Strub, A.

Archive of European Integration

¡ρ 'ujk E U R 7 O O. e REPRINi ' · * ■ » & EUROPEAN ATOMIC ENERGY COMMUNITY ­ EURATOM im ûm^ m«tì LIGHT PIPES TO BE MOUNTED WITHOUT SILICON GREASE A. STRUB i«3Wj l i Λ BW iß Joint Nuclear Research Center Ispra Establishment ­ Italy Reactor Physics Department Reprinted from THE REVIEW OF SCIENTIFIC INSTRUMENTS Vol. 35, No. 10 ­ 1964 ^!&ζΝ*.ϋ., fffï;*"'tt',*'W ι il'mnftwrniffli PBI&'ÜP a" iffiBfl I? ii**ll ■ ■ This document was prepared under the sponsorship of the Commission of the European Atomic Energy Community (EURATOM). Neither the EURATOM Commission, its contractors nor any person acting on their behalf : bei! ÏÏ mwM una ISBØmmmWIm 0 — Make any warranty or representation, express or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this document, or that the use of any information, apparatus, method, or process disclosed in this document may not infringe privately owned rights; ° | p H ^ ^ ^ ^ ^ ^ ^ ^ ^ H 2° — Assume any liability with respect to the use of, or for damages resulting from the use of any information, apparatus, method or process disclosed in this document. .;·» Ætf.'ii»**»;.* y;3? lijft" This reprint is intended for restricted distribution only. It reproduces, by kind permission of the publisher, an article from "THE REVIEW OF SCIENTIFIC INSTRUMENTS", Vol. 35, No. 10 - 1964, 1268-1269. For further copies please apply to American Institute of Physics — 335 East 45 Street, New York 17, N.Y. (USA). m m ¡m Dieser Sonderdruck ist für eine beschränkte Verteilung bestimmt. Die Wiedergabe des vorliegenden in „THE REVIEW OF SCIENTIFIC INSTRUMENTS", Vol. 35, No. 10 - 1964, 1268-1269 erschienenen Aufsatzes erfolgt mit freundlicher Genehmigung des Herausgebers. Bestellungen weiterer Exemplare sind an American Institute of Phy-- 335 East 45 Street, New York 17, N.Y. (USA), zu richten Ce tiré-à-part est exclusivement destiné à une diffusion restreinte. Il reprend, avec l'aimable autorisation de l'éditeur, un article publié dans «THE REVIEW OF SCIENTIFIC INSTRUMENTS», Vol. 35, No. 10 - 1964, 1268-1269. Tout autre exemplaire de cet article doit être demandé à American Institute of Physics — 335 East 45 Street New York 17, N.Y. (USA). Questo estratto è destinato esclusivamente ad una diffusione limitata. Esso è stato riprodotto, per gentile concessione dell'Editore, da «THE REVIEW OF SCIENTIFIC INSTRUMENTS», Vol. 35, m No. 10 - 1964, 1268-1269. Ulteriori copie dell'articolo debbono essere richieste a American Institute of Physics — 335 East 45 Street, New York 17, N.Y. (USA). Deze overdruk is slechts voor beperkte verspreiding bestemd. Het artikel is met welwillende toestemming van de uitgever overgenomen uit „THE REVIEW OF SCIENTIFIC INSTRUMENTS", Vol. 35, No. 10 - 1964, 1268-1269. Meer exemplaren kunnen besteld worden bij American Institute of Physics — 335 East 45 Street, New York 17, N.Y. (USA). mm EUR 700.e R E P R I N T MATERIAL FOR MAKING RESILIENT LIGHT P I P E S TO BE MOUNTED WITHOUT SILICON GREASE by A. STRUB. European Atomic Energy Community - EURATOM. Joint Nuclear Research Center - Ispra Establishment (Italy). Reactor Physics Department. Reprinted from "The Review of Scientific Instruments", Vol. 35, No. 10 - 1964 - pp. 1208-1269. For a special neutron scintillation counter a light pipe material was required which could replace Plexiglas or Lucite. This material had to be elastic in order to make a good fit between a scintillation crystal and photomultiplier even when mechanical dimensions (distance, parallelism) were somewhat undefined. Furthermore, the use of silicon grease for making an optical seal was to be avoided. Silicoloid 201, an embedding compound for electronic components and circuits, fulfills these requirements. I t has a refractive index of 1.408 and a reasonable transmission for scintillation light wavelengths. In addition, it is easy to mold into any shape and it adheres well to glass, forming stable but removable