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					  MUST II: Large solid angle light charged particle telescope
   for inverse kinematics studies with radioactive beams.
     E. Pollacco°, E. Atkin°, F. Auger°, P. Baron°, J.P. Baronick, Y. Blumenfeld,
         A. Boujrad, A. Drouart°, P. Edelbruck, L. Lavergne, L. Leterrier,
                    L. Olivier, B. Raine, A. Richard, M. Rouger°,
               P. Roussel-Chomaz, F. Saillant, M. Tripon, E. Wanlin
                              °DSM/DAPNIA/SPhN, CEA Saclay, 91191Gif-sur-Yvette, France
                            Institut de Physique Nucléaire, IN2P3-CNRS, 91406 Orsay, France
                         *GANIL, IN2P3-CNRS / DSM-CEA , BP 55027, 14021 Caen Cedex 5, France

Abstract. Over the past four years we have studied (p,p'), (d,p) ,(d, 3He) and other reactions using radioactive beams in inverse
kinematics to obtain spectroscopic information for nuclei away from the valley of stability After a general overview of the
experimental method we will describe our ongoing MUST II development. This is to build a very compact (1000cm 3) three stage
telescope with an active area of 100cm² with position resolution of 0.70.7 mm² and time of flight measurement. The mass
identification and energy dynamic range is of 0.4 to 80 MeV.A up to alpha particles. The compactness of the array is assured
through the use of an ASIC development to measure the time of flight and energy. The large solid angle coverage of 2.6sr and
compactness of this array will allow it to be used in particle-gamma coincidence experiments.

                   INTRODUCTION                                    important to increase the detector geometrical
                                                                   efficiency.
    Direct reactions with light ions (protons, deuterons,              Also of importance is the beam emittance because
… alphas) on stable nuclei were shown to be an                     of kinematic effects. To obtain reasonable energy
important tool in nuclear spectroscopy. Reactions like             resolution the angular resolution better than 0.5° is
elastic and inelastic give matter radii and deformation.           required. For available beams this implies that beam
The transfer of one or two nucleon (d,p), (d, 3He),                tracking to better than 1 mm is vital. A final comment
(t,p)… give spin and parity as well as spectroscopic               about exotic beams, is their purity which is not often
strength. These elements were central in building our              guaranteed. Therefore the coincident detection of the
present macro and micro vision of the nucleus. It is               projectile-like fragment in a spectrometer, is frequently
therefore natural to extend this method to unstable                crucial to obtain a clean background in the light ion
species. Unstable nuclei far outnumber stable ones and             energy spectra. In pickup reactions, like (p,d), the
we enquire to what extent do our present models, tuned             kinematics for the detected light ejectiles are forward
on stable systems, will apply to highly neutron rich or            focused making it relatively easy to cover a large
proton rich ensembles. Inklings of modified structure is           fraction of the solid angle. This is not so true with the
given by the neutron rich Li and Be isotopes [1].                  interesting (d,p) reaction. The p cover a wider angular
Neutron haloes in nuclei like 6He, 11Li and 19C are                domain with steep kinematic variations. Thus for an
structures that were not expected and still need to be             acceptable telescope position resolution, the distance to
understood [2].                                                    target has to be increased at the expense of solid angle.
    Direct reaction measurements with exotic nuclei are            Energy resolution deteriorates with the combined
performed in inverse kinematics using CH2 or CD2                   effects of target thickness, beam emittance and total
targets. Liquid or solid H2 and gaseous T2 have been               angular resolution which are limiting factors for this
employed. The beams of course are not intense. With                experimental technique. Values better than 300 keV are
cross sections of a few mb/sr and reasonable target                difficult to achieve even when applying thin targets and
thickness, present detection systems require beams                 beam tracking. A solution is to perform gamma-particle
intensities better than 104 particles/s . Hence it is              coincidence measurements.          Highly efficient Ge
                                                                   detectors like ExoGam coupled to position sensitive
detectors will be used at GANIL. Thick targets can then          The standard flight distance is 15cm and the
be employed which offsets the loss in efficiency.            geometry of the telescope ensemble is highly dictated
    Elastic and inelastic scattering measurements are        by this choice. Thus, the mechanics of the telescopes is
fundamental and are often a prerequisite to transfer         a truncated pyramid with a base 1313cm² with a
reaction analysis. The complication with this reaction is    vertex at 15 cm and an “active” face of 1111cm². To
the relatively low threshold that is required and values     allow      set-up   flexibility   in    gamma-particle
of 0.4 MeV.A with particle identification, is a must. In     measurements, the CsI can be removed. In
our experiments we have opted for a time of flight           back/forward angle measurements a typical ensemble
method.                                                      has high and smooth solid angular coverage as shown
    In many ways MUST II is defined to have features         in fig. 1.
very similar to that of MUST [3]. Our original ambition
was to increase the active area to cover symmetrically
and widely the forward/backward angles without
modifying the electronic structure. However with the
opportunity to found the electronics in ASIC
(applications specific integrated circuit) form presented
a significant reduction in the volume occupied by the
electronics behind the telescope and the number of
cable/connectors runs. This solution opens the
possibility to perform particle-gamma, measurements
that permit only limited volume around the Ge clovers.
Further, although ASIC developments are costly, the
cost per channel is inexpensive, making the prospect
for future increase of solid angle possible. Presently,
MUST II is an ongoing project where we have opted
for an ASIC solution for the front-end electronics. It
consists of six telescopes that multiply the solid angle
coverage of the MUST ensemble by a factor of three.
The large phase space coverage will make it possible to
measure low yield reactions and open the prospect to
study several reactions simultaneously ((p,d), (p,t),
(p,2p) etc to bound or unbound states.

