Ionisation Chambers and Secondary Emission Monitors at the

Ionisation Chambers and Secondary Emission Monitors at the

Project
Document No.
Ionisation Chambers and Secondary Emission
Monitors at the PROSCAN Beam Lines
Title
P24/DR84-618.0
Document type
Author
Co-Author(s)
R. Dölling
Poster
Das folgende Poster wurde gezeigt am 12th beam instrumentation workshop (BIW06), May
1st - 4th, 2006, Fermilab, Batavia, Illinois, USA. Das zugehörige Paper wird publiziert in
AIP conference proceedings und ist zugleich Proscan-Document P24/DR84-617.0.
Keywords (max. 5): Diagnostik, Messergebnisse
Org.
Distribution List
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Pages
Attachments
Date: 15.5.06
PSI
Project Archive: W. Roser/WBGA/C36a
(electronic + paper version!)
Seite 1 von 10
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10
IONISATION CHAMBERS
AND SECONDARY EMISSION MONITORS
AT THE PROSCAN BEAM LINES
R. Dölling, PSI, Villigen, Switzerland
PROSCAN,
the dedicated new medical facility at PSI
using proton beams for the treatment of deep
seated tumours and eye melanoma, is now in
the commissioning phase. Air filled ionisation chambers in several configurations are
used as current monitors, profile monitors,
halo, position and loss monitors at the
PROSCAN beam lines. Similar monitors
based on secondary emission are used for
profile and current measurements in the
regime where saturation deteriorates the
accuracy of the ionisation chambers.
• proton beam 250 MeV, 1 to 500 nA
• <10 nA after degradation (230 to 70 MeV)
• fast changes of beam energy (~50ms)
eye
treatment
material
irradiation
cyclotron
degrader
gantry 2
control
point
tomography
control
point
gantry 1
insertable profile monitor
thin profile & current monitors
halo monitors
external ionisation chambers
BPM
collimators & slits
stoppers
dipoles/steerer/quadrupoles
Multistrip Ionisation Chamber Profile Monitors
• chamber filled with ambient air
• plane separation 4 mm, anode voltage +600 V
• intercepting
variant with variable strip configuration
• 68 thick-film metallised strips (each plane)
• read-out of 16 or 32 channels per plane
• adaptation of strip pitch by combining strips
central board:
front side: horizontal profile
back side: vertical profile
• 2-D beam profile
• measurement during actuator movement
• vertical position from potentiometer
• pixels connected by inner layer of PCB
connectors for
2x 68 signals
(outer boards:
inner side: high voltage
outer side: ground)
beam
selectable pitch (with
16 channels)
1 cm
switching boards
variant with a line of pixels
Thin Profile And Current Monitors
•
•
•
•
•
•
•
2 current measurements, horizontal & vertical beam profile
6 µm titanium foils, plane separation 2 mm
foils soldered to ceramic board
fast venting of vacuum chamber must be prevented
as secondary emission monitor: yield 0.052
as ionisation chamber in air: yield 48 (@250 MeV)
IC: anode voltage +2000 V, charge collection time ~12 µs
IC saturation due to recombination
• takes place at small beam diameters and higher beam currents:
the efficiency is a function of
(plane separation)4 * (beam current density)
(high voltage)2
• effect found less severe than expected:
calculation according to
Mie, G., „Der elektrische Strom in ionisierter Luft in einem ebenen Kondensator“,
Ann. Phys. (Leipzig) 13, 857-889 (1904).
variant: small IC current monitor
• 2 current measurements (5 foils) in front of
Gantry1, Gantry 2, Optis
• 6 µm Ti foils, plane separation 2.5 mm
• foils between PCB (fixed with glue)
• anode voltage +2000 V
Position Monitors In Front Of Gantries
•
•
•
•
in air
main part of beam not intercepted
position information from beam core and beam halo
anode voltage +2000 V
thick profile monitor
(retracted from beam)
vacuum air
position and
halo monitor
circular anodes
at +2000 V
4 segment
electrode
(for halo)
current monitor
5 foils of 6 µm Ti
+2000 V at #1, 3, 5
output from #2, 4
beam
4 segment
electrode
(for position)
contact rods
are shown
collimator
Ø30 mm
window
collimator Ø8 mm
fast stopper
(retractable)
position determination
• the beam position is determined from the signal currents
Ileft, Itop, Iright, Ibottom as x = (x ′ + y ′) 2 , − y = (y ′ − x ′) 2
with
− Ileft − Itop + Iright + Ibottom
− Ileft + Itop + Iright − Ibottom
, y′ = k
x′ = k
Ileft + Itop + Iright + Ibottom
Ileft + Itop + Iright + Ibottom
and k , k
directions.
a measure for the beam width in both diagonal
• comparison with thick MSIC profile monitor in front:
A focused beam was swept horizontally over the monitors by a
steering magnet. k and k are fitted by requiring horizontal
slopes in the points of symmetry which define the monitor
axis. A discrepancy between the readings of ~1 mm in both
directions was observed although all monitors were aligned
within 0.1 mm. Azimuthal asymmetry of the 2-dimensinal
beam profile is a possible explanation. The noise of the
position readings is of the order of 0.2 mmpp. This is
satisfactory, especially when taking into account that very few
strips of the MSIC contribute to the profile.
• The use of only roughly estimated k and k leads to a
distortion of the curves around the points of symmetry.
Nevertheless, this "position" information can still be used as
input to a beam centring procedure.
Beam current 1.8 nA. Beam energy 220 MeV. Beam width horizontally ~12 mm, vertically ~7 mm full width quarter maximum
Halo Monitors and external IC
• halo monitors are placed around (beam-pipe) bellows at each
quadrupole doublet or triplet (i. e. at large beam diameter)
• for an online check of beam "tune"
• for loss control
Signal levels when the beam is steered far off-axis.
Beam current 1.4 nA. Beam energy 230 MeV.
1 cm
beam halo
4-segment circular electrode
HV electrode
beam axis
ANORDNUNG DER SEGMENTE DES POSTERS
Multistrip IC Profile Monitors
Title und Abstract
2
1
variant with variable strip configuration
3
Position Monitor in front of gantries
6
position determination
Thin Profile And Current Monitors
7
4
IC saturation due to recombination
Halo monitors and external IC
5
8