473
Nuclear Instruments and Methods in Physics Research A301 (1991) 473-481
North-Holland
DAPHNE : a large-acceptance tracking detector for the study
of photoreactions at intermediate energies
G. Audit, A. Bloch, N. D'Hose, V. Isbert, J. Martin, R. Powers, D. Sundermann,
G. Tamas and P.A. Wallace
DPhN/SEPN, CEN Saclay, 91191 Gif stir Yvette, France
S. Altieii
a,
A . Braghieri
a,b,
F. Fossati
a,b,
P. Pedroni
a
arid T. Pirielli
a,b
INFN, Sezione di Pavia, Via Bassi 6, 27100 Pavia, Italy
b Dipartimento di Fisica Nucleare e Teorica dell' Università di Pavia, Via Bassi 6, 27100 Pavia, Italy
J. Bechade, P.H. Carton, S. Conat, D. Foucaud and M. Goldsticker
DPhN/STEN, CEN Saclay, 91191 Gaf sur Yvette, France
Received 20 November 1990
A large-acceptance (94% of 4m sr) hadron detector capable of handhng multiparticle final states is described. The track
reconstruction capability, energy resolution, particle identification capability and neutral-particle detection efficiency of the detector
are discussed and the results of tests shown . Tests have been performed both with cosmic rays and in a realistic experimental
situation using a 500 MeV photon beam impinging on hydrogen and deuterium targets .
1. Introduction
The availability of high-intensity tagged photon
beams has led to an increased interest in the use of
intermediate-energy photonuclear reactions as a tool for
the study of the nucleus in the baryonic resonance
region .
The experiments currently envisaged range from inclusive measurements of total-photoabsorptmon cross
sections to the study of exclusive photoproduction or
photodisintegration processes on light nuclei . The ideal
detector for a total-absorption measurement would produce a signal in response to at least some of the products of every nuclear reaction induced in the target. A
practical detector whose performance approaches this
ideal would have as large an angular acceptance as
possible to minimise the necessary extrapolation to 41r
sr, a wide momentum acceptance and a reasonable
efficiency for the detection of neutrons and rr o as well
as charged particles.
The more exclusive experiments would also benefit
from large acceptances, since these would permit a
sizeable region of the phase space of the final state to be
explored, especially in the case of experiments where
the trigger requires a coincidence between two or more
particles in the final state. However, the selection of a
definite final state usually requires the additional capability to identify charged particles and determine their
momenta and trajectories.
In the momentum domain under considerations protons, pions and electrons can be distinguished by dE/dx
techniques for which the resolution provided by plastic
scintillators is adequate . However, rather than rely on
the scintillators for the determination of the particle
energies it was decided to base the reconstruction of the
reaction kinematics upon the angular information proTable 1
Principal characteristics of DAPHNE
Angular acceptance
Polar angle
Azimuthal angle
21' < 0 < 159' 94% of 4m
0 ° < 0 < 360'
Charged-particle detection thresholds (1 g/CM Z target)
Pions
Protons
T=12MeV (p= 60MeV/c)
T = 23 MeV (p = 220 MeV/c)
Maximum energy of particles stopped to scintillators A, B and C
Pions
0 = 90,
0=21°
Protons
8=90 °
0 = 21'
0168-9002/91/$03 .50 C 1991 - Elsevier Science Publishers B.V . (North-Holland)
T = 57 MeV
T=120 MeV
T=125 MeV
T = 225 MeV
(p =138 MeV/c)
(p=219 MeV/c)
(p=500 MeV/c)
(p = 668 MeV/c)
474
G Audit et al. /DAPHNE
30cm
SIDE
VIEW
,
SCINTILLATORS
E
FRONT
Fig . 1 . Schematic view of the detector .
vided by a high resolution tracking detector, in this case
comprising a set of multiwire proportional chambers
(MWPCs) . This technique avoids the need to use a
magnetic field within the detector in order to achieve
good momentum resolution . It is particularly suitable
for the study of photoreactions on very light nuclei
producing two or three charged particles in the final
state : for example the reactions yd - ppm - and y 3 He
- ppn . Nonetheless, additional information from scintillator pulse-height signals with moderately good resolution would be valuable since it would further overdetermine the kinematics and improve the reliability of
the calculation of the "best fit" momenta and trajectories for each event .
