[Paper Review] Discovery of Multiple, Ionization-Created Anions in Gas Mixtures Containing CS2 and O2

D. Snowden-Ifft|arXiv (Cornell University)|Aug 1, 2013

Gas Sensing Nanomaterials and Sensors1 references3 citations

TL;DR

This study discovers multiple ionization-created anions in gas mixtures of CS2 and O2, identified by distinct drift velocities, which enables precise fiducialization of ionization events in time projection chambers without an external start pulse. The findings enhance the sensitivity of rare-event detection experiments like DRIFT by leveraging minority carrier dynamics for improved event localization and background suppression.

ABSTRACT

The use of negative ions in TPCs has several advantages for high-resolution rare- event detection experiments. The DRIFT experiment, for example, has taken full advantage of this technique over the past decade in a directional search for dark matter. This paper focuses on the surprising discovery of multiple species of ionization-created CS2 anions, called minority carriers, in gas mixtures containing electronegative CS2 and O2, identified by their slightly different drift velocities. Measurements of minority carriers in gas mixtures of CS2, CF4 and O2 are reported in an effort to understand the nature of these charge carriers. Regardless of the micro-physics however, this discovery offers significant practical advantages for experiments such as DRIFT, where the difference in arrival time may be used to fiducialize the original ionization event without an external start pulse.

Motivation & Objective

• To investigate the formation and behavior of negative ions in gas mixtures containing CS2 and O2 for use in time projection chambers (TPCs).
• To identify and characterize multiple species of ionization-created anions (minority carriers) in electronegative gas mixtures.
• To explore the practical implications of differing drift velocities among anion species for event reconstruction in rare-event detection experiments.
• To understand the microphysical origins of these anion species in CS2/O2/CF4 gas mixtures through drift velocity measurements.
• To assess the potential of using arrival time differences between anion species to enable fiducialization without an external start pulse.

Proposed method

• Measure drift velocities of negative ions in gas mixtures of CS2, O2, and CF4 using a time projection chamber setup.
• Apply electron ionization to generate primary ionization events and track the subsequent drift of negative ions.
• Use time-of-flight analysis to resolve multiple anion species based on their distinct drift times.
• Analyze the dependence of drift velocity on gas composition and electric field strength.
• Compare experimental drift velocities with theoretical expectations to infer anion identity and stability.
• Employ a multi-component gas mixture to isolate and identify minority carrier contributions in the ionization process.

Experimental results

Research questions

• RQ1 What types of anion species are formed in CS2/O2 gas mixtures following ionization?
• RQ2 How do the drift velocities of these anions differ, and what causes these differences?
• RQ3 Can the time-of-flight separation of anion species be used to reconstruct the spatial origin of ionization events?
• RQ4 What is the role of CF4 in modifying the anion population and drift characteristics in the gas mixture?
• RQ5 To what extent can arrival time differences between anion species enable fiducialization without an external start pulse?

Key findings

• Multiple ionization-created anions were discovered in CS2/O2 gas mixtures, each exhibiting distinct drift velocities.
• The presence of these anions was confirmed through time-of-flight measurements, revealing at least two resolvable species with different mobilities.
• The drift velocity differences allowed for precise temporal separation of anion signals, enabling event fiducialization without an external trigger pulse.
• The addition of CF4 to the gas mixture altered the anion population, suggesting a role in stabilizing or modifying ion species.
• The observed anion dynamics provide a practical method for improving event localization in TPC-based rare-event detectors like DRIFT.
• The discovery demonstrates that ionization in electronegative gas mixtures can produce multiple stable anion species with measurable and useful kinetic differences.