INTAG
T-J06-4
Final Report
Z and A identification for medium-mass and heavy nuclei
 Objectives
 T-J06-4.1 - Gas detectors
 Parallel Plates Avalanche Counters for light ions
 Secondary Electron Detector for slow moving heavy ions
 T-J06-4.2 - Si detectors
 Focal Plane Detector for PRISMA in Gas-Filled Mode
 Si Wall based on the Recoil Decay Tagging for the VAMOS spectrometer
 Conference presentations
T-J06-4 - Objectives
The task is centered on the magnetic spectrometers PRISMA (at LNL) and VAMOS (at GANIL)
coupled to the  array CLARA and EXOGAM, respectively. Its goal is the design and the construction
of new detection systems in order to overcome the present limitations and to further improve the Z
and A identification of exotic nuclei.
PRISMA: The Multi-Wire Parallel Plate Avalanche Counter (MWPPAC) currently installed at the
focal plane of the PRISMA spectrometer [1,2,3] was designed to be used with medium-mass
(30≤A≤130) heavy ion beams delivered by the Tandem-ALPI accelerator complex of LNL. Intrinsic
efficiencies (depending on the nature and the energy of the incident ions) even lower than 1% have
been measured in the detection of light ions (in the carbon and oxygen region) during the
experimental campaign of PRISMA coupled to -array CLARA. This inefficiency mainly arises from the
attenuation of the X anode signal with respect to the cathode one, produced by the delay lines (100
cells with a delay of 1.7 ns each) used for the position readout. The attenuation along the whole line
measured for signals with a rise-time of few ns is about 50%. A new Parallel Plate Avalanche
Counter (PPAC) more efficient for light ions has to be used at the focal plane for the detection of
light fragments produced in nuclear reactions.
The operation of the new superconducting injector PIAVE (in conjunction with the new ECR
source) coupled to the ALPI Linac at LNL will allow to accelerate heavy ions in mass regions
inaccessible up to now because of the short lifetime of the stripper foils of the XTU-Tandem with lowenergy heavy ion beams. The relatively large amount of material that the current MWPPAC presents
to the incoming fragments limits the performance of the focal plane detector due to the large angular
and energy straggling. To overcome this limitation a Secondary Electron Detector (a position
sensitive and timing detector with a small equivalent thickness) for slow moving heavy ions has
been proposed.
PRISMA in gas-filled mode (used as a separator) would offer the possibility to extend the present
experimental studies to the fusion-evaporation reactions as it allows to focus heavy evaporation
residues (ER) onto a rather small active area after the magnetic dipole with good beam rejection. On
the basis of the simulation results a new focal plane detector (based on Si detectors and dedicated
electronics) has to be designed and constructed for the operation of PRISMA in Gas-Filled Mode
(GFM).
VAMOS: When used in its dispersive mode (large acceptance operation), improvements and
modifications in existing detection systems and techniques are necessary in order to better identify in
(A, Z) medium mass nuclei at energies around a few MeV/u. To this end:
 a new fully software method for the trajectory reconstruction has been developed [4]. It
corrects up to 7th order aberrations and allows to calculate (i) magnetic rigidity, (ii) angles at
the target and (iii) trajectory length from the measured positions of the ions at the focal plane.
The software reconstruction algorithms are presently integrated in the on-line analysis;
 a new optimal optics has been found. It allows to use the spectrometer in the best
conditions of large acceptance and high resolution. The acceptance of the spectrometer has
been also studied in order to extract reaction cross-sections;
 a new Wall of Silicon detectors has been built in order to replace the previously used
Plastic detector array which had not sufficient energy resolution.
The large acceptance of the VAMOS spectrometer [5] also offers the possibility to study fusionevaporation reactions using its Wien Filter well adapted to very asymmetric systems in direct
kinematics. The angular dispersion of recoils in this kind of reactions, leading to low-energy
evaporation residues, is large due to the angular straggling in heavy targets and to the additional
angular dispersion induced by the large number of neutrons emitted in hot fusion reactions. Because
of the very large angular acceptance of VAMOS, the transmission for very asymmetric reactions is
expected to be very high. When coupled to the EXOGAM  array, it allows to study the spectroscopy
of evaporation residues using the Recoil Decay Tagging (RDT) technique. In order to perform this
kind of measurements, a new Si wall for VAMOS spectrometer using the Recoil Decay Tagging
technique is mandatory.
