VITESS Module Detector

The detector module simulates a basic 2D detector array, which may have a rectangular or cylindrical geometry. The coordinate system (x,y,z) is defined by the preceding module (the origin being e.g. the centre of a sample) and is not altered by the detector module. The centre of the detector is given in spherical coordinates, with q [0;180°] the angle between the position vector and the +x-axis and f [0;360°] the angle between the projection of the position vector to the yz-plane and the +y-axis (q>0;f=0 points along the +y-axis,  q>0;f=90° along the +z-axis, a.s.o.). The distance (length of the position vector pointing from the origin to the detector surface centre)  has also to be specified as well as the total height, width and thickness of the detector and the  rows and columns defining the size and number of the position sensitive areas.  

Due to this definitions the centre of the rectangular detector can be every point in space, while the surface is always perpendicular to the position vector mentioned above.
The cylindrical detector is restricted to one orientation (the cylinder axis  will always point along the +z-axis of the coordinate system)
and the centre (=middle of the cylinder arch) of this detector type always lies in the z=0 plane . The arch centre is defined with the angles described above and due to the restrictions  f can only  be  0° or 180°, the cylinder centre itself is always fixed at the origin position (==> cylinder radius = distance to detector centre). The covered angular range is calculated from two input values: distance of the detector from the origin (= cylinder radius)  and the width (=arch) of the cylinder segment. The angular range is then bordered by
(detector centre +-width/(2*radius)).

Now the focus will be put at some neutron probability issues: First of all it is tested whether the neutron intersects the detector, if not the neutron is discarded. If the neutron intersects the detector the flight path through the detector is determined and the neutron will be written to the output accordingly. The position of detection l1 along the flight path of the neutron in the detector will be given by a Monte Carlo choice and the neutron's probability weight  P is multiplied by an exponential factor which considers appropriately the position dependent probability of detection and the efficiency of the detector. In addition a wavelength dependent factor F is multiplied (it will be treated more differentiated for different types of detectors in future VITESS versions):
F= min(0.5 + 0.1*wavelength;1.0)

All this may be deactivated by the "monitor only"-option, i.e. the probability weight of the neutron is not altered by the detector module
and the neutrons are registered always with Efficiency 1 and each point of  the flightpath within the detector is considered with equal chance to be the point of detection. The monitor option might lead to better statistics because of the deactivation of the realistic detector thickness MonteCarlo procedure).

From version 1.7 on, the detector writes the same parameter set for the trajectories as all the other modules. The transformation to scattering angles is performed in the 'eval' modules now.
In the "monitor only"-option, the incoming flight direction and the intersection point with the detector surface are written to the output.
In the "usage normal" mode, the position of detection and the direction between the centre of the sample and the point of detection are written. The position of detection (and thus the direction) maybe altered by switching the 'grid' on. In this case, each trajectority is treated as having hit the centre of the detector cell.
With 'TOF option'=no, the path length to the point of detection (inside the detector) is set to zero, but the efficiency of detection is nevertheless treated.
 

Input parameters:

geometry
 The geometry parameter specifies the  geometry of the detector.
 There are rectangular or cylindrical detectors.

height [cm]
 Height of the detector in cm.

width [cm]
 Full width of a flat detector in cm.In case of a cylindrical detector it is the length of the cylinder arch under consideration.

thickness [cm]
 Thickness of the detecting material in cm.

efficiency
Efficiency of the detector, range: 0<Efficiency<0.99999.

theta
 Angle theta [0;180 deg]  of the middle of the detector.  Theta is defined as the angle between the position vector (pointing from
the origin to the detector centre) and the +x-axis.

phi
 Angle phi [0;360 deg] of the middle of the detector, i.e. the angle between the projection of the position vector to the yz-plane and the +y-axis. For cylindrical geometry phi must be 0 oder 180!

distance [cm]
 Distance of the centre of the detector surface to the origin (0,0,0) in cm. In case of a cylindrical detector this is the cylinder radius.

number of columns
 Number of columns of the detector.

number of rows
 Number of rows partitioning the detector height.

repetition
 'repetition' specifies the number of neutron data sets generated for each trajectory arriving at the detector, i.e. the detection process is repeated 'repetition' times. This can improve statistics, but be aware that these are not independent events.

usage
 If the 'usage' option 'monitor only' is selected, the detector geometry is only used as a monitor, i.e. the weight of the trajectory and the flight direction are unchanged, the intersection point with the detector surface is written to the output. Otherwise thickness, efficiency and wavelength are used to calculate a count rate that can be expected in experiments.

detector grid
 If the detector grid is switched off, the exact position of neutron detection and the direction from the centre of the sample to this point are written to the output file; otherwise the centre of the cell and the direction to this point are written (provided usage is set to 'normal'.)

TOF option
 By setting the 'TOF option' to 'no', a zero flight path from the detector surface to the point of detection is assumed. The detector efficiency is considered.

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!!!!! Please make sure that no neutron from the input stream starts already within the detector (having a finite thickness) itself. In case of cylindric geometry: All neutrons initially have to lie within the cylinder of radius = distance to detector. !!!!!!
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Last modified: Tue May 8 17:08:06 MET DST 2001