Measurements of Neutron Cross Section of the 243 Am(n,γ) 244 Am Reaction

Transcription

1 Measurements of Neutron Cross Section of the (n,γ) 44 Am Reaction Yuichi HATSUKAWA, Nobuo SHINOHARA, Kentaro HATA Nuclear Chemistry Laboratory Tokai-mura, Naka-gun, Ibaraki-ken The effective thermal neutron cross section of (n,γ) 44 Am reaction was measured by the activation method. Highly-purified target was irradiated in an aluminum sule by using a research reactor JRR-3M. The tentative effective thermal neutron cross sections are 3.9 b, and b for the production of 44g Am and 44m Am, respectively. 1. Introduction Minor actinides are produced by successive neutron ture reactions of nuclear fuel and accumulated in a high burn up reactor. The minor actinides caused severe problems in nuclear waste management. The nuclide is one of the important minor actinides, because of this alpha emitter has long half-life(738 year), a large amount of its activity remains in waste a long period. To reduce such radioactivity, nuclear transmutation system by reactor or proton accelerator has been actively investigated(1,). It is necessary for the system to obtain the accurate neutron cross sections for determining the transmutation rate of the nuclide. In this study, the effective thermal neutron cross section of (n,γ) 44 Am reaction was measured by the activation method. Experiment For target preparation, 1 µl of highly-purified americium solution containing 3 kbq of was put into a small quartz tube and evaporated to dryness. The tube was heat-sealed and housed in an aluminum sule together with a flux-monitor wire of Co/Al-alloy. The target contained in an alminum sule shown in Fig.1 was irradiated during a 1 hour period in the HR-pipe of the JRR-3M reactor at JAERI. The irradiation position is characterized with a thermal neutron flux of 1 x 1 14 n/cm s. After the irradiation, the quartz tube was measured by using a HP Ge detector with 1.8 Aluminum Spacer Aluminum Media Irradiation Target Al foil Co/Al neutron flux monitor wire Quartz glass tube containing the43am Fig. 1 Hydraulic type sule of the target irradiated in JRR-3M.

2 Counts/Channel Gamma Spectrum of Irradiated 74.7 kev kev 44 Am 511 kev kev 44 Am kev 44 Am kev 4 Na 173 kev DE 4 Na Channel Number Fig. Gamma spectrum of the irradiated 43Am target. kev energy resolution at 1.3 MeV. After γ-ray measurements, the target was dissolved in water and a radiometric source for alpha spectroscopy was prepared by dropping the americium solution onto a tantalum disk and evaporating it. The alpha particle spectrum of the source was measured with a silicon surface barrier detector. 44m Am N 1m σ 1m ( n,γ ) + ( n, f ) N λ σ m f σ σ g ( n,γ ) ( n, f ) 44g Am N 1g λ 1m λ 1g σ 1g ( n,γ ) + ( n, f ) 44 Cm N λ ( n,γ ) + n, f σ Fig. 3 Decay and growth of Am and Cm isotopes in neutron irradiation of the 43Am target.

3 3. Results and Discussions Figure shows a γ-ray spectrum obtained from the irradiated targets, where the γ-rays of 44g Am can be seen at 154, 744 and 898 kev. The cross section for the (n,γ) 44g Am reaction was deduced from the intensities of 44g Am γ-rays and determined to be 3.9 b. The 44 Cm nuclide is formed by the decays of the nuclides 44m Am and 44g Am that are produced by the neutron ture reaction of as given in Fig. 4. In order to obtain the cross section of the (n,γ) 44m Am, the α-particle spectrum of the irradiated target shown in Fig.4 was analyzed and determined the activity of 5.8 MeV peak of 44 Cm. The cross section for the formation of 44m Am was calculated according to the following formulas: We define that N (), i t σ i and σ i are the atom number, the orption cross section and the ture cross section for the nuclide i at irradiation time t, respsctively. The timederivatives of atom numbers of the actinide nuclides during irradiation are given by dt 1 m dt = N () t λ + φσ t = N () t φσ N () t λ + φσ m 1 m 1m 1m 1 g = N () t φσ N () t λ + φσ dt g 1 g 1g 1g m = λ dt 1m N () t N () t 1m m ( λ + φσ ) g = λ () () dt g N g t N g t ( λ + φσ 1 1 ) N () t = N () t + N () t m g (1) () (3) (4) (5) (6) where the notation of each nuclide i is given in Fig.3. φ and λ i are the netron flux and the decay constant, respectively. The solution of these differential equations are A t N( t) Ne 7 A t A1 m t N φσ m ( e e ) N1m () t = () 8 A + A1m A m t A t N φλ1mσ m A ( e e )+ A1m( e e ) A e e Nm() t = ( A A1m) ( A + A ) ( A1m + A ) A g t A t N φλ1gσ g A ( e e )+ A1g( e e ) A e e Ng() t = A A A A A A = [ ] 1 A t A t A t A1 m t 1 A t A t A t A g t 1 [ ] + + 1g 1g ( 9) ( 1 ) where Ai is Ai = ri + φσ i. 1 6 Irradiated target

4 Irradiated target Counts/Channel MeV MeV MeV 44 C m MeV 4 C m Channel Channel Number Fig. 4 Alpha spectrum of the irradiated 43Am target The cross sections of (n,γ) 44 Am reaction obtained from the calculated atom number of 44m Am is 84.4 b. The present value of 3.9 b for (n,γ) 44g Am reaction is good agreement with the previous one of 3.8 ±.4 b (4). On the other hand, the cross section of the (n,γ) 44m Am reaction measured in this study (84.4 b) is not consistent with the value of 75.1 ± 1.8 b (4) but agree with that of 83 ± 6 b measured by Garrilov et al. (5) The neutron cross sections for in JENDEL-3- should be evaluated newly on the basis of these results. References (1) Taube, M: Nucl. Sci. Eng., 61, 1(1976) ()Bowman, C.D., Lisowski, P.W., Arthur, E.D.,: Proc. of nd Int. Symp. Advanced Nuclear Energy Research-Evolution by Accelerator-, Jan. 4, 199, Mito, Japan, p. 149, JAERI. (3)Kocherov, N., McLaughlin, P.K.(eds.): INDC(SEC)-14, (1993). (4)Lederer, C.M., Shirley, V.S.: Table of Isotopes, (8th ed.) (1996), John Wiley & Sons, New York (5)Garrilov, V.D., Goncharov, V.A., Ivanenko, V.V., Kustov, V.N., and Smirnov, V.P.:

5 Atomnaya Energiya, 41, 185 (1976)