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A spectrometer for 1 GeV electrons using scintillation detectors
Nuclear Instruments and Methods in Physics Research A324 (1993) 191-197 North-Holland
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
A spectrometer for 1 GeV electrons using scintillation detectors M. Anghinolfi, P. Corvisiero, G. Coscia, L. Mazzaschi, V. Mokeev, G. Ricco, M. Ripani, M . Taiuti and A. Zucchiatti Istituto Nazionale di Fisica Nucleare and Dipartimento di Fisica via Dodecaneso 33, 16146 Genova, Italy
N. Bianchi, E. De Sanctis, P. Levi Sandri, V. Lvcherini, V. Mvccifora, E. Polli and A.R. Reolon Istituto Nazionale di Fisica Nucleare Laboratori Nazionali di Frascati, Italy
P. Rossl
Istituto Nazionale di Fisica Nucleare ISS, Italy
G. Gervino
Istituto Nazionale di Fisica Nucleare, Torino, Italy
Received 1 June 1992 The design criteria of a scintillator telescope for high energy electrons, consisting of plastic scintillators, a BGO preshower detector, a segmented BGO calorimeter and an aerogel Cherenkov counter are discussed. The performance of the telescope is described using the data collected in an inclusive electron scattering experiment around 1 GeV, with emphasis on the efficiency to electrons, which is enhanced by the analysis of the complete information provided by such a multidetector system . 1. Introduction The electromagnetic interaction is an ideal probe to study the hadronic structure of the nucleus. It is described by quantum electrodynamics, it is sufficiently weak to be computed in a perturbative approach and it is sensitive to the entire nuclear volume . The electron scattering has so far supplied a very accurate and organic information on the charge distribution, on single particle state characteristics and also on nucleon excited states and their behaviour in nuclear matter. Magnetic spectrometers have normally been used to detect scattered electrons. Their most characteristic performance is the high momentum resolution (Op/p 10 -3 -10 -4 ), and limited momentum acceptance (1 .1
and could allow the simultaneous acquisition of multiple informations from the constituent detectors. For the study of inclusive electron scattering up to ' .~ GeV [1] off an oxygen target at the Jet-Target facility of the INFN Frascati National Laboratories we have developed such a spectrometer . This has represented a good solution to the space constraints of our experimental area and has resulted particularly suitable to the kinematical conditions of our experiment where the proton and pion background was a severe problem. 2. The scintillation telescope The schematic layout of our telescope is shown in fig. 1. The reaction products are intercepted in order by a thin single plastic scintillator, an aerogel Cherenkov counter, a segmented thick plastic scintillator, a BGO preshower counter and a matrix of 20 BGO crystals arranged (5 x 4) in a carbon fibre basket . The first element is a thin plastic scintillator (labeled (a) in fig. 1) of size 2.5 x 3 .0 x 0.2 cm3. This detector distinguishes charged particles from neutral particles
0168-9002/93/$06.00 C 1993 - Elsevier Science Publishers B.V . All rights reserved
192
M. Anghmolfi et al. / A spectrometer for 1 GeVelectrons
(n, -rr°, y) which do not release energy by ionization . Its function is to better define, together with the segmented plastic scintillator (labeled (b) in fig. 1) the solid angle of the spectrometer . The second component of the spectrometer is an aerogel Cherenkov detector with a refraction index n = 1.045 . This special material has allowed the realization of a compact detector of square cross section and 5 X 5 X 4 cm 3 size, obeying the stringent mechanical constraints of our apparatus. It is used to separate medium and low energy electrons against the high pion and proton background which, at laboratory angles greater than 60° can be an order of magnitude larger than the electron contribution . The third element is a plastic scintillator, (b), consisting of four separate elements, each measuring 1 .1 X 4 X 1 cm 3 ; it reduces the uncertainty on the angle of emission of charged particles through its subdivision in sectors and therefore improves the precision by which we know the momentum transfer . It could also be used for particle mass selection by the E-dE/dx technique in the energy range of our experiments . The fourth element is a preshower detector consisting of a BGO slab measuring 2.5 X 10 X 10 cm3. Its purpose is to separate electrons from charged pions. In fact the energy deposited in this device is markedly different when we observe the development of an electromagnetic shower caused by an electron or when we detect an ionizing pion . The bulk of the spectrometer is a BGO sector which is part of a 4 ,rr electromagnetic calorimeter [2]. It consists of 20 BGO crystals, 21 radiation lengths thick,
having the shape of a cut pyramid with trapezoidal bases of approximate size 2 X 2 cm z and 6 X 6 CM Z. The BGO sectors are cointained in a carbon fibre basket in optically and mechanically separated cells. In this configuration only the six central crystals in the basket are interested by the reaction products . The 14 BGO crystals at the edge of the basket detect the transversal development of showers improving up to a few percent the total energy resolution . Besides giving the complete electron energy spectrum up to 1 .5 GeV the granularity of this detector can be used as a further method for distinguishing pions and electrons through the different distribution of energy amongst the crystals, in the case of an electromagnetic shower or in the case of partial ionisation followed by hadron interactions in the BGO. 3. Design criteria The code GEÀNT3 [3] has been extensively used [4,5] to study the response of some of the spectrometer elements to assess their performances and define their design . 3.1 . Preshower counter
The distinctive quantity for the electromagnetic shower is the radiation length Xo , which for BGO has a value of 1 .13 cm, while for the production of hadrons is the nuclear absorption length .t which in BGO is of the order of a few 10 cm . A detector thickness longer
Fig. 1. The general layout of our experimental apparatus. Two scintillation telescopes viewing the oxygen jet target are shown and their main parts are indicated.
