METAL FOIL DETECTORS FOR THE RADIATION AND BEAM POSITION MONITORING

1
METAL FOIL DETECTORS FOR THE RADIATION AND BEAM
POSITION MONITORING
EPS2003
V. Pugatch, Yu. Vassiliev, V. Aushev, N. Tkatch
ABSTRACT To monitor charge particle fluences
distribution various methods have been developed.
We report here on the approach based on the so
called Metal Foil Detectors (MFD). MFD is a
device which measures a flux of charged
particles by means of the charge integration
originated in the thin metal foil due to the
Secondary Electron Emission initiated by the
incident particles. The radiation profile is
reconstructed from the charge distribution in a
set of metal foils covering the area under the
study. Their size has to be optimized to provide
a reasonable profiling, collection of a charge
which should be big enough to trigger the
read-out charge integrators. The foils have to
be thin to provide no deterioration of the
tracks, and, on the other hand, thick enough for
mechanical stability.
Rate sharing
Introduction The first monitor based on
Secondary Electron Emission (SEE) initiated by
the incident particles has been build in the old
1955. Since then various types of the SEE beam
profile monitors were build, yet, to our
knowledge the principle has not been explored for
the radiation monitoring with a comparatively
large area sensors, placed in the
atmosphere. Concerning radiation hardness and
thermal features of the SEE monitors stable
operation was established under the impact of the
average beam density of 150 ?A/cm2 and fluence of
1020 protons/cm2 as well as low temperature
dependence has been observed. It was established
that the yield of the SEE exceeds by factor of 10
directly produced ?-electrons. The above
mentioned features of SEE served us a basis for
the designing Metal Foil Detectors (MFD) for the
charge particle fluxes monitoring. Several MFD
were build and successfully tested at KINR
(Kiev), DESY (Hamburg), MPIfK (Heidelberg) and
CERN (Geneva) during the last years. MFD BPIT
at 21 Mev proton beam For the purpose of the
silicon strip-detectors irradiation hardness
study we have built and tested the MFD
Beam-Profile-Imaging-Target (MFD BPIT) at MPIfK
(Heidelberg). To simulate a non-uniform radiation
load at experiments HERA-B and LHCb 21 MeV
protons scattered by the 50 ?m thick Au-target
are used. To exclude the uncertainties in the
accumulated fluence due to the beam instabilities
it was decided to equip the setup by a monitor
which should measure the distribution of the
irradiation load alongside the detector area (50
x 70 mm2). Different target materials and BPIT
prototypes were studied. It was established that
a sensitivity of the system could be improved if
a separating foil was inserted between two
read-out foils and a low positive voltage was
applied. The charge distribution in the BPIT grid
made out of thin foils and introduced directly
into the 21 MeV proton beam being read-out by
sensitive charge integrators from every X- and
Y-foils of the grid should reflect the incident
beam profile. We assume that this charge emerges
mainly due to SEE- and ?-electrons. Thus, the
method developed by us is applicable for beams of
any type charged particles with energies higher
than few MeV/nucleon. The output frequency of the
charge integrators reaches a value of 500 Hz per
1 pA of the 21 MeV proton beam which should be
also sufficient for the feedback purposes aimed
at the positioning or focusing of the low
intensity radioactive beams. The test experiment
with a 12.5?m thick Ni-foil (2x15 mm2) positioned
at very forward scattering angles has confirmed
that the rate of a charge integrator (100 Hz per
1 pA calibrating current) connected to the foil
perfectly followed the expected Rutherford
scattering behaviour. The obtained values of the
proton fluences agreed well with calculations.
The BPIT with sizes large enough to cover nearly
the whole silicon detector area has been made out
of 10 Ni-foils (12.5 ?m thick , 4 mm wide , 50 mm
long) placed on a PCB with a pitch of 5mm.
As an example Fig.5 shows the distribution of the
vertices over 4 inserted targets. Number of
vertices at each target is in a good agreement
with the IR distribution provided by CIs.
MFD at 920 GeV proton beam Another way to
reconstruct the beam profile was realized by us
for the HERA-B experiment, which operates a set
of 8 target ribbons (wires) surrounding the 920
GeV proton beam in two planes with a typical
dimension of 125 -500 ?m (Fig. 2), along the
beam axis and 50 ?m, in the radial direction to
provide the Interaction Rate (IR) of 5-40 MHz. In
addition to the data from charge integrators
connected to the targets we used the Silicon
Vertex Detector (SVD) data to get the beam
profile in a plane perpendicular to the beam
axis. At nominal target position of 4 beam ?, (?
