Nuclear Instruments and Methods in Physics Research A 641 (2011) 19–32
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in
Physics Research A
journal homepage: www.elsevier.com/locate/nima
Calibration and monitoring of the MEG experiment by a proton beam
from a Cockcroft–Walton accelerator
J. Adam n,o, X. Bai k, A. Baldini a, E. Baracchini l, C. Bemporad a,b, G. Boca e,f, P.W. Cattaneo e, G. Cavoto g,
F. Cei a,b, C. Cerri a, M. Corbo a,b, N. Curalli a,b, A. de Bari e,f, M. De Gerone c,d, T. Doke p, S. Dussoni c,d,
J. Egger n,1, K. Fratini c,d, Y. Fujii k, L. Galli a,b, G. Gallucci a,b, F. Gatti c,d, B. Golden l, M. Grassi a,
D.N. Grigoriev q, T. Haruyama m, M. Hildebrandt n, F. Ignatov q, T. Iwamoto k, P.-R. Kettle n, B.I. Khazin q,
O. Kiselev n, A. Korenchenko r, N. Kravchuk r, A. Maki m, S. Mihara m, W. Molzon l, T. Mori k, D. Mzavia r,2,
H. Natori k, D. Nicolo a,b, H. Nishiguchi m, Y. Nishimura k, W. Ootani k, M. Panareo i,j, A. Papa a,b,
R. Pazzi a,b,2, G. Piredda g, A. Popov q, F. Renga g,h, S. Ritt n, M. Rossella e, R. Sawada k, F. Sergiampietri a,
G. Signorelli a,, F. Tenchini a,b, C. Topchyan l, Y. Uchiyama k, R. Valle c,d,3, C. Voena g, F. Xiao l,
A. Yamamoto m, Yu.V. Yudin q, D. Zanello g,1
a
INFN Sezione di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy
Dipartimento di Fisica dell’Universita , Largo B. Pontecorvo 3, 56127 Pisa, Italy
INFN Sezione di Genova, Via Dodecaneso 33, 16146 Genova, Italy
d
Dipartimento di Fisica dell’Universita , Via Dodecaneso 33, 16146 Genova, Italy
e
INFN Sezione di Pavia, Via Bassi 6, 27100 Pavia, Italy
f
Dipartimento di Fisica dell’Universita , Via Bassi 6, 27100 Pavia, Italy
g
INFN Sezione di Roma, Piazzale A. Moro 2, 00185 Roma, Italy
h
Dipartimento di Fisica dell’Universita ‘‘Sapienza’’, Piazzale A. Moro 2, 00185 Roma, Italy
i
INFN Sezione di Lecce, Via per Arnesano, 73100 Lecce, Italy
j
Via per Arnesano, 73100 Lecce, Italy
Dipartimento di Fisica dell’Universita,
k
ICEPP, University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
l
University of California, Irvine, CA 92697, USA
m
KEK, High Energy Accelerator Research Organization 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
n
Paul Scherrer Institute PSI, CH-5232 Villigen, Switzerland
o
Swiss Federal Institute of Technology ETH, CH-8093 Zürich, Switzerland
p
Research Institute for Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan
q
Budker Institute of Nuclear Physics, 630090 Novosibirsk, Russia
r
Joint Institute for Nuclear Research, 141980, Dubna, Russia
b
c
The MEG Collaboration
a r t i c l e i n f o
abstract
Article history:
Received 5 January 2011
Received in revised form
22 March 2011
Accepted 23 March 2011
Available online 13 April 2011
The MEG experiment at PSI searches for the decay m-eg at a level of 1013 on the branching ratio
BRðm-eg=m-tot), well beyond the present experimental limit (BR r 1:2 1011 ) and is sensitive to the
predictions of SUSY-GUT theories. To reach this goal the experiment uses one of the most intense
continuous surface muon beams available ( 108 m=s) and relies on advanced technology (LXe
calorimetry, a gradient-field superconducting spectrometer as well as flexible and powerful trigger
and acquisition systems). In order to maintain the highest possible energy, time and spatial resolutions
for such detector, frequent calibration and monitoring, using a Cockcroft–Walton proton accelerator,
are required. The proton beam is brought to the centre of MEG by a special bellows insertion system
and travels in a direction opposite to the one of the normal m-beam. Protons interact with a lithium
tetraborate (Li2B4O7) nuclear target and produce one g (17.6 MeV) from the reaction 73 Liðp, gÞ 84 Be or two
12
coincident gs (11.67 and 4.4 MeV) from the reaction 11
5 Bðp, g1 Þ 6 C . The 17.6 MeV g is used for
Keywords:
Calibration
Cockcroft–Walton accelerator
Beam monitoring
g-rays
Corresponding author. Tel.: þ 39 050 2214 425; fax: þ39 050 2214 317.
E-mail address: giovanni.signorelli@pi.infn.it (G. Signorelli).
Retired.
2
Deceased.
3
Present address: Lames Holding S.r.l., 16043 Chiavari, Italy.
1
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.nima.2011.03.048
20
J. Adam et al. / Nuclear Instruments and Methods in Physics Research A 641 (2011) 19–32
calibrating and monitoring the LXe calorimeter (sEg =Eg ¼ 3:85 7 0:15% at 17.6 MeV) while the coincident 11.67 and 4.4 MeV gs are used to measure the relative timing of the calorimeter and the
spectrometer timing counters (sDt ¼ 0:450 7 0:015 ns).
& 2011 Elsevier B.V. All rights reserved.
1. Introduction
3. Experimental set-up and performance
Lepton flavour violation (LFV) in the neutral lepton sector and
neutrino oscillations are now established facts. We are nevertheless very far from understanding the dynamics behind these
phenomena. In the Standard Model (SM) with finite but tiny
neutrino masses, flavour violating decays are predicted to be
immeasurably small, hence any such observed flavour violating
signal must be based on new physics beyond the SM. Supersymmetric (SUSY) and supersymmetric grand unified theories
(SUSY-GUT) can naturally house finite neutrino masses and can
also predict rather large branching ratios for lepton flavour
violating decays [1]. In particular, in the charged lepton sector,
the decay m þ -e þ g is predicted to be one of the most sensitive
reactions for observing LFV-effects. The current upper limit on the
branching ratio for this decay was obtained in 1999, by the MEGA
experiment: BR r 1:2 1011 at 90% CL [2].
The MEG (Mu to E Gamma) Collaboration [3], made of Institutions
from Italy, Japan, Russia, Switzerland, United States, aims at a
sensitivity of 1013 on the m þ -e þ g branching ratio, a level
within the prediction bands of many SUSY and SUSY-GUT theories.
The experiment has been taking-data at Paul Scherrer Institute (PSI)
since September 2008, using one of the most intense DC muon beam
currently available in the world. The experiment has recently reached
an upper limit BR r 2:8 1011 at 90% CL [4] on the m þ -e þ g
branching ratio from its first three months of beam time.
