Ionization quenching factors and W-values of low-energy H$_2^+$ and He$^+$ ions in Ar gas at low pressure measured with a bulk resistive MICROMEGAS

TL;DR

This work measures the ionisation quenching factors (IQF) for low-energy ${\rm H}_2^+$ and ${\rm He}^+$ ions in a low-pressure Ar/CO$_2$ mixture, spanning 2.5–5 keV and 75–150 mbar. Employing a bulk MICROMEGAS detector and calibrated X-ray sources, the authors quantify the ionisation efficiency and determine $W$-values, revealing a strong energy dependence and no observed pressure effect in the studied range. The results show notable deviations from SRIM predictions, especially at the lowest energies, highlighting limitations in current stopping-power models for low-energy ions in gases. The derived $W$-values and IQF energy trends provide critical input for accurate ion energy reconstruction in spaceborne ENA detectors such as SWEATERS, advancing detector calibration for microdosimetry, dark matter searches, and space weather instrumentation.

Abstract

The ionization quenching factor, the fraction of an ion's initial kinetic energy lost through ionization in a medium, was measured for H$_2^+$ and He$^+$ ions within the 2.5-5 keV energy range in an Ar/CO$_2$ gas mixture at pressures between 75 and 150 mbar. The mixture was contained in the active volume of a MICROMEGAS type Micro Pattern Gaseous Detector (MPGD), which was connected to an ion source via a high vacuum system. The results showed a significant decrease in ionization as the ion energy decreased, although no dependence on gas pressure was observed within the low pressure range studied. Additionally, significant deviations from the predictions of SRIM (Stopping and Range of Ions in Matter) simulations were found. These measurements also allowed the first determination of $W$ values (the average energy required to create an electron-ion pair in the gas) for H$_2^+$ and He$^+$ ions in Ar at this particular energy range, which had not been previously explored for these projectile-target combinations. These findings are essential for accurate ion energy reconstruction in low-pressure MPGDs, with particular relevance for space-based energetic neutral atom detection, as proposed by the Italian Space Agency's SWEATERS project.

Figures (10)

• Figure 1: The Ion Beam Facility (IBF) directs an ion beam from the Ion Source (IS) toward the Main Chamber (MC), as indicated by a green arrow. The MC vacuum conditions are maintained using a Turbo Pump (TP) and a Scroll Pump (SP), and monitored with a Pressure Gauge (PG) and a Residual Gas Analyzer (RGA). The MM is mounted on a Positioning System (PS) that allows motion in all six degrees of freedom.
• Figure 2: Mechanical support frame of the MM detector connected to the beam pipe of the IBF. The $^{55}$Fe radioactive source (in its orange protective container) is positioned in the lateral window.
• Figure 3: Example of digitized waveform acquired when an X-ray from a $^{55}$Fe source is detected by the MM at a gas pressure of 75 mbar. The red line represents the fitted function (Eq. \ref{['equ:FitFunction']}). An estimate of the collected charge is given by the height of the fitted peak, indicated by the blue dot.
• Figure 4: Pulse height distributions measured with the MM (gas pressure at 150 mbar) irradiated with 5 keV He$^+$ (a) and H$_2^+$ (b) ions. The red lines represent Gaussian fits.
• Figure 5: Spectrum of $^{55}$Fe (a) and $^{109}$Cd (b) measured with the MM held at 150 and 1000 mbar respectively.