ARTICLE IN PRESS

Nuclear Instruments and Methods in Physics Research A 521 (2004) 367–377

Studies of proximity focusing RICH with an aerogel radiatorusing ﬂat-panel multi-anode PMTs (Hamamatsu H8500)
T. Matsumotoa,*, S. Korparb, I. Adachic, S. Fratinab, T. Iijimad, R. Ishibashie,
H. Kawaie, P. Križanb, S. Ogawaf, R. Pestotnikb, S. Saitohc, T. Sekia,
T. Sumiyoshia, K. Suzukic, T. Tabatae, Y. Uchidaf, Y. UnnoeaDepartment of Physics Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo, Japanb
Jožef Stefan Institute, Ljubljana, SloveniacHigh Energy Accelerator Research Organization (KEK), Tsukuba, Japand
Nagoya University, Nagoya, Japane
Chiba University, Chiba, JapanfToho University, Funabashi, Japan

Received 25 August 2003; received in revised form 10 November 2003; accepted 12 November 2003

Abstract
A proximity focusing ring imaging Cherenkov detector using aerogel as the radiator has been studied for upgrading
the Belle detector at the KEK-B-factory. We constructed a prototype Cherenkov counter using a 4  4 array of 64channel ﬂat-panel multi-anode PMTs (Hamamatsu H8500) with a large effective area. The aerogel samples were madewith a new technique to obtain a higher transmission length at a high refractive index ðn ¼ 1:05Þ: Multi-channel PMTs
are read-out with analog memory chips. The detector was tested at the KEK-PS p2 beam line in November, 2002. To
evaluate systematically the performance of the detector, tests were carried out with various aerogel samples using pion
beams with momenta between 0.5 and 4 GeV=c: The typical angular resolution was around 14 mrad; and the average
number of detected photoelectrons was around 6. We expect that pions and kaons can be separated at a 4s level at
4 GeV=c:
r 2003 Elsevier B.V. All rights reserved.
PACS: 29.40.Ka
Keywords: Aerogel; Flat-panel PMT; Ring imaging Cherenkov counter; Proximity focusing; Particle identiﬁcation; Belle

1. Introduction
Silica aerogel is a unique material with arefractive index ðnÞ in the range between gases*Corrsesponding author. Tel.: +81-426-77-2500; fax: +81426-77-2483.
E-mail address: matumot@bmail.kek.jp (T. Matsumoto).

and liquids or solids. Its refractive index can beeasily controlled from n ¼ 1:01 to 1.06. As a result,
the refractive index of the aerogel can be chosensuch that for a given momentum interval in the few
GeV=c region charged pions radiate Cherenkov
photons, while kaons stay below the Cherenkov
radiation threshold [1]. In the Belle experiment at
KEK [2], a threshold type Cherenkov detector

0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2003.11.384

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T. Matsumoto et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 367–377(Belle-ACC) [3] which uses aerogel as a radiator, isoperated, providing at 3:5 GeV=c a kaon identiﬁcation efﬁciency of 88% with a pion misidentiﬁcation probability of 8% [4].
A new production method of hydrophobic
aerogel with a high-transmission length and n in
the interval between 1.01 and 1.03 was developed
during the construction period of Belle-ACC [5].
The improvement in quality allows the use of an
aerogel radiator in a ring imaging Cherenkov
counter (RICH) [6]. In the HERMES experiment at
DESY, a RICH counter is used with a dualradiator (aerogel and gas), and mirrors to focus the
Cherenkov photons [7]. A similar detector is also
designed for the LHCb experiment at CERN [8].
We are studying the feasibility of a RICH
counter with an aerogel radiator for the BelleACC in the forward end-cap region [9]. Since thispart is now optimized for the pion/kaon separation needed for tagging of the B ﬂavor, andcovering the momentum range below 2 GeV=c;
separation at high-momentum region of around
4 GeV=c is not adequate. This kinematic region is,
however, very important for the studies of twobody decays such as B-pp; Kp: In order toachieve a p=K separation for a wider momentum
range, a ring imaging-type of detector is needed.
Due to spatial restrictions, such a counter has to
be of the proximity focusing type. To cover theidentiﬁcation in the lower momentum region(around 0:7 GeV=c) as well as in the region up to
4 GeV=c; the aerogel has to have a refractive index
around n ¼ 1:05: The ﬁrst beam test of such a
detector was carried out in 2001 at the KEK-PS p2
beam line [10]. These tests used an array of multianode PMTs (Hamamatsu R5900-M16) for photodetection. The detected number of photoelectrons
was 2.7 per ring for a 2 cm thick aerogel tile withn ¼ 1:05; and the Cherenkov angle resolution perphoton was 10 mrad: These results were consistent
with expectations. The number of detected
photons was, however, rather low, partly because
only 36% of the detector surface was covered by
the photo-cathodes, and partly because the transmission length of the aerogel with n ¼ 1:05 could
not be made large enough. For the second beam
test, we improved the aerogel transmission by
optimizing the materials used in the production

