Aerogel and Its Applications to RICH Detectors
INFN, Sez. Bari,
Istituto Nazionale di Fisica Nucleare,
via Amendola 173,
I-70126 Bari, Italy
Beam test results show that the ``new generation" aerogel
has attractive features and appears an interesting candidate as radiator in
Ring Imaging Cherenkov (RICH) detectors.
The challenging applications envisaged in the LHCb experiment at CERN
and in the HERMES experiment at DESY will be reviewed.
The hadron identification in the momentum domain of few GeV/c
represents a challenge for Cherenkov detectors, in fact
traditional gas and liquid radiators have a refractive
index either smaller of 1.0018 (
C5F12) or larger than 1.27
To avoid the use of gases at high pressure or in unmanageable liquified form,
possible way to partially close the gap in refractive indices
is represented by silica aerogel that can be produced
in a fairly wide range from n=1.004 to n=1.1.
After the unsuccesful attempt of Linney and Peters  in 1972 to
use compressed silica powder to obtain a material with n<1.2, soon
abandoned due to the very poor
transparency of their radiators, in 1973 Cantin et al.  adopted
silica aerogel in Cherenkov counters.
Although aerogel was discovered in 1931 , a
time and cost effective fabrication method was found only in the late 1970s,
decided to store rocket fuels in porous materials.
Since then, an explosive growth of specific application in the scientific
community has stimulated new techniques for the production
of aerogel with remarkable optical quality.
Indeed aerogel is now currently produced in a sol-gel chemical process that
provides a very transparent
hydrophobic polymer gel structure while old aerogel was fabricated in a way
to lead to seedy hydrophillic colloidal structures.
The ``breakthrough" in aerogel fabrication promoted more advances in the use
of this material
in Cherenkov detectors, as V.I. Vorobionov 
and H. v. Hecke  pointed out in 1991 and 1993, respectively.
Nonetheless, the major merit of the rapid progress of aerogel in real RICH
be ascribed to J. Seguinot and T. Ypsilantis, who revised the van Hecke
proposal in the light of currently available photodetector technology and
envisaged an appealing
application in the LHCb experiment .
The outstanding potential of their detector design inspired the
upgrade of HERMES at DESY .
In the next section, the chemical and physical properties of silica aerogel
will be briefly reviewed,
results from beam tests at CERN-PS are then presented in Sec. 3.
An outlook of the experiments LHCb and HERMES will be
given in Secs. 4 and 5, respectively. Finally, Sec. 6 is
devoted to conclusions.
Aerogel is a manmade material that could have a density as low as
three times that of air.
It essentially consists of grains of amorphous SiO2 with sizes
ranging from 1 to 10 nm linked together in a three-dimensional structure filled
by trapped air.
The huge number of such tiny primary particles determines
an internal surface close to 1000 m2/g that plays the fundamental
role in the aerogel chemical and physical behavior.
It exists a simple relationship between the resultant
index of refraction and the aerogel density
in g/cm3 :
Density values lying between 0.003 g/cm3 and 0.55 g/cm3 are
in principle available, corresponding to refractive indices of
and n=1.11 (
In the aerogel preparation, the starting phase is
the hydrolysis and condensation of silicon alkoxides in presence of
an alcoholic solvent.
Aerogel is then obtained by removing
the solvent in a quite complicated way because if the liquid were
simply left to evaporate, then adhesion and capillary forces would shrink
the gel into a very dense material. Therefore,
in order to prevent the collapse of the porous structure,
the pressures and temperatures
during liquid extraction must be raised above the
triple point of the solvent.
In the past, this operation required high temperatures and high pressures.
Moreover, the demanding control of the correct quantity of solvent
made possible only the production of silica aerogel within a limited range of
refractive indices .
In the final treatment, the aerogel needed to be
baked at several hundred degrees Celsius in order to dry off the residual
adsorbed solvent. The final product had surfaces not
particularly clean and flat and the baking treatment makes it
very hydrophillic, consequently aerogel samples used in the
experiments had to be often rebaked during their operative life.
