- Email: [email protected]
Linear polydiorganosiloxanes as plastic bases for radiation hard scintillators
Nuclear Instruments North-Holland
and Methods
in Physics
Research
Linear polydiorganosiloxanes scintillators J. Harmon, Deparrmem Received
J. Gaynor,
V. Feygelman
309
B53 (1991) 309-314
as plastic bases for radiation hard
and J. Walker
of Physics, University of Florida, 215 Williamson Hall, Gainsville, FL 32611.2085,
4 June 1990 and in revised form 31 August
USA
1990
Melt-processible (spinnable) polydiorganosiloxanes have been tested. The polymers offer advantages over thermosetting siloxane systems. Radiation damage to the optical properties of this polymer have been measured other known optical polymers. Most importantly, intramolecular proton transfer dyes (e.g. 3-Hydroxyflavone) shifted fluorescence in the thermoplastic, unlike that in comparable thermosetting systems. A plastic scintillator should find application in the harsh radiation environment of the Superconducting Super Collider.
1. Introduction Polymer scintillators have been used for forty years to detect elementary particles in high energy accelerators. The Superconducting Super Collider (SSC) requires scintillator which is stable at doses up to 10 Mrad [l]. Commercially available plastic scintillators used in today’s detectors degrade optically after only 1 Mrad doses. After exposure to 1 Mrad, a 1 cm block of polystyrene or polyvinyltoluene exhibits significant absorption at wavelengths up to 600-700 nm. Although some of the color centers are eliminated with time, a finite [2] radiation induced absorption of about 5% loss in transmission per centimeter at 500 nm after 15 Mrads remains after 10 months of annealing at room temperature. This is true whether the irradiation occurs in an air or argon environment. Since most projected needs of a scintillator at the SSC are in the form of fibers about a meter long, even a small loss of transparency in a 1 cm path length transforms into a less than desirable operating characteristic at 1 Mrad for commercially available scintillator. Our early work resulted in the identification of the most radiation hard optically transparent plastic known: thermosetting (cross-linked) polyorganosiloxanes. This plastic shows < 0.5% loss in transmission per centimeter at 500 nm after exposure to 18 Mrad in an argon environment [3,4]. For this reason, we believe polydiorganosiloxane is the plastic of choice at the SSC and in other optical detectors exposed to high doses of radiation. Commercially available poly(dimethy1 diphenyl siloxanes) form cross-linked, non-heat-processible materials. Thus they cannot be spun or drawn into fibers by conventional techniques. 0168-583X/91/$03.50
0 1991 - Elsevier Science
Publishers
previously explored and are smaller than exhibit large Stokes based on this system
Polyorganosiloxanes are composed of one or more of the following monomer units: BIock 1. These siloxanes are available in liquid form. Two liquid prepolymers are combined in the presence of platinum catalysts to yield a cross-linked elastomeric network.
CH -Si-0
I
-Si-0
CH
’
-Si-O-
I CH3 Block 2. Such cross-linked structures exhibit no melt flow. They are not suitable for heat processing by fiber drawing or spinning. Additional peroxide-cure siloxane systems are also commercially available, but again yield unspinnable cross-linked structures. This spurred our development of linear heat processible polyorganosiloxanes. The above monomers can be combined in linear sequences. ESi-H -
+ ESi-CH=CH, -Si-CH,-CHrSi
Block 3. The ratio of m/n are key elements in determining the refractive index, fluor solubility and light output. It is an increase in phenyl content that increases each of these properties. The phenyl content and monomer sequence determine transparency and viscosity. The purpose of this study was to identify a linear siloxane which optimizes all of these considera-
B.V. (North-Holland)
310
J. Harmon et al. / Polydiorganosiloxanes for scintillators
tions. In particular, we aimed at identifying a scintillator base in which 3HF is soluble and exhibits tautomeric fluorescence.
Abbe refractometer, using a helium-neon light source (633 nm). 2.2. ScintilIator
-Si-
or
I CH,
Pure homopolymers of diphenyl siloxane are rigid thermoplastic structures. They are, however, highly crystalline in nature. The mismatch in refractive indices between the crystalline and amorphous regions results in light scattering and therefore opacity. Copolymers of dimethyl diphenyl or methylphenyl diphenyl siloxane are transparent if crystallinity is impeded by disruption of stereoregularity due to the presence of dimethyl or methylphenyl units. We optimized phenyl content and monomer sequence to yield high viscosity, transparent scintillator base materials.
