USE OF SiO2 AEROGELS WITH n = 1.05 - 1.25 AS CERENKOV DETECTORS

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USE OF SiO2 AEROGELS WITH n = 1.05 - 1.25 AS CERENKOV DETECTORS

I. Rasmussen

To cite this version:

I. Rasmussen. USE OF SiO2 AEROGELS WITH n = 1.05 - 1.25 AS CERENKOV DETECTORS.

Journal de Physique Colloques, 1989, 50 (C4), pp.C4-221-C4-226. �10.1051/jphyscol:1989436�. �jpa- 00229519�

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Colloque C4, Suppl6rnent au n 0 4 , Tome 24, a v r i l 1989

U S E OF S i O , AEROGELS WITH n = 1 . 0 5

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Danish Space Research Institute, Lundtoftevej 7 ; DK-2800 Lyngby, Denmark

R6sume : Nous presentons les etudes que nous avons real isees sur le developpement des detecteurs Cerenkov r6alises i partir d'aerogels dont l'indice de refraction varie entre 1,05 et 1,25. Notre conclusion est que ce materiau est parfaitement apte i 6tre utilise comme compteur Cerenkov de haute precision dans les futures missions spatiales dont le but est d'etudier les rayons cosmiques.

Abstract - We present here our work on the development of aerogel Cerenkov detectors with refractive indices in the range 1.05 - 1.25. Our conclusion is that this material is well suited for use in high precision Cerenkov counters on future space missions aimed at cosmic ray studies.

1 - INTRODUCTION

The Danish Space Research Institute has participated in many experimental investigations of the charge and mass composition of high energy cosmic rays. The cosmic rays contain important information about the synthesis of the elements in stellar interiors. In order to obtain this information it is necessary to perform precise measurements of the charge, mass and energy of the fully ionized nuclei constituting the cosmic rays. Near the earth - inside the magnetosphere - these measurements have to be performed at high kinetic energies primarily

in the range 0.5 - 10 GeV/nucleon.

We have, in collaboration with groups in France and the United States, developed experimental methods based upon the use of Cerenkov detectors (Refs. 1-4). From the formula

giving the number N of photo electrons (p.e.) produced by a particle of charge Z and momen- tum/nucleon p in terms of the number of p.e. produced by a singly charged relativistic par- ticle and the threshold momentum

where n is the refractive index of the Cerenkov material one can determine the momentum resolution of the measurement due to p.e. statistics

Using a measurement of momentum/nucl eon combined with e.g. either a rigidity or energy deter- mination to determine the mass composition, the precision of the momentum measurement must be

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989436

Light yield

/

F i g u r e 1. Cerenkov l i g h t y i e l d F i g u r e 2. Momentum r e s o l u t i o n

( c o n t r i b u t i o n s f r o m o t h e r sources than Cerenkov l i g h t are neglected).

The l i g h t y i e l d i s shown i n Fig. 1. The corresponding p r e c i s i o n i n momentum d e t e r m i n a t i o n i s shown i n F i g . 2. From t h i s i t can be seen t h a t i f we want t o have

we can use a d e t e c t o r w i t h a l i g h t y i e l d N(1,m) o f 10 photo e l e c t r o n s from t h r e s h o l d p, up t o 1.3 x p,. I f t h e d e t e c t o r produces 100 p.e. (and t h a t i s a h i g h l i g h t y i e l d ) i t can be used i n t h e range up t o 1.9 x p,.

refractive index

The narrow range o f h i g h p r e c i s i o n means t h a t i t i s v e r y important t o be a b l e t o have detec- t o r s w i t h c o r r e c t t h r e s h o l d . Figure 3 shows t h e t h r e s h o l d as a f u n c t i o n o f r e f r a c t i v e index.

I t can be seen t h a t t h e energy range o f i n t e r e s t corresponds t o a range 1.01 - 1.4 i n r e f r a c - t i v e index. T h i s range o f i n d i c e s can be obtained i n aerogels. From 1.01 - 1.05 by chemical methods (Ref.5) and above using h i g h temperature s i n t e r i n g .

