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Nuclear Instruments and Methods in Physics Research A 596 (2008) 311–316
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in
Physics Research A
journal homepage: www.elsevier.com/locate/nima
A threshold Cherenkov detector for Kþ =pþ separation using silica aerogel
R. Siudak a,b, A. Budzanowski b, A. Chatterjee f, H. Clement j, E. Dorochkevitsch j, J. Ernst a, P. Hawranek d,
F. Hinterberger a,, R. Jahn a, L. Jarczyk d, R. Joosten a, K. Kilian c, Di. Kirillov e, S. Kliczewski b, D. Kolev i,
M. Kravcikova g, H. Machner c, A. Magiera d, G. Martinska h, F. Massmann a, J. Munkel a, N. Piskunov e,
D. Protic c, J. Ritman c, P. von Rossen c, B. Roy f, I. Sitnik e, I. Slepnev e, J. Smyrski d, R. Tsenov i, K. Ulbrich a,
J. Urban h, G. Vankova i, G.J. Wagner j, R. Ziegler a
a
Helmholtz-Inst. f. Strahlen- u. Kernphysik, Universität Bonn, Germany
Institute of Nuclear Physics, PAN, Cracow, Poland
c
Institut für Kernphysik, Forschungszentrum Jülich, Jülich, Germany
d
Institute of Physics, Jagellonian University, Cracow, Poland
e
Laboratory for High Energies, Joint Institute for Nuclear Research, Dubna
f
Nuclear Physics Division, BARC, Bombay, India
g
Technical University Kosice, Kosice, Slovakia
h
University Kosice, Kosice, Slovakia
i
Physics Faculty, University of Sofia, Sofia, Bulgaria
j
Physikalisches Institut, Universität Tübingen, Germany
b
a r t i c l e in fo
abstract
Article history:
Received 29 May 2008
Received in revised form
31 July 2008
Accepted 13 August 2008
Available online 28 August 2008
A new threshold Cherenkov detector has been built for the detection of charged pions in the focal plane
of a magnetic spectrograph. Silica aerogel with refractive index of n ¼ 1:05 is applied as a radiator. The
detector exhibits a very high detection efficiency for pions with momenta X900 MeV=c. Using two
Cherenkov detectors in series a very high pion suppression factor of 105 was achieved.
& 2008 Elsevier B.V. All rights reserved.
Keywords:
Threshold Cherenkov detector
Silica aerogel
Reaction pp ! Kþ ðLpÞ
Kþ =pþ separation
1. Introduction
We describe a threshold Cherenkov detector which was
developed for a high resolution measurement of the reaction pp !
Kþ ðLpÞ near the Lp threshold [1]. In order to achieve a high
missing mass resolution the kaons are detected in the focal plane of
the magnetic spectrograph BIG KARL [2–4] at the Cooler Synchrotron COSY [5]. The momenta of the kaons are in the range of
900–1070 MeV=c. For the particle identification of the kaons a
threshold Cherenkov detector was necessary in order to suppress
the huge background of pions. Using silica aerogel with an index of
refraction n ¼ 1:05 only pions emit Cherenkov light, whereas the
velocities of protons and kaons are below the threshold velocity.
The Cherenkov detector was designed using the experience from a
previous experiment [6,7] performed at SATURNE II with the
spectrometer SPES4. Concerning the radiator material and the
Corresponding author. Fax: +49 228 73 2505.
E-mail address: [email protected] (F. Hinterberger).
0168-9002/$ - see front matter & 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2008.08.129
reflecting foil of the light collection box we refer to the threshold
Cherenkov detector developed for the BELLE experiment [8–10] and
the HERMES experiment [11]. The threshold Cherenkov detector
was designed with the aim to achieve a very high detection
efficiency for pions and a good homogeneity of the detection
efficiency over the focal plane of the magnetic spectrograph.
In Section 2 the design of the Cherenkov detector is sketched. The
Monte Carlo simulations of the detector are described in Section 3.
Experimental results are presented and discussed in Section 4.
