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Response of MEDEA BaF2 detectors to 20–280 MeV photons
Nuclear Instruments and Methods in Physics Research A329 (1993) 173-178 North-Holland
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
Response of MEDEA BaF2 detectors to 20-280 MeV photons a, P. Finocchiaro a, C. Maiolino ~, G. Bellia a,b, R . Alba a, R. Coniglione a, A. Del Zoppo E. Migneco a,b, P. Piattelli a, P. Sapienza a, N. Frascaria e, I. Lhenry e, J .C. Roynette ', T. Suomijärvi `, N. Alamanos d, F. Auger d, A. Gillibert d, D. Pierroutsakou d, J.L. Sida d and P.R. Silveira Gomes d
Iststuto Nazionale di Fisica Nucleare, Laboratorio Nazionale del Sud, Catanea, Italy
b Dipartemento dc Fisica dell'Universuà, Catanta, Italy `Institut de Physique Nucleaire, IN2 P3 -CNRS, Orsay, France d DAPNM, Saclay, France Received 9 December 1992
The response function of MEDEA BaF2 crystals to high energy photons, up to 280 MeV, has been studied using monochromatic y-rays from the in flight annihilation of positron beams. The experimental response functions are compared to the results of Monte Carlo simulations based on the EGS3 code and parametrized over the whole investigated energy range.
1. Introduction Hard photons, as well as sub-threshold pions and ,q's, seem to be very promising probes for the study of the nuclear dynamics in heavy ion collisions and the equation of state of nuclear matter [1]. Therefore the detection of 1'-rays up to energies of several hundreds of MeV has become an experimental goal of great interest . The MEDEA detection system [2] has been developed as a detector for light charged particles and ,y-rays up to some hundreds of MeV. It is a 4,r multielement array constituted by 180 BaF2 crystals, arranged in the shape of a ball covering the polar angles between 30° and 170°, and a forward wall of 120 phoswich detectors, extending from 30° down to 10°. The properties of BaF2 as a scintillating material have been intensively studied in the last years [3-13] due to the very appealing characteristics of its scintillation light, which exhibits two components : the first one centered at a wavelength of - 310 nm and having a decay constant of - 600 ns and the second one centered at a wavelength of - 220 nm and having a decay constant of - 600 ps . The presence of such a fast component gives an excellent time resolution that has been measured to be about 400 ps even for large volume detectors [4]. Moreover the relative amount of the two components is a function of the incident radiaElsevier Science Publishers B.V.
tion (y, p, d, t, a); the fast component is less intense for charged particles than for y-rays and decreases with increasing particle charge and mass . This allows to use a pulse shape analysis to discriminate between -y-rays and light charged particles and among light charged particles [2,8,9] . In this paper we report on a study of the response of MEDEA BaF2 detectors to nearly monochromatic photons of energy up to 280 MeV. The aim of such a study is to obtain a detailed description of the lineshapes in order to perform a reliable unfolding of the experimental -y-ray spectra from heavy ion collision experiments . 2. Experimental procedures 2 .1. Photon beams
Hard photons in the energy range 20-280 MeV were obtained from the in flight annihilation of positron beams delivered by the ALS accelerator facility at Saclay [14]. Two runs were performed at beam energies of Ebeam = 300 MeV and Ebeam = 190 MeV to obtain several calibration points spanning a broad dynamic range. The beam spot was - 5 mm in diameter on the target with an angular spread of 0.5 mrad. The beam energy resolution was DE/E - 0.25 X 10 -2 . The angu-
G . Belha et al. / Response of MEDEA BaF2 detectors
174
lar straggling due to multiple scattering of the e+ beam inside the annihilation target was estimated to be about 0.7 mrad at p = 300 MeV/c. The monochromatic -y-rays were selected by requiring the coincidence between two BaF2 detectors placed on opposite sides of the beam . The angles of the two detectors relative to the beam were adjusted according to the kinematics . The experimental setup, according to geometrical conditions, determined constraints that selected two -y-energy ranges, one at low energy (soft energy) E Y f, = 20-50 MeV and the other at the complementary energy (hard energy) E, ha d = Ebedm - Eysot, . The detector on the soft energy side was placed at about 2 m from the target while the detector on the hard energy side was at about 5 m. The photon beams were collimated by means of a slide system to a spot of - 5 mm diameter on the center of the front surface of the crystal . The detection angles and the corresponding photon energies and uncertainties are listed in table 1 . 2.2. The BaFZ detectors
The BaF2 MEDEA modules have the shape of truncated pyramids grouped in five different shapes named from type A to type E. The first four shapes (from A to D) are 20 cm thick (about 10 radiation lengths), while the type E is 12 cm thick. The crystals are wrapped with three layers of a 50 wm thick Teflon foil and with one layer of 50 wm thick aluminized Mylar, and coupled to the photomultiplier tube (PMT) with a silicon grease transparent to UV radiation. A complete description of the detectors is given in ref. [2]. The full energy and the single escape peaks of the 4.43 MeV y line from an Am-Be radioactive source were used to evaluate the energy resolution of each detector and equalize the gain of the ADCs . Each detector was irradiated in turn on both the soft and the hard side. The PMT bias voltage was kept on during each displacement and the gain was checked at each time with the Am-Be source . For each couple of photon energies determined by the kinematic conditions only coincidence events between the two detectors on the hard and soft sides were recorded .