M. Anghmolfi et al. / A spectrometer for 1 GeV electrons 400
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superposition with pions is of the order of 30% . For a thickness of 3 .5 cm there is a further enlargement of the pion spectrum and the contribution of the pionnucleus interactions becomes evident . As a result of these simulations the thickness of 2 .5 cm has been chosen .
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Fig. 2 . The simulated response of the preshower counter to pions and electrons produced at 30° by a 1 GeV electron beam on an oxygen jet target . The assumed preshower thickness is 1 .5 cm (a), 2.5 cm (b), 3 .5 cm (c) .
than Xo and shorter than A will make high the probability that an electromagnetic shower is started in this detector while very few hadrons are produced . The combined response to electrons and pions produced on oxygen at 30° by a 1 GeV electron beam, has been studied for different thicknesses of the detector using appropriate cross sections [6] to give the correct relative population of the two particles at different energies . In fig . 2 are reported the energy spectra simulated for 1 .5, 2.5 and 3 .5 cm thickness, respectively . For 1 .5 cm the energy released by pions corresponds to the ionization energy loss slightly widened by straggling . For electrons there are energy depositions larger than the amount expected from the ionization, since many electrons have given rise to an electromagnetic shower . The region where electrons and pions are superimposed contains unfortunately about 65% of the total electrons . When we increase the thickness to 2.5 cm the pion signal peaks at a higher average ionization energy loss and is enlarged due to the increased straggling . Very few higher energy events, due to pionnucleus interactions in the detector, are seen . The electron spectrum is much broader in this case and the
The main BGO detector has been designed as part of a 41T calorimeter and already optimized [2] for the confinement of electromagnetic shower up to 2 GeV . Each sector has been equipped with selected photomultipliers [7] linear up to 80 mA of peak current and provides a longitudinal response uniform within 3% on the average [8] . The overall resolution of the complete 4 ,tr BGO calorimeter, including the statistical contribution, the intrinsic width due to energy escape from the detector boundaries, a temperature variation within 1°C and the measured nonuniformity has been calculated 2% (FWHM) at 1 GeV . The limited solid angle of our setup causes only six crystals to detect the reaction products while the remaining 14 crystals detect the development of showers. In this configuration we expect a value of about 3 .5% FWHM at 1 GeV . We have focused attention on the information provided by the granularity of the BGO array, using the different distribution of energy amongst crystals in the case of an electromagnetic shower and in the case of a hadronic interaction . In fact the size of the basket is large enough to contain the electromagnetic shower but still appreciably shorter than the nuclear absorption length . Furthermore since the hadron energy in an inclusive electron scattering is concentrated below some hundred MeV, it is highly probable that only one nuclear interaction takes place within the BGO volume, the rest of the energy being released by ionisation . We have simulated, in the design stage of this apparatus [5,9] several granularity functions . The simplest one consists in counting the number of crystals where an energy above a threshold value Eth (3 MeV in our case) has been released . Fig . 3 reports the simulated distribution of this granularity function for electrons, protons and pions produced on oxygen at 30° by a 1 GeV electron beam, using appropriate cross sections to give the correct relative population of the two particles at different energies . The energy released by pions and protons is evidently more localized than for electrons . Other functions have been simulated for this apparatus : however for the analysis of our experimental data the granularity was sufficient to identify high energy pions and more complex functions were not used .
M. Anghinolfi et al. / A spectrometer for 1 GeV electrons
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with good efficiency . This detector fits well in the area between the two plastic scintillators, which are sufficiently separated to sharply define the solid angle of the apparatus.
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Fig. 3. The simulated granularity distribution observed in the matrix of 20 BGO crystals for electrons (a), protons (b) and pions (c), produced at 30° by a 1 GeV electron beam on an oxygen jet target. 3.3 . The Cherenkou counter A strong selection of electrons against the pion and proton background is given by the Cherenkov counter. This is described in detail in an accompanying report [10] . Two conditions have been considered : a cut-off energy of a few hundred MeV for charged pions of mass m,r and the largest possible detection efficiency for electrons. The cut-off energy fixes an upper limit to the refractive index n of the Cherenkov material through the equation : n<
T,+m,
V T,(T, +2m,)
= 1 .05 .
