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A silicon rubber scintillation compound for complex geometry radiation detectors
N U C L E A R I N S T R U M E N T S AND METHODS
IO7
(I973) 3 3 3 - 3 3 5 ;
© N O R T H - H O L L A N D P U B L I S H I N G CO.
A SILICON RUBBER S C I N T I L L A T I O N C O M P O U N D FOR C O M P L E X G E O M E T R Y RADIATION D E T E C T O R S A. BUSSINI, A. J. DEAN, R. DI RAFFAELE, G. G E R A R D I and A. IGIUNI*
Laboratorio di Fisica Cosmica e Tecnologie Relative, Consiglio Nazionale delle Ricerche, Milano-Palermo, Italy Received 3 October 1972 A scintillation compound which cures at room temperature is described. The scintillation efficiency and optical transmission for different compounds is discussed.
I. Introduction
2. Scintillator efficiency
In order to reduce the number of background events initiated by gamma ray induced electromagnetic showers in the material of a balloon borne low energy (1-10) MeV gamma ray astronomical telescope1), a semi-active shielding system was found to be necessary. This consisted of a sandwich arrangement which was constructed from alternate coaxial cylinders of scintillation material and lead. Due to their dimensions (approximatly 0.5 m diameter, 1 m long, wall thickness 1 cm) and multiplicity (seven), it was considered advantageous to develop a different constructional technique than that of the use of plastic scintillators, which, for counters of this geometry, is expensive and fragile with respect to surface and volume cracks. For large " t h i n " scintillators light collection by total internal reflection is the most efficient method2'a). The resulting scintillator must therefore have highly reflecting surfaces and be very transparent to the scintillation light. With this problem in mind two coaxial perspex cylinders of wall thickess 3 mm separated by a distance of 6 mm were constructed. The space between the two cylinders was filled with a scintillation compound of high efficiency and transparency. The counters were viewed via light guides by photomultipliers situated at the two ends of the cylinders. Since these detectors were for application in a non-pressurized gondola, a liquid, with associated risks of leakage, could not be used. Thus a scintillation compound was developed, which would solidify at room temperature and did not chemically attack perspex. Several possibilities were investigated, the most acceptable of which is discussed here in detail, and consisted of a mixture of commercially available NE224 liquid scintillator (from Nuclear Enterprises) and transparent silicon rubbers.
The scintillation efficiencies of the various compounds were measured in small identical perspex cells. The cell was placed inside a diffusive light guide above a Philips 54-AVP 5" photomultiplier (fig. 1). A 137Cs gamma ray source was used to activate the scintillator and the pulses analysed in a Laben 1024 multichannel analyser. The scintillation efficiency, with respect to the Sll photocathode of the photomultiplier used, was considered proportional to the channel number of the half height of the Compton edge. Care was taken to keep the experimental conditions constant during each measurement series. Pulses from a piece of Nuclear Enterprises NE-102A plastic scintillator served as a reference. The measurements taken in this way were found to be reproducible to about 1% accuracy. The scintillation efficiency was measured as a function of the concentration of NE224 and with respect to the quantity of hardener used to solidify the resulting compound.
* ]stituto Nazionale Fisica Nucleare, Via Celoria 16, Milano, Italy.
Fig. 1. Experimental method for measuring the relative scintillation efficiency.
