First proof-of-principle experiment with the post-accelerated isotope separator on-line beam at BRIF: measurement of the angular distribution of ²³Na + ⁴⁰Ca elastic scattering Enhanced Publication

First proof-of-principle experiment with the post-accelerated isotope separator on-line beam at BRIF: measurement of the angular distribution of 23 Na + 40 Ca elastic scattering 1 first-author 1 corresp guobing@ciae.ac.cn guobing@ciae.ac.cn guobing@ciae.ac.cn 1 2 corresp cjlin@ciae.ac.cn cjlin@ciae.ac.cn cjlin@ciae.ac.cn 1 1 1 1 1 1 1 1 1 1 1 1 3 1 1 1 1 1 1 1 1 4 5 1 1 1 1 4 1 1 1 2 1 2 1 1 1 corresp wpliu@ciae.ac.cn wpliu@ciae.ac.cn wpliu@ciae.ac.cn 1 China Institute of Atomic Energy (CIAE), P.O. Box 275(10), Beijing 102413, China China Institute of Atomic Energy (CIAE), P.O. Box 275(10), Beijing 102413, China Department of Physics, Guangxi Normal University, Guilin 541004, China Department of Physics, Guangxi Normal University, Guilin 541004, China College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China College of Nuclear Science and Technology, Beijing Normal University, Beijing 100875, China School of Physics, Beihang University, Beijing 100191, China School of Physics, Beihang University, Beijing 100191, China Key Laboratory of Advanced Nuclear Materials and Physics, Beihang University, Beijing 100191, China Key Laboratory of Advanced Nuclear Materials and Physics, Beihang University, Beijing 100191, China The reaction dynamics of exotic nuclei near the drip line is one of the main research topics of current interest. Elastic scattering is a useful probe for investigating the size and surface diffuseness of exotic nuclei. The development of rare isotope accelerators offers opportunities for such studies. To date, many relevant measurements have been performed at accelerators using the projectile fragmentation technique, while the measurements at accelerators using isotope separator on-line (ISOL) systems are still quite scarce. In this work, we present the first proof-of-principle experiment with a post-accelerated ISOL beam at the Beijing Radioactive Ion Beam Facility (BRIF) by measuring the angular distribution of elastic scattering for the stable nucleus 23 Na from the doubly magic nucleus 40 Ca at energies above the Coulomb barrier. The angular distribution measured by a silicon strip detector array in a scattering chamber using the ISOL beam at BRIF is in good agreement with that measured by the high-precision Q3D magnetic spectrograph using the non-ISOL beam at nearly the same energy. This work provides useful background for making BRIF a powerful tool for the investigation of the reaction dynamics of exotic nuclei. BRIF Exotic nuclei Elastic scattering Angular distribution 1 Introduction 1 Light loosely bound nuclei near the drip line are characterized by very low nucleon binding energies, typically one order of magnitude smaller than those for their stable counterparts located along the β -stability valley. These nuclei can exhibit exotic properties, such as a halo structure. The reaction dynamics of light exotic nuclei near the drip line is one of the main research topics of current interest and has attracted considerable interest in the low-energy nuclear physics community [ 1 1 - 3 3 1-3 ]. Elastic scattering is a useful probe for investigating the size and surface diffuseness of exotic nuclei by comparing the similarities and differences in the angular distributions of the reactions induced by loosely bound and tightly bound nuclei [ 4 4 ]. To date, several measurements have been made of the elastic-scattering angular distributions of neutron-rich nuclei, such as the 2 n -halo nuclei 6 He [ 5 5 ] and 11 Li [ 6 6 ], neutron skin nucleus 8 He [ 7 7 , 8 8 ], 8 Li [ 9 9 ], and the 1 n -halo nucleus 11 Be [ 10 10 - 12 12 10-12 ]. In recent years, the elastic scattering of proton-rich nuclei has also been investigated (e.g., 7 Be, 8 B, 9 C + Pb [ 4 4 , 13 13 , 14 14 ], 17 F + 12 C and 14 N [ 15 15 ], 17 F + 58 Ni [ 16 16 ], 17 F + 89 Y [ 17 17 ], and 17 F + 208 Pb [ 18 18 , 19 19 ]). These experiments have greatly deepened our understanding of the reaction dynamics and their interplay with the exotic structure of weakly bound nuclei. However, more investigations are needed. The development of rare isotope accelerators offers opportunities to explore the structure of unstable exotic nuclei. There are two rare complementary isotope accelerators in China. One is the Radioactive Ion Beam Line in Lanzhou (RIBLL) [ 20 20 ] using the projectile fragmentation (PF) technique, which was established by the Institute of Modern Physics, Chinese Academy of Sciences. Since the delivery of the first beam in 1997, a series of experiments have been conducted for the study of, for example, the clustering structure in neutron-rich nuclei [ 21 21 , 22 22 ], two-proton emission in proton-rich nuclei [ 23 23 , 24 24 ], and revalidation of the isobaric multiplet mass equation [ 25 25 ]. In addition, the elastic-scattering angular distributions of a few proton-rich nuclei [ 4 4 , 13 13 , 17 17 ] and the 1 n -halo nucleus 11 Be [ 12 12 ] have been measured using silicon detector arrays [ 26 26 , 27 27 ] at RIBLL. The other accelerator is the Beijing Radioactive Ion Beam Facility (BRIF) [ 28 28 , 29 29 ] using the isotope separator on-line (ISOL) technique, which was established by the China Institute of Atomic Energy. BRIF consists of a 100-MeV 200- μ A compact proton cyclotron [ 30 30 ], an ISOL system with a mass resolution of 20,000 [ 31 31 ], and a 13-MV tandem accelerator for post-acceleration. BRIF was commissioned in 2015 and successfully produced a few unstable ion beams such as 20,21,22 Na and 37,38 K. Recently, the exotic β – γ – α decay mode was directly observed for the first time with a pure 20 Na ISOL beam at BRIF [ 32 32 ]. BRIF is a potential high-precision facility for investigating the nuclear reaction dynamics of unstable nuclei, as it can deliver almost pure unstable ion beams. However, an experiment with a post-accelerated ISOL beam at BRIF has not been performed to date. Therefore, a proof-of-principle experiment with a post-accelerated stable 23 Na beam is highly desirable. For this purpose, the angular distribution of 23 Na + 40 Ca elastic scattering was measured using two techniques. One is based on a silicon detector array with a post-accelerated 23 Na beam using the ISOL technique at BRIF (ISOL beam); the other is based on a high-precision Q3D spectrograph with the 23 Na beam directly from the traditional negative-ion source and accelerated by the HI-13 tandem accelerator (non-ISOL beam). The excellent consistency of these two results demonstrates that BRIF can be a powerful tool for investigating the reaction dynamics of exotic nuclei. 2 Experimental setup and procedure 1 The experiment with the post-accelerated ISOL beam was performed at BRIF. First, a non-ISOL 23 Na beam directly from an ion source was accelerated by the HI-13 tandem accelerator as the pilot beam and transported to the R60 scattering target chamber to determine the beam optics. Second, the proton cyclotron served as the driving accelerator. The 23 Na beam was then produced with a 100-MeV 10-e μ A proton beam from the cyclotron bombarding the MgO target. Reaction products diffusing out of the thick target were ionized by an ion source and separated by a two-stage ISOL system for subsequent post-acceleration. Finally, the ISOL 23 Na beam was accelerated by using the HI-13 tandem accelerator and transported to the R60 scattering target chamber for the angular distribution measurement with the same optical systems as used for the pilot beam. The energy and the current of the ISOL 23 Na beam were 83.25 MeV and 0.5 enA, respectively. The setup of the target chamber is illustrated in Fig. 1 Fig. 1 . Two micro-channel plates (MCPs), which can present a fast timing signal, were used