Position-sensitive detectors (P.S.D.) for neutron diffraction

NUCLEAR

INSTRUMENTS

AND

METHODS

I26

(I975)

29-42;

@) N O R T H - H O L L A N D

PUBLISHING

CO.

POSITION-SENSITIVE DETECTORS (P.S.D.) FOR N E U T R O N DIFFRACTION R, A L L E M A N D , J. B O U R D E L and E. R O U D A U T

Centre d't~tudes Nucldaires de Grenoble (CEN-G), B.P. 85 Centre de Tri 38041 Grenoble Cedex, France P. C O N V E R T , K. IBEL and J. J A C O B E

lnstitut Laue-Langevin (ILL), B.P. 156 Centre de Tri 38042 Grenoble Cedex, France J. P. C O T T O N and B. F A R N O U X

Centre d't~tudes Nucldaires de Saclay, B.P. 2 91190 Gif-sur- Yvette, France Received 14 February 1975 Several types of position-sensitive detectors for thermal neutrons in a gaseous environment (BFa, aHe), called multidetectors, have been designed, constructed, and tested on several neutrondiffraction experiments. They have made possible a considerable saving in time in relation to conventional diffractometers. The following instruments are described: - A curved multidetector with 400 cells: position-sensitive in a single dimension for the study of powder spectra; angular aperture: 8 0 , angular resolution: 12' in 20, radius o f curvature: 150 cm.

- A plane multidetector with 4096 cells: position-sensitive in two dimensions X, Y (64 x 64 cells at I 0 m m spacing), intended for small-angle scattering studies. - A plane multidetector permitting position sensing in polar coordinates: this consists o f 29 concentric circles at 10 m m spacing and 36 angular sectors. The main results obtained for each o f these are given. The technological solutions used have made it possible to achieve excellent stability o f the characteristics with time.

1. Introduction

in the case of an analysis with a multi-detector, the dimensions of the samples form a physical limit to the angular resolution. There is no advantage in the position resolution of the detector being better than the diameter of the samples. These are generally from several millimetres to 1 cm. Thus taking account of the angular sector to be explored (of the order of 80 120 °) and of the desirable angular resolution (approx. 10'), it seems that the multi-detectors should be large instruments.

Since intense neutron flux have been available, the techniques of scattering and diffraction of neutrons have become methods complementary to the scattering of X-rays. The conventional equipment comprises a diffractometer which explores the space to be analysed point by point, but this method requires long periods of measurement. The equipment described here, called a "multidetector", senses the position and measures the intensity of the neutron beams diffracted over the whole of the space to be explored. This results in a very considerable time saving and in simple operation, as the diagram is visible immediately. This equipment thus opens up prospects of new studies in the analysis of organic substances which deteriorate rapidly with time or under irradiation, and in the analysis of samples which can only be prepared in small quantities; they thus offer a new field of applications for low-power research reactors.

2.1. DETECTION METHOD

The evaporation of thin foils of l°B or 235U, for example, as a neutron-detection medium was not used, because the self-absorption of charged particles in the layer considerably limits the efficiency of detection and it is difficult to achieve homogeneous deposits over large areas. The scintillation systems (I°B-ZnS, 6Li-ZnS) are efficient as regards detection, but it is difficult to adapt them to large position-sensitivity detectors. Gaseous environments (~ He, I°BF3) have the advantage of good detection efficiency and of permitting the use of large detectors; 3He permits the use of pressures higher than BF 3, because the latter is more electronegative. In fact, this advantage cannot be fully utilised for large volumes, because technological difficulties in-

2. Choice of system of detection and of position sensing The choice of a method of detection must take account of the experimental conditions. In particular 29