process. The active area fraction of the photon
detector was increased by employing recently
developed ﬂat-panel PMTs, Hamamatsu H8500.
Although this type of PMT is not immune tomagnetic ﬁeld, and therefore cannot be applied in
the Belle spectrometer, we consider this device as
an intermediate step in our development. The
paper is organized as follows. We ﬁrst present theexperimental set-up with ﬂat-panel PMTs, brieﬂyreview the improvement in aerogel production,
describe the measurements, and ﬁnally discuss theresults.

2. The experiment set-up
2.1. Flat-panel PMT
The photon detector for the tested prototype
RICH counter employed 64 channel multi-anode
PMTs (Hamamatsu H8500, so called ﬂat-panel
PMT) because of their large effective area. Sixteen
PMTs were used in a 4  4 array and aligned witha 52:5 mm pitch, as shown in Fig. 1. The surface ofeach PMT is divided into 64 ð8  8Þ channels witha 6:0  6:0 mm2 pixel size. Therefore, the effective
area of photon detection is increased to 84%. At
the back of each PMT, an analog memory board is
attached to read out multi-channel PMT signals,
as described below. Among 16 PMTs, 8 PMTs
were delivered in January, 2002, and the remaining
PMTs were delivered in October, 2002. Since the
manufacture method of the PMT was still underdevelopment, they exhibit a large variation inquantum efﬁciency and gain. The quantum efﬁciency at 400 nm varies between 16% and 25%;
the gain varies from 1  106 to 6  106 when the
maximal allowed high voltage of 1100 V isapplied to the photo-cathode.1 The PMTs fromthe later batch show a slightly better performance.
2.2. Aerogel radiator
The hydrophobic form of the aerogel
radiator from Novosibirsk [11] is known to havea long transmission length. However, we prefer1

Hamamatsu Photonics K.K.

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60

Transmission length (mm)

50

40

30
Matsushita (DMF)
Matsushita (MtOH)

20

Chiba-U. (MtOH)
10

Novosibirsk (PrOH)
2001 year (MtOH)

0

1

1.01

1.02

1.03

1.04

1.05

1.06

1.07

Refractive index

Fig. 1. Photon detector, an array of 16 H8500 PMTs, mounted
at a 52:5 mm pitch.

hydrophobic aerogel to the hydrophilic one for theapplication to a collider experiment. With a low
refractive index ðn ¼ 1:0121:03Þ; such an aerogel
was developed for the Belle-ACC, and is characterized by a high-transmission length (B40 mm
at a wavelength of 400 nm) which was notachieved before. However, the transmission lengthof aerogel with a higher refractive index of n ¼
1:05 fell below one-half the value compared to the
aerogel with n ¼ 1:03: Therefore, we reexamined
the aerogel production technique in a joint
development with Matsushita Electric WorksLtd. As a result, we found that the importantfactors determining the transmission length are thesolvent and selection of the precursor to be used
for its production. Originally, we used methylalcohol for the solvent, and methyl-silicate as aprecursor [5]. When we applied di-methyl-formamide (DMF),2 and changed the supplier of the
precursor, we could improve the transmission
length of the aerogel.

