We have developed new machining and polishing techniques that have previously been applied to large scintillating crystal arrays for high energy physics experiments such as the Barium Fluoride Electromagnetic Calorimeter for the GEM Detector at SSCL, the LCsI Electromagnetic Calorimeter for the BaBar Detector at PEP-II B Factory at SLAC and the 110,000 crystal CMS Lead Tungstate Electromagnetic Calorimeter at LHC at CERN. We discuss earlier results achieved with diamond machining and polishing methods and present new results on diamond machining of lead tungstate crystals. Additionally we present new results on mechanical properties of lead tungstate including toxicological data important for the safe handling and processing of this material

Fuchs, B. A.

Shi, X.

Wuest, C. R.

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UCRLJC- 120027 PREPRINT *I Precision Machining, Polishing and Measurement of Mechanical and Toxicological Properties of Lead Tungstate Crystals for the CMS Electromagnetic Calorimeter c. R Wuest This paper was prepared for submiHal to the International Conference on Inorganic Scintillators and Their Applications Delft, Netherlands August 28-!%ptember I, 1995 August 1995 Thisba preprint of a paper intended forpublicationin a journalorproceedinga S*ce channea mav be made before uublication, thii preprint is made available with the und&tand& that it will not be cited or kpd-u&d without the permission of the author. MASTER DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein t o  any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. DISCLAIMER Portions of this document may be illegible electronic image products. Images are produced from the best available original document. t b ,PRECISION MACHINING, POLISHING AND MEASUREMENT OF MECHANICAL AND TOXICOLOGICAL PROPERTIES OF LEAD TUNGSTATE CRYSTALS FOR THE C M S  ELECTROMAGNEITCCALORMEER Craig R Wuest, Baruch A. Fuchs, Xiaorong Shi Lawrence Livermore National Laboratory P. 0. Box 808 Livermore, CA 94551 USA We have developed new machining and polishing techniques that have previously been a lied to large scintillating crystal arrays for hi h energy physics ex eriments such as the Barium%uoride Electromagnetic Calorimeter at LHC at C RN. We discuss earlier res ts achieved with diamond machinin and polishing methods and present new results on diamond machining of lead tungstate crystals. Wdditionally we present new results on mechanical properties of lead tungstate including toxicological data important for the safe handling and processing of this material. Electroma etic Calorimeter for the 8 EM Detector at SSCE, the CsI Electromagnetic Calorimeter for the Ba r ar Detector at PEPII B Factor at SLAC and the 110,000 c stal CMS Lead Tungstate H l7INTRODUCTION the surface is crit- flat surface is re- ECAL will use to the develop- LEAD TUNGSTATE SURFACE PREPARATION AND ANALYSIS Surface preparation is critical to the erformance of crystals for a number of reasons. First, an im roperly prepared (machined, grouncf polished, lapped) crystal suffers from induced stresses selves in the formation of cracks (crazing) over long times, or more quickl when subjected to ex- ishing and lapping techniques that gently bring the surface to a final finish. These techniques have been developed at LLNL for barium fluoride and also applied to cerium fluoride, lead fluoride and an i! deformations in the first few hundred microns of the surface. These stresses can manifest them- tremes of heat, radiation, humidity, etc.. Surface stresses can be minimize B using well-known pol- c Figure 1. Photograph (179x) of single point diamond turning of PWO showing regions of both plastic flow and fracture. PWO. Improper surface reparation can introduce contaminants into the surface of the crystal. Under certain conditions gese contaminants can migrate into the bulk of the crystal and cause areas of radiation susceptibilit . Because these scintillator materials emit their light typically in the blue or near UV, surface ?Gh is especially important for ood light transport ro erties. We have previously described surface preparation tecfkques explored at ELI&,, including ion beam millin diamond turning with a single point diamond tool, and various polishing/lap ing techniques [B. In general we have found that ion beam milling provides the best crystalline surface, however, the Unirormity of the surface, as well as the surface firush is not very good. In terms of sur- face finish, diamond-turned surface finishes can be quite impressive - 6 8, RMS finishes have been demonstrated on barium fluoride. However, RBS analysis of diamond-turned surfaces reveal that they are amorphous, presumably due to plastic flow of the material under the action of the dia- mond tool. Figure l shows a diamond turned sample of PWO. We observe that the material exhibits localized areas of plastic flow interspersed with areas of fracture. We conjecture that improved PWO growing and annealing methods might lead to fully diamond-turnable crystal