A cold neutron small angle scattering device for technological applications

NUCLEAR INSTRUMENTS

AND METHODS 134

(1976) 369-378; © N O R T H - H O L L A N D P U B L I S H I N G CO.

A COLD N E U T R O N SMALL ANGLE SCATTERING DEVICE FOR TECHNOLOGICAL APPLICATIONS C . P . GALOTTO, P. PIZZI, H. WALTHER

FIAT, D.C.R., Laboratori Centrali, Torino, Italy V. ANGELASTRO, N. CERULLO and G. CHERUBINI

CAMEN, S. Piero a Grado, Pisa, Italy Received 12 November 1975 and in revised form 3 February 1976 The neutron small angle scattering device set up at the Camen Galileo Reactor in Pisa is described in detail. In particular the performance of the liquid propane cold source, guide tube, wavelength selector, detection and electronic data processing system are examined. The system allows low angle measurements at scattering vectors between ~ 1 0 -a and 2 x 10 -1 A -1. Some technological applications related to structural modifications occurring in turbine blades, dislocations evolution in fatigued polycrystalline nickel and ferritic phase formation in welded AISI 304, are shortly reported.

1. Introduction Small angle scattering with neutrons has become, during the last ten years, an important tool for the investigation of structural problems in solid state physics, chemistry and biologyt). The development of experimental (and theoretical) methods has now reached a state, that makes technological applications attractive as well 2'3). This is due to the fact that information on structural "long range order", heterogeneities like precipitates, microvoids and dislocations can be obtained from the small angle diffraction pattern, and that the evolution of such heterogeneities is of primary importance in iheat treatment procedures for alloy improving and degeneration processes in service like yielding, creep, thermal- and mechanical fatigue as well. For obtaining such information it is essential to use long wavelength neutrons (instead of X-rays) in order to avoid double Bragg scattering, which would obscure the true small angle effects in metals < 5). At the same time neutrons make it possible to obtain this information in a non-destructive way owing to their low absorption cross section in most of the metals. Neutron small angle scattering extends the field of non-destructive testing virtually down to very small iheterogeneities into a dimensional range between l0 and 10.000 A, where it becomes possible to control the degeneration processes with the sample still far away from failure, and where a determination of residual sample life seems possible. The attractiveness of the method however, is restricted by the main disavantage of neutrons: their low

intensity at high generation costs as available in common research reactors. This latter circumstance necessitates an economically optimized scattering device. The results of the basic development work on the single device components (cold neutron sources, guide tubes, counting systems) carried out in the last ten years give criteria which are reasonable enough to design it. The here described device for cold neutron small angle scattering is entirely destined for the development of technological methods of applications. It has been built for the 7 MW reactor A V O G A D R O (SORIN-Saluggia) and has been transferred later to the 5 MW reactor G A L I L E O (CAMEN-Pisa), where it has been operating since 1973.

2. Description of the scattering device The experimental arrangement is shown in fig. 1. It consists of a cold neutron source at the periphery of the reactor, which "illuminates" the rectangular entrance cross section of a neutron guide tube. From the exit of the tube, the neutrons pass through a wavelength selector and a rectangular slit system (D1, D2) before they reach the sample. Scattering is measured with a bidimensional multicellular counter. After a brief review of the general aspects of this device, the components are discussed in more detail. 2.1. GENERAL ASPECTS

The primary neutron intensity is, scattered from the sample into the solid angle I2, is given by6): --=dis rd£s £i,cl, io e x p ( - £ r D ) D ~ - - + dQ [dK2 4re _]

(1)

370

c . P . GALOTTO et al.

Anelastic Scatterlng~ r. 210 Cold ~

1

il

ii l:

Reac t o r ~ / /

i

\

i

_3m..]rf

Wavelength Sample Selector Holder Shielding Tank

soppo,,j

Elastic

14.6m.

/

gldimenslonal Multldetector

C)

/

Scattering/ 21.6m.

