The FT- Cal crystal assembly: tooling and technical procedures

Transcription

1 CLAS12 Note 10/20/2013 The FT- Cal crystal assembly: tooling and technical procedures F.Parodi, M.Battaglieri, A.Celentano, R. De Vita, S.Fegan, G.Ottonello Istituto Nazionale di Fisica Nucleare e Universita di Genova Via Dodecaneso Genova Italy Abstract In this note we describe tools and procedures developed to assemble the active elements (crystal and light sensor) of the electromagnetic calorimeter of the Forward Tagger for CLAS12 (FT- Cal). Each PbWO4 crystals need to be wrapped by a reflecting sheet, glued to a Large Area APD and the assembly finished with a front nose that holds an LED, part of the Light Monitoring System. 1. The crystal Assembly The building block of the calorimeter is the single lead tungstate crystal. Each crystal, 15 x 15 x 200 mm 3 in size, is coupled on one side to a photo- sensor (Hamamatsu S Large Area APD) and to the other side to an LED used as pulsed light source for calibration. Figure 1 shows the crystal assembly together with the preamplifiers attached to the APD. To achieve the maximum light collection efficiency, the APD covers almost the whole area of the downstream end of the crystal, so the LED for monitoring will be hosted on the upstream end. The LAAPD is welded to a small connection board that host, on the other side a connector for the preamp. All these parts are glued to a PEEK support. The whole assembly is then wrapped within a thin reflective sheet, heat- preformed. The wrapping is glued to the front PEEK LED holder and to the back APD PEEK holder. Moreover few strips of kapton tape are used to seal the longitudinal patches and provide the required robustness. We chosen the 3M Vikuiti VM 2000 reflective film. This material is non conductive, has a reflectivity higher than aluminated Mylar and is robust enough to hold the weight of a crystal.

2 Figure 1. Single crystals assembly: from the front to the back, the PEEK holding nose, the VM2000 wrapped crystal, the LAAPD in the PEEK housing, the FT- Orsay preamp. 2. Crystal assembly procedures Each crystal is prepared as follow: 1. A rectangle of 3M Vikuiti VM 2000 reflective film is heat- formed using an appropriate tool to fit the crystal shape. 2. The reflective film is fixed in a mold (see Figs. 2 and 3), with sharp edges of the inner part to obtain good quality angles in the 90 degrees bents. The mold is backed in the oven at T=105 o C for few minutes. Two batches of five tools (Fig. 4) are routinely used to speed up the procedure.

3 Figure 2. The aluminum mold used to heat- form the VM2000 sheet. On the left a wrapping after the backing procedure. Figure 3. Another picture of the wrapping tool

4 Figure 4. The battery of 5 tools to speed up the wrapping foil heat- formation. 3. The APD is welded to a PCB glued to a PEEK support and using Huntsman Araldite (see Fig. 5). Figure 5. The APD is welded to a small board hosting a connector for the preamp on the other side. The PCB and the APD is the glued to a PEEK support.

5 4. The APD surface is then glued to crystal face by Dow Corning 3145 Adhesive/Sealant. The glue takes about 8h to harden. For mechanical reasons, the APD needs to be precisely positioned on the crystal face. A tool to provide the required precision and simplify the gluing procedure has been designed and manufactured. Figure 6 shows the tool with some details of the mechanical assembly. As for the wrapping, two batteries of five tools have been designed and manufactured to speed up the gluing procedure (see Fig. 7). Figure 6. The tool to center and position the APD respect to the face of the crystal. The APD after being connected to the PCB and the PEEK is glued with optical cement. Figure7. The battery of 5 tools to glue the APD to the crystal face.

6 5. The crystal + APD + PCB + PEEK on the back side and the LED- holder PEEK nose on the front side is then wrapped into the heat- formed VM 2000 reflective sheet, using the same tool previously used for the film forming. The reflecting film is glued only on the PEEK block with Huntsman Araldite The Araldite takes about 8h to fully harden. Figure 8 shows the wrapped crystals after opening the tool. The assembly is now ready. Figure 8. The final phase of the assembly: the front and the back PEEK holders are glued to the VM2000 wrapping providing a robust and light- tight system.

7 The resulting assembly presents pins on front and backside (see Fig. 9), used to hold the crystal when attached to a front and back Cu plate. The whole self- standing structure is robust enough to provide the required mechanical resistance. Figure 10 shows two crystals fully assembled. Figure 9. The front and the backside of the crystal assembly. The PEEK LED holder and the connector for the preamps are respectively shown on the left and right pictures. Figure 10. Two crystals fully assembled.

8 3. Tools design All tools developed for the wrapping and gluing procedures have been designed by the Mechanic Service of the INFN- Genova Unit. The tools have been produced by the Mechanic Workshop of the INFN- Genova Unit. Figures 11, 12, 13 and 14 show some of the CAD and final drawings of the tools. Figure 11. CAD of the wrapping tool. Figure 12. CAD of the APD positioning and gluing tool.

9 Figure 12. Example of final drawing of the APD positioning and gluing tool. Figure 13. Detail of the the APD positioning and gluing tool.