2
Matsushita Electric Works Ltd. has a Japanese patent (No.
2659155) for usage of DMF as solvent to make aerogel.

Fig. 2. Transmission length at 400 nm and refractive index at
405 nm for the aerogel samples used in the test.

Fig. 2 shows the refractive indices of aerogel andthe relation to transmission length for samples
which were used in this beam test. The refractive
index was determined by measuring the deﬂection
angle of laser light (laser diode: 405 nm) at acorner of each aerogel tile; the transmission length
was measured with a photo-spectrometer (Hitachi
U-3210). In addition to the samples produced with
the new technique at Matsushita Electric WorksLtd. and Chiba University, samples from BINP
(Novosibirsk) were tested [11]; for comparison, we
also tested the samples used in the previous beam
test. The thicknesses of the prepared aerogel
samples ranged from 10 to 25 mm: Various
thicknesses of up to about 50 mm were tested bystacking these samples. Note that in the productionof the aerogel samples at BINP propenol was used as
the solvent, and the resulting aerogel was hydrophilic. Also note that the Matsushita aerogel samples
produced with the new technique have a very similartransmission length as the BINP samples. The
transmission length for nB1:05 samples used in
the ﬁrst beam test was around 15 mm; but wasincreased to 45 mm for the Matsushita sample with
the new production method.

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2.3. Beam set-up
For the beam test, pions with momenta between
0.5 and 4 GeV=c were used. Besides the RICH
detector under study, counters for triggering,
tracking and particle identiﬁcation were employed.
The set-up of the aerogel RICH is shown in Fig.
3. Two RICH counters were placed in a lightshield box and tested simultaneously. Each RICH
was composed of a layer of aerogel radiator and aphoto-detection plane, parallel to the radiator faceat a distance of 20 cm: The upstream Cherenkov
counter was the detector under study; the downstream counter was the one employed in theprevious beam test. Since the latter uses a wellknown photo-detector, the multi-anode PMTs
Hamamatsu R5900, we regarded it as a reference.
Particle identiﬁcation was done to remove
particles other than pions. Two CO2 gas Cherenkov counters in the beam line were used to
exclude electrons. Also, an aerogel counter was
equipped and used to exclude protons for the highmomentum region. This detector was also used to
exclude muons for the low-momentum region
around 0:5 GeV=c:
The particle trajectories were measured withmulti-wire proportional chambers (MWPC) at the
upstream and downstream ends of the light-shieldbox. These 5  5 cm2 MWPCs, with 20 mm diameter, gold-plated tungsten anode wires at 2 mm
pitch and with 90% Ar þ 10% CH4 gas ﬂow, wereread out by delay lines on the x and y cathodestrips.
The trigger signal was generated as a coincidence of signals from several 5  5 cm2 plastic
scintillation counters and anode signals from the
MWPCs to ensure valid tracking information.
2.4. Readout electronics
For the beam test, a new read-out system wasdesigned by using analog memory chips. The
analog memory chip is based on a chip developedby Ikeda [12] for a cosmic-ray experiment. We
borrowed the chips from National Space Development Agency of Japan (NASDA), and developed the chip control system. In the analog
memory chip, the signals of 32 channels arepreampliﬁed, sampled in 1 ms intervals, and stored
in an 8 steps deep analog pipeline. Fig. 4 shows aschematic view of the readout system with theseanalog memories. Two 32 channel analog memories are attached to each 64 channel PMT. The
memories corresponding to four PMTs are controlled by a 256 channel memory controller. Whenthe gate pulse is formed from the trigger signal, acontrol signal is sent from the controller to theanalog memories. The difference in the value ofthe analog memory between the latest and the ﬁrst
memory content is fed to the output. The obtained
output values of 256 channels are clocked into one
signal train with a period of 10 ms per channel.

