New methods for ACCULINNA-II were developed at FLNR

FLNR scientists developed some new experimental methods and approaches to study properties of exotic nuclei at ACCULINNA-II – U400m complex. First experiments with 6Не, 8Не, 9Li, 10Ве, 26P and 27S radioactive beams arouse hope for a number of nuclear physics problems to be solved: production and study of light nuclei spectra (Z = 1÷16) beyond the limits of nucleon stability, study of these nuclei rare decay modes, production of the unknown before heavy isotopes of elements with atomic numbers Z = 94 – 107 and measurements of fission barriers of those elements. Developed methods may be used to study some problems of astrophysics, on the origin of some elements in the Universe, for example.

“Development and application of new experimental methods at ACCULINNA-2 fragment separator” cycle of works was awarded the second JINR Prize for 2020 in category Physics Instruments and Methods. Co-author list includes: A.A. Bezbakh, M.S. Golovkov, А.V. Gorshkov, S.A. Krupko, I.A. Muzalevskii, G.M. Ter-Akopian, A.S. Fomichev, V. Chudoba, G. Kaminski (all employed in FLNR JINR) and E.Yu. Nikolskii (NRC KI, FLNR JINR)

In 2017, a new unique facility, the ACCULINNA-2 fragment separator located at the U-400M heavy-ion accelerator, was launched at FLNR. Commissioning of the separator, performed by means of the fragmentation of the 15N primary beam on the 2 mm beryllium target, demonstrated that the achieved properties of the produced secondary beams of radioactive ions (RI) are fully consistent with the stated parameters. In 2018, methodical works aimed at to perform full scale experiments with RI beams 6Не, 8Не, 9Li, 10Ве, etc. [1-5] were carried out.

Flagship experiment was aimed at search for 7Н system characterized by extremely large neutron excess. The reaction 8He (26 MeV per nucleon) + d → 3He + 7H was chosen for population of the 7Н states. A registration of low-energy 3Н recoils in coincidence with fast tritons (E ~ 70 ± 30 MeV), emitted at very forward angles in a narrow cone Theta ≤ 6° was crucial for the identification of the 7Н events. It was necessary to detect such tritons with angular resolution ~ 0,5° and energy resolution better than 2%. For this purpose, the triton telescope, consisting of the 1.5 mm thick silicone double side detector and 4×4 array of CsI(Tl)+PMT detectors, was developed. In order to improve determination of energy losses of the 3Не recoils in 20 μm silicon detectors, distributions of their thicknesses were measured with accuracy better than 0.2 μm (detailed review is in [3]). It allowed one to clearly identify all hydrogen, helium and lithium isotopes produced in 8He + d interactions by dE-E method with low threshold of ~1 MeV.

The feasibility of the 7Н studies in the 2Н(8Не,3Не)7Н reaction at the ACCULINNA-2 fragment separator was based on:

  • methods of the triton energy and emission angle measurement in coincidence with 3Не and selection of true events related to 7Н (7Н → t + 4n) using the so called «kinematic triangle»;
  • the use of the cryogenic deuterium gas target, providing high experimental resolution of ~ 1 MeV FWHM achieved in studies of the 7Н excitation spectrum, and well measured low background;
  • possibility of calibration of applied missing mass method in reference reaction 2Н(10Bе,3Не)9Li performed at the same setup;
  • high quality of RI beams at ACCULINNA-2 fragment separator (intensity, purity, focusing at physical target).

Taking advantage of the developed methods, a series of 3 experimental runs was carried out, and high quality data for the unknown 6,7Н systems were acquired [6-8].

