Patent Number: 047602660
Section: description

Referring now to the figures of the drawings in detail and first, particularly, to FIGS. 1 and 2 thereof, there is seen a housing 2 which encloses a neutron source 1 and is formed of a hemispherically constructed shell 3 having free ends 4 that are extended in the form of a cylinder. A disc 5 formed of neutron-permeable material such as aluminum, extends perpendicular to the wall of the cylinder and bridges the free ends. The cylindrical and the hemispherical parts of the housing 2 are also formed of aluminum. The inner surface of the wall of the housing 2 is provided with a lining 6 of neutron-moderating material such as polyethylene. A moderator housing 7 is centrally disposed in the hemispherical part of the housing 2. The moderator housing 7 is spherical, is maintained in position by aluminum brackets 8 and contains a filling of moderating material. If plastic or synthetic material is used for the moderator, no moderator housing is needed. The moderator is then merely a plastic or synthetic ball. A canal or channel 9, which is constructed as a stepped hole, penetrates the center of the moderator housing or ball 7 and extends from one side of housing 2 to the other, penetrating the wall thereof. The neutron source 1, which is supported by a shoulder 10 of the canal or stepped hole, is introduced from the end of the stepped hole having the larger diameter. In the vicinity of the housing wall, the stepped hole is constructed as a funnel 11 in order to facilitate the introduction of the neutron source 1. Disposed between the moderator housing 7 and the inner surface of the wall of the housing 2 are several spherical half shells or cups 12 being shown in FIGS. 1, 2 and 4 or parabolic half shells or cups 12' being shown in FIG. 5 and being formed of moderating material, which maintain a distance from each other as well as from the inner surface of the wall of the housing 2 and from the moderator ball 7. This creates intermediate spaces 13 in which the neutrons are reflected by the surfaces of the cups 12. A secondary moderation of epithermal and fast neutrons takes place in the half shells. The neutrons experience their primary moderation in the moderator ball 7. The half shells 12 are fastened to the brackets 8 extending between the moderator housing 7 and the inner surface of the wall of the housing 2. This assures that the spacings, once selected, are maintained. Annular openings 14 of the half shells 12, which are best seen in FIG. 4, form a plane which runs through the center of the moderator housing 7 and parallel to the disc 5 of the cylindrical housing part 4. The disc 5 forms a neutron outlet opening 15 of the housing 2. The operating mode of the device will now be explained by way of a neutron spectrum of a Californium-252 source with the aid of FIG. 3. The relative frequency H is plotted on the ordinate and the energy in MeV is plotted on the abscissa. As may be seen from the branch 16 of the curve, the mean neutron energy of this source approximates 2 MeV. The source configuration is such that the fraction of primary neutrons emitted by the source 1 and having the greatest relative frequency in the associated source spectrum of the source 1 corresponding to a mean neutron energy of 2 MeV (FIG. 3), is primarily slowed down to thermal energy ranges in the moderator ball 7. The shaded region 17 of the spectrum contains primary neutrons of higher energy (E&gt;2 MeV) having thermalizations which proceed stepwise according to the half shells 12. For instance, the primary neutrons having energy intervals in the source spectrum only slightly above 2 MeV, are slowed down in the half shell adjacent the moderator housing 7. The neutrons of the next higher energy intervals are slowed down in the central half shells and those with an even higher energy are slowed down in the outer half shell. Primary neutrons of maximum energy experience their slowdown in the vicinity of the moderator lining 6 of the housing 2. In each interspace 13 furthermore the reflection of the thermal neutrons by the surfaces of the cups 12 and their dispersion in the direction toward the neutron outlet opening 15 of the housing 2 takes place. The reflection can be increased by enclosing the half shells 12 in an aluminum sheath if the housing 2 is disposed in a water seal. The number and thickness of the moderator half shells is determined in dependence on the respective source spectrum of the neutron source 1. A further improvement of the thermal neutron yield is achieved in conjunction with a collimator 18 which is connected directly to the neutron outlet opening 15 and which has a collimator inlet side 20 with a plastic or synthetic plating 21 of predetermined or given thickness. This assures that the higher energy neutrons (E&gt;E.sub.therm) traveling from the moderator housing 7 in the direction of the neutron outlet opening 15, and hence not slowed down by the half shells, also experience a slowdown and reflection. Beyond this, the plastic plating 21 of the collimator 18 reduces an absorption of thermal neutrons which, without such a plating, would take place due to the absorbing effect of the collimator material at the collimator side 20. The lining 6 of housing 2 is extended to the collimation inlet side 20 in one continuous taper forming a part of the plating and thus additionally intensifying the secondary moderation and the reflection. Therefore, the plastic platings 21 and 6 are in effect an additional source of thermal neutrons. A predetermined or given free space 22 in the region between the plane of the openings 14 of the moderator housing 7 and the plastic platings 21 and 6 is thus acted upon by a greatly increased thermal neutron flow. Converting the unoriented neutron flow in the free space 22 into an extracted or isolated, oriented neutron flow is accomplished in a known manner by means of the collimator. Collimators are required, for example, when objects are to be bombarded by an oriented neutron flow in order to determine their internal structure. The collimator 18 has two collimation paths 19 indicated in phantom lines in FIG. 2. The foregoing is a description corresponding in substance to German Application No. P 35 34 760.0 dated Sept. 28, 1985, the International priority of which is being claimed for the instant application. Any material discrepancies between the foregoing specification and the aforementioned corresponding German application are to be resolved in favor of this latter.