High altitude test of RPCs for the Argo YBJ experiment
ABSTRACT A 50 m2 RPC carpet was operated at the YanBaJin Cosmic Ray Laboratory (Tibet) located 4300 m a.s.l. The performance of RPCs in detecting Extensive Air Showers was studied. Efficiency and time-resolution measurements at the pressure and temperature conditions typical of high mountain laboratories, are reported.
arXiv:hep-ex/9909034v1 21 Sep 1999
High Altitude test of RPCs for the Argo YBJ
The ARGO-YBJ Collaboration
C. Baccia, K.Z. Baob, F. Baronec, B. Bartolic, P. Bernardinid,
R. Buonomoc, S. Bussinoa, E. Callonic, B.Y. Caoe,
R. Cardarellif, S. Catalanottic, A. Cavalieref, F. Cesaronid,
P. Cretia, Danzengluobug, B. D’Ettorre Piazzolic,
M. De Vincenzia, T. Di Girolamoc, G. Di Sciascioc,
Z.Y. Fengh, Y. Fue, X.Y. Gaoi, Q.X. Gengi, H.W. Guog,
H.H. Hej, M. Hee, Q. Huangh, M. Iacovaccic, N. Iuccia,
H.Y. Jaih, F.M. Konge, H.H. Kuangj, Labacireng, B. Lib,
J.Y. Lie, Z.Q. Liui, H. Luj, X.H. Maj, G. Mancarellad,
S.M. Marik, G. Marsellad, D. Martellod, D.M Meig,
X.R. Mengg, L. Milanoc, A. Morsellif, J. Mui, M. Panareod,
M. Parisia, G. Pellizzonia, Z.R. Pengj, C. Pintod,
P. Pistillia,E. RealifR. Santonicof 1, G. Severinoℓ, P.R. Shenj,
C. Stanescua, J. Suj, L.R. Sunb, S.C. Sunb, A. Surdod,
Y.H. Tanj, S. Vernettoℓ, C.R. Wange, H. Wangj, H.Y. Wangj,
Y.N. Weib, H.T. Yangj, Q.K. Yaob, G.C. Yuh, X.D. Yueb,
A.F. Yuang, H.M. Zhangj, J.L. Zhangj, N.J. Zhange,
T.J. Zhangi, X.Y. Zhange, Zhaxisangzhug, Zhaxicireng,
1E-mail address email@example.com; fax number + 39 06 2023507.
Preprint submitted to Elsevier Preprint3 February 2008
aINFN and Dipartimento di Fisica dell’Universit` a di Roma Tre, Italy
bZhenghou University, Henan, China
cINFN and Dipartimento di Fisica dell’Universit` a di Napoli, Italy
dINFN and Dipartimento di Fisica dell’Universit` a di Lecce, Italy
eShangdong University, Jinan, China
fINFN and Dipartimento di Fisica dell’Universit` a di Roma ”Tor Vergata”, Italy
gTibet University, Lhasa, China
hSouth West Jiaotong University, Chengdu, China
iYunnan University, Kunming, China
jIHEP, Beijing, China
kUniversit´ a della Basilicata, Potenza, Italy
ℓIstituto di Cosmogeofisica del CNR and INFN, Torino, Italy
A 50 m2RPC carpet was operated at the YanBaJin Cosmic Ray Laboratory (Ti-
bet) located 4300 m a.s.l. The performance of RPCs in detecting Extensive Air
Showers was studied. Efficiency and time resolution measurements at the pressure
and temperature conditions typical of high mountain laboratories, are reported.
Key words: Gamma-Ray Astronomy; Extensive Air Shower; ARGO-YBJ; RPCs
The aim of the ARGO-YBJ experiment is the study of cosmic rays, mainly γ-
radiation, in an energy range down to about 100 GeV, by detecting small size
air showers with a ground detector. This very low energy threshold, which is
below the upper limit of the next generation satellite experiments, is achieved
in two ways:
1-By operating the experiment at very high altitude to better approach the
level where low energy air showers reach their maximum development. The
choice of the YangBaJing (YBJ) Cosmic Ray Laboratory (Tibet, China, 30.11◦
N, 90.53◦E.), 4300 m a.s.l, was found to be very appropriate.
