Activation cross-sections for 158Dy(n,p)158Tb, 156Dy(n,α)153Gd and 160Dy(n,p)160Tb reactions induced by neutrons at 14.7 MeV

ARTICLE IN PRESS Applied Radiation and Isotopes 67 (2009) 1892–1896

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Activation cross-sections for 158Dy(n,p)158Tb, 156Dy(n,a)153Gd and 160 Dy(n,p)160Tb reactions induced by neutrons at 14.7 MeV Junhua Luo a,b,, Xinxing Wang a, Zhenlai Liu a, Fei Tuo b, Xiangzhong Kong b a b

Department of Physics, Hexi University, Zhangye 734000, China School of Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, China

a r t i c l e in f o

a b s t r a c t

Article history: Received 5 July 2008 Received in revised form 25 April 2009 Accepted 25 May 2009

The cross-sections for the 158Dy(n,p)158Tb, 156Dy(n,a)153Gd and 160Dy(n,p)160Tb reactions induced by 14.7 MeV neutrons were measured in this work and calculated by a previously developed formula. Measurements were corrected for gamma-ray attenuations, random coincidence (pile-up), dead time and fluctuation of neutron flux. Nuclear model calculations using the code HFTT, which employs the Hauser-Feshbach (statistical model) and exciton model (precompound effects) formalisms, were undertaken to describe the formation of the products. Crown Copyright & 2009 Published by Elsevier Ltd. All rights reserved.

PACS: 25.40.h 24.60.Dr 24.10.i 23.60.+e Keywords: Neutrons Nuclear reaction Cross-section Activation technique Model calculation Dysprosium

1. Introduction Experimental data of neutron-induced reactions in the energy range around 13–15 MeV are needed to verify the accuracy of nuclear used in the calculation of cross-sections. Furthermore, the data are of considerable importance for practical applications, such as for integral calculations on the first wall, blanket and shield of a conceptual fusion power reactor. The data for gas production via neutron induced reactions are of great importance in the domain of fusion reactor technology, particularly of nuclear transmutation rates, nuclear heating and radiation damage due to gas formation. A lot of experimental data on neutron induced cross-sections for fusion reactor technology applications have been reported and great efforts have been devoted to compilations and evaluations (CINDA-A, 2000; Mclane et al., 1988). The variations in the cross-sections with the neutron energy are also of great interest for studying the excitation of nuclei to different energy levels and subsequent decay to ground state, either

 Corresponding author at: Department of Physics, Hexi University, Zhangye 734000, China. Tel.:+86 9368282065. E-mail address: [email protected] (J. Luo).

directly or through different energy levels including metastable states. Dysprosium (Dy) is one of the rare-earth isotopes. The neutron total cross-section of dysprosium (Dy) is of great importance not only for the design and development of nuclear reactors but also for the basic study of neutron interaction with nuclei. Dy is a useful absorbing material for the control rods of thermal reactors due to its large neutron cross-sections in the thermal neutron energy region. The reaction cross-sections of 160Dy(n,p)160Tb at the neutron energies around 14 MeV were investigated by various studies (Qaim, 1976; Weigel et al., 1975; Kong et al., 1998), but most measurements were obtained before 1980. Furthermore, there were large discrepancies in those data. The large discrepancies are probably due to the implementation of different methods, detectors, target materials and to the adoption of different nuclear parameters. Thus, it is necessary to measure them again and obtain excitation functions around the neutron energies of 14 MeV. For 158Dy(n,p)158Tb and 156Dy(n,a)153Gd reactions of dysprosium isotopes, the cross-section s has not been reported in the literature. Concerning the cross-sections of 162Dy(n,p)162Tb, 163 Dy(n,p)163Tb, 162Dy(n,a)159Gd, 164Dy(n,a)161Gd, 156Dy(n,2n)155 Dy and 158Dy(n,2n)157Dy reactions in neutron energies

0969-8043/$ - see front matter Crown Copyright & 2009 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.apradiso.2009.05.013