30

R. A L L E M A N D

crease considerably when the pressure increases. We have therefore chosen to use I°BF3 which also has the advantage of being considerably less expensive than 3He. 2.2. METHOD OF OPERATION There is a choice between the method of direct collection of primary charge (pulse ionisation chamber) and the proportional system. The system of direct collection of charge is very flexible as regards the shape and dimensions of the position-sensitive electrodes, but has two disadvantages: 1) The signals received are of low amplitude. For the I°BF 3, the maximum charge which can be collected (Q ..... ) is of the order of 10 - 1 4 C. On the other hand, the noise of the charge-sensitive preamplifiers increases at the same time as the parasitic entry capacities. With multi-detectors, these reach several hundred pF, and as a result the signal-to-noise ratio does not exceed a few units. 2) The amplitude-distribution spectrum of the pulses is not very wide: for a chamber with plain, parallel electrodes, all the amplitudes from 0 to Q .... are equally probable. The physical results presented below are for detectors working in direct charge collection, but new instruments are operating on the proportional system. 2.3. THEORETICALPOSITIONALRESOLUTION In a gas detector, the theoretical position resolution is proportional to the track of the ionising particles (7Li and c( in the case of the reaction l°B(n,~)TLi), if the ionisation density is constant along their path. An ideal position detector, therefore, determines the centroid of the charge created by ionisation. Since the distribution in space of ionising particles is isotropic, the theoretical resolution is deduced directly from the projection of the centroid sphere onto the plane of the electrodes. At a pressure of 1 bar of BF 3 the limit of resolution in position is close to 2.5 m m (fwhm). 2.4. PRINCIPLE OF POSITIONSENSING The high number of detection cells (several hundreds to several thousands) makes it impossible to consider the simple juxtaposition in space of individual detectors. Whatever the method of operation (direct charge collection or proportional system), each nuclear event is detected simultaneously on two electrodes, and the position is sensed by the coincidence between two

et al.

signals. The electrodes are interconnected to form lines and columns of a matrix, each point of which represents one detection cell. An electronic analogue channel is associated with each line and each column, thus a multi-detector containing/7 2 cells is connected to 2n electronic channels1). In the direct charge-collection method, signals are received from the anode and cathode electrodes. In the proportional system, it is technologically more simple to divide the cathode for each cell into two parts and thus to achieve the position sensing by coincidence between the two signals induced on the cathodes. The geometrical arrangement of the electrodes is shown in fig. I (ref. 2). The spacing of the wires is thus identical to that of the cathodes. The avalanche process occurs in the immediate vicinity of the wire. The amplitude of the signals received on the cathodes is therefore proportional to the solid angle at which each cathode is seen from the point on the wire where the multiplication occurred. The position is sensed by the coincidence between the two cathode signals with the greatest amplitude3). The position resolution is limited along the X axis by the space between the wires. Along the Y axis the wires do not introduce any physical limitation. It is therefore possible to achieve the theoretical resolution if the electronics enables the centroid of the charges created by the avalanche effect to be determined, the centroid being found, for example, by interpolation from the amplitudes of the signals received on successive cathodes4). To increase the position resolution along the X axis, it is necessary to reduce the spacing between the wires.

induced

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wires plane

pulses

,

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y~Y3

IX- cathodes strips

Fig. 1. Principle of the position-sensing method in a proportional chamber. Position sensing is carried out on the basis of the coincidence between the two largest signals: X3 and Y.,.

POSITION-SENSITIVE

DETECTORS

The spacing between the cathodes is then greater than that between the wires, and the method of interpolation of signals can be applied again. 3. Linear multidetector with 400 cells

3.1. DESCRIPTION This instrument consists of a matrix of 2 0 x 2 0 elementary detectors arranged in a single container with BF 3 5). Fig. 2 shows the principle of electrode interconnections. The associated electronics include: - 40 analogue channels, each formed by a low-noise charge amplifier, followed by a second stage amplifier and a sensitive discriminator. A logical system providing for the coincidence and coding in position of each event. A diagram of the electronics is shown in fig. 3. Two prototypes have been produced and are at present in operation, the first in the neutron-diffraction laboratory at CEN/Grenoble, the second at the lnstitut Laue Langevin (ILL). Both of these operate by direct charge collection; an exploded view of the second detector, which, from a technological point of view, is an improved version of the first, is shown in fig. 4. The general characteristics of these instruments are as follows:

X Xl ~U

II I ~ 1

II I H - I I

ps of

/20anodes

X ~20groupsof II I---I---I I II t/H III t--I---I I / 2o cathodes

/

LNeutron position :(Xi,Yj) Fig. 2. 400-cell linear multidetector. Principle o f matrix interconnection o f the anode and cathode electrodes.

