- Email: [email protected]
MAYA, a gaseous active target
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 573 (2007) 145–148 www.elsevier.com/locate/nima
MAYA, a gaseous active target C.E. Demonchya,1, W. Mittiga, H. Savajolsa, P. Roussel-Chomaza, M. Chartierb, B. Juradoa, L. Giota,2, D. Cortina-Gilc, M. Caaman˜oc,, G. Ter-Arkopiand, A. Fomichevd, A. Rodind, M.S. Golovkovd, S. Stepantsovd, A. Gilliberte, E. Pollaccoe, A. Obertellie, H. Wanga,3 a GANIL, Bvd Henri Becquerel BP 55027, 14076 Caen Ce´dex 5, France Physics Department, University of Liverpool, Oxford Street, L69 7ZE, UK c Universidade de Santiago de Compostela, 15706 Santiago de Compostela, Spain d FLNR/JINR, P.O. Box 79 101 000, Dubna, Moscow, Russia e C.E.A./D.S.M./D.A.P.N.I.A./S. Ph N. Saclay, 91191 Gif-sur-Yvette Ce´dex, France b
Available online 27 November 2006
Abstract The recent improvements concerning the production of exotic beams in facilities such as SPIRAL at GANIL allow us to explore new areas of the nuclear chart, away from the line of stability. However, the intensities for these beams are still relatively low (few thousands of particles per second) and the cross-sections of the most of the reactions of interest are also low (of the order of milibarns, or lower). In order to increase the reaction rates with a thick target, without loss of resolution, we developed the MAYA detector, based on the active target concept. MAYA is essentially a Time-charge Projection chamber where the gas plays also the role of the target. The segmented cathode allows particle identification, three-dimensional tracking and complete reconstruction of kinematics of binary reactions with beam particles impinging on the gas inside the detector. The parameters of the chamber, such as gas type and pressure, electric field and amplification regime, are modifiable depending on the characteristics of the reaction of interest, allowing a low-energy detection threshold, and an optimum thickness. r 2006 Elsevier B.V. All rights reserved. PACS: 29.40.Cs; 29.40.Gx; 25.60. t Keywords: Ionization chamber; Active target; Tracking detector; Binary reaction; Exotic beam
1. Introduction With the recent improvements in production and on-line acceleration of radioactive nuclei, in facilities as Louvainla-Neuve, Ganil-Spiral, ORNL, Triumf-Isac and RexIsolde, exotic beams are available for experiments at energies typically in the 0.1–10 MeV/nucleon domain. The availability of these beams opens new areas of the nuclear chart, away from the stability line for experiments using Corresponding author. Tel.: +34 981 563100; fax: +34 981 520676.
E-mail addresses: [email protected], [email protected] (M. Caaman˜o). 1 Present address: CENBG Chemin du Solarium, Le Haut-Vigneau, BP 120, F-33175 Gradignan Ce´dex, Bordeaux, France. 2 Present address: GSI, D-64291 Darmstadt, Germany. 3 Present address: SINAP, Shanghai, PRC. 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.11.025
resonant reactions, direct transfer, or pick-up reactions. However the intensities are still low, as they are also the typical cross-sections of the reactions of interest. Solutions to this problem may involve increasing the thickness of the target thereby getting higher reaction rates but with less resolution and/or increasing the detection efficiency. The concept of active target includes these solutions, and overcomes the problem of resolution. Active targets, such as bubble chambers, were developed many years ago in high energy physics. They use part of the detection system as reaction target. In the domain of secondary beams, the archetype is the detector IKAR [1,2], developed for GSI energies. The use of IKAR was limited to H2 at a pressure of 10 atm. Another example, MSTPC [3,4], uses a flash ADC readout of wedge signals.
