Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 53

26

Si beam production and thermal durability test of gas target window

M. J. Kim,∗1 K. Y. Chae,∗1 S. Hayakawa,∗2 S. M. Cha,∗1 N. N. Duy,∗1 G. M. Gu,∗1 M. S. Kwag,∗1 N. R. Ma,∗2
N. K. Uyen,∗1 H. Shimizu,∗2 and H. Yamaguchi∗2∗1∗2Department of Physics, Sungkyunkwan University
Center for Nuclear Study, University of Tokyo- 56 -10090

Time of Flight (ns)The study of the αp process is important for understanding nucleosynthesis in x-ray burst. The process,which is composed of alternating (α, p) and (p, γ) reactions, is considered the bridge between the CNO cycle and rp process. Owing to technical issues, mostof the astrophysical (α, p) reactions on radioactive isotopes are less understood experimentally, including the26Si(α, p)29 P reaction. The energy levels of 30 S fallingin the Gamow window of the reaction at x-ray burst
temperatures (T = 1 − 3 GK, Ex = 10 − 14 MeV) are
crucial for the 26 Si(α, p)29 P reaction rate. Many energylevels were identified through previous works,1) but thespins of levels above Ex = 10 MeV are not constrained inmany cases. Additionally, no energy levels above Ex =12.4 MeV have been measured so far. Thus, we thus plan
to measure the 26 Si(α, α)26 Si elastic resonant scattering.
We simultaneously expect that the 26 Si(α, p)29 P reaction is also directly observable, which requires a rather
high-intensity 26 Si beam. The primary beam intensity
is, however, limited to only approximately 200 pnA toprevent the deposited heat from breaking the F0 gas target window (Havar foil). Therefore, other types of foils
were tested during the beam time.
We took 1.5 days out of the approved 12 days ofbeamtime to test the 26 Si beam production at the Center for Nuclear Study Radioactive Ion Beam Separator (CRIB)2) of the University of Tokyo in December
2019. A primary beam of 24 Mg8+ was accelerated by
the AVF cyclotron at 7.5 MeV/nucleon and with a maximum intensity of 400 pnA, bombarding the cryogenic
3
He gas target of 0.94 mg/cm2 . The 3 He gas was confined with windows of 3-µm-thick Ti foil (ϕ = 20 mm),which has a lower thickness and higher heat conductivity than the commonly used Havar foil, with the expectation of a higher injecting beam intensity. The
beam of the radioactive isotope 26 Si was produced by the3He(24 Mg, 26 Si)n reaction. We installed a 0.2-mg/cm2 thick carbon foil as a charge stripper to obtain a higher
population of the highest charge state 26 Si14+ . The magnetic rigidity was set to 0.61208 Tm to select the desired
beam energy of 5.2 MeV/nucleon. The voltage of thesubsequent Wien filter was ±85.5 kV. A 26 Si purity of59% and a beam intensity of about 4.7 × 104 particles
per second were achieved at the F3 focal plane, which aremuch higher than those of the previously produced 26 Si
beam at CRIB.3) The particle identification plot at F3
is shown in Fig. 1. This improvement may be attributed
to the higher primary beam intensity, the thinner gas
target window foil, the newly installed charge stripper,
and the higher Wien filter voltage.
We also investigated the thermal durability of other26

Si14+807025Al13+

6050-20020F3 PPAC a - X position (mm)Fig. 1. Particle identification spectrum obtained at the F3
focal plane. The 26 Si beam is clearly separated from otherbeam particles.

materials of the gas target window through primarybeam injection. We installed Mo (100% molybdenum,thickness: 3 µm, thermal conductivity: 138 W/m/K,melting point: 2896 K), Ti (100% titanium, thickness:
3 µm, thermal conductivity: 21.9 W/m/K, melting
point: 1941 K), Havar (thickness: 2.5 µm, thermal conductivity: 13 W/m/K, melting point: 1753 K) and CuBe(98% copper, 2% beryllium, thickness: 3 µm, thermal
conductivity: 105 W/m/K, melting point: 1143 K) foils
held by copper frames on a rotating wheel4) in the F0chamber. The temperature of each window was monitored by thermography. The foils were exposed toa 24 Mg8+ primary beam up to an intensity of about2800 enA, at which all the foils were pin-holed on thebeam-spot position but the Mo foil endured. The typical beam size is measured to be about 2 mm. We also
tested those foils mounted on the gas-filled target cellwith liquid-nitrogen cooling-except the CuBe foil. We
exposed the Ti foil to a 2600 enA 24 Mg8+ beam and theHavar and Mo foils to 1800 enA beams. The result showsthat only the Mo foil resisted breakage, suggesting that itis the most durable material. This is consistent with thehighest thermal conductivity and melting point among
the tested materials. We will determine the upper limitof the heat deposition rate through further investigation.
We anticipate that a higher intensity of the secondary
26Si beam could be realized when Mo foils are used at
the F0 gas cell.
References

1) S. Almaraz-calderon et al., Phys. Rev. C 86, 065805 (2012).
2) Y. Yanagisawa et al., Nucl. Instrum. Method Phys. Res.
A 539, 74 (2005).
3) H. S. Jung et al., Phys. Rev. C 85, 045802 (2012).
4) D. Kahl, Master’s thesis, McMaster University (2008).