seals without grease. EUR 700.e R E P R I N T MATERIAL FOR MAAING RESILIENT LIGHT P I P E S TO BE MOUNTED WITHOUT SILICON GREASE by A. STRUB. European Atomic Energy Community - EURATOM. Joint Nuclear Research Center - Ispra Establishment (Italy). Reactor Physics Department. Reprinted from "The Review of Scientific Instruments", Vol. 35, No. 10 - 1904 - pp. 1208-1209. For a special neutron scintillation counter a light pipe material was required which could replace Plexiglas or Lucite. This material had to be elastic in order to make a good fit between a scintillation crystal and photomultiplier even when mechanical dimensions (distance, parallelism) were somewhat undefined. Furthermore, the use of silicon grease for making an optical seal was to be avoided. Silicoloid 201, an embedding compound for electronic components and circuits, fulfills these requirements. I t has a refractive index of 1.408 and a reasonable transmission for scintillation light wavelengths. In addition, it is easy to mold into any shape and it adheres well to glass, forming stable but removable seals without grease. EUR 700.e R E P R I N T MATERIAL FOR MAKING RESILIENT LIGHT P I P E S TO BE MOUNTED WITHOUT SILICON GREASE by A. STRUB. European Atomic Energy Community - EURATOM. Joint Nuclear Research Center - Ispra Establishment (Italy). Reactor Physics Department. Reprinted from "The Review of Scientific Instruments", Vol. 35, No. 10 - 1904 - pp. 1208-1209. For a special neutron scintillation counter a light pipe material was required which could replace Plexiglas or Lucite. This material had to be elastic in order to make a good fit between a scintillation crystal and photomultiplier even when mechanical dimensions (distance, parallelism) were somewhat undefined. Furthermore, the use of silicon grease for making an optical seal was to be avoided. Silicoloid 201, an embedding compound for electronic components and circuits, fulfills these requirements. I t has a refractive index of 1.408 and a reasonable transmission for scintillation light wavelengths. In addition, it is easy to mold into any shape and it adheres well to glass, forming stable but removable seals without grease. Reprinted from T H E REVIEW OF SCIENTIFIC INSTRUMENTS, Vol. 35, No. 10, 1268­1269, October, 1964 Printed in U. S. A. Material for Making Resilient Light Pipes to be Mounted Without Silicon Grease A. STRUB Reactor Physics Department, Euratom CCR, Ispra, Italy (Received 30 April 1964; and in final form, 1 June 1964) For a special neutron scintillation counter a light pipe material was required which could replace Plexiglas or Lucite. This material had to be elastic in order to make a good fit between a scintillation crystal and photomulti­plier even when mechanical dimensions (distance, parallelism) were somewhat undefined. Furthermore, the use of silicon grease for making an optical seal was to be avoided. Silicoloid 201, an embedding compound for electronic components and circuits, fulfills these requirements. It has a refractive index of 1.408 and a reasonable transmission for scintillation light wavelengths. In addition, it is easy to mold into any shape and it adheres well to glass, forming stable but removable seals without grease. INTRODUCTION T70R a special application we have constructed ­*­ a large­surface neutron counter, using 20 photo­multipliers of 7.6­cm cathode diameter (Du Mont 6363), together with 20 6LiI(Eu) crystals, 2 mm thick and 7.6 cm in diameter. Because of the small thickness of the crystals and the considerable nonuniformity of photocathode sensitivity, conical Plexiglas light pipes were used to diffuse the scintillation light before it arrived at the phototubes (Fig. 1). Optical coupling between crystal, pipe, and photo­cathode was provided by silicon grease, following the well­known techniques. I t proved to be rather difficult, how­ever, to obtain perfect optical seals, for the following reasons : The scintillation