To explore branching ratios at the 10 13 level one primarily
needs a high intensity muon beam that can stop in a target of
minimal thickness. With a muon decay rate of 108 m=s, sensitivities to branching ratios t 1012 can be reached in some
months of data-taking. Furthermore, the use of a direct current
muon beam helps to minimize the rate of accidental coincidences,
as in the case of the pE5 surface muon channel at PSI [5]. Here
muons are created from pions decaying at rest on the surface of
the production target, fed by the world’s most intense continuous
proton cyclotron.
In order to discriminate a m þ -e þ g signal from background,
one requires the highest possible resolutions in energy, emission
angle and timing for both the positron and the g-ray. Therefore a
high sensitivity experiment demands the use of innovative
detector technologies.
In Table 1 the performances of previous m þ -e þ g experiments
are compared with the expectations of MEG, for which the goal is
to gain two orders of magnitude with respect to the present limit.
This is clearly an experimental challenge and required a long R&D
phase (see, for instance, Ref. [10]) in order to obtain adequate
detector performances, as also listed in Table 1. A layout of the
MEG detector is shown in Fig. 1. The pE5 surface muon beam is
brought to rest in a thin (205 mm) polyethylene target, after
passing through a degrader and a Wien filter which eliminates
almost all of the contaminating beam positrons from the production target (at a separation level of 7:5s). The muon momentum is
set to 28 MeV=c, where the surface muon production rate
reaches its maximum.
The stopping rate Rm is tuned to 3 107 stopping muons/s to
achieve the best sensitivity and signal-to-noise ratio. The muon
stopping target is slanted by 20.51 with respect to the beam
direction in order to minimize the positron path in the material
and reduce the multiple scattering in the target.
The positron momentum is measured by a superconducting
iron-free magnetic spectrometer, with an axial gradient field
housing a system of sixteen ultra-thin radial drift chambers. The
gradient field of the COBRA spectrometer (COnstant Bending
RAdius) has the advantage of sweeping away low momentum
particles, which would contribute to a high drift chamber occupancy and dead time, more efficiently than a pure solenoidal field.
Moreover, monochromatic positrons describe trajectories with an
almost constant projected bending radius, independent of their
emission angle at the target.
2. Signal and backgrounds in the MEG experiment
The signal for the m þ -e þ g decay at rest is given by a positron
and a g-ray, emitted simultaneously and back-to-back, with the
muon mass equally shared between the two particles each with
52.8 MeV kinetic energy. The background originates from two
different contributions:
(1) the correlated background, given by the muon radiative
decay (inner bremsstrahlung) process m þ -e þ ne n m g. The
expected number of background events of this type is directly
proportional to the muon decay rate Rm ;
(2) the uncorrelated background, given by the accidental
coincidence of a positron from the normal Michel decay
m þ -e þ ne n m and a g-ray from radiative decay or positron
annihilation in flight in the materials of the experiment. This
background is dominant for positron and g-energies energies in
the signal window and its rate depends quadratically on Rm .
Table 1
The performances of previous m þ -e þ g experiments compared with the expectation of MEG. All the quoted resolutions are FWHM.
Place
Year
DEe =Ee ð%Þ
DEg =Eg ð%Þ
Dteg (ns)
Dyeg
Upper limit
References
SIN
1977
8.7
9.3
1.4
–
o 1:0 109
[6]
TRIUMF
1977
10
8.7
6.7
–
o 3:6 109
[7]
LANL
1979
8.8
8
1.9
37 mrad
o 1:7 1010
[8]
LANL
1986
8
8
1.8
87 mrad
o 4:9 1011
[9]
LANL
1999
1.2a
4.5a
1.6
17 mrad
o 1:2 1011
[2]
PSI
2012
0.8
4.0
0.15
12 mrad
a
Shows an average of the numbers given in Ref. [2].
13
o 1 10
MEG [3]
J. Adam et al. / Nuclear Instruments and Methods in Physics Research A 641 (2011) 19–32
21
Fig. 1. Layout of the MEG experiment.
The current obtained energy and angular resolutions of the
spectrometer are sE =E 0:7% and sy 11 mrad, sf 7 mrad.4
The positron timing is measured by two double-layer arrays of
plastic scintillators, placed on either sides of the spectrometer
(Timing Counters: TC). The external layer of each is equipped with
15 scintillating bars (one bar: 4 4 79.6 cm), while the
internal layer is composed of scintillating fibres (one fiber:
0.6 0.6 156 cm). A measured timing resolution of st 70 ps
was achieved almost uniformly along the bars and for a large
range of impact angles.
The g-rays, after crossing the very thin magnet superconducting coils with a 80% transmission probability, are detected by a
900 l volume liquid xenon LXe detector, where their energy,
direction and timing are measured. This detector material was
chosen because of its large light yield (comparable with that of
NaI), its homogeneity and its fast scintillation light decay time
( 45 ns for g-rays and 22 ns for a-particles). The LXe calorimeter is viewed by 846 Hamamatsu 2 in. PMTs [11], specially
manufactured to operate at cryogenic liquid temperatures. The
LXe purity is one of the crucial parameters for obtaining the
desired performance. A liquid phase purification system was
developed which employs a pump with a flow of 70 l/h, molecular
filters and an O2-getter. Further purification is performed in the
gaseous phase, taking advantage the thermodynamic equilibrium
of gaseous and LXe at the top of the calorimeter vessel.
A FPGA-FADC based digital trigger system was specifically
developed for the MEG experiment. The trigger is organized in a
tree-structure of different types of digital boards, which process
the fast signals coming from the LXe calorimeter and the TC system
to perform a fast estimate of the r-energy, timing and direction and
of the positron timing and direction; all of this information is then
combined to select events with a candidate m þ -e þ g decay
signature. Starting from a stopping muon beam intensity of 3
107 m=s we obtain a data acquisition trigger rate of R 5 Hz.
The signals coming from all detectors are sampled by a
switched capacitor array ASIC named ‘‘Domino Ring Sampler’’
version 4 (DRS4) which has been designed specifically for this
experiment [12]. The drift chamber signals are sampled with
800 MHz, while all PMTs are sampled at 1.6 GHz, with a signal-tonoise ratio of 11.4 bits. The waveform recording is of crucial
importance, enabling rejection of accidental superimposition of
4
Taking the z-axis as the beam-axis, y is defined as the polar angle, while f is
the azimuthal angle.
low energy events within the acquisition timing window (‘‘pileup’’), which can mimic signal events.
The experimental sensitivity is evaluated by computing the
expected background in the signal region on the basis of experimental or predicted resolutions and of the muon stopping rate.