2. Experimental details 2.1. General Electronic absorption and emission (uncorrected) spectra were recorded on a Hewlett Packard model 8452A spectrophotometer and a model 1046A fluorescence detector, respectively. A DuPont 910 differential scanning calorimeter equipped with a cooling unit was used to determine glass transition temperatures. Light output measurements were made in a lightproof box into which a photomultiplier tube (RCA model 8850, ten stages) had been installed. Type 68 adhesive (Norland Products, Inc.) was used to affix samples to the phototube. An Ortec model 556 power supply was used for the phototube. The alpha source was 241Am. Irradiation of the plastic was performed with a 6oCo gamma ray source at a dose rate of 140 krad/h. The refractive index of the plastic was tested with an
lamp as a
base poIymers
Polyorganosiloxanes used in this study were prepared by two methods. In the first method, Michigan Molecular Institute (MMI) synthesized a copolymer of diphenyl siloxane and dimethyl siloxane units containing 85 wt.% diphenyl units. MM1 developed a statistical polymerization procedure using a secondary butyl lithium catalyst to ensure a triblock polymer sequence [5,6]. In the second method, a random sequence structure composed of diphenyl siloxane and methylphenyl siloxane units (50/50) was synthesized by both MM1 and Petrarch. For purposes of comparison, we also studied commercially available, cross-linked elastomer (20% diphenyl siloxane) which was obtained from Petrarch. 3HF was introduced to the polymer by solution incorporation (statistical sample), heat compounding (random sample) or into prepolymer before polymerization (commercial cross-linked structure).
3. Results and discussion This investigation of the properties of thermoplastic siloxanes resulted in the identification of siloxane polymers that can be spun into scintillating fibers. In the statistical approach dimethyl diphenyl substituents were sequenced in AAA and BBB type order rather than in large blocks. An attempt was made to impede crystallinity in high diphenyl content siloxane by introducing a low amount of dimethyl units (15% by weight). DSC analysis of this polymer did not evidence a sharp glass transition temperature, although random polymers containing similar phenyl contents have glass transition temperatures < - 20°C [7]. It is believed that the triad sequences provide a level of molecular association that imports structural integrity. The polymer was cast into self-supporting films and spun into fibers. Thin films of this polysiloxane exhibited 80% light transmission at wavelengths greater than 550 nm. The shape of the transmission spectra evidences Rayleigh scattering. Copolymers containing longer block sequences, however, are almost totally opaque. In fig. 1 the siloxane transmission spectrum is compared to that of polystyrene film immediately after irradiation. Both films were irradiated at a dose of 10 Mrad in air. The siloxane film shows no radiation damage. The polystyrene film of the same thickness shows a significant decrease in transmission under these conditions (5 and 2% at 350 and 450 nm, respectively). In ref. [2] the
J. Harmon et al. / Polydiorganosiloxanes
for scintillators
311
I....,..,,,,,,,,.,,.,,,,,,,,,.,.,..,,,,,,.
250
Fig. 1. Transmission
300
spectra
350
400
of statistical
siloxane:
450
500 WAVELENGTH
(1) before
550
600
(2) after
10 Mrad;
650
0
700
Inn)
irradiation;
PS: (3) before
irradiation;
(4) after
irradiation.
extent to which the passage of time anneals the loss of transmission in polystyrene is discussed. Polymers synthesized by the random method were
,....I.
190
240
.,I.
290
Fig. 2. Transmission
significantly more transparent than those made by the statistical method. The transmission spectrum of a 1 cm thick sample is shown in fig. 2. This polymer contains
.,.,..,..*.,
340
390
spectrum
I,...,,...,,...,,.,.,.,..,
440
490
540 690 WAVELENGTH (nm) of 1 cm thick sample of random
640 siloxane
690
740
copolymer.