to measure the time of flight (TOF) of the beam. A quadrant silicon detector (QSD) with a thickness of 1000 μ m, which was placed at the end of the target chamber, was used to measure the total energy of the beam. The ISOL beam from BRIF can be confirmed by a combination of the TOF and energy. After the development of the 23 Na beam was confirmed, the upstream MCP and QSD were removed to avoid interference with the measurement of the angular distribution, while the downstream MCP was left to monitor the beam. The target was made by evaporation of natural CaF 2 with a thickness of 118 μ g/cm 2 on a 200 μ g/cm 2 thick Au foil. The silicon detector array was placed 10 cm downstream of the target. The detector array consists of four detector telescopes, as shown in Fig. 2 Fig. 2 . The first and second layers of each telescope are two single-sided silicon strip detectors (SSSDs) with thicknesses of 20 and 300 μ m, respectively. Each SSSD has a typical energy resolution of 0.3% and has 16 strips with a width of 3 mm, spaced 0.1 mm apart. These two SSSDs were placed in the horizontal and vertical directions, respectively. The third layer is a QSD with a thickness of 1000 μ m and an area of 50 × 50 mm 2 for potential light reaction products such as protons and α particles. Such a configuration can identify reaction products, including light and heavy ions. The angular coverage range of the present detector array in the laboratory system ranges from 10.2° to 39.6°, as shown in Fig. 3 Fig. 3 . In this experiment, the energy of the 23 Na beam at the center of the target was 82.8 MeV. The scattered 23 Na ions were detected in the first two layers of the telescope. The 20- μ m-thick SSSD was used for the measurement of energy loss Δ E , while a 300- μ m-thick SSSD was used to measure the residual energy E r . The experimental spectra of the scattered 23 Na particles from 40 Ca and 197 Au are shown in Fig. 4 Fig. 4 . Both the two-dimensional spectrum of energy loss versus the total energy and the one-dimensional spectrum of the total energy are presented. The 23 Na particles from 40 Ca can be clearly distinguished from those from 197 Au. To verify the ISOL-based technique at BRIF, the same angular distribution was also measured with a well-developed technique using a high-precision Q3D spectrograph with the non-ISOL 23 Na beam directly from the traditional negative-ion source and accelerated by the HI-13 tandem accelerator at the China Institute of Atomic Energy. The experimental setup and procedures were identical to those reported in our previous work [ 33 33 , 34 34 ]. The experimental target was made by evaporation of natural CaF 2 with a thickness of 18 μ g/cm 2 on a 30 μ g/cm 2 thick Au foil. In this experiment, the energy of the 23 Na beam was 84 MeV, and its energy at the center of the target was 83.9 MeV (which was almost the same as the measurement at BRIF). 3 Data analysis and results 1 The differential cross section of a reaction can be expressed as <math display="block" id="M1"><mrow><mi>σ</mi><mo>=</mo><mfrac><mi>N</mi><mrow><mi>I</mi><msub><mi>N</mi><mtext>s</mtext></msub><mi>Ω</mi></mrow></mfrac><mn>,</mn></mrow></math> where N and I represent the numbers of emitted ions and incident ions, respectively, N s denotes the number of target atoms per unit area, and Ω represents the solid angle. In the present work, we used a relative measurement method to reduce the uncertainties from the solid angle and beam intensity. The cross section of 23 Na + 197 Au elastic scattering at the present energy is equal to the Rutherford cross section; therefore, the ratio of the measured 23 Na + 40 Ca elastic-scattering cross section to the corresponding Rutherford cross section can be calculated by using <math