Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 50 (2017)

Channeling eﬀect in ultra-thin monolithic silicon telescopes
F. Parnefjord Gustafsson,∗1 B. Dolachay,∗2 V. Ho Phong,∗3 S. Nishimura,∗3 and T. Ikeda∗3
Determining the cross section of the 8 Li(α, n)11 B reaction in the low energy regime Ecm < 2 MeV has
been of particular interest in recent decades.1,2) In apotential inhomogenous Big Bang scenario, this reaction could bridge the mass gap at A = 8, leading tothe evolution of heavier nuclei in the very early universe. Furthermore, full network calculations indicatethat this reaction could play a vital role in enhancing
the r-process nucleosynthesis in supernovae.3,4)
Multiple experiments have been performed to determine the low-energy cross section using the inverse
kinematics of a 8 Li beam incident on a 4 He gas target.2,5,6) However, data are scarce and show remarkable discrepancy below Ecm < 1.7 MeV owing to difﬁculties with the accurate identiﬁcation of the lowenergy reaction products.2) A new method was proposed at RIKEN to study the low-energy resonances
using ultra-thin ΔE-E monolithic silicon telescopes(MSTs)1) . Each telescope consists of ﬁve ultra-thin∼1 μm (ΔE) silicon detector pads on a ∼500-μm-thick(E) silicon detector. Experiments performed in 2005
show promising results for identifying the reaction ateven a low energy of Ecm ∼= 0.5 MeV.7)
To assure optimal detector performance for the ﬁnal
cross-section measurement, it is crucial to determine
the MST characteristics and response for particles incoming at diﬀerent angles of incidence. Ultra-thin silicon detectors are particularly prone to the channelingeﬀect, which degrades the detector signal for particles
incident along the detector crystal planes or axis by signiﬁcantly reducing the stopping power. Moreover, theexact detector characteristics remain unknown, anddetermining the thickness of the two dead layers in
the telescope could reveal the detector threshold forparticle identiﬁcation.
The tandem Pelletron accelerator at the Nishina
Center, RIKEN was used to accelerate 4-MeV 11 B, 9MeV 63,65 Cu, and 7- and 25.5-MeV 197 Au ions into theMST. The telescope was attached to a rotation stage
within the target chamber allowing accurate remoteangular control with respect to the ion beam. Approximately 30000 coincidence events in the ΔE and E
detector were collected at varying angles of incidence
for each selected ion beam.
Our study reveals the inﬂuence of the channeling effect in the ΔE-E particle identiﬁcation spectrum withrespect to detector orientation (see Fig. 1). A tailextending from the 4-MeV 11 B main peak in the spectrum was observed at speciﬁc detector angles, and it
∗1
∗2
∗3

Department of Physics, Lund University
Department of Physics, University of Paris-Sud
RIKEN, Nishina Center- 209 -

was attributed to the channeling eﬀect. The channeling ratio was deﬁned as the ratio of the number ofevents in this tail (Nchan ) to the number of events
within the main peak (Npeak ). By determining thechanneling ratio as a function of detector angle the silicon crystal orientation could be deduced. Our results
indicate that the channeling eﬀect can be signiﬁcantly
reduced by orienting the silicon detector at 10◦ facing
the anisotropic silicon crystal orientation in between
the crystal axis [111] and 231.

Fig. 1. Channeling ratio versus detector angle θ for 4-MeV
11B. The ΔE-E particle identiﬁcation spectrum is displayed for the [111] silicon crystal axis and at 10◦ withthe minimal channeling ratio. The peaks with highchanneling ratio are indicated with the corresponding
silicon crystal axis.

References
1) M. Kurata-Nishimura et al., Nucl. Phys. A 758, 162(2005).
2) M. La Cognata et al., Phys. Lett. B 664.3, 157 (2008).
3) M. Terasawa et al., Astrophys. J. 562, 470 (2001).
4) T. Kajino et al., Nucl. Phys. A 704, 165c (2002).
5) Y. Mizoi et al., Phys. Rev. C 6206, 5801 (2000).
6) S. Cherubini et al., Eur. Phys. J. A 20, 355 (2004).
7) S. Nishimura et al., RIKEN Accel. Prog. Rep. 39, 43(2006).