Assuming Rm ¼ 3 107 m=s, one obtains an accidental background
of 6 10 14 events per muon decay; the corresponding 90% CL
upper limit on the m þ -e þ g branching ratio, in the case of no
events observed in four years of data taking, is 1.7 10 13.
4. Calibration and monitoring considerations of MEG
The calibration and monitoring (C&M) methods are the key to
success for the difficult measurement MEG wants to perform.
MEG is an ambitious experiment in terms of the sensitivity
necessary in studying the process m-eg. This demands the
highest precision in measuring the four-vectors of the m-decay
products; moreover the precision must be coupled with a high
running stability of all detectors, under high beam intensity and
its possible time variations. These requirements suggested the
integration of a 1000 keV C–W (Cockcroft–Walton) accelerator
into the experiment. It was used to excite two nuclear reactions
(discussed later): 73 Liðp, gÞ 84 Be and 115 Bðp, gÞ 126 C. The introduction of
such an accelerator for an elementary particle experiment is
rather unusual. It is of importance to stress that the use of the
C–W allows the C&M of both of the LXe-detector and the
magnetic spectrometer over long time periods, including accelerator shutdown periods or periods of beam-time dedicated to
other experiments. This has the advantage that independent
tuning of the detector can be undertaken, allowing for a more
efficient usage of the allocated MEG beam-time. It is worth
defining the meaning of ‘‘calibration and monitoring’’ in our case
and to try to list the quantities one has to measure and monitor,
mainly with respect to the ones of the LXe detector. By ‘‘calibration’’ one refers to the determination of the photomultipliers
relative quantum efficiencies QE, of the PMT amplifications, of the
degree of purity of the LXe, of the intensity and spectrum of the
Xe light emission, as well as of the optical parameters of both the
liquid and gaseous Xe (refractive index, Rayleigh scatteringlength, absorption-length [3,10,13]). It also refers to the determination of other important global quantities like the calorimeter
energy calibration and resolution as a function of the g-ray energy
and impact point, the time resolution, the resolution on the
position of the impact point and the LXe-detector’s capacity for
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J. Adam et al. / Nuclear Instruments and Methods in Physics Research A 641 (2011) 19–32
separating two particles in time and in space. By ‘‘monitoring’’
one essentially means the check of the stability of all important
quantities, as often and as completely as feasible, in conditions
which are as close as possible, or similar to the normal running
conditions of MEG (COBRA magnet at full field, high beam
intensity, minimal modifications to the MEG set-up.). Another
important calibration method, relevant to liquid scintillator
calorimeters and in particular to liquid cryogenic noble gas
detectors, is based on the use of multiple a-sources distributed
in the detector sensitive volume. For the MEG experiment we
developed 241Am point sources deposited on thin (100 mm diameter) gold-plated tungsten wires permanently suspended in the
volume as well as sources fixed on the surfaces of the large vessel
containing the LXe [14]. The method is valuable in measuring the
relative QEs of all PMTs surrounding the sensitive LXe volume, for
determining the LXe optical properties of the UV scintillation light
and for checking the stability of the calorimeter properties during
the experiment.
Table 2
Characteristics of the MEG C–W.
Proton beam properties
MEG C–W
Energy (keV)
Energy spread (FWHM) (keV)
Angular divergence (FWHM) (mrad mrad)
Spot size at 3 meter (FWHM) (cm cm)
Energy setting reproducibility (%)
Energy stability (FWHM) (%)
Range of the average current (mA)
Current stability (%)
Current reproducibility (%)
Duty cycle (%)
300–1000
o 0:5
o3 3
o3 3
0.1
0.1
1–10
3
10
100
5. The Cockcroft–Walton accelerator project
The Cockcroft–Walton (C–W) accelerator is in operation in the
MEG experiment for calibrating, monitoring, and tuning the
performance of the Liquid Xenon, the relative inter-bar timing of
the TCs and the relative timing between the TC and LXe detector.
These calibration measurements are necessary on a frequent
basis and losses to the normal data-taking time should therefore
be minimized. For ease of operation the following requirements
were met:
(i) a separate radiation-safety monitored area with controlled
access;
(ii) an automated, controlled beam-pipe insertion bellows system for the introduction of the LiF (or the Li2B4O7) target as
well as the transportation of the proton beam to the centre of
the COBRA spectrometer;
(iii) a beam transport system, consisting of vacuum pipes, a set of
two horizontal and two vertical steering magnets (parallel
displacement), with axial injection.
Fig. 2. A view of the Cockcroft–Walton accelerator.
These measures minimize the effect of the COBRA stray magnetic
field on the C&M accelerator and allow interventions on the
accelerator to be performed, when necessary, in a separate area,
without interfering with the MEG experiment.
5.1. The Cockcroft–Walton characteristics
The accelerator which is coupled to MEG is a 1 MeV C–W of
recent production [15]. Its performance is listed in Table 2.
5.2. The positioning of the C–W accelerator
A picture of the C–W accelerator is shown in Fig. 2. The C–W
accelerator is placed in a separate area, independently radiation
surveyed, in which it can be opened, closed and tested. At the
moment of performing a calibration, the C–W accelerator must be
turned-on, conditioned and tuned. Since the accelerator is in a
separate area, these operations can take place in parallel with the
normal MEG running. Close to the accelerator, a system of two
horizontal-deflecting and two vertical-deflecting magnets (parallel beam displacement) were installed allowing an axial injection
into the solenoid to hit the centre of the target (see Fig. 4.) The
problem of injecting a proton beam into COBRA to reach a target
at the COBRA centre is very similar to the one of the normal
m-beam. The particle momenta are similar and so are the optical
Fig. 3. Layout of the MEG and C–W experimental areas.
properties of the beam. The p-beam has to reach the target under
vacuum. The p-beam is introduced into the spectrometer from
downstream, in the opposite direction to the normal m-beam. The
present layouts of the downstream-side of the MEG experiment
and of the C–W area are shown in Fig. 3.
J. Adam et al. / Nuclear Instruments and Methods in Physics Research A 641 (2011) 19–32
23
Fig. 4. A scheme of the proton beam optics, of the control elements and of the bellows system. The figure is not to scale and shows the logical sequence of all the elements.
5.3. The proton-beam line and remote target insertion system
Both the accelerator and the beam line have optical elements
for optimizing the beam at the target. The accelerator is placed
some 5 m downstream of the COBRA magnet centre. The optics of
the p-beam was studied by a Monte Carlo simulation and it did
not present any problems. The COBRA magnet has a fringing field
which reaches the accelerator, though, at a distance 4 3:5 m from
COBRA centre, this is reduced to acceptable levels. A bellows
system is required to introduce the nuclear target into COBRA at
the magnet centre (see Fig. 4). Upstream of the bellows system, a
rigid pipe section connects the bellows system to the accelerator,
while downstream of the bellows system, another rigid pipe
section, containing the target at its end, is used to enter the
COBRA magnet. In order to enter the controlled He-atmosphere
inside COBRA, a larger diameter synthetic-rubber bellows insertion system, which is part of the downstream COBRA end-cap, is
utilized. This is coupled to the C–W bellows system and simultaneously driven to the centre of the detector. The speed of
insertion, together with a pressure-control system, guarantees
that the pressure differential on the thin cathode foils of the
tracking chambers inside of COBRA, as well as the beam vacuum
window in the upstream end-cap, does not exceed 10 Pa during
insertion or extraction.
