Method for precision measurement of the Range -- Energy relation for protons in pure hydrogen gas
The available experimental information on the Range-Energy relation for protons stopped in hydrogen gas is summarized in the SRIM software package. The estimated precision of this data is several percents. Here we describe a possibility to measure this relation with 0.1$\%$ precision in the course of an electron-proton elastic scattering experiment to be performed in the 720 MeV electron beam at the Mainz Microtron (MAMI). This experiment, aimed at precision measurement of the proton charge radius, exploits a large hydrogen "active target" to detect the recoiled protons and a forward tracker to detect the scattered electrons. The angles of the scattered electrons are measured with $2\cdot10^{-4}$ absolute precision. Also, the electron beam momentum is known at MAMI with $2\cdot10^{-4}$ absolute precision. This gives a possibility to determine with 0.1$\%$ absolute precision the four-momentum transfer $Q^2$ which, in its turn, is a direct measure of the recoiled proton energy $T_{p}$: $Q^2 = 2M_{p}T_{p}$, where M$_{p}$ is the proton mass. From this point of view, this experimental setup can be considered as a source of protons with well defined energies inside an "active target", which is a specially designed hydrogen high-pressure Time Projection Chamber (TPC). The design of the TPC allows to measure with high precision the energy of the protons corresponding to some selected values of the proton range. In this way, the Range-Energy relation can be established for the proton energies from 1 MeV to 9.3 MeV with 0.1$\%$ absolute precision, the maximal energy being limited by the size of the TPC and by the hydrogen gas pressure.