Performance of hybrid photon detector prototypes ... - LHCb - CERN

Nuclear Instruments and Methods in Physics Research A 442 (2000) 164}170
Performance of hybrid photon detector prototypes with 80%
active area for the rich counters of LHCB
E. Albrecht, M. Alemi, G. Barber, J. Bibby, M. Campbell, A. Duane,
T. Gys*, J. Montenegro, D. Piedigrossi, R. Schomaker, W. Snoeys, S. Wotton,
K. Wyllie
Abstract
* Corresponding author. Tel.: ##41-22-767-83-07; fax:
##41-22-767-32-00.
E-mail address: thierry.gys@cern.ch (T. Gys)
EP Division, CERN, CH-1211 Geneva 23, Switzerland
University of Milan, Italy
Imperial College London, UK
University of Oxford, UK
Federal University of Rio de Janeiro, Brazil
Delft Electronic Products B.V., Roden, The Netherlands
University of Cambridge, UK
We report on the ongoing work towards a hybrid photon detector with integrated silicon pixel readout for the ring
imaging Cherenkov detectors of the LHCb experiment at the Large Hadron Collider at CERN. The photon detector is
based on an electrostatically focussed image intensi"er tube geometry where the image is de-magni"ed by a factor of &5.
The anode consists of a silicon pixel array, bump-bonded to a binary readout chip with matching pixel electronics. The
performance of full-scale prototypes equipped with 61-pixel anodes and external analogue readout is presented. The
average signal-to-noise ratio is &11 with a peaking time of 1.2 s. The tube active-to-total surface ratio is 81.7%, which
meets the LHCb requirements. The spatial precision is measured to be better than 90 m. A cluster of three such tubes
has been installed in the LHCb RICH 1 prototype where Cherenkov gas rings have been successfully detected. Progress
towards the encapsulation of new pixel electronics into a tube is also reported. In particular, the status of the
development of a binary readout chip with a peaking time of 25 ns and a low and uniform detection threshold is
summarized. 2000 Elsevier Science B.V. All rights reserved.
PACS: 85.60.Ha; 41.90.#e; 29.40.Ka; 85.40.e
Keywords: Hybrid photon detectors; Electron optics; Cherenkov detectors; Microelectronics
1. Introduction
Particle identi"cation in the LHCb detector [1]
is based on two Ring Imaging Cherenkov (RICH)
0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 2 1 6 - 4
detectors with one silica aerogel and two #uorocarbon
gas radiators. The Cherenkov photons with
wavelength from 250 to 600 nm must be detected
with high e$ciency over a total area of 2.9 m and
at a granularity of 2.5 mm2.5 mm. The time resolution
should be better than 25 ns to cope with the
LHC bunch-crossing rate. The photodetectors will
be subjected to the fringe "eld (between 50 and

E. Albrecht et al. / Nuclear Instruments and Methods in Physics Research A 442 (2000) 164}170 165
100 G without magnetic shielding) of the LHCb
dipole magnet. They will be exposed to a maximum
radiation dose of 3 krad per year.
A major e!ort is going into the development of
a photon detector system which provides a large
fraction of active area (&70%) at an acceptable
cost. For this purpose, two photon detector options
are presently considered: Hybrid Photon Detectors
(HPDs) and Multi-Anode Photo-Multiplier Tubes
(MAPMTs). Commercially available examples
of both kinds exist, but do not fully meet the
LHCb requirements. In particular, they su!er from
a rather poor active-to-total area ratio (450%).
An extensive R&D programme is being carried out,
oriented towards the manufacturing of HPDs customized
for this application. For the MAPMTs, the
envisaged solution is to individually couple them to
a small lens in order to increase their area coverage.
2. The pixel hybrid photon detector
The present R&D work follows one approach
to HPD design referred to, within LHCb, as the
pixel-HPD [2]. Another approach, called the pad-
HPD, is described elsewhere in these proceedings
[3]. The development of the pixel-HPD is being
carried out in close collaboration with industry [4].
It is based on an electrostatically focussed tube
design, de-magnifying by a factor of 4}5 the photocathode
image onto a small silicon detector array
with &500 m 500 m pixels (see Fig. 1). This
detector is in turn bump-bonded to a fast, binary
readout chip with matching pixel electronics integrated
inside the vacuum envelope of the tube, which
results in a high signal-to-noise ratio and a limited
number of feed-throughs from the tube. This concept
has already been demonstrated by the successful
realization of the Imaging Silicon Pixel Array
(ISPA) tube [5].
A half-scale pixel-HPD prototype, based on
a 40 : 11 mm electrostatically focussed electron optics
and equipped with a fast (100 ns peaking time),
binary pixel readout chip, has already been developed
[6]. The tube has successfully detected
Cherenkov air rings produced by high-energy pions
in prototypes of the LHCb RICH system [7,8].
