Design and test of a high-speed
beam monitor for hardon therapy
H. Pernegger on behalf of Erich Griesmayer
Fachhochschule Wr. Neustadt/Fotec Austria
(H. Frais-Koelbl, E. Griesmayer, H. Kagan, H. Pernegger)
MedAustron
• Austrian medical accelerator facility
• Cancer treatment and non-clinical
research with protons and C-ions
Protons
Conventional X-Ray Therapy
CERN RD42 meeting
1 cm
Ion-Therapy
C-Ions
H. Pernegger , E. Griesmayer
1 cm
2
Layout
Synchrotron
Injector
2 Experimental rooms
Preliminary layout
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4 Treatment rooms
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Parameters
• Proton & Carbon Beam
– Energy: 60-240 MeV protons and 120-400 MeV/u C-ions
– Intensity: 1x1010 protons (1,6 nA) and 4x108 C-ions (0,4 nA)
– Beam size: 4x4 mm2 to 10x10 mm2
• Setup
– 4 fixed beams and 2 gantries
– Field sizes: 40x40 cm2, 25x25 cm2, 4x4 cm2 (fixed beams), 20x20
cm2 (gantries)
– Active scanning
– Extraction period: 1 s to 10 s
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High speed beam monitor
• Initial goal: Develop a detector for beam diagnostic
• Measure intensity & structure of extracted beam by
counting individual particles (no integration)
– Short pulses with good time resolution for high-speed counting
– Resolve beam time structure (measure number of extracted particles
for each revolution)
– 1D or 2D position sensitivity to provide beam profile
• Rates: counting single particles at rates close to the
GHz/channel-range
Protonen:
p
p
8
1.0  10
   1.0  10
2
s  mm
s  mm 2
C-Ionen :
C
C
6
4.0  10
   4.0  10
2
s  mm
s  mm 2
6
4
Maximum rates up to 6x larger during RF cycle
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Beam Monitor Concept
• Segmented CVD diamond as
detector material
– High drift velocity + short charge
lifetime give short signals
– Radiation hard
– Variable segmentation possible on
thin solid stage detector
• RF-amplifier and parallel counting
– Direct amplification of ionization
current pulse (no current integration)
– discriminator and pulse counter to
parallel readout
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Test of first prototype
• Tested a first prototype of detector and electronics at
Indiana University Cyclotron Facility
– Tested with protons (worse case: smaller signal)
– Tested in energy range for proton therapy (55-200MeV)
– Variable intensity
• Main focus: measure analog signal characteristics
–
–
–
–
Signal time properties
Amplitude properties
Energy scan and dE/dx in diamond
Efficiency
• Tested with first prototype of
– 2 samples of CVD diamond
– First prototype for analog amplification stages
– First tests of digital electronics (in progress)
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Setup and Samples used for tests
• 2 diamond samples with different pad size +
scintilator as “telescopes”
– 2.5 x 2.5 mm2 (in trigger) CCD = 190 mm, D= 500 mm
– 7.5 x 7.5 mm2 (for analog measurements) CCD = 190 mm,
D= 500 mm
– Trigger scintilator (5x3mm2)
trigger
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measured
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RF amplifier stage
• 3-stage current amplification
• Parameters (per stage)
– Bandwidth 2GHz
– Amplification 20dB, Noise 2.7dB
• Some signal estimates:
– Max. current peak from diamond 1.7mA for MIP
– Max (theor.) SNR expected for 55 to 200 MeV protons: 20:1 to
8:1
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Digital readout stage
• Disriminator
• Counting & readout
– Discriminate on voltage and
time difference
– Baseline restoration with
delay line
– Implemented in PECL
– Count in fast 8-bit and latch
to 24 bit counters
– Allows to store full “pulse
trains” for fast rate vs time
measurement in SDRAM
u
t
u
ana log
in
PECL
out
t
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Measured pulses
• Single signals in diamond (protons at 55 MeV)
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Signal Time Properties
• Rise time : 340ps Duration: 1.4ns
• Average pulse shape
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• Pulse duration (FWHM)
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Diamond signal amplitudes
• Amplitudes in the full energy range
– r.m.s. noise = 18 mV
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dE/dx in CVD Diamond
• Compare measured signal to calculated dE/dx behaviour
in diamond
– Normalized at 104 MeV for uncertainty in absolute calibration
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First results on Signal-to-Noise
200 MeV
104 MeV
55 MeV
• Measured most probable S/N ranges from 15:1 to 7:1
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Preliminary results on Efficiency
• Defined as signal with
– amplitude > 3 x snoise
– tsignal in +/- 3ns window of trigger time
100%
90%
• Measured efficiency of 99% to 94% (noise limited)
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Next steps
• Diamond and dedicated electronics seems to be ideally suited for
beam diagnostics
• Achieved very promising results for beam diagnostics with
protons
– SNR 7:1 to 15:1 in the typical energy range for proton therapy
– Risetime of 350ps and pulse width 1.4ns
– Efficiency 94% to 99% (electronics noise limited)
• Since then
– Worked on optimizing SNR for even lower signals (MIP range) and
achieved lower noise with modified electronics
– Recently tested with C ions (3 weeks ago) and large surface (3x1cm
pad)
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