First Optical Observation of Negative Ion Drift at Surface Pressure

Abstract

We report the first observation of Negative Ion Drift (NID) at surface pressure of $900 \pm 7$ mbar at Laboratori Nazionali del Gran Sasso in a He:CF$_4$:SF$_6$ mixture using an optically read out Time Projection Chamber (TPC) within the CYGNO/INITIUM project. We present the first PMT waveform analysis in the NID regime, interpreting the temporal light pattern through a model that combines track geometry and charge transport. The inferred drift velocities correspond to mobilities of O(cm$^2$ V$^{-1}$ s$^{-1}$), consistent with negative ion transport. The observed linear scaling of the time extension mean with drift distance reveals the presence of a faster minority charge carrier population in addition to the dominant SF$_6^-$ species, drifting at a $\sim$25\% higher velocity under external inputs. These results demonstrate multi-species negative ion drift operation at surface pressure in a He:CF$_4$:SF$_6$ mixture and open a concrete path toward large scale, low diffusion optical TPCs for rare event searches.

Figures (6)

• Figure 1: Example of raw scientific CMOS camera images acquired with MANGO exposed to a $^{241}$Am source and operated with He:CF$_4$ 60:40 (left) and He:CF$_4$:SF$_6$ 59:39.4:1.6 (right) at LNGS surface pressure. Images are displayed at the same pixel scale with no rebinning, with a scale bar shown for reference.
• Figure 2: Charge signal and PMT response from an alpha track for electron drift (ED, left) and negative ion drift (NID, right) operation at LNGS surface pressure. Top: GEM3 preamplifier output, showing the fast and compact charge arrival in ED (a) and the millisecond scale arrival in NID (b). Bottom: Corresponding ED (c) and NID (d) PMT waveforms, highlighting the compact electron drift signal versus the sparse, extended time structure characteristic of negative ion transport. The dashed lines represent the end and start of the signal, as identified by the algorithm illustrated in Sec. \ref{['subsec:pmt650']}.
• Figure 3: Averaged $E_d \langle \Delta T \rangle$ as a function of the drift distance $Z_{\rm av}$ with a linear fit superimposed ($\chi^2/\mathrm{ndf}=1.69/3$).
• Figure S1: Example of NID PMT waveform analysis: (a) original NID waveform with start and end timestamps obtained with the illustrated procedure overlaid as black dashed lines, (b) NID waveform peaks above 6 times the waveform RMS, (c) rebinned histogram of top right waveform, (d) selected NID waveform $\Delta T$ interval between black dashed lines obtained defining the beginning (end) of the signal when two consecutive bins are above (below) 10 mV.
• Figure S2: Rescaled observable $E_d\langle \Delta T \rangle$ as a function of the average drift distance $Z_{\rm av}$ at $E_d = 400$ V/cm for original data (black circle) and after injection of 10% randomly distributed fake peaks (downward triangles). Linear fits are superimposed, with full line for original data and dashed line for data with random fake peaks injection. The fitted slope parameters are statistically compatible between them within uncertainties and reported explicitly in the text.