VESPA

The fundamental idea behind the so-called neutron vibrational spectroscopy [1] (NVS) technique is analogous to that exploited in optical spectroscopy [2] (IR and Raman): a sampling probe, carrying energy larger than that of the internal excitations, is directed at the sample.

The resulting neutron energy loss upon excitation of a vibrational mode gives direct information on the vibrational energy level structure of the sample. In general, vibrational spectroscopy [3] is a fundamental technique used constantly in educational, research, and industrial laboratories all over the world. Its applications in the investigation of solids and liquids, soft matter, complex fluids, and biomaterials are well-known. Indeed, it has become an essential tool in medical applications, forensics, environmental compliance, and quality control to cite but a few common uses.

Fundamentally, vibrational spectroscopy probes potential energy surfaces and interatomic interactions. Alone or in combination with other techniques, vibrational spectroscopy permits the identification of bonds and functional groups, as well as the transformations that occur when bonds are broken and made in chemical reactions (e.g., in catalysis or thermal decomposition). The vibrational density of states is of great interest in itself, being related to various thermodynamic properties such as specific heat or entropy. The vibrational spectrum is affected by configurational changes in molecules thereby also providing structural information.

NVS exploits the large incoherent scattering cross section of the hydrogen nucleus. Proton dynamics or vibrations connected to the movement of H atoms can be easily detected spectroscopically, even if hydrogen is dissolved at very low concentrations in materials composed mostly of heavier atoms. For this reason, the technique attracts a high interest in the scientific community operating in the fields of chemistry, materials science, physics, and biology, with a particular emphasis on applications.

[1] P. C. H. Mitchell, S. F. Parker, and A. J. Ramirez-Cuesta, Vibrational Spectroscopy With Neutrons (World Scientific, Singapore, 2005).
[2] M. Hesse , H. Meier, B. Zeeh, Spectroscopic Methods in Organic Chemistry(Thieme, Stuttgart, 2007).
[3] M. Diem, Introduction to Modern Vibrational Spectroscopy (Wiley-Interscience, Hoboken (NJ), 1993).

The VESPA spectrometer can be conceptually divided into the following functional blocks:

• Neutron optics system
• Chopper system
• Analyzers, filters, and detectors
• Shielding
• Shutters
• End-station
• Beam stop

However, given the introductory character of the present explanation, we are going to deal only with the three main parts, also reported in Fig. 2.

Neutron Optics System

The VESPA neutron optics system will be able to perform an efficient transport of a large spectral bandwidth (0.4 Å<l<4.7 Å) in order to give rise to a homogeneously illuminated sample area (35´35 mm², angular divergence better than 2^o) at 59 m. The neutron guide will stretch from 2.00 m to 58.50 m (i.e. up to 0.50 m upstream the sample area) close to the target-to-sample centre line. In the VESPA layout the guide is planned so as to allow a direct line of sight between moderator and sample. Approximately the first 3.5 m of the guide (i.e. the “feeder”) will be inserted in the monolith, followed downstream by about 2 m of straight guide crossing the chopper area (see below). After that, a straight tapered guide (with m=4) of about 51 m will transport neutrons up to the end of the optical system. So, in total, the full guide length will be of about 56.50 m. Another part of the optical system will be a set of jaws (horizontal and vertical) in order to define the beam size before the sample.

Chopper System

The VESPA chopper system (right inset in Fig. 2) is a central, and probably the most significant, part of the instrument. Used choppers can be classified according to their function into: three pulse shaping choppers (PSC, 3 pairs of counter-rotating discs), one frame overlap chopper (FOC, 1 disc), which can be also understood as a band width chopper (BWC), and one sub-frame overlap chopper (s-FOC, 1 pair of counter-rotating discs). Together (using two PSC’s out of three at the same time) they will form a complex wavelength frame multiplication (WFM) chopper system enabling the planned performances of the spectrometer. The rotation speeds will be indeed very low (154, 28, and 42Hz) if measured against the state-of-the-art capability, but the system will have a peculiarity: the pulse shaping chopper sets, the part closest to the source at a distance ranging from 6.50 to 7.44 m, has to allow the simultaneous use of two chopper pairs leaving the third parked open. For example, the 1^st (at 6.50 m) and 2^nd (at 6.80 m), while the 3^rd (at 7.44 m) stays open (high-resolution mode), or the 1^st and the 3^rd with the 2^nd staying open (low-resolution mode).

