Instrument Definition File¶

Downloaded Instrument File Definitions¶

As well as the instrument definitions that ship with Mantid, mantid can also download updates or new instrument definitions when it is started. This is a great way of keeping the files current without needing a new release of Mantid for each instrument change.

Mantid does not overwrite files in the MantidInstall instrument directory when downloading files, it writes to another location that is a little different on windows and linux.

For windows:

%appdata%\mantidproject\instrument

For Linux:

~/.mantid/instrument

You should not edit files there, or add new ones as the may be deleted or overwritten. If you have a change to an instrument definition you wish to use then edit a copy in the MantidInstall instrument directory, but updated the valid-from date so mantid will pick that one up in preference. Or if you just wish to force a particular instrument definition for a particular workspace just run LoadInstrument for that workspace.

Geometry shapes¶

For information on how to define geometric shapes see HowToDefineGeometricShape.

Top level <instrument>¶

<instrument> is the top level XML element of an IDF. It takes attributes, three of which must be included. An example is

<instrument xmlns="http://www.mantidproject.org/IDF/1.0"
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xsi:schemaLocation="http://www.mantidproject.org/IDF/1.0 http://schema.mantidproject.org/IDF/1.0/IDFSchema.xsd"
            name="ARCS"
            valid-from="1900-01-31 23:59:59"
            valid-to="2100-01-31 23:59:59">

Of the attributes in the example above

• xmlns, xmlns:xsi, xsi:schemaLocation are required attributes that can be copied verbatim as above
• name is (at present) optional, although it is recommended to specify something sensible
• valid-from is compulsory and is the date from which the IDF is valid from (+). This date must be larger than or equal to 1900-01-31 23:59:01
• valid-to may optionally be added to indicate the date to which the IDF is valid to. If not used, the file is permanently valid. (+)

(+) Both valid-from and valid-to are required to be set using the ISO 8601 date-time format, i.e. as YYYY-MM-DD HH:MM:SS or YYYY-MM-DDTHH:MM:SS 2. Valid ranges may overlap, provided the valid-from times are all different. If several files are currently valid, the one with the most recent valid-from time is selected.

Using <component> and <type>¶

Use the element to define a physical part of the instrument. A requires two things

1. It must have a type=”some type” attribute. This specify the ‘type’ of the component and this type must be specified somewhere in the IDF using: .
2. It must contain at least one <location> element. If multiple <location> are specified then this is essentially a shorthand notation for defining multiple components of the same type at different locations.

Here is an example

<component type="slit" name="bob">
  <location x="10.651"/>
  <location x="11.983"/>
</component>

<type name="slit"></type>

Which defined two slits at two difference locations. Optionally a <component> can be given a ‘name’, in the above example this name is “bob”. If no ‘name’ attribute is specified the name of the <component> defaults to the ‘type’ string, in the above this is “slit”. Giving sensible names to components is recommended for a number of reasons including #. The ‘Instrument Tree’ view of an instrument in MantidPlot uses these names #. when specifying <parameter>s through <component-link>s these names are used.

<type name="pixel" is="detector">
  <cuboid id="app-shape">
    <left-front-bottom-point x="0.0025" y="-0.1" z="0.0"  />
    <left-front-top-point  x="0.0025" y="-0.1" z="0.0002"  />
    <left-back-bottom-point  x="-0.0025" y="-0.1" z="0.0"  />
    <right-front-bottom-point  x="0.0025" y="0.1" z="0.0"  />
  </cuboid>
</type>

<component type="neutron moderator"> <location z="-10.0"/> </component>
<type name="neutron moderator" is="Source"/>

<component type="some sample holder"> <location /> </component>
<type name="some sample holder" is="SamplePos" />

Using detector/monitor IDs¶

Any component that is either a detector or monitor must be assigned a unique detector/monitor ID numbers (note this is not spectrum ID numbers but detector/monitor ID numbers). There are at least two important reason to insist on this.