2-By utilizing a full coverage detector to maximize the number of detected
particles for a small size shower.
The choice of the detector is subject to the following requirements. The search
Front end board
Fig. 1. Layout of the CLUSTER prototype which has been tested. Each RPC is
subdivided in 10 Pads. The details of the Pad are also shown.
for point sources requires the accurate reconstruction of the shower param-
eters, mainly the direction of the primary particle, in order to suppress the
isotropic background. This can be obtained by a diffuse sampling on the arrival
times of the shower front particles with nanosecond accuracy. Moreover the
full coverage concept requires an extremely large active detector area which
is only achievable with a very reliable and low cost detector. Robustness is a
further important requirement for a detector to be operated far away from the
facilities available in ordinary laboratories. The use of Resistive Plate Cham-
bers (RPCs) has been envisaged to meet these requirements. Indeed, RPCs
offer noticeable advantages owing to low cost, large active area, excellent time
resolution and possibility of an easy integration in large systems.
The ARGO-YBJ detector consists of a single RPC layer of ∼ 5000 m2and
about 95% coverage, surrounded by a ring of sampling stations which recover
edge effects and increase the sampling area for showers initiated by > 5 TeV
The trigger and the DAQ systems are built following a two level architecture.
The signals of a set of 12 contiguous RPCs, referred to as CLUSTER in the
following, are managed by a Local Station. The information from each Local
Station is collected and elaborated in the Central Station. According to this
logic a CLUSTER represents the basic detection unit.
A CLUSTER prototype of 15 RPCs, shown in fig 1, has been put in opera-
tion in the YBJ Laboratory in order to check both the performance of RPCs
operated in a high altitude laboratory and their capability of imaging a small
portion of the shower front.
In this paper the results concerning the performance of 2 mm gap, bakelite
RPC detectors operated in streamer mode at an atmospheric depth of 606
g/cm2are described. Data collected by the carpet and results from their anal-
Insulatin PET film
Grounded aluminium foil
Polystyrene foam plate
40 µm Al foil
glued on 40 m
Fig. 2. Cross section of the detector with details of the strip panel
ysis will be presented elsewhere.
2The experimental set up
The detector, consisting of a single gap RPC layer, is installed inside a dedi-
cated building at the YBJ laboratory. The set up, shown in fig. 1, is an array
of 3x5 chambers of area 280 × 112cm2each, laying on the building floor and
covering a total area of 8.5×6.0m2. The active area of 46.2m2, accounting for
a dead area due to a 7mm frame closing the chamber edge, corresponds to a
90.6% coverage. The RPCs, of 2 mm gas gap, are built with bakelite electrode
plates of volume resistivity in the range (0.5÷1) 1012Ω·cm, according to the
standard scheme reported in . The RPC signals are picked up by means
of aluminum strips 3.3 cm wide and 56 cm long which are glued on a 0.2
mm thick film of plastic material (PET
allows to work out the strips by milling a full aluminum layer. The strips
are embodied in a panel, consisting of a 4 mm thick polystyrene foam sheet
sandwitched between the PET film and an aluminum foil used as a ground
reference electrode. The detector cross section is given in fig. 2.
2) used as a robust support which
A rigid polystyrene foam plate is used to avoid the direct contact of the RPCs
with the concrete floor. The strip panel lays on top of the detector with the
strips oriented in the direction of the detector short side as shown in fig. 1 . At
the edge of the detector the strips are connected to the front end electronics
and terminated with 50 Ω resistors. The opposite end of the strips, at the
center of the detector, is not terminated. The RPC bottom electrode plate is
connected to a negative high voltage so that the strips, facing the grounded
plate, pick up a negative signal. A grounded aluminum foil (see fig. 2) is used
to shield the bottom face of the RPC and an extra PET foil, on top of the
aluminum, is used as a further high voltage insulator.
