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Multinucleon transfer measured with the magnetic spectrometer Prisma
Nuclear Physics A 746 (2004) 195c–199c
Multinucleon transfer measured with the magnetic spectrometer Prisma F. Scarlassaraa b , S. Beghinib, G. Montagnoliab , B.R. Beherac , L. Corradic, E. Fiorettoc, A. Gadeac , A. Latinac, A.M. Stefaninic, S. Szilnerc ∗ , M. Trottac † Y.W. Wuc ‡ a
Universit`a di Padova, Dipartimento di Fisica ”G. Galilei”, via Marzolo 8, Padova, Italy
b
Istituto Nazionale di Fisica Nucleare, Sezione di Padova, Via Marzolo 8, Padova, Italy
c
Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Legnaro, Viale dell’Universit`a 1, Legnaro (Padova) Italy The magnetic spectrometer Prisma for heavy ions, at the Laboratori Nazionali di Legnaro, is a device with large acceptance and good resolution. Installation of the spectrometer and its detectors has been recently completed. Preliminary results of the first measurements on multinucleon transfer reactions are reported. 1. THE SPECTROMETER The magnetic spectrometer Prisma (see figure 1), installed at Legnaro National Laboratory [1], has been designed for detection of heavy ions (A=100-200) produced in reactions at the Tandem-Alpi accelerator and, thanks to its features, is a natural candidate for use with radioactive beams which will be produced in the proposed SPES facility. A Prisma-like device has also been considered by the Eurisol study group for use in a next generation radioactive beam facility [2]. The main features of Prisma are a large acceptance (solid angle ∼ 80 msr, i.e. ±6 o horizontal and ±11o vertical) and momentum acceptance (± 10%) coupled to a mass resolution of 1/300 and an energy resolution up to 1/1000. Prisma can achieve this performance by means of the ”trajectory reconstruction”, a concept that is a hybrid between particle tracking in high energy physics and a classical spectrometer for use in low energy nuclear physics. The optical design is very simple: it consists of a magnetic quadrupole singlet (30 cm diameter, 50 cm effective length) placed at 50 cm from the target, and a magnetic dipole (60o bending angle, 100 cm width, 20 cm gap and 1.2 m curvature radius). The information required for mass reconstruction is: entrance angle, exit position, time of flight and energy. The entrance angle is defined through a start detector placed at 25 cm from the target; it detects the secondary electrons emitted when the ion traverses a thin carbon foil, after amplification by a large area micro-channel plate yielding X and Y position and timing information. ∗
Present addres: Ruder Boskoviˇc Institute, Zagreb, Croatia Present address: Istituto Nazionale di Fisica Nucleare, Sezione di Napoli ‡ Present address: China Institute of Atomic Energy, Beijing, China †
0375-9474/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2004.09.147
196c
F. Scarlassara et al. / Nuclear Physics A 746 (2004) 195c–199c
Figure 1. The Prisma spectrometer in experimental hall I at the XTU Tandem-ALPI accelerator of LNL, here at 90o to the beam direction. The large quadrupole and dipole magnets are clearly seen, on the rotating platform. Presently, the area around the target position is occupied by the gamma-detector array Clara [4]
The position resolution reported for this detector used to be about 1mm in both directions, but a new read-out, based on delay line, has significantly improved the resolution and the response linearity. It is essential to use strip targets (1 mm wide) in order not to spoil the horizontal angle resolution, which is the single most critical parameter. The stop signal is provided by a large area MWPPAC detector (13 × 100 cm 2 ) placed at the focal plane and divided into 10 sections. This detector also yields X and Y exit information with 1 mm resolutions. The time of flight (ToF) has an intrinsic resolution around 300 ps. A large ionization chamber (IC), placed 60 cm downstream of the MWPPAC completes the detector setup. Its anode is segmented transversally (10 sections for a total of 100 cm) to avoid count rate problems, and longitudinally (4 sections, total depth 120 cm) for ∆E measurement. All 40 sections are in the same gas volume. A two dimensional E-∆E spectrum from the ionization chamber is shown in 2 (left) while the right panel of 2 is a X-ToF spectrum taken in the same reaction from a single MWPPAC section. The spectrum, gated with a small region in the entrance position spectrum, shows the single masses (actually mass to charge state ratio, M/Q) as distinct lines: this is the principle upon which mass reconstruction is based. Prisma can rotate around the target point from −20 o to +130o to the beam direction: a very important requirement for the study of two-body reactions like multi nucleon transfer.