333
I
• ~
CS 137 S RAY SOURCE SAMPLE CELL WHITE PAINTED DIFFUSIVE LIGHT GUIDE
I
~ TO
PIIA
334
A. B U S S I N I et al.
Fig. 2 shows the variation of the scintillation efficiency with the concentration, by volume, of NE224 for two values of the quantity of catalyst employed. Also shown is the efficiency of the pure liquid scintillator with 80% anthracene and the curve should the NE224 be mixed with an inert transparent solution. It is noticed that for higher NE224 concentrations the experimental curves lie progressively below the uncontaminated value; this effect is more pronounced with a larger quantity of hardener. The degradation of the scintillation efficiency is probably a quenching effect of the catalyst on the NE224 as, although the light absorbtion is slightly dependent on the quantity of hardener, in a small cell of a few cubic centimeters this effect would not be important. When the fraction of NE224 was taken below about 50% a deposit was formed upon solidification which rendered the compound of little practical use. These lower concentrations were not considered as they are less efficient for the production of light, the silicon rubber costs more by volume than the liquid scintillator and the NE224 is the more transparent component to the scintillation light. Fig. 3 shows the effect of the quantity of hardener on the scintillation efficiency for several different NE224 concentrations. In each case it is seen that the scintillation efficiency is reduced with an increased quantity of hardener. The magnitude of the effect is more pronounced for a higher NE224/silicon rubber ratio. Thus at lower NE224 concentrations the quantity of hardener used is less critical and enables a relatively hard compound to be made which sets in a few hours. For large fractions of NE224 the quantity of catalyst used is
600
"1'1
x 40ROPS/30 c.c. total of CATALYST • 8 DROPS/ 30 r.c. total of CATALYST - - - - - N E 224 +INERT TRANSPARENT LIQUID
500
a
I
'1'1'1'1"1'1
important required.
if the
highest efficiency scintillator
is
3. Optical transmission
The optical transmission of a representative compound is shown in fig. 4 as a function of wavelength. The transmission was measured in a Cary-14 Recording Spectrophotometer using standard perspex cells which presented two centimeters of the compound in the beam of light. The short wavelength cut off is due to the liquid scintillator. The peak of the emission of the scintillation light is at 4250 An). The calculated attenuation lengths at this wavelength were of the order of 2 m; in fact little different from that of the pure liquid scintillator for high NE224 concentrations. The exact values of the attenuation lengths were difficult to
,,,,
400
I,
i I ll,,,,l,, , 70 ~o
HE 2 2 4
30 ~RTV
602
_
• 80~o
NE224
20~RTV
602
_
•
HE224
lO,~'flTV602
90~o
_
o
~
-
35E
=
a.
300
,,,,
I,,,,I,,It
5
ll,-
10
15
DROPS/30 c~. TOTAL COMPOUNO
Fig. 3. Variation o f scintillation efficiency with quantity of hardener for different solutions ( R T V - 6 0 2 . silicon rubber). i
I
I
I
''1''''1''''1'
1.(3
40C
~30C z
0.5 20C
m.
loc
I
1,1,1,1,1
90
80
70
60
50
,I, 40
HE 2 2 4 / , T V .
,I 30
I
20
602
Fig. 2. Scintillation efficiency vs concentration o f NE224 for two quantities o f catalyst. T h e silicon rubber used here is RTV-602 from the General Electric C o m p a n y .
3500
I ,
,
,Jl
4000
,
,
,
I I ,
,
4500 WAVELENGTH (A)
,
,
I ,
5000
J
Fig. 4. Optical transmission o f a representative c o m p o u n d (80% NE224; 4 drops hardener/30 cm ~ total c o m p o u n d ; 2 cm thick). Corrected for reflection losses.