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2. Experimental Cross-sections were measured via neutron activation and identification of the radioactive products. This technique is very suitable for investigating low-yield reaction products and closely spaced low-lying isomeric states, provided that their lifetimes are not too short. The details have been described over the years in many publications (Luo et al., 2007a, b; Rahman and Qaim, 1985; Bostan and Qaim, 1994; Cserpa´k et al., 1994; Nesaraja et al., 2003). Here we give some salient features relevant to the present measurements. 2.1. Samples and irradiations

where Md, Mn and Ma are the masses of deuteron, neutron and alpha particle, respectively. The effective D–T neutron energy at irradiation position was determined by the Nb/Zr method (Pavlik et al., 1982; Nethaway, 1978; Lewis and Zieba, 1980). The measured neutron energy was shown in Fig. 1 together with the calculation using Eq. (1). The uncertainty in the neutron energy at 5 cm was estimated to be 200 keV from a consideration of the sample sizes, d+ beam diameter of about 3–4 mm, and the uncertainty in the Nb/Zr method (Lewis and Zieba, 1980). 2.3. Measurement of radioactivity After having been irradiated, the samples (Dy2O3) and monitor foils (Al) were cooled for about 60 days and 10 h, respectively, and the gamma ray activity of 158Tb, 153Gd, 160Tb, and 24Na was determined by a CH8403 coaxial high-purity germanium detector made in the People’s Republic of China with a relative efficiency of 20% and an energy resolution of 3 keV at 1332 keV. The efficiency of the detector was calibrated using the standard gamma source, Standard Reference Material 4275 from the National Institute of Standards and Technology, Washington, D.C., USA. An absolute efficiency calibration curve was obtained at 20 cm from the

15.0 14.8 Neutron Energy (MeV)

13.5–14.8 MeV have been reported by several authors (Qaim, 1974, 1976; Kong et al., 1998; Sakane et al., 1996; Wille and Fink, 1960; Bari, 1982; Coleman et al., 1959; Jaskola et al., 1968; Khurana and Hans, 1959, 1960; Khurana and Govil, 1965). In the present work, the cross-sections of the 158Dy(n,p)158Tb, 156Dy(n,a)153Gd and 160 Dy(n,p)160Tb reactions were measured at neutron energy 14.7 MeV and a gamma-ray counting technique was applied using high-resolution gamma-ray spectrometer and data acquisition system. Pure Dy2O3 was used as target material. The reaction yields were obtained by absolute measurement of the gamma activities of the product nuclei using a coaxial high-purity germanium detector. The neutron energy in this measurement was determined by cross-section ratios for the 90Zr(n,2n)89m+gZr and 93Nb(n,2n)92mNb reactions (Lewis and Zieba, 1980). The total cross-section of the 158Dy(n,p)158Tb, 156Dy(n,a)153Gd and 160 Dy(n,p)160Tb reactions were compared with the comprehensive evaluation data in ENDF/B-VII.0 library, and with values of model calculations including the pre-equilibrium contribution. The cross-sections of 158Dy(n,p)158Tb and 156Dy(n,a)153Gd reactions were first reported here.

1893

Nb/Zr method Ed = 125keV

14.6 14.4 14.2 14.0 13.8 13.6 13.4 13.2 -150

-100

-50 0 50 Angle (degree)

100

150

Fig. 1. Angular dependence of d–T neutron energy. The solid circles show experimental data determined by the Nb/Zr method (Lewis and Zieba, 1980).

103.18 keV

218.22 keV 298.58 keV

197.04 keV

105

104 Counts

About 3 g of Dy2O3 powder of natural isotopic composition (499.99% pure) was pressed at 10 ton/cm2, and a pellet, 0.2 cm thick and 2.0 cm in diameter, was obtained. Three such pellets were prepared. Monitor foils of Al (99.999% pure, 0.04 mm thick) of the same diameter as the pellets were then attached in the front and at the back of each sample. Irradiation of the samples was carried out at the ZF-300-II Intense Neutron Generator at Lanzhou University and lasted 22 h with a yield 1 to 3  1012 n/s. Neutrons were produced by the T(d,n)4He reaction with an effective deuteron beam energy of 125 keV and beam current of 20 mA. The tritium–titanium (T–Ti) target used in the generator was 0.9 mg/cm2 thick. The neutron flux was monitored by a uranium fission chamber so that corrections could be made for small variations in the yield. The groups of samples were placed at 01 angle relative to the beam direction and centered about the T–Ti target at distances of 5 cm. Cross-sections for 27Al(n,a)24Na reaction were selected as monitors to measure the reaction cross-section on several Dy isotopes.

944.09 keV

2.2. Determination of the incident neutron energy In the D–T reaction (Q value of 17.6 MeV), induced by deuterons of energy Ed, the kinetic energy En of the neutrons emitted at angle y can be estimated (Curtis, 1969) from the following:

103

200 ðEn Þ1=2 ¼

ðM d M n Ed Þ1=2 cos y þ ðM d M n Ed cos2 y þ fM a þ M n g½M a Q þ Ed ðM a  M n ÞÞ1=2 Ma þ Mn

(1)

400

600 800 γ-ray energy (keV)

1000

1200

Fig. 2. The g-ray spectrum of about 60 days after the end of the irradiation.

