Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

The spin polarization of proton target in SHARAQ04 experiment
T.L. Tang,*1 S. Kawase,*1 Y. Kubota,*1 H. Tokieda,*1 K. Yako,*1 T. Fuji,*1 S. Michimasa,*1 K. Kisamori,*1 M. Takaki,*1H. Miya,*1 C.S. Lee,*1 R. Yokoyama,*1 M. Kobayashi,*1 S. Ota,*1 S. Shimoura,*1 T. Kawahara,*2 T. Wakui,*3J. Zenihiro,*4 H. Matsubara,*4 M. Dozono,*4 H. Takeda,*4 K. Kusaka,*4 N. Fukuda,*4 D. Kameda,*4 H. Suzuki,*4M. Sasano,*4 T. Uesaka,*4 S. Sakaguchi,*5 T. Wakasa,*5 J. Yasuda,*5 T. Fukunaga,*5 T. Noro,*5 Y. Maeda,*6 S.H. Hwang,*7W.Y. Kim,*7 S. Stepanyan,*7 D. Beaumel,*8 A. Obertelli,*9 A. Galindo-Uribarri,*10 E. Padilla-Rodal*11A (p,2p) knockout reaction was used in the SHARAQ04
experiment to extract the spectroscopic factor of orbital
protons, and a spin-polarized proton target was used toextract the analyzing power for distinguishing the spin-up(𝐽𝐽> ) and spin-down (𝐽𝐽< ) orbits of the knockout proton. The
analyzing power can be determined using the cross section
left-right asymmetry and spin polarization. We present theanalysis and results of the spin polarization.
The measurement of the spin polarization of the polarized
proton target1,2) was conducted using two methods. The
nuclear magnetic resonance (NMR) was used for quick andconstant monitoring, and proton-proton elastic scattering
was used for measuring the absolute magnitude of the spinpolarization.
A 260 MeV pure proton beam was used in proton-proton
elastic scattering runs. The spin polarization was reversed
during the measurement to eliminate of any systematicasymmetry. A pair of recoil protons was correlated and theopening angle was 180o in the center of mass frame, whichis equal to 86.3o to 90o in the laboratory frame. The recoil
protons were tracked by two MWDCs3) locateddownstream4). The MWDCs were arranged 30o to the beamaxis and 1022 mm away from the target. The MWDCs
covered a forward angle of 20o – 70o, which is equivalent toapproximately 40o – 140o in the center of mass frame. Theposition resolution of the MWDCs is approximately 0.1 mm
for X and 0.2 mm for Y.
The NMR method was applied by inserting an NMR coilto surround the target crystal. From the measurement, it wasshown that the spin polarization of the spin-down runs wasapproximately 78% smaller than that of the spin-up runs
during proton-proton elastic scattering 2).
The target crystal was made of naphthalene (C10H8). The
beam profile was much larger than the size of the target sothat significant amount of the beam was incident on thetarget holder (hydrogen-free plastics) and surroundingstructures. These two factors provided us a broad
background signal on the opening angle.
________________________________________________*1 Center of Nuclear Study, Tokyo University*2 Toho University*3 CYRIC, Tohoku University*4 RIKEN Nishina Center*5 Kyushu University*6 University of Miyazaki*7 Kyungpook National University*8 IPN Orsay*9 CEA Orsay*10 Oak Ridge National Laboratory*11 ICN, Nuiversidad Nacional Autonoma de Mexico

By assuming that the reaction point is along the z-axis ofthe lab frame, we calculated the weighted reaction position(Z-vertex) from the MWDCs’ tracking result. The Z-vertex
was gated by an opening angle with a central gate (84o-89o)
and a side gate (81.5o-84o or 89o-91.5o). The central gatecontained the elastic scattering peak and background, while
the side gate contained only background because theopening angle is not allowed by kinematics and detector
acceptance (Figure 1).

Figure 1. Weighted Z-vertex (reaction vertex on z-axis)
with opening angle gate (upper right corner). Elastic
scattering runs #7 to #15 were used2).
We divided the cover angle into five angle sections in thecenter of mass frame. The yield for each section is𝛽𝛽

𝑌𝑌𝛽𝛽𝑥𝑥 (𝛥𝛥𝛥𝛥) = 𝑓𝑓𝐿𝐿𝛽𝛽 𝜖𝜖𝑥𝑥 𝜎𝜎(𝛥𝛥𝛥𝛥)𝛥𝛥𝛥𝛥(1 + 𝑠𝑠𝑥𝑥 𝐴𝐴𝑦𝑦 (𝛥𝛥𝛥𝛥)𝑃𝑃𝛽𝛽 )𝛽𝛽

𝐿𝐿𝛽𝛽 = 𝑁𝑁𝑇𝑇 ∑ 𝑁𝑁𝐵𝐵 (𝑖𝑖)𝜆𝜆𝛽𝛽 (𝑖𝑖)

(1)

𝑠𝑠𝐿𝐿↑ = 𝑠𝑠↓𝑅𝑅 = 1 = −𝑠𝑠↓𝐿𝐿 = −𝑠𝑠𝑅𝑅↑
where 𝑥𝑥 is left or right, 𝛽𝛽 is spin-up or down, 𝑓𝑓 is thefraction of beam on the target, 𝐿𝐿 is the integrated
luminosity, 𝑁𝑁𝑇𝑇 is the number of proton in target, 𝑁𝑁𝐵𝐵 (𝑖𝑖𝑖 isthe number of beam protons in the 𝑖𝑖 -th run, 𝜆𝜆 is thelifetime of the DAQ system, 𝜖𝜖 is the detector efficiency,𝜎𝜎(Δ𝜃𝜃) is the differential cross section in angle section Δ𝜃𝜃,ΔΩ is the solid angle on the angle section Δ𝜃𝜃, 𝑠𝑠 is a sign,
𝐴𝐴𝑦𝑦 is the analyzing power and 𝑃𝑃 is the spin polarization ofthe target. We used the spin-up and spin-down asymmetry
to avoid the efficiency non-uniformity of MWDCs. The
final spin polarization is a weighted mean from each angle
section. The spin-up polarization was 16% ± 14%. The spinpolarization is as expected and similar to the results of aprevious experiment performed under a similar conditions5).
An asymmetry of the cross section distribution can beobtained for large statistics.
References

1) T.L. Tang, et al., RIKEN Accel. Prog. Rep. 46, 162 (2012).
2) T.L. Tang, et al., CNS Annual Rep. 59 (2012)3) H. Okamura et al., Nucl. Instr. and Meth. A, 406, 78 (1998).
4) S. Kawase, et al., RIKEN Accel. Prog. Rep. 46, 30 (2012).
5) S. Sakaguchi, et al., Phys Review C, 84, 024604 (2011)- 189 -