In this report we outline the study of the development of calorimeter detectors using bright scintillating crystals. We discuss how timing information with a precision of a few tens of pico seconds and below can significantly improve the reconstruction of the physics events under challenging high pileup conditions to be faced at the High-Luminosity LHC or a future hadron collider. The particular challenge in measuring the time of arrival of a high energy photon lies in the stochastic component of the distance of initial conversion and the size of the electromagnetic shower. We present studies and measurements from test beams for calorimeter based timing measurements to explore the ultimate timing precision achievable for high energy photons of 10 GeV and above. We focus on techniques to measure the timing with a high precision in association with the energy of the photon. We present test-beam studies and results on the timing performance and characterization of the time resolution of LYSO-based calorimeters. We demonstrate time resolution of 30 ps is achievable for a particular design

Anderson, D.

Apresyan, A.

Bornheim, A.

Duarte, J.

Hassanshahi, M. H.

Peña, C.

Ronzhin, A.

Spiropulu, M.

Trevor, J.

Xie, S.

Caltech Authors - Main

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LYSO based precision timing calorimeters
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17th International Conference on Calorimetry in Particle Physics (CALOR2016) IOP Publishing
IOP Conf. Series: Journal of Physics: Conf. Series 9 8 (2017) 012023  doi :10.1088/1742-6596/928/1/012023
LYSO based precision timing calorimeters
A Bornheim1, A Apresyan1, A Ronzhin2, S Xie1, J Duarte1, M
Spiropulu1, J Trevor1, D Anderson1, C Pena1 and M H Hassanshahi3
1Caltech, 1200 E. California Blvd., 91125 Pasadena, USA
2FNAL, Kirk Road and Pine Street, 60510 Batavia, IL, USA
3Sharif University, Azadi Avenue, Tehran, Iran
E-mail: bornheim@hep.caltech.edu
Abstract. In this report we outline the study of the development of calorimeter detectors
using bright scintillating crystals. We discuss how timing information with a precision of a
few tens of pico seconds and below can significantly improve the reconstruction of the physics
events under challenging high pileup conditions to be faced at the High-Luminosity LHC or
a future hadron collider. The particular challenge in measuring the time of arrival of a high
energy photon lies in the stochastic component of the distance of initial conversion and the
size of the electromagnetic shower. We present studies and measurements from test beams for
calorimeter based timing measurements to explore the ultimate timing precision achievable for
high energy photons of 10 GeV and above. We focus on techniques to measure the timing with
a high precision in association with the energy of the photon. We present test-beam studies and
results on the timing performance and characterization of the time resolution of LYSO-based
calorimeters. We demonstrate time resolution of 30 ps is achievable for a particular design.
1. Introduction
High energy particle collider experiments are facing ever more challenging conditions, operating
at todays accelerators, such as the Large Hadron Collider (LHC), capable of providing
instantaneous luminosities of 1034 cm−2s−1 and above. The high center of mass energy, the
large number of simultaneous collisions of beam particles in the experiments and the very
high repetition rates of the collision events pose huge challenges. Current upgrade plans for
the LHC, the High Luminosity LHC (HL-LHC), future energy upgrades of the LHC or next
generation hadron colliders such as the Future Circular Collider (FCC) will push the boundaries
even further. These operational conditions result in extremely high particle fluxes, causing
very high occupancies in the particle physics detectors operating at these machines. A precise
timing information with a precision of around 10 ps and below is seen as a major aid in the
reconstruction of the physics events under such challenging conditions.
The rate of simultaneous interactions per bunch crossing (pileup) at the HL-LHC is projected
to reach an average of 140 to 200. The large amount of pileup increases the likelihood of confusion
in the reconstruction of events of interest, due to the contamination from particles produced in
different pileup interactions. The ability to discriminate between jets produced in the events of
interest, especially those associated with the vector boson fusion processes, and jets produced by
pileup interactions, will be degraded, the missing transverse energy resolution will deteriorate,
and several other physics objects performance metrics will suffer.
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17th International Conference on Calorimetry in Particle Physics (CALOR2016) IOP Publishing
IOP Conf. Series: Journal of Physics: Conf. Series 9 8 (2017) 012023  doi :10.1088/1742-6596/928/1/012023
One way to mitigate pileup confusion effects, complementary to precision tracking methods,
is to perform a time of arrival measurement associated with a particular layer of the calorimeter,
allowing for a time assignment for both charged particles and photons. Such a measurement with