.I/interface Computer channels J'~ Coincidence dmt:lctl;r J~ 20 analogueV] logic unit I "Multichannel 20ancdogue[

k

channels J Fig. 3. 400-cell linear multidetector. installation.

analyser Block diagram

o f the

FOR NEUTRON

31

DIFFRACTION

CEN/G Prototype Number of cells Spacing between cells Angular opening (in 20) Radius of curvature Useful height of cells BF 3 filling pressure Useful thickness Detection efficiency (for 2 = 1.8 A) Operating voltage

ILL Prototype

400 5.2 mm

400 5.2 mm

80 ° 150 cm 7.5 cm 900 tort 1.5 cm

80 ~ 150 cm 2.5 cm 900 torr 11 cm

18% 1.5 kV

70% 2 kV

3.2. RESULTS6) 3.2.1. Response linearity Fig. 5 represents the response of 5 cells corresponding to the same cathode when the detector is moved behind a beam of monoenergetic neutrons 0.7 mm wide. The variation in the counting rate between the cells is less than 2%. The count-rate level corresponding to the base of each response curve is due to the scattering of the neutrons in the entry window (4 mm of aluminium) and particularly in the glass-fibre support of the electrodes. This curve was obtained with the C E N G prototype for which the neutrons have to pass through the electrode supports as shown in fig. 6. This effect was greatly reduced on the ILL prototype by the geometrical arrangement of the electrodes shown in fig. 4. Fig. 7 is the response of the detector cells to the level of the junction between two cathodes. The loss in efficiency observed is due to the loss of the events on the cathodes. The signals induced have in fact a smaller amplitude at this point than at the centre of the cathodes, and a certain number of events are eliminated by the amplitude discriminator. This systematic error is corrected by computer on the basis of a table of coefficients determined by calibration with samples which scatter virtually isotropically (vanadium and water). After correction the response linearity is better than 3%. 3.2.2. Accuracy of positioniny qf Bragy reflections The minimum angular resolution of a Bragg reflection is approximately 0.1 °. It is detected by at least two cells, which permits positioning to better than _+0.05°. In practice, for the majority of cases, it can be positioned to -t-0.02 ° by simple interpolations. Table 1 gives a comparison between the theoretical and

32

R. A L L E M A N D et al.

~12 Penetration joint of stainless \ ~ steel into a[Lrmn,~n \

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3. lO4

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5

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15

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25 Position ( mm )

Fig. 5. 400-cell multidetector: response of 5 adjacent cells to a c o l l i m a t e d ne ut ron be a m m o v i n g in relation to the detector. Beam width: 0.7 mm. Spacing between each me a s ure me nt : 0.5 ram.

POSITION-SENSITIVE

DETECTORS

FOR

NEUTRON

TABLE 1 Theoretical

h

1 2 3 4 3 4 5 4

k

1 2 1 0 3 2 I 4

and

/

TABLE 3

m e a s u r e d lattice s p a c i n g s for a g e r m a n i u m s a m p l e (,% = 1.2840 A). 0 (observed)

d (measured)

d (theoretical)

11-~335 18°73 22'~10 27°00 29'65 3377 3 6 15 39 9 0

3.267 2.0003 1.7055 1.4141 1.2977 1.1550 1.0883 1.0001

3.266 2.000 1.706 1.414 1.298 1.1547 1.0888 1.0000

I 0 1 0 1 2 1 0

33

DIFFRACTION

C o m p a r i s o n b e t w e e n the m e a s u r e m e n t s o f the s t r u c t u r e f a c t o r F o f an a l u m i n a s a m p l e t a k e n w i t h a 400-cell m u l t i d e t e c t o r a n d with a diffractometer.

