We report on a windowless, high-density, gas flow target at Jefferson Lab that was used to measure rp, the root-mean-square charge radius of the proton. To our knowledge, this is the first such system used in a fixed-target experiment at a (non-storage ring) electron accelerator. The target achieved its design goal of an areal density of 2×1018 atoms/cm2, with the gas uniformly distributed over the 4 cm length of the cell and less than 1% residual gas outside the cell. This design eliminated scattering from the end caps of the target cell, a problem endemic to previous measurements of the proton charge radius in electron scattering experiments, and permitted a precise, model-independent extraction of rp by reaching unprecedentedly low values of Q2, the square of the electron's transfer of four-momentum to the proton.
@article{arxiv.2103.01749,
title = {The PRad Windowless Gas Flow Target},
author = {J. Pierce and J. Brock and C. Carlin and C. Keith and J. Maxwell and D. Meekins and X. Bai and A. Deur and D. Dutta and H. Gao and A. Gasparian and K. Gnanvo and C. Gu and D. Higinbotham and M. Khandaker and N. Liyanage and M. Meziane and E. Pasyuk and C. Peng and V. Punjabi and W. Xiong and X. Yan and L. Ye and Y Zhang},
journal= {arXiv preprint arXiv:2103.01749},
year = {2021}
}