Figure 3. The t h r e s h o l d i n momentum/nucleon and k i n e t i c energy/nucleon as a f u n c t i o n o f r e f r a c t i v e index.

As t h e cosmic r a y s a r r i v e i n a low i n t e n s i t y i s o t r o p i c f l u x , we a l s o need t o have l a r g e (> .I m2ster) d e t e c t o r s i n order t o c o l l e c t enough data. A t t h e Danish Space Research I n s t i - t u t e we have, t h e r e f o r e , developed methods t o c o n s t r u c t mosaics o f c a r e f u l l y machined blocks o f aerogel w i t h index s e l e c t e d f r e e l y i n t h e range 1.05 -1.25.

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2 - PRODUCTION OF HIGH INDEX AEROGELS

I n t h i s paper I w i l l concentrate on t h e p r o d u c t i o n o f SiO, aerogels w i t h r e f r a c t i v e index i n

1 .OO 1.05 1.10 1.15 1.20

10 -

I

t h e range 1.1 t o 1.25. These aerogels a r e produced by s i n t e r i n a b a s i c material with n = 1.05. This b a s i c material was obtained from t h e group i n Sweden ( )

S

The blocks a r e placed in a f a i r l y l a r g e (230 l i t r e ) k i l n and heated t o a temperature of 1000°C under microprocessor c o n t r o l . The standard heating curve used i s shown i n Figure 4.

To s i m p l i f y t h e process, we have always used t h e same temperature p r o f i l e and only t h e dura- t i o n a t plateau temperature ( e i t h e r 950°C o r 100O0C) has been v a r i a b l e . The r e l a t i o n s h i p bet- ween t h e d u r a t i o n a t plateau and t h e r e s u l t i n g r e f r a c t i v e index i s shown on Fig. 5.

Temperature OC Index

A T=plateau time A

1.25 -

1.05 1 1 1 1 ~ 1 1 ' 1 ' 1 1 1 +

0 20 40 6 0 Duration Hours 0 200 400 600 800 1000 1200

Minutes at plateau

Figure 4. Heating curve. Figure 5. Index a s a function of time a t max. temperature.

When we s i n t e r e d our f i r s t blocks, we encountered a problem: The blocks became curved. The g e l l i n g process i n Sweden (Ref. 5) t a k e s place i n shallow aluminium t r a y s , s o t h e aerogel ( o r more p r e c i s e l y t h e a l c o g e l ) has 5 s i d e s i n c o n t a c t with t h e t r a y and one s u r f a c e open t o t h e methanol. The g e l l i n g process obviously l e a v e s some mechanical t e n s i o n i n t h e block which causes them t o curve around t h e open s u r f a c e when heated.

The s o l u t i o n t o t h i s problem i s t o heat t h e blocks under p r e s s u r e . We place t h e blocks i n a ceramic ( s i l i m a n i t ) box with a f l o a t i n g l i d . By applying a p r e s s u r e of between 8 and 11 g/cm2 we o b t a i n f l a t blocks. Below 8 g/cm2 t h e blocks l i f t t h e l i d . Above 11 g/cm2, t h e f r i c t i o n from t h e ceramic prevents t h e c o n t r a c t i o n of t h e blocks and they crack i n t o several pieces

.

After t h e s i n t e r i n g process we have t o machine t h e blocks t o remove t h e remaining d e v i a t i o n s from t h e d e s i r e d r e c t a n g u l a r shape. I f t h e blocks a r e t o be used i n high p r e c i s i o n measure- ments t h e path l e n g t h of t h e p a r t i c l e s must be c o n t r o l l e d t o b e t t e r than 1%. We t h e r e f o r e machine t h e p i e c e s t o a square shape e i t h e r 140x140~20 mm3 o r 120x120~18 mm3 with a t o l e r a n c e of

-

Although t h e high d e n s i t y aerogel material i s strong - i t can withstand bulk pressures of several hundred kg/cm2 - i t i s a l s o very hard and b r i t t l e and t h u s d i f f i c u l t t o f i x f o r mechanical treatment. We have b u i l t a s p e c i a l vacuum t o o l t o hold t h e blocks while t h e s u r f a - ces a r e machined with a f l y - c u t t i n g technique. The c u t t i n g t o o l s used a r e worn out very quickly a s l u b r i c a n t s cannot be used due t o t h e high absorption i n t h e aerogel material.

