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Ri beam factory project
NUCLEAR PHYSICS A ELSEVIER
Nuclear Physics A588 (1995) 253c-258c
RI BEAM FACTORY PROJECT Isao Tanihata RIKEN, 2-1 Hirosawa, Wako, Saitama 351-01, Japan The RI Beam Factory is proposed at RIKEN which is the project to construct a superconducting ring elotron (SRC) and a double storage-cooler ring (MUSES). Radioactive beams of up to 500A MeV for light nuclei and 150A MeV for the heaviest nuclei are supplied as secondary beams. Electrons, stable nuclei, highly charged ions in addition to radioactive nuclei can be stored in the storage ring. The MUSES provides various method for the use of beams such as in colliding mode, in merging mode, and in internal target mode. A few of the new nuclear physics opportunities are presented.
1. Outline of the Facilities
The a c c e l e r a t o r facility has the tentative configuration shown in Fig. 1. The accelerator system is composed of a s u p e r c o n d u c t i n g ring cyclotron (SRC), a double storage ring (MUSES), and an electron linac. A beam from the existing ring cyclotron is injected Fig. I Conceptualview of the new Acceleratorsin RI beam factory. into the SRC. Light nuclei are accelerated to 500A MeV and the heaviest nuclei are accelerated to 15114 MeV with highest intensity. Higher energy beams of heaviest nuclei will also be available if one is satisfied with a lower intensity. For example, one gets 200A MeV if beam intensity is about 1/5 of the maximum intensity, or even higher energy if one uses 1/10 of the maximum available intensity (But it is still as high as 100 pnA). The beam energy after the SRC is shown in Fig. 2. The heavy ion beams obtained from the SRC will be converted into RI beams by the RIPS II. Then, the separated beams (as well as the primary beam) are sent to various experimental facilities. One of these is the direct transmission to the laboratory and another is the injection to the MUSES. The MUSES stores an accelerated beam of electrons, molecules or clusters for use in colliding or merging mode of collisions with the RI or other beam in the other ring. 0375-9474/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SSDI 0375-9474(95)00148-4
254c
I. Tanihata / Nuclear Physics A588 (1995) 253c-258c
With these accelerators, the RI Beam Factory project aims at supplying 500 the isotopes of all elements as beams with an energy up to 500 MeV per nucleon for light nuclei and 150 MeV per nucleon for Uranium to facilitate 400 progress in a wide variety of research. This project also aims at enabling collisions among unstable nuclear beams ~, 300 and other beams (electrons, highly charged atoms, molecular ions, clustered ions, etc.) to pioneer new types of research in the utilization of ions. ~ 200 . On the
the
Beam
Selection
Energy of
m
B
m
m
and the
100-
Accelerator The central interest of the RI Beam 0 - H O Ar Kr Xe Bi U Factory is to study the ground states and i 8b ' ' ' 120 A' 200 240 low-excited states of unstable nuclei far from the stability line through reactions Fig. 2 Beam energy at the Superconducting using RI beams and to develop a variety Ring Cyclotron. The maximum energy is of applications of unstable nuclei. The for the ions with the highest intensity charge conditions required for the facility state. include: 1. A large number of unstable nuclei should be available efficiently as beams. 2. The beam energy should be high enough so that the reaction mechanism is simple enough. 3. High intensity beams should be supplied cost-effectively. The consideration of the above conditions and the projectile fragmentation cross section, the momentum spread, and the binding effect require the beam energy higher than 100 MeV per nucleon. In general, as represented by the target thickness, an increase in energy can increase the secondary beam production efficiency gradually and make the reaction mechanism easier to understand. Higher energy, however, increases the cost of the accelerator. Currently, the present ring cyclotron provides beams with energies up to 135A MeV for lighter nuclei and up to 20A MeV for the heaviest nuclei. In general, the maximum heavy-ion acceleration energies using cyclotrons are similar to this value and synchrotrons are used to obtain higher energies. Cyclotrons provide a continuous beam operation and therefore they are able to accelerate beams from ion sources with a 100% duty factor. On the other hand, synchrotrons are based on pulse operation and thus have low duty cycles and they can use only small time portion of the beams obtained from the ion sources. If the beam can be pulsed at the ion source and if the integrated number of the usable ion can be made equal to the continuously obtained beam intensity, synchrotrons can be used to accelerate up to
L Tanihata / Nuclear Physics A588 (1995) 253c-258c
255c
the space-charge limit and provide beams of pretty high intensity. Even in this case the intensity may be lower than the beams obtained from cyclotrons by one or two orders of magnitude. Realistically, the peak intensity of current ion-source under the pulsed operation is only a few times higher than the average intensity of continuous operation. Therefore a further loss of intensity due to the duty factor is unavoidable. Since the duty factor of the pulsed operation is 1% at most, the difference in beam intensity of at least three or more orders of magnitude arises between a cyclotron and a synchrotron. Under these circumstances, if an energy above 100 MeV can be obtained, it is suitable to use a cyclotron because it can efficiently produce the secondary beams and it makes full use of the intensity from an ion source. It is believed that the cyclotron can accelerate uranium nuclei to 150A MeV or higher if the present superconducting technology is applied properly. The RI Beam Factory aims at accelerating uranium beams with an energy to 150A MeV or higher. The intensity of the pulsed beams injected in synchrotrons remains weak with the current technology. However, if there is a revolutionary discovery in the technology for pulsing beams, the proposed superconducting cyclotron also provides best solution for a future higher-energy accelerator. In particular, at higher injection energy the spacecharge limit of the synchrotron increases than the one at a low energy (because the spacecharge limit is proportional to ~2T3). As the result, the combination of a superconducting cyclotron and a synchrotron will be the best way to obtain higher-energy and higherintensity beams in the future.
