Methods which use neutron diffraction are of constant interest, due to the development of the theory and practice of the analysis of materials in a condensed state (the Nobel Prize in physics was awarded to C. G. Shull and B. N. Brockhouse in 1994). The paper by Peterson and Levy [1] was one of the first to mention single-crystal neutron diffractometry. With the appearance of four-circle goniometers in the 1960s in the USA, France and Russia, which enable the diffraction from single crystals to be recorded in all possible crystallographic directions, neutron diffractometers began to be constructed [2–6]. As new structural problems arose, special systems of programs and methods for calculating structural characteristics were developed.

Neutron diffractometry methods were primarily employed to investigate the features of the magnetic substructure of substances and materials, in which particularly high accuracy in determining the structural parameters was not required. It was later shown that these methods can be important and an independent means of constructing relations (the composition – structure – property – degree of dispersion) and, together with other methods, was capable of developing in many priority areas of science and technology. As a result, neutron single-crystal diffractometers began to be optimized [3–6] up to the level of an x-ray diffractometer. The increased accuracy of the analogous x-ray equipment is due to the highly accurate values of the wavelength of x-ray radiation, and also due to the use of certified procedures for measuring standard samples, produced for tests and checks of x-ray diffractometers [7–19]. At the same time, one can use the results of neutron measurements for a more reliable analysis of samples containing light atoms and, consequently, one can determine structural characteristics, including atomic characteristics, and the structure of materials with such components.

In this paper, we present the results of measurements using the SyntexP1 goniometer made by Syntex on neutron single-crystal diffractometer equipment, designed with the VVR-ts research reactor at the L. Ya. Karpov Scientific-Research Physicochemical Institute at Obninsk. For comparison we present the results of measurements carried out using the same goniometer on a laboratory x-ray single-crystal diffractometer. Both series of measurements were made on a test sample from a batch of standard samples based on the diffraction properties of quartz.

The Neutron Single-Crystal Diffractometer. The diffraction of neutrons (subatomic particles) is based on the fact that they can occur as de Broglie waves, which are reflected from the nuclei of the atoms of the material being analyzed. The wavelength is in the region of the maximum of the neutron spectrum and is of the order of 0.1 nm. This value is identical with the interatomic distances in crystals, which hence also enables neutrons to be diffracted.

Neutrons are electrically neutral, and neutron radiation penetrates most deeply into a material. As a result, one can use a large volume of samples, analyzing materials which form components of different engineering devices. The wavelength can also be regulated using monochromators, which select different energy sections from the reactor radiation.

In 1971, a four-circle neutron diffractometer, based on the SyntexP1 goniometer, was introduced into the neutron-diffraction equipment of the 3rd horizontal channel of the VVR-ts reactor and was the first such diffractometer in Russia. It was capable of making an automatic search, accurately defining and determining the angular positions of the maxima of diffraction reflections, determining the parameters of the unit cell of a single crystal, calculating the angular positions of the diffraction reflections from the determined parameters for automatic collection of the intensities, and automatic measurement of the neutron intensities of a single crystal and recording them on an electronic carrier.

The diffractometer equipment consists of the following: a source of neutron radiation, a monochromatizing radiation unit, an instrument for detecting a reflected beam of neutrons, and for controlling and processing data, a goniometer, and a panel for the manual control of the goniometer. The operation of the goniometer in the automatic mode is controlled by a data control and processing unit. Equatorial geometry is used to measure the intensity, for which the detector and the incident neutron beam are in the horizontal plane. In its construction and programming, the neutron diffractometer is similar to an x-ray diffractometer with the same goniometer. The main difference is that the x-ray detector is replaced by a neutron detector. However, whereas diffractometers for x-ray structural analysis are more standardized laboratory devices, in the case of neutron structural analysis neutron diffractometers in neutronographic equipment are more individual in their construction in each neutron research center.

For experiments on neutron diffraction, a nuclear fission reactor is necessary as the source. Hence, all types of neutron equipment must have physical and biological protection from gamma and neutron radiation of the primary beam of neutrons. Also, to increase the sensitivity and accuracy of the results obtained, the neutron monochromator units must be additionally protected from radiation. The secondary beam of neutrons, reflected from the crystal monochromator, requires the construction of a unit which provides protection from induced radioactivity of the materials of the structure and of the samples being investigated.

In all the neutron centers with stationary reactors, neutron diffractometers have been developed for analyzing polycrystalline and single-crystal materials, which are placed directly inside the experimental reactor room at the output of the collimated neutron beam from the monochromator crystal. For the detector, heavy protection from background neutrons of the reactor room is provided, as a result of which the diffractometer equipment becomes very large. To maintain the accuracy of the readout of angular quantities for such a diffractometer it was necessary to develop a special mass goniometer, capable of supporting a heavy load. The accuracy of the results of measurements of the parameters of the unit cell of crystals in these devices is reduced. At the same time, among the advantages of this type of neutron diffractometer we must mention that it is possible to mount other heavy additional equipment on it, for example, a helium cryostat with a solenoid to produce fields to act on the samples or a high-temperature furnace.

We used a different approach [20] at the Karpov Institute. First, the diffractometer was placed in a special protected unit, representing the chamber, welded from steel sheets and having an entrance door. The hollow walls of the chamber were filled with water to which boric acid had been added. The wall thickness of the chamber was 300 mm and the volume of the filled space inside the tank was 1200 ∞ 1200 ∞ 1800 mm. This protection enabled the SyntexP1 goniometer to be used, as employed in an x-ray diffractometer. For the neutron diffractometer, the x-ray detector was replaced by a highly effective end neutron counter based on a gas mixture of 10BF3.

