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The SKIRT project
advanced radiative transfer for astrophysics
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The SKIRT project
advanced radiative transfer for astrophysics
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#include <XRayAtomicGasMix.hpp>
Public Types | |
| enum class | BoundElectrons : int { None , Free , FreeWithPolarization , Good , Exact } |
 Public Types inherited from MaterialMix | |
| enum class | DynamicStateType { None , Primary , Secondary , PrimaryIfMergedIterations } |
| enum class | MaterialType { Dust , Electrons , Gas } |
Public Member Functions | |
| const vector< double > & | abundancies () const |
| bool | hasPolarizedScattering () const override |
| bool | hasScatteringDispersion () const override |
| double | indicativeTemperature (const MaterialState *state, const Array &Jv) const override |
| double | mass () const override |
| MaterialType | materialType () const override |
| double | opacityAbs (double lambda, const MaterialState *state, const PhotonPacket *pp) const override |
| double | opacityExt (double lambda, const MaterialState *state, const PhotonPacket *pp) const override |
| double | opacitySca (double lambda, const MaterialState *state, const PhotonPacket *pp) const override |
| void | peeloffScattering (double &I, double &Q, double &U, double &V, double &lambda, Direction bfkobs, Direction bfky, const MaterialState *state, const PhotonPacket *pp) const override |
| void | performScattering (double lambda, const MaterialState *state, PhotonPacket *pp) const override |
| BoundElectrons | scatterBoundElectrons () const |
| double | sectionAbs (double lambda) const override |
| double | sectionExt (double lambda) const override |
| double | sectionSca (double lambda) const override |
| vector< StateVariable > | specificStateVariableInfo () const override |
| double | temperature () const |
| Public Member Functions inherited from MaterialMix | |
| virtual double | asymmpar (double lambda) const |
| virtual Array | emissionSpectrum (const MaterialState *state, const Array &Jv) const |
| virtual DisjointWavelengthGrid * | emissionWavelengthGrid () const |
| virtual Array | emissivity (const Array &Jv) const |
| virtual bool | hasContinuumEmission () const |
| virtual DynamicStateType | hasDynamicMediumState () const |
| virtual bool | hasExtraSpecificState () const |
| virtual bool | hasLineEmission () const |
| virtual bool | hasNegativeExtinction () const |
| virtual bool | hasPolarizedAbsorption () const |
| virtual bool | hasPolarizedEmission () const |
| virtual bool | hasPolarizedScattering () const |
| virtual bool | hasResonantScattering () const |
| virtual bool | hasScatteringDispersion () const |
| virtual bool | hasStochasticDustEmission () const |
| virtual double | indicativeTemperature (const MaterialState *state, const Array &Jv) const |
| virtual void | initializeSpecificState (MaterialState *state, double metallicity, double temperature, const Array ¶ms) const |
| bool | isDust () const |
| bool | isElectrons () const |
| bool | isGas () const |
| virtual bool | isSpecificStateConverged (int numCells, int numUpdated, int numNotConverged, MaterialState *currentAggregate, MaterialState *previousAggregate) const |
| virtual Array | lineEmissionCenters () const |
| virtual Array | lineEmissionMasses () const |
| virtual Array | lineEmissionSpectrum (const MaterialState *state, const Array &Jv) const |
| virtual double | mass () const =0 |
| virtual MaterialType | materialType () const =0 |
| virtual double | opacityAbs (double lambda, const MaterialState *state, const PhotonPacket *pp) const =0 |
| virtual double | opacityExt (double lambda, const MaterialState *state, const PhotonPacket *pp) const =0 |
| virtual double | opacitySca (double lambda, const MaterialState *state, const PhotonPacket *pp) const =0 |
| virtual vector< SnapshotParameter > | parameterInfo () const |
| virtual void | peeloffScattering (double &I, double &Q, double &U, double &V, double &lambda, Direction bfkobs, Direction bfky, const MaterialState *state, const PhotonPacket *pp) const =0 |
| virtual void | performScattering (double lambda, const MaterialState *state, PhotonPacket *pp) const =0 |
| virtual double | sectionAbs (double lambda) const =0 |
| virtual double | sectionExt (double lambda) const =0 |
| virtual const Array & | sectionsAbs (double lambda) const |
| virtual const Array & | sectionsAbspol (double lambda) const |
| virtual double | sectionSca (double lambda) const =0 |
| virtual vector< StateVariable > | specificStateVariableInfo () const =0 |