Light-shield box

main RICH

Aerogelradiator

trackreference RICH

Cherenkov
photons

MWPC

MWPC
20 cm

20 cm
Flat-panel PMT(H8500) array

Fig. 3. Experimental set-up.

Multi-anode PMT(R5900-M16) array

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HV(-1100V)

AM(32ch)

PMT
64ch(H8500)AM(32ch)Driver filter

Ext. power
(5V)

N ch

Analog Memory

RC

371

Analog out

Power

AM control+5V
GND

AM(32ch)

PMT
64ch(H8500)AM(32ch)AM control

Analog Memory
Controller

Analog out
AM control
+5V
GND

ADC
Control pulse
AM(32ch)

PMT
64ch(H8500)AM(32ch)Analog out

Ext.
TriggerBusy

delay 1.5ms

Trigger Gatefrom all system

AM control
+5V
GND

F I/O
Software vetoAM(32ch)Analog out

OutputVME I/O Register
Input

AM control
+5V
GND

Gate

VME ADC

Input

Ditto

Ethernet
cable

Ditto
Ditto

VME BusAM(32ch)

PMT
64ch(H8500)Sparc board
Ext. Hard disk

Fig. 4. Schematics of the read-out system for the ﬂat-panel PMTs.

Each analog memory controller outputs the serial
signal together with synchronized control signals.
These signals are then read by a 12-bit VME ADC(DSP8112, MTT Co.) with a conversion time of
5 ms:

2.5. Reference RICH
A reference RICH was instrumented with multianode PMTs, Hamamatsu R5900-M16, the same
photon detector as used in the previous test [10].
The quantum efﬁciency of the PMTs is around
26% (at 400 nm), and the gain was around 6  106with 900 V applied to the photo-cathode. The
PMTs were grouped in a 2  6 array at a 30 mm
pitch. Due to a limited number of available PMTs
and read-out channels, only a part of theCherenkov ring was covered with photon detectors.

3. Measurement and results
Most of the test measurements were performed
with a p beam at 3 GeV=c: To systematicallyevaluate the detector performance, data weretaken with different aerogel samples with varioustransmission lengths and thicknesses. Data werealso taken by varying the p momentum in therange from 0.5 to 4:0 GeV=c:
A few typical events are displayed in Fig. 5. The
hits on PMTs can be associated with the expected
position of the Cherenkov ring. The hit near thecenter of the ring is due to Cherenkov radiation
generated by the beam particle in the PMT
window. The distribution of accumulated hits isshown in Fig. 6. Cherenkov photons from theaerogel radiator are clearly seen with a low
background level. The background hit distribution
on the photon detector is consistent with theassumption that it originates from Cherenkov

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100

100

50

50
y (mm)y (mm)

372

0

-50

-50

-100

-100
-100

-50

0
x (mm)

50

100

100

100

50

50
y (mm)y (mm)

0

0

-50

-100

-50

0
x (mm)

50

100

-100

-50

0
x (mm)

50

100

0

-50

-100

-100
-100

-50

0
x (mm)

50

100

Fig. 5. Some examples of event hit patterns for 3 GeV=c pions. The circle corresponds to the Cherenkov ring as expected from themeasured beam particle track. The dot corresponds to the impact point of the track upon the PMT window.