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surface of the germanium crystal. At this distance the coincidence losses can be considered to be negligible. In our case, however, we needed to calibrate the efficiency at 2 cm, which was the actual counting position used due to the weak activity of the sample. Therefore, we selected a set of single g-line sources and placed them at two positions (20 and 2 cm) successively to measure their efficiency ratios to be able to evaluate the efficiency ratio curve as a function of energy. The absolute efficiency calibration curve at 2 cm was obtained from the calibration curve at 20 cm and the efficiency ratio curve. The error in the absolute efficiency curve at 2 cm was estimated to be 2–3%, while the error of the activity of the standard source is 1.0%. A typical gamma-ray spectrum is shown in Fig. 2. The decay characteristics of the product radioisotopes and the natural abundances of the target isotopes under investigation are summarized in Table 1 (Browne and Firestone, 1996). 2.4. Calculation of cross-sections and their uncertainties The measured cross-sections can be calculated by the following formula (cf. Luo et al., 2005):

sx ¼

½SIg ZKMD0 ½lAFCx s0 ½SIg ZKMDx ½lAFC0

the following: fs ¼

fg ¼

mh 1  expðmhÞ

(5)

ðD þ ðh=2ÞÞ2

(6)

D2

Here m (cm1) is the linear attenuation coefficients in Dy2O3 for gamma rays at each of the photon energies E, h (cm) is the thickness of the sample and D is the distance from the measured sample to the surface of the germanium crystal. The main error sources in our work result from counting statistics (1–15%), standard cross-sections uncertainties (1%), detector efficiency (2–3%), weight of samples (0.1%), self-absorption of gamma-ray (0.5%) and the coincidence sum effect of cascade gamma-rays (0–5%), the uncertainties of irradiation, cooling and measuring times (0.1–1%), etc. Some other minor error contribution from the parameters of the measured and standard nuclei, such as uncertainties of the branching ratio of the characteristic gamma rays, uncertainties of the half life of the radioactive product nuclei and so on, have also been considered.

(2) 3. Nuclear model calculations

where the subscript m represents the term corresponding to the monitor reaction and subscript x corresponds to the measured reaction. e is the full-energy peak efficiency of the measured characteristic gamma-ray, Ig is the gamma-ray intensity, Z is the abundance of the target nuclide, M is the mass of sample, D ¼ elt1  elt2 is the counting collection factor, t1, t2 are the time intervals from the end of the irradiation to the start and end of counting, respectively, A is the atomic weight, C is the measured full energy peak area, l is the decay constant, K is the neutron fluence fluctuation factor: " #, L X Fi ð1  elDti ÞelT i FS (3) K¼ i

where L is the number of time intervals into which the irradiation time is divided, Dti is the duration of the ith time interval, Ti is the time interval from the end of ith interval to the end of irradiation, Fi is the neutron flux averaged over the sample during Dti, S ¼ 1  elT is the growth factor of the product nuclide, T is the total irradiation time, F is the neutron flux averaged over the sample during the total irradiation time T, F is the total correction factor of the activity: F ¼ fc  fs  fg

(4)

where fc, fs and fg are correction factors for the self-absorption of the sample at a given gamma-energy, the coincidence sum effect of cascade gamma-rays in the investigated nuclide and in the counting geometry, respectively. Coincidence summing correction factor fc was calculated by this method (McCallum and Coote, 1975). In turn, the gamma-ray attenuation correction factors fs in the Dy2O3 foil and the geometry correction fg were calculated by

The total cross-sections 158Dy(n,p)158Tb, 156Dy(n,a)153Gd and Dy(n,p)160Tb reactions were calculated using the code HFTT (Huang et al., 1989). HFTT employs the Hauser–Feshbach (statistical model) and preequilibrium exciton model (precompound effects) formalisms. The formation cross-section and the inverse reaction cross-section were calculated using optical model. Local and global phenomenological optical potential was applied. The optical parameters given by Becchetti et al. (1969, 1972), Lohr and Haeberli (1974) and McFadden and Satchler (1966) for neutrons, protons and alpha particles were used. The code HFTT is mainly used for calculations of the cross-sections of nuclear reactions induced by light ions on medium heavy target nuclei. Only a few input parameters are needed to run the code. The energy and mass number dependence of the effective matrix element for internal transition rate was accepted as |M|2 ¼ KA3E1, where A is the mass number and E is the excitation energy of the compound nucleus. The free parameter K of internal transition matrix element was taken as 260 MeV3 for 158 Tb, 300 MeV3 for 153Gd and 700 MeV3 for 160Tb. 160

4. Results and discussion The cross-sections measured in our work are summarized in Table 2. The cross-section for the 27Al(n,a)24Na reaction, 111.970.5 mb at 14.770.2 MeV incident neutron energy, was obtained by interpolating the values of Wagner et al. (1990) and used as the monitor to measure the 158Dy(n,p)158Tb, 156 Dy(n,a)153Gd and 160Dy(n,p)160Tb reaction cross-sections.