To meet these requirements, we have constructed
DAPHNE (Detecteur à grande Acceptance pour la
PHysique photoNucleaire Experimentale) : a largeacceptance tracking detector for intermediate-energy
hadrons comprising a vertex detector surrounded by a
segmented calorimeter .
The detector comprises three principal parts,
arranged as a set of coaxial cylinders . In the center
there is a vertex detector which is surrounded by a
charged-particle detector consisting of several layers of
scintillator which is itself surrounded by a leadaluminium-scintillator sandwich designed to detect
neutral particles (see fig . 1 and table 1) . It follows from
the cylindrical symmetry of the detector that the coverage of the azimuthal angular range is complete ; the
lengths of the cylinders are so arranged as to subtend a
range of angles in the polar direction 0 from 21° to
159' which corresponds to 94% of 4m sr.
VIEW
cathode strip readout [1] . Each MWPC has inner and
outer cylindrical walls made from 1 mm thick Rohacell
covered in 25 Wm Kapton film . The interior surfaces are
laminated with aluminium strips (0 .1 Wm thick, 4 mm
wide and separated by 0 .5 mm) which form the cathode .
The anode surfaces consist of arrays of 20 Wm diameter tungsten wires stretched parallel to the cylinder axis
at 2 mm intervals around the circumference . The anode
wires are maintained at a tension of 60 g .
The inner and outer cathode strips are wound helically in opposite directions at angles of ±45° with
respect to the anode wires . The anode-to-cathode gap is
4 mm and a mixture of Ar (74 .5%), ethane (25%) and
freon (0 .5%) is used as a filling gas . The cathode strips
are each provided with an Alcatel 1757 charge amplifier
and a LeCroy FERA 4300B ADC to record the analogue charge signals. The anode wires are each provided
with a LeCroy PCOS 2735PC amplifier/ discriminator,
the signals from which are registered in a set of pattern
units . The geometrical characteristics of each chamber
are reported in table 2 .
2.2 . The chargedparticle detector
A three layer plastic-scintillator telescope is used to
identify charged particles and measure their energies .
The layers, which are labelled A, B and C in fig . 1, are
10, 100 and 5 mm thick, respectively .
Table 2
Geometncal characteristics of the MWPCs
Chamber
2 . General description of the detector
2.1 . The vertex detector
The central section of the detector is devoted to
charged-particle tracking. It consists of three coaxial
cylindrical multiwire proportional chambers with
Length [mm]
Internal radius [mm]
External radius [mm]
Number of wires
Number of int . strips
Number of ext . strips
1
2
3
360
60
68
192
60
68
560
92
100
288
92
100
760
124
132
384
124
132
G . Audit et al. / DAPHNE
Each layer consists of 16 longitudinal bars of
NE102A plastic scintillator (see fig . 1) assembled in
such a way that their cross sections are 16-sided regular
polygons inscribed inside circles of 156, 172 and 275
mm radii, respectively . The A scintillators are viewed
from one end by RTC 2012B photomultipliers, while
the B and C scintillators are viewed from both ends by
Philips XP2262 and EMI 9903 KB photomultipliers,
respectively.
The A and B layers taken together serve as a dE/dx
charged-particle identification telescope. Signals from
the C layer serve to indicate that a particle has passed
through the B layer without stopping. If the particle
stops in the B layer, one may determine its energy from
the pulse-height signal . The measurable energy range
depends on particle flight direction : table 1 shows the
thresholds and largest measurable energies for charged
pions and protons at and 0 = 21 ° and 0 = 90 ° .
The signals from the C scintillators, together with
those from the subsequent layers, can be used to obtain
a less precise energy determination for those particles
which pass through the B layer but still stop inside the
detector. In this case the scintillator-absorber "sandwich" acts as a range telescope .