2. The Cherenkov detector design
In order to identify the Kþ particles the huge background of
scattered pions can be suppressed using aerogel threshold
Cherenkov detectors. The scheme of the Cherenkov detector is
shown in Fig. 1. The light collection box has 820 mm 117 mm
cross-section and 266 mm depth. A 80 mm thick wall of silica
aerogel tiles with an index of refraction n ¼ 1:05 is used as a
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Cherenkov threshold based on the simulation framework GEANT4
[14]. The program yields the number of photons produced by a
charged particle traversing the silica aerogel and measured by
photomultipliers. The optical properties of the silica aerogel and
reflector foil as well as the quantum efficiency of photomultipliers
used in the simulations are described in this section. The
simulations help to find the most effective and simple detector
geometry and the necessary aerogel thickness.
3.1. Optical properties of silica aerogel
Fig. 1. Scheme of a Cherenkov detector unit. The 8 cm thick radiator consists of
eight layers of silica aerogel tiles (n ¼ 1:05). The inner surfaces of the light
collection box are covered by a Whitestar reflector foil R. The Cherenkov light is
detected using seven 5 in. photomultiplier tubes (PMT) which are arranged
vertically above and below the detector midplane.
radiator. Seven photomultiplier tubes (PMT) with a diameter of
5 in. are arranged vertically above and below the detector
midplane. In total, two Cherenkov detectors have been built using
seven RCA 8854 (now BURLE 8854) and seven Philips XP2041Q
phototubes, respectively, for the light collection. The effective
diameter of the photocathode amounts to 114 mm (BURLE 8854)
and 110 mm (Philips XP2041Q), respectively. The inner surfaces of
the light collection box are completely covered with a highly
reflecting Whitestar reflector foil R (Gore-tex) [12]. The side walls
of the light collection box are made from 12 mm thick aluminum.
The front and rear windows are thin and made of 0.5 mm thick
Whitestar reflector foil, 0.025 mm thick aluminized mylar foil and
0.1 mm thick carbon foil.
The silica aerogel tiles were produced by the advanced
technology research laboratory of Matsushita Electric Works
[13]. The silica aerogel is hydrophobic. Therefore, the optical
quality of the radiator is not affected by the ambient humidity.
The refractive index amounts to n ¼ 1:05; the density is
0:18 g=cm3 . The details of the optical properties of our silica
aerogel are discussed in Section 3.1. The radiator wall of silica
aerogel consists of 56 silica aerogel tiles. The tile size is
characterized by 113 mm 113 mm cross-section with 10 mm
depth. In x-direction seven aerogel tiles are arranged to form a
layer with 791 mm 113 mm cross-section. There are in total
eight aerogel layers in z-direction which are staggered in
x-direction by about 3 mm in order to minimize the inhomogeneity which is caused by the gaps. The stack of aerogel tiles is fixed
using Al retainers which are covered by a soft foam and the
reflecting foil. The effective area of the Cherenkov detector is
sufficiently large to cover the focal plane size of the magnetic
spectrograph BIG KARL.
3. Monte Carlo simulations of the threshold Cherenkov detector
Monte Carlo simulations were performed in order to estimate
the detection efficiency for particles above and below the
The optical properties of silica aerogel are characterized by the
refractive index nðlÞ, the Rayleigh scattering length Lsc ðlÞ and
the absorption length Labs ðlÞ. The refractive index nðlÞ and the
scattering length Lsc ðlÞ are rather well known. However,
the absorption length is not so well known. In order to determine
the Rayleigh scattering length Lsc ðlÞ and the absorption length
Labs ðlÞ the manufacturer measured the transmittance TðlÞ of a
10.33 mm thick silica aerogel tile. The transmittance TðlÞ accounts
for the residual light from aerogel in the forward direction. The
transmittance data are listed in Table 1. From these measurements
the Rayleigh scattering length Lsc ðlÞ, the absorption length Labs ðlÞ
and the attenuation length Ltot ðlÞ have been deduced assuming
the following equations [8]:
Labs ¼
Lsc ¼
l2
(1)
a
l4
1
Ltot ðlÞ
(2)
b
¼
1
Lsc ðlÞ
TðlÞ ¼ exp þ
s
1
Labs ðlÞ
Ltot ðlÞ
.
