Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 47 (2014)

Ion-optical measurements using uranium primary beam with
diﬀerent charge states
H. Takeda,∗1 T. Kubo,∗1 N. Fukuda,∗1 D. Kameda,∗1 H. Suzuki,∗1 N. Inabe,∗1 and T. Ohnishi∗1Transfer matrix elements are essential for various
ion-optical diagnoses, especially for trajectory reconstruction1) to improve the particle identiﬁcation powerof the BigRIPS. Thus, it is very important to measure
the transfer matrix elements precisely to realize thebest possible performance of the BigRIPS.
Primary beams of heavy ions such as uranium could
be unique tools for measuring the transfer matrix elements because they are distributed into several charge
states after passing through materials such as targets, degraders, and detectors. Such beams with different charge states have the following characteristics:(1) Small widths at position x and angle a.
(2) Discrete peaks in magnetic rigidity (δ) spectrum.
(Each peak is narrow and the peak position isdeﬁnitely known.)(3) Same velocity for all the charge states.
Some ion-optical parameters can be measured precisely by utilizing these characteristics. Indeed, weperformed such measurements in 2007 during the ﬁrst
BigRIPS commissioning by using uranium beams.
Positions of a 238 U beam were measured with parallel plate avalanche counters (PPACs) installed in theBigRIPS foci F3, F5, and F7. The ion-optical systemfrom F3 to F7 is a four-bend achromatic spectrometer with an intermediate dispersive focus F5. Figure 1
shows a spectrum of horizontal position x at F7 versus x at F5 of the 238 U beam after it passes throughthe PPACs at F3. Diﬀerent charge states generated by
the F3 PPACs had diﬀerent positions at F5 due to theF3-F5 dispersion (x|δ)35 , as indicated by red (dashed)lines. The measured result was 31.5 mm/%, which is100

92+91+90+ (F5-F7)

50∆Q = +289+F7-X (mm)∆Q = +188+0∆Q = 0-50∆Q = −1-100∆Q = −2(at F5)-100

92+-5091+90+089+F5-X (mm)5088+ (F3-F5)

100

Fig. 1. Spectrum of F7-x vs. F5-x measured with a 238 U
primary beam after it passes through PPACs at F3.
Red and blue lines have information on the F3-F5 dispersion and the F5-F7 magnification, respectively. TheF3-F7 dispersion and the F5-F7 dispersion can be deduced from green lines.
∗1RIKEN Nishina Center

Fig. 2. Measurement of (�|δ) with primary beams. TOF
from F3 to F7 was measured for each charge state by
selecting the charge state with the F5 slits. The solid
curve represents a quadratic fitting, and the slope corresponds to (�|δ).

consistent with the designed value of 31.7 mm/% calculated by COSY INFINITY. Charge state transition
because of the PPACs occurred at F5 also. Events connected by each blue (solid) line in Fig. 1 have the samecharge state between F5 and F7. They have very diﬀerent positions at F5 but have the same magnetic rigidity. This situation allows measurement of the F5-F7
magniﬁcation (x|x)57 , which corresponds to the slopeof the blue lines. The measured and COSY results
were 1.069 and 1.080, respectively. Each green (dashdot) line in Fig. 1 indicates events of the same charge
state transition (ΔQ) at F5. The vertical distances of
the green lines indicate the F5-F7 dispersion (x|δ)57 .
The measured and COSY results were -32.9 mm/% and-34.2 mm/%, respectively. Because the slope of thegreen lines is (x|δ)37 /(x|δ)35 , the green lines become
horizontal due to the achromaticity of the F3-F7 optical system. Indeed, the F3-F7 dispersion of -1.1 mm/%deduced from the slope of the green lines was small.
Charge states are also useful to measure the (�|δ) parameter, where � is a ﬂight-path-length diﬀerence withrespect to the central orbit. Because all the charge
states have the same velocity, � can easily be deducedby the time-of-ﬂight (TOF) measurements. Figure 2shows the TOFs from F3 to F7 as a function of δ
measured by plastic scintillators at F3 and F7 with238
U primary beam. Diﬀerent charge states were generated by the F3 scintillator. BigRIPS was tuned sothat the 90+ charge state came at the center of F3,
F5, and F7. 89+ , 90+ and 91+ charge states were selected individually with slits at the dispersive focus F5.
The (TOF|δ) value was obtained as -0.0498 ns/% by aquadratic ﬁtting, indicated by a solid curve in Fig. 2.
Then, (�|δ) was estimated as -10.2 mm/%, while theCOSY result was -6.53 mm/%.
References1) N. Fukuda et al.: Nucl. Instr. Meth. B317, 323 (2013).

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