The four detector types from A to D were tested and their performances are reported . 2 .3. Electronics
The cabling and electronics used were identical to that described in ref. [2] in order to reproduce exactly the experimental arrangement adopted for the MEDEA detectors. The PMT anode signal from each detector was divided into two by a resistive splitter . One of the splitter outputs was sent to a constant fraction discriminator providing a logical signal to be used for timing and triggering purposes . The other splitter output was further splitted into three signals by another resistive splitter, with weights 0.60, 0.35 and 0.05. These outputs were sent to charge to digital converters (QDCs) for the partial and total integration of the signals . The first splitter output, integrated within 20 ns, was used to obtain the fast component information . The other two splitter outputs, integrated within 700 ns, give two different energy ranges (E) and (Ea ), the second one being seven times broader than the first. This structure provides energy measurements over a wide range (E a range), while preserving a good resolution for low energy gammas (E range) . In the present measurements the PMT gains were adjusted to full scales of about 50 MeV and 350 MeV y-ray energy in the E and Ea ranges, respectively . The relation between values measured by each couple of QDCs in the overlap region was checked to be linear within 0.5% . This allows to reduce measured values in the two ranges to a common extended range. 3. Data reduction and analysis In order to reject spurious coincidences, mainly due to positron bremsstrahlung, the events were sorted off-line according to the A-time correlation spectrum, selecting the events within a 2 ns wide time window around the time coincidence peak, 1 .3 ns FWHM wide . Fig. 1 shows some experimental spectra associated to both hard and soft incident photons (full circles) for different crystal types and incident energies . The observed peaks have an asymmetric shape that can be
Table 1 Detection angles and corresponding photon energies and widths . The indices ys and 1'h refer to the parameters of the soft- and hard--y rays respectively. (Energies are in MeV, angles in mrad and momenta in MeV/c) p
300 300 190
EY,
0 EYS
01,
20 44 24 .5
0.38 1.06 0 .44
219.0 141 .0 191.1
EYh
280.9 256.9 166.3
AE yh 2.4 3.3 1 .6
BYh
15 .5 24 .1 28 .1
G. Bellia et al. / Response of MEDEA BaF2 detectors
17 5
reasonably fitted with a Gaussian smoothly joined on both sides to exponential tails. The best fit of the experimental data shows that there is an almost linear dependence of the centroids versus the incident photon energy . This relation holds for all the four crystal shapes . 3.1 . Expected peak shape
0 U
At low photon energy (E_Y <_ 10 MeV), even with finite detector size, it is possible to recognize in the experimental spectra events associated to full deposition of the incident energy in the crystal. A linear dependence of the centroids of the full energy peaks on the incident -y-ray energy is observed due to the small specific energy loss in the crystal of leptons from photoelectric, Compton and pair production interactions; the associated ionization density is so small that the light quenching is negligible . Thus, the scintillation light output in an event, and consequently the associated measurable charge, is proportional to the energy deposited in that event. Fig. 2 shows, as an example, the response of a type B crystal to 4.43 MeV -y-rays (full circles) from an Am-Be source . Neutrons from the source are detected by the crystal and give rise to a background in the spectrum . The histogram is the simulation of the response to 4.43 Mev photons, realized by means of a program based on the code EGS3 ; an almost parabolic line has been added to take into account the neutron background . The two peaks in the
Energy (MeV) Fig. 2. B-type crystal response to the 4.43 MeV -y-rays of the Am-Be source . The histogram is the simulated spectrum . A background has been added to the simulated spectrum to take into account for the neutrons emitted by the Am-Be source .
charge spectrum correspond to full energy and single escape events . For Ey >_ 10 MeV bremsstrahlung of the secondary leptons in the BaF2 crystal is expected to dominate the ionization process and the leakage of the electromagnetic shower out of the crystal may affect the observed spectra. Accordingly the position of the centroids is related to the most probable deposited energy. Several simulations were performed to understand the be-
i-1
E, = 20 MeV B-type
10
20
100
200
30
O U
300
100
200
300
Deposited Energy
100
(MeV)
200
300
Fig. 1. Spectra for both hard and soft photons for the B, C and D-type crystals . Histograms are the simulated data obtained as described in the text . For sake of comparison all the spectra have been normalized to the peak maximum. The energy calibration procedure is described in section 3.4 .