assemblies were to be mounted so close to each other that it became impossible to observe visually whether the silicon seal was free of air inclusions. There was a non­negligible difference of length (±3mm) and of cathode parallelism from one phototube to another, and this led us to use a somewhat cumbersome ring assembly (Fig. 1) in which the different parts were held together by springs. The mechanical stress produced by these springs sometimes caused instabilities of the optical coupling with time and with temperature. Although the large­surface counter was built and used successfully, we decided later to try to fabricate improved light pipes using Silicoloid 201, a new transparent potting and embedding compound for electrical and electronic crystal rubber ball spring rubber. FIG. 1. Sectional λ­iew of a 7.6­cm­diam 6LiI(Eu) crystal mounted on a Du Mont 6363 multiplier using a conical Plexiglas light pipe in a light­tight housing. circuits which has been developed recently by ICI..1 This compound is supplied as a solventless liquid which, upon the addition of a special curing agent, quickly cures at room temperature to a completely stable and resilient solid with very good light transparency and electrical insulation properties. For our purposes the use of Silicoloid presents the follow­ing advantages: Silicoloid 201 can be molded into any desirable shape (no machining or polishing is necessary). The cured compound is elastic like a soft rubber, and it can thus be made to match other components having mechanical dimensions which are not subject to very close tolerances. Pieces of Silicoloid 201 show a good adhesion (air­tight fit) to glass and other clean dry sur­faces, which makes the use of silicon grease for optical couplings unnecessary. In spite of this adhesion power, the pieces can be removed without a special releasing agent and reset simply by applying them under slight pressure. OPTICAL PROPERTIES OF SILICOLOID 201 Silicoloid 201 has a refractive index of 1.408, which is nearly the same index as Plexiglas. The most important property for our purposes is, of course, the ability of Silicoloid 201 to transmit scintillation 1 -0.8 0.6-0.2 0 -Plexiglass 2 Silicoloid 2 c S-ll response of pholomulhpliGi—-1 1 cm-/ / J / ' / ' / ' / ' ¿> >-/— ^ ^--^^ ( / / ^ \ OPTICAL TRANSMISSION ¡r ^ I \ r \ \ \ \ \ \ \ V 6000 wavelength m Ä FIG. 2. Optical transmission versus wavelength, measured for 2­cm­thick samples of Plexiglas and Silicoloid 201. Dashed line shows typical S­ll response curve of a photocathode. imper i a l Chemical Industries Ltd., Nobel Division, Silicones Department, Stevenson, Ayrshire, Scotland. 1268 1269 L I G H T P I P E S „500-400-300 R = 15 % 200-100 80 120 relative pulse height FIG. 3. Amplitude spectrum obtained with thermal neutrons ir-radiating a 7.6-cm-diam and 2-mm-thick 6LiI(Eu) crystal, coupled to a Du Mont 6363 multiplier with one of the conical Silicoloid 201 light pipes. Peak width is 15%. light without prohibitive attenuation. As such data were not available, the light transmission was measured for some representative samples with a Zeiss refractometer. The results for 2-cm-thick samples of Plexiglas and of Silicoloid 201, plotted together with a classical S-ll photomultiplier response curve2 of a Du Mont 6363 tube, are shown in Fig. 2. From these results it can be seen that the difference in light transmission between Plexiglas and Silicoloid 201 is not very significant for the sample thicknesses (2 cm) and wavelengths (~4500 Å) considered. FABRICATION OF LIGHT PIPES After several trials with glass and plastic, we have con-structed a mold with aluminum which gave excellent results. This mold consists of a central part (the interior surface of which must be very well polished) and of two rings which serve to close the mold from the side with glass disks and rubber seals. Since escaping air bubbles during evacuation tend to expand the liquid, a conical reservoir