Time Projection Chamber with GEM

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Last Updated: Friday, 26 August 2016 18:47

A compact time projection chamber (TPC) has been designed, built and tested, for beam monitoring at the Dafne Beam Test Facility (BTF). The Facility provides electron and positron beams in a wide range of intensity, from single particle up to 1010 particles per pulse, and energy, from a few tens of MeV up to 800MeV .

TPG structure

   The main idea in developing this detector is to place a standard triple GEM detector parallel to the particle beam and to use it as a time projection chamber, by enlarging the drift gap.In this way the depth of the material crossed by the particle is particularly small (less than 0.2%X0) and the beam position measurement could be more precise, in the z coordinate along the drift O(100 microns), by measuring the time of arrival of the electron clusters.
tpc_gem  
 The 128 readout pads, as shown in the picture below, have  a dimension of 3x6 mm2 organized always in a matrix 8x16 pads but with a total active area of 5x5 cm2. With this pattern readout, the track z position is measured 8 times per event with a final precision of O(50 micron) and a good resolution O(1mm) is obtained in the other two coordinates X and Y.
The TPC, working without a magnetic field, has been switched on with a gas mixture Ar CO2 (70/30) at a Gain 104: just opening a random time gate of 8 ms, clean cosmic tracks like in the picture can be seen on the online display through the scalers readout.
tpc_fpga   In order to have a more flexible system, a new  mother board has been designed and realized with an FPGA, that will be able to analyze the LVDS signals coming from the FEE board, both with scalers and multihit TDC with a resolution of 2 ns, sufficient to record the time drift of electrons along the 4 cm drift. The high rate capability of GEM technology allows the use of this monitor not only for single tracks events but also for multi particle beam.
 
The use of different gas mixture is under study to increase the range of particles per beam. Thanks to a small material budget crossed by the particles and the 3D reconstruction of the particles track, it's use for ion beam monitor in hadrotherapy is very promising and under study.
 
     
Performances    
single_track_web

 

The  performances of this detector have been measured  at BTF in Frascati. In this picture is visible the online monitor.

On the first row the three views of one electron 500 MeV .

On the second row the history plots of fews minutes.

On the third row :

  • the horizontal beam profile ;
  • the number of hits (is visible the peak of one and two electrons)
The particle position is sampled 8 times with a residual of 100 microns for each hit,and the final track position resolution is 50 microns.
     
double_track_web  

In this picture is visible the online monitor.

On the first row the three views of two electrons 500 MeV .

On the second row the history plots of fews minutes.

On the third row :

  • the horizontal beam profile ;
  • the number of hits
  • the vertical beam profile
     
1000_tracks_web  

On the first row the three views of 1000 electrons 500 MeV in a bunch of 10 ns.

On the second row the history plot of fews minutes.

On the third row :

  • the horizontal beam profile ;
  • the number of hits
  • the vertical beam profile
     
  This viedo shows  the online monitor during the BTF test when a beam of  one or two electrons goes through the chamber
     

 

TPC GEM as tracker monitor for channeling experiment.

Recently this compact TPC has been used at H8 test beam at CERN for proton channeling studies. The chamber was placed 50 meters downstream the crystal.  On the right  it's possible to see the two views (side and front) of the main and the channeled beam. The two beam are separated by 3.6 mm.
Just below the online beam profiles without T0 corrections. 

On the left the two beams images taken with two medipix with 50x50 micron2 pixels.