The pixel ionisation chamber: a detector for beam monitor and dosimetry

1
The pixel ionisation chamber a detector for beam
monitor and dosimetry

• A.Boriano 1,2, F.Bourhaleb 2,3, R. Cirio 2, M.
  Donetti 2,4,
• F. Marchetto 2, C. Peroni 2, C.J. Sanz Freire 2
• 1 ASP, Torino, Italy
• 2 University and INFN, Torino, Italy
• 3 ICSC WorldLab, Lausanne, Switzerland
• 4 TERA Foundation, Novara, Italy
• partially supported by IBA, Louvaine- la-Neuve,
  Belgium
• PTCOG XXXV PMRC Tsukuba November 15 2001

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Improving the Magic Cube (3D dosimeter for
hadron beams)
The Magic Cube can be bought at Physalus
(furio.gramatica_at_physalus.it)
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The pixel chamber

• parallel plate ionization chamber
• anode segmented in 1024 square pixels
• pixel dimension 7.5 X 7.5 mm2
• sensitive area 24 X 24 cm2
• 1 mm water equivalent thickness
• front-end electronics, located around the
  chamber, perform analog to digital conversion

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Front-end electronics

• located around the chamber
• we have developed a full custom chip (TERA05)
• every chip has 64 channels that convert the
  collected charge in digital output

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TERA05

• Very Large Scale Integration (VLSI)
• i?f conversion
• output ? Qint
• 100 fCltcharge quantumlt800 fC
• Imax 4 ?A
• 64 channels
• 16 bit wide counters
• multiplexed digital output
• dead-time free readout
• max read out 10 MHz

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Linearity at 100 fC charge quantum

• 20 pA lt I lt 0.6 ?A
• Linearity better than 0.7

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Linearity at 600 fC charge quantum

• 10 pA lt I lt 2 ?A
• Linearity better than 0.3

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Spread of charge quanta

• I 49.96 nA
• 26 chips

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Reproducibility of charge quantum

• I 49.96 nA
• Qc 600 fC
• 24 ºC lt T lt 27 ºC
• 1 month
• 6 measures

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Data acquisition

• connection detector-data acquisition by twisted
  pair flat cables (100 m maximum lenght)
• max rate transfer 10 MHz 40 Mb/s
• read out transfer time 50 ?s
• read out cycle total time 100 ?s
• real time operating system

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Beam test at GSI
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Spatial resolution
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Homogeneity of response
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Beam test at PSI
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Comparison with PSI proton beam transverse profile
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very good for IMRT with photons too !(cfr.
collaboration with IBA/Wellhofer/Scanditronix)
Field shape varying with time 6 MV Elekta
accelerator at S.Anna hospital in Torino 1.2x1.2
cm2 square field going to 12.4x12.4 cm2 square
field
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Conclusions

• parallel plate pixel ionization chamber
• big sensitive area
• 1024 independent measurements
• VLSI electronics dead-time free readout
• one dead-time free readout of all 1024 channels
  every 100 ?s
• ?GOOD MONITOR FOR SCANNED BEAMS
• Next step might be a real-3D-Magic Cube (?
  dosimeter)