Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 56 (2023)
RIKEN
Accel. Prog. Rep. 56 (2023)

Silicon tracker array for RIB experiments at SAMURAI†
A. I. Stefanescu,∗1,∗2,∗3 V. Panin,∗4 L. Trache,∗1 D. Tudor,∗1,∗2 I. C. Stefanescu,∗1,∗2 J. Tanaka,∗3 H. Otsu,∗3T. Motobayashi,∗3 A. Spiridon,∗1 and T. Uesaka∗3 for HI-p Collaboration
A silicon tracker for use in front of the SAMURAI
spectrometer at the RIBF facility of the Nishina Center of RIKEN, Wako, Japan was designed, built andtested.1) The tracker consists of two pairs of singlesided microstrip detectors that determine the (x, y)
position at two locations along the beamline after thetarget. The system provides particle identiﬁcationfrom protons up to Z ∼ 50 heavy fragments with energies 100–350 MeV/nucleon. GLAST-type 325 µm
thick detectors were used. The inherent problems
stemming from the large granularity, the required verylarge dynamic range and counting rate were handledusing two matched ASICs: a dual-gain preampliﬁer
DGCSP and the HINP16 pulse processing system.
Combining the HINP16 ASICs with the newly developed dual-gain preampliﬁers, produces 2 × 4 × 128= 1024 channels. (Lo/Hi gain × 4 Si each with 128
channels.) This system yields a very large dynamicrange of ∼104 . It can work in a self-triggering mode orin slave mode where it requires an external trigger andcan tolerate very large input capacitance. The large
dynamic range, provided by the dual-channel preampliﬁer, makes it possible to measure a wide range ofnuclear charges. Parts of the system were tested withbeams from the HIMAC facility in Chiba, Japan, toshow that it can measure energy losses from 100 keV(protons) to 600–900 MeV (for heavy fragments up toZ = 50), which makes it appropriate for the studies
with radioactive ion beams at intermediate energies.
The whole system was used and characterized in
two RIBF experiments (NP1412-SAMURAI29 andNP1406-SAMURAI24), using the SAMURAI magnetic spectrometer. The silicon detection system wassuccessfully used to track protons and heavy fragments
simultaneously in the breakup of the proton-rich nuclei, (like 9 C and 66 Se) to reconstruct the reaction vertex, the emerging angles of the particles, the momentum distributions of the protons and the relative energy spectra. We run the system with 1.2 × 103 Hz
132
Xe beam rate during the HIMAC test and with
4 × 104 Hz of 9 C beam, and we assume that the maximum rate can be higher. However, the damage ofthe detector may become important. The problem becomes more complex because the center of the detector
system takes a larger rate of hits and will lead to local
damages. And it is more important for heavier (sec-ondary) beams. This will be addressed by replacing
the detectors when necessary.
To achieve the science objectives of the experimentswith proton-rich radioactive beams in which we have tomeasure the proton and the heavy ion remnant, the essential tracking performances are: the atomic number
identiﬁcation, the vertex reconstruction resolution, theangular resolution and the momentum reconstruction
resolution. All these are required to determine with agood energy resolution the missing mass spectra. We
illustrate these with the case of the proton breakupof 9 C at 160 MeV/nucleon. The physics objectives
are to disentangle the E1 and E2 contributions to theCoulomb dissociation process on a lead target and toevaluate the astrophysical factor, S18 (0) by separating
the contribution of the resonance at 918 keV. The resolutions obtained with the silicon system are resulting
from the incoming beam phase space characteristics,
the strip pitch size (0.684 mm), the distance between
target and the ﬁrst pair of detectors and the distance
between the two pairs of detectors. With the 4 silicondetectors used in the conﬁguration presented in the article, the resolution of the longitudinal distance (σ =35 mm) is suﬃcient to separate the spurious events induced by the beam in the other materials around thetarget, as illustrated in Fig. 10(b) of Ref. 1). The angular resolution obtained for the laboratory opening
angle is 3.5 mrad and the largest contribution is coming from the proton straggling in the ﬁrst two siliconlayers (straggling in 0.650 mm Si ≈ 2.8 mrad). By using these silicon detectors for proton tracking together
with the proton drift chambers mounted at the exitof the SAMURAI superconducting magnet, we got for
proton momentum resolution: ∆P/P ≈ 0.065%. Considering these resolutions, we expect that at 1 MeV
relative energy of p-8 B we get the energy resolution of≈100 keV, about 2.5 times better than without the silicon tracker, which is suﬃcient to determine the crosssection at energies below the resonance and to evaluate
S18 (0) by extrapolation.
This silicon tracker dramatically extends the research opportunities with the SAMURAI spectrometer
especially for systems with two or more charged particles, which had been too diﬃcult with the standard
detectors equipped in SAMURAI.
Reference†∗1∗2∗3∗41) A. I. Stefanescu et al., Eur. Phys. J. A 58, 223 (2022).

Condensed from the article in Eur. Phys. J. A 58, 223 (2022)
Horia Hulubei National Institute for R&D in Physics andNuclear Engineering
Doctoral School of Physics, University of Bucharest
RIKEN Nishina Center
GSI Helmholtzzentrum für Schwerionenforschung GmbH- 92 -