Current activities of LPEN

Since the middle of the first decade a group of LPEN focused on research in foreign centers in works with ion Penning traps. The ion trap is a highly sensitive and high-precision device that has broadly multifunctional applications in fundamental and applied research. I t has no equal in mass spectrometry on these parameters (see K. Blaum, Yu.N. Novikov and G. Werth, Contemporary Physics 51 (2010) 149).

To date LPEN actively participates in many existing European projects with ion Penning traps: ISOLTRAP (ISOLDE, CERN), SHIPTRAP (GSI, Darmstadt), JYFLTRAP (Jyvaskyla) and TRIGATRAP (Mainz). Currently the scientists of the laboratory are collaborating within the project PENTATRAP at the Max-Planck Institute for Nuclear Physics in Heidelberg.

The main activity of LPEN is the study of exotic nuclei and exotic physical phenomena using ion traps.
(see Booklet)

Physical tasks that laboratory researchers are working on

The definition of mass landscape of nuclides of superheavy elements

Direct mass measurements of isotopes of elements No and Lr at the setup SHIPTRAP initiated the establish of the mass surface of the region of superheavy (see M. Block et al., Nature 463 (2010), 785; and E. Minaya Ramirez, et al. Science 337 (2012), 1207). The cryogenic ion chamber and a new method of phase imaging for mass measurements invented by S. Elissev have been used. These pioneer works made by German scientists in collaboration with LPEN, which do not have analogues in world practice, were awarded the 2013 international prize G. Flerov.

The analysis of data in 2019 gave the mass values for ^251-254No, ^254-256Lr and ²⁵⁷Rf and consequently a part of the mass landscape of the superheavy nuclides till the darmstadtium (Z=110) wit the use of alpha-decay chains. This led to the possibility of determining the values of gaps in the shell structure responsible for stabilizing factors in superheavy nuclides. Data on the landscape of the mass surface obtained by us show the “quasimagicity” of the neutron numbers N=152 and N=162 in nuclides, and lead to the conclusion that there are small Islands of stability that arise at the approaches to the supposed Island of stability of superheavy elements. This conclusion was directly confirmed by experiments on SHIPTRAP. Another interesting result was the direct measurement of the mass of a number of isomeric states of elements No and Lr.
The collaboration is going to continue the research in the future.
The search for new candidates for the measurement of the neutrino mass in the process of electron capture by the nucleus

Studies were initiated at the setup ISOLTRAP/ISOLDE at CERN (see S. Eliseev et al., Phys. Lett. B 693 (2010), 426). ¹⁹⁴Hg was determined as one of the possible candidates. The research program continues on the project IS-473/ISOLDE. Meanwhile, despite expectations another nuclide, ¹³¹Cs, is not suitable for determining the mass of neutrinos based on the measured decay energy
(see J. Karthein et al. Hyperfine Interaction 240 (2019) 61).
The search for candidate for the neutrinoless double electron capture by the nucleus

The large-scale systematic measurements of the mass difference between stable nuclides have been perfomed to identify the nuclide pairs with energetically degenerate states, which can be related with neutrinoless transition (see S. Eliseev, Yu. Novikov, and K. Blaum, J. Phys. G 39 (2012) 124003). The presence of such process would indicate a violation of the lepton number conservation law and Majorana-type of neutrinos. ¹⁵²Gd was found as the most likely candidate for the study of the resonance effect. Possibility of multiple resonance in ¹⁵⁴Dy was observed for the first time. The work is performed at the setup SHIPTRAP with the cooperation of German scientists with LPEN and St. Petersburg State University. (S. Eliseev, Yu.N. Novikov, K. Blaum, Ann. der Phys. 525 (2013) 707).
Study of possibility of using of an orbital electron capture by the nuclide to identify the relic neutrinos and heavy sterile neutrinos

