High pressure record for neutrons broken again!

The Geophysical Laboratory neutron group has tested a new design of a poly-crystalline diamond supported diamond cell (PCD-DAC), that has allowed diffraction data to be collected at 78.3 GPa.
The new device uses a massively supported anvil designed by Boehler & Guthrie, and retains optical access to the sample.

The data quality in the cell is excellent (see previous post for example of a short, 10 minute, dataset).

Two anvils and gasket after eventual "blowout" one of the anvils was reduced to a small pile of diamond dust.

 

 

 

Key background source identified.

During recent IDT time it was discovered that downstream collimating jaws are a significant source of background. The line-of-sight to detectors has to traverse 0.5" B4C plate, so this had in the past been overlooked. The addition of substantial amounts of borated poly and further B4C plates blocking this line-of-sight has halved background levels when a DAC is in place.

Figure shows the raw focused counts from a 10 minute dataset from a ~50ug sample of (our favourite calibrant) ice VII in a diamond anvil cell at 10 GPa.  The clear Bragg peaks are from the sample. The signal to background level is actually better than that achieved with the much larger volume Paris-Edinburgh cells.

UT Austin researchers test Low temperature DAC set-up

front to back (Marshall, Lin & Guthrie) in SNAP cave

 Afu Lin, Junjie Wu and Luke Marshall from UT Austin along with Malcolm Guthrie (GL) tested the first iteration of the low-temperature membrane set-up on SNAP.

Much of the beamtime was spent optimising the windows and collimation on the CCR tank but, eventually, count rates were achieved that were only 10% higher than without the tank. With further design work, it's likely that this can be improved.

Marshall and Wu with inner CCR attachment on cold head

The cell was found to cool rapidly to 53K and was stable. We were then able to pressurise the sample, but found this caused the temperature to increase to ~70K.

With input from Lin, clear design improvements to the cooling connections were conceived and sub 30K operation is likely in the future

New membrane driver for DACs available

A new membrane driver for DAC's on SNAP has been developed and built.  Early commissioning has shown that it performs to specification and high quality data can be measured. The relatively low mass of this driver is expected to allow rapid cooling of the DAC to temperatures below 30 K. Cooling tests will be conducted in the next 1-2 weeks.

Images show the membrane and clamp system. (The upper image also shows new diffracted beam collimator developed by M. Guthrie and J. Molaison)
 

IDT members Reini Boehler, Stas Sinogeikin and Malcolm Guthrie, in collaboration with Facility staff Jamie Molaison and Neelam Pradhan contributed to the membrane design

IDT proposal deadline

The deadline for submitting proposals for the next cycle (October 1st through December 19th  2012) is   Monday July 2nd (at 9am). The user office has informed us that 15 days of beamtime are allocated on SNAP for IDT experiments.

Please submit your proposals through the IPTS system. As per usual, the IDT is open to considering new members prepared to devote time to pushing the limits of high-pressure neutron diffraction on SNAP (click here if you're interested in becoming a member)

Maximum pressure achieved on SNAP increases again!

In collaboration with the instrument staff, members of the Carnegie IDT team (Guthrie, Boehler, Li, Machida) have increased the maximum pressure for measuring structural refinement-quality data to 57 GPa. At this (unofficial) record-breaking pressure Bragg intensities were clearly visible down to ~0.75 Å and full structural refinements have conducted.

On increasing the pressure further, there was a failure of the carbide seat (at an estimated 60 GPa). In ongoing developments, these seats will be replaced with a sintered diamond variant, with the aim of extending the maximum pressure further into the 100 GPa range.  

The figure shows a Rietveld fit to a typical data set from a test sample of D2O at around 3 GPa measured on SNAP.

Zr gasket provides new window at high d

For crystalline systems with structure at high d-spacings 7-10 Å, including many magnetic systems, it has been so far necessary to use detectors in a low-angle geometry, with concurrent loss in resolution. Another possibility has been to use the bandwidth choppers to use very long wavelength neutrons to probe this structure with increased resolution. However, the substantial attenuation of the gasket and pressure vessel has made this unfeasible.

Xiaojia Chen has used IDT time to successfully test a pure Zr gasket in place of the more conventional TiZr alloys. The low absorption cross-section of Zr (0.185 barn at 2200m/s) versus Ti (6.09 barn at 2200m/s) means that a much stronger sample signal is accessible with the new gaskets. These measurements use a vertical geometry with the beam entering and exiting through the gasket and both detectors at 90 degrees. Testing has shown that the resulting data shows sharp strong Bragg peaks on very low backgrounds.

Pellet mass calculator

Jamie has released a mass calculator for typical pellets used with the PE toroidal anvils...you may find this useful:

http://neutrons.ornl.gov/instruments/SNS/SNAP/calc.shtml

IDT time allocated for 2012-A

The IDT allocation committee have met and decided on the beamtime allocations. We received requests for 62 days of beamtime with a total of 24 days available this cycle. In making the allocations, the committee followed the core priorities of the IDT. Here's looking forwards to a successful cycle!

Magnetic order in new polymorph of Mn3O4

As part of the ongoing development of low-temperature capabilities on 
SNAP, IDT member Shigeto Hirai has determined the magnetic structure of 
the orthorhombic Mn3O4 measured at ambient pressure 
and below the Neel temperature. As part of an IDT focus on 
low-temperature capabilities for SNAP, Shigeto aims to extend these 
measurements combining high pressure and lower temperature.

Figure shows Shigeto's Rietveld fit to the new magnetically ordered low temperature phase.

IDT Proposal deadline today!

Today is the deadline for submitting proposals for IDT beamtime. There are 24 days available in the upcoming cycle from Feb-June 2012.