CHIN. PHYS. LETT. Vol. 32, No. 4 (2015) 042501 Residual Nuclides Induced in Cu Target by a 250 MeV Proton Beam * ZHANG Hong-Bin(张宏斌)1 , ZHANG Xue-Ying(张雪荧)1** , MA Fei(马飞)1 , JU Yong-Qin(鞠永芹)1 , GE Hong-Lin(葛红林)1 , CHEN Liang(陈亮)1 , ZHANG Yan-Bin(张艳斌)1 , WEI Ji-Fang(魏计房)2 , LI Yan-Yan(李严严)1,3 , LUO Peng(骆鹏)1 , WANG Jian-Guo(王建国)1 , WAN Bo(万波)1,3 , XU Xiao-Wei(许晓伟)1,3 , ZHOU Bin(周斌)4 1 2 Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000 North China Sea Environmental Monitoring Center, State Oceanic Administration, Qingdao 266033 3 University of Chinese Academy of Sciences, Beijing 100049 4 Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049 (Received 28 October 2014) Residual nuclide production is studied experimentally by bombarding a Cu target with a 250 MeV proton beam. The data are measured by the off-line 𝛾-spectroscopy method. Six nuclides are identified and their cross sections are determined. The corresponding calculated results by the MCNPX and GEANT4 codes are compared with the experimental data to check the validity of the codes. A comparison shows that the MCNPX simulation has a better agreement with the experiment. The energy dependence of residual nuclide production is studied with the aid of MCNPX simulation, and it is found that the mass yields for the nuclides in the light mass region increase significantly with the proton energy. PACS: 25.40.Sc, 25.70.Mn, 24.10.Lx DOI: 10.1088/0256-307X/32/4/042501 High-energy and high-current proton accelerators can be used in many fields, such as the environmental sciences, medicine, and nuclear waste transmutation and energy amplification.[1−3] Generally, uncontrolled particle loss in an accelerator must be kept to a very low level, while there is still no way to completely prevent the activation of the accelerator components. To avoid suffering radiation damage, it is necessary to estimate the radioactive products induced in the accelerator components by a high-energy proton beam. A common approach to predict nuclide production is based on a simulation with Monte-Carlo transport codes, such as MCNPX,[4] PHITS,[5] GEANT4[6] and FLUKA.[7] Several experimental studies on the production of radioactive nuclides in target irradiated by protons have been carried out.[8−10] However, it is still necessary to accumulate more experimental data to check the applicability of all kinds of codes. Recently, such accelerator facilities have been started or planned to be built in China, including the China spallation neutron source (CSNS) and the China initiative accelerator driven sub-critical system (CIADS). For the safety and shielding of these facilities, we must have good predictions for the activation production that comes from the accelerator components, especially for the copper that is a main element of accelerator components. It was thus decided to measure the activation yields by irradiating a Cu target with a high energy proton beam. In the present work the cross section data for a Cu target are obtained by an off-line 𝛾spectrometry method, and they are compared with the results simulated by MCNPX2.7.0 and GEANT4 codes. Our irradiation experiment was performed at the heavy ion research facility and cooling storage ring (HIRFL-CSR)[11] in Lanzhou, China. Protons with energy of 250 MeV were used to bombard a thin Cu plate target. The proton beam spot was less than 20 mm in diameter at the target position. A schematic view of the experimental setup is shown in Fig. 1. The diameter and thickness of the Cu target were 10 cm and 50 µm, respectively. A natural Pb cylinder with 100 mm diameter and 100 mm length was used as the beam dump behind the Cu target. The primary protons were stopped in the Pb cylinder after passing through the Cu target. An ionization chamber was placed in front of the Cu target to monitor the beam intensity. The high-purity Al foil, which was 100 mm in diameter and 800 µm in thickness, was placed in front of the ionization chamber to record the total number of protons. The total number of protons in the process of irradiation was given by activation analysis via the yield of the reaction 27 Al(p, 3p1n)24 Na. The cross section has been reported to be 10.6 mb.