A research project has been initiated to study the effects of ionising radiation on various grades of plastic scintillators. Various scintillator materials were sourced from Eljen, Amcrys and Saint Gobain.
Three grades of Eljen plastic scintillator materials, namely EJ-200, EJ-208 and EJ-260, were received for irradiation studies. The size of the plastic scintillator bars received were 200mm in length, 25mm wide and 10mm thick (Figure 1a). A section of the Amcrys material, UPS923A, was received via CERN. The thickness of UPS923A shown in Figure 1b is 20mm.
For the proton irradiation studies, polished specimens approximately 10mm2 and 0.25mm thick were required. A polishing procedure has to be developed for preparing these relatively soft polymer materials to the required dimensions. The Eljen materials are a polyvinyl toluene (PVT) base polymers, whilst Amcrys is a polystyrene (PS) base material. Both types of materials can be prepared using metallographic techniques, however, the challenge is the required sample thickness of 250µm.
A “Buehler IsoMet” low speed cut-off machine (Figure 2a) was used to cut a 10mm section from the length of the bar. The cutting operation was carried out using a diamond blade with cutting fluid as a lubricant as shown in Figure 2b.
A cutting speed of 60rpm with a load of 150g was used for sectioning the plastic scintillators. A photograph of the cut section is shown in Figure 3.
The cut sections were then sliced into thin sections using a “Wells” diamond wire saw (Figures 4a and 4b). Thin sections of approximately 0.5mm were cut using 0.3mm diameter diamond wire with a 50g load, and a wire speed of 0.15m/sec. This cutting operation is a slow process which takes 2 to 3 hours per sample.
Aluminium sample holders were manufactured for holding the thin specimens during the polishing operation. The design of the sample holder is specific to the “Struers” polishing machine. A slot, 11mm2 and 0.25mm deep, was cut into the holder to provide support for the thin plastic section to be polished. A rough sketch and photograph of the sample holder is shown in Figure 5.
The next stage in the mounting process was to find a suitable adhesive or glue for attaching the specimen onto the holder. The adhesive has to have sufficient strength to hold the specimen during the mechanical polishing operation, but must be easily removed in low alcohol solvents thereafter. According to Eljen’s property data sheets for the plastic scintillator materials, only low alcohol solvents such as ethanol, methanol and isopropanol can be used. Other types of solvents would attack the plastic material.
During the preparation trials, it was established that any force applied to the thin plastic section during the removal from the holder results in permanent deformation. Therefore, the adhesive must either dissolve in a low alcohol solvent, or be softened enough to remove the plastic specimen without bending it. Different types of adhesives and adhesive tapes were tried without much success. The bonding was either too strong or too weak. Eventually, conductive carbon tape which is used for mounting scanning electron microscope (SEM) specimens was tried, and showed some success. The carbon tape was strong enough to firmly hold the specimen during the polishing operation, and required about 30min. of ultrasonic cleaning with ethanol or methanol to soften sufficiently. Leaving the mounted specimen to soak for a few hours was also found to be ideal for removing the plastic specimen. Figure 6 is a photograph of the carbon tape.
The cut sections which were approximately 0.5mm (500µm) thick, and were required to be polished on both faces to a thickness of 250µm. Polishing was carried out using a “Struers” metallographic polishing machine (Figure 7). Metallographic preparation usually involves grinding stages using various grit sizes of abrasive papers, followed by various polishing stages, sequentially progressing from coarse to very fine. The relatively soft plastic thin sections did not require any grinding stages as it was much too aggressive, and were polished using two polishing stages (medium and fine).
The medium polishing stage was carried out using 6µm diamond suspension on a “Struers MD-Dac” polishing cloth, which is a satin woven acetate cloth. The fine polishing was carried out using 1µm diamond suspension on a soft synthetic fibre cloth (Struers MD-Nap). The polishing parameters are given in Table 1. Using these parameters, the material removal rate was determined as 8 to 12µm/min. for the medium stage, and 3 to 5µm/min. for the fine polishing stage.
The specimens were cleaned with liquid soap and water after each polishing stage, rinsed with ethanol and dried using a hot air dryer. To remove the specimens from the sample mount, the mount was placed in a beaker of ethanol and ultrasonically cleaned for 30min. A photograph of the ultrasonic bath is shown in Figure 8.
Images of the thin plastic section before and after polishing is shown in Figures 9a and 9b. The example shown in Figure 9 is grade EJ-208, which opaque after the cutting operation, and transparent after the polishing procedure.
The specimen thickness for each grade of material is summarised in Table 2, and represented in graphical form in Figure 10 to show the scatter of the thickness values.
The Proton Microprobe has a hexagonal carousel (Figure 11) for mounting 6 specimens. Aluminium sample holders were manufactured for supporting the thin plastic specimens during irradiation. The sample holders consisted of two aluminium discs, approximately 2mm thick, with a 7mm diameter hole in the centre (Figures 12a and 12b). The polished plastic scintillator material is sandwiched between the two discs as shown in Figure 12b.
The carousel assembly in the Proton Microprobe chamber with the sample holder attached, is shown in Figure 13.