Estimation and analysis of the data show that the nuclide ¹⁵⁷Tb can be used to identify the presence of relic neutrinos in the process of their resonance capture (J. Vergados and Yu.N. Novikov, J. Phys. G 41 (2014) 125001). Work of LPEN with foreign colleagues also shown that a set of nuclides can be used as a test in the terrestrial laboratory conditions to identify sterile neutrinos in the mass range 1-100 keV (P. Filianin, K. Blaum et al., J. Phys. G 41 (2014) 095089). It is believed that they are candidates for warm dark matter in the universe. The project provides for the study of experimental possibilities with the use of ion traps in combination with a cryogenic microcalorimetry.
Measurements of mass difference of nuclides for the purposes of cosmochronology

This project, carried out jointly with Max Planck Institute for Physics and group of SHIPTRAP at GSI, was initiated with a measurement of the mass difference between ¹⁸⁷Re-¹⁸⁷Os, in which the parent nucleus ¹⁸⁷Re can be used to determine the age of the Universe due to its very long lifetime. However, the probability of the reverse process from thermally populated excited states of ¹⁸⁷Os, which can greatly disrupt the balance of natural abundance of Re/Os, depends on the accuracy and reliability of measurements of the mass difference in this pair (D. Nesterenko, S. Eliseev et al., Phys. Rev. 90 (2014) 042501). In these measurements we used a new method for the determination of the cyclotron frequency of ions that rotate in a magnetic field based on the phase analysis of their motion in the Penning trap. This method, recently developed, allows to increase the accuracy of mass determination and resolving power (S. Eliseev et al., Phys. Rev. Lett. 110 (2013), 082501).

A similar picture can be observed for other pairs of nuclides, which are interested for astrophysics, the mass of which are measured or are to be measured (P. Filianin et al., Physics Letters B 758 (2016) 407-411, K.Takahashi, K. Blaum and Yu.Novikov. The Astrophysical Journal, 819:118 (2016) March 10).

The resulting mass difference value of ¹⁸⁷Re-Os, equal to 33 eV according to our SHIPTRAP measurements, was improved by an order of magnitude (up to 3 eV) by measurements at the new PENTATRAP facility in Heidelberg (the material is being prepared for publication in 2020). An important factor in these studies is the good coincidence of the mass difference data obtained by the traps with the data on the decay energies of ¹⁸⁷Re obtained by cryogenic microcalorimetry, which opens the possibility of using the latter to determine the mass of antineutrino.

The development of new projects where the LPEN is involved

Precision measurements of the smallest of the existing decay energies of beta-transformations

The project aims to precision measurements of the mass difference between ground states of the mother nuclide and excited states of the daughter nuclide with the expected lowest energies of beta-transitions known in the literature. The analysis selected nuclides, which are proposed to measure at first. The reasons are the possibility of using these nuclides for alternative determination of neutrino mass, as well as verification of the theory of beta-decay at very low energies (lower than 1 keV). Similar complementary projects are approved by Program Committees JYFLTRAP (Finland) and ISOLDE (CERN) with the allocation of beam time in the respective centers.
Ultra-precision mass measurements. Project ECHo, devoted to the determination of the neutrino mass in the process of electron capture in nuclide ¹⁶³Ho

The experimental part of the project ECHo consists of two complementary sub-projects: ultra-precision measurement of the mass difference between ¹⁶³Ho-¹⁶³Dy in the trap PENTATRAP and calorimetric measurements of the atomic spectrum of the de-excitation after the capture of an orbital electron by the nucleus. To determine the mass of a neutrino, which is currently known with poor accuracy (in contrast to the mass of an antineutrino), it is necessary to measure the calorimetric spectrum, which includes all types of deexcitation of excited atomic states that occur after the departure of the neutrino. The neutrino mass can be extracted from the analysis of this spectrum, but with the knowledge of the difference between the masses of holmium and dysprosium. The accuracy of latter will give an error in determining the neutrino mass (see L. Gastaldo et al. Eur. Phys. J. Special Topics 226 (2017) 1623). This mass difference, determined reliably and with great accuracy, can be provided by the new ultra-precision PENTATRAP installation, which is launched at the Max Planck Institute for nuclear physics.
Ultra-precision mass measurements. PENTATRAP project