was mounted on the top of the evacuation hole. The fabrication of a light pipe involves the following steps: (1) Mixing of the required quantity of Silicoloid 201 and of curing agent (0.25-0.5%) in a glass cup. (2) Air 2 See photomultiplier manuals of Du Mont or RCA. removal by evacuation in a desiccator with a water pump. (3) Filling of the mold. The mold must be absolutely clean and dry. (4) Air removal as above. This must be achieved before the liquid loses its flow characteristics, otherwise some air bubbles will remain in the mold. (5) Curing at ambient temperature for at least two and, pref-erably, three days. We have observed that heating the mold in order to accelerate curing sometimes leads to destruction of the glass disks. This is due to the expansion of Silicoloid 201 while curing. (6) Opening of the mold and releasing of the glass disks. This requires a certain skill and patience. Breaking of glass disks causes traces on the light pipe surface. In order to become familiar with the techniques, we have also tried to mold other shapes than the one suitable for our light pipes. A rod of about 2 cm in diameter and 20 cm long was made, using as a mold a glass tube the inner surface of which was clad with an aluminium foil. To allow for expansion during curing, the mold was first filled to a level about 1 cm less than the desired length of the rod. The last centimeter of liquid was added after two days in order to obtain a flat top surface. After curing, the glass tube was destroyed, and thus the rod could be released from the aluminum foil very easily. There was no evidence of the molding in two steps. RESULTS OBTAINED WITH A CONICAL LIGHT PIPE We have compared several conical light pipes made of Silicoloid 201 with similar light pipes made of Plexiglas. The pipes were placed between a Lil(Eu) crystal and a Du Mont 6363 multiplier. Silicon seals were used with the Plexiglas, while the Silicoloid pipes were set up without grease. In the latter case, we observed that only a slight mechanical pressure was necessary for removing all air inclusions. After applying the pressure for some minutes, the Silicoloid 201 cones remained in place and gave a good optical coupling after the mechanical pressure had been removed. The seals remained unaltered after several months of use in a portable scintillation probe. Figure 3 shows the pulse-height spectrum obtained with thermal neutrons and a 7.6-cm-diam, 2-mm-thick Lil(Eu) crystal coupled to the multiplier with one of the Silicoloid light pipes. The peak width is 15%, which represents an improvement over the 18% width measured with the same crystal and a Plexiglas pipe. We think that further interesting applications of Silicoloid light pipes are possible, especially in cases other than scintillation counting where light intensity is higher and where attenuation may, therefore, be tolerated. ACKNOWLEDGMENTS The author wishes to thank Dr. A. Baehr for having made the transmission measurements and A. Di Chiano for his assistance in fabricating and testing the light pipes. ;To disseminate knowledge is to disseminate prosperity — I mean | general prosperity and not individual riches — and with prosperity [disappears the greater part of the evil which is our heritage from \ darker times. Alfred Nobel CDNA00700ENC 

MATERIAL FOR MAKING RESILIENT LIGHT PIPES TO BE MOUNTED WITHOUT SILICON GREASE. EUR 700.e

¡ρ 
'ujk E U R 7 O O. e 
REPRINi 
' · * ■ » & 
EUROPEAN ATOMIC ENERGY COMMUNITY ­ EURATOM 
im 
ûm^ 
m«tì LIGHT PIPES TO BE MOUNTED WITHOUT 
SILICON GREASE 
A. STRUB 
i«3Wj 
l i 
Λ 
BW 
iß 
Joint Nuclear Research Center 
Ispra Establishment ­ Italy 
Reactor Physics Department 
Reprinted from 
THE REVIEW OF SCIENTIFIC INSTRUMENTS 
Vol. 35, No. 10 ­ 1964 
^!&ζΝ*.ϋ., 
fffï;*"'tt',*'W 
ι il'mnftwrniffli PBI&'ÜP a" 
iffiBfl I? 
ii**ll 
■ ■ 
This document was prepared under the sponsorship of the Commission of the European 
Atomic Energy Community (EURATOM). 