790
312
J. Harmon et al. / Polydiorganosiloxanes
for scintillators
maximum, 520 nm. This meets the requirements for efficient light transmissions through long fibers as needed at the new generation of proton colliders [9]. One of the objectives of this study was to incorporate 3HF into polydiorganosiloxanes. Commercially available siloxanes undergo chemical reactions during cure. This presents the problem that catalysts and reaction products can attack the fluors which are incorporated into the matrix during cure. We have found, for example, that platinum-cure siloxanes which contain hydride groups react with 3HF and result in non-fluorescent material. In additional, the hydride group affords an H bonding site. Any unreacted hydride groups can impede intramolecular proton transfer in unreacted 3HF molecules. Peroxide-cure systems do not undergo this reaction. Tautomeric fluorescence of 0.05 wt.% 3HF in peroxidecured 20 wt.% diphenyl siloxane has been observed. However, after one month, recrystallization of the dye took place as evidenced by blue-shifted crystalline fluorescence and microscopic analysis. This is depicted in the emission spectrum shown in fig. 3. The peak at 485 nm appeared after 1 month. 0.15% 3HF was incorporated into the statistical matrix. A fluorescence spectrum was recorded using surface. excitation geometry. The characteristic tautomeric emission fluorescence maximum at 520 nm [lo] is evident (fig. 4). This fluor remained in solution in the polymer matrix since the high phenyl content in the thermoplastic siloxane im-
50 mole % diphenyl and 50 mole I% methylphenyl siloxane units arranged in random sequence. The triad sequence which provides rigidity is not present. However, the phenyl content is high enough that a glass transition temperature is evidenced at 7O C. It is a highly viscous liquid (1000000 cps at room temperature). We show in a forthcoming publication that such viscous liquids form stable fibers when clad with thermoplastic materials. We are now in the process of extensively studying these random copolymers. The high phenyl content of these siloxanes enhance dye solubility. The phenyl substituents chemically and physically resemble the aromatic ring structural units of the fluors. This assures that the adhesive forces between the fluor and polymer are like the cohesive forces between the fluors or between polymer units [8]. In low phenyl content siloxanes (20% by weight) 0.05 wt.% 3-hydroxyflavone (3HF) dissolves initially, but recrystallizes from the siloxane matrix after a 1 month period, whereas 0.15 wt.% 3HF (possibly higher) remains in solution in the 85 wt.% diphenyl siloxane statistical polymer. We have solubilized as much as 1.2% 3HF in the random copolymer siloxane. Intramolecular proton transfer dyes exhibit a large Stokes shift in the absence of H-bonding interactions. Due to this larger Stokes shift there is very low absorption at the emission wavelength. For example, we measured the extinction coefficient of 3HF in toluene and found it to be as low as 0.11 mol-’ cm at the emission
!---
0420
I..
470
.
.
I
.
.
.
520 WAVELENGTH hml
.
5701
*
.
.
.
I.
620
Fig. 3. Blue-shifted crystalline fluorescence and tautomeric fluorescence in low phenyl content siloxane.
J. Harmon et al. / Polydiorganosiloxanes
313
for scintillators
0”“““““’ 350
300
400
450 WAVELENSTH
500
550
\ __A 600
lnml
Fig. 4. Fluorescence excitation and emission spectra of 3HF in polysiloxane (solid curves) and emission spectrum in polystyrene
(dotted curve).
parts the system with enhanced dye solubility. This spectrum is compared to the spectrum of a 3HF/polystyrene fiber in fig. 4. The siloxane thermoplastic similar to polystyrene functions as an unperturbing medium for an intramolecular proton transfer dye. As a rule, increasing the phenyl content increases the refractive index. Commercially available 20 wt.% diphenyl siloxane has a refractive index of 1.49. The refractive index of the statistical polymer is 1.54, and
the random polymer 1.58. This compares to 1.59 for polystyrene used in commercially scintillator. PMMA, a conventional cladding material, has a refractive index of 1.49. When PMMA is used to clad polystyrene the numerical aperture is 0.55 and with the random siloxane, 0.52. This ensures good light pipe trapping in optical fiber. Light output studies were conducted on thin films of scintillator made from the random copolymer and 3HF. Polystyrene/3HF films were used as controls. The minimum film thickness for americium a-particle containment was determined to be = 1 mil. At thicknesses l-5 mil, light output is constant. Fig. 5 is a plot of light output versus 3HF concentration-for the siloxane and polystyrene films with thickness > 1 mil. The results are essentially identical for both polymers. This demonstrates that high phenyl content siloxanes are comparable to standard polystyrene scintillator bases in light output.