The second important measurements at ACCULINNA-2 were aimed at studies of low-lying states of 10Li system populated in the 9Li(d,p)10Li → n + 9Li reaction at 29 MeV per nucleon [4]. The key feature of this experiment was developed method of detection of proton, emitted at backward direction in laboratory system in coincidence with 9Li and neutrons emitted at forward angles. For precise measurement of neutron energy and their emission angle, specially designed array of 48 mono-crystal stilbene scintillator detectors [Bezbakh A.A. et al., Instrum. Exp. Tech., 2018 vol.61, No.5, p.631] was used. One of the important issues in detection of 9Li was the developed ToF detector consisted of thin plastic scintillator EJ-212 (125 μm) and four PMTs Hamamatsu R7600. This detector placed at distance of 39 cm downstream the physical target allowed one to identify the components of the secondary beam (d, 6He, 9Li и 12Be), and moreover, to separate incoming beam 9Li from one produced in the 10Li decay by dE-ToF method. To check the experimental resolution of the whole setup and normalization of the 9Li missing mass spectrum, the reference measurement of 6He(d,p)7He with triple coincidence p-6He-n was performed at the same setup. Based on the well-known properties of the 7Не ground state (E = 410 keV, Г = 150 keV), it was found, that the determination of the neutron energy on the 2 m base by time of flight provides the energy resolution of the reconstructed excitation spectrum of ~ 180 keV (FWHM). In addition, the angular analysis of the 7Не decay products at energies of 2-4 MeV showed the asymmetry in the emission angles of the reaction products with respect to the transferred momentum. This triple coincidence method was also applied in 9Не system studies, performed in 9Не system studies, performed in 8He(d,p)9He reaction with a 8Не secondary beam at energy of 26 MeV/nucleon. Basing on the analysis of the collected data, it is expected to get new data on the structure of the studied systems.

New methods and experimental approaches, commissioned in the first experiments at the ACCULINNA-2 facility, provide the opportunity to observe new neutron-rich nuclei in the range 94 < Z < 107 with neutron-rich beams in multi-nucleon transfer reactions at threshold energy of 4-6 MeV per nucleon as well. This area is not available by means of other methods, though it is of special interest as a possible way to move towards the island of stability. Using the given methods, one can measure lifetime, fission barriers, and decay modes of such nuclei [5].

Development of experimental methods at ACCULINNA-2 coupled to U-400M facility offers new opportunities for resolving a number of tasks using RI beams with energies ~ 5÷50 MeV/nucleon: production and study of light nuclei (Z = 1÷16) lying beyond the boundaries of nucleon stability, investigation of rare decay modes, production of new isotopes of heavy elements (Z = 94÷107), and measurements of their fission barriers in the reactions of multi-nucleon transfers at the RI beams9Li, 10Be, 14,16C 17,19N, 20,22O, etc., up to 24,26Ne.