Table 1 Reactions and associated decay data of activation products. Reaction Dy(n,p)158Tb

Abundance of target isotope (%)

Q-value (MeV)

Mode of decay (%)

Half-life of product

Eg (keV)

Ig (%)

944.09 218.22 103.18 298.58 197.04 1368.6

44.0 0.933 21.4 25.5 5.02 100

158

0.10

0.153

EC (83.4)

180 y

156

0.06 2.34

8.006 1.053

EC (100) b (100)

241.6 d 72.3 d

3.13

b (100)

14.959 h

Dy(n,a)153Gd Dy(n,p)160Tb

160

27

Al(n,a)24Na

100

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Table 2 Summary of cross section measurements. Reaction

This work

Literature values

En (MeV) s (mb)

En (MeV)

14

This work Qaim, (1976) Weigel et al., (1975) Kong et al., (1998) ENDF/B-VII.0 HFTT

12 Cross section (mb)

These cross-section measurements are shown in Figs. 3–5 along with model calculations. A gamma-ray of 944.09 keV (Ig ¼ 44.0%) emitted in the 158Tb decay and 103.08 keV (Ig ¼ 21.4%) gamma-ray emitted in the 153 Gd decay were used to deduce the values of the 158Dy(n,p)158Tb and 156Dy(n,a)153Gd reaction cross-sections, respectively. In the

1895

10 8 6 4

Dy(n,p)158Tb 14.770.2 7.670.4 Dy(n,a)153Gd 14.770.2 2.070.1 160 Dy(n,p)160Tb 14.770.2 2.770.3

s (mb)

Reference

2

158

160

Dy (n,p)160Tb

156

27

24

Al(n,a) Na

14.770.3 9.371.0 14.2070.45 1.870.2 14.770.2 7.270.8 14.770.2 111.970.5

Qaim (1976) Weigel et al. (1975) Kong et al. (1998) Wagner et al. (1990)

158

10

12 14 16 Neutron energy (MeV)

18

20

Dy(n,p)158Tb

15 Cross section (mb)

8

Fig. 5. The measured cross-sections of the 160Dy(n,p)160Tb reaction at neutron energy 14.7 MeV compared to the previous measurements, the evaluation data from ENDF/B-VII.0 library, and the results of nuclear model calculation using the code HFTT.

20

10

5

This work ENDF/B-VII.0 HFTT

0 10

11

12

13

14

15

16

17

18

19

20

Neutron energy (MeV) Fig. 3. The measured cross-sections of the 158Dy(n,p)158Tb reaction at neutron energy 14.7 MeV compared to the evaluation data from ENDF/B-VII.0 library, and the results of nuclear model calculation using the code HFTT.

6

case of 158Dy(n,p)158Tb reaction (see Fig. 3), our result is in agreement with the nuclear model calculation using the code HFTT, but our value and calculated cross-section are lower than evaluated data from ENDF/B-VII.0. Regarding the 156Dy(n,a)153Gd reaction, from Fig. 4 can be seen that there are large discrepancies between the evaluated data from ENDF/B-VII.0 and the theoretical (HFTT) cross-sections; present measured cross-section is between those of the evaluated data and results of the HFTT. It can be seen from Table 2 and Fig. 5 that for the cross-sections of 160 Dy(n,p)160Tb reaction, our measurement is in very good agreement with the nuclear model calculation using the code HFTT, but present value, result of Weigel et al. (1975), and calculated cross-section are lower than those of ENDF/B-VII.0, Qaim (1976) and Kong et al. (1998). Regarding the large discrepancies between the present result and the previous ones by Kong et al. (1998), the probable reason is the different plan of irradiation and cooling time and adoption of slightly different nuclear parameter (Ig ¼ 25.5% and 26.8%, respectively). It must be pointed that the cross-sections of 158Dy(n,p)158Tb and 156 Dy(n,a)153Gd reactions were first reported here.

