RIKEN Accel. Prog. Rep. 50 (2017)

Ⅱ-9. Instrumentation

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T.Fujita,∗1A.Enokizono,∗1,∗2K.Adachi,∗1S.Ichikawa,∗2N.Uchida,∗1T.Ohnishi,∗2K.Kasama,∗3K.Kurita,∗1S.Sasamura,∗1T.Suda,∗2,∗3T.Tamae,∗2,∗3K.Tsukada,∗2,∗3M.Togasaki,∗1K.Namba,∗3M.Hara,∗2M.Hori,∗1T.Hori,∗2K.Matsuda,∗3K.Yamada,∗1M.Watanabe,∗2andM.Wakasugi∗2The self-conﬁning RI ion target (SCRIT) electron scatteringfacilityhasbeenconstructedtorealizeelectronscatteringoﬀunstablenuclei1) . In2014,aluminositymonitor
(LMon) was installed to measure the absolute luminosity
value of electron scattering in the SCRIT experiment2 3).Theluminosityisdeterminedas
d2 N
d2 σ= L·dΩdE
dΩdE

(1)

where N is the number of scattered electrons, L is the luminosity and σ is scattering cross section.The angular distribution of scattered electrons is measured by the windowframe spectrometer for electron scattering(WiSES)4) . The
required accuracy of the luminosity value is expected to beless than 5% which is comparable with the systematic uncertainty of WiSES.
To determine the luminosity with a total uncertainty lessthan 5%, it is necessary to measure the bremsstrahlung photon counts, energies and spatial distribution very preciselyand accurately. The photon counts and energies are measured by a CsI calorimeter array. The CsI calorimeter consits of 7 pure CsI scintillators. The spatial distributions of
the bremsstrahlung photons are measured by two layers ofﬁber scintillators. Each layer consits of 16 ﬁber scintillators and the cross section of each ﬁber is 3×3 mm2 . In theexperiment, L is calculated as
L=NBrems. 1·[cm−2 sec−1 ]σBrems. ε(2)where NBrems. is the number of bremsstrahlung photons persecond and σBrems. is the caluculated bremsstrahlung crosssection5) . Because LMon is located 7 m downstream ofthe vertex position and there are structures in the path ofthe photons such as a cryopanel in the beam chamber, itis necessary to evaluate the transmittance. ε is estimatedas the number of photons detected with LMon divided by
the number of radiated photons in the SCRIT calculated by
GEANT4 simulations. It is important to understand several electron beam conditions such as electron beam energy
and beam shape. In order to determine the parameters to behandled in the simulation, we compared experimental dataand simulation data.
From 2015 to 2016, the experiment of electron scattering
for 132 Xe and 208 Pb targets had been performed at electron
∗1
∗2
∗3

Department of Physics, Rikkyo University
RIKEN Nishina Center
ELPH Tohoku University

beam energies Ee = 150 MeV, 200 MeV and 300 MeV.
Figure 1 shows the experimental spatial distributions together with that calculated with GEANT4 simulations at
E e = 150 MeV. The red and blue closed circles in theupper panels in Fig. 1 represent the spatial distributions
with and without target ions (Ion In and Ion Out), respectively. The lower panels show the spatial distributions ofbremsstrahlung photons from 208 Pb estimated by subtracting Ion Out from Ion In. The systematic uncertainty of εis estimated by varying the initial beam parameters withinχ2 /ndf between exprimental data and GEANT4 spatial distributions. With a careful study of the initial beam condition, it has become possible to reproduce the spatial distributions of data by GEANT4 simulation, and the systematic uncertainty of ε was reduced to ∼ 6% , signiﬁcantly
less than the uncertainty of ∼ 10% achieved in a previousstudy. The key point to increase the reproducibilty and toreduce the systematic uncertainty is to understand the electron beam size according to the lattice function of the electron storage ring. In Fig. 1, the beam size is σH ∼ 2.13 mm,σV ∼ 0.60 mm and the spread of incident angle in the horizontal direction is +0.4 mrad in σH at the center of theSCRIT device.

Fig. 1. Horizontal (X ﬁber) and vertical (Y ﬁber) spatial distributions. The histograms show simulation results.

In summary, we found that a careful treatment of the initial beam condition is key to reproduce the spatial distribution of bremsstrahlung photons and to reduce the systematicuncertainty in the determination of ε.
References
1) M. Wakasugi et al., Nucl. Instr. Meth. B317, 668 (2013).
2) S. Yoneyama et al., RIKEN Accel. Prog. Rep. 48, 227 (2014).
3) A. Enokizono et al.: In this progress report.
4) K. Tsukada et al.: In this progress report.
5) Y. S. Tsai, Rev. Mod. Phys. 46 815-851 (1974).

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