Excitation functions of 109Ag(n,2n)108mAg, 151Eu(n,2n)150mEu and 159Tb(n,2n)158Tb reactions from threshold to 15 MeV

Appl. Radiat. Isot. Vol.47, No. 5/6, pp. 569-573, 1996 Copyright © 1996ElsevierScienceLtd S0969"8043(96)00006-1 Printed in Great Britain.All rights reserved 0969-8043/96 $15.00+ 0.00

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Excitation Functions of l°9Ag(n,2n)l°8mAg, 151Eu(n,2n)lS°mEu and 59Tb(n,2n) 58Tb Reactions from Threshold to 15 MeV S. M. Q A I M 1., F. C S E R P A K 2 a n d J. C S I K A I 2 qnstitut fiir Nuklearchemie, Forschungszentrum Jiilich GmbH, 52425 Jiilich, Germany and 2Institute of Experimental Physics, Kossuth University, 4001 Debrecen, Hungary (Received 14 September 1995)

Cross-sections were measured for the formation of long-lived products ~°SmAg(~= 433y), Js°mEu(T½= 36.9y) and 15STb(~= 180y)in (n,2n) reactions on z°gAg,151Euand 15gTb,respectively,over the neutron energy range of 10.8-12.2MeV. In the case of silver the target material consisted of 99.26% enriched 1°gAg.The l°9Ag(n,2n)l°SmAgreaction cross-section was also measured at 14.5 MeV using silver of natural isotopic composition. All measurements were done using the activation technique in combination with high resolution ),-ray spectrometry. A re-normalization of our previous results in the 8.7-10.7 MeV energy region for the lSlEu(n,2n)lS°mEuand 15gTb(n,2n)mSSTbreactions was done. The experimental data were compared with results of nuclear model calculations based on compound-precompound theory. Except for some small deviations in the energy region below l0 MeV, the experimental and theoretical data for the 151Eu(n,2n)lS°mEuand t59Tb(n,2n)lSSTbreactions agree. In the case of t°gAg(n,2n)l°SmAgreaction, good agreement is found between experiment and theory over the whole investigated energy range.

Introduction In an effort to develop low activation materials for fusion reactor technology, the IAEA established a co-ordinated research programme (CRP) on crosssections relevant to the generation of long-lived activities. A total of 16 reactions of major concern were identified and several laboratories started the joint effort (cf. Wang DaHai, 1990). The elements Ag, Eu and Tb, occurring as impurities in potential blanket materials, would lead to long-lived l°SmAg(T~= 433y), 15°mEu(T½= 36.9y) and mTb(T½= 180y) via i°gAg(n,2n)-, 151Eu(n,2n)- and mTb(n,2n) reactions, respectively. As far as their formation cross-sections are concerned, some information existed in the literature around 14 MeV, and several new measurements were done under the CRP (cf. Wang DaHai, 1992; Pashchenko, 1993). Vonach and Wagner (1993) evaluated all the available data in the 13-15 MeV range, normalizing them to the same half-life, y-ray abundance and monitor reaction cross-section. The recommended cross-sections at 14.5 MeV have an overall uncertainty of 4-7% and serve as a good base for work in other energy regions. Parallel to the experimental work, nuclear model calculations were performed by several groups. The *To whom all correspondence should be addressed.

theoretical results were normalized to the evaluated experimental data at 14.5 MeV (mentioned above), and then an average of the various results was taken. This way the full excitation functions of the above mentioned three reactions were made available (cf. Chadwick et al., 1993). In contrast to the 14 MeV region, very little experimental information was available near the thresholds of the reactions. A preliminary report by Meadows et aL (1993) and somewhat more detailed measurements by us (cf. Qaim et al., 1992) provided some data around 10MeV. Those data were, however, considerably lower than the averaged theoretical curves (see above). The discrepancy could be either due to faulty experiments or deficiencies in nuclear model calculations. In general, the model calculations can reproduce the (n,2n) excitation functions very well. In the case of l°9Ag(n,2n) t°SmAg and mEu(n,2n)lS°mEu reactions, however, since isomeric states are involved, the model calculation may not satisfactorily reproduce the excitation function. It was therefore considered very important to do some accurate measurements near the thresholds of the reactions. The present work deals with cross-section measurements using somewhat better sample dimensions than in our previous study (cf. Qaim et al., 1992). Furthermore, we normalized our previous data taking into 569

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account a recent energy calibration of the beam from the compact cyclotron CV28 at Jiilich.

