Research Report of Laboratory of Nuclear Science, Tohoku University Vol. 41 (2008)

31(LNS Experiment : #2576)

Development of a Cryogenic Target System for the FORESTExperiments
R. Hashimoto1 , H. Fujimura1 , T. Ishikawa1 , H. Kanda2 , J. Kasagi1 ,S. Kuwasaki1 , M. Sato1 , H. Yamazaki1 , and H. Shimizu1
1

Laboratory of Nuclear Science, Tohoku University, Sendai 982-0826
2

Department of Physics, Tohoku University, Sendai 980-8578

A new cryogenic target production system for a 4π γ-ray detector FOREST has been installed inthe GeV-γ experimental hall. The achieving temperature at the target is 4.5 K. It can maintain solid orliquid H2 /D2 . The target system enables us to make up solid or liquid hydrogen in a short time ∼2 hours
after 3 hour-precooling.

§ 1. A New Cryogenic H2 /D2 Target Production System
We have constructed a 4π γ-ray detector named FOREST [1] in the GeV-γ experimental hall atLaboratory of Nuclear Science. The FOREST is used for the study of elementary reactions such asN (γ, π 0 )X. A hydrogen/deuterium (H2 /D2 ) target is important in our study. We need to construct theH2 /D2 target ﬁtted to the FOREST detector. At ﬁrst, we constructed a solid hydrogen target in a small
vacuum chamber, using a cryostat. Then, we have made many tests under various conditions with a
new vacuum chamber, which is planned to be used in our experiments. We have succeeded in making
a solid hydrogen target. The new cryogenic H2 /D2 target system consists of three parts: a gas coolingsystem, a heat shield defending a cryostat from heat radiation, and a vacuum chamber. Figure 1 showsa schematic view of the system.

CFRP

Heat shield

Target cell
Target pipe
Vacuum chamber

Refrigerator

Fig.1. Schematic view of a target system.

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1.1 Gas cooling system
A gas cooling system is mainly comprised of two parts: a 1000 mm long target pipe and a 2-stage
Gifford-McMahon refrigerator. The target pipe made of aluminium is mounted on the top of the 2ndstage of the refrigerator. The target cell is located at the end of the pipe. A solid or a liquid H2 /D2 target
is made up in the cell.
We used, at ﬁrst, a target pipe made of an A6063 aluminium alloy and a Sumitomo Heavy IndustriesSRDK-408D-W71C cryocooler (31 W cooling power at 40 K for the 1st stage and 1.0 W cooling power at
4.2 K for the 2nd stage). Our target pipe is so long that we can not cool down the cell enough for making
solid H2 /D2 because of thermal conductivity of A6063 and refrigerator’s power shortage, so we replaced
the target pipe and the refrigerator with new ones. A Sumitomo Heavy Industries SRDK-415D-W71C
is used for a new refrigerator. Its cooling power is 35 W at 50 K for the 1st stage and 1.5 W at 4.2 K forthe 2nd stage. We also made a new target pipe made of 4N pure-aluiminium which has 10 times higher
thermal conductivity than A6063. Figure 2 shows thermal conductivity as a function of temperature forsome materials [2]. Pure-aluminium is proper to our target system since it has good thermal conductivity
at our operation temperature ∼10 K.

Pure-Al
Al-alloyFig.2. Thermal conductivities as a function of temperature for some materials. Pure-aluminium
has high thermal conductivity at our operation temperature ∼10 K.

1.1.1 Target cell with the target pipe
A size of the target pipe is 1000 mm long, 65 mm and 61 mm for outer and inner diameters, respectively. A target cell with a thickness of 40 mm is located at the end of the target pipe, and the innerdiameter of the cell is also 61 mm. The center of the cell is 920 mm downstream from the refrigerator.