The detection efficiency is directly related to the number of photoelectrons which, for a fixed radiator length and a given ß of the particle, increases with n [11] . Due to our geometrical and mechanical constraints we choose an aerogel Cherenkov radiator. In particular we adopted n = 1.045 that fixes a detection threshold of only 1.25 MeV for electrons and = 400 MeV for pions. The 4 cm thickness of the aerogel resulted ideal to detect the Cherenkov light produced
In a complex detector like this several operations have been necessary to insure the achievement of the expected performance, in particular on the 4 x 5 BGO matrix as regards temperature control, intercalibration and stability. Temperature control on BGO crystals is a stringent requirement since it is known that the gain variation of BGO with temperature is of the order of 1.28% per degree [12] . Our carbon fibre basket is surrounded by a copper jacket in which a thermostatic liquid at the temperature of 20°C circulates . Even with external temperature variations of several degrees this system insures a stability of the BGO crystal matrix within at most ±0 .5°C as measured during several days of experimental run by a VME based monitoring system . The equalization of all crystals in the matrix is an essential requirement for the correct particle energy measurement as well as for an efficient use of the granularity related functions. In our case we have performed two different kinds of equalization : a low energy one based on a Am-Be 4.4 MeV -y-ray source and a high energy one based on cosmic ray signals in the crystals . The first one is evidently simpler and faster while the latter works at energies much closer to the real detector energy range. For the equalization of the BGO crystal with cosmic rays we have taken advantage of the symmetry of the 4 ,rr structure to which the crystals belong . Since the BGO crystals in the 4 x 5 matrix all focus to a common center we evaluated by a Monte Carlo code the expected count rates and spectra for a simultaneous double coincidence of each crystal in the vertically standing basket with a unique plastic scintillator placed in the focal position [9]. In practice this focal detector was a 4 cm diameter, 1 cm thick disc . An equalization within approximately ± 2% is sufficient for an inclusive electron scattering experiment . This was achieved with an integral above 100 counts in the cosmic ray Landau peak since the width of this distribution is around 20%. Our count rate estimates were of = 3.4 counts/ h with some 30% less counts in the edge crystals . An acquisition of two days was sufficient to a statistically significant measurement. In practice the crystals were equalized beforehand with an Am-Be source . The equalization process with cosmic rays required then between 2 and 3 iterations and gave a distribution of average values as in fig. 4a . All points are within a ±3% band and the standard deviation of these points is ±2 .7%
M. Anghinolfi et al. / A spectrometer for 1 GeV electrons
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Fig . 4 . The response of the 20 BGO crystals after equalisation with cosmic rays (a) and with an Am-Be gamma source (h) .
well in range with the required equalization precision . A further check with an Am-Be source in these conditions reproduces the cosmic ray results as seen in fig . 4b . For long term stability control we mounted on each crystal a green (HLMP-1540) LED . All of them were driven by a unique pulse generator and read in coincidence with the pulser gate . The system long term stability was within ±1% . The preshower detector has required a simpler calibration procedure [9] . We produced a uniform surface response by equalizing the six photomultipliers associated to this detector by means of a 6° Co . We also set the high voltages for the best energy resolution . Calibration of this detector was determined collecting the cosmic ray energy spectra in a triple coincidence with two small plastic scintillators for cosmic rays crossing the detector through its shorter (2 .5 cm) or its longer (9 .6 cm) side . Tuning of the plastic scintillator consisted first of all in equalizing the response of the phototubes with the use of a 'Sr source and subsequently in setting appropriate high voltage values to insure at the same time the best timing performance and the compatibility of the anode charge with the linearity range of the ADCs .
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The Cherenkov detector was tested using the high energy c'-e - pairs produced by a 1200 MeV bremsstrahlung gamma beam on a gold radiator [10] . The charge distribution of anode signals had a width corresponding to an average of 10 photoelectrons produced per electron or positron crossing the aerogel . The efficiency of the counter was measured to be higher than 97% in any instance. The data handling [9] has been designed mostly around standard commercial modules . The logic was designed using standard NIM modules and the linear part of the chain used CAMAC based FERA ADCs with DMA reading of a 16 Kbyte stack memory. The anode signal of each crystal has been resistively splitted into three parts corresponding approximately to 90, 1 and 10% of the signal . The 90% part and the 1% part were directly analysed by FERA ADCs to detect respectively low energy (upper limit 12 MeV) and full energy depositions (upper limit 1 GeV) in the individual crystals . The remaining part was linearly added to the contributions of the other crystals to provide a total energy pulse (upper limit 1 .5 GeV) for the basket . A fourfold coincidence between the plastic scintillators, the BGO preshower and the total energy in the BGO calorimeter was the fast trigger for the FERA . Details of this rather conventional electronics can be found elsewhere [9] . 5 . Experimental results Inclusive electron scattering off an oxygen jet target [13] has been measured for beam energies going from 600 MeV to 1200 with telescopes located between 32 and 100° according to the chosen energy . The analysis of data taken at Ee = 1 GeV with the telescope at 60° to the beam direction is well representative of the performance of our apparatus. In fact the energy is high enough to approach the acceptance range of our electronics and test it close to its limits . Besides this, the kinematics and dynamics of the inclusive electron scattering are such to produce at 60° a critical condition where the electron signal has to be extracted from a proton and pion background four times larger . This allows us to demonstrate how the use of the complete information provided by our detectors can give a clean separation of the events of interest . Fig . 5 shows a bidimensional plot of the energy released in the preshower detector and in the BGO calorimeter . Three regions emerge from this plot . The electrons are distributed almost uniformly over the plane . Their electromagnetic shower begins within the preshower counter at a penetration depth which can be anywhere between 0 and 2.5 cm . The energy released in the preshower can be therefore larger than expected from the ionization energy loss alone up to about 200 MeV .
