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Developments in spectrometers for radioactive ion beams
Nuclear Physics A 701 (2002) 172c–178c www.elsevier.com/locate/npe
Developments in spectrometers for radioactive ion beams G. de France GANIL, BP 5027, F-14076 Caen cedex 5, France
Abstract Severe limitations might reduce the potentiality expected from the use of radioactive ion beams if R&D efforts are not done. The design of the future spectrometers intended to be used with these beams integrates new difficulties arising from the beam characteristics. In this talk the performances expected from this new generation of detectors and the new ideas in large efficiency and large acceptance spectrometers will be briefly described. 2002 Elsevier Science B.V. All rights reserved.
1. Introduction
The physics motivations at the origin of the developments around the production of exotic nuclei in many laboratories all around the world are very ambitious. Simultaneously and in order to fully explore the future possibilities, the efforts in the production of new species must be accompanied by substantial developments in detection techniques. Indeed, as it will be shown, the constraints from the expected physics are severe and R&D efforts are mandatory. The performances of the existing recoil or γ spectrometers are far from being compatible with these constraints in terms of efficiency, modularity and quality. In this contribution, the design specifications arising from the expected physics with Radioactive Ion Beams (RIBs) will be summarized for both recoil and γ spectrometers and the general trends in developments in both cases will be described. As typical examples, the VAMOS recoil spectrometer [1] and the EXOGAM γ -ray array [2] will be described.
E-mail address: [email protected] (G. de France). 0375-9474/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 9 4 7 4 ( 0 1 ) 0 1 5 6 8 - 8
G. de France / Nuclear Physics A 701 (2002) 172c–178c
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2. Development in recoil spectrometers The physics case assigned to the next generation of recoil spectrometers to be used with RIBs is the study of the reaction mechanisms at, or close to the Coulomb barrier energy. To get some design specifications out from the physics case, we can divide the wide range of reaction mechanisms into three distinct categories which lead to different constraints: • Elastic and inelastic scattering, transfer reaction in inverse kinematics. These mechanisms will be used to study the interaction potentials, e.g., or the structure of halo nuclei (i.e., the distribution of matter in nuclei). The ejectiles must be detected to reduce the background and to obtain a good energy and angular resolutions. This already indicates that the device will have to cope with several other detectors. Because of the inverse kinematics, a beam rejection capability must be designed and the clear identification of the mass and charge of the recoils is necessary. There is in this case no particular need for a very large acceptance since the recoils are strongly peaked around 0◦ . • Deep inelastic scattering. These reactions will be used to produce efficiently new nuclei. Their specificity is the large angular distribution of the recoiling nuclei (up to 90◦ ) which imposes severe conditions on the design. As in the case of fusion– evaporation reactions, the cross sections are expected to be larger when using RIBs as compared to stable beams. Because of this it will be possible to study in details the N/Z equilibration process. To match these experimental conditions, an appropriate device must therefore have a good mass and charge identification. Apart from the fact that the recoiling nuclei must be detected up to 90◦ with respect to the beam direction, their recoiling energy range is about 1–5 MeV per nucleon. • Fusion–evaporation reaction. This is the favorite mechanism to populate N = Z nuclei or neutron rich isotopes starting from an exotic beam at low energy, especially with beams from fission induced reactions. The energy of the compound recoiling nuclei are even lower that in the previous case and mass and charge must be measured to identify the recoils. Again, the beam must be rejected since the product of the fusion is forward peaked and with quite a large distribution in some cases. To summarize, it appears that a very efficient ‘tagger’ is more appropriate than a standard spectrometer. It is also very clear that such a device has to work in coincidence with many other auxiliary detectors such as a γ -ray spectrometer, a charged particle array, an electron spectrometer or all of them together. Thus the main characteristics is a very large acceptance (efficiency), the ideal device being an event-by-event high-resolution 4π tagger. Several possibilities have been studied by the several working groups in the VAMOS community, for example. The first one is a gas-filled dipole with a magnetic field of up to 10 T and the target inside to get the 4π detection. In this case, the reaction products are recoiling in the magnetic field and generate vortices which have radii proportional to the magnetic rigidity of the products. Recoil coincidences are feasible with Time Projection Chambers (TPCs). However, the number of vortices generated would have required a very sophisticated software for the offline analysis to disentangle the trajectories. It would