This a l s o means t h a t c a r e must be taken t o control t h e aerogel d u s t produced during t h e machining.

(I' A i r g l a s s A/B, Box 150, 24500 S t a f f a n s t o r p , Sweden.

3 - SELECTION OF INDEX

As i n d i c a t e d i n Fig. 4, t h e s i n t e r i n g process i s n o t very w e l l determined. Although t h e c o r r e l a t i o n between p l a t e a u time and r e s u l t i n g index i s monotonic t h e r e i s a l a r g e spread i n the i n d i c e s o f blocks produced under i d e n t i c a l c o n d i t i o n s . Even blocks from t h e same r u n show t y p i c a l v a r i a t i o n s o f An = t .01. T h i s could o f course be caused by d i f f e r e n t c o n d i t i o n s a t d i f f e r e n t places i n t h e k i l n . However, no such c o r r e l a t i o n was found.

It i s t h e r e f o r e important t o have an easy method o f s e l e c t i n g blocks w i t h t h e c o r r e c t index.

The l a s e r d e f l e c t i o n method used a t lower i n d i c e s does n o t work a t h i g h e r i n d i c e s . The op- t i c a l appearance o f t h e blocks can b e t t e r be described as t r a n s l u c e n t than as transparant.

However, as we machine t h e blocks t o t i g h t t o l e r a n c e s we can o b t a i n a p r e c i s e estimate o f t h e i r d e n s i t y p from t h e i r weight.

Using the standard r e l a t i o n

v a l i d f o r amorphous s i l i c a aerogels we can then determine t h e r e f r a c t i v e index

.

+

Refractive index 1.25 A

1

0 .25 .50 .75 1.00 Density g/crn3

F i g u r e 6. Index o f r e f r a c t i o n as f u n c t i o n F i g u r e 7. L i g h t y i e l d contours. The num-

o f aerogel d e n s i t y . b e r s i n d i c a t e decreasing y i e l d

i n %.

4 - UNIFORMITY OF THE MATERIAL

Two kinds of n o n - u n i f o r m i t i e s have t o be considered, a) l a r g e - s c a l e v a r i a t i o n s which can be c o r r e c t e d i f t h e y are n o t t o o b i g and r a p i d l y changing and b) s m a l l - s c a l e v a r i a t i o n s which appear as f l u c t u a t i o n s i n t h e s i g n a l .

Fig. 7 shows t h e r e s u l t s o f a c a l i b r a t i o n o f 3 blocks stacked t o g i v e a 6 cm pathlength.

Over a l a r g e p a r t o f t h e area t h e v a r i a t i o n s a r e l e s s than l%/cm which can be c o r r e c t e d f o r even w i t h a moderate p r e c i s i o n i n p o s i t i o n determinations.

A t t h e edges, t h e v a r i a t i o n s are more r a p i d and t h i s area must be discarded when h i g h p r e c i - s i o n i s needed. The v a r i a t i o n s a r e assumed t o be due t o index v a r i a t i o n s . These v a r i a t i o n s are already present i n t h e 1.05 m a t e r i a l and are n o t s i g n i f i c a n t l y enlarged. The hexagon blocks used i n t h e HEAO-C2 experiment (Ref. 2) show s i m i l a r l i g h t y i e l d v a r i a t i o n s . It i s expected t h a t l a r g e r b l o c k s would g i v e a more u n i f o r m area.

With respect to the hall-scale variations analysis of the same 6 cm block (Ref. 4) shows that the small-scale index variations are below An = 0.00025. This upper limit is determined by assigning to index variations all fluctuations not described by known mechanisms.

