VITESS Detector Modules

There are two VITESS modules to simulate detectors:
The module screen monitors the neutrons arriving on a flat or banana shaped detector surface considering only the binning of the detector cells.
In contrast, the module detector simulates a 3D detector array consisting of rectangular or cylindrical elements. It considers detection efficiency and different resolution effects
In the screen module, the spatial distribution of the neutrons on the screen is written to an output file, while the detector module needs to be followed by a monitor module for that; but it enables writing event files.

VITESS Module 'screen' to monitor intensity distribution on a cylindrical or flat detector

The module screen monitors the spatial distribution of neutrons arriving on a infinitely thin detector surface for 2 simple geometries:
(a) a rectangular shaped detector centered around the beam direction
(b) a banana shaped or cylindric detector with vertical symmetry axis, vertically centered around the beam axis.
The screen gives directly an output file of the intensity distribution over the detector, i.e. a monitor function is included.

Input parameters

VITESS Module 'detector' for a realistic simulation of a detector (array)

The detector module simulates a 3D 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 ϑ [0;180°] the angle between the position vector and the +x-axis and φ [0;360°] the angle between the projection of the position vector to the yz-plane and the +y-axis (ϑ>0;φ=0 points along the +y-axis, ϑ>0;φ=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, columns and layers defining the size and number of the position sensitive areas.

Overall detector geometry
The centre of a rectangular (flat) detector can be every point in space and is defined by the distance and the angles ϑ and φ. The surface can be incline w.r.t. the position vector by the angles ϑ_n and φ_n defining the direction of the surface normal in the coordinate system which has the detector position as x-axis. The surface is perpendicular to the position vector for ϑ=0°.
The cylindrical detector is restricted to three orientations (the cylinder axis will always point along one of the axes of the coordinate system) and the centre (=middle of the cylinder arch) of this detector type always lies in the 0-plane perpendicular to the cylinder axis (i.e. z=0 for a cylinder around the z-axis). The arch centre is defined with the angles described above and the cylinder centre itself is always fixed at the origin position (==> inner cylinder radius = distance to detector arch centre). Due to these restrictions, the following constraints apply to φ and ϑ for a cylinder axis along the
- z-axis: φ=0° or 180° defines +y or -y direction
- y-axis: φ=0° or 180° defines +z or -z direction
- x-axis: ϑ=90° (will be set to 90° if other input is given), only φ defines the arch position
The extension of the detector in direction of the cylinder axis is given by the input variable height for all cylinder orientations, and the covered angular range is calculated from the width and the distance of the detector from the origin (= cylinder radius). The angular range is then bordered by (detector surface centre +-width/(2*radius)).

Tube detector
A flat detector can be build from horizontally or vertically arranged tubes with circular or rectangular cross-sections. In the flight path of the neutron through the detector, only the detector volume containing active material is taken into account in the calculation of interaction probability and the scattering position; dead zones between circular tubes as well as the tube walls (specified by the wall thickness) are treated as vacuum. Different layers of detector tubes can be shifted against each other (see fig. 1(b)).
The tubes option is not possible for cylindical geometry.
Figure 1: Side view of tube detector (a: circular, b: circular shifted, c: rectangular)

Interaction probability
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 interaction along the flight path of the neutron in the detector will be given by a Monte Carlo choice and the neutron's probability weight is multiplied by the interaction probability calculated from the total scattering cross-section σ, the particle density N and the length until scattering l as
Nσ ⋅ exp(-Nσ ⋅l) ⋅ L/N_rep ⋅ ε_mod          (1).
The total length in the detector L divided by the number of repetitions N_rep is used as normalization and represents the distance over which the probability is assumed to be constant.
ε_mod is an optional modifyer that can be used to take losses due to e.g. secondary particle production into account, since σ contains only the interaction of neutrons with the absorber/converter material. The wavelength dependence of this interaction is calculated by a linear approximation from the tables at NNDC (http://www.nndc.bnl.gov/sigma/index.jsp?as=10&lib=endfb7.1&nsub=10). Currently implemented are ³He, ¹⁰B and ⁶Li. The particle density N is calculated from either gas pressure and temperature in case of ³He and BF₃, or directly given as input variable atom density for solid materials. The latter case assumes two converter layers per detector layer, i.e. the absolute converter thickness is 2 ⋅ number of layers ⋅ solid lauer thickness.
If other is chosen as absorber/converter material, a wavelength independent interaction probability is calculated from ε_mod, which then serves not as a modifyer but gives the overall efficiency.
Alternatively, the wavelength-dependent detection efficiency can be given via an input file. In this case, the choice of active detector material will be ignored and the efficiency taken from this file, hence it is independent of the path of the neutron through the detector material.

Detector signal
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. This point of detection given as detector signal is a 3-dimensional pixel center determined by the number of rows, columns and layers in combination with the detector height, width and thickness. These "pixels" can represent either a physical pixel size (e.g. a tube diameter) or a digital pixel size caused by the readout. Note that for detection with solid B or Li, two converter layers per detector layer are assumed, so each detection position (layer center) has two possible detection times (layer borders) for monochromatic neutrons with equal time-of-flight zero point.
If a non-zero resolution is given in either dimension, the interaction point of the neutron with the detector is smeared by a gaussian function before being shifted to the pixel center. (The standard deviation of this gaussian is calculated from the FWHM given as input.) A spatial uncertainty caused by e.g. the extension of the secondary particle cloud can thus be taken into account.
All this as well as the probability calculation may be deactivated by the monitor only option, i.e. the true interaction point is written and the probability weight of the neutron is not altered by the detector module, so the neutrons are registered always with efficiency 1 and each point of the flightpath within the active detector material 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.) In the monitor only option, the incoming flight direction and the interaction point with the detector are written to the output.
In the grid off mode, the neutron detection position is not shifted to the 3-dimensional pixel center. Other than with the monitor only mode, the resolution is taken into account; if the resolution is set to 0 in all dimensions, the grid off mode yields the same output position as the monitor only mode but with the neutron weight changed according to the detection probability. The neutron direction is set to the normalized vector of the written position (like in the usage normal mode).

Detector array
A complex detector shape can be build out of several detector parts by using the detector module several times in a row and ticking the array box for all but the last detector part. The first and intermediate detector parts will then pass neutrons that don't intersect with them on to the next detector module. The last part of the detector array discards neutrons that have not been detected by any of the detector parts. Neutrons detected in different sub-detectors of an array can be distinguished by usage of the add color value. Note that this adds the given value to the color property of the neutrons to allow distinction on several levels; if only a seperation of different detector parts is needed, make sure the color of incoming neutrons is the same (e.g. set it to 0 with the module spin_reset).
Figure 2: example of an detector array consiting of 4 detector parts, two flat and two cylindrical

Output file
If a filename is given as Output filename, a file containing the 3D position, time, probability weight and colour of each neutron will be written (event mode output). In a detector array, only the last detector writes this file, which then contains the neutrons detected in all sub-detectors. (The sub-detector in which a neutron has been detected can be distinguished by use of the add color option in each detector module.)

--------------------------------------

!!!!! 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. !!!!!!
--------------------------------------

Note: Visualization of a cylindrical detector is only possible for width < 2 π distance (round down amply)

Input parameters