This option can for instance be used to study two-track resolution.
In order to prevent summing of signals computed with different characteristics, one may not use the ADD option with the first signal after:
[By default, signals are not summed. The option is reset to NEW at each signal calculation.]
Use of this keyword implies the ION-TAIL model of ion tail calculation.
[By default, n_angle is set to 2.]
The spread is to be provided as a probability distribution in terms of the angle PHI (in radians) between the incidence angle of the electron and the angle at which the ions start to drift away from the wire.
In the ION-TAIL model, ions start only from discrete angles around the wire. The number of such angles can be set with ION-ANGLES. The function that you specify is integrated around the nearest angles using Newton-Raphson integration with ANGULAR-INTEGRATION-POINTS sampling points.
The spread function may be specified as FLAT, in which case the avalanche is assumed to wrap uniformly around the wire. You may also specify NOANGULAR-SPREAD, in which case the ions will be drifted back from the nearest angular sampling point only.
Use of this keyword implies the ION-TAIL model of ion tail calculation.
Example:
Global sigma 30
signal cross ion-angles 1000 ang-spread 'exp(-((phi*180/(pi*{sigma}))^2))'
This example assumes a Gaussian angular spread with a sigma of 30\°. The number of discretisation points is set to 1000 in this example.
References:
Attachment coefficients are taken into account by simply multiplying the multiplication factor computed from Townsend coefficients with the loss factor computed from attachment coefficients. This leads to incorrect statistics: an electron lost through attachment will not be able to multiply later, while an electron that has already multiplied will usually only be partially lost through attachment. This asymmetry is important since in many gas mixtures the attachment coefficients dominate the Townsend coefficient at fields slightly smaller than those found at the onset of multiplication. For gas mixtures in which attachment is important, it is therefore advisable to use the AVALANCHE_SIGNAL procedure instead of the SIGNAL command.
This option can only be specified if attachment data has been entered in the gas section.
[This option is initially on and is remembered from one SIGNAL statement to the next.]
[This option is initially on and is remembered from one SIGNAL statement to the next.]
The averaging is done with an 2*n_average+1 point Newton-Raphson integration over a time bin centred at the point in time indicated in the output, interpolating the signal vector with a polynomial order set with INTERPOLATION-ORDER.
This parameter can also be set with the SIGNAL-PARAMETERS command.
[By default, 5 points are used, i.e. n_average is set to 2.]
Direct and indirect signals are shown separately by PLOT-SIGNALS, they can also separately be retrieved by the GET_SIGNAL procedure.
When the CROSS-INDUCED option is on, Garfield computes both the direct and the indirect currents in all electrodes that are currently SELECTed for read-out. When CROSS-INDUCED is off, then Garfield computes only the direct signals.
[The option is initially switch on.]
In this model, the avalanche develops along the path of the primary electron. A mean avalanche profile is computed from the Townsend and attachment coefficients. This profile is scaled to a size as given by the AVALANCHE command, which also determines the fluctuations. Point-to-point correlations in the avalanche size are absent in this model.
This model is to be preferred in case the avalanche region is substantial or when the integrated charge is important. This model must also be used in case the electrons hit other electrodes than wires (planes, tube, solids). Otherwise, the simpler ION-TAIL and NONSAMPLED-ION-TAIL models will be faster.
Ion tail calculation requires the possibility of drifting ions from the vicinity of electrodes. To enable this, one may have to switch CHECK-ALL-WIRES off.
Use the AVALANCHE_SIGNAL procedure if the lateral extent of the avalanche is of importance and if you wish to have a more accurate growth model.
[The default is ION-TAIL.]
[Diffusion is included by default, if diffusion data is present.]
The electron pulse is computed by following the avalanche process along the electron drift-line, this option therefore requires the presence of Townsend coefficients. Attachment coefficients, if present, will also be taken into account. Also the INTERPOLATE-TRACK option is not compatible with ELECTRON-PULSE.
[An electron pulse is by default not included.]
The ion pulse is computed by tracing the ionisation electrons from their production point. The model currently doesn't foresee multiplication processes along the ion path. As a result, the signal due to this process is negligibly small in all chambers where amplification of the primary electrons occurs.
