Abstract
The Time Projection Chamber (TPC) is an ideal candidate to finely study the charged particle ionization in a gaseous medium. Large volume TPCs can be readout with a suitable number of channels offering a complete 3D reconstruction of a charged particle track, that is the sequence of its energy releases in the TPC gas volume. Moreover, He-based TPCs are very promising to study keV energy particles as nuclear recoils, opening the possibility for directional searches of Dark Matter (DM) and the study of Solar Neutrinos (SN).In this paper we report the analysis of the data acquired with a small TPC prototype (named LEMOn) built by the CYGNO collaboration that was exposed to a beam of 450 MeV electrons at the Beam Test Facility of National Laboratories of Frascati. LEMOn is operated with a He-CF4 mixture at atmospheric pressure and is based on a Gas Electron Multipliers amplification stage that produces visible light collected by the high granularity and very good sensitivity of scientific CMOS camera. This type of readout – in conjunction with a fast light detection – allows a 3D reconstruction of the electrons tracks. The electrons are leaving a trail of clusters of ionizations corresponding to a few keV energy release each. Their study leads to predict a keV energy threshold and 1–10 mm longitudinal and 0.1–0.3 mm transverse position resolution (sigma) for nuclear recoils, very promising for the application of optically read out TPC to DM searches and SN measurements.
Highlights
Large Time Projection Chambers (TPC) have been employed in various high energy physics and nuclear physics experiments [1, 2, 3, 4]
The fast photomultiplier [53] (PMT) signal waveform was acquired using as external trigger from the timing signal of the BTF line synchronized with the electrons arrivals
The standard deviation of the distribution of the residuals of ts at each Z can be converted into the standard deviation of the BTF electrons Z position using the gas mixture drift velocity that was simulated with Garfield [63] to be 6 cm/μs for an electric field of 500 V/cm
Summary
Large Time Projection Chambers (TPC) have been employed in various high energy physics and nuclear physics experiments [1, 2, 3, 4]. LEMOn has an optical readout of a triple-GEM structure based on one sCMOS camera to image energy releases in He-based gas mixtures kept at atmospheric pressure This would allow to host larger target mass for DM searches than previous low pressure TPC prototypes and experiments. We study a method to measure the longitudinal position in the drift volume based on the electron diffusion, as already proposed for an electronic readout (see for example [35, 36]) This is very important to fully define a fiducial gas volume in a DM search, in a way to discard the background events generated by the detector components, in particular the cathode and the amplification system (as the GEMs)[37, 38]. LEMOn was accommodated over a remotely controlled table in order to scan the Z coordinate (with a 0.2 mm precision)
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