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M. Anghinolfi et al. / A spectrometer for 1 GeV electrons -
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Fig. 5 . The bidimensional plot obtained from the preshower and the BGO calorimeter collected energy . The three main regions corresponding to electrons, pions and protons are discussed in the text .
Fig. 6. The distribution of the signals collected by the Cherenkov detector . The energy scale is m channels and pions and protons are basically concentrated at channel zero. Chan nel 80 has been selected as the discrimination level for electrons.
The energy released by pions in the preshower counter is much more defined and clusters around 15-30 MeV. This is due essentially to the ionisation process since the pions are at the minimum of their specific energy loss and, as pointed before, the probability of a nuclear interaction is rather low. The proton specific energy loss is not minimum but depends on the proton energy . Low energy protons loose in the preshower around 70 MeV and about 30 MeV when their total energy is maximum. Some of these events scatter from the ionisation limit to lower energies, due to proton nuclear interaction in BGO. This effect is masked for pions due to the much narrower range of their energy loss in the preshower. We could treat in the same way the energy deposited in the segmented plastic scintillator and the energy released in the BGO preshower counter to distinguish from their behaviour different types of particles . However this information would not be independent from the previous one and affected by the poorer energy resolution of the plastic scintillator . Consequently the main function of the plastic scintillator in our analysis is the determination of the scattering angle while the other detectors provide particle identification . In fig. 6 is shown the light output of the Cherenkov detector, triggered by the fourfold coincidence of the other detectors. Only electrons give signals above zero while pions and protons are almost entirely below the detection energy threshold. As seen from fig. 6, a cut at channel 80 discriminates very well the electrons with
an efficiency above 97%. This value has been determined by fitting the broad electron peak with a Gaussian function extrapolated to zero. Fig. 7 refers to events that have produced a signal above channel 80 in the Cherenkov counter. Electrons
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Fig. 7. The bidimensional plot obtained from the preshower and the BGO calorimeter collected energy, for all events that have produced a signal above channel 80 in the Cherenkov counter . Three graphical cuts (CUTI, CUT2, CUT3) identify respectively medium and high energy electrons, low energy electrons, and medium energy (T, >_ 400 MeV) pions.
M. Anghmolfi et al . / A spectrometer for 1 GeV electrons
0 U
electrons unadvertedly discarded, we end up with a reconstruction efficiency higher than 97 .5% . The Cherenkov detection efficiency was measured to be not lower than 97% including the experimental errors . This puts to the electron detection efficiency of our telescope a lower limit of 95% ; a more realistic value being expected between 96 and 98% .
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Fig . 8 . The granularity distributions of events belonging to the region identified by CUTI (a), CUT2 (b), and CUT3 (c) .
For the study of inclusive electron scattering up to 1 .5 GeV off an oxygen target at the Jet-Target facility in the Frascati National Laboratories of the Istituto Nazionale di Fisica Nucleate, we have developed a scintillation telescope that covers a solid angle of 30 msr and allows the simultaneous acquisition of multiple informations from the constituent detectors . Such a detector has given results well in agreement with numerical simulations . It has proved to be particularly indicated in the kinematical and dynamical conditions of our experiment, where the proton and pion background are larger than the electron signal . Its detection efficiency for electrons is higher than 95% in any instance . The granularity of the main BGO calorimeter can be used for a sharp rejection of unwanted background events not discriminated by our simple aerogel Cherenkov detector .
Acknowledgements populate the entire plane but it is evident that the Cherenkov information is still not sufficient to exclude from this plot energetic pions . Our system of detectors can provide the necessary extra information . Some graphical cuts can be performed as indicated and the granularity computed for each selected area, obtaining the results reported in figs . 8a-8c . When we compare these measured quantities one to another and to the simulations of fig . 3, it is evident that the area defined by CUT3 refers essentially to pions and areas defined by CUT1 and CUT2 refer to high and low energy electrons, respectively . The addition of the granularity gives to this telescope the necessary separation capability when different particles remain confused in our simple Cherenkov counter . Our system has operated with a signal to background level just above 20% . In this severe condition the electron detection efficiency was determined first by the Cherenkov threshold and secondly by the possible presence of electron counts in the granularity distribution of fig . 8c . Comparing figs . 8a and 8c it appears that medium energy electrons produce preferably a granularity above 7 . Even if we assume that all cases with granularity greater than 7 in fig . 8c are
We ackowledge with appreciation the collaboration of the technical staff of both the INFN Genova Section and the INFN Frascati Laboratories .
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
M . Anghinolfi et al ., Report LNF 90/026 (1990) . M . Anghinolfi et al ., Report LNF 90/084(R) (1990) . R . Brun et al., Report CERN/DD/EE/84-1 . L. Mazzaschi et al., Nucl . Instr . and Meth. A305 (1991) 391 . M. Bogliardi, Thesis, University of Genova (1991) unpublished . J .W. Lightbody Jr. e t al ., Computer Phys . (1988) 57 . C . Bernini et al., Report INFN/TC/89/13 (1989) . A. Zucchiatti et al ., Nucl. Instr . and Meth . A317 (1992) 492 . G. Coscia, Thesis, University of Genova (1991) unpublished . M . Anghinolfi et al ., Report INFN/TC-92/15 . J . Carlson et al., Nucl. Instr . and Meth . 166 (1979) 425 . A. Zucchiatti et al ., Nucl. Instr . and Meth . A281 (1989) 341 . M . Taiuti et al., Nucl . Instr . and Meth . A297 (1990) 354.