Analyzers, Filters and Detectors

VESPA will use highly oriented pyrolytic graphite (HOPG) crystals (left inset in Fig. 2) to determine the final neutron energy, as fixed by the selected Bragg angle. Such a configuration requires a high-pass filter downstream from the HOPG analyzer to remove higher order harmonics before detection. A beryllium filter can be used to fulfil this task. However, this solution works only if the Bragg angle is so large that the first harmonic (l₁) exceeds the beryllium threshold at l_co=3.966 Å, but so small that the second harmonic (l₁/2) does not exceed the same threshold. Thus we have chosen Bragg angles between 40^o and 60^o, so that the minimum reflected wavelength will be l₁(40°)=4.313 Å>l_co, while one half of the maximum reflected wavelength will be l₁(60°)/2=2.905 Å<l_co. The configuration of the spectrometer includes two backscattering analyzers sets, placed at scattering angles (not to be confused with Bragg angles!) of 150°(named “external”) and 130°(named “internal”). In this configuration two types HOPG analyzers can be distinguished, using Bragg angles of 60°and 40°, respectively. These analyzers will select neutrons whose first harmonics are: l₁(40°) = 4.313 Å and l₁(60°) = 5.811 Å, corresponding to the following final neutron energies: E₁(40°) = 4.4 meV and E₁(60°) = 2.4 meV, which are fully compatible with the transmission threshold of a beryllium filter (i.e. 5.2 meV). Let us now see some further details:

1) internal analyzer set (scattering angle: 130°, Bragg angle: 40°): each HOPG crystal will have a trapezoidal shape with a lower base width of 8 cm, an upper base width of 12 cm and height of 10 cm. The solid angle covered by a single HOPG slab, with respect to the centre of the sample, will be 0.0612 sr. Each slab will illuminate 20 detector tubes (see below for details) through a beryllium filter (cubic, 12 cm edge) forming a module (also known as “analyzer-filter-detector module”). Considering 7 modules (one has been removed to leave some space for the sample environment), a total solid angle of 0.428 sr will be provided;

2) external analyzer set (scattering angle: 150°, Bragg angle: 60°): assuming the same HOPG crystal shape and 17 tubes, the resulting solid angle, with respect to the centre of the sample, will be 0.0212 sr, providing a total solid angle (8 analyzer-filter-detector modules) of 0.170 sr.

Thus the solid angle collected by the whole system of analyzers (backscattering internal and external sets) will amount to about 0.598 sr.

Vibrational spectrometers are normally equipped with standard commercial detectors like, for example, ³He tubes (e.g. 200 mm long). In our case thin cylindrical tubes (e.g. with a 7.9 mm diameter) will be particularly suitable since the reduced value of the neutron travelling path into the tube is highly beneficial to the time-of-flight (i.e. energy transfer) instrumental resolution. However, in this case the gas pressure has to be quite higher (up to 12 bar) than the standard values in order to cope with the detection efficiency.  As for the diffraction capabilities, since the requested performances are simply standard, similar cylindrical ³He tubes (but shorter, e.g. 100 mm) will be adequate.

In a nutshell:

• VESPA will be the only spectrometer in the ESS instrument suite focused on molecular vibrations in chemistry and materials science.
• VESPA will have huge benefits over the traditional optical spectroscopy (i.e. IR and Raman).
• VESPA will provide exceptional flux in the “fingerprint region” of spectroscopy matching the existing neutron vibrational spectrometers.
• VESPA will offer a constant energy resolution up to 0.8% of the incoming neutron energy.
• VESPA will be able to trade neutron flux for energy transfer resolution.
• VESPA will enable time-resolved and in situ techniques in chemistry.
• The long pulse of the ESS will be fully exploited via an efficient use of the wavelength frame multiplication technique.

The VESPA instrument will exploit the wavelength frame multiplication technique in order to cover a wide range of energy transfers, 0<hw<500 meV, “in one shot”. The full neutron spectrum is collected in a single ESS pulse, making kinetic or parametric experiments feasible and easily interpreted.

Simulation results, using an optimized neutron guide, show that VESPA will be able to provide an excellent neutron flux up to 115-185 meV (depending on the resolution setting), covering most of the so-called vibrational “fingerprint region” (60-220 meV), which is the crucial interval in neutron vibrational spectroscopy for the scientific community intended to be served. The two-backscattering-bank configuration of the secondary spectrometer provides a large solid angle coverage (i.e. 0.6 sr) which could be even increased with further inclusion of forward-scattering banks.

The instrument resolution was accurately calculated showing that VESPA will be able to compete with corresponding neutron vibrational spectrometers operating at present, achieving over most of its energy transfer range two constant relative energy resolution values, either hDw/E₀=0.8% (high resolution mode) or hDw/E₀=2.5%  (low resolution mode), with E₀ being the incoming neutron energy.

Moreover, the possibility of improving the resolution at the expenses of the count rate or, on the contrary, sacrificing the resolving power to increase the instrument flux at the sample position, is a feature that makes VESPA unique among all indirect-geometry inelastic instruments worldwide.

Thanks to its high energy resolution and significant count rate, VESPA will allow the study of small amounts of materials, making it possible to investigate samples in extreme environments (such as high pressure and in situ) and compounds that are complex to be synthesized. 

