Brand-new extremely light straw-tube detector with a nonwoven graphite-textile

The COMET experiment at J-PARC aims to search for a lepton-flavour violating process of muon to electron conversion, with a branching-ratio sensitivity of $10^{-17}$. The expected signal of this process is monochromatic 105 MeV single electron. To distinguish such a low energy signal, a material budget of detector is essential since the detection accuracy is primarily limited by multiple scattering. To realise the required LOW material budget, a vacuum-compatible ultra-thin-wall straw tracker, 20 µm-thick Mylar straw with 70 nm Al cathode, has been developed employing ultrasonic-welding technique. This was reported in VCI2016, and the detector performances such as detection efficiency and intrinsic spacial resolutions were reported in VCI2019. In parallel to 20 µm straw production, further thinner straw, ie. 12 µm-thick, was developed for the COMET upgrade (COMET Phase-II). Details of R&D on 12 µm straw were reported in VCI2022.

In the process of developing the 12 µm straw, it became clear that it would be fundamentally difficult to make it any thinner using the current straw manufacturing method based on ultrasonic welding. Our R&D showed that the limit is around 10–12 µm. Then, the brand-new extremely light straw was developed with a nonwoven graphite-textile. This was enabled by a collaboration with the nano-tech textile science.

In this article, detailed R&D of the brand-new nonwoven graphite straw will be presented, in addition to the current status of straw-tube tracker construction for the COMET experiment.