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
A spectrometer for 1 GeV electrons using scintillation detectors M. Anghinolfi, P. Corvisiero, G. Coscia, L. Mazzaschi, V. Mokeev, G. Ricco, M. Ripani, M . Taiuti and A. Zucchiatti Istituto Nazionale di Fisica Nucleare and Dipartimento di Fisica via Dodecaneso 33, 16146 Genova, Italy
N. Bianchi, E. De Sanctis, P. Levi Sandri, V. Lvcherini, V. Mvccifora, E. Polli and A.R. Reolon Istituto Nazionale di Fisica Nucleare Laboratori Nazionali di Frascati, Italy
P. Rossl
Istituto Nazionale di Fisica Nucleare ISS, Italy
G. Gervino
Istituto Nazionale di Fisica Nucleare, Torino, Italy
Received 1 June 1992 The design criteria of a scintillator telescope for high energy electrons, consisting of plastic scintillators, a BGO preshower detector, a segmented BGO calorimeter and an aerogel Cherenkov counter are discussed. The performance of the telescope is described using the data collected in an inclusive electron scattering experiment around 1 GeV, with emphasis on the efficiency to electrons, which is enhanced by the analysis of the complete information provided by such a multidetector system . 1. Introduction The electromagnetic interaction is an ideal probe to study the hadronic structure of the nucleus. It is described by quantum electrodynamics, it is sufficiently weak to be computed in a perturbative approach and it is sensitive to the entire nuclear volume . The electron scattering has so far supplied a very accurate and organic information on the charge distribution, on single particle state characteristics and also on nucleon excited states and their behaviour in nuclear matter. Magnetic spectrometers have normally been used to detect scattered electrons. Their most characteristic performance is the high momentum resolution (Op/p 10 -3 -10 -4 ), and limited momentum acceptance (1 .1
and could allow the simultaneous acquisition of multiple informations from the constituent detectors. For the study of inclusive electron scattering up to ' .~ GeV [1] off an oxygen target at the Jet-Target facility of the INFN Frascati National Laboratories we have developed such a spectrometer . This has represented a good solution to the space constraints of our experimental area and has resulted particularly suitable to the kinematical conditions of our experiment where the proton and pion background was a severe problem. 2. The scintillation telescope The schematic layout of our telescope is shown in fig. 1. The reaction products are intercepted in order by a thin single plastic scintillator, an aerogel Cherenkov counter, a segmented thick plastic scintillator, a BGO preshower counter and a matrix of 20 BGO crystals arranged (5 x 4) in a carbon fibre basket . The first element is a thin plastic scintillator (labeled (a) in fig. 1) of size 2.5 x 3 .0 x 0.2 cm3. This detector distinguishes charged particles from neutral particles
0168-9002/93/$06.00 C 1993 - Elsevier Science Publishers B.V . All rights reserved
192
M. Anghmolfi et al. / A spectrometer for 1 GeVelectrons
(n, -rr°, y) which do not release energy by ionization . Its function is to better define, together with the segmented plastic scintillator (labeled (b) in fig. 1) the solid angle of the spectrometer . The second component of the spectrometer is an aerogel Cherenkov detector with a refraction index n = 1.045 . This special material has allowed the realization of a compact detector of square cross section and 5 X 5 X 4 cm 3 size, obeying the stringent mechanical constraints of our apparatus. It is used to separate medium and low energy electrons against the high pion and proton background which, at laboratory angles greater than 60° can be an order of magnitude larger than the electron contribution . The third element is a plastic scintillator, (b), consisting of four separate elements, each measuring 1 .1 X 4 X 1 cm 3 ; it reduces the uncertainty on the angle of emission of charged particles through its subdivision in sectors and therefore improves the precision by which we know the momentum transfer . It could also be used for particle mass selection by the E-dE/dx technique in the energy range of our experiments . The fourth element is a preshower detector consisting of a BGO slab measuring 2.5 X 10 X 10 cm3. Its purpose is to separate electrons from charged pions. In fact the energy deposited in this device is markedly different when we observe the development of an electromagnetic shower caused by an electron or when we detect an ionizing pion . The bulk of the spectrometer is a BGO sector which is part of a 4 ,rr electromagnetic calorimeter [2]. It consists of 20 BGO crystals, 21 radiation lengths thick,
having the shape of a cut pyramid with trapezoidal bases of approximate size 2 X 2 cm z and 6 X 6 CM Z. The BGO sectors are cointained in a carbon fibre basket in optically and mechanically separated cells. In this configuration only the six central crystals in the basket are interested by the reaction products . The 14 BGO crystals at the edge of the basket detect the transversal development of showers improving up to a few percent the total energy resolution . Besides giving the complete electron energy spectrum up to 1 .5 GeV the granularity of this detector can be used as a further method for distinguishing pions and electrons through the different distribution of energy amongst the crystals, in the case of an electromagnetic shower or in the case of partial ionisation followed by hadron interactions in the BGO. 3. Design criteria The code GEÀNT3 [3] has been extensively used [4,5] to study the response of some of the spectrometer elements to assess their performances and define their design . 3.1 . Preshower counter
The distinctive quantity for the electromagnetic shower is the radiation length Xo , which for BGO has a value of 1 .13 cm, while for the production of hadrons is the nuclear absorption length .t which in BGO is of the order of a few 10 cm . A detector thickness longer
Fig. 1. The general layout of our experimental apparatus. Two scintillation telescopes viewing the oxygen jet target are shown and their main parts are indicated.