5m.

6. "1 m.

\,l

•~

~

Guide Tube Support

Fig. 1. Cold neutron scattering device.

where D = sample thickness, ~ine = macroscopic incoherent scattering cross section *, ~ s = macroscopic coherent scattering cross section *, ~ r = macroscopic removal cross section *, i0 = incident intensity, and where d~s/dg2(K), that is the macroscopic differential scattering cross-section, represents the structure sensitive scattering law, to be measured (in the x, y detector plane) as a function of the scattering vector K = (Kx, Ky, 0) with ]g[ =

sin 10 4n--, 2

* Cross section density is the more correct expression.

0

= scattering angle,

2

= neutron wavelength. The intensity that has not reacted with the sample

ip = i o e x p ( - Z r D ) is measured for normalizing purposes. The principle of the arrangement for small angle scattering is shown in fig. 2. The quality of the arrangement is evidently dependent on the m i n i m u m resolvable scattering vector components in the x, y detector plane Kx, m i n - 2re Xmin , Ky, min = . . . .

2

L2

(2)

COLD

NEUTRON

SMALL

ANGLE

on the error of the K~, K r measurements

AKx K~

_

A2 + L__2_2FDl,x+D2,x + D2,x+D3,x~, 2 x L L~ L 2 .J

AK, ..... Ky

(3)

and on the neutron current A/o(2) = qbcp(2) a~_.____2yDz,~ D2,y G(2) T(2) z(2) A2 (4) 4n

SCATTERING

371

DEVICE

a small quantity (-,~250cm 3) is temporarily exposed to radiation in the proper cold neutron source. In this system radiolytic decomposition in the cold source is such that occlusion occurred for the first time after ~ 3000 h. Propane circulation is accomplished, as shown in fig. 3, by alternative pressurization of two Dewar vessels D1 and D2. For pressurization, N2-has is used from the same 20001 tank that contains liquid N 2 for cooling the propane in a heat exchanger when circulating from one of the vessels to the source and back to the other vessel.

available within the "illuminated" sample cross section l)z,~ Dz,r, the angular divergency

Liquid propane circuit

---!con,to,_,

ax c~r = (O 1,x + O2, ~) (O 1,y + 02, ~,)/L2, and the wavelength interval A2. This current also depends on the following quantities: qi = thermal neutron flux at the reactor periphery, q~(~) = wavelength distribution, G ( ~ ) = gain factor of the cold neutron source, T(2) = transmission of the guide tube, r ( 2 ~ = transmission of the wavelength selector.

D2D1=. liquid . . propane . . . .dewar O 3 - emelgeney dewar for C~Ha

................ E =heat exchangar

[ ------ J

'~

'

e v : solenoid valves (t>~ o p e n , I~1 closed )

2.2. THE PROPANE COLD NEUTRONSOURCE Cold neutron sources have been installed in many reactors. They mainly use, as moderating substance, liquid hydrogen 7-15) or liquid deuterium 16-19). The use of hydrocarbons is connected with problems of radiolytic decomposition and was therefore excluded for reactor powers exceeding 500 kW 2 0 - - 2 3 ) . Though they do not offer high absolute gain factors, hydrocarbons have interesting advantages: a) minor safety problems, b) high gain-to-cost ratios. In the present case, the radiolysis problem was resolved by using a great amount of liquid propane ( ~ 50 1) in a closed circuit ( ~ 5 1/min) from which only

ENTRANCE S L

SAMPLE I ~

x1 i

DETECTOR SLITSD3,x

I

MC: moderator chamber

Reactor

N2

Tank

Fig. 3. Liquid propane cold source loop. 6.10

4

I

I

F

I

I

with

l

source cold s o u r c e

cold

without

-Io.

\

II1

.....

_z i

BIDIMENSIONAL MULTIDETECTOR 1°.

I~

L1

~

-- L 2

--

Fig. 2. Principle of the S.A.N.S. measurements.

