On the Mechanism of Townsend Avalanche for Negative Molecular Ions

TL;DR

This paper investigates the initiation mechanism of Townsend avalanches in negative ion time projection chambers, revealing that in CS$_2$ vapor it is likely caused by collisional electron detachment, with more complex mechanisms in Nitromethane-CO$_2$ mixtures.

Contribution

It provides the first detailed understanding of avalanche initiation in NITPCs, highlighting the role of collisional detachment in CS$_2$ and complex mechanisms in mixtures.

Findings

Avalanche in CS$_2$ vapor is likely initiated by collisional electron detachment.

Mechanism in Nitromethane-CO$_2$ mixtures appears more complex.

Enhanced understanding of gas gain mechanisms in negative ion TPCs.

Abstract

Time projection chambers drifting negative ions (NITPC) instead of electrons have several advantages. A NITPC can operate at very high reduced drift fields without diffusion runaway, and the readout digitization sampling rate requirement is considerably relaxed due to the low drift speed of negative ions. The initiation of Townsend avalanches to allow gas gain in these devices has not been understood until now. It is shown here that the avalanche in low pressure CS$_2$ vapor is most likely initiated by collisional detachment of the electron from the negative molecular ion. In mixtures of Nitromethane vapor with CO$_2$ the mechanism appears to be more complex.