• Data stored in files need to have a way to be linked to detectors/monitors defined in the IDF. For example, at the ISIS facility, data are recorded together with unique detector ID numbers. Hence the job here to match the IDs in the data file with the IDs of the IDF. Where unique IDs are not stored with the data the creator of an IDF have some flexibility to chose these ID numbers since the data themselves does not contain such number. However a link between the IDs and spectra in a workspace still needs to be made. By default the LoadInstrument algorithm, see in particular the RewriteSpectraMap parameter of this algorithm, will map the detector/monitor IDs with spectrum numbers as follows: the detector/monitor IDs in the IDF are ordered from smallest to largest number and then assigned in that order to the spectra in the workspace used to hold the data in Mantid.
• Mantid needs to have a way to associate data which the detectors/monitors of the instrument, which is do this using the detector IDs. Although not mandatory it is recommended to give memorisable names to collection of detectors/monitors or individual detectors/monitors that a user is likely to want to refer. This allow a user to refer to a collection of detectors by name rather than trying to remember a sequence of IDs. Note the counts in a histogram spectrum may be the sum of counts from a number of detectors and Mantid, behind the scene, use the IDs to keep track of this.

The <idlist> element and the idlist attribute of the elements is used to assign detector IDs. The notation for using idlist is

<component type="monitor" idlist="monitor-id-list">
  <location r="5.15800" t="180.0" p="0.0" /> <!-- set to ID=500 in list below -->
  <location r="5.20400" t="180.0" p="0.0" /> <!-- set to ID=510 -->
  <location r="5.30400" t="180.0" p="0.0" /> <!-- set to ID=520 -->
  <location r="5.40400" t="180.0" p="0.0" /> <!-- set to ID=531 -->
  <location r="6.10400" t="180.0" p="0.0" /> <!-- set to ID=611 -->
  <location r="6.24700" t="0.000" p="0.0" /> <!-- set to ID=612 -->
  <location r="6.34700" t="0.000" p="0.0" /> <!-- set to ID=613 -->
  <location r="6.50000" t="0.000" p="0.0" /> <!-- set to ID=650 -->
</component>

<type name="monitor" is="monitor"/>

<idlist idname="monitor-id-list">
  <id start="500" step="10" end="530" />  <!-- specifies IDs: 500, 510, 520, 530 -->
  <id start="611" end="613" />            <!-- specifies IDs: 611, 612 and 613 -->
  <id val="650" />                        <!-- specifies ID: 650 -->
</idlist>

As can be seen to specify a sequence of IDs use the notation <id start=”500” step=”10” end=”530” />, where if the step attribute defaults to step=”1” if it is left out. Just specify just a single ID number you may alternatively use the notation <id val=”650” />. Please note the number of ID specified must match the number of detectors/monitors defined.

Creating Rectangular Area Detectors¶

There is a shortcut way to create 2D arrays of detector pixels. Here is an example of how to do it:

<component type="panel"  idstart="1000" idfillbyfirst="y" idstepbyrow="300">
  <location r="0" t="0" name="bank1">
  </location>
</component>

<component type="panel"  idstart="100000" idfillbyfirst="y" idstepbyrow="300">
  <location r="45.0" t="0" name="bank2">
  </location>
</component>

<!-- Rectangular Detector Panel. Position 100 "pixel" along x from -0.1 to 0.1
     and 200 "pixel" along y from -0.2 to 0.2 (relative to the coordinate system of the bank) -->
<type name="panel" is="RectangularDetector" type="pixel"
    xpixels="100" xstart="-0.100" xstep="+0.002"
    ypixels="200" ystart="-0.200" ystep="+0.002" >
</type>

<!-- Pixel for Detectors. Shape defined to be a (0.001m)^2 square in XY-plane with tickness 0.0001m -->
<type is="detector" name="pixel">
  <cuboid id="pixel-shape">
    <left-front-bottom-point y="-0.001" x="-0.001" z="0.0"/>
    <left-front-top-point y="0.001" x="-0.001" z="0.0"/>
    <left-back-bottom-point y="-0.001" x="-0.001" z="-0.0001"/>
    <right-front-bottom-point y="-0.001" x="0.001" z="0.0"/>
  </cuboid>
  <algebra val="pixel-shape"/>
</type>

