Dr Valery Nesvizhevsky on his serendipitous discovery

Researchers at the Institut Laue Langevin recently made a serendipitous discovery while reviewing 60-year old research into the lifetime of neutrons. They inadvertently stumbled on a new scientific tool to accurately measure the movement of nanoparticles moving along a membrane. This month we find out more from neutron scientist Dr Valery Nesvizhevsky. How did the discovery of this new technique come about? UCN (ultracold neutrons) were discovered in 1969, and since the very first observation, it was clear they play a special role in physics. Any of the four fundamental interactions – electromagnetic, weak, strong and gravity – affects them to some extent; their energy is so small that they are sensitive to even minor perturbations. UCN are elastically reflected from most materials and can be stored in closed traps for periods of up to many minutes; therefore such perturbations are accumulated and enhanced. To summarise: UCN are excellent probes for studies in fundamental physics! And, indeed, UCN were sensitive probes in many famous fundamental experiments like searches for the neutron electric dipole moment or measurements of the neutron lifetime. They were also the unique tool used to observe gravitational quantum states of matter, an experiment in which I was strongly involved. Other applications of UCN were less obvious: although surface or material studies with UCN were discussed and explored, the available UCN densities were too low. Application of UCN is justified only if they are a really unique tool for a particular problem. While measuring neutron lifetime, or other fundamental properties of neutrons, physicists observed surprisingly high UCN losses in trap walls. Some losses were expected because of reactions with nuclei or inelastic scattering of UCN in trap wall materials; however the losses were orders of magnitude larger than expected! People even called them the anomalous losses of UCN. About a decade after the first observation of UCN, a participant of that first experiment and a guru of most scientists in the field including myself, Alexander Strelkov found a reason for the enhancement of UCN losses: hydrogen, with its huge up-scattering cross-section, in the surface. With this knowledge, many people made great progress by means of eliminating a significant fraction of hydrogen or covering trap surfaces with in-situ prepared hydrogen-free layers. This was a “classical age” of UCN physics and a great number of newcomers entered the field with new ideas, techniques and motivations. However, at a significantly lower level, the problem of anomalous UCN losses still appeared to be present; many major players continued thinking that hydrogen was the only danger, and that the problem of extra losses could be solved by simply cleaning the surface. Results and interpretations of experiments were highly contradicting – something was clearly out of control. Numerous “false truths” were accumulating during the following decades, the “dark ages” of UCN physics. The new phenomenon of UCN up-scattering on weakly bound nanoparticles or nanodroplets, which we discussed in the recent paper in the Crystallography Reports, explains part of the extra losses of UCN from traps, and thus might provide an mean of “renaissance“ for the field by removing at least some of major accumulated contradictions. And anyway, it is a unique method to study motions of nano-sized objects in the surface vicinity. When did you realise you could use UCN in this way? It would not be fair to say that we have realised only now that we could use UCN in this way. Since the very first observation in 1999, we hypothesised this application. However, our ideas about nanoparticles weakly bound to surfaces were too vague, our experimental data too inaccurate, and the resistance of UCN community to accepting this new hypothesis was too strong. Finally, our arguments were not convincing enough, even for ourselves. Nevertheless, we had no doubts that we saw the phenomenon of small changes in energy of UCN in traps. Thus we continued studying it phenomenologically, without yet digging deep into its nature. UCN up-scattering on liquid surfaces; UCN up-scattering on nano-powders; UCN up-scattering on solid surfaces: after initial rejection of our results by many groups active in the field, they started accepting them; they started performing their own experiments and confirming our results. Alternative interpretations – like another manifestation of hydrogen on surface, various false methodical effects, and even various extensions of quantum mechanics – were removed experimentally one after another. This process took several years. And then only recently all conditions met in order to make a qualitatively new step. The principle condition was a new team of collaborators with the deep complementary knowledge necessary to clearly formulate the physical model, to draw precise quantitative conclusions and to perform precision experiments. There is no surprise that this work was done on the PF2 instrument at the ILL (Institut Laue-Langevin), the Mecca of UCN physicists for several decades. Not only is high UCN density is crucial for this measurement – and the competitive stimulating international environment characteristic of the ILL – but so is the unique instrumentation developed over years for precision gravitational neutron spectroscopy. Today we not only realise the potential of this technique, but also propose various concrete realisations. So, after explaining all that, contradicting to the first sentence: we have