When the calibration is performed, the bellows systems with
the target pipe are fully extended into COBRA, placing the target
at the COBRA centre. Once finished, the bellows systems are fully
retracted, with the larger diameter bellows residing inside the
end-cap, while the target pipe is fully retracted outside of the
end-cap. The rigid pipe section entering COBRA has a length of
223 cm. It can be moved by 221 cm in total (the stroke). The
bellows system which allows the stroke has a length of 360 cm. A
general picture of the C–W bellows system is presented in Fig. 5.
It was built by combining five bellows sections (stainless steel
bellows by MEWASA AG [16]). The system is guaranteed for a
lifetime of 500 kcycles. The operation of the system is computer
controlled and allows fully remote insertion and extraction. There
are several redundant safety systems built-in to ensure fault-free
manipulation.
Fig. 5. The C–W beam line and bellows system for introducing the nuclear target
into COBRA; a detail view of the bellows.
6. Beam diagnostics and intensity measurements
Luminescence based beam diagnostics, in various forms, is
widely used at accelerators [17,18]. After the acquisition of the
MEG C–W accelerator, this method was immediately applied for
setting-up the proton beam-line from the accelerator to the
COBRA magnet. The beam could be optimized by observing the
beam spot at various positions with a TV camera. The radiator
used was a quartz crystal, which emits a bluish fluorescence light.
The method was simple and fast, consisting of introducing the
crystal into the beam by means of a remotely controlled compressed-air actuator. The fluorescence light emitted from the
crystal placed at an angle of 451 to the incoming proton beam,
Fig. 6. TV camera and quartz crystal pneumatic actuator.
24
J. Adam et al. / Nuclear Instruments and Methods in Physics Research A 641 (2011) 19–32
is recorded, through a plexiglass window placed at 901 to the
beam, by a simple TV-camera (see Figs. 6 and 7). The properties of
the MEG muon and proton beams are reported in Table 3. The
quartz crystal is covered by a thin tungsten wire net, which is
grounded to avoid the accumulation of beam charge and consequent sparking on the crystal.
The light emitted by the crystal under proton bombardment
(see example in Fig. 8) proved to be a linear function of the C–W
current, as shown in Fig. 9. An example of the beam focusing
procedure is presented in Fig. 10, where the varying dimensions
of the proton beam-spot are shown as a function of the C–W
extraction voltage; this parameter is related to the ion-source of
the accelerator, and it is used to modify the beam focusing. The
beam intensity distribution (in ADC units, 256 maximum) was
obtained by a MATLAB based image processing procedure and the
beam spot intensity contours are displayed for a full set of
extraction voltages (7, 8, 9, 10, 11, 11.7, 12, 13, 14 kV) at a C–W
energy of 500 kV. The different intensity contours in the pixel
plane (0.2 real millimeters match into one pixel of 5.6 mm)
correspond each to a difference of five ADC units. An optimum
beam focus, with a s 8 mm, is reached at an extraction voltage
of approximately 10 kV. This can be seen in Fig. 11, where the
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
s2x þ s2y in milli-
beam intensity distribution s (defined as s ¼
meters in the image plane) is plotted as a function of the C–W
extraction voltage.
One remaining problem was that of exactly positioning the
proton beam on the nuclear target at the centre of the COBRA
magnet. The presence of stray magnetic fields along the beamline, associated with the iron content of the experimental hall
floor, requires a final check of the spot at the target. Here optical
methods and TV cameras cannot be employed due to the high
COBRA magnetic field. We therefore had to rely on a different
approach: a rotating target support and a pixel centring device
mounted at the end of the proton beam line. Normally the target
is kept at an angle of 451 relative to the beam direction, but can be
rotated into a parking position, out of the beam, when one uses
the pixel centring device. This is achieved by circulating a current
in a coil mounted at the back of the target support. The current
generates a magnetic moment, producing a torque in the COBRA
magnetic field which rotates the target (see Fig. 12). The pixel
device has thin copper pixels deposited on vetronite. The beam
position is reconstructed from the current readings of each
independent pixel. The system is computer interfaced and an
example of the current pattern is shown in Fig. 13. The proton
beam intensity measurement can in principle be performed on
any insulated beam intersecting device, such as the beam shutter
or the quartz crystal holder, which can be inserted by a remotely
controlled compressed air system. However these provide only a
rough measurement, since they are affected by the COBRA
fringing field and their structure cannot avoid a loss of electrons
originating from the proton collisions. A precise measurement is
provided by a suitably designed Faraday cup, containing a guardring to prevent the loss of electrons. The Faraday cup is also
inserted by a remotely controlled compressed air system.
Fig. 7. Detail of the quartz crystal actuator. The crystal has a diameter of 4 cm.
light integral (a.u)
2.5
2
1.5
1
0.5
Table 3
Main properties of the MEG experiment particle beams.
Beam
Units
Momentum
MeV/c
K.E.
MeV
Velocity
Beta
Intensity
Particles/s
Current
mA
C–W p
MEG m þ
43.3
28.0
1
3.65
0.046
0.256
6.25 1012
3.0 107
1
4.8 10 6
0
0
0.5
1
1.5
2
proton current (µA)
2.5
3
Fig. 9. Fluorescence light emission from the quartz crystal, as a function of the
proton beam current.
Fig. 8. Light from quartz with beam defocussed (left) and focussed (right).
25
J. Adam et al. / Nuclear Instruments and Methods in Physics Research A 641 (2011) 19–32
25
120
50
45
120
20
80
40
100
35
80
30
80
60
25
60
15
10
20
20
40
10
20
40
60
80
150
80
60
100
40
160
100
120
140 120
90
100
120 100
100
80
80
60
60
40
50
80
70
60
80
50
60
40
40
30
20
20
40
20
20
10
10
0
12
0
10
0
12
0
20
40
60
80
20
40
60
80
20
10
0
12
0
10
0
12
0
10
0
12
0
100
20
20
5
20
40
60
80
5
200
40
40
15
20
120
60
10
0
12
0
40
80
20
40
60
80
60
100
20
40
60
80
100
100
120
45
70
120
120
40
120
40
100
35
100
35
60
100
50
80
30
80
60
20
30
40
40
15
20
20
30
80
25
40
60
45
20
10
10
25
60
20
40
15
20
10
5
10
0
12
0
20
40
60
80
10
0
12
0
20
40
60
80
10
0
12
0
20
40
60
80
5
Fig. 10. Beam-spot contours as a function of the C–W extraction voltage (7, 8, 9, 10, 11, 11.7, 12, 13, 14 kV). The pixel number in the pixel-plane is indicated along the
x,y-axes. The 0.2 real millimeters match into one pixel of 5:6 mm.