[12] The irradiation was performed for about 24 h to make the beam fluence enough. A combination of the activity analysis of Al foil and the calibration of ionization chamber enabled us to obtain the absolute beam current. The average beam intensity was 2.8 × 107 pps. After the end of the irradiation experiment, the 𝛾 activities of the Cu target and Al foil were immediately measured with a high purity germanium (HPGe) detector. The relative efficiency of the HPGe detector was about 65% and energy resolution was 1.90 keV at 1.33 MeV. The absolute efficiency was calibrated with the standard sources 60 Co, 133 Ba, 137 Cs, and 152 Eu. The irradiated Cu sample was measured several times to detect the nuclides with different halflives. The gamma spectra, obtained in three measurements: 8.1 d, 56 d and 133 d after the end of irradia- * Supported by the National Natural Science Foundation of China under Grant Nos 11305229, 11105186, 91226107 and 91026009, and the Strategic Priority Research Program of Chinese Academy of Sciences under Grant No XDA03030300. ** Corresponding author. Email: [email protected] © 2015 Chinese Physical Society and IOP Publishing Ltd 042501-1 CHIN. PHYS. LETT. Vol. 32, No. 4 (2015) 042501 tion, are shown in Fig. 2. Analysis of the gamma spectra and identification of radionuclides were performed by the GAMMA-W code,[13,14] which was applied to calculate the net 𝛾peak areas via an unfolding algorithm by using a leastsquare fit. Six radioactive nuclides were clearly recognized by different characteristic gamma rays from measured gamma spectra, see Table 1. All of the types of products are denoted as independent 𝐼, cumulative positron 𝐶 + or cumulative electron 𝐶 − .[1] As listed in Table 2, the half-lives of these identified nuclides vary from a few days to a few hundreds of days, and no nuclide with short (or very long) halflife was found. There is also no light nuclide, such as 7 Be (𝑇1/2 ≈ 53.12 D), to be found in the experiment, and all of the mass numbers of the measured nuclides are large and are not far from Cu (𝐴 ≈ 64 amu). This means that the nuclides are produced dominantly by the primary protons and then by the secondary particles.[15] Ionization chamber Beam dump Cu target Al Foil Proton beam Fig. 1. A schematic view of the experimental setup. 103 (a) 102 0 10 103 tcount=2 h tdecay=8.1d (b) 57 Co 102 101 100 103 Cross sections (mb) Number of counts 101 tcount=49h tdecay=56d (c) 102 101 1000 tcount=48h tdecay=133d 2000 4000 6000 8000 54 Co 56 Mn Co 51 Cr 52 Mn Present work (250 MeV) MCNPX (250 MeV) GEANT4 (250 MeV) Ref. [16] (200 MeV) 50 Channel number 58 51 52 53 54 55 56 57 58 59 Mass numbers Fig. 2. Examples of gamma spectra at different times after the end of irradiation. Fig. 3. Comparison of the measured and calculated cross sections of residual nuclides. Table 1. Features of production and decay of measured radionuclides produced in Cu. Residual nuclides 51 Cr Type of yield 𝐶+ Half-life (d) 27.702 52 Mn 𝐼 5.591 54 Mn 𝐼 312.12 56 Co 𝐶+ 77.27 57 Co 𝐶+ 271.79 58 Co 𝐼 70.82 The peak level of the gamma decay spectrum is associated with the decay of the nuclide during the time of irradiation, cooling, and measurement, thus the total counts 𝐶 of gamma-ray peak area could be expressed as Gamma-ray energy (keV) 320.08 744.23 935.54 1434.07 834.84 846.76 1238.27 122.06 136.47 810.77 For each nuclide, the yield 𝑁 (0) (i.e., number of activated nuclei) at the end of irradiation could be determined according to the relation ∫︁ 𝑡irr 𝑁 (0) = 𝐶 = (𝑁 (𝑡d ) − 𝑁 (𝑡d + 𝑡c ))𝜖𝛾 𝐼𝛾 = 𝑁 (0)𝑒−𝜆𝑡d (1 − 𝑒−𝜆𝑡c )𝜖𝛾 𝐼𝛾 , Branching ratio(%) 9.86 90.60 94.50 100 99.98 99.93 66.07 85.60 10.68 99.45 (Φ𝜎𝑁𝑡 𝐷𝑒−𝜆𝑡 )d𝑡 