ARTICLE IN PRESS 146
C.E. Demonchy et al. / Nuclear Instruments and Methods in Physics Research A 573 (2007) 145–148
We developed MAYA [5,6] as an active target for the domain of low energies and for the use of various gases as different types of target. In order to cover the most of the interesting direct reaction dynamics, MAYA is meant to have a 4p solid angle of detection and a big effective target thickness without loss of energy resolution. It is ideally suited for the study of binary reactions induced by secondary beams from Spiral at Ganil in the energy domain of 2–25 MeV/nucleon. In the next section the design of MAYA is described in detail. The general data analysis procedure is presented in Section 3. 2. Description of the detector MAYA is essentially a Time-Charge Projection chamber where the gas inside plays the role of the target. In Fig. 1 we can see a general description of its layout. The active zone of the detector is a volume of 28 26 20 cm3 filled with gas. The stainless-steel container of the detector, with the 1 cm diameter Mylar window at the entrance, was tested for gas pressures up to 3 atm. The type of gas (H2, D2, C4H10, etc.) is chosen depending on the reactions of interest. The gas is surrounded by a cathode plane on the upper side of the volume and the zone of amplification and detection on the lower side, separated by a Frisch grid. Below the Frisch grid, at a modifiable distance, it is the plane of the anode proportional wires. These wires are parallel to the beam, therefore their diameter can be different in the region of the beam to adjust for the different ionization densities of beam and recoil particles.
The spacing of the wires is chosen to ensure that the charge collected is not affected by interaction between neighbor wires. The plane of anode wires is above the lower cathode. This is segmented into 35 35 hexagonal pads with 5 mm sides. The size of the pads is chosen on the basis of a good resolution for the calculation of ranges of about 10 cm to an accuracy of better than 1%, with a reasonable number of electronic channels being read. The hexagonal structure was chosen in order to have good conditions for the reconstruction of trajectories independent of their directions. The acceleration of the electrons in the vicinity of the wires induces a signal in the pads below. The distance between the wires and the pads determines the size of the induction pattern. The pads are arranged in rows parallel to the rows of wires, with each set of wires corresponding to the same line of pads connected to the same preamplifier. Finally, the plane of pads is connected to a set of Gassiplex chips. These are 16 channel analogue multiplexed ASICs developed at CERN. The induced signals in the pads are recorded and kept in memory by the Gassiplex chips through a track-and-hold procedure, which starts with a signal provided by the wires, until they are sent to the data acquisition. The electrons from atoms ionized in the gas drift to the zone of amplification in an electric field applied between the upper cathode and the proportional wires. The Frisch grid is kept grounded. The homogeneity of the electric field is maintained using metallic field strips that cover the sides of the detector, except for the back side where they are replaced by field wires because of those particles escaping in forward direction. The applied voltage, depending mainly on the gas pressure and the detection energy threshold required, can be set up to 15 kV in the upper cathode, and 5 kV in the proportional wires. Inverse kinematics generates recoil particles with a large energy. High-energy light particles, such as protons, cannot be stopped in a reasonable gas volume and pressure. For such escaping particles there is a 20 cm 25 cm wall made by twenty 5 cm 5 cm Cesium–Iodide (CsI) crystal detectors of 1 cm depth. In addition Silicon (Si) detectors of 5 cm 5 cm and 500 mm of thickness may be placed, for dE-E particle identification purposes. Further possible modifications, depending on the experiment, include beam shielding, a beam stopper in front of the CsI wall, diamond detectors for monitoring the incoming beam or displacement of the beam axis. 3. Data analysis
Fig. 1. General scheme of the detector MAYA. The active zone is surrounded by the cathode on the upper side, and the zone of amplification and detection on the lower side. This zone includes the Frisch grid, the amplification anode wires and the segmented cathode with the Gassiplex electronics below. The homogeneity of the applied electric field is assured with the strips along the sides. At the back side there is a wall made with twenty Cesium–Iodide (CsI) detectors.