• The “panel” type defined above has the special “is” tag of “RectangularDetector”. The same type definition then needs these attributes specified:
  • type: point to another type defining your pixel shape and size.
  • xpixels: number of pixels in X
  • xstart: x-position of the 0-th pixel (in length units, normally meters)
  • xstep: step size between pixels in the horizontal direction (in length units, normally meters)
  • ypixels: number of pixels in Y
  • ystart: y-position of the 0-th pixel (in length units, normally meters)
  • ystep: step size between pixels in the vertical direction (in length units, normally meters)
• Detectors of the type specified (“pixel” in the example) will be replicated at the X Y coordinates given. The usual rotation and translation of the panel will rotate the pixels as needed.
• Each instance of a “panel” needs to set these attributes, at the <component> tag, in order to specify the Pixel IDs of the 2D array.
  • idstart: detector ID of the first pixel
  • idfillbyfirst: set to true if ID numbers increase with Y indices first. That is: (0,0)=0; (0,1)=1, (0,2)=2 and so on. Default is idfillbyfirst=”y”.
  • idstepbyrow: amount to increase the ID number on each row. e.g, if you fill by Y first,and set idstepbyrow = 100, and have 50 Y pixels, you would get: (0,0)=0; (0,1)=1; ... (0,49)=49; (1,0)=100; (1,1)=101; etc.
  • idstep. Default to 1. Set the ID increment within a row.
• DO NOT also specify an “idlist” attribute for rectangular detectors, as it will not be used.
• Advantages of using a Rectangular Detector tag instead of defining every single pixel:
  • The data will be displayed as a bitmap in the instrument 3D view, making rendering much faster.
  • Smaller IDF and faster instrument loading times.
  • No need to make a script to generate the pixel positions.
• Disadvantages/Limitations:
  • Must have constant pixel spacing in each direction.
  • Must be rectangular shape.

Using <location>¶

The <location> element allows the specification of both the position of a component and a rotation or the component’s coordinate system. The position part can be specified either using standard x, y and z coordinates or using spherical coordinates: r, t and p, which stands for radius, theta and phi, t is the angle from the z-axis towards the x-axis and p is the azimuth angle in the xy-plane 3. Examples of translations include

<component type="something" name="bob">
  <location x="1.0" y="0.0" z="0.0" name="benny" />
  <location r="1.0" t="90.0" p="0.0"/>
</component>

The above two translations have identical effect. They both translate a component along the x-axis by “1.0”. Note that optionally a <location> can be given a name similarly to how a <location> can optionally be given a name. If a ‘name’ attribute is not specified for a <location> element it defaults to the name of the <component>.

The rotation part is specified using the attributes ‘rot’, ‘axis-x’, ‘axis-y’, ‘axis-z’ and these result in a rotation about the axis defined by the latter three attributes. As an example the effect of

<location rot="45.0" axis-x="0.0" axis-y="0.0" axis-z="1.0"/>

is to set the coordinate frame of the this component equal to that of the parent component rotated by 45 degrees around the z-axis.

Both a translation and rotation can be defined within one <location> element. For example

<location x="1.0" y="0.0" z="0.0" rot="45.0" axis-x="0.0" axis-y="0.0" axis-z="1.0"/>

will cause this component to be translation along the x-axis by “1.0” relative to the coordinate frame of the parent component followed by a rotation of the coordinate frame by 45 degrees around the z-axis as demonstrated in the figure below.

Location-element-transformation.png

Any rotation of a coordinate system can be performed by a rotation about some axis, however, sometime it may be advantageous to think of such a rotation as a composite of two or more rotations. For this reason a <location> element is allowed to have sub-rotation-elements, and an example of a composite rotation is

<location r="4.8" t="5.3" p="102.8" rot="-20.6" axis-x="0" axis-y="1" axis-z="0">
  <rot val="102.8">
    <rot val="50" axis-x="0" axis-y="1" axis-z="0" />
  </rot>
</location>

The outermost is applied first followed by the 2nd outermost operation and so on. In the above example this results in a -20.6 degree rotation about the y-axis followed by a 102.8 degree rotation about the z-axis (of the frame which has just be rotated by -20.6 degrees) and finally followed by another rotation about the y-axis, this time by 50 degrees. The ISIS NIMROD instrument (NIM_Definition.xml) uses this feature.

