NEUTRON FLUX IN THE EXPOSURE ROOM OF THE TRIGA MARK II REACTOR IN LJUBLJANA

Transcription

1 Internatonal Conference Nuclear Energy n Central Europe 2000 Golf Hotel, Bled, Slovena, September 11-14, 2000 NEUTRON FLUX IN THE EXPOSURE ROOM OF THE TRIGA MARK II REACTOR IN LJUBLJANA Edvard S. Krštof Jožef Stefan Insttute Reactor Physcs Dvson Jamova 39, 1000 Ljubljana, Slovena Edvard.Krstof@js.s ABSTRACT In ths paper a descrpton of the neutron flux at a partcular poston n the exposure room of the Ljubljana TRIGA reactor s gven. Neutrons emergng from the graphte reflector enter the exposure room through the empty horzontal shaft. At the rradaton poston samples of materals that are well known n reactor metrology were rradated. After rradaton gamma ray spectra were measured and from the saturaton actvtes the neutron spectrum was calculated. 1 INTRODUCTION The exposure room s often used for rradaton wth moderated fluxes less than 10 8 cm -2 s -1. For example, n the last few years rradaton of mnerals, nuclear track detectors, and bologcal samples were carred out. Also some rradaton were performed by use of the secondary nduced fast neutrons from the fsson plate. Anyhow, for an evaluaton of the rradaton data the neutron spectrum must be known. For ths reason such known reactons as 197 Au(n,γ) wth bare, 197 Au(n,γ) under boron or cadmum layers, and a few threshold reactons were utlzed. After every rradaton γ-ray spectra of each sample were measured. From the saturaton actvtes the neutron flux spectrum was calculated. 2 IRRADIATION SET-UP A smplfed horzontal and vertcal cross-secton through the essental part of the devce are shown n the Fg.1 and Fg.2 respectvely. The rradaton poston (S) s about 20cm ahead of the outlet aperture. A sample for rradaton s fxed on a sledge, whch s raled down n the rradaton poston through an alumnum gude. Irradaton of the sample usually begns by lowerng the sledge nto the room, and fnshes by lftng t back.

2 Edvard S. Krštof page 2 of 5 Fgure 1 Smplfed horzontal cross-secton through the shaft. Letters GRR, GR, and S denote the graphte reflector, the thermalzng graphte, and the sledge wth the sample, respectvely. Fgure 2 Smplfed vertcal cross-secton through the shaft n the vcnty of the outlet aperture. Letters SH, G, and S denote the shutter, the alumnum gude, and the sledge wth the sample, respectvely. Proceedngs of the Internatonal Conference Nuclear Energy n Central Europe, Bled, Slovena, Sept , 2000

3 Edvard S. Krštof page 3 of 5 3 SAMPLES AND EVALUATION All rradaton were performed at full reactor power of 250kW. Each radaton was montored by an approxmately 50µm thck en fol wth the dameter of 0.8cm. As prncpal detecton materal for neutrons n the low and ntermedate energy regon was chosen. Namely, the sotope 197 Au has very favourable decay propertes and ts neutron cross sectons are relable. In the hgh energy regon the threshold reactons 115 In(n,n'), 64 Zn(n,p), 27 Al(n,α) and 27 Al(n,p) were utlzed. All together about 25 samples were rradated. Characterstcs of the samples, whch were evaluated up to now, are gathered n the Table 1. Neutron reacton cross secton data are taken from the JEF-2 Cross Secton Data [1]. These data are processed nto 20 groups per decade structure wth a flat weghtng spectrum. Other data necessary for the evaluaton are taken from [2,3,4]. Table 1 Detecton materal Reacton Flterng substance Superfcal densty [g cm -2 ] 197 Au(n,γ) none Au(n,γ) boron Au(n,γ) boron Au(n,γ) boron Au(n,γ) boron Au(n,γ) boron znc 64 Zn(n,p) none 0 alumnum 27 Al(n,p) none 0 ndum 115 In(n,n') none 0 alumnum 27 Al(n,α) none Au(n,γ) cadmum Au(n,γ) slver PRELIMINARY RESULTS The measured neutron flux spectrum s shown n the Fg.3. It s strongly nfluenced by the propertes of the graphte. The pt at 2.7 MeV les n the resonance regon of the sotope 12 C. The peak at thermal energes s placed at ev, but ts dstrbuton s not Maxwellan. At energes below 1 mev, where the Bragg scatterng can not occur, the spectrum s enhanced [5]. Let a be the measured saturaton actvty of the th sample, and α the correspondng saturaton actvty calculated by use of the spectrum, shown n the Fg.3. The value of the expresson ε 1 1 N( N 1) N 1 α a a , N 12 (1) could serve as a measure of the relablty of the calculaton. The total flux Φ tot equals Proceedngs of the Internatonal Conference Nuclear Energy n Central Europe, Bled, Slovena, Sept , 2000

4 Edvard S. Krštof page 4 of 5 φ tot 20MeV φ( E) de 10 5 ev 8.8 (1 ± 0.04) 10 7 cm 2 s 1 (2) The error n ths value contans the number ε 1 as well the contrbuton because of the uncertantes of all the nduvdual γ-spectrometrc measurements. A tral ncluson of some addtonal samples shows that the neutron dfferental flux s n the low energy regon perhaps more varegated as ϕ(e) presented n the Fg.3. Consequently the value of the total flux Φ tot mght somewhat dffer from that gven n the expresson (2), whereby the number ε 1 must reman essental the same. Fgure 3 Normalzed neutron flux spectrum ψ(e) ϕ(e)/c, where C , so that: 20MeV 100keV ψ ( E) de 1. (3) 5 CONCLUSION Ths work confrms that t s possble to measure the neutron spectrum n our exposure room n the way presented. The spectrum s strongly nfluenced by the propertes of the graphte. The pt at 2.7 MeV les n the resonance regon of the zotope 12 C. The peak at thermal energes s not Maxwellan. In the cold neutron regon the flux s enhanced. An ncluson of addtonal samples wll probably reveal further detals of the dfferental neutron flux spectrum. Proceedngs of the Internatonal Conference Nuclear Energy n Central Europe, Bled, Slovena, Sept , 2000

5 Edvard S. Krštof page 5 of 5 6 ACKNOWLEDGEMENT I would lke to thank Bojan Huzjan, Janez Jezeršek, Darko Kavšek, Matjaž Koželj, Bojan Oman, Gvdo Pregl, Matjaž Ravnk and Marko Rosman for knd support. 7 REFERENCES [1] JEF-PC, A personal Computer program for Dsplayng Nuclear Data from the Jont Evaluated Fle Lbrary,1997, OECD Nuclear Energy Agency, Issy-les-Moulneaux, France. [2] J.H.Baard, W.L.Zjp, H.J.Nolthenus, Nuclear Data Gude for Reactor Neutron Metrology, Kluver Academc Publshers,1989, Dordrecht, The Nederlands. [3] N.G.Gusev and P.P.Dmtrev,Quantum radaton of Radoactve Nucldes, 1979, Pergamon Press, Kronberg-Taunus, Germany. [4] W.Seelman-Eggelbert et al, Karlsruher Nukldkarte, 6 th edton 1995, Kernforschungszentrum Karlsruhe, Germany. [5] W.Schneder, Neutronenmesstechnk, pp , Jülch 1973, Germany. Proceedngs of the Internatonal Conference Nuclear Energy n Central Europe, Bled, Slovena, Sept , 2000