3.1. Event signals from MAYA The detector MAYA gives information for a complete reconstruction of the kinematics of those reactions that take place inside the filling gas. The information comes mainly from the tracking of the particles involved through
ARTICLE IN PRESS C.E. Demonchy et al. / Nuclear Instruments and Methods in Physics Research A 573 (2007) 145–148
the projection of the charge in the segmented cathode and the dE-E identification in the Si–CsI detectors. In a typical event the beam particles enter MAYA through the Mylar window, usually after passing through a drift chamber or other detectors for monitoring purposes. The beam particle then hits an atom in the gas and makes a reaction. If it is a binary reaction recoil and scattered particles appear with trajectories defining a plane. The gas is ionized by the charged particles involved in the reaction. The electrons drift from the trajectories of the particles to the proportional wires, guided by the electric field applied in the active volume. They reach the space between the Frisch grid and the proportional wires, where they are accelerated by the field gradient between the grid and the wires. The accelerated electrons ionize the gas around the wires, creating positive ions which drift towards the cathode pads just below. The image charge induced is collected and recorded individually for each pad. The charge collected in the wires is also recorded giving a timing signal for the measurement of the drift time relative to another signal typically from the drift chamber monitoring the beam, as well as timing for the track-and-hold of the pads (for the Gassiplex chips). Depending on the kinematics of the reaction and the settings of MAYA, the recoil and scattered particles can either be stopped inside the gas or escape towards the Si–CsI wall where they are stopped and their energy is measured. 3.2. Translation from signals into data The mapping of the charge collected in each pad gives a pattern of the particle trajectories projected onto the cathode plane. Fig. 2 shows a typical event in the charge matrix of the pads. The analysis of the matrices usually
147
starts with searching for maximal charges. The exact position of these maxima is determined by also using the charges in the surrounding pads. These positions are then used in order to get the trajectories of the particles. The crossing point of the trajectories is identified as the position of the vertex of reaction. From the fitted trajectories the projected angles are calculated. The particles that stop inside the gas leave a charge deposition profile from which the projected range is extracted. The ranges and angles calculated from the matrix of the pads should be referred to the angle of the reaction plane in order to get the real values instead of projected ones. The determination of the plane angle is made through the drift times measured upto each wire collecting charge, translated into distance using the drift velocity. The limitation of MAYA to detect and identify binary reactions is mainly due to this procedure. Since the signals in the wires are not sensitive to the position along the longitudinal axis or to the time profile of the electron cascade there is no possibility to distinguish between two different trajectories crossing the same wire. The identification of the particles stopping inside MAYA may be done with range–charge relation: RpQ2/(MZ2), where the range, R, is proportional to the square of the charge deposited, Q, divided by the mass M and the square of the nuclear charge, Z. The particles leaving the gas volume are eventually stopped in the Si–CsI detectors at the end of MAYA. Depending on the detector, CsI, or Si, the identification of the particles may be done in different ways. In the case of a Si detector, a dE-E procedure is usually performed, taken as dE the energy loss inside the gas, or a signal collected in some wires of the Frisch grid. In the case of a CsI detector the signal released has two components; one of them is sensitive to the mass and charge of the ionizing particle. The identification of the particles stopping inside the gas, and those which are stopped in the Si–CsI wall, with their energies, and the calculation of the trajectories and their angles, allow us to a complete characterization of the reaction. 3.3. Limitations of MAYA
Fig. 2. A typical event as seen from the projection in the segmented cathode. Each number represents the charge collected in 32 32 pads. The last two columns are the charge, and drift time for each row of pads and wires. A 3.9 MeV/A 8He beam coming from the left hits a proton of the gas inside MAYA, which is stopped in the CsI wall. The recoil He stops inside the gas.
Even if MAYA is meant to cover 4p solid angle of detection, there are some general considerations to remark. The calculation of the angle of the reaction plane needs, in the worst case, at least two groups of wires to be fired in order to perform the fit of the drift times. This requirement results in a limitation of the angle of the reaction plane, which depends on the distance between the groups of wires and the angles and ranges of the particles. Depending on the reaction type, this is around a 5% of events lost. However this limitation in the azimuthal angle does not represent any loss of kinematical information, and it still allows MAYA to be a extremely valuable tool for measuring low energy recoils covering a kinematical region not accessible with standard solid targets.
ARTICLE IN PRESS 148
C.E. Demonchy et al. / Nuclear Instruments and Methods in Physics Research A 573 (2007) 145–148
References [1] A.A. Vorobyov, et al., Nucl. Instr. and Meth. A 119 (1974) 509. [2] A.A. Vorobyov, et al., Nucl. Instr. and Meth. A 270 (1988) 419. [3] Y. Mizoi, et al., Nucl. Instr. and Meth. A 431 (1999) 112.
[4] Y. Mizoi, et al., Phys. Rev. C 62 (2000) 065801. [5] P. Gagnant, et al., Report Ganil 27.2002. [6] C.E. Demonchy, Thesis T 03 06, University of Caen, France, December 2003.