176
G. Bellia et al. / Response of MEDEA BaF2 detectors
haviour of the BaF 2 crystals when they are hit by high energy y-rays, with the aim of determining a relation between the centroids of the experimental spectra and the corresponding deposited energies . 3.2. Computer simulations
The detector response to high energy photons has been simulated by a Monte Carlo calculation based on the EGS3 code . 10 000 y events for each detector shape and each incident energy value were generated to simulate the BaF2 detector -y-ray response . In these calculations the detector geometry and the characteristics of the photon beam, such as energy spread, angular distribution and collimation, and the kinematical coincidence constraints were taken into account. Individual leptons and photons of the electromagnetic shower produced by the incident Y-ray were followed down to 200 keV and 1 keV energy, respectively, lower energies being considered as deposited inside the crystal. Photon statistics and electronic noise, not accounted for by the EGS3 code, were introduced in the calculation by folding the deposited energy spectrum with a Gaussian distribution having FWHM(E) = k(E) t/2 , where the constant k = 0.146 was determined by fitting the energy line shapes measured with radioactive sources. At energies lower than 10 MeV the distributions exhibit structures which correspond to the full energy, single escape and double escape events with widths which are essentially due to the simulation of the photon statistics and electronic noise. At incident energies greater than about 20 MeV the distributions are strongly dependent on the sideward and backward leakage of the -y-shower created by the sec-
ondary processes that take place in the interaction of high energy -y-rays. A certain amount of energy is lost which depends on the -y-ray impact direction and on the interactions inside the detector . The incomplete collection of the total incident energy by the detector gives rise to spectra whose maximum values do not correspond to the incident energy and whose widths reflect the leakage distribution . The contribution of the simulated photon statistics and electronic noise was found negligible . In fig. 1 histograms are the simulated spectra obtained according to the outlined procedure. They are compared with experimental data calibrated assuming that the energy corresponding to the centroid of the experimental charge spectra is the most probable deposited energy . The agreement between the experimental and simulated spectra is satisfactory. The lineshape widths are well reproduced in the hard energy range where the FWHM values are about 15% for the A, B and C shapes and about 25% for the D shape. In the soft energy range the experimental FWHM values are about 20% for all detector shapes, at variance with the calculated values of about 10-15% . 3.3. Line shape analysis
The analysis of the experimental and simulated responses were executed assuming a Gaussian shape with two exponential tails to take into account the incomplete collection of Compton scattered gammas and pair electrons. The fit of the simulated spectra over the whole investigated energy range shows that the high energy tail gives a negligible contribution to the total peak area . The line-shape parameter values
100
simulated too
h)
loo
IOU
dO0
WO
Ioo
sn
Incident energy (Me \r) Incident energy (NIeV) Fig. 3. (a). Uncalibrated centroids of the experimental distributions vs incident photon energies for the four investigated crystal shapes . Values are normalized to the 4.43 MeV y-line of the Am-Be source. Full diamonds refer to A and B shapes; full circles to C shape and full squares to D shape. (b). Simulated most probable deposited energy vs incident photon energy.
G . Bellia et al. / Response of MEDEA BaF2 detectors
show a simple functional dependence on the incident energy. This allows energy interpolation and a reliable reconstruction of the corresponding detector response . 3 .4. Energy calibration
Fig. 3a shows the centroids of the available uncalibrated experimental distributions vs the incident energy values . Data from different shapes are normalized to the 4.43 MeV point to focus the attention on the almost linear dependence between the centroids of the available experimental distributions and the incident energy . Fit errors are smaller than the points . Fig. 3b shows the centroids of the simulated deposited energy distributions vs the incident energy values. We note that the slope related to A, B and C shapes are quite the same while the one related to the D shape reflects the larger y-shower sideward leakage due to the smaller tranverse size of the D type detector. The centroids of the simulated responses show a linear dependence on the incident energy as demonstrated in fig. 3b . Again the fit errors are smaller than the symbols. From the comparison between experimental and simulated data we deduce that possible deviations from linearity, if any, introduced by the scintillation light collection and by the analog electronics associated to the detector (not accounted