Instead of the heavy protection from the background neutrons of the reactor room, a thin (1 mm) cadmium protection from induced gamma radiation was deposited on the counter. The background in the chamber was several pulses per minute. Even more importantly, the range of measured reflections over an angular scale of 2θ was extended to 140° by a sharp reduction in the transverse dimensions of the neutron detector, i.e., they were extended to a range similar to the range of an x-ray diffractometer.

Second, the protection of the crystal in the monochromator and collimators from radiation contains steel tanks filled with water with the addition of boric acid (see the figure 1). Inside the tank there is space for a stack of lead. Slowed neutrons are introduced from the reactor through channel 1 and the first collimator 2, which are surrounded by special protection of water, paraffin, boron carbide and lead. Collimator 2 is made in the form of a steel tube of rectangular cross section 2000 × 50 × 50 mm and enclosed in a protective jacket of paraffin, boron carbide and iron. Neutrons of monochromatic wavelength, reflected from the monochromator crystal 7 at an angle of 90°, are incident, through the second collimator 3, on the single-crystal sample 4, placed on the goniometer head of the neutron diffractometer. The second collimator 3 is made of sheet cadmium, and also has a rectangular cross section of 1000 × 12 × 25 mm. A collimator-limiter of the beam, with an internal diameter of 10 mm and a divergence of about 2°, is placed in front of the detector.

Fig. 1
figure 1

Sketch of the neutron diffractometer equipment (a section at the level of the horizontal reactor channel [20]): 1) channel; 2, 3, 5) collimators; 4) sample; 6, 7) monochromator crystals.

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The second crystal of the monochromator 6 – a lead single crystal, is the first crystal of the monochromator, which allows a single beam of monochromator neutrons to pass through collimator 5 for investigations at a small monochromatic angle of 11°. This arrangement of the monochromator signals leads to an increase in the utilization factor of the neutron beam from the horizontal channel of the reactor.

Moreover, we optimized the geometry of the neutron radiation incident on the sample: the primary neutron beam, followed by the monochromator crystal, followed by the secondary neutron beam. According to calculations [2, 21] we chose the optimum focusing conditions for the whole field of reflections of the Ewald sphere. The monochromatization angle θm = 45°. Using collimators we achieved similar divergence of the primary and secondary radiations (about 40 min). These measures enabled the half-width of the reflections to be reduced considerably over the whole measured range, which is 3–4 times less than the time taken to measure the intensities by reducing the angular scanning interval of each reflection. Also, by reducing the overlap of the reflections in the recorded inverse diffraction space we increased the maximum values of the parameters of the unit cell of the crystal lattice of the substance being analyzed.

Basic Parameters and Characteristics of the Equipment. The flux of monochromatic radiation is approximately 106 n/(cm3·min); reflection with Miller indices (331) of the monochromator crystal (a single crystal of copper) in the transmission position; the wavelength of the monochromatic wave λ = 1.167Å. The contribution of the second harmonic: λ/2 ~ 2%, Δλ/λ ≤ 2%; instrumental background 1–5 pulses/min; the range of angles on the readout scale 2θ = 20–140°; the researcher chooses the scanning rate in the range 0.5–10 deg/min; the interval of the angular scanning range of a reflection for further automatic selection of the data is determined experimentally by the operator after preliminary measurements of the width of the reflections.

We added a cryostat to the diffractometer equipment, in order to investigate samples at low temperatures in the nitrogen range (down to −150°C), and a furnace for operations at temperatures up to 300°C.

An increase in the accuracy of the measured interplane distances occurred both due to an increase in the angle of reflection, and to a reduction in the half-width of the reflection. This enabled us to measure the parameters of the crystal lattice with an accuracy exceeding that obtained using neutron single-crystal diffractometers employed at the present time.

As is well known, the accuracy with which the structural parameters can be determined is reduced, together with a reduction in the symmetry of the unit cell. An increase in the accuracy and the volume of the data on this equipment was confirmed by the results of measurements of the parameters of the unit cell and a calculation of the structure of a crystal with a lower orthorhombic symmetry of the unit cell [22].

To test this structure, we measured the characteristics of the diffraction pattern of samples from a single batch for standard samples based on the diffraction properties of quartz. The measurements were made in an independent region for neutronographic equipment and over the whole Ewald sphere (all three independent regions) for x-ray equipment. The data were processed using CSD and XTL software. The results of a determination of the structural characteristics of both equipments are shown in the table.

A high level of agreement between the results of a determination of the structural parameters of quartz was reached using both equipments, employing the same kind of goniometer. The difference does not exceed 3σ. Hence, we have shown that the neutron diffractometer equipment has been optimized up to the capabilities of an x-ray diffractometer. These data (see the table 1) also agree with the results of more detailed measurements on other samples of this batch of synthetic quartz, which were carried out in order to certify this batch as a standard sample of the diffraction properties of quartz [23, 24].

Table 1 Parameters of the Unit Cell and the Relative Coordinates of the Atoms from the Data of x-Ray and Neutron Experiments in the Crystal Structure of a Single Crystal of Quartz SiO2 (the spatial group of this material is P3121)
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Note that in this reactor we later employed a lighter goniometer made by the Huber Company. In view of the successful results obtained from many years of measurements on different types of materials, this equipment was given the status of unique research equipment. As a result of highly accurate measurements on single crystals and the use of methods of processing the diffraction data, which considerably expanded the information content of the diffraction pattern, when operating this equipment we obtained new information on traditional and new materials. Additional information on the equipment employing the Huber goniometer, and the results obtained, are presented in [25].