# Presentation: 004113

## Polarized Beam Operation of the Hybrid Spectrometer at the pulsed Spallation Neutron Source.

- _Outline_
- HYSPEC: project timeline and place in the SNS instrument suite
- HYbrid SPECtrometer’s layout and principal features
- Polarized beam setup: principle and components
- Performance optimization of the (Fe/Si) transmission polarizer
- Summary 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)

## 2000
Concept of the Hybrid Spectrometer proposed at BNL
2001
Direct Geometry Hybrid Spectrometer presented to SNS EFAC
HYSPEC Instrument Development Team (IDT) formed 
2002
HYSPEC IDT filed Letter of Intent with SNS
HYSPEC proposal submitted to DOE
2003
DOE CD0, HYSPEC is approved as part of the SING project
2004
HYSPEC’s placement approved, design&engineering: real work begins 
2005
DOE CD2, HYSPEC’s performance baseline approved
2006
DOE CD3, construction begins 
2011
CD4, commissioning & beginning of operation

- Concept of the Hybrid Spectrometer proposed at BNL
- 2001
- Direct Geometry Hybrid Spectrometer presented to SNS EFAC
- HYSPEC Instrument Development Team (IDT) formed
- 2002
- HYSPEC IDT filed Letter of Intent with SNS
- HYSPEC proposal submitted to DOE
- 2003
- DOE CD0, HYSPEC is approved as part of the SING project
- 2004
- HYSPEC’s placement approved, design&engineering: real work begins
- 2005
- DOE CD2, HYSPEC’s performance baseline approved
- 2006
- DOE CD3, construction begins
- 2011
- CD4, commissioning & beginning of operation

- HYSPEC’s timeline

## HYSPEC Teams.

- _S. M. Shapiro_, co-PI	BNL
- _I. Zaliznyak_, co-PI	BNL
- D. Abernathy	SNS
- L. Daemen		LANL
- B. Gaulin		McMaster
- J. Gardner		BNL
- V. Ghosh		BNL
- M. Greven		Stanford
- M. Hagen		SNS
- K. Hirota		ISSP
- V. Kiryukhin	Rutgers
- Y. Lee		MIT
- C. Majkrzak	NIST
- R. MQueeney	Ames/Iowa St. U.
- S. Nagler		ORNL
- R. Osborn		ANL
- L. Passell		BNL
- L. P. Regnault	ILL
- J. Rhyne		U. Missouri
- J. Tranquada	BNL
- G. Xu		BNL
- A. Zheludev	ORNL

_*IDT Members 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*

_*Instrument Design Team*_

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

*S. M. Shapiro, PI (BNL)*
*I. Zaliznyak, PI (BNL)*
*L. Passell (BNL) *
*R. McQueeney (Ames/Iowa St. U.) *
*J. Rhyne  (LANL) *
*J. Tranquada (BNL) *

_*IDT Executive Committee*_

## 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-Focussing _Hy_brid _Spec_trometer
-  E = 2.5 - 90 meV
-  Q = 0.1 – 8 Å-1

- epithermal

- subthermal

- thermal

## Neutron spectrum produced by SNS vs reactor

## 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) *

## Layout of the SNS instrument suite - 2004 

## HYSPEC layout and principal features

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

- T0 Chopper
- T2 Chopper
- Monochromator
- Goniometer
- Radial Collimator
- or Bender Polarizers
- Flight Chamber (Ar/He filled)
- Detectors

## HYSPEC layout in the polarized beam mode

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

- Heusler crystal monochromator,
- vertically focussed

- 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 18-20 supermirror-bender transmission polarizers, each 2 cm wide, 5 cm thick and 15 cm high,
- 3.7 meV < Efpol < 15-25 meV

## 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.*

## MC simulation (NISP) of 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

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

- Straight beam

- Deflected beam

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

- Straight beam

- Deflected beam

## A somewhat similar concept: D7 at ILL

* *_Important differences of the proposed HYSPEC setup_
* **optimized for thermal neutrons *
*=>  **optimized for using the straight-through transmitted beam *
*=> **allows low-polarization-efficiency mode extending to high energies!** *
* **both spin states are measured by the detector array*

## Summary and open questions

- Heusler monochromator provides polarized incident beam
- Scattered beam polarization is determined by the array of transmission polarizers
- straight-through transmitted beam is always measured
- polarization sensitivity covers thermal neutron energies up to ~30 meV
- all scattering angles are covered, ~2/3 of detectors are efficiently used
- price in intensity paid for using 20’ collimators also buys lower background and  somewhat better q-resolution
- Fe/Si, Co/Si, other?
- Optimization of the polarizer geometry for 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?

## Spallation Neutron Source (SNS) at ORNL

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

## SNS accumulator ring built by BNL

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