The front end electronics that has been used in the present test is not the
one envisaged for the final experiment, which will be described elsewere, but
is an already existing 16 channel circuit  developed for RPCs working in
streamer mode. The circuit contains 16 discriminators with about 50 mV
voltage threshold and gives the following output signals:
• The Fast OR of the 16 discriminators with the same input-to-output delay
(10 ns) for all the channels, which is used for time measurements and trigger
purposes in the present test.
• Serial read out of each channel that could be used for a strip by strip read
out. This possibility however is beyond the purposes of the present test.
The circuit is mounted on a 50×15cm2G10 board which is fixed on top of the
strip panel near to the edge of the detector as shown in fig. 1. The length of the
board is approximately tuned with the width of 16 strips so that very short
wires (a few cm) can be used for connecting each strip to the corresponding
input electrode on the board.
The 16 strips connected to the same front end board are logically organized in
a PAD of 56×56cm2area. Each RPC is therefore subdivided in 10 PADs which
work like independent functional units. The PADs are the basic elements which
define the space-time pattern of the shower; they give indeed the position and
the time of each detected hit. The fast OR signals of all 150 pads are sent
through coaxial cables of the same length to the carpet central trigger and
read out electronics.
The trigger logics allows to select events with a pad multiplicity in excess of a
given threshold. At any trigger occurrence the times of all the pads are read
out by means of multihit TDCs of 1 ns time bin, operated in common STOP
mode. Each TDC has 32 input channels and can store up to 16 events per
channel. The multiple hit operation is particularly important in detecting the
core of high energy showers where several particles can fall on the same pad
in a time interval of hundreds of nanoseconds. The trigger signal is used as
the common STOP signal. For each event the trigger multiplicity, the set of
all pads which produced the trigger and the times of all pads of the carpet are
As the carpet consists just of a single layer detector, a direct measurement
of the detection efficiency and time resolution requires the use of an auxiliary
”telescope” which can clearly define a cosmic ray impinging on it. The set
up was therefore completed with a small telescope consisting of 3 RPCs of
50 × 50cm2area with 16 pick up strips 3 cm wide connected to front end
electronics boards similar to the ones used in the carpet. The 3 RPCs were
overlapped one on the other and the triple coincidence of their fast OR signals
was used to define a cosmic ray crossing the telescope.
The gas system consisted of a central mixing station using three mass flowme-
ters that measured the gas composition with the require accuracy, better than
1% for all the components, and 5 parallel gas lines each feeding 3 RPCs in
series. The gas sharing among the 5 input lines was equalized using identical
high impedance capillar pipes in series with each line and the regular gas flow
was monitored by bubblers put at the exit of each line. An open gas circuit was
used, as only a modest amount of gas, about 15 l/h corresponding to 4 vol-
ume changes per day, was needed during about 2 months of carpet operation.
Three gas components were used: Argon, iso-Butane C4H10and TetraFluo-
roEthane C2H2F4that will be indicated in the following as Ar, i-But and TFE
The High Voltage system consisted of five 10 kV supplies each one feeding
3 RPCs in parallel. The operating voltage was settable to the wanted value
within 10 V accuracy and the operating current was monitored with a 1 µA
sensitivity instrument. A further two channel HV supply with 10 nA sensitivity
current monitor was used to feed the auxiliary telescope.
3 Data taking and experimental results
The peculiar working conditions of the mountain YBJ laboratory are not
only a very low average pressure of about 600 mbar, corresponding to an
atmospheric vertical depth of 606 g cm−2, but also a temperature that could
be particularly low in winter even inside the laboratory.
The measurements described in this paper were performed in the 2ndhalf of
February 1998 with an external temperature ranging between -20 and -5oC
and in the 1st half of May when the temperature was in the range -5 +15oC.
The internal temperature was kept, by using some heaters, between +4 and
+8oC in the first run and around 16-18oC in the second. The laboratory
temperature and pressure were monitored during all data taking.
