# Status Report on

# Studies of Compensating Planar Dual Readout Calorimetry

G. Mavromanolakis, A. Para, N. Saoulidou, H. Wenzel, Shin-Shan Yu,
Fermilab

Tianchi Zhao, University of Washington

# Motivation 

ILC will be a source of high precision measurements which will allow
determination of the nature and the detailed structure of the 'Physics
beyond the Standard Model'. It is expected that many heavy particles
will be found at the LHC with cascade decays down to the LSP. Jets and
W/Z bosons will play the role similar to gammas in the nuclear decays;
hence a precise jet energy measurement will have the same impact as the
high precision gamma spectroscopy in nuclear physics, or in the studies
of excited states of charmonium.

The need for the improvements of the jet energy resolution is evident;
the ways to improve the 'traditional' calorimetry are few. The principal
reason for the poor energy resolution of hadron calorimetry comes from
the fluctuations of the energy lost to nuclear breakup. It results in
the observable energy to be reduced by about the 30%. This overall
(energy dependent) calibration factor(function) can be calibrated-out
for single hadrons, but event-by-event fluctuations of this energy
deficit leads to a constant term in the energy resolution.

In case of hadronic jets, where the observed signals from different
particles are typically mixed up, the difference in response of a
calorimeter to charged and neutral pions introduces another, usually
dominant, term in the jet energy resolution. Another major contribution
to jet energy resolution arises in the usual case when 'electromagnetic'
and 'hadronic' sections of a calorimeter have different
response/calibration.

There are two principal methods for major improvement of jet energy
resolution proposed so far:

-   Particle Flow Calorimetry, which reduces the above-mentioned
    contributions to energy resolution by reducing the role of the
    'hadron' calorimetry to that of measuring the relatively small
    fraction of jet energy carried by neutrons and neutral kaons

-   Compensating calorimetry, which attempts to correct the energy
    deposited by hadrons for the nuclear-binding energy loss. To achieve
    good resolution for single hadrons this compensation must be
    performed on the event-by-event basis, to maintain good jet energy
    resolution it must not affect the measurement of neutral pions
    energy.

# Compensating Calorimetry

Observable signals produced in the calorimeter by mono-energetic hadrons
vary substantially from one shower to another, reflecting fluctuations
of a number of nucleons liberated from the nuclei of the medium.
Improvement of the energy resolution can be attained by making a
correction for the energy lost for overcoming the binding energy. This
correction must be done on the event-by-event basis and it must reflect
the number of the nuclei broken in a particular shower. There are two
experimental sources of such a correction:

-   Number of slow neutrons in the shower is proportional to the number
    of broken nuclei. By a judicious weight given to the signals created
    by slow neutrons one can accomplish, on average, a correction to the
    observed energy which will be compensated for the invisible energy.
    Such a calorimeter can be realized in practice by a judicious choice
    of the relative thicknesses of the absorber and scintillation
    layers.

-   Number of the broken nuclei is anti-correlated with the
    electromagnetic fraction of a hadron shower. The latter can be
    determined from the relative amount of scintillation and Cherenkov
    light as the latter will reflect primarily the contribution of
    electrons in the electromagnetic cascades. This method is known as
    Dual Readout Calorimetry.

# Dual Readout Calorimetry

For an illustration of the principles of dual readout calorimetry let's
consider the energy deposited via ionization in a large homogeneous
block of lead glass by 10 GeV electrons and pions.

# ![](media/image1.wmf){width="5.0in" height="3.38125in"}

Figure 1 Fraction of energy of the incoming particle deposited in the
homogeneous block of material (lead glass) for electrons and pions.

As it is well known, in the case of the electron-induced showers the
total ionization energy is, with high precision, equal to that of the
incoming particle. In the case of the pion-induced showers, even in this
total-absorption calorimeter case, some 30% of the incoming particle
energy remains undetected and the fluctuations are very large.

![](media/image2.wmf){width="5.0in" height="3.386111111111111in"}

Figure 2 Correlation between the total observed ionization energy and
the electromagnetic component of the shower, as measured by the
Cherenkov component. The calibration factor K is determined by the
requirement that K×E~Cherenkov~ = E~ionization~ for electrons.