Signals due to primary ions are classified as cross-induced and you therefore have to ensure the CROSS-INDUCED flag is on when setting the ION-PULSE option.
[An ion pulse is by default included.]
This option can not be used together with ELECTRON-PULSE nor with DETAILED-ION-TAIL.
[Default: Even if a prepared track is available, it will by default not be used for the signal calculation.]
The parameter n_order should not be chosen large since especially electron pulses rise very fast. This can easily give rise to interpolated values of the wrong sign.
This parameter can also be set with the SIGNAL-PARAMETERS command.
[A value of 1 is therefore recommended, and is also default.]
The number of electron incidence angles for which a separate ion tail is calculated can be chosen with this keyword. A value of 1 would be suitable for cylindrically symmetric detectors, while a value of order 10-50 would be appropriate if one wishes to study stereo effects.
You may specify the number of angles as NOSAMPLING (or INFINITE) to indicate that ions should start from the point where the electron hit the wire. This choice implies the use of the NONSAMPLED-ION-TAIL model. Otherwise, using this keyword implies the use of the ION-TAIL model.
Separate ion tails are kept for the different wires on which avalanches are occurring and for the different wires on which the induced current is measured. A large setting therefore implies that a large volume of data has to be stored.
[Default: 50]
You may in this model choose to spread the ions that are produced in the avalanche around the wire of. This can be achieved via the ANGULAR-SPREAD keyword.
This model, for efficiency reasons, keeps ion tails from a set of angles in memory. The number of such angles can be set with ION-ANGLES. If such sampling is not desired, then you should opt for the NONSAMPLED-ION-TAIL model, which however does not offer the possibility of spreading the avalanche around a wire.
Only electrons that hit a wire and some selected solids generate a current (direct or cross induced) in this model. DETAILED-ION-TAIL should be used if the electrons hit other electrodes such as planes, the tube and solids in general.
Ion tail calculation requires the possibility of drifting ions from the vicinity of electrodes. To enable this, one may have to switch CHECK-ALL-WIRES off.
[This is the default.]
To save CPU time, only steps are considered in which at least a certain fraction of the total number of ions is produced.
This fraction should be set to 0 for chambers filled with, for instance, liquid Helium where the avalanche develops over a large part of the electron drift-line.
For conventional gaseous counters, 10\<SUP\>-3\</SUP\> to 10\<SUP\>-4\</SUP\> would be a more appropriate choice.
Using this keyword implies the use of the DETAILED-ION-TAIL model.
[The fraction is initially set to 0.]
Since all electrons from a cluster are treated independently, and since options like INTERPOLATE-TRACK can not be used in conjunction with MONTE-CARLO-DRIFT-LINES, use of this option tends to make the computations longer.
You may have to adjust the Monte Carlo parameters in the INTEGRATION-PARAMETERS statement when using this option.
[Default is RUNGE-KUTTA-DRIFT-LINES]
This option is the reverse of ADD.
[This is default.]
This model does not take the spatial extent of the avalanche into account, for which the DETAILED-ION-TAIL model should be used, nor does it provide angular spread of the ions around a wire.
Ion tail calculation requires the possibility of drifting ions from the vicinity of electrodes. To enable this, one may have to switch CHECK-ALL-WIRES off.
[The default is ION-TAIL.]
Runge Kutta integration is easier to use than Monte Carlo stepping in that the integration parameters are more tolerant.
The parameters controlling the accuracy are adjusted for chambers that are several centimetres large. When studying much smaller structures, at the \μm scale, one may wish to request more accuracy.
The Runge Kutta algorithm is well suited for smooth fields, such as those obtained with analytic potentials. The field computed from field maps is sometimes not even continuous, and one should in such cases prefer the Monte Carlo algorithm.
[The initial default is RUNGE-KUTTA-DRIFT-LINES.]
This parameter can also be set with the SIGNAL-PARAMETERS command.
[By default, AVERAGE-SIGNAL is used with 5-point integration.]
[By default, AVERAGE-SIGNAL is used with 5-point integration.]
Formatted on 21/01/18 at 16:55.