𝑡0 (1) where 𝑡d is cooling time (i.e., the time from the end of irradiation to the beginning of the measurement), 𝑡c is the time of measurement, 𝜖𝛾 is the detector efficiency, 𝐼𝛾 is the intensity of the 𝛾 transition per decay, and 𝜆 = ln 2/𝑇1/2 is the disintegration constant. = Φ𝜎𝑁𝑡 𝐷 (1 − 𝑒−𝜆𝑡irr ), 𝜆 (2) where the parameter Φ is the average beam intensity, 𝑁𝑡 is the atomic density of the Cu foil, 𝐷 is the thickness of the Cu foil, and 𝑡irr is the irradiation time. Combining Eqs. (1) and (2), the cross section of 042501-2 CHIN. PHYS. LETT. Vol. 32, No. 4 (2015) 042501 radioactive nuclide can be derived as follows: 𝜎= 𝐶𝜆𝑒𝜆𝑡d . 𝐼𝛾 𝜖𝛾 Φ𝑁𝑡 𝐷(1 − 𝑒−𝜆𝑡c )(1 − 𝑒−𝜆𝑡irr ) (3) According to Eq. (3), the cross sections of the six radioactive nuclides are determined, and the specific results are given in the experimental column of Table 2. Errors which include the systematic and statistic uncertainties are in the range 8.5–20.5%. The similar work by Titarenko et al.[16] at a proton-energy of 200 MeV is reasonably in agreement with our work with the exception of 57 Co and 58 Co, which deviates from ours by nearly 50%, see Fig. 3. Table 2. Cross sections of residual nuclides from experiment and simulation by MCNPX and GEANT4. Residual nuclides Half-life 47 Ca 4.536 day 330 day 1.4×1017 year −a 27.702 day − − 5.591 day 3.74×106 year 312.12 day − − 2.73year − − − 77.27 day 271.79 day 70.82 day − 5.271 year − 7.6×104 year − − − 101.1 year − 3.333 hour 9.74 minute 12.7 hour 49 V 50 V 50 Cr 51 Cr 52 Cr 53 Cr 52 Mn 53 Mn 54 Mn 55 Mn 54 Fe 55 Fe 56 Fe 57 Fe 58 Fe 56 Co 57 Co 58 Co 59 Co 60 Co 58 Ni 59 Ni 60 Ni 61 Ni 62 Ni 63 Ni 64 Ni 61 Cu 62 Cu 64 Cu a Here Experiment 12.43±2.55 4.00±0.57 15.80±2.50 10.09±1.37 24.25±2.11 27.30±2.57 Cross sections (mb) MCNPX 1.63±0.20 3.94±0.31 3.57±0.29 5.29±0.35 8.24±0.44 12.73±0.55 3.45±0.29 5.53±0.36 15.18±0.59 17.60±0.64 7.61±0.42 10.44±0.50 24.59±0.76 30.92±0.85 11.05±0.51 5.08±0.35 11.00±0.51 30.90±0.85 33.80±0.89 26.60±0.79 10.26±0.47 15.28±0.60 36.16±0.93 64.18±1.23 34.18±0.90 39.07±0.96 11.91±0.36 10.24±0.49 28.82±0.82 66.81±1.26 28.81±0.76 GEANT4 3.17±0.27 7.56±0.42 7.11±0.41 7.94±0.43 15.12±0.60 19.06±0.67 4.98±0.34 12.00±0.53 27.28±0.80 21.44±0.71 8.53±0.45 19.56±0.70 33.54±0.89 29.72±0.84 10.28±0.49 5.03±0.34 20.95±0.70 41.60±0.99 35.76±0.92 20.93±0.70 6.07±0.38 20.50±0.70 37.98±0.95 56.83±1.16 35.05±0.91 38.13±0.95 6.17±0.38 7.35±0.42 20.50±0.70 53.24±1.12 21.00±0.70 −− stands for stable nuclides. The Monte Carlo simulations for the production of residual products in the Cu target were performed by the MCNPX code 2.7.0[4] and GEANT4 code.[6,17] In the MCNPX simulation, the Bertini,[18] pre-equilibrium and RAL[19] models were used in the intra-nuclear cascade (INC) stage, pre-equilibrium stage, and in the process of evaporation, respectively. The binary cascade model[17] was chosen in the GEANT4 simulation. In Table 2, we list the major nuclides with large cross sections (𝜎 > 1 mb) calculated by MCNPX and GEANT4 codes. The calculation only considers the direct production of the certain nuclide from the proton-induced reaction. For the experimental results, 52 Mn, 54 Mn and 58 Co are independent nuclides which are directly produced in the reaction. The nuclides of 51 Cr, 56 Co and 57 Co, except for the direct proton induced production, may also come from the decay of 51 Mn, 56 Ni and 57 Ni, respectively. However, these mother nuclides are not observed in Table 2, which means that their calculated cross sections are less than 1 mb. Therefore, we conclude that the decay contribution from mother nuclides could be negligible. Except for the nuclides which were identified in experiment, the rest of the nuclides can be divided into three categories: stable nuclides, short-lived radionuclides, and long-lived ones. They are all unable or very difficult to be detected, and this is why only six radionuclides were measured when there was not enough beam intensity