| virtual const Array & | thetaGrid () const |
| virtual UpdateStatus | updateSpecificState (MaterialState *state, const Array &Jv) const |
| Public Member Functions inherited from SimulationItem | |
| template<class T > | |
| T * | find (bool setup=true) const |
| template<class T > | |
| T * | interface (int levels=-999999, bool setup=true) const |
| virtual string | itemName () const |
| void | setup () |
| string | typeAndName () const |
| Public Member Functions inherited from Item | |
| Item (const Item &)=delete | |
| virtual | ~Item () |
| void | addChild (Item *child) |
| const vector< Item * > & | children () const |
| virtual void | clearItemListProperty (const PropertyDef *property) |
| void | destroyChild (Item *child) |
| virtual bool | getBoolProperty (const PropertyDef *property) const |
| virtual vector< double > | getDoubleListProperty (const PropertyDef *property) const |
| virtual double | getDoubleProperty (const PropertyDef *property) const |
| virtual string | getEnumProperty (const PropertyDef *property) const |
| virtual int | getIntProperty (const PropertyDef *property) const |
| virtual vector< Item * > | getItemListProperty (const PropertyDef *property) const |
| virtual Item * | getItemProperty (const PropertyDef *property) const |
| virtual string | getStringProperty (const PropertyDef *property) const |
| int | getUtilityProperty (string name) const |
| virtual void | insertIntoItemListProperty (const PropertyDef *property, int index, Item *item) |
| Item & | operator= (const Item &)=delete |
| Item * | parent () const |
| virtual void | removeFromItemListProperty (const PropertyDef *property, int index) |
| virtual void | setBoolProperty (const PropertyDef *property, bool value) |
| virtual void | setDoubleListProperty (const PropertyDef *property, vector< double > value) |
| virtual void | setDoubleProperty (const PropertyDef *property, double value) |
| virtual void | setEnumProperty (const PropertyDef *property, string value) |
| virtual void | setIntProperty (const PropertyDef *property, int value) |
| virtual void | setItemProperty (const PropertyDef *property, Item *item) |
| virtual void | setStringProperty (const PropertyDef *property, string value) |
| void | setUtilityProperty (string name, int value) |
| virtual string | type () const |
Protected Member Functions | |
| XRayAtomicGasMix () | |
| ~XRayAtomicGasMix () | |
| void | setupSelfBefore () override |
| Protected Member Functions inherited from MaterialMix | |
| MaterialMix () | |
| Configuration * | config () const |
| Random * | random () const |
| void | setupSelfBefore () override |
| Protected Member Functions inherited from SimulationItem | |
| SimulationItem () | |
| virtual bool | offersInterface (const std::type_info &interfaceTypeInfo) const |
| virtual void | setupSelfAfter () |
| virtual void | setupSelfBefore () |
| Protected Member Functions inherited from Item | |
| Item () | |
Private Types | |
| using | BaseType = MaterialMix |
| using | ItemType = XRayAtomicGasMix |
Private Member Functions | |
| int | indexForLambda (double lambda) const |
| void | setScatteringInfoIfNeeded (PhotonPacket::ScatteringInfo *scatinfo, double lambda) const |
Private Attributes | |
| vector< double > | _abundancies |
| ScatteringHelper * | _com |
| ArrayTable< 2 > | _cumprobscavv |
| vector< double > | _lambdafluov |
| Array | _lambdav |
| ScatteringHelper * | _ray |
| BoundElectrons | _scatterBoundElectrons |
| Array | _sigmaextv |
| Array | _sigmascav |
| double | _temperature |
| vector< double > | _vthermscav |
Friends | |
| class | ItemRegistry |
The XRayAtomicGasMix class describes the material properties of neutral atomic gas in the X-ray wavelength range, taking into account the effects of photo-absorption, fluorescence, and scattering by bound electrons. To avoid the use of this material mix outside of the regime for which it has been designed, all cross sections are forced to zero below 4.3 eV and above 500 keV (corresponding approximately to a wavelength range from 2.5 pm to 290 nm).
The class assumes a gas containing a mixture of non-ionized elements with atomic numbers from 1 (hydrogen) up to 30 (zinc). The spatial density distribution of the gas is established by setting the hydrogen density. The relative abundances of the 30 elements and the temperature of the gas can be configured by the user as constant properties. In other words, the abundances and the temperature are considered to be spatially constant (for a given medium component), while the overall density can obviously vary across space as usual.