174c
G. de France / Nuclear Physics A 701 (2002) 172c–178c
have been very difficult to obtain a clean trigger and this is vital, in particular, when running in coincidence with other devices. Finally, TPCs are not very well suited for the heaviest elements due to additional straggling in the entrance window. Therefore this technical solution has been rapidly rejected for VAMOS. The second solution which has been examined was a superconducting solenoid associated to a large gap magnetic sector field. This idea was already developed for SUSAN, a UK proposal which never came into reality. The main advantage of such a solution is the very large solid angle obtained thanks to the superconducting solenoid, and the good performances in mass and charge resolution (A/A ∼ 0.5%; Z/Z ∼ 3.3%) expected from stable beams. But the huge field from the solenoid degrades seriously the energy resolution of the photomultiplier tubes and the position resolution of the start detector. Furthermore the calculated performances with stable beams could not be maintained with RIBs. Therefore, it appears that the more suitable solution for VAMOS corresponding to the technical specifications mentioned above was a ‘standard’ association of quadrupoles and dipole with the addition of a velocity filter (see Fig. 1). However to get a very large solid angle (about one order of magnitude larger than the existing devices like the Argonne FMA and others, i.e., close to 100 msr for VAMOS) strong fields are necessary. The resolution will be achieved if, and only if, a powerful trajectory reconstruction can be done and a lot of work has been initiated in this field. This will be possible also because the counting rates with RIBs is not too large (∼ 106 pps). VAMOS has another specificity which is its flexibility. VAMOS stands for VAriable MOde
Fig. 1. Sketch of the VAMOS spectrometer.
G. de France / Nuclear Physics A 701 (2002) 172c–178c
175c
Table 1 The various VAMOS working modes Mode
Quadrupoles
Dipole
Wien filter
Type
1 2 3
On On On
Off On On
Off Off On
SOLENO type large acceptance facility SUSAN type dispersive facility Beam rejection and/or A/Q selection
Spectrometer and indeed, there are essentially three distinct working modes for VAMOS which are summarized in Table 1.
3. Development in γ -ray spectrometers The physics case related to the use of γ -ray detectors implies also severe constraints on the design specifications of the arrays. One of the major goal is to reach a large photopeak efficiency. In addition, there are specific properties of γ -rays which must be considered into some details: multiplicity, energy, recoil velocity of the emitting nuclei are some of these. On the side of the beam, the limitations come essentially from the low (or very low) intensity and the radioactive nature of the beam. Which means that we demand not only a very large photopeak efficiency but also a very good signal-to-noise ratio. In addition to the reaction mechanisms listed above, we should mention the case of the Coulomb excitation of exotic nuclei which will be used extensively to get some knowledge on the structure of a nucleus. In many cases we will use an inverse kinematics which implies an important Doppler broadening of the full energy peak. Convoluted to the possibly important background this reinforces the requirement of the very good energy resolution. Dealing with unknown species, we also need to detect the emitting isotopes in coincidence with the γ -rays. Therefore in most cases the coupling with auxiliary detectors such as mass spectrometers, light charged particle arrays, neutron detectors, etc., will be vital. This is a huge task for the designers to make mechanically and electronically compatible several arrays in an efficient way. EXOGAM is one array among others (like MINIBALL [3], e.g.) which are designed in such a way. It is built up from 16 large segmented clover detectors [4,5] each being surrounded with an anti-Compton shield having a very special design as will be shown later on. The clover detectors are built from 4 distinct crystals put together in a very compact way in a common can. The photopeak efficiency of a clover is characterized by the individual efficiency of the four crystals and also by the addback factor (Fab). If the total efficiency of the whole clover measured as being one detector is ph (clover) and if the individual efficiency of the crystals are ph (i), i = 1–4, then Fab = ph (clover)
4
ph (i).
