Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, Japan

PASJ2015 FROM13

PRESENT STATUS OF MUON ROTATING TARGET AT J-PARC/MUSE*Shunsuke Makimura, Shiro Matoba, Naritoshi Kawamura, KEK, Ibaraki, Japan
Satoshi Onizawa, Yukihiro Matsuzawa, NAT, Ibaraki, Japan
Masato Tabe, Seekel, Ibaraki, Japan
Yasuo Kobayashi, Hiroshi Fujimori, Yutaka Ikedo, Akihiro Koda, Kenji Kojima, Yasuhiro Miyake,Jumpei Nakamura, Koichiro Shimomura, Patrick Strasser, Ryosuke Kadono, KEK, Ibaraki, JapanAbstractA pulsed muon beam with unprecedented intensity will
be generated by a 3-GeV 333-microA (1-MW) proton
beam on a muon target made of 20-mm thick isotropic
graphite
atJ-PARC/MUSE(MuonScience
Establishment). The energy deposited by the 1-MW
proton beam is estimated to be 3.9kW on the muon target.
The first muon beam was successfully generated in
September of 2008. Gradually upgrading the beam
intensity, continuous 300-kW proton beam has been
operated by a fixed target method without replacements
till June of 2014. However, the lifetime of the fixed target
will be less than 1 year by the proton-irradiation damageof the graphite in case of 1-MW proton beam operation.
To extend the lifetime, the muon rotating target, in which
the radiation damage is distributed to a wider area, hadbeen developped. In the rotating target, the lifetime ofbearing will have a dominant influence on the lifetime of
the muon target. The disulfide tungsten are introduced assolid lubricant of the bearings. The muon rotating target
was installed in September of 2014 and has been stableyutilized up to 500-kW proton beam operation. In thisreport, the present status of the muon rotating target will
be described.

activated by proton irradiation, the handling of the usedmuon target must be performed at a remote handlingroom, Hot cell [10]. The remote controlled tasks require a
lot of time, cost and manpower. Therefore the extensionof the lifetime is essential for stable operation of ourfacility. To extend the lifetime, the developments of themuon rotating target, in which the radiation damage isdistributed to a wider area, had been started since 2008 inparallel with the proton beam operation. While thelifetime of graphite becomes long enough, the lifetime of
the bearing is supposed to be the critical issue for themuon rotating target. The development of the muon
rotating target has already been completed as the resultsof the analyses, the detailed designs, and durability tests
with a heating and rotating mock-up. The muon rotating
was installed in September of 2014. Figure 1 shows
pictures of the rotating target assembly on the left and therotating target on the down-right. The schematic cutting
section is shown on the up-right. In this article, the muon
rotating target will be described.

INTRODUCTION
The muon production target is placed on the upstreamof the neutron mercury target at J-PARC/MLF/MUSE(Japan Proton Accelerator Research Complex/ Materials
and Life Science Experimental Facility/ Muon ScienceEstablishment) [1, 2]. A pulsed muon beam with
unprecedented intensity will be generated by a 3-GeV
333-microA proton beam on a muon target made of 20mm thick isotropic graphite, IG-430 (Toyo Tanso Co.
LTD. [3]). The energy deposited by the 1-MW proton
beam is estimated to be 3.9kW on the muon target by
PHITS [4, 5]. The first muon beam was successfully
generated on September 26th, 2008. Gradually upgrading
the beam intensity, 300-kW proton beam has been
operated continuously since January of 2013 [6]. The
current muon target with a fixed target method, hereafter
described as “muon fixed target”, has been utilized
without replacements since the first muon beam
generation till May of 2014. Proton irradiation gives
radiation damage to material properties of graphite [7, 8, 9].
In particular, the effect on the dimension of graphite isserious. The lifetime of the muon fixed target is estimatedto be 1 year by the simulation under the 1-MW proton
irradiation on graphite. Because the muon target is highly

Figure 1: Pictures of the rotating target assembly on theleft and the rotating target in the down-right. The
schematic cutting section is shown in the up-right.

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Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, Japan

PASJ2015 FROM13

DEVELOPMENTS OF ROTATINGTARGET
Basic Design of Rotating Target
The current muon fixed target will break down by theradiation damage of the graphite in less than 1 year by 1MW proton beam on the simulation. To extend thelifetime, the developments of the muon rotating target, inwhich the radiation damage is distributed to a wider area,
had been started since 2008 in parallel with the proton
beam operation. A horizontal shaft, which is a rotation
axis of the rotating target, is parallel to the proton beamline. Because a motor device of the rotating target must belocated 2.4 m above the beam line level due to highradiation, the rotating motion is transmitted into thehorizontal shaft through a vertical long shaft and a pair ofbevel gears. The rotating body is composed of a graphitewheel, a wheel support, and the horizontal shaft supported
by two horizontal bearings. The two bearings are attached
to a cooling jacket in which water piping is embedded.
The outer and inner diameters of the graphite wheel are
336 mm and 230 mm, respectively. The temperature of
the graphite wheel was estimated to be 940 Kelvins. The
temperature of the hottest bearing was 390 Kelvins. Therotation speed of the rotating target could be determinedfrom the evaluation of the maximum temperature
gradients inside the graphite. Consequently, it wasdetermined to be 15 rounds per a minute (abbreviated asrpm afterwards). For the rotating target, the bearings
supplied from JTEKT CO., LTD [11] were selected,
considering the actual performance in the past
accelerators field.