The RPCs of the test carpet were operated in streamer mode  as foreseen for
the final experiment. This mode delivers  large amplitude saturated signals,
and is less sensitive than the avalanche or proportional mode  to electro-
magnetic noise, to changes in the environment conditions and to mechanical
deformations of the detector. On the other hand the larger rate capability
achievable in avalanche mode  is not needed in a cosmic ray experiment.
The first task to be carried out was the optimization of the gas mixture and
the search for the detector operating point in the YBJ laboratory conditions.
This was accomplished by means of the auxiliary telescope, before the start
up of the carpet test. The efficiency of the RPC in the central position of the
telescope (RPC2 in the following) was measured as the ratio of the number of
triple coincidence events to the number of double coincidences of the other two
RPCs. Three gas mixtures were tested which used the same components, Ar,
i-But and TFE, in different proportions: TFE/Ar/i-But = 45/45/10; 60/27/13
and 75/15/10. In the three cases the ratio Ar/TFE was changed to a large
extent, living the i-But concentration relatively stable.
TFE is an heavy gas with good quenching properties . An increase of TFE
concentration at expenses of the Ar concentration should therefore increase
the primary ionization thus compensating for the 40% reduction caused by the
lower gas target pressure (600 mbar) and reduce the afterpulse probability. For
each of the three gases a voltage scan was made for RPC2, leaving the other
two RPCs at a fixed operating voltage, and the following measurements were
made: RPC2 counting rate and current, double and triple coincidence rate.
The detection efficiency vs the operating voltage for the three gases is shown
in fig. 3 The reduction of the Argon concentration in favor of TFE results in
a clear increase of the operating voltage as expected from the large quenching
action of TFE. The data shown in fig. 3 are consistent with an increase of
30-40 V in operating voltage for a 1% reduction of the Argon concentration
in the mixture. In spite of the different operating voltages all three gases
approach the same efficiency of about 90% which include the inefficiency due
to geometrical effects. A more systematic study of the plateau efficiency is
presented below, in connection with the carpet test.
Fig. 4 shows the RPC2 current and counting rate vs the operating voltage
for the three gases. A small current linearly increasing with the voltage is
measurable well below the point where the RPC start to show a significant
counting rate. We interpret this as a leak current not flowing through the RPC
gas and not taking part in the detector working mechanism.
The ratio of the operating current to the counting rate gives the charge per
count delivered in the RPC gas, which is shown in fig. 5 as a function of the
operating voltage for the three gases. Here the small term corresponding to the
current leaks, as mentioned above, is subtracted to the total current. The data
presented in fig. 5 show that the higher is the TFE fraction, the lower is the
High Voltage (kV)
Fig. 3. Detection efficiency of one RPC of the auxiliary telescope vs operating voltage
for 3 gases: TFE/Ar/i-But=45/45/10 (+); 60/27/13 (*) and 75/15/10 (◦)
High Voltage (kV)
Fig. 4. RPC2 operating current and counting rate vs voltage for the three gases al-
ready mentioned in fig 3: TFE/Ar/i-But=45/45/10 (+); 60/27/13 (*) and 75/15/10
Fig. 5. Charge delivered per count for the 3 gases vs operating voltage:
TFE/Ar/i-But=45/45/10 (+); 60/27/13 (*) and 75/15/10 (◦)
charge delivered in the gas by a single streamer. Concerning the optimization
of this parameter the following points should be noted.
• The signal charge, in streamer mode operation, is anyway much above the
achievable threshold of the front end electronics. This is particularly true for
the final front end electronics that will be used for the experiment. Therefore
a larger detector signal is not an advantage in this respect.
• A lower operating current, on the contrary, is an advantage even if in a
cosmic ray experiment the currents are expected to be modest.