Experimental Samples and irradiations Measurements in the 10-12 MeV range were done using quasi-monoenergetic neutrons produced via the 2H(d,n)3He reaction on a D2 gas target at the cyclotron CV28 in Jiilich or the MGC-20 in Debrecen. Each sample was placed at a distance of 1 crn from the beam stop of the gas target, and irradiated several times in 0 ° direction relative to the deuteron beam, with fresh AI monitor foils each time. The total irradiation time for each sample was about 25 h. Irradiations with 14.5 MeV neutrons were done using a D-T neutron generator in Debrecen. In the case of silver, ~°gAg powder (99.26% enriched, supplied by Oak Ridge National Laboratory, U.S.A.) was pressed at 10 t cm -2 to obtain a thin pellet of 1.3 cm ~ (wt ~0.16g). Three such pellets were prepared and each of them was sandwiched between two AI foils. In one irradiation the primary deuteron energy was 8.25 MeV and in the other 9.62MeV. The third sample was irradiated with background neutrons (Ed = 8.94 MeV, gas target evacuated). All the irradiations were done at Jfilich. For measurement at 14.5 MeV, 1.9 cm ~ x 0.05 cm thick high purity Ag samples were irradiated at Debrecen. In studies on europium, three Eu203 samples (99.999% pure, Koch-Light, England) were pressed at 10 t era-2 to obtain 2 cm ~ x 0.4 em thick pellets (wt ~ 4.3 g). Each one of them was placed in an Al-capsule and sandwiched between two AI foils. Two samples were irradiated at Jiilich and the third one at Debrecen, all with D-D neutrons. In work on terbium, a Tb407 sample (99.999% pure, Heraeus, Germany) was pressed as in the case of Eu203. A 2 cm ~ x 0.4 cm thick pellet (wt ,-, 4.2 g), placed in an Al-capsule and sandwiched between two A1 foils, was used. Here also the irradiation was done at J/ilich.

Measurement of radioactivity The radioactivity of the monitor reaction product (24Na) was determined via Ge(Li) detector y-ray spectrometry. The peak area analysis was done using the program Maestro II developed for IBM compatible computers. Measurements on ~°SmAg(T½= 433y; E v = 434 KeV, I v=90.5%; E v = 6 1 4 K e V , I v=89.8%; E v=723 KeV, I v = 90.8%) were done using a specially stable and low background counting system available at Jiilich. It consisted of a 140era 3 HPGe detector connected to a stabilized ADC and a 16 K Spectrum ACE card. The detector was shielded with a 15 cm thick special Pb wall (the outer 12 cm made of low activity Pb [ < 50 Bq/kg] and the inner 3 cm of North Sea Pb [ < 3 Bq/kg]). Furthermore, the lead shielding had an inner lining of 0.5 cm Cu and 0.5 cm plexiglass. Each sample was counted for about 3 weeks during which the accumulated peak area reached a counting statistics error of <2.5%. The peak area analysis was done using the program Gamma Vision. The areas under the above mentioned three peaks, when corrected for the y-ray intensities and the efficiency of the detector, agreed within about 8%. We therefore took an average of the three values and adopted an error of 8% in the peak area. The samples irradiated with 14.5 MeV neutrons were counted at Debrecen using a HPGe detector. The Eu203 and Tb407 samples, whether irradiated at Jfilich or Debrecen, were counted at Debrecen. For this purpose either a HPGe or a Ge(Li) detector coupled to a 4 K Spectrum ACE card was used. Each sample was counted for about 3 days during which counting statistics with errors of < 3% for J~*Eu and < 1% for 158Tb were achieved. The peak area analysis was done using the PC version of the G A M A N A L spectrum analyzer program. The decay data used were: t5°~Eu(T~ = 36.9y; E v = 334 KeV, I v = 96.0%; Er. = 439 KeV, I.: = 80.3%); ~SSTb(~ = 180y; E v = 944 KeV, I v = 43.9%). These values are slightly different than those used by us previously (cf. Qaim et al., i 992) but are in accordance with the values adopted during the CRP (cf. Vonach and Wagner, 1993).

