- Email: [email protected]
Temperature treatment of plastic scintillator affects radiation hardness
372
Nuclear Instruments and Methods m Physics Research A301 (1991) 372-375 North-Holland
Letter to the Editor
Temperature treatment of plastic scintillator affects radiation hardness K.F. Johnson and H.L. Whitaker
The Florida State University, Tallahassee, FL, USA
Received 4 October 1990 It is observed that thermal treatment of plastic scmtillator can sensitize it to radiation damage . The new generation of hadron colliders now being planned (LHC and SSC) will generate intense radiation fields in the interaction regions [1]. This characteristic has added a new requirement for detectors to be located at the intersection points, namely, survivability in intense fields. In particular, the great utility of plastic scintillator, has made the investigation of its radiation sensitivity an active area of study. A major impediment to the systematic investigation of the effects of irradiation on plastic is the number of relevant variables. It is known, for instance, that total dose, dose rate, temperature, gas environment during irradiation, gas environment after irradiation all affect the severity of damage. The type of damage, usually classified as decreased local light yield
or decreased attenuation length, is also differently affected by the above factors. This complexity had made comparisons of results between groups difficult and uncertain. There are surprising, and sometimes, dismaying results in the literature . An example is the finding by Giokaris of very low dose radiation damage of the CDF beam-beam counters, reported at the Workshop on Radiation Hardness of Plastic Scintillator [2]. Another paper also reported damage occurring at a very low total dose [3]. The scintillator types were different in these two reports and in both cases it seems that an additional aggravating factor was at work . This Letter addresses these difficulties by identifying yet another variable which may be the aggravating factor here . The authors have found that simple storage
UNTREATED AND TREATED SAMPLES BEFORE IRRADIATION
ô Z O N tn 19
N Z Q I -
AFTER IRRADIATION
TEMPERATURE TREATED SAMPLE AFTER IRRADIATION (stored at 70C for 10 hours in air)
SCSN-38, 1 CM THICK
WAVELENGTH (nanometers)
Fig. 1 . Effect of temperature treatment on the transparency of SCSN-38; the 1-cm thick samples were simultaneously irradiated to 1 Mrad . 0168-9002/91/$03 .50 © 1991 - Elsevier Science Publishers 13N. (North-Holland)
K F Johnson, H. L. Whitaker / Temperature treatment
of some scintillators at increased temperatures prior to irradiation severely reduces their radiation hardness . The three types of scintillator tested to date all emit in the blue and can be obtained in either plate or fiber form . SCSN-38 and SCSN-81T are "standard" polystyrene based plastics produced by Kuraray Corporation and have been available for some years . RH-1 is a new development from Bicron Corporation, specifically intended to be highly radiation resistant . Indeed, RH-1 fiber is the most radiation hard blue emitting fiber ever tested [4] . Both the fluor system and base material of RH-1 are proprietary . The test for effects of warm storage is extremely simple. Two pieces of scintillator are cut from adjacent sections of the same cast plate. One piece is kept at 70'C in air for 10 h . Three hours after retrieving the piece from the oven, it and its companion are irradiated, in this case at the FSU 3-MeV electron accelerator, to about a megarad . At each stage, transmission spectrophotometry is done . Figs . 1 and 2 show the results . The curves show the transmission as a function of wavelength. Radiation damage expresses itself as increased absorption in the blue, gradually reaching toward the red . Fig . 1, for example, shows that the additional absorption due to the thermal treatment after irradiation is about 50% per centimeter . In no case did the temperature affect the optical properties of the plastic (curves 1 and 2 in each figure are identical) before irradiation . However, irradiation uncovers a large difference in radiation sensitivity (curves 3 and 4) .