In 1988, a new fabrication process was
developed at Lawrence Livermore National Laboratory, later on adopted
by the russian
team lead by A. P. Onuchin in collaboration with the Boreskov Institute
of Catalysis in Novosibirsk to produce very transparent silica
aerogel with refractive index in the range 1.005-1.055 .
Oppositely to the
aforementioned method, called ``one-step," the new one treats the starting gel
into two succesive steps: the alcohol (mainly methanol) within the gel is,
in the first step, replaced by liquid
CO2 that undergoes a supercritical drying in the second
step without damaging the aerogel .
The ``two-step" process
is much safer than the simply extraction of supercritical methanol, in fact
CO2 has a lower critical point (31oC and
1050 psi) than methanol (240oC and
and does not pose an explosion hazard as
But the breakthrough in the fabrication process
occured only few years ago, in the framework of the Belle experiment
at the B-factory in Japan, when
silica aerogel with very low refractive index was produced by means
of a revolutionary technique . The National Laboratory for
Physics (KEK) in Japan in collaboration with Matsushita Electric Works
developed a method based on the old single-step process but
adopting the basic philosophy of the two-step method. The aerogel is
baked under supercritical conditions after replacing the alcohol with CO2by avoiding in this way the complication of the alcohol distillation.
This aerogel is hydrophobic, due to a treatment of the surface of the aerogel
pores, and highly transparent, but it loses
this property if a
baking process is applied to improve its
Moreover, KEK aerogel has been found to be radiation hard at least up to
9.8 mrad of gamma ray dose .
The granular structure of aerogel with a typical length scale of few nm
determines its optical properties. Indeed, the behavior of visible light in aerogel is dominated by Rayleigh scattering
which increases as the fourth power of the frequency.
The bluish haze that surrounds aerogel samples is
an effect of the Rayleigh scattering since short wavelengths are the most
severely affected by the continuos scattering mechanism.
absorption does not play a significant role in the visible region
(the intensity drops to 1/e only
after several cm), while weak absorbances appear in the infrared.
The measured transmittance t
of an aerogel sample of thickness L, as function of the light wavelength
in the range from 300 nm to 700 nm, is fairly fitted
by the expression:
where C characterizes the aerogel clarity,
and A is the measured transmission in the long-wavelength region.
a good optical quality have A and C close to
the value of 1 and 0 respectively.
The typical values of the parameters A and C have been reported in
Table where samples with n=1.03 produced with the aforementioned
fabrication methods are compared.
Silica aerogel optical parameters
When the Rayleigh scattering occurs, the directionality of the Cherenkov
radiation is completely lost. Therefore,
the major concern associated with the design and construction of a RICH detector
with an aerogel radiator is whether the
Cherenkov photons that traverse the aerogel without any scattering
are in sufficient number
to allow the measurement of their emission angle with the expected accuracy.
The fraction N of photons of wavelength
cross undeflected an aerogel sample of
thickness L, characterized by the optical parameters A and C, is given
Although the C parameter of Russian samples is smaller than that of aerogel
fabricated with other methods, the KEK aerogel has the big advantage to be
hydrophobic and therefore it ages less in the long term.
By assuming the best optical parameters, the fraction
of photons of 350 nm that
have not undergone any scattering
inside a 3 cm thick sample is about 60%, but N raises up
to almost 85% for photons of 500 nm.
These simple calculations show that the useful production of Cherenkov
light is limited to the visible. This therefore places high demands on
photon detection. A large area multicell hybrid photodiode (HPD) with a bialkali
photocathode seems the
most promising candidate to detect and resolve single photoelectrons
from aerogel .
Moreover, HPDs ensure operational stability on a long term since they are
devices without intrinsic gain .
In the framework of Hermes and LHCb RICH detector development,
investigations have been carried out to prove the feasibility
of detecting a single event Cherenkov ring produced in aerogel
Aerogel samples with n=1.03, procured from KEK, were
tested at the PS-T9 beam facility at CERN with 10 GeV/c negative pions.
