Microscopic simulation of xenon-based optical TPCs in the presence of molecular additives

Abstract

We introduce a simulation framework for the transport of high and low energy electrons in xenon-based optical time projection chambers (OTPCs). The simulation relies on elementary cross sections (electron-atom and electron-molecule) and incorporates, in order to compute the gas scintillation, the reaction/quenching rates (atom-atom and atom-molecule) of the first 41 excited states of xenon and the relevant associated excimers, together with their radiative cascade. The results compare positively with observations made in pure xenon and its mixtures with CO2 and CF4 in a range of pressures from 0.1 to 10 bar. This work sheds some light on the elementary processes responsible for the primary and secondary xenon-scintillation mechanisms in the presence of additives, that are of interest to the OTPC technology.

Helium–Xenon mixtures to improve the topological signature in high pressure gas xenon TPCs

“…After one meter of drift, a point-like ionization deposit becomes a cloud distributed as a gaussian of 10 mm sigma in the direction perpendicular to the electric field (transverse) and 4 mm in the parallel direction (longitudinal). This situation is far from ideal and can be largely improved by adding molecular electron coolants to the gas [5,6] or by positive-ion detection [7]. As a reference, the thermal diffusion limit which can be found in [8] gives a diffusion factor of ∼1.5 mm/ √ m for a field of 250 V/cm, which is very close to the ∼2.5 mm/ √ m value obtained for instance in Xe/CO 2 mixtures [6].…”

Gaseous and dual-phase time projection chambers for imaging rare processes

“…This is one thousand times less than the information conveyed in the ionization sector, thereby making a calorimetric measurement specially challenging. Present measurements and simulations indicate that W sc (defined as the average energy that it takes to produce a scintillation photon) is fairly constant for primary electrons/x-rays [129,130]. Compared to W I , their values span over a broader range, from 15 to 70 eV, with experimental uncertainties being larger too.…”

“…As an example, a 30 keV x-ray produced in xenon gas under a (not unreasonable) 1% photo-detector coverage at 30% quantum efficiency (QE) results in approximately one photon recorded, on average (see, e.g. [129]). This is one thousand times less than the information conveyed in the ionization sector, thereby making a calorimetric measurement specially challenging.…”

“…17 Provided the scintillation probability approaches 1 in pure noble gases, W sc W ex,γ (i.e., the number of emitted photons is very similar to the number of excited species). A useful (but not general) description in the presence of additives is the 'triplet dominance model' ( [129], [132]), under which: 16 Lighter noble gases display increasingly longer lifetimes [136,137]. 17 In liquid, ∆ε/Wex must be interpreted as the number of excitons, the formal treatment being analogous to that in gas phase.…”

“…(Figure adapted and reprinted from [30]). Bottom: path diagram for xenon scintillation (from [129]). This partly artistic diagram indicates the three type of states involved, that appear associated to different radii (from right to left: unbound ≡ atom, loosely bound, and strongly bound or 'excimer'), each identified with boxes.…”