Measurements of 181Ta(n,2n)180Ta reaction cross-section at the neutron energy of 14.78 MeV

Indian Journal of Pure & Applied Physics Vol. 58, April 2020, pp. 228-233 Measurements of 181Ta(n,2n)180Ta reaction cross-section at the neutron energy of 14.78 MeV Bhargav K Sonia*, Siddhart Parasharia, S Mukherjeea, Rajnikant Makwanaa, M Mehtab, R Chauhana, S V Suryanarayanac, I Pashad, L S Danuc, H Naike, B K Nayakc, J Varmuzaf & K Katovskyf aDepartment of Physics, Faculty of Science, The M S University of Baroda, Vadodara 390 002, India bInstitute for Plasma Research, Gandhinagar 382 428, India cNuclear Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India dDepartment of Physics, Bangalore University, Bengaluru 560 056, India eRadio Chemistry Division, Bhabha Atomic Research Center, Mumbai 400 085, India fDepartment of Electrical Power Engineering, Brno University of Technology, Brno, Czech Republic Received 17 February 2020 The cross-section of the 181Ta(n,2n)180Ta reaction has been measured with respect to the 197Au(n,2n)196Au monitor reaction at the incident neutron energy of 14.78± 0.20 MeV, using neutron activation analysis and off-line γ-ray spectrometric technique. The present measurement has been done at the energy where discrepant measured results are available in the EXFOR data library. The result has been compared with evaluated data libraries JEFF-3.3 and ENDF/BVII.1. The present result has also been supported by theoretical predictions of nuclear model code TALYS1.8 and TALYS1.9. The uncertainty and the correlations among the measured cross-section has been studied using co-variance analysis. Keywords: Neutron Activation Analysis (NAA), 3H(2H, n)4He reaction neutron, Off-line γ-ray spectrometry, Co-variance analysis, TALYS1.8, TALYS-1.9. 1 Introduction Nuclear reaction cross-section data are very useful for upcoming nuclear technologies like Accelerator Driven Subcritical System (ADSs) and Advance Heavy Water Reactor (AHWER)1,2. Tantalum (Ta) is regarded as a control-rod material for lead-bismuth cooled fast reactors. Moreover, neutron-induced activation cross-sections of tantalum are needed3 as for the decommissioning of light-water reactors4. Also, it is very crucial for both nuclear fission and fusion applications. It has a major constituent of the low activation ferritic-martensitic steel Eurofer which is qualified for future fusion reactors, it means of test irradiations in the International Fusion Irradiation Facility (IFMIF). Tantalum is a candidate material of the spallation target5 to be used for accelerator driven subcritical systems (ADSs). The cross-section of 181Ta(n,2n)180Ta reaction around 14 MeV has been reported by various authors6-10, but most of them were obtained before 1992, furthermore, there was disagreement in those data with theoretical nuclear code TALYS-1.8 and TALYS-1.911. The natural tantalum consists of two isotopes, namely, 180mTa (0.012%) and 181Ta (99.988%). 181Ta is a stable isotope, while 180mTa is in the meta stable state that decays to 180Ta by isomeric transition. From the compilation of EXchange FORmat12 (EXFOR) data library it can be seen that, very few cross-section data are available for the 181Ta(n,2n)180Ta reaction from threshold to 20 MeV neutron energies. In the present work, we have measured the crosssection datum for the 181Ta(n, 2n)180Ta nuclear reaction for the incident particle energy of 14.78 ± 0.20 MeV. The measured cross-sections have also been compared with available different level density models of the theoretical nuclear code TALYS-1.8 TALYS-1.911. The present work is based on experimental measurement of the cross section in which the uncertainties have been calculated by using the covariance analysis13,14. This covariance analysis —————— *Corresponding author (Email: [email protected]) SONI et al.: MEASUREMENTS OF 181Ta(n,2n)180Ta REACTION CROSS-SECTION 229 help in identifying the error and the relative correlation among the different quantities used in the cross-section measurement. The present work was carried out to improve the evaluated nuclear data library on 181Ta(n,2n)180Ta reaction by considering the recent knowledge on experimental and theoretical nuclear physics. 