NEUTRONS VERSUS PHOTONS IN SOLID STATE PHYSICS

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NEUTRONS VERSUS PHOTONS IN SOLID STATE PHYSICS

J. Winter

To cite this version:

J. Winter. NEUTRONS VERSUS PHOTONS IN SOLID STATE PHYSICS. Journal de Physique

Colloques, 1987, 48 (C9), pp.C9-31-C9-37. �10.1051/jphyscol:1987904�. �jpa-00227199�

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JOURNAL DE PHYSIQUE

Colloque C9, suppl6ment au 11'12, Tome 48, d6cembre 1987

NEUTRONS VERSUS PHOTONS I N S O L I D STATE PHYSICS

J. WINTER

CEN Grenoble, Departement de Recherche Fondamentale, BP 85X, F-38041 Grenoble Cedex, France

Dans cet article, une comparaison entre les Btudes principalement structurales sur. la mati6re condensee utilisant soit le neutron, soit les rayons X. est faite.

Cette comparaison est actuellement en pleine evolution, compte tenu du developpement des sources utilisant le rayonnement synchrotron.

On se livre h quelques speculations sur 1'Bvolution future.

ABSTRACT

The comparaison between condensed matter studies using neutrons and photons is a well known subject which can be found in many text books.

However the occurence of synchrotron source is strongly changing the situation.

Some personal considerations about the possible future evolutions will be given.

I

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INTRODUCTION

The problem we want to study is to understand the basic properties of matter on a microscopic scale.

Condensed matter is a collection more a less organized of atoms, molecules or ions, or simply a large ensemble of nucleus and electrons.

To study such a system, tinere is two basically different technics.

The first consists on performing macroscopic measurements : conductivity, susceptibility, specific heat,...

These methodes although important are indirect structure measurements. One has to assume something about the strbucture, from which a prediction about these quantities is deduced and compared to experiment.

The other method I will focused on is to send a beam of particles with definite energy and momentum and to measure changes of momentum or energy or both after the interaction with the sample.

If we assume a small absorption and a perturbative treatment (a doubtful approximation sometimes) we directly obtain the response of the system X(q,w), where q is the change in momentum and w the change in energy.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987904

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If we are only interested in the static (or average) structure we only look to change in momentum.

More points will be precised later.

Now come the first question, what particles we will use. In principle any type of particLe easy to produce in sufficient amount in a well defined state is good. In practice there are four candidates, photons, electrons, neutrons, protons ; charged particles created some problem due to the long range of the Coulomb interaction leading to large absorption.

In that respect the proton has to be avoided having all disavantages being charged and heavy thus changing (or even destroying) the sample.

It is well known that electrons are widely used using thin sample (It is also fair to note that charged particles may be easily focused by magnetic or electric lenses, this is not so simple for neutrons or photons in the X ray range)

.

Now let us start the competition between the two remaining particles.

I1

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ACTE I : NEUTRONS VERSUS PHOTONS, THE THEORIST VIEW

It is now time to precise quantitatively the vaiue used for momentum (or wavelength) and energy.

For the wavelength due to the bragg relation one has to use wavelength

0

comparable to lattice spacing, that is to say ranging from 1 to 10 A.

0

Here comes the first large difference for Neutron 1.4 A correspond to E = 1/40 eV (300°K), but for X-rays 12 KeV correspond to 1,44

i.

The energy is considerably higher.

Knowing that excitation in condensed matter are of order 30G°K (or even much smaller for system with large molecules), we conclude that neutrons are better suited for inelastic studies.

b) Interaction, cross section, absorption

The inzeractions are of a very differente nature, but turn out to be of the same order of magnitude (let us forget first the spin) the neutrons interact by strong forces with the nucleus, the cross section being of the order 27~ b2 with b

2' 10-12 cm. It is a point interaction (no q dependance). The X-rays interact via electromagnetic interaction with the electronic cloud.

Consequently the scattering is :

i) q dependant due to the size of the electronic cloud ii) increasing with 2 .

A complication arise due to the spin of the neutron.

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F i r s t l y f o r nucleus with s p i n t h e n u c l e a r i n t e r a c t i o n is s p i n dependant. A s n u c l e a r s p i n a r e d i s o r d e d t h e s p i n dependant n u c l e a r i n t e r a c t i o n produces i n c o h e r e n t s c a t t e r i n g .

Secondly t h e neutron magnetic moment i n t e r a c t with t h e e l e c t r o n cloud i n magnetic system ( g i v i n g a s f o r t h e X-rays a form f a c t o r ) .

These p r o p e r t i e s a r e summarized i n t a b l e 1 (1).

The c o n c l u s i o n from t h e t h e o r i s t p o i n t of view i s c l e a r l y neutron i s t h e b e s t .

ENERGY 1.4 CROSS SECTION

ABSORPTION

EX CO

S P I N DEPENDANT SCATTERING

MAGNCTIC SCATTER1 NG

FORM FACTOR

INTENSITY VERY CRUDE ESTIMATF.

X-RAYS

NONE

NONE OR 1 0 -

YES

TABLE 1

NEUTRON 25 NEV VARIABLE SOMETIMES LARGE FOR LOW Z : H, D

1 EXCEPTIONS 8. CD, HEJ , GD

LARGE INCOllERENT SCATTERING

LARGE

NO. EXCEPT FOR MAGNETIC SCATTERING

I11

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^ACTE^I1^:EXPERIMENTAL CONSIDERATION

I n Acte I1 we w i l l i g n o r e t h e p o s s i b i l i t y of u s i n g synchrotron r a d i a t i o n . I n t a b l e I1 we compare some experimental f e a t u r e s of both t e c h n i c s .