photons which were Rayleigh scattered in theradiator.
The pulse-height distribution of the Cherenkov
photons detected in one of the ﬂat-panel PMT isshown in Fig. 7. The raw data were corrected asfollows. A common-mode ﬂuctuation of the baseline was subtracted and signals due to cross-talk in
the read-out electronics were removed. The signal
mainly containing one photoelectron is clearly
separated from the pedestal peak. Note, however,that this distribution differs considerably fromtube to tube because of the large variation in
performance, as described before. For further
analysis we also applied a threshold cut tosuppress the pedestal noise contribution.
3.1. Cherenkov-angle resolution for single photons
Fig. 8(a) shows a typical distribution of theCherenkov angle for single photons. The angularresolution was obtained from a ﬁt of this distribution with a Gaussian signal and a linear function
for the background. Fig. 9 shows the resolution in
the Cherenkov angle for the p beam at 3 GeV=c
and 20 mm thick aerogel samples. The resolution
was around 14 mrad; independent of the refractive
index. The main contributions to the resolution of
the Cherenkov angle come from the uncertainty inthe emission point and from the pixel size of the
PMT. The ﬁrst contribution
pﬃﬃﬃﬃﬃ is estimated to be
semp ¼ d sin yc cos yc =L 12; where d is the aerogel thickness, yc is the Cherenkov angle and L isthe distance from an average emission point in the
aerogel to the surface of the PMT.
pﬃﬃﬃﬃﬃ The second
contribution is spix ¼ a cos2 yc =L 12; where a isthe pixel size. The measured variation of theresolution with the thickness of aerogel is shown in
Fig. 10. By comparing the measured resolution
and the expected values, we observed a rather
good agreement. There was, however, a discre-

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0.6
10

2

0.4

θy (rad)

0.2

0

10

-0.2

-0.4
1
-0.6
-0.6

-0.4

-0.2

0

0.2

0.4

0.6

θx (rad)Fig. 6. Distribution of PMT hits in the Cherenkov x; y spacefor 3 GeV=c pions.

Flat-panel PMT (H8500)
HV = -1100 V

140
120

Entries

100
80
60
40
20
0
0

250

500

750 1000 1250 1500 1750 2000

Pulse height (ADC ch)
Fig. 7. Pulse-height distribution for the ﬂat-panel PMT(H8500) for the hits in the region within 3s of the mean
Cherenkov angle. Data were corrected with the proceduredescribed in the text. In this ﬁgure, the pulse-height distribution
for the high sensitive PMT is shown.

pancy between the two, which can be accounted
for by a contribution of about 6 mrad: The
discrepancy could arise from the effect of aerogel(non-ﬂat aerogel surface and possible non-uniformities in the refractive index due to position
variation and chromatic dispersion), which are373

subject to further investigation. The uncertainty inthe track direction is expected to be negligible at
3 GeV=c; but increases considerably at low momenta ð0:5 GeV=cÞ due to the effect of multiple
scattering, as can be seen in Fig. 11.
3.2. Photoelectron yieldFig. 8(b) shows a typical distribution of thenumber of hits within 73s from the average
Cherenkov angle. The number of hits for the signal
region was estimated by subtracting the background from the ﬁts to the Cherenkov-angle
distribution. The number of detected photonsðNpe Þ depends on the aerogel thickness and theeffectR d R of scattering. It is expressed as Npe ¼
C 0 eðlÞl2 sin2 yc expðx=LðlÞ cos yc Þ dl dxE
C 0 sin2 yc L cos yc ð1  expðd=L cos yc ÞÞ where L
is the transmission length of the aerogel at an
average wavelength of 400 nm and eðlÞ is thequantum efﬁciency of the PMT. Fig. 12 shows thedependence of the number of detected photons on
the aerogel thickness. As expected from the above
expression, the number of photons does not
linearly increase with the aerogel thickness, but
saturates due to the scattering effect in aerogel.
Fig. 13 shows the dependence of the number ofphotons with transmission length. From the ﬁgure
the beneﬁt of the improvement in the transmission
length of the n ¼ 1:05 aerogel from around 15 mm;
as used in the previous beam test, to 45 mm using
the new production technique becomes evident.
The dependence on the pion momentum, displayedin Fig. 14, is ﬁtted with the form expected from theCherenkov relations, and shows a good agreement. For pions with momenta above 1 GeV=c;
the number of detected Cherenkov photons was
typically around 6 for aerogel samples with n ¼
1:05:
The performance of the RICH counter understudy was compared in the same set-up with theperformance of the reference counter with a wellknown photon detector, Hamamatsu R5900-M16
multi-anode PMTs. Since the two counters have a
different active area fraction (84% for the ﬂatpanel PMTs, and 36% for the R5900-M16 PMTs)
and a different acceptance, the comparison of thephoton yields was made by normalizing to the full