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Two 12.5 μm aramid ﬁlms are epoxied upstream and downstream of the target cell to separate target region from vacuum. A pre-cooled pipe combines the cell with compressed H2 /D2 gas cylinders. Because ofa high cooling power of the refrigerator and good thermal conductivity of 4N pure-aluminium, achieving
temperature of the target cell is 4.5 K.
1.2 Heat shield
The cryostat is protected from heat radiation by the heat shield having two components: a Cu shield
and a cylindrical Al shield. The Cu shield, mounted on the top of the 1st stage of the refrigerator, is 4 mm
thick and has high thermal conductivity. The cylindrical Al shield, connected to the Cu shield, surrounds
the target pipe. The total length of the Al shield measures 1775 mm. The shield thickness is 0.5 mm
around the target cell.
1.3 Vacuum chamber
The cryostat and the heat shield are placed in the vacuum chamber. The vacuum chamber comprises
a vacuum cylinder and a vacuum duct made of carbon ﬁber reinforced plastic (CFRP) [3]. The target
pipe and the Al shield are placed in the CFRP duct, which is employed to reduce the conversion rate ofoutgoing photons from the target cell. The CFRP has a disadvantage in the large emissivity of thermalradiation. Emissivity is about 1 for CFRP although that is about 0.1 for most of metals. The target pipe
and the heat shield are surrounded with super insulation ﬁlms (a 6.45 μm thick corrugated aluminized
Mylar sheet: an aluminium deposited layer is 0.05–0.06 μm in thickness and a polyester sheet is 6.4 μmthick). They reduce the heat radiation from the CFRP to the heat shield and also from the heat shield tothe target pipe.

§ 2. Gas Handling System
The gas ﬂowing should be controlled ﬁnely at making, holding or vaporizing a cryogenic target byusing some valves, vacuum gauges and vacuum pumps. Vacuum pumps are also used for vacuuming achamber. Figure 3 shows schematic view of a gas handling system with a vacuum system.
The valve A and the scroll pump B (Edwards XDS5 Dry Pump) are used for purging an initial gas
line with a target material. Valves B and C protect a puriﬁer from vacuuming and air inﬂowing. Thevalve D which is a needle valve of a ﬂow meter (Koﬂoc RK-1250) is mainly used for controlling gas
pressure. Gas pressure in the gas line (nearly equal to in the cell) is measured by a vacuum gauge (MKSBaratron 122A) placed between the ﬂow meter and the target cell.
A turbo molecular pump (Oerlikon Leybold Vacuum TURBO VAC 361) is combined with the vacuumchamber and the scroll pump A (Busch Fossa FO-0018B) is used for pre-vacuuming. The degree ofvacuum in the chamber is measured by a Pirani gauge (ULVAC GP-1S with a sensor WPB-10) and aionization vacuum gauge (ULVAC GI-M with a sensor M-13). The vacuum chamber is connected to abuffer tank (always evacuated) through a safety valve. If the pressure in the chamber is 0.05 atm higher
than in the buffer tank, the safety valve automatically opens and the gas ﬂowing from the chamber tothe tank occurs. For discharging a target material gas, the scroll pump B and the valve G are used.

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H2 cylinderBC

PurifierD2 cylinder

N2 cylinderA

Buffer tank

D

Flow meter

EF

Vacuum gauge

Safetyvalve

Vacuumchamber

Opened in target
chamber
Pre-cooled with 1ststage of refrigerator

Target cellScroll pump AScroll pump B

Turbo moleculer pumpGFig.3. Gas handling system.
A compressed nitrogen cylinder is installed on the gas line between the ﬂow meter and the vacuum
gauge. It’s used for breaking a vacuum in the chamber and the gas line. The nitrogen ﬂowing is handledwith valves E and F.

§ 3. Temperature Controlling System
It is important for safety operation to monitor and control temperatures of the target cell and theheat shield. A temperature controller (LakeShore model 331S), a temperature monitor (LakeShore
model 218S), sensors (CX-1030-SDC, CX-1070-SD) and heaters (HTR-25) are used for a temperature
controlling system. Figure 4 shows positions of temperature sensors and heaters. Sensors A and B areused as a reference point to maintain the cell temperature.