SILICON RUBBER S C I N T I L L A T I O N C O M P O U N D
calculate since the absorption in the 2 cm sample was very low. There are two points of interest which arise from the relative values of the transparency. The first of these is the fact that the transmission was found to decrease slightly with an increasing quantity of catalyst, and was probably due to absorption in the hardener itself which was a yellowish liquid. The transparency was reduced with decreasing NE224 concentration and may be explained as the absorption in the silicon rubber which was the less transparent of the two components at wavelengths greater than 4000 ,~. The transparency of the various compounds, however, compares favourably with commercial plastic scintillators. 4. Production details The NE224 and the silicon rubber were mixed together under clean conditions, the hardener was then added and thoroughly stirred in. The resulting liquid was of very low viscosity, almost like water, and solidified in a period more dependent on the amount of rubber than that of the hardener. Using the commercially suggested quantity of catalyst for the total volume, the solidification period varied from hours at 50% to days at 90% NE224 for a temperature of 20°C. An excessive quantity of hardener prevented final setting. The hardness of the resultant scintillation compound depended on the quantity of silicon rubber used but was not very different from the pure silastic even with 90% NE224 by volume. Several types of commercially available silicon rubbers were tested: Emerson & Cuming "Eccosil 2 C N " , I.C.I. "Silicoid 201 ", General Electric " R T V 602". The results obtained showed small variation from one sample to another. 5. Applications This type of scintillator is ideal for forming scintillators of complex geometry as it has a relatively long life in a liquid state of low viscosity. Thus there is no great problem for the removal of air bubbles. The
335
mixture solidifies at room temperature so that an oven is not required. It has a high scintillation efficiency and very good transparency. The above mentioned factors together with a refractive index close to 1.5 make it ideal for use with perspex containers in which total internal reflection may be used for the collection of the scintillation light. The perspex provides a mechanically strong and highly polished reflecting surface which is chemically robust. Thin sheet scintillator may be made in this way at a reduced cost; however, the economy may be improved for more complex geometries where difficult and costly fabrication processes of the plastic scintillator would be required. An example is the above mentioned anticoincidence cylinders for the low energy gamma-ray telescope. The perspex cylinders, which formed the walls, were made from commercially obtainable perspex thin sheet which was bent into a cylindrical shape at a temperature of about 150 °C. The ends were sealed using machined perspex rings welded by means of a chloroform based glue. Care was taken to ensure that the scintillation compound was isolated from the outside world as some evaporation of the NE224 was observed over a period of months in the unsealed state. The resulting cost was about $250 for each completed cylindrical scintillator. The authors wish to express their gratitude to Prof. G . P . S . Occhialini for the facilities of the Laboratory and to Vismara Spa, Milano who constructed the perspex cylinders. References 1) A. Bellomo, P. Coffaro, A. J. Dean, M. Fatta, G. Gerardi, F. Madonia, A. Russo and L. Scarsi, IAU Syrup. no. 41 (1970) p. 254. 2) D. Brini, L. Peli, O. Rimondi, and P. Veronesi, Suppl. Nuovo Cimento 2, no. 10 (1955) 1048. z) D. Crabb, A. J. Dean, J. G. McEwen and R. J. Ott, Nucl. Instr. and Meth. 45 (1966) 301. 4) Nuclear Enterprises Catalogue (1970) p. 4.
IO7
(I973) 3 3 3 - 3 3 5 ;
© N O R T H - H O L L A N D P U B L I S H I N G CO.
A SILICON RUBBER S C I N T I L L A T I O N C O M P O U N D FOR C O M P L E X G E O M E T R Y RADIATION D E T E C T O R S A. BUSSINI, A. J. DEAN, R. DI RAFFAELE, G. G E R A R D I and A. IGIUNI*
Laboratorio di Fisica Cosmica e Tecnologie Relative, Consiglio Nazionale delle Ricerche, Milano-Palermo, Italy Received 3 October 1972 A scintillation compound which cures at room temperature is described. The scintillation efficiency and optical transmission for different compounds is discussed.
I. Introduction
2. Scintillator efficiency
In order to reduce the number of background events initiated by gamma ray induced electromagnetic showers in the material of a balloon borne low energy (1-10) MeV gamma ray astronomical telescope1), a semi-active shielding system was found to be necessary. This consisted of a sandwich arrangement which was constructed from alternate coaxial cylinders of scintillation material and lead. Due to their dimensions (approximatly 0.5 m diameter, 1 m long, wall thickness 1 cm) and multiplicity (seven), it was considered advantageous to develop a different constructional technique than that of the use of plastic scintillators, which, for counters of this geometry, is expensive and fragile with respect to surface and volume cracks. For large " t h i n " scintillators light collection by total internal reflection is the most efficient method2'a). The resulting scintillator must therefore have highly reflecting surfaces and be very transparent to the scintillation light. With this problem in mind two coaxial perspex cylinders of wall thickess 3 mm separated by a distance of 6 mm were constructed. The space between the two cylinders was filled with a scintillation compound of high efficiency and transparency. The counters were viewed via light guides by photomultipliers situated at the two ends of the cylinders. Since these detectors were for application in a non-pressurized gondola, a liquid, with associated risks of leakage, could not be used. Thus a scintillation compound was developed, which would solidify at room temperature and did not chemically attack perspex. Several possibilities were investigated, the most acceptable of which is discussed here in detail, and consisted of a mixture of commercially available NE224 liquid scintillator (from Nuclear Enterprises) and transparent silicon rubbers.