surfaces. In the meantime another polishin techni ue - pitch lapping with diamond abrasives - pro- vides the best combination of surface &h (11-20 8, RMS) and surface crystallinity for most crys- tals. The technique is applied after more standard olishing techniques and is a simple lap pre- pared with a low meltin temperature synthetic pit$. Grooves are formed in the pitch in a pattern to allow cutting fluids, a % rasives and ground material to be washed awa during the lapping pro- cess. The key to the process is a final polish with an abrasive of very d o r m l y  sized diamond, 0.5 ,urn or 0.25 ,mi diameter, imbedded in the pitch. In addition, a non-aqueous cutting fluid such as low viscosi oil, or ethylene glycol is used to uniformly disperse the diamond and to c v  away waste materix Water is not a good fluid for diamond polishing because of the tendency of hamond to ag- glomerate in water. These polishing techniques are simple to im lement and are essentially exten- have successfu6y worked with our counterparts at the Shanghai Institute of Ceramics and the Beijing Glass Research Institute to develo tlus ca ability [3]. Fi re 2 shows exam les of im roper sions of standard polishin techniques already in ractice in tK e US and elsewhere. These tech- niques are easil transferre % to industry both in the e S and abroad. LLNL engineers and physicists (as received from the crystal grower) anaproper rdiamond-loa r ed pitch lap) polis L 8  * g of P 0. MEASUREMENTS OF PWO MECHANICAL PROPERTIES Measurements of PWO mechanical pro erties are being made at LLNL to better define the C M S  ECAL structural and thermal systems. #e have measured the coefficient of thermal expansion of a small - 1 cm x 1 c m  x 0.5 cm crystal sample cut from a lar er crystal. Measurements were made using a DuPont 9900 Thermal Mechanical Analysis system an % the sam le was cycled from dry ice tem- perature (-38 degrees Centigrade) to +lo0 degrees Centigrade. In &e sample we prepared, the crys- tal's rinciple, or "a" axis is about 19 degrees off from the actual growth axis, which corresponds to the 8 lrection of the 0.5 cm sample dimension. Thus we measure a superposition of com onents of lin- ear expansion along the two principle axes of the crystal. The coefficient of thermaPexpansion is linear m the range of 15 degrees to 30 degrees, the ran e of expected minimum and maximum tem- perature in the ECAL. The results give a coefficient of aermal expansion of 10.0 x 10d/OC and 11.5 x 1 0 6 / O C  for the two 1 cm long sides in the range of 15 - 30 OC. This can be compared with measure- ments from the Shanghai Institute of Ceramics which give 9.58 x 104/OC at 100 OC perpendicular to Figure 2. Photographs (x179) of two polished PWO surfaces. On the left is the as-received stal with large surface scratches and inclusions of polishing debris. Poor surfaces such as this z i b i t  lar e amounts of subsurface dama e and stress. The large scratches and pits make a lication of diamond-loaded pitch lapping. The surface roughness is -15-20 A RMS. the "a" axis and 22.7 x 10-6/oC arallel to the "a" axis. Earlier work by Krishna Rao and spectively, at 30 OC [4]. Our result, measured perpen 'c ar to the growgaxis and 19 degrees offset from the "a" axis is consistent with previous measurements. We have measured the Young s Modulus of compression of a right circular cylinder of PWO with dimensions 1 cm in diameter and 2 cm lon The sample was prepared with the cylinder axis in buildin u in localized regions of the crystal due to non-uniform loading, the sam le was prepared poun test machine using a 20k pound load cell set to the 500 ound load range. A s herical seat was mstalled at the bottom platen to correct for any remaining eviation from aralle&m between the top and bottom surfaces. Three MTS 0.3 inch 15% extensometers were insta ed on the test sample to provide average strain data. Figure 3 shows the setup. Three runs were made at room tem erature with a constant crosshead travel of 0.002 inches (50 microns)/minute. The first run loaded &e sam- ple to 250 ounds. The second run inadvertentl began with the crosshead loading the sample to 2100 poun g s rior to the error being detected. &e sample developed a slight internal fracture but did not heal!? The third run took the sample to 500 pounds to determine if the modulus had changed. The results of the third run are shown in Fi e 3. No large change in modulus was ob- psi and 9.27 x 106 psi (62.1 GPa and 64.0 GPa), respectivelr For comparison the modulus of alu- minum alloy is 10 x 106 psi and lead silicate lass is 9 x 10 psi. Our result is comparable to mea- hig K, quality reflective coatings J ficult. On the right is a PWO surface polished at %% NL using 83 Deshpande give 8.13 x 10-6/oC an B 19.73 x 10-6/oC e endicular and arallel to the "a" axis, re- the direction of the growth axis of the crysta F from whch it was taken. To prevent stress from L 8 e r  with very parallel faces. The sample was measured on an Instron K ode1 1125, 20k asa7 83 served between the first and third runs and the results P or the modulus were measured to be 9.0 x lo6 surements made by researchers at CERN and ?? cole Polytechnique [5]. MEASUREMENTS OF LEAD SOLUBILITY FROM LEAD TUNGSTATE We have also made measurements of lead solubility in cutting oil. Two ex eriments have been ranging in size from about 1-20 microns. The chi s were immersed in 20 cc of cutting oil. The samp e was aged for 43 days at room temperature anzthen analyzed. In the analysis the cutting oil was separated from the solid material by centrifugation at 4000 m for 10 minutes. 