18

Research Report of Laboratory of Nuclear Science, Tohoku University Vol. 41 (2008)(LNS Experiment : #2578)

Development of a Compact Silica Aerogel Čerenkov Counter
K. Suzuki1 , K. Futatsukawa2 , T. Ishikawa1 , H. Kanda2 , M. Kaneta2 ,H. Kawai3 , M. Kawai2 , T. Kawasaki2 , S. Kuwasaki1 , K. Maeda2 , Y. Okada1 ,H. Shimizu1 , M. Tabata3 , Y. Tajima4 , K. Tsukada5 , and H. Yamazaki1
1

Laboratory of Nuclear Science, Tohoku University, Sendai, 982-0826
234

Department of Physics, Tohoku University, Sendai, 980-8578

Graduate School of Science, Chiba University, Chiba, 263-8522

Networking and Computing Science Center, Yamagata University, Yamagata, 990-8560
5

Advanced Meson Science Laboratory, RIKEN Nishina center, Wako, 351-0198

A compact silica aerogel Čerenkov counter with ﬁne-mesh type photo-multiplier tubes has beendeveloped to reject e+ e− pair created events at a trigger level. The silica aerogel is covered with a highly
reﬂective material to lead emitted Čerenkov photons into the photo-cathodes. The efﬁciency of e+ e−event identiﬁcation is obtained to be more than 97%.

§ 1. Introduction
A magnetic spectrometer has been constructed in the 2nd experimental hall at Laboratory of Nuclear Science (LNS) to study photo-production of the K 0 meson [1], π + π − mesons [2], and the Θ+baryon [3]. Many electrons and positrons are generated by the electro-magnetic interaction in a target.
To keep the data taking efﬁciency at a realistic level for a high intensity photon beam, it is important toveto e+ e− events at a trigger level.
An e+ e− veto counter should be placed just behind the target to response the e+ e− event efﬁciently
since electrons and positrons are bent inside a space with high magnetic ﬂux density. One of π + π −mesons to which a K 0 meson decays has a momentum of 1 GeV/c at maximum. It is necessary thatthe refractive index should be 1.01 or less so that 1 GeV/c π mesons can not emit Čerenkov photons. It
should consist of low amounts of substance so that the emitted particles are not much inﬂuenced withthem, due to the energy loss and multiple scattering.
The available space for the counter thickness is limited and is about 50 mm. To construct thecounter in a compact size with a few materials, silica aerogel has been selected. Diffuse reﬂectors havebeen also selected to let the Čerenkov photons reach a photon detecting device. Fine-mesh type photomultiplier tubes (PMT) are employed since the magnetic ﬂux density measures 0.4 T near the target. We
have selected the PMT with a quartz window, Hamamatsu H8409-70QB , because the most Čerenkov
photons have a short wave length.

19§ 2. Aerogel Čerenkov counter
The size of each silica aerogel piece is 132 (H) × 132 (W) × 22 (T) mm3 and its refractive index
is 1.01. Two pieces are stacked to generate Čerenkov photons enough to get a high efﬁciency of e+ e−
veto. Four PMTs are coupled to the aerogel in such a way that most of the charged particles produced
at the target can not go through the PMT materials. The two of them are located on the top of it, andthe others are on the bottom. Aerogel pieces are covered with a diffuse reﬂector to make the emittedČerenkov photons go into photo-cathodes of PMTs.
We have tested two types of diffuse reﬂectors. One is a Millipole membrane ﬁlter, HAWP00010,
with a thickness of 150 μm, a pore size of 0.45 μm, and a porosity of 79%. The other is a GORE-TEX
DRP backlight reﬂector sheet with a thickness of 500 μm. The whole counter is wrapped with a sheet of150 μm thick black paper [4]. Figure 1 shows the aerogel Čerenkov counter we made.

AerogelPMTPMTReflector

PMTPMTFig.1. The silica aerogel Čerenkov counter. The left and right panels show the counter before
and after assembly, respectively.

§ 3. Experiment
The detection efﬁciency for the counter has been measured by using a 457 MeV electron beam [5], atLNS, which is able to produce Čerenkov photons. Two experimental sets, with aerogel and without aerogel, have been performed. The experimental setup is shown in Fig. 2. We have detected small amountsof Čerenkov photons without aerogel because Čerenkov photons are also generated in the air [6]. Another experiment has been carried out with a 80 MeV proton beam at Cyclotron Radio Isotope Center(CYRIC), Tohoku University [7], to conﬁrm no detection of Čerenkov photons due to low energy protons.
We have used two scintillating ﬁber hodoscopes, x and y, to make a trigger signal, which is acoincidence signal between x and y as
(x ﬁber OR) ⊗ (y ﬁber OR).
The pedestal data have been also taken by a clock trigger.