Neither the EURATOM Commission, its contractors nor any person acting on their behalf : bei! 
ÏÏ mwM una ISBØmmmWIm 0 — Make any warranty or representation, express or implied, with respect to the accuracy, 
completeness, or usefulness of the information contained in this document, or that the use 
of any information, apparatus, method, or process disclosed in this document may not 
infringe privately owned rights; ° | p H ^ ^ ^ ^ ^ ^ ^ ^ ^ H 
2° — Assume any liability with respect to the use of, or for damages resulting from the use of 
any information, apparatus, method or process disclosed in this document. 
.;·» Ætf.'ii»**»;.* y;3? lijft" 
This reprint is intended for restricted distribution only. It reproduces, 
by kind permission of the publisher, an article from "THE REVIEW 
OF SCIENTIFIC INSTRUMENTS", Vol. 35, No. 10 - 1964, 
1268-1269. For further copies please apply to American Institute of 
Physics — 335 East 45 Street, New York 17, N.Y. (USA). m m 
¡m 
Dieser Sonderdruck ist für eine beschränkte Verteilung bestimmt. Die 
Wiedergabe des vorliegenden in „THE REVIEW OF SCIENTIFIC 
INSTRUMENTS", Vol. 35, No. 10 - 1964, 1268-1269 erschienenen 
Aufsatzes erfolgt mit freundlicher Genehmigung des Herausgebers. 
Bestellungen weiterer Exemplare sind an American Institute of Phy-
- 335 East 45 Street, New York 17, N.Y. (USA), zu richten 
Ce tiré-à-part est exclusivement destiné à une diffusion restreinte. Il 
reprend, avec l'aimable autorisation de l'éditeur, un article publié dans 
«THE REVIEW OF SCIENTIFIC INSTRUMENTS», Vol. 35, 
No. 10 - 1964, 1268-1269. Tout autre exemplaire de cet article doit 
être demandé à American Institute of Physics — 335 East 45 Street 
New York 17, N.Y. (USA). 
Questo estratto è destinato esclusivamente ad una diffusione limitata. 
Esso è stato riprodotto, per gentile concessione dell'Editore, da 
«THE REVIEW OF SCIENTIFIC INSTRUMENTS», Vol. 35, 
m 
No. 10 - 1964, 1268-1269. Ulteriori copie dell'articolo debbono essere 
richieste a American Institute of Physics — 335 East 45 Street, 
New York 17, N.Y. (USA). 
Deze overdruk is slechts voor beperkte verspreiding bestemd. Het artikel 
is met welwillende toestemming van de uitgever overgenomen uit 
„THE REVIEW OF SCIENTIFIC INSTRUMENTS", Vol. 35, 
No. 10 - 1964, 1268-1269. Meer exemplaren kunnen besteld worden 
bij American Institute of Physics — 335 East 45 Street, New York 17, 
N.Y. (USA). 
mm 
EUR 700.e 
R E P R I N T 
MATERIAL FOR MAKING RESILIENT LIGHT P I P E S TO BE 
MOUNTED WITHOUT SILICON GREASE by A. STRUB. 
European Atomic Energy Community - EURATOM. 
Joint Nuclear Research Center - Ispra Establishment (Italy). 
Reactor Physics Department. 
Reprinted from "The Review of Scientific Instruments", 
Vol. 35, No. 10 - 1964 - pp. 1208-1269. 
For a special neutron scintillation counter a light pipe material was required 
which could replace Plexiglas or Lucite. This material had to be elastic in 
order to make a good fit between a scintillation crystal and photomultiplier 
even when mechanical dimensions (distance, parallelism) were somewhat 
undefined. Furthermore, the use of silicon grease for making an optical seal 
was to be avoided. Silicoloid 201, an embedding compound for electronic 
components and circuits, fulfills these requirements. I t has a refractive index 
of 1.408 and a reasonable transmission for scintillation light wavelengths. In 
addition, it is easy to mold into any shape and it adheres well to glass, forming 
stable but removable seals without grease. 