Strong correlation between the fraction of the incoming particle energy
observed as the scintillation light and the amount of Cherenkov light is
illustrated in Fig. 2. If both, the scintillation and Cherenkov light
were recorded for the hadronic showers than this correlation can be used
to correct the observed scintillation light (compensate for the nuclear
binding energy losses) and the resulting energy resolution will be only
limited by the width of the correlation.

### Dual readout? Why Planar?

Principles of dual readout calorimetry have been experimentally
established by the DREAM collaboration with fiber-based geometry. This
design is also used by the 4-th concept ILC collaboration, hence the
obvious question arises, why should one purse an alternative calorimeter
design? Here are some reasons:

-   Planar geometry of dual readout calorimeter is probing the
    underlying phenomena in a different way, hence it will provide
    additional cross-checks for the physics mechanism of compensation

-   Planar implementation allows the calorimeter to have uniform
    detector/absorber structure throughout the entire volume, hence
    avoid the degradation of the resolution from combination of two
    different devices

-   Lead glass planar calorimeter offers excellent electron and photon
    energy resolution

-   Planar implementation admits fine segmentation of the front section
    to meet or exceed requirements of the EM calorimeters. If this
    segmentation is a sub-division of the 'hadronic' part of the
    calorimeter it accomplished without compromising the above-mentioned
    advantage

-   It allows fine transverse segmentation if it required by some
    physics requirement, but it is likely to require rather crude
    segmentation by itself.

### Dual Readout? Why not PFA?

PFA is widely believed to be best (the only?) method to attain very high
energy resolution for hadronic jets. This may be even true, although no
convincing method of experimental demonstration of the PFA performance
has been proposed, yet. PFA energy resolution is dominated by the
confusion term, with the next contribution, that of the hadron
calorimeter being in practice negligible. The confusion term related to
the spatial extent of a hadronic shower being the figure of merit.
Ultimate spatial resolution is of no use, wheres making hadronic shower
nearly pencil-like can help a lot. This situation is very similar to
that of very forward jet calorimetry at the LHC and the solution may be
the similar: Cherenkov based calorimetry. If the only signals used are
those produced by electrons inside the EM component of the hadron shower
the visible shower size shrinks to a very small radial extent, dependent
on the effective Moliere radius at the expense of a very poor energy
resolution, of the order of ![](media/image3.wmf).

It may well be, therefore, that the planar dual readout calorimeter
represents the optimal detector design for the PFA. This is a unique
situation for two reasons:

-   Typical PFA-optimized calorimeters have mediocre-to-poor performance
    as stand-alone calorimeters, thus requiring a commitment to the
    detector design long before the performance can be experimentally
    demonstrated (if ever). In case of a dual readout planar calorimeter
    the fall-back position is a very comfortable one.

-   Improvements in the PFA and dual readout calorimeters energy
    resolution are probably weakly correlated. This opens up a
    possibility of both methods used at once with even further
    improvement in the jet energy resolution.

# Studies of Dual Readout Calorimetry: Outline of a Program 

Dual readout calorimeter, as described before, offers a very attractive
possibility for the future experiments in general and for the ILC in
particular. We propose to undertake a systematic study of practical
implementations of such a calorimeter and to examine contributions to
the energy resolution from various sources including detector geometry,
sampling fluctuations, inevitable presence of structural materials etc.
We also propose to undertake studies to develop cost-effective
implementations of such detectors. It is important to point out that the
idea of the dual readout calorimetry is more than 20 years old. Its
practical implementation was made possible only recently by the advances
of integrated electronics circuitry and the advent of novel solid
state-based photodetectors: Geiger-mode Avalanche Photodiodes.

Our current research program consists of three components, described
below:

-   Simulation studies of various detector configurations and
    optimization of the detector design

-   R&D efforts on development of cost-effective detecting medium

-   Evaluation and characterization of the GM-APD's

We believe that the construction of a final detector with high jet
energy resolution requires experimental demonstration of the claimed
performance. This is a tall order, but an important one to satisfy. Test
beam demonstration of high resolution requires a large detector with the
design very close to the final one; small 'details' can make a
significant difference when the resolution is very good.