irradiation test of the WS2 by an electron beam wasperformed. In the irradiation test, 80 pieces of the sinteredcompacts of the WS2 were irradiated by 2-MV, 1-mA
electron beam in Takasaki Advanced Radiation ResearchInstitute, Japan Atomic Energy Agency [12]. The lubricants
were disassembled from the bearings and were set in jigs
for irradiations. The jigs were placed in a vacuum vessel
with pressure of 5 Pa. During the electron irradiation, thetemperatures of the jigs were monitored to be from 310 to340 Kelvins. The radiation damage given to the pieces oftheWS2 can be evaluated from 50 to 100 MGy throughthe pre-irradiation with CTA dose-meter. The irradiatedpieces of the WS2 were assembled into 8 bearings again.
Then the durability tests under the similar conditions tothe beam line were performed in JTEKT. Then noirradiation effect was observed in the durability tests.
Figure 2 shows the pictures of the disassembled bearing
on the up-left, the irradiation jig including only thecompacts of the solid lubricant on the up-right, and theelectron-irradiation apparatus on the down.

Bearings and Solid Lubricants
When the rotating target method is applied, the lifetimeof graphite will be more than 30 years. Then, the lifetimeof bearing will have a dominant influence on the lifetimeof the muon target. The bearings of the rotating target arelocated in high vacuum of 10-4 Pa, at high temperature of390 Kelvins, and under high radiation dose of 100
MGy/year. Though disulfide molybdenum or silver isgenerally used in these conditions, the sintered compact
of disulfide tungsten is used for our target. Hereafter,disulfide molybdenum, silver, and disulfide tungsten areabbreviated as MoS2, Ag, and WS2 respectively. In caseof the MoS2 or the Ag, it is coated on the components ofthe bearing, such as balls, an inner ring, an outer ring, andretainers. On the other hand, the sintered compact of WS2
is inserted as separators of the balls. In case of the WS2,
the quantity of the lubricants is expected to be muchlarger than the MoS2 and the Ag. Actually, according to
the evaluation formula in JTEKT catalogue, the lifetimes
for our bearings in cases of the MoS2, the Ag, and theWS2 are 1100 hours, 5800 hours, and 110000 hours,respectively. Actually the proton beam has been operated
for 5000 hours in a year. In case of the WS2, the lifetimeof the bearings can be evaluated to 22 years. However, it
has not been proven that the WS2 is used as the solid
lubricant under high radiation dose. Therefore, anFigure 2: Pictures of the disassembled bearing on theup-left, the irradiation jig including only the compactsof the solid lubricant on the up-right, and the electronirradiation apparatus on the down.

Heating and Rotating Tests by Mock-up
A mock-up, which could perform the rotating andheating tests, was fabricated to confirm the durability ofthe rotating target and the lifetimes of the bearings. In themock-up, the maximum temperature of the bearing, themaximum temperature of the graphite, and the- 262 -Proceed iA unggutemperature difference inside the wheel support are larger
than the ones in the actual beam line to confirm thedurability of the rotating target and the lifetimes of thebearings. As the results of simulations for both cases, thespecifications of the sheath heater, which covered thewhole graphite was decided. In the actual beam line, thetemperature of the bearing and the graphite wheel, and thetemperature difference inside the wheel support wereevaluated to be 390 Kelvins, 940 Kelvins, and 550Kelvins, respectively. In the mock-up, they were 400
Kelvins, 1020 Kelvins, and 620 Kelvins, respectively. Therotation speed can be as slow as 15 rpm in the beam line.
However, if the heating and rotating tests were performedat the speed, it would take one year to confirm whether
the lifetime of the bearing is one year. Therefore, in themock-up, the rotating target could be rotated at amaximum speed of 500 rpm. The resistance of thebearings was carefully monitored through the current of
the AC servo-motor for the rotation. From the incrementof the motor current, the wear of the bearing can beknown. Finally the continuous heating and the rotating
test at a speed of 300 rpm for 220 hours was successfullycompleted. The accumulated number of the revolutions in
the tests corresponded to the number at the actual
operation speed of 15 rpm for 4400 hours. Thus, thedevelopments, such as the analyses, the detailed designs,
and durability tests with a heating and rotating mock-up
were completed [13].