4. Conclusions
0.0
0.2
0.4
0.6
0.8
1.0
3HF CONCENTRATION
Fig. 5. Light output versus 3HF concentration: for polystyrene +, and 0 for polysiloxane.
The present study indicates that random siloxane copolymers with high phenyl content show promise as plastic scintillator media, though considerable development work remains to be done. They offer advantages over commercially available cross-linked systems. In
314
J. Harmon et al. / Poiydiorganosiloxanes for scintiiiators
particular, chemical reactions do not occur during fluor incorporation, ensuring that red-shifted tautomeric fluorescence is not impeded in proton transfer fluors. Heat proeessibility, enhanced fluor solubility and the high refractive index of these polymers (1.58) are additional advantages which point out the need for an extensive study of these polymers.
Acknowledgements The authors thank Dale P. Meier of Michigan Molecular Institute for synthesizing the statistical polymer. This work was supported by DOE DE-FGOS86ER40272.
References [l] Snowmass Workshop on Physics in the 1990’s, Snowmass, CO, ed. S. Jensen (World Scientific, Singapore, 1986).
[2] R. Clot&
Data presented at the Tallahassee Conference on Radiation Effects on Plastic Scintillator, March 1990. [3] M. Bowen, S. Majewski, J. Walker, C. Zorn, Preliminary Results with a Polysiloxane-Based, Radiation Resistant Plastic Scintillator, ibid. pp. 783-798; K. Walker, Data presented at the Tallahassee Conference on Radiation Effects on Plastic Scintillator, March 1990. [4] C. Zorn et al., Nucl. Instr. and Meth. A273 (1988) 108. [S] D. Meier et al., ACS Preprints 26 (1985). [6] E.E. Bostic, ACS Polymer Preprints 10 (1965) 877. [7] D. Meier, private communication. [8] H.R. Allcock and F.W. Lumpe, Contemporary Polymer Chemistry (Prentice-Hall, NJ, 1981) p. 332. [9] H. Leutz, Scintillating fibers for particle tracking, ICFA Instrumentation Bulletin, 6189 pp. 6-13. [lo] D. McMorrow and M. Kasha, J. Phys. Chem. 88 (1984) 2235.
and Methods
in Physics
Research
Linear polydiorganosiloxanes scintillators J. Harmon, Deparrmem Received
J. Gaynor,
V. Feygelman
309
B53 (1991) 309-314
as plastic bases for radiation hard
and J. Walker
of Physics, University of Florida, 215 Williamson Hall, Gainsville, FL 32611.2085,
4 June 1990 and in revised form 31 August
USA
1990
Melt-processible (spinnable) polydiorganosiloxanes have been tested. The polymers offer advantages over thermosetting siloxane systems. Radiation damage to the optical properties of this polymer have been measured other known optical polymers. Most importantly, intramolecular proton transfer dyes (e.g. 3-Hydroxyflavone) shifted fluorescence in the thermoplastic, unlike that in comparable thermosetting systems. A plastic scintillator should find application in the harsh radiation environment of the Superconducting Super Collider.
1. Introduction Polymer scintillators have been used for forty years to detect elementary particles in high energy accelerators. The Superconducting Super Collider (SSC) requires scintillator which is stable at doses up to 10 Mrad [l]. Commercially available plastic scintillators used in today’s detectors degrade optically after only 1 Mrad doses. After exposure to 1 Mrad, a 1 cm block of polystyrene or polyvinyltoluene exhibits significant absorption at wavelengths up to 600-700 nm. Although some of the color centers are eliminated with time, a finite [2] radiation induced absorption of about 5% loss in transmission per centimeter at 500 nm after 15 Mrads remains after 10 months of annealing at room temperature. This is true whether the irradiation occurs in an air or argon environment. Since most projected needs of a scintillator at the SSC are in the form of fibers about a meter long, even a small loss of transparency in a 1 cm path length transforms into a less than desirable operating characteristic at 1 Mrad for commercially available scintillator. Our early work resulted in the identification of the most radiation hard optically transparent plastic known: thermosetting (cross-linked) polyorganosiloxanes. This plastic shows < 0.5% loss in transmission per centimeter at 500 nm after exposure to 18 Mrad in an argon environment [3,4]. For this reason, we believe polydiorganosiloxane is the plastic of choice at the SSC and in other optical detectors exposed to high doses of radiation. Commercially available poly(dimethy1 diphenyl siloxanes) form cross-linked, non-heat-processible materials. Thus they cannot be spun or drawn into fibers by conventional techniques. 0168-583X/91/$03.50
0 1991 - Elsevier Science
Publishers
previously explored and are smaller than exhibit large Stokes based on this system
Polyorganosiloxanes are composed of one or more of the following monomer units: BIock 1. These siloxanes are available in liquid form. Two liquid prepolymers are combined in the presence of platinum catalysts to yield a cross-linked elastomeric network.