F. Scarlassara et al. / Nuclear Physics A 746 (2004) 195c–199c
197c
Figure 2. Two dimensional spectra from the ionization chamber (left, E-∆E) and the MWPPAC (right, X-ToF). The reaction is 56 Fe + 124 Sn with the Fe-like ions detected at 70o to the beam axis.
2. EXPERIMENT AND FIRST RESULTS The tests of Prisma with the complete detector setup have been performed recently, and the design resolutions, both in mass and energy, have been practically demonstrated. 3 shows the energy spectrum obtained in a recent experiment with a 40 Ca beam on 208 Pb corresponding to an energy resolution of 0.3%. This is actually a position spectrum gated on M=40 and Z=20 with the condition of a narrow entrance cone (gate on entrance position). The elastic peak is clearly separated from the inelastic octupole excitations of both projectile and target. One has to keep in mind that the energy resolution of the ”direct” IC measurement is about 2%, mostly a consequence of the energy straggling in the mylar windows of the MWPPAC and IC. There should be room left for improvement, in consideration of the rather broad cuts operated in the entrance angle. The first measurements of multi-nucleon transfer with Prisma were performed recently. The purpose of this research is twofold. On the one hand, it aims at a better understanding of the reaction mechanism, in particular the effects of nucleon correlations (e.g. pair transfer). On the other hand, the experiments aim to investigate the production of neutron rich nuclei. Transfer is one of the few mechanisms allowing to populate nuclei in this little known region, together with fission and deep inelastic. Unfortunately, the production of neutron rich nuclei driven by sequential transfer is in competition with neutron evaporation due to the high residual excitation energy of many of the product nuclei, especially so in view of the dropping neutron binding energy. In the experiment, beams of 56 Fe, 54 Cr and 64 Ni were accelerated onto 124 Sn targets
198c
F. Scarlassara et al. / Nuclear Physics A 746 (2004) 195c–199c
Figure 3. Energy spectrum of for explanation.
40
Ca scattered around 90o from a
208
Pb target. See text
at energies slightly above the Coulomb barrier (7% to 10%). Also, a 36 S beam was accelerated onto a 238 U target. Angular distributions were measured between 65 o and 105o , after which longer runs with good statistics were performed at the grazing angles for a detailed study of the mass and charge distribution of the product nuclei. Products of such reactions may allow to address important issues like the shell closure at N=32-34 for Ca and Ti nuclei [3]. The data analysis is in progress, but the examples reported in 4 ( 56 Fe+124 Sn) show both a satisfactory mass resolution (up to about 1/280, close to the design value of 1/300). It is important to remember that such spectra do not represent the whole statistics; rather, they are obtained by considering one MWPPAC section (out of 10) with a suitable cut in the entrance position spectrum. The cut is a compromise between the need for statistics and the requirements of mass resolution (see also 2 and comments). 3. CONCLUSIONS The magnetic spectrometer Prisma is starting to show its potential as a powerful instrument for nuclear physics, in particular two body reactions. Its characteristics make it an ideal apparatus to couple to a γ-detector array which, at the time of the conference, was already nearing completion. Such a setup named Clara (for clover array [4]) is made of 24 clover detectors from the Euroball array, mounted around the target point of Prisma and rotating with the spectrometer. A measurement campaign is planned during 2004.