The translation part of a <location> element can like the rotation part also be split up into a nested set of translations. This is demonstrated below

<location  r="10" t="90" >
  <trans r="8" t="-90" />
</location>

This combination of two translations: one moving 10 along the x-axis in the positive direction and the other in the opposite direction by 8 adds up to a total translation of 2 in the positive x-direction. This feature, for example, is useful when the positions of detectors are best described in spherical coordinates with respect to an origin different from the origin of the parent component. For example, say you have defined a <type name=”bank”> with contains 3 pixels. The centre of the bank is at the location r=”1” with respect to the sample and the positions of the 3 pixels are known with respect to the sample to be at r=”1” and with t=”-1”, t=”0” and t=”1”. One option is to describe this bank/pixels structure as

<component type="bank">
  <location />
</component>

<type name="bank">
  <component type="pixel">
    <location r="1" t="-1" />
    <location r="1" t="0" />
    <location r="1" t="1" />
  </component>
</type>

However a better option for this case is to use nested translations as demonstrated below

<component type="bank">
  <location r="1"/>
</component>

<type name="bank">
  <component type="pixel">
    <location r="1" t="180"> <trans r="1" t="-1" /> </location>
    <location r="1" t="180"> <trans r="1" t="0" /> </location>
    <location r="1" t="180"> <trans r="1" t="1" /> </location>
  </component>
</type>

since this means the bank is actually specified at the right location, and not artificially at the sample position.

Finally a combination of <trans> and <rot> sub-elements of a <location> element can be used as demonstrated below

<location x="10" >
  <rot val="90" >
    <trans x="-8" />
  </rot>
</location>

which put something at the location (x,y,z)=(10,-8,0) relative to the parent component and with a 90 rotation around the z-axis, which causes the x-axis to be rotated onto the y-axis.

Most of the attributes of have default values. These are: x=”0” y=”0” z=”0” rot=”0” axis-x=”0” axis-y=”0” axis-z=”1”

<facing x="0.0" y="0.0" z="0.0"/>
or
<facing r="0.0" t="0.0" p="0.0"/>

<facing val="none"/>

<type name="door">
  <component type="standard-tube">
    <location r="2.5" t="19.163020" name="tube1"/>
    <location r="2.5" t="19.793250" name="tube2"/>
    <location r="2.5" t="20.423470" name="tube3"/>
    <location r="2.5" t="21.053700" name="tube4"/>
    <location r="2.5" t="21.683930" name="tube5"/>
  </component>
</type>

<component type="door">
  <location x="0">
    <exclude sub-part="tube1"/>
    <exclude sub-part="tube3"/>
  </location>
  <location x="1" />
  <location x="2" />
  <location x="3">
    <exclude sub-part="tube3"/>
  </location>
</component>

Using <locations>¶

Most instruments have detectors which are ordered in some way. For a rectangular array of detectors we have a shorthand notation. The <locations> tag is a shorthand notation to use for a linear/spherical sequence of detectors, as any of the position coordinates or the coordinate rotation angles of a <location> tag are changing.

For example a <locations> element may be used to describe the position of equally distanced pixels along a tube, in the example below along the y variable

<locations y="1.0" y-end="10.0" n-elements="10" name="det"/>

The above one line of XML is shorthand notation for

<location y="1.0" name="det0"/>
<location y="2.0" name="det1" />
<location y="3.0" name="det2" />
<location y="4.0" name="det3" />
<location y="5.0" name="det4" />
<location y="6.0" name="det5" />
<location y="7.0" name="det6" />
<location y="8.0" name="det7" />
<location y="9.0" name="det8" />
<location y="10.0" name="det9" />

As is seen n-elements is the number of <location> elements this <locations> element is shorthand for. y-end specifies the y end position, and the equal distance in y between the pixels is calculated in the code as (‘y’-‘y-end’)/(‘n-elements’-1). Multiple ‘variable’-end attributes can be specified for the <locations> tag, where ‘variable’ here is any of the <location> attributes: x, y, z, r, t, p and rot. The example below describes a list of detectors aligned in a semi-circle:

<locations n-elements="7" r="0.5" t="0.0" t-end="180.0" rot="0.0" rot-end="180.0" axis-x="0.0" axis-y="1.0" axis-z="0.0"/>