(3)
(4)
Here, s is the thickness of the aerogel tile (s ¼ 10:33 mm).
The program Minuit of the CERN program library [15] is used in
order to determine the parameters a and b in a nonlinear leastsquare fit.
Since the uncertainties of the transmittance data measured by
the manufacturer are not known, we estimate the uncertainties
using a method described by Bevington [16]. We assume a
constant error s for each data point. The value of s is deduced
from the sample variance s2 of the nonlinear least-square fit
P
s2 s2 ¼ Ni¼1 ½T meas ðli Þ T fit ðli Þ2 =ðN nÞ. Here, N is the number
of data points, n the number of fit parameters and N n denotes
the number of degrees of freedom. The resulting error s amounts
to 0.0087. Taking this estimated error into account the nonlinear
least-square fit yields the parameters a and b and the estimated
uncertainties of the parameters a and b,
a ¼ ð9:18 1:69Þ 1013 m
b ¼ ð7:54 0:24Þ 1025 m3 .
(5)
(6)
2
red
This method yields automatically a reduced chi-square w ¼
w2 =ðN nÞ ¼ 1:0. The resulting fit values are denoted by T fit;1 ;
Labs;1 and Lsc;1 in Table 1.
However, in view of more recent direct measurements of the
absorption length Labs ðlÞ and scattering length Lsc ðlÞ of n ¼ 1:03
silica aerogel by the HERMES collaboration [11] the l-dependence
2
of the ansatz Labs ðlÞ ¼ l =a of Eq. (1) is questionable. According to
Ref. [11], the absorption length Labs is nearly constant between
about 320 and 900 nm and it drops down on a logarithmic scale
between 320 and 200 nm. It should be mentioned that the silica
aerogel investigated by HERMES was from the same producer as
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Table 1
Transmittance T, absorption length Labs and scattering length Lsc of n ¼ 1:05 silica
aerogel
l (nm)
T meas
T fit;1
Labs;1
Lsc;1 (cm) T fit;2
(cm)
250
300
350
400
450
500
550
600
650
700
750
800
0.1359
0.3436
0.5408
0.6849
0.7850
0.8522
0.8935
0.9247
0.9417
0.9544
0.9651
0.9701
0.1170
0.3441
0.5508
0.6952
0.7892
0.8500
0.8901
0.9172
0.9361
0.9495
0.9594
0.9667
6.8
9.8
13.3
17.4
22.1
27.2
33.0
39.2
46.0
53.4
61.3
69.7
Labs;2
0.5
1.1
2.0
3.4
5.4
8.3
12.1
17.2
23.7
31.8
42.0
54.3
0.0987
0.3501
0.5721
0.7093
0.7850
0.8374
0.8703
0.8908
0.9047
0.9146
0.9231
0.9282
2.8
9.2
19.6
22.0
18.1
18.4
18.4
18.1
17.9
17.9
18.4
18.4
0.5
1.1
2.0
3.5
5.6
8.5
12.4
17.6
24.3
32.7
43.0
55.7
used in our detector and it has very similar transmittance
although it has different refractive index. In order to test the
effect of a nearly constant absorption length Labs ðlÞ as measured
by HERMES we perform a second fit taking directly Labs ðlÞ from
HERMES (Fig. 7 of the HERMES paper [11]) and fitting only the free
parameter b of Eq. (2). The fit yields
b ¼ ð7:35 0:43Þ 1025 m3 .
Table 2
Quantum efficiency q:e: of BURLE 8854 and Philips XP2041Q PMT
E (eV)
l (nm)
BURLE
q:e:
Philips
q:e:
4.96
4.13
3.54
3.10
2.76
2.48
2.25
2.07
250
300
350
400
450
500
550
600
0.13
0.19
0.23
0.22
0.17
0.11
0.04
0.01
0.13
0.22
0.26
0.26
0.21
0.13
0.05
0.01
Lsc;2 (cm)
(cm)
(7)
The absorption lengths from HERMES, denoted by Labs;2, and the
fit values T fit;2 and Lsc;2 from the second fit are listed in Table 1.
