Patent Number: 043022855
Section: summary

FIELD OF THE INVENTION The present invention relates to highly sensitive nuclearphysical means for quantitative determination of an impurity content in various materials, more specifically, for determination of content of lightweight elements and gas impurities such as oxygen, nitrogen, silicon and the like; in particular it concerns neutron activation analysis installations. The invention may be used at metallurgical and chemical plants, at general metallurgy and machine-building factories, in various branches of industry such as aviation, electronics and the like, as well as in agriculture. It may also be used to advantage for research in solid-state physics and material studies and in monitoring of semi-finished products. Use of aluminum, magnesium and their alloys as well as oxygen-free copper and titanium-magnesium alloys is preferable in the production of lightweight metals and alloys. Use of niobium, molibdenum, tantalum, tungsten, rhenium and like elements as well as special steels and alloys is preferable in the production of refractory and heat-resistant metals. PRIOR ART Known in the art are neutron activation analysis installations (cf. J. L. Duggan and I. L. Morgan "Industrial Applications of Small Accelerators", IEEE Transactions on Nuclear Science, 1975, NS-22, No. 3, pp 1216-1228) comprising a neutron generator whose target chamber communicates through a transport means with a test sample receiving and loading assembly which, in its turn, communicates with a test sample impurity concentration measuring unit. Such installations are used for quick determination of impurities, primarily, oxygen whose minimum concentration in the material is 5.multidot.10.sup.-3 % by mass. Materials having such an oxigen content may not be regarded as pure or highly pure. The known neutron activation analysis installations may not be used with pure and highly pure materials having an oxigen content of 1.multidot.10.sup.-3 % by weight, maximum, a disadvantage associated with the fact that the surface of test samples is contaminated before or during the analysis. The test sample surface contamination, say, whith oxigen may be due to its sorption from the atmosphere (or vacuum medium), moisture or oil vapour condensation and mechanical impurities from transport means. For example, the test sample surface may be heavily contaminated due to the injection of 16.sub.N recoil nuclei in oxygen determination via the 16.sub.O(n,p) 16.sub.N reaction from the atmosphere and the surface of object adjacent the sample during irradiation. The formed 16.sub.N radioactive recoil nucleus acquires kinetic energy sufficient to get onto the surface of the irradiated sample. A maximum energy of the 16.sub.N recoil nucleus is 1.8 MeV, the path in metals being 1.5 to 2.mu., while the path in the air is 4.4 mm. This activity source characterizes an imaginary quasi-oxygen content and not an actual oxygen content in the sample insofar as no difference can be made between the imaginary and actual oxygen in registration of the 16.sub.N activity. It is obvious that any treatment of the sample surface before irradiation does not exclude the effect of surface contamination on the analysis results. Also, in the event of sample surface removal after irradiation account should be taken of the total time spent on the treatment of the irradiated sample in contamination removal in view of the fact that a determination sensitivity may be degraded. Since the half life of the 16.sub.N isotope is 7.14 s, the sample surface treatment time should not exceed 1 to 1.5 half-life periods, i.e., it should be 10 s, maximum. Another known method involves the etching of irradiated samples in an aggressive medium for surface removal in doing oxygen content neutron activation analysis (cf. F. Dugain, M. Andre, A. Speecke "Radiochemical Radioanalytical Letters", 4, 121, 35, 1970). With the aforesaid method, the samples are etched manually by performing the following steps: placing the irradiated sample in a vessel containing an etching solution; holding the sample in the vessel as long as needed; removing the etched sample from the vessel; and transferring it into a vessel containing water for washing. The total treatment time amounts, in this case, to 20-30 s. Serious disadvantages of the aforesaid method are manual etching, a rather long sample treatment time, an increased radiation hazard, sample etching in still water causing sorption of radioactive nuclei from the etching solution, and also incomplete removal of the etching solution from the sample surface. Also known in the art is a neutron activation analysis installation for determining an oxygen content in highly pure substances (cf. USSR Inventor's Certificate No. 409,555 filed in 1973). As distinct from the aforementioned installation it includes an additional device by means of which the sample is etched after irradiation from a neutron generator. This additional device (irradiated sample surface layer removal unit) represents a rectangular teflon unit having four successively arranged vertical dead channels communicating with one another through guide cavities (slips) whose number suits the number of reagents required to treat the sample. The extreme channels are, respectively, provided whith sample inlet and outlet ports. Connections are incorporated in the channels to deliver the reagents. The vertical channels contain cylindrical