i=1
This ratio is larger than 1 because a Compton scattering between two crystals, giving one background count in each of the crystals, may give one photopeak event when added. In
176c
G. de France / Nuclear Physics A 701 (2002) 172c–178c
the case of the clover Fab ∼ 1.5. This quantity is an indirect measure of the quality of the assembly of the crystals since dead material between crystals would degrade it severely. The array is designed to reach a total photopeak efficiency of 20% for a 1.3 MeV γ -ray energy and multiplicity 1. The main developments in the field of γ -ray detection systems are in Ge detectors, BGO shield design, and development in electronics. For the Ge detectors, and since up to now no alternative materials compete with Ge, the main ongoing developments are related to the elaboration of larger hyper-pure Ge crystals (more than 14 cm long and 9 cm diameter) and to the electric segmentation of the detectors. The suppression shield for EXOGAM is very modular and can accomodate several configurations. There are three distinct parts: the back cather, the side catcher and the side shield (see Fig. 3).
Fig. 2. The EXOGAM spectrometer with 16 segmented clovers.
Fig. 3. The EXOGAM Compton suppression shield.
G. de France / Nuclear Physics A 701 (2002) 172c–178c
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When assembled together, the back and side catchers cover the rear part of the Ge detectors. It is also possible to add the 3rd part so that a shield surrounds completely a clover as it is usually done with Compton suppressed Ge detectors. The clovers are designed in such a way that in a compact assembly of 16 detectors (see Fig. 2) the front part of the clovers are touching each other over 3 cm. Looking from the target, the γ -rays ‘see’ only Ge except 8 empty triangles. This configuration privileges the efficiency because it optimizes the solid angle coverage with Ge and limits dead material and BGO shield. In addition, it is possible to run in ‘inter-clover addback’. This consists in adding back the energies deposited in neighbouring clovers. The gain in efficiency is of the order of 10%. This is the configuration which must be used when the multiplicity is low (typically less than 10). Above this, the probability that two distinct γ -rays interact in a single clover is too high, and in a traditional analysis, this leads to a loss in efficiency and signal-to-noise ratio. In such conditions, it is possible to pull back individually each detectors independently of all the other ones at any distance between about 6 and 20 cm from the target. This limits the loss in efficiency due to pile-up. At medium spin this represents a gain in efficiency even though there is a loss in solid angle and in the fact that there is no inter-clover addback. Finally, the most promising development for future arrays is the progress in the γ -ray tracking [6] thanks to the today possible 3D electric segmentation of the Ge detectors. It consists basically in following the track of a γ -ray in the Ge, on the basis of the segments which have fired, from the 1st interaction point up to its complete absorption (or loss). The final goal is to eliminate completely the anti-Compton shields and associated material (collimators, etc.) to end up with a full segmented Ge ball around the target (or implantation). However, this ‘ideal’ γ -ray spectrometer will be possible only if the efficiency of the trajectory reconstruction inside the Ge is large enough at low as well as to high spin. This task is very difficult and ambitious from several aspects: reliable 3D segmentation, retracing software, number of electronics channels (several thousands), etc. EXOGAM and MINIBALL are the first steps towards this ultimate Ge array since (1) they will use the preamplifier pulse shape analysis technique associated to flash ADCs and DSPs to determine the initial interaction point of the γ inside the Ge detectors and (2) in the compact geometry, EXOGAM is already a Ge shell.
4. Conclusion The next generation of spectrometers will be operational very soon from now and this is an exciting period for experimental physicists. In both examples (recoil and γ spectrometers), the long term solution is clearly based on a powerful and precise trajectory reconstruction. The task is ambitious but really promising. I thank W. Mittig, R. Anne and H. Savajols for their help in preparing the VAMOS part of this talk.
References [1] H. Savajols et al., Nucl. Phys. A 654 (1999) 1027c. [2] G. de France, Acta Phys. Polon. B 30 (5) (1999) 1661.
178c
[3] [4] [5] [6]
G. de France / Nuclear Physics A 701 (2002) 172c–178c
D. Habs et al., Part. Nucl. Phys. 38 (1997) 1. G. Duchêne et al., Nucl. Instrum. Methods A 432 (1999) 90. S.L. Shepherd et al., Nucl. Instrum. Methods A 434 (1999) 373. M.A. Deleplanque et al., Nucl. Instrum. Methods A 430 (1999) 292.