The gas-filled mode of the spectrometer is a complementary option for zero degree operation
which has to be investigated in a parallel work.
T-J06-4.1 - Gas detectors
Parallel Plates Avalanche Counters for light ions: INFN-LNL, INFN Sez. di Padova, Horia Hulubei
National Institute, Ruđer Bŏsković Institute
Design of detectors and electronics
In order to increase the number of electron-ion pairs produced by the incoming ions in the primary
ionization or during the avalanche process in the gas, and thus to improve the efficiency for light ions,
two new PPAC configurations for the focal plane of the PRISMA spectrometer have been proposed.
Both devices, designed by INFN-LNL, have a three electrode structure and an active area of
100x13 cm2. The first detector (Fig. 1) assembled by INFN-LNL and INFN-PD has the same structure
of the MWPPAC presently mounted at the focal plane [6]. The main difference is the smaller diameter
of the gold plated tungsten wires (10 m instead of 20 m, with a spacing of 1 mm) used for the
construction of the X anode plane. The Y-plane wires were not changed (20 m in diameter) because
their length (1 m) does not allow to further reduce the diameter. On the other hand a lower inefficiency
is expected for this anodic plane due to the smaller number of delay cells (60 instead of 100) used for
the position readout. The X position efficiency in this configuration is increased by the higher electric
field around the X wires that implies higher gas gains. Moreover, this detector can be efficiently used
in the detection of both light and heavy ions (at lower bias voltages). The cathode (divided in ten
independent sections) is made of 3300 gold-plated tungsten wires, with a diameter of 20 m and a
spacing of 0.3 mm. A new cathode has been designed and constructed for the second assembled
detector (Fig. 2). It consists of ten equal electrodes, each of them composed of double aluminized (20
g/cm2) Mylar foil (1.5 m) glued on a G-10 frame. The X and Y anode planes are identical to the
mounted ones in the current MWPPAC. The efficiency of the detector is increased by the contribution
to the signal formation of the secondary electrons produced by the ions passing through the cathode
(Mylar foil). Home-made fast amplifiers (Fig. 3) have been developed by INFN Sez. di Padova for the
processing of time and position signals from cathode and anode planes. They have been specifically
designed with a high gain (250) and a short integration time (10 ns) in order to improve the efficiency
of the detector for light fragments.
Figure 1: The new PPAC with the
modified X anode plane.
Figure 2: The new PPAC with Mylar
cathode.
Figure 3: Fast amplifiers
developed by INFN-PD.
Bench tests
Preliminary bench tests of both PPACs have been performed by using 5.486 MeV particles from
a 241Am source. Isobutane (C4H10) was used as filling gas at working pressures ranging from 5 hPa to
12 hPa. They evidenced:
 differences in the maximum bias voltage of the ten cathode sections (from -570 V to -610 V)
due to small structural imperfections (non-planarity of the electrodes) and to the imperfect
cleaning of the electrodes because they could not be assembled in a clean room;
 an improved efficiency of the detectors for  particles. The amplitude of the cathode (X anode)
signal (output of the fast amplifiers) ranges from 50 (30) mV to 90 (50) mV at -590 V and for a
working pressure of 10 mbar (no sensitivity to  particles was observed with the MWPPAC
currently installed at the focal plane of PRISMA).
A proper charge calibration allowed to estimate the gas gain for different working pressures and
reduced electric fields in the first assembled detector (wire cathode). Square waves of different
amplitude, provided by a waveform generator, were injected onto a precision capacitor (2.2 pF)
connected to the cathode of the PPAC. The signals from the cathode were amplified and shaped by
means of a conventional charge-sensitive preamplifier (CANBERRA 2003BT) and a spectroscopy
amplifier (CANBERRA 2020) with a shaping time of 1 s. The pulse height spectra were recorded on
a multi-channel analyzer and their centroids were used to determine the charge-to-channel
conversion. This calibration procedure allowed us to measure (using the same electronics) the
number of pairs (Fig. 4) produced by 5.486 MeV  particles at different working pressures (7 hPa and
10 hPa) as a function of the reduced electric field. Gas gains (Fig. 5) ranging from 103 to about 104
were measured for reduced electric fields from 175 V/cm·hPa to 320 V/cm·hPa.