in our experiment. A comparison between the measured and calculated cross sections of residual nuclides is shown in Fig. 3. It is obvious that the crosssection values simulated by MCNPX are more consistent with the experimental data, where the maximum difference is less than 35%. For the GEANT4 simulation, however, the calculated results are greater than the experimental data, especially for 52 Mn production. It is obviously found that the MCNPX simulation could give better predictions on residual nuclide production than GEANT4 code. We also compare the simulated cross sections as functions of mass number and charge number between MCNPX with GEANT4, 042501-3 CHIN. PHYS. LETT. Vol. 32, No. 4 (2015) 042501 Cross sections (mb) Cross sections (mb) as shown in Fig. 4. It can be found that the values calculated by GEANT4 are higher overall than the results by MCNPX, while they both have similar trends of cross sections with the increasing mass number or charge number. 102 101 100 10-1 10-235 (a) MCNPX GEANT4 40 102 101 100 10-1 10-2 18 45 50 55 60 Mass numbers 65 (b) MCNPX GEANT4 20 22 24 Charge numbers 26 28 30 Cross sections (mb) Cross sections (mb) Fig. 4. Production distribution of the calculated products as functions of (a) mass number and (b) charge number. 102 101 100 10-1 10-2 20 (a) 102 101 100 10-1 10-2 30 40 50 1200 MeV 800 MeV 400 MeV 250 MeV 60 Mass numbers References 70 (b) 10 15 20 Charge numbers 25 the lighter nuclides in that the yields increase strongly with the projectile energy. This means that we must evaluate the contribution of the production of light nuclides as the energy of the proton beam increases. All of the above calculations have considered the contribution from the thick Pb cylinder (i.e., beam dump) and lab setup. The residual nuclide production, regardless of the interference of the thick Pb cylinder, is also calculated. The results show that the contribution of secondary particle from beam dump could be negligible. In summary, we have studied the production of residual nuclides via the reaction of 250 MeV protons bombarding Cu foil. Six radioactive nuclides are identified in our experiment by using the off-line 𝛾-spectrometry method. The experimental data are compared with Monte Carlo simulations performed by the MCNPX and GEANT4 codes, and the comparison suggests that MCNPX could give good predictions on the productions of the residual nuclides. We also calculate the cross sections of unmeasured nuclides to obtain the mass-yield distribution. Finally, we study the energy dependence of residual nuclide production in a Cu target by using the MCNPX code. The simulation shows that the light nuclides would play a more important role as the energy of proton beam increases. We gratefully acknowledge the support and assistance of the accelerator operation staff at HIRFLCSR. 1200 MeV 800 MeV 400 MeV 250 MeV 30 Fig. 5. Comparison of the mass-yield distribution trends in different proton energies. Considering the good agreement between the measured data with the calculated results from MCNPX2.7.0, we try to study the energy dependence of proton-induced nuclides of the Cu target on the basis of the MCNPX simulation. Figure 5 shows the massyield distributions for calculated residual products induced in the Cu target by protons with energies of 250, 400, 800, and 1200 MeV, respectively. For each energy point, as shown in Fig. 5, the yields of the residual products increase with the mass number. However, the distributions become flatter and the yields of light fragments are enhanced with the increase of the proton energy. Furthermore, for the products in the mass region with 𝑍 ≥ 25 and 𝐴 ≥ 50, the cross sections are almost equal and do not depend strongly on the proton energy. In contrast, a clear trend is observed for [1] Gloris M, Michel R, Sudbrock F et al 2001 Nucl. Instrum. Methods Phys. Res. Sect. 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