Patent Number: 043022855
Section: description

DETAILED DESCRIPTION OF THE INVENTION Referring to FIG. 1 a neutron activation analysis installation forming the subject of the present invention is designed for quantitative determination of the chemical composition of various materials and quick non-destructive test in the production of metals, alloys, semiconductor and other materials to obtain a desired chemical composition of semi-finished and finished products. The proposed installation comprises a neutron generator 1 whose target chamber 2 communicates through a transport means 3 with a test sample receiving and loading assembly 4 which, in its turn, communicates with an impurity concentration measuring unit 5. The impurity concentration measuring unit includes a detector 6, measuring equipment 7 and a minicomputer 8 and communicates with the receiving and loading unit 4 over a through channel 9. The channel 9 has a through lateral port 10 communicating on one side with the input of an irradiated sample surface layer removal unit 11, an irradiated sample distribution assembly 12 being arranged on the other side of the port 10, the distribution assembly 12 represents an air cylinder 13 (FIG. 2) with a hollow rod 14 having a bar 15 arranged along the axis thereof, said bar carrying on its end a sample receiver 16. The bar 15 is disposed in a manner allowing its rotation along the longitudinal axis thereof and reciprocating motion through the port 10 in the through channel 9. In one extreme position the bar 15 does not reach the channel 9 (ref. I of FIG. 2 shown with a dashed line) leaving it vacant to enable a sample 17 falling from a capsule 18 (FIG. 2) of the receiving and loading assembly 4 to pass along the through channel 9 into the detector 6 (FIG. 1). In the intermediate position (ref. II of FIG. 2) the sample receiver 16 secured to the bar 15 is found in the channel 9, thus blocking the latter. In the other extreme position (ref. III shown with a dashed line) the bar 15 with the sample receiver 16 gets into the surface layer removal unit 11 after passage via the port 10 in the through channel 9. The reciprocating motion of the bar 15 with the sample receiver 16 and the 180-degree turn of the bar 15 in the through channel 9 are accomplished by supplying air to the air cylinder 13 through holes a and b. Under its pressure the rod 14 with the bar 15 carrying the sample receiver 16 moves in either direction. The bar 15 is turned about its axis by means of a turning mechanism composed of a piston 19 contained within the rod 14 encompassing the bar 15, secured on the bar 15 with a key 20 in a manner allowing sliding motion along the axis of the bar 15 and coupled to the rod 14 by means of a carrier 21 which is rigidly fixed on the piston 19 in a manner allowing its motion through a screw slot 22 in the rod 14. The air displacing the piston 19 is supplied into the cavity of the rod 14 from the air cylinder 13 through a hole "c". A lever 23 enables installation of the bar 15 in position I or II. In the event of an oxygen content determination the detector 6 may, for example, represent a device based on two scintillation units with large lead-shielded NaI (T1) crystals. The measuring equipment 7 may include five discriminating amplifiers, four recomputation devices, and two coincidence circuits. In doing oxygen content analysis, for example, use is made of two sample activity measuring channels, one neutron flux monitoring channel used during irradiation of samples, and one neutron flux test channel. In doing nitrogen content analysis use is made of five channels and two coincidence circuits. The minicomputer 8 processing measurement data may include a keyboard computer and a matching unit which interrogates scales, feeds data into the computer and initiates computation instructions in accordance with the preset algorithm. For example, an oxygen content in the sample in accordance with the preset algorithm is determined from the formula ##EQU1## where .eta..sub.x =oxygen content in the test sample, % by weight; .eta..sub.0 =oxygen content in the reference sample, % by weight; PA1 N.sub.x =number of counts for the sample; PA1 N.sub.1x =number of background counts for the sample; PA1 N.sub.0 =number of counts for the reference sample; PA1 N.sub.10 =number of background counts for the reference sample; PA1 M.sub.x =number of sample monitor counts; PA1 M.sub.0 =number of reference sample monitor counts; PA1 M.sub.x =weight of the sample, g; PA1 M.sub.0 =weight of the reference sample, g; and PA1 K=coefficient accounting for a difference in absorption of 16.sub.N isotope gammas in the test and reference samples. For clarity, FIG. 3 presents a perspective view of the irradiated sample distribution assembly 12 with the surface layer removal unit 11 and a portion of the channel 9. Turning now to FIG. 4 the irradiated sample surface layer removal unit 11 comprises at least three communicating chambers 24, 25, 26 arranged successively in the direction of reciprocating motion of the bar 15, the position of the last chamber 24 corresponding to extreme position III of the bar 15. The sample receiver 16 is protected by a cylindrical guard 27 to preclude the penetration of a reagent from one of the chambers (24 to 26) to the other during backward motion of the bar 15. Two vessels 28 containing the reagents are provided for each chamber (24 to 26), one vessel being used to treat the sample with a required reagent, while the other vessel is used for draining the reagent. The air cylinder 13 is provided with air locks 29 whose cavities communicate with the cavity of the air cylinder 13 through ports 30 disposed along the periphery thereof. The number of the air locks 29 suits the number of partitions between the chambers 24 to 26, each lock being designed to brake and stop the bar 15 with the sample receiver 16 in one of the chambers (25 or 26) during its backward motion by discharging the air through outlet connections d, e, respectively. The ports a, b and connections d, e are closed and opened by electromagnetic valves 31 a, b, d and e (letter designations of the valves correspond to letter designations of the respective holes). The operation of the valves 31 is controlled by a timer (not shown in the drawings) activated on signals from a photosenser 32 arranged in the channel 9. Referring to FIG. 5 the sample 17 is contained within the receiver 16 placed in one of its extreme position in the chamber 24. In FIG. 6 the ports 30 are distributed along the periphery of the air cylinder 13 and the air lock 29 is shown with the outlet connection e. The neutron activation analysis installation forming the subject of the present invention operates in the following manner. Before operation, it is necessary to estimate the purity of the test sample 17 as regards an impurity content. If the impurity content has a concentration exceeding 5.multidot.10.sup.