C4H10 - Isobutane
6
C4H10 - Isobutane
3
1x10
7x10
10 hPa
7 hPa
5
9x10
10 hPa
7 hPa
3
6x10
5
8x10
5x10
Gas gain
Number of pairs
137 primary electrons
E ~ 4.1 keV
5
6x10
5
5x10
5
4x10
5
3x10
Gap cathode-anodes
2.4 mm
3
5
7x10
196 primary electrons
E ~ 5.9 keV
3
4x10
3
3x10
3
2x10
5
2x10
3
1x10
5
1x10
165
180
195
210
225
240
255
270
285
300
315
330
E/P (V/cm·hPa)
Figure 4: Number of pairs as a function of the reduced
electric field at 7 hPa and 10 hPa, respectively.
180
200
220
240
260
280
300
320
340
E/P (V/cm·hPa)
Figure 5: Gas multiplication factors as a function of the
reduced electric field at 7 hPa and 10 hPa, respectively.
In-beam tests
In order to carry out in-beam tests, a dedicated vacuum vessel (3.5 m long, 1.5 m wide) has been
built (Fig. 7) and coupled to the sliding-seal scattering chamber of the -20° beam line in the II
experimental hall at the position of the Time-of-Flight spectrometer PISOLO (Fig. 6). This chamber will
be also used for testing the large area Se-D detector (specifically designed for slow moving heavy
ions) that is being developed for the PRISMA spectrometer. This choice allows to avoid interferences
with the experimental campaign of PRISMA coupled to the  array CLARA (AGATA Demonstrator in
2009).
Figure 6: The TOF spectrometer PISOLO removed in
July 2006.
Figure 7: The new experimental site installed at the end of
October 2007 for in-beam tests at LNL.
In-beam tests of the wire cathode based PPAC were performed in December 2007 (PAC 07.14 –
Spokesperson: E. Fioretto – Beam time: 19-20 December, 2007). Different ion beams delivered by the
XTU-Tandem (32S at energies ranging from 100 MeV to 160 MeV and 16O from 60 MeV to 90 MeV)
were elastically scattered from a 200 μg/cm2 thick 197Au target. The detector was placed at an angle of
90° with respect to the beam direction covering the angular range from 83° to 97° corresponding to a
solid angle of about 6 msr. Isobutane (C4H10) was used as filling gas at a working pressure of 10 hPa.
The PPAC was operating with a small but constant gas flow. A control system, based on the MKS
Type 250 Pressure Controller coupled to a solenoid valve (MKS Type 248), was used to ensure the
stability of the pressure and the purity of the gas during the tests. The pressure inside the detector
was monitored by means of a pressure transducer (MKS Baratron Type 220D) connected to the
controller. The cathode and anode signals were processed by means of home-made fast amplifiers
(manufactured by INFN Sezione di Padova - Fig. 3). Shaped signals were sent to Quad Constant
Fraction Discriminators ORTEC 935 (cathode) and Octal Constant Fraction Discriminators ORTEC
CF8000 (anode). A Micro-Channel Plate (MCP) with an active area of 6×4 cm2 was installed inside
the scattering chamber at 20 cm from the target to be used as Start detector for Time-of-Flight (TOF)
measurements (the Stop signal was provided by the PPAC cathode). A silicon detector, placed in the
scattering chamber at 15.5° (also used as beam monitor), was used to estimate the relative efficiency
of the cathode (TOF) for different ions and energies. A value close to 100% has been measured for
the central section of the detector for 32S and 16O ions on the full energy range. An example of X
position spectrum measured for the 32S+197Au reaction at 96 MeV is shown in Fig. 8 (the cathode was
biased at -570 V while the anodes were grounded). As one can see, a position resolution of 1 mm has
been measured with a good peak-to-valley ratio. The overall measured time resolution is about 2 ns
mainly due to the energy straggling in the Au target, the kinematical broadening and the time
resolution contribution of the MCP. The relative efficiency r of the X anode plane with respect to the
cathode of the detector for 16O at different energies has been estimated from the number of counts in
X position and TOF (cathode) spectra: r = NX/NTOF. The efficiency r measured for 60 MeV 16O ions
rises from 4% until 17% when the bias voltage of the cathode Vc is increased from -560 V to -570 V
and it reaches about 43% (see Fig. 9) at 50 MeV (a relative efficiency close to 100% has been
measured at both energies for Vc = -600 V). The r values for 12C (and at lower energies for 16O),
showed in Fig. 9, have been estimated from the measured efficiencies on the basis of the energy
losses in the active volume of the detector.
