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Production of high-intensity RIB at SPES
Nuclear Physics A 834 (2010) 754c–757c www.elsevier.com/locate/nuclphysa
Production of high-intensity RIB at SPES A. Andrighettoa , L. Biasettoa , M. Manzolaroa , D. Scarpaa , J. Montanoa , J. Stanescua , P. Benettib , I. Cristofolinic , M.S. Carturana , P. Colombod , P. di Bernardoe , M. Guerzonif , G. Meneghettid , B. Monellic , G. Pretea , G. Puglierina , A. Tomasellib , P. Zanonatoe . a
INFN Laboratori di Legnaro, Viale dell’Universit`a 2, 35020 Legnaro (Pd), Italy.
b
Dipartimento di Chimica Generale and INFN, Via Bassi 6, 27100 Pavia, Italy.
c
Dipartimento di Ingegneria Meccanica, Via Mesiano 77, 38050 Trento, Italy.
d e f
Dipartimento di Ingegneria Meccanica, Via Venezia 1, 35131 Padova, Italy.
Dipartimento di Scienze Chimiche, Via Marzolo 1, 35131 Padova, Italy.
INFN Sezione di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy.
The SPES Project at INFN Laboratori Nazionali di Legnaro [1] is now in its early construction phase. SPES is an ISOL type Radioactive Ion Beam (RIB) [2] facility for the production of neutron-rich radioactive nuclei by uranium fission. RIBs will be produced by proton induced fission on a UCx multi foil direct target at a rate of 1013 fps, more than one order of magnitude larger than the currently available beam intensities. The recent developments on the production, ionization and acceleration of RIBs at SPES are hereafter presented. 1. THE SPES PROJECT AT LNL In the ISOL type SPES facility, a cyclotron proton driver provides beams, up to E= 70 MeV and total current up to 750μ A, to the production target where the nuclear reactions take place. The exotic species are ionized nearby at 60 keV to +1 charge state. Successively, the RIBs are extracted from the target-ion source system, mass separated by a small Wien filter and transported to the first focal plain. The front end and the Wien filter are placed on a 250 kV platform. After the first separation stage, out of the irradiation bunker, a high resolving power (1:20000) spectrometer performs the isobar separation. To optimize the successive re-acceleration of the exotic ions, the beam is fed to a charge breeder (CB) housed on another 250 kV platform. The outgoing multi charge beam, further selected to remove the contaminants coming from CB itself, is injected in the PIAVE superconductive RFQ, which represent the first re-acceleration stage. The final acceleration step will take place in the ALPI superconducting linear accelerator, where, the final RIBs, with energy up to 11 MeV/A, are sent to experimental set-ups for nuclear physics studies. 0375-9474/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2010.01.137
A. Andrighetto et al. / Nuclear Physics A 834 (2010) 754c–757c
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Figure 1. The in-target isotope yields.
2. THE TARGET ISOTOPE PRODUCTION In an ISOL facility, like SPES, the working core is constituted of the production target and the ion source [3]: they have to be carefully designed and optimized in order to obtain the desired high production rate of RIBs. Due to the low pressure of the environment, the power deposited by the proton beam in the target by means of electromagnetic and nuclear interactions can be removed only by thermal radiation towards the surrounding container box. In order to optimize both heat dissipation and fission fragments evaporation, the SPES target consists of seven, 40 mm diameter and 1.3 mm thick, co-axial disks housed in a cylindrical graphite box [4]. In choosing the target material, which should stand the highest possible beam current, it is crucial to optimize the beam-target combination with respect to the highest production cross section and lowest amount of contaminants. UCx , uranium dicarbide dispersed in an excess of graphite, is widely recognized as the reference material for the production of neutron-rich radioactive beams [5]. The preparation of the SPES UCx disks is based on the carbon-thermal reduction of UO2 powders in excess of graphite. The powders are mixed and ground in order to obtain a homogeneous mixture; these powders are uniaxially cold pressed. Finally the heat treatment is performed in a dedicated vacuum furnace. The final bulk density of the disks turns out to be about 3 g/cm3 , while the U:C atomic ratio is close to 1:4. The calculated in target fission rate in all the 7 disks approaches 1013 fps. [4]. The distribution of the fission products for the mass numbers 80 < A < 160 is shown in Figure 1. The isotope in-target production for some interesting isotopes (Ag, Sn, Cs) reaches values up to 1011 aps. All fission fragments produced are indicated in red color in the Figure 2. The 132 Sn isotope, being a double-magic nucleus, is one of the radioactive nuclei of interest. Its in target production yield is here estimated to be 1010 aps.