CH -Si-0
I
-Si-0
CH
’
-Si-O-
I CH3 Block 2. Such cross-linked structures exhibit no melt flow. They are not suitable for heat processing by fiber drawing or spinning. Additional peroxide-cure siloxane systems are also commercially available, but again yield unspinnable cross-linked structures. This spurred our development of linear heat processible polyorganosiloxanes. The above monomers can be combined in linear sequences. ESi-H -
+ ESi-CH=CH, -Si-CH,-CHrSi
Block 3. The ratio of m/n are key elements in determining the refractive index, fluor solubility and light output. It is an increase in phenyl content that increases each of these properties. The phenyl content and monomer sequence determine transparency and viscosity. The purpose of this study was to identify a linear siloxane which optimizes all of these considera-
B.V. (North-Holland)
310
J. Harmon et al. / Polydiorganosiloxanes for scintillators
tions. In particular, we aimed at identifying a scintillator base in which 3HF is soluble and exhibits tautomeric fluorescence.
Abbe refractometer, using a helium-neon light source (633 nm). 2.2. ScintilIator
-Si-
or
I CH,
Pure homopolymers of diphenyl siloxane are rigid thermoplastic structures. They are, however, highly crystalline in nature. The mismatch in refractive indices between the crystalline and amorphous regions results in light scattering and therefore opacity. Copolymers of dimethyl diphenyl or methylphenyl diphenyl siloxane are transparent if crystallinity is impeded by disruption of stereoregularity due to the presence of dimethyl or methylphenyl units. We optimized phenyl content and monomer sequence to yield high viscosity, transparent scintillator base materials.
2. Experimental details 2.1. General Electronic absorption and emission (uncorrected) spectra were recorded on a Hewlett Packard model 8452A spectrophotometer and a model 1046A fluorescence detector, respectively. A DuPont 910 differential scanning calorimeter equipped with a cooling unit was used to determine glass transition temperatures. Light output measurements were made in a lightproof box into which a photomultiplier tube (RCA model 8850, ten stages) had been installed. Type 68 adhesive (Norland Products, Inc.) was used to affix samples to the phototube. An Ortec model 556 power supply was used for the phototube. The alpha source was 241Am. Irradiation of the plastic was performed with a 6oCo gamma ray source at a dose rate of 140 krad/h. The refractive index of the plastic was tested with an
lamp as a
base poIymers
Polyorganosiloxanes used in this study were prepared by two methods. In the first method, Michigan Molecular Institute (MMI) synthesized a copolymer of diphenyl siloxane and dimethyl siloxane units containing 85 wt.% diphenyl units. MM1 developed a statistical polymerization procedure using a secondary butyl lithium catalyst to ensure a triblock polymer sequence [5,6]. In the second method, a random sequence structure composed of diphenyl siloxane and methylphenyl siloxane units (50/50) was synthesized by both MM1 and Petrarch. For purposes of comparison, we also studied commercially available, cross-linked elastomer (20% diphenyl siloxane) which was obtained from Petrarch. 3HF was introduced to the polymer by solution incorporation (statistical sample), heat compounding (random sample) or into prepolymer before polymerization (commercial cross-linked structure).
