New study device of the media, propagating the neutrons

摘要

887,197. Moderating neutrons. COMMISSARIAT A L'ENERGIE ATOMIQUE. July 18, 1958 [July 22, 1957], No. 23149/58. Class 39(4) Apparatus for investigating the neutron propagating characteristics of a medium comprises means for supporting a sample of the medium, neutron source means for irradiating the sample from a surface substantially completely surrounding the sample or for simulating irradiation from a surface substantially completely surrounding the sample, and means for detecting the resulting neutron flux at points within the sample. In the arrangement shown in Fig. 1 the medium 1 may comprise a cylindrical beryllium oxide-uranium lattice arranged between two guard sections 30, 31 on a fixed support. The guard section may comprise graphite-hydrogen-uranium lattices, the hydrogen being in the form of polyethylene tubes. The fixed structure is surrounded by a rotatable tower 4 driven by a motor 8. The tower carries two diametrically opposed neutron sources 12, 13 which are reciprocated up and down by a drive taken through a coupling and reversing arrangement from a gear 23 meshing with a rack 24. It is arranged that the sources move from top to bottom in 12.25 revolutions of the tower and then remain stationary for 3 revolutions; thus a complete cycle is half an integral number of revolutions and the sources 12, 13 exchange positions every cycle thereby ensuring perfect compensation for any minor inequalities between the sources. Each revolution may take 30 seconds, and a complete cycle of 30.5 revolutions thus takes about 15 minutes. The detectors may be rods of manganese or indium, the induced activity of which is measured to give the integrated flux. Each neutron source block, Fig. 7, comprises a sodium-beryllium source 40 surrounded by a bismuth shield 41 for screening the gamma rays and a cylindrical block 42 of uranium which also acts as a gamma ray shield and retards fast neutrons by inelastic collisions. In front of the source is a beryllium oxide stack 39 containing uranium rods 35-38 to act as a multiplier, and behind the source is a graphite reflector 43 and a layer of paraffin 44. Two sheets of cadmium 45, 46 line the side surfaces of the block to absorb thermal neutrons.