The experimental setup is schematically shown in
Schematic layout of the test setup at CERN for the imaging of the
Cherenkov light produced in silica aerogels with n=1.03. Two detector schemes
were consecutively implemented on the mirror focal plane: (a) a single phototube
mounted on a motorized stage, and (b) an array of 114 photomultipliers.
It consists of a black painted light-tight aluminum box
flushed with nitrogen at atmospheric pressure,
which contains the aerogel sample and an angled
spherical mirror used to focalize
the light from the radiator on the photodetector entrance window.
The mirror has a focal length of 45 cm, and thus, aerogel rings
with a diameter d of 11 cm are expected in the mirror focal plane.
Smaller rings, with 2 cm, are instead
produced by particles in nitrogen between the aerogel and the mirror.
Two different systems employing either a single phototube (PM)
or a matrix of 114
phototubes have been used to detect the emitted light.
In the first part of the test,
a Hamamatsu 1332Q PM with a bialkali photocathode of one inch was
installed on a linear motorized translator in order to scan horizontally
across the mirror
focal plane. The number of counts, referred to the same number of triggers,
registered by the PM during the
scanning of the focal plane, is reported in Fig. as a
function of the position.
In order to suppress the noise and mantain sensitivity
to single photoelectrons,
a threshold in the pulse height was placed between the pedestal and
the one-photoelectron (p.e.) peak of the PM spectrum.
The stronger peak in Fig. , obtained with the PM nearly
in the center of the focal plane,
corresponds to the Cherenkov ring produced in nitrogen.
Another peak is clearly visible when the PM is displaced 11.6 cm from the
position of the maximum of the first peak.
The same enhancement is seen moving the PM in the opposite direction.
These enhancements are originated from the unscattered Cherenkov light
produced in the radiator and prove that KEK aerogel preserves the direction
of an appreciable fraction of Cherenkov photons.
From the values of the mirror focal length and from the observed radius
of the aerogel ring, a refractive index of 1.03 has been calculated,
in good agreement with the nominal value provided by the
KEK group (n=1.028).
The resolution of the peaks in Fig. is dominated by the size
of the PM photocathode.
Scan with one PM across the mirror focal plane. The first
peak, on the left-hand side,
is due to the unresolved Cherenkov ring from nitrogen, while the second
peak corresponds to Cherenkov photons produced in the aerogel. The counts
between the two peaks are very few as expected.
In the second part of the test, the array of 114 one-inch PM's
allowed to perform the electronic imaging of the full Cherenkov pattern.
A threshold was applied to the signal of each of the 114 PM's
recorded, corresponding to about 2
of the pedestal distribution.
In Fig. , the PM hit map containing a large number of
overlapped events is reported. The nitrogen and aerogel rings are clearly
visible over a
very low background.
From the positions and the number of PM's fired in each event,
the average radius of the ring, the average number of
Cherenkov photoelectrons per ring and the average
background due to photons emerging from the aerogel after having
undergone one or more scatterings, were calculated . The
following results, referred to a 2.5 cm thick sample, were found:
(1) a ring radius value of
(2) an average number of 14.9 p.e. per ring; and
(3) an average background of 2.4 p.e. per ring.
The photomultiplier hit map over the mirror focal plane obtained
by overlapping events from a full run. The Cherenkov ring from aerogel is
clearly visible, while the ring from nitrogen is poorly resolved by the three
central photomultipliers. The sparse background hits show the good optical
quality of the tested aerogel sample.
LHCb is an experiment designed to study CP violation in B decays.
It is conceived
as a collider-mode forward spectrometer which will be operational at the
LHC start-up. The proposed layout (Fig. ) features
an accurate momentum reconstruction and particle identification since
precision determination of the CKM unitarity triangle angles
requires an excellent
pion/kaon separation over the momentum range
from 1 GeV/c to 150 GeV/c. A detailed description of LHCb can be found
in Ref. , in the
following only those aspects of the setup which concern particle identification
will be addressed.