The above one line of XML is shorthand notation for

<location r="0.5" t="0"   rot="0"   axis-x="0.0" axis-y="1.0" axis-z="0.0"/>
<location r="0.5" t="30"  rot="30"  axis-x="0.0" axis-y="1.0" axis-z="0.0"/>
<location r="0.5" t="60"  rot="60"  axis-x="0.0" axis-y="1.0" axis-z="0.0"/>
<location r="0.5" t="90"  rot="90"  axis-x="0.0" axis-y="1.0" axis-z="0.0"/>
<location r="0.5" t="120" rot="120" axis-x="0.0" axis-y="1.0" axis-z="0.0"/>
<location r="0.5" t="150" rot="150" axis-x="0.0" axis-y="1.0" axis-z="0.0"/>
<location r="0.5" t="180" rot="180" axis-x="0.0" axis-y="1.0" axis-z="0.0"/>

If name is specified, e.g. as name=”det” in the first example, then as seen the <location> elements are given the ‘name’ plus a counter, where by default this counter starts from zero. This counter can optionally be changed by using attribute name-count-start, e.g. setting name-count-start=”1” in the above example would have named the 10 <location> elements det1, det2, ..., det10.

When one <locations> tag was used in ISIS LET_Definition.xml the number of lines of this file reduced from 1590 to 567.

Using <parameter>¶

Parameters which do not change or are changed via <logfile> should be stored using this element inside the IDF, however parameters which may need to be accessed and changed manually on a regular basis should be stored in a separate parameter file.

<parameter> is used to specify a value to a parameter which can then be extracted from Mantid. One usage of <parameter> is to link values stored in log-files to parameter names. For example

<parameter name="x">
  <logfile id="trolley2_x_displacement" extract-single-value-as="position 1" />
</parameter>

reads: “take the first value in the “trolley2_x_displacement” log-file and use this value to set the parameter named ‘x’.

The name of the <parameter> is specified using the ‘name’ tag. You may specify any name for a parameter except for name=”pos” and name=”rot”. These are reserved keywords. Further a few names have a special effect when processed by Mantid

• “x”, “y”, and “z” overwrite the x, y and z coordinate respectively of the element of the component the is a sub-element of.
• “r-position”, “t-position” and “p-position” like “x”, “y” and “z” overwrite the x, y, z coordinates but specified using spherical coordinates (as defined ). Note that the parameters “t-position” and “p-position” are ignored if the parameter “r-position” is not also set for the same component. If only “r-position” is set, say to r-position=”10.0”, than the component will be set to (x,y,z)=(0,0,10.0) i.e. theta and phi default to zero where not specified.
• “rotx”, “roty” and “rotz” rotate the component’s coordinate system around the x-axis, y-axis and z-axis respectively in units of degrees. If any of these are specified they re-define the rotation for the component. You can specify two or three of these to create any rotation. Regardless of what order rotx, roty and rotz is specified in the IDF the combined rotation is equals that obtained by applying rotx, then roty and finally rotz.
• “Efixed”. If specified the ConvertUnits algorithm uses this value in unit conversion
• “SplitInto”. How many MD boxes to split into when converting to MD.
• “SplitThreshold”. The threshold number of MDEvents in an MDBox before splitting into a new MDBox. Concerns convert to MD.
• “MaxRecursionDepth”. The maximum depth of the MDBox tree when converting to MD.
• “offset-phi”. Effective boolean for turning on/off Phi offsets by PI. Set to Always to apply.

The value of the parameter is in the above example specified using a log-file as specified with the element <logfile>. The required attribute of <logfile> is

• id - the logfile name minus the file extension and the ISIS raw file name. For example the id for the logfile ‘CSP78173_height.txt’ is ‘height’.

Optional attributes of <logfile> are:

• extract-single-value-as - specify which value (or values) from the logfile should be used to. This attribute takes any of the following strings
  • mean (default)
  • position n where n is an integer
  • first_value The first value in the run
  • last_value The last value in the run
  • median The median value in the run
  • minimum The minimum value in the run
  • maximum The maximum value in the run
• eq - the values in the log-file may not directly specify the parameter you want to set in the IDF. A simple example is where the values in the logfile are in units of mm, whereas the unit of length in the IDF is meters. Hence for this case by setting eq=”0.001*value” the values in the logfile are automatically converted to meters. A more complicated example is where the height of a detector is recorded in a log-file as the angle between from the horizontal plane to the detector in unit of degrees. Say the distance between the sample (which is assumed to be in the horizontal plane) and the detector is 1.863m then by specifying eq=”1.863*sin(value*0.0174533)” the values in the log-file are automatically converted into the height of the detector from the horizontal plane in units of meters. Note pi/180=0.0174533 in “sin(value*0.0174533)” above is to transform degrees to radians.

