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Pumped liquid scintillator-filled halar fibres: An approach to radiation-resistant calorimeter readout
Nuclear Instruments and Methods in Physics Research A286 (1990) 345-347 North-Holland
345
Letter to the Editer PUMPED LIQUID SCINTILLATO
-FILLED HALAR FIBRES: AN APPROAC TO RADIATION-RESISTANT CALORIMETER READOUT J .L . PINFOLD, J-G . BOUTIN and D . DUMAS Physics Department, Carleton Univershy, Ottawa, Ontario, Canada KIS SB6
Received 9 January 1989 and in revised form 29 June 1989
A prototype liquid scintillator-filled halar fibre is described and the results of some preliminary tests designed to demonstrate the feasibility of its use in calorimeter readout are given .
The planning of the next generation of colliders is giving rise to a growing demand for radiation hardness in the materials to be used as sensitive detecting media . For example, calorimeter readout systems based on plastic scintillators are widely used in high energy particle detectors. However, it has been known for some time that radiation can adversely affect the light output of plastic scintillators, both in the case of long term low level exposures [1], and relatively high level short term exposures [2] . Halar [3] is a fluoropolymer and, although not a scintillator itself, a hollow piece of halar filled with liquid scintillator can act as a guide Ior scintillation light . The main advantage of halar is that a radiation standard sample will withstand radiation doses of up to 200 Mrad, whereas comparable materials such as PTFE (Teflon) and FEP [4] become brittle after doses of the order of 10 Mrad or less. Halar can be further treated to maintain useful properties for doses of up to 1 Grad. Other advantages of halar are : resistance to chemical attack, strength, flexibility, and its ability to be machined or welded . A summary of the main properties of halar is given in table 1 . Note that all radiation measurements referred to here are made using a 6° Co source. Further tests are being plar .,ed to study the radiation resistance of halar when subjected to irradiation with
protons and neutrons. Calorimeter readout using liquid scintillator-filled teflon tubes has been employed previously [5] . However, in that case, the liquid scintillator was not circulated continuously. Teflon tubes employed in this calorimeter became brittle after a radiation dose of approximately 4 Mrad . The use of liquid scintillator-filled quartz glass tubes has been studied by Raghavan [6] and others [7] . The refractive index of a halar block was me-cured, by reilection, to be 1 .46 . A scintillating fibre light guide was formed from a halar tube with inner diameter 2 .37 0168-9002/90/$03 .50 <(:) Elsevier Science Publishers E.V . (North-Holland)
mm filled with BC-505 liquid scintillator with a refrac-
tive index of 1 .506. The cone of acceptance for the
scintillation light is given by : A = 2nT (1 - n 2/n t ), where A is the angle of the cone of acceptance, n ,
is the
Table 1 The main properties of halar extracted from the technical data
summaries of Allied Corporation, plastics division. The refractive index of halar was determined by the authors. Property
Values
Mechanical :
Tensile strength at yield [psi] Flexural modulus [psi]
4500 2.4x 10 5
Electrical :
Dielectric strength 0.001 in . thick [V/0 .001 in .]
Volume resistivity [9/cm] Dielectric constant 60-10 6 Hz
2000 1 015 2.5
Chemical : Resistance to sulphuric acid at 212° C (60% and 98ßô),
nitric acid, aqua regia, sodium hydroxide
Thermal:
Melting point [ ° C]
1Brittleness temperature [ ° C]
Maximum service temperature [ ° C]
no attack ::40 < -76
150-170
Other:
Radiation resistancc [Mrad] from a
6U
Co source For radiation treated halar [Mrad], from a ")Co source Specific gravity Moisture absorption Optical :
Refractive index (by refl,~ction)
200 up to 1000 1.68
< 0.1%
1 .46
346
J. L. Pinfold et al. / Pumped liquid scintillator filled halar fibres
Fig. 1 . Schematic diagram of the apparatus : (a) light-tight box, (b) scintillating fibre; (c) source ; (d) PMT; (e) power supply ; (f) printer; (g) qVt interface; (h) LeCroy qVt; (i) oscilloscope. refractive index of the scintillator, and n 2 is the refractive index of the surrounding material . Assuming that light is detected from only one end of the fibre, the cone of acceptance is 0.06-r sr . The light output of the halar fibre was compared to that of a conventional, Bicron BC-400, plastic scintillating fibre, using the apparatus depicted in fig. 1 . The diameter of the fibre was measured to be 2.00 mm . Its len¢tb was about 110 cm. The refractive index of the plastic is 1 .58 and, since the plastic was surrounded by air during all tests, the cone of acceptance was 0.73m sr . The