J

I-.

rain.

A__i

pt~d

21 10I.

, °

8

t 9o

,

i '115

, RQ

o,

NEUTRON WAVELENGTH ACA)

Fig. 4. Neutron spectra at the exit of the guide tube.

c . P . GALOTTO et al.

372

Thickness optimization of the cold neutron source has been performed theoretically with a three-group treatment (fast-thermal--cold neutrons) in a two-layer moderator (warm water-cold propane), and experimentally in a mock-up by measuring neutron temperatures2). As a result, propane thicknesses in the range between 12-20 mm are virtually equivalent; the smaller thickness of 12 mm has been preferred because of its smaller heat dissipation. Fig. 4 shows two neutron spectra measured after the neutron guide tube, with and without cold propane (100K) in the source. The ratio of both intensity distributions defines the gain factor G(2), which is shown in fig. 5.

2.3. THE NEUTRONGUIDETUBE Totally reflecting guide tubes are now commonly used in neutron scattering devices24-26), and a detailed description of the design criteria is available27). Total reflection28) is obtained as in optics within the limiting angle 5o = x / [ 2 ( 1 - n ) ] = 2

x/(Na¢oh/2rO,

(5)

when the refraction index for neutrons (with respect to the vacuum) is n < 1. This is the case of most of the materials, as long as the coherent scattering length density Na¢~is positive. The important advantages of using curved neutron guides are: a) to lead the neutron beam away from the reactor with simultaneous shielding against the background of fast neutrons and 7-rays;

b) to achieve a certain wavelength preselection. As schematized in fig. 6 the guide tube used here has a rectangular cross-section, (ax=2 cm, ay= 10 cm), a length of L = 2 1 . 6 m and is slightly curved (p=493 m). It consists of mirror glas plates and has been optimized with the usual method 26'27) for the transmission of neutrons with 2 = 1 0 A , for which 5 o = 1 0 -2 rad. Its specific function is to conduct the neutrons which emerge from a surface of (ax+21o5o)(ay+21o5o) of the cold source and enter the tube, to the entrance slit D i, where they are available with a maximum angular divergency of 5~ = 5y = 2 50. The transmission of the guide tube, as defined by eq. (4), can be formulated by T(;t) = 7"1(2)

R"P,

(6)

where T1(2) = transmission without reflection and gravitational losses ( R = 1, P = 1), R = reflectivity, n = number of reflections, P = gravitational loss factor. Gravitational losses result to be rather negligible (P=0.99). The function T 1(2)R n, calculated with the usual C O L D NEUTRON SOURCE

~v

~

"

;ENTRANCE

I

L,T

Fig. 6. Guide tube schematization. bent radius 1.2

0 Z'

Ip-493m.

t u b e w i d t h a= 2 c m .

i 0.8

~

1t=o.94

0

I NEUTRON WAVELENGTH 6

10

lU

_

-

x= 1

X(~,) RO

0

4

8

12

16

20

WAVELENGTH A ( ~ )

Fig. 5. Intensity gain factor as function of the neutron wavelength.

Fig. 7. Guide tube transmissionfunctionvs neutron wavelength.

COLD NEUTRON SMALL ANGLE SCATTERING

~ ........ ~

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2

4

5

6

8

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10 11 12 NEUTRON WAVELENGTH ). ( ~ )

13

14

Fig. 8. N e u t r o n spectra at different positions o f the guide tube exit.

methods26), is shown in fig. 7. For the case R=0.94, its dependence on the x-coordinate of the crossI

I

I

I

0°5

0.6 x/a

0,7

0,8 i

o w i t h v a c u u m (..10"=tort) & with helium (~0,2 atm )

lo"

,._~..--"

10 s

0

0,1

0,2

0,3

0,4

0,9

Fig. 9. Total neutron intensity as function of the exit position x/a of the guide tube.

