Neutron scattering determination of local order in amorphous and liquid systems using a position sensitive detector

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Neutron scattering determination of local order in amorphous and liquid systems using a position sensitive

detector

J.P. Ambroise, M. C. Bellissent-Funel, R. Bellissent

To cite this version:

J.P. Ambroise, M. C. Bellissent-Funel, R. Bellissent. Neutron scattering determination of local order in amorphous and liquid systems using a position sensitive detector. Revue de Physique Appliquée, Société française de physique / EDP, 1984, 19 (9), pp.731-734. �10.1051/rphysap:01984001909073100�.

�jpa-00245247�

Neutron scattering determination of local order in amorphous

and liquid ^systems using

^a

position ^{sensitive detector}

J. P. Ambroise, M. C. Bellissent-Funel and R. Bellissent

Laboratoire Léon Brillouin, ^C.E.N. Saclay, ⁹¹¹⁹¹ Gif-sur-Yvette Cedex, ^France

Résumé. ²⁰¹⁴ Cet article décrit le diffractomètre

7C2

^{installe sur un des deux canaux de la source} chaude du réacteur

Orphée

^à Saclay. Notre but essentiel sera de mettre l’accent sur les

problèmes

dus à l’association d’un multi- détecteur linéaire avec différents types d’environnement. La détermination de l’ordre à courte distance des sys- tèmes désordonnés

implique

l’usage ^de

porte-échantillons

pour les

liquides,

^de fours pour les

systèmes

^à

point

de fusion élevés, ^etc... Même dans le cas des

amorphes,

^{il est souvent} nécessaire d’utiliser un cryostat pour réduire les excitations

thermiques qui

amortissent les anneaux

qui

caractérisent l’ordre local ou pour ^se placer ^au-dessous

de la transition

paramagnétique,

par

exemple.

Nous donnons quelques

exemples d’expériences déjà

^réalisées

dans différents domaines sur ce

spectromètre

^{et nous tenterons de}

dégager

^des

perspectives

nouvelles pour ^cet

appareil.

Abstract. ²⁰¹⁴ This paper consists of ^a

description

^{of the two axis} spectrometer

7C2

^{on the hot source of the reactor}

Orphée

^at Saclay. Our main purpose will be ^to

emphasize

^the

problems

^{due to} the association of the

position

sensitive detector with various types ^{of the}

sample

environment. In order to determine the local order on disor- dered systems ^{we need to use} containers for the liquid state, furnaces for

high

melting

point

systems, ^{etc... Even} for

amorphous

materials, ^{the use of a} cryostat ^is often necessary ^to get ^{rid of}

paramagnetic scattering

^{or to reduce}

thermal excitations

broadening

^the structure. We shall give ^some examples ^of

experiments

which have already

been realized on this spectrometer ^{and we shall} try ^to

point

^{out some future}

possible developments.

1. Introduction.

Due to the low

absorption

^{of neutrons}

by

^most

elements,

^{thermal neutron}

scattering

has been inten-

sely

^used

during

^the past ^fifteen years for structural studies which necessitated the use of various containers. More

recently, high

^{flux reactors and}

position

sensitive detectors

(PSD)

^have

provided

^us

with a

facility

^for

studying amorphous samples

even in

relatively

^small

quantities.

In order to obtain accurate structure factors a

precise

determination of the scattered

intensity

^is

needed over a wide _range of the momentum transfer

Q given by :

for elastic

scattering

^{of neutron} of incident

wavelength À,

^{at a diffusion}

angle

^{2 0. A}

high counting

^{rate has}

been obtained

by

^{the use of a PSD}

[1, 2].

^{The wide}

range of a momentum transfer which is necessary to obtain an accurate

pair

correlation function

by

Fourier transform of the structure factor has been obtained

by

^{the use of a hot source.}

2.

7C2 Spectrometer

^{and PSD.}

The

7C2

spectrometer has been built on one of the two beams of the hot ^source of the reactor

Orphée

at

Saclay.

^{The use} of three monochromators in a

rotating

shield allows both discrete or continuous variation of the incident

wavelength.

Our standard

position corresponding

^{to a}

monochromating angle

of 20°

gives 0.5,

^{0.7 and l.l}

A wavelengths.

^{The flux}

is in the range from 106 to 2.2 ^{x 10’ N}

^x

^{cm i 2}

^{x s -1}

following

^the

wavelength

and the used collimators.

We

give

^{in table 1 a}

multiplication

factor due to the hot source at a temperature ^{of 1} 100 °C for the three

wavelengths

^used.

This source may be removed for a

given

^{run of the}

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:01984001909073100

732

Table I.

reactor in order to increase the flux at

high wavelength

if _necessary.

