II-9. Instrumentation

RIKEN Accel. Prog. Rep. 54 (2021)

RIKEN Accel. Prog. Rep. 54 (2021)

Heat durability test of molybdenum foil for the new CRIB cryogenicgas target

A cryogenic gas target was developed and used forover 10 years1) for high-intensity low-energy secondary
beam productions at CRIB (Center for Nuclear Study
RI Beam separator.2) ) The basic design comprises a
stainless-steel gas cell with a 80-mm-long vs. ϕ20-mm
cylindrical aperture, where the target gas is conﬁned
by 2.5-µm-thick Havar foil windows at the beam entrance and exit sides. The gas supply duct and gas cell
are cryogenically cooled by liquid nitrogen. A forcedgas ﬂow system is employed through the entire gas line
to avoid density reduction under high heat deposition
caused by beam irradiation. The maximum intensityof the primary beam irradiation is limited by the heat
durability of the Havar foil, which is known to be 2 W.
Therefore, the gas target is always operated at a suﬃciently low heat deposition of the primary beam onto
the Havar foil windows to avoid damage.
Currently, we are developing a 26 Si RI beam withan intensity of 5×104 pps or higher for future measurements of the α-resonant scattering and 26 Si(α, p)reaction at CRIB (Exp. No. NP1812-AVF55). The
highest beam intensity of 26 Si achieved at CRIB
is 1.2 × 104 pps3) with a 24 Mg primary beam at
7.5 MeV/nucleon and 0.2 particle µA (pµA), which
is limited by the 2-W heat deposition on the Havar
windows. The above goal is possible once we achieve
the maximum heat deposition of 8 W or higher. To
this end, we consider the following design changes: (1)Using a foil material with a higher heat conductivity
and/or higher melting point than those of Havar foil;(2) Redesigning the gas cell with a higher cooling performance.
In the previous test experiment (Exp. No. NP1812AVF55-01), we conﬁrmed the advantages of molybdenum (Mo) and titanium (Ti) over Havar foil. The former has better high-temperature property due to its
higher melting point and heat conductivity, and thelatter has lower heat deposition because the minimum
required vacuum-tight thickness oﬀers smaller energy
loss of the beam passing through it. Next, we tested theheat durability of 2- and 3-µm-thick Mo foil mounted
on the currently used gas target cell in the Detector
Develop Beam Time (Exp. No. DD20-01). The target
cell was ﬁlled with 4 He gas up to 400 Torr, cooled byliquid nitrogen with forced circulation. An 16 O beam
at 6.8 MeV/nucleon was injected into the gas target
with the maximum beam intensity of 1.4 pµA. The
time course of the maximum temperature monitoredby the thermography is shown in Fig. 1 for diﬀerent∗1∗2Center for Nuclear Study, University of Tokyo
National Astronomical Observatory of Japan

Maximum thermography temperature (°C)

S. Hayakawa,∗1 N. Ma,∗1 H. Shimizu,∗1 and H. Yamaguchi∗1,∗280t

Foil pinholed
Hole expanding
Target gas released completely

3 µm Mo, 1.4 pµAt
3 µm Mo, 1.1 pµAt
2 µm Mo, 1.3 pµAt
2 µm Mo, 1.0 pµA
Foil not broken, beam off

7060
50

Foil suddenly
exploded

40

Foil not broken, beam off

30
20

Room temperature
Beam onset
00:00

00:02

00:04

00:0600:08

00:10

Time from beam onset (min)Fig. 1. Time course of maximum temperatures.

foil thicknesses and beam intensities. Note that theread temperature tends to be considerably lower thanthe expected value because of the limited spatial resolution of the thermography for the beam spot size. For
an overall trend, the thicker foil is more easily heated
up because of the larger energy loss of the beam particle than the thinner one. We believe that the processof each foil above 1.3 pµA was diﬀerent; the temperature of the thinner one (blue line) increased gradually and the foil suddenly exploded at some point. In
contrast, the thicker one (red line) showed a pinhole
created right after the beam injection started, and theread temperature was decreasing as the hole expanded
to loose the gas completely at the end. Despite such
diﬀerent trends, the breaking intensity was found to besimilar (1.3–1.4 pµA), which can be attributed to themechanical strength of the thinner foil becoming too
weak to hold the gas even at a lower temperature where
the thicker foil is still stable (even with a pinhole). We
conclude that both the 2- and 3-µm-thick Mo foils canresist a beam intensity of 1.0–1.1 pµA or a heat deposition of 6–7 W at least; thicker foils are preferred forstable operation.
The remaining issue is the more eﬃcient heat releasefrom the Mo foil to the cryogenic gas cell body; thiscan be achieved by a design change (2). This is important to achieve further heat duration (>8 W) andfor a more stable long-duration operation. This will betested by another Detector Development Beam Time(Exp. No. DD20-02).
References1) H. Yamaguchi et al., Nucl. Instrum. Methods Phys. Res.
A 589, 150 (2008).
2) Y. Yanagisawa et al., Nucl. Instrum. Methods Phys. Res.
A 539, 74 (2005).
3) H. S. Jung et al., Phys. Rev. C 85, 045802 (2012).

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