Ⅱ-9. Instrumentation

RIKEN Accel. Prog. Rep. 48 (2015)

Evaluation of eﬀects of a large energy deposition on Deuterium
gaseous active target for a high-intensity ion beam injection
C.S. Lee,∗1,∗2 S. Ota,∗1 M. Takaki,∗1 Y. Kiyokawa,∗1 Y.N. Watanabe,∗2,∗3 H. Tokieda,∗1 and J. Zenihiro∗2
A deuterium gaseous active target, CNS Active Target (CAT)1,2) , has been under development for measuring the Isoscalar Giant Monopole Resonance (ISGMR)
in unstable nuclei via the (d, d’) reaction. In order toextract the ISGMR component by using the multipole
decomposition analysis, a wide angular coverage of thedistribution of the diﬀerential cross-section is required.
The CAT consists of GEM-TPC and Si detectors. In
the center-of-mass frame, the scattering angular regionof recoil particle of θCM around 2 degrees was coveredby the GEM-TPC, while the region of θCM ≥ 5 degrees was covered by the Si detectors. In the previous
experiment2) at HIMAC using a 132 Xe beam with ahigh intensity of around 1 MHz at 115-MeV/u incident energy, we experienced instabilities of the CAT; asudden jump of the current value of the ﬁeld cage and aﬂuctuation of the baseline of Si signals were observed.
The instabilities can be considered as the eﬀect of highenergy-loss density in the active gas target caused bya heavy ion beam. For evaluating of such eﬀect, webombarded the CAT with a very intense 3-MeV proton(H+ ) beam provided by a pelletron in RIKEN. In thispaper, the experiment and the results are reported.

Fig. 1. A schematic view of the setup at pelletron, RIKEN.

Figure 1 shows a schematic view of the experimental setup. The CAT was connected to the exitof the pelletron beam port with a 12-µm-thick Almylar foil. An Al ﬂange of the chamber at the exit of
the CAT was used as a Faraday Cup (FC). A copper
plate was employed to collect the charges multipliedby three thick GEMs (THGEMs) for evaluating theeﬀective gas gain. The current outputs of the FC andthe Cu plate were measured using picoammeters (ADVANTEST TR8641). The voltage and current valuesof each high-voltage supply were recorded using a multimeter (KEITHLEY 2701). Four Si detectors with aneﬀective area of 9×9 cm2 and thickness of 0.5 mm weresurrounding the ﬁeld cage. The Si signals were ampliﬁed using a preampliﬁer (mesytec MPR-32) and readout using an oscilloscope. The energy of the H+ beam
was 3 MeV, and its intensity was varied from 1 to 60
∗1
∗2
∗3

Center for Nuclear Study, The University of Tokyo
RIKEN Nishina Center
Department of Physics, The University of Tokyo

pA. The GEM-TPC was operated at an eﬀective gas
gain of about 60 in 0.4-atm H2 .
Baseline ﬂuctuations were observed in the signalsof the Si detectors for a beam intensity of 20-pA, asshown in the left panel in Fig. 2. This beam condition provides an energy-loss density equivalent to a132
Xe beam with an intensity of 1.2 MHz and energy
115 MeV/u. The ﬂuctuation disappeared, when, withthe same beam condition, a 10-µm-thick Al-mylar foil
was placed in front of each Si detector. Therefore, weunderstand that the ﬂuctuation is possibly due to radioactive noise. At 60-pA 3-MeV H+ beam injection,
the ﬂuctuations became smaller.

Fig. 2. Preamplifier outputs of Si detectors for a 20-pA
3-MeV H+ beam injection without Al-mylar foil cover(left) and with Al-mylar foil cover (right).

Figure 3 shows the variation of current in the Cu
plate and ﬁeld cage during the 20-pA 3-MeV H+ beam
injection. The current of the ﬁeld cage increased by
4-5 µA immediately after the injection of the highintensity beam and gradually increased during thebeam irradiation. This may be explained by the effect of the initial ions from the beam and the ion backﬂow (IBF) from the THGEMs. Although the ﬁeld cage
had tripped (the current limit was 1 mA) several times,
a sudden jump of the current value seldom happened.
This may be caused by the diﬀerence between a heavyion and a light-ion, such as δ-ray creation. The eﬀectof δ-rays on both Si detectors with Al-mylar covers
and the GEM-TPC will be evaluated by using a highintensity heavy-ion beam in the near future.

Fig. 3. Current variation of the Cu plate (left) and the fieldcage (right) during a 20-pA 3-MeV H+ injection.

References1) S. Ota et al.: CNS Annual Report 2011, CNS-REP-90(2013) 70-71.
2) S. Ota et al.: CNS Annual Report 2013, CNS-REP-93(2015) 49-50.

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