The scintillation efficiencies of the various compounds were measured in small identical perspex cells. The cell was placed inside a diffusive light guide above a Philips 54-AVP 5" photomultiplier (fig. 1). A 137Cs gamma ray source was used to activate the scintillator and the pulses analysed in a Laben 1024 multichannel analyser. The scintillation efficiency, with respect to the Sll photocathode of the photomultiplier used, was considered proportional to the channel number of the half height of the Compton edge. Care was taken to keep the experimental conditions constant during each measurement series. Pulses from a piece of Nuclear Enterprises NE-102A plastic scintillator served as a reference. The measurements taken in this way were found to be reproducible to about 1% accuracy. The scintillation efficiency was measured as a function of the concentration of NE224 and with respect to the quantity of hardener used to solidify the resulting compound.
* ]stituto Nazionale Fisica Nucleare, Via Celoria 16, Milano, Italy.
Fig. 1. Experimental method for measuring the relative scintillation efficiency.
333
I
• ~
CS 137 S RAY SOURCE SAMPLE CELL WHITE PAINTED DIFFUSIVE LIGHT GUIDE
I
~ TO
PIIA
334
A. B U S S I N I et al.
Fig. 2 shows the variation of the scintillation efficiency with the concentration, by volume, of NE224 for two values of the quantity of catalyst employed. Also shown is the efficiency of the pure liquid scintillator with 80% anthracene and the curve should the NE224 be mixed with an inert transparent solution. It is noticed that for higher NE224 concentrations the experimental curves lie progressively below the uncontaminated value; this effect is more pronounced with a larger quantity of hardener. The degradation of the scintillation efficiency is probably a quenching effect of the catalyst on the NE224 as, although the light absorbtion is slightly dependent on the quantity of hardener, in a small cell of a few cubic centimeters this effect would not be important. When the fraction of NE224 was taken below about 50% a deposit was formed upon solidification which rendered the compound of little practical use. These lower concentrations were not considered as they are less efficient for the production of light, the silicon rubber costs more by volume than the liquid scintillator and the NE224 is the more transparent component to the scintillation light. Fig. 3 shows the effect of the quantity of hardener on the scintillation efficiency for several different NE224 concentrations. In each case it is seen that the scintillation efficiency is reduced with an increased quantity of hardener. The magnitude of the effect is more pronounced for a higher NE224/silicon rubber ratio. Thus at lower NE224 concentrations the quantity of hardener used is less critical and enables a relatively hard compound to be made which sets in a few hours. For large fractions of NE224 the quantity of catalyst used is
600
"1'1
x 40ROPS/30 c.c. total of CATALYST • 8 DROPS/ 30 r.c. total of CATALYST - - - - - N E 224 +INERT TRANSPARENT LIQUID
500
a
I
'1'1'1'1"1'1
important required.
if the
highest efficiency scintillator
is
3. Optical transmission
The optical transmission of a representative compound is shown in fig. 4 as a function of wavelength. The transmission was measured in a Cary-14 Recording Spectrophotometer using standard perspex cells which presented two centimeters of the compound in the beam of light. The short wavelength cut off is due to the liquid scintillator. The peak of the emission of the scintillation light is at 4250 An). The calculated attenuation lengths at this wavelength were of the order of 2 m; in fact little different from that of the pure liquid scintillator for high NE224 concentrations. The exact values of the attenuation lengths were difficult to
,,,,
400
I,
i I ll,,,,l,, , 70 ~o
HE 2 2 4
30 ~RTV
602
_
• 80~o
NE224
20~RTV
602
_
•
HE224
lO,~'flTV602
90~o
_
o
~
-
35E
=
a.
300
,,,,
I,,,,I,,It
5
ll,-
10
15
DROPS/30 c~. TOTAL COMPOUNO
Fig. 3. Variation o f scintillation efficiency with quantity of hardener for different solutions ( R T V - 6 0 2 . silicon rubber). i
I
I
I
''1''''1''''1'
1.(3
40C
~30C z
0.5 20C
m.
loc
I
1,1,1,1,1
90
80
70
60
50
,I, 40
HE 2 2 4 / , T V .