A very important p o i n t i s i n t e n s i t y c o n s i d e r a t i o n s . I n t a b l e I we mention t h a t i n a very crude way t h e r e a r e

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more photons t h a n n e u t r o n s ( b r i l l a n c e e s t i m a t i o n ) . This f a c t o r is q u i t e l a r g e , however one has t o r e a l i z e t h a t what m a t t e r s is n o t t h e i d e a l b r i l l a n c e , geometry and s i z e of an X-rays o r neutron

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JOURNAL D E PHYSIQUE

experiment a r e completely d i f f e r e n t . Indeed t h e b e s t cornparaison i s the number o f c o u n t s p e r u n i t time o r t h e time needed t o perform an experiment with a r e a s o n a b l e s i g n a l t o n o i s e r a t i o . Also t h e r e a much more X-rays tube t h a n r e a c t o r s .

I b e l i e v e t h a t now t h e chance a r e more o r l e s s even depending s t r o n g l y of c o u r s e o f t h e type of experiment, we a r e c o n s i d e r i n g . Recent s t r u c t u r e measurements on high T c superconductors u s i n g n e u t r o n s and X-rays o b t a i n e d comparable r e s u l t s , a t t h e same time.

NGUTRONS

SOURCE

X-RAYS

DETECTORS

REACTORS (DEDICATED OR HOT)

+ SPALLATION SOURCES

LARGE w KT

(fiOWOCHROHATOR CRYSTAL OR TIME OF FLIGHT) TUNABLE BY CHANGING T trlnrn 1)

B F ~

HE 3

(INDIRECT. RESULT OF A NUCLEAR REACTIOIO

SHALL (A R O W )

TUBE (STANDARD -

ROTATING ANODE)

IIONOCHROHATAC

LARGE. TONS I IIETERS l o 6 FRANCS

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IV

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^ACTE^I11^:IMF'ACT OF SYNCHROTRON RADIATION

A charged particle with relativistic velocity when deviated by a nucleus or a static magnetic field produces electromagnetic radiation.

This radiation has the following properties (2).

a) pulsed in time b) polarized

c) large energy spectrum (schown on Figure 2) d) very large number of photons (see Figure 3).

For convenience one will distinguish two type of synchrotrons.

a) Low energy E < 1.5 GeV. The cut off fr3quency (see Figure 2) is of the order of 10 or higher. These relatively small rings have many applications in solid state physic, the best known being photoemission.

b) High energy E > 1,5 GeV. They produce X-rays in an energy range comparable to standard X-rays tube, but with intensity or brillance considerably higher (see Figure 3).

There are now in Europe, USA and Japon project of large rings in the 6 GeV range. An important improvement already tested is the so-called "insertion devices".

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BRILLICE VERSUS WAVE LENGTH

EXIWLE FOR L

-

^I^GEY ^W

-

^OT

-

^1.16

^;

I

^YEAR

In the first experiments, the solid state physicist used colliding rings built by elementary particles physicists, and the photons were coming from the electron beam in the bending magnets. The value of the field in these magnets is determined for controling the electron trajectories and consequently cannot be changed.

The insertion devices produce oscillating magnetic field using straight section of the ring. More will be said about this subject during the conference.

Let us only mention as seen on Figure 3 that undulators increase drastically the brillance of the source (but change the energy spectrum).

Due to the drastic changes of intensity some of the conclusions of chapter I and I1 have to be reconsidered. I will only mention one example, the magnetic scattering.

I said that neutrons were by far the best tool for studying magnetic scattering. There is a very small possibility of scattering X-rays by a magnetic lattice (the cross section is reduced by roughly six order of magnitude). Indeed the experiment (very difficult) has been done using X-rays tube 4 3 ) . Using synchrotro~l radiation magnetic scattering with good signal ratio has been obtained (4).

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V

-

^FUTURE

-

ÂCTEÎVÂM)^V

It is probably too early to measure today the impact of synchrotron machine in the 6 GeV range with insertion devices. With already existing machines the X-rays is clearly gaining over the neutron in several fields.

The question to ask is do we expect improvement on the neutron side. It is certainly possible in principle to build reactor with larger flux. However, due to safety considerations (and other considerations) I am convirlced that new reactors with higher flux will not be built very soon.

A new way is developping now in several countries (UK, USA, JAPON), the use of spallation sources.

A strong pulse of protons is sent on a heavy element target. The spallation of heavy nucleus produces neutrons.

The flux during the pulse is large even after reducing the neutron energy, Such a source is well adapt to study using time of flight technics, and also well adapted for relatively large energy neutrons (0.1 to 1 eV). Again it is too soon to draw definitive conclusion about the impact of these new sources.

BIBLIOGRAPHY

(1) G.E. BACON, Neutron Diffraction Oxford at the Clarendon Press.

(There exist several editions, the book is relatively old but convenient for non specialist).

(2) Handbook on Syrlchrotron Radiation

-

North Holland, 1983, p. 3.

Synchrotron Radiation a powerful1 tool in science by E.E. KOCH, D.E. EASTMAN and Y. FARGE.

(3) F. de BERGEVIN and M. BRUNEL Phys. Letters, 39A, 141, (1972)

(4) D. GIBBS; D.E. MONCTON, K.L. dVAMICO, J. BOHR and J.B. GRIEI Phys. Rev. Lett., 55, 234, (i985).