&DRIFT: TRACK

FROM

The coordinates of the starting point of the track.

This would usually be a set of 3\ numbers. If you omit the z-coordinate, 0 is assumed.

[No default, in cm.]

The coordinates of the end point of the track. If the particle undergoes multiple scattering or energy loss (HEED model), then the end point merely serves to compute the initial direction of the particle.

When both TO is used and DIRECTION and RANGE, then the information provided with TO is taken.

This would usually be a set of 3\ numbers. If you omit the z-coordinate, 0 is assumed.

[No default, in cm.]

DIRECTION

The direction in which the particle starts moving from the starting point.

When using DIRECTION, you must also use RANGE. You should not use TO however in this case, since TO overrules the information from DIRECTION and RANGE.

This would usually be a set of 3\ numbers. If you omit the z-coordinate, 0 is assumed.

[No default, in cm.]

RANGE

This is the maximum distance the particle is allowed to travel from the starting point. The range is measured by projection onto the DIRECTION, i.e. the additional length that results from multiple scattering is not taken into account.

The RANGE should be chosen sufficiently large to avoid having the particle cut prematurely, but not too large either - this would cause overflow in HEED's internal buffers.

[No default, in cm.]

FIXED-NUMBER

Requests the simple model in which an electron or an ion is generated at equally spaced points along the track.

The number of points can be specified after the LINES keyword. The number of clusters per cm, which can be entered in the gas section with PARAMETERS, is not used.

Each deposit contains a single electron or ion, a cluster size distribution, if entered, is not used by this model.

The track is straight in this model.

If this keyword is used, it should be the last keyword on the command line. The model is implied by the use of the LINES keyword.

Additional information on:

LINES

EQUAL-SPACING

Requests a clustering model in which the distance between adjacent clusters is constant, and equal to

1 / mean

where "mean" is the mean number of clusters per cm as entered with the MEAN option of the PARAMETERS statement in the gas section.

The cluster size distribution will usually be taken from a CLUSTER statement, but can also be derived from data entered with PARAMETERS.

Both the cluster size distribution and the number of clusters must be specified as described above. Heed is not called to fill in missing data.

If this keyword is used, it should be the last keyword on the command line.

EXPONENTIAL-SPACING

Requests a model in which the number of clusters generated along a straight track is Poisson distributed with a mean of:

mean * length

where "mean" is the mean number of clusters per cm as entered with the MEAN option of the PARAMETERS statement in the gas section and where "length" is the distance between the start and end points of the track.

The spacing between the clusters is exponentially distributed.

The cluster size distribution will usually be taken from a CLUSTER statement, but can also be derived from data entered with PARAMETERS.

Both the cluster size distribution and the number of clusters must be specified as described above. Heed is not called to fill in missing data.

If this keyword is used, it should be the last keyword on the command line.

WEIGHTED-DISTRIBUTION

This model generates single electron or ion deposits (and not clusters) at positions that follow a user specified distribution. The model can be used to simulate certain background conditions.

The deposits are generated in random sequence.

If this keyword is used, it should be the last keyword on the command line. The use of this model is implied by the keywords WEIGHTING-FUNCTION and SAMPLES.

Additional information on:

WEIGHTING-FUNCTION

SAMPLES

SINGLE-CLUSTER

A single cluster is generated on each track. The cluster size is taken from the information entered in the gas section with the CLUSTER or the PARAMETERS statement.

This model differs from FIXED-NUMBER with a number of samples set to 1 in that the cluster of SINGLE-CLUSTER is at a random location while it would be in the middle of the track when using FIXED-NUMBER.

This model can be seen as a rough approximation to photon from e.g. an \<SUP\>55\</SUP\>Fe source, provided an adequate cluster size distribution is entered.

If this keyword is used, it should be the last keyword on the command line.

HEED

This option requests the Heed program to take care of cluster generation. This program simulates the ionisation of the gas molecules by a particle.

The particle knocks an electron out of an atom. The electron may have sufficient energy to cause further ionisations (in that case we speak of a \δ-electron) or it may start drifting towards the anode. The atom returns to its ground state by either emitting an Auger electron or by fluorescence photons. The Auger electrons are treated like the ionisation electrons. The photons can be absorbed in other atoms.

When using the Heed interface, you must define the gas mixture with a HEED command in the gas section. You should specify on the TRACK statement what kind of particle traverses the chamber. Clustering information entered in the gas section through the PARAMETERS and CLUSTER statements is not used when clustering is performed hy Heed.

Since Heed will occasionally generate \δ-electrons, which are not located on the track, it is advisable to switch on the COMPUTE-IF-INTERPOLATION-FAILS integration parameter if you intend to use commands that perform track preparation.

If this keyword is used, it should be the last keyword on the command line. The use of this model is implied by the specification of a particle type, charge or kinetic energy and by the options related to multiple scattering and \δ-electrons.

Reference: I.B.\ Smirnov, NIM\ A\ 554 (2005) 474.