The desired momentum range for pion/kaon separation cannot be spanned by one
refractive index setting; therefore,
two focused RICH detectors with three radiators have been proposed.
The first RICH is placed upstream the dipole magnet to allow the identification
of particles in the low momentum region from 1 to 60 GeV/c. It is
based on the innovative idea to
implement aerogel and
C4F10 radiators in the same focusing system by
positioning the aerogel radiator close to the gas vessel entrance window and
tilting the 2 m focal length
mirror to bring the image out of the beam aperture in order to reduce secondary
Photons are detected via
an array of HPDs located on each side of the RICH detector
The innovative set-up of the
LHCb RICH with aerogel and gas radiators.
The second RICH has a 2 m long CF4 gas radiator to identify high momentum
particles between 16 and 150 GeV/c. A system of mirrors
transfers the ring images to the HPD array located in such a way that it is not
traversed by particles.
Anticipated performances of the RICH systems
are listed in Table .
Expected performances of LHCb RICH detectors with n=1.03 aerogel radiator
C4F10 gas radiators. The following factors are listed:
momentum thresholds for pions and kaons,
maximum Cherenkov emission angle,
contributions to the angle resolution from the uncertainty of the
photon emission-point, from the radiator
chromatic dispersion and from photon detector spatial resolution (assuming
2.5x2.5 mm2 pixel size), total
angle resolution per photoelectron, and the momentum upper
limit of 3
The particle identification layout proposed has a high discrimination power,
but the innovative technical solution envisaged need crucial tests to
determine their feasibility.
Hermes is an internal gas-target experiment designed to investigate the nucleon
spin structure functions at HERA .
An open spectrometer has been built
with the aim to measure the scattered electron and the leading hadrons coming
from the target fragmentation with a momentum and angle resolution
of 1% and 1 mrad (at 4 GeV/c), respectively.
The spectrometer consists of a conventional dipole magnet of 1.3 Tm and many
tracking chambers for an accurate event reconstruction
The scattered primary electron is identified with an efficiency greater than
(with less than
of hadron contamination) by the combination
of a lead-glass
a preshower, and a transition radiation detector. The threshold
Cherenkov detector that formerly allowed the identification of pions
above a threshold of
3 GeV/c using
C4F10 was replaced by a LHCb-like dual radiator
RICH during the summer of 1998 .
In this way, the unique opportunity to provide valuable
information on the flavor dependence of the spin structure functions
and estimates of the strange sea polarization is fully exploited by
pions, kaons, and protons in the momentum
3 to 20 GeV/c.
Reconstruction of an event in the HERMES spectrometer. The scattered
positron has been recognized by hits in the transition radiation detector (TRD),
the preshower counter (PRE) and the electromagnetic calorimeter (CAL).
Hydrophobic, crack-free, very transparent aerogel samples are now
Loss of photons due to the absorption and scattering processes in the
bulk material have been minimized by using innovative production techniques.
Aerogel ageing effects due to exposure to atmosphere
can be alleviated by proper handling and storage.
This new silica aerogel can be an ideal medium to be employed in RICH
detectors as radiator.
Test beam studies of aerogel gave very promising results indicating that
KEK-aerogel has the required optical quality, and therefore,
it is suitable to be used
for RICH detectors. Moreover results show that the background
from scattered photons is low.
LHCb and HERMES experiments have already planned to implement an aerogel RICH
in their setup. Indeed,
a RICH device with aerogel and an array of visible light photodetectors
shows an attractive conceptual
simplicity due to the modest service and maintenance
needs. The major drawback of this technique is the high detector cost per
unit of surface. In fact, the commercial available devices (HPD and multi-anode
photomultipliers) have suitable performances but they suffer of large inactive
area and high cost. The developments of cheap hybrid phototubes with large
active area is underway at CERN in collaboration with INFN-Bari and
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