3.2. Optical properties of the reflector foil
The reflector foil is a 0.5 mm thick Whitestar Gore-Tex
membrane. The reflectivity R of this diffusely reflecting foil is
very high. It is given by the manufacturer as Rðl ¼ 400 nmÞ ¼
0:987 and Rðl ¼ 700 nmÞ ¼ 0:981. We assume a linear dependence on the wave length l in the range 250–800 nm. The diffuse
reflection of photons is described by the cosine Lambert’s law. The
amount of specular reflection is negligibly small.
3.3. Quantum efficiency of the photomultiplier
The photocathodes of the RCA 8854 (now BURLE 8854) and
Philips XP2041Q phototubes are bialkali cathodes yielding a
rather high quantum efficiency q:e: in the wavelength range
250–500 nm. The quantum efficiencies are taken from the PMTs
data sheets. They are listed in Table 2.
313
and the application of mirrors was tested in the simulations as
shown in Fig. 2(a)–(c).
In Fig. 2(a) the simplest configuration with the PMTs placed
just above and below the aerogel stack is presented. Increasing the
number of detected photons was obtained by moving the PMTs in
the forward direction of the incoming pions as is shown in
Fig. 2(b). An application of mirrors (Fig. 2(c)) improves again
somewhat the detected photon number but increases also the
costs and causes additional complexity. Therefore, the detector
geometry from Fig. 2(b), described in Section 2, was decided to be
optimal for our purpose.
An example of the simulated distribution of the total number
of photoelectrons (sum over photoelectrons detected by seven
photomultipliers) at pion momenta of 960 MeV=c for the detector
geometry described in Section 2 is shown in Fig. 3. For
comparison, the results for two sets of the aerogel absorption
length Labs ðlÞ and scattering length Lsc ðlÞ as presented in Table 1
and discussed in Section 3.1 are shown. Both sets yield very
similar photoelectron distributions. This is due to the fact that the
quantum efficiency of PMTs with bialkali photocathodes is high
only in the range of 250–500 nm where the differences between
those two descriptions of aerogel optical properties are small.
A mean photoelectron number of about 20 is predicted for the
detector geometry described in Section 2 with 8 cm thick aerogel.
The radiator thickness plays a crucial role with respect to the
detected number of photoelectrons. The simulated dependence of
the mean photoelectron number on the aerogel thickness for
pions at 960 MeV=c is shown in Fig. 4. An aerogel thickness of 8 cm
was chosen for our detector (see Fig. 1) to get a required detector
efficiency and a pion suppression factor larger than 104 .
4. Experimental results
3.4. Simulation results
The detector geometry considered in the simulations is
described in Section 2. Modifications of this detector geometry
as shown in Fig. 2 were also studied. The response of the detector
for pions with momenta larger than 900 MeV=c, incoming
perpendicularly to the surface of entrance foil and homogeneously
distributed over the full detector area was investigated. The
simulated pion distribution corresponds to the real situation in
the focal plane of the spectrograph BIG KARL, which is
characterized by a small angle dispersion in the horizontal plane
and a nearly point-to-parallel imaging in vertical direction. Silica
aerogel with refractive index of n ¼ 1:05 and optical properties
discussed in Section 3.1 was used in the simulations. The optical
properties of reflecting foil and quantum efficiency of photomultipliers are described in Sections 3.2 and 3.3, respectively.
In order to find the optimum geometry of the light collection
box the position of aerogel with respect to the photomultipliers
The reaction of interest pp ! Kþ ðLpÞ is measured at forward
direction using the magnetic spectrograph BIG KARL [2–4]. The
momentum of the incoming proton beam amounts to about
2:735 GeV=c. The scattered particles are detected in the momentum range 900–1070 MeV=c. Two threshold Cherenkov detectors
are located in the focal plane of the magnetic spectrograph.
