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&SIGNAL: SIGNAL


ADD

Signals from repeated simulations will be summed, electrode by electrode. This option is the reverse of NEW.

This option can for instance be used to study two-track resolution.

The ADD option can not be used after:

[By default, signals are not summed.]


ANGULAR-INTEGRATION-POINTS

When computing the weight assigned by the ANGULAR-SPREAD function to each of the ION-ANGLES, the function is integrated using the Newton-Raphson technique with 2*n_angle+1 points.

Use of this keyword implies the ION-TAIL model of ion tail calculation.

[By default, n_angle is set to 2.]


ANGULAR-SPREAD

In the ION-TAIL model, you have the possibility to spread the ions that are produced in the avalanche, around the wire.

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 degrees. The number of discretisation points is set to 1000 in this example.

References:

[1]
G. Charpak et al., NIM 148 (1978) 471-482
[2]
T.J. Harris & E. Mathieson, NIM 154 (1978) 183-188
[3]
E. Mathieson & T.J. Harris, NIM 159 (1979) 483-187
[4]
Harry van der Graaf, PhD thesis, TU Delft (1986)

ATTACHMENT

Attachment coefficients will be taken into account for signal calculations. They occur at two instances:


AVALANCHE

Enables the avalanche setting chosen with AVALANCHE. NOAVALANCHE leads to a fixed multiplication factor of 1.

Both ELECTRON-PULSE and DETAILED-ION-TAIL require Townsend coefficients. They use these coefficients, provided they are available, regardless of the setting of this option.


AVERAGE-SIGNAL

Switching on this option makes that the total induced charge corresponds closely to the integral of the signal that is output by the program. This is less trivial than it may sound since signals can contain structure on a much smaller time scale than the binning of the signal.

The averaging is done with an 2*n_average+1 point Newton- Raphson integration over a time bin centered at the point in time indicated in the output.

[By default, 5 points are used, i.e. n_average is set to 2.]


CROSS-INDUCED

Requests the computation of the signal induced in all electrodes that are currently SELECTed for read-out, by all ionisation electrons and avalanche ions that drift in the chamber.

The part of the signal that is due to ionisation electrons that hit the electrode and avalanche ions that are drifting away from the electrode is called the "direct" component, the remainder of the signal is called "indirect". The two components are shown separately by e.g. PLOT-SIGNALS.

If CROSS-INDUCED is off, then Garfield computes only the direct signals. If the field is derived from a field map, the classification of signals is based on the solids. In the absence of solids, there can not be direct signals.

The option can be used in conjunction with DETAILED-ION-TAIL.

[The option is initially switch off.]


DETAILED-ION-TAIL

Adds an ion tail to the computed signal according to a more detailed model in which the ions do not necessarily start at the wire surface or in the vicinity of other electrodes. Rather, they start where they are produced during the electron avalanche.

This model is to be prefered 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.

[The default is ION-TAIL.]


DIFFUSION

Varies the arrival times of the individual electrons from the clusters according to a Gaussian distribution.

[Diffusion is included by default, if diffusion data is present.]


ELECTRON-PULSE

Adds an electron pulse to the computed signal.

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.]


INTERPOLATE-TRACK

Enables the use of the prepared track, see PREPARE-TRACK.

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.]


INTERPOLATION-ORDER

In order to average the signal over a time bin, the signal is interpolated with polynomials of order n_order, and then integrated using the Newton-Raphson technique over 2*n_average+1 points.

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.

[A value of 1 is therefore recommended, and is also default.]


ION-ANGLES

When the ION-TAIL model has been selected, then the shape of the ion tail is stored for a series of electron incidence angles. For many applications, this is reasonable since:

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 occuring 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]


ION-TAIL

Adds an ion tail to the computed signal according to a simplified model in which the ions are assumed to come from the wire surface rather than from the area near the wire in which the avalanche is developing.

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.]


ION-THRESHOLD

In the detailed ion tail model, the ions are traced from the point where they were produced. This is done on a step-by-step basis of the electron drift line that generated the ions.

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**-3 to 10**-4 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.]


MONTE-CARLO-DRIFT

Uses the Monte_Carlo drifting routines rather the the default Runge_Kutta_Fehlberg integration routines. This option is useful if diffusion can cause electrons starting from the same starting point to reach significantly different end points.

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, 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]


NEW

Means that summing of signals over repeated simulations does not take place.

This option is the reverse of ADD.

[This is default.]


NONSAMPLED-ION-TAIL

A variant of the ION-TAIL model, in which the ions start drifting from the point where the electrons hit the wire.

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-DRIFT

Requests the use of the Runge_Kutta_Fehlberg integration technique for drift lines.

Runge Kutta integration is easier to use than Monte Carlo stepping in that the integration parameters are more tolerant.

The parameters controling the accuracy are adjusted for chambers that are several centimeters large. When studying much smaller structures, at the micron 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.]


SAMPLE-SIGNAL

If this option is switched on, the signal that the program returns corresponds to the current at the point in time indicated in the output. Any fine structure smaller than the binning is lost.

[By default, AVERAGE-SIGNAL is used with 5-point integration.]


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Formatted on 0099-12-08 at 15:52.