The purpose of this study is to optimize a design of such a detector and
to establish technologies necessary for its construction. As an
intermediate step we plan to construct and test in the beam a smaller
prototype aimed at the demonstration of the performance of this
jet-oriented calorimeter for photons and electrons, including the
pointing accuracy and spatial resolution. While the segmentation of this
small prototype will likely be driven by the EM-oriented requirements,
its materials and structure must be such that by appropriate grouping of
signals it will be identical to the main body of the calorimeter.

At some later stage, once the optimal detector configuration is
identified we envisage detailed engineering studies including
integration with the rest of the experiment.

## Detector optimization studies

Given the high precision of the target calorimeter we have divided the
optimization studies into several steps with increasing complexity. This
will help, hopefully, to understand sources of various contributions to
the energy resolution and their relative interplay. It is interesting to
note, that all stages considered correspond to build-able detectors and
that the choice of the final detector design may depend on
considerations other than the ultimate energy resolution.

We have concentrated on the calorimeters using lead glass as the primary
material. Its high density, approaching 6 g/cm^3^, allows for very
compact calorimeter design. Lead glass is a good Cherenkov radiator, it
can also be doped with organic or inorganic scintillators to provide the
second readout. While this was an obvious starting point, it is quite
possible that some other materials may be finally used.

### Step 1: Large Homogeneous Calorimeter

A calorimeter consists of a single large block of an active material
with separate readout of the scintillation and Cherenkov light
components. A possible realization of such a detector could be a lead
glass block doped with (relatively slow) scintillator and the timing of
the light signal serving as a discriminator of the two components.

Mechanism of dual readout-based compensation was illustrated in Fig. 2.

![](media/image4.wmf){width="5.041666666666667in" height="3.4375in"}

Figure 3 Observed energy, scaled by the beam energy, of 20 GeV pions in
a large total absorption calorimeter (black histogram). Read histograms
illustrate the effect of the correction using the correlation between
scintillation and Cherenkov light.

Fig. 3 illustrates the result of the measurement with a dual readout
calorimeter: the mean energy is equal to the beam energy (linearity!)
and the relative energy resolution is significantly improved. It is
worth noticing that the corrected response is much better described by a
Gaussian curve than the uncorrected one. It is important to notice that
the shape of the correlation is very weakly dependent on the pion energy
and in the following we will use a single correction function derived
for pions at 5 GeV.

![](media/image5.png){width="5.0in" height="3.392361111111111in"}

Figure 4 Scaled energy resolution ![](media/image6.wmf) for pion-induced
showers as a function of pion energy. Black points are for the total
ionization measurement, red points are the measurements corrected using
the Cherenkov component.

Resulting corrected energy resolution for single charged pions scales
with the pion energy like ![](media/image7.wmf) in contrast to the
uncorrected measurement which shows an early onset of the departure
related to a constant term in energy resolution.

The measurement of pion energy using the Cherenkov-corrected
scintillation light shows no significant non-linearity: the resulting
mean energy is equal to the primary particle energy and, at the same
time, it is same for the neutral and charged pions. This is of critical
importance for the jet energy measurement; it removes the dependence on
the jet fragmentation.

The results mentioned above are for single particles detected in a large
calorimeter. One might expect that the event-by-event correction will be
less effective when an ensemble of particles (jet) is measured at once.

![](media/image8.png){width="6.1097222222222225in"
height="4.097222222222222in"}

Figure 5 Scaled jet energy resolution for corrected and uncorrected
measurement as a function of jet energy. Jets are defined as ensembles
of particles, with 20% of them, on average, being neutral pions. 'Basic'
jets are constructed with a 'typical' fragmentation function, 'Low' jets
consist of 5 GeV particles, 'High' jets consist of 20 GeV particles
only.

Fig. 5 demonstrates that the Cherenkov-based correction works well even
in the jet environment. Jet energy is determined using the scintillation
and Cherenkov signals summed over the entire collection of particles,
yet the energy resolution remains better than![](media/image9.wmf)and it
depends very little on the actual jet fragmentation model. This is in
contrast with the uncorrected measurement, where the energy resolution
depends significantly on the jet composition.