Nuclear Instruments and Methods in Physics Research A 573 (2007) 145–148 www.elsevier.com/locate/nima
MAYA, a gaseous active target C.E. Demonchya,1, W. Mittiga, H. Savajolsa, P. Roussel-Chomaza, M. Chartierb, B. Juradoa, L. Giota,2, D. Cortina-Gilc, M. Caaman˜oc,, G. Ter-Arkopiand, A. Fomichevd, A. Rodind, M.S. Golovkovd, S. Stepantsovd, A. Gilliberte, E. Pollaccoe, A. Obertellie, H. Wanga,3 a GANIL, Bvd Henri Becquerel BP 55027, 14076 Caen Ce´dex 5, France Physics Department, University of Liverpool, Oxford Street, L69 7ZE, UK c Universidade de Santiago de Compostela, 15706 Santiago de Compostela, Spain d FLNR/JINR, P.O. Box 79 101 000, Dubna, Moscow, Russia e C.E.A./D.S.M./D.A.P.N.I.A./S. Ph N. Saclay, 91191 Gif-sur-Yvette Ce´dex, France b
Available online 27 November 2006
Abstract The recent improvements concerning the production of exotic beams in facilities such as SPIRAL at GANIL allow us to explore new areas of the nuclear chart, away from the line of stability. However, the intensities for these beams are still relatively low (few thousands of particles per second) and the cross-sections of the most of the reactions of interest are also low (of the order of milibarns, or lower). In order to increase the reaction rates with a thick target, without loss of resolution, we developed the MAYA detector, based on the active target concept. MAYA is essentially a Time-charge Projection chamber where the gas plays also the role of the target. The segmented cathode allows particle identification, three-dimensional tracking and complete reconstruction of kinematics of binary reactions with beam particles impinging on the gas inside the detector. The parameters of the chamber, such as gas type and pressure, electric field and amplification regime, are modifiable depending on the characteristics of the reaction of interest, allowing a low-energy detection threshold, and an optimum thickness. r 2006 Elsevier B.V. All rights reserved. PACS: 29.40.Cs; 29.40.Gx; 25.60. t Keywords: Ionization chamber; Active target; Tracking detector; Binary reaction; Exotic beam
1. Introduction With the recent improvements in production and on-line acceleration of radioactive nuclei, in facilities as Louvainla-Neuve, Ganil-Spiral, ORNL, Triumf-Isac and RexIsolde, exotic beams are available for experiments at energies typically in the 0.1–10 MeV/nucleon domain. The availability of these beams opens new areas of the nuclear chart, away from the stability line for experiments using Corresponding author. Tel.: +34 981 563100; fax: +34 981 520676.
E-mail addresses: [email protected], [email protected] (M. Caaman˜o). 1 Present address: CENBG Chemin du Solarium, Le Haut-Vigneau, BP 120, F-33175 Gradignan Ce´dex, Bordeaux, France. 2 Present address: GSI, D-64291 Darmstadt, Germany. 3 Present address: SINAP, Shanghai, PRC. 0168-9002/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2006.11.025
resonant reactions, direct transfer, or pick-up reactions. However the intensities are still low, as they are also the typical cross-sections of the reactions of interest. Solutions to this problem may involve increasing the thickness of the target thereby getting higher reaction rates but with less resolution and/or increasing the detection efficiency. The concept of active target includes these solutions, and overcomes the problem of resolution. Active targets, such as bubble chambers, were developed many years ago in high energy physics. They use part of the detection system as reaction target. In the domain of secondary beams, the archetype is the detector IKAR [1,2], developed for GSI energies. The use of IKAR was limited to H2 at a pressure of 10 atm. Another example, MSTPC [3,4], uses a flash ADC readout of wedge signals.