section has been examined. For low x/a values, the transmission mechanism is mainly determined by "zig-zag" reflections (from one plate to the other), for high values x/a " g a r l a n d " reflections (along the outer plate ~nly) are preferred. Fig. 8 shows a comparison between the spectra of neutrons emerging at different x/a positions. For the lower wavelengths, where neutrons have smaller reflection angles ~0, and especially for low x/a values where these neutrons should arrive at zig-zags, the transmission results are much lower than indicated by calculation. This (unexpected) cutoff at low wavelengths, although to be explained by more or less avoidable errors of abutment, curvature or alignment, offers the possibility of avoiding double Bragg scattering without a wavelength selector or a filter. For the longer wavelengths (2 > 9 A), a rather uni-

374

c.P.

GALOTTO

et al.

Fig. 10. Mechanical velocity selector.

form emission is obtained, not far from the expectations. The resulting emission profile of the integrated neutron intensity is reported in fig. 9, where the effect of He in the tube is also shown. The practical design of the neutron guide is based on the joining of precurved glass tube elements, 120 cm long, which are positioned inside 360 cm long vacuum tight steel tube elements. The glass tube aligment is adjustable externally. The steel tube elements are hung on a strong support beam (fig. 1) in such a way that the whole system can be regulated vertically to compensate for possible ground assessments (similar to those of the Pisa tower). 2.4. T H E WAVELENGHT SELECTOR* Wavelength selection is performed by a "helical slit type" selector, similar to the usual ones 29 31) consisting of a rotating cylinder with the axis parallel to the neutron beam and the helical slits incided along the surface. Instead of a full cylinder, single disks at suitable distances are used (fig. 10) and the slits are * This selector has been designed by C C R - E U R A T O M Va).

(Ispra-

formed by 258 blades of a neutron absorbing Mg-Cdalloy on each disk. Different disk configurations can be assembled in this way on the same shaft with different effective selector lengths L, so that transmission ~ and resolution A2/2 can be varied according to z = ZoO - ~ y L / 4 a ) ,

for

a/L _> ~r,

(7)

A2/2 = Vo/OoRo (0~y+ a/L), where ro = geometrical transmission (0.71), a = slit width (5 ram), Vo = neutron velocity (400 m/s at = 10 A), Ro = av. disk radius (287 mm), o3 = angular velocity. The practical layout of the selector is such, that for 4o = 10 A and , x = 10 -2 a resolution of A2/2=0.05 is obtained for Lmax = 640 m m (11 disks), and A2/2o = O. 1 for Lml n = 174 m m (3 disks). The speed of the rotor can be varied between 2500 and 10.000 rev/min with a precision of 1°/°°. An increase of the transmission as compared with that given in eq. (7) can be achieved in certain cases by placing totally reflecting guide tubes between the rotating disks2).

COLD NEUTRON

SMALL ANGLE

The total system (fig. 10) operates under vacuum (10 - 2 torr) so that special cooling- and refrigeration

systems were necessary in addition to a continuous monitoring of excessive vibrations. An external steelwood sandwich structure provides protection in the case of disintegration. The whole assembly, including the concrete base, is movable horizontally on rails into, and out of, the neutron beam. Presently a 5-disk configuration ( L = 3 1 0 m m ) is used; its resolution and its transmission as calculated with eqs. (7) and (8) (~y=2~o = 2 x 10 -2) are illustrated in fig. 11, together with the values measured by a time-of-flight technique. As can be noted from the figure, the measured transmission is somewhat lower than the expected values, especially at the lower wavelengths ( 2 < 1 0 A ) and for the wider entrance slit (D1 ,x = 10 mm). This is due to the earlier mentioned fact, that the transmission of the neutron guide, because of its tolerances, shows "cutoff" at the very small angles, which is more effective on the lower wavelengths. The intensity of the neutrons entering the wavelength selector is therefore not constant over the angular interval x y = 2 ~ o, as assumed in eq. (7), which, as a consequence, overestimates the transmission especially for the lower wavelengths.

z O

a

D1 = 4 m m .

a

D 1 : 1 0 mm.

