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 (1000cm3) 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.
important to increase the detector geometrical
efficiency.
Also of importance is the beam emittance because
of kinematic effects. To obtain reasonable energy
resolution the angular resolution better than 0.5° is
required. For available beams this implies that beam
tracking to better than 1 mm is vital. A final comment
about exotic beams, is their purity which is not often
guaranteed. Therefore the coincident detection of the
projectile-like fragment in a spectrometer, is frequently
crucial to obtain a clean background in the light ion
energy spectra. In pickup reactions, like (p,d), the
kinematics for the detected light ejectiles are forward
focused making it relatively easy to cover a large
fraction of the solid angle. This is not so true with the
interesting (d,p) reaction. The p cover a wider angular
domain with steep kinematic variations. Thus for an
acceptable telescope position resolution, the distance to
target has to be increased at the expense of solid angle.
Energy resolution deteriorates with the combined
effects of target thickness, beam emittance and total
angular resolution which are limiting factors for this
experimental technique. Values better than 300 keV are
difficult to achieve even when applying thin targets and
beam tracking. A solution is to perform gamma-particle
coincidence measurements.
Highly efficient Ge
detectors like ExoGam coupled to position sensitive
INTRODUCTION
Direct reactions with light ions (protons, deuterons,
… alphas) on stable nuclei were shown to be an
important tool in nuclear spectroscopy. Reactions like
elastic and inelastic give matter radii and deformation.
The transfer of one or two nucleon (d,p), (d,3He),
(t,p)… give spin and parity as well as spectroscopic
strength. These elements were central in building our
present macro and micro vision of the nucleus. It is
therefore natural to extend this method to unstable
species. Unstable nuclei far outnumber stable ones and
we enquire to what extent do our present models, tuned
on stable systems, will apply to highly neutron rich or
proton rich ensembles. Inklings of modified structure is
given by the neutron rich Li and Be isotopes [1].
Neutron haloes in nuclei like 6He, 11Li and 19C are
structures that were not expected and still need to be
understood [2].
Direct reaction measurements with exotic nuclei are
performed in inverse kinematics using CH2 or CD2
targets. Liquid or solid H2 and gaseous T2 have been
employed. The beams of course are not intense. With
cross sections of a few mb/sr and reasonable target
thickness, present detection systems require beams
intensities better than 104 particles/s . Hence it is
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
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The standard flight distance is 15cm and the
geometry of the telescope ensemble is highly dictated
by this choice. Thus, the mechanics of the telescopes is
a truncated pyramid with a base 13×13cm² with a
vertex at 15 cm and an “active” face of 11×11cm². To
allow
set-up
flexibility
in
gamma-particle
measurements, the CsI can be removed. In
back/forward angle measurements a typical ensemble
has high and smooth solid angular coverage as shown
in fig. 1.
detectors will be used at GANIL. Thick targets can then
be employed which offsets the loss in efficiency.
Elastic and inelastic scattering measurements are
fundamental and are often a prerequisite to transfer
reaction analysis. The complication with this reaction is
the relatively low threshold that is required and values
of 0.4 MeV.A with particle identification, is a must. In
our experiments we have opted for a time of flight
method.
In many ways MUST II is defined to have features
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
time resolution expected are 50 keV and 250 ps for
alphas of 5.48 MeV. Two Si(Li) detectors of thickness
4.5 mm will be used to cover the 100 cm². Each crystal
of ~10×5 cm² will be segmented into 8 pads.
Resolution aimed for is 120 KeV and a dynamic range
for protons up to 32 MeV. The CsI crystals are
segmented to shadow the Si(Li) from a point target and
are of length 3 cm to stop 80 MeV protons. The light
output is read by 2×2cm² photodiodes. An energy
resolution about 6% is expected for alphas of 5.48
MeV.