Photo-absorption by an atom is the process where the energy of a photon is used to liberate a bound electron from one of the electron shells of the atom. This class supports photo-absorption by any of the 30 elements in the gas for any of the electron shells, i.e. up to the K, L, M, or N shell depending on the atomic number of the element.
Fluorescence (in this context) is the process where an electron from a higher energy level "falls" into an empty space created by photo-absorption, emitting a new photon with a different energy. For each electron shell and for each possible fluorescence transition towards that shell, the yield defines the probability that such fluorescence event occurs after an electron has been liberated in that shell. This class supports all fluorescent lines that have line energies \(E > 0.1 \, \text{keV}\), for all elements in the gas. These are 20 lines: K \(_{\alpha2}\), K \(_{\alpha1}\), K \(_{\beta3}\), K \(_{\beta1}\), K \(^{II}_{\beta5}\), K \(^{I}_{\beta5}\), L \(_{\beta4}\), L \(_{\beta3}\), L \(_{\beta10}\), L \(_{\beta9}\), L \(_{\eta}\), L \(_{\beta17}\), L \(_{\beta1}\), L \(_{\gamma5}\), L \(_{\ell}\), L \(_{t}\), L \(_{s}\), L \(_{\alpha2}\), L \(_{\alpha1}\), L \(_{\beta6}\) (i.e. all transitions from higher shells towards the K and L shells, except for \(L \to L\) lines).
Because fluorescence only occurs as the result of a photo-absorption event, this class implements fluorescence as a form of scattering (where the wavelength of the photon being scattered changes) as opposed to emission. This allows both photo-absorption and fluorescence to be treated during primary emission. A possible drawback is that the weaker fluorescence lines will be represented by a fairly small number of photon packets.
Electrons bound to the atoms in the gas scatter incoming X-ray photons. This process can be elastic (Rayleigh scattering) or inelastic (bound-Compton scattering). The user can select one of five implementations of bound-electron scattering. In increasing order of accuracy and computational effort, these are:
Configuring the simulation
In addition to a medium component configured with the material mix represented by this class, simulations will usually include primary sources and possibly a dust medium. As indicated above, there is no need to include secondary emission, so the simulation mode can be set to "ExtinctionOnly" and there is no need to store the radiation field. The resulting continuum spectrum and absorption and emission features can be recorded by a single instrument configured with a high-resolution wavelength grid, or separate instruments can be configured with wavelength grids to resolve specific features of interest.
To study fluorescence lines seperated from the background continuum, configure the instrument to record flux components seperately and consider the scattered flux component, which includes the fluorescence lines in addition to any flux scattered from other media components. To ensure that low-intensity lines are properly included in this flux, set the advanced property minWeightReduction in the PhotonPacketOptions section to a value of 1e10 or so. With the default value of 1e4, low-intensity fluorescence photon packets are killed before having a chance to register in the instruments.
The input model must define the spatial distribution of the hydrogen number density \(n_\mathrm{H} = n_\mathrm{HI} + n_\mathrm{HII} + 2\,n_\mathrm{H2}\), i.e. including atomic, ionized, and molecular hydrogen. Note the factor 2 in this equation; \(n_\mathrm{H}\) in fact specifies the number of hydrogen protons per volume rather than the number of hydrogen-like particles per volume.
If this material mix is associated with a geometric medium component, the geometry defines the spatial density distribution \(n_\mathrm{H}\). Alternatively, if the material mix is associated with a subclass of ImportedMedium, the spatial density distribution is read from an input file. In that case, the ski file attributes importMetallicity, importTemperature, and importVariableMixParams must be left at 'false'. For example, if bulk velocities are also imported for this medium component (i.e. importVelocity is 'true'), the column order would be
\[ ..., n_\mathrm{H}, v_\mathrm{x}, v_\mathrm{y}, v_\mathrm{z} \]
The relative abundances of the 30 elements in the gas and the temperature of the gas are configured in the ski file as constant properties. In other words, the abundances and the temperature are considered to be spatially constant (for a given medium component). If the list of abundances is left empty in the ski file, default abundancies are used, taken from Table 2 of Anders & Grevesse (1989), the default abundance table in Xspec. In the default list, the abundance of hydrogen is set to unity. However, it is acceptable to specify a hydrogen abundance lower than one, for example to model an ionized hydrogen fraction.
