Chapter 36
Designing Radiation-Hard Plastic Scintillators
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Stan Majewski and Carl Zorn Physics Division, Continuous Electron Beam Accelerator Facility, 12,000 Jefferson Avenue, Newport News, VA 23606
Plastic scintillators are a ubiquitous component of modern par ticle physics detectors. With the planned construction of a new generation of high intensity, high energy, colliding beam accelera tors, it is known that unprecedented high levels of radiation will be present in the experimental halls. Some components of the de tector (specifically those utilizing scintillator) will have to endure annual doses of 10 to 10 Gy or more. Standard scintillators will not survive such conditions. This is a survey of research started several years ago with the goal of developing plastic scintillators appropriate to such an environment. 4
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Particle physics has the goal of trying to decode the interrelationship of matter and energy at its deepest level. The experimental tools used are essentially huge microprobes that analyze the decay products of immensely high energy processes. Their essential purpose is to (a) track the path of charged particles, (b) identify the particles, and (c) measure the momenta and energy of the particles. The energy of the particles is measured by calorimeters. An ideal calorimeter would be a material that could both absorb and detect the particle's presence. Crystalline scintillators, such as Nal(Tl) are one such device, but are too expensive to be made in the quantities needed, and have too slow a response time (in general). For a variety of reasons, the best compromise has been the sampling caloiimetei. This device consists of alternate series of a high-Ζ material (lead is the most common) and a particle-sensitive medium. The former initiates electromagnetic or hadionic showers in the material. These are the result of the initial particle's interaction with the high-Ζ material and result in a forward spray of particles carrying the energy and momentum of the incident particle. This spray of particles also passes through the the sensitive medium, resulting in excitation of the material, and
0097-6156/91/0475-0569S06.00/0 © 1991 American Chemical Society
Clough and Shalaby; Radiation Effects on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
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RADIATION EFFECTS ON POLYMERS
subsequent detection of the de-excitation products. A variety of sampling media are possible, but the most popular is the plastic scintillator. It has the advantage of being cheap, fast in response (10 Gy) is the loss in light output. The cause of this loss is rooted in two principal components: (i) the permanent destruction of fluorescent components, and (ii) the permanent change in the spectral transmission of the base plastic where thefluor(s)emit. The latter has a subtle aspect to it in that it can simulate the first. A study (3) of several common scintillators found that after irradiation, the resultant loss in light output could not be explained by either damage to thefluors(which was minimal) nor to direct absorption of the emission light by the base plastic (as the change in transmission of the scintillator was minimal). Rather, the base plastic was found to have formed new absorption centers deep in the UV corresponding to those wavelengths where the primary fluor radiatively transferred energy to the secondary. Another study (4) came to the same conclusion, but by studying radiation damage to liquid scintillators. There are a variety of parameters of concern when irradiating scintillators, but the three key ones are (i) absorbed dose, (ii) dose rate, and (ui) the presence or absence of oxygen. The effect of thefirstis obvious, but the latter two are interrelated. Although not all the details have been worked out, there is enough 4
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Clough and Shalaby; Radiation Effects on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
36. MAJEWSKI & ZORN
Radiation-Hard Plastic Scintillator Design571
data to indicate the general trend. For some regime of dose rates (>10 Gy/hr), it seems that the observed level of damage is independent of dose rate when irradiations are carried out in air (5) for polystyrene-based scintillators. But at very low dose rates, the damage level climbs to very high levels (6) (observed in acrylic-based scintillator). At least one reason for this seems to lie in the presence of oxygen. At the low dose rates, oxygen is able to participate fully in the permanently damaging reactions (7). For a variety of reasons, there is a current revived interest in using scintillators in the form of opticalfibers.These consist of a doped polystyrene core surrounded by a thin cladding of lower refractive index plastic, namely acrylic. Although it is believed that the polystyrene will probably not be affected additionally at lower dose rates, there is a need to check if the radiation sensitive acrylic cladding has any additional effects upon the optical performance of thefiber(8). At the higher dose rates, one is able to observe an annealing (or recovery) phenomena. That is, after a high rate irradiation, an observed level of damage is reached. If the material is exposed to oxygen, the damage level decreases over a period of time (depending upon temperature, plastic, dye(s), oxygen level, etc.) that is on the order of several weeks or months to a plateau of permanent residual damage. There is experimental evidence to indicate that over some range of dose rates (10-10 Gy/hr), the residual damage is independent of dose rate (9). This may even be the case for very low dose rates (say · >·· >·