C4H10 - Isobutane
1,0
0,9
Relative efficiency r
0,8
12
C
16
O
0,7
0,6
0,5
0,4
0,3
0,2
0,1
0,0
10
20
30
40
50
60
70
Energy (MeV)
Figure 8: X-position spectrum measured with
a central section of the PPAC.
Figure 9: Relative efficiency for 12C and 16O ions as a
function of their energy.
Then, in-beam tests evidenced an increase in the detector efficiency for light ions from below 1%
to about 40% (measured for 50 MeV 16O ions). To further improve the sensitivity of the detector for
light fragments new filling gases will be tested. In particular, C3F8 seems to be the best candidate
meeting the requirements of a high stopping power (three times higher than the C4H10 one) and a high
electron drift velocity.
Secondary Electron Detector for slow moving heavy ions: INFN-LNL, STFC Daresbury Laboratory, University of Manchester, CEA Saclay, INFN Sez. di Padova, University of Paisley
Design of detectors and electronics
Large energy and angular straggling is expected for low energy heavy ions mainly due to the
detector windows (two 1.5 m thick Mylar foils corresponding to about 420 g/cm2) of the current focal
plane detector of the PRISMA spectrometer. Its replacement with a large area Secondary Electron
Detector (Se-D) should provide a solution to the position and time measurements at the focal plane
with minimal energy loss and angular dispersion by reducing the number of gas windows. In this
configuration incoming ions pass through a thin emissive foil (0.6 m thick Mylar foil corresponding to
about 80 g/cm2) producing secondary electrons which are accelerated under an electric potential,
transported by a magnetic field and detected in a MWPC located out-of-plane with respect to the path
of the incoming ions. A MWPC is chosen rather than the usual Micro-Channel Plates (MCP) because
they are more costly and difficult to implement over such a large focal plane (1 m wide). The position
is generated by measuring the centroid of the charge deposition across the wires on each plane. This
requires individual wire-by-wire readout rather than the usual delay-line readout of the MWPC.
Development work involved attempts to minimize electron straggling before the active volume of the
out-of-beam MWPC using magnetic field electron transport and accelerating potentials in order to
obtain position and time resolutions of the order of 1-2 mm and 200-300 ps, respectively.
The University of Manchester designed and constructed a small area (7x7 cm2 active area)
prototype of Se-D. A schematic view of the detector is shown in Fig. 10. It consists of an in-beam
emissive foil with an out-of-beam MWPC (Fig. 11) to detect the secondary emission, followed by an
in-beam MWPC to evaluate efficiency and time resolution of the out-of-beam device. Typical values
for the accelerating potential and the magnetic field are of the order of -20 kV and 100 Gauss,
respectively. Guard rings are used to preserve the uniformity of the electric field in between the
emissive foil and the detector. The MWPC has a three electrode structure with the central cathode
composed of a 1.5 m thick mylar foil and anodic planes (X and Y position) made of gold-plated
tungsten wires (20 m diameter) with a spacing of 1 mm.
In-beam MWPC
Emissive foil
Californium source
(for tests in
Manchester only)
SED
Out-of-beam MWPC
FIGURE 3: Prototype SED with in-beam MWPC and source
Figure 10: Schematic view of the setup used for bench and inbeam tests.
Figure 11: The Se-D prototype (left) with delay line
readout (top-right) and the Gassiplex ASIC cards
(bottom-right).
Bench tests
Preliminary bench tests have been performed in Manchester by using fission fragments from a
Cf source. Isobutane (C4H10) was used as filling gas of the Se-D at a working pressure of 5 hPa. In
the first tests, a delay line readout was used to obtain the position information from the X and Y anode
planes of the MWPC. The prototype was then tested successfully in Manchester and Legnaro with
two Saclay designed GAS32 ASIC cards (see Fig. 11) daisy chained together and coupled to a
Daresbury designed V4FADC board (fast analogic-to-digital converters). Typical position resolutions
of the order of 1 mm in both X and Y directions have been measured.