Automatic sample changer

For higher throughput when measuring standard samples (i.e. dry powders at ambient pressure) kept at low temperature (say, T<20 K), a sample changer with a least 35 slots will be developed. The basic concept is inspired by the current TOSCA sample changer which uses a sort of computer-controlled “conveyor belt” equipped with a camera for the sample lock-in in the correct position.

Dedicated closed-cycle refrigerator (CCR)

The VESPA sample area is designed to allow fast, reproducible changes of CCR’s (or  helium cryostats), since most of the measurements are operated at low temperature (i.e. T<20 K). While these are routinely performed by VESPA on a large set of samples making use of the automatic sample changer (see above), in some cases this is not possible (e.g. in case of sensitive samples and/or in connection with special sample environment requirements). So, in these cases, it can be useful to cool down a new sample outside the experimental cave in a spare CCR, while the previous sample is still under measurement, and then just exchanging the two CCR’s, which can be done in a few seconds. Having more than one CCR is also a big advantage if the primary system fails, as most of the VESPA vibrational experiments require low temperature.

Gas flow cells and gas manifold

Flow-through gas cells, together with a gas flow control and monitoring system, are gaining a lot of interest in the last years in connection with neutron scattering. This type of equipment allows neutron powder diffraction data to be collected on samples at various temperatures depending on the cell material (e.g., using quartz, from 10 K up around 1300 K) when exposed to a selected gaseous atmosphere, e.g. H₂, O₂, CO₂, CO. In the future, on novel vibrational spectrometers like VESPA, these types of measurement will be also routinely performed in vibrational spectroscopy. The flow rate from the gas manifold to the cell and the corresponding chemical composition will be computer controlled by means of a system which will synchronize the gas dosing with the neutron data acquisition.

Photo-lamp and battery cells

The dedicated VESPA CCR (see above) could include an appropriate setup for: (a) a laser light source for sample photo-excitation, and (b) built-in four-point electrode connections for measuring the electric properties of neutron-irradiated materials. In the former case a non-standard, powerful cooling unit for the CCR will be needed in order to remove the heat load emanated by the photo-excitation laser inserted in the cryostat. As for the latter item, built-in electrode connectors are necessary to perform perfectly shielded (but still relatively easy) in-situ electrochemical measurements, such as impedance and conductivity. Finally, a new concept for the construction of batteries with conventional electrochemical performance, but allowing in situ neutron spectroscopic measurement is to be developed: a cell (c) comprised of a wound cathode, electrolyte and anode stack will be designed and prepared. The conventional H-containing components of the cell will have to be replaced by H-free equivalents (i.e. CFC, deuterated liquids etc.).

Anvil cell and high-pressure cell

The use of high pressure cells for inelastic neutron scattering studies is not new, since earlier He-gas cells (up to 6 kbar), and later aluminium-oxide clamp cells (up to 30 kbar), have been employed since the mid ‘70s. Obviously these types of cells will be also used on VESPA. However, anvil cells, once applied to neutron scattering by the well-known Paris-Edinburgh group, have made diffraction experiments up to 100-250 kbar possible on samples about 30-80 mm² large. Today on extremely intense vibrational spectrometers, like e.g. VESPA, these pressures will be certainly reachable during inelastic neutron scattering studies, since the requested sample volume will be reduced by two orders of magnitude in comparison with that routinely used on the standard vibrational instruments (i.e. ~1000 mm²).

• D. Colognesi
  Consiglio Nazionale delle Ricerche 
• L. Di Fresco
  Milano-Bicocca University
• M. Hartl
  European Spallation Source
• K. Jones
  Science and Technology Facility Council
• M. Chowdhury
  Science and Technology Facility Council
• S. F. Parker
  Science and Technology Facility Council
• G. Gorini
  Milano-Bicocca University
• C. Vasi
  Consiglio Nazionale delle Ricerche
• M. Zanetti
  Milano-Bicocca University
• F. Masi
  Milano-Bicocca University
• A. De Bonis
  Milano-Bicocca University
• S. Bellissima
  Consiglio Nazionale delle Ricerche
• L. del Rosso
  Consiglio Nazionale delle Ricerche
• G. Scionti

  Università della Calabria

   

• M. Zoppi, A. Fedrigo, M. Celli, D. Colognesi, VSI@ESS: Case study for a Vibrational Spectroscopy Instrument at the European Spallation Source, Eur. Phys. J. Web of Conf.83, 03021 (2015).
• A. Fedrigo, D. Colognesi, M. Bertelsen, M. Hartl, K. Lefmann, P. P. Deen, M. Strobl, F. Grazzi, and M. Zoppi, The neutron vibrational spectrometer for the European Spallation Source, Rev. Sci. Instrum.87, 065101 (2016).