The layout of the magnetic spectrograph BIG KARL is shown in
Fig. 5. The charged particle tracks are measured by two stacks of
multiwire drift chambers (MWDC). The DE signals of the
scintillator hodoscopes and the time-of-flight (TOF) information
of a 5 m long TOF setup made from the scintillator hodoscopes are
used for the particle identification. An example of the TOF
spectrum of particles detected at 960 MeV=c is shown in Fig. 6.
Very pronounced peaks for pions and protons can be identified in
the spectrum. The measured kaons are expected to appear at the
right side of the pion peak in the TOF spectrum. The 5 m path is
not long enough for the separation of kaons and pions using the
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PMT
PMT
Aerogel
Mirror
PMT
π+
Fig. 2. Scheme of possible geometries of the threshold Cherenkov detector.
Focal plane detectors
MWDC
D2
Q3
Scintillator
Hodoscope
50000
40000
Counts
Cherenkov
30000
D1
Q2a
Q2
20000
TARGET
10000
Q1
3m
e
ret
ll
wa
nc
co
0
10
0
20
30
40
50
Npe
Fig. 3. Simulated distribution of total number of photoelectrons N pe from
Cherenkov detector for pions at 960 MeV=c with two different sets of aerogel
absorption lengths and scattering lengths presented in Table 1. Solid line
represents simulation results with parameters Lsc;1 and Labs;1 and dashed line
with Lsc;2 and Labs;2 (see Table 1).
20
mean Npe
16
12
8
4
0
0
2
4
6
8
10
aerogel thickness (cm)
Fig. 4. Simulated mean number of photoelectrons as a function of aerogel
thickness for 960 MeV=c pions.
Fig. 5. Layout of the magnetic spectrograph BIG KARL. The charged particles are
measured in the focal plane using two stacks of multiwire drift chambers, two
threshold Cherenkov detectors and two scintillator hodoscopes.
TOF information. In order to identify the Kþ particles, the huge
background of scattered pions is suppressed using the signal from
the threshold Cherenkov detector. Pions with TOF signals smaller
than the peak maximum ( 17 ns) do not contaminate any other
particles and they can be used to determine the pion suppression
factor for the tested Cherenkov detector.
Signals from seven PMTs of the Cherenkov detector were read
through charge ADC’s, triggered by coincidences between two
layers of the scintillator hodoscopes. Each single ADC spectrum
was normalized to photoelectron number using the location of
single photoelectron peak. Next for every event, the sum over all
seven ADC readouts was performed and total number of
photoelectrons N pe was deduced. The events with track position
jxjp380 mm and jyjp40 mm from the Cherenkov box center,
corresponding to the size of the magnetic spectrograph focal
plane, were included in the efficiency determination. An example
of the distribution of total number of photoelectrons N pe for pions
at 960 MeV=c is shown in Fig. 7. An average experimental value of
Npe 20 is found to be very close to the value predicted by the
simulations (see Fig. 3).
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315
106
x103
400
pions
protons
pions
104
Counts
F (Npe)
protons
300
102
200
1
0
100
10
20
30
Npe (photoelectrons)
40
50
Fig. 8. Spectrum of N pe for protons and pions at 960 MeV=c. First large peak at
N pe ¼ 0 corresponds to ADC pedestal.
0
15
10
20
TOF (ns)
25
30
1
Fig. 6. TOF spectrum from pp ! X reaction measured at 960 MeV=c momentum.
pions
Misidentification M
10−1
20000
Counts
15000
10−2
10−3
protons
10−4
10000
10−5
5000
0
10
20
30
Nth
pe (photoelectrons)
40
50
Fig. 9. Misidentification of pions (solid line) and protons (dashed line) from the
Cherenkov detector.
0
0
20
40
60
Npe (photoelectrons)
80
Fig. 7. Measured distribution of total number of photoelectrons N pe from
Cherenkov detector for pions at 960 MeV=c.