display="block" id="M2"><mrow><mtext>d</mtext><mi>σ</mi><mo>/</mo><mtext>d</mtext><msubsup><mi>σ</mi><mtext>r</mtext><mrow><mtext>C</mtext><mi>a</mi></mrow></msubsup><mo>=</mo><mfrac><mrow><msup><mi>N</mi><mrow><mtext>C</mtext><mi>a</mi></mrow></msup><mo>×</mo><msubsup><mi>N</mi><mtext>s</mtext><mrow><mtext>A</mtext><mi>u</mi></mrow></msubsup><mo>×</mo><mtext>d</mtext><msubsup><mi>σ</mi><mtext>r</mtext><mrow><mtext>A</mtext><mi>u</mi></mrow></msubsup></mrow><mrow><msup><mi>N</mi><mrow><mtext>A</mtext><mi>u</mi></mrow></msup><mo>×</mo><msubsup><mi>N</mi><mtext>s</mtext><mrow><mtext>C</mtext><mi>a</mi></mrow></msubsup><mo>×</mo><mtext>d</mtext><msubsup><mi>σ</mi><mtext>r</mtext><mrow><mtext>C</mtext><mi>a</mi></mrow></msubsup></mrow></mfrac><mn>,</mn></mrow></math> where N Ca and N Au are the numbers of the scattered 23 Na ions from 40 Ca and 197 Au, respectively; <math id="M3"><mrow><msubsup><mi>N</mi><mtext>s</mtext><mrow><mtext>C</mtext><mi>a</mi></mrow></msubsup></mrow></math> and <math id="M4"><mrow><msubsup><mi>N</mi><mtext>s</mtext><mrow><mtext>A</mtext><mi>u</mi></mrow></msubsup></mrow></math> are the numbers of Ca and Au atoms per unit area of the target, respectively; and <math id="M5"><mrow><mtext>d</mtext><msubsup><mi>σ</mi><mtext>r</mtext><mrow><mtext>C</mtext><mi>a</mi></mrow></msubsup></mrow></math> and <math id="M6"><mrow><mtext>d</mtext><msubsup><mi>σ</mi><mtext>r</mtext><mrow><mtext>A</mtext><mi>u</mi></mrow></msubsup></mrow></math> are the Rutherford differential cross sections of 23 Na + 40 Ca and 23 Na + 197 Au, respectively. It can be seen that the uncertainties from the solid angle and beam intensity can be removed through the present relative measurement method. In Fig. 5 Fig. 5 , we show the experimental angular distribution of 23 Na + 40 Ca elastic scattering with the ISOL-based technique at BRIF and a well-developed technique at a high-precision Q3D spectrograph, together with the optical model calculations with the single-folding potential [ 35 35 ]. These two experimental results were in good agreement. This presents strong evidence that BRIF can be used for such measurements. 4 Conclusion and perspective 1 In this work, we performed the first proof-of-principle experiment with a post-accelerated 23 Na beam using the ISOL technique at BRIF (ISOL beam) by measuring the angular distribution of 23 Na + 40 Ca elastic scattering. 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Xu , Application of the Bruyeres Jeukenne-Lejeune-Mahaux model potential to composite nuclei with a single-folding approach . Phys. Rev. C 83 , 064619 ( 2011 ). doi: 10.1103/PhysRevC.83.064619 http://doi.org/10.1103/PhysRevC.83.064619 Application of the Bruyeres Jeukenne-Lejeune-Mahaux model potential to composite nuclei with a single-folding approach All authors contributed to the study conception and design. Experiment was carried out by Wei Nan, Cheng-Jian Lin, Lei Yang, Dong-Xi Wang, Yang-Ping Shen, Hui-Ming Jia, Yun-Ju Li and Xin-Yue Li. Targets were made by Qi-Wen Fan. Data analysis was performed by Wei Nan, Bing Guo, Cheng-Jian Lin, Yang-Ping Shen, Dan-Yang Pang, and Wei-Ping Liu. The ISOL beam was developed by Bing Tang, Bao-Qun Cui, Tao Ge, Yin-Long Lyu, Li-Hua Chen, Jian-Cheng Liu, Rui-Gang Ma, Xie Ma, Ying-Jun Ma and Tian-Jue Zhang. The first draft of the manuscript was written by Wei Nan and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript. 10.1007/s41365-021-00889-9 This work was supported by the National Natural Science Foundation of China (Nos. 11490561, 11635015, 11961141003, 11805280, 11975316, 12075045, 12005304, U1867212, and U1867214), the National Key Research and Development Project (Nos. 2016YFA0400502 and 2018YFA0404404), the Continuous Basic Scientific Research Project (No. WDJC-2019-13), and the Leading Innovation Project (Nos. LC192209000701 and LC202309000201). 2021-04-21 2021-04-21 2021-04-29 2021-05-24 2021-05-01 2021-12-20 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2021 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-021-00889-9 http://dx.doi.org/10.1007/s41365-021-00889-9 . 2021 Nuclear Science and Techniques 05 32