3. A Few Example of the Experimental Opportunities The most important feature of research in the RI Beam Factory is that it makes it possible to select nuclei in a wide range of proton and neutron numbers freely and to study the structures of those nuclei. It not only increases the number of nuclei that can be studied by a factor of 10 but also introduce unprecedented variables. It allows the following quantities to be varied over a wide range: a. Isospin b. Nucleon binding energy c. Density difference between nucleons These provide us with a new field that is "nuclear physics of the asymmetric weakly bound state and non-uniform density". In particular, isospin is regarded to be one of the most important axes of the future research of nuclei. Although many experiments can be made, here, I present a few possible studies of nuclear matter distributions and some possibilities of unique usage of the MUSES. Measurements of Nucleon Density Distributions and Proton-Neutron DecoupUng. The determination of density distributions of unstable nuclei gives a special interest. As observed by the neutron halo and the neutron skin, the decoupling of proton and neutron distributions is expected for nuclei far from the stability line. The beam energy at the RI Beam Factory is set to be capable of measuring the nucleon distribution radius of unstable isotopes of all elements up to uranium (2500 nuclides or more). So it is expected to provide a systematic data in a very wide range. In addition, the internal
256c
/. Tanihata / Nuclear Physics A588 (1995) 253c-258c
nucleon momentum distribution, detailed information of density distributions, and the correlation between nucleons will be studied by measuring the momentum distribution of nuclei produced by the fragmentation reactions. For a nucleus with a sufficient beam intensity (>10 4/S), the nucleon distribution can be determined more precisely (for ~2000 nuclei) through elastic-proton-scattering experiments using a storage ring and an internal gas target. Studies of the central density of nuclei with extreme N/Z ratio are of great interest for studying the saturation properties of asymmetric nuclear matter. The RI Beam Factory is capable of storing electron beams in one of the rings of the MUSES and storing unstable nuclei in the other ring. Then electron scatterings on unstable nuclei are possible. Although the luminosity restricts the determination of higher-order moment of the density distributions, it is possible to determine the rootmean-square radii of a considerable number of unstable nuclei (600 nuclides). But this method has difficulties when it is applied to exotic nuclei because the number of stored nuclei is very much limited due to small production rates and short lifetimes. Isotope shift measurement is 4xl~ I also possible by storing ions of unstable nuclei having several "~ c~ electrons (particularly lithium-like c~ ions). Since one deals with "~ 6x104[: electrons in inner shells in such cases, a large isotope shift is ~ 0 expected.too An ordinary laser beam ~E~ 04f is low in energy for this . 1.5xl measurement. However x-rays of the optimum energy can be ~ 2x1031. generated and used in measurement if one uses an undulator with X I I I ~ I I continuously variable pitch in the ,~ electron storage ring. As this ~ 3x103 I" method uses the light absorption ~'~ ~, 210U process by atoms, it has a large ~ 0 106 107 cross-section of 10 l0 - l012 b and 105 X ray energy (eV) \ 8 2 ions! thus it can be applied to unstable nuclei with low intensity. By the Fig. 3 Simulated x-ray resonance spectra of Li same reason, it can be applied like ions of U isotopes. The measurement can be directly to secondary beams made even with only 82 stored ions. without storing them, provided that the electron beams are strong enough. In addition, a nuclear spin and a nuclear moment can also be determined by measuring the hyperfine structure of the spectrum at the same time. The simulated resonance spectra are shown in Fig. 3.
27u 1
oE
'
' = 5ij l ^A '
ol
F
I
I
I
I
I
i
'213U 1
I
I
I
Experiments at MUSES
The MUSES is a unique experimental system which provides various collision modes between any two among heavy-ion, RI beam, electron, and internal gas jet target. The most interesting among then are the electron vs RI beam and the x-ray vs RI beam scatterings as already presented above.