3. Results and discussion This investigation of the properties of thermoplastic siloxanes resulted in the identification of siloxane polymers that can be spun into scintillating fibers. In the statistical approach dimethyl diphenyl substituents were sequenced in AAA and BBB type order rather than in large blocks. An attempt was made to impede crystallinity in high diphenyl content siloxane by introducing a low amount of dimethyl units (15% by weight). DSC analysis of this polymer did not evidence a sharp glass transition temperature, although random polymers containing similar phenyl contents have glass transition temperatures < - 20°C [7]. It is believed that the triad sequences provide a level of molecular association that imports structural integrity. The polymer was cast into self-supporting films and spun into fibers. Thin films of this polysiloxane exhibited 80% light transmission at wavelengths greater than 550 nm. The shape of the transmission spectra evidences Rayleigh scattering. Copolymers containing longer block sequences, however, are almost totally opaque. In fig. 1 the siloxane transmission spectrum is compared to that of polystyrene film immediately after irradiation. Both films were irradiated at a dose of 10 Mrad in air. The siloxane film shows no radiation damage. The polystyrene film of the same thickness shows a significant decrease in transmission under these conditions (5 and 2% at 350 and 450 nm, respectively). In ref. [2] the
J. Harmon et al. / Polydiorganosiloxanes
for scintillators
311
I....,..,,,,,,,,.,,.,,,,,,,,,.,.,..,,,,,,.
250
Fig. 1. Transmission
300
spectra
350
400
of statistical
siloxane:
450
500 WAVELENGTH
(1) before
550
600
(2) after
10 Mrad;
650
0
700
Inn)
irradiation;
PS: (3) before
irradiation;
(4) after
irradiation.
extent to which the passage of time anneals the loss of transmission in polystyrene is discussed. Polymers synthesized by the random method were
,....I.
190
240
.,I.
290
Fig. 2. Transmission
significantly more transparent than those made by the statistical method. The transmission spectrum of a 1 cm thick sample is shown in fig. 2. This polymer contains
.,.,..,..*.,
340
390
spectrum
I,...,,...,,...,,.,.,.,..,
440
490
540 690 WAVELENGTH (nm) of 1 cm thick sample of random
640 siloxane
690
740
copolymer.
790
312
J. Harmon et al. / Polydiorganosiloxanes
for scintillators
maximum, 520 nm. This meets the requirements for efficient light transmissions through long fibers as needed at the new generation of proton colliders [9]. One of the objectives of this study was to incorporate 3HF into polydiorganosiloxanes. Commercially available siloxanes undergo chemical reactions during cure. This presents the problem that catalysts and reaction products can attack the fluors which are incorporated into the matrix during cure. We have found, for example, that platinum-cure siloxanes which contain hydride groups react with 3HF and result in non-fluorescent material. In additional, the hydride group affords an H bonding site. Any unreacted hydride groups can impede intramolecular proton transfer in unreacted 3HF molecules. Peroxide-cure systems do not undergo this reaction. Tautomeric fluorescence of 0.05 wt.% 3HF in peroxidecured 20 wt.% diphenyl siloxane has been observed. However, after one month, recrystallization of the dye took place as evidenced by blue-shifted crystalline fluorescence and microscopic analysis. This is depicted in the emission spectrum shown in fig. 3. The peak at 485 nm appeared after 1 month. 0.15% 3HF was incorporated into the statistical matrix. A fluorescence spectrum was recorded using surface. excitation geometry. The characteristic tautomeric emission fluorescence maximum at 520 nm [lo] is evident (fig. 4). This fluor remained in solution in the polymer matrix since the high phenyl content in the thermoplastic siloxane im-
50 mole % diphenyl and 50 mole I% methylphenyl siloxane units arranged in random sequence. The triad sequence which provides rigidity is not present. However, the phenyl content is high enough that a glass transition temperature is evidenced at 7O C. It is a highly viscous liquid (1000000 cps at room temperature). We show in a forthcoming publication that such viscous liquids form stable fibers when clad with thermoplastic materials. We are now in the process of extensively studying these random copolymers. The high phenyl content of these siloxanes enhance dye solubility. The phenyl substituents chemically and physically resemble the aromatic ring structural units of the fluors. This assures that the adhesive forces between the fluor and polymer are like the cohesive forces between the fluors or between polymer units [8]. In low phenyl content siloxanes (20% by weight) 0.05 wt.% 3-hydroxyflavone (3HF) dissolves initially, but recrystallizes from the siloxane matrix after a 1 month period, whereas 0.15 wt.% 3HF (possibly higher) remains in solution in the 85 wt.% diphenyl siloxane statistical polymer. We have solubilized as much as 1.2% 3HF in the random copolymer siloxane. Intramolecular proton transfer dyes exhibit a large Stokes shift in the absence of H-bonding interactions. Due to this larger Stokes shift there is very low absorption at the emission wavelength. For example, we measured the extinction coefficient of 3HF in toluene and found it to be as low as 0.11 mol-’ cm at the emission
!---
0420
I..