a precision of about 20 to 30 ps, when unambiguously associated to the corresponding energy
measurement, will significantly reduce the inclusion of pileup particles in the reconstruction of
the event of interest given that the spread in collision time of pileup interactions is about 200
ps. The association of the time measurement to the energy measurement is crucial, leading
to a prototype design that calls for the time and energy measurements to be performed in
the same active detector element. We have demonstrated in the past that full absorption
crystal calorimeters as well as crystal sampling calorimeters can achieve such a precise timing
measurement [1, 2].
2. Experimental Setup
Our previous results on precision timing with LYSO based calorimeters [2] were obtained with
multi-channel plate photo multiplier (MCP-PMT) as photo detectors. MCP-PMTs feature a
signal rise time on the order of 100 ps. For laser pulses with a FWHM of 50 ps we achieved a
differential timing resolution of 7 ps [3], corresponding to a single device timing resolution of 5
ps. This performance is limited by the DRS based digitizer we use as a DAQ system, operating
at 5 GS/s. In the studies described in this paper we use silicone multi pixel photon counters
(SiPM). While they feature a slower rise time they allow a very good timing performance for
large, coherent signals [4]. For our setup we use four different types of SiPMs with 10, 15 and
25 µm pixel size and 1 × 1 mm and 3 × 3 mm sensor size [5]. They are all read out with a
DRS based digitizer through a clipping circuit as described in [4]. We do not amplify the out-
put signal of the SiPMs, only exploiting the very large light yield of the LYSO scintillator and
the intrinsic amplification of the SiPMs. Using the same fast laser pulses and DAQ system we
obtain a differential timing resolution between two SiPMs of better than 10 ps, very similar to
the MCP-PMT performance.
The experimental setup we use for the calorimetric timing measurements consists of a single cell
of a sampling calorimeter with 29 layers of LYSO crystal and tungsten absorber. The lateral
dimensions are 14× 14 mm2. The total depth of the cell is about 11.5 cm with the LYSO plates
having a thickness of 1.5 mm. The scintillation light from the LYSO plates is extracted with
four wave length shifting (WLS) fibers. The same cell has been used to measure the timing per-
formance in comparison to the timing performance of a single cube of LYSO [2]. More details
of the calorimetric performance of the Shashlik configuration are discussed in references [6] and
[7].
In addition to plastic WLS fibers we also tested quartz capillaries filled with liquid wave length
shifter using DSB as a wave length shifting agent [8]. To optically couple the quartz capillaries
to the SiPMs we use a clear plastic fiber light guide which is connected to the end of the quartz
capillary with a metal sleave tube. The same clear fiber coupler is used for the plastic WLS
fibers to maintain equivalent light collection efficiency. The ratio of the light collection efficiency
between the plastic fibers and the quartz capillaries approximately scales with the ratio of the
diameter of the plastic fiber and the liquid core of the quartz capillary. This ratio is about 3 for
the fibers and capillaries we used.
As a timing reference we use a Photek 240 MCP-PMT. It is placed behind the calorimeter cell
and detects secondary shower particles escaping from the Shashlik calorimeter cell as we did in
our previous studies [2]. To extract the time of flight we measure the time difference between
the reference counter and the calorimeter cell. The time stamp is extracted from a Gaussian
fit to the peak of the reference counter pulse which resembles a Gaussian shape. For the time
stamp from the calorimeter cell we perform a straight line fit to the rising edge of the pulse and
extract the time at half of the maximal signal amplitude.
31234567890
17th International Conference on Calorimetry in Particle Physics (CALOR2016) IOP Publishing
IOP Conf. Series: Journal of Physics: Conf. Series 9 8 (2017) 012023  doi :10.1088/1742-6596/928/1/012023
We measure the timing performance of the calorimeter cell with high energy electrons in a range
between 20 GeV and 200 GeV in the CERN North Area test beam. The impact point of the
electrons onto the calorimeter cell is measured with a fiber hodoscope with a precision of bet-
ter than 1 mm. The timing results are extracted from events impacting in the center of the
calorimeter cell in an area of 2x2 mm. As we are using a single calorimeter cell the containment
outside this region is limited which affects the timing measurement.
3. Experimental Results
The timing precision extracted from a pulse improves as the rise time of the pulse gets shorter
and as the signal over background improves. The rise time of the pulses from the shashlik
calorimeter cell are driven by the time constants of the wave length shifter as we demonstrated
previously [2]. The rise time of the LYSO scintillation signal and the rise time of the previously
used MCP-PMT is much faster than the WLS time constants. As SiPMs feature a slower rise
time than MCP-PMTs we studied the signal rise time with our new setup. In figure 1 we show