• In a cosmic ray experiment, on the other hand, the analog measurement
of the hit density, which is achievable either from amplitude measurements
of the strip signals or by sampling the operating current in appropriate
time intervals, is an interesting possibility to be exploited for studing the
shower core at energies as high as about 100 TeV. Indeed, according to a
MonteCarlo simulation of the final experiment, the digital read out of pads
near the shower core, is expected to saturate at about 15-20 TeV. In this
respect a lower delivered charge extends the dynamic range achievable for
the analog measurement.
We decided therefore to operate the test carpet with the gas mixture corre-
sponding to the highest fraction of TFE.
The tests performed on the carpet were essentially the same as for the auxil-
iary telescope. Fig. 6 shows the operating efficiency for the ORed pads 2-3-7-8
High Voltage (kV)
Fig. 6. Detection efficiency vs operating voltage for one of the carpet RPCs (•).
The same curve for a 2 mm gap RPC operating at sea level is also reported (◦) for
of one RPC of the carpet. The efficiency was measured using cosmic ray sig-
nals defined by the triple coincidence of the RPCs of the auxiliary telescope
which was placed on top of the carpet and centered on the corner among four
pads. The counting rate of the same pads OR signal, together with the RPC
current, are reported in fig. 7 vs the operating voltage. The results of the gas
with TFE=45% are also reported for comparison. A rather flat counting rate
plateau is observed corresponding to a rate of about 400 Hz for a single pad of
area 56×56 cm2. The residual slope of the plateau is mostly due to afterpulses
occurring after the end of the 250 ns shaped discriminated signals which pro-
duce a double counting of the signal due to the same CR track. The rate and
efficiency curves rise in the same voltage interval as expected.
The time jitter distribution of the pad signals was obtained by measuring the
delay of the fast OR signal with respect to RPC2 in the trigger telescope. This
distribution is shown in fig. 8 for the four pads. The average of the standard
deviations is 1.42 ns corresponding to a resolution of 1.0 ns for the single RPC
if we account for the fact that the distributions in fig. 8 show the combined
jitter of two detectors.
In the detection of extensive air showers however, the primary cosmic ray
direction is measured by comparing the times of hits due to different particles
of the shower. The space-time distribution of the shower hits allows to fit the
front of the shower that can be assumed to a good approximation to be a
plane. The time residual distribution of the individual shower particles with
respect to the front is reported in fig. 9. The long tail of delayed hits is due to
particles arriving much after the shower front.
High Voltage (kV)
Fig. 7. Counting rate (full triangles and circles) and operating current (open trian-
gles and circles) vs Voltage of one RPC of the carpet. Results are presented for the
gas with 45 % (triangles) and 75% (circles) of TFE respectively. The rate shown
refers to 4 ORed Pads out of the 10 pads of the RPC.
4 Discussion of the results
The use of RPCs for the detection of Extensive Air Showers in high altitude
laboratories poses some basic questions that the present test contributes to
• how do the operating voltage and plateau efficiency scale with the pressure
for the streamer mode operation
• how does the detector time resolution compare with the intrinsic jitter of
the shower front.
With the purpose of answering the first question a 2 mm gap RPC was op-
erated at sea level with the same gas, TFE/Ar/i-But=75/15/10, used for the
30 3540 45
3035 40 45
3035 40 45
Fig. 8. Time jitter distribution of 4 pads of the carpet. The telescope RPC2 signal
is used as common stop. The operating voltage is 7.4 KV
YBJ carpet. The detection efficiency vs operating voltage in fig. 6, compared
to the operation at 600 mbar pressure in YBJ, shows an increase of about 2.5
kV in operating voltage.
The effect of small changes of temperature T and pressure P on the operating
voltage can be accounted for  by rescaling the applied voltage Va according
to the relationship
V = VaP0
where Po and To are arbitrary standard values, e.g.1010 mb and 293 K re-
spectively for a sea level laboratory. However, starting from the YBJ data, the
above formula predicts an operating voltage at sea level which is considerably
larger than the experimental one.