470
.
.
I
.
.
.
520 WAVELENGTH hml
.
5701
*
.
.
.
I.
620
Fig. 3. Blue-shifted crystalline fluorescence and tautomeric fluorescence in low phenyl content siloxane.
J. Harmon et al. / Polydiorganosiloxanes
313
for scintillators
0”“““““’ 350
300
400
450 WAVELENSTH
500
550
\ __A 600
lnml
Fig. 4. Fluorescence excitation and emission spectra of 3HF in polysiloxane (solid curves) and emission spectrum in polystyrene
(dotted curve).
parts the system with enhanced dye solubility. This spectrum is compared to the spectrum of a 3HF/polystyrene fiber in fig. 4. The siloxane thermoplastic similar to polystyrene functions as an unperturbing medium for an intramolecular proton transfer dye. As a rule, increasing the phenyl content increases the refractive index. Commercially available 20 wt.% diphenyl siloxane has a refractive index of 1.49. The refractive index of the statistical polymer is 1.54, and
the random polymer 1.58. This compares to 1.59 for polystyrene used in commercially scintillator. PMMA, a conventional cladding material, has a refractive index of 1.49. When PMMA is used to clad polystyrene the numerical aperture is 0.55 and with the random siloxane, 0.52. This ensures good light pipe trapping in optical fiber. Light output studies were conducted on thin films of scintillator made from the random copolymer and 3HF. Polystyrene/3HF films were used as controls. The minimum film thickness for americium a-particle containment was determined to be = 1 mil. At thicknesses l-5 mil, light output is constant. Fig. 5 is a plot of light output versus 3HF concentration-for the siloxane and polystyrene films with thickness > 1 mil. The results are essentially identical for both polymers. This demonstrates that high phenyl content siloxanes are comparable to standard polystyrene scintillator bases in light output.
4. Conclusions
0.0
0.2
0.4
0.6
0.8
1.0
3HF CONCENTRATION
Fig. 5. Light output versus 3HF concentration: for polystyrene +, and 0 for polysiloxane.
The present study indicates that random siloxane copolymers with high phenyl content show promise as plastic scintillator media, though considerable development work remains to be done. They offer advantages over commercially available cross-linked systems. In
314
J. Harmon et al. / Poiydiorganosiloxanes for scintiiiators
particular, chemical reactions do not occur during fluor incorporation, ensuring that red-shifted tautomeric fluorescence is not impeded in proton transfer fluors. Heat proeessibility, enhanced fluor solubility and the high refractive index of these polymers (1.58) are additional advantages which point out the need for an extensive study of these polymers.
Acknowledgements The authors thank Dale P. Meier of Michigan Molecular Institute for synthesizing the statistical polymer. This work was supported by DOE DE-FGOS86ER40272.
References [l] Snowmass Workshop on Physics in the 1990’s, Snowmass, CO, ed. S. Jensen (World Scientific, Singapore, 1986).
[2] R. Clot&
Data presented at the Tallahassee Conference on Radiation Effects on Plastic Scintillator, March 1990. [3] M. Bowen, S. Majewski, J. Walker, C. Zorn, Preliminary Results with a Polysiloxane-Based, Radiation Resistant Plastic Scintillator, ibid. pp. 783-798; K. Walker, Data presented at the Tallahassee Conference on Radiation Effects on Plastic Scintillator, March 1990. [4] C. Zorn et al., Nucl. Instr. and Meth. A273 (1988) 108. [S] D. Meier et al., ACS Preprints 26 (1985). [6] E.E. Bostic, ACS Polymer Preprints 10 (1965) 877. [7] D. Meier, private communication. [8] H.R. Allcock and F.W. Lumpe, Contemporary Polymer Chemistry (Prentice-Hall, NJ, 1981) p. 332. [9] H. Leutz, Scintillating fibers for particle tracking, ICFA Instrumentation Bulletin, 6189 pp. 6-13. [lo] D. McMorrow and M. Kasha, J. Phys. Chem. 88 (1984) 2235.