Presentation on theme: "TRIUMF UCN workshop, 2007 Solid state physics experiments with UCN E. Korobkina."— Presentation transcript:
TRIUMF UCN workshop, 2007 Solid state physics experiments with UCN E. Korobkina
Possible Solid state Experiments with UCN UCN scattering – study of slow motion of large biological molecules Anderson localisation nano eV energy transfer – study of thermal acoustical waves in the bulk and surface waves Study of molecular monolayers containing Hydrogen surface content study with prompt gamma's at room temperature study of T-dependence of UCN inelastic scattering with UHV cryostat further development of the combined method using UCN prompt gamma-analysis at low temperatures
Surface study with (UCN, gamma) analysis Idea and first experimental test RRC KI, 1993, UCN curved guide First study, Be and Stainless steel samples ILL, 1996, PF2 Routine study of materials commonly used in experiments with UCN ILL, 1998-1999, PF2
Surface study with (UCN,gamma) analysis - basic principles What we could measure: total loss probability, tot Probability of inelastic scattering, ie probability of the radiative capture by individual isotopes on the surface, cap
Surface study with (UCN,gamma) analysis - experimental layout, ILL, 1999 Ti Calibration curve UCN density - 0.03 per cm 3 - count rate - 0.5 per sec
Surface study with (UCN, gamma) analysis Cl H, cap
Surface study with (UCN, gamma) analysis – inelastic scattering
What could we derive from ie and cap of Hydrogen –Measurement of the sample with different amount of Hydrogen allowed us to calculate the absolute value of Inelastic scattering cross section of the surface Hydrogen from the linear fit of the experimental data – Since the inelastic cross section is an integral function of density of Hydrogen excitations G() Inelastic cross section could tell us about surface state of Hydrogen
Study of the temperature dependence of UCN losses with UHV cryostat, ILL, 2001 4K K 4K 77K UCN cryostat cryopump cryostat UCN UCN Shutte r Guide switcher UCN detector UCN valve Dry pumps Cooling down to 4K with LHe Metal sealing with annealed Al Two pumps - dry pump and cryopump Pressure < 10 -9 mbar Heating up to 200°C through LHe bath of the UCN cryostat Negligible absorption at 4K - warming and cooling showed the same data within 10% of statistical accuracy.
Study of the temperature dependence of the UCN losses rate
2001 Success of data evaluation was due to use of the data of ERDA analyze made at ISL, HMI. ERDA analyze provide us the atomic density of all surface compounds of UCN storage bottle including Hydrogen that was of great importance! Analyzing our data with the data of ERDA, neutron scattering and prompt (UCN,gamma) analyze we showed, that only model of the thin water- containing film can explain experimental data. Another model ( sub-barrier penetration into the bulk ) was reliably ruled out, closing room for the speculation about anomaly in UCN interaction with the bulk. Published in Phys. Rev. B 70 (2004) 035409
UCN interaction with surface of materials Details of the UCN upscattering have been the subject of some controversy, with a range of models proposed to explain the observed losses. Nevertheless, there are two basic models to be considered first: subbarrier model, I.e. upscattering on hydrogen dissolved in the substrate surface layer 10 nm thick, E UCN < V F sub 1/v model, I.e. upscattering in the hydrogenated thin film, V F film < E UCN < V F sub To calculate actual profile of Fermi-potential V F we need to know the chemical content of the surface layer. In present study we used ERDA analysis at ISL facility of Hahn- Meitner Institute to detect surface density of Hydrogen and another elements in the bulk and surface layers. it was found that in the top layer 7.2 1017 at/cm2, ( 100Å) there are H - 8% (5.8 1016 at/cm2), Cu -55%, C- 10%, O-25%. The deuterium was found in the amount of 0.6 10 15 at/cm2 8% of H allowed us to consider both models
Calculation of UCNs loss rate in the subbarier model ( H-Cu compound) The model can explain neither T-dependence nor absolute value of the UCN loss rate!!! The loss probability per collision Chemical content (c H =8%) was studied with ERDA analysis. Comparison of the low temperature parts: experiment Calculation (H-Cu)
Loss probability for UCN upscattering on H 2 O, model of the hydrogenated film, 1/v low Loss probability in the model of the hydrogenated film with Fermi- potential below UCN energy, V F < E UCN Calculated upscattering cross section
UCN upscattering cross section vs temperature for harmonic oscillator The model of harmonic oscillator helps to understand that only frequencies below 0 L < 10meV can contribute significantly to the low temperature part of the cross section, i.e. at T<80K. The larger mass of oscillator, the lower 0 L. Here M=1/7
Field where UCN can compete with NIS instruments: inelastic scattering in surface nanolayers Study of ie (T) with UCN is similar to the specific heat measurement. The difference is high selective sensitivity to Hydrogen excitations and an emphasis on the thin surface layer 10 nm that is intermediate between first monolayers, which are accessible to normal methods of surface physics, and a true bulk matter ( 0.5 m) accessible with NIS instruments. Our data analyses have demonstrated a high sensitivity of the UCN upscattering cross section to low frequencies < 8 meV. We also showed that in the range < 10 meV the upscattering intensity from a hydrogenated film with low Fermi-potential is orders of magnitude higher than the interaction with H dissolved in metal. Thus we can use UCN to study low frequency excitations and low dimensionality in nanometers thick films with V F <E UCN (polymers, ice deposited on metal foil) on the metal substrate practically without background signal from the metal. The sensitivity to Hydrogen (UCN storage technique) with new high density UCN sources could be as good as 10 15 atoms/cm 2. The stability of the surface and amount of interacting Hydrogen in the course of measurement can be monitored in situ with the prompt (n, ) technique.