M. Anghmolfi et al. / A spectrometer for 1 GeV electrons 400
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superposition with pions is of the order of 30% . For a thickness of 3 .5 cm there is a further enlargement of the pion spectrum and the contribution of the pionnucleus interactions becomes evident . As a result of these simulations the thickness of 2 .5 cm has been chosen .
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Fig. 2 . The simulated response of the preshower counter to pions and electrons produced at 30° by a 1 GeV electron beam on an oxygen jet target . The assumed preshower thickness is 1 .5 cm (a), 2.5 cm (b), 3 .5 cm (c) .
than Xo and shorter than A will make high the probability that an electromagnetic shower is started in this detector while very few hadrons are produced . The combined response to electrons and pions produced on oxygen at 30° by a 1 GeV electron beam, has been studied for different thicknesses of the detector using appropriate cross sections [6] to give the correct relative population of the two particles at different energies . In fig . 2 are reported the energy spectra simulated for 1 .5, 2.5 and 3 .5 cm thickness, respectively . For 1 .5 cm the energy released by pions corresponds to the ionization energy loss slightly widened by straggling . For electrons there are energy depositions larger than the amount expected from the ionization, since many electrons have given rise to an electromagnetic shower . The region where electrons and pions are superimposed contains unfortunately about 65% of the total electrons . When we increase the thickness to 2.5 cm the pion signal peaks at a higher average ionization energy loss and is enlarged due to the increased straggling . Very few higher energy events, due to pionnucleus interactions in the detector, are seen . The electron spectrum is much broader in this case and the
The main BGO detector has been designed as part of a 41T calorimeter and already optimized [2] for the confinement of electromagnetic shower up to 2 GeV . Each sector has been equipped with selected photomultipliers [7] linear up to 80 mA of peak current and provides a longitudinal response uniform within 3% on the average [8] . The overall resolution of the complete 4 ,tr BGO calorimeter, including the statistical contribution, the intrinsic width due to energy escape from the detector boundaries, a temperature variation within 1°C and the measured nonuniformity has been calculated 2% (FWHM) at 1 GeV . The limited solid angle of our setup causes only six crystals to detect the reaction products while the remaining 14 crystals detect the development of showers. In this configuration we expect a value of about 3 .5% FWHM at 1 GeV . We have focused attention on the information provided by the granularity of the BGO array, using the different distribution of energy amongst crystals in the case of an electromagnetic shower and in the case of a hadronic interaction . In fact the size of the basket is large enough to contain the electromagnetic shower but still appreciably shorter than the nuclear absorption length . Furthermore since the hadron energy in an inclusive electron scattering is concentrated below some hundred MeV, it is highly probable that only one nuclear interaction takes place within the BGO volume, the rest of the energy being released by ionisation . We have simulated, in the design stage of this apparatus [5,9] several granularity functions . The simplest one consists in counting the number of crystals where an energy above a threshold value Eth (3 MeV in our case) has been released . Fig . 3 reports the simulated distribution of this granularity function for electrons, protons and pions produced on oxygen at 30° by a 1 GeV electron beam, using appropriate cross sections to give the correct relative population of the two particles at different energies . The energy released by pions and protons is evidently more localized than for electrons . Other functions have been simulated for this apparatus : however for the analysis of our experimental data the granularity was sufficient to identify high energy pions and more complex functions were not used .
