Sample environment for neutron scattering experiments at ISIS

1. Introduction

In the last decade neutron scattering facilities have experienced a booming popularity that can be explained by astonishing new opportunities offered for users of neutron scattering facilities due to considerable progress in a number of areas such as neutron optics, neutron detection, large and complex dataset analysis, neutron scattering instrumentation and sample environment (SE). SE is particularly rapidly gaining importance because the majority of frontier science neutron scattering experiments are performed under special SE conditions [2]. Since the last Sample Environment Workshop in Oxford [9] there is an ongoing discussion about tendencies in this area and statistical data that can shed light on the role of SE at different neutron scattering facilities. In order to fuel this discussion further we are going to start from reviewing the current situation with SE and giving some numbers which can illustrate the state of SE at the ISIS neutron scattering and muon spectroscopy facility. Currently the ISIS facility operates 27 neutron and 7 muon Instruments. This allows us to run more than 30 experiments per day simultaneously and 800 experiments per year. About 95% of all experiments require some kind of SE. About 2/3 of all experiments use cryogenic SE. Gas handling, high pressure and high temperature SE are used in ∼ 25 % and high magnetic field SE in ∼ 10 % al all experiments.

One of the most labour demanding areas of ISIS SE is experiments below 1K often combined with high magnetic fields. These experiments cover quite a broad range of subjects such as: superconductivity, quantum criticality, low temperature magnetism, quantum topological effects, spintronics, quantum fluids and solids etc.

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Fig. 1.

The number of experiments below 1K per year.

In Fig. 1 we can see that the number of experiments below 1K was growing since 2008 and in last 4 years it has settled around 80 experiments per year (2014 was a year of long shutdown). It is important to note that 2/3 of sample environments below 1K are used in muon spectroscopy experiments. This tendency could be explained by the fast turnover of muon experiments which sometime take just few hours to collect the data required.

The low temperature experiments are not only labour intensive; together with high magnetic field SE they are also in the list of top consumers of liquid helium. In order to address this issue the ISIS SE team has started a helium recovery project about 3 years ago. The helium recovery system has been successfully commissioned earlier this year. The details of this system and ISIS collaboration with other neutron scattering facilities in this area can be found in Richard Down’s paper submitted to the same issue of the conference proceedings.

Another resource demanding area of SE is a range of experiments that requires gas handling, high pressure and in-situ chemistry [8]. In the last decade the number of these experiments has grown just by 30 % and reached an average of ∼ 90 experiments per year. At the same time the complexity of this kind of SE has increased immensely. In the early years of 21st century this kind of experiments have usually been using one gas and one sample in a furnace or cryostat. Today they are using complex mixtures of gases (sometime at high pressure), require few sample-changes during one experiment and need a broad temperature range spanning from cryogenic temperatures to thousands of kelvins. There is also demand for simultaneous spectroscopic analysis of reaction products or in-situ Raman spectroscopy. All of these challenging tendencies could be mitigated by implementing automation, computer control, optimisation of operational procedures and planning process whenever possible.

SE for soft matter experiments is the youngest, but fastest developing member of neutron scattering SE family. Soft matter is usually used for describing a group of materials that are easily deformed by thermal fluctuations and external forces such as liquids, colloids, polymers, foams, gels, granular materials, liquid crystals, and a number of biological materials. Ten years ago this kind of SE was represented by individually made user equipment sets for relatively rare soft matter neutron scattering experiments. Today the ISIS facility has got a dedicated team of engineers and technicians which are responsible for experimental support of soft matter experiments. An extraordinary variety of methods and vast selection of hardware used in the soft matter SE present the main challenge for organising of experimental operation support. The most efficient way to address this challenge would be a collaboration between different neutron sources when each participant concentrates on one or two tasks and share design, test results and operational experience with other partners. A very good example of such collaboration is the WP20-NMI3-II collaboration “Advanced Neutron Tools for Soft and Bio-Materials” [1]. Another way to improve and optimise soft matter SE could be standardisation of the hardware, software and operation protocols in order to achieve certain level of compatibility between different instruments and neutron scattering facilities.

Here we are going to review a few examples of recent ISIS SE related research and development projects in all three areas mentioned above. This paper is explicitly focused on ISIS facility, that is why we would like to emphasise that reader should be aware that alternative (or similar) systems have been successfully developed at other neutron scattering facilities.

Pressure generation at cryogenic temperatures presents a problem for neutron scattering experimental techniques due to the volume of sample required. We have developed compact pressure cell [7] also known as a “Sputnik cell”, with relatively large sample volume in which the load to anvils is generated by a bellow which can be operated in a pressure range of up to 35 MPa. A specially designed sample stick allows operating of the cell in the variable temperature insert (Ø100 mm) of the standard pulse tube refrigerator top loading cryostat [4]. The pressure of the sample is determined by the in situ ruby luminescence optical spectrometer. The cell has reached 5 GPa at temperature 4.5 K with a 2 mm diameter culet, using gem sapphire anvils and an annealed beryllium copper gasket. Load and on-line tests have been done at the WISH beamline of the ISIS pulsed neutron source to verify the cell performance.