The various current reading devices can be calibrated by means of
the Faraday cup.
7. The nuclear calibration reactions used
The main calibration method for defining the energy scale and
for guaranteeing the stability of the LXe detector is the one based
on the 17.6 MeV g-ray production by the reaction 73 Liðp, gÞ 84 Be [19].
Another important reaction is 115 Bðp, ggÞ 12
6 C [19], mainly used for
the relative timing of the MEG detectors. Previous to its implementation in MEG, we did a preliminary test of the proposed
method using the Van de Graaff accelerator of the Legnaro INFN
National Laboratories. We measured the reaction rates at different energies and with various target thicknesses. The quality of
the g-lines, in view of their use for energy calibration, was also
studied by examining their widths, the signal to background
ratios, the presence of unwanted tails in the energy distribution.
7.1. Protons on lithium
The reaction 73 Liðp, gÞ 84 Be is resonant at Ep ¼ 440 keV, with a
resonance-width G 15 keV. It produces a 17:6 MeVg-line, an
energy factor three times smaller than that of the g-rays from the
m-eg decay, but in an interesting region for C&M. This reaction,
which is excitable by very low-energy protons, is highly exothermic and is almost unique in providing high-energy g-rays with a
large peak cross-section (speak 5 mb), since, for this particular
reaction, the normally preferred particle emission (i.e.:
a-emission) is depressed [20,21]. The reaction also has a nonresonant component, but its cross-section, at energies larger or
smaller than 440 keV, drops by a factor 4100. Since the
7
8
3 Liðp, gÞ 4 Be reaction has unique properties, it was used to C&M a
previous version of the m-eg experiment [22].
It is worth pointing out that, for accelerators of energy higher
than the 1000 keV of our C–W, other reaction channels open, with
an unwanted increase in the background level of the experiment.
At the resonance for 73 Liðp, gÞ 84 Be the expected reaction rate
26
J. Adam et al. / Nuclear Instruments and Methods in Physics Research A 641 (2011) 19–32
depends on the cross-section integrated over the 440 keV resonance as well as on the total number of 73 Li nuclei reached by the
protons. One operates in the so-called ‘‘thick target’’ mode
corresponding to an effective target thickness of a fraction of a
mm. The resulting 17.6 MeV g-rays are isotropic with a convenient
rate of 1 MHz. The 73 Li reaction also produces a less intense and
wider 14.6 MeV g-line. (Other lines at lower energies correspond
to fluorine if a LiF target is used.)
The NACRE information on the 73 Liðp, gÞ 84 Be reaction [19] is
given in the form of the S-factor (MeV b), and in the form of crosssection, both as a function of the centre of mass total kinetic
30
25
σ (mm)
20
15
Fig. 14. 7Li cross-section for g-production as a function of Tp [19].
10
5
0
0
2
4
6
8
10
12
14
RF extraction voltage (kV)
16
18
20
Fig. 11. Beam-spot dimensions as a function of the C–W extraction voltage.
Fig. 12. Scheme of the movable target and of the pixel centring device. The pipe
diameter is 10 cm. The drawing is not to scale.
Fig. 15. Resonance in the 7Li cross-section for g-production as a function of
Tp; Tp ¼ 440 keV at the resonance, GR 12:5 keV.
Fig. 13. The pixel system mounted on a vacuum flange (left). Monitoring of the proton beam position by the pixel system (right).
27
J. Adam et al. / Nuclear Instruments and Methods in Physics Research A 641 (2011) 19–32
energy TnIN and of the centre of mass proton kinetic energy Tnp.
Some information on the angular distribution for the almost
isotropic g-emission is also available. It is convenient to use the
cross-section as a function of the proton laboratory kinetic energy
Tp (see Fig. 14). The resonant part of the cross-section, as
measured by us at the Legnaro INFN National Lab. is presented
in Fig. 15.
We give, as an example in Table 4, the computation of the
energies of the g-rays, emitted at the LAB polar angle 01 (relative to
the incident proton direction) and at 901, as a function of Tp for the
reaction 73 Liðp, gÞ 84 Be. Kinematic variables in the LAB (unstarred) and
CM (starred) systems and the total cross-section values are also
reported. From the close examination of the table one obtains an
estimate for the displacement of the g-lines when Tp is far from the
resonance energy, due to the effect of Doppler shifts and, consequently, for the possible deformation of the g-line when protons are
slowed-down in a thick target. Because of the cross-section behaviour as a function of Tp, the effects mentioned are more important
for boron than for lithium. The isotopic composition of elements and
in particular the one of Li can be found in Ref. [23]. Li has two main
isotopes (6Li¼7.59%, 7Li¼ 92.41%). In the case of the use of a LiF
target, with density rLiF ¼ 2:635 g=cm3 and NLiF ¼6.11 1022 cm 3,
the g-ray spectrum generated by protons hitting such target, at
resonance, is presented in Fig. 16 and complies with the ones
available in literature [27]. The sharp line at Eg0 ¼ 17:6 MeV and
the broad line at Eg1 ¼ 14:6 MeV are produced in a ratio
g0 =ðg0 þ g1 Þ ¼ 0:72 70:07. One can also see some g-production from
19
F and some naturally occurring radioactivity lines.
Table 4
Kinematic variables and 7Li cross-section (see text).
Tp (keV)
250.0
350.0
450.0
550.0
650.0
750.0
850.0
950.0
Tp ðkeVÞ
TIN
ðkeVÞ
s (mbarn)
Eg ð03 Þ MeV
Eg ð903 Þ MeV
191.15
267.61
344.07
420.53
496.99
573.45
649.91
726.37
218.60
306.04
393.48
480.92
568.36
655.80
743.24
830.68
0.016
0.070
2.011
0.062
0.069
0.055
0.071
0.082
17.50
17.60
17.70
17.79
17.89
17.98
18.07
18.17
17.45
17.54
17.63
17.72
17.80
17.89
17.98
18.06
Fig. 16. The g-spectrum from the proton reaction on a LiF target.
7.2. Protons on fluorine
Initially a lithium target in the form of lithium fluoride was
used, therefore the g-lines emitted by fluorine from the reaction
19
Fðp, agÞ 16 O are always associated with those emitted by lithium
under proton bombardment (see Fig. 16). It is worth noting that
the 19 F g -line at about 6 MeV is actually a composite line
corresponding to three g-energies: 6130, 6917, 7117 keV. The
6130 keV line is dominant; the three lines are somewhat differently populated as a function of the incident proton energy [28].
