5th Conference on Nuclear and Particle Physics EG0700131 19-23 Nov. 2005 Cairo, Egypt ACTIVATION ANALYSIS BASED ON SECONDARY NUCLEAR REACTIONS S.MUKHAMMEDOV, A.KHAYDAROV Institute for Nuclear Physics ofUzRAS pgt Ulugbek, Tashkent, Uzbekistan 702132 Abstract. Various types of analytical techniques founded on achievements of nuclear physics are used. There are two directions of the using of the main sources of the nuclear projectiles at development of the nuclear methods. In the first, the particles from the source are used directly for the excitation of nuclear reactions. In the second, the particles from the source are used for the generating of "intermediate particles" of other types which are used in turn for excitation of secondary nuclear reactions. In our research the neutrons are used for the generating of secondary charged particles which serve for excitation of nuclear reactions on elements with small atomic numbers. There are two variants in which both types of neutrons, as thermal, so and fast neutrons are used: 1) The triton flow is produced by thermal neutrons flux, which excites the nuclear reaction 6Li (n, a)T on lithium; 2) The recoil protons are produced as the result of (n, p) elastic or inelastic scattering interaction of fast neutrons with nucleus of light elements, for example, hydrogen. In this work the theoretical base of the application of secondary nuclear reactions excited by recoil protons was investigated. 1NRODUTION The traditional neutron activation analysis (NAA) based on (n,/)-neutron capture nuclear reaction has been developed into a reliable and powerful analytical technique, allowing the determination of over 60 chemical elements with good accuracy and low detection limits [1]. Considering all possibilities of activation and a radiochemical separation of the indicator radionuclide, the majority of the elements of this group can be determined at the ppb concentration level and below."However, for solving a number of analytical problems NAA technique is not well suited or it cannot be used at all. An important limitation is that all light, some medium and heavy elements (for example, H, Be, Li, B, C, N, 0, Ti, Nb, Pb, etc) cannot be determined even at ppm concentration level by this method. Accurate determination of lithium, oxygen and other light elements in sub microgram level is of importance in geochemical and material studies. Such examples are great many. On such instances, several non-traditional reactor activation analysis can be used which have increasingly been developed and applied to several fields of semiconductor industry, biology, geology in recent years [2, 3]. These techniques can be -named as the nuclear reactor based charged particle activation analysis. We have been distinguished two possibilities of the application of a nuclear reactor as charged narticles source for the develonment of the nuclear Li" thermal neutronflux fast neutron flux FigUre 1 .Means of production of charged particles f><™ we have considered the --^""trottrl^-ites the f . The triton flow is J^J^ToSr* um - - - — « ** unclear reaction 'WlwV °" N, S, and Mg PL detemunat.on of o.he, "g^T" £££ of' („, p, elastic and inelas ,c Ti S and other elements. investigation of the theoretical base ot mi-^rf^r^rfSSS ^ reactions excted by THE Ten a neutron collides with. ■£*-£ Jtt2*J VTeV P)-coUiS1ons o, M^^]X the development of nuclear 4, m^****» ***T^^^^tm -actors * It's a well know, the fast neutron specl™ ^ ^ ^ ^ could be placed in widely energetic range ^fromse expressed as following [7]: _ ZQ. 2 KiEhae^s/icEJ' <„ Where, N„(E)-The lot of fast neutrons with the energy of E on the unit of energy 2 interval, n/cm x.MeV; sh -Hyperbolic sinuses; E -Neutron energy; a,b,c-the characteristic constants for every U isotope which satisfied following correlation: 235 It is showed that for U a=0.453, b=l,035; c=2.29 [8] By the using of the IBM program we carried out the numerical calculation of energetic distribution of fast neutrons. Results obtained for the nuclear reactor fast neutrons ofWWR-SM type are presented in table 1. Table l.Thee lergetic distribution of nuclear reactor fast neutrons Eo, *N0(E) Eo, *N0(E) Eo, *Nn(E) **Nn(E) **Nn(E) **Nn(E) MeV MeV MeV 0.1 0,203 0.989 1 0.348 0.693 10 0.868,10"J 0.107.10"^ 0.2 0.269 0.965 2 0.240 0.396 11 0.389.10"J 0.476.10"J 0.3 0,308 0.936 3 3 0.139 0.210 12 0.173.I0" 0.210.10"* 0.4 0.332 0,904 4 0.742.10"' 0.106 13 0.761.10"* 0.910.10"* 0.5 6:347 0.870 5 1 