F. Scarlassara et al. / Nuclear Physics A 746 (2004) 195c–199c
199c
Figure 4. M/Q spectrum from the reaction 56 Fe+124 Sn. The spectrum is obtained by gating a single MWPPAC section on the entrance detector (narrow solid angle) and on Z=24 (-2p xn) in the E-∆E spectrum. REFERENCES 1. A.M.Stefanini et al., LNL Ann. Rep. 1999, LNL-INFN (Rep) - 160-00, p.159 2. LNL-INFN (REP) 181/02 - June 2002. A.Bracco and A.Pisent Editors. 3. M.Honma et al., Phys. Rev. C65 (2002) 061301 R R.V.F.Janssens et al., Phys. Lett. B510 (2002) 55 4. A.Gadea et al., LNL Ann. Rep. 2002, p. 150; Eur. Jour. Phys. A, in press
Multinucleon transfer measured with the magnetic spectrometer Prisma F. Scarlassaraa b , S. Beghinib, G. Montagnoliab , B.R. Beherac , L. Corradic, E. Fiorettoc, A. Gadeac , A. Latinac, A.M. Stefaninic, S. Szilnerc ∗ , M. Trottac † Y.W. Wuc ‡ a
Universit`a di Padova, Dipartimento di Fisica ”G. Galilei”, via Marzolo 8, Padova, Italy
b
Istituto Nazionale di Fisica Nucleare, Sezione di Padova, Via Marzolo 8, Padova, Italy
c
Istituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Legnaro, Viale dell’Universit`a 1, Legnaro (Padova) Italy The magnetic spectrometer Prisma for heavy ions, at the Laboratori Nazionali di Legnaro, is a device with large acceptance and good resolution. Installation of the spectrometer and its detectors has been recently completed. Preliminary results of the first measurements on multinucleon transfer reactions are reported. 1. THE SPECTROMETER The magnetic spectrometer Prisma (see figure 1), installed at Legnaro National Laboratory [1], has been designed for detection of heavy ions (A=100-200) produced in reactions at the Tandem-Alpi accelerator and, thanks to its features, is a natural candidate for use with radioactive beams which will be produced in the proposed SPES facility. A Prisma-like device has also been considered by the Eurisol study group for use in a next generation radioactive beam facility [2]. The main features of Prisma are a large acceptance (solid angle ∼ 80 msr, i.e. ±6 o horizontal and ±11o vertical) and momentum acceptance (± 10%) coupled to a mass resolution of 1/300 and an energy resolution up to 1/1000. Prisma can achieve this performance by means of the ”trajectory reconstruction”, a concept that is a hybrid between particle tracking in high energy physics and a classical spectrometer for use in low energy nuclear physics. The optical design is very simple: it consists of a magnetic quadrupole singlet (30 cm diameter, 50 cm effective length) placed at 50 cm from the target, and a magnetic dipole (60o bending angle, 100 cm width, 20 cm gap and 1.2 m curvature radius). The information required for mass reconstruction is: entrance angle, exit position, time of flight and energy. The entrance angle is defined through a start detector placed at 25 cm from the target; it detects the secondary electrons emitted when the ion traverses a thin carbon foil, after amplification by a large area micro-channel plate yielding X and Y position and timing information. ∗
Present addres: Ruder Boskoviˇc Institute, Zagreb, Croatia Present address: Istituto Nazionale di Fisica Nucleare, Sezione di Napoli ‡ Present address: China Institute of Atomic Energy, Beijing, China †
0375-9474/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2004.09.147
196c
F. Scarlassara et al. / Nuclear Physics A 746 (2004) 195c–199c
Figure 1. The Prisma spectrometer in experimental hall I at the XTU Tandem-ALPI accelerator of LNL, here at 90o to the beam direction. The large quadrupole and dipole magnets are clearly seen, on the rotating platform. Presently, the area around the target position is occupied by the gamma-detector array Clara [4]
The position resolution reported for this detector used to be about 1mm in both directions, but a new read-out, based on delay line, has significantly improved the resolution and the response linearity. It is essential to use strip targets (1 mm wide) in order not to spoil the horizontal angle resolution, which is the single most critical parameter. The stop signal is provided by a large area MWPPAC detector (13 × 100 cm 2 ) placed at the focal plane and divided into 10 sections. This detector also yields X and Y exit information with 1 mm resolutions. The time of flight (ToF) has an intrinsic resolution around 300 ps. A large ionization chamber (IC), placed 60 cm downstream of the MWPPAC completes the detector setup. Its anode is segmented transversally (10 sections for a total of 100 cm) to avoid count rate problems, and longitudinally (4 sections, total depth 120 cm) for ∆E measurement. All 40 sections are in the same gas volume. A two dimensional E-∆E spectrum from the ionization chamber is shown in 2 (left) while the right panel of 2 is a X-ToF spectrum taken in the same reaction from a single MWPPAC section. The spectrum, gated with a small region in the entrance position spectrum, shows the single masses (actually mass to charge state ratio, M/Q) as distinct lines: this is the principle upon which mass reconstruction is based. Prisma can rotate around the target point from −20 o to +130o to the beam direction: a very important requirement for the study of two-body reactions like multi nucleon transfer.