realised only recently how UCN could be really used in this way. You were originally investigating a 60-year-old experiment on the lifetime of neutrons – why? I was originally investigating experiments on the neutron lifetime for a simple reason: when I graduated from university and came to work in a laboratory working with UCN, the neutron lifetime experiment was the most interesting and exciting one. Therefore I was extremely grateful to my supervisor for proposing this experiment as a subject of my PhD. Motivations of my senior colleagues were much deeper. First, the neutron lifetime experiment is probably the most elegant application of UCN in fundamental physics. If UCN losses in trap walls were negligible then the experiment is extremely simple and convincing: just measure the number of UCN in the trap as a function of storage time, and the characteristic time of the exponential decay of this number will be the value you are searching for. In case of finite probability of UCN losses in the trap walls, the procedure is much more complex and we have to control and take into account these losses precisely. Second, at that time the precise value of the neutron lifetime was of extreme interest and importance. It was the crucial parameter needed for estimating the number of generations of quarks and leptons within the Standard model, as well as a crucial parameter for stellar models. However, as soon as the accuracy of 3 s (~0.3 %) was achieved in three independent experiments, and these results agreed with each other rather well, our motivations a bit moved towards another problem: namely towards understanding extra UCN losses. And this problem appeared to be even more difficult to solve. On the other hand, I am convinced that its proper solution is the necessary condition to further improve the accuracy of neutrons lifetime experiments. How can other scientists use this technique? The new technique allows one to measure thermal and other motions of nano-sizes objects. Saying that, we can ignore the “zero” approximation in any internal structure of these objects; only the translational motion of the object is of importance. Nanoparticles, nanodroplets, macromolecules, or viruses all interact with UCN in a similar way. In the “zero” approximation, we deal with the classical billiard-ball collisions of a nano-object with an ultracold neutron, in which the UCN energy can increase or decrease. This change in energy for nearly each collision event can be measured using a gravitational UCN spectrometer. With better precision, you can resolve the “fine structure” corresponding to the shapes of nano-objects, to other-than-translations degrees of freedom, to the details of the interaction of a nano-object with the surface and/or with the neighbours etc. This means that the range of applications of the new method is fairly broad. In a properly designed measurement, we are not restricted by the nature of a nano-object. Only its size or, more precisely, its mass is of importance. The reason for such selective sensitivity of the method to a certain mass lies in the fact that we can measure only certain range of values of the UCN energy changes. If the nano-object is too large, if moves too slowly and thus the energy change is too small, it could not be resolved with a finite energy resolution of the UCN gravitational spectrometer. If the nano-object is too small, if moves too fast and thus the energy change is too large, such up-scattered neutrons cannot be trapped and efficiently transported to the UCN detector. Moreover, the cross-section of neutron scattering decreases enormously with the nano-object size (as its 6^th power). Some constraints arise from the fact that UCN experiments are done in vacuum (otherwise UCN are lost quite rapidly due to their scattering on gas molecules). However, special sample chambers could be designed even for biological objects. Another constraint arises from the fact that measurements have to be performed in specially designed UCN spectrometers; however, such devices can be built or bought. The main constraint is due to the fact that UCN, in particular high densities of UCN, are available worldwide in a very few places. Therefore, at least at present, scientists who wish to explore this new technique, should collaborate with existing UCN groups providing corresponding experience, equipment and access to UCN. What’s next for you and your research team? Well, these are only first steps. The list of open questions is still much longer than the list of solved problems. Will it be possible to increase statistical sensitivity of such experiments in order to make them more accessible for routine experiments? Will it be possible to build even more efficient gravitational spectrometers optimised for this kind of studies? Will it be possible to resolve and use the “fine structure” mentioned above? What information about the interaction potentials can be extracted from such measurements? What information about other-than-translational degrees of freedom can be extracted? What is the origin and peculiarities of naturally growing nano-objects in each particular case? And what processes govern the behaviour of nano-sized objects in the surface vicinity? To what extend this phenomenon affects results in fundamental physics experiments, in particular, in the neutron lifetime? How broad could the range of applications of this new technique be, outside of physical adsorption and fundamental physics experiments? Can it be really useful for surface chemistry or biology? We are confident we can answer at least some of these questions.