Litrani, a Monte-Carlo for optical photons

1. Litrani, a Monte-Carlo for optical photons Built upon ROOT. Use the old, simpler geometry of ROOT. A photon strictly obeys the Maxwell equations along its path, no simplification. Detailed description of materials and wrappings. Dielectric constant and absorption length may be anisotropic. Possibility of diffusion length and wavelength shifting. Possibility to handle thin slices. Various sources of light. F.X. Gentit

2. Litrani, usage and documentation • Delivered with its source code, running on Windows XP and Linux. • Activated using short CINT macros. CINT is the interpreter language of ROOT. • Very detailed documentation (~280 web pages !) on the web at: • http://gentit.home.cern.ch/gentit/litrani/ • Examples of use provided. • Physics behind the program described on the web. • Most users of Litrani could use it without exchanging mails with me. • Users everywhere in the world. Thanks to the interaction of many users, very well debugged. F.X. Gentit

3. Litrani: Parameter dependency • For all physical quantities depending upon a variable parameter, like • Index of refraction, depending upon wavelength • Absorption length, depending upon wavelength • Gain profile of APD, depending upon depth • And so on • Litrani uses the very flexible and powerful application SplineFit, described on the web at: • http://gentit.home.cern.ch/gentit/splinefit/ F.X. Gentit

4. Litrani: Materials • A material in Litrani has the following properties: • Index of refraction or dielectric tensor: the index of refraction may be anisotropic. In addition, they depend upon wavelength. • The absorption length also may be anisotropic and is dependent upon wavelength. • The material may have a wavelength dependent diffusion length, possibly with wavelength shifting. • Sensibility: the material may or not be a detector. • The material may have a fluorescent spectrum when crossed by a particle or a gamma. Any type of fluorescent spectrum may be described. • dE/dx characteristics when crossed by a particle, radiation length, Moliere radius and critical energy when crossed by an electromagnetic shower. • Possibility of describing position dependent damage factor due to irradiation. F.X. Gentit

5. Litrani: Wrappings • When a face is covered by some wrapping, there may be, between the face and the wrapping, a slice of some material, for instance air, allowing total reflection. • The wrapping has a complex index of refraction. The absorption is governed by the real part of the index. The Maxwell equations are strictly solved, without simplification. • One can add supplementary absorption for instance for simulating a dirty wrapping. • When the photon is not absorbed, it is either reflected or diffused, according to a wavelength dependent probability given in the definition of the wrapping. Diffusion is calculated exactly as reflection, except that it occurs on a plane of any orientation. F.X. Gentit

6. Litrani: Shapes • The shapes of Litrani are the same as those of GEANT: • Any shape with 8 vertices and 6 plane faces: • TBRIK, TGTRA, TPARA, TTRAP, TTRD1, TTRD2 • Cylinders with or without hole (TTUBE) • Cones, truncated or not (TCONE) • Notice the following limitations • Litrani only allows a flat geometry: it is impossible to define a volume inside an other volume. Hopefully, this limitation will disappear when I switch to the new ROOT geometry, in an unpredictable future. This limitation has as counterpart an advantage: the description of the setup in Litrani is simple. But the drawback is that some complicate setups cannot be described by Litrani. • It is impossible in Litrani to describe a lens. It will remain so even with the new geometry! For that, look at specialized programs. F.X. Gentit

7. Litrani: Faces • Each face of a shape may be either partially or totally in contact with an other face of an other shape, or covered with some wrapping. Each face may have a different type of wrapping. Each face may be divided into sub-faces. If a contact exists between 2 shapes, one can insert a thin slice between the two. • A face may be polished or unpolished. • When unpolished, the normal to the surface at the point hit by the photon is randomly tilted with respect to the true normal of the surface, by the angles q, f, generated randomly according to the distribution sinq dq df, with 0< q < qM. So qM (0° <= qM < 90°) can adjust the roughness of the grinding. F.X. Gentit

8. Litrani: Sources of light described • Spontaneous photons, with any kind of emission spectrum. • Optical fibre • Beam of particles generating light along their path, with or without Cerenkov light • Gamma around 1 Mev (Thanks to David Wahl). Compton effect and photo-electric effect are simulated. (Pair creation due to appear soon). • Electromagnetic showers (crude description according to the formulae proposed in the Review of Particle Physics). F.X. Gentit

9. Litrani: Detectors • General type of surface detectors. • Phototubes. Variation of quantum efficiency as a function of wavelength is described. • General type of volume detectors. • Avalanche Photo-Diodes. The handling of APD goes until the generation of electrons and simulation of the electronic pulse, taking into account the gain profile as a function of depth. • PIN diodes. F.X. Gentit

10. Litrani: conclusion • In general and for sometimes good reasons, Monte-Carlo of optical photons are not used to get precise results, but more for grossly understanding the behaviour of a setup. This is due to the difficulty in describing precisely some details of the setup. Even with this restriction, a Monte-Carlo of optical photon may be extremely useful. • I want to point on a striking exception to the above rule: a work done by David Wahl using a mix of Litrani predictions and lab measurements, to get a precise measure of the proportion of diffusion versus absorption in a crystal: • The Monte-Carlo refractive index matching technique for determining the input parameters for simulation of the light collection in scintillating crystals • NIM Section A: Volume 570, Issue 3, 21 January 2007, Pages 529-535 • I work now in association with David Wahl for improving Litrani. F.X. Gentit