# Presentation: 004112

## Polarized Beam Mode for the Hybrid Spectrometer (HYSPEC) at the Spallation Neutron Source.

- _Outline_
- BNL’s HYSPEC project and its place in the SNS instrument suite
- HYbrid SPECtrometer’s layout and principal features
- Polarized beam setup: principle, specific features and components
- Performance and optimization of the (Fe/Si) transmission polarizer for different neutron energies
- Summary, work in progress and open questions

- Igor Zaliznyak
_*Neutron Scattering Group, Brookhaven National Laboratory*_
_*HYSPEC Instrument Design Team*_
- V. Ghosh, L. Passell and S. Shapiro (BNL), M. Hagen (SNS/BNL)

## Spallation Neutron Source (SNS) at ORNL

- _[http://www.sns.gov/](http://www.sns.gov/)_
- http://www.sns.gov/partnerlabs/partners.htm

## March, 2004
M. Hagen (instrument scientist) and W. Leonhardt (engineer) join the project
May, 2003
DOE CD0, part of the SING project
December, 2002
HYSPEC proposal submitted to DOE
January, 2002
HYSPEC IDT filed Letter of Intent with SNS
Fall, 2001
Instrument Development Team formed 
Workshop on the Hybrid Spectrometer held at BNL
Refined HYSPEC concept presented to EFAC
March, 2001
Draft proposal of a Direct Geometry Hybrid Spectrometer first presented to EFAC, received positive reply
December, 2000
Completed review of the possible instrument designs
Concept of the Hybrid Spectrometer formulated and adopted

- M. Hagen (instrument scientist) and W. Leonhardt (engineer) join the project
- May, 2003
- DOE CD0, part of the SING project
- December, 2002
- HYSPEC proposal submitted to DOE
- January, 2002
- HYSPEC IDT filed Letter of Intent with SNS
- Fall, 2001
- Instrument Development Team formed
- Workshop on the Hybrid Spectrometer held at BNL
- Refined HYSPEC concept presented to EFAC
- March, 2001
- Draft proposal of a Direct Geometry Hybrid Spectrometer first presented to EFAC, received positive reply
- December, 2000
- Completed review of the possible instrument designs
- Concept of the Hybrid Spectrometer formulated and adopted

- HYSPEC timeline: history of the project

## HYSPEC Instrument Development Team and Design Team.

- _S. M. Shapiro_, co-PI	BNL
- _I. Zaliznyak_, co-PI		BNL
- G. Shirane		BNL
- J. Tranquada		BNL
- L. Passell		BNL
- D. Abernathy		SNS
- L. Daemon		Los Alamos
- M. Greven		Stanford
- B. Gaulin		McMaster
- V. Kiryukhin		Rutgers
- S.-H. Lee		NIST
- Y. Lee			MIT
- R. MQueeney		Ames/Iowa St.
- S. Nagler		ORNL
- R. Osborn		ANL
- J. Rhyne			U. Missouri
- A. Zheludev		ORNL

_*IDT Members (US) and their Affiliations*_

*I. Zaliznyak (BNL)*
*S. M. Shapiro (BNL)*
*L. Passell (BNL) *
*V. J. Ghosh (BNL) **Monte-Carlo simulations*
*W. Leonhardt  (BNL) **Project Engineer*
*M. Hagen (SNS/BNL) **Instrument scientist*

_*HYSPEC Instrument Design Team*_

**http://neutrons.phy.bnl.gov/hyspec**

## HYSPEC’s place in the SNS inelastic instruments suite.

- 10-100 eV Multichopper Spectrometer
-  E = 2 - 20 meV
-  Q = 0.1 - 4 Å-1

_*High energy transfer*_
- 10-1000 meV Fermi Chopper Spectrometer
-  E = 10 - 1000 meV
-  Q = 0.1 – 22 Å-1

_*High intensity at moderate resolution and medium energy transfer + polarized beam*_
- Crystal Monochromator _Hy_brid _Spec_trometer
-  E = 2.5 - 90 meV
-  Q = 0.1 – 8 Å-1

- epithermal

- subthermal

- thermal

## Comparison of the HYSPEC performance with other inelastic instruments planned for the SNS

*MCSTAS simulations by HYSPEC IDT (V. Ghosh), with different re-scaling for ARCS and SEQUOIA *

*CNCS, ARCS and HRCS intensities were re-scaled to*
*the same, coarser energy resolution as HYSPEC (this over-estimates their actual intensity)*

*MC simulations by SNS (G. Granroth and D. Abernathy) *

## Current status of the SNS instrument suite

## HYSPEC layout and principal features

*To get more information, and for the project updates, please, visit *
- http://neutrons.phy.bnl.gov/hyspec

- T0 Chopper
- Disc Chopper
- Monochromator
- Goniometer
- Radial Collimator,
- or Bender Polarizers
- Flight Chamber (evacuated or Ar/He filled)
- Detectors

## HYSPEC layout in the polarized beam mode

- 18-20 transmission polarizers
- 2cm x 5cm (WxL) with 20’ Soller collimators upfront