M. Anghinolfi et al. / A spectrometer for 1 GeV electrons
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Fig. 3. The simulated granularity distribution observed in the matrix of 20 BGO crystals for electrons (a), protons (b) and pions (c), produced at 30° by a 1 GeV electron beam on an oxygen jet target. 3.3 . The Cherenkou counter A strong selection of electrons against the pion and proton background is given by the Cherenkov counter. This is described in detail in an accompanying report [10] . Two conditions have been considered : a cut-off energy of a few hundred MeV for charged pions of mass m,r and the largest possible detection efficiency for electrons. The cut-off energy fixes an upper limit to the refractive index n of the Cherenkov material through the equation : n<
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The detection efficiency is directly related to the number of photoelectrons which, for a fixed radiator length and a given ß of the particle, increases with n [11] . Due to our geometrical and mechanical constraints we choose an aerogel Cherenkov radiator. In particular we adopted n = 1.045 that fixes a detection threshold of only 1.25 MeV for electrons and = 400 MeV for pions. The 4 cm thickness of the aerogel resulted ideal to detect the Cherenkov light produced
In a complex detector like this several operations have been necessary to insure the achievement of the expected performance, in particular on the 4 x 5 BGO matrix as regards temperature control, intercalibration and stability. Temperature control on BGO crystals is a stringent requirement since it is known that the gain variation of BGO with temperature is of the order of 1.28% per degree [12] . Our carbon fibre basket is surrounded by a copper jacket in which a thermostatic liquid at the temperature of 20°C circulates . Even with external temperature variations of several degrees this system insures a stability of the BGO crystal matrix within at most ±0 .5°C as measured during several days of experimental run by a VME based monitoring system . The equalization of all crystals in the matrix is an essential requirement for the correct particle energy measurement as well as for an efficient use of the granularity related functions. In our case we have performed two different kinds of equalization : a low energy one based on a Am-Be 4.4 MeV -y-ray source and a high energy one based on cosmic ray signals in the crystals . The first one is evidently simpler and faster while the latter works at energies much closer to the real detector energy range. For the equalization of the BGO crystal with cosmic rays we have taken advantage of the symmetry of the 4 ,rr structure to which the crystals belong . Since the BGO crystals in the 4 x 5 matrix all focus to a common center we evaluated by a Monte Carlo code the expected count rates and spectra for a simultaneous double coincidence of each crystal in the vertically standing basket with a unique plastic scintillator placed in the focal position [9]. In practice this focal detector was a 4 cm diameter, 1 cm thick disc . An equalization within approximately ± 2% is sufficient for an inclusive electron scattering experiment . This was achieved with an integral above 100 counts in the cosmic ray Landau peak since the width of this distribution is around 20%. Our count rate estimates were of = 3.4 counts/ h with some 30% less counts in the edge crystals . An acquisition of two days was sufficient to a statistically significant measurement. In practice the crystals were equalized beforehand with an Am-Be source . The equalization process with cosmic rays required then between 2 and 3 iterations and gave a distribution of average values as in fig. 4a . All points are within a ±3% band and the standard deviation of these points is ±2 .7%
M. Anghinolfi et al. / A spectrometer for 1 GeV electrons
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well in range with the required equalization precision . A further check with an Am-Be source in these conditions reproduces the cosmic ray results as seen in fig . 4b . For long term stability control we mounted on each crystal a green (HLMP-1540) LED . All of them were driven by a unique pulse generator and read in coincidence with the pulser gate . The system long term stability was within ±1% . The preshower detector has required a simpler calibration procedure [9] . We produced a uniform surface response by equalizing the six photomultipliers associated to this detector by means of a 6° Co . We also set the high voltages for the best energy resolution . Calibration of this detector was determined collecting the cosmic ray energy spectra in a triple coincidence with two small plastic scintillators for cosmic rays crossing the detector through its shorter (2 .5 cm) or its longer (9 .6 cm) side . Tuning of the plastic scintillator consisted first of all in equalizing the response of the phototubes with the use of a 'Sr source and subsequently in setting appropriate high voltage values to insure at the same time the best timing performance and the compatibility of the anode charge with the linearity range of the ADCs .
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The Cherenkov detector was tested using the high energy c'-e - pairs produced by a 1200 MeV bremsstrahlung gamma beam on a gold radiator [10] . The charge distribution of anode signals had a width corresponding to an average of 10 photoelectrons produced per electron or positron crossing the aerogel . The efficiency of the counter was measured to be higher than 97% in any instance. The data handling [9] has been designed mostly around standard commercial modules . The logic was designed using standard NIM modules and the linear part of the chain used CAMAC based FERA ADCs with DMA reading of a 16 Kbyte stack memory. The anode signal of each crystal has been resistively splitted into three parts corresponding approximately to 90, 1 and 10% of the signal . The 90% part and the 1% part were directly analysed by FERA ADCs to detect respectively low energy (upper limit 12 MeV) and full energy depositions (upper limit 1 GeV) in the individual crystals . The remaining part was linearly added to the contributions of the other crystals to provide a total energy pulse (upper limit 1 .5 GeV) for the basket . A fourfold coincidence between the plastic scintillators, the BGO preshower and the total energy in the BGO calorimeter was the fast trigger for the FERA . Details of this rather conventional electronics can be found elsewhere [9] . 5 . Experimental results Inclusive electron scattering off an oxygen jet target [13] has been measured for beam energies going from 600 MeV to 1200 with telescopes located between 32 and 100° according to the chosen energy . The analysis of data taken at Ee = 1 GeV with the telescope at 60° to the beam direction is well representative of the performance of our apparatus. In fact the energy is high enough to approach the acceptance range of our electronics and test it close to its limits . Besides this, the kinematics and dynamics of the inclusive electron scattering are such to produce at 60° a critical condition where the electron signal has to be extracted from a proton and pion background four times larger . This allows us to demonstrate how the use of the complete information provided by our detectors can give a clean separation of the events of interest . Fig . 5 shows a bidimensional plot of the energy released in the preshower detector and in the BGO calorimeter . Three regions emerge from this plot . The electrons are distributed almost uniformly over the plane . Their electromagnetic shower begins within the preshower counter at a penetration depth which can be anywhere between 0 and 2.5 cm . The energy released in the preshower can be therefore larger than expected from the ionization energy loss alone up to about 200 MeV .
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M. Anghinolfi et al. / A spectrometer for 1 GeV electrons -
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Fig. 5 . The bidimensional plot obtained from the preshower and the BGO calorimeter collected energy . The three main regions corresponding to electrons, pions and protons are discussed in the text .
