Photoelectron track length distributions measured in a negative ion time projection chamber

TL;DR

This study measures and analyzes photoelectron track length distributions in specific gas mixtures using a negative ion time projection chamber, providing the first quantitative analysis of such distributions relevant for optimizing TPC and NITPC detectors.

Contribution

It presents the first quantitative analysis of photoelectron track length distributions in gases, fitting the data with lognormal and power-law models, and relates these to detector optimization.

Findings

Track lengths follow a lognormal distribution at fixed energies.

Power-law relationship between mean track length and energy is established.

Measured power-law exponents are approximately 1.2 to 1.3 for different gas mixtures.

Abstract

We report photoelectron track length distributions between 3 and 8 keV in gas mixtures of Ne+CO2+CH3NO2 (260:80:10 Torr) and CO2+CH3NO2 (197.5: 15 Torr). The measurements were made using a negative ion time projection chamber (NITPC) at the National Synchrotron Light Source (NSLS) at the Brookhaven National Laboratory (BNL). We report the first quantitative analysis of photoelectron track length distributions in a gas. The distribution of track lengths at a given energy is best fit by a lognormal distribution. A powerlaw distribution of the form, f(E)=a(E/Eo)n, is found to fit the relationship between mean track length and energy. We find n=1.29 +/- 0.07 for Ne+CO2+CH3NO2 and n=1.20 +/- 0.09 for CO2+CH3NO2. Understanding the distribution of photoelectron track lengths in proportional counter gases is important for optimizing the pixel size and the dimensions of the active region in electron-drift time projection chambers (TPCs) and NITPC X-ray polarimeters.