of plastic scintillator affects radiation hardness
373
The size of the effect is quite surprising . 70 ° C is well below the glass transition temperature (about 105 ° C) of the base materials . There does not seem to be an obvious chemical reaction which would account for the effect . A plausible explanation is that at slightly elevated temperatures, the diffusion of air into the plastic is accelerated . If this is coupled with a mechanism which "fixes" oxygen in place, then when the plastic is irradiated after returning to room temperature, it would be supersaturated with oxygen and, possibly, more easily damaged . Although additional diffusion of air into the plastic may play a role, that it cannot be the only cause of the increase in radiation sensitivity is shown by fig. 3 . Here we see the results of storing a piece of scintillator in vacuum at 70'C for 26 h . Again for comparison, a simultaneously irradiated room temperature air-stored piece is shown . Clearly, no air can have diffused into the plastic, but the damage to the heat treated sample after irradiation is still greater than to the untreated sample . Not shown are very similar results for SCSN-38 . It is of interest whether pretreatment at still lower temperatures has an effect . Fig. 4 illustrates what happens to SCSN-81T after 77 h at only 54 ° C . There is clearly additional damage . We hasten to point out that temperatures near 50 ° C can sometimes be found inside surface bean-dines, for example at FNAL in the summer . It is not excluded that extended storage at even lower temperatures could have a similar effect . If the effect holds true for month-long storage at low temperatures,
UNTREATED AND TREATED SAMPLES BEFORE IRRADIATION
80
ROOM TEMPERATURE STORED SAMPLE AFTER IRRADIATION
ô Z O
TEMPERATURE TREATED SAMPLE AFTER IRRADIATION (stored at 70C for 10 hours in air)
60
20
RH-1, 0 5 CM THICK
600
WAVELENGTH
700
800
(nanometers)
Fig . 2 . Effect of temperature treatment on the transparency of RH-1 ; the 0 .5-cm thick samples were simultaneously irradiated to 0 .6 Mrad .
374
KF Johnson, HL . Whitaker / Temperature treatment ofp1asttc scrntrllator affects radiation hardness UNTREATED AND TREATED SAMPLES BEFORE IRRADIATION 80 3 ROOM TEMPERATURE STORED SAMPLE AFTER IRRADIATION
ô Z O tn Z Q tY H
60
-4 TREATED SAMPLE AFTER IRRADIATION (26 HOURS IN VACUUM AT 70C) 40
SCSN-81 T, 1 CM THICK
20
300
400
500
WAVELENGTH (nanometers) Fig. 3 . Effect of temperature treatment in vacuum on the transparency of SCSN-81T; the 1-cm thick samples were simultaneously irradiated to 1.6 Mrad .
then unexpectedly high radiation sensitivity of scintillator-based devices, such as discussed in refs . [2,3] would be explained.
The consequence of these observations for the systematic study of radiation damage to plastic scintillator or for the fabrication of a radiation resistant device, is
UNTREATED AND TREATED SAMPLES I BEFORE IRRADIATION
3
ROOM TEMPERATURE STORED SAMPLE AFTER IRRADIATION
- 4 TEMPERATURE TREATED SAMPLE AFTER IRRADIATION (stored at 54C for 77 hours in air)
WAVELENGTH
(nanometers)
Fig. 4 . Effect of temperature treatment on the transparency of SCSN-38; The 1-cm thick samples were simultaneously irradiated to 1 .6 Mrad .
KF. Johnson, H.L. Whitaker / Temperature treatment ofplastic scintillator affects radiation hardness
that the entire history of the scintillator from the moment it is drawn or cast (under inert gas?) is relevant . This is rarely available, and the lack of this information will introduce uncontrolled variations into an investigation and may prevent a device from functioning as planned. For example, temperature extremes experienced during shipping of scintillator, or during the process of fabrication of a device, could drastically diminish the radiation resistance of the device from that calculated from measurements on samples . These measurements were motivated by the failure of a calorimeter to perform as predicted during a radiation damage test [5] . The calorimeter was a lead/ scintillating fiber calorimeter specifically designed to withstand several megarads of dose . The active medium was a fiber of the same RH-1 material mentioned above . During fabrication of the calorimeter, because of time pressure, it was heated to 70 ° C for 36 h to accelerate curing of an adhesive, with dire consequences. There may be other fabrication procedures, such as high-speed polishing or embedding of sctntillator in a low melting lead eutectic, which might similarly sensitize scmtillator .
375
Acknowledgements The authors would like to thank Dr . C. Hurlbut of Bicron Corporation and Dr. T. Shimizu of Kuraray Corporation for scintillator samples, Mr . W. Phillips of FSU for his efforts in irradiating the samples, Drs. R. Clough (Sandia) and S. Malewski (CEBAF) for valuable discussions and V. Hagopian (FSU) for supporting this work.
References [1] D. Groom, Proc. Workshop on the Radiation Hardness of Plastic Scintillator, Florida State University, Tallahassee, FL, USA, in press. [2] N Giokaris, ibid [3] C.S Lindsey et al ., Nucl . Instr. and Meth . A254 (1987) 212 . [4] C . Zorn, Proc . Workshop on the Radiation Hardness of Plastic Scintillator, Florida State University, Tallahassee, FL, USA, 1990, in press, V. Hagopian et al., 1990 Snowmass Summer Study. [5] K.F . Johnson, D.W . Hertzog et al ., m preparation.