Additional information on:

particle	MASS
CHARGE	ENERGY
MULTIPLE-SCATTERING	DELTA-ELECTRONS
TRACE-DELTA-ELECTRONS	ENERGY-CUT
SWITCH-ELECTRON-TO-CHARGED

EQUAL-FLUX-INTERVALS

Generates points along the track spaced by equal flux intervals. The spacing of drift-lines from such points indicates the electric field strength.

The number of points can be set with the FLUX-LINES keyword.

The flux is computed by integrating the electric field component that is in viewing plane and perpendicular to the track.

If the flux changes sign over the track, then points are only generated over the parts of the track where the flux is positive if the total flux over the track is positive. Conversely, if the total flux is negative, points are generated only in areas where the flux is negative.

The cluster size is set to 1 and the cluster energy to 0 in this model.

If this keyword is used, it should be the last keyword on the command line. The use of this model is implied by the presence of the FLUX-LINES keyword.

Additional information on:

FLUX-LINES

SRIM

Generates electron clusters along a track, trying to be compatible with the energy loss, range and straggling tables computed by the SRIM program. Such tables must have been read on beforehand, in the gas section, using the SRIM command. Also the A and Z parameters of the gas must have been set in the gas section.

When using this clustering model, you have to specify the kinetic energy. You may also specify the mass, and charge, of the particle that traverses the chamber, even though this data is contained in, and extracted from, the SRIM tables.

The SRIM tables contain in addition the following data:

a lost of energies at which losses and straggling have been computed;
average energy lost per unit distance via electromagnetic processes, for each energy;
average energy lost per unit distance via nuclear processes, for each energy;
projected path length, for each energy;
longitudinal straggling, for each energy;
transverse straggling, for each energy.

Note that these tables contain only average quantities, not their distributions. The interface tries to generate individual tracks which statistically reproduce the above quantities. It does this by starting with a particle with the given energy and a direction derived from the beginning and ending point of the track.

Then, iteratively,

the electromagnetic and nuclear energy loss per unit length are interpolated at the current particle energy;
a step is made with a length over which the particle will produce on average a certain number of electrons (see GROUPING), the length is computed by dividing the electromagnetic energy loss by the work function of the medium;
the longitudinal and transverse scatter are interpolated for the current particle energy;
the scatter is scaled to the length of the step to be taken;
the position is updated for step length and scatter;
although in principle also the direction of the particle changes as a result of scattering, no change in direction is applied since this tends to lead to much overestimated overall straggling;
a randomised actual electromagnetic energy loss over the step is computed using as mean the electromagnetic energy loss interpolated earlier, plus a Monte-Carlo generated fluctuation according to the selected fluctuation-model;
the particle energy is diminished by the actual electromagnetic and average nuclear energy over the step.

Iteration ends when either the maximum length of the track has been reached, or when the particle has no energy left.

Example: displaying the Bragg curve

&DRIFT
area -1 -10 30 10
track 0 0 20 0 ...
    energy 10 MeV ...
    charge 2 ...
    grouping=auto ...
    fluctuation-model=gaussian

Call book_histogram(hx,100,0,8)
For i From 1 To 100 Do
   Call new_track
   Do
      Call get_cluster(x,y,z,n,e,done)
      If done Then Leave
      Call fill_histogram(hx,x,n)
   Enddo
Enddo
Call plot_histogram(hx,`x [cm]`,`Deposits vs x`)
Call plot_end

The histogram hx is booked for a range of [0\ cm, 8\ cm] - we can't use an automatic range here since the range will be determined from the first few entries and the x-coordinates will come in sequence.

Additional information on:

LATERAL-STRAGGLING	LONGITUDINAL-STRAGGLING
FAST-VAVILOV	GROUPING
ENERGY	MASS
CHARGE	particle
FLUCTUATION-MODEL

TRIM

Generates clusters according to a file generated by the TRIM program and read using the TRIM command in the &GAS section.

Interface written by James Butterworth.

CONSTANT-FLUX-INTERVALS

Generates points along the track spaced by a given flux interval. The spacing of drift-lines from such points indicates the electric field strength.

The flux between 2 points can be specified following the FLUX-INTERVAL keyword.

The flux is computed by integrating the electric field component that is in viewing plane and perpendicular to the track.

The cluster size is set to 1 and the cluster energy to 0 in this model.

If this keyword is used, it should be the last keyword on the command line. The use of this model is implied by the presence of the FLUX-INTERVAL keyword.

Additional information on:

FLUX-INTERVAL

representations

The track is plotted using the following representations:

the track location is drawn with TRACK, a point track uses the polymarker representation, a line track the polyline representation;
when using the Heed model, the \δ- and Auger-electrons are shown with DELTA-ELECTRON and AUGER-ELECTRON while the photons are shown with PHOTON.

The track location and, when using the Heed model also the clusters, are shown usually as last elements of a figure to ensure that they are not hidden under drift-lines.

In other models than Heed, the track location is plotted but the cluster locations are not marked.

Go to the top level, to &DRIFT, to TRACK, to the topic index, to the table of contents, or to the full text.

Formatted on 21/01/18 at 16:55.