The PRad Windowless Gas Flow Target

TL;DR

This paper introduces a novel windowless, high-density gas flow target at Jefferson Lab, enabling precise, model-independent measurement of the proton's charge radius by reducing scattering-related uncertainties in electron scattering experiments.

Contribution

The paper presents the first use of a windowless gas flow target in a fixed-target electron scattering experiment, achieving high density and uniform distribution to improve proton radius measurements.

Findings

Achieved areal density of 2×10^18 atoms/cm^2

Reduced residual gas outside the target to less than 1%

Enabled measurements at very low Q^2 for precise radius extraction

Abstract

We report on a windowless, high-density, gas flow target at Jefferson Lab that was used to measure $r_p$, 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$\times$10$^{18}$ atoms/cm$^2$, 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 $r_p$ by reaching unprecedentedly low values of $Q^2$, the square of the electron's transfer of four-momentum to the proton.