plastic fibre was read out by a photomultiplier tube (PMT). A lead shield was placed in front of the PMT in order to reduce background pulses . A similar setup was used for the halar tube . A small pillbox-shaped plex.,glas tank was mounted at one end of the board to keep the halar tube filled with liquid scintillator . The free face of the tank was then coupled to the PMT. The entire assembly was mounted in a light-tight box. Collimated electrons emitted from a 106 Ru source with energies mostly in the range of 1-2 MeV were used to excite the sctntillators. Pulses from the photomultiplier tube were fed into the q-input of a 16-bit LeCroy qVt . The internal discriminator of the qVt was set to -17.5 mV to just cut out the high rate photomultiplier noise. In this mode a 256-channel pulse height spectrum was accumulated for each one-hour run. Data were collected for several points along the length of the halar and plastic fibres . Background spectra were recorded at regular intervals. For both the halar and the plastic fibres, the pulse height spectrum at around 70 cm from the source was due entirely to background . The variation in the average number of photoelectrons detected per second with distance of the source from the PMT window is show in fig. 2. The liquid and plastic scintillator used had a light output of approximately 60% that of anthracene . However, the cone of acceptance for the plastic scintillator is approximately 12 times that of the halar, and consequently more light is transmitted by the plastic fibre, despite the larger diameter of the halar tubing . As a result, the average pulse height obtained from the pla,tic scintillator was larger than that of halar. As the source was mo-cd, away from the PMT, the number of
photoelectrons/s registered by the qVt for halar was seen to fall faster than that for the plastic, as can be seen in fig. 2. However, the effective attenuation length for the liquid scintillator-filled halar was not observed to be appreciably shorter than that of the scintillator plastic fibre. The transmission efficiency of liquid scintillator-filled halar fibres can quite easily be improved by using a liquid scintillator with a high refractive index. A liquid scintillator of refractive index 1.62 [6] has already been developed for a similar application . The surface finish of polished halar compared with the inner surface of the fibres used in this work indicated that the optical quality of the internal wall of the halar fibre could be improved. In this letter we have described a prototype liquid scintillator-filled halar fibre and some preliminary test designed to demonstrate the feasibility of its use in calorimeter readout. Our approach to the radiation hardness problem would be to continually refresh the scintillating material by pumping liquid scintillator along networks of halar vessels from a large central 300 w c 0 200 -I
M
U
a) â)
0 0
0 0
100-
0
0 0
10
20
500 400c 0 L U (2)
â)
0 0
300-
a0
0
40
0 50
60
Distance (cm)
70
Plastic .
0 0
200100 -1 0
10
0
n
0 -T -r-,--,.- r--r- ---~ -* 20 30 40 50 ' 60 70
Distance (cm)
Fig . 2. Plot of distance against the average number of photoelectrons per second (averaged over 2 one-hour runs for each point shown) for : (a) liquid scintillator-filled halar; (b) scintillating plastic fibres .
J. L. Pinfold et al. /
Pumped liquid scintillator
reservoir . The scintillators would then be read out in the conventional way. The obvious advantages are : firstly, the scintillator and its container are much less prone to radiation damage ; and secondly, one can adjust the properties of the calorimeter readout easily by employing a liquid scintillator with a different loading. As far as we are aware, the use of liquid scintillator circulating in semi-flexible halar plastic fibres is a new approach to the problem of radiation-resistant calorimeter readout . The way seems clear to construct a prototype calorimeter element with an active medium that in principle can be used in environments where the integrated radiation dose ranges up to 1 Grad. Acknowledgement We would like to thank Chuck Hurlbut of Bicron for valuable discussions and scintillator samples .
filled halar fibres
347
References [I] Y. Sirois and R. Wigmans, Nuel. Instr. and Meth. A240 (1985) 262. [2] C.S. Lindsay et al., Nucl. Instr . and Meth. A254 (1987) 212. [3] Halar Fluoropolymer Resin Technical Data, Allied Fibres and Plastics, P.O. Box 2332R, Morristown, NJ 07960, USA . [4] DuPont Product Information . [5] L. Bachman et al., Nucl. Instr . and Meth. 206 (1983) 85. [6] R.S. Raghavan, int . Conf. on Neutrino Physics and Astrophysics, Maui, Hawaii, July 1-8, 1981, p. 27. [7] D.R. Potter, Proc . Workshop on New Solid State Devices for High Energy Physics, LBL (1985) .
