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A hydrophobic, highly transparent silica aerogel with a refractive index of $\sim $1.15 was developed using sol–gel polymerization, pin drying, and supercritical carbon dioxide solvent extraction technologies. A total of 96 monolithic tiles with dimensions of 11 cm $\times $ 11 cm $\times $ 1 cm were mass produced with a high crack-free yield for use as Cherenkov radiators to be installed in the proximity-focusing ring-imaging Cherenkov (RICH) detector. The RICH detector, containing 36 aerogel tiles, will be installed in the High Energy Light Isotope eXperiment (HELIX) spectrometer and used to measure the velocity of cosmic-ray particles. HELIX is a balloon-borne experimental program designed to measure the chemical and isotopic abundances of light cosmic-ray nuclei. A water-jet cut test of the aerogel tiles and a gluing test of the trimmed tiles with dimensions of 10 cm $\times $ 10 cm $\times $ 1 cm in an aluminum frame were successful in the context of integration into the radiator module.

HELIX (High Energy Light Isotope eXperiment) is a balloon-borne experiment designed to measure the chemical and isotopic abundances of light cosmic ray nuclei. Detailed measurements by HELIX, especially of $^{10}$Be from 0.2 GeV/n to beyond 3 GeV/n, will provide an essential set of data for the study of propagation processes of the cosmic rays. HELIX consists of a 1 Tesla superconducting magnet with a high-resolution gas tracking system, time-of-flight detector, and a ring-imaging Cherenkov detector. The instrument is scheduled to have a long-duration balloon flight out of McMurdo Station during NASA's 2020/21 Antarctic balloon campaign. Here, we discuss the scientific goals and the design of the experiment, and report on its current status.

HELIX (High Energy Light Isotope eXperiment) is a balloon-borne experiment designed to measure the chemical and isotopic abundances of light cosmic ray nuclei, especially the $^{\textrm{10}}\textrm{Be}/^{\textrm{9}}\textrm{Be}$ ratio over the energy range from 0.2 GeV/n to beyond 3 GeV/n. This is a key measurement for constraining cosmic-ray propagation models. The detector is a mass spectrometer, which is built around a 1 Tesla superconducting magnet and a high-resolution tracking system to determine particle rigidity. Time-of-flight counters and a ring-imaging Cherenkov detector (RICH) are used to measure velocities. The proximity-focused RICH consists of a radiator made of aerogel tiles (refractive index approximately 1.15) and a detector plane of silicon photomultipliers. For discrimination of the $^{\textrm{9}}\textrm{Be}$ and $^{\textrm{10}}\textrm{Be}$ isotopes at high energy, the refractive index of the aerogel must be known to a precision of 0.07%. Given the manufacturing tolerances in the production process, the index must be mapped over the lateral extent of aerogel tiles on a fine grid. In this contribution, we describe and show initial results from procedures developed for this task. These include laser-deflection and electron-beam measurements.

This paper reports the successful fabrication of silica aerogel Cherenkov radiators produced in the first batches from a 96-tile mass production performed using pin-drying technique in our laboratory. The aerogels are to be used in a ring-imaging Cherenkov detector in the spectrometer of a planned balloon-borne cosmic-ray observation program, HELIX (High Energy Light Isotope eXperiment). A total of 36 transparent, hydrophobic aerogel tiles with a high refractive index of 1.16 and dimensions of 10 cm $\times $ 10 cm $\times $ 1 cm will be chosen as the flight radiators. Thus far, 40 out of the 48 tiles fabricated were confirmed as having no tile cracking. In the first screening, 8 out of the first 16 tiles were accepted as flight-qualified candidates, based on basic optical measurement results. To fit the aerogel tiles into a radiator support structure, the trimming of previously manufactured prototype tiles using a water-jet cutting device was successful., Submitted to Nucl. Instrum. Methods Phys. Res. A (NIMA Proc. Special Issue: RICH 2018), 5 pages, 6 figures