- vertically focusing
- Heusler crystal monochromator

- neutron spin flipper

## HYSPEC polarization analysis: principle and experimental demonstration on SPINS at NIST

*S.-H. Lee, C. F. Majkrzak, Physica B ****267-268****, 341 (1999)*

- Heusler

*Polarized beam Measurement with a Position Sensitive Detector (PSD)*

## HYSPEC polarization analysis: experimental demonstration with PSD on SPINS

*Nuclear and magnetic scattering intensities in La**5/3**Sr**1/3**NiO**4*

*I. A. Zaliznyak and S.-H. Lee, *
*in Modern Techniques for Characterizing Magnetic Materials, ed. Y. Zhu* *(to be published by Kluwer Academic, 2004)*

## HYSPEC setup for polarization analysis

-  Polarized incident beam is supplied by reflection from the vertically focusing Cu2MnAl (Heusler alloy) crystal monochromator
- 10 meV < Eipol < 90 meV
-  Polarization analysis of the scattered neutrons is performed by a set of 18-22 supermirror-bender transmission polarizers, each 2 cm wide, 5 cm thick and 15 cm high,
- 3.7 meV < Efpol < 15-25 meV

## A somewhat similar concept: D7 at ILL

* **Important distinctions of the HYSPEC*
* **optimized for using the straight-through transmitted beam*
* **both spin states are measured by the detector array*

## Most important question: can we expect the transmission polarizers to work up to 15-25 meV?

*Performance of an optimized Fe/Si transmission polarizer for ~15 meV C. Majkrzak, Physica B 213&214 (1995)*

*Yes, but fine-tuning of the polarizer tilt angle is necessary.*

## Optimizing the geometry of a single-bounce transmission polarizer

- _Defining parameters are:_
* **θ**c**(up)** and **θ**c**(down)** *
* **L, length*
* **d, channel width*
-  *, tilt angle*
* **β**, **bend angle*
* **L ≈ 2R *sin*(**β**/2)≈ R **β*

- _Optimization considerations and constraints_
-  *θ**c**(up)** = 3.0 **θ**c**(Ni)**, **θ**c**(down)** = 0.6 **θ**c**(Ni)**,** => best we can imagine for now*
* **L**≈ 50 mm** => maximum length is constrained by the transmission through Si *
* **d ≤ R(1- **cos**β**) **≈ L**β**/2 ≈ 0.25 mm** => to remove the line-of-sight*
* **polarizer bend angle **β** => mechanically constrained, currently use **0.57°*
* **polarizer tilt angle **** => must be optimized*
- _Simple optimization condition for a single-bounce device_
*(**** + **β**) = **θ**c**(up)** = 3.0 **θ**c**(Ni)*

## Optimizing the polarizer tilt angle at E = 3.7 meV

-  = 0.15°

-  = 0.3°

-  = 0.8°

-  = 1.2°

- Neutron beam profiles on the detector

- 20’ collimator
- in front

## Optimizing the polarizer tilt: E = 3.7 meV is quite “forgiving”

- Straight beam

- Deflected beam

## Optimizing the polarizer tilt angle at E = 10 meV

-  = 0.1°

-  = 0.3°

-  = 0.4°

-  = 0.5°

- Neutron beam profiles on the detector

- 20’ collimator
- in front

## Optimizing the polarizer tilt angle at E = 20 meV

-  = 0.0°

-  = 0.2°

-  = 0.3°

-  = 0.4°

- Neutron beam profiles on the detector

- 20’ collimator
- in front

## Optimizing the polarizer tilt: fine tuning is needed for higher energies

- Straight beam

- Deflected beam

## MC simulation (NISP) of the HYSPEC operation in the polarized beam mode: beam separation

- Simulation for the bender geometry optimized for E=14.7 meV (C. Majkrzak, 1995) Sample-to-detector distance LSD is 4.5 m

## The spatial separation of two polarizations for different sample-to-detector distances

- θc(up) = 3.0 θcNi,
- θc(down) = 0.6θcNi.

- The two polarizations only become sufficiently separated that they can be measured cleanly in the adjacent detector tubes for values of the secondary flight path LSD > 4.0m.

- LSD = 3.0m

- LSD = 3.5m

- LSD = 4.0m

- LSD = 4.5m

## Summary, work in progress, and open questions

- Heusler monochromator provides polarized incident beam
- Scattered beam polarization is determined by an array of transmission polarizers
- Fe/Si, Co/Si, other?
- straight-through transmitted beam is always measured
- all scattering angles are covered
- most of the detectors are efficiently used
- price in intensity for using 20’ collimators also buys lower background and a somewhat better q-resolution
- Optimization of the polarizer geometry for the broadband operation
- important to use the optimized tilt angle for every Ei, and E-range
- curvature choice (possibly straight stack)?
- fine tuning: length, channel width, collimation in front.
- Effect of a coarse (2-3 degrees) radial collimator behind the polarizers?

## Neutron spectrum produced by SNS vs reactor

## SNS accumulator ring built by BNL

- _[http://sns.bnl.gov/](http://sns.bnl.gov/)_
- http://sns.bnl.gov/ap_group/ring.html