Developments in spectrometers for radioactive ion beams G. de France GANIL, BP 5027, F-14076 Caen cedex 5, France
Abstract Severe limitations might reduce the potentiality expected from the use of radioactive ion beams if R&D efforts are not done. The design of the future spectrometers intended to be used with these beams integrates new difficulties arising from the beam characteristics. In this talk the performances expected from this new generation of detectors and the new ideas in large efficiency and large acceptance spectrometers will be briefly described. 2002 Elsevier Science B.V. All rights reserved.
1. Introduction
The physics motivations at the origin of the developments around the production of exotic nuclei in many laboratories all around the world are very ambitious. Simultaneously and in order to fully explore the future possibilities, the efforts in the production of new species must be accompanied by substantial developments in detection techniques. Indeed, as it will be shown, the constraints from the expected physics are severe and R&D efforts are mandatory. The performances of the existing recoil or γ spectrometers are far from being compatible with these constraints in terms of efficiency, modularity and quality. In this contribution, the design specifications arising from the expected physics with Radioactive Ion Beams (RIBs) will be summarized for both recoil and γ spectrometers and the general trends in developments in both cases will be described. As typical examples, the VAMOS recoil spectrometer [1] and the EXOGAM γ -ray array [2] will be described.
E-mail address: [email protected] (G. de France). 0375-9474/02/$ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 5 - 9 4 7 4 ( 0 1 ) 0 1 5 6 8 - 8
G. de France / Nuclear Physics A 701 (2002) 172c–178c
173c
2. Development in recoil spectrometers The physics case assigned to the next generation of recoil spectrometers to be used with RIBs is the study of the reaction mechanisms at, or close to the Coulomb barrier energy. To get some design specifications out from the physics case, we can divide the wide range of reaction mechanisms into three distinct categories which lead to different constraints: • Elastic and inelastic scattering, transfer reaction in inverse kinematics. These mechanisms will be used to study the interaction potentials, e.g., or the structure of halo nuclei (i.e., the distribution of matter in nuclei). The ejectiles must be detected to reduce the background and to obtain a good energy and angular resolutions. This already indicates that the device will have to cope with several other detectors. Because of the inverse kinematics, a beam rejection capability must be designed and the clear identification of the mass and charge of the recoils is necessary. There is in this case no particular need for a very large acceptance since the recoils are strongly peaked around 0◦ . • Deep inelastic scattering. These reactions will be used to produce efficiently new nuclei. Their specificity is the large angular distribution of the recoiling nuclei (up to 90◦ ) which imposes severe conditions on the design. As in the case of fusion– evaporation reactions, the cross sections are expected to be larger when using RIBs as compared to stable beams. Because of this it will be possible to study in details the N/Z equilibration process. To match these experimental conditions, an appropriate device must therefore have a good mass and charge identification. Apart from the fact that the recoiling nuclei must be detected up to 90◦ with respect to the beam direction, their recoiling energy range is about 1–5 MeV per nucleon. • Fusion–evaporation reaction. This is the favorite mechanism to populate N = Z nuclei or neutron rich isotopes starting from an exotic beam at low energy, especially with beams from fission induced reactions. The energy of the compound recoiling nuclei are even lower that in the previous case and mass and charge must be measured to identify the recoils. Again, the beam must be rejected since the product of the fusion is forward peaked and with quite a large distribution in some cases. To summarize, it appears that a very efficient ‘tagger’ is more appropriate than a standard spectrometer. It is also very clear that such a device has to work in coincidence with many other auxiliary detectors such as a γ -ray spectrometer, a charged particle array, an electron spectrometer or all of them together. Thus the main characteristics is a very large acceptance (efficiency), the ideal device being an event-by-event high-resolution 4π tagger. Several possibilities have been studied by the several working groups in the VAMOS community, for example. The first one is a gas-filled dipole with a magnetic field of up to 10 T and the target inside to get the 4π detection. In this case, the reaction products are recoiling in the magnetic field and generate vortices which have radii proportional to the magnetic rigidity of the products. Recoil coincidences are feasible with Time Projection Chambers (TPCs). However, the number of vortices generated would have required a very sophisticated software for the offline analysis to disentangle the trajectories. It would