756c
A. Andrighetto et al. / Nuclear Physics A 834 (2010) 754c–757c
Figure 2. The main isotopes that will be ionized and extracted in the SPES project.
3. THE IONIZATON PROCESS The desired exotic species must be extracted from the target, ionized and accelerated to make a RIB [6]. This process is time demanding and usually unsuitable for atoms having half lives shorter than a few tens of ms. The hot cavity ion source chosen for the SPES project was designed at ISOLDE [7]. The source has the basic structure of the standard high temperature RIB ion sources employed for on-line operation. The isotopes produced in the target diffuse in the target material and after that effuse through the transfer tube into the ionizer cavity where they undergo surface, laser or plasma ionization. The ionizer cavity is a W tube resistively heated to near 2200◦ C. For alkalis and some of the rare earth elements, high ionization efficiencies can be easily achieved at this temperature by the surface ionization technique. For most part of the others elements, as schematized in Figure 2, the laser resonant photo-ionization, using the same hot cavity cell, is the powerful method to achieve a sufficiently selective exotic beams. The aim is to produce a beam as pure as possible (chemical selectivity) also for these metal isotopes. As first step for the R&D of the photo-ionization process for SPES, dye lasers will be used to generate resonant light source. In order to investigate the proper ionization path for the element of interest, a Hollow Cathode Lamp (HCL) is current using as atomizer. This device is a good and simple atoms reservoir for preliminary studies. A dedicated plasma ion source (FEBIAD type), which will be used to produce halogen and noble gas elements, is also under development. 4. THE OFF-LINE TEST In order to test the target off-line, a full scale prototype has been developed at LNL. In this prototype SiC disks replace the UCx ones. A dedicated electrical-thermal Finite Element Model of the target-ion source complex, able to reproduce the electrical and thermal behavior of the target and its heating tantalum housing, was written with the finite code ANSYS; the model, thoroughly validated by temperature and potential difference measurements, will be used to optimize these devices in a virtual environment. At the same time, the off-line testing of the target front-end is also in progress. The front end,
A. Andrighetto et al. / Nuclear Physics A 834 (2010) 754c–757c
757c
Figure 3. The SPES front-end.
see Figure 3, is developed on the ISOLDE design. In the SPES front-end, stable atoms are introduced, by means of a mass marker capillarity technique, close to the ion source device. They diffuse toward the ionizer where they are charged to the +1 state and finally accelerated up to 60 kV through the extraction electrode. After this step, the position of beam centroid in the transverse plane is corrected by four electrical steerers. In the final stage on the front-end, a triplet of electrostatic quadruples is responsible for the focusing of the beam to a desired downstream point. It is expected that the SPES surface ion source will produce a beam with a transverse emittance of about 6π*mm*mrad and a beam spot, in the focal plane, below 10 mm in diameter. 5. CONCLUSIONS The SPES project is one of the main nuclear physics developments in Italy for the next years. SPES is an up to date project in the field of nuclear physics and in particular in the field of RIBs; the project, once realized, will represent an important step in the direction of the constitution of the European-project-facility EURISOL. Before starting with the hardware installation, the R&D program will continue and mainly focus on the target and on the ion source development and optimization. REFERENCES 1. SPES Technichal Design Report - www.lnl.infn.it/ spes/TDR2008/. 2. J.Khalili, E. Roeckl, The Euroschool Lect. on Phys. with Exotic Beams, Vol. II, Springer (2006). 3. A. Andrighetto, S. Cevolani, and C. Petrovich, Eur. Phys. J., A25 (2005) 41-47 4. A. Andrighetto, C.M. Antonucci, S. Cevolani, C. Petrovich and M. Santana Leitner, Eur. Phys. J., A30 (2006)591 5. A. Andrighetto, C.M. Antonucci, et al., Eur. Phys. Journal. S.T. ; 150 (2007)273 6. A. Andrighetto, L. Biasetto, M. Manzolaro et al AIP 1099 (2009)728 7. J. Lettry, Proceedings of the 1999 Particle Accelerator Conference, New York, 1999.
