Small structures, Big facilities

I am writing this as I embark on a journey from Copenhagen to Chicago for a 24-hour experiment. Luckily, I am going to be in the city longer than I will be flying, but only just. Traveling over 4,000 miles may seem like a long way to go for an experiment, and it is. I perform small-angle scattering experiments for a living though, and sometimes this is just what needs to be done. My previous post on Softbites was all about the fundamentals of X-ray and neutron scattering, but I didn’t give an indication of what an experiment is actually like. This post focuses on the practicalities. What are the experimental facilities like? What do you have to do to access them?

The first X-ray and neutron sources (developed by Röntgen and Chadwick, both of whom duly received Nobel Prizes for their discoveries) could fit on a desktop; examples are shown in Figure 1. Unfortunately, these historical apparatuses produce insufficient X-ray or neutron intensity for the kind of experiments that actually make use of the radiation. To enable these experiments, large-scale facilities have been built across the world, which produce much higher fluxes. For example, a next generation X-ray source will be 1015 times more brilliant than one of these historical X-ray tubes, and a new neutron source will have a flux 1018 times greater than that produced by Chadwick. The greater intensity of X-rays and neutrons generated at these large-scale facilities not only makes measurements possible but enables smaller and smaller samples to be analyzed at higher and higher throughputs.

Early apparatuses to produce X-rays and neutrons.
Figure 1. Early apparatuses to produce X-rays and neutrons. (Left) Crookes tube with concave cathode, the early way to produce X-rays. (Image from the Science Museum Group Collection, copyright The Board of Trustees of the Science Museum) (Right) James Chadwick’s Neutron Chamber device. (Image from the Department of Physics, The Cavendish Laboratory, University of Cambridge)

Intense and well-defined X-ray beams are primarily produced by the acceleration of electrons in machines called synchrotrons, which emit X-rays when magnets are used to bend electrons as they travel around the ring. Neutron beams are produced either by a nuclear reactor or at a spallation source, which emit neutrons from a metal target that has been bombarded by high energy protons. None of these are on the scale of a typical laboratory though, and instead, they require large-scale facilities.

These large-scale facilities are definitely required for some small-angle scattering experiments. The larger a particle is, the smaller an angle that it will scatter at. Small-angle scattering measurements do indeed measure very small angles (less than 1°). This means that large distances between samples and detectors are required to measure scattering at these very small angles. There are instruments with 40 meter vacuum tanks and even 100 meter vacuum tanks. Examples of some long instruments are shown in Figure 2. This means that the size of the structures you are studying (ångströms, nanometers or micrometers) is very out of proportion with the size of the facilities you use (multiple meters).

ng instruments for neutron and X-ray scattering.
Figure 2. Long instruments for neutron and X-ray scattering. (Top) The new 40m long detector tank of the small angle neutron scattering (SANS) instrument D11, at the Institut Laue–Langevin. (Image from Peter Lindner) (Bottom) Inside view of the 34 m detector tube at ID02 at the ESRF. The detector that measures scattered X-rays travels along the tube. (Image from T. Narayanan)

The ability of some instruments to access very long length scales from scattering at very small angles is the exact reason for my journey to the Advanced Photon Source (APS) near Chicago. For this experiment, I am interested in studying nanoparticles with diameters of several hundred nanometers but with interactions between particles that span distances much longer than this. To study interactions at the scale of micrometers, we need to measure scattering at the APS’s ultra-small-angle X-ray scattering instrument.

As you might guess given my long journey from Copenhagen to Chicago, there are a limited number of these instruments in the world. Even if there is a facility nearby, only a small number of samples can normally be measured rapidly, and even that might require waiting weeks or months. It is not possible to easily do test measurements at facilities, like you can with laboratory-based equipment. There are two ways to access time on these instruments, which as it is “time” on instruments using “beams” of X-rays or neutrons, is literally called “beamtime”.

Commercial access is more rapid, but it is costly (on the order of tens of thousands of dollars a day). Academic access costs less, and tends to be funded by the facility or a national fund, but you have to compete to get access this way. To gain beamtime via the academic route, I had to design and propose an experiment and have it accepted by a panel. Competing for and booking time at large-scale facilities is common in other fields, like astrophysics or particle physics. However, it is not typically necessary for soft matter experiments.

After being granted access by the panel for some beamtime on my desired instrument, the planning begins. The first thing to do is to schedule a time for the measurement with a local scientist at the facility. This might be during a weekend or a holiday, and I may have to be there overnight. I then need to update my radiation safety and awareness trainings and tests, which enables me to enter the facility, and book my travel. Finally, I prepare the samples, which I send to the facility a few weeks before my experiment. Once I arrive at the facility and familiarize myself with operating the instrument, I will finally perform my measurements. If all goes well, I will come home with a mountain of data to analyze. This time I am looking at nanoparticles, but depending on the materials I bring, I may reveal the structure of a protein or the size and shape of a micelle or the interactions between components in a complex mixture. There are a lot of possibilities in studying soft matter and biology. However, even if I haven’t found all the answers,  I should hopefully have enough preliminary data to write my next proposal. This may sound like a lot of work, but given the capabilities available on the instruments at large-scale facilities, we can really push the limits of what is achievable in soft material characterization. There are over 100 neutron and X-ray facilities around the world, and one may be near you. If not, you too may get to travel thousands of miles for a brief window of beamtime.

The featured image at the top is an impression of Science Village Scandinavia, which is designed to surround the new MAX IV X-ray synchrotron and ESS neutron source being constructed in Lund, Sweden. (Image from COBE.)

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