5. Conclusions 156Dy

(n,)153Gd

5 Cross section (mb)

0

4 3 2 This work ENDF/B-VII.0 HFTT

1 0 8

10

12 14 16 Neutron energy (MeV)

18

20

Fig. 4. The measured cross-sections of the 156Dy(n,a)153Gd reaction at neutron energy 14.7 MeV compared to the evaluation data from ENDF/B-VII.0 library, and the results of nuclear model calculation using the code HFTT.

The accuracy of the cross-sections obtained in the early years was limited, because there are seven different isotopes (156Dy(0.06%), 158Dy(0.10%), 160Dy(2.34%), 161Dy(18.9%), 162 Dy(25.5%), 163Dy(24.9%) and 164Dy(28.2%)) for dysprosium and overlapping of the gamma-peaks from different isotopes could not be neglected in the gamma-spectrum measurements with the use of low resolution power NaI(Tl) spectrometers or Ge(Li) detectors. The decay data, such as half life and branching ratio of the radioactive product nuclei were less accurate at that early time. In recent years, the reliability of the results has been improved with the use of high-resolution Ge semiconductor detectors and the more accurate values of the above-mentioned parameters. A good plan of irradiation and cooling time is also important and in our experiment we selected different irradiation times according to the half life of the product nuclei. We have measured the activation cross-sections for 158 Dy(n,p)158Tb, 156Dy(n,a)153Gd and 160Dy(n,p)160Tb reactions induced by 14.770.2 MeV neutrons. Our work gives more accurate measurement of these reaction cross-sections since we

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used more new nuclear data for decay characteristics of the product nuclei and the natural abundances and we also used a high-purity germanium detector (HPGe) which has better energy resolution than GeLi detectors used by early experimenters. In addition, the present measurements were performed in the Low Background Laboratory of Lanzhou University and disturbance from environmental radiation was reduced to a very low level. In conclusion, our data would improve the quality of the neutron cross-section database.

Acknowledgments We would like to thank the Intense Neutron Generator group at Lanzhou University for performing the irradiations. This work was supported by the Program for Long Yuan Young Innovative Talents of Gansu Province, China, and Scientific Research Start up Outlay of High-Position Talent in Hexi University in Gansu Province, China. References Bari, A., 1982. 14.8 MeV neutron activation cross-sections for (n,p) and (n,a) reactions of some rare earth nuclides. J. Radioanal. Chem. 75, 189. Becchetti, F., Greenlees, G.W., 1969. Nucleon-nucleus optical-model parameters, A440, Eo50 MeV. Phys. Rev. 182, 1190. Becchetti, F., 1972. Polarixation Phenomena in Nuclear Reactions, The University of Wisconcin Press, p. 682. Bostan, M., Qaim, S.M., 1994. Excitation functions of threshold reactions on 45Sc and 55Mn induced by 6 to 13 MeV neutrons. Phys. Rev. C 49, 266. Browne, E., Firestone, R.B., 1996. Table of Isotopes. Wiley, New York. CINDA-A, 2000. The Index to Literature and Computer Files on Microscopic Neutron Data. International Atomic Energy Agency, Vienna. Coleman, R.F., Hawker, B.E., O‘Connor, L.P., Perkin, J.L., 1959. Cross sections for (n,p) and (n,a) reactions with 14.5 MeV neutrons. Proc. Phys. Soc. (London) 73, 215. Cserpa´k, F., Suda´r, S., Csika, J., Qaim, S.M., 1994. Excitation functions and isomeric cross section ratios of the 63Cu(n,a)60Com,g, 65Cu(n,a)62Com,g, and 60 Ni(n,p)60Com,g processes from 6 to 15 MeV. Phys. Rev. C 49, 1525. Curtis, L.F., 1969. Introduction to Neutron Physics. D. Van Nostrand Company, Princeton, NJ. ENDF/B-VII.0 (Kim, H.I., Mughabghab, S. F., Herman M.W., Oblozinsky P.), 2006. Evaluated nuclear data file (ENDF). Database version of July 23, 2008, USA, /http://www-nds.iaea.org/exfor/endf.htmS. Huang, F.-Z., Shi, Z.-M., Bao, S.-L., 1989. Calculations of excitation functions with the code HFTT. Univ. Pekinensis 25, 289 (in Chinese). Jaskola, M., Osakiewicz, W., Turkiewicz, J., Wilhelmi, Z., Zemlo, L., Rogulski, Z., Madej, M., Stodolska, A., Zych, B., Glowacka, L., 1968. (n,a) reactions on heavy nuclei induced by 14 MeV neutrons. Nucl. Phys. A 110, 11. Khurana, C.S., Hans, H.S., 1959. Measurements of (n,p),(n,a) and (n,2n) total crosssections at 14 MeV. Nucl. Phys. 13, 88.

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