                   GEOMETRY
    Each telescope consists of a Si double-sided strip
detector, Si(strip), followed by a Si(Li) and CsI crystal.
The Si(strip) is of dimension 1010cm² with 128 strips
on either face. The crystal is an n-type low resistivity
(~ 6 KOhm-cm) to be biased to twice the depletion
voltage to allow a high field strength over the full
thickness of 300µm. The masks for this detector are not
much different from those of MUST with the exception
that the inter-strip will be 56 µm. Overall energy and       Fig 1. Geometry of telescope (top) and Efficiency vs
time resolution expected are 50 keV and 250 ps for           laboratory angle for the Si(Strip)
alphas of 5.48 MeV. Two Si(Li) detectors of thickness
4.5 mm will be used to cover the 100 cm². Each crystal                        ELECTRONICS
of ~105 cm² will be segmented into 8 pads.
Resolution aimed for is 120 KeV and a dynamic range              The electronic hardware of MUST II consists of
for protons up to 32 MeV. The CsI crystals are               three basic units. MATE, MUFEE and MUVI. MATE,
segmented to shadow the Si(Li) from a point target and       (Must Asic for Time and Energy) delivers the E and T
are of length 3 cm to stop 80 MeV protons. The light         from the detectors. A total of 18 MATEs/telescope are
output is read by 22cm² photodiodes. An energy              distributed on two quasi identical card MUFEE (MUst
resolution about 6% is expected for alphas of 5.48           Front End Electronics) connected with the detectors via
MeV.                                                         8 cm Kapton bands. Data transfer, high tension and
communication are done through 25 channel                                pulser inputs, gain, shaping and inverse current
connectors. A single width unit MUVI, (MUst in VXI)                      measurement) are satisfied via the industrial protocol
in VXI standard, assures the slow control and data                       I²C.
coding for the six telescopes. With the exception of                         The principal results derived from simulation for
MUVI, the general philosophy is that each telescope in                   the strip detector are as follows (capacitance 65pF, dc
the reaction chamber is electrically independent.                        20 namp,). (The slow controlled dynamic range and
                                                                         resolution for the Si(Li) and CsI are given in italics);
                            MATE                                             Energy range: 50 MeV, 250 MeV
                                                                             Energy resolution (fwhm): 16 KeV, 90 KeV (filter
    The ASIC MATE has 16 channels per chip and                               rc-cr 1µs, 3µs) Track and Hold, T/H.
process signals delivered from silicon strip detectors,                      TAC range 300 ns.
Si(strip) pads and photodiodes. The architecture                             Time jitter (fwhm): 240 ps (protons 6 MeV filter
delivers three types of information for each channel:                        rc-cr 30 ns)
    1.Value of the energy losses from particles hitting                      Threshold range: ± 1.0 MeV, on 8 bits quantization
the telescope.                                                               Power consumption: 35 mW.
    2.Value of time of flight from a leading edge                            Readout: 2MHz serial. All channels read at request.
discriminator with adjustable threshold and Time to                      MATE uses a BICMOS technology A.M.S. 0.8µm. The
amplitude converter, TAC, with a common stop.                            first submission was in May 2002 and characterization
    3.Value of DC leakage current for monitoring                         will start in October 2002.
purposes.
The choice of discriminators is a leading edge. Current                                           MUFEE
pulses were simulated and showed that the time
resolution is sufficient to separate the 3He and 4He over                   The main function of MUFEE is to process the
the Si (strip) E-dynamic range. Namely, the differential                 physical signals from the detectors; each MUFEE
walk for different particle types of the same energy is                  processing 128 strips of one side of the Si(strip) and 16
negligible in comparison to the time of flight.                          physical signals from the Si(Li) or CsI detector.