257c
1. Tanihata /Nuclear Physics A588 (1995) 253c-258c
The other interesting use is the internal gas target for proton and other light particle collisions. This mode is important not only for RI beams but also for stable heavy ions. Because of the cooling of the beam as well as the inverse kinematical conditions, an experimental resolution can be extremely good. Because of an easiness of the target, highly resolution study can be made for all elements. Other unique use of the MUSES is the merging beam mode. This mode provides a method to use the secondary beam for low energy reaction studies. In general, the secondary beam method of RI beam production is considered not to be suited for low energy reaction studies mainly due to its unavoidable energy spread. However, in the merging mode, two high-energy beams can be used to have a low energy collision in their center of mass. The luminosities of various modes of operation for some selected beams are shown in Table I. Nuclei with neutron skins or halos are expected to give special effects on fusion reactions. When nuclei approach each other, the cores usually repel each other by the Coulomb force and this repulsion forms the Coulomb barrier which reduces the fusion cross-section. On the other hand, fusion may occur more easily in the reaction of nuclei with neutron skins or halos because fusion may start when the cores are still far away. However, nuclei with skins or halos are weakly bound and thus the reaction Q values are high. This leads to the opposite prediction that the nucleus cannot fuse without emitting many nucleons because of the high excitation of the final state. Enhancement or not, the change in the fusion cross-section due to the presence of a neutron layer is gaining attention.
Table I. Luminosities with RI b e a m s
# from RIPS
Luminosity (cm-2 s -1)
T1/2 (s)
equivalent/s
RI+ gas jet
RI + e
RI +A
(merging) 6He llBe 150
1013 1013 1013
17F
1013
39Ca
1012
49Sc 56Ca
5.1012 5.104
0.8
2.1026
6.1023
13.8
9.1029 2.1031 9.1030
3.1027 6.1028
1.6.1019 9.1022 2.1024
3.1028
9.1023
7.1028
2.1026
7.1021
1.5.1033 2.1021
4.1030 6.1018
1.5.1026 7.1021
5.1031 1.5'1029 6.1025 2.1028 2.1026 6.1023 1.7 x 1029 cm2.s
5.1024 2.1021 2.1019
122 64.7 0.86 3400 1? 5.105
56Ni 2.1011 64Ge 5.109 63 132Sn 4.107 40 HI+ HI colliding mode at 1.2A GeV: HI+ HI merging mode : 1.1 x 1026.s
Nuclear Physics A588 (1995) 253c-258c
RI BEAM FACTORY PROJECT Isao Tanihata RIKEN, 2-1 Hirosawa, Wako, Saitama 351-01, Japan The RI Beam Factory is proposed at RIKEN which is the project to construct a superconducting ring elotron (SRC) and a double storage-cooler ring (MUSES). Radioactive beams of up to 500A MeV for light nuclei and 150A MeV for the heaviest nuclei are supplied as secondary beams. Electrons, stable nuclei, highly charged ions in addition to radioactive nuclei can be stored in the storage ring. The MUSES provides various method for the use of beams such as in colliding mode, in merging mode, and in internal target mode. A few of the new nuclear physics opportunities are presented.
1. Outline of the Facilities
The a c c e l e r a t o r facility has the tentative configuration shown in Fig. 1. The accelerator system is composed of a s u p e r c o n d u c t i n g ring cyclotron (SRC), a double storage ring (MUSES), and an electron linac. A beam from the existing ring cyclotron is injected Fig. I Conceptualview of the new Acceleratorsin RI beam factory. into the SRC. Light nuclei are accelerated to 500A MeV and the heaviest nuclei are accelerated to 15114 MeV with highest intensity. Higher energy beams of heaviest nuclei will also be available if one is satisfied with a lower intensity. For example, one gets 200A MeV if beam intensity is about 1/5 of the maximum intensity, or even higher energy if one uses 1/10 of the maximum available intensity (But it is still as high as 100 pnA). The beam energy after the SRC is shown in Fig. 2. The heavy ion beams obtained from the SRC will be converted into RI beams by the RIPS II. Then, the separated beams (as well as the primary beam) are sent to various experimental facilities. One of these is the direct transmission to the laboratory and another is the injection to the MUSES. The MUSES stores an accelerated beam of electrons, molecules or clusters for use in colliding or merging mode of collisions with the RI or other beam in the other ring. 0375-9474/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SSDI 0375-9474(95)00148-4
254c
I. Tanihata / Nuclear Physics A588 (1995) 253c-258c
With these accelerators, the RI Beam Factory project aims at supplying 500 the isotopes of all elements as beams with an energy up to 500 MeV per nucleon for light nuclei and 150 MeV per nucleon for Uranium to facilitate 400 progress in a wide variety of research. This project also aims at enabling collisions among unstable nuclear beams ~, 300 and other beams (electrons, highly charged atoms, molecular ions, clustered ions, etc.) to pioneer new types of research in the utilization of ions. ~ 200 . On the
the
Beam
Selection
Energy of
m
B
m
m
and the
100-