2 .3 . Neutralparticle detection
The 100 mm thick B scintillators provide a modest
neutron detection capability whose efficiency for 500
MeV/c neutrons ranges from about 10% to 20%, depending upon the neutron emission angle and therefore
the path length of the particle in the scintillator . Efficient photon detection, and therefore efficient m° detection, requires high-Z material in which to convert the
photons into electron-positron pairs which are then
detectable in the scintillators . To this end the telescope
has been surrounded with lead-scintillator "sandwich"
elements . These consist of a 5 mm Pb converter and two
layers of 5 mm thick plastic scintillator (labelled as D
and E in fig . 1), separated by a 6 mm layer of aluminium .
The sandwich modules follow the same 16-element pattern as the inner layers . All the scintillator elements are
fitted with EMI 9903KB photomultipliers at either end .
The D and E layers are operated m coincidence in order
to reduce the counting rate due to low-energy background .
The 16-fold segmentation provides azimuthal positional information for the neutral particles, while the
longitudal coordinate may be determined by measuring
the difference in propagation time between the two
signals coming from the photomultipliers mounted on
each end of the scintillators . However, it is not practical
to determine the energy of neutral particles by pulseheight or time-of-flight techniques .
475
2 .4 . Associated electronics
In order to exploit the versatility of the detector, a
flexible electronic trigger has been designed which permits on-line discrimination between various classes of
events .
It is possible to trigger both on charged and neutral
particles. In the case of charged particles it is possible to
reduce the electromagnetic background from electrons
and photons or to select proton events by setting a
threshold on the weighted sum of the pulse-height signals from the A and B layers .
Several triggers may be used in parallel, depending
on the requirements of the experiment . For example,
during a measurement of the process y 3 He - ppn a
trigger requiring two high-energy charged particles may
be used to select events which result in a ppn final state
as opposed to the much more plentiful charged-pion
photoproduction events . Meanwhile, a rate-divided
single charged-particle trigger may be used to record a
fraction of the events resulting in fast charged pions in
order to monitor the experiment .
Signals coming from the different detectors are
analysed using standard ECL-CAMAC electronics. The
pulse-height amplitudes are recorded using LeCroy
FERA 4300B ADCs and the timing signals from the
scintillators are digitized using a set of LeCroy FERA
4303 TFC and ADC modules . All the FERA data
words, both from the MWPCs and the scintillators, are
read out in compressed mode and buffered in a VMEbus
high-speed memory together with the data from the
pattern units which record the anode wire hits before
being transferred to a SUN workstation for storage and
display.
3. Test results
The performance of the detector has been tested
both in an experimental situation and with cosmic rays
and has also been simulated numerically using the
GEANT3 package [2] .
3.1 . Vertex detector efficiency and resolution
The tracks of charged particles are reconstructed
from the coordinates of the points of intersection of the
tracks with the three MWPCs . Within each chamber
both the azimuthal (a) and longitudinal (z) coordinate
of the avalanche (see fig . 2) are evaluated from the
centroid of the charge distribution induced on the
cathode strips . The location of the hit wire(s) is used to
resolve the ambiguity which arises from the fact that
each pair of inner and outer strips cross each other
twice . The resolution in the longitudinal coordinate
depends on the particle flight direction : if the track
47 6
G. Audit et al / DAPHNE
intersects the chamber at a large angle with respect to
the perpendicular direction, then the charge distribution
collected by cathode strips becomes broader and the
position determination is less precise .
In real operational conditions it is necessary to take
into account defects arising from the possible malfunctioning of some of the wires or strips . However, since
there is a degree of redundancy in the information
provided by the ensemble of anode wires and internal
and external cathode strips it is possible to make corrections for individual dead wires or strips in many cases .
Tests made with a collimated 0 source show that the
anode efficiency for minimum-ionising particles is well
above 99% over the whole sensitive area of the MWPCs .
The impact-point reconstruction efficiency for each
MWPC, again measured with a 0 source, was found to
be about 98% . Cosmic-ray data indicate that the global
reconstruction efficiency after all corrections for defective wires and strips have been made is typically about
96% for each MWPC .