![](media/image10.png){width="6.1097222222222225in"
height="4.143055555555556in"}

Figure 6: Jet energy resolution as a function of jet energy of corrected
and uncorrected measurement for 'typical' jets and the artificial
ensemble of jets consisting of charged hadrons only.

Fig. 6 shows that in the traditional calorimeters with e/π response
different from unity the jet energy resolution is dominated by the
fluctuations in the π^o^ component of the jet. In case of the dual
readout calorimetry the situation is quite different; in fact the
sizeable electromagnetic component of the jet tends to improve slightly
the energy resolution.

We conclude that a homogeneous calorimeter with dual readout would
provide a very attractive option for detector optimized for jet
spectroscopy. If built on the basis of lead glass doped with
scintillator it would be a relatively compact detector, as the density
of lead glass may approach 6 g/cm^3^. It is imortant to note that while
the requirements on the detector granularity are extremely weak in such
a case, very fine segmentation is not at all precluded, if desirable for
other reasons.

The principal challenges in the construction of such a detector will
likely involve:

-   Identification of a suitable scintillating dopant, preferentially
    with longer time constant of the order of tens of nanoseconds

-   Efficient light collection

-   Photodetectors capable of separation of the 'prompt' and the 'slow'
    components of light

### Step 2: Longitudinally Segmented Calorimeter; Case I - Uniform Medium

In this step we investigate how the performance of the dual readout
degrades when the calorimeter is subdivided longitudinally into several
layers: Cherenkov detector, Scintillator and Absorber (structural
material). We expect several sources of additional fluctuations to
appear here, in comparison with the previous step:

-   Observed signals (Cherenkov and scintillation) will reflect
    additional (sampling) fluctuations. They are present even in the
    absence of structural materials.

-   The Cherenkov signals and the scintillation signals (one used to
    'correct' the other) reflect physical signals in different locations
    in the detector

It would be natural to use plastic scintillator to detect scintillation
light. This would, however, introduce another source of complexity
related to the fact that the majority of neutrons would be deposit their
energy in the scintillator. This configuration, will be studied in the
next step.

Investigations of the longitudinally segmented detector have just
commenced. The main questions we want to address are:

-   What is the optimal geometry of the detector layers to achieve the
    best energy resolution? What is the contribution of the sampling
    fluctuations to the energy reaolution?

-   What is the optimal procedure of combining the scintillation and
    Cherenkov information? Segmented readout opens up possibility of
    'local' rather than 'global' correction.

In the following we indicate some of the initial insights:

![](media/image11.wmf){width="5.0in" height="5.1in"}

Figure 7 Sampling fluctuations contribution to the energy resolution as
a function of the thickness of the active detector for different
thicknesses of the interleaved inactive (absorber).

We have simulated a detector consisting of 1 mm thick lead glass and
collected the ionization and Cherenkov energy loss in every layer. In
the subsequent analysis we group these thin layers into 'scintillation',
'Cherenkov' and 'passive' layers to study various detector geometries.

Fig. 7 shows the contribution of sampling fluctuations (for 10 GeV pion
showers) to the energy resolution as a function of the thickness of the
'scintillation' layer. When scintillator layer is 'thick enough' its
actual thickness is unimportant and the energy resolution scales
approximately like ![](media/image12.wmf), where d is the thickness of
the inactive layer. We observe that the energy resolution is not only a
function of the sampling fraction, or the sampling frequency alone. In
particular we notice that fluctuations of energy depositions in a very
thin scintillation layer have a significant contribution to the energy
resolution.

The compensation algorithm in a segmented detector can be more
complicated, but as a first step we use the same one as used for the
homogeneous detector: correct the total amount of scintillation light
with the help of total amount of the Cherenkov light. It appears that
this compensation method works quite well, moreover it appears that the
compensating information is fully contained in a very thin layer of
Cherenkov radiator adjacent to the scintillator layer. As an example,
energy resolution of a detector consisting of alternating layers of 30
mm scintillator and 20 mm Cherenkov detector is nearly the same as the
detector consisting of 30 mm scintillator, 2 mm Cherenko0v and 18 mm
structural material.