EUR 700.e 
R E P R I N T 
MATERIAL FOR MAAING RESILIENT LIGHT P I P E S TO BE 
MOUNTED WITHOUT SILICON GREASE by A. STRUB. 
European Atomic Energy Community - EURATOM. 
Joint Nuclear Research Center - Ispra Establishment (Italy). 
Reactor Physics Department. 
Reprinted from "The Review of Scientific Instruments", 
Vol. 35, No. 10 - 1904 - pp. 1208-1209. 
For a special neutron scintillation counter a light pipe material was required 
which could replace Plexiglas or Lucite. This material had to be elastic in 
order to make a good fit between a scintillation crystal and photomultiplier 
even when mechanical dimensions (distance, parallelism) were somewhat 
undefined. Furthermore, the use of silicon grease for making an optical seal 
was to be avoided. Silicoloid 201, an embedding compound for electronic 
components and circuits, fulfills these requirements. I t has a refractive index 
of 1.408 and a reasonable transmission for scintillation light wavelengths. In 
addition, it is easy to mold into any shape and it adheres well to glass, forming 
stable but removable seals without grease. 
EUR 700.e 
R E P R I N T 
MATERIAL FOR MAKING RESILIENT LIGHT P I P E S TO BE 
MOUNTED WITHOUT SILICON GREASE by A. STRUB. 
European Atomic Energy Community - EURATOM. 
Joint Nuclear Research Center - Ispra Establishment (Italy). 
Reactor Physics Department. 
Reprinted from "The Review of Scientific Instruments", 
Vol. 35, No. 10 - 1904 - pp. 1208-1209. 
For a special neutron scintillation counter a light pipe material was required 
which could replace Plexiglas or Lucite. This material had to be elastic in 
order to make a good fit between a scintillation crystal and photomultiplier 
even when mechanical dimensions (distance, parallelism) were somewhat 
undefined. Furthermore, the use of silicon grease for making an optical seal 
was to be avoided. Silicoloid 201, an embedding compound for electronic 
components and circuits, fulfills these requirements. I t has a refractive index 
of 1.408 and a reasonable transmission for scintillation light wavelengths. In 
addition, it is easy to mold into any shape and it adheres well to glass, forming 
stable but removable seals without grease. 

Reprinted from T H E REVIEW OF SCIENTIFIC INSTRUMENTS, Vol. 35, No. 10, 1268­1269, October, 1964 
Printed in U. S. A. 
Material for Making Resilient Light Pipes to be Mounted Without Silicon Grease 
A. STRUB 
Reactor Physics Department, Euratom CCR, Ispra, Italy 
(Received 30 April 1964; and in final form, 1 June 1964) 
For a special neutron scintillation counter a light pipe material was required which could replace Plexiglas or 
Lucite. This material had to be elastic in order to make a good fit between a scintillation crystal and photomulti­
plier even when mechanical dimensions (distance, parallelism) were somewhat undefined. Furthermore, the use of 
silicon grease for making an optical seal was to be avoided. Silicoloid 201, an embedding compound for electronic 
components and circuits, fulfills these requirements. It has a refractive index of 1.408 and a reasonable transmission 
for scintillation light wavelengths. In addition, it is easy to mold into any shape and it adheres well to glass, forming 
stable but removable seals without grease. 
INTRODUCTION 
T70R a special application we have constructed 
­*­ a large­surface neutron counter, using 20 photo­
multipliers of 7.6­cm cathode diameter (Du Mont 6363), 
together with 20 6LiI(Eu) crystals, 2 mm thick and 7.6 cm 
in diameter. 
Because of the small thickness of the crystals and the 
considerable nonuniformity of photocathode sensitivity, 
conical Plexiglas light pipes were used to diffuse the 
scintillation light before it arrived at the phototubes 
(Fig. 1). 