The local resolution in the longitudinal direction was
measured with the collimated 0 source and was found
to be about 200 li m (FWHM) at 0 = 90' . However, the
value obtained for the whole chamber was rather poorer
because of the existence of local mechanical defects
such as misalignments among wires or a variable anodeto-cathode gap, etc . These effects can be seen in fig . 3,
where the longitudinal coordinate difference 0(z) between the "best fit" impact position obtained from all
the signals induced m the three MWPCs by a cosmic
ray and that determined from the strips of a single
MWPC is plotted against 0 .
The cosmic-ray data were used to determine correction factors for the calculated a-and z-coordinates which
take into account the smoothly varying effects such as
misalignments between the cathode and anode planes
inside each MWPC and among the different chambers .
When all these corrections are included, a global resolution of 350 g.m (FWHM) was obtained for the case of
perpendicular incidence, 0 = 90' .
0 .9
E
E
x
3
N
4
0 .6
04
03
0
0
10
20 30 40 50 60 70 80 90 100
0 (deg .)
Fig . 3 . 0(zt" - z`P) (FWHM) as a function of 0.
The polar angular resolution, which is essentially
dependent upon the longitudinal coordinate, was also
measured directly with cosmic rays. It was obtained
from the difference between the two fitted 0 values
obtained from the incoming and outgoing tracks produced by the cosmic rays as they traverse the chambers .
The result is shown in fig . 4, where it can be seen that,
as expected, the resolution 0(B) depends on the polar
angle with a minimum value at 0=90' of 0 .7'
(FWHM).
The vertex reconstruction resolution was determined
by calculating the minimum distance between the the
two straight lines fitted to the particle tracks, and the
result, plotted as a function of 0, is shown in fig . 5 .
The azimuthal-angle (q)) resolution was calculated in
the same way ; the resulting distribution is constant with
and, because of the wire spacing, discrete with a
FWHM of 2° .
3.2 . Position-resolution and attenuation measurements for
scintillators
Fig . 2 . Coordinate system used m the MWPC analysis .
The position resolution obtainable from the scintillators as well as their light attenuation characteristics
were measured with cosmic rays using the information
from the MWPCs to define the trajectories of the particles.
Fig . 6 shows the result of the position-resolution
tests for the B layer after correction for the effect of the
finite rise time of the scintillator signals . The position
G. Audit et al / DAPHNE
477
200
175
1.2
150
1.1
10
3
0
a
125
-6--
0 .9
aV
100
0.8
0.7
E
z 75
0.6
50
0
25
0
10
20 30 40 50 60 70 80 90 100
0 (deg .)
Fg . 4. 0(B) (FWHM) as a function of 0.
resolution (FWHM) is about 5 cm ; the same result was
also obtained for the D and E layers .
The results of the attenuation measurements for the
A and B layers are shown in figs. 7 and 8.
28
rí
0
-200 -150 100 -50 0
50 100 150 200
Position (mm)
Fig. 6. Position resolution for B-layer detectors.
3.3. Chargedparticle response and energy resolution
The dE/dx telescope energy resolution was obtained during the initial tests made on all the sections of
the A, B and C layers.
The scintillators were placed in the focal plane of the
ALS "700" magnetic spectrometer [31 and their individ-
2.4
2
E
0.8
0.4
0
0
20
40
60
0 (deg .)
80
100
Fig. 5 . Vertex-reconstruction resolution as a function of 0.
Position (mm)
Fig. 7. Position dependence of the pulse-height response for
the A layer. The curve is a fit to the experimental points with
an attenuation constant of 1580+120 mm .
478
G. Audit et al. /DAPHNE
300
m
ó
08
200
C
C
0 .6
U
U
0 .4
100
0 .2
0
-400 -300 -200 -100
0
100
200 300 400
0-
-
r
70
Position (mm)
Fig. 8. Position dependence of the ratio B1/B2 between the
two signals from the photomulthpliers mounted at each end of
the B scintillators. The curve is a fit to the experimental points
with an attenuation constant of 2700+30 mm .
ual response to monochromatic charged particles was
measured using protons photoproduced on a solid target
by a bremsstrahlung photon beam .