2 Experimental Techniques The measurements of the 181Ta(n,2n)180Ta and 197Au(n,2n)196Au reactions were carried out at the Cockcroft–Walton voltage multiplier accelerator of Purnima at Bhabha Atomic Research Center (BARC), Mumbai, India by using Neutron Activation Analysis (NAA) technique followed by γ-ray spectroscopy. D+ ions were used to produce suitable energies of neutrons. The D+ ions were produced in an RF ionsource and then accelerated up to 300 kV, the accelerated D+ ions were incident on titanium–tritium (TiT) target to produce 14.1 MeV neutrons through the 3H(2H,n)4He (Q-value=17.59 MeV) reaction15. In the present experiment the D+ ions were accelerated to 99.71 keV which was impinge on titanium–tritium (TiT) target. This collision produces neutrons of energy 14.78 ± 0.20 MeV in the laboratory frame through the 3H(2H,n)4He reaction, in at nearly forward angles. A Schematic diagram of the experimental set up for the neutron irradiation set up is given in Fig. 1. In our irradiation set up, the Ta and Au sample of area about 1 × 1 cm2 were separately wrapped with Al foil of thickness 0.011 mm to shield the radioactive contamination during irradiation. The weights of natTa and natAu metal foils were 239.12 mg and 334.30mg with 99.97 % purity, respectively. The stack of Ta-Au placed at a distance of 1.5 cm from the neutron generating (T-Ti) target at zero degree angle relative to the beam direction. The stack foils of Ta-Au were irradiated for 1.5 h. After the irradiation, samples were allowed to cool for 0.3 h to accumulate the radiation dose. The radioactive samples of Ta and Au along with Al wrapper were mounted on different Perspex plates and then taken for γ-ray spectrometry. The γ-ray counting of the irradiated samples (Ta & Au) were carried out using a lead shielded precalibrated 185-cc Baltic HPGe detector coupled with the PC-based 4096 channel analyzer. In the γ-ray counting, dead time of the detector was always kept less than 4% by keeping the mounted samples at a suitable distance from the detector end cap. The data acquisition was done using a CAMAC based LAMPS (Linux Advance Multi Parameter System) software. A standard 152Eu source was used for the energy and efficiency calibration. The resolution of the detector system during counting was measured as 1.8 keV at 1332 keV of 60Co. A typical gamma ray spectrum obtained from the irradiated natTa sample at 14.48 ± 0.20 MeV neutron energy is shown in Fig. 2. 3 Data Analysis 3.1 Estimation of HPGe detector efficiency The efficiency (ε) of the HPGe detector system is determined by using the standard 152Eu source with the help of their known characteristic γ-ray energies. The variation in the efficiency with the γ-ray energy is totally independent with the detector geometry, but its absolute value depend on geometry16. The geometry dependent efficiency of the HPGe detector is given by the relation: 𝜀= 𝐾𝑐𝐶 𝐴0𝐼𝛾𝑒−𝜆𝑇 𝛥𝑡 … (1) where C is the count of the photo peak of the characteristic 152Eu γ-ray spectrum, A0 is source Fig. 1 — Schematic diagram showing the arrangements used for the neutron irradiation. Fig. 2 — Gamma ray spectrum from irradiated natural Ta isotope at 14.78 ± 0.20 MeV neutron energy. INDIAN J PURE & APPL PHYS, VOL. 58, APRIL 2020 230 activity (6659.21 ± 81.60 Bq as on 1 October 1999), Iγ is the branching intensity of γ-rays were retrieved from NuDat 2.717 database, λ is the decay constant of product nucleus, T is the time interval between the source manufacturing data and observation. The correction factor (Kc) in the measured efficiency due to coincidence summing were estimated using Monte Carlo simulation code EFFTRAN18. The measured values of efficiency along with the polynomial fitting are shown in Fig. 3. 3.2 Neutron flux calculation In relative cross-section measurement, the accurate neutron flux calculation is essential. For the resent work, 197Au(n, 2n)196Au monitor reaction was taken for the measurement of the neutron flux. The daughter product of reaction196Au has a half-life of 6.169 ± 0.006 d19. The neutron flux of the reaction has been calculated by using the following relation: 𝛷= 𝐶𝑜𝑏𝑠 λ 𝑡𝑐 𝑡𝑟 𝑁0𝐼𝛾εσ(1−𝑒−λ𝑡𝑖)(1−𝑒−λ𝑡𝑐)(𝑒−λ𝑤) … (2) where Cobs is the counts observed from the measured natAu γ-ray spectrum, λ is decay constant of 197Au nucleus (s−1),N0number of target atom, Iγ branching intensity (87 ± 3 %) of γ-ray of 196Au nucleus, ε efficiency of the detector related (196Au = 355.7 keV) to chosen γ-ray, σ cross-section of the 197Au(n, 2n)196Au reaction at neutron energy of 14.78 MeV from EXFOR12 data library, tr clock time (s), tc counting time (s), tw cooling time (s), ti =irradiation time (s), Φ is bombarded neutron flux(n cm−2). The spectroscopic data related to the above reaction is given in Table 1. This measured value of the neutron flux from the monitor reaction have been used for estimation of the cross-section. 