# Presentation: 281810

## Choosing the Right 

*Neutron Spectrometer*
- Dan Neumann
- NIST Center for Neutron Research
- dan@nist.gov       www.ncnr.nist.gov

## Main Messages of the Week

-   Neutron scattering experiments measure the number of neutrons scattered by a sample as a function of the wavevector change (**Q***)* and the energy change (********** of the neutron
-   Expressions for the scattered neutron intensity involve the positions and motions of atomic nuclei or unpaired electron spins
-   The scattered neutron intensity as a function of **Q** and ********** is proportional to the space and time Fourier transform of the probability of finding one or two atoms separated by a particular distance at a particular time

## What is required to do an inelastic 

- What is required to do an inelastic
- neutron scattering experiment

- A source of neutrons
- A method to prescribe the wavevector (**k****i**) of the neutrons incident on the sample
- A well-chosen sample
- A method to determine the wavevector (**k****f**) of the scattered neutrons
- A detector

- NSE measures |**k**| directly by attaching a “clock” to each neutron

## Methods of specifying & measuring ki and kf

- Methods of specifying & measuring** ****k****i**** **and** ****k****f**

- SPINS, FANS, Backscattering

**1.  Bragg Diffraction**

**2.  Time-of-flight**

- DCS, Backscattering (??)

**3.  Larmor Precession**

- Spin Echo

## Why are there so many different spectrometers?

- Neutron scattering is an intensity limited technique.  Thus the detector coverage and resolution MUST be tailored to the science.

- Uncertainties in the neutron wavelength and direction imply that **Q** and ******** can only be defined with a certain precision.
- The total signal in a scattering experiment is proportional to the resolution volume *i.e.* better resolution leads to lower count rates

- Courtesy of R. Pynn

## Different spectrometers cover different regions of phase space

## Things to consider

**Things to consider**

- DCS		          Backscattering		  Spin Echo

## Things to consider

**Things to consider**

- Is your sample polycrystalline or amorphous?
- 	Does ONLY the magnitude (not the direction) of **Q** matter?
- Is the expected **Q**-dependence of the scattering weak?
-     This often means that you want to look at a large region of **Q****-********** space
- 	or that you can sum the data over a large region of **Q****-********** space
- 	If YES, consider instruments with large analyzer areas
- 		(FANS, DCS, Backscattering)

## Things to consider

**Things to consider**

- Now consider the energies (********) or time scales of interest (******t** ~1/****)
- 		********10-20 meV**  **-**  **use FANS (or some other spectrometer 							designed for vibrational spectroscopy)
- 		********************20-30 eV  -  use Backscattering
- 		in between  -  use DCS (or some other cold neutron TOF spectrometer)		
** **BUT – check to make sure that the length scale of the motions that you’re interested in is within the range of the instrument.  As a simple example of this, consider the Backscattering spectrometer.  (**Q** ~ 2/**L**)
- 		**Q**min = 0.25 A-1 	=> **L**max~ 25 A
- 		**Q**max = 1.75 A-1  => **L**min~ 3.5 A

- REMEMBER - **Q**min and **Q**max are inversely proportional to the
- 		incident neutron wavelength

## Things to consider after choosing DCS

-  wavelength 
-  chopper slot widths W

**Things to consider after choosing DCS**

## Sample “Design” for 

- Sample “Design” for
- DCS & Backscattering

- Does the sample contain H?
- 	Remember: Neutrons LOVE H!!
- Create a sample where the “interesting” portions of the sample are hydrogenated and the “uninteresting” portions are deuterated.

## I/Io = exp –(nD)

-     => Fill the beam with sample
- 	maximum beam size is usually given in the instrument description
- 	3 cm X 10 cm for DCS (or 1.5 cm X 10cm)     3 cm X 3 cm  for Backscattering
- If possible, use cylindrical samples (rather than flat plate)
- 	Remember - For incoherent, quasielastic
- 		scattering the transmission of the
- 		beam should be ~90%
- 	Often annular is the best sample geometry

- I/Io = exp –(nD)

- Sample “Design” for
- DCS & Backscattering

## Things to consider

- If the resolution of backscattering is “not good enough” or if you are only interested in a “limited” region of **Q** space  (typically small **Q**),
- 	use NSE (low **Q**, long times)

- These cases typically involve coherent scattering which tends to peak around 2/(the relevant length scales in your sample)

- Remember – slower motions
- usually imply larger length scales.
- Many atoms moving together
-        => Coherent scattering

## Sample “Design” for NSE

- Create a sample where the “interesting” portions
- of the sample have a different SLD than the “uninteresting” portions
- 	Typically this means deuterating the major phase
- 	in order to reduce the incoherent background

- D2O
- (deuterated)

- C10D22
- (deuterated)

- AOT
- (hydrogenated)

- SLD core 	6.410-6 Å-2
- SLD shell 	1.610-6 Å-2
- SLD solvent 	6.510-6 Å-2

- http://www.ncnr.nist.gov/resources/sldcalc.html

## Sample “Design” for NSE

-     => Fill the beam with sample
- Typically use flat plate samples (at small angles)
- Rule of thumb - the transmission should be ~70%

- I/Io = exp –(nD)

## Triple Axis Spectrometers

**Triple Axis Spectrometers**

- Triple axis spectrometers are typically used when either the _*direction*_ of **Q** is important or the interesting region of **Q****-****** space is of limited extent.

- _Collimation(') __	__  __****__ __	__rel. signal      FWHM ._55-80-80-80 	 4 Å 	   1.00 	     0.28 meV
- 55-40-40-40 	 4 Å 	   0.24 	     0.17 meV
- 69-80-80-80 	 5 Å 	   0.26	     0.13 meV
- 84-80-80-80 	6.1 Å 	   0.03	     0.05 meV

## Sample “Design” for Triple Axis

**Sample “Design” for Triple Axis**

- Single Crystals yield the most information

- Increase the intensity by increasing the amount of sample
- If you have a powder, use a cylindrical container (rather than flat plate)
- 	Annular may be the best sample geometry

- Almost all experiments on triple-axis spectrometers involve coherent scattering
- => sample should be deuterated (if it contains H at all)

## General Sample “Design”

- Try to avoid isotopes that are strongly absorbing

- 6Li   10B   113Cd

- http://www.ncnr.nist.gov/resources/n-lengths

- For a complete listing go to

## General Sample “Design”

- The most important thing is:

- Know as much about your sample as possible
- The types of things that you might want to know include:
- 	What’s the structure (in a general sense)?
- 	Are there any phase transitions (or a glass transition)?
- 	What isotopes are present?

## Things to consider

**Things to consider**

- Microemulsion

## DCS vs. SPINS

- DCS – incoherent scattering, surveys
- SPINS – limited region of **Q****-**********

## Applying for beam time

**Applying for beam time**

- The use of the neutron scattering instrumentation that you’ve used over the past week is open to all qualified users based on peer-reviewed proposals. Calls for proposals are issued about twice per year.
- The next deadline for new proposals will be in the Fall of 2003.

- http://www.ncnr.nist.gov/programs/CHRNS/CHRNS_prop.html

- Further information on submitting proposals can be found at:

## Acknowledgements

- Our administrative staff (Julie Keyser)
- The experiment teams
- Alexei Sokolov and Bruce Gaulin

**Acknowledgements**

**Thanks for coming**