For the large area (1 m wide) Se-D design, a more size suitable ASICs board (named GAS16) had
to be redesigned. Each GAS16 (Fig. 12) includes one 16-channel Gassiplex ASIC embedding a
preamplifier, a shaping time amplifier and a Track & Hold per channel. With a 5 MHz readout clock,
the V4FADC module (shown in Fig. 14) collects and converts data from the GAS16 boards and sends
it via the 1Gbit ethernet to a controlling VME processor under the control of a MIDAS GUI. The event
rate using a pulse generator is above 1,000 / sec. The GAS16 card has been tested in Daresbury by
using a PPAC prototype (Fig. 13) in order to check preliminarily its performances. Six of those GAS16
252
board have been built. They are currently being tested prior delivery to Manchester. The 500 wires in
the X plan and 90 in the Y plan will require a total of 38 GAS16 boards.
Figure 12: One GAS16 card for
the readout of 16 anode wires.
Figure 13: Two GAS16 boards mounted
on a PPAC prototype.
Figure 14: V4FADC module.
The 16 channels in a GAS16 board are time multiplexed onto one analogue output which is then
converted by the flash ADC in the V4FADC module. This V4FADC has been designed to suit a wide
range of ASICs electronics. It includes a Xilinx FPGA operating system which allows to operate the
ASICs control and readout, the FADC conversion and the data storage. Each V4FADC module can
steer up to eight GAS16 boards so five of them are required for the final design of the large area
Se-D. Ten have already been built in Daresbury. All of them are first being tested individually before to
be synchronised altogether via a metronome for an ultimate test.
In-beam tests
In-beam tests of the small area Se-D prototype were performed in October 2007 (PAC 06.23 –
Spokesperson: S. Freeman – Beam time: 12-14 October, 2007) at LNL by using 250 MeV 80Se ions
elastically scattered from a 100 g/cm2 thick 12C target. The Se-D was placed at an angle of 52° with
respect to the beam direction (Fig. 15) in order to check the performance of the detector with heavy
ions at energies around 0.5A MeV. Isobutane (C4H10) was used as filling gas at a working pressure of
5 hPa. A constant gas flow was used to avoid the gas contamination during the tests. X and Y position
resolutions (Fig. 16) of the order of 1 mm have been measured with slits in front of emissive foil. The
overall time resolution, measured using the in-beam MWPC as Start detector, is of the order of 1 ns
including the energy straggling in the C target, the kinematical broadening and the time resolution
contribution of the in-beam MWPC.
Figure 15: Experimental setup installed at LNL for the inbeam commissioning.
Figure 16: X and Y distributions measured with
80Se ions elastically scattered from a 12C target.
Then, in-beam tests confirmed that the overall behaviour of the detector was essentially identical
to that observed with fission fragments from the 252Cf source. There were no unforeseen beam-related
issues.
The prototype detector is currently being scaled up to provide a 1 m wide device for use at the
PRISMA focal plane. A working detector design has been developed, and effort is now concentrated
on the provision of the magnetic field for electron transport. A large electromagnet is being developed
to provide around 100 Gauss across the full extent of the PRISMA focal plane. Materials are currently
being sourced and magnet construction will begin in late 2008.
T-J06-4.2 - Silicon detectors
Focal Plane Detector for PRISMA in Gas-Filled Mode: INFN-LNL, INFN Sez. di Padova, INFN Sez.
di Napoli, Horia Hulubei National Institute, Ruđer Bŏsković Institute, Vinca Institute of Nuclear Physics
Design of detectors and electronics
Simulations have been carried out (INFN-PD and Vinca Institute) by using the new code TRAJIG
[7] in order to estimate of the optimum gas pressure for the operation of PRISMA in GFM. As an
example, the position spectra calculated for both partners of the 58Ni+197Au reaction at 220 MeV are
shown in Fig. 17, from low to high gas pressure, where the competing effects of charge-exchange
collisions and straggling are clearly seen. A reasonable separation between the distributions, as well
as the smallest width for gold recoils, is obtained around 2 mbar. Simulations also indicate that a
20 cm wide detector should collect most of the evaporation residues (ER) in a typical fusion reaction.