for by EGS3), are confined within the experimental uncertainty (±5%). Based on this result, a linear relation can be assumed between the most probable measured charge and the calculated most probable deposited energy. Accordingly, the conversion between charge spectra and deposited energy spectra reported in fig. 1 is fully justified. It is worthwhile to remark that even for the large volume BaF2 crystals used here impinged by collimated photon beams, the most probable fraction of the incident energy escaping the detector amounts to about 13% with a non-negligible dispersion as mentioned above. The transfer of the detector response data of this work to future experiments is not trivial . In fact reference photon energies obtainable from radioactive sources are confined to E Y < 6 MeV. The extrapolation of an energy calibration performed accordingly suffers from poor accuracy since a possible loss of linearity in the analogic electronics associated to the detectors cannot be monitored. Therefore it is necessary to have a high energy reference point that may be given by cosmic rations. The arrow in fig. 3a indicates the position of the centroid of the experimental data related to cosmic muons collected with a B shape detector using the same setup. In this measurement the B shape detector was positioned with its axis in the vertical direction and along the axis of another small BaF2 detector posi-
17 7
tioned at a distance of about 15 cm . Only coincidence events were recorded . The spectrum of the deposited energy consists of a narrow bump (FWHM - 20%) whose centroid corresponds to the energy deposited by cosmic muons traversing the full detector thickness. The value of about 6.5 MeV per cm is found, in good agreement with data for the BaF2 crystals [7,13] and with a simulation performed by using the code GEANT3 [15] . A possible calibration procedure is then to use radioactive sources and cosmic muons energy references . 4. Conclusions The experimental response function of MEDEA BaF2 detectors to collimated photons of E Y _< 280 MeV is satisfactorily reproduced by EGS3 calculations simulating the experimental geometry. This result supports the use of the EGS3 code to generate the BaF2 response function for real experimental situation where photons impinge on the whole front surface of the detector . The results shown indicate that even for the large volume BaF2 crystals of the MEDEA detector the detector response plays an important role in determining the incident photon spectra in real experiments . A simple parametrization in terms of a gaussian smoothly joined to two exponential tails describes well the data over the whole range of investigated photon energies and this can be used, in combination with an appropriate unfolding procedure, to restore complex photon spectra. References [1] C. Detraz, C. Esteve, C. Gregoire, D. Guerreau and B. Tamain (eds .), Proc . Third Int. Conf. on Nucleus-Nucleus Collision, Saint Malo, France, June 6-11, 1988, Nucl. Phys . A488 (1988) . [2] E. Migneco, C. Agodi, R. Alba, G. Bellia, R. Coniglione, A. Del Zoppo, P. Finocchiaro, C. Maiolino, P. Piattelli, G. Raia and P. Sapienza, Nucl . Instr. and Meth . A314 (1992) 31 . [3] N. Laval, M. Moszynski, R. Allemand, E. Cormoreche, P. Guinet, R. Odru and J. Vacher, Nucl . Instr. and Meth . 206 (1983) 169. [4] K. Wisshak and F. Käppeler, Nucl . Instr. and Meth. 227 (1984) 91 . [5] F.A . Beck, Instrumentation for Heavy Ion Nuclear Research, Nucl. Sci. Res. Conf . Series Vol. 7, ed . D. Shapira (Harwood, 1985) p. 129. [6] K. Wisshak, F. Käppeler and H. Miiller, Nucl . Instr . and Meth . A251 (1986) 101 . [7] R. Novotny, R. Riess, R. Hingmann, H. Str6her, R.D . Fischer, G. Koch, W. Kiihn, V. Metag, R. Mühlhans, U.
178
[8] [91 [101 [11] [121
G. Bellia et al. / Response of MEDEA BaF2 detectors Kneissl, W. Wilke, B. Haas, J.P. Vivien, R . Beck, B . Schoch and Y . Schutz, Nucl . Instr. and Meth . A254 (1987) 54 . E. Dafm, Nucl . Instr . and Meth. A254 (1987) 54. C. Agodi, R . Alba, G . Bellia, R . Coniglione, A . Del Zoppo, C. Maiolino, E . Migneco, P . Piattelli, P . Sapienza and Yan Chen, Nucl. Instr . and Meth . A269 (1988) 595 . W . Karle, M . Knopp and K.-H. Speidel, Nucl . Instr . and Meth . A271 (1988) 507. T. Matulewicz, E. Grosse, H . Grein, H . Emling, R. Kulessa, F .M . Baumann, G . Domogala and H . Freiesleben, Nucl . Instr . and Meth . A274 (1989) 501 . T. Matulewicz, E . Grosse, H . Emling, R . Freifelder, H .
Grein, W. Henning, N . Herrmann, R . Holzmann, R . Kulessa, R .S. Simon, H .J . Wollersheim, B . Schoch, J . Vogt, M . Wilhelm, J .V . Kratz, R . Schmidt and R .V .F . Janssens, Nucl . Instr . and Meth . A289 (1990) 194 . [13] J . Clayton, W . Benenson, N . Levinsky, J .D. Stevenson, M .F. Mohar, E . Hallin, J .C . Bergstrom, H .S . Caplan, R .E . Pywell, D .M . Skopik and J.M . Vogt, Nucl . Instr. and Meth . A305 (1991) 116 . [14] H . Beil and R . Bergere, Saclay Note CEA-N-2144 (1980) ; and refs. therein . [151 R . Brun, F . Bruyant, M . Maire, A .C . McPherson and P. Zanarini, GEANT3 User's guide, CERN-DD/EE/84-1 (1987).