Cu shield 251Al shieldBPipeA

4Cell

heater 1 with the cellheater 2 with the gas pipe
Fig.4. Positions of temperature sensors and heaters. To maintain the cell temperature, thesensors A and B are referred.

§ 4. Making and Holding a Cryogenic Target
Thanks to excellent thermal conductivity of 4N pure-aluminium, a cryogenic target can be made up
quickly. Using a temperature control system, both the solid phase and the liquid are selectable. The gaspressure and the cell temperature are kept stable for a long time.

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4.1 Making a cryogenic H2 /D2 target
To make up solid hydrogen, a temperature of the target cell should be less than 10 K. Because ofgood thermal conductivity of 4N pure-aluminuium, the cell temperature is cooled down below 10 K after
3 hour-precooling, and it takes only 2 hours to make up solid hydrogen. We can easily change targets
from hydrogen to empty or deuterium without changing the cooling system. Figure 5 shows a photo of
the solid hydrogen target.

Fig.5. A photo of the solid hydrogen target.
It is important to keep the temperature of a gas-inlet above the triple point of the target material,13.8 K for hydrogen and 18.7 K for deuterium, so that the gas ﬂowing is not blocked by frozen H2 /D2 in
the gas-inlet. The gas pressure should be maintained at a proper value, which should be kept between
the melting and the boiling point. The target material in the gas-inlet should be maintained in theliquid phase by using the temperature controller. For example we make a liquid target, operations areconducted at 14–15 K of temperature and about 240 torr of gas pressure for hydrogen, at 19–21 K andabout 240 torr for deuterium. Figure 6 shows trends of the cell temperatures and the gas pressureduring making the liquid hydrogen target. The temperatures of the cell and the gas pressure were wellcontrolled.
4.2 Holding a cryogenic target
The cryogenic solid target is maintained for a long time because the cooling system has a sufﬁcient
power. The liquid H2 /D2 target can be also kept in safety. Figure 7 shows trends of the temperatures andthe gas pressure of the target cell with liquid deuterium during a week beam time. The heater power iscontrolled with the sensor A attached to a lower part of the target cell.

Acknowledgment
The authors express many thanks to Mr. K. Matsuda. He has given many advices and suggestions
at every stage of this work, and has also supported in manufacturing the target production system. They
are also grateful to Prof. S. Ishimoto from KEK. He has given much information on pure-alminium andmany techniques for making solid hydrogen. They acknowledge Dr. T. Nakabayashi and Mr. Y. Saito who
have contributed to the early stage of this work.

3616860making

200150100
5000

1000 2000 3000 4000 5000

starting operation

10

making completed
and holding liquid

12starting operation

Temperature (K)14

Gas pressure (Torr)

250

makingmaking completed
and holding liquid

Sensor A
Sensor B

1000 2000 3000 4000 5000

Time (sec)Time (sec)Fig.6. Trends of the cell temperatures and the gas pressure during making the liquid hydrogentarget. It takes about 2 hours to make a liquid hydrogen target

164

Sensor ASensor B

19.7
19.6

Gas pressure (Torr)

Temperature (K)

19.9
19.8

19.5
19.4
19.3
19.2
19.1
19.0
0100

200300400Time (sec ×10 )
3500

600

163162
161
160
159
158
157
0100

200300400500Time (sec ×10 )600

3Fig.7. Trends of the temperatures and the gas pressure of the target cell with liquid D2 in aweek. The vertical broken lines are drawn every 24 hours. Each value is stable for a longtime.

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References[1]T. Ishikawa et al.: in this Research Report of LNS, 41 (2008) 7.

[2]

S. Ishimoto: private communication.

[3]

H. Kanda et al.: Research Report of LNS, Tohoku University 37 (2004) 27.