Construction and characterization of a seven-chip GridPix X-ray detector for solar axion searches

TL;DR

The paper presents the construction and characterization of a seven-GridPix X-ray detector designed for solar axion searches with the CAST experiment. It integrates hardware vetoes (scintillators, septem/line veto using surrounding GridPixes) and a software-based likelihood classifier to achieve ultra-low backgrounds while maintaining sensitivity to very low-energy X-rays. Calibration with a variable X-ray source and a long CAST data-taking campaign demonstrate significant background suppression, with a final rate around $7.92\times10^{-6}$ keV$^{-1}$ cm$^{-2}$ s$^{-1}$ in the 0–8 keV range and a combined efficiency of about 61.6%. The authors also outline a future upgrade to GridPix3 (Timepix3) that could provide simultaneous ToA and ToT measurements, enabling full removal of FADC-triggered vetoes and further improvements in background rejection and energy threshold.

Abstract

In the scope of solar axion searches, detectors which are able to measure low energetic X-rays with high efficiency are required. For this purpose a detector based on the GridPix technology was built for the CAST experiment at CERN. The GridPix is a pixelised readout ASIC (Timepix) with a Micromegas-like gas amplification stage (grid) built photolithographically on top. In order to reduce the detector's background level, several hardware and software vetoes were implemented. Hardware-wise, these vetoes consist of a ring of six GridPixes around a central GridPix, a readout of the induced grid signal, and two scintillators. On the software side, multiple approaches to distinguish between background events and X-ray photons are implemented. Here, also the hardware features, like the six surrounding GridPixes, are used. The new detector was tested in a long ($3500\,\text{h}$) background data taking campaign. The performance of the new vetoes was evaluated. The detector performance itself, for low energetic X-rays, was also evaluated with a variable X-ray generator using eight different energies from 0 to $10\,\text{keV}$. The efficiency for very low energetic X-rays and the energy resolution was determined.

Figures (19)

• Figure 1: Comparison of the axion spectrum for different possible production mechanisms. This figure uses $g_\text{ae} = 1.0e-13$ and $g_\text{a\textgamma} = 1e-12GeV^{-1}$ for which axion-electron $g_{ae}$ production is dominant.redondo2013solarJvO_axionElectron
• Figure 2: Exploded view of the detector showing the individual components.
• Figure 3: SEM image of the grid above a Timepix ASIC. The grid and the pixels underneath the holes can be seen. Darker areas on the grid indicate the positions of the pillars holding the grid. Courtesy of gridpix-izm-berlin.
• Figure 4: Layout of the 300 Norcada windows used at CAST in 2017/18. The strongback has a thickness of 200µm and thus is opaque to X-rays of desired energies.
• Figure 5: Comparison of two different window setups. A $2µ m$ thick Mylar window as it was used in the GridPix detector at CAST in 2014/15 krieger2018search and the 300nm SiN window setup used on this detector. As a reference the solar axion flux for an axion-electron coupling $g_{ae}$ dominated model is shown, highlighting the significant gain in transmission in the relevant energy ranges below $3keV$Schmidt_xrayAttenuation_2022JvO_axionElectron.