the rise time of the pulses as a function of the signal amplitude. We observe that the the rise time
Figure 1. Rise time of the pulses
as a function of the amplitude for
the four different SiPMs. The
data recorded with the plastic
fibers and the quartz capillaries are
distinguished as dots and squares.
For each of the two data sets with
plastic fibers and capillaries there
are 5 measurements corresponding
to beam energies of 20, 50, 100, 150
and 200 GeV.
of the pulses becomes shorter for larger pulses, reaching around 2 ns for the largest amplitudes
in each channel. The amplitude dependency of the rise time is similar between different types
of SiPMs but reaches is shortest value at different signal amplitudes. The bias voltage of the
SiPMs is set to the same voltage, no individual optimization has been performed. The absolute
signal amplitude in the SiPMs is different for the same beam energy which is expected as pixel
count, sensor size and the light guide coupling efficiency varies. The clipping circuit used for
all SiPMs is the same while the capacitance of the SiPMs is not. In figure 2 we show the time
resolution of the four individual fibers as a function of their respective signal amplitude. Lines
indicate the trend of the time resolution. We see a similar trend as for the rise times. However
we also see that the improvement of the timing resolution is not just due to the faster rise times.
The increased signal amplitude and the resulting improvement in the signal over background
results in additional improvement.
For the same amplitude we find the same timing resolution with the capillaries and the plas-
tic WLS fibers, both using DSB WLS agent. This further supports our previous findings that
the the light propagation in the optical elements of a scintillation based calorimeter dos not
significantly affect the performance at the level of 50 ps.
The light extraction efficiency of capillaries with liquid WLS remains sufficiently high for dose
rates of 100 Mrad and beyond and for fluences of 1014protons/cm2 and beyond [7]. The energy
41234567890
17th International Conference on Calorimetry in Particle Physics (CALOR2016) IOP Publishing
IOP Conf. Series: Journal of Physics: Conf. Series 9 8 (2017) 012023  doi :10.1088/1742-6596/928/1/012023
Figure 2. Time resolution
achieved with the calorimeter cell
using the signal of each of the
SiPMs individually. The data for
each SiPM consists of two sets, one
with the DSB WLS plastic fiber
and one with the capillaries with a
liquid DSB based WLS. The time
resolution improves with increasing
signal amplitude. The best time
resolution per fibre is around 60 ps
for all of the channels. The ampli-
tude at which this performance is
achieved varies.
resolution performance of the LYSO/tungsten cell is not limited by photo statistics but the
sampling fraction. The timing performance at lower energies could be improved by increasing
the raw signal. This could be achieved by using larger diameter capillaries.
4. Summary
We present timing performance studies on a LYSO/tungsten Shashlik calorimeter cell with
wave length shifting fiber and capillary readout with SiPMs as photo detectors and a DRS
based fast digitizer data acquisition. We find a timing performance which reaches around 60
ps per individual readout fiber and better than 50 ps when combining the information from
four fibers. Our results demonstrate that timing resolution of a few 10 ps can be achieved with
scintillator based calorimeters even when complex optical readout schemes are employed. SiPMs
are suitable as photo detectors for precision timing calorimeters.
4.1. Acknowledgments
Supported by funding from California Institute of Technology High Energy Physics under
Contract DE-SC0011925 with the United States Department of Energy. We thank Randy Ruchti
from the University of Notre Dame for providing the wave length shifting capillaries. We thank
CERN for providing access to the test beam facilities and our colleagues from Rome and ETH
for fruitful collaboration and help with the DAQ system.
References
[1] Calorimeters for Precision Timing Measurements in High Energy Physics, A. Bornheim, CALOR2014,
doi:10.1088/1742-6596/587/1/012057
[2] On timing properties of LYSO-based calorimeters, D. Anderson, A. Apresyan, A. Bornheim, J. Duarte, C.
Pena, A. Ronzhin, M. Spiropulu, J. Trevor, S. Xie, NIM-A, doi:10.1016/j.nima.2015.04.013, 2015
[3] Precision Timing Calorimetry for High Energy Physics, A. Bornheim et al, 13th Pisa meeting on Advanced
Detectors, Elba 2015, NIM-A doi:10.1016/j.nima.2015.11.129
[4] Timing Performance of new Hamamatsu Silicon Photomultipliers, A. Mangu et al, IEEE2014
[5] www.hamamatsu.com
[6] Longevity of the CMS ECAL and Scintillator-Based Options for Electromagnetic Calorimetry at HL-LHC, H.
Li, CMS Collaboration, CMS-CR-2015-127
[7] A Crystal Shashlik Electromagnetic Calorimeter for Future HEP Experiments, R. Zhu, CALOR2016, these
proceedings
[8] Studies of Wavelength-Shifting Liquid Filled Quartz Capillaries for Use in a Proposed CMS Calorimeter, B.
W. Baumbaugh, R. Ruchti, et al, IEEE/NSS2015, Conference Record.


LYSO based precision timing calorimeters

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LYSO based precision timing calorimeters

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