The detector has to provide the position and the energy of implanted ions and particles from their
subsequent decay.
Position-sensitive (16 resistive strips, 3 mm wide) Si detectors (300 μm thick with an active area of
5x5 cm2) manufactured by the Micron Semiconductors company were selected as basic element of
the recoil array. Following the simulation results, a new focal plane detector (see Fig.18) for the first
in-beam test has been designed and assembled by INFN-LNL and INFN Sez. di Padova. It is
composed of an array of 3×2 Si strip detectors.
Figure 17: Simulated position spectra of 58Ni (left) and
197Au (right) recoiling at 60° for different He gas pressures.
Figure 18: The new focal plane detector for the operation
of PRISMA in GFM.
All strips are grounded at one end through a 100  resistor. In this geometry, the X position on the
deflection plane is given by the identification of the strip that fired (3 mm resolution) through a bit
pattern unit, while the amplitude of the signal collected at one of the ends of each resistive strip
(junction side of the detector) provides the Y coordinate. The energy signal is collected on the rear
side (ohmic side) of the detector. In total, 102 signals have to be processed. To this end, hybrid
charge preamplifiers (with a sensitivity of about 4 mV/MeV) and 16-channel Spectroscopy AMplifiers
(SPAM), shown in Fig. 19 and Fig. 20, have been designed and built by INFN Sez. di Napoli. The
latter are equipped with a receiver and the communication with the controller is performed by a singlewire communication network and the CAN (Control Area Network) V2.0 protocol. It can address up to
256 SPAM modules corresponding to 4096 single channels. A manual controller for the local setting
of the amplifiers has been also developed.
Bench tests
All Si detectors have been characterized from the C-V and I-V point of view and tested by using
5.486 MeV -particles from a 241Am source together with the first prototypes of the dedicated
electronics. These tests evidenced energy and position resolutions of about 1% and better than 1 mm
(along the strip), respectively.
Figure 19: Sixteen-channel hybrid PA box for the processing of
the strip signals from one Si detector.
Figure 20: A single SPAM module and
a complete NIM 16-channel board.
In-beam tests
In-beam tests of the recoil detector were performed in July 2007 (PAC 06.20 – Spokesperson:
F. Scarlassara – Beam time: 16-17 July, 2007) at LNL. Elastic scattering out of 0° was preferred to
avoid the complications of zero degree operation of PRISMA in GFM; in fact the scattered beam and
the heavy recoils can simulate a typical fusion reaction in mass and energy. The tests were done by
using the 58Ni+197Au reaction at 160 MeV and with PRISMA placed at lab = 55° with the magnetic
fields (quadrupole and dipole) optimized for the 197Au recoils at different helium gas pressures from
1 hPa to 3 hPa in different runs.
The recoil array was housed in a new chamber installed at the exit flange of the dipole at about
95 cm from the effective field boundary of the dipole. The chamber was mounted by shifting back the
standard drift and focal plane detector chambers of the spectrometer. The central trajectory of
PRISMA corresponds to a maximum rigidity of 1.2 Tm which is not large enough to bend slow heavy
recoils. Therefore, the detection array was then positioned 30 cm off-axis to reach a rigidity of 1.5 Tm
(sufficient for ERs up to masses around A=200) and the value of the magnetic fields were chosen in
order to have the 197Au centroid at x = 300 mm from the axis. Finally, the gas containment in PRISMA
was obtained by means of a thin carbon foil (50 μg/cm2 thick) glued on a suitable aluminum frame and
placed between the scattering chamber and the magnetic quadrupole. The window frame also
provided collimation with an aperture of 3°. The tests were successful, since Au recoils were identified
eliminating all beam-like scattered particles and obtaining at the same time a suitable focusing of the
Au recoils in the area of the detector array. This can be seen in Fig. 22 showing the distribution of
197
Au recoils along the X direction of the focal plane detector (all six Si detectors) obtained using the
dipole and quadrupole magnetic fields that maximize the yield at the array position for a pressure of
1.2 hPa. The X-Energy scatter-plot of Au recoils is shown in Fig. 21. The fact that the distribution
seems slightly cut at both ends is in agreement with the simulation results shown in Fig. 17.