SCATTERING

DEVICE

375

2 . 5 . TH E DETECTION SYSTEM

After collimation and wavelength selection the neutron beam reaches the sample (fig. 1). Positioning and scanning of the sample in a vacuum tight chamber can be performed from outside through horizontal and vertical movements. Between the sample chamber and the tube leading to the detector, a pneumatic valve is installed, so that the vacuum in the tube can be maintained whilst the sample is interchanged. The intensity of the scattered (and the transmitted) neutron beam is measured by a multicellular BF aproportional counter with 64 horizontal and 64 vertical cathodes, so that anyone of the 4096 cells has a crosssection of 1 cm by 1 cm *). This cross section forms the detector slits D 3 (fig. 2), unless decreased by Cd diaphragms. In the central part of the multidetector this is necessary in order to achieve, according to eq. (3), also for smaller x,y-values, a not too high resolution width of the scattering vector K: Evidently the single terms of eq. (3) should give equal contributions in an optimized case. Coming from the multidetector, the amplified signals are addressed, by means of a binary code (corresponding to the x,y-system), to a minicomputer, where they are accumulated. Whilst the accumulated bidimensional pulse distribution is continuously visualized, a first elaboration of the data can be performed on the minicomputer.

8

3. Applications

4

Small angle scattering is obtained from structural heterogeneities when they introduce local fluctuations Aq of the scattering length density t/

~1 = bN,

o 8

L

9

10

L

11

12

13

WEVELENGTH A (.*~)

o DI= 4mm. .a8 0

_ A Dl-lOmm.

with b = nuclear + magnetic scattering length, N = atomic density. Such fluctuations can be caused by both positional (AN) and/or compositional (Ab) deviations from the normal lattice periodicities of the matrix. The scattering amplitude A can be expressed by the Fourier transform of A~/, and the scattering cross section by

dS,~_ 1 AA*, dO V 7

8

9

10

11

12

I3

WAVELENGTH X ( X )

Fig. 11. R e s o l u t i o n and t r a n s m i s s i o n o f the wavelength selector.

* This m u l t i d e t e c t o r has been designed a nd built by C E A C E N G at Grenoble.

376

c.P.

GALOTTO et al.

TABLE 1 Neutron small angle scattering between radiography and bragg scattering

Radiography

Small angle scattering

Bragg scattering

Phenomena Angles of observation Heterogeneities

Transmission Zero Macroscopic flaws, etc.

Dimensions Parameters

106 .~. Single section and thickness Quality-control maintenance

Diffraction Small Precipitates,microvoids, dislocations l0 s A 104+ 10 A Concentration Concentration, dimensions, distances Control of heat treatments and degeneration processes, residual lifetime determination

X- and y-rays, neutrons

Neutrons, (X-rays)

Applications Radiation

Refraction Small Dispersed phases, etc.

Diffraction Wide Atoms ~5 A lnteratomic distance, crystallite dimensions

Analyses of phases, composition, anisotropy and stress X-rays, electrons, neutrons

where

3.2. DEGENERATION OF TURBINE BLADES

V = sample volume.

Small angle scattering from Ni-super-alloys, such as ! N C O N E L X-750 or I N C O N E L 700, is mainly due to its precipitated strengthening phase y' 32,33) and occurs in the form of spherical particles of the intermetallic compound Nia(Al, Ti). Peak strengths are achieved with these particles (as barriers against the moving dislocations), when their diameter is optimal (in the range of 100 to 1000 A and with a nucleation density of the order of 1016 c m - 3) 34). The evolution of the average particle diameter (which is thermodynamically unstable at high temperature and under load) is subjected to a non-destructive control. Fig. 12 shows the results in the case of precipitation heat treatment of I N C O N E L X-750 at 843°C. It is noted that the increase of the particle radius, as measured by neutron small angle scattering, follows the time law predicted by the classical theory of Lifshitz and Wagner35).