However, this tube has been manufactured with
Fig. 1. Schematic design of a 72:18 mm pixel-HPD prototype
tube. The electron optics are based on a tetrode structure with
cross-focussing. On the anode is mounted a silicon detector
comprising 1024 pixels and their associated binary front-end
electronics.
existing parts and its active area does not exceed
50%. In addition, the pixel readout chip encapsulated
in the tube is not optimal for single photoelectron
detection. To improve these aspects, larger
tubes as well as new pixel electronics are under
development.
3. Large tube developments
3.1. Phosphor screen prototype
Detailed design studies of the electron optics
indicated that the desired active-area fraction
(&80%) could be achieved using electron optics
based on a tetrode structure. The electrodes are
also shaped in such a way that the tube performance
is una!ected by the proximity of other tubes
or magnetic shielding. The baseline dimensions of
the tube are 72 mm active input diameter and
18 mm output diameter and it is nominally operated
at 20 kV. The optical input window is spherical
and made of quartz, and the photocathode is
a multi-alkali S20 type.
A "rst prototype has been produced with a phosphor
screen anode coupled to a CCD camera [2].
From electron optics measurements, the tube active
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166 E. Albrecht et al. / Nuclear Instruments and Methods in Physics Research A 442 (2000) 164}170
area is 81.7%. The de-magni"cation is 0.210 onaxis
and 0.247 at the edge. The corresponding standard
deviations of the Point Spread Function
(PSF) at the anode plane are &33 and &54 m,
respectively.
3.2. 61-pixel HPD prototypes
Three HPD versions of the 72 : 18 mm tube have
also been manufactured. The quantum e$ciencies
(which include light re#ection losses at the entrance
face of the quartz window and its transmission) of
these prototypes range from 22% to 26% at
270 nm and from 16% to 20% at 400 nm. Each
anode is currently equipped with a 61-pixel silicon
detector read out externally with the analogue VA2
chip (1.2 s peaking time) [9]. The pixels are hexagonal
with dimension of 2 mm #at-to-#at. These
tubes were "rst evaluated in the laboratory using
a blue Light Emitting Diode (LED) source operated
in pulsed mode. A typical photoelectron response
is shown in Fig. 2. The superimposed "tting
curve is calculated following a procedure similar to
the one detailed elsewhere in these proceedings
[10]. The average signal-to-noise ratio is &11 at
nominal tube operation (20 kV tube high voltage,
60 V silicon detector bias voltage).
The de-magni"cation law of the tube has been
measured precisely by mounting the LED source
on a XY translation table system. The source was
90 mm distant from the quartz window of the tube.
A cylindrical collimator (20 mm long, 0.2 mm in
diameter) limited the source divergence to
&5 mrad, resulting in a light disk &0.9 mm in
diameter on the window axis. The translation
tables were moved horizontally and vertically over
the full active diameter of the tube. The density of
measurement points was increased around the pixel
boundaries. The results are shown in Fig. 3. Each
translation table position, known to within 1 m,
has been assigned a radial coordinate relative to the
tube axis. Since the entrance face of the window is
spherical, a refraction e!ect occurs o!-axis, and the
resulting radial coordinate on the photocathode
(r ) is smaller. This coordinate corresponds to the
horizontal axis on the "gure. Similarly, the arrival
points of the photoelectrons on the silicon pixel
detector have been assigned a radial coordinate, r ,
Fig. 2. Typical photoelectron spectrum recorded from a fullscale
prototype tube operated at 20 kV and read out with
external analogue electronics (1.2 s peaking time). A "t tothe
data is indicated by the solid line and yields a photoelectron
average of 1.42.
relative to the tube axis. This coordinate is the
vertical axis of Fig. 3. More precisely, r was deter-
mined by calculating the centre-of-gravity of the
pixel cluster created by the light spot image. Each
point (r , r ) (black dot in Fig. 3) is distributed
around a staircase function, as expected from the
pixel geometry.
For a cross-focussing design of the electron optics,
the de-magni"cation law can be modelled, to
a good approximation, as
r "r #r. (1)
is the linear de-magni"cation and is related to
the edge, or pin-cushion, distortion. r and r are
expressed in mm, in mm. The design values for
and are 0.216 and 0.710 mm, respectively
[2]. The experimental values can be derived from
a staircase function "t to the data. For the horizontal
scan, the "t procedure is as follows. The light
spot image on the pixel array is the result of the
convolution between the tube PSF and the LED
"nite spot size scaled by the de-magni"cation factor.
This image is modelled as a two-dimensional

E. Albrecht et al. / Nuclear Instruments and Methods in Physics Research A 442 (2000) 164}170 167
Fig. 3. Measured de-magni"cation laws (dashed lines) of the
HPD electron optics resulting from a horizontal (top) and vertical
(bottom) scan of a LED spot over the photo-cathode active
diameter. The solid lines are the results of a staircase function "t
to the data (black dots).