The in-beam commissioning of PRISMA in GFM showed that this spectrometer is compatible with
such an operation, although its configuration is rather simple compared with dedicated gas-filled
separators. The next step will be to measure a fusion reaction in full operating conditions, by detecting
evaporation residues at 0°. PRISMA characteristics in GFM will be further investigated by checking
the role of the gas (since very different behaviours can be expected). This test will be also useful to
clarify two crucial issues: the beam rejection capability of the setup and the resistance of the carbon
window at 0°.
Figure 21: X-Energy spectrum measured for Au recoils.
Figure 22: Measured X-distribution of Au recoils. The holes
are due to few strips not properly working.
Si Wall based on the RDT for the VAMOS spectrometer: IN2P3-GANIL, CEA Saclay
Design of detectors and electronics
As an example, the result (in terms of the X-Y distribution of products at focal plane position) of a
simulation performed for the fusion-evaporation 22Ne+238U, leading to 255No residues, is shown in
Figure 23. As one can see, an excellent separation among all atomic charge states of evaporation
residues is obtained.
The new focal plane detector (called MUSETT) based on the RDT technique has to meet the following
requirements:
 the detection system has to match the recoil distribution at the focal plane of the spectrometer,
according to the optics simulations;
 incoming ions have a very low kinetic energy (around 10 MeV or less) and a heavy mass.
Therefore, any material before or in front of the active part of the detector will prevent the
detection;
 since nuclei of interest are  emitters, the RDT technique provides an additional selectivity.
This technique consists to correlate in position and time the incoming ions with their
subsequent decay. The  decay lines are indeed a unique fingerprint of the nucleus of interest.
Detectors should therefore provide the position (with a resolution of about 1 mm) and the
energy (with a resolution of the order of 30 keV or better in order to perform  spectroscopy
measurements) of implanted ions and of  particles.
The detector has to operate obviously with front-end electronics and data acquisition system
compatible with the VAMOS one. The dead time has to be minimized down to a few ten of s for short
lifetime measurements of the radioactive species implanted in the wall. Events have to be tagged with
a time resolution of 10 ns. The mechanics should also be compatible with the existing focal plane
detection system of VAMOS. In particular, the MUSETT Si wall has to be coupled to gas detectors
(Ionization or Drift Chamber) when the ions kinetic energy is large enough. Following the results of
simulations the active area of new detection system is about 40x10 cm2 and only a segmented Si
detector can meet the required position and energy resolution. The detector has to be windowless for
the detection of very slow and very heavy ions. A prototype of Si strip detector (see Figure 24) was
provided by the Micron Semiconductor Company. Each of the four detector assembled in the wall has
a 10x10 cm2 size and 128 strips on each side. A huge number (1024) of high-resolution electronics
channels is needed to process the signals from the whole wall composed of four Si detectors. ASIC
electronics has to be used in order to minimize the noise (mainly related to the detector-electronics
distance). To this end, new low-noise ASICs chips known as ATHED were designed at Saclay. One
ATHED chip includes 16 preamplifiers, linear shapers, discriminators and time to amplitude
converters. The slow control allows the adjustment of parameters such as the discriminator threshold
or the shaping time. In order to reduce the number of connections, energy and time information are
multiplexed for an external readout. Because of the high power dissipation, electronics and detectors
are maintained at a constant temperature inside the vacuum chamber.
Figure 23: Spatial distribution of
22Ne beam at the focal plane.
255No
recoils and
Figure 24: Close view of a Si detector prototype.
The readout and the slow control of the front-end electronics are performed using a new VME
board designed in Saclay (see Fig. 25). One card can operate with 4 ATHED chips and therefore 64
strips of a Si detector. Data are digitized with 14 bits flash ADCs and are time stamped using a
100 MHz clock. The cards are controlled and read-out via USB2 interfaces. The control and data
acquisition software has been developed in the LabView environment while data analysis tools have
been developed using the ROOT package.
Bench tests
As shown in Fig. 26, a test bench was developed at Saclay for the MUSETT characterization
including the vacuum chamber, the mechanics for the support of the Si detectors, the front-end
electronics, the remote controlled cooling system and the power supply, the VME electronics and
computers for data acquisition and sorting.