The uniformity of the high index SiO, - aerogels is thus good enough to allow high precision measurements.

5 - LIGHT YIELDS

If aerogels should be usable in high precision Cerenkov detectors they must have a reaso- nably high light yield and the light must predominantly be produced by the Cerenkov mecha- nism.

From our analysis of the accelerator runs we can place an upper limit on the scintillation contribution to the light of 1%.

In the diffusion box used both in our HEAO satellite experiment (Ref. 2) and in our HEIST balloon instrument (Ref, 3) we have light yields from aerogel? with index 1.015, 1.053 and 1.10. Calculating the figure of merit pe/(thickness x (1-n- ) ) we get, respectively, 20, 25 and 23. It can thus be seen that the sintering process does not strongly affect the amount of light produced in the material although the visual appearance is greatly changed.

As the figure of merit indicates, there is some absorption of light in high index aerogels.

We are at present analyzing a series of measurements to determine this effect more precisely.

One more problem affecting the light yield deserves to be mentioned. The large surface area of the material makes it very easily affected by contamination. We have observed large decre- ases in 1 ight yield as a function of time of the order of 1-2%/month (private communication R.A. Mewaldt). The light decrease is much larger than observed in other experiments (Ref. 6) as well as being in marked contrast to the stability observed in space (Ref. 7). It remains to be seen whether the more rapid change is caused by using BaSO, instead of millipore or whether the sintering process makes the aerogel more sensitive to contamination.

6 - CONCLUSIONS

The main conclusions from this study are:

- it is possible to produce Cerenkov counters with any needed refractive index between 1.05 and 1.25,

- the uniformity of the material is good enough to allow the determination of particle momenta with the high precision needed in studies of the mass composition of cosmic rays, and

- the aerogel blocks are stable, strong and easy to handle in particular in a low contamina- tion space environment.

We plan to continue this work by trying to use larger pieces of aerogel material as a basis for the sintering process. It is our hope that this will allow us to produce blocks where the variations in index of refraction can be reduced considerably. Larger blocks will also mean fewer interfaces in the mosaic counters, thereby improving the data collection efficien- cy.

We also plan a more detailed study of the sintering process by analyzing aerogel material of different densities using S.A.X.S. and Scanning Tunneling Microscope techniques. This should lead to a better understanding of the level of uniformity in the bulk material which can be achieved with sintering methods.

7. - ACKNOWLEDGMENTS

Many people have contributed to the work described in this paper. I would in particular like to thank S. Laursen for help in building up the facilities and during tests of the materials in accelerator runs and balloon flights. I would also like to thank my collaborators at Caltech, in particular A. Buffington, J.E. Grove, R.A. Mewaldt, S. Schindler and E.C. Stone for help in testing and analyzing the performance of the aerogels at the Bevalac and in balloon flights.

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REFERENCES

/1/ Cantin, M., Engelmann, J.J., Koch, L., Masse, P., Lund, N. and Byrnak, B., Proc. 14th I n t e r n a t i o n a l Cosmic Ray Conference, Munich, 9, 3188 (1975).

/2/ Bouffard, M., Engelmann, J.J., Koch, L., Soutoul, A., Lund, N., Peters, B. and Rasmussen, I .L., Astrophysics and Space Sciences 84, 3 (1982).

/3/ B u f f i n g t o n , A., Lau, K., Laursen, S., Rasmussen, I.L., Schindler, S.M. and Stone, E.C., Proc. 18th I n t e r n a t i o n a l Cosmic Ray Conference, Bangalore, 2, 49 (1983).

/4/ Rasmussen, I .L., Laursen, S., B u f f i n g t o n , A. and Schindler, S.M., Proc. 18th I n t e r n a t i o - nal Cosmic Ray Conference, Bangalore, 8, 77 (1983).

/5/ Henning, S., Thesis, U n i v e r s i t y o f Lund, Lund, Sweden, 1979.

/6/ Poetz, G., i n Aerogels, Proc. o f 1 s t I n t e r n a t i o n a l Symposium,

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