As an example, the energy spectrum of a B layer
element for 500 MeV/c protons incident on the centre
of the scintillator is shown in fig. 9. The energy resolution was found to be AE/E=10ß'o (FWHM), where E
is the kinetic energy of the incoming proton .
The overall detector response to different particles
was also checked by means of the following reactions
which were studied using the ALS-tagged photon beam
y+d-p+n.
(3)
The kinematics of these two-body reactions are completely determined from knowledge of the photon energy and the charged-particle emission angle. This fact
permitted the detector response to protons and charged
pions of known energy to be determined over a very
large range of energies . As an example, the correlation
between kinetic energy and angle for protons produced
in reaction (1) at EY = 400 MeV is shown in fig . 10
where the expected monotonic relationship characteristic of two-body reactions can clearly be seen. In this
case, having taken into account the effects due to target
absorption and gamma-ray energy uncertainty, the proton energy resolution is equal to that observed in the
spectrometer.
E (MeV)
ti
100
3.4. Chargedparticle identification
For dE/dx particle identification it is necessary to
take into account the changes in the particle's path
length within the A layer which occur as the angle 0
varies. This is done by using the information from the
MWPCs to normalize the measured energy values (dE)
to those corresponding to perpendicular incidence on
the detector surface, i.e. dE' =dE sin 0. The 0-value is
150
140
130
(1)
(2)
I
90
Fig. 9. Response of the B detector to 500 MeV/c protons.
[41 :
Y+P-P+Tr °,
y+p-n+Tr + ,
1
80
a
100
80
60
40
20
0
0
02
0.4
06
08
1
1 .2
14
0 (rad)
Fig. 10 . Energy-angle correlation for recoil protons from Tr
production on hydrogen at Ev = 400 MeV. The region inside
the dashed curves represents the limits (±3a) obtained from a
GEANT3 simulation .
E (MeV)
Fig. 11 . Experimental E-dE/dX plot for charged particles
photoproduced on 6 Li at E. in the region 150-300 MeV. The
region inside the dashed curves represents the limits (±3a)
obtained from a GEANT3 simulation .
directly evaluated from the MWPC readout and its
precision permits the uncertainty in the A measurement
to be limited to that due to the finite resolution of the
scintillators .
Fig. 11 shows a two-dimensional plot of dE' plotted
against E for charged particles produced in photoreactions on a 6 Li target . The E-signal was determined from
100
75
50
25
E
E
Z (mm)
Fig. 13 . The same as in fig. 12 but in a plane parallel to the
beam axis.
the pulse-height signal from the B layer and only those
particles which stopped in the B layer are shown. Both
d E'- and E-values have been corrected to take into
account light-attenuation effects in the scintillators. The
separation between electron and pion zones is clearly
seen and, in addition, pions and protons are well separated throughout the energy range covered by the B
layer.
In the same fig. it may be seen that the particle
distributions in the E-dE' plane lie inside the limits
obtained by a GEANT3 Monte Carlo simulation in
which scintillator resolution effects were included .
The quality of the vertex reconstruction system is
shown in figs . 12 and 13 which display the reconstructed vertex coordinates for reactions on deuterium
when two charged particles were detected in the final
state, i.e . or pp or pm - .
T
-25
-50
-75
-l00
-200
-150
-100
-50
0
50
100
150
200
X (m m)
Fig. 12 . ZH target profile in a plane perpendicular to the beam
axis reconstructed using vertex coordinates from reactions contaimng pp or pm - in the final state.
Table 3
irt ° and neutron experimental detection efficiencies for the
reactions y + p -+ p + m o and m + p -" n + ,R+ at E,, =400
MeV, compared to GEANT3 predictions. The numbers given
correspond to thresholds of 1 MeV for the B layer and 0.3
MeV for the D and E layers ; the m ° detection-efficiency figure
assumes the detection of either one or both decay photons
Detection efficiency [%]
,T O
n
Experiment
61 .5 ±1 .0
20 .2+0 .5
GEANT3
61 .6+0 .5
20 .0+0 .4
480
G. Audit et at. / DAPHNE
60
50
0
40
0
000
0
0
0
0
0=21°
0
0=90°
Values reconstructed
10
Momenta
0
0
200
400
v
800
600
ET (MeV)
Fig . 14. y-ray efficiency as a nction of energy at 0 = 90 ° and
6=
as calculated
by GEANT3 .
fu21'
3.5. Neutralparticle efficiency
Using the reactions (1), (2) and (3), we may experimentally determine the detection efficiency for neutrons
and neutral pions over a large range of energies .
