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Question Tracking problem with policapillary tubes and tori 

Forum: Geometry
Date: 22 May, 2006
From: Alexey Tkachenko <ayut@hea.iki.rssi.ru>

Dear all.

Our group is developing an application to simulate transportation of gamma rays across polycapillary X-ray optics. We are currently using G4 v.8.0 on Debian platform with gcc 3.3.5.

We are using G4Tubs for LeadGlass fibers and filling it by dense-packed G4Tubs channels of Air. The diameter of fiber is 1 mm. The diameter of channels is 20 micron. The distance between two channels is about 100 nm. The fiber and channels have the same length. We are shooting gamma to these channels at a small angle (~1 arc. min). For reflection of gamma we are using the process based on G4OpBoundaryProcess. This routine was taken as a starting point and extended to describe the propagation of an X ray from one medium to another (e.g. air to leadglass surface) and decides either if the X ray will be reflected (total reflection), or absorbed. For testing we force gamma to backscatter and we have encountered the following tracking problem.

For central channels our reflection is working properly and we have the following tracking information.

**********************************************************************************************
* G4Track Information:   Particle = gamma,   Track ID = 1,   Parent ID = 0
**********************************************************************************************
Step#    X         Y        Z     KineE   dEStep  StepLeng TrakLeng Volume      Process
  0  -4.28 mum -2.67 mum   70 cm  20 keV   0 eV    0 fm      0 fm   World       initStep
  1  -4.25 mum -2.67 mum   70 cm  20 keV   0 eV  100 mum   100 mum  World       Transportation
  2   9.64 mum -2.67 mum 65.2 cm  20 keV   0 eV  4.77 cm  4.78 cm   Fiber_holes Transportation
  3   9.64 mum -2.67 mum 65.2 cm  20 keV   0 eV     0 fm  4.78 cm   Klens_fiber Transportation
  4  -4.25 mum -2.67 mum   70 cm  20 keV   0 eV  4.77 cm  9.56 cm   Fiber_holes Transportation
  5  -1.26 mm  -2.67 mum     5 m  20 keV   0 eV    4.3 m   4.4 m    OutOfWorld  Transportation
----------------------------------------------------------------------------------------------

Here the "Fiber_holes" is a air channel and the "Klens_fiber" is a leadglass fiber. The gamma changes momentum to the negative after step 2 and comes back to the channel.

But for non-central channel in some geometrical configurations the gamma does not return to the channel after backscattering and is absorbed in the glass fiber.

**********************************************************************************************
* G4Track Information:   Particle = gamma,   Track ID = 1,   Parent ID = 0
**********************************************************************************************
Step#    X         Y        Z     KineE   dEStep   StepLeng TrakLeng Volume      Process
  0    217 mum -2.67 mum   70 cm  20 keV   0 eV     0 fm      0 fm   World       initStep
  1    217 mum -2.67 mum   70 cm  20 keV   0 eV   100 mum   100 mum  World       Transportation
  2    231 mum -2.67 mum 65.2 cm  20 keV   0 eV  4.77 cm   4.78 cm   Fiber_holes Transportation
  3    231 mum -2.67 mum 65.2 cm  0 eV  15.9 keV 5.77 mum  4.78 cm   Klens_fiber phot
:----- List of 2ndaries - #SpawnInStep=  1(Rest= 0,Along= 0,Post=1), #SpawnTotal=1 ------------
:    231 mum -2.67 mum  65.2 cm   4.14 keV        e-
:----------------------------------------------------------- EndOf2ndaries Info ---------------

This tracking error is unstable and depends on position of channels in the fibre. For example for configuration of channels drawing on pic.1 http://hea.iki.rssi.ru/~ayut/geant/polycap_1.gif the reflection is working properly, but for pic.2 http://hea.iki.rssi.ru/~ayut/geant/polycap_2.gif the gamma is absorbed (The fiber is a blue. The channels is a green. We were shoting gamma to red channel.). The initial position and momentum of gamma remained fixed.

We see this problem both for G4Tubs and for G4Torus based geometry.

Why are we getting these results? Are we doing something wrong?

Thanks in advance.

Alexey Tkachenko

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1 Feedback: Re: Tracking problem with policapillary tubes and tori   (Peter Gumplinger - 24 May, 2006)
(_ More: Re: Tracking problem with policapillary tubes and tori   (Alexey Tkachenko - 25 May, 2006)
(_ None: Re: Tracking problem with policapillary tubes and tori   (Makoto Asai - 25 May, 2006)
(_ Feedback: Re: Tracking problem with policapillary tubes and tori   (Alexey Tkachenko - 25 May, 2006)
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