![](media/image13.wmf){width="4.822916666666667in"
height="3.2395833333333335in"}

Figure 8 Jet energy resolution as a function of jet energy for a
calorimeter with 3 cm thick scintillator, 2 mm thick Cherenkov radiator
and 18 mm thick passive material

In a manner similar to the homogeneous detector case we have studied the
energy resolution for the ensembles of particles (a.k.a. jets). As shown
in Fig. 8 even in the presence of significant structural material the
'global' correction allows to attain the energy resolution of the order
of ![](media/image14.wmf).

In a segmented detector there is more information available in a form of
individual scintillation and Cherenkov energy depositions. As the
compensation aims at correcting the 'hadronic' response to the level of
'electromagnetic' one may expect further improvement of the resolution
by a judicious use of depth-dependent information. As the first step we
tried to apply a 'correction' to the scintillation signals by defining
the total energy as:

![](media/image15.wmf)

where i denotes the layer number, E^sc^ and E^Ch^ are the observed
energy depositions and the correction function β is determined to
optimize the overall energy resolution.

Initial studies indicate that further improvement to the resolution can
be realized, as shown in Fig. 9

![](media/image16.wmf){width="3.6145833333333335in"
height="2.4895833333333335in"}

Figure 9 Energy resolution for single particles om a calorimeter with 3
mm scintillator and 2 mm cherenkov layers. Red curve is the scintillator
energy alone, blue curve is the dual readout-corrected with the total
amount of light, green line is for the dual readout and local correction

### Step 3: Longitudinally Segmented Calorimeter; Case II -- Plastic Scintillator

Shower development depends to a certain degree on the material. From the
calorimetric point of view the probably the most important material
difference is related to the hydrogen (or other light elements) content
of the material. The primary difference between the scintillating glass
discussed before and the plastic scintillator case lies in the fact that
most of slow neutrons are likely to deposit their kinetic energy in the
plastic, thus providing some degree of compensation. We expect to extend
the studies of local compensation to the plastic scintillator case and
to learn about possible advantages of combined compensation mechanism
involving neutrons and Cherenkov light.

### Step 4: Longitudinally and Transversally Segmented Calorimeter 

Dual-readout compensation in a homogeneous calorimeter did not require
any segmentation. With segmented detector a global correction using dual
readout is clearly an attractive avenue. The power of such a correction
is reduced by the fact that two observed components: scintillation and
Cherenkov light do not necessarily correspond to the same physical
signals. In the case of longitudinally segmented calorimeter a 'local'
correction dependent on the local scintillation-to-Cherenkov ratio has
further improved the energy resolution, despite the fact that the light
collected in any given layer is, in general, a mixture of signals
produced by 'electromagnetic' and 'purely hadronic' shower components.
One may expect that extension of the weighting technique, but involving
a weighting function defined for a single cell, rather than an entire
plane, will lead to further improvement of the resolution.

## Hardware-oriented R&D

Detector performance and optimization studies are the primary focus of
our studies and this is primarily related to the scarcity of available
resources and manpower. It is very important to keep adequate connection
with reality, to make sure that the concepts under investigations
correspond the some realize-able detectors. Conversely, we utilize our
optimization studies to identify critical areas which need to be
addressed.

So far we have identified the following areas:

-   Light collection and light yield.

-   Development of scintillator-doped (lead) glass

-   Production of cost-effective lead glass

-   Photodetectors permitting seamless light read out of voxel-ized
    detector

On the light yield front, we have started to build an infrastructure and
cosmic ray test stands to evaluate the number of collected photons, but
we do not have definite answers, so far. On the other hand we notice
that related measurements were performed as a part of the detector R&D
for TESLA. Thesis of Ralph Dollan reports that a minimium ionizing
particle traversing a 1×1×4 cm crystal of SF57 lead glass read out with
the waveshifting fiber produces 19±7 photons emerging from the fiber.

In a similar vein, we did not make any progress towards the production
of scintillating glass, but we notice that there are groups pursuing
this subject with new colliding beam detectors in mind. For example, a
group from Northwestern Polytechnical University, Xian has reported
interesting results of doping glass with organic activators p-TP and/or
POPOP.

