Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

Field measurement of SCRIT electron spectrometer
T. Tamae, *1,*2 T. Miyamoto, *1,*2 and T. Suda*1,*2
A large-acceptance magnetic spectrometer for electron
scattering on unstable nuclei was constructed, and its
magnetic field was measured. The analysis of the field
distribution and the development of a program of trackreconstruction are in progress.
The design1) and construction2), 3) of the spectrometer
have been reported elsewhere. The spectrometer is designed3to have a reasonably good resolution of p / p : 10 , alarge solid angle of about 100 msr, and a large scattering
coverage of 30-60°.
The spectrometer consists of a single window-frame
dipole magnet with an aperture of 29 cm in height (y), 1.4 m
in length (z) and 1.71 m in width (x). As the spectrometerhas no focal plane, the momentum and vertex position of
scattered electrons are fixed through the trackreconstruction using the information of drift chambers, as
shown in Fig. 1. This implies that the knowledge of theprecise distribution of the magnetic field is essential foranalysis.

Fig. 2. Contour plot of field strength for a center fieldof 8,017 gauss. Numbers attached to lines show theratio of the field strength to the central field strength.
The order of lines in the figure is same as the order oflines in the legend. Light-grey region: aperture regionof the magnet; dark-grey region: coil; black region:
return yoke.
the probe orientation. They were normalized to the NMR
values.
The field strength for case (c) is contoured in Fig. 2;
numbers attached to lines show the ratio of the fieldstrength to the central field strength. The figure shows thatthe field is almost uniform all the way to the coils, which isa unique merit of the window-frame magnet. The measured
field along the electron beam at z = -155 cm is about -3gauss; the leakage field of 3 G gives a closed orbit
distortion (COD) of 0.1 mm at the SCRIT (max. 1.5 mm inthe ring). These values are within acceptable levels for thering operation.
The field strengths were also compared to calculations
using the program OPERA-3D. The difference between themeasured and calculated field strengths is much less than
10-3 in the aperture region, but it becomes larger at thefringe field region, especially at positions close to magnetic
poles and field clamps. The estimation of errors for theelectron momentum and vertex positions due to inaccuracyof the field distribution is in progress.

Fig. 1. Layout of the spectrometer, detectors, and SCRIT(Self-Confining Radioactive Isotope ion Target).
The magnetic field distribution was measured using aHall probe at 37,118 points in 1/8 of the entire volume of
the aperture and connecting regions. The absolute
magnetic-field values were calibrated using an NMR set ata fixed position in the mid-plane. The field map wasobtained at three strengths ((a) 4,096, (b) 7,628, and (c)8,017 G at the NMR). The measured values werecorrected for the drift of a Hall probe and for inaccuracy of*1*2

References

1) T. Adachi et al.: Accel. Prog. Rep. 45, 144 (2012).
2) T. Suda et al.: Accel. Prog. Rep. 46 184 (2013).
3) T. Suda et al.: in this progress report.

RIKEN Nishina Center
Research Center for Electron-Photon Science (ELPH),
Tohoku University- 196 -