Activation cross-sections measurement of Au-197 using quasi-monoenergetic neutrons below 36 MeV

Transcription

1 Nuclear Physics Institute Řež Activation cross-sections measurement of Au-197 using quasi-monoenergetic neutrons below 36 MeV M.Honusek, P. Bém, V.Burjan, M.Götz, V.Kroha, J.Novák, E.Šimečková Nuclear Physics Institute Řež, Czech Republic U.Fischer, S.P.Simakov Institut für Reaktorsicherheit, Forchsungszentrum Karlsruhe Gmbh, Germany 1

2 1. Introduction 2. Experimental conditions 3. Statistical properties of experimental data 4. Neutron spectra and CS data 5. Cross-sections and C/E calculations 6. Conclusions 2

3 1. Introduction Quasi-monoenergetic neutron source based on reaction p+li7 is used. We used Au as a monitor foil in different experiments at 13 energies during 4 years. Proton beam energies MeV. Isotopes obtained in the reactions (n,2n), (n,3n), (n,4n) and (n,γ) on Au197 are studied using γ -spectroscopic technique. Cross-sections and C/E (EAF 2007) values for (n,2n), (n,3n) and (n,4n) reactions are obtained. Preliminary. 3

4 2. Experimental conditions The proton beam energies: 19.8, 22.0, 22.1, 25.1, 27.1, 27.2, 29.5, 30.1, 32.1, 35.0, 36.5, 36.7, 37.5 MeV Beam profile (intensity of proton beam x time) was measured by digital technique. Typical proton beam current of 3 μa. 4

5 Quasi-monoenergetic neutron source p+ 7 Li source reaction 7 Li(C backing) target cooling by 5 o C alcohol stream beam-power of 600 W reliably operated proton beam energy range MeV flux density ~ 10 8 n/cm 2 /s calculated (in peak) for 30 MeV 1 μa proton beam at minimum t-s distance of 50 mm 5

6 Cyclotron U 120M of NPI Řež carbon disc T 8 x 20 mm proton beam cooling T 4 mm Ta foils T 0.05 mm Irradiated foils at distances 48 mm and 88 mm from Li foil Li foil T 2 x 40 mm 35 mm 6

7 Irradiated samples are investigated by means of gamma-spectroscopy method Two calibrated HPGe detectors of 23 and 50 % efficiency and FWHM of 1.8 kev at 1.3 MeV. Activated isotopes were identified on the basis of T 1/2, γ-ray energies and intensities. Cooling times of gamma measurement ranged from minutes to approx. 100 day. Diameter of Au foils 14 mm, thickness 0.05 mm The isotope activities normalized to 1 μa proton beam were obtained for both positions of foils (48 and 88 mm from Li foil). 7

8 3. Statistical properties of experimental data The effects of angular distributions were discussed many times, e.g. in Bi experiments M.Honusek et al., NEMEA 5, Ljubljana The effect - ratios of activities for 48 mm and 88 mm distances of Au foils from Li foil is clearly seen. Dependence on reaction threshold is evident. See figs. Geometrical ratio is

9 Au197(n,4n)Au194 Mean Int.err. 1.7 %, ext. err. 1.8 % Au197(n,3n)Au195 Mean Int.err. 2.6 %, ext. err. 2.7 % 9

10 Au197(n,2n)Au196 Mean Int.err. 1.3 %, ext. err. 1.1 % Au197(n,2n)Au196m Mean Int.err. 1.6 %, ext. err. 1.9 % 10

11 Au197(n,γ)Au198 Mean Int.err. 1.2 %, ext. err. 1.0 % The angular correlations are important. Therefore we decided to use experimental data measured at 88 mm distance only. The experimental study of angular distributions is necessary. 11

12 4. Neutron spectra and CS data Spectra consist of - quasi-monoenergetic part corresponding to the reactions to g.s. and MeV state in 7Be - low-energy tail generated a) by reactions on 7Li leading to further excited states in 7Be and other reactions on 7Li b) by reactions of protons on carbon stopper - thermal neutrons not taken into account 12

13 p+7li - typical spectrum (taken from INS Tokyo) 13

14 We used neutron spectra measured using TOF technique at INS Tokyo (facility is similar to Řež apparatus). Y.Uwamino et al., NIM A389 (1997) 463. The spectra from INS Tokyo were measured at 20,25,30,35 and 40 MeV proton beam energies. We used larger set of proton energies. The shifted INS spectra were used. This procedure and detailed description of experimental apparatus were published in our previous works M.Honusek et al., NEMEA 4, Prague 2007, proceedings p. 39 P.Bém et a., NDS, Nice 2007, 14