Another option for specifying a value for a parameter is to use the notation:

<parameter name="x">
  <value val="7.2"/>
</parameter>

Here a value for the parameter with name “x” is set directly to 7.2. The only and required attribute of the <value> element is ‘val’.

For a given <parameter> you should specify its value only once. If by mistake you specify a value twice as demonstrated in the example below then the first encountered <value> element is used, and if no <value> element is present then the first encountered <logfile> element is used.

<parameter name="x">
  <value val="7.2"/>
  <logfile id="trolley2_x_displacement" extract-single-value-as="position 1" />
</parameter>

In the above example <value val=”7.2”/> is used.

Accessing <parameter>¶

Parameters are by default accessed recursively. Demonstrated with an example:

<component type="dummy">
  <location/>
  <parameter name="something"> <value val="35.0"/> </parameter>
</component>

<type name="dummy">
  <component type="pixel" name="pixel1">
    <location y="0.0" x="0.707" z="0.707"/>
    <parameter name="something1"> <value val="25.0"/> </parameter>
  </component>

  <component type="pixel" name="pixel2">
    <location y="0.0" x="1.0" z="0.0"/>
    <parameter name="something2"> <value val="15.0"/> </parameter>
  </component>
</type>

this implies that if you for instance ask the component with name=”pixel1” what parameters it has then the answer is two: something1=25.5 and something=35.0. If you ask the component name=”dummy” the same question the answer is one: something=35.0 and so on.

Using string <parameter>¶

This is a special category of parameters where the value specified for the parameter is string rather than a double. The syntax is

<parameter name="instrument-status" type="string">
  <value val="closed"/>
</parameter>

Using fitting <parameter>¶

This is a special category of parameters, which follows the same syntax as other but allows a few extra features. Fitting parameters are meant to be used when raw data are fitted against models that contain parameters, where some of these parameters are instrument specific. If such parameters are specified these will be pulled in before the fitting process starts, where optionally these may, for instance, be specified to be treated as fixed by default. To specify a fitting parameter use the additional tag type=”fitting” as shown in the example below

<parameter name="IkedaCarpenterPV:Alpha0" type="fitting">
  <value val="7.2"/>
</parameter>

It is required that the parameter name uses the syntax NameOfFunction:Parameter, where NameOfFunction is the name of the fitting function the parameter is associated with. In the example above the fitting function name is IkedaCarpenterPV and the parameter name is Alpha0.

To specify that a parameter should be treated as fixed in the fitting process use the element as demonstrated in the example below

<parameter name="IkedaCarpenterPV:Alpha0" type="fitting">
  <value val="7.2"/>
  <fixed />
</parameter>

A parameter can be specified to have a min/max value, which results in a constraint being applied to this parameter. An example of this is shown below

<parameter name="IkedaCarpenterPV:Alpha0" type="fitting">
  <value val="7.2"/>
  <min val="4"/> <max val="12"/>
</parameter>

The min/max values may also be specified as percentage values. For example:

<parameter name="IkedaCarpenterPV:Alpha0" type="fitting">
  <value val="250"/>
  <min val="80%"/> <max val="120%"/>
  <penalty-factor val="2000"/>
</parameter>

results in Alpha0 being constrained to sit between 250*0.8=200 and 250*1.20=300. Further this example also demonstrates how a can be specified to tell how strongly the min/max constraints should be enforced. The default value for the penalty-factor is 1000. For more information about this factor see FitConstraint.