Photo-absorption cross section
The total photo-absorption cross section per hydrogen atom for this material mix is obtained by accumulating the photo-absorption cross sections for all shells and for all individual elements, weighted by element abundancy, and convolved with a Gaussian profile reflecting each element's thermal velocity. Because the abundancies and the temperature are fixed, this calculation can be performed during setup and the result stored, discretized on a high-resolution wavelength grid for later retrieval.
Verner and Yakovlev (1995, 1996) provide analytic fits to the photo-absorption cross sections \(\sigma_{ph}(E)\) as a function of photon energy \(E\) for the ground-state shells of the first 30 atomic elements:
\[\begin{aligned} \sigma_{ph}(E) &= \begin{cases} 0 & E < E_\mathrm{th} \\ \sigma_0 \, F(y) & E_\mathrm{th} \le E < E_\mathrm{max}\\ 0 & E_\mathrm{max} \le E \end{cases}, \\ x &= \frac{E}{E_0} - y_0, \; \; \; \; y = \sqrt{x^2 + y_1^2},\\ F(y) &= \left[(x-1)^2+y_{\rm w}^2 \right]y^{-Q} \left(1+ \sqrt{(y/y_{\rm a})} \right )^{-P}, \\ Q&=5.5+l-0.5P, \end{aligned} \]
with \(E_\mathrm{th}\) the tabulated ionization threshold energy, \(E_\mathrm{max}\) the tabulated maximum energy for the formula to be valid, \(\sigma_0\), \(E_0\), \(y_{\rm w}\), \(y_{\rm a}\), \(P\), \(y_0\) and \(y_1\) seven tabulated fitting parameters, and \(l\) the subshell orbital quantum number ( \(l=0, 1, 2, 3\) for s, p, d, f orbitals respectively).
Fluorescence cross section
The total fluorescence cross section per hydrogen atom for this material mix is obtained similarly, but now including only the K and L shell photo-absorption cross sections for each element, multiplied by the appropriate fluorescence yields in addition to the element abundancy.
Electron scattering
As described above, this class provides several implementations for the scattering of X-rays photons by the electrons bound to the atoms. These implementations involve four types of scattering: free-electron Compton scattering, bound-electron Compton scattering, smooth Rayleigh scattering, and anomalous Rayleigh scattering. Note that, in all cases, the listed cross sections must be multiplied by the abundance of the corresponding element relative to hydrogen.
For free-electron Compton scattering, the cross section for an element with atomic number \(Z\) is given by \(Z\,\sigma_\mathrm{C}\), where \(\sigma_\mathrm{C}\) is the (wavelength-dependent) Compton cross section for a single free electron. The implementation of the scattering events is delegated to the ComptonPhaseFunction class; see there for more information on the cross section and phase function for free-electron Compton scattering.
The other scattering implementations depend on quantities that are available for each element \(Z\) in tabulated form as a function of the incoming photon energy \(E\) or as a function of the dimensionless momentum transfer parameter \(q\),
\[q= \frac{E}{12.4 \, \mathrm{keV}} \cdot \sin(\theta/2), \]
with \(\theta\) the scattering angle.
For bound-electron Compton scattering, the cross sections \(\sigma_{CS, Z}(E)\) are available as a table. The normalised scattering phase function for element Z is given by
\[ \Phi_{CS, Z}(\theta, E)= \frac{3}{4}\, \frac{\sigma_T}{\sigma_{CS, Z}(E)}\Big[C^3(\theta, E) + C(\theta, E) -C^2(\theta, E)\sin^2\theta\Big] \cdot S_Z(q), \]
with tabulated incoherent scattering functions \(S_Z(q)\) and the Compton factor \(C(\theta, E)\) defined as
\[ C(\theta, E) = {\Big[{1+\frac{E}{m_ec^2}(1-\cos \theta)\Big]}}^{-1}. \]
Also, inelastic bound-Compton scattering will change the photon energy by the Compton factor, just as for free-electron scattering.
For smooth Rayleigh scattering, the cross sections \(\sigma_{RSS, Z}(E)\) are available as a table. The normalised scattering phase function for element Z is given by
\[ \Phi_{RSS, Z}(\theta, E)= \frac{3}{4}\, \frac{\sigma_T}{\sigma_{RSS, Z}(E)}\Big[ 1 + \cos^2\theta \Big] \cdot F_Z^2(q), \]
with tabulated atomic form factors \(F_Z(q)\), which converge to \(Z\) at small \(q\) and decrease to zero for large \(q\).