Gaussian function centred at r and with a standard
deviation assumed to be constant over the
silicon detector. As long as the whole light spot is
con"ned within one pixel, the value of r is assigned
to be the horizontal coordinate of the pixel centre.
In this case, there is no precise relation between
r and r . Near to the pixel boundaries, however,
the light spot is integrated over two pixels, and the
horizontal coordinate of its centre-of-gravity r will
change according to the error function. Consequently,
the point of in#ection r of this function
corresponds exactly to the pixel boundary. The
staircase function "t will be the sum of several error
functions centred at the pixel boundaries. Since the
coordinates of the pixel boundaries on the silicon
detector are known with high accuracy and are
equal to integer multiples of half the pixel size
(1 mm), these points of in#ection can be folded into
relation (1) to derive the experimental values of
and . A similar procedure is used for the vertical
scan, taking into account the hexagonal shape of
the pixels. The "ts (solid lines in Fig. 3) yield the
following values for and : (0.208$0.003) and
(0.64$0.13)10 mm for the horizontal scan,
and (0.212$0.003) and (0.40$0.12)10 mm
for the vertical scan. The resulting quadratic laws
are shown as dashed lines in Fig. 3. The standard
deviations of the spot image are (102$26) m
(horizontal scan) and (109$20) m (vertical scan).
There are two contributions to these values of :
the LED "nite spot size on the pixel detector
(&900 m0.208/12+54 m for the horizontal
scan) and the PSF standard deviation. Since
they sum in quadrature, the PSF standard deviation
is calculated to be &87 m (horizontal scan).
Taking into account the parameter errors, this in
reasonable agreement with the values measured on
the phosphor screen prototype.
3.3. Pixel-HPD cluster tests in the LHCb RICH 1
prototype
Beam tests were performed at CERN in the T1-
X7b facility which provides 120 GeV/c negative
pions. The Cherenkov counter used in the test
beam is a full-scale prototype [9] of the LHCb
RICH 1 detector (see Fig. 4). The beam particles
enter the prototype box along a tube of 90 mm
internal diameter and the beam axis intersects the
mirror centre at an angle of 183 to the mirror axis.
The detector plane is "xed at a distance of 1143 mm
from the mirror centre. Initial alignment relative to
the particle beam axis was performed using a laser
beam. The Cherenkov radiator is a C F gas
contained within a volume of length ¸"1000 mm
between the entrance window and the mirror. The
spherical glass mirror used in this con"guration has
a focal length of 1117 mm and a diameter of
112 mm. The re#ectivity of the mirror was measured
to be &88% at 600 nm, falling to &70%
at 200 nm. The radiator volume is sealed with
a 25 mm-thick quartz plate. The 3 pixel-HPD cluster
(total detection area +135 cm) is positioned at
the focal plane with a 2 mm gap between the tubes.
The mirror position is adjusted to focus the
Cherenkov rings on the 3 photon detectors. All
three tubes were operated at nominal high voltage
(20 kV) and silicon detector bias (60 V).
Fig. 5 shows an accumulated data set of Cherenkov
rings produced by 120 GeV/c negative pions in
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168 E. Albrecht et al. / Nuclear Instruments and Methods in Physics Research A 442 (2000) 164}170
Fig. 4. Full-scale prototype of the LHCb-RICH 1 detector used for the pixel-HPD cluster tests.
Fig. 5. Accumulated data set of Cherenkov rings produced by
120 GeV/c negative pions traversing a C F gas radiator. The
circle is to guide the eye.
the C F radiator maintained at a pressure of
&1115 mbar, corresponding to a Cherenkov angle
of &55.5 mrad. In the "gure, every pixel chan-
nel is represented by a hexagon, the size of which is
&10 mm #at-to-#at at the level of the optical
input window. The level of shading is proportional
to the pixel content and the number of photons
observed in each pixel is also indicated. The total
number of events recorded for this particular run
is 10 000. The observed photoelectron number
N corresponds to a "gure of merit per tube
N +N /(¸sin ) between 170 and 190 cm,
with a &10% error. This result includes corrections
to take into account light re#ection losses on
the 25 mm-thick quartz plate (&7%) and the geometrical
coverage (each tube covers &703 of the
full ring). On the contrary, corrections related to
light losses due to the mirror size and re#ectivity, as
well as dead DAQ channels (e.g. 5 pixels in the
bottom right tube) have not been made. Detailed
analyses of the test beam data are in progress and
will be reported in a future publication.