A value for a parameter may alternatively be set using a look-up-table or a formula. An example demonstrating a formula is

<parameter name="IkedaCarpenterPV:Alpha0" type="fitting">
  <formula eq="100.0+10*centre+centre^2" unit="TOF" result-unit="1/dSpacing^2"/>
</parameter>

‘centre’ in the formula is substituted with the centre-value of the peak shape function as known prior to the start of the fitting process. The attributes ‘unit’ is optional. If it is not set then the peak centre-value is directly substituted for the centre variable in the formula. If it is set then it must be set to no one of the units defined in Unit Factory, and what happens is that the peak centre-value is converted to this unit before assigned to the centre variable in the formula.

The optional ‘result-unit’ attribute tells what the unit is of the output of the formula. In the example above this unit is “1/dSpacing^2” (for the ‘result-unit’ this attribute can be set to an algebraic expression of the units defined in Unit Factory). If the x-axis unit of the data you are trying to fit is dSpacing then the output of the formula is left as it is. But for example if the x-axis unit of the data is TOF then the formula output is converted into, it in this case, the unit “1/TOF^2”. Examples where ‘unit’ and ‘result-unit’ are used include: CreateBackToBackParameters and CreateIkedaCarpenterParameters.

An example which demonstrate using a look-up-table is

<parameter name="IkedaCarpenterPV:Alpha0" type="fitting">
  <lookuptable interpolation="linear" x-unit="TOF" y-unit="dSpacing">
    <point x="1" y="1" />
    <point x="3" y="100" />
    <point x="5" y="1120" />
    <point x="10" y="1140" />
  </lookuptable>
</parameter>

As with a formula the look-up is done for the ‘x’-value that corresponds to the centre of the peak as known prior to the start of the fitting process. The only interpolation option currently supported is ‘linear’. The optional ‘x-unit’ and ‘y-unit’ attributes must be set to one of the units defined in Unit Factory. The ‘x-unit’ and ‘y-unit’ have very similar effect to the ‘unit’ and ‘result-unit’ attributes for described above. ‘x-unit’ converts the unit of the centre before lookup against the x-values. ‘y-axis’ is the unit of the y values listed, which for the example above correspond to Alpha0.

Using <component-link>¶

Allow <parameter>s to be linked to components without needing <parameter>s to be defined inside, as sub-elements, of the components they belong to. The standard approach for defining a parameter is

<component type="bank" name="bank_90degnew">
  <location />
  <parameter name="test"> <value val="50.0" /> </parameter>
</component>

where a parameter ‘test’ is defined to belong to the component with the name ‘bank_90degnew’. However, alternatively the parameter can be defined using the notation in the an example below. Note that if more than one component e.g. have the name ‘bank_90degnew’ then the specified parameters are applied to all such components.

<component type="bank" name="bank_90degnew">
  <location />
</component>

<component-link name="bank_90degnew" >
  <parameter name="test"> <value val="50.0" /> </parameter>
</component-link>

<component-link> is the only way parameters can be defined in a parameter file used by the LoadParameterFile algorithm.

If there are several components with name ‘bank_90degnew’ but you want specified paramentes to apply to only one of them, then you can specify the name by a path name.

<component-link name="HRPD/leftside/bank_90degnew" >
  <parameter name="test"> <value val="50.0" /> </parameter>
</component-link>

The path name need not be complete provided it specifies a unique component. Here we drop the instrument name HRPD.

<component-link name="leftside/bank_90degnew" >
  <parameter name="test"> <value val="50.0" /> </parameter>
</component-link>

Using <combine-components-into-one-shape>¶

The standard way of making up geometric shapes as a collection of parts is described here: HowToDefineGeometricShape. However, <combine-components-into-one-shape> offers in some circumstances a more convenient way of defining more complicated shapes, as for example is the case for the ISIS POLARIS instrument. This tag combining components into one shape as demonstrated below:

<component type="adjusted cuboid"
  <location />
</component>

<type name="adjusted cuboid" is="detector">
  <combine-components-into-one-shape />
  <component type="cuboid1">
    <location name="A"/>
    <!-- "A" translated by y=10 and rotated around x-axis by 90 degrees -->
    <location name="B" y="10" rot="90" axis-x="1" axis-y="0" axis-z="0" />
  </component>
  <algebra val="A : B" />