ARTICLE IN PRESS 146
C.E. Demonchy et al. / Nuclear Instruments and Methods in Physics Research A 573 (2007) 145–148
We developed MAYA [5,6] as an active target for the domain of low energies and for the use of various gases as different types of target. In order to cover the most of the interesting direct reaction dynamics, MAYA is meant to have a 4p solid angle of detection and a big effective target thickness without loss of energy resolution. It is ideally suited for the study of binary reactions induced by secondary beams from Spiral at Ganil in the energy domain of 2–25 MeV/nucleon. In the next section the design of MAYA is described in detail. The general data analysis procedure is presented in Section 3. 2. Description of the detector MAYA is essentially a Time-Charge Projection chamber where the gas inside plays the role of the target. In Fig. 1 we can see a general description of its layout. The active zone of the detector is a volume of 28 26 20 cm3 filled with gas. The stainless-steel container of the detector, with the 1 cm diameter Mylar window at the entrance, was tested for gas pressures up to 3 atm. The type of gas (H2, D2, C4H10, etc.) is chosen depending on the reactions of interest. The gas is surrounded by a cathode plane on the upper side of the volume and the zone of amplification and detection on the lower side, separated by a Frisch grid. Below the Frisch grid, at a modifiable distance, it is the plane of the anode proportional wires. These wires are parallel to the beam, therefore their diameter can be different in the region of the beam to adjust for the different ionization densities of beam and recoil particles.
The spacing of the wires is chosen to ensure that the charge collected is not affected by interaction between neighbor wires. The plane of anode wires is above the lower cathode. This is segmented into 35 35 hexagonal pads with 5 mm sides. The size of the pads is chosen on the basis of a good resolution for the calculation of ranges of about 10 cm to an accuracy of better than 1%, with a reasonable number of electronic channels being read. The hexagonal structure was chosen in order to have good conditions for the reconstruction of trajectories independent of their directions. The acceleration of the electrons in the vicinity of the wires induces a signal in the pads below. The distance between the wires and the pads determines the size of the induction pattern. The pads are arranged in rows parallel to the rows of wires, with each set of wires corresponding to the same line of pads connected to the same preamplifier. Finally, the plane of pads is connected to a set of Gassiplex chips. These are 16 channel analogue multiplexed ASICs developed at CERN. The induced signals in the pads are recorded and kept in memory by the Gassiplex chips through a track-and-hold procedure, which starts with a signal provided by the wires, until they are sent to the data acquisition. The electrons from atoms ionized in the gas drift to the zone of amplification in an electric field applied between the upper cathode and the proportional wires. The Frisch grid is kept grounded. The homogeneity of the electric field is maintained using metallic field strips that cover the sides of the detector, except for the back side where they are replaced by field wires because of those particles escaping in forward direction. The applied voltage, depending mainly on the gas pressure and the detection energy threshold required, can be set up to 15 kV in the upper cathode, and 5 kV in the proportional wires. Inverse kinematics generates recoil particles with a large energy. High-energy light particles, such as protons, cannot be stopped in a reasonable gas volume and pressure. For such escaping particles there is a 20 cm 25 cm wall made by twenty 5 cm 5 cm Cesium–Iodide (CsI) crystal detectors of 1 cm depth. In addition Silicon (Si) detectors of 5 cm 5 cm and 500 mm of thickness may be placed, for dE-E particle identification purposes. Further possible modifications, depending on the experiment, include beam shielding, a beam stopper in front of the CsI wall, diamond detectors for monitoring the incoming beam or displacement of the beam axis. 3. Data analysis
Fig. 1. General scheme of the detector MAYA. The active zone is surrounded by the cathode on the upper side, and the zone of amplification and detection on the lower side. This zone includes the Frisch grid, the amplification anode wires and the segmented cathode with the Gassiplex electronics below. The homogeneity of the applied electric field is assured with the strips along the sides. At the back side there is a wall made with twenty Cesium–Iodide (CsI) detectors.