Fig 1. Geometry of telescope (top) and Efficiency vs
laboratory angle for the Si(Strip)
ELECTRONICS
The electronic hardware of MUST II consists of
three basic units. MATE, MUFEE and MUVI. MATE,
(Must Asic for Time and Energy) delivers the E and T
from the detectors. A total of 18 MATEs/telescope are
distributed on two quasi identical card MUFEE (MUst
Front End Electronics) connected with the detectors via
8 cm Kapton bands. Data transfer, high tension and
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communication are done through 25 channel
connectors. A single width unit MUVI, (MUst in VXI)
in VXI standard, assures the slow control and data
coding for the six telescopes. With the exception of
MUVI, the general philosophy is that each telescope in
the reaction chamber is electrically independent.
pulser inputs, gain, shaping and inverse current
measurement) are satisfied via the industrial protocol
I²C.
The principal results derived from simulation for
the strip detector are as follows (capacitance 65pF, dc
20 namp,). (The slow controlled dynamic range and
resolution for the Si(Li) and CsI are given in italics);
Energy range: 50 MeV, 250 MeV
Energy resolution (fwhm): 16 KeV, 90 KeV (filter
rc-cr 1µs, 3µs) Track and Hold, T/H.
TAC range 300 ns.
Time jitter (fwhm): 240 ps (protons 6 MeV filter
rc-cr 30 ns)
Threshold range: ± 1.0 MeV, on 8 bits quantization
Power consumption: 35 mW.
Readout: 2MHz serial. All channels read at request.
MATE uses a BICMOS technology A.M.S. 0.8µm. The
first submission was in May 2002 and characterization
will start in October 2002.
MATE
The ASIC MATE has 16 channels per chip and
process signals delivered from silicon strip detectors,
Si(strip) pads and photodiodes. The architecture
delivers three types of information for each channel:
1.Value of the energy losses from particles hitting
the telescope.
2.Value of time of flight from a leading edge
discriminator with adjustable threshold and Time to
amplitude converter, TAC, with a common stop.
3.Value of DC leakage current for monitoring
purposes.
The choice of discriminators is a leading edge. Current
pulses were simulated and showed that the time
resolution is sufficient to separate the 3He and 4He over
the Si (strip) E-dynamic range. Namely, the differential
walk for different particle types of the same energy is
negligible in comparison to the time of flight.
MUFEE
The main function of MUFEE is to process the
physical signals from the detectors; each MUFEE
processing 128 strips of one side of the Si(strip) and 16
physical signals from the Si(Li) or CsI detector.
MATE
hold
I leak i
selidi
selid
energy i
seleni
Idfuite
Idleak
i
Rf
Si
Stripinini i
Idf
Idl
Filtre &
Filter
&
Track
&&
Hold
Ampli
cf
in VIC
time i
Filter
Ampli
Ampli
+discri
-
OU
Or
selti
Th
TAC
stop
starti
OU
ininj
in Th
Res
Thr
seuilp Thr
seuilm side
hyst
inhibit
i
Tstart
ResetStart
Fig 2. MATE schematic diagram
MUFEE has I²C driven internal pulse generator to
allow the different functions to be tested and the
physical calibration of the E and T channels. A single
injection capacitor is used so the injected charge is the
same for all channels of ASIC. An external pulse
generator input is also made available. Numerical
In spite of the relatively large dynamic range
requested, MATE process bipolar signals in energy and
time channels and therefore software adaptable for both
sides of the strip detector. The programming functions
of MATE (discriminator levels, inhibits of channels,
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and is specifically studied to allow a high density of
channels with a minimum of acquisition dead time, DT.
Each telescope is independent and connected to (one of
six) cards CAS residing in MUVI. CAS delivers the
slow control for discriminator, pulser, current reading
etc. It receives the hit signal and distributes the stop
signal for the TACs, T/H, clock readout and pulser
trigger. Four lines are dedicated to analogical
differential outputs at 2 MHz that carry the signal train.
The architecture of CAS is presented in fig 3. The
coding block allows the pattern of signals on four
channels to be coded. Each channel is composed of 2
differential converters giving a numeric resolution of
14 bits in 400 ns. The FPGA+DSP in CAS will cover
important functions, such as the suppression of zero
readings, the I²C communication, and sliding scale.