LPEN participated in constraction and commisioning of the setup of five-traps tower PENTATRAP, which is being realized with German colleagues at Max Planck Institute for nuclear physics in Heidelberg (J. Repp et al., App. Phys. B 107 (2012) 983; C. Roux et al., App. Phys. B 107 (2012) 995). The idea of creating this unique system of traps is related to the need to reduce the systematic error, the value of which limits the accuracy of determining the masses in traps, especially those operating on accelerator and reactor beams. A significant improvement in systematic accuracy is achieved by simultaneous measurement of cyclotron frequencies for the studied and reference source with the placement of this pair sequentially in different traps.

Launched in mid-2018, the PENTATRAP system has already allowed the mass of xenon calibration isotopes to be determined with record relative accuracy (2x10^-11) (A. Rischka et.al. Phys. Rev. Lett. 124 (2020) 113001). This accuracy is two orders of magnitude higher than the accuracy obtained on traps for radioactive nuclides, and many orders of magnitude higher than the precision of all other mass spectrometry methods.

At the turn of 2019-2020 a new phenomenon was experimentally discovered - a long-lived highly excited, high-spin atomic isomerism. When measuring the mass of the highly charged ion ¹⁸⁷Re²⁹⁺, a state with an energy of 202.2(17) eV was observed that lived for more than one week. A similar metastable level with an energy of 207(3) eV was observed in the isoelectronic state ¹⁸⁷Os³⁰⁺ (R. Schuessler et al., Nature 581 (2020) May 7). This type of atomic (ionic) isomerism resembles the high-spin isomerism in nuclei. Its study is just beginning and may open the gates to a new physics. In addition, the detected multi-charge isomer can be used as the most accurate frequency (time) standard.
Study of the properties of the isomeric state of ²²⁹Th^m as a possible nuclear frequency reference.

The existence of the ²²⁹Th isomer with an energy of 8.3 eV opens up intriguing possibilities for the development of technology and basic science. The low transition energy gives hope for the use of laser methods to populate the nuclear transition and opens the way for the development of a more accurate and possibly more compact (solid-state) frequency standard. Currently, in cooperation with the metrology institute VNIIM (St.Petersburg), the PNPI is considering the possibility of creating a stand for studying decay modes and searching for the possibility of settling the isomeric state of thorium-229.
The project of Penning trap PITRAP for studies of nuclides produced at the reactor PIK

LPEN, having a great experience with ion traps abroad, is going to create the first Russian Penning trap for precision mass measurements (Y.I. Gusev et al., Preprint PNPI (2014), Yu. Novikov et al. PNPI report F-310 (2016)). According to the project the Penning trap can be used in the “on-line” mode on the beam of the products of fission by neutrons from the reactor, as well as in the “off-line” mode using stable or long-lived nuclides produced at the reactor. Developed in the project ion-optical system adjusted to two possibilities of transporting the ion beam to the tandem of two Penning traps (preparation and measurement trap): to the gas jet system (D. Simonovski et al. Atomiv Energy 125 (2018) 338) as well as to the mass separator.

The setup PITRAP, placed in the experimental hall of the PIK reactor, will combine the advantages of using a high flux of neutrons with high sensitivity (at the level of single particles) of the trap. The expected high yields of exotic nuclides will achieve nuclide pathway of the astrophysical process of rapid capture of neutrons occurring in Supernovae bursts (r-process) (Yu. Novikov et al. Euroasian J. Phys. 3(1) (2019) 63-70). Determination of the true path of this process in a terrestrial laboratory can be carried out only as a result of mass measurement (total binding energy) of nuclides in the assumed region of the r-process, that will be carried out by a Penning trap.

The project uses the development of setup TRIGATRAP with the Penning trap installed at TRIGA reactor in Mainz (Germany), setup SHIPTRAP with the Penning trap, placed after the mass separator of heavy ions SHIP in Darmstadt (Germany). LPEN supports a fruitful cooperation with these groups for many years.