GasDB

Overview

This database is filled with both simulations and measurements. At the moment, only simulation runs performed with $\magboltz$, configured and run through $\garfield$ on our local computing cluster, are available. The corresponding $\magboltz$ versions are given within the run names of the simulations. Measurements are done in-house with various detectors of the Gas Monitoring Chamber type (T2K-style GMC and HPGMC). Some data was imported from publications, see the run names for the original sources.

Usage

The workflow is from left to right:

Select the desired gases one by one from the dropdown list, then click the "Submit Gases" button.
- Uncheck "Strict" if additional gases are allowed in the mixture, as available.
Select a specific mixture from the dropdown list, then click the "Submit Mixture" button.
- You can type in the field to filter the mixtures and only display those which match your input.
Select a run from the dropdown list, then click the "Add Run to List" button to add it to your plotting and download list.

By default, no range restrictions are applied to $E$, $p$, or $B$. If desired, these have to be set before submitting the gas mixture.

Runs on the runs list can be downloaded. Error bars displayed in the plots correspond to the total errors, but exported data distinguishes between statistical and systematic errors. The exported data is in human readable format and can be read into Python with our example project example.py.

For online plotting, a number of options are available, most of which are self-explanatory. Density scaling corrections for comparing electron swarm parameters at different pressures or temperatures are a useful preselection possibility, since simulation runs can contain scans in the gas density. More details on density corrections can be found under Density Scaling.

Density Scaling

For a given gas mixture, all swarm parameters depend in one way or another on the gas density. Given sufficient distance from phase boundaries of the mixture components, the ideal gas law can be used to factor in the change in gas density: \begin{align} pV &= nRT \\ n &\propto \frac{p}{T} \end{align} While the electric field itself is not modified by gas density, the field between microscopic collisions of electrons with the gas components scales inversely with the distance of gas components.

The following table lists the density scalings of swarm parameters available in this database. A density-corrected version of the swarm parameters is available in the $x$- and $y$-axis variable dropdown lists.

Density corrections for the electric field and the gas parameters that are available in this database. At the same $\frac{ET}{p}$, the drift velocity is independent of $T$ and $p$, while diffusion is suppressed and gas gain increases as $\frac{p}{T}$ increases. Table taken and expanded from doi:10.1103/PhysRevD.102.033005. For a more complete table of density corrections, see doi:10.1016/j.nima.2017.09.024.
Swarm Parameter	Density Correction
Electric Field Strength	$E\cdot\frac{T}{p}$
Drift Velocity	$v_\textrm{d}$
Diffusion Coefficients	$\difflt\cdot\sqrt{\frac{p}{T}}$
First Townsend Coefficient	$\alpha\cdot \frac{T}{p}$
Attachment Coefficient	$\eta\cdot \frac{T}{p}$

Drift and Diffusion

Primary ionization electrons are generally extracted from a bulk gas volume to a readout plane by an electric field. For many detectors, the electric field is parallel to a magnetic field, but perpendicular configurations of $\vec{E}$ and $\vec{B}$ are also sometimes used. This database is mostly filled with $B=0~\mathrm{T}$ simulations and measurements, but $\magboltz$ can be run with any angle between $\vec{E}$ and $\vec{B}$.

Drifting, free electrons undergo rapid collisions with almost isotropic scattering. However, the center of the electron cloud effectively moves along the field lines with a much slower velocity, dubbed the drift velocity $v_d$. In the case when $\vec{E}||\vec{B}$, the drift velocity (by convention) only has a component in $z$.

As the electron cloud drifts it undergoes diffusion. This diffusion can be split into two components, one perpendicular (transverse) to the direction of motion $\difft$, and one along (longitudinal) the direction of motion $\diffl$. Magnetic fields affect the transverse component, reducing the spread by a factor relating to microscopically curved tracks of the cyclotron frequency $\omega$ and the mean time between collisions $\tau$:

$$ \frac{\difft(\omega)}{\difft(\omega=0)} = \frac{1}{1+\omega^2\tau^2} $$

Gas Multiplication

After propagation through a drift space, electron clouds are usually multiplied by avalanche ionization in strong electric fields. The amplification is characterized by the first Townsend coefficient $\alpha$, that is the mean path between ionizing collisions. Counteracting to the amplification, free electrons can also be captured. Analogous to $\alpha$, $\eta$ is the mean free path between electron-absorbing collisions in a gas.

The gas gain $G$ for a given field configuration with extent $x_0$ to $x_1$ can be calculated by:

$$ G=\exp\left(\int^{x_0}_{x_1}(\alpha-\eta)dx\right). $$

Swarm Parameter	Density Correction
Electric Field Strength	\(E\cdot\frac{T}{p}\)
Drift Velocity	\(v_\textrm{d}\)
Diffusion Coefficients	\(\difflt\cdot\sqrt{\frac{p}{T}}\)
First Townsend Coefficient	\(\alpha\cdot \frac{T}{p}\)
Attachment Coefficient	\(\eta\cdot \frac{T}{p}\)