15 Isotope T 1/2 reaction Threshold (MeV) Au h (n,4n) Au d (n,3n) Au d (n,2n) Au196m 9.6 h (n,2n) The cross-section data for the analysis were taken from EAF 2007 Total cross-sections are used except second isomeric state in Au196m 15

16 4.Cross-section and C/E calculations The results for close proton energies are grouped together. Neutron peak maxima: 17.9, 18.9, 21.9, 23.9, 27.1, 29.3, 32.6 and 35.4 MeV. The limits of corresponding neutron energy intervals (boundaries of peak) cover the energy interval from 16.1 to 36.4 MeV CS values are calculated step by step. (CS)n is calculated on the basis of (RR )n (RR )n = (RRe)n (RRc)n-1 RRe experimental activity of foil normalized to 1μC (and 1 kg) RRc the similar value calculated for given CS and neutron spectrum 16

17 Error analysis RRe: Error of proton-beam current from Uwamino 10 % Error of our proton-beam current 5 % Error of activity measurement min. 3 % Minimum error of RRe 12 % RRc: Correlations. We assumed 10 % for each step. (RR )n = (RRe)n (RRc)n-1 Absolute errors: err(rr ) = sqrt(err(rre)^2 + err(rrc)^2) 17

18 Au197(n,4n)Au194 Integral characteristics C/E values 18

19 Au197(n,4n)Au194 Cross-sections (RR )n = (RRe)n (RRc)n-1 (RRc)n-1 is small Data of Uwamino et al., NSE 111, 391, 1992 are shown as well (from EXFOR). 19

20 Au197(n,4n)Au194 Fig. from R.Forrest et al., UKAEA FUS 547, 2008 Cross section (b) 4.0E E E E+00 LAS75 LAS75 LAS75 TOH92 BRC77 Data of Uwamino 92 are not included. The green points come from Uwamino et al., NIM A389 (1997) 463? 8.0E E E E E E E E E E E+07 Energy (ev) Final 20

21 Au197(n,3n)Au195 Integral characteristics C/E values 21

22 Au197(n,3n)Au195 Cross-sections (RR )n = (RRe)n (RRc)n-1 (RRc)n-1 is up to ~ 30 % of (RRe)n 22

23 Au197(n,3n)Au195 Fig. from R.Forrest et al., UKAEA FUS 547, E E+00 KGU80 LAS75 GIT72 LAS75 LAS75 AEP89 LAS77 Cross section (b) 1.5E E+00 EXFOR data up to 30 MeV. The same tendency. 5.0E E E E E E E E+07 Energy (ev) Final 23

24 Au197(n,2n)Au196 Integral characteristics C/E values 24

25 Au197(n,2n)Au196 Cross-sections (RR )n = (RRe)n (RRc)n-1 (RRc)n-1 is comparable with (RR )n 25

26 Au197(n,2n)Au196 Convolution integrals Parts belonging to quasimonoenergetic peak are clearly seen 26

27 Au197(n,2n)Au196 27

28 Au197(n,2n)Au196 Fig. from R.Forrest et al., UKAEA FUS 547, 2008 Cross section (b) 2.5E+00 Systm SIU02 n KTO88 PIT84 2.0E+00 RI 03 n BUC92 KTO90 g NPL81 g AUB84 n AUB82 n RI E+00 BRC81 n IBJ84 g MOH82 g BRC81 g SIU89 KOS00 FEI83 1.0E+00 ANL87 AUB84 n TOH92 n TOH92 g RI 03 RI 03 n KOS82 5.0E-01 AEP89 RI 99 n RI 99 LAS77 LAS61 LAS61 n 0.0E+00 JAE84 JAE88 n 5.0E E E E E E E+07 JAE88 g LAS75 LRL60 n Energy (ev) BRC02 Final LRL60 g GEL75 Data of Uwamino 92 are not included. The green points come from Uwamino et al., NIM A389 (1997) 463? Data of Uwamino 92 important part of EXFOR. New evaluation. K.I.Zolotarev, IAEA INDC (NDS) 0526 (2008). Error at 30 MeV is 4.3 %. 28

29 6. Conclusions - Large set of experimental data is analyzed - Preliminary data on C/E ratios and cross-sections in reactions 197Au(n,4n)Au194, 197Au(n,3n)Au195 and 197Au(n,2n)Au196 are given - The near future: analysis of 197Au(n,2n)Au196m reactions - Errors of (n,2n) reaction data are large - Limited possibilities if cross-sections En<20 MeV are large and unknown with required uncertainties 29

30 To improve the data: - higher precision of current measurement - future measurement of our p+7li spectra using TOF we believe in -analysis of angular distributions. We need the spectra at foil positions. - good cross-section data En<20 MeV - extend the distance foil - Li? The activity may be enough Similar method was used in calculation of some cross-sections of Bi, Honusek et al., NEMEA 5, Ljubljana Complete evaluation in near future. 30