The 19F cross-section is presented in Fig. 17 [29]. When using a
proton energy Tp ¼500 keV on a thick LiF target, protons are
effectively captured by the 7Li 440 keV resonance but also by the
main 19F resonances at Tp ¼340 and 225 keV, following the proton
energy loss in the thick target. The 19F cross-section is roughly
constant in the energy region around the 440 keV 7Li-resonance,
therefore the 19F g-lines provide an effective proton-flux normalization when scanning the 440 keV 7Li-resonance.
7.3. Protons on boron
The 11B cross-section is presented in Fig. 18 [19,30]. One can
notice a large resonance at Tp ¼ 163 keV and a rising cross-section
at larger Tp. One important observation is that if one excites the
11
12
5 Bðp, 1 Þ 6 C
12
reaction (followed by 12
6 C - 6 C þ g2 ) with protons
far from the resonance energy, as with Tp 500 keV, hitting a
thick boron target, then a large fraction of the (nominal) 16.11
(g0 ) and 11.67 MeV (g1 ) g-rays will be produced at proton
energies higher than the resonant energy. When the (g1 ) is
emitted it will always be accompanied by the carbon de-excitation line (g2 ) at 4.44 MeV. Energy conservation implies higher
g-ray energies. This will spoil the (g0 )- and the (g1 )-lines by
introducing a large high energy tail. The 4.44 MeV line is left
unchanged since it corresponds to a precise 12C-level (and
Doppler effects are small). The isotopic abundance of B can be
found in Ref. [23]; there are two main isotopes (10B ¼19.9%,
11
B¼80.1%). The density is rB ¼ 2:46 g=cm3 and NB ¼1.37
1023 cm 3. The g-ray spectrum generated by protons hitting the
B target is presented in Fig. 19. The sharp line at Eg0 ¼ 16:11 MeV
and the much more abundant line at Eg1 ¼ 11:67 MeV are
g
Fig. 17.
19
F cross-section for g-production as a function of Tp [29].
28
J. Adam et al. / Nuclear Instruments and Methods in Physics Research A 641 (2011) 19–32
Fig. 18.
11
B cross-section for g0 - and g1 -production as a function of Tp [19,30].
targets had a limited lifetime and were slowly damaged by the
proton beam, when proton intensities of the order of 1 or 2 mA
were used. Since the target is positioned at the centre of our
spectrometer, where water cooling is not advisable, we are
limited to the use of a low proton beam current. The final choice
for low-cost individual LiF and B targets were, in the case of LiF,
optical UV-windows in thick crystal form from ALKOR [24]. These
had dimensions of 40 mm diameter and 4 mm thickness. Even
though these crystals showed optical signs of radiation damage,
we were able to prove that there was no degradation in time of
the Li g-ray lines. In the case of boron, durable targets of boron
carbide (B4C, produced by FELDCO international [25]) were used,
which had a negligible contribution from carbon g-rays. The use
of separate targets was however superseded by the need for a
combined Li and B target, as the advantage of the relative timing
calibrations became apparent and hence the time required for the
change of targets had to be reduced.
The selection of a combined target material, having both
suitable mechanical and thermal properties, led to the use of
lithium tetraborate (Li2B4O7), crystal disks manufactured by Jinan
Crystals [26]. We studied the relative yield of lithium and boron
g-lines as a function of the proton beam energy. Since the
calorimeter energy calibration depends on a sharp g-line at a
fixed energy, we used the lithium reaction at its resonant energy.
It was also verified that the quality of the lithium 17.6 MeV g-line
from a lithium fluoride and from a lithium tetraborate target was
identical. As the boron reaction is only used for timing and less
dependent on the g-line quality, we used the boron reaction at
relatively high proton energies (750–1000 keV) where the g-ray
yield is much larger than the one at the boron 163 keV resonance.
Lithium and boron calibrations were performed on alternate days,
with a duration of about 20 min each (30 k-events for lithium,
10 k-events for boron). The preparation of the calibrations, such
as the C–W tuning and the positioning of the targets at the centre
of the experiment, was performed in parallel with other aspects of
the experiment and did not require extra allotted time.
8. The study of the LXe calorimeter
The MEG experiment has so far collected data during the last
three months of 2008 and the last two months of 2009. Further
runs started in the summer 2010 and are expected to last two
years. During the periods the calorimeter properties were studied
daily by calibration methods, in particular by the use of the C–W
accelerator. As an example of the quality and reliability of MEG
data during the run, we shall discuss the time evolution of the
calorimeter behaviour as derived from calibration data.
Fig. 19. The g-ray spectrum from the proton reaction on a B-target [30].
8.1. The development of LXe calorimetry
produced in a ratio g0 =ðg0 þ g1 Þ ð3:5 70:07Þ 102 . The Eg ¼
12
4:44 MeV, by the C de-excitation, is emitted in coincidence
with the 11.67 MeV line. In the spectrum one can observe other
weak lines and escape peaks. As already stated, the coincident
Eg ¼ 4:44 and 11.67 MeV lines are used for timing. It is interesting
to note that these lines also provide a method of testing our
detector’s capability of distinguishing double g-rays entering the
MEG calorimeter.
7.4. Choice of targets and calibration programme
Targets for the MEG calibrations were optimized in a series of
successive steps. Initially we produced LiF or B targets by depositing thick and thin layers of LiF and B on a copper support. Such
The characteristics of the MEG calorimeter were briefly presented
in Section 3. The design of the final calorimeter for the experiment
was based on preliminary studies performed on calorimeter prototypes of smaller dimensions. The studies extended over several years
and were important for obtaining a practical experience in the
handling of such cryogenic detectors. The properties of LXe as a
scintillator and a first determination of relevant quantities such as the
LXe absorption length were discussed and presented in several
publications [10,13,14]. We refer to those for all basic aspects of
LXe calorimetry. It is advisable to recall briefly the relevant characteristics of xenon calorimetry by scintillation light. Pure liquid
xenon emits light in the ultraviolet at l ¼ 178 nm. The scintillation
light yield is similar to that of NaI. The average time constant for light
emission is 20 ns for ions and 50 ns for electromagnetic radiation
(electrons and gs), thus enabling particle discrimination. The light
J. Adam et al. / Nuclear Instruments and Methods in Physics Research A 641 (2011) 19–32
propagation in liquid xenon follows the usual exponential law:
x=latt
IðxÞ ¼ I0 e
ð1Þ
29
8.2. The determination of the calorimeter energy scale and energy
resolution.
where ldif is the diffusion length (photon elastic scattering, coinciding
with Rayleigh scattering lR 45 cm) and labs is the absorption
length (photon disappearance). Pure LXe is expected to be essentially
transparent to its own radiation. The LXe refractive index is 1:6.