3.3 Neutron activation analysis The experimental data were evaluated by using the delayed gamma-ray neutron activation analysis (DGNAA) technique. This technique is based on the measurement of the cross-section by irradiating the samples with neutrons in which, the rate of creation of the daughter isotope depends on the number of nuclei available in the target and the incident neutron flux20. This proportionality is given by the cross-section of the reaction. The activated sample during neutron irradiation emits characteristic γ-rays having an adequately long half-life and γ-ray branching abundances. By using the following equation, the cross-section of the reaction can be calculated: σ= 𝐶𝑜𝑏𝑠 λ 𝑡𝑐 𝑡𝑟 𝑁0𝐼𝛾ε𝛷(1−𝑒−λ𝑡𝑖)(1−𝑒−λ𝑡𝑐)(𝑒−λ𝑤) … (3) where all the symbols have their meanings similar to Eq. (2). In the Eq. (3), the peak area of the 197Au(n,2n)196Au reaction from the γ-ray spectrum was measured using LAMPS software. The number of target nuclei (N0) was calculated using the mass of Ta target. ε efficiency of the detector at 93.32 keV γ-ray energy was calculated by extrapolation, Φ is calculated neutron flux from 197Au(n, 2n)196Au reaction were used in the Eq. (3). Other standard parameters of the reactions were taken from theNuDat17 data library. 3.4 Covariance analysis In the present experiment, the cross-sections were measured relative to the monitor reaction, and a Fig. 3 — The measured efficiency with the 152Eu source keeping at 2cm from the detector head. Table 1 — Nuclear Spectroscopic data used in the present measurements Reaction Threshold energy (MeV) Spin state Jπ Half-life Decay mode (%) Eγ (KeV) Iγ (%) 197Au(n,2n)196Au 8.113 ± 0.296 (3/2)+ 6.1669 ± 0.0006 d ε(93) + β-(7) 355.71 ± 0.6 87 ± 3 181Ta(n,2n)180Ta 7.618 ± 0.135 1+ 8.154 ± 0.006 h ε(85) + β-(15) 93.32 ± 0.2 4.5 ± 0.4 SONI et al.: MEASUREMENTS OF 181Ta(n,2n)180Ta REACTION CROSS-SECTION 231 common detector setup was used for recording of the γ-ray spectra from Au and Ta irradiated sample foil. Therefore, the measured cross-section is in correlation with the monitor reaction and with the efficiency of the detector used in the present measurement. The covariance analysis13,14 was performed using a ratio method14. In this analysis, first we calculate the relative correlations and covariance among the efficiencies of the standard (152Eu) and the sample (196Au and 180Ta) gamma lines. By using these correlation factors, which came in the first step, we deduce the correlation factors and the related covariance among the measured cross-section. The partial uncertainty contributing in the efficiency measurement given in Table 2, by using it the correlation and co-variance matrix for the efficiencies are calculated and it shown in Tables 3 and 4. The uncertainty in the measured cross-section of the estimated by multiplying the fractional uncertainties in various parameters to obtain cross-section of 181Ta(n,2n) 180Ta reaction and was found to be 15%, which is the least value for the present cross-section measurement by considering the error from each of the quantity used in the calculations. 4 Results and Discussion The cross sections of the 181Ta(n,2n)180Ta reaction have been measured with respect to197Au(n,2n)196Au monitor reaction at the neutron energy of 14.78 ± 0.20 MeV, which is obtained as 1.83±0.15 (barns) and plotted in Fig. 4. The efficiency of HPGe detector was carried out using a standard 152Eu source and the cross section datum of the reaction were determined using NAA and off-line γ-ray spectrometry technique. We have estimated the uncertainties in the measurement by considering various attributes in the data using the covariance analysis and correlations between them. The theoretical nuclear codes TALYS-1.8 and TALYS-1.911 were used for the analysis and prediction of nuclear reaction cross-section values based on the different level density model available in it. The comparison of the 181Ta(n,2n)180Ta reaction cross-section with the evaluated data from JEFF-3.321 