Similarly, for anomalous Rayleigh scattering, the cross sections \(\sigma_{RSA, Z}(E)\) are available as a table. The normalised scattering phase function for element Z is now given by
\[ \Phi_{RSA, Z}(\theta, E)= \frac{3}{4}\, \frac{\sigma_T}{\sigma_{RSA, Z}(E)}\Big[ 1 + \cos^2\theta \Big] \cdot \Big[\big(F_Z(q) + F'_Z(E)\big)^2 + {F''_Z}^2(E)\Big], \]
with the same atomic form factors \(F_Z(q)\) as before, and tabulated real and imaginary anomalous scattering functions \(F'_Z(E)\) and \(F''_Z(E)\).
Performing scattering
The function performing an actual scattering event randomly selects one of the supported scattering channels (i.e. scattering by an electron bound to one of the supported elements or one of the 20 fluorescent line transition for one of the supported elements). The relative probabilities for these transitions as a function of incoming photon packet wavelength are also calculated during setup. The selected transition determines the scattering mechanism. For bound electrons, Rayleigh or Compton scattering is used. For fluorescence, the emission direction is isotropic, and the outgoing wavelength is the fluorescence wavelength. In both cases, a random Gaussian dispersion reflecting the interacting element's thermal velocity is applied to the outgoing wavelength.
Thermal dispersion
The thermal dispersion appropriate for a given interaction depends on the (constant) temperature configured for this material mix and on the mass of the interacting atom. For electron scattering and fluorescence events, the implementation is straightforward. Once a channel has been randomly selected, the magnitude of the interacting atom's thermal velocity is taken from a precalculated table and a velocity vector is sampled from the Maxwell distribution.
For absorption, the situation is much more involved. In principle, the full cross section curve for each ionization transition must be convolved with a Gaussian kernel of appropriate width. In practice, the cross section curves are very smooth except for the step at the threshold energy. The effect of the convolution is therefore limited to energies near the threshold energy, replacing the infinitely steep step by a sigmoid function. Considering that the convolution of a step function with a Gaussion is given by the error function, this class uses the following approximation. With \(E_\mathrm{th}\) the threshold energy and \(E_\mathrm{s}\) the energy dispersion corresponding to the thermal velocity of the atom being ionized, the cross section near \(E_\mathrm{th}\) is replaced by
\[ \sigma_{ph}'(E) = \frac{\sigma_{ph}(E_\mathrm{th} +2 E_\mathrm{s})} {2} \, \left[ 1+ \mathrm{erf} ( \frac{E - E_\mathrm{th}} {E_\mathrm{s}} ) \right] \qquad \mathrm{for} \; E_\mathrm{th} -2 E_\mathrm{s} < E < E_\mathrm{th} +2 E_\mathrm{s}. \]
The sigmoid is scaled by the value of the actual cross section at the end of the interval, achieving good continuity at that point in most cases, including all important transitions.
When an item of this type is used, the names provided by the conditional value expression "GasMix" are inserted into the name sets used for evaluating Boolean expressions.
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The enumeration type indicating the implementation used for scattering by bound electrons.
None : "ignore bound electrons" .
Free : "use free-electron Compton scattering" .
FreeWithPolarization : "use free-electron Compton scattering with support for polarization" .
Good : "use smooth Rayleigh scattering and exact bound-Compton scattering" .
Exact : "use anomalous Rayleigh scattering and exact bound-Compton scattering" .
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Default constructor for concrete Item subclass XRayAtomicGasMix : "A gas mix supporting photo-absorption and fluorescence for X-ray wavelengths" .
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The destructor destructs the phase function helpers that were created during setup.
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inline |
This function returns the value of the discoverable double list property abundancies : "the abundancies for the elements with atomic number Z = 1,...,30" .
The minimum value for this property is "[0" .
The maximum value for this property is "1]" .
This property is required only if the Boolean expression "false" evaluates to true after replacing the names by true or false depending on their presence.
This property is displayed only if the Boolean expression "Level2" evaluates to true after replacing the names by true or false depending on their presence.
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This function returns true if this material mix supports polarization during scattering events. For this class, the function returns true if the scatterBoundElectrons property has been set to FreeWithPolarization and false otherwise.
Reimplemented from MaterialMix.