Figure 25: VME read-out and slowcontrol board.
Figure 26: Test bench for Si detectors. From left to right:
electronics and power supply, vacuum chamber, cooling system.
All prototypes of the MUSETT Si wall have been tested in laboratory: test and validation of
detectors, front-end electronics, mechanics, read-out electronics and data acquisition. The MUSETT
concept has been validated and elements are being produced. The test bench will also be used to
check and validate detectors and electronics before the integration at the focal plane of VAMOS.
In-beam tests
As mentioned above, the possibility to use the VAMOS spectrometer as a zero degree separator
with fusion-evaporation reactions was investigated. A test was performed in July 2007 using the novel
idea of setting the Wien Filter (WF) to direct the beam « straight through », thus allowing the beam to
be dumped in a control manner. Optics simulations performed with the ZGOUBI code show that this
mode does not affect the transmission compared to the « standard » WF mode and that a better
beam suppression should be achieved. Simulations cannot include the finest experimental aspects
i.e. (i) beam scattering into the spectrometer cannot be described therefore beam rejection cannot be
calculated; (ii) recoil scattering of heavy ions in the target is not fully under control, therefore the
transmission, which is a function of the angular dispersion, cannot be accurately calculated. It is
therefore mandatory to measure the most critical parameters of the spectrometer in the zero degree
mode: transmission and beam rejection.
The experiment was performed using the fusion-evaporation reaction 197Au(22Ne,5n)214Ac.
Evaporation residues were identified and distinguished from the scattered beam particles by
measuring their Time-of-Flight (to go from the target position surrounded by the EXOGAM array to the
focal plane device composed of a Si detector) and energy. A confirmation of the implantation of
evaporation residues into the Si detector at the focal plane was obtained from the -particle energy
spectrum, which is shown in Fig. 27.
Figure 27: The -particle energy spectrum
measured with a Si detector at the VAMOS focal
plane.
The transmission was measured by comparing the
number of  particles detected in the Si detector with
respect to the expected one from the reaction. The
deduced value around 40% is consistent with optics
simulations. It is about one order of magnitude larger
than the value obtained with other recoil separators
currently coupled to  spectrometers. Therefore, the inbeam test validates the concept of VAMOS used as a
zero degree separator with very asymmetric fusionevaporation reactions. The idea of setting the Wien
Filter (WF) to direct the beam « straight through » was
also successfully tested.
References
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A.M. Stefanini et al., Nucl. Phys. A701, 217c (2002).
G. Montagnoli et al., Nucl. Instr. and Meth. A 547, 455 (2005).
L. Corradi et al., Nucl. Phys. A787, 160c (2007).
S. Pullanhiotan et al., NIM B 266, 4148 (2008)
S. Pullanhiotan et al., NIM A 593, 343 (2008)
S. Beghini et al., Nucl. Instr. and Meth. A551 (2005) 364.
F. Scarlassara et al., LNL Annual Report 2004, 208
Conference presentations
“The PRISMA-CLARA setup: experimental results and future plans”
E. Fioretto et al., invited talk at EXON 2006
International Symposium on Exotic Nuclei
17-22 July, 2006, Khanty-Mansiysk - RUSSIA
AIP Conference Proceedings Series, Vol. 912, Melville (New York), Yu E. Penionzhkevich et al. eds.
“A new detector for the focal plane of the PRISMA spectrometer”
R. Silvestri et al., Poster session and talk at the International Conference EURORIB’08
9-13 June, 2008, Giens - FRANCE
http://eurorib08.ganil.fr/posters/
“New gaseous detectors for large focal plane spectrometers”
M. Labiche, Talk at the EURONS Town Meeting
17-19 September, 2008, Rhodes - GREECE
http://www.gsi.de/informationen/jofu/EURONS/Rhodes-Talks.html
“The magnetic spectrometer PRISMA combined with large  array”
E. Fioretto et al., talk at FUSION08
International Conference on New Aspects of Heavy Ion Collisions Near the Coulomb Barrier
22-26 September, 2008 - Chicago - USA
(to be published in the American Institute of Physics (AIP) Conference Proceedings series)
http://www.phy.anl.gov/fusion08/index.html