Bragg reflection

0 1 2 0 0 1

1 1 2 1 2 2

I 2 2 2 2 3

0

F ° multidetector

10.72 14.70 17.39 18.10 21.88 23.84

0.05874-0.6% 0.06774-0.8% 0.1999+0.5% 1 ±0.1% 0.35494-0.4% 0.88204-0.2%

F ~ diffractometer

0.066312% 0.0663±3% not measured 1 ±0.6% 0.35994-2% 0.87704-0.8%

TABLE 2 M e a s u r e m e n t o f the s t r u c t u r e f a c t o r F o f a c o p p e r s a m p l e . h

k

l

0

l (measured)

I (standardised)

I (theoretical)

F2

I 2 2 3 2

1 0 2 l 2

1 0 0 1 2

17.98 20.89 30.31 36.21 38.13

2 407 500 1338000 1 374400 2 047 000 581 200

100 ±0.2 55.6 zE0.15 57.1 ± 0 . 2 5 85.024-0.30 24.15±0.30

100 56.30 57.17 85.45 23.95

2.3764- 5 2.3444- 7 2.371±11 2.3744- 9 2.3944-29

4.10 4

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s

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Cell n: 22

L

Position (ram)

Fig. 7. 400-cell m u l t i d e t e c t o r : r e s p o n s e o f s e v e r a l a d j a c e n t cells at the level o f the j u n c t i o n b e t w e e n 2 c a t h o d e s as a f u n c t i o n o f the m o v e m e n t s o f a c o l l i m a t e d n e u t r o n b e a m . B e a m w i d t h : 0.7 ram.

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BRAGG ANGLE

TIME

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SOLLER SLIDES

GIE.OXIDE POWDER DIAGRAM WITH GONIOIdETER

24

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19

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dr

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Fig. 8. Geoxide powder diagrams from diffractometer a n d multidetector.

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SOLLER SLIDES

(e)

BRAOG ANGLE

GE.OXIDE POWDER DIAGRAM WITH /.0()CELLS MULTIDETECTOR.

> Z

> tt-

4~

taJ

POSITION-SENSITIVE

D E T E C T O R S FOR N E U T R O N D I F F R A C T I O N

The factor R:

measured lattice spacing of germanium and indicates an accuracy of better than 10-3.

R = ~ {I(calculated)-l(observed)} /(calculated)

3.2.3. Measurement off intensities of Braq9 re[tections The analysis of spectra from different known samples (copper powder, international sample of alumina, alumina and CoNiP sample) has made it possible to deduce the structure factor (F) and to compare them with those measured with a conventional diffractometer.

is 1.5% with the nmltidetector and 3.7% with the diflYactometer. Thus, with an equivalent factor R, the time saved with the multidetector in comparison with the diffractometer is of the order of 15-20 times on the prototype and should reach 30-40 with the new model.

Table 2, for a sample of copper powder, shows that the intensity values measured are correct as the F z values are constant to within the measured statistical accuracy. Table 3 shows the comparison for alumina between the values of F 2 deduced from measurements made with the multidetector and with a conventional diffractometer, and shows the agreement between the results. The graphs in fig. 8 show the results obtained with the specimen of germanium-oxide powder, one using a conventional diffractometer in 25 hours, the other using the 400-cell multidetector in 2.5 hours.

4. Two-dimensional (X, Y) multidetector with 4096 cells This instrument is used at lnstitut Laue-Langevin for the study of small-angle neutron scattering. It operates by direct charge collection. A new detector of identical characteristics but operating on the proportional system is under test. 4.1. DESCRIPTION 7) It consists of a matrix of 64 x 64 elementary detectors arranged in a single welded container. Fig. 9 is an exploded view of the multidetector. Each cell is formed with a double detection space to improve

S Aturn~nium membrane of 1ram thickness

2 Low neutron absorbing gas . . . . . . . . . . . . . . . . to equatise pressore across the membrane 3 Gas det~ctlen vo(ume(BF3) 1 Structural w~r~ow of a[t,mr,~n~um 2.5 -nmthc~ness to P~ov Je mechanical rioidity

35

6

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Metat e[ectr.~£~es deposited on glass by PhotodemlcaL e~chng

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[

~J t nG I 9

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Sorrier jo~'~t bet,,*,LeenDI(ver L, ar'd staintess stee(

E[ectnca[ connecbons thrr~gh 9~ass meta seat

Fig. 9. Exploded view of 4096-cell multidetector.

36

R. A L L E M A N D

et al.

