Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 50 (2017)

Isochronism of Rare RI Ring
Y. Yamaguchi,∗1 M. Wakasugi,∗1 Y. Abe,∗1 A. Ozawa,∗1,∗2 F. Suzaki,∗1 D. Nagae,∗1 S. Ohmika,∗1,∗3 S. Naimi,∗1
∗1,∗3
∗1,∗2
∗1,∗3
∗1,∗3
∗1,∗2
∗1 ∗1
∗1,∗3
Suzuki,
Uesaka,Yamaguchi
for forthethe
Rare
RI RI
Ring
Collaboration
Z. Z.
Ge,Ge,S. S.
Suzuki,T.T.
Uesaka,T.T.
Yamaguchi,
andRareRing
Collaboration
A precise isochronous condition for reference particles is essential to precisely determine the mass ofnuclei using the Rare RI Ring (R3)1) ; therefore, oneof the main purposes of a machine study for R3 is theisochronous tuning with beams. An isochronous ﬁeld
is formed by a combination of the edge-angle and gradient magnetic ﬁeld of dipoles that are equipped with10 trim coils.
In the ﬁrst machine study, a ﬁrst-order isochronousﬁeld tuned to a 78 Kr primary beam was successfully
formed by adjusting the trim coils. The time-of-ﬂight(TOF) spectrum after extraction as a function of momentum can be used to verify the isochronism, as
shown in Fig. 1. Although it is natural that the sec-Fig. 2. TOF spectra of 78 Ge after extraction as a functionof momentum. Upper and lower ﬁgures show the results
obtained before and after adjusting the isochronism by
the second order, respectively.

Fig. 1. TOF spectrum of 78 Kr after extraction as a functionof momentum. The red line shows a quadratic ﬁtting
curve.

ondary component remains in the distribution, theTOF width is broad overall. Because TOF width affects the experimental uncertainty of mass determination, it must be reduced as much as possible. Before
conducting the third machine study,2) we investigatedthe cause of the TOF broadening using simulation. Itwas found that the non-uniformity of the pole face ofdipoles inﬂuence the TOF width. However, it could
be improved by correcting the secondary component
remaining in the distribution.
In the third machine study, we performed theisochronous tuning with 78 Ge, which was produced by
in-ﬂight U ﬁssion. The upper panel of Fig. 2 shows theTOF of extracted particles as a function of momentum
before adjusting the isochronism by the second order.
The full width of the TOF is almost as large as that of78
Kr case. From this ﬁgure, it seems that isochronism
is not satisﬁed on the high momentum side, in contrast
∗1
∗2
∗3

RIKEN Nishina Center
Institute of Physics, University of Tsukuba
Department of Physics, Saitama University- 183 -

to the low momentum side; thus, we tuned mainly theouter trim coils of R3. This resulted in the disappearance of the secondary component and the reductionof TOF width simultaneously as shown in the lower
panel of Fig. 2. Consequently, the isochronism withthe full momentum of 0.6% was improved. The TOF
width after projection on the y-axis direction is 3.6 ns
in sigma, and therefore, the degree of isochronism isapproximately 5 ppm.
This value is at the level of the main magneticﬁeld ﬂuctuation of R3 during the measurement timeof about 1 day. There is no strong correlation between
the TOF data and the ﬂuctuation of magnetic ﬁeld;
however, it may be necessary to reduce the ﬂuctuationof the magnetic ﬁeld to further improve the isochronism. Another possibility is that the isochronism willfurther improve by third- or higher-order correction.
The optimum value would be 1 ppm or less, which isquite challenging and requires step-by-step improvement.
References
1) A. Ozawa et al., Prog. Theor. Exp. Phys. 2012, 03C009(2012).
2) D. Nagae et al., In this report.