The PSD described here is a linear banana type multidetector

[3].

^It consists of 32 blocks of 20

cells,

each block

being

^a cathode with anode wires of

40 gm

in diameter. The

corresponding voltages

^are

- 2 700 V and + 2 900 V

respectively.

^The

filling

gas is

lOBF3

^{at a pressure of 1 bar.} The measured

efficiency

^{was about 30}

%

^{at 03BB = 1.1}

^Á

^{and 12}

%

^at

03BB ⁼ 0.5

Á.

The 640 cells with a linear resolution of 5.2 mm at a radius of 1.5 m allows one to cover

128° with an

angular

step ^{of 0.2°. Due to} the absence of collimation between the

sample

and the detector each cell of the PSD receives neutrons from a solid

angle

^{2 Te.}

Therefore,

^{in order to} reduce the back-

ground,

^the

incoming

^{beam is} surrounded

by

^{a vacuum}

tube coated

by

^{neutron absorbers.}

3.

Température

^devices.

The temperature ^{devices are a vanadium furnace} which

provides

temperatures up ^to 1 200 °C and

a

cryostat

of which the lower temperature ^{is about} 1.4 K. Both devices are

represented

^on

figure

^1.

In order to reduce the

background

^{of the}

furnace,

we have used a vanadium

heating cylinder

^{of 0.1 mm}

thickness. In this _way, the scattered

intensity

^{for the}

furnace remains _very

low ;

moreover as it is

isotropic,

the transmission corrections are easy ^to carry ^out.

An aluminium tail

cryostat

has been tested but appears ^to be _very

unsatisfactory

^{due to the conta-}

mination of the

smoothly varying

^{structure factor of}

disordered

systems by

^the

badly

^resolved

Bragg peaks

^of aluminium. A new

cryostat

^{with a vanadium} tail will be _very soon available.

4. Structural studies at room,

high

^{or low}

temperatures.

This

part is

^concerned

by

^three

typical experiments

which

exhibit

some new

developments

^{in the use of}

the

spectrometer.

1. Studies on small

samples.

2.

High temperature

^studies.

3. Low

temperature

^studies.

4.1 STRUCTURE OF AMORPHOUS S1LICON a-Si. ^- These

experiments

^{are the}

beginning

^{of a more}

general study

^{of the structure of}

a-SiHx

which is also

being

studied

by

^other

techniques [4, 5]. They

^{have been}

performed using

^a very small

sample (0.2

g in 0.1

cm3)

Fig. ^{1. -} Sample environment on

7C2.

a ^- monochromator shielding,

b - evacuated collimator,

c - ),,/2 filter,

d - monitor,

e - variable

diaphragm,

f - internal collimators,

- Cryostat ^Furnace

g - collimator h -

LN2

^screen

i - vanadium screen

j -

vanadium tail resistor

k - sample

1 - Cd

shielding,

m - vacuum chamber : 0 ^{= 600} mm,

n ^- beam stop,

o ^- detector

shielding,

p ^- 640-cell PSD.

of silicon of which the coherent

scattering

^cross

section is

fairly low,

^{2 barns. In order to minimize} the

scattering

^{of the}

sample

^{holder we have used a} vanadium

cylinder

^{of 20 mm}

height,

^{5 mm} in diameter and

only

^{0.02 mm} in thickness.

The measured intensities for the

background,

^the

container and the

sample

in its container are shown

Fig. 2. 1. - Vertical section of the cryostat ^{a -}

zircalloy

window, ^{b - vacuum chamber :} 0 ^{= 600} mm, ^{c -} colli- mators and cadmium masks, d - standard orange cryostat with adaptator

flange,

^{e - vanadium} tail, ^{f -} sample ⁱⁿ

its container, g ^- copper thermal

bridging,

^{h -} angular

positioning

pins.

Fig. ^{2.2. -} Vertical section of the furnace. a ^-

zircalloy

window, ^{b - vacuum} chamber : 0 ^{= 600 mm, c - colli-}

mators and cadmium masks, ^{d - vanadium} resistor, ^{e -} upper and lower boron nitride masks, ^{f -} sample ^{in its con-} tainer, g ^- power supply connections, h ^- stainless steel

centering

ring, ^{i -} insulating

flanges, j

^{- copper} cooling ring.

in

figure

^{3. It} should be noticed that the scattered

intensity

for the vanadium container is _very low

compared

^{to the}

background.

^{However the total}

background

^{is of the same order of}

magnitude

^{as the}

sample scattering.

Therefore such a

sample

appears ^to be the smallest

possible

^to

study

^{on this} spectrometer ^{if a} reasonable

precision

^{is to} be obtained for the structure factor determination.