               MATE                                                                hold
                                                                                              I leak i         selidi
                                                                                                               selid
                          Idleaki
                          Idfuite
                                Rf                      Filtre &
                                                        Filter &                              energy i         seleni
                                                                                      &
                                                                                  Track&
                    Idf
                    Idl             cf                   Ampli                     Hold
      Si                                                                                                        in VIC
      Strip in i
           in i
                                                                                            time i              Th
                                                                                                                selti
                                            Filter            +discri               Or
                                                                                    OU        TAC
                                             Ampli
                                            Ampli             -

                                                                                                                stop

                                                                            OU                                     i
                                                                                                               start

               ininj Th
               in            Res         Thr    Thr
                                         seuilp seuilm side   hyst           i
                                                                        inhibit   Tstart   ResetStart
                             et

Fig 2. MATE schematic diagram

    In spite of the relatively large dynamic range                       MUFEE has I²C driven internal pulse generator to
requested, MATE process bipolar signals in energy and                    allow the different functions to be tested and the
time channels and therefore software adaptable for both                  physical calibration of the E and T channels. A single
sides of the strip detector. The programming functions                   injection capacitor is used so the injected charge is the
of MATE (discriminator levels, inhibits of channels,                     same for all channels of ASIC. An external pulse
                                                                         generator input is also made available. Numerical
information for the slow control of the MATEs is                                                                                  and is specifically studied to allow a high density of
carried on the standard serial bus, I²C.                                                                                          channels with a minimum of acquisition dead time, DT.
     To insure good immunity against electromagnetic                                                                              Each telescope is independent and connected to (one of
perturbations, all control signals are carried in Low                                                                             six) cards CAS residing in MUVI. CAS delivers the
Voltage Differential Signal (LVDS) except the STOP-                                                                               slow control for discriminator, pulser, current reading
TDC signal that is in Low Voltage Positive Emitter                                                                                etc. It receives the hit signal and distributes the stop
Coupled Logic (LVPECL) to minimize the timing                                                                                     signal for the TACs, T/H, clock readout and pulser
jitter. For the same reason, the two analog lines,                                                                                trigger. Four lines are dedicated to analogical
carrying E and T travel in differential current through                                                                           differential outputs at 2 MHz that carry the signal train.
twisted pair. The fan-in to form the trigger and the                                                                                  The architecture of CAS is presented in fig 3. The
distribution of the detector tensions, STOP-TDC and                                                                               coding block allows the pattern of signals on four
T/H are also on the card.                                                                                                         channels to be coded. Each channel is composed of 2
     Of importance is the relatively large thermal energy                                                                         differential converters giving a numeric resolution of
(~15Watt/telescope) generated by the electronics in                                                                               14 bits in 400 ns. The FPGA+DSP in CAS will cover
vacuum. This is drained via a heat exchanger                                                                                      important functions, such as the suppression of zero
sandwiched by the two MUFEE boards. The MATEs                                                                                     readings, the I²C communication, and sliding scale.
are cooled directly by a conductive interface material                                                                                Three trigger modes are available. The common DT
(Gap Pad). The temperature is monitored via a sensor                                                                              mode is estimated to correspond to10% dead time at
on the card. Mechanically, MUFEE is of                                                                                            1KHz. The mode semi autonome allows the liberation
dimension130130 mm².                                                                                                             of all telescopes that are not triggered. This option cuts
                                                                                                                                  down the acquisition time by a factor of two. Finally
                                                         MUVI                                                                     the mode autonome, is characterized by removal of
                                                                                                                                  GMT function and each CAS functions independently.
