Inverse Geometry Inelastic Neutron Scattering Spectrometer BJN
Leader: Chudoba D.M.
The BJN (Bajorek-Janik-Natkaniec) instrument is an inelastic neutron scattering (INS) spectrometer in inverse geometry. The development and construction of an advanced competitive INS spectrometer at IBR-2 is necessary to significantly expand the range of problems to be solved and to effectively implement the possibilities of neutron spectroscopy for studying atomic and magnetic dynamics of condensed matter. The availability of such an instrument will considerably broaden the field of INS application, since, having a record luminosity, it will allow experiments with samples that have small scattering cross sections or are available in small quantities, which in general will lead to a significant increase in potential research objects and enhance the competitiveness and efficiency of investigations conducted at FLNP.
Fig. 1. FLNP physicists, whose studies were the first in the world to lead to the creation of a new research area – inelastic neutron scattering spectrometry in inverse geometry 
Main scientific areas and objects of research
Neutron scattering makes it possible to obtain unique information about the properties of condensed matter. This is due to a number of properties of the neutron and the nature of its interaction with matter. In the process of inelastic scattering, neutrons can excite atomic/molecular vibrations or enhance stochastic processes through the exchange of energy and momentum. For inelastic incoherent scattering, the intensity is proportional to the space-time transformations of the Fourier autocorrelation function 
, which describes the probabilities of the particle being in position 
and the time t when the same particle was in =0 at t = 0. This allows us to obtain information about the dynamics of the studied object at the microscopic level.
It should be noted that the use of neutrons as a probe for studying atomic and magnetic dynamics has a number of unique features:
- ‘direct’ spectroscopic observation of atomic/magnetic excitations as a function of energy and momentum transfer;
- possibility to observe the evolution of the dynamics of the substance under study during a phase transition, for example, the behavior of the soft acoustic mode near the temperature of structural instability;
- energy transfer regions of several hundred meV (thousands cm-1) can be achieved quite easily;
- due to the exceptionally large cross section for incoherent scattering of hydrogen nuclei (80.26 barn), measurements of vibrational spectra of hydrogen-containing objects can be very efficiently carried out using the INS method;
- the existence of a neutron's magnetic moment makes it possible to study magnetic dynamics;
- high penetrating power of the neutron for most materials allows us to obtain detailed information about the dynamics of the object under study in bulk, which is a significant advantage over optical and X-ray methods;
- scattering cross section depends on isotopes, therefore isotopic substitution is used to refine spectroscopic information.
Main research topics:
Investigation of structural phase transitions at the microscopic level;
Investigation of proton diffusion processes in systems with various types of hydrogen bonds;
Investigation of proton dynamics in molecular crystals over a wide range of energy transfers;
Investigation of associative interactions of chemical particles, including systems with the formation of hydrogen bonds of various types;
Investigation of magnetic dynamics in compounds with 4f and 3d transition metals.
Main research objects:
Molecular crystals and their phase derivatives;
Pharmaceutical preparations in bulk and in the form of "micronized" or "amorphized" powders;
New biologically active compounds, including nanostructured ones;
Energy storage materials;
Intermetallic compounds of 4f and 3d transition metals;
Catalysts;
Photonic materials for industrial applications;
Nanocomposite materials.
Conceptual design of the spectrometer
A time-of-flight spectrometer in inverse geometry is a relatively inexpensive and very effective tool for neutron spectroscopic studies and is almost ideally suited for a pulsed neutron source (IBR-2). The general conceptual view and schematic layout of the instrument are shown in Fig. 2.
Fig. 2. General conceptual view and schematic layout of the BJN instrument 
The primary spectrometer consists of two main elements:
- Mirror neutron guide (~100 m) with focusing end part (~25 m)
Neutron optics will be optimized for a wavelength range of 0.5-4.2 Å (i.e., for energy transfer values of 0-330 MeV). Beam size at sample position – 3×3 cm.
- System of choppers.
- Suppression of delayed neutron background.
- Selection of transmission band to eliminate overlap of neutron pulses.
The secondary spectrometer comprises a system of analyzers (cooled beryllium filters, HOPG), and a detector system. The basic concept of the secondary spectrometer is shown in Fig. 3.
Fig. 3. Basic concept of the secondary spectrometer of the BJN instrument 
The optimal choice of material for the reflective surface of the analyzer is highly oriented pyrolytic graphite (HOPG), which has a high (~ 70%) reflectivity at an energy of ~ 4.5 meV for reflection (002). A beryllium filter cooled to ~100 K, which is designed to suppress high-order reflections from graphite crystals, will be installed between the sample position and the HOPG analyzer.
Expected results
One of the advantages of the new spectrometer at the IBR-2 reactor will be its high luminosity. This feature will lead to better, almost lossless use of the neutron beam, which will significantly reduce the duration of experiments and the possibility of working with samples of small mass. In general, the proposed instrument will be the best in terms of the efficiency of using neutrons. The results obtained with the new spectrometer will be on par with those obtained with the leading inelastic neutron scattering instruments in Europe.
Expected characteristics of the new INS spectrometer compared to the existing NERA spectrometer NERA BJN Explanatory notes Analyzer area 15×3×25 1125 cm2 4×4×2100 33600cm2 Ratio of entrance and exit areas of the neutron guide 16×5cm2/5×5cm2 3.2 20×20cm2 / 3×3cm2 44.44 Gain in flux density (without taking into account the quality of the neutron guide) 44.44/3.2 = 14 Solid angle ~ 0.2 sr ~ 6.4 sr Gain in solid angle 30 Luminosity ratio between NERA and BJN 30×14 = 420 Measurements will be possible with samples of ~ 10-20 mg