  <!-- this bounding box is used for this combined into one shape-->
  <bounding-box>
    <x-min val="-0.5"/>
    <x-max val="0.5"/>
    <y-min val="-5.0"/>
    <y-max val="10.5"/>
    <z-min val="-5.0"/>
    <z-max val="5.0"/>
  </bounding-box>

</type>

<type name="cuboid1" is="detector">
  <cuboid id="bob">
    <left-front-bottom-point x="0.5" y="-5.0" z="-0.5"  />
    <left-front-top-point  x="0.5" y="-5.0" z="0.5"  />
    <left-back-bottom-point  x="-0.5" y="-5.0" z="-0.5"  />
    <right-front-bottom-point  x="0.5" y="5.0" z="-0.5"  />
  </cuboid>

  <!-- this bounding box is not used in the combined shape -->
  <!-- Note you would not normally need to add a bounding box
       for a single cuboid shape. The reason for adding one
       here is just to illustrate that a bounding added here
       will not be used in created a combined shape as in
       "adjusted cuboid" above -->
  <bounding-box>
    <x-min val="-0.5"/>
    <x-max val="0.5"/>
    <y-min val="-5.0"/>
    <y-max val="5.0"/>
    <z-min val="-0.5"/>
    <z-max val="0.5"/>
  </bounding-box>
</type>

which combines two components “A” and “B” into one shape. The resulting shape is shape is shown here:

CombineIntoOneShapeExample.png

Note for this to work, a unique name for each component must be provided and these names must be used in the algebra sting (here “A : B”, see HowToDefineGeometricShape). Further a bounding-box may optionally be added to the to the type. Note the above geometric shape can alternatively be defined with the XML (Mantid behind the scene translates the above XML to the XML below before proceeding):

<component type="adjusted cuboid">
  <location />
</component>

<type name="adjusted cuboid" is="detector">
  <cuboid id="A">
    <left-front-bottom-point x="0.5" y="-5.0" z="-0.5"  />
    <left-front-top-point  x="0.5" y="-5.0" z="0.5"  />
    <left-back-bottom-point  x="-0.5" y="-5.0" z="-0.5"  />
    <right-front-bottom-point  x="0.5" y="5.0" z="-0.5"  />
  </cuboid>
  <!-- cuboid "A" translated along y by 10 and rotated around x by 90 degrees -->
  <cuboid id="B">
    <left-front-bottom-point x="0.5" y="10.5" z="-5.0"  />
    <left-front-top-point  x="0.5" y="9.5" z="-5.0"  />
    <left-back-bottom-point  x="-0.5" y="9.5" z="-5.0"  />
    <right-front-bottom-point  x="0.5" y="10.5" z="5.0"  />
  </cuboid>
  <algebra val="A : B" />
</type>

<combine-components-into-one-shape> for now works only for combining cuboids. Please do not hesitate to contact the Mantid team if you would like to extend this.

This applies when defining any geometric shape, but perhaps something which a user has to be in particular aware of when defining more complicated geometry shapes, for example, using the <combine-components-into-one-shape> tag: the coordinate system in which a shape is defined can be chosen arbitrary, and the origin of this coordinate system is the position returned when a user asked for its position. It is therefore highly recommended that when a user define a detector geometric shape, this could be simple cuboid, that it is defined with the origin at the centre of the front of the detector. For detector shapes build up of for example multiple cuboids the origin should be chosen perhaps for the center of the front face of the ‘middle’ cuboid. When Mantid as for the position of such a shape it will be with reference to coordinate system origin of the shape. However, sometimes it may simply be inconvenient to build up a geometry shape with an coordinate system as explained above. For this case, and for now only when using <combine-components-into-one-shape> it possible to get around this by using the element <translate-rotate-combined-shape-to>, which takes the same attributes as a <location> element. The effect of this element is basically to redefine the shape coordinate system origin (in fact also rotate it if requested).

Using <defaults>¶

Used for setting various defaults.

<components-are-facing x="0.0" y="0.0" z="0.0" />

<offsets spherical="delta" />

<reference-frame>
  <!-- The z-axis is set parallel to and in the direction of the beam. the
       y-axis points up and the coordinate system is right handed. -->
  <along-beam axis="z"/>
  <pointing-up axis="y"/>
  <handedness val="right"/>
</reference-frame>

<angle unit="radian"/>

<length unit="meter"/>