0.377.10"' 0.517.10" 14 0.332.1Q"" 0.393. ID* 1 0.6 0.355 0.835 (i , 0.185.10' 0.246.10*' 15 0.144. lO"" 0.I68.10"4 J 0.7 0.35X 0.79') 7 O.B8f>.10" 0.1 15.10"' 16 O.fjiy.lO"1"" 0.704.1 f)""1 0.8 0.357 0.763 8 0.415.10"= 0.529.10-' 17 6.265.10"5 0.286,10"1, 0.9 0.354 0.728 9 J J 5 0.191.10" 0.240.10" IS OJI3.10" 0.107.10"' *M fc **N„(E) -the sum of fast neutrons with energy higher 2 MeV. ■'■■ 'X 1 \ "^u3 m . It «* \ W" d 1M \ ., t 11 t I i * t t la fl 14 ll la E0I1.MeV E|pMeV Figure 2.Energetic distribution of fast neutrons (Left- differential, right- total) 31 Figure 2 presents the graphic of energetic distribution of fast neutrons. It is visible that the share of fast neutrons with energy ofhigher lOMeV is very small (in order of lf/MO"*). Therefore these are least importance for .the producing of recoil protons. The recoil protons with energy of more 2 MeV are importance for activation analysis, because the useful nuclear reactions can be excited at these energies of particles. B. Energetic distribution of recoil protons The cross section of (n, p) - process on hydrogen is 5- 4.5 b in the energy range of reactor fast neutrons [6]. We have obtained that the cross section of elastic scattering of fast neutrons on hydrogen nuclei for this energy range is decreased of 10 times, approximately. 1'he numbers of recoil protons are expressed by N,(E) = ]™xjE)NM(E>£ (2) 0 Where, n-the numbers of hydrogen atoms in a target; ^ anp(E)-The excitation functions of n-p elastic scattering process, cm"; N„(E)-the function of distribution of fast neutrons, MeV'cm" s" ; We estimated differential value of the number of secondary recoil protons by the use of following formula: F,(E) = ^ = ^(E)N„(E) m This expression presents the energetic distribution of recoil protons. The excitation function anp(E) of elastic scattering of fast neutrons on hydrogen nucleus can be expressed as the sum of two exponential functions: E AB ff,(B)=aI«-A +a1e- m Where, a, (3 - Constants being following values: a, = 4.23 ■-> ff, = 1,245 -> P,= 0.4 -► fit = 0.0608 Therefore the expression (4) could be written as 4E 00608E a„p(E)=4.23e-°- +1.245e- (5) By the uniting formulas (4) and (5) we can obtain following expression for the proton distribution cE * -32- By the use of the expression (6) we have calculated the energetic distribution of recoil protons for the case of the use of polystyrene of lg weight as a target and the WWR-SM nuclear reactor with power equal 1 MW. Our results are listed in Table 2. Table 2/The energetic distribution of recoil protons (protons/s.MW) E0 FptE) Eo %(£), b <y:%° FP(E) 1U 2 3.0 4,0.10 8 0.9 2,1.10s !H 3 2.3 1,8.I0 9 0.8 9,3.10' 4 1.8 7,4.10" 10 0.7 3,7.107 5 1.5 3,1,10" 11 0.64 1,6.107 6 1.2 1,3.10* 12 0.60 7,0.106 s 1 1.0 5,1.10 13 0.54 2,7.106 It is visible the recoil protons number is sufficiently high.The energetic distribution of recoil'protons satisfies the exponential law: 873£ Fp(E)=2.29-10"e-°' (7) Preliminary investigations showed the detection limits of proton activation analysis for light elements could be better than lOppm C. The production of recoil protons on a nuclear reactor The recoil protons are produced using of two possible ways. In the first, proton- forming material (for instance, polystyrene, paraffin, resin et al) disposes on surfaces of irradiated samples. For instance, before irradiation a sample is turned with hydrogen containing material. In second case, a powder investigated sample is carefully mixed with hydrogen containing material. Depending on way used in^practice, equations of activation analysis will be a miscellaneous. This way is more practical since each atom of the defined element (for instance, lithium) is situated near the ensemble of "proton- forming" target atoms. This circumstance promotes to increase the probability of their interaction, hereunder to increase the probability of analytical radionuclide. So we shall consider this way and in the further we shall use it for undertaking analysis. D. Equation for activation analysis Recoil protons having energy E>2 MeV exited the analytical reactions of the type (p, n), (p,«), (ptan). The threshold energy of first reaction is not less 1.8 MeV. The second has not a threshold, but proton must possess the certain energy required for issue of some part to reaction products for overcoming of Coulomb barrier of nucleus, Cconsequently, coming from need of activation aanalysis in formula (2) the integral limits necessary to change.