3.1. GENERALPOSITIONINGOF THE TECHNIQUE Small angle scattering, in view of its application in industrial metallurgy, can be positioned between two intensively used classical methods: radiography and Bragg scattering. As shown in table 1, in fact, the observation angles, the dimensions of the observable heterogeneities and the types of measurable parameters are to be placed intermediately. Also the kind of the prospected applications appears intermediate: small angle scattering suggests large possibilities of controlling degeneration processes at a state still far away from failure, by observing the early evolution of the responsible heterogeneities and defects. Whilst some of such information could be obtained in principle also from the line width of the Bragg reflections, radiography only indicates a component failure in the ultimate state, or of course a manufacture defect. The position so assigned recalls also the reasons for using neutrons instead of X-rays: on one side, in fact, a high wavelength is necessary in order to avoid (double) Bragg scattering, on the other side the used radiation should be as penetrating as for radiography. Both requirements exclude virtually the use of X-rays in a field where neutrons make it possible to fill an enormously large gap, especially in non-destructive testing. A few examples of practical interest are briefly illustrated here whilst references are given for more details.

T

1000

/

r J I J rJl I ' I I ~=95-t lj3

o<

t III 1

(T: 843°C)

50O 400

<

300

--

2OO

100 2

3

4

5

10

2

3

TIME

4

5

t (h)

10 z m,

Fig. 12. Gamma prime radius growth for INCONEL X-750 superalloys as function of the ageing time.

COLD NEUTRON

SMALL ANGLE

Treatment at 843°C (,,,24 h) is normally followed by a second treatment at 704°C ( ~ 2 0 h ) , which stabilizes the precipitate dimensions for a very long time. A further increase of the radius was obtained (fig. 13) beyond 16 000 h of operation of turbine blades in a power station, until failure occured at 65 000 h. It is interesting to note that no effect is obtained near the base of the blades, where the temperatures are low. In fig. 13 are also shown comparatively the results from electron microscopy in the case of an accelerated creep experiment a6) which indicate an analogous dimensional evolution. Similar effects are observed in aircraft turbine blades. As shown in fig. 14, neutron small angle scattering may indicate the dimensional evolution profile and therefore

SCATTERING

DEVICE

the part of the blade with the faster degeneration process (curves for O, 274, 394 and 912 h of operation). It may furthermore indicate the effect of a temperature excursion, as occurred in the blades with 51 and 395 h. 3.3. FATIGUE Fatigue in metals is closely connected with the formation and migration of dislocations. Variations of small angle scattering from such dislocations can therefore be used to characterize the fatigue process. Current theories of dislocation s c a t t e r i n g a7-39) allowed to calculate from scattering data the incremental dislocation density p for Ni samples stressed in tensional fatigue assuming that no other scattering i 6

10

t wit

I, o o

E i~l

i lil I i~ii .ADIUS

=/3 ;EiO.T

rl

II>u) ~_ 4 <[ n-

/Jl

l

t

"""

/'/

CREEP

tf

I

S,.MICR.

~

I

AGING

I

a43Oc n To4iOc

"2 am

~/

l

Y

J

1 o o

A

j//'

r

i

J ll/ , ,ooo

.As.

6

377

6

! 10

8

12

14

18

18

L/., ~ Fig. 15. Dislocation density evolution as function of the square of applied stress for fatigued polycrystalline nickel.

I III

I i I 10

i 102

1II

I III

103

I II 104

TIME

(h)

__

105 it'

AlSl 304 [STEEL_ A=IO~ Direct

Fig. 13. G a m m a prime particle evolution for I N C O N E L X-750 during ageing and service as turbine blades. Qualitative comparison with E. M. for N I M O N I C 80 A under creep.

1

A

#

,, - 1 3 ' 4 8 " ,, - 1 1 ' 1 2 "

80

=.