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This function returns true, indicating that a scattering interaction for this material mix may (and usually does) adjust the wavelength of the interacting photon packet.
Reimplemented from MaterialMix.
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private |
This function returns the index in the private wavelength grid corresponding to the specified wavelength. The parameters for converting a wavelength to the appropriate index are stored in data members during setup.
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This function returns an indicative temperature of the material mix when it would be embedded in a given radiation field. The implementation in this class ignores the radiation field and returns the (spatially constant) temperature configured for this material mix.
Reimplemented from MaterialMix.
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This function returns the mass of a hydrogen atom.
Implements MaterialMix.
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This function returns the fundamental material type represented by this material mix, which is MaterialType::Gas.
Implements MaterialMix.
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This function returns the absorption (i.e. extinction minus scattering) opacity at the given wavelength and material state, using the abundances and temperature configured for this material mix. The photon packet properties are not used.
Implements MaterialMix.
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This function returns the extinction opacity at the given wavelength and material state, using the abundances and temperature configured for this material mix. The photon packet properties are not used.
Implements MaterialMix.
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This function returns the scattering (i.e. bound electrons plus fluorescence) opacity at the given wavelength and material state, using the abundances and temperature configured for this material mix. The photon packet properties are not used.
Implements MaterialMix.
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This function calculates the contribution of the medium component associated with this material mix to the peel-off photon luminosity, polarization state, and wavelength shift for the given wavelength, geometry, material state, and photon properties. The contributions to the Stokes vector components are stored in the I, Q, U, V arguments, which are guaranteed to be initialized to zero by the caller. If there is wavelength shift, the new wavelength value replaces the incoming value of the lambda argument.
The function first calls the private setScatteringInfoIfNeeded() function to establish a random scattering channel and atom velocity for this event. In case the selected channel is bound electron scattering, the peel-off bias weight and wavelength shift are determined by the Compton phase function and the selected atom velocity. For fluorescence, the peel-off bias weight is trivially one because emission is isotropic and unpolarized, and the outgoing wavelength is determined by Doppler-shifting the rest wavelength of the selected fluorescence transition for the selected atom velocity.
Implements MaterialMix.
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This function performs a scattering event on the specified photon packet in the spatial cell and medium component represented by the specified material state and the receiving material mix. It first calls the private setScatteringInfoIfNeeded() function to establish a random scattering channel and atom velocity for this event.
In case the selected channel is bound electron scattering, the outgoing direction and adjusted wavelength are determined by the Compton phase function and the selected atom velocity. For fluorescence, emission is isotropic, so the outgoing direction is randomly chosen from the isotropic distribution. The outgoing wavelength is determined by Doppler-shifting the rest wavelength of the selected fluorescence transition for the selected atom velocity.
Implements MaterialMix.
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This function returns the value of the discoverable BoundElectrons enumeration property scatterBoundElectrons : "implementation of scattering by bound electrons" .
The default value for this property is given by the conditional value expression "Good" .
This property is displayed only if the Boolean expression "Level3" evaluates to true after replacing the names by true or false depending on their presence.
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This function returns the absorption (i.e. extinction minus scattering) cross section per hydrogen atom at the given wavelength and using the abundances and temperature configured for this material mix.
Implements MaterialMix.
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This function returns the extinction cross section per hydrogen atom at the given wavelength and using the abundances and temperature configured for this material mix.
Implements MaterialMix.
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This function returns the scattering (i.e. bound electrons plus fluorescence) cross section per hydrogen atom at the given wavelength and using the abundances and temperature configured for this material mix.
Implements MaterialMix.
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This private function draws a random scattering channel and atom velocity and stores this information in the photon packet's scattering information record, unless a previous peel-off stored this already.
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This function precalculates relevant cross sections and relative contributions over a high-resolution wavelength grid.
Reimplemented from MaterialMix.
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This function returns a list of StateVariable objects describing the specific state variables used by the receiving material mix. For this class, the function returns just the descriptor for the number density.
Implements MaterialMix.
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This function returns the value of the discoverable double property temperature : "the temperature of the gas or zero to disable thermal dispersion" .
This property represents a physical quantity of type "temperature" .
The minimum value for this property is "[0" .
The maximum value for this property is "1e9]" .
The default value for this property is given by the conditional value expression "1e4" .
This property is displayed only if the Boolean expression "Level2" evaluates to true after replacing the names by true or false depending on their presence.