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use of plastic scintillating fibers i n SSC detectors. These would be i n the same form as optical fibers: a central (scintillating) core surrounded by a thin cladding of lower refractive index w i t h an overall diameter of 0.5-1.0 m m . A s the sampling element of electromagnetic calorimeters, the fibers face their greatest challenge in terms of radiation-resist ance. A l t h o u g h most research has concentrated on the problem of improving the fibers' resistance to irradiation v i a the improvement of the fibers' attenuation characteristics (15,16), recent simulation work (21) has indicated that due to the relatively concentrated longitudinal deposition of en-
Clough and Shalaby; Radiation Effects on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
36. MAJEWSKI & ZORN
Radiation-Hard Plastic Scintillator Design575
ergy within the calorimeter, the intrinsic scintillation light output losses have the greatest effect upon the performance of the calorimeter. In contrast, with hadronic calorimeters, their fundamentally larger size and the large longitudinal spread of the energy deposition will make it essential to maximize the attenuation performance of the fiber. The problem of intrinsic light output loss after irradiation translates into two requirements: (i) the use of intrinsically radiation-hardfluors,and (ii) increasing the concentration of the secondary fluor (if no sufficiently large Stokes shift primary is available) in order to reduce the amount of primary emission absorbed by the damaged plastic. Figure 3 displays the result of irradiating severalfibers,each with radically different concentrations of the fluor 3HF. A short section of fiber (between 8 and 9 inches from the PMT) was protected by a lead shield during irradiation. This portion remains as "virgin" scintillating fiber. Hence the size of the "step-function" after irradiation indicates the relative loss in intrinsic scintillation efficiency. (Any differences in output among the fibers was normalized out at the start of the step.) As can be seen infigure3, increasing the concentration of the secondary fluor can significantly decrease this loss in light output. This effect can also be interpreted as due to the saturation in light output with high concentrations of dopant. If one sits on the plateau in light output versus fluor concentration, then destruction of a given amount offluorwill not affect the overall light output as would happen at a small concentration of fluor. Of course, other effects will have to be weighed when using this method, namely, (a) if the destroyedfluoradds additional absorption centers, and (b) the effect of increased self-absorption on the attenuation character of the scintillator due to the higher concentration of dopant.
Summary [1] Shifting the emission wavelength from the blue to the green via the use of large Stokes shift fluors (most notably 3HF) can significantly improve the radiation tolerance of plastic scintillators in terms of changes in attenuation after irradiation (and recovery). [2] In addition, increasing the concentration of the fluor can decrease the loss in scintillation efficiency after irradiation. This will have to take into account any self-absorption increase due to the larger concentration. [3] It may yet be possible to produce radiation-resist ant, blue-emitting scintillators by synthesizing radiation-hard plastics that are also suitable as base materials for scintillators. By keeping the emission in the blue, then only standard (and cheap) photo-detectors need be utilized. This also circumvents the problem that the longer wavelength fluors have larger decay times (i.e., the scintillator is slower). [4] Most tests of radiation tolerance have been based upon accelerated, high dose rate experiments. Although there is good evidence to suggest that for polystyrene-based scintillators, a correspondance can be made from high dose rates to realistic rates, it will be necessary to make low dose rate tests in order to verify this hypothesis, particularly with regard to scintillating optical fibers.
Clough and Shalaby; Radiation Effects on Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1991.
576
RADIATION EFFECTS ON POLYMERS 8
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