LU m < 60 m

u.

10 s

beam ASilO'

o W e l d e d zone __ n H.A.Z. i

(

m z

40

Z

--~ a 20

\ 274h 912 h

3 9 5 h S.T.

394 h

100

200 RADIUS OF GYRATION

300 ~'g ( ~ )

400

0

P

Fig. 14. Distribution of ~," particle dimensions in I N C O N E L 700 turbine blades at different service times and temperatures.

5 SCATTERING

10 ANGLE

15 • {rain.)

Fig. 16. Direct beam broadening from AISI 304 stainless steel for welded and heat affected zone.

378

c.P.

GALOTTO et al.

sensitive v a r i a t i o n s take place4°'41). T h e results are plotted in fig. 15 as a f u n c t i o n o f 0-2, the s q u a r e o f the applied p e r i o d i c p e a k stress; they all refer to law cycle n u m b e r s N with values still far f r o m r u p t u r e (5 + 10% o f lifetime). I n such a n early state o f fatigue, small angle scattering resulted i n d e p e n d e n t o f N. T h e results show that p is p r o p o r t i o n a l to 0 -2 a s theoretically predicted42), a n d the i n t e r c e p t value 0-= 24.5 k g / m m 2 (for p = 0 ) is in g o o d a g r e e m e n t with the fatigue limit o f Ni. 3.4. HEAT TREATMENT OF STEELS F l u c t u a t i o n s o f the n u c l e a r a n d m a g n e t i c scattering l e n g t h density are a c o n s e q u e n c e o f t h e r m a l a n d m e c h a n i c a l t r e a t m e n t o f steels. Austenitic, m a r t e n s i t i c a n d p r e c i p i t a t i o n h a r d e n i n g steels have b e e n m e a s u r e d after different t h e r m a l t r e a t m e n t s ; the observed scattering effects h a v e been e x p l a i n e d by n e u t r o n refraction 1'4a) which occurs with large particles with different index o f refraction f r o m the matrix. A n e x a m p l e is given in fig. 16 where the b r o a d e n i n g o f the direct b e a m o b t a i n e d f r o m " w e l d e d " a n d " h e a t affected z o n e " o f A I S I 304 stainless steel is shown*. This effect has b e e n i n t e r p r e t e d as ferrite particle refractions at a m e a n distance o f 20/~m, as c o n f i r m e d by electron microscopy. T h a n k s are due to the F I A T a n d C A M E N M a n a g e g e m e n t for p e r m i s s i o n to p u b l i s h this w o r k a n d in p a r t i c u l a r to D r O. M o n t a b o n e a n d A d m i r a l G. B. Giuliana. T h e e n t h u s i a s t i c c o o p e r a t i o n o f E. A n s a l d i , B. Giorgi, A. R a s t a l d o , P. L. T a r d i t i a n d P. M a r a c c h i o n i w h o helped in the s t a r t i n g u p o f the system a n d perf o r m i n g the experiments, is gratefully a c k n o w l e d g e d . T h e design o f the w a v e l e n g t h selector was d u e to A. B. Vincenzi. This w o r k was p a r t i a l l y s p o n s o r e d by the I t a l i a n National Research Council (C.N.R.).

References ~) W. Schmatz, T. Springer, J. Schelten and K. Ibel, J. Appl. Cryst. 7 (1974) 96. 2) H. Walther, P. Pizzi and A.B. Vincenzi, Un dispositivo per la diffrazione di neutroni freddi, Memoria presentata all' Giornate dell'Energia Nucleare 1970, Milano (10/12 December 1970); N. Cerullo and H. Walther, I1 dispositivo per esperienze con neutroni freddi presso il reattore G. Galilei del CAMEN, Estratto dagli atti del "Convegno sull'impiego dei neutroni freddi nello studio della materia", S. Piero a * Work performed in collaboration with CEA-CENG of Grenoble, France.

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