Production of high-intensity RIB at SPES A. Andrighettoa , L. Biasettoa , M. Manzolaroa , D. Scarpaa , J. Montanoa , J. Stanescua , P. Benettib , I. Cristofolinic , M.S. Carturana , P. Colombod , P. di Bernardoe , M. Guerzonif , G. Meneghettid , B. Monellic , G. Pretea , G. Puglierina , A. Tomasellib , P. Zanonatoe . a
INFN Laboratori di Legnaro, Viale dell’Universit`a 2, 35020 Legnaro (Pd), Italy.
b
Dipartimento di Chimica Generale and INFN, Via Bassi 6, 27100 Pavia, Italy.
c
Dipartimento di Ingegneria Meccanica, Via Mesiano 77, 38050 Trento, Italy.
d e f
Dipartimento di Ingegneria Meccanica, Via Venezia 1, 35131 Padova, Italy.
Dipartimento di Scienze Chimiche, Via Marzolo 1, 35131 Padova, Italy.
INFN Sezione di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy.
The SPES Project at INFN Laboratori Nazionali di Legnaro [1] is now in its early construction phase. SPES is an ISOL type Radioactive Ion Beam (RIB) [2] facility for the production of neutron-rich radioactive nuclei by uranium fission. RIBs will be produced by proton induced fission on a UCx multi foil direct target at a rate of 1013 fps, more than one order of magnitude larger than the currently available beam intensities. The recent developments on the production, ionization and acceleration of RIBs at SPES are hereafter presented. 1. THE SPES PROJECT AT LNL In the ISOL type SPES facility, a cyclotron proton driver provides beams, up to E= 70 MeV and total current up to 750μ A, to the production target where the nuclear reactions take place. The exotic species are ionized nearby at 60 keV to +1 charge state. Successively, the RIBs are extracted from the target-ion source system, mass separated by a small Wien filter and transported to the first focal plain. The front end and the Wien filter are placed on a 250 kV platform. After the first separation stage, out of the irradiation bunker, a high resolving power (1:20000) spectrometer performs the isobar separation. To optimize the successive re-acceleration of the exotic ions, the beam is fed to a charge breeder (CB) housed on another 250 kV platform. The outgoing multi charge beam, further selected to remove the contaminants coming from CB itself, is injected in the PIAVE superconductive RFQ, which represent the first re-acceleration stage. The final acceleration step will take place in the ALPI superconducting linear accelerator, where, the final RIBs, with energy up to 11 MeV/A, are sent to experimental set-ups for nuclear physics studies. 0375-9474/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2010.01.137
A. Andrighetto et al. / Nuclear Physics A 834 (2010) 754c–757c
755c
Figure 1. The in-target isotope yields.
2. THE TARGET ISOTOPE PRODUCTION In an ISOL facility, like SPES, the working core is constituted of the production target and the ion source [3]: they have to be carefully designed and optimized in order to obtain the desired high production rate of RIBs. Due to the low pressure of the environment, the power deposited by the proton beam in the target by means of electromagnetic and nuclear interactions can be removed only by thermal radiation towards the surrounding container box. In order to optimize both heat dissipation and fission fragments evaporation, the SPES target consists of seven, 40 mm diameter and 1.3 mm thick, co-axial disks housed in a cylindrical graphite box [4]. In choosing the target material, which should stand the highest possible beam current, it is crucial to optimize the beam-target combination with respect to the highest production cross section and lowest amount of contaminants. UCx , uranium dicarbide dispersed in an excess of graphite, is widely recognized as the reference material for the production of neutron-rich radioactive beams [5]. The preparation of the SPES UCx disks is based on the carbon-thermal reduction of UO2 powders in excess of graphite. The powders are mixed and ground in order to obtain a homogeneous mixture; these powders are uniaxially cold pressed. Finally the heat treatment is performed in a dedicated vacuum furnace. The final bulk density of the disks turns out to be about 3 g/cm3 , while the U:C atomic ratio is close to 1:4. The calculated in target fission rate in all the 7 disks approaches 1013 fps. [4]. The distribution of the fission products for the mass numbers 80 < A < 160 is shown in Figure 1. The isotope in-target production for some interesting isotopes (Ag, Sn, Cs) reaches values up to 1011 aps. All fission fragments produced are indicated in red color in the Figure 2. The 132 Sn isotope, being a double-magic nucleus, is one of the radioactive nuclei of interest. Its in target production yield is here estimated to be 1010 aps.