~E COUNTS for each position

6.104

5.10 4

II"N

Cell N*

'\ Cell N* 4

4 .104 "7 0

3.10 4

/

2.10 4

104

L

0 0

10

20

Position (mm)

~ ~_ 30

Fig. 10. (X, Y) multidetector: response o f 4 adjacent cells to a collimated neutron b e a m m o v i n g in relation to the detector. Beam width: 0.7 ram. Spacing between m e a s u r e m e n t s 1 m m .

efficiency. The entrance window has a double wall: this makes it possible to have a small thickness o f window (3 ram), and therefore to reduce the neutron scattering, while retaining very good mechanical characteristics. The principal characteristics are as follows: - n u m b e r of cells: 4096 (64 x 64), - spacing between cells: 10 mm, detection space: (2 x 10) mm, filling pressure: 900 torr of BF3, - operating voltage: 1500 V, - detection efficiency: 50% for 2 = 6 A, m a x i m u m counting rate: 5 x 104 c/s, - background noise: reactor shutdown: 1.5 c/hour per cell. -

-

-

which involves the elimination of some events by discrimination o f amplitude. The limiting effect of this is the measurement of a Bragg reflection and here no inconvenience is caused as in general the reflection is detected over several cells. 4.3. DESCRIPTION OF THE INSTALLATION AT INSTITUT LAUE-LANGEVIN

The instrument 8) (fig. 1 l) of a total length o f 80 m, is designed for the investigation of structures in the range of several tens to several thousand Angstroms, achieved by the analysis of the angular distribution of neutrons scattered into small angles. Curved neutron guides connect it with a cold source within the core o f

The associated electronics include 128 analogue channels. A logic system is provided for the coincidence and coding in position o f each event. 4.2. RESULTS OBTAINED The response curves of the different ceils to a collimated neutron beam are shown in fig. 10. The response linearity is g o o d as regards sensitivity and position. The neutron scattering at the entry window is represented by the counting rate appearing on the cells adjacent to the irradiated cell. There is an apparent loss of efficiency at the cathode junctions of approximately 5%. This loss is due to the reduction in amplitude of the cathode signals at this point, a reduction

Multidetector positions

Fig. 11. Perspective o f the n e u t r o n small-angle C a m e r a D I I at the High Flux reactor in Grenoble, situated in the external hall. Selection o f the a n g u l a r spread o f the p r i m a r y beam within the first 40 m in front o f the sample. Detector positions up to 40 m behind the sample.

POSITION-SENSITIVE

D E T E C T O R S FOR N E U T R O N

the reactor. The full distance of 40 m between monochromator and sample may be used for collimation for high-resolution work, but beam intensity and angular deviation of neutrons incident upon the sample can be changed by appropriate choice of the movable neutron guides. The detector is mounted in an evacuated tube of 40 m length with 5 possible positions: 40 m, 20 m, 10 m, 5 m, and 2 m. In addition, the sample may be positioned at distances of 0.66 m and 1.33 m in front of the detector. The instrument is coupled on-line to a computer for data acquisition and for apparatus control. A visualization unit permits the instantaneous observation of the pattern of scattered neutrons. A typical example of the use of the instrument is the scattering of proteins in solution9). One of the numerous proteins investigated so far is the low-density lipoprotein of human plasma 1o). A highly concentrated gel of the proteins gives rise to a pronounced interparticle effect, i.e., a depression of intensities at small angles and a ring corresponding to the mean interparticle distance. Fig. 12 shows the perspective of

DIFFRACTION

37

intensities of scattered neutrons, as seen on the live display of the visualization unit. Each point corresponds to one of the 64 × 64 channels of the detector. The primary beam has been suppressed by a beam catcher. One important field of application is that of lowangle diffraction studies. Experiments have been done with myeline, collagen, chromatin, and Tobacco Mosaic Virus (TMV). The T M V is rodlike and consists of a helical array of subunits. The particles can be induced to form oriented gels. The neutron experiments tt) aim at structural information on the base moiety position of the viral R N A and on molecular boundaries of protein subunits. Fig. 13 shows a contour map of the equatorial line which is due to the helical arrangement, using raw data without correction for the response. A logarithmic scale was chosen with a ratio of \/2 in intensity between neighbouring contours. The beam stop is placed in the centre, and the fiTst three maxima can be seen. This apparatus, which has been installed since about 1972, has permitted measurement of 14000 spectra during approximately 18 hours operation per day.