Neutron energies and flux densities The neutron energy effective at each sample was calculated using the Monte Carlo program NEUT (cf. Birn, 1992). The neutron flux densities in front and at the back of the sample were determined via the monitor reaction 27Al(n,ct)24Na(T½= 15.0h; Er = 1368 KeV; I v = 100%), whose cross-sections were taken from the literature (IRDF, 1990). In our previous work on europium and terbium (cf. Qaim et al., 1992), relatively long cylindrical samples were used and therefore a special flux averaging method had to be applied. In contrast, in the present work the sample dimensions were smaller and more welldefined. The average flux density was therefore obtained by taking an arithmetic mean of the values in front and at the back of the sample.

Corrections for interferences Three types of corrections were considered in the present measurements to take account of the possible interferences. (a) Effect of background neutrons. Our recent studies showed (cf. Gr~illert et al., 1994) that the neutron background is appreciably decreased when the collimator and beam stop are made of tungsten. The gas targets both at Jfilich and Debrecen now make use of tungsten. For the high threshold (n,2n) processes investigated in this work, therefore, no empty gas target irradiations were performed. In the case of the monitor reaction 27Al(n:t)24Na, however, a gas out correction of about 5% was necessary at the highest incident deuteron energy of about 10 MeV.

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(b) Effect of (n,~) process. In measurements on the Treatment of previous data J°9Ag(n,2n)l°a~Ag reaction, especially at energies After the publication of our previous paper (Qaim around 10MeV, the 1°TAg(n,7)l°8mAg process could et al., 1992) it was realized that, due to a major repair interfere seriously. This was avoided through the use of the compact cyclotron CV28, the deuteron energies of highly enriched l°gAg as target material. In each recorded via adjustment of cyclotron parameters irradiation the thermal neutron flux (4~th) was were somewhat high and a normalization factor had determined via the J°gAg(n,7)]~°mAg process induced to be used (cf. Birn and Qaim, 1994). Recent accurate in the sample itself as well as via the 197Au(n,),)lgSAu deuteron energy measurements (cf. Korm~ny, 1994) process induced in an Au foil placed in front of confirmed that observation. We therefore recalcuthe sample. The two values agreed within 8%, and lated the older data on 15~Eu(n,2n)~5°mEu and the averaged qSth amounted to < 1% of ~bfast. Based ~59Tb(n,2n)J58Tb reactions using the new energy on this value and the isotopic composition of the scale. The lowest energy point in the case of enriched ~°~Ag sample, the interference from the ~59Tb(n,2n)lSSTb reaction, however, had such an in]°TAg(n,~/)l°8mAg process to the J°gAg(n,2n)l°SmAg creased error that it appeared meaningless to include reaction was found to be negligibly small (<0.5%). it in the present evaluation. The effect of thermal neutrons during 14.5 MeV neutron irradiation was corrected by using the Results and Discussion HSIn(n,),)H6In reaction and by measurement of the relative change in the ~°SmAg activity at different The results of present measurements are summarpoints of an extended sample. ized in Table I. Our previous data (cf. Qaim et al., (0 Effect of y-background. Due to the very low 1992) corrected for energy and other related quancount rates of the three products under investigation, tities (e.g. monitor cross-section and consequently a check of the ~-background was mandatory. All neutron flux density), are given in Table 2. measurements were started after a "cooling time" of The data for the three reactions, 1°gAg(n,2n) l°SmAg, 15JEu(n,2n)lS°mEu and several months so that the undesired shorter-lived viz. activities had decayed out. Furthermore, special care ~59Tb(n, 2n)JSSTb, now available near the thresholds, was taken to analyse the weak ~,-rays from some are shown in Figs 1-3, respectively. The cross-secstronger "f-rays present in their vicinity. tions reported recently by Yu Weixiang et al. (1995) are also given. In the higher energy region, however, Calculation of cross-sections and estimation of errors for each reaction only the evaluated value at The count rate was converted to decay rate using 14.5 MeV is shown since it carries a high confidence the ~/-ray emission probability and the efficiency of level. It is worth mentioning that our cross-section the detector (incorporating self-absorption, geometry value for the I°gAg(n,2n)~°SmAgreaction at 14.5 MeV and pile-up corrections). A correction for the coinci- (cf. Table 1) agrees well with the evaluated value. dence loss was applied only for ~5°mEu, as described The averaged results of nuclear model calculations earlier (cf. Qaim et al., 1992). The cross-sections were (cf. Chadwick et al., 1993) are reproduced in Figs 1-3. then calculated using the well-known activation In the case of l°gAg(n,2n)l°SmAg reaction (Fig. l) the equation. experimental and theoretical data agree very well. The principal sources of errors and their magni- Also for the 151Eu(n,2n)lS°mEu and 159Tb(n,2n)]~STb tudes while using extended samples were described reactions there are now no serious discrepancies earlier (cf. Qaim et al., 1992). In the present work the between the experimental and theoretical data; only errors were slightly smaller since the samples had in the low energy region there appear to be still some better dimensions and the counting geometry was disagreements. Below 10MeV, for both reactions well defined. We estimate that the uncertainty in the some slight adjustments of the evaluated curves may efficiency of the detector for the present counting therefore be necessary. geometry was about 4% and that in the averaging of Our measurements near the thresholds of the three neutron flux < 2 % . All of the other uncertainties reactions under consideration confirm the general were the same as given earlier. The total error, validity of nuclear model calculations. The results of obtained by summing all the individual errors in this CRP have thus shown that the approach of quadrature, was estimated to be between 9 and 15%. performing accurate measurements in the 14MeV