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Cherenkov angle

Number of hits

900

<θ> = 0.322 rad

800

σθ = 14.8 mrad

<Npe > = 6.2

350

700

300

600

250
Entries

Entries

400

500
400

200
150

300
100

200

50

100
0

0

0.2

0.4
θ (rad)

(a)

0

0.6

0

10

(b)

20
N

Fig. 8. (a) Distribution over the Cherenkov angle for single photons, and (b) the number of detected photons per ring, for a 20 mm
thick aerogel radiator sample with n ¼ 1:056 and a transmission length of 30 mm:

30

20
18

25

16

20

12

σθ (mrad)σθ (mrad)

14

10

15

8

10

6
4

n= 1.056
n= 1.051

5

2
0
1.02

n= 1.047
1.03

1.04
1.05
Refractive index

1.06

1.07

Fig. 9. Typical single photon resolution for 20 mm thickaerogel samples.

active surface. While the ﬂat-panel yield for aparticular case was 6.2, which resulted in 7.8 if
extrapolated to the full active area, the corresponding number for the R5900-M16 was 12. It
appears that this difference is mainly due to therather low quantum efﬁciency and ampliﬁcation ofsome of the ﬂat-panel tubes employed. This, in

0

0

10

20
30
40
thickness (mm)5060Fig. 10. Angular resolution as a function of the aerogel
thickness. The symbols correspond to the data for the different
samples and the curves are function described in the text.

turn, causes inefﬁciencies in single photon detection with a given threshold. If the best tube in theset is normalized to the full acceptance, thecorresponding number increases to 10; and wewould expect about 8 photons per ring.

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375

10

25
8

Npe (corrected)σθ (mrad)

20

15

10

5

0

0.5

1

1.5 2 2.5 3 3.5
Momentum (GeV/c)4n ∼ 1.052

n=1.056
n=1.0470

6n ∼ 1.03
4

4.5

5

Fig. 11. Angular resolution as a function of the charged
particle momentum for two different samples. The symbols
correspond to the data and the curves are ﬁts including theeffect of multiple-scattering.

0

0

10

20
30
40
50
Transmission length (mm)60

Fig. 13. Number of detected photons per Cherenkov ring ðNpe Þ
for 20 mm thick aerogel samples as a function of thetransmission length. Npe is corrected for the refractive index
to n ¼ 1:05 and 1.03, respectively. The symbols correspond to
the data and the curves are ﬁts described in the text.

10

8
7

8

6

5

Npe

Npe

6

4

4
3

n= 1.056, Λ = 30.0 mm

22

n= 1.051, Λ = 18.3 mm0

n=1.056
1

0

10

20
30
40
thickness (mm)50

n=1.047

60

Fig. 12. Number of detected photons per Cherenkov ring as afunction of the aerogel thickness. The symbols correspond to
the data and the curves are ﬁts described in the text.

3.3. Particle identification
Finally, we estimate the performance of pion/kaon separation in the momentum range of

0

0

0.5

1

1.5 2 2.5 3 3.5
Momentum (GeV/c)

4

4.5

5

Fig. 14. Number of detected photons per Cherenkov ring as afunction of the charged particle momentum. The symbols
correspond to the data and the curves are ﬁts described in thetext.

around 4 GeV=c; which is of considerable importance for the Belle experiment. If we take into
account a typical measured value for the single-

