We have been developing silica aerogels for use as RICH radiators used in the HELIX (High-Energy Light Isotope eXperiment) spectrometer. The HELIX program is a balloon-borne cosmic-ray experiment designed to measure the mass of light cosmic-ray isotopes (in particular, those of beryllium). The main objective is to explore the propagation mechanism of cosmic rays by measuring the relative abundances of key light cosmic-ray isotopes. The first flight is scheduled during NASA's 2019/20 Antarctic long-duration balloon campaign.
The HELIX instrument consists of a 1-T superconducting magnet, a gas drift chamber, time-of-flight counters, and an aerogel RICH system. The HELIX RICH is used for measuring the velocity of cosmic-ray isotopes in an energy range from 1 to 3 GeV/nuc. The system is a proximity-focusing RICH with an expansion length of 50 cm, and a focal-plane composed of a silicon-photomultiplier module array with a pixel size of 6 mm. Hydrophobic aerogels with a refractive index, n of 1.15 were chosen as the radiators. The requirements for the aerogels include tile dimensions of 10 $\times $ 10 $\times $ 1 cm and refractive index and thickness uniformities of ~1% across the tile for the precise determination of the particle velocity. The dimensions of the radiator module are 60 $\times $ 60 cm; thus, a total of 36 tiles are needed.
Developing highly transparent aerogels with an ultrahigh index exceeding 1.10 without cracking is a serious challenge. In late 2016, we began the experimental fabrication of aerogels specific for HELIX. We determined that the production of n = 1.15 tiles is feasible using the pin-drying technology. In early 2018, we succeeded in prototyping highly transparent (a transmittance of ~73% for 400-nm wavelength) aerogels close to the flight-qualified radiators. We have begun the mass production of 92 tiles as candidates for the flight aerogels at the end of March 2018. Here, progress in the development and mass production of high optical quality aerogels is presented.