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH Section A
Response of MEDEA BaF2 detectors to 20-280 MeV photons a, P. Finocchiaro a, C. Maiolino ~, G. Bellia a,b, R . Alba a, R. Coniglione a, A. Del Zoppo E. Migneco a,b, P. Piattelli a, P. Sapienza a, N. Frascaria e, I. Lhenry e, J .C. Roynette ', T. Suomijärvi `, N. Alamanos d, F. Auger d, A. Gillibert d, D. Pierroutsakou d, J.L. Sida d and P.R. Silveira Gomes d
Iststuto Nazionale di Fisica Nucleare, Laboratorio Nazionale del Sud, Catanea, Italy
b Dipartemento dc Fisica dell'Universuà, Catanta, Italy `Institut de Physique Nucleaire, IN2 P3 -CNRS, Orsay, France d DAPNM, Saclay, France Received 9 December 1992
The response function of MEDEA BaF2 crystals to high energy photons, up to 280 MeV, has been studied using monochromatic y-rays from the in flight annihilation of positron beams. The experimental response functions are compared to the results of Monte Carlo simulations based on the EGS3 code and parametrized over the whole investigated energy range.
1. Introduction Hard photons, as well as sub-threshold pions and ,q's, seem to be very promising probes for the study of the nuclear dynamics in heavy ion collisions and the equation of state of nuclear matter [1]. Therefore the detection of 1'-rays up to energies of several hundreds of MeV has become an experimental goal of great interest . The MEDEA detection system [2] has been developed as a detector for light charged particles and ,y-rays up to some hundreds of MeV. It is a 4,r multielement array constituted by 180 BaF2 crystals, arranged in the shape of a ball covering the polar angles between 30° and 170°, and a forward wall of 120 phoswich detectors, extending from 30° down to 10°. The properties of BaF2 as a scintillating material have been intensively studied in the last years [3-13] due to the very appealing characteristics of its scintillation light, which exhibits two components : the first one centered at a wavelength of - 310 nm and having a decay constant of - 600 ns and the second one centered at a wavelength of - 220 nm and having a decay constant of - 600 ps . The presence of such a fast component gives an excellent time resolution that has been measured to be about 400 ps even for large volume detectors [4]. Moreover the relative amount of the two components is a function of the incident radiaElsevier Science Publishers B.V.
tion (y, p, d, t, a); the fast component is less intense for charged particles than for y-rays and decreases with increasing particle charge and mass . This allows to use a pulse shape analysis to discriminate between -y-rays and light charged particles and among light charged particles [2,8,9] . In this paper we report on a study of the response of MEDEA BaF2 detectors to nearly monochromatic photons of energy up to 280 MeV. The aim of such a study is to obtain a detailed description of the lineshapes in order to perform a reliable unfolding of the experimental -y-ray spectra from heavy ion collision experiments . 2. Experimental procedures 2 .1. Photon beams
Hard photons in the energy range 20-280 MeV were obtained from the in flight annihilation of positron beams delivered by the ALS accelerator facility at Saclay [14]. Two runs were performed at beam energies of Ebeam = 300 MeV and Ebeam = 190 MeV to obtain several calibration points spanning a broad dynamic range. The beam spot was - 5 mm in diameter on the target with an angular spread of 0.5 mrad. The beam energy resolution was DE/E - 0.25 X 10 -2 . The angu-
G . Belha et al. / Response of MEDEA BaF2 detectors
174
lar straggling due to multiple scattering of the e+ beam inside the annihilation target was estimated to be about 0.7 mrad at p = 300 MeV/c. The monochromatic -y-rays were selected by requiring the coincidence between two BaF2 detectors placed on opposite sides of the beam . The angles of the two detectors relative to the beam were adjusted according to the kinematics . The experimental setup, according to geometrical conditions, determined constraints that selected two -y-energy ranges, one at low energy (soft energy) E Y f, = 20-50 MeV and the other at the complementary energy (hard energy) E, ha d = Ebedm - Eysot, . The detector on the soft energy side was placed at about 2 m from the target while the detector on the hard energy side was at about 5 m. The photon beams were collimated by means of a slide system to a spot of - 5 mm diameter on the center of the front surface of the crystal . The detection angles and the corresponding photon energies and uncertainties are listed in table 1 . 2.2. The BaFZ detectors
The BaF2 MEDEA modules have the shape of truncated pyramids grouped in five different shapes named from type A to type E. The first four shapes (from A to D) are 20 cm thick (about 10 radiation lengths), while the type E is 12 cm thick. The crystals are wrapped with three layers of a 50 wm thick Teflon foil and with one layer of 50 wm thick aluminized Mylar, and coupled to the photomultiplier tube (PMT) with a silicon grease transparent to UV radiation. A complete description of the detectors is given in ref. [2]. The full energy and the single escape peaks of the 4.43 MeV y line from an Am-Be radioactive source were used to evaluate the energy resolution of each detector and equalize the gain of the ADCs . Each detector was irradiated in turn on both the soft and the hard side. The PMT bias voltage was kept on during each displacement and the gain was checked at each time with the Am-Be source . For each couple of photon energies determined by the kinematic conditions only coincidence events between the two detectors on the hard and soft sides were recorded .
