PlomBoxear: A Device for Open Source Metrology to Fight Lead Contamination in Drinking Water This project aims to translate research funded by a 2017 STFC GCRF Foundation Award to deliver an  open-source, low-cost device to measure lead (Pb) contamination in drinking water.  This project draws on an area of research supported by the STFC core science programme: ultra-precise radioactivity measurement and calorimetry developed to search for dark matter.  The predecessor GCRF project built upon this research to develop detection of Pb in water using interactions of radioactivity emitted from Pb atoms in CMOS sensors, together with data acquisition (DAQ) via a mobile phone.  This project aims to translate these developments to application in a low-cost device, and demonstrate its utility for measuring Pb contamination in water in a pilot study in Argentina.  Development and validation activities will be carried out across the UK, Argentina and México.  The motivation for growing capacity for and developing new Pb measurement techniques is that Pb pollution is the #1 cause of loss of life expectancy in the world, and currently 2 of the top 10 worst pollution problems in the world are associated with Pb [1]. This is a problem both in LMICs and in high income countries- examples like the Flint, Michigan, USA, water safety breaches demonstrate that water quality in violation of World Health Organization (WHO) guidelines can happen anywhere, and information about water quality, and its variation in time, is not widespread. Even 'clean' technologies, like electric cars, employ traditional Pb-based batteries; the disposal of these batteries is a leading contributor to Pb pollution in drinking water, particularly in LMICs.  Currently, Pb in water measurements require expensive, specialized equipment to reach relevant sensitivity levels for human health- even 0.1 parts per million contamination with Pb produces devastating biological impacts. The device we propose to develop, the PlomBox, will use a custom sensor assembly box that plugs into a mobile phone to acquire and analyze the data.  We aim for the box cost to be <10 GBP.  The goal is to make widely-distributed metrology and real-time, crowd-sourced monitoring of Pb levels in drinking water possible for the first time.  This technology could have a transformative impact on the public health problems associated with Pb pollution.  The World Health Organization (WHO) limit for Pb in drinking water is 10 parts per billion (ppb).  Reaching this level of sensitivity is an incredibly challenging problem, which we address by leveraging the precision calorimetry techniques for Pb in silicon (Si) detectors demonstrated in the predecessor GCRF project, together with biological techniques to detect trace impurities using engineered bacteria demonstrated by the U. of Buenos Aires collaborators.   We request support for 873k over 18 months to support PDRAs and Technicians in Argentina (5), México (3), and the UK (3), travel, and consumables costs to fabricate the PlomBox prototypes, deploy them in a field test, cross-validate against Germanium (Ge) detector gamma spectroscopy and inductively coupled plasma mass spectrometry (ICP-MS).  [1] http://www.worstpolluted.org/ The Global Challenges Research Fund (GCRF) supports cutting-edge research to address challenges faced by developing countries. The fund addresses the UN sustainable development goals. It aims to maximise the impact of research and innovation to improve lives and opportunity in the developing world.