Table I. Activationcross-sectionsmeasured in this work ICaAg(n,2n)lt~mAg ISlEu(n,2n)~'~mEu IS~Tb(n,2n)lSSTb En*(MeV) a (rob) E,*(MeV) a (mb) E~"(MeV) o"(mb) 10.86+0.27 276-+36 10.83_+0.30 849,+102 10.83_+0.30 1410_+145 12.17_+0.30 451 - + 5 8 11.20,+0.30 952_+142 14.50_+0.25 6 9 7 - + 6 0 12.14+0.30 982-+127 *The deviationgives the uncertaintyof the mean neutron energy,

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~ 301) 200 iO0 0

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l0 II 12 Neutron energy [ M e V ]

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Fig. I. Excitationfunction of mAg(n,2n)~°emAg process.At 14.5 M e V only the evaluated value of Vonach and Wagner (1993) is shown since it carries a high confidence level(our value of 697 + 60 m b being in excellent agreement). The theoreticalcurve was taken from Chadwick et al. (1993).

1500

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This work Our recalculated data Yu Weixiang Eval. 14.5MeV

/

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07 . . . . . . . .

~

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9 .........

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ISIEu(n'2n)lS0mEu

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Fig. 2. Excitation function of ISIEu(n,2n)IS°~Eu process. At 14.5 MeV only the evaluated value of Vonach and Wagner (1993) is shown since it carries a high confidence level. The theoretical curve was taken from Chadwick et al. (1993).