345
Letter to the Editer PUMPED LIQUID SCINTILLATO
-FILLED HALAR FIBRES: AN APPROAC TO RADIATION-RESISTANT CALORIMETER READOUT J .L . PINFOLD, J-G . BOUTIN and D . DUMAS Physics Department, Carleton Univershy, Ottawa, Ontario, Canada KIS SB6
Received 9 January 1989 and in revised form 29 June 1989
A prototype liquid scintillator-filled halar fibre is described and the results of some preliminary tests designed to demonstrate the feasibility of its use in calorimeter readout are given .
The planning of the next generation of colliders is giving rise to a growing demand for radiation hardness in the materials to be used as sensitive detecting media . For example, calorimeter readout systems based on plastic scintillators are widely used in high energy particle detectors. However, it has been known for some time that radiation can adversely affect the light output of plastic scintillators, both in the case of long term low level exposures [1], and relatively high level short term exposures [2] . Halar [3] is a fluoropolymer and, although not a scintillator itself, a hollow piece of halar filled with liquid scintillator can act as a guide Ior scintillation light . The main advantage of halar is that a radiation standard sample will withstand radiation doses of up to 200 Mrad, whereas comparable materials such as PTFE (Teflon) and FEP [4] become brittle after doses of the order of 10 Mrad or less. Halar can be further treated to maintain useful properties for doses of up to 1 Grad. Other advantages of halar are : resistance to chemical attack, strength, flexibility, and its ability to be machined or welded . A summary of the main properties of halar is given in table 1 . Note that all radiation measurements referred to here are made using a 6° Co source. Further tests are being plar .,ed to study the radiation resistance of halar when subjected to irradiation with
protons and neutrons. Calorimeter readout using liquid scintillator-filled teflon tubes has been employed previously [5] . However, in that case, the liquid scintillator was not circulated continuously. Teflon tubes employed in this calorimeter became brittle after a radiation dose of approximately 4 Mrad . The use of liquid scintillator-filled quartz glass tubes has been studied by Raghavan [6] and others [7] . The refractive index of a halar block was me-cured, by reilection, to be 1 .46 . A scintillating fibre light guide was formed from a halar tube with inner diameter 2 .37 0168-9002/90/$03 .50 <(:) Elsevier Science Publishers E.V . (North-Holland)
mm filled with BC-505 liquid scintillator with a refrac-
tive index of 1 .506. The cone of acceptance for the
scintillation light is given by : A = 2nT (1 - n 2/n t ), where A is the angle of the cone of acceptance, n ,
is the
Table 1 The main properties of halar extracted from the technical data
summaries of Allied Corporation, plastics division. The refractive index of halar was determined by the authors. Property
Values
Mechanical :
Tensile strength at yield [psi] Flexural modulus [psi]
4500 2.4x 10 5
Electrical :
Dielectric strength 0.001 in . thick [V/0 .001 in .]
Volume resistivity [9/cm] Dielectric constant 60-10 6 Hz
2000 1 015 2.5
Chemical : Resistance to sulphuric acid at 212° C (60% and 98ßô),
nitric acid, aqua regia, sodium hydroxide
Thermal:
Melting point [ ° C]
1Brittleness temperature [ ° C]
Maximum service temperature [ ° C]
no attack ::40 < -76
150-170
Other:
Radiation resistancc [Mrad] from a
6U
Co source For radiation treated halar [Mrad], from a ")Co source Specific gravity Moisture absorption Optical :
Refractive index (by refl,~ction)
200 up to 1000 1.68
< 0.1%
1 .46
346
J. L. Pinfold et al. / Pumped liquid scintillator filled halar fibres
Fig. 1 . Schematic diagram of the apparatus : (a) light-tight box, (b) scintillating fibre; (c) source ; (d) PMT; (e) power supply ; (f) printer; (g) qVt interface; (h) LeCroy qVt; (i) oscilloscope. refractive index of the scintillator, and n 2 is the refractive index of the surrounding material . Assuming that light is detected from only one end of the fibre, the cone of acceptance is 0.06-r sr . The light output of the halar fibre was compared to that of a conventional, Bicron BC-400, plastic scintillating fibre, using the apparatus depicted in fig. 1 . The diameter of the fibre was measured to be 2.00 mm . Its len¢tb was about 110 cm. The refractive index of the plastic is 1 .58 and, since the plastic was surrounded by air during all tests, the cone of acceptance was 0.73m sr . The plastic fibre was read out by a photomultiplier tube (PMT). A lead shield was placed in front of the PMT in order to reduce