Very transparent liquid scintillator (LS) is chosen as the antineutrino target of the Daya Bay Reactor Neutrino Experiment. The antineutrino detectors are immersed in 2,000 tons of ultra-pure water. To avoid contamination of the LS or water by the detector materials, hundreds of tests have been done at Institute of High Energy Physics in Beijing and Brookheaven in U.S. since February 2007 to select the materials.
In the industry of pure water, it is well known that stainless steel will pollute water, although at low levels. The leaching rate is about 30 ng/cm2/h. The leaching rate of polypropene (PP) and polyethene (PE) is 0.14 and 0.29 times that of stainless steel. Teflon and quartz are much better, about 0.07 of stainless steel. In our compatibility tests, materials are immersed in liquid scintillator. The UV-vis absorption spectrum of the liquid is monitored over time to know if there is any impurity leaching out from materials. We found that most plastics pollute the liquid scintillator apparently. Only a small portion of fluoro-plastics has invisible leaching. It is specially interesting that high density polyethene (HDPE) is found to have only a little pollution to linear alkylbenzene (LAB), while linear low-density polyethene (LLDPE) has a lot. Probably due to different polymerization techniques, the LLDPE has more unpolymerized monomers inside, which will leach out when soaked in solvent.
Disposable plastics items are widely used in biological and pharmacological laboratories. Andrew Holt, an assistant professor of pharmacology at the University of Alberta, found that their plastic tubes can leach out two additives, the disinfectant (DiHEMDA) into water and the lubricant oleamide into methanol and DMSO, which make their results unintelligible. He reported on Science magazine in Nov. 2008 that chemicals leaching from plastic lab equipment could be compromising the accuracy of biological studies.
We conclude that 1) chemicals leaching from plastics could compromise the accuracy of particle physics experiments, but it is not a big deal worth an announcement. If it is compromising the accuracy of biological studies, that’s worth a Science article. 2) A study, maybe not original, or even a common sense, could be very valuable if it does help a lot of people.
P.S. I asked my colleage Zhiyong Zhang if they encountered similar cases in their biology-related researches. He said yes! Several years ago, Shanghai Institute of Applied Physics surveyed the lab equipments in the market and measured the leaching impurity with ICP-MS. They identified a product with the least leaching. To avoid contaimination in their experiments, Zhiyong bought 10,000 of those tubes!
不存在干净的容器
大亚湾反应堆中微子实验的中微子探测器使用极为透明的液体闪烁体，中微子探测器则浸泡在纯净水中。为了防止探测器材料污染纯净水和液体闪烁体，自2007年2月起，数百个材料与纯水或液体闪烁体的化学兼容性试验在北京的高能物理所和美国的布鲁克海汶实验室进行。
在纯净水工业中，不锈钢会析出杂质污染水是常识性的，尽管不大。析出量大约是30ng/cm2/h。聚丙烯与聚乙烯的析出量约为不锈钢的0.14和0.29倍。聚四氟乙烯和石英好得多，析出量为不锈钢的0.07倍。在我们的兼容性实验中，材料被泡在液体闪烁体中，通过监测液体闪烁体的紫外可见吸光度随时间的变化来判断是否有杂质析出到液体中。我们发现除了少数氟塑料对液体闪烁体的污染难以察觉外，大部分的塑料对液体闪烁体的污染是明显的。特别有意思的是，高密度聚乙烯对烷基苯的污染很小，可是线性低密度聚乙烯则非常明显。因此我们估计，由于聚合方法不同，线性低密聚乙烯存在较多的未聚合单体，浸泡在溶剂中时易于析出。
廉价的一次性塑料器皿在全世界的生物实验室中被广泛使用。加拿大阿尔伯特大学的药理学副教授安德鲁•霍尔特发现，所用的塑料试管会析出两种生产塑料所用的添加剂：消毒剂DiHEMDA和润滑剂油酸酰胺，这些析出物使他的实验结果莫明其妙。2008年11月他在《科学》杂志上发表文章，称“从塑料实验容器中溶出的化学物质可能会使生物、医学研究的精确性大打折扣”。
我们可以得到如下结论：1）从塑料容器中析出的化学物质可能会使粒子物理实验的精确性大打折扣，不过不值得发表；如果使生物、医学研究的精确性大打折扣，那就值得发表在《科学》上。2）一项成果，也许不是原创性的，甚至是常识性的，如果能够帮助很多人，那也是很重要的成果。
后记：我问我的同事张智勇是否碰到过这种情况。他说多年前就注意到了，上海应用物理所检测了多个国内厂家的产品，最后选择了一个杂质含量最低的厂家，通过他们买了这个厂家的1万个瓶子。