3.1. Event signals from MAYA The detector MAYA gives information for a complete reconstruction of the kinematics of those reactions that take place inside the filling gas. The information comes mainly from the tracking of the particles involved through
ARTICLE IN PRESS C.E. Demonchy et al. / Nuclear Instruments and Methods in Physics Research A 573 (2007) 145–148
the projection of the charge in the segmented cathode and the dE-E identification in the Si–CsI detectors. In a typical event the beam particles enter MAYA through the Mylar window, usually after passing through a drift chamber or other detectors for monitoring purposes. The beam particle then hits an atom in the gas and makes a reaction. If it is a binary reaction recoil and scattered particles appear with trajectories defining a plane. The gas is ionized by the charged particles involved in the reaction. The electrons drift from the trajectories of the particles to the proportional wires, guided by the electric field applied in the active volume. They reach the space between the Frisch grid and the proportional wires, where they are accelerated by the field gradient between the grid and the wires. The accelerated electrons ionize the gas around the wires, creating positive ions which drift towards the cathode pads just below. The image charge induced is collected and recorded individually for each pad. The charge collected in the wires is also recorded giving a timing signal for the measurement of the drift time relative to another signal typically from the drift chamber monitoring the beam, as well as timing for the track-and-hold of the pads (for the Gassiplex chips). Depending on the kinematics of the reaction and the settings of MAYA, the recoil and scattered particles can either be stopped inside the gas or escape towards the Si–CsI wall where they are stopped and their energy is measured. 3.2. Translation from signals into data The mapping of the charge collected in each pad gives a pattern of the particle trajectories projected onto the cathode plane. Fig. 2 shows a typical event in the charge matrix of the pads. The analysis of the matrices usually
147
starts with searching for maximal charges. The exact position of these maxima is determined by also using the charges in the surrounding pads. These positions are then used in order to get the trajectories of the particles. The crossing point of the trajectories is identified as the position of the vertex of reaction. From the fitted trajectories the projected angles are calculated. The particles that stop inside the gas leave a charge deposition profile from which the projected range is extracted. The ranges and angles calculated from the matrix of the pads should be referred to the angle of the reaction plane in order to get the real values instead of projected ones. The determination of the plane angle is made through the drift times measured upto each wire collecting charge, translated into distance using the drift velocity. The limitation of MAYA to detect and identify binary reactions is mainly due to this procedure. Since the signals in the wires are not sensitive to the position along the longitudinal axis or to the time profile of the electron cascade there is no possibility to distinguish between two different trajectories crossing the same wire. The identification of the particles stopping inside MAYA may be done with range–charge relation: RpQ2/(MZ2), where the range, R, is proportional to the square of the charge deposited, Q, divided by the mass M and the square of the nuclear charge, Z. The particles leaving the gas volume are eventually stopped in the Si–CsI detectors at the end of MAYA. Depending on the detector, CsI, or Si, the identification of the particles may be done in different ways. In the case of a Si detector, a dE-E procedure is usually performed, taken as dE the energy loss inside the gas, or a signal collected in some wires of the Frisch grid. In the case of a CsI detector the signal released has two components; one of them is sensitive to the mass and charge of the ionizing particle. The identification of the particles stopping inside the gas, and those which are stopped in the Si–CsI wall, with their energies, and the calculation of the trajectories and their angles, allow us to a complete characterization of the reaction. 3.3. Limitations of MAYA
Fig. 2. A typical event as seen from the projection in the segmented cathode. Each number represents the charge collected in 32 32 pads. The last two columns are the charge, and drift time for each row of pads and wires. A 3.9 MeV/A 8He beam coming from the left hits a proton of the gas inside MAYA, which is stopped in the CsI wall. The recoil He stops inside the gas.
Even if MAYA is meant to cover 4p solid angle of detection, there are some general considerations to remark. The calculation of the angle of the reaction plane needs, in the worst case, at least two groups of wires to be fired in order to perform the fit of the drift times. This requirement results in a limitation of the angle of the reaction plane, which depends on the distance between the groups of wires and the angles and ranges of the particles. Depending on the reaction type, this is around a 5% of events lost. However this limitation in the azimuthal angle does not represent any loss of kinematical information, and it still allows MAYA to be a extremely valuable tool for measuring low energy recoils covering a kinematical region not accessible with standard solid targets.
ARTICLE IN PRESS 148
C.E. Demonchy et al. / Nuclear Instruments and Methods in Physics Research A 573 (2007) 145–148
References [1] A.A. Vorobyov, et al., Nucl. Instr. and Meth. A 119 (1974) 509. [2] A.A. Vorobyov, et al., Nucl. Instr. and Meth. A 270 (1988) 419. [3] Y. Mizoi, et al., Nucl. Instr. and Meth. A 431 (1999) 112.
[4] Y. Mizoi, et al., Phys. Rev. C 62 (2000) 065801. [5] P. Gagnant, et al., Report Ganil 27.2002. [6] C.E. Demonchy, Thesis T 03 06, University of Caen, France, December 2003.