,I 30
I
20
602
Fig. 2. Scintillation efficiency vs concentration o f NE224 for two quantities o f catalyst. T h e silicon rubber used here is RTV-602 from the General Electric C o m p a n y .
3500
I ,
,
,Jl
4000
,
,
,
I I ,
,
4500 WAVELENGTH (A)
,
,
I ,
5000
J
Fig. 4. Optical transmission o f a representative c o m p o u n d (80% NE224; 4 drops hardener/30 cm ~ total c o m p o u n d ; 2 cm thick). Corrected for reflection losses.
SILICON RUBBER S C I N T I L L A T I O N C O M P O U N D
calculate since the absorption in the 2 cm sample was very low. There are two points of interest which arise from the relative values of the transparency. The first of these is the fact that the transmission was found to decrease slightly with an increasing quantity of catalyst, and was probably due to absorption in the hardener itself which was a yellowish liquid. The transparency was reduced with decreasing NE224 concentration and may be explained as the absorption in the silicon rubber which was the less transparent of the two components at wavelengths greater than 4000 ,~. The transparency of the various compounds, however, compares favourably with commercial plastic scintillators. 4. Production details The NE224 and the silicon rubber were mixed together under clean conditions, the hardener was then added and thoroughly stirred in. The resulting liquid was of very low viscosity, almost like water, and solidified in a period more dependent on the amount of rubber than that of the hardener. Using the commercially suggested quantity of catalyst for the total volume, the solidification period varied from hours at 50% to days at 90% NE224 for a temperature of 20°C. An excessive quantity of hardener prevented final setting. The hardness of the resultant scintillation compound depended on the quantity of silicon rubber used but was not very different from the pure silastic even with 90% NE224 by volume. Several types of commercially available silicon rubbers were tested: Emerson & Cuming "Eccosil 2 C N " , I.C.I. "Silicoid 201 ", General Electric " R T V 602". The results obtained showed small variation from one sample to another. 5. Applications This type of scintillator is ideal for forming scintillators of complex geometry as it has a relatively long life in a liquid state of low viscosity. Thus there is no great problem for the removal of air bubbles. The
335
mixture solidifies at room temperature so that an oven is not required. It has a high scintillation efficiency and very good transparency. The above mentioned factors together with a refractive index close to 1.5 make it ideal for use with perspex containers in which total internal reflection may be used for the collection of the scintillation light. The perspex provides a mechanically strong and highly polished reflecting surface which is chemically robust. Thin sheet scintillator may be made in this way at a reduced cost; however, the economy may be improved for more complex geometries where difficult and costly fabrication processes of the plastic scintillator would be required. An example is the above mentioned anticoincidence cylinders for the low energy gamma-ray telescope. The perspex cylinders, which formed the walls, were made from commercially obtainable perspex thin sheet which was bent into a cylindrical shape at a temperature of about 150 °C. The ends were sealed using machined perspex rings welded by means of a chloroform based glue. Care was taken to ensure that the scintillation compound was isolated from the outside world as some evaporation of the NE224 was observed over a period of months in the unsealed state. The resulting cost was about $250 for each completed cylindrical scintillator. The authors wish to express their gratitude to Prof. G . P . S . Occhialini for the facilities of the Laboratory and to Vismara Spa, Milano who constructed the perspex cylinders. References 1) A. Bellomo, P. Coffaro, A. J. Dean, M. Fatta, G. Gerardi, F. Madonia, A. Russo and L. Scarsi, IAU Syrup. no. 41 (1970) p. 254. 2) D. Brini, L. Peli, O. Rimondi, and P. Veronesi, Suppl. Nuovo Cimento 2, no. 10 (1955) 1048. z) D. Crabb, A. J. Dean, J. G. McEwen and R. J. Ott, Nucl. Instr. and Meth. 45 (1966) 301. 4) Nuclear Enterprises Catalogue (1970) p. 4.