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).
The data written to the output stream are nine double variable having the following structure:
1. total neutron time (whole history of the neutron is considered) in ms
2. neutron wave length in Å
3. neutron probability (whole "probability history" is considered)
4.-6. the exact position of detection as x- ,y- ,z- coordinate [cm] of the neutron.
7.-9. the middle of the hit detector element, given in the spherical coordinates defined above: 7.: q [rad] (in the case of elastic scattering (provided that origin = centre of sample and that the x-direction corresponds to no scattering) it is equivalent to the scattering angle). 8.: f [rad] 9.: r=distance to the middle of the detector element surface [cm]
If the option "no detector grid" is activated, the variables 7.-9. will contain the exact position of the neutron in spherical coordinates.
!!!Only "evaluation (elast or inelast)" or "writeout" are useful subsequent
modules!!!
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 rate
The neutron repetition rate specifies the number of neutron data sets
generated for each scattered neutron.
A larger neutron repetition rate will give better statistics in the
spectrum.
usage
If 'monitor only' is selected, use detector geometry only as a monitor,
i.e. the probability of the neutron is unchanged.
detector grid
If the detector grid is switched off, the exact neutron position is
written to the output file.
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!!!!! Please make sure that no neutron from the input stream starts already
within the detector (having a finite thickness) itself.
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