[Paper Review] Sealed operation, and circulation and purification of gas in the HARPO TPC

M. Frotin, Philippe C. Gros+10 more|arXiv (Cornell University)|Dec 10, 2015

Particle Detector Development and Performance8 references3 citations

TL;DR

This paper demonstrates sealed operation of the HARPO time projection chamber (TPC) for five months, showing measurable gas degradation due to impurities; a lightweight circulation and purification system (CAPS) rapidly restored initial gas properties, including drift velocity, gain, and electron attenuation, confirming long-term stability for space-based gamma-ray telescopes.

ABSTRACT

HARPO is a time projection chamber (TPC) demonstrator of a gamma-ray telescope and polarimeter in the MeV-GeV range, for a future space mission. We present the evolution of the TPC performance over a five month sealed-mode operation, by the analysis of cosmic-ray data, followed by the fast and complete recovery of the initial gas properties using a lightweight gas circulation and purification system.

Motivation & Objective

• Address the challenge of maintaining high gas purity in sealed TPCs for long-duration space missions.
• Investigate gas degradation mechanisms—particularly electron attachment and contamination—during extended sealed operation.
• Develop and validate a lightweight, low-power gas circulation and purification system (CAPS) for in-situ recovery of gas quality.
• Demonstrate that gas parameters critical to TPC performance (drift velocity, gain, attenuation) can be fully restored after degradation.
• Establish the feasibility of stable, long-term TPC operation in space using minimal, autonomous gas purification.

Proposed method

• Operated the HARPO TPC in sealed mode for five months, using cosmic rays to monitor gas performance parameters.
• Employed a simple turbine-based gas circulation system (CAPS) with Oxisorb cartridges to remove H₂O and O₂ down to <5 ppb O₂ and 30 ppb H₂O.
• Monitored gas pressure, temperature, and flow using calibrated sensors and a Bronkhorst mass flowmeter.
• Measured drift velocity, gain, and electron attenuation from cosmic-ray data, normalizing to a reference run after fresh gas fill.
• Used Garfield++ simulations to model electron attachment and predict time constants for purification recovery.
• Conducted high-resolution mass spectrometry (R=2800) on gas samples before and after purification to quantify impurities.

Experimental results

Research questions

• RQ1 To what extent does gas impurity accumulation degrade key TPC performance parameters during sealed operation?
• RQ2 Can a lightweight, low-power gas circulation and purification system (CAPS) fully restore initial gas quality and TPC performance?
• RQ3 What are the dominant contaminants affecting electron drift and amplification in a sealed TPC over time?
• RQ4 How do changes in gas composition (e.g., O₂, H₂O, isobutane) correlate with measurable changes in drift velocity and electron attenuation?
• RQ5 What is the characteristic time constant for purification recovery, and how does it compare to simulation predictions?

Key findings

• Over five months of sealed operation, electron attenuation increased due to oxygen contamination, with O₂ rising from <20 ppm to 180 ppm.
• The drift velocity decreased by approximately 6.0 ± 0.5% after gas pressure dropped from 2.1 to 1.9 bar and isobutane decreased from 5.1% to 4.5%.
• After activating CAPS, electron attenuation recovered to initial levels with a characteristic purification time constant of τ_purif = 105 ± 4 hours.
• O₂ levels dropped below the detection limit (<20 ppb) after purification, while CO and CO₂ levels decreased significantly, indicating outgassing from plastics as a source.
• The isobutane concentration decreased by ~15% after purification and stabilized, though the cause remains unexplained.
• Drift velocity and gain fully recovered after purification, with drift velocity returning to 106% of its initial value, consistent with pressure and composition corrections.