A large change of pressure produces a proportional change in the gaseous
target mass per unit surface, like a change of the gas gap size. The operating
voltage as a function of the gap is studied in  for 1.5, 2, and 3 mm gap
RPCs, in the case of the binary gas mixture TFE/i-But=97/3. The result is
shown in fig. 10 where the operating voltage in streamer mode is defined as
that giving 50% streamer probability with respect to the plateau.
Hits (× 106)
Fig. 9. Time residual distribution for all 150 Pads. The trigger signal is used as
1.2 1.41.6 1.82.0 2.22.4 220.127.116.11 3.2
Sea level operating point
YBG Operating Voltage
Gas gap (mm)
Fig. 10. Operating voltage vs gas gap for the binary gas TFE/i-But=97/3 (upper
curve) and for the YBJ gas TFE/Ar/i-But= 75/15/10 (lower curve)
The data which refer to the same pressure of 1010 mbar and temperature
of 293 K, show that the voltage do not scale proportionally to the gap, the
electric field (voltage/gap) being larger for thinner gaps. Indeed the avalanche
to streamer transition occurs when the gas amplification, eαg/αg, exceeds a
given threshold. The larger is the gap the smaller is the α value and therefore
the electric field that is needed for reaching the streamer threshold. A zero
constraint parabolic fit of the three experimental points is also reported in
fig. 10. The fitted curve, which refers to the binary gas, can be scaled to the
YBJ gas, TFE/Ar/i-But=75/15/10 at 20OC temperature, using the point at
sea level (8.6 kV at 1010 mbar and 32OC, rescaled to 20OC according to the
above formula) and assuming that the ratio of the operating voltages for the
two gases is the same for all gap sizes. The result is the lower curve in fig. 10
which represents the operating voltage vs gap for the YBJ gas and fits well
the YBJ operating point, 6.12 kV, if we assume that a 2mm gap at the YBJ
pressure of 603 mbar is equivalent to a 1.2 mm gap at 1010 mbar. The above
assumption is based on the fact that, in the ideal gas approximation, the mass
per unit surface of the gaseous target, which fixes the operating voltage for
each gas, is given by the parameter gap · pressure/temperature.
Fig. 6 also shows that the plateau efficiency measured at YBJ is 3-4% lower
than at sea level. Although a lower efficiency is expected from the smaller
number of primary clusters at the YBJ pressure, we attribute most of the
difference to the underestimation of the YBJ efficiency. At the YBJ level
indeed the ratio of the cosmic radiation electromagnetic to muon component
is about 4 times larger than at sea level. A spatial tracking with redefinition of
the track downstream of the carpet would eliminate the contamination from
soft particles, giving a more accurate and higher efficiency. On the other hand
the lower efficiency could hardly be explained with the gas lower density. The
number of primary clusters in the YBJ test, estimated around 9, is the same
as in the case of some gas, e.g. Ar/iBut/CF3Br=60/37/3, that was frequently
used at sea level with efficiency of 97-98% .
The time residual distribution in fig. 9 shows a long tail due to delayed particles
traveling well behind the shower front. The gaussian fit, disregarding this tail,
gives a standard deviation of 1.6 ns to be compared with the RPC intrinsic
time resolution of 1.0 ns. Taking into account the additional uncertainties due
to the propagation time of the signal traveling along a strip of 56 cm and to
the impact point of the shower particle which can be everywhere inside the
PAD we get a total RPC jitter of 1.3 ns. The residual jitter of the shower front
can be estimated to be: σshower= 0.9ns.
This is valid for high energy showers selected by the multiplicity trigger as
in the case reported in fig. 9 At lower energies the shower jitter increases
The authors are endebted to G. Aielli (Universit` a di Roma “Tor Vergata”) for
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List of Figures
1 Layout of the CLUSTER prototype which has been tested.
Each RPC is subdivided in 10 Pads. The details of the Pad
are also shown.3
2 Cross section of the detector with details of the strip panel4
3Detection efficiency of one RPC of the auxiliary telescope vs
operating voltage for 3 gases: TFE/Ar/i-But=45/45/10 (+);
60/27/13 (*) and 75/15/10 (◦)8