background pulses . A similar setup was used for the halar tube . A small pillbox-shaped plex.,glas tank was mounted at one end of the board to keep the halar tube filled with liquid scintillator . The free face of the tank was then coupled to the PMT. The entire assembly was mounted in a light-tight box. Collimated electrons emitted from a 106 Ru source with energies mostly in the range of 1-2 MeV were used to excite the sctntillators. Pulses from the photomultiplier tube were fed into the q-input of a 16-bit LeCroy qVt . The internal discriminator of the qVt was set to -17.5 mV to just cut out the high rate photomultiplier noise. In this mode a 256-channel pulse height spectrum was accumulated for each one-hour run. Data were collected for several points along the length of the halar and plastic fibres . Background spectra were recorded at regular intervals. For both the halar and the plastic fibres, the pulse height spectrum at around 70 cm from the source was due entirely to background . The variation in the average number of photoelectrons detected per second with distance of the source from the PMT window is show in fig. 2. The liquid and plastic scintillator used had a light output of approximately 60% that of anthracene . However, the cone of acceptance for the plastic scintillator is approximately 12 times that of the halar, and consequently more light is transmitted by the plastic fibre, despite the larger diameter of the halar tubing . As a result, the average pulse height obtained from the pla,tic scintillator was larger than that of halar. As the source was mo-cd, away from the PMT, the number of
photoelectrons/s registered by the qVt for halar was seen to fall faster than that for the plastic, as can be seen in fig. 2. However, the effective attenuation length for the liquid scintillator-filled halar was not observed to be appreciably shorter than that of the scintillator plastic fibre. The transmission efficiency of liquid scintillator-filled halar fibres can quite easily be improved by using a liquid scintillator with a high refractive index. A liquid scintillator of refractive index 1.62 [6] has already been developed for a similar application . The surface finish of polished halar compared with the inner surface of the fibres used in this work indicated that the optical quality of the internal wall of the halar fibre could be improved. In this letter we have described a prototype liquid scintillator-filled halar fibre and some preliminary test designed to demonstrate the feasibility of its use in calorimeter readout. Our approach to the radiation hardness problem would be to continually refresh the scintillating material by pumping liquid scintillator along networks of halar vessels from a large central 300 w c 0 200 -I
M
U
a) â)
0 0
0 0
100-
0
0 0
10
20
500 400c 0 L U (2)
â)
0 0
300-
a0
0
40
0 50
60
Distance (cm)
70
Plastic .
0 0
200100 -1 0
10
0
n
0 -T -r-,--,.- r--r- ---~ -* 20 30 40 50 ' 60 70
Distance (cm)
Fig . 2. Plot of distance against the average number of photoelectrons per second (averaged over 2 one-hour runs for each point shown) for : (a) liquid scintillator-filled halar; (b) scintillating plastic fibres .
J. L. Pinfold et al. /
Pumped liquid scintillator
reservoir . The scintillators would then be read out in the conventional way. The obvious advantages are : firstly, the scintillator and its container are much less prone to radiation damage ; and secondly, one can adjust the properties of the calorimeter readout easily by employing a liquid scintillator with a different loading. As far as we are aware, the use of liquid scintillator circulating in semi-flexible halar plastic fibres is a new approach to the problem of radiation-resistant calorimeter readout . The way seems clear to construct a prototype calorimeter element with an active medium that in principle can be used in environments where the integrated radiation dose ranges up to 1 Grad. Acknowledgement We would like to thank Chuck Hurlbut of Bicron for valuable discussions and scintillator samples .
filled halar fibres
347
References [I] Y. Sirois and R. Wigmans, Nuel. Instr. and Meth. A240 (1985) 262. [2] C.S. Lindsay et al., Nucl. Instr . and Meth. A254 (1987) 212. [3] Halar Fluoropolymer Resin Technical Data, Allied Fibres and Plastics, P.O. Box 2332R, Morristown, NJ 07960, USA . [4] DuPont Product Information . [5] L. Bachman et al., Nucl. Instr . and Meth. 206 (1983) 85. [6] R.S. Raghavan, int . Conf. on Neutrino Physics and Astrophysics, Maui, Hawaii, July 1-8, 1981, p. 27. [7] D.R. Potter, Proc . Workshop on New Solid State Devices for High Energy Physics, LBL (1985) .