350 ?m) the IR is very sensitive to the beam
shape and its fluctuations (Fig.3), and the
knowledge of the beam profile is of a paramount
importance for the target steering. The feedback
data for steering are derived from Scintillator
Hodoscopes (overall IR) and CIs (partial
contribution into the IR)
HERA-B Target Setup
Y (cm)
Al
Pd
W
Target size ?m a1 Al 50500 b1 C
100500 i1 W d 50 o1 Ti d 50 a2
Pd d 50 b2 Ti d 50 i2 C
100500 o2 C 100500
P - beam
C
Ti
C
C
Ti
X (cm)
Z (cm)
Figure 5. Vertices distribution over 4 targets
surrounding the 920 GeV proton beam (IR 30
MHz). X and Y are the coordinates of the targets
in the SVD coordinate system the beam axis is
shifted down by 1 mm and closer by 3.2 mm to the
inner target from the SVD axis.
Figure 3. Correlations IR, ChI, beam and target
position, Primary vertices.
Figure 2. Reconstructed vertices from 8 targets
Wire Charge Integrators Vertices Above
26.06 ? 0.08 26.6 ? 0.7 Below
24.26 ? 0.10 25.9 ?
0.7 Inner 23.49 ? 0.06
21.4 ? 0.7 Outer 26.20 ? 0.07
26.1 ? 0.7
Another essential feature of the Luminosity
monitoring by the MFD is that this is nearly
background free method. Indeed, due to small size
of the targets the response of the corresponding
ChI to any MIP flux is small in comparison with a
signal obtained by the beam-target interactions.
The Fig. 3 illustrates the evolution of the rates
in Hodoscopes (black line), ChI-Inner2 (pink) and
background counters (red line). The upper plot
shows red Inner2 target position, blue X
beam position and green dots Primary vertices
reconstructed off-line in the Inner2 target. The
unusual behavior of beam movement was perfectly
compensated by the Target steering program and
nicely seen by the MFD online.
Table 1. Relative rate sharing among wires
obtained with the Charge Integrators and Primary
vertices counting
Luminosity Monitoring
The relative sharing of the IR among the inserted
targets is illustrated by Table 1 for the case of
the overall IR 30 MHz. There is a perfect
linear relation between the number of the
reconstructed vertices and CIs values for IR per
individual target up to 10 MHz (till multiple
interactions per bunch crossing make an impact on
the vertices reconstruction efficiency).
An application of the MFD Luminosity Monitor for
the HERA-B Minimum Bias runs are shown on the
Fig. 4. The red dots are the ratio of Luminosity
measured by the MFD 1 (upper) and 2 (lower) to
the ones measured by Hodoscopes. X-axes are the
physics run . Green lines are expected
values. The typical fluctuations of the ratio is
few percent. As far as MFD have the order of
magnitude higher acceptance (50) then Hodoscopes
(1) they are more sensitive to any changes of
the beam position and/or material like different
VDS pots position on the beam way. Most of the
MB runs 2002-2003 show perfect (within 2)
stability of the delivered luminosity measured by
the MFD, Hodoscopes, Lumi-counters and another
parts of the HERA-B detector like ECAL and RICH.
Conclusions The reliable results on the beam
profiling were obtained with the help of the MFD
BPIT at two different proton beam energies of 21
MeV and 920 GeV. The achieved sensitivity of the
BPIT allows for an on-line beam-profile
measurements for beam currents exceeding 1 pA.
The developed MFD proved to be a reliable new
tool for the charged particle radiation
monitoring in a wide range of applications. The
advantages of the MFD are extremely low mass of
the detecting material, low cost, simple
structure, low operating voltage (20 V), simple
read-out electronics (charge integrators and
scalers) and very high radiation tolerance.
Figure 1. Left part BPIT (50 x 50 mm2). Proton
beam axis is perpendicular to the BPIT plane.
Right part 21 MeV proton beam profile measured
by the BPIT. Beam axis is between strips 5 and 6
Right part of the Figure 1 shows the beam profile
measured by the BPIT positioned at the beam axis
250 mm downstream of the 50 ?m thick Au-target.
Soild line - Gaussian fit green circles - GEANT
simulation. Integrating the incident proton beam
current we have evaluated number of SEE- and
?-electrons emitted by individual foils (Y-scale
of the Fig.1. On average, every proton kicks out
one electron.
Figure 4. Luminosity Monitoring of MB runs by MFD