The Rayleigh scattering length and the refractive index are related
[10]. The parameters outlined for pure LXe are heavily affected by the
presence of contaminants (mainly O2 and H2O), so it is important to
reach the maximum LXe purity and to keep it stable during the
experiment.
Xenon is kept liquid in the cryostat at 165 K and 0.12 MPa
during normal operations. The LXe active volume is 800 l (see
Fig. 20), the total amount of LXe being approximately 900 l. A
diagram of the liquid xenon cryogenic system is shown in Fig. 21.
Xe can be stored either as a gas in eight tanks with a total volume
of 2448 l, at a pressure of 70 bar or, as a liquid, in a 1000 l dewar
using the cryogenic equipment in common with the cryostat. The
inner and outer cryostat vessels form a C-shape, in which a
similar photomultiplier support structure fits (see Fig. 22). Both
the inner and the outer vessels have very thin steel windows
(thickness about 3 mm) to minimize the interactions of g-rays
from m-decay. A honeycomb structure is fixed to the inner thin
window to prevent a shape distortion, from concave to convex,
when vacuum is applied in the outer vessel and LXe is present in
Fig. 20. The LXe cryostat and the LXe dewar.
Fig. 22. The PMT support structure mounted inside the cryostat inner vessel.
where latt is the light attenuation length. The attenuation is the result
of light diffusion and of light absorption:
1
latt
¼
1
labs
þ
1
ð2Þ
ldif
Fig. 21. Diagram of the MEG LXe cryogenic system.
30
J. Adam et al. / Nuclear Instruments and Methods in Physics Research A 641 (2011) 19–32
the inner one. The honeycomb is a thin carbon fiber structure
characterized by a high mechanical rigidity.
Xe is liquified by cooling the cryogenic vessel. Both the cryostat
and the dewar are equipped with a pulse-tube cryocooler and
liquid nitrogen (LN2 pipes). The pulse-tube cryocoolers and the
LN2-pipes are placed at the top of both the cryostat and dewar. The
cryostat is precooled to reduce the duration of the cooling phase,
by LN2 circulating in pipes attached to the lateral walls of its inner
volume. During the liquefaction process the pulse-tube cryocooler
and the LN2 circulation system are switched on. At the end of the
process the LN2 flow is switched off and the pulse-tube cryocooler
alone maintains the Xe in the liquid state.
As previously mentioned LXe is transparent to its own scintillation light, but possible contaminants, such as water or oxygen at
the ppm level, could produce considerable absorption.
Purification in the final calorimeter is achieved by using a
molecular filter and a gas purifier operating in the liquid phase.
The coupling of a cryogenic centrifugal pump to the filter and
purifier achieved a purification corresponding to 40 ppb from an
initial water impurity of 250 ppb, in only 5 h [31,32].
As an example of the importance of the Xe purity and of the
usefulness of the calibration method based on the C–W to follow
the evolution of the calorimeter response during the 2008 MEG
data taking, we present in Fig. 23 the variation of the light
emission yield corresponding to the Li 17.6 MeV g-line, as a
function of time. The increase in the light yield and several steps
present in the plot are associated with the period of operation of
the purification methods. At the end of the data-taking period the
LXe calorimeter reached a resolution of sEg =Eg ¼ 3:857 0:15% at
the 17.6 MeV g-line, as can be seen in Fig. 24.
XEC spectrum
300
250
200
150
100
50
0
0
2000
4000
6000
8000
qsum2
10000
12000
14000
Fig. 24. LXe calorimeter resolution at the 17.6 MeV g-line. The events in the 5000–
9000 region are due to the a-particles from the calibration wire sources.
9. The relative time tuning of the experiment
The fine-tuning of the relative timing between the LXe
calorimeter and the timing-counters is important in obtaining
the best possible background rejection in the MEG experiment. To
this purpose MEG exploits, for example, the radiative m-decay, in
which a g-ray is emitted in addition to the electron and the
neutrinos. The MEG Cockcroft–Walton provides both a complementary and simpler approach. The 115 Bðp, g1 g2 Þ 12
6 C is the only
proton reaction on a natural element which is highly exothermic
and which produces two usable coincident g-rays in the final
state. One of these photons can be detected in the calorimeter,
while the other might interact in front of or in the timingcounters, so also being detected. Coincidences can be obtained
with a high rate, allowing a temporal connection between the
calorimeter and the timing-counters to be made in a reproducible
22000
Gas Purification
20000
Fig. 25. The g-ray spectra (in MCA units) from a lithium tetraborate (Li2B4O7)
target, as a function of Tp.
way. The detected particles, their energies and their paths would
however be different to those of the m-eg, or radiative m-decay
channels, but nevertheless in a precise relation to them. Timewalk corrections due to the lower energies of the boron g-rays
were determined and applied.
Liquid Purification
photoelectrons
18000
9.1. The boron experimental data
16000
14000
12000
10000
8000
6000
30/04/08
17/06/08
05/08/08
22/09/08
Date
10/11/08
28/12/08
Fig. 23. LXe light emission yield as a function of the calendar date.
We measured the g-ray spectra emitted by the lithium tetraborate (Li2B4O7) target above the 163 keV boron resonance, for
400 rTp r 1000 keV. The spectra are shown in Fig. 25 and one can
observe that the choice of proton energy ensures the dominance
of selected g-lines: at 500 keV, just above the 7Li 440 keV
resonance, the 7Li 17.6 MeV g-line is dominant and contributions
of the 11B lines are visible at lower g-energies; at 1000 keV the 11B
11.7 MeV g-line is dominant, together with the 12 C 4.4 MeV deexcitation line. In contrast, below the Li-resonance at
Tp ¼400 keV, the total rate is low and associated with only 11B.
The quality of the 7Li 17.6 MeV line at Tp ¼500 keV, from LiF (red)
J. Adam et al. / Nuclear Instruments and Methods in Physics Research A 641 (2011) 19–32
and from Li2B4O7 (black) is compared in Fig. 26. One can see how
similar the two spectra are at around 17.6 MeV. A large fluorine
contribution is present in the LiF data, while a small contribution
from boron is visible around 11.7 and 4.4 MeV in the Li2B4O7 data.
Oxygen does not contribute appreciably to the g-rate.
A study of the boron g-coincidences in the LXe calorimeter and
in the TC shows that while the g-energy energy is correctly
determined in the LXe calorimeter, the 4.4 and 11.7 MeV g-rays
interact mainly by Compton effect in the plastic scintillator TC
bars. Thus the corresponding energy spectrum measured in the TC
consists of two wide partially superimposed Compton electron
energy distributions. A scattered plot of the TC energy (in
arbitrary units) vs. the LXe calorimeter energy for coincident
g-events is presented in Fig. 27.