Excitation functions of t°gAg(n,2n)l°SmAg, 'SIEu(n,2n)lS°mEu and 159Tb(n,2n)lSSTb reactions

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158Tb

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Neutron energy [MeV] Fig. 3. Excitation function of Z59Tb(n,2n)tSSTbprocess. Other details are the same as in Fig. 2. Table 2. Cross-sections obtained after renormalization of previous data (cf. Qaim et al., 1992) Cross-section (mb) En*(MeV) 9.18 +_0.24 9.60 ___0.24 10.10 _+0.24

ISIEu(n,2n)l~°mEu JSgTb(n,2n)lSSTb 164 +_23 333 + 32 250 _+28 547 + 60 335 + 32 828 + 99

energy range (where higher n e u t r o n fluxes are available) a n d using the evaluated value for normalization of the nuclear model calculationai results, provides a fairly reliable excitation function of a n (n,2n) reaction, even if a n isomeric state o f the p r o d u c t nucleus is involved. However, whether the same applies to other types o f reactions like (n,p), (n,n',p), (n,~), ( n , n ' ~ ) etc. is a r a t h e r open question. Acknowledgements--We would like to thank Professor (Emeritus) G. St6cklin for his continued support of the German-Hungarian cooperation in the field of nuclear data. This work was done under the auspices of the IAEA-CRP on the Generation of Long-lived Radionuclides of Importance in Fusion Reactor Technology. It was partly supported by the Hungarian Research Foundation (OTKA T 016713).

References Birn I. (1992) NEUT--Ein Programm zur Berechnung von Neutronenspektren erzeugt durch die D(d,n)3He-Reak tion in einem Gastarget am Zyklotron. KFA Jiilich, Internal Report INC-IB-1/92. Birn I. and Qaim S. M. (1994) Excitation functions of neutron threshold reactions on some isotopes of germanium, arsenic and selenium in the 6.3 to 14.7 MeV energy range. Nucl. Sci. Engng 116, 125. Chadwick M. B., Gardner M., Gardner D., Grudzevich O. T., Ignatyuk A. V., Meadows J. W., Pashchenko A., Yamamuro N. and Young P. G. (1993) Intercomparison of theoretical calculations of important activation cross

sections for fusion reactor technology. Preprint UCRLJC-115239, cf. also Pashchenko (1993), p. 123. Gr~.llert A., Csikai J. and Qaim S. M. (1994) Improved gas-cell D-D neutron sources. Nucl. Instr. Meth. A337, 615. IRDF (International Radiation Dosimetry File) (1990) Issued by the International Atomic Energy Agency, Vienna, as a computer file. Korm~iny Z. (1994) A new method and apparatus for measuring the beam energy of cyclotron beams. Nucl. Instr. Meth. A337, 258. Meadows J., Smith D., Greenwood L., Haight R., Ikeda Y. and Konno C. (1993) Results from the Argonne, Los Alamos, JAERI collaboration, cf. Pashchenko (1993), p. 13. Pashchenko A. B. (1993) Activation cross sections for the generation of long-lived radionuclides of importance in fusion reactor technology. Proc. Second Res. Coord. Meet., Del Mar, April 1993, IAEA Vienna, Report INDC (NDS)-286/L. Qaim S. M., Cserp~k F. and Csikai J. (1992) Cross sections of 151Eu(n,2n)lS°mEu and 159Tb(n,2n)tSSTb reactions near their thresholds. Appl. Radiat. Isot. 43, 1065. Vonach H. and Wagner M. (1993) Evaluation of some activation cross sections for formation of long-lived activities important for fusion reactor technology, cf. Pashchenko (1993), p. 67. Wang DaHai (1990) Activation cross sections for the generation of long-lived radionuclides of importance in fusion reactor technology. Proc. Consult. Meet., Argonne, September 1989, IAEA Vienna, Report INDC (NDS)-232/L. Wang DaHai (1992) Activation cross sections for the generation of long-lived radionuclides of importance in fusion reactor technology. Proc. First Res. Coord. Meet., Vienna, November 1991, IAEA Vienna, Report INDC (NDS)263/G + Sp. Yu Weixiang, Zhao Wenrong, Cheng Jiangtao and Lu Hanlin (1995) Cross sections of 1°gAg(n,2n)~°8"Ag, 151Eu(n,2n)JS°mEu, 159Tb(n,2n)lSSTband 179Hf(n,2n)lTSmHf reactions at 9.5 and 9.9 MeV, Report to the Third Research Coordination Meeting of the CRP on Activation Cross Sections, St Petersburg, June 1995, to be published as IAEA report.