756c
A. Andrighetto et al. / Nuclear Physics A 834 (2010) 754c–757c
Figure 2. The main isotopes that will be ionized and extracted in the SPES project.
3. THE IONIZATON PROCESS The desired exotic species must be extracted from the target, ionized and accelerated to make a RIB [6]. This process is time demanding and usually unsuitable for atoms having half lives shorter than a few tens of ms. The hot cavity ion source chosen for the SPES project was designed at ISOLDE [7]. The source has the basic structure of the standard high temperature RIB ion sources employed for on-line operation. The isotopes produced in the target diffuse in the target material and after that effuse through the transfer tube into the ionizer cavity where they undergo surface, laser or plasma ionization. The ionizer cavity is a W tube resistively heated to near 2200◦ C. For alkalis and some of the rare earth elements, high ionization efficiencies can be easily achieved at this temperature by the surface ionization technique. For most part of the others elements, as schematized in Figure 2, the laser resonant photo-ionization, using the same hot cavity cell, is the powerful method to achieve a sufficiently selective exotic beams. The aim is to produce a beam as pure as possible (chemical selectivity) also for these metal isotopes. As first step for the R&D of the photo-ionization process for SPES, dye lasers will be used to generate resonant light source. In order to investigate the proper ionization path for the element of interest, a Hollow Cathode Lamp (HCL) is current using as atomizer. This device is a good and simple atoms reservoir for preliminary studies. A dedicated plasma ion source (FEBIAD type), which will be used to produce halogen and noble gas elements, is also under development. 4. THE OFF-LINE TEST In order to test the target off-line, a full scale prototype has been developed at LNL. In this prototype SiC disks replace the UCx ones. A dedicated electrical-thermal Finite Element Model of the target-ion source complex, able to reproduce the electrical and thermal behavior of the target and its heating tantalum housing, was written with the finite code ANSYS; the model, thoroughly validated by temperature and potential difference measurements, will be used to optimize these devices in a virtual environment. At the same time, the off-line testing of the target front-end is also in progress. The front end,
A. Andrighetto et al. / Nuclear Physics A 834 (2010) 754c–757c
757c
Figure 3. The SPES front-end.
see Figure 3, is developed on the ISOLDE design. In the SPES front-end, stable atoms are introduced, by means of a mass marker capillarity technique, close to the ion source device. They diffuse toward the ionizer where they are charged to the +1 state and finally accelerated up to 60 kV through the extraction electrode. After this step, the position of beam centroid in the transverse plane is corrected by four electrical steerers. In the final stage on the front-end, a triplet of electrostatic quadruples is responsible for the focusing of the beam to a desired downstream point. It is expected that the SPES surface ion source will produce a beam with a transverse emittance of about 6π*mm*mrad and a beam spot, in the focal plane, below 10 mm in diameter. 5. CONCLUSIONS The SPES project is one of the main nuclear physics developments in Italy for the next years. SPES is an up to date project in the field of nuclear physics and in particular in the field of RIBs; the project, once realized, will represent an important step in the direction of the constitution of the European-project-facility EURISOL. Before starting with the hardware installation, the R&D program will continue and mainly focus on the target and on the ion source development and optimization. REFERENCES 1. SPES Technichal Design Report - www.lnl.infn.it/ spes/TDR2008/. 2. J.Khalili, E. Roeckl, The Euroschool Lect. on Phys. with Exotic Beams, Vol. II, Springer (2006). 3. A. Andrighetto, S. Cevolani, and C. Petrovich, Eur. Phys. J., A25 (2005) 41-47 4. A. Andrighetto, C.M. Antonucci, S. Cevolani, C. Petrovich and M. Santana Leitner, Eur. Phys. J., A30 (2006)591 5. A. Andrighetto, C.M. Antonucci, et al., Eur. Phys. Journal. S.T. ; 150 (2007)273 6. A. Andrighetto, L. Biasetto, M. Manzolaro et al AIP 1099 (2009)728 7. J. Lettry, Proceedings of the 1999 Particle Accelerator Conference, New York, 1999.