  1. A. S. Fomichev, A. A. Bezbakh, S. G. Belogurov, R. Wolski, E. M. Gazeeva, A. V. Gorshkov, L. V. Grigorenko, B. Zalewski, G. Kaminski, S. A. Krupko, I. A. Muzalevskii, E. Yu. Nikolskii, Yu. L. Parfenova, S. I. Sidorchuk, R. S. Slepnev, G. M. Ter-Akopian, V. Chudoba, and P. G. Sharov, “The first experiments with the new ACCULINNA-2 fragment separator”, Bulletin of the Russian Academy of Sciences: Physics, 83 (2019) 385–391.
  2. G. Kaminski, B. Zalewski, S. G. Belogurov, A. A. Bezbakh, D. Biare, V. Chudoba, A. S. Fomichev, E. M. Gazeeva, M. S. Golovkov, A. V. Gorshkov, L. V. Grigorenko, D. A. Kostyleva, S. A. Krupko, I. A. Muzalevsky, E. Yu. Nikolskii, Yu. L. Parfenova, P. Plucinski, A. M. Quynh, A. Serikov, S. I. Sidorchuk, R. S. Slepnev, P. G. Sharov, P. Szymkiewicz, A. Swiercz, S. V. Stepantsov, G. M. Ter-Akopian, R. Wolski, “Status of the new fragment separator ACCULINNA-2 and first experiments”, Nucl. Instrum. Methods Phys. Res. B 463 (2020) 504-507.
  3. I. A. Muzalevskii, V. Chudoba, S. G. Belogurov, A. A. Bezbakh, D. Biare, A. S. Fomichev, S. A. Krupko, E. M. Gazeeva, M. S. Golovkov, A. V. Gorshkov, L. V. Grigorenko, G. Kaminski, O. Kiselev, D. A. Kostyleva, M. Yu. Kozlov, B. Mauyey, I. Mukha, E. Yu. Nikolskii, Yu. L. Parfenova, W. Piatek, A. M. Quynh, V. N. Schetinin, A. Serikov, S. I. Sidorchuk, P. G. Sharov, R. S. Slepnev, S. V. Stepantsov, A. Swiercz, P. Szymkiewicz, G. M. Ter-Akopian, R. Wolski, B. Zalewski, “Detection of the low energy recoil 3He in the reaction 2H(8He,3He)7H”, Bulletin of the Russian Academy of Sciences: Physics, 84 (2020) 500-504.
  4. A. A. Bezbakh, S. G. Belogurov, D. Biare, V. Chudoba, A. S. Fomichev, E. M. Gazeeva, M. S. Golovkov, A. V. Gorshkov, G. Kaminski, S. A. Krupko, B. Mauyey, I. A. Muzalevskii, E. Yu. Nikolskii, Yu. L. Parfenova, W. Piatek, A. M. Quynh, A. Serikov, S. I. Sidorchuk, P. G. Sharov, R. S. Slepnev, S. V. Stepantsov, A. Swiercz, P. Szymkiewicz, G. M. Ter-Akopian, R. Wolski, B. Zalewski, “Study of 10Li low energy spectrum in the 2H(9Li,p) reaction”, Bulletin of the Russian Academy of Sciences: Physics, 84 (2020) 491-494.
  5. G. M. Ter-Akopian, Yu. Ts. Oganessian, A. A. Bezbakh, A. S. Fomichev, M. S. Golovkov, A. V. Gorshkov, S. A. Krupko, E. Yu. Nikolskii, S. I. Sidorchuk, S. V. Stepantsov, R. Wolski, “Radioactive-ion beams for the fission study of heavy neutron-rich nuclei”, Physics of Atomic Nuclei, 2020, Vol. 83, No. 4, 497–502.
  6. A. A. Bezbakh, V. Chudoba, A. V. Gorshkov, S. A. Krupko, S. G. Belogurov, D. Biare, A. S. Fomichev, E. M. Gazeeva, L. V. Grigorenko, G.Kaminski, O. Kiselev, D. A. Kostyleva, I. Mukha, I. A. Muzalevskii, E. Yu. Nikolskii, Yu. L. Parfenova, A. M. Quynh, A. Serikov, S. I. Sidorchuk, P. G. Sharov, R. S. Slepnev, S. V. Stepantsov, A. Swiercz, P. Szymkiewicz, G. M. Ter-Akopian, R. Wolski, B. Zalewski, M. V. Zhukov, “Evidence for the first excited state of 7H”, Physical Review Letters 124 (2020) 022502.
  7. I. A. Muzalevskii, A. A. Bezbakh, E. Yu. Nikolskii, V. Chudoba, S. A. Krupko, S. G. Belogurov, D. Biare, A. S. Fomichev, E. M. Gazeeva, A. V. Gorshkov, L. V. Grigorenko, G. Kaminski, O. Kiselev, D. A. Kostyleva, M. Yu. Kozlov, B. Mauyey, I. Mukha, Yu. L. Parfenova, W. Piatek, A. M. Quynh, V. N. Schetinin, A. Serikov, S. I. Sidorchuk, P. G. Sharov, N. B. Shulgina, R. S. Slepnev, S. V. Stepantsov, A. Swiercz, P. Szymkiewicz, G. M. Ter-Akopian, R. Wolski, B. Zalewski, M. V. Zhukov, “Resonant states in 7H: Experimental studies of the 2He(8He,3He) reaction”, Physical Review C 103 (2021) 044313.
  8. E. Yu. Nikolskii, I. A. Muzalevskii, A. A. Bezbakh, V. Chudoba, S. A. Krupko, S. G. Belogurov, D. Biare, A. S. Fomichev, E. M. Gazeeva, A. V. Gorshkov, L. V. Grigorenko, G. Kaminski, O. Kiselev, D. A. Kostyleva, M. Yu. Kozlov, B. Mauyey, I. Mukha, Yu. L. Parfenova, W. Piatek, A. M. Quynh, V. N. Schetinin, A. Serikov, S. I. Sidorchuk, P. G. Sharov, N. B. Shulgina, R. S. Slepnev, S. V. Stepantsov, A. Swiercz, P. Szymkiewicz, G. M. Ter-Akopian, R. Wolski, B. Zalewski, M. V. Zhukov, “The 6H states studied in the 2He(8He,α) reaction and evidence of extremely correlated character of the 5H ground state”, Submitted to Physical Review C (2021) [arXiv:2105.04435].