Although protons are below Cherenkov threshold, a small
fraction of them produced a Cherenkov signal with total number
of photoelectrons larger than zero as is shown in Fig. 8. It could be
due to proton induced d-electrons with velocity above the
Cherenkov threshold, random coincidences of photomultiplier
noises with the trigger or light produced in some other than
aerogel components of the light collection box. Analysis of the
photoelectron distributions for pions and protons (as in Fig. 8)
allows one to deduce the Cherenkov detector efficiency for
particles above and below Cherenkov threshold. All particles with
Npe larger than the threshold value Nth
pe can be treated as pions.
The pion efficiency Ep can be deduced from the photoelectron
spectrum for pions, F p ðN pe Þ,
R1 p
F ðN pe Þ dNpe
Nth
pe
p
.
(8)
E ¼ R1
p
F
ðNpe Þ dNpe
0
th
For given Nth
pe some protons will have N pe 4N pe so they will be
treated as pions. In case of protons we call it proton misidentification Mp . Proton misidentification can be calculated from photoelectron distribution for protons, F p ðNpe Þ,
R1 p
F ðN pe Þ dNpe
Nth
pe
p
.
(9)
M ¼ R1
p
F
ðNpe Þ dN pe
0
The misidentification of pions, M p , can be defined as
Mp ¼ 1 Ep .
(10)
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1
pions
protons
105
pions
10−1
10−2
Counts
Misidentification M
104
103
10−3
102
10−4
protons
kaons
10
10−5
1
10
0
10
20
40
total (photoelectrons)
Nth,
pe
60
80
Fig. 10. Misidentification of pions (solid line) and protons (dashed line) from two
Cherenkov detectors.
The pion misidentification M p is connected with the pion
suppression factor S through the relation
Mp ¼
1
.
S
(11)
The misidentification distributions for pions and protons at
960 MeV=c are shown in Fig. 9. We assume that the misidentifications of kaons and protons are of the same order of magnitude.
Proper choice of threshold N th
pe , for which the high number of
pions is properly identified, and low number of protons misidentified as pions, provides a high pion suppression factor.
However, as can be seen in Fig. 9, to get necessary pion
suppression factor S 105 one should use Nth
pe 1. To improve
the situation a second Cherenkov detector was built with
the same design as the first one. The sum of photoelectron
numbers from both detectors (N th;total
) is used in the final pion
pe
identification
th;2
Nth;total
¼ N th;1
pe
pe þ N pe .
20
TOF (ns)
30
Fig. 11. TOF spectrum from pp ! X reaction measured at 960 MeV=c momentum.
Solid line—full spectrum, dotted line—TOF spectrum in coincidence with
Cherenkov, dashed line—TOF distribution vetoed with Cherenkov. The searched
kaon peak is indicated.
cooler synchrotron COSY (Jülich, Germany). Silica aerogel with
refractive index n ¼ 1:05 and thickness of 8 cm was used as a
radiator in each detector. The inner surface of the light collection
box was covered by high reflectivity Whitestar Gore-Tex foil. The
Cherenkov detectors were applied in kaon/pion separation
measured in the momentum range of 90021070 MeV=c. A very
high pion suppression factor of 105 was achieved. At the same
time, the misidentification of the particles with velocity below
Cherenkov threshold was found to be less than 1%.
Acknowledgments
We thank the operating team of COSY for excellent beam
support. This work was supported by the Bundesministerium für
Bildung und Forschung, BMBF (06BN108I), and Forschungszentrum Jülich GmbH, COSY FFE (41520742).
(12)
Misidentification distributions for pions and protons at
960 MeV=c for two Cherenkov detectors are shown in Fig. 10. As
can be seen, a very high pion suppression factor of about 105 and a
low (below 1%) misidentification of protons with velocity below
the Cherenkov threshold was achieved with Nth;total
5 for two
pe
Cherenkov detectors. In Fig. 11 the decomposition of the TOF
spectrum with and without coincidences with two Cherenkov
detectors is presented. The Nth;total
¼ 5 threshold is used in pion
pe
identification with the Cherenkov detector. The clearly visible
kaon peak after pion suppression is indicated in the figure.
5. Summary
Two new threshold Cherenkov detectors were built and used in
the focal plane of the magnetic spectrograph BIG KARL at the
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