174c
G. de France / Nuclear Physics A 701 (2002) 172c–178c
have been very difficult to obtain a clean trigger and this is vital, in particular, when running in coincidence with other devices. Finally, TPCs are not very well suited for the heaviest elements due to additional straggling in the entrance window. Therefore this technical solution has been rapidly rejected for VAMOS. The second solution which has been examined was a superconducting solenoid associated to a large gap magnetic sector field. This idea was already developed for SUSAN, a UK proposal which never came into reality. The main advantage of such a solution is the very large solid angle obtained thanks to the superconducting solenoid, and the good performances in mass and charge resolution (A/A ∼ 0.5%; Z/Z ∼ 3.3%) expected from stable beams. But the huge field from the solenoid degrades seriously the energy resolution of the photomultiplier tubes and the position resolution of the start detector. Furthermore the calculated performances with stable beams could not be maintained with RIBs. Therefore, it appears that the more suitable solution for VAMOS corresponding to the technical specifications mentioned above was a ‘standard’ association of quadrupoles and dipole with the addition of a velocity filter (see Fig. 1). However to get a very large solid angle (about one order of magnitude larger than the existing devices like the Argonne FMA and others, i.e., close to 100 msr for VAMOS) strong fields are necessary. The resolution will be achieved if, and only if, a powerful trajectory reconstruction can be done and a lot of work has been initiated in this field. This will be possible also because the counting rates with RIBs is not too large (∼ 106 pps). VAMOS has another specificity which is its flexibility. VAMOS stands for VAriable MOde
Fig. 1. Sketch of the VAMOS spectrometer.
G. de France / Nuclear Physics A 701 (2002) 172c–178c
175c
Table 1 The various VAMOS working modes Mode
Quadrupoles
Dipole
Wien filter
Type
1 2 3
On On On
Off On On
Off Off On
SOLENO type large acceptance facility SUSAN type dispersive facility Beam rejection and/or A/Q selection
Spectrometer and indeed, there are essentially three distinct working modes for VAMOS which are summarized in Table 1.
3. Development in γ -ray spectrometers The physics case related to the use of γ -ray detectors implies also severe constraints on the design specifications of the arrays. One of the major goal is to reach a large photopeak efficiency. In addition, there are specific properties of γ -rays which must be considered into some details: multiplicity, energy, recoil velocity of the emitting nuclei are some of these. On the side of the beam, the limitations come essentially from the low (or very low) intensity and the radioactive nature of the beam. Which means that we demand not only a very large photopeak efficiency but also a very good signal-to-noise ratio. In addition to the reaction mechanisms listed above, we should mention the case of the Coulomb excitation of exotic nuclei which will be used extensively to get some knowledge on the structure of a nucleus. In many cases we will use an inverse kinematics which implies an important Doppler broadening of the full energy peak. Convoluted to the possibly important background this reinforces the requirement of the very good energy resolution. Dealing with unknown species, we also need to detect the emitting isotopes in coincidence with the γ -rays. Therefore in most cases the coupling with auxiliary detectors such as mass spectrometers, light charged particle arrays, neutron detectors, etc., will be vital. This is a huge task for the designers to make mechanically and electronically compatible several arrays in an efficient way. EXOGAM is one array among others (like MINIBALL [3], e.g.) which are designed in such a way. It is built up from 16 large segmented clover detectors [4,5] each being surrounded with an anti-Compton shield having a very special design as will be shown later on. The clover detectors are built from 4 distinct crystals put together in a very compact way in a common can. The photopeak efficiency of a clover is characterized by the individual efficiency of the four crystals and also by the addback factor (Fab). If the total efficiency of the whole clover measured as being one detector is ph (clover) and if the individual efficiency of the crystals are ph (i), i = 1–4, then Fab = ph (clover)
4
ph (i).