Three trigger modes are available. The common DT
mode is estimated to correspond to10% dead time at
1KHz. The mode semi autonome allows the liberation
of all telescopes that are not triggered. This option cuts
down the acquisition time by a factor of two. Finally
the mode autonome, is characterized by removal of
GMT function and each CAS functions independently.
For the last two modes the reconstruction of each event
is done through the time stamping of CENTRUM The
triggers and acquisition with CENTRUM will facilitate
the integration of MUST II with other detection
systems.
information for the slow control of the MATEs is
carried on the standard serial bus, I²C.
To insure good immunity against electromagnetic
perturbations, all control signals are carried in Low
Voltage Differential Signal (LVDS) except the STOPTDC signal that is in Low Voltage Positive Emitter
Coupled Logic (LVPECL) to minimize the timing
jitter. For the same reason, the two analog lines,
carrying E and T travel in differential current through
twisted pair. The fan-in to form the trigger and the
distribution of the detector tensions, STOP-TDC and
T/H are also on the card.
Of importance is the relatively large thermal energy
(~15Watt/telescope) generated by the electronics in
vacuum. This is drained via a heat exchanger
sandwiched by the two MUFEE boards. The MATEs
are cooled directly by a conductive interface material
(Gap Pad). The temperature is monitored via a sensor
on the card. Mechanically, MUFEE is of
dimension130×130 mm².
MUVI
The ensemble of the E and T information (3072 for
6 telescopes) is sent to acquisition system based on
VXI-C. For MUST II and two tracking detectors CATS
[4] the configuration will include a MUVI, a trigger
(GMT) coupled to a bit pattern and scalars unit (U2M).
CENTRUM provides a time and event stamping. The
back plane data transfer for CENTRUM and
distribution of resources for the visual inspection of
signals is done by GAMER. Four QDCs (XDC3214)
will code the CATS data. The slot∅ is coupled to a
VIC8250 for connection to VME or VXI or coupling to
a CPU allowing an Ethernet access.
CONCLUDING REMARKS
Direct reactions are an important tool and the
development of MUST II will make it possible to
exploit lower yield reactions in this domain. Other
reaction studies that require similar specifications will
benefit from such a development. Examples are
resonant elastic scattering [5], breakup reactions [6]
and energy measurement in magnetic spectrometers[7].
We hope to perform our first tests in 2003 with MUST
II and first measurement in 2004.
MUST2 / MUVI / CAS: block diagram
STOP
I2C
STOP
Logic LVPECL
STOP
Logic LV
ENSTOP
Trigger signals + STOP
TRIG
SIGTRIG
JTAG DSP
Logic LV
Slow control
Logic LV
JTAGs
I2Cint
JTD
JTF
ACQ
ANA
Logic LVDS
MUFEE Acquisition
4 channels
I to V + ADC + EG
SIGACQ
DAG EG
ETAT
FPGA
+
DSP
TST
I_TST int
SYN_TST, ENVTST
Tests:
V_TST, SYN_TST
STATE
REFERENCES
1. M. Labiche et al., Phys. Rev. C 60 (1999) 027303
2. A. Lagoyannis et al., Phys.Lett. B518 (2001) 27
3. Y. Blumenfeld et al., NIM A366 (1999) 298
4. S. Ottini-Hustache et al., NIM A431 (1999) 476
5. J. Gómez del Campo et al., Phys. Rev. Lett. 86
(2001) 43, reference therein.
6. A. Wuossma et al., Ann. Rev. Nucl. Part. Sci.
Astrophys. 45 (1995)1
7. E. C. Pollacco et al., contribution to CAARI 2002
LP DSP
ADCs
ADINSANA
Test current
I_TST
Logic LV
Acquisition signals
FROM / TO MUVI
FROM / TO MUFEE
JTAG FPGA
INSLOG1
INSLOG2
Logic LV
INSLOG 1&2
INSLOG
Analog
INSANA 1 & 2
INSANA
VTST
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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