Fig. 12. Perspective of intcnsities on the live display o f the D I 1 instrument in Grenolzle. Each point corresponds to one o f the 64 x 64 channels of the detector. Scattering pattern of a concentrated gel of low-density lipoprotein.

38

R. A L L E M A N D I. i ~ .

et al.

[ Ir;Q.,m4l

o , lrt 2S-JU~?4 14:sIz31 N~PT. v

"It

Fig. 13. C o n t o u r m a p o f raw data. R a t i o ~ / 2 in intensity represented by n e i g h b o u r i n g contours. Diffraction pattern o f an oriented gel o f tobacco-mosaic virus.

5. PSD in polar coordinates (p, 0) In general systems, of which the atoms are correlated over considerable distances, give rise to a central isotropic scattering of neutrons the intensity I of which decreases in accordance with the equation:

I(p)~-- [p[-~,

with a>~ 1,

(1)

where p is the distance of the point under consideration from the centre of the multidetector. A PSD in the form of concentric rings centred on the incident beam is well suited to the study of such phenomena: On the one hand, the measured intensity J(p), which varies as: J(p)= pl(p), (ref. 2) introduces a

POSITION-SENSITIVE DETECTORS FOR NEUTRON DIFFRACTION

39

Fig. 14. View of the (p,O) electrodes of the multidetector. compensation in counting rate of the law of variation of I(p), which optimises the statistical accuracy of the measurements for a given time. - On the other hand, the geometry of the cells miniraises the angular divergence which would be introduced by reorganisation of the cells of a PSD space X, Y. The study of non-isotropic phenomena (or the verification of isotropy) makes it necessary at the same time to know the response by angular sectors. In particular it is interesting to be able to follow the development of an isotropic scattering towards an anisotropic scattering as a function of the stresses on the sample. Depending on whether the scattering is slightly or strongly anisotropic, the angular sectors are arranged on either side of two perpendicular axes (its anisotropic axes) or in relation to the axis of the counter. 5.1. DESCRIPTION12) This instrument which is in operation at the Centre d'Etudes Nucl6aires at Saclay consists of 29 concentric circles at 1 cm spacing and 36 angular sectors arranged 3 by 3. The mechanical part is identical with that of the multidetector described previously; only the electrodes shown in fig. 14 are different. The operation is by direct charge collection with a single detection space of 15 mm. The BF3 filling pressure is 900 torr, which gives a detection efficiency close to 30% for 2 = 4 A. The

background noise inherent in the equipment is of the order of I count per hour per cm 2. 5.2. RESULTS Figs. 15a and b are an example of results obtained with this equipment. They represent the intensity scattered by a sample of polyethylene labeled with deuterium, at different temperatures. Figs. 16a and b represent the intensity scattered by a polystyrene gel with the nodes labeled with deuterium. These samples had been previously studied with a conventional spectrometer13). 6. Technological design of multidetectors

Great importance was attached to the technology and the manufacturing processes in order to be able to guarantee their characteristics: long life, stability of performance, homogeneity of response between cells, accuracy of angular resolution, possibility of regeneration of filling gas, etc. 6.1. THE CONTAINERS These are made of pure aluminium (type A5 or A9). The structure is entirely welded, either by argon-arc or by electronic beam, depending on the shape of the sections. To permit any repairs or modifications the final welding of the container can be made and broken three or four times.

40

R. ALLEMAND et a!. l

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£

~6

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v

7. 4

~2 ._E

10

20

30 e(r-m)

~0

-Cl-

20

30 e(cm)

-b ~

Fig. 15. Intensity scattered by a deuterated-polyethylene sample for different temperatures. Average incident wavelength: 2m = 12 A, Distance from sample to counter: 5 m; (O) temperature T = 137 ~'C, ( 0 ) temperature T = 87 C , ( ~ ) background noise from a heavywater sample of the same thickness. (a) Curves produced by the multidetector. (b) Curves of the response of the multidetector corrected in accordance with the relationships (l) and (2) in section 5.

