The built-in redundancy of the physics prototype calibration and readout system allows several
different monitoring concepts to be applied and cross-checked. The auto-calibration capabilities
of new developed photodetectors (SiPM, see below) can be of so manifold use, from production
quality control to commissioning diagnostics, linearity correction and temperature monitoring.
A. Novel photodector SiPM and its calibration
The scintillator based hadron calorimeter (AHCAL) relies on
the use of novel multi-pixel Geiger mode silicon
photo-sensors (SiPMs). The SiPM is a pixilated avalanche
photo-diode operated in limited Geiger mode (see figure on
the left). The detector surface of l x l mm2 is divided into 1156
pixels. We gained experience with this photodetector building
small test calorimeter MiniCal with about 100 SiPM channels
[see MiniCal].
The analog output is obtained by adding the response of all
pixels fired as independent digital counters. The SiPM are operated at 2-3 volts above breakdown
voltage (typically at 55 V). By being the internal pixel capacitance Cpixel of typically 50 fF the
charge collected for one photo-electron signal is ~160 fC (or ~106 electrons). The SiPM offers a
very fast response with a typical rise time of the order of a nanosecond. The fall time of the signal
depends on the pixel quenching resistor and can be tuned to the needs of the experimental
application (typical values are between 2-150 ns). The dynamic range is determined by the finite
number of pixels and is ~200 pC.
SiPMs have virtues as well as drawbacks. The left figure above displays the SiPM signal as a
function of the number of photoelectrons released from the photocathode. The signal becomes
saturated at high light intensities as a result of finite number of pixels. At low light intensities one
can distinguish individual photoelectrons on the photocathode equally spaced. This allows
obtain the gain. SiPMs with their high intrinsic gain are sensitive to temperature and bias voltage
fluctuations. The typical values are -1.7% per temperature increase of 1°C and 2.5% per 0.1V
change of bias voltage. Therefore, calibration and monitoring systems (see next section) are of
central importance. Their layout has important consequences for mechanical structures and
electronics specifications. Careful evaluation of present running experience and characterization
of different hardware solutions must enter into final conceptual design choices, and it may be
appropriate to pursue more than one option during the next R&D cycle.
B. Calibration and Monitoring Board
The task of calibration is to obtain gain and measure the nonlinearity of each SiPM. The aim of
monitoring is to record response of a SiPM on long time scale and provide correction factors for
gain which changes due to the temperature, bias voltage fluctuations and also due to non
predictable situations. The Calibration and Monitoring Board (CMB) provides short steered
pulses for UV LEDs and the light is carried by clear fibres to each scintillator tile.
The pulses are provided by a LED driver which has fast rise and fall edges of the rectangular
signal of ~ 10 ns width. The amplitude can be tuned from zero to 6 V. Each LED is monitored by
a PIN diode. The corresponding light intensity in the tile changes from several photons to the
equivalent of light produced by the passage of up to 100 minimum ionizing particles. The low
intensity light is used to observe single photoelectron peaks in the SiPM which are equally
spaced and allow obtaining gain of each SiPM. With a sweeping from low up to high intensities
the saturation curve of a SiPM can be measured.
Each detection plane with 216 SiPMs has on a side one CMB which is divided to two printed
boards. The upper one is shown in the figure with circuits for 6 LEDs. The LED driver is
controlled by DAQ via CAN bus which allows to control parameters like enabling, pulse width
and amplitude. The board contains also the temperature readout from sensors connected to the
photodetectors.
C. Operational experience and calibration
The HCAL was assembled and commissioned at DESY, where also an initial calibration of the
active layers was obtained in the electron test beam. In 2006, together with ECAL and TCMT,
the stack with 23 instrumented layers was exposed to electron and hadron beams of 6-45 GeV
and 6-120 GeV, respectively, in the H6 beam line at the CERN SPS. In addition high intensity
muon beams were available for calibration.
All calorimeter cells have been calibrated with muons. The MIP signal AMIP is used as a scale for
the deposited energy and to set the noise suppression threshold of ½ MIP, which yields a MIP hit
efficiency of about 95%. The noise hit occupancy is then about 10-3, corresponding to ~0.5 GeV
on the electromagnetic (em) energy scale. The gain Apixel is measured with low intensity LED
light and used for non-linearity correction. The energy per cell in units of MIPs is then obtained
from the formula
E [MIP] = A/AMIP ∗ F (Npixel) with Npixel = A/Apixel
where A is amplitude measured in ADC counts. The non-linearity correction F depends only on
the amplitude in units of pixels and is 1 for small amplitudes (all scale factors are absorbed in the
MIP calibration factor). The function F is the inverse of the normalized response function and
can be approximated as F = −N/Npixel * log (1−Npixel/N) for a total of N active pixels on the SiPM.
In practice F is obtained from the test bench measurements. The conversion from MIPs to
deposited energy depends on the incident particle type and is taken from simulations or using the
known beam energy as reference. MIP and pixel scale are subject to temperature variations of a
few percent per Kelvin. The redundant monitoring system offers various possibilities for
correcting these effects, using gain, LED reference signals or direct temperature measurement.