbridge, MA 01566 USA (e-mail: [email protected]).R. G. Downing is with NOVA Scientific, Inc.660 Main St. P.O. Box 928Sturbridge, MA 01566 USA (e-mail: [email protected]).the peak transmission and rocking curve comparable toconventional collimators [2]. The collimators can be madeeven more compact if the thickness of the spacers can bereduced to a few microns. The structural dimensions ofmicrochannel plates (with typical pore size of 6-15 µmseparated by 2-3 µm walls) meet these requirementsperfectly. Microchannel plates are currently widely used inelectron multiplying and photon counting imagingapplications such as intensified imaging tubes [4],astrophysical imaging and spectroscopic detectors [5],fluorescence imaging [6], as well as in X-ray focusingapplications [7].Modification of the MCP glass mixture by adding aneutron-absorbing material without changing the remainderof the manufacturing process has been suggested by Fraserand Pearson [8] and successfully implemented for thermal[8]- [11] and fast [12] neutron detection. The same modified-glass microchannel plates can be used as very compact andhighly efficient thermal neutron collimators. The latter factwas first confirmed by our computer simulations and thenverified by experimental measurements. In our first attemptswe used microchannel plates doped with 157Gd and 155Gdisotopes, which has a very high thermal neutron capture crosssection of 259000 and 61100 barns, respectively (natural Gdcontains 15.7 % and 14.8 % of 157Gd and 155Gd isotopes,respectively).In contrast to Soller slit collimators, where off-angleneutrons are absorbed by a thin layer coated on a non-absorbing substrate, the neutron absorption in MCP occursinside the entire volume of Gd-doped microchannel platewalls. One of the advantages of microchannel platecollimators is the fact that they can collimate the neutronbeam in two orthogonal planes simultaneously, versus onlyone plane collimation for Soller slits. Although the glassmixture can contain only a limited proportion of Gd atoms,this deficiency is compensated in MCP collimators by thepossibility to produce structures with very large aspect ratios.The ratio of pore length to its width L/D for currenttechnology can be as high as 250:1 (Fig.1.a). Moreover, if theundoped core-glass mixture can be made to be transparentenough to neutrons then collimators with much higher aspectratio can be produced. In that case the pores of the MCP canbe left filled with the core glass and L/D ratio can be as highas ~2000 (Fig.1.b). We can also make the walls ofmicrochannel plates from undoped glass and have the poresfilled with neutron absorbing glass, so that the walls of theMCP are transparent to neutrons and the pores are serving asthe absorbers for collimation. Our numerical model ofVery Compact High Performance MicrochannelPlate Thermal Neutron CollimatorsAnton S. Tremsin, David F. R. Mildner, W. Bruce Feller, R. Gregory DowningTIEEE Nuclear Science Symposium and Medical Imaging Conference, Portland, Oregon, October 20032collimation efficiency of these doped microchannel platesindicates that they can be a very attractive alternative to theexisting neutron collimators.Fig. 1.a. A section of a circular pore microchannel plate with core glassetched out (not to scale).Fig. 1.b. A section of a circular pore MCP with core glass not yet etched (notto scale).II. MCP MANUFACTURING TECHNOLOGYThe MCP manufacturing process typically starts withappropriate clad glass and a suitable soluble core glass. Forneutron collimation applications one of the glass mixtures isto be doped with neutron absorbing material (NatGd in ourfirst attempts). These glasses are fused together in a drawprocess. The resulting fibers, consisting of two different glassmixtures, are stacked into a predefined pattern, fused again,and drawn for the second time. At this point, the fusedbundles are stacked for the second time and fused togetherinside a solid border glass to provide a strong