,3

A:

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-a_

Fig. 16. Intensity scattered by a polystyrene gel with the nodes labelled with deuterium. (O) Intensity scanered by the sample, (O) intensity scattered by an unmarked sample. Incident wavelength 2 = 6.0 A, distance from sample to counter 2.15 m. (a) Curves obtained with the mu[tidetector. (b) Curve obtained from the difference between the 2 curvcs in fig. 16a, corrected for the response of the multidetector.

6.2. ENTRY WINDOW T h i s is o f a l u m i n i u m . Its t h i c k n e s s v a r i e s f r o m 0.5 to 6 m m d e p e n d i n g o n t h e t y p e o f d e t e c t o r . F o r large i n s t r u m e n t s [ d e t e c t o r (X, Y) o r (p,0)], a s y s t e m w i t h t w o e n v e l o p e s (fig. 9) m a k e s it p o s s i b l e t o h a v e g o o d m e c h a n i c a l p r o p e r t i e s while retaining a small w i n d o w thickness.

The internal envelope (item 5 on fig. 9) is kept in pressure equilibrium between the useful volume of BF 3 and tile equilibrium volume filled with CO2 by means of a deformable metal blower. The convex external envelope (item 1 in fig. 9), which is practically nondeformable, absorbs pressure variation.

P O S I T I O N - S E N S I T I V E D E T E C T O R S FOR N E U T R O N D I F F R A C T I O N

6.3. THE ELECTRODES The electrode support is generally made of a sodiumcalcium glass without boron. The actual electrodes are made by metallic evaporation on glass under vacuum, followed by chemical photoetching. This method ensures great stability of the geometrical dimensions, and a good transparency to neutrons (fig. 14). For multidetectors using the proportional method, the wires are positioned and tensioned by leaf springs made of Cu-Be alloy or of Dilver obtained by chemical etching. This method makes it possible to achieve an excellent geometric accuracy. 6.4. ELECTRIC OUTLETS

These are made by glass-metal seals (glass Dilver P). They are mounted on the aluminium container with the aid of a hot-press-bonded aluminium-stainless-steel joint. 6.5. THE PUMPING APERTURE Its diameter is 80 mm. After removal of gas it is closed by an iron cap which plunges into an annulus of tin kept molten during closure. 6.6. THE DETECTIONGAS The quality of the enriched boron trifluoride (or in some cases 3He) is checked before and during filling. The quality criterion for BF 3, which is produced in the laboratory by desorption of a complex CaF2BF 3, is the resolution of the neutron peak on the pulse-height spectrum measured with a cylindrical test detector of diameter gZf 20 mm operating under a pressure of 400 tort. In such conditions the fwhm resolution should be better than 5%. A test detector is placed in the container of each multidetector to check the quality of the gas during the life of the instrument. 6.7. NEUTRON SHIELDING Apart from the external shielding which is necessary for eliminating the general ambient neutron background, cadmium shielding is placed inside the container on the opposite side from the entry window as regards useful volume, to eliminate the neutrons back-scattered by the rear surface of the container. The layer of cadmium is placed in a sheath of aluminium because of the high vapour pressure of cadmium. However, for multidetectors with a pressure-equalising region containing CO2 (fig. 9), the cadmium is placed within this space.

41

6.8. MATERIALS USED

These must be compatible with the filling gas and easily capable of being degassed. Possible materials are: aluminium, stainless steel, mild steel, nickel, glass and Teflon in small quantities. 6.9. CONDITIONSOF DEGASSINGAND FILLING After several masses of argon rinses, degassing generally takes place at 200"C over several days under secondary high vacuum. After cooling the quality of the vacuum in the container of the multidetector before the filling operation is close to 10 -8 torr. For multidetectors with a double-envelope facility, filling takes place synchronously between the two volumes with the help of a high-sensitivity differential manometer (sensitivity 0. I torr). 7. Conclusion