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Furthermore, the 500-kW and the 600-kW operation had
been performed for 2 weeks and for 1 hour, respectively.
During the proton beam operation, the temperatures of thecooling jacket, the horizontal shaft, and the graphite aremonitored. Because it is difficult to measure thetemperatures of the rotating body directly, the temperaturerises of the thermos-couples through thermal radiation arerelatively monitored. The temperature of the horizontal
shaft is continuously monitored by inserting
thermocouples into a hollow core at the center of theshaft. When the thermocouples were calibrated without
rotation and a direct thermal contact in the mock-up, themeasurement through the thermal radiation is similar tothat with the thermal contact. The temperatures of thegraphite are also measured by the thermos-couples
through thermal radiation. The thermos-couples areisolated by thermal shielding and are irradiated only by
the graphite. However the effect of the heat generation by
proton beam irradiation must be carefully confirmed withthe actual operation. Table 1 demonstrates the comparisonof the measurement with the simulation for the typicalbeam power.

INSTALLATION OF ROTATING TARGETAND OPERATON BY THE TARGETThe muon fixed or rotating target is rigidly fabricatedinto the target rod. The target rod is integrated into theplug shield through four screws, which can be handled
remotely. The assembled components are called target
assembly. Because the muon target is highly activated byproton irradiation, the used muon target must be
controlled by remote handling. The used target assembly
will be transported by the shielding vessel, named muontransfer cask, and stored in the tentative storage pod at theLarge Apparatus Handling Room, which is located at thefloor level of 10 m [10]. Because the muon target must stayin vacuum for corrosion control, storage vessels for themuon target assembly were prepared. The muon fixedtarget, which had been used for five years, wassuccessfully replaced with a new rotating target in
September of 2014. Then, the used muon fixed target wasactivated up to 400 mSv/hour with 20-cm distance.
Simultaneously, there was no crack on the target throughobservation of remote-controlled digital cameras. Figure
3shows pictures of the muon fixed target, which had beenutilized for five years on the up-left, the storage vessels
on the up-right, and the replacement by the muon transfer
cask on the down. Since then, the proton beam operationby the muon rotating target has been successfully
performed without the increment of the rotation-motor
current. So far, the 300-kW and 400-kW proton beam
operation had been performed for three months.

-Figure 3: Pictures of the muon fixed target, which hadbeen utilized for five years on the up-left, the storage
vessels on the up-right, and the replacement by the muontransfer cask on the down.

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Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, Japan

PASJ2015 FROM13

Table 1: The Comparison of the Measurement with theSimulation for the Typical Beam Power
Shaft
Simulation
Shaft
Measurement
Graphite
Simulation
Grap. (T. Rad.)
Measurement

300 kW

500 kW

1 MW

71 degC

84 degC

115 degC

78 degC

95 degC-400 degC

475 degC

620 degC

45 degC

60 degC-assembly, JTEKT CO., LTD for selection of the bearings,and Dr. Ohshima and Mr. Hanaya for giving the valuableadvices with respect to the electron-irradiation tests inTakasaki Advanced Radiation Research Institute, Japan
Atomic Energy Agency.

REFERENCES

CONCLUSION
The muon fixed target had been utilized without
replacements since the first muon beam generation till
June of 2014. However, the lifetime of the muon fixed
target will be less than 1 year by the 1 MW protonirradiation damage of graphite. To extend the lifetime, themuon rotating target, in which the radiation damage isdistributed to a wider area, had been developed. Themuon rotating target was installed in September of 2014.
Since then, the proton beam operation by the muon
rotating target has been successfully performed without
the increment of the rotation-motor current.

ACKNOWLEDGMENT
We would like to thank Hitachi-Zosen CO., LTD [14] fordesigning the heating and rotating mock-up, Irie-Koken
CO., LTD [15] for designing the muon rotating target[1] http://msl.kek.jp/folder_Eng/index_en.html[2] S. Makimura et al.: Nucl. Instrum. Methods Phys.
Res. A 600 (2009) 146.
[3] http://www.toyotanso.co.jp/index_en.
[4] H. Iwase, K. Niita, T. Nakamura: J. Nucl. Sci.
Technol. 39 (2002) 1142.
[5] N. Kawamura et al.: Nucl. Instrum. Methods A 600(2009) 114.
[6] S. Makimura, et al.: Proceedings of the 8th Annual
Meeting of Particle Accelerator Society of Japan,(2011) 1188.
[7] S. Makimura et al.: J. Nucl. Mater. 450 (2014) 110.
[8] H. Matsuo: Graphite 150 (1991) 290.
[9] T. Maruyama et al.: Journal of Nuclear Materials
195(1992)44.
[10] S. Makimura et al.: Proceedings of the 7th Annual
Meeting of Particle Accelerator Society of Japan,(2010) 479.
[11] http://www.jtekt.co.jp/e/index.html[12] http://www.taka.jaea.go.jp/index_e.html[13] S. Makimura et al., J. Radioanal. Nucl. Chem., online first, DOI: 10.1007/s10967-015-3949-0.
[14] http://www.hitachizosen.co.jp/english/index.html[15] http://www.ikc.co.jp/e_index.html- 264 -