(1)

20

Fiber(x)yzx
Fiber(y)

PMT 1
PMT 2

PMT 3
PMT 4eFig.2. Experimental setup at LNS. Two scintillating ﬁber hodoscopes are used for a trigger
counter. The silica aerogel Čerenkov counter is located in front of the two hodoscopes.

§ 4. Data analysis
ADC data of four PMTs have been obtained under every experimental setup. Figure 3 shows ADC

Counts/channel

distributions for electrons with a GORE-TEX sheet where aerogel is placed in the counter.

1010PMT 13PMT 2PMT 3PMT 421010500 1000 1500 0500 1000 1500 0ADC500 1000 1500 0500 1000 1500

Fig.3. ADC distributions for electrons with a GORE-TEX sheet where aerogel is placed in thecounter. The dotted histograms show ADC distributions with the ﬁber trigger, and thesolid histograms below 300 show the distributions with the clock trigger.

Each peak of pedestals corresponding to solid histograms has been ﬁtted by a gauss function. The
peak value μ and standard deviation σ of the ﬁtting function have been determined. Two counting ratios
are deﬁned as
R1 =
R2 =

NADC>μ+3σ
× 100, and
NTrig
multi
NADC>μ+3σ

NTrig

× 100.

(2)
(3)

multi
NTrig is the total number of taken events. NADC>μ+3σ or NADC>μ+3σ
is the number of events of which

21”one” or ”more than one” ADC value exceeds μ + 3σ. In Table 1, the counting ratios R1 and R2 aresummarized.

R1R2Table 1. The counting ratio R1 and R2 for the electron and proton beams.
for electrons
for protons
Millipole ﬁlter GORE-TEX sheet Millipole ﬁlter GORE-TEX sheet
w/ aerogel
97.9±0.3
99.1±0.3
13.0±0.1—
w/o aerogel
30.9±0.274.5±0.2
15.5±0.1—
w/ aerogel
82.6±0.3
90.7±0.3
0.7±0.0—
w/o aerogel
4.4±0.1
33.0±0.1
1.0±0.0—The ratio R2 with aerogel and the Millipole ﬁlter gives the lowest efﬁciency 0.7% for the protonbeam, and 82.6% for the electron beam. In the planned experiments, e+ e− pairs pass through theČerenkov counter even in the magnetic ﬁeld, since it is placed just downstream of the target. Therefore
the e+ e− detection efﬁciency would be about 97% for the counter with the Millipole ﬁlter and 99% with
the GORE-TEX sheet. The determination method R2 , in which multiple response events are used, issuitable for rejecting e+ e− events without rejecting proton events.

§ 5. Conclusion
The detection efﬁciency for one electron is 82.3% for the counter with the Millipole ﬁlter and 90.7%
with the GORE-TEX sheet. The fraction of which protons are mistaken for electrons is 0.7% with theMillipole ﬁlter. The efﬁciency for e+ e− events would be 97% with the Millipole ﬁlter and 99% with theGORE-TEX sheet.

Acknowledgments
We are grateful to Prof. Y. Sakemi and Dr. M. Itoh for assistance of the test experiment at CYRIC.
One of the authors (T. I.) acknowledges the ﬁnancial support from Tohoku University: Dean’s Grant for
Exploratory Research (Graduate School of Science).

References[1]

K. Tsukada et al.: Phys. Rev. C 78 (2008) 014001; K. Tsukada et al.: Research Report of Laboratory of Nuclear Science 38 (2005) 1.

[2]

K. Hirose et al.: Research Report of Laboratory of Nuclear Science 39 (2006) 7.

[3]

T. Ishikawa: talk at LNS Nuclear Working Group Meeting (2005); T. Ishikawa: talk at LNS Nuclear
Working Group Meeting (2006).

[4]

K. Suzuki: Internal GeV-γ analysis note No. 49 (2007).

[5]

T. Ishikawa et al.: Research Report of Laboratory of Nuclear Science 39 (2006) 39; T. Ishikawa:Internal GeV-γ analysis note No. 78 (2008).

[6]

K. Suzuki: Internal GeV-γ analysis note No. 52 (2007).

[7]

K. Suzuki: Internal GeV-γ analysis note No. 51 (2007).