The four detector types from A to D were tested and their performances are reported . 2 .3. Electronics
The cabling and electronics used were identical to that described in ref. [2] in order to reproduce exactly the experimental arrangement adopted for the MEDEA detectors. The PMT anode signal from each detector was divided into two by a resistive splitter . One of the splitter outputs was sent to a constant fraction discriminator providing a logical signal to be used for timing and triggering purposes . The other splitter output was further splitted into three signals by another resistive splitter, with weights 0.60, 0.35 and 0.05. These outputs were sent to charge to digital converters (QDCs) for the partial and total integration of the signals . The first splitter output, integrated within 20 ns, was used to obtain the fast component information . The other two splitter outputs, integrated within 700 ns, give two different energy ranges (E) and (Ea ), the second one being seven times broader than the first. This structure provides energy measurements over a wide range (E a range), while preserving a good resolution for low energy gammas (E range) . In the present measurements the PMT gains were adjusted to full scales of about 50 MeV and 350 MeV y-ray energy in the E and Ea ranges, respectively . The relation between values measured by each couple of QDCs in the overlap region was checked to be linear within 0.5% . This allows to reduce measured values in the two ranges to a common extended range. 3. Data reduction and analysis In order to reject spurious coincidences, mainly due to positron bremsstrahlung, the events were sorted off-line according to the A-time correlation spectrum, selecting the events within a 2 ns wide time window around the time coincidence peak, 1 .3 ns FWHM wide . Fig. 1 shows some experimental spectra associated to both hard and soft incident photons (full circles) for different crystal types and incident energies . The observed peaks have an asymmetric shape that can be
Table 1 Detection angles and corresponding photon energies and widths . The indices ys and 1'h refer to the parameters of the soft- and hard--y rays respectively. (Energies are in MeV, angles in mrad and momenta in MeV/c) p
300 300 190
EY,
0 EYS
01,
20 44 24 .5
0.38 1.06 0 .44
219.0 141 .0 191.1
EYh
280.9 256.9 166.3
AE yh 2.4 3.3 1 .6
BYh
15 .5 24 .1 28 .1
G. Bellia et al. / Response of MEDEA BaF2 detectors
17 5
reasonably fitted with a Gaussian smoothly joined on both sides to exponential tails. The best fit of the experimental data shows that there is an almost linear dependence of the centroids versus the incident photon energy . This relation holds for all the four crystal shapes . 3.1 . Expected peak shape
0 U
At low photon energy (E_Y <_ 10 MeV), even with finite detector size, it is possible to recognize in the experimental spectra events associated to full deposition of the incident energy in the crystal. A linear dependence of the centroids of the full energy peaks on the incident -y-ray energy is observed due to the small specific energy loss in the crystal of leptons from photoelectric, Compton and pair production interactions; the associated ionization density is so small that the light quenching is negligible . Thus, the scintillation light output in an event, and consequently the associated measurable charge, is proportional to the energy deposited in that event. Fig. 2 shows, as an example, the response of a type B crystal to 4.43 MeV -y-rays (full circles) from an Am-Be source . Neutrons from the source are detected by the crystal and give rise to a background in the spectrum . The histogram is the simulation of the response to 4.43 Mev photons, realized by means of a program based on the code EGS3 ; an almost parabolic line has been added to take into account the neutron background . The two peaks in the
Energy (MeV) Fig. 2. B-type crystal response to the 4.43 MeV -y-rays of the Am-Be source . The histogram is the simulated spectrum . A background has been added to the simulated spectrum to take into account for the neutrons emitted by the Am-Be source .
charge spectrum correspond to full energy and single escape events . For Ey >_ 10 MeV bremsstrahlung of the secondary leptons in the BaF2 crystal is expected to dominate the ionization process and the leakage of the electromagnetic shower out of the crystal may affect the observed spectra. Accordingly the position of the centroids is related to the most probable deposited energy. Several simulations were performed to understand the be-
i-1
E, = 20 MeV B-type
10
20
100
200
30
O U
300
100
200
300
Deposited Energy
100
(MeV)
200
300
Fig. 1. Spectra for both hard and soft photons for the B, C and D-type crystals . Histograms are the simulated data obtained as described in the text . For sake of comparison all the spectra have been normalized to the peak maximum. The energy calibration procedure is described in section 3.4 .