pistons with receiving frames on ends thereof, into which the irradiated sample is successively rolled. The pistons with frames are lifted by two air cylinders which are connected in pairs to the respective pistons. The aforesaid installation has been generally unsatisfactory due to the fact that a rather long time is spent while the irradiated sample moves from the inlet port via all the channels to the outlet port, a limitation resulting in low response and intolerable sample activity loss, which, in its turn, drastically degrades the determination sensitivity. Moreover, the known installation does not permit analyzing conventional samples whithout etching insofar as no provision is made therein for direct communication between the test sample receiving and loading assembly and the impurity concentration measuring unit bypassing the irradiated sample surface layer removal unit. Also, the known installation has been open to the objection that its reliability is comparatively low because of the need to use several moveable cylinders with frames alternately receiving the sample and difficulties encountered in making the frames moving in a boiling acid mechanically strong. The sample is moved from one channel to another over slips filled with reagents by gravity, a limitation preventing the analysis of randomly shaped samples whose density is close to 1 g/cm.sup.3. BRIEF DESCRIPTION OF THE INVENTION It is an object of the present invention to expand a concentration measurement range by the use of a neutron activation analysis installation. Another object of the invention is to enhance an impurity determination sensitivity. A further object of the invention is to reduce a sample treatment time after irradiation. A still further object of the invention is to improve the construction of a neutron activation analysis installation with a view to increasing its reliability. The foregoing objects are accomplished by that in a neutron activation analysis installation comprising a neutron generator whose target chamber communicates through a transport means with a test sample receiving and loading assembly communicating, in its turn, with a test sample impurity concentration measuring unit, and also an irradiated sample surface layer removal unit, according to the invention, the receiving and loading assembly is in communication with the impurity concentration measuring unit over a channel having a through lateral port communicating on one side with the input of the irradiated sample surface layer removal unit, an irradiated sample distribution assembly being arranged on the other side of the port, said assembly representing an air cylinder with a hollow shaft having a bar located along the axis thereof and mounting on its end a sample receiver, said bar being disposed in a manner allowing its rotation about the longitudinal axis thereof and reciprocating motion through the port in the channel so that in one extreme position the bar does not reach the channel leaving it vacant, in the intermediate position the sample receiver is found in the channel blocking the latter, and in the other extreme position the sample receiver passes through the port in the channel getting into the surface layer removal unit. Preferably the mechanism turning the bar about its axis represents a piston contained within a hollow rod encompassing the bar, secured thereon in a manner allowing sliding motion along the latter and coupled to the rod by means of a carrier rigidly connected with the piston and installed in a manner allowing its motion through a screw slot in the rod. To enhance sensitivity and reliability of the installation, the irradiated sample surface layer removal unit preferably comprises at least three communicating chambers arranged successively in the direction of reciprocating motion of the bar, the position of the last chamber in the direction of progressive motion of the bar corresponding to the extreme position of the bar, while the air cylinder mounts air locks to suit the number of partitions between the communicating chambers. The neutron activation analysis installation forming the subject of the present invention permits high-accuracy quantitative determination of an impurity and macrocomponent content in various materials, an advantage associated with the fact that the effect of surface contamination on analysis results is excluded. An actual impurity content within the sample is, thus, determined and the probability of a systematic error is substantially reduced. The hereinproposed installation providing means for impurity determination within a wide concentration range (from tens to 1.multidot.10.sup.-5 % by weight) allows its use with conventional initial materials, whether contaminated or highly pure, without any design modifications. Furthermore, the possibility of analyzing various materials regardless of their properties in solid, powder and liquid phases close to a production site or research ground makes the hereinproposed installation sufficiently versatile to meet production, research and technological needs. Samples of virtually any shape having an indefinitely low density may be analyzed in the installation forming the subject of the present invention due to the fact that the chambers in the irradiated sample surface layer removal unit are arranged successively in the direction of reciprocating motion of the bar carrying the receiver with the irradiated sample.