# Presentation: 580421

## A High Performance Instrument for the Single Crystal Spectroscopy at the Pulsed SNS.

* **What is the spectrometer we want and what is its place in the proposed SNS instrument suite?*
* **Main criteria for the optimal design of an instrument at the pulsed source. *
* **Hybrid (crystal monochtomator) versus chopper spectrometer: general considerations and Monte-Carlo results.*

-  L. Passel, V.Ghosh, Brookhaven National Laboratory
-  L. L. Daemen, P. Seeger, Los Alamos National Laboratory

- Igor Zaliznyak

- Outline

## Neutron spectrometer for studies of the low-energy coherent excitations in single crystals.

- Common requirements for a single crystal neutron spectrometer:
* **Transmission function for both primary (monochromator) and  secondary (analyzer) spectrometers should be close enough to 1 within the acceptance range, and vary smoothly over a substantial energy range, typically from 5meV to 100meV. *
* **Both spectral (energy resolution) and angular (~ wavevector resolution) acceptances of the monochromator and analyzer should be flexible and easily adjusted, typical resolutions are 1% to 5%.*
* **Scattering volume seen by a detector should be well defined and easily adjustable depending on the sample size, to minimize the background.*
* **Efficient use of the large incident neutron beam by focusing it on the sample is very important, and should be previewed.*

## SNS workshop on Inelastic Neutron Scattering: where is the instrument we want?

## What is a flagship inelastic instrument at a spallation neutron source?

- MAPS at ISIS

- Proposed chopper spectrometer at SNS

* **SNS should be equipped with the top of the line instruments, which will be the best performers in their class*
* **SNS spectrometers should rely on the latest technology, and feature original and innovative design  *

- Direct geometry chopper spectrometer?

## What is it that we want to study?

-  *Coherent low-energy excitations in single crystals*
* **Different coherent static phases, ordered and weakly disordered states*
* **Magnetic vs structural scattering: polarization-dependent cross-section*
* **Reasonably small samples*

- CuGeO3 sample from Masa Arai group, used for measurements on MAPS

## How does the instrument we discuss fit SNS floor layout?

- Follows a long, 30 m to 50 m neutron guide.

- 10-1000meV direct geometry high resolution (~1%) chopper spectrometer (Abernaty) is more compact (at ~11m guide).

## What are the general guidelines for the optimal design of an instrument at the pulsed spallation source?

-  Pulsed nature of the neutron beam should be used
-  The monochromatic “useful” neutron current through the sample should be maximized
-  Solid angle accepted by the secondary spectrometer should be made as large as possible
-  The direct view of the source by the sample should be avoided
-  Scattering volume seen by a detector should be reduced to minimum, determined by the sample size

*Note: once the time structure of the neutron beam is used for either the monochromator or the analyzer, no additional gain will result from using it for both.*

## Optimization of the neutron optics for the chopper spectrometer: MC results.

- Basic layout for the direct geometry disc chopper spectrometer

- Optimized layout: curved guide loses ~6%, tapered section gain <1.4

## Two layouts for the low-energy direct geometry spectrometer: TOF vs PG crystal monochromator. 

## MC simulation for the PG(002) monochromator transmission and focusing properties.

- Perfectly collimated beam

- Beam from 3c supermirror guide

- Ei=14.7meV

## Punchline: pro’s and  con’s of a crystal monochromator hybrid spectrometer.

*Advantages*
-  The flux at the sample is maximized for all incident neutron energies by using the segmented crystal with vertical focusing
-  Moving the sample and its environment out of the direct beam should significantly reduce the background in the detector bank
-  The wavevector acceptance of the analyzer is very flexible and easily adjustable in the crystal reciprocal space (avoid the blank spots, etc.)
-  The wavevector resolution may be easily controlled by adding collimators, a very high Q-resolution is possible
-  The spectrometer is open for various kinds of the focussing options at the monochromator: 2D focusing, focusing with the asymmetrically cut perfect crystals, etc.
-  The spectrometer can be fitted with the polarization analysis using the available technology: a Heusler crystal monochromator for  the high-energy incident neutrons and an array of bender polarizers for the scattered neutrons.
-  The pulsed time structure of the neutron beam is used in full

## Punchline: pro’s and  con’s of a crystal monochromator hybrid spectrometer.

*Disadvantages*
-  Need to move an extremely bulky back end with detector bank, covering a significant solid angle
-  Coupling between the resolution functions of the primary and secondary spectrometers
-  Smaller than 1 and energy dependent reflectivity of the monochromator?
-  Smaller solid angle coverage of the detector bank?