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vTAS is a Three Axis Spectrometer simulator written in Java. It is aimed at making easier the understanding of the behavior of the instrument and its limits. It supports both the classical TAS geometry and that of the multiplexer FLATCONE (ILL). vIMPS and vUFO are part of the vTAS suite but address other multiplexed TAS geometries.

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vTAS - A Virtual Three Axis Spectrometer

by Martin Böhm, Alain Filhol and Yannick Raoul


Using a Three Axis Spectrometer (TAS) is not easy since one has constantly translate parameters from real space to reciprocal space and vice versa
The goal of vTAS is to provide an interactive and graphical display of both the instrument configuration and the corresponding measured space thus making easier the understanding of the behavior of the instrument and its limits.
vTAS supports both the classical TAS geometry and the multiplexed geometry of FLATCONE. Other geometries are available through the vTAS suite described below.

Simulation mode: vTAS allows experimentation with sample parameters, for example the Miller indices of the unit cell, whilst maintaining a representation of the instruments physical state, for example the location of the walls around the instrument, and its angular limits.
The instrument resolution function is computed using the Cooper Nathan's method. A yet untested McStas like Monte Carlo approach is also available but for the bundled version only.

Data viewer mode: vTAS is capable of reading ILL TAS data files. It displays both the data and the corresponding instrument configuration.


Ref.: "The vTAS suite: a simulator for classical and multiplexed three-axis neutron spectrometers"
       M. Boehm, A. Filhol, Y. Raoul, J. Kulda, W. Schmidt, K. Schmalzl,
       Nuclear Inst. and Methods in Physics Research, A (2013) 697, 40-44.


The vTAS suite is now a single application vTIU regrouping three simulators:

  • vTAS: classical TAS geometry and  multiplexed geometry of FLATCONE.
  • vIMPS: multiplexed geometry of IN8-IMPS
  • vUFO: multiplexed geometry of IN12-UFO

An early version of vTAS was developped by Alain Bouvet in 1998.
The current vTAS project was funded by the ILL and NMI3.

Running the application

Requires Java 2 v1.6 or higher.
Make sure that Java is installed and activated <>.
Activating Java is sometimes tricky. On some computers you may also need to clear the Java cache. On some computers there is not even an error message when Java is not responding!
The JNLP mechanism will give you the opportunity to install a standalone version of the application on your hard drive.
Prior installing a new version it is recommended to delete the old preference files.
     E.g. for Mac OS X users, delete folder ~/Library/Preferences/ILL/vTAS.

Click HERE to launch vTAS v4.51 in a browser page
or Download the signed application: vTIU.jar

Release date: 29 janvier 2013
Report bugs to You may have to register first.

Mac users should read this : OS X 10.9 or higher, Safari 6 or higher !

Mac : if the Java runtime tool is missing, install it from here : <>

Mac test

Special keys and options

Here what you need to know for a good experience with the vTAS interface.

  • Instrument and scattering panes

    • right click+drag: paning
    • [Mac OS X] cmd-key+click+drag: paning
    • mouse wheel: zoom in/out at mouse position
    • click+drag : [FLATCONE] to modify vectors kf and Q
    • opt-key+click+drag : [FLATCONE] to change the active channel

  • Scattering pane

    • when tau+q option is active: press the shift key to display the "q" constrain direction

  • Instrument resolution function (only for the bundled-vTAS version, not for the web version)
    First, put the focus on the reciprocal space pane, then:

    • The R key will launch a McTAS-like Monte Carlo simulation. The calculation may take several seconds, be patient ! (Warning: this is an unverified option)
    • The BS key will erase the plot


vTAS is a free application. Use it at your own risks. Neither the authors nor the ILL endorse or accept any responsibility for problems or wrong results you may obtain with it. No support is provided but feedback is warmly welcome -> <filhol(at)>.



vUFO now implements instrument resolution calculation through both the Cooper Nathan's method. It also implements the McTAS approach, i.e. a Monte Carlo simulation.


vIMPS by Martin Boehm, Emmanuel Farhi, Alain Filhol and Yannick Raoul.

vUFO by Emmanuel Farhi, Alain Filhol, Yannick Raoul and W. Schmidt


Yannick Raoul (ILL) fully rewrote the mathematical part and most of the core and GUI functions of vTAS. The application is now functional and new options were added:

  • TAS data file (ILL format): vTAS can automatically display the data and instrument configuration read from a TAS file.
  • FLATCONE: this is a multiplexer (several analysers and detectors) which can be installed on ILL TAS spectrometers. vTAS fully supports this instrument geometry and the corresponding data file format.
  • Instrument zone: this can be edited through a dedicated dialogue.
  • On line help: in progress.


  • IMPS support: this is the multiplexer of IN8 (ILL)
  • UFO support: this is an advanced multiplexer designed for IN12 (Julich-ILL). Its geometry being quite complexe, a simulation tool like vTAS should prove to be highly valuable or even essential to users.
  • 4D resolution: an early version of vTAS implemented a limited support for 4D resolution calculation and display. Our goal is to implement it again in a more correct and complete manner.


Peter Braden (University of St. Andrews, UK) : As the codebase had become very messy, completely reimplemented the program to modern software engineering principles, expanded on the visualisations offered, and improved the usability of the interface. Report

Mac OS X, Eclipse 3.3.0, J2SE 5.0 (JAVA 1.5).


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Noëlle Le Delliou (UFR IMA, Grenoble, France) resurrected the old code, updated the inteface using Swing, and added the ability to simulate walls around the instrument area. Development using: Mac OS X, Netbeans 4.1, JAVA Report

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First developed by Alain Bouvet in 1999 using an early Mac OS implementation of JAVA/AWT. Unfortunately the application was hardly usable due its buggy AWT interface. Report

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Updates: 9 Sept 2009, 20 Jan. 2011, 13 march 2013, A.Filhol, <filhol(at)>