Nuclear Instruments and Methods m Physics Research A301 (1991) 372-375 North-Holland
Letter to the Editor
Temperature treatment of plastic scintillator affects radiation hardness K.F. Johnson and H.L. Whitaker
The Florida State University, Tallahassee, FL, USA
Received 4 October 1990 It is observed that thermal treatment of plastic scmtillator can sensitize it to radiation damage . The new generation of hadron colliders now being planned (LHC and SSC) will generate intense radiation fields in the interaction regions [1]. This characteristic has added a new requirement for detectors to be located at the intersection points, namely, survivability in intense fields. In particular, the great utility of plastic scintillator, has made the investigation of its radiation sensitivity an active area of study. A major impediment to the systematic investigation of the effects of irradiation on plastic is the number of relevant variables. It is known, for instance, that total dose, dose rate, temperature, gas environment during irradiation, gas environment after irradiation all affect the severity of damage. The type of damage, usually classified as decreased local light yield
or decreased attenuation length, is also differently affected by the above factors. This complexity had made comparisons of results between groups difficult and uncertain. There are surprising, and sometimes, dismaying results in the literature . An example is the finding by Giokaris of very low dose radiation damage of the CDF beam-beam counters, reported at the Workshop on Radiation Hardness of Plastic Scintillator [2]. Another paper also reported damage occurring at a very low total dose [3]. The scintillator types were different in these two reports and in both cases it seems that an additional aggravating factor was at work . This Letter addresses these difficulties by identifying yet another variable which may be the aggravating factor here . The authors have found that simple storage
UNTREATED AND TREATED SAMPLES BEFORE IRRADIATION
ô Z O N tn 19
N Z Q I -
AFTER IRRADIATION
TEMPERATURE TREATED SAMPLE AFTER IRRADIATION (stored at 70C for 10 hours in air)
SCSN-38, 1 CM THICK
WAVELENGTH (nanometers)
Fig. 1 . Effect of temperature treatment on the transparency of SCSN-38; the 1-cm thick samples were simultaneously irradiated to 1 Mrad . 0168-9002/91/$03 .50 © 1991 - Elsevier Science Publishers 13N. (North-Holland)
K F Johnson, H. L. Whitaker / Temperature treatment
of some scintillators at increased temperatures prior to irradiation severely reduces their radiation hardness . The three types of scintillator tested to date all emit in the blue and can be obtained in either plate or fiber form . SCSN-38 and SCSN-81T are "standard" polystyrene based plastics produced by Kuraray Corporation and have been available for some years . RH-1 is a new development from Bicron Corporation, specifically intended to be highly radiation resistant . Indeed, RH-1 fiber is the most radiation hard blue emitting fiber ever tested [4] . Both the fluor system and base material of RH-1 are proprietary . The test for effects of warm storage is extremely simple. Two pieces of scintillator are cut from adjacent sections of the same cast plate. One piece is kept at 70'C in air for 10 h . Three hours after retrieving the piece from the oven, it and its companion are irradiated, in this case at the FSU 3-MeV electron accelerator, to about a megarad . At each stage, transmission spectrophotometry is done . Figs . 1 and 2 show the results . The curves show the transmission as a function of wavelength. Radiation damage expresses itself as increased absorption in the blue, gradually reaching toward the red . Fig . 1, for example, shows that the additional absorption due to the thermal treatment after irradiation is about 50% per centimeter . In no case did the temperature affect the optical properties of the plastic (curves 1 and 2 in each figure are identical) before irradiation . However, irradiation uncovers a large difference in radiation sensitivity (curves 3 and 4) .