Improvements in the available flux at neutron sources is making it increasingly feasible to obtain refineable neutron diffraction data from samples smaller than 1 mm³. The signal is typically too weak to introduce any further sample environment in the 50–100 mm diameter surrounding the sample due to the high ratio of background to sample signal, such that even longer count times fail to reveal sample peaks. Many neutron diffraction instruments incorporate collimators to reduce parasitic scattering from the instrument and from any surrounding material from larger pieces of sample environment, such as cryostats, but conventional collimation is limited in the volume it can concentrate on due to difficulties in producing tightly spaced neutron absorbing foils close to the sample, and in integrating this with neutron instruments. In order to solve this problem for the “Sputnik cell” we have designed a novel compact 3D laser ‘printed’ collimator which removes these limitations, and is shown to improve the ratio of signal to background, opening up the feasibility of using an additional sample environment for neutron diffraction from small sample volumes [18]. The photo of the collimator is presented in Fig. 2.

The commissioning test data have shown a large reduction in the incoherent background, and a decrease in the intensity of the powder peaks of the gasket and sample. Comparing the relative intensities of the sample to the gasket shows that the sample signal becomes order of magnitude more intense relative to the gasket. Analysis of the background signal also shows an increase of signal to noise of approximately 2–3 times. The compactness of the design also allows this concept to be integrated with existing sample environment, with designs that can be tailored to individual detector geometries without the need to alter the setup of the instrument.

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Fig. 2.

3D laser printed collimator. Gadolinium paint was used as a neutron absorbing material.

Internal stresses in materials have a considerable effect on material properties including strength, fracture toughness and fatigue resistance. The ENGIN-X beamline is an engineering science facility at ISIS optimized for the measurement of strain and stress using the atomic lattice planes as a strain gauge [19]. Nowadays the rapidly rising interest in mechanical properties of engineering materials at low temperatures has been stimulated by dynamic development of cryogenic industry and advanced applications of superconductor technology.

In order to provide broad temperature range sample environment for ENGIN-X beam applications the ISIS SE team together with instrument scientists have developed a high temperature furnace and cryogenic testing chamber for neutron scattering measurements of internal stresses in engineering materials at elevated [6] and cryogenic temperatures respectively [17].

In recent years the situation has dramatically changed by the growing demand for a new generation of superconducting magnets produced from superconducting materials which require an operational temperature below 10 K. First the cryogenic load frame which allowed access to this temperature range has been designed and commissioned by JAEA facility [20]. The system has reached the base temperature of 5 K, however mechanical load applied to the sample in this load frame is limited by 10 kN that significantly restricts the area of the system applications. Recently we designed, manufactured and commissioned a new cryogenic testing chamber for neutron scattering measurements of internal stresses in engineering materials under the load of up to 100 kN in the temperature range from 6.5 K to 300 K. Complete cooling of the system from room to base temperature takes around 90 minutes. The photo of the experimental set-up is presented in Fig. 3. The cryostat design [12] is similar to the one described in [17] and based on the idea of cooling only sample grips with the sample, keeping the hydraulic loading rig at room temperature. The main difference is the use of two stages of cryo-coolers, instead of single stage ones used in the previous design. The first stages of each of the two cryo-coolers are thermally connected to the infrared radiation shield. They are also connected through copper braids to 100 K thermal anchoring points on the left and right grips. The sample holder grip is thermally linked to the second stage of the cryo-cooler. The design of left and right sides of the grips assembly is completely symmetric.

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Fig. 3.

Experimental set-up for measurement of internal stress in engineering materials under loads. The uniaxial load up to 100 kN is provided by Hydraulic loading rigs.

Excessive cooling power at the first stages of cryo-coolers allowed us to incorporate a couple of high temperature superconductor current leads into the system. This allows us to supply high current up to 200 A to the sample under load, simultaneously measuring stresses in the sample. In first friendly user system commissioning experiment we have managed to measure a dependence of the second generation HTS tape sample critical current as a function of the sample’s internal strain.

The Pearl High Pressure Facility (Pearl) is a medium resolution high-flux diffractometer optimized for data collection from the Paris-Edinburgh (PE) pressure cell [3]. Pearl has been specifically designed for in situ studies of materials at high pressure. In recent years, upgrades to the instrument have led to improvements in data quality and the range of achievable pressures and temperatures: currently 0.5–28 GPa and 80–1400 K. However growing demand for high pressure experiments at significantly low temperatures [13] persuaded the SE team together with Perl scientists to initiate the development of compact, cryo-cooler based variable temperature insert for the Paris-Edinburgh press presented in Fig. 4. This system designed and developed at ISIS is capable of varying the sample temperature over 17.5–300 K range. The insert utilises two powerful 1.5W @ 4.2 K Sumitomo cryo-coolers to cool just the sample and the standard profile tungsten carbide anvils. In fact, we are using the same engineering solution that was utilised in the design of Engin X cryogenic stress rig described above. The variable temperature insert assembly is thermally insulated from the P–E press body by zirconia-cored seats and backing disks and PTFE insulation. The anvil is mounted in the cooling ring with a zirconia cored seat and backing disc. The temperature of the P–E press cylinder housing is maintained close to ambient by the means of a separate constant-temperature circuit. The whole installation is mounted on a Tomkinson flange (vacuum flange standard for ISIS facility) and is designed to operate in all standard ISIS instrument vacuum tanks. The sample temperature and pressure can be monitored and controlled remotely from the ISIS instrument computer used for neutron data acquisition. Due to the requirements to have both high pressure and neutron beam access to the sample, direct cooling and warming of the sample is impossible and the only available position for attaching the heat exchangers is external to cylindrical surface of anvils. Therefore the heat exchanger consists of two halves which can be clamped on respective anvils using screws. Whole assembly can be set up on the bench and then mounted into the press.