4. Pixel electronics developments
A front-end binary pixel chip for the LHCb
RICH must meet stringent requirements. In

particular, the ampli"er must have a shaping time
of 425 ns, and the discriminator apply a threshold
of (2000e with a pixel-to-pixel RMS spread of
(200e to uniformly identify the low signals
(&5000e) generated by single photoelectrons.
Both of these requirements have been met in recent
pixel developments. For example, a test chip fabricated
in a 0.25 m commercial CMOS process
[11] exhibits a minimum threshold of 1500e with
an RMS spread of 160e without adjustments to
individual pixels. A 3-bit adjustment per pixel reduces
this spread to 25e. Operation of a chip with
such a low and uniform threshold will improve the
e$ciency of detecting single photoelectrons by
minimizing the e!ects of charge sharing at the pixel
boundaries and photoelectron back-scattering at
the silicon detector surface [6]. The test chip has
also been irradiated and is still fully operational
after a 30 Mrad dose of X-rays. It incorporates
some digital circuitry complying with the LHCb
readout architecture.
A full chip is currently under design, mainly as
a collaborative e!ort between the ALICE pixel
tracker project and the LHCb RICH group, for
which there are many similar requirements. The
basic pixel size is 50 m300 m. The chip architecture
has been designed in such a way as to allow
the chip to be operated in a ALICE or a LHCb
mode. In the latter mode, the discriminators of
8 pixels will be OR-ed together, e!ectively creating
large pixel channels of 400 m300 m, which is
close to the LHCb requirement, whilst reducing the
hit occupancy seen by the front-ends and decreasing
the risk of any corresponding pulse pile-up. The
number of columns (32) in the chip is common to
the global LHCb architecture, and allows the data
from a chip to be read out in &800 ns through 32
parallel lines at a rate of 40 MHz.
5. Conclusions
E. Albrecht et al. / Nuclear Instruments and Methods in Physics Research A 442 (2000) 164}170 169
Within the framework of the LHCb RICH R&D
work, full-scale (72:18 mm) HPD prototypes have
been recently manufactured. They are equipped
with 61-pixel silicon detector anodes and external
analogue readout. The average signal-to-noise ratio
is &11. The tubes exhibit a high active-to-total
area ratio of &81.7%. The spatial precision at the
anode is measured to be &90 m. A cluster of
three such tubes has been installed in the LHCb
RICH 1 prototype for beam tests purposes.
Cherenkov rings produced by charged particles
traversing a C F gas radiator have been successfully
detected.
The performance achieved from recent developments
in pixel electronics meet the analogue frontend
requirements of LHCb. In particular, the low
discrimination thresholds measured on these chips
imply that the detection of single photoelectrons
can be close to optimum. The design of a mixedsignal
pixel chip is underway, compatible with operation
in the ALICE and LHCb experiments. The
aim is to encapsulate this chip within a 72 : 18 mm
HPD tube.
Acknowledgements
The authors acknowledge H.-J. Hilke for his
support and the industrial collaborators for their
e$cient contribution. The work reported here
would not have been possible without the close
involvement of D. Websdale and the technical support
provided by colleagues from several institutes
associated to the LHCb RICH detector developments.
The authors also thank O. Ullaland,
G. Mallot and R. Forty for discussions and comments.
References
[1] The LHCb Collaboration, LHCb Technical Proposal,
CERN/LHCC 98-4, LHCC/P4, 20 February 1998.
[2] M. Alemi et al., Development of hybrid photon detectors
with integrated silicon pixel readout for the RICH
counters of LHCb, Proceedings of the IEEE 1998 Nuclear
Science Symposium and Medical Imaging Conference,
November 8}14, 1998, Toronto, Canada, IEEE Trans.
Nucl. Sci., (2000) in press.
[3] A. Braem et al., Nucl. Instr. and Meth. A 442 (2000) 128.
[4] Delft Electronic Products (DEP) B.V., P.O. Box 60,
Dwazziewegen 2, NL-9300 AB Roden, Netherlands.
[5] T. Gys et al., Nucl. Instr. and Meth. A 355 (1995) 386.
[6] M. Alemi et al., First operation of a hybrid photon
detector prototype with electrostatic cross-focussing and
SECTION V.

170 E. Albrecht et al. / Nuclear Instruments and Methods in Physics Research A 442 (2000) 164}170
integrated silicon pixel readout, Nucl. Instr. and Meth.
A (2000) in press.
[7] M. Alemi et al., Nucl. Phys. B (Proc. Suppl.) 78 (1999) 360.
[8] E. Albrecht et al., Nucl. Instr. and Meth. A 433 (1999) 159.
[9] E. Albrecht et al., Nucl. Instr. and Meth. A 411 (1998) 249.
[10] C. Damiani et al., Nucl. Instr. and Meth. A 442 (2000) 136.
[11] M. Campbell et al., IEEE Trans. Nucl. Sci. 46 (3) (1999)
156.