A simulation shows that g-rays reaching the LXe calorimeter are
detected with good efficiency ( 98%), while a much smaller
fraction ( 21%) of the g-rays, within the solid angle of the TC,
corresponds to interactions. A g-ray is detectable if the
31
corresponding energy loss is 4 1 MeV. The overall efficiency for
the detection of all the double boron g-events, generated over the
full solid angle and surviving all analysis selections (to be discussed) is of the order of 1.6 10 3, corresponding to a possible
acquisition rate of 20 Hz. The selected events provide a sufficiently precise determination of the relative timing of the calorimeter vs. the TC.
9.2. Measuring the time in the TC
Prior to the description of the LXe-TC relative timing, let us
briefly recall how the arrival time of the positron at the TC is
determined.
A schematic representation of a TC bar is shown in Fig. 28. The
time t0 and t1 measured by the two photomultipliers at each end
of a bar of length L, can be written as:
c0
h
þ b0
t0 ¼ TTC þ pffiffiffiffiffiffi þ
A0 veff
c1
Lh
t1 ¼ TTC þ pffiffiffiffiffiffi þ
þ b1
veff
A1
ð3Þ
ð4Þ
where TTC is the arrival time of the positron at the the timing
counter bar,
pffiffiffiffiffimeasured with respect to an arbitrary reference. The
terms ci = Ai , where Ai is the i-th PMT signal amplitude, represent
the time walk corrections, h is the distance of the i-th impact
point from the inner PMT, veff the effective light velocity in the bar
and bi represent any electronic time offset in the same bar. The
effective velocity is assumed to be constant along each bar
(a good approximation). It is straightforward to extract the impact
time of the positron at the TC from the above relations:
!
t0 þ t1 1
c0
c1
L
b0 þ b1
p
ffiffiffiffiffi
ffi
p
ffiffiffiffiffi
ffi
TTC ¼
þ
ð5Þ
þ
2
2veff
2
2
A0
A1
Note that the term L=ð2veff Þ þðb0 þ b1 Þ=2 is a constant kj characteristic of each TC bar and is thus an inter-bar time offset that needs
to be determined and monitored.
9.3. Tuning the LXe-TC relative timing
Fig. 26. The g-spectrum, at Tp ¼ 500 keV, from LiF (red) and Li2B4O7 (black);
logarithmic scale. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
40
35
TC Energy (a.u.)
30
The study and the accurate monitoring of the LXe-TC timing is
performed using boron C–W events yielding two photons in the
final state, as previously mentioned. The energy of the g-rays in the
LXe calorimeter is required to be less than 8 MeV; namely events
are selected where the low energy g-ray enters the calorimeter and
the high energy g-ray hits the TC. We define Tgg as the time
difference of the two g-rays from the boron reaction, measured by
the LXe calorimeter and the TC with reference to the target:
Lg,TC
Lg,LXe
Tgg ¼ Tg,LXe
Tg,TC
ð6Þ
c
c
where Tg,LXe is the time measured by the LXe calorimeter, Lg,LXe =c is
the g-ray time-of-flight from the target (assumed to be point-like
and at the origin of the apparatus reference system) to the g-ray
impact point on the calorimeter front face. Tg,TC is the TC time of
25
20
15
10
5
0
0
2
4
6
8
10
12
14
16
18
20
LXe Energy (MeV)
Fig. 27. Scatter plot of energies measured in the LXe calorimeter and in the timing
counter, for coincident 4.4 and 11.7 MeV g-events.
Fig. 28. A schematic representation of a particle crossing a TC bar.
32
J. Adam et al. / Nuclear Instruments and Methods in Physics Research A 641 (2011) 19–32
0.5
250
200
mean of ∆Tγγ (ns)
Events/0.25ns
Mean = (21.770 ± 0.012)ns
Sigma = (0.449 ± 0.014)ns
150
100
0
-0.5
-1
-1.5
-2
50
-2.5
0
0
0
5
10
15
20
25
30
35
40
45
5
50
the first bar hit, using the algorithm previously described, and
Lg,TC =c is the g-ray time-of-flight from the target to the TC. The
impact point at the TC is derived from the known TC radial position
and the z-coordinate measured by the bar PMT time difference. The
time Tg,LXe is, at this stage, already corrected for the time offsets
between different calorimeter PMTs. Thus, given that the two
g-rays are emitted simultaneously, for g-rays impinging on the
bar j of the TC we can write:
Tgg,j ¼
b0,j þ b1,j
L
þ TB ¼ kj þTB
2veff ,j
2
ð7Þ
where TB is time offset between LXe and TC, given that the two
sub-detectors times are measured by two independent clocks.
Fig. 29 shows the Tgg,j distribution for a bar (bar 17).
9.4. Inter-bar offset determination using the Boron events
We took advantage of the frequent (twice or three times per
week) C–W data-taking planned for the LXe calorimeter calibrations. As already described in Section 7.3, the boron reaction
yields two simultaneous g-rays, almost isotropically emitted, of
energy 11.7 MeV (g1 , the high energy photon) and 4.4 MeV (g2 ,
the low energy photon). The distribution for each bar j is fitted by
a Gaussian function, in the range of 1.2 ns around its maximum.
The mean value of the Gaussian is, by definition, the offset kj þTB.
If one arbitrarily chooses, say, bar 17 as the reference, the interbar offset of the j-th bar is the quantity (kj–k17). The tails of the
distribution are due to cosmic ray contamination, as verified by
data-taking with the C–W accelerator off and the same event
selection. The systematic effects on the evaluation of the offsets
due to cosmic rays are very small. Additional systematic effects
related to the fitting procedure were found to be negligible. The
inter-bar time offsets were rather stable during the three month
duration of the MEG data-taking. This can be seen in Fig. 30 where
the inter-bar offsets are shown as a function of the TC bar number
and for various periods.
10. Conclusions
Energy and timing calibrations using a proton beam from a
Cockcroft–Walton proton accelerator were fully exploited during
the 2008 and 2009 MEG data-taking. They were essential for
satisfying the strict stability requirements of the experiment and
15
20
25
30
TC bar number
Tγγ (ns)
Fig. 29. Tgg,j distribution for a single bar (bar 17) with the Gaussian fit
superimposed.
10
Fig. 30. The inter-bar offsets (relative to bar 17) vs. the TC bar number for
different periods (corresponding to different colors). (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of
this article.)
led to a better understanding of the complex processes associated
with the use of such a detector.
Acknowledgements
Preliminary measurements on the nuclear reactions of interest
for the MEG experiment were performed at the INFN Legnaro
National Laboratories. We thank the laboratory personnel for the
effective collaboration. We would also like to express our thanks
to the PSI infrastructure groups involved with the construction
and commissioning of the MEG C–W area.
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