0.2 gram of oil was anic material. The total volume of material was then eva orated to approximately 0.5 g to which ume of 10 ml, the presence of lead was determined by inductively coupled plasma mass s ectrometry exposed oil. Both measurements have an uncertain posed to cutting oil in a polyethylenex8ottle. Small amounts of exposed and unex osed oil will be measured over regular periods approximately once per month for the next few m o n k .  Occupational performed. The first ( ualitative) measurement was made using chips of PW 8 mixed with hydro- P carbon cutting oil (Dra a eo1 7) used in the diamond turning study. The sample included 0.5 cc of chi s then wet ashed in concentrated HNO3/H2SO,in a closed Te ?i; on digestion vessel to destroy any or- f00 ng of thallium was added as an internal standard. Fo 8 owing volume adjustment to a final vol- (ICP-MS). A clean Sam le of unused cutting oil was also measured in the same manner P or compan- son. The results show iR at the amount of lead is 160 ng/g for the unexposed oil and 530 ng/g for the In a second ongoing quantitative e eriment a P x 0 cube, 1 cm on a side, is being continuously ex- of about 10%. , I 0 , ~ “ ’ ~ ’ ~ ” ~ ‘ ” ‘ ~ ~ ” ~ ! ‘ ~ ~ ” ’ ~ ~ ‘ l  0 0.00005 0.0001 0.0001 5 Strain (inlln) Figure 3. Photograph of the Young‘s Modulus test setup and results of a 500 pound load run. health and safety regulations at LLNL do not specify a limit on skin contact with lead because ele- mental lead is not readily absorbed through the skin. Exposure limits are available for ingestion of lead. The US EPA limit for lead exposure in drinking water is 15 ng/g. Environmental regulations at LLNL also define lead wastes as containing greater than 200 ng/g of lead [6]. The oil waste can also contain or ano-lead compounds, which might be absorbed through the skin or inhaled. Thus we see that the P fv 0 sohbility in cutting oil poses a problem for handling and disposal by LLNL stan- dards. Pro er collection, processin and disposal of this lead-containing hydrocarbon oil waste is . necessary. frotective gloves should % e used to prevent ingestion via hand to mouth. CONCLUSIONS LLNL is contributing to the understanding of PWO as part of the effort to build the CMS ECAL sub-system. We have seen that PWO is a material that challenges our abilities to produce a high quality electroma etic calorimeter. Our diverse program of study encompasses many aspects im- constants and ersonnel and environment sa ety. We intend to continue this program of study dur- ing the R&D p ase of CMS to develo machines for production factories and to provide input to me- chanical and thermal models of the CAL. a p P  portant to this ef !? ort including materials ro erties, machining, polishing, fundamental physical E i ACKNOWLEDGEMENTS We gratefully acknowledge the work of Phil Miller and Terry Duewer for lead solubility mea- surements, Scott Preuss for measurements of Young’s Modulus, and Mervyn Lawrence for measure- ments of thermal expansion. This work was performed under the auspices of the US De artment of Energy by the Lawrence Livermore National Laboratory under Contract W-7405-EN& REFERENCES 1. 2. 3. 4. 5. 6.  C. R. Wuest, et al., “Precision Machining and Polishing of Scintillating Crystals for Large Calorimeters and Hodoscopes,’’ UCRL-JC-114014 REV1, Proc. Spring Meeting Materials Research Society, San Francisco, CA, April, 1994. “The Compact Muon Solenoid Technical Proposal,” CERN/LHCC 94-38, December, 1994. Z. W. Yin, ”Research and Development Works on BaF2 Crystals in Shanghai Institute of Ceramics,” Proc. Spring Meefing Malerials Research Society, San Francisco, CA, April, 1994. K. V. Krishna Rao, V. T. Deshpande, ”Thermal Expansion and Chemical Bond,” Proc. 6 f h  Int. Symp. Thermal Expansion, Plenum Press, .43-56 (1978) and “Proceedings of the May, 1995 C M S  ECAL Engineering Group Meeting at CE&, ” unpublished. M. Lebeau and J. Badier, private communication. ”LLNL 1994-1995 Discharge Permit,” issued by the City of Livermore, CA. 

Precision machining, polishing and measurement of mechanical and toxicological properties of lead tungstate crystals for the CMS electromagnetic calorimeter

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Precision machining, polishing and measurement of mechanical and toxicological properties of lead tungstate crystals for the CMS electromagnetic calorimeter

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