EXPERT project main objectives

The joint proposal EXPERT is suggested by the consortium GSI (Darmstadt, Germany) – FLNR JINR (Dubna, Russia) – University of Warsaw (Warsaw, Poland) – PTI (Ioffe Physics-Technical Institute of Russian Academy of Science, St. Petersburg, Russia) – KI (National Research Center “Kurchatov Institute”, Moscow) and the structure is open for other institutes to be involved. It is aimed at studies of the nuclear landscape beyond the proton and neutron drip-lines and intends to push researches up to limits of nuclear existence. By combining the EXPERT instrumentation in different scenarios, phenomena of radioactivity, resonance decays, beta-delayed decays and exotic excitation modes can be studied via observations of particle emissions, including the 2p, 4p, n, 2n, 4n channels. Therefore the main objectives of the EXPERT proposal are:

  • Exotic 2p radioactivity studies and search for novel types of radioactive decays: 4p, 2n, 4n.
  • Studies of p, 2p, 4p, n, 2n, 4n resonance decays coupled with spectroscopy of continuum.
  • Quest to discover the limits of existence of nuclear structure. Search for systems located far beyond the drip-lines aimed to answer for the basic question: “Where is the borderline between a resonant behavior and continuum response of nuclear matter”?
  • Studies of beta-delayed particle (multi-particle) emission from exotic isotopes near and beyond the drip-lines.

For the systems which ground states decay by (multy-) nucleon emission the proposed setup covers two important lifetime ranges of 1 s – 100 ns, and 1 ps – 100 ns by applying the implantation-decay and decay-in-flight techniques, respectively. For the short-lived systems the resonance properties and information about continuum dynamics is extracted on the basis of the angular correlations between the products. The above types of measurements are augmented with information about gamma-deexcitation and betta-delayed particle emission of the decay products.

Below is a schematic layout of the proposed experiments for exploratory studies of nuclei beyond the proton and neutron drip-lines. The illustrated scenario suggests a population of two-proton (green) or two-neutron (orange) precursor in a secondary reaction of one-nucleon knockout by using radioactive beam. Theoretical/MC simulation framework is mentioned in this graph as a component of the proposal required in most considered experimental scenarios.

EXPERT components and subsystems

There are three main components for measurements of decays-in-flight of exotic nuclei:
  1. Radiation-hard silicon strip detectors SSDs. These compact and universal beam detectors of the SuperFRS provide information on time-of-flight, position and energy loss of ions, and they will be used for tracking of the secondary beam impinging the secondary target.
  2. Micro-strip silicon (Si) tracking detectors. The detectors are essential for applications of tracking technique for studies radioactive decays-in-flight and provide information on trajectories of all charged decay products, which is sufficient for determination of half-life values in the range of 1 ps – 100 ns as well as on decay energies and angular correlations of decay products.
  3. The NeuRad (Neutron Radioactivity) fine-resolution detector of neutrons. Together with Si detectors, this small-size 40x40x100 cm3 neutron detector can provide precise information on angular correlations of decay neutrons with a charged fragment, which is used to derive the decay energy of exotic radioactive decays (e.g., an unobserved yet phenomenon of neutron radioactivity is suggested to be probed in the decay energy range of 0.1-100 keV).
The EXPERT components augmenting the tracking subsystem are:
  1. The GADAST (Gamma-ray Detectors Around Secondary Target) array. It measures gamma-rays and light particles emitted instantaneously after secondary reaction. In the context of the proposal it could allow to disentangle the decays channels with a heavy fragment resulted in an excited state (and thus instantaneously de-excited by gamma emission).
  2. The OTPC (Optical Time Projection Chamber) for radioactivity studies by the implantation-decay method. The detector measures trajectories of all charged fragments of radioactive precursors with lifetimes in the range 1 s – 100 ns.
  3. Theoretical/Simulation framework. In order to obtain physics results, the information provided by tracking/angular measurements needs the detailed theoretical analysis followed by Monte Carlo simulations. Moreover, solid theoretical predictions like the first theory of 2p and 2n radioactivity make strong motivation for performing high-risk pioneering studies.

GADAST - Gamma Detectors Around Secondary Target

The GADAST array measures gamma-rays and light particles emitted instantaneously after secondary reaction. In the context of the proposal it could allow to disentangle the decays channels with a heavy fragment resulted in an excited state (and thus instantaneously de-excited by gamma emission).

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NeuRad - Neutron Radioactivity

The NeuRad (Neutron Radioactivity) fine-resolution detector of neutrons. Together with Si detectors, this small-size 40x40x100 cm3 neutron detector can provide precise information on angular correlations of decay neutrons with a charged fragment, which is used to derive the decay energy of exotic radioactive decays (e.g., an unobserved yet phenomenon of neutron radioactivity is suggested to be probed in the decay energy range of 0.1-100 keV).

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OTPC - Optical Time Projection chamber

The OTPC detector consists of a set of parallel 20 cm 20 cm electrodes forming several electric field regions. Active volume of the detector has a length of 30 cm. A whole volume of the OTPC was filled with a gas mixture of 95% of He and 5% of N2 at atmospheric pressure. Such gas mixture was chosen to have long enough tracks of low energy decay products of 8He.

Incoming ions as well as their charged decay products produced ionization electrons along their trajectories in the gas filling the chamber. These primary ionization electrons were transported with a drift velocity of Vd = 0.5 cm/mks towards two stage charge amplification structures. First of them was formed by three gas electron multiplier (GEM) foils, second one by two closely spaced wire-mesh electrodes. At the final amplification stage ultraviolet and visible photons were emitted. A visible part of this light spectrum was recorded by a 2/3” 1M pixel CCD camera with light amplification and a 5” photomultiplier (PMT). The data acquisition was triggered by a coincidence of a TOF signal and a PMT signal which provided a signature of a 8He ion entering the OTPC detector. At the arrival of the trigger signal, the beam from the separator was switched off and the exposure of the camera was initiated for a period of 1–1.2 s. At the end of this time interval the camera image, the digitized PMT signal, and the dE – TOF information of the triggering ion were saved on a hard disk. While the camera registered the projection of a particle’s track on the electrode plane, the shape (width) of the time distribution of a PMT signal provided information on the track’s projection in the direction normal to the image plane (dLz = Vd*dt). The combination of information contained in the camera image and in the recorded drift time profile allows a complete reconstruction of particle’s momentum. In the applied procedure the energy and particle emission angle were determined by fitting the projected theoretical ionization density distributions simulated by using SRIM code to the light intensity distributions measured by the CCD and PMT. Fit parameters were a normalization factor, the energy and the emission angle. The experimentally determined response function of the OTPC detector was taken into account when comparing the calculated and measured profiles.

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