download report - Sapienza

Scientific Report 2007-2009
Particle physics
P23. Dual readout calorimetry with crystals
In recent years, dual-readout calorimetry has emerged
as a promising new solution for the need to detect
both leptons and hadrons with excellent accuracy in
high-energy particle physics experiments [1]. The Dual
Readout Method (DREAM) is based on a simultaneous
measurement of different types of signals which provide
complementary information about details of the shower
development. It has been argued and experimentally
demonstrated that a comparison of the signals produced
by Čerenkov light and scintillation light makes it possible
to measure the energy fraction carried by the electromagnetic
shower component, f em , event by event. Since
the event by event fluctuation in f em is the main limitation
for the energy resolution in hadronic calorimeters,
this may lead to an important improvement in the performance
of hadron calorimeters. The first calorimeter
of this type was based on a copper absorber structure,
equipped with two types of active media. Scintillating
fibers measured the total energy deposited by all the
shower particles, while Čerenkov light, generated only by
charged relativistic particles, was produced in undoped
optical fibers.
The signals from certain high-density crystals
(PbWO 4 , BGO) can also be unraveled into Čerenkov
and scintillation components; such crystals, when used in
conjunction with the fiber calorimeter mentioned above,
can offer in principle the same advantages for hadronic
shower detection and, at the same time, provide accurate
energy resolution for the electromagnetic component.
Figure 1: The average time structure of the UV signals from
200 GeV π + in BGO crystal. The long tail is due to the
scintillation component,while the prompt peak represents the
Cerenkov contribution(a). The ”contamination” of scintillation
light in a narrow time window ∆t around the prompt
peak (b).
We have performed a series of measurements in the
H4 beam line of the SPS at CERN [2] providing a beam
of high energy electrons and pions. Our detector was
a high-density Bi 4 Ge 3 O 12 (BGO) crystal with a length
of 24 cm mounted on a rotating table. The light produced
by particles traversing this crystal was read out
by two photomultiplier tubes Hamamatsu R5900U, 10-
stage, bialkali photocathode, borosilicate window, located
at the opposite ends. The light generated in the
crystal was UV filtered at one side before being read out
to reduce the scintillation contribution.
Figure 2: The Čerenkov/scintillation ratio in the UV signals
from the BGO crystal, for a gate of 10 ns around the prompt
peak, as a function of the orientation of the crystal with respect
to the beam. Data for 50 GeV electrons and 200 GeV
π + .
The purpose of these tests was to split the crystal
signals into their scintillation and Čerenkov components.
We exploited the following differences between these
components: 1) differences in time structure. Čerenkov
light is prompt, while the scintillation mechanism is
characterized by one or several time constants. 2) differences
in directionality. Contrary to scintillation light,
which is emitted isotropically, Čerenkov light is emitted
at a characteristic angle by the relativistic (shower)
particles that traverse the detector. We measured the
signals for different orientations of the crystal with
respect to the particle beam. In Figure 1 the prompt
Čerenkov signal is superimposed to the slow scintillation
component, allowing an easy separation of the two
contributions. In Figure 2 the ratio between Čerenkov
and scintillation light is plotted as function of the angle
of the crystal with respect to the beam direction; the
peak around the angle of Čerenkov emission is clearly
visible. At present additional studies with a large BGO
matrix [3] and with different type of crystals [4] are
performed.
References
1. R. Wigmans, New Journal of Physics 10 (2008) 025003.
2. N. Akchurin, et al., N.I.M. A595 (2008) 359.
3. N. Akchurin, et al., N.I.M. A610 (2009) 488.
4. N. Akchurin, et al., N.I.M. A604 (2009) 710.
Authors
G. Ciapetti, F. Lacava, D. Pinci 1 , C. Voena 1
Sapienza Università di Roma 130 Dipartimento di Fisica