Improvement of the storage time: Pure metal Helicoflex Cu sealing Surface treatment controlled by ERDA analysis Low temperature valve New modification of UHV cryostat
The UCN bottle was mounted in Berlin, then assembled cryostat was shipped to Grenoble. –Right - view of the polished surface of the bottle inside The UCN bottle was mounted in Berlin, then assembled cryostat was shipped to Grenoble. –Right - view of the polished surface of the bottle inside ILL test, July 2004
Temperature dependence of UCN losses, ILL run 2004 The new experimental data shows that our treatment indeed removed the low temperature UCN upscattering below 77K ! This means we indeed have no water contamination and no vacuum contaminations The new construction of the bottle and UCN shutter allowed us approach very close the theoretical limit of the storage time, i.e. loss rate due to the beta-decay and absorption by Copper nuclei.
Progress in theory of the UCN interaction with surface Rigorous theory of UCN coherent scattering recently was developed by S.Belyaev and A.Barabanov, Eur. Phys. J. B15(2000). It takes into account second order terms significant for multiple scattering effects. Theory verifies the use of the optical theorem for phonon scattering, whereas for liquids and amorphous materials new effects are predicted.
Layout of the UCNS facility Low gamma radiation heating Low cooling power cryogenic High efficiency of cold neutron moderation- cold flux averaged over SD 2 volume 0.5x10 12 n/cm 2 /s UCN production rate (0.75- 1.5)x10 4
Gregg S. Kottas, Laura I. Clarke, Dominik Horinek, and Josef Michl Chem. Rev. 2005, 105, 1281-1376 Nanotechnology related study of artificial molecular rotors The study deals with man-made molecular rotors from the point of view of their potential utility for molecular machinery
in two-dimensional surface mounted systems or three- dimensional crystals, in random or ordered systems. single molecules –solution or vapor phase Artificial molecular rotors rotator stator axis torsional motion
Dipolar Rotors U = -p E At present are studied by capacitance measurement
How UCN can help study of re-orientational wells on different substrates (Si- and Al 2 O 3 ) dipole and non-dipole rotors Surface phonon bath of the substrate surface interactions with the local environment
Self-Assembled Monolayers Silane SAMs have particular technological importance in modifying water-loving oxides to water-hating surfaces with negligible increase in thickness (< 1nm). Inexpensive, relatively green process. Used for corrosion prevention, lubrication… inert tail group reactive head group Self-assembled monolayer (SAM): spontaneously- formed film one molecule thick Si R'R' R'R' R'R' R Silane =
increasing molecular density (decreasing temperature) Structural morphology of self-assembled monolayers is still under active study: disordered rotator phasecrystalline forms Phase transitions in SAMs? Are these changes in dynamics continuous (reflecting the exponential decrease in hopping due to decreasing temperature) or do new energy scales appear (closer to a phase transition)?. Presence/ absence of rotator phases has been correlated with changes in frictional properties.
Side chain rotation in linear polymers side-chain motion backbone motion Rotator motion in a SAM (a quasi-two dimensional polymeric sheet) is analogous to side chain rotation in linear polymers. At right, the ability of the side chains to interdigitate in a drop-cast sample, makes them strongly interacting – altering their dynamics with temperature, as opposed to a spin-coated samples where each side chain is independent. This in turn alters the backbone motion. Similar effects should be observable in self-assembled monolayers. Dielectric Spectroscopy of Polyguanidine Polymers, Eva R. Garland, Derrick Stevens, Laura Guy, Hong- Zhi Tang, Bruce M. Novak, and Laura I. Clarke, submitted.
UCN cryogenic (n,gamma) method to study molecular monolayers
Summary At present we have modified apparatus which has been tested with UCN fully developed methods of measurement interesting samples for calibration and study We are waiting for powerful UCN source