of plastic scintillator affects radiation hardness
373
The size of the effect is quite surprising . 70 ° C is well below the glass transition temperature (about 105 ° C) of the base materials . There does not seem to be an obvious chemical reaction which would account for the effect . A plausible explanation is that at slightly elevated temperatures, the diffusion of air into the plastic is accelerated . If this is coupled with a mechanism which "fixes" oxygen in place, then when the plastic is irradiated after returning to room temperature, it would be supersaturated with oxygen and, possibly, more easily damaged . Although additional diffusion of air into the plastic may play a role, that it cannot be the only cause of the increase in radiation sensitivity is shown by fig. 3 . Here we see the results of storing a piece of scintillator in vacuum at 70'C for 26 h . Again for comparison, a simultaneously irradiated room temperature air-stored piece is shown . Clearly, no air can have diffused into the plastic, but the damage to the heat treated sample after irradiation is still greater than to the untreated sample . Not shown are very similar results for SCSN-38 . It is of interest whether pretreatment at still lower temperatures has an effect . Fig. 4 illustrates what happens to SCSN-81T after 77 h at only 54 ° C . There is clearly additional damage . We hasten to point out that temperatures near 50 ° C can sometimes be found inside surface bean-dines, for example at FNAL in the summer . It is not excluded that extended storage at even lower temperatures could have a similar effect . If the effect holds true for month-long storage at low temperatures,
UNTREATED AND TREATED SAMPLES BEFORE IRRADIATION
80
ROOM TEMPERATURE STORED SAMPLE AFTER IRRADIATION
ô Z O
TEMPERATURE TREATED SAMPLE AFTER IRRADIATION (stored at 70C for 10 hours in air)
60
20
RH-1, 0 5 CM THICK
600
WAVELENGTH
700
800
(nanometers)
Fig . 2 . Effect of temperature treatment on the transparency of RH-1 ; the 0 .5-cm thick samples were simultaneously irradiated to 0 .6 Mrad .
374
KF Johnson, HL . Whitaker / Temperature treatment ofp1asttc scrntrllator affects radiation hardness UNTREATED AND TREATED SAMPLES BEFORE IRRADIATION 80 3 ROOM TEMPERATURE STORED SAMPLE AFTER IRRADIATION
ô Z O tn Z Q tY H
60
-4 TREATED SAMPLE AFTER IRRADIATION (26 HOURS IN VACUUM AT 70C) 40
SCSN-81 T, 1 CM THICK
20
300
400
500
WAVELENGTH (nanometers) Fig. 3 . Effect of temperature treatment in vacuum on the transparency of SCSN-81T; the 1-cm thick samples were simultaneously irradiated to 1.6 Mrad .
then unexpectedly high radiation sensitivity of scintillator-based devices, such as discussed in refs . [2,3] would be explained.
The consequence of these observations for the systematic study of radiation damage to plastic scintillator or for the fabrication of a radiation resistant device, is
UNTREATED AND TREATED SAMPLES I BEFORE IRRADIATION
3
ROOM TEMPERATURE STORED SAMPLE AFTER IRRADIATION
- 4 TEMPERATURE TREATED SAMPLE AFTER IRRADIATION (stored at 54C for 77 hours in air)
WAVELENGTH
(nanometers)
Fig. 4 . Effect of temperature treatment on the transparency of SCSN-38; The 1-cm thick samples were simultaneously irradiated to 1 .6 Mrad .
KF. Johnson, H.L. Whitaker / Temperature treatment ofplastic scintillator affects radiation hardness
that the entire history of the scintillator from the moment it is drawn or cast (under inert gas?) is relevant . This is rarely available, and the lack of this information will introduce uncontrolled variations into an investigation and may prevent a device from functioning as planned. For example, temperature extremes experienced during shipping of scintillator, or during the process of fabrication of a device, could drastically diminish the radiation resistance of the device from that calculated from measurements on samples . These measurements were motivated by the failure of a calorimeter to perform as predicted during a radiation damage test [5] . The calorimeter was a lead/ scintillating fiber calorimeter specifically designed to withstand several megarads of dose . The active medium was a fiber of the same RH-1 material mentioned above . During fabrication of the calorimeter, because of time pressure, it was heated to 70 ° C for 36 h to accelerate curing of an adhesive, with dire consequences. There may be other fabrication procedures, such as high-speed polishing or embedding of sctntillator in a low melting lead eutectic, which might similarly sensitize scmtillator .
375
Acknowledgements The authors would like to thank Dr . C. Hurlbut of Bicron Corporation and Dr. T. Shimizu of Kuraray Corporation for scintillator samples, Mr . W. Phillips of FSU for his efforts in irradiating the samples, Drs. R. Clough (Sandia) and S. Malewski (CEBAF) for valuable discussions and V. Hagopian (FSU) for supporting this work.
References [1] D. Groom, Proc. Workshop on the Radiation Hardness of Plastic Scintillator, Florida State University, Tallahassee, FL, USA, in press. [2] N Giokaris, ibid [3] C.S Lindsey et al ., Nucl . Instr. and Meth . A254 (1987) 212 . [4] C . Zorn, Proc . Workshop on the Radiation Hardness of Plastic Scintillator, Florida State University, Tallahassee, FL, USA, 1990, in press, V. Hagopian et al., 1990 Snowmass Summer Study. [5] K.F . Johnson, D.W . Hertzog et al ., m preparation.