The technological methods used and the precautions taken during manufacture have made it possible to achieve excellent stability of characteristics as a function of time. No deterioration in performance (associated for example with pollution of the filling gas) has been observed during periods now amounting to two years. The results described in this paper were obtained with detectors operating by direct charge collection. This method of operation permits a high degree of freedom in the shape of the electrodes (curved shapes, structure in polar coordinates, etc.) but has the disadvantages already mentioned: a mediocre signal-tonoise ratio, and a pulse-height spectrum which is continuous and has no neutron peak. More recently, instruments operating by the proportional method and using the methods of position sensing described in this article have been designed and constructed in our laboratory, in particular two multidetectors of 400 cells each, and two two-dimensional multidetectors (X, Y) of 4096 cells. The physical results obtained will be published in the near future. Utilising the experience gained in fields of technology and production of different prototypes, our present studies are aimed at improving the resolution in position by the use of new position-sensing methods and more sophisticated electronic processing of the data. The authors wish to thank Prof. Bertaut, Prof. Maier-Leibnitz, Prof. Mossbauer and Mr Cordelle, Director of LET1, for having allowed and encouraged these studies. Their thanks are also due to Messrs. Axmann, Cribier, Gariod, and Jacrot, who have always

42

R. A L L E M A N D et al.

shown a great deal of interest in this field. They also wish to thank all the technicians who assisted in the construction of this equipment, particularly MMr. Benoit, Dumas, Ferruit, Rambaud, Rustique, Sermay and also to everybody of the technological group of CEN-G. They are very grateful to MMr Meardon and Gray for the English translation. References 1) R. Allemand, J. Jacobe and E. Roudaut, French Patent no. 148.589 (18 April 1968), Dispositif d6tecteur de neutrons (Position-sensitive neutron detector). 2) R. Allemand, C. Brey, J. J. Gagelin and M. Laval, Proc. 21st round table on Functional exploration by radioactive isotopes (Strasbourg, May 1970), D6tecteur stationnaire 5_ gaz pour la cartographie d'organes ~ partir d'6metteurs X ou 7' de faible 6nergie (Stationary gas detector for plotting organs on the basis o f low-energy X ov 7 emitters). ~) R. Allemand, C. Brey and J. 3acobe, Patent no. 6917042 (23 May 1969), Dispositif d6tecteur de localisation de rayonnement (Position-sensitive radiation detector). 4) G. Charpak, A. Jeavons, F. Sau[i and R. Stubbs, High accuracy measurements o f the centre o f gravity o f avalanches in proportional chambers, R a p p o r t C E R N 73-11 (September 1973).

5) L E T I / M C T E Note (30.11.70), Devis technique du multid6tecteur 400 cellules pour ILL (Technical specification for 400-cell multidetector for ILL), Note LETI no. 70-3050. 6) E. Roudaut, P. Convert, R. Allemand, J. Bourdel and J. Jacobe, L E T I / M C T E / N U Note d'6tude no. 72 (31.1.73), Caract6ristiques et essais physiques d'un multid6tecteur 400 cel[ules en diffraction neutronique (Physical characteristics and tests on a 400-cell multidetector for neutron diffraction). 7) L E T I / M C T E Note (2.8.70), Devis technique du multid6tecteur X, Y 5_ 4096 cellules pour ILL (Technical specification for 4096-ce[I X, Y multidetector for ILL), Note LETI no. 70-2080. 8) W. Schmatz, T. Springer, J. Schelten and K. Ibel, J. Appl. Cryst. 7 (1974) 96. 9) H. B. Stuhrmann, J. Appl. Cryst. 7 (1974) 173. 10) H. B. Stuhrmann, V. Luzzati, L. Mateu, C. Sardet, A. Tardieu, L. Aggerbeck and A. M. Scanu, to be published. 11) E. Mandelkow, K. C. Holmes, H. Fuess, B. Jacrot and K. lbel, unpublished results. a2) L E T I / M C T E note 70-1640 (16.6.70), Devis technique du mu[tid6tecteur (p,O) pour le C E N / S A C L A Y (Technical specification for (p,O) multidetector for C E N / S A C L A Y ) . 1.3) j, p. Cotton, B. Farnoux, G. Jannick, C. Picot and G. S. Summerfield, J. Polymer. Sci. (C) 42 (1973) 807. 14) j. Jacobe and P. Convert, Note interne ILL, Multicompteurs neutrons (ILL internal note: Neutron multidetectors) no. ST 71-140.