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

mie=f g 477 keV

f UCN=J g 477 keV eUCN

J UCN e g

mcap=f gf UCN

=J g eUCNJ UCN e g

Surface study with (UCN,gamma) analysis - experimental layout, ILL, 1999

Ti Calibration curve

UCN density - 0.03 per cm3

- 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()

mie =a+bmcapH »bmcap

H ;

s ie »bsaH=0 . 33 b

s ie T =4pb2∫ e−2W T G w n w,T wE 0

dw ;

Inelastic cross section could tell us about surface state of

Hydrogen

Study of the temperature dependence of UCN losses with UHV cryostat, ILL, 2001

4KK

4K

77K

77K

UCNcryostat

cryopumpcryostat

UCN

UCN Shutter

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, EUCN < VF

sub

• 1/v model, I.e. upscattering in the hydrogenated thin film, VF

film < EUCN < VF

sub To calculate actual profile of Fermi-potential VF 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 7.21017 at/cm2, ( 100Å) there are

H - 8% (5.81016 at/cm2), Cu -55%, C- 10%, O-25% . The deuterium was found in the amount of 0.61015 at/cm2

8% of H allowed us to consider both models

Calculation of UCN’s loss rate in the subbarier model ( H-Cu compound)

The model can explain neither T-dependence

nor absolute value of the UCN loss rate!!!

h T =∑ic i s

iup T

2λ∑lc lb l

»cHsH up T

2λ <b>

The loss probability per collision

h exp 300 /hexp 77 » 7

s ie 300 / s ie 77 » 14

Chemical content (cH=8%) was studied with ERDA analysis.

Comparison of the low temperature parts:•experiment

•Calculation (H-Cu)

Loss probability for UCN upscattering on H2O,model of the hydrogenated film, 1/v low

h film=2s ie EUCN nH ;nH =N H d

Loss probability in the model of the hydrogenated film with Fermi-potential below UCN energy, VF < EUCN

Calculated upscattering cross section

s ie 300 / s ie 77 » 7

s ie 300 =4 . 2b

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 VF<EUCN (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 1015 atoms/cm2 .

• 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 SD2 volume 0.5x1012 n/cm2/s

UCN production rate (0.75-1.5)x104

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-dimensionalsurface 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 = -pE At present are studied by capacitance measurement

50 100 150 2000.0

1.0x10-4

2.0x10-4

3.0x10-4

lithium niobate, 100 Hz

Dis

sipa

tion

Fac

tor

Temperature (K)

before film deposition

dipolar film

How UCN can help study of

re-orientational wells on different substrates (Si- and Al2O3)

dipole and non-dipole rotorsSurface 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

reactivehead group

Self-assembled monolayer (SAM): spontaneously- formed film one molecule thick

SiR' R' R'

R

Silane =

increasing molecular density(decreasing temperature)

Structural morphology of self-assembled monolayers is still under active study:

disordered rotator phase crystalline 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

150 200 250 300 350 400

0.0

2.0x10-4

4.0x10-4

6.0x10-4

0.0

5.0x10-5

1.0x10-4

1.5x10-4

2.0x10-4

Dis

sip

atio

n F

act

or

(ta

n ()

)

Temperature (K)

Drop-cast Spin-coated

1 kHz

side-chain motion backbone motion

C

N

N

n

Rotator motion in a SAM (a quasi-two dimensional polymeric sheet) is analogous to side chain rotation in linear polymers.

C

N

N

n

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