Table 3 gives the rr o and neutron detection efficiencies determined from analysis of the reactions (1) and
(2) with Ey = 400 MeV . The predictions of a GEANT3
Monte Carlo program are shown in the same table . The
version of the GEANT3 code used for this purpose had
been modified to treat low-energy hadronic interactions
correctly [5] . It may be seen that the agreement with the
experimental values is fairly good.
60
ó
T
V
c
w
w
0
0
40
30
0
20
0
0
10
0
0
E,, =
01 =
02 =
0,=
Pion
20
50
Data observed
Photon
Protons
a
30
Table 4
Example of the reconstruction of the kinematics of a
d(y, ppir - ) event using only the angular information obtained
from the tracking detector
200
400
600
0
0
800
0
0 =21°
0
0=90°
1000
P (MeV/c)
Fig. 15 . Neutron efficiency as a function of momentum at
0 = 90 ° and 0 = 21 ° as calculated by GEANT3 .
Pl
P2
Pm
Invariant
masses
MP MPZ MPlP2
479 ± 1 MeV
(90 ±0.3) °
(30 ±0 .5) °
(61 .9+0 .4)'
01=(180 ±1) °
42= (30 ±1)°
0, = (336.3 ± 1)°
= 400±3 .1 MeV/c
= 400±5 .4 MeV/c
= 281±3 .2 MeV/c
=1313±2 .5 MeV
=Il79+2 .3 MeV
=1996+2 .9 MeV
Figs . 14 and 15 show the results produced by the
GEANT3 code for the gamma and neutron efficiencies
over a range of energies for particles emanating from
the centre of the target at angles 0 = 90 ° and 21 ° . The
efficiencies were calculated using the same detection
thresholds as assumed in table 3 .
4 . Conclusions
A large-solid-angle detector has been designed to
study multiparticle reactions induced by an intermediate-energy photon beam .
The performance of the detector has been found to
meet its design expectations. In particular, it is possible
to precisely determine the tracks of charged particles
and to discriminate clearly between protons, pions and
electrons. The reconstruction of the trajectories of the
emitted particles will in many cases permit a very
accurate reconstruction of the kinematics of the reaction. An example of this procedure applied to the
reaction yd - ppir - is shown in table 4, where a maximum-likelihood algorithm has been used to determine
the momenta of the three particles and hence calculate
the three two-body invariant masses .
The detector has already been proved suitable for
practical use with ALS photon beam and in the future it
will be operated in conjunction with the Mainz MAMI-B
tagged bremsstrahlung beam which will provide 10 7
photons/s at energies ranging up to 850 MeV .
The combination of DAPHNE'S large acceptance
and the high intensity of the tagged photon beam available at MAMI-B will lead to an improvement in both
the quality and the quantity of data to be obtained on
exclusive photoreaction processes .
G. Audit et al. / DAPHNE
Acknowledgements
The authors would like to thank PSI (formerly SIN)
technical staff for the help and the assistance given in
the construction of MWPCs. One of us, P.A .W ., wishes
to acknowledge the support of the SERC in the form of
a NATO postdoctoral fellowship .
References
48 1
[1] M. Kobayashi et al ., Nucl Instr. and Meth . A245 (1986) 51
and references therein.
[2] R . Brun et al ., GEANT3 User's Guide, CERN DD/EE/
84-1(1987).
[3] P.E . Argan et al., Nucl . Phys . A237 (1975) 447.
[4] P.E . Argan et al ., Nucl . Instr. and Meth . 228 (1984) 120.
[5] P. Pedroni, INFN Report BE-88/3 (1988).