Fig. 6. The distribution of the signals collected by the Cherenkov detector . The energy scale is m channels and pions and protons are basically concentrated at channel zero. Chan nel 80 has been selected as the discrimination level for electrons.
The energy released by pions in the preshower counter is much more defined and clusters around 15-30 MeV. This is due essentially to the ionisation process since the pions are at the minimum of their specific energy loss and, as pointed before, the probability of a nuclear interaction is rather low. The proton specific energy loss is not minimum but depends on the proton energy . Low energy protons loose in the preshower around 70 MeV and about 30 MeV when their total energy is maximum. Some of these events scatter from the ionisation limit to lower energies, due to proton nuclear interaction in BGO. This effect is masked for pions due to the much narrower range of their energy loss in the preshower. We could treat in the same way the energy deposited in the segmented plastic scintillator and the energy released in the BGO preshower counter to distinguish from their behaviour different types of particles . However this information would not be independent from the previous one and affected by the poorer energy resolution of the plastic scintillator . Consequently the main function of the plastic scintillator in our analysis is the determination of the scattering angle while the other detectors provide particle identification . In fig. 6 is shown the light output of the Cherenkov detector, triggered by the fourfold coincidence of the other detectors. Only electrons give signals above zero while pions and protons are almost entirely below the detection energy threshold. As seen from fig. 6, a cut at channel 80 discriminates very well the electrons with
an efficiency above 97%. This value has been determined by fitting the broad electron peak with a Gaussian function extrapolated to zero. Fig. 7 refers to events that have produced a signal above channel 80 in the Cherenkov counter. Electrons
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Fig. 7. The bidimensional plot obtained from the preshower and the BGO calorimeter collected energy, for all events that have produced a signal above channel 80 in the Cherenkov counter . Three graphical cuts (CUTI, CUT2, CUT3) identify respectively medium and high energy electrons, low energy electrons, and medium energy (T, >_ 400 MeV) pions.
M. Anghmolfi et al . / A spectrometer for 1 GeV electrons
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electrons unadvertedly discarded, we end up with a reconstruction efficiency higher than 97 .5% . The Cherenkov detection efficiency was measured to be not lower than 97% including the experimental errors . This puts to the electron detection efficiency of our telescope a lower limit of 95% ; a more realistic value being expected between 96 and 98% .
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For the study of inclusive electron scattering up to 1 .5 GeV off an oxygen target at the Jet-Target facility in the Frascati National Laboratories of the Istituto Nazionale di Fisica Nucleate, we have developed a scintillation telescope that covers a solid angle of 30 msr and allows the simultaneous acquisition of multiple informations from the constituent detectors . Such a detector has given results well in agreement with numerical simulations . It has proved to be particularly indicated in the kinematical and dynamical conditions of our experiment, where the proton and pion background are larger than the electron signal . Its detection efficiency for electrons is higher than 95% in any instance . The granularity of the main BGO calorimeter can be used for a sharp rejection of unwanted background events not discriminated by our simple aerogel Cherenkov detector .
Acknowledgements populate the entire plane but it is evident that the Cherenkov information is still not sufficient to exclude from this plot energetic pions . Our system of detectors can provide the necessary extra information . Some graphical cuts can be performed as indicated and the granularity computed for each selected area, obtaining the results reported in figs . 8a-8c . When we compare these measured quantities one to another and to the simulations of fig . 3, it is evident that the area defined by CUT3 refers essentially to pions and areas defined by CUT1 and CUT2 refer to high and low energy electrons, respectively . The addition of the granularity gives to this telescope the necessary separation capability when different particles remain confused in our simple Cherenkov counter . Our system has operated with a signal to background level just above 20% . In this severe condition the electron detection efficiency was determined first by the Cherenkov threshold and secondly by the possible presence of electron counts in the granularity distribution of fig . 8c . Comparing figs . 8a and 8c it appears that medium energy electrons produce preferably a granularity above 7 . Even if we assume that all cases with granularity greater than 7 in fig . 8c are
We ackowledge with appreciation the collaboration of the technical staff of both the INFN Genova Section and the INFN Frascati Laboratories .
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M . Anghinolfi et al ., Report LNF 90/026 (1990) . M . Anghinolfi et al ., Report LNF 90/084(R) (1990) . R . Brun et al., Report CERN/DD/EE/84-1 . L. Mazzaschi et al., Nucl . Instr . and Meth. A305 (1991) 391 . M. Bogliardi, Thesis, University of Genova (1991) unpublished . J .W. Lightbody Jr. e t al ., Computer Phys . (1988) 57 . C . Bernini et al., Report INFN/TC/89/13 (1989) . A. Zucchiatti et al ., Nucl. Instr . and Meth . A317 (1992) 492 . G. Coscia, Thesis, University of Genova (1991) unpublished . M . Anghinolfi et al ., Report INFN/TC-92/15 . J . Carlson et al., Nucl. Instr . and Meth . 166 (1979) 425 . A. Zucchiatti et al ., Nucl. Instr . and Meth . A281 (1989) 341 . M . Taiuti et al., Nucl . Instr . and Meth . A297 (1990) 354.