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Fig. 4.

Low temperature PE cell based on Cryo-coolers: (1) – GM cold-heads; (2) – infrared radiation shield; (3) – PE cell; (4) – thermal links; (5) – sample between two anvils.

Currently the system is in the commissioning stage of the project. In the first cool-down test the system has achieved based temperature of 17.5 K (thermometer was mounted at the sample position) in a little less than 3 hours of cooling from room temperature.

As it was mentioned in the introduction, the soft matter SE covers a very broad area of science and technology disciplines. Limited by the scope of this publication we considered to present here just one example: the SE used in cryopreservation experiments. Currently this is still quite an exotic area of soft matter neutron scattering, but in the recent few years an interest in this field has grown dramatically.

Development of new cryopreservation strategies has major potential in medicine and agriculture, and is critical to the conservation of endangered species that currently cannot be preserved [5]. The nucleation and growth of crystalline ice during cooling, and further crystallization processes during re-warming are considered to be key processes determining the success of low temperature storage of biological objects. Neutron scattering analysis combined with computer modelling allows the study of cell-damaging ice formation during fast cooling (vitrification) and subsequent warming of cryoprotective solutions (warming phase is a major challenge for successful recovery from vitrification) [10,11].

ISIS sample environment team has developed an experimental set-up and temperature protocol similar to the one described in [16] for studying of cryoprotective solutions in SANDALS diffractometer especially built for investigating the structure of liquids and amorphous materials. Two cubic centimetres of the sample solution were sealed inside the experimental container made of TiZr alloy with null coherent neutron scattering cross section. The sample and its container were crash-cooled to 77 K by immersing in liquid nitrogen. The time required to reach thermal equilibrium was less than 40 seconds. The cooling rate of this process can be estimated as 5.6 K s − 1 . After keeping the sample in the liquid nitrogen for another two minutes, the container was quickly withdrawn and loaded into the variable temperature insert of a top-loading cryostat based on pulsed tube refrigerator [4] which was already installed on the neutron beam line. In order to prevent condensation of air and water vapour during the loading of the sample into the cryostat, high purity helium gas flowed through the insert and the insert heat-exchanger temperature was controlled at ∼ 100 K . Once the sample container was installed the insert was evacuated and a low pressure of helium gas was added back in to provide a thermal link between the insert heat-exchanger and the sample container. The temperature of the insert was quickly reduced to 77 K and then controlled at this level with temperature stability ± 0.1 K during the subsequent neutron scattering measurements. The temperature of the sample cell achieved 77 K within ∼ 5 min , however another half an hour was allowed for sample stabilization before the neutron diffraction data were collected. After the collection of neutron diffraction data at this temperature was completed the sample cell was warmed up by 20 K, stabilised at the next temperature and neutron diffraction measured again. The temperature range of the experiment was from 80 K to 225 K. The neutron scattering background signal from the empty TiZr container, empty cryostat and empty instrument were subtracted from the experimental data, using separate measurements, and the data put on an absolute scale of differential scattering cross section by comparison with scattering from a slab of vanadium.

Recent spectacular demonstration of advantages of QENS method used for study of intracellular water has opened new opportunities for bio-medical research [14]. The human breast cancer cells used in the study were grown from cell lines created from removed tumour tissue. The cells were grown and then incubated with cisplatin for 48 hours at the Research Complex at Harwell, and were then immediately taken to the TOSCA and OSIRIS instruments at ISIS. However in many cases samples can be produced only in user’s laboratory and after that stored and transported in cryogenic storage. Recently we have received exactly this kind of request: study of cryopreserved alginate-encapsulated liver cell (HepG2) spheroids used in artificial liver bio-transplant [15] using high-resolution QENS spectroscopy. The sample is produced in London Royal Free Hospital bio-laboratory, cryopreserved and then transported to ISIS. This experiment requires a new sample environment kit which allows cryopreservation, cold transportation and cold loading of the sample in containers compatible with QENS measurements. The experimental research in this area is in progress.

6. Conclusion

In this paper we discussed some tendencies in operations and development of SE used in neutron scattering and muon spectroscopy experiments at ISIS. The importance of international collaboration between neutron, muon and other beam line user facilities, universities and industry was emphasised. We also reviewed the ongoing ISIS research and development projects in different areas of SE such as: cryogenics, high pressure and soft matter. All these projects are currently active and we will keep SE community informed about their progress.