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376

photon angular resolution, sc B14 mrad; and thenumber of detected photons Npe B6; typical for
20 mm thick aerogel samples with n ¼ 1:05; we canestimate theCherenkov
angle resolution per trackﬃ
pﬃﬃﬃﬃﬃﬃﬃ
to be sc = Npe ¼ 5:7 mrad: This naive estimate isalso conﬁrmed by the direct measurement shownin Fig. 15. Here, the track-by-track Cherenkov
angle is calculated by taking the average of theangles measured for hits around the predicted
position of the Cherenkov ring. From this we candeduce that at 4 GeV=c; where the difference of
Cherenkov angles for pions and kaons is 23 mrad;a 4s separation between the two is possible. As an
additional cross check, we have also collected datawith the pion beam of 1:1 GeV=c; which can beused to represent a kaon beam of 4 GeV=c (apartfrom a slightly larger sigma due to multiple
scattering). As can be seen from Fig. 15, the two
peaks are well separated. Thus, the proximity
focusing aerogel RICH seems to be promising for
the upgrade of the Belle PID system at the forward
region.

225
Pseudo K

200

π (4.0 GeV/c)

from
π (1.1 GeV/c)

175

Entries

150
125
100
75

4. Conclusions
We report on the test beam results of aproximity focusing RICH using aerogel as theradiator. To obtain larger photoelectron yields, weused ﬂat-panel multi-anode PMT with a large
effective area, and aerogel samples produced witha recently developed method which have a higher
transmission length than before. We also developed a multi-channel PMT read-out system using
analog memory chips. A clear Cherenkov ringfrom the aerogel radiator could be observed, andthe number of photons was enhanced compared tothat in previous tests. We performed a systematic
study of the detector using various samples of the
aerogel. The typical angular resolution was around
14 mrad and the number of detected photoelectrons was around 6. The pion/kaon separation at
4 GeV=c is expected to be around 4s:
However, we still have some issues which haveto be solved for implementation in the Belle
spectrometer. The most important item is thedevelopment of a PMT which can be operated
under a strong magnetic ﬁeld ð1:5 TÞ: An example
of a candidate for such a device is a multi-anodehybrid photodiode (HPD) or hybrid avalanche
photodiode (HAPD). Of course, for a goodcandidate, its ability to efﬁciently detect single
photons on a large active area has to be demonstrated. The other item is mass production of the
aerogel tiles. While we have demonstrated that the
new production method signiﬁcantly increases thetransmission length of the n ¼ 1:05 aerogel, theproduction method has to be adapted to stablegood-quality manufacturing. We will study these
items at the next stage towards construction of areal detector.

50

Acknowledgements

25
0
0.2

0.22 0.24 0.26 0.28
θ (rad)0.3

0.32 0.34

Fig. 15. Cherenkov angle per track for pions of 4.0 and
1:1 GeV=c: Pions at 1:1 GeV=c are used to represent the kaon
beam of 4 GeV=c: The angular resolutions for 4.0 and
1:1 GeV=c are 5.4 and 6:7 mrad and two peaks are separated
by 4:2s:

We would like to thank the KEK-PS members
for operation of accelerator and for providing thebeam to the p2 beam line. We also thank H. Ikeda(KEK) and the Meisei Co. for their help in
preparing the read-out electronics, the Matsushita
Electric Works Ltd. for the good collaboration indeveloping the new aerogel type, and Hamamatsu

ARTICLE IN PRESS
T. Matsumoto et al. / Nuclear Instruments and Methods in Physics Research A 521 (2004) 367–377

Photonics K.K. for their support in equipping thephoton detector. We also thank A. Bondar (BINP,
Novosibirsk) for providing us excellent aerogel
samples, and Dr. T. Goka of NASDA for
providing us their read-out chips. One of theauthors (T.M.) is grateful to Fellowships of theJapan Society for the Promotion of Science (JSPS)
for Young Scientists. This work was supported inpart by a Grant-in-Aid for Scientiﬁc Researchfrom JSPS and the Ministry of Education,
Culture, Sports, Science and Technology underGrant No. 13640306, 14046223, 14046224, and inpart by the Ministry of Education, Science andSports of the Republic of Slovenia.

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