Optical coupling between crystal, pipe, and photo­
cathode was provided by silicon grease, following the well­
known techniques. I t proved to be rather difficult, how­
ever, to obtain perfect optical seals, for the following 
reasons : 
The scintillation assemblies were to be mounted so 
close to each other that it became impossible to observe 
visually whether the silicon seal was free of air inclusions. 
There was a non­negligible difference of length (±3mm) 
and of cathode parallelism from one phototube to another, 
and this led us to use a somewhat cumbersome ring 
assembly (Fig. 1) in which the different parts were held 
together by springs. The mechanical stress produced by 
these springs sometimes caused instabilities of the optical 
coupling with time and with temperature. 
Although the large­surface counter was built and used 
successfully, we decided later to try to fabricate improved 
light pipes using Silicoloid 201, a new transparent potting 
and embedding compound for electrical and electronic 
crystal rubber ball 
spring 
rubber. 
FIG. 1. Sectional λ­iew of a 7.6­cm­diam 6LiI(Eu) crystal mounted 
on a Du Mont 6363 multiplier using a conical Plexiglas light pipe in 
a light­tight housing. 
circuits which has been developed recently by ICI..1 This 
compound is supplied as a solventless liquid which, upon 
the addition of a special curing agent, quickly cures at 
room temperature to a completely stable and resilient 
solid with very good light transparency and electrical 
insulation properties. 
For our purposes the use of Silicoloid presents the follow­
ing advantages: Silicoloid 201 can be molded into any 
desirable shape (no machining or polishing is necessary). 
The cured compound is elastic like a soft rubber, and it 
can thus be made to match other components having 
mechanical dimensions which are not subject to very 
close tolerances. Pieces of Silicoloid 201 show a good 
adhesion (air­tight fit) to glass and other clean dry sur­
faces, which makes the use of silicon grease for optical 
couplings unnecessary. In spite of this adhesion power, 
the pieces can be removed without a special releasing agent 
and reset simply by applying them under slight pressure. 
OPTICAL PROPERTIES OF SILICOLOID 201 
Silicoloid 201 has a refractive index of 1.408, which is 
nearly the same index as Plexiglas. 
The most important property for our purposes is, of 
course, the ability of Silicoloid 201 to transmit scintillation 
1 -
0.8 
0.6-
0.2 
0 -
Plexiglass 2 
Silicoloid 2 c 
S-ll response of 
pholomulhpliGi—-
1 1 
cm-
/ / J 
/ ' / ' 
/ ' 
/ ' 
¿> >-/— ^ ^--^^ 
( / / ^ \ OPTICAL TRANSMISSION ¡r ^ 
I \ 
r \ 
\ 
\ \ \ 
\ 
\ 
V 6000 wavelength m Ä 
FIG. 2. Optical transmission versus wavelength, measured for 
2­cm­thick samples of Plexiglas and Silicoloid 201. Dashed line shows 
typical S­ll response curve of a photocathode. 
imper i a l Chemical Industries Ltd., Nobel Division, Silicones 
Department, Stevenson, Ayrshire, Scotland. 
1268 
1269 L I G H T P I P E S 
„500-
400-
300 
R = 15 % 
200-
100 
80 120 
relative pulse height 
FIG. 3. Amplitude spectrum obtained with thermal neutrons ir-
radiating a 7.6-cm-diam and 2-mm-thick 6LiI(Eu) crystal, coupled 
to a Du Mont 6363 multiplier with one of the conical Silicoloid 201 
light pipes. Peak width is 15%. 
light without prohibitive attenuation. As such data were 
not available, the light transmission was measured for 
some representative samples with a Zeiss refractometer. 
The results for 2-cm-thick samples of Plexiglas and of 
Silicoloid 201, plotted together with a classical S-ll 
photomultiplier response curve2 of a Du Mont 6363 tube, 
are shown in Fig. 2. 