i=1
This ratio is larger than 1 because a Compton scattering between two crystals, giving one background count in each of the crystals, may give one photopeak event when added. In
176c
G. de France / Nuclear Physics A 701 (2002) 172c–178c
the case of the clover Fab ∼ 1.5. This quantity is an indirect measure of the quality of the assembly of the crystals since dead material between crystals would degrade it severely. The array is designed to reach a total photopeak efficiency of 20% for a 1.3 MeV γ -ray energy and multiplicity 1. The main developments in the field of γ -ray detection systems are in Ge detectors, BGO shield design, and development in electronics. For the Ge detectors, and since up to now no alternative materials compete with Ge, the main ongoing developments are related to the elaboration of larger hyper-pure Ge crystals (more than 14 cm long and 9 cm diameter) and to the electric segmentation of the detectors. The suppression shield for EXOGAM is very modular and can accomodate several configurations. There are three distinct parts: the back cather, the side catcher and the side shield (see Fig. 3).
Fig. 2. The EXOGAM spectrometer with 16 segmented clovers.
Fig. 3. The EXOGAM Compton suppression shield.
G. de France / Nuclear Physics A 701 (2002) 172c–178c
177c
When assembled together, the back and side catchers cover the rear part of the Ge detectors. It is also possible to add the 3rd part so that a shield surrounds completely a clover as it is usually done with Compton suppressed Ge detectors. The clovers are designed in such a way that in a compact assembly of 16 detectors (see Fig. 2) the front part of the clovers are touching each other over 3 cm. Looking from the target, the γ -rays ‘see’ only Ge except 8 empty triangles. This configuration privileges the efficiency because it optimizes the solid angle coverage with Ge and limits dead material and BGO shield. In addition, it is possible to run in ‘inter-clover addback’. This consists in adding back the energies deposited in neighbouring clovers. The gain in efficiency is of the order of 10%. This is the configuration which must be used when the multiplicity is low (typically less than 10). Above this, the probability that two distinct γ -rays interact in a single clover is too high, and in a traditional analysis, this leads to a loss in efficiency and signal-to-noise ratio. In such conditions, it is possible to pull back individually each detectors independently of all the other ones at any distance between about 6 and 20 cm from the target. This limits the loss in efficiency due to pile-up. At medium spin this represents a gain in efficiency even though there is a loss in solid angle and in the fact that there is no inter-clover addback. Finally, the most promising development for future arrays is the progress in the γ -ray tracking [6] thanks to the today possible 3D electric segmentation of the Ge detectors. It consists basically in following the track of a γ -ray in the Ge, on the basis of the segments which have fired, from the 1st interaction point up to its complete absorption (or loss). The final goal is to eliminate completely the anti-Compton shields and associated material (collimators, etc.) to end up with a full segmented Ge ball around the target (or implantation). However, this ‘ideal’ γ -ray spectrometer will be possible only if the efficiency of the trajectory reconstruction inside the Ge is large enough at low as well as to high spin. This task is very difficult and ambitious from several aspects: reliable 3D segmentation, retracing software, number of electronics channels (several thousands), etc. EXOGAM and MINIBALL are the first steps towards this ultimate Ge array since (1) they will use the preamplifier pulse shape analysis technique associated to flash ADCs and DSPs to determine the initial interaction point of the γ inside the Ge detectors and (2) in the compact geometry, EXOGAM is already a Ge shell.
4. Conclusion The next generation of spectrometers will be operational very soon from now and this is an exciting period for experimental physicists. In both examples (recoil and γ spectrometers), the long term solution is clearly based on a powerful and precise trajectory reconstruction. The task is ambitious but really promising. I thank W. Mittig, R. Anne and H. Savajols for their help in preparing the VAMOS part of this talk.
References [1] H. Savajols et al., Nucl. Phys. A 654 (1999) 1027c. [2] G. de France, Acta Phys. Polon. B 30 (5) (1999) 1661.
178c
[3] [4] [5] [6]
G. de France / Nuclear Physics A 701 (2002) 172c–178c
D. Habs et al., Part. Nucl. Phys. 38 (1997) 1. G. Duchêne et al., Nucl. Instrum. Methods A 432 (1999) 90. S.L. Shepherd et al., Nucl. Instrum. Methods A 434 (1999) 373. M.A. Deleplanque et al., Nucl. Instrum. Methods A 430 (1999) 292.