F. Scarlassara et al. / Nuclear Physics A 746 (2004) 195c–199c
197c
Figure 2. Two dimensional spectra from the ionization chamber (left, E-∆E) and the MWPPAC (right, X-ToF). The reaction is 56 Fe + 124 Sn with the Fe-like ions detected at 70o to the beam axis.
2. EXPERIMENT AND FIRST RESULTS The tests of Prisma with the complete detector setup have been performed recently, and the design resolutions, both in mass and energy, have been practically demonstrated. 3 shows the energy spectrum obtained in a recent experiment with a 40 Ca beam on 208 Pb corresponding to an energy resolution of 0.3%. This is actually a position spectrum gated on M=40 and Z=20 with the condition of a narrow entrance cone (gate on entrance position). The elastic peak is clearly separated from the inelastic octupole excitations of both projectile and target. One has to keep in mind that the energy resolution of the ”direct” IC measurement is about 2%, mostly a consequence of the energy straggling in the mylar windows of the MWPPAC and IC. There should be room left for improvement, in consideration of the rather broad cuts operated in the entrance angle. The first measurements of multi-nucleon transfer with Prisma were performed recently. The purpose of this research is twofold. On the one hand, it aims at a better understanding of the reaction mechanism, in particular the effects of nucleon correlations (e.g. pair transfer). On the other hand, the experiments aim to investigate the production of neutron rich nuclei. Transfer is one of the few mechanisms allowing to populate nuclei in this little known region, together with fission and deep inelastic. Unfortunately, the production of neutron rich nuclei driven by sequential transfer is in competition with neutron evaporation due to the high residual excitation energy of many of the product nuclei, especially so in view of the dropping neutron binding energy. In the experiment, beams of 56 Fe, 54 Cr and 64 Ni were accelerated onto 124 Sn targets
198c
F. Scarlassara et al. / Nuclear Physics A 746 (2004) 195c–199c
Figure 3. Energy spectrum of for explanation.
40
Ca scattered around 90o from a
208
Pb target. See text
at energies slightly above the Coulomb barrier (7% to 10%). Also, a 36 S beam was accelerated onto a 238 U target. Angular distributions were measured between 65 o and 105o , after which longer runs with good statistics were performed at the grazing angles for a detailed study of the mass and charge distribution of the product nuclei. Products of such reactions may allow to address important issues like the shell closure at N=32-34 for Ca and Ti nuclei [3]. The data analysis is in progress, but the examples reported in 4 ( 56 Fe+124 Sn) show both a satisfactory mass resolution (up to about 1/280, close to the design value of 1/300). It is important to remember that such spectra do not represent the whole statistics; rather, they are obtained by considering one MWPPAC section (out of 10) with a suitable cut in the entrance position spectrum. The cut is a compromise between the need for statistics and the requirements of mass resolution (see also 2 and comments). 3. CONCLUSIONS The magnetic spectrometer Prisma is starting to show its potential as a powerful instrument for nuclear physics, in particular two body reactions. Its characteristics make it an ideal apparatus to couple to a γ-detector array which, at the time of the conference, was already nearing completion. Such a setup named Clara (for clover array [4]) is made of 24 clover detectors from the Euroball array, mounted around the target point of Prisma and rotating with the spectrometer. A measurement campaign is planned during 2004.
F. Scarlassara et al. / Nuclear Physics A 746 (2004) 195c–199c
199c
Figure 4. M/Q spectrum from the reaction 56 Fe+124 Sn. The spectrum is obtained by gating a single MWPPAC section on the entrance detector (narrow solid angle) and on Z=24 (-2p xn) in the E-∆E spectrum. REFERENCES 1. A.M.Stefanini et al., LNL Ann. Rep. 1999, LNL-INFN (Rep) - 160-00, p.159 2. LNL-INFN (REP) 181/02 - June 2002. A.Bracco and A.Pisent Editors. 3. M.Honma et al., Phys. Rev. C65 (2002) 061301 R R.V.F.Janssens et al., Phys. Lett. B510 (2002) 55 4. A.Gadea et al., LNL Ann. Rep. 2002, p. 150; Eur. Jour. Phys. A, in press