Production in general, and cost effective production in particular, of
lead glass tiles presents a significant problem. Lead glass was a
workhorse of high energy physics for decades, but its relatively high
cost was one of the impeding factors. This cost is dominated by the cost
of cutting and polishing of the blocks to provide superb optical
quality. Already some 25 years ago a dedicated studies have demonstrated
that a cost-effective casting and/or extrusion production methods can be
utilized to produce lead glass with adequate quality for a possible use
in large colliding beam detectors. Environmental concerns have reduced
the spectrum of potential vendors, though. We have contacted several
potential lead glass manufacturers. The most advanced discussions have
been conducted with the Shanghai Xinhu Glass Co., LTD in Shanghai. The
preliminary discussions of our needs and the working of their production
line indicate that relatively inexpensive production of tiles of the
required dimensions. Cost estimates make little sense at the present
stage, though, before the adequacy of dimensional tolerances and optical
quality is demonstrated.

The greatest amount of effort we have dedicated to photodetectors. For a
magnetic environment of the expected detectors and for the purpose of
hermetic light read out inside the large volume there is the only one
possibility: Geiger-mode Avalanche Photodiodes. These novel
photodetectors are, in fact, an enabling technology for the planar dual
readout calorimetry. These detectors are still in their infancy, hence
their evaluation and characterization is of great importance. On one
hand we need to validate their applicability for our purposes, on the
other hand it is still possible to contribute to their evolution.

We have constructed a GM APD testing laboratory with a goal of
systematic evaluation and characterization of detectors produced by
different vendors. The testing facility is fully automated and run under
the control of LabView. At present we are conducting and analyzing
static tests to determine the breakdown characteristics of the diodes as
a function of the operating conditions.

![](media/image17.jpeg){width="5.0in" height="3.8652777777777776in"}

Figure 10 I-V curves for different types of detectors as a function of
temperature. For each of the three detectors there are 8 curves taken at
temperatures ranging from 0 to 30 degrees C.

Fig. 10 illustrates the range of characteristics of the available
detectors. The operating voltages, gains, noise levels and the
temperature sensitivity vary considerably.

Static characteristic of the detector is used to establish the operating
point as a function of the temperature. This is the input to the dynamic
studies, which are conducted in two steps: with and without the external
light input.

The setup for the dynamical characterization of the detectors consists
of transconductance amplifier, digital scope, LabView data acquisition
and ROOT-based analysis program. We have established an automated system
for measurement of rates and gains of the detectors as a function of the
operating conditions. We have also commissioned a laser-based
variable-intensity light source and we are planning to develop a
complete characterization of the detector output as a function of rate
and amplitude of the light signal.

Dual readout calorimetry we may require separation of fast and slower
light components. On one had silicon-based photodetectors have excellent
timing resolution, on the other hand they do exhibit significant
afterpulsing, as shown in Fig. 11. Good understanding of the origin and
the scale of these phenomena is necessary to provide feedback to the
manufacturers to optimize the detector design on one hand and to
understand their implications for our application on the other hand.

![](media/image18.jpeg){width="5.0in" height="3.8631944444444444in"}

Figure 11 Examples of single output waveforms produced by a silicon
photodetector in the absence of external light. Dark count rates are far
too low to account for the frequency of the multiple signals observed.
These signals are afterpulses induced by the trigger signal.

# Manpower and resources

The work presented here was conducted by a group of people and Fermilab
and University of Washington working on a part-time fashion. The total
efforts intgegrates to about 1.0 FTE. The hardware effort at the
University of Washington is supported by the LCRD grant of \$17K. At
Fermilab this effort benefits from a support by the Particle Physics
Division.

While we find these studies are quite interesting, it is quite clear
that the resources available are not adequate to bring the planar dual
readout calorimeters to a full maturity. We are very happy to notice,
however, that the interest in this approach to calorimetry is growing.
Our efforts have been joined lately but the INFN groups from Pisa and
Trieste and by University of Iowa. We are also aware of the dual readout
calorimetry collaboration *in statu nascendi* in Italy. The 4-th concept
collaboration pursues different implementation of a dual readout
calorimeter, but there is a considerable of overlap.