    The ensemble of the E and T information (3072 for                                                                             For the last two modes the reconstruction of each event
6 telescopes) is sent to acquisition system based on                                                                              is done through the time stamping of CENTRUM The
VXI-C. For MUST II and two tracking detectors CATS                                                                                triggers and acquisition with CENTRUM will facilitate
[4] the configuration will include a MUVI, a trigger                                                                              the integration of MUST II with other detection
(GMT) coupled to a bit pattern and scalars unit (U2M).                                                                            systems.
CENTRUM provides a time and event stamping. The
back plane data transfer for CENTRUM and                                                                                                   CONCLUDING REMARKS
distribution of resources for the visual inspection of
signals is done by GAMER. Four QDCs (XDC3214)                                                                                         Direct reactions are an important tool and the
will code the CATS data. The slot is coupled to a                                                                                development of MUST II will make it possible to
VIC8250 for connection to VME or VXI or coupling to                                                                               exploit lower yield reactions in this domain. Other
a CPU allowing an Ethernet access.                                                                                                reaction studies that require similar specifications will
                                                                                                                                  benefit from such a development. Examples are
                                                                                                                                  resonant elastic scattering [5], breakup reactions [6]
                                 MUST2 / MUVI / CAS: block diagram
                                                                                                                                  and energy measurement in magnetic spectrometers[7].
               STOP          Logic LVPECL
                                                  ENSTOP
                                                                  STOP               Logic LV
                                                                                Trigger signals + STOP
                                                                                                         TRIG                     We hope to perform our first tests in 2003 with MUST
                                 STOP
                                                                   SIGTRIG                                                        II and first measurement in 2004.
                                                                    JTAG DSP                             JTD
                               Logic LV         I2Cint                               Logic LV
                     I2C                                                                                 JTF
                             Slow control                                             JTAGs
                                                                                                                                                    REFERENCES
                                                                                                                 FROM / TO MUVI




                                                                    JTAG FPGA
   FROM / TO MUFEE




                                               SIGACQ
                             Logic LVDS                                  ETAT       Logic LV
                     ACQ                                                                                 STATE
                           MUFEE Acquisition                                    Acquisition signals
                                                           FPGA                                                                   1. M. Labiche et al., Phys. Rev. C 60 (1999) 027303
                                                             +
                                4 channels
                                                DAG EG      DSP                                          LP DSP                   2. A. Lagoyannis et al., Phys.Lett. B518 (2001) 27
                     ANA                        ADCs
                           I to V + ADC + EG
                                                                    INSLOG1
                                                                                                                                  3. Y. Blumenfeld et al., NIM A366 (1999) 298
                                             ADINSANA                                Logic LV
                                               I_TST int
                                                                    INSLOG2        INSLOG 1&2
                                                                                                         INSLOG                   4. S. Ottini-Hustache et al., NIM A431 (1999) 476
                              Test current
                                I_TST                                                                                             5. J. Gómez del Campo et al., Phys. Rev. Lett. 86
                                                                                     Analog
                                                                                                         INSANA
                                                                                  INSANA 1 & 2                                    (2001) 43, reference therein.
                                          SYN_TST, ENVTST
                     TST       Tests:
                           V_TST, SYN_TST                                                                 VTST                    6. A. Wuossma et al., Ann. Rev. Nucl. Part. Sci.
                                                                                                                                  Astrophys. 45 (1995)1
                                                                                                                                  7. E. C. Pollacco et al., contribution to CAARI 2002
Fig 3. MUVI schematic diagram.
   The units mentioned above, with the exception of
MUVI, correspond to basic elements of the GANIL
acquisition system. MUVI is a MUST II development

				
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posted:9/8/2012
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