-3 % by weight and no surface removal is required after irradiation prior to measuring the sample activity, the lever 23 should be set to position I so that the through channel 9 is unblocked to allow passage of the sample to the detector 6. The test sample 17 is enclosed in the capsule 18 which is then placed in the receiving and loading assembly 4. The transport means 3 is used to deliver the sample 17 from the receiving and loading assembly to the target chamber 2 of the neutron generator 1 wherein the sample 17 is irradiated. The same transport means 3 delivers the irradiated sample 17 to the receiving and loading assembly 4 whence it goes over the through channel 9 to the detector 6. The sample activity is measured by the measuring equipment 7, the impurity content is calculated from formula (1) using the minicomputer 8 and a presentation of the result is provided. If the sample 17 is pure or highly pure having, for example, an oxygen content less than 5.multidot.10.sup.-3 % by weight, the lever 23 should be set to position II so that the sample receiver 16 blocks the through channel 9. The transport means 3 delivers the sample 17 with the capsule 18 to the target chamber 2 of the neutron generator 1 wherein it is irradiated. At a preset time after irradiation the transport means 3 delivers the sample 17 with the capsule 18 to the receiving and loading assembly 4. Therefrom the sample 17 removed from the capsule 18 passes over the through channel 9 to the sample receiver 16. As the sample 17 passes over the channel 9, the photosensor 32 furnisches a signal causing the electromagnetic valve 31 a to open. From said valve the compressed air is supplied through the port a to the air cylinder 13, thus pushing the rod 14 with the internal bar 15 whose end mounts the receiver 16 with the sample 17 via the through port 10 into the irradiated sample surface layer removal unit 11. The receiver 16 with the sample 17 is placed in the extreme chamber 24 under the inlet connection coupled to the vessel 28 filled with the reagent required to treat the irradiated sample 17 with a view to removing its surface layer. When the receiver 16 is installed in the chamber 24, the running reagent in a uniform manner the surface layer from the irradiated sample 17 after which it is drained into the second vessel 28 through the outlet connection. At a preset time after the treatment of the sample in the chamber 24 is completed the compressed air is supplied through the open electromagnetic valve 31b and the respective port b to the air cylinder 13, thus pushing the rod 14 with the bar 15 and the receiver 16 which is stopped in the next chamber 25 to enable further treatment or washing of the sample 17 in the receiver 16 with running reagent or water. As this happens, the guard 27 closes the port through which the chambers 24 and 25 communicate. To stop the receiver in the chamber 25, the rod 14 is braked by discharging the air from the air lock 29 through the connection e and the electromagnetic valve 31e. At a preset time after the treatment of the sample 17 in the chamber 25 is completed, the valve 31e closes and the air coming through the port b pushes the rod 14 until the receiver 16 with the sample 17 stops in the chamber 26 to enable further treatment and blowing of the sample 17 with air. The receiver 16 with the sample 17 is stopped in the chamber 26 by discharging the air from the second air lock 29 through the connection d and the electromagnetic valve 31g. In this case, the guard 27 closes the two ports through which the chambers 24 to 26 communicate. At a preset time after the treatment of the sample 17 in the chamber 26 is completed, the valve 31d closes and and the compressed air is supplied through the valve 31b and the port b to the air cylinder 13, thus pushing the rod 14 until the receiver 16 with the sample 17 enters the through channel 9. When the receiver 16 with the sample 17 is placed in the through chanel 9, the compressed air is supplied from the air cylinder 13 through the port c to the cavity of the rod 14, thus pushing the piston 19 which slides along the axis of the bar 15. Since the piston 19 is coupled to the rod 14 by means of the carrier 21 rigidly fixed on the piston 19 in a manner allowing its motion through the screw slot 22 in the rod 14, the bar 15 with the sample receiver 16 makes a 180-degree turn thanks to the screw slot 22 whereby the sample 17 is removed from the receiver 16 and supplied to the detector 6 over the through channel 9. Next, the sample activity is measured by the equipment 7 and the minicomputer comutes in accordance with the preset algorithm (say, formula (1) an oxygen content and feeds the test data to a printer or a display unit. The minicomputer also allows computing errors of a randomly chosen set of data or a single test. The entire impurity determination process is invariably short, say, from 1.5 to 3 min in doing oxygen content analysis, its duration being dependent upon the half-life of a given radioisotope. A short impurity determination time permits monitoring the entire process of fabricating semi-finished products to a high accuracy. Another advantage of the proposed neutron activation analysis installation over the prior art is that it holds much promise as regards sensitivity, accuracy, use of a still greater number of elements for impurity content analysis, automation of the entire test process and fast data output by sound signalling, visual presentation or printing. The aforesaid advantage is associated with the use of a high-current neutron generator with a minimum flux of 5.multidot.10.sup.12 neutron/s employing deuterium-tritium beams and a tritium-fed target and also of a minicomputer and up-to-date integrated circuits, which is generally a space-saving factor allowing further miniaturization. Using neutron moderators the hereinproposed installation is capable of operating not only with direct-action accelerators generating monochromatic .about.14 MeV neutrons but also with slow and thermal reactors. Furthermore, the installation forming the subject of the present invention makes it possible to do volume, surface and correlation analyses.