From these results it can be seen that the difference in 
light transmission between Plexiglas and Silicoloid 201 is 
not very significant for the sample thicknesses (2 cm) and 
wavelengths (~4500 Å) considered. 
FABRICATION OF LIGHT PIPES 
After several trials with glass and plastic, we have con-
structed a mold with aluminum which gave excellent 
results. This mold consists of a central part (the interior 
surface of which must be very well polished) and of two 
rings which serve to close the mold from the side with glass 
disks and rubber seals. Since escaping air bubbles during 
evacuation tend to expand the liquid, a conical reservoir 
was mounted on the top of the evacuation hole. 
The fabrication of a light pipe involves the following 
steps: (1) Mixing of the required quantity of Silicoloid 
201 and of curing agent (0.25-0.5%) in a glass cup. (2) Air 
2 See photomultiplier manuals of Du Mont or RCA. 
removal by evacuation in a desiccator with a water pump. 
(3) Filling of the mold. The mold must be absolutely 
clean and dry. (4) Air removal as above. This must be 
achieved before the liquid loses its flow characteristics, 
otherwise some air bubbles will remain in the mold. (5) 
Curing at ambient temperature for at least two and, pref-
erably, three days. We have observed that heating the 
mold in order to accelerate curing sometimes leads to 
destruction of the glass disks. This is due to the expansion 
of Silicoloid 201 while curing. (6) Opening of the mold and 
releasing of the glass disks. This requires a certain skill and 
patience. Breaking of glass disks causes traces on the light 
pipe surface. 
In order to become familiar with the techniques, we 
have also tried to mold other shapes than the one suitable 
for our light pipes. 
A rod of about 2 cm in diameter and 20 cm long was 
made, using as a mold a glass tube the inner surface of 
which was clad with an aluminium foil. To allow for 
expansion during curing, the mold was first filled to a 
level about 1 cm less than the desired length of the rod. The 
last centimeter of liquid was added after two days in 
order to obtain a flat top surface. 
After curing, the glass tube was destroyed, and thus the 
rod could be released from the aluminum foil very easily. 
There was no evidence of the molding in two steps. 
RESULTS OBTAINED WITH A CONICAL LIGHT PIPE 
We have compared several conical light pipes made of 
Silicoloid 201 with similar light pipes made of Plexiglas. 
The pipes were placed between a Lil(Eu) crystal and a 
Du Mont 6363 multiplier. Silicon seals were used with 
the Plexiglas, while the Silicoloid pipes were set up without 
grease. In the latter case, we observed that only a slight 
mechanical pressure was necessary for removing all air 
inclusions. After applying the pressure for some minutes, 
the Silicoloid 201 cones remained in place and gave a good 
optical coupling after the mechanical pressure had been 
removed. The seals remained unaltered after several 
months of use in a portable scintillation probe. 
Figure 3 shows the pulse-height spectrum obtained with 
thermal neutrons and a 7.6-cm-diam, 2-mm-thick Lil(Eu) 
crystal coupled to the multiplier with one of the Silicoloid 
light pipes. The peak width is 15%, which represents an 
improvement over the 18% width measured with the same 
crystal and a Plexiglas pipe. 
We think that further interesting applications of 
Silicoloid light pipes are possible, especially in cases other 
than scintillation counting where light intensity is higher 
and where attenuation may, therefore, be tolerated. 
ACKNOWLEDGMENTS 
The author wishes to thank Dr. A. Baehr for having 
made the transmission measurements and A. Di Chiano for 
his assistance in fabricating and testing the light pipes. 
;To disseminate knowledge is to disseminate prosperity — I mean 
| general prosperity and not individual riches — and with prosperity 
[disappears the greater part of the evil which is our heritage from 
\ darker times. 
Alfred Nobel 
CDNA00700ENC 


MATERIAL FOR MAKING RESILIENT LIGHT PIPES TO BE MOUNTED WITHOUT SILICON GREASE. EUR 700.e

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