Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 50 (2017)26m

Al beam production with CRIB

D. Kahl,∗1 H. Yamaguchi,∗2 H. Shimizu,∗2 K. Abe,∗2 O. Beliuskina,∗2 S. M. Cha,∗3 K. Y. Chae,∗3 Z. Ge,∗4S. Hayakawa,∗2 M. S. Kwag,∗3 D. H. Kim,∗5 J. Y. Moon,∗6 S. Y. Park,∗5 and L. Yang∗2The γ-ray emission associated with the radioactive
decay of 26 Al is one of the key pieces of evidence indicating stellar nucleosynthesis is an ongoing process
in our Galaxy. Its ground state 26g Al has a half-lifeof 7 × 105 yr owing to its high spin J π = 5+ , andit predominately decays to the ﬁrst excited state in
26Mg, which promptly de-excites by emitting a characteristic 1.809 MeV γ-ray. The situation is complicatedby a low-lying isomeric state 26m Al at 228 keV; thisJ π = 0+ isomer decays with a half-life of 6.3 s directly
to the ground-state of 26 Mg.
Little is known about the 26m Al(p, γ) reaction rate,and in a sensitivity study on 26 Al yields from massive
stars, the (p, γ) rate on the isomeric state was assumed
to be the same as for the ground state1) , despite thelarge spin-parity diﬀerence. At present, there is only
limited information on J π and no proton partial widths
Γp are known for low-spin resonances above the proton
threshold in 27 Si.
In preparation for a measurement of proton elastic scattering, we undertook a two-day machine test
to check the production yield, purity, and phase-spaceparameters of 26m Al at the Center for Nuclear Study
low-energy RIB separator (CRIB)2) . A primary beamof 26 Mg8+ was accelerated to 6.77 MeV/u and impinged on the CRIB cryogenic production target3) ,which was ﬁlled with 260 Torr of H2 gas at 90 K
(0.7 mg·cm−2 ) and sealed with 2.5 μm Havar foils,
producing the RIB of interest via the 1 H(26 Mg, 26 Al)nreaction. These conditions were chosen to optimize
production of 26m Al over 26g Al based on the literaturecross-sectional data4) . A sample particle identiﬁcation
plot at the achromatic focal plane F2 is shown in Fig. 1.
26Al13+ production yield was optimized at 119.6 ± 1.5
MeV. After passing through a Wien ﬁlter, the 26 Al
purity was 80–90% at the experimental focal plane F3,
with 23 Na as the main contaminant. The typical intensity of 26 Al was 1 × 105 pps (at 25 pnA). The setup
at the experimental focal plane consisted of two parallel plate avalanche counters (PPACs), a 7.5 mg·cm−2
CH2 target, and a silicon telescope (SSDs).
A key purpose of the machine test was to determine the isomeric purity 26m Al/26 Al in the cocktail
beam. Although CRIB does not have the capability todistinguish between 26g,m Al for individual ions, a previous measurement of 26g Al(p, p) showed no resonant
∗1∗2
∗3
∗4∗5
∗626

Al13+

120
26Mg

12+
23Al12+

Na11+

2680
26Mg

pileupResidual Energy (MeV)

26100

60

10+

23NaMg

11+

23Na

10+

109+

40

20

11020
30
4050
Relative Flight Time of Radio Frequency (ns)60Fig. 1. Particle identiﬁcation of the beam at the focal plane
F2. The abscissa shows relative ﬂight time and the ordinate shows the residual ion energy. The optical settings used to produce the histogram are not optimizedfor 26 Al but are useful for illustrative purposes.

structure5) . The previous work reduces our inability
to uniquely identify the state of 26 Al event-by-event tobackground subtraction, provided we have a reliable
determination of the isomeric purity.
To determine the isomeric purity, we performed adecay study by pulsing the primary beam in an on/oﬀ
mode with a duty cycle of 12 s. When the beam wason it was implanted into the CH2 target, and when
the beam was oﬀ, we measured β + particles with theSSDs. Considering the long half-life of 26g Al and that23
Na is stable, it is reasonable to assume that all β +particles emitted from the target when the beam wasoﬀ can be associated with the decay of 26m Al (whichdoes not decay by electron capture). Our preliminary
measured half-life for the decay associated with theseparticles is 6.1 s; this value is close to the known 26m Al
half-life of 6.3 seconds considering we have yet to evaluate our error. The shape of our preliminary β + -decay
spectrum was also consistent with the expectation for26m
Al. We tentatively calculate an isomeric purity of≈ 40% from the test run data. We anticipate that thedeveloped 26m Al beam is satisfactory to observe low-proton resonances with large widths in the future.
References
1) C. Iliadis et al., Astrophys. J. Supp 193, 16 (2011).
2) Y. Yanagisawa et al., Nucl. Instrum. Methods in Phys.
Res., Sect. A 539, 74 (2005).
3) H. Yamaguchi et al., Nucl. Instrum. Methods in Phys.
Res., Sect. A 589, 150 (2008).
4) R.T. Skelton et al., Phys. Rev. C 35, 45 (1987).
5) S.T. Pittman et al., Phys. Rev. C 85, 065804 (2012).

School of Physics and Astronomy, University of Edinburgh
Center for Nuclear Study, University of Tokyo
Department of Physics, Sungkyunkwan University
RIKEN Nishina Center
Department of Physics, Ewha Womans University
Institute for Basic Science and Wako Nuclear Science Center- 81 -