A large focal plane tracking detector for low energy
high mass ions.
E.C. Pollacco, C. Mazur, E. Bougamont, P. Bourgeois,
A. Drouart, L. Nalpas, M. Riallot
DSM/DAPNIA, CEA Saclay, 91191Gif-sur-Yvette, France
Abstract. For measurements with low energy radio active or stable beams it is important to reconstruct trajectories for ions in
large solid angle spectrometers. In this paper we describe a large area (106 × 420 mm _) tracking secondary electron detectors,
Se-D. In Se-D, the electrons emitted by an aluminized Mylar foil are captured by an electromagnetic field and focused on an
electron gaseous detector to give a time and 2-D position resolution (sigma) of typically 130 nsec and 0.8 mm.
has been shown to give a higher electron yield then
carbon. The electrons emitted towards e-D are
accelerated by a field of 1 KV/mm by placing a grid at
10 mm from the foil. For F ission Fragments, FF
typically 100 electrons are emitted [4]. The electron
cloud drifting through a field free distance of 200 mm
have a “circular” distribution of a diameter ~10 mm on
e-D. To reduce the size of the electron spot a magnetic
field of 100 Gauss is applied perpendicular to the
emission foil [2]. Fig 2 shows the effect of introducing
the magnetic field.
In this paper we give succinct results for a large
area (106 × 420 mm_) Se -D to be placed at the focal
plane of the spectrometer VAMOS at GANIL,
Caen[1]. It is a time and position (x-y) detector for
slow moving (0.3 to 1 MeV/A) heavy ions ( Z>10 ).
The general principle of the operation of Se-D is well
known [2,3]; the particles to be tracked pass through a
thin foil. The electron cloud that is stripped off from
the foil by the transit of the ions, are focussed by
electric and magnetic fields and dispatched to the
electron detector, e-D. The e-D gives a position and
timing information (fig 1).
B = 0 gauss
B= 100 Gauss
Résolution (FWHM): X=3.7 mm - Y= 4.8 mm
Fig 2 X-Y image of two holes with B=0 and 100 G.
The e-D is a low pressure (~ 4 Torr) multi wire
avalanche counter [3] where the amplification occurs
over two regimes: the parallel plate region where the
field is constant followed by a strong field provided by
the 10 µm wires set at ~ 600 Volts (see fig. 1). The xposition is derived from the set of 108 strips. The yposition signals are induced on wires of 50µm
stretched along the 420 mm. Three Timing signals, T
Fig 1. Diagram showing the working incident heavy
ions on the emission foil, secondary emission and the
amplification “process”.
The secondary electrons are emitted from a
aluminised Mylar foil of 0.9 µm of dimension 150×420
mm. The aluminised (100 nm) surface exposed to air
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
259
MeV.A 76 Ge beam are consistent with these values.
The time resolution performed with the prototype
detector [3] using a plastic scintillator as time-zero
detector give an intrinsic resolution of 0.13 µsec for FF
and 420 nsec for 6.11 MeV alpha particles. Time
measurements on the full scale detector are in progress.
Efficiencies are 100 and 60% for FF and alphas
respectively.
Further R&D is principally directed towards
reducing the thickness of the emission foil to reduce
the angular straggling of the incident particles. To
extend the range of ions to lower charge or higher
energies, coating of the foils with more emissive
compounds like CsI and other alternatives are being
considered. A larger Se -D is being evaluated for the
spectrometer PRISMA at Legnaro, Italy.
The
coupling of a Se-D with large area double sided Si strip
detectors [8] will highly enhance the capacity of
spectrometers like VAMOS and PRISMA.
are taken from the anode. The entrance window is an
aluminised 0.9 µm foil held flat by a fine mesh.
Preamplification and shaping of the 48 y and 108 x
signals is assured by an ASIC (GASSIPLEX)[5].
Boards that hold and protecting the chips (operate in
vacuum) were developed for the project. The serial
read-out greatly simplifies the cabling, operation and
reduces the costs of the front-end electronics and
acquisition. Further, the electronic noise contribution
to the measured position resolution is small [6]. The
track and hold needed for the GASSIPLEX is provided
by the sum of the T signals. Charge calibration is
obtained by injecting calibrated pulses on the anode
plains. Acquisition dead time of ~10% at 2 KHz is
measured.
For FF typically 5 charges from the pads (or wire
bunches) are collected. The position is extracted by
using a Gaussian fit on the induced charge signals. The
resolution for FF is 0.8mm in the x and y planes.
Measurements performed very recently with a 2.6
REFERENCIES
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7.
H. Savajols, VAMOS collaboration, Nucl. Phys. A654 (1999) 1027c.
O.H. Odland, et al., Nucl. Inst and Meth. A 378 (1996) 149.
A. Drouart, et al. Nucl. Inst and Meth. A 477 (2002) 401.
E. Pollacco et al., to be published .
J.C Santiard, CERN ER-MIC, Geneva.
L. Nalpas et al., to be published
E. Pollacco et al., see CAARI 2002.
Fig 3 CAD view of Se-D. Frame, B corresponds to the magnet and e-D is indicated by A
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