176
G. Bellia et al. / Response of MEDEA BaF2 detectors
haviour of the BaF 2 crystals when they are hit by high energy y-rays, with the aim of determining a relation between the centroids of the experimental spectra and the corresponding deposited energies . 3.2. Computer simulations
The detector response to high energy photons has been simulated by a Monte Carlo calculation based on the EGS3 code . 10 000 y events for each detector shape and each incident energy value were generated to simulate the BaF2 detector -y-ray response . In these calculations the detector geometry and the characteristics of the photon beam, such as energy spread, angular distribution and collimation, and the kinematical coincidence constraints were taken into account. Individual leptons and photons of the electromagnetic shower produced by the incident Y-ray were followed down to 200 keV and 1 keV energy, respectively, lower energies being considered as deposited inside the crystal. Photon statistics and electronic noise, not accounted for by the EGS3 code, were introduced in the calculation by folding the deposited energy spectrum with a Gaussian distribution having FWHM(E) = k(E) t/2 , where the constant k = 0.146 was determined by fitting the energy line shapes measured with radioactive sources. At energies lower than 10 MeV the distributions exhibit structures which correspond to the full energy, single escape and double escape events with widths which are essentially due to the simulation of the photon statistics and electronic noise. At incident energies greater than about 20 MeV the distributions are strongly dependent on the sideward and backward leakage of the -y-shower created by the sec-
ondary processes that take place in the interaction of high energy -y-rays. A certain amount of energy is lost which depends on the -y-ray impact direction and on the interactions inside the detector . The incomplete collection of the total incident energy by the detector gives rise to spectra whose maximum values do not correspond to the incident energy and whose widths reflect the leakage distribution . The contribution of the simulated photon statistics and electronic noise was found negligible . In fig. 1 histograms are the simulated spectra obtained according to the outlined procedure. They are compared with experimental data calibrated assuming that the energy corresponding to the centroid of the experimental charge spectra is the most probable deposited energy . The agreement between the experimental and simulated spectra is satisfactory. The lineshape widths are well reproduced in the hard energy range where the FWHM values are about 15% for the A, B and C shapes and about 25% for the D shape. In the soft energy range the experimental FWHM values are about 20% for all detector shapes, at variance with the calculated values of about 10-15% . 3.3. Line shape analysis
The analysis of the experimental and simulated responses were executed assuming a Gaussian shape with two exponential tails to take into account the incomplete collection of Compton scattered gammas and pair electrons. The fit of the simulated spectra over the whole investigated energy range shows that the high energy tail gives a negligible contribution to the total peak area . The line-shape parameter values
100
simulated too
h)
loo
IOU
dO0
WO
Ioo
sn
Incident energy (Me \r) Incident energy (NIeV) Fig. 3. (a). Uncalibrated centroids of the experimental distributions vs incident photon energies for the four investigated crystal shapes . Values are normalized to the 4.43 MeV y-line of the Am-Be source. Full diamonds refer to A and B shapes; full circles to C shape and full squares to D shape. (b). Simulated most probable deposited energy vs incident photon energy.
G . Bellia et al. / Response of MEDEA BaF2 detectors
show a simple functional dependence on the incident energy. This allows energy interpolation and a reliable reconstruction of the corresponding detector response . 3 .4. Energy calibration
Fig. 3a shows the centroids of the available uncalibrated experimental distributions vs the incident energy values . Data from different shapes are normalized to the 4.43 MeV point to focus the attention on the almost linear dependence between the centroids of the available experimental distributions and the incident energy . Fit errors are smaller than the points . Fig. 3b shows the centroids of the simulated deposited energy distributions vs the incident energy values. We note that the slope related to A, B and C shapes are quite the same while the one related to the D shape reflects the larger y-shower sideward leakage due to the smaller tranverse size of the D type detector. The centroids of the simulated responses show a linear dependence on the incident energy as demonstrated in fig. 3b . Again the fit errors are smaller than the symbols. From the comparison between experimental and simulated data we deduce that possible deviations from linearity, if any, introduced by the scintillation light collection and by the analog electronics associated to the detector (not accounted for by