Very Cold Moderator Development at LENS David V. Baxter and W. M. Snow Indiana University A. Bogdanov, V. P. Derenchuk, H. Kaiser, C. M. Lavelle, M. A. Lone, M. B. Leuschner, H. O. Meyer, H. Nann, R. Pynn, N. Remmes, T. Rinckel, Y. Shin P. Sokol

OUTLINE What/why is LENS? Neutronic design; what is unique about LENS? Why is LENS ideal for very cold netron development? Conclusions

What is LENS? Low Energy Neutron Source: based on low-energy (p,nx) reactions (Ep<13MeV) in Be. The source is tightly coupled to a cold moderator (e.g. solid CH4 at 3K<T<30K). LENS will have a variable pulse width (from ~10 ms to more than 1.0 ms). In long-pulse mode, LENS will have a time-averaged cold neutron intensity suitable for SANS and other materials research. A small number of scattering instruments will be developed to utilize these neutrons. Budget : $14 M+ for construction; $1.5M/yr operations

LENS Missions Materials research Education LENS Instrumentation development

The Facility Timeline-Source Phase I (Early ’05: 7MeV, 7mA, 0.2% DF; 1011 n/s) Moderator studies: Benchmarking LENS performance, lower T, different materials, … Simple diffraction experiments Phase II (July ‘07: 7MeV, 20mA, 0.5% DF; 1012 n/s) Moderator composition studies/neutronic improvements Start pushing to lower spectral temperatures ->VCNS Initial SANS measurements Total cross section measurements Phase III (Jan. ’08: 13 MeV, 1013n/s) SANS studies Development of SESAME instrument Eventual power (13MeV, 50mA, 5% DF; 1014 n/s)

The Facility Timeline-Instruments SANS Commissioning of final instrument July 2007 Experiments on confined water, polymer networks, fractal structures in geological samples, nano-particles Sept. 2007. SESAME Magnetic component development Summer 2007 Initial construction Oct. 2007 Initial commissioning Jan. 2008 Total elastic scattering instrument Conceptual design Summer 2007 Possible construction to begin in Fall 2007 (not yet fully funded) Micro-SANS Seeking funding Others …

IUCF

IUCF

Facility Layout: 2005 - 2006

Protons in linac: 15 Dec. 2004 DTL Power RFQ power Proton Current

Neutrons in 2-D Detector: 15 Dec. 2004

Target Moderator Reflector (TMR)

Target Moderator Reflector (TMR)

Empty Moderator Spectrum Detector at 5.7 m Y550 numbers for this figure are 206 n/cm^2.uC (measured) and 180 n/cm^2.uC (MCNP) Ratio 1.14

Cryogenics inserted in TMR2

Moderator Assembly PT-410 Al Poly CH4 50 cm Water

Spectra Captured every 10 Minutes 5-point low-pass filter applied Data have been passed through a 5-point smoothing algorithm

Phase II Structure of Methane Circle at the center represents an essentially freely rotating molecule, all others are hindered rotors that librate in place.

Calculated Cross Section of Methane in Phase II From Grieger, J. Chem. Phys. 109, 3161 (1998).

Shin kernel: Total cross section The NJOY curve is the cross section computed by NJOY from the frequency spectrum Shin’s kernel produces. It does not adequately account for the elevated cross section at lower temperatures due to the temperature dependence of the total spin projection (and presumably this accounts for the discrepancy at frequencies below 4meV).

Shin kernel- freq. spectrum

Shin kernel: 20 K CH4 spectra

MCNP kernel (Y. Shin) Free Rotational and Phonon modes Tunneling, Librational and Phonon modes

Methane-Argon Moderator: Excitation Spectrum Prager et. al., J. Chem. Phys. Vol. 95, 569-575 (1991)

Facility Layout: 2007

Scattering Hall Update: 2007

Scattering Hall Update: 2007 22 Feb 2007 29 March 2007

Pictures of Upgrade: SANS

Conclusions LENS has been producing cold neutrons for some time, and has recently done so at its second target station. Neutronic performance is within 20% of predictions at low E (cold and thermal). Spectral temperature of <30K has been realized, and work is underway to reduce this. This will be a major goal for the project over the next 3 years. Future improvements to neutronics should increase cold flux by more than 30% (more on this tomorrow). We have started to explore new materials, and are looking for more ideas in this area!