and ENDF/B-VII.122 libraries, literature data6-10from Table 2 — Uncertainity contributing in the efficiency measurement Energy (KeV) Partial uncertainty (× 103) Total uncertainty (× 103) C Iγ N0 T1/2 (σεii) 121.8 0.501621 2.786854 5.657249 0.017541 6.230154 244 0.692513 1.564532 3.351265 0.010125 3.651169 344 0.265107 1.653215 2.452846 0.007545 3.016710 411 0.993210 0.874145 1.718635 0.005123 2.187452 778.9 0.340125 0.603151 1.094105 0.003214 1.321628 867 0.923510 0.635410 1.011235 0.003151 1.532147 964 0.253974 0.398471 0.879845 0.002591 1.012105 1112 0.213503 0.403814 0.782146 0.002255 0.901768 1212 0.942130 0.384639 0.652149 0.001898 1.132546 1299 0.738596 0.334648 0.662456 0.001947 1.087452 1408 0.121142 0.251646 0.251486 0.001911 0.689526 Table 3 — Co-variance matrix (× 100) for the detector efficiency 0.004132 0.001852 0.001467 0.001546 0.000824 0.000987 0.001014 0.000522 0.004317 0.000474 0.000654 0.000321 0.000287 0.000197 0.000170 0.000584 0.000125 0.000256 0.000174 0.000214 0.000227 0.000519 0.000312 0.000122 0.000156 0.000864 0.000412 0.000365 0.000352 0.000170 0.000156 0.000287 0.000197 0.000150 0.000101 0.000094 0.000325 0.000215 0.000321 0.000256 0.000132 0.000120 0.000156 0.000099 0.00035 0.000369 0.000174 0.000214 0.000198 0.000095 0.000321 0.000200 0.000410 0.000325 0.000156 0.000312 0.000235 0.000864 0.000789 0.000523 0.000400 0.000345 0.000215 0.000113 0.000049 0.000035 Table 4 — Measured efficiency with correlation matrix Eγ (KeV) Efficiency Correlation Matrix 355.71 0.059811 ± 0.0008 1 93.32 0.104731 ± 0.0012 0.91 1 INDIAN J PURE & APPL PHYS, VOL. 58, APRIL 2020 232 EXFOR12, as well as the theoretically prediction nuclear code TALYS-1.911 within 8-24 MeV are shown in Fig. 4. It is observed from Fig. 4, that the 181Ta(n,2n)180Ta reaction cross-section of present measurement at the neutron energy of 14.78 ± 0.20 MeV is in excellent agreement with the ldmodel-3 which is generalized superfluid model of the TALYS1.911 and in close agreement with the previous experimental data of Frehaut et al.9. However, the prediction of the L. R. Veeser et al.8, as well as evaluated from JEFF-3.321, and ENDF/B-VII.122 libraries are slightly higher. It suggests that more experimental data are needed to be obtained and compared with the different nuclear models. A detailed covariance analysis has also been carried out in order to find out the exact values of the uncertainties in the measured data. Covariance analysis is one of the methods which can help us to calculate the uncertainty in the measured datum by propagating the error in each quantity used. 5 Conclusions The experimental cross-section for the 181Ta(n, 2n)180Ta reaction have been measured at the neutron energy of 14.78 ± 0.20 relative to the 197Au(n,2n)196Au monitor reaction by using the NAA technique and off-line γ-ray spectrometry technique. The efficiency of HPGe detector system was calculated by using 152Eu standard source along with coincidence summing effect. The polynomial fitting is chosen to estimate the efficiency of unknown γ-ray energies. The uncertainty in the present measurement calculated with the help of covariance analysis was found to be 15%. The measurement has been compared with the literature data6-10, JEFF-3.321, ENDF/B-VII.122and the theoretical modular code TALYS-1.8 and TALYS-1.911. A comparison of the present result shows a good agreement with literature data Frehaut et al.9 as well as with ldmodel-3 of the TALYS-1.911code. The present work highlights that the nuclear reaction datum can be measured within the uncertainty of ≈15% by using the 3H(2H, n)4He reaction neutron generator. The cross-section data presented in this work are essential for the data libraries, and for the future ADSs and IFMIF reactor technology. Acknowledgement One of the authors (SM) thanks to the DAE-BRNS for the sanction of a major research project (Sanction Number: 36(6)/14/22/2016-BRNS). The authors are thankful to the staff of the BARC-PURNIMA facility, Mumbai for their excellent operation of the accelerator and other supports during the experiment. The author (BKS) would like to thank Dr Ambar Chatterjee for giving the valuable suggestion regarding analysis of LAMPS software and author also thankful to The M. S. University of Baroda for providing a teaching assistance-ship. 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