EGS3), are confined within the experimental uncertainty (±5%). Based on this result, a linear relation can be assumed between the most probable measured charge and the calculated most probable deposited energy. Accordingly, the conversion between charge spectra and deposited energy spectra reported in fig. 1 is fully justified. It is worthwhile to remark that even for the large volume BaF2 crystals used here impinged by collimated photon beams, the most probable fraction of the incident energy escaping the detector amounts to about 13% with a non-negligible dispersion as mentioned above. The transfer of the detector response data of this work to future experiments is not trivial . In fact reference photon energies obtainable from radioactive sources are confined to E Y < 6 MeV. The extrapolation of an energy calibration performed accordingly suffers from poor accuracy since a possible loss of linearity in the analogic electronics associated to the detectors cannot be monitored. Therefore it is necessary to have a high energy reference point that may be given by cosmic rations. The arrow in fig. 3a indicates the position of the centroid of the experimental data related to cosmic muons collected with a B shape detector using the same setup. In this measurement the B shape detector was positioned with its axis in the vertical direction and along the axis of another small BaF2 detector posi-
17 7
tioned at a distance of about 15 cm . Only coincidence events were recorded . The spectrum of the deposited energy consists of a narrow bump (FWHM - 20%) whose centroid corresponds to the energy deposited by cosmic muons traversing the full detector thickness. The value of about 6.5 MeV per cm is found, in good agreement with data for the BaF2 crystals [7,13] and with a simulation performed by using the code GEANT3 [15] . A possible calibration procedure is then to use radioactive sources and cosmic muons energy references . 4. Conclusions The experimental response function of MEDEA BaF2 detectors to collimated photons of E Y _< 280 MeV is satisfactorily reproduced by EGS3 calculations simulating the experimental geometry. This result supports the use of the EGS3 code to generate the BaF2 response function for real experimental situation where photons impinge on the whole front surface of the detector . The results shown indicate that even for the large volume BaF2 crystals of the MEDEA detector the detector response plays an important role in determining the incident photon spectra in real experiments . A simple parametrization in terms of a gaussian smoothly joined to two exponential tails describes well the data over the whole range of investigated photon energies and this can be used, in combination with an appropriate unfolding procedure, to restore complex photon spectra. References [1] C. Detraz, C. Esteve, C. Gregoire, D. Guerreau and B. Tamain (eds .), Proc . Third Int. Conf. on Nucleus-Nucleus Collision, Saint Malo, France, June 6-11, 1988, Nucl. Phys . A488 (1988) . [2] E. Migneco, C. Agodi, R. Alba, G. Bellia, R. Coniglione, A. Del Zoppo, P. Finocchiaro, C. Maiolino, P. Piattelli, G. Raia and P. Sapienza, Nucl . Instr. and Meth . A314 (1992) 31 . [3] N. Laval, M. Moszynski, R. Allemand, E. Cormoreche, P. Guinet, R. Odru and J. Vacher, Nucl . Instr. and Meth . 206 (1983) 169. [4] K. Wisshak and F. Käppeler, Nucl . Instr. and Meth. 227 (1984) 91 . [5] F.A . Beck, Instrumentation for Heavy Ion Nuclear Research, Nucl. Sci. Res. Conf . Series Vol. 7, ed . D. Shapira (Harwood, 1985) p. 129. [6] K. Wisshak, F. Käppeler and H. Miiller, Nucl . Instr . and Meth . A251 (1986) 101 . [7] R. Novotny, R. Riess, R. Hingmann, H. Str6her, R.D . Fischer, G. Koch, W. Kiihn, V. Metag, R. Mühlhans, U.
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G. Bellia et al. / Response of MEDEA BaF2 detectors Kneissl, W. Wilke, B. Haas, J.P. Vivien, R . Beck, B . Schoch and Y . Schutz, Nucl . Instr. and Meth . A254 (1987) 54 . E. Dafm, Nucl . Instr . and Meth. A254 (1987) 54. C. Agodi, R . Alba, G . Bellia, R . Coniglione, A . Del Zoppo, C. Maiolino, E . Migneco, P . Piattelli, P . Sapienza and Yan Chen, Nucl. Instr . and Meth . A269 (1988) 595 . W . Karle, M . Knopp and K.-H. Speidel, Nucl . Instr . and Meth . A271 (1988) 507. T. Matulewicz, E. Grosse, H . Grein, H . Emling, R. Kulessa, F .M . Baumann, G . Domogala and H . Freiesleben, Nucl . Instr . and Meth . A274 (1989) 501 . T. Matulewicz, E . Grosse, H . Emling, R . Freifelder, H .
Grein, W. Henning, N . Herrmann, R . Holzmann, R . Kulessa, R .S. Simon, H .J . Wollersheim, B . Schoch, J . Vogt, M . Wilhelm, J .V . Kratz, R . Schmidt and R .V .F . Janssens, Nucl . Instr . and Meth . A289 (1990) 194 . [13] J . Clayton, W . Benenson, N . Levinsky, J .D. Stevenson, M .F. Mohar, E . Hallin, J .C . Bergstrom, H .S . Caplan, R .E . Pywell, D .M . Skopik and J.M . Vogt, Nucl . Instr. and Meth . A305 (1991) 116 . [14] H . Beil and R . Bergere, Saclay Note CEA-N-2144 (1980) ; and refs. therein . [151 R . Brun, F . Bruyant, M . Maire, A .C . McPherson and P. Zanarini, GEANT3 User's guide, CERN-DD/EE/84-1 (1987).