4.2. ^- The

following study

^concems the local order of the

liquid Li4Pb

system

[6, 7].

^It has been undertaken to determine the structure factor variations versus

temperature.

The results shown in

figure

⁴

respectively

represent the scattered

intensity

^{for the}

sample

^{at 950}

OC,

^a vanadium

container,

^the vanadium furnace described in part

2,

^the apparatus

background,

^{and the same}

Fig. ^{3. -} Amorphous ^{silicon. a -}

background,

^{b - back-} ground ⁺ container, ^{c -}

background

^{+ container +} sample.

Fig. ^{4. -}

Li4Pb.

^{a -} background ^{with cadmium} mask,

b ^- background, ^{c -} background ⁺ furnace, ^{d - back-} ground ^{+ furnace +} container, ^{e -} background ^{+ fur-}

nace + container + sample.

background

^{with a} cadmium mask in the

sample position.

^A

comparison

between the two last curves

shows that most of the

background

^{is not}

coming

from the neutron beam. Therefore this

background

will be difficult to reduce. However since the

scattering

coming

from all instrumental sources are much

734

lower than the

sample scattering,

^{such an}

experiment

will

provide

^{us with a}

good precision

^{in the structure}

factor determination.

4. 3. ^- The last

study

^{concems the} short range order structure of the vitreous system

LiCI,

³

H20

^{at low}

temperature. ^{It has been}

performed by

^J.

Dupuy

Fig. ^{5. -} LiCI, ³

H20.

^{a -} cryostat + vanadium contai- ner, b ^- cryostat ⁺ vanadium container + sample.

and J. F. Jal

(Département

^de

Physique

des Matériaux de

Lyon).

^{We show on}

figure

^{5 data from such a}

measurement. It consists of the

intensity

^{curve for}

a vanadium container in an aluminium tail cryostat and a curve of the scattered

intensity

^{from the}

sample.

These curves

clearly highlight

^the

importance

^of

the

intensity by

^the

Bragg peaks

of the aluminium tail of the cryostat. ^Moreover this effect will be even more troublesome if we use the shorter

wavelength (0.7

^{or 0.5}

Á)

^{which is} often useful for measurements

on disordered systems.

5. Conclusion.

The use of a PSD for structural studies on

amorphous

and

liquid

systems appears ^to be a very efficient method because it

provides

^{one with the}

good

accuracy

on the whole _range of momentum transfer which is necessary ^to obtain the structure factors of the disordered systems.

Due to the _very

important

part

played by

^the

temperature ^on the local order of

amorphous

^and

liquid

systems ^a furnace and a

cryostat

appear ^to

provide

the necessary environment of such a spectro-

meter.

The vanadium furnace which has been used _appears to be _very convenient for measurements from room

temperature up ^to 1 000 °C and even up ^to 1 250 °C

using

^{a vanadium screen. An} aluminium-tail cryostat has been tested in

preliminary experiments

^but

coherent

scattering

of aluminium is not consistent with the use of the PSD.

Future

development

^{of the}

spectrometer

^must involve the

replacement

^{of the} cryostat

by

^{a vanadium}

tail cryostat

already

^{used at ILL}

[8].

Moreover

isotopic

substitution measurements would be more accurate

using

^a

high

temperature

sample changer.

^A

preliminary study

of this device is

currently

in _progress.

References

[1] ALLEMAND, R., BOURDEL, J., ROUDAUT, E., CONVERT, P., IBEL, K., JACOBE, J., COTTON, J. P. and FAR- NOUX, B., Nucl. Instrum. Methods 126 (1975)

29-42.

[2] CONVERT, ^P., Thèse, ^Grenoble (1975).

[3]

AMBROISE, J. P. and BELLISSENT, R., ^ILL

Workshop

on Position Sensitive Detectors, Grenoble, ^11-

12 octobre 1982. Ed. Convert, ^{P. and} Forsyth, ^B.

(Academic Press, London) ^1983.

[4] BELLISSENT, R., CHENEVAS-PAULE, ^{A. and} ROTH, M., J. Non-Cryst. Solids 59-60 (1) (1983) ^229.

[5] BELLISSENT, R., CHENEVAS-PAULE, A., LAGARDE, ^P.

and RAOUX, D., ^J. Non-Cryst. Solids 59-60 (1) (1983) 237.

[6] RUPPERSBERG, H. and EGGER, H., ^{J. Chem.}

Phys.

⁶³ (1975) ^4095.

[7] RUPPERSBERG, ^{H. and} REITER, H., ^J.

Phys.

^{F 12} (1982)

1311.

[8] BROCHIER, D., Private Communication.