Accelerator The central interest of the RI Beam 0 - H O Ar Kr Xe Bi U Factory is to study the ground states and i 8b ' ' ' 120 A' 200 240 low-excited states of unstable nuclei far from the stability line through reactions Fig. 2 Beam energy at the Superconducting using RI beams and to develop a variety Ring Cyclotron. The maximum energy is of applications of unstable nuclei. The for the ions with the highest intensity charge conditions required for the facility state. include: 1. A large number of unstable nuclei should be available efficiently as beams. 2. The beam energy should be high enough so that the reaction mechanism is simple enough. 3. High intensity beams should be supplied cost-effectively. The consideration of the above conditions and the projectile fragmentation cross section, the momentum spread, and the binding effect require the beam energy higher than 100 MeV per nucleon. In general, as represented by the target thickness, an increase in energy can increase the secondary beam production efficiency gradually and make the reaction mechanism easier to understand. Higher energy, however, increases the cost of the accelerator. Currently, the present ring cyclotron provides beams with energies up to 135A MeV for lighter nuclei and up to 20A MeV for the heaviest nuclei. In general, the maximum heavy-ion acceleration energies using cyclotrons are similar to this value and synchrotrons are used to obtain higher energies. Cyclotrons provide a continuous beam operation and therefore they are able to accelerate beams from ion sources with a 100% duty factor. On the other hand, synchrotrons are based on pulse operation and thus have low duty cycles and they can use only small time portion of the beams obtained from the ion sources. If the beam can be pulsed at the ion source and if the integrated number of the usable ion can be made equal to the continuously obtained beam intensity, synchrotrons can be used to accelerate up to
L Tanihata / Nuclear Physics A588 (1995) 253c-258c
255c
the space-charge limit and provide beams of pretty high intensity. Even in this case the intensity may be lower than the beams obtained from cyclotrons by one or two orders of magnitude. Realistically, the peak intensity of current ion-source under the pulsed operation is only a few times higher than the average intensity of continuous operation. Therefore a further loss of intensity due to the duty factor is unavoidable. Since the duty factor of the pulsed operation is 1% at most, the difference in beam intensity of at least three or more orders of magnitude arises between a cyclotron and a synchrotron. Under these circumstances, if an energy above 100 MeV can be obtained, it is suitable to use a cyclotron because it can efficiently produce the secondary beams and it makes full use of the intensity from an ion source. It is believed that the cyclotron can accelerate uranium nuclei to 150A MeV or higher if the present superconducting technology is applied properly. The RI Beam Factory aims at accelerating uranium beams with an energy to 150A MeV or higher. The intensity of the pulsed beams injected in synchrotrons remains weak with the current technology. However, if there is a revolutionary discovery in the technology for pulsing beams, the proposed superconducting cyclotron also provides best solution for a future higher-energy accelerator. In particular, at higher injection energy the spacecharge limit of the synchrotron increases than the one at a low energy (because the spacecharge limit is proportional to ~2T3). As the result, the combination of a superconducting cyclotron and a synchrotron will be the best way to obtain higher-energy and higherintensity beams in the future.
3. A Few Example of the Experimental Opportunities The most important feature of research in the RI Beam Factory is that it makes it possible to select nuclei in a wide range of proton and neutron numbers freely and to study the structures of those nuclei. It not only increases the number of nuclei that can be studied by a factor of 10 but also introduce unprecedented variables. It allows the following quantities to be varied over a wide range: a. Isospin b. Nucleon binding energy c. Density difference between nucleons These provide us with a new field that is "nuclear physics of the asymmetric weakly bound state and non-uniform density". In particular, isospin is regarded to be one of the most important axes of the future research of nuclei. Although many experiments can be made, here, I present a few possible studies of nuclear matter distributions and some possibilities of unique usage of the MUSES. Measurements of Nucleon Density Distributions and Proton-Neutron DecoupUng. The determination of density distributions of unstable nuclei gives a special interest. As observed by the neutron halo and the neutron skin, the decoupling of proton and neutron distributions is expected for nuclei far from the stability line. The beam energy at the RI Beam Factory is set to be capable of measuring the nucleon distribution radius of unstable isotopes of all elements up to uranium (2500 nuclides or more). So it is expected to provide a systematic data in a very wide range. In addition, the internal
256c
/. Tanihata / Nuclear Physics A588 (1995) 253c-258c
nucleon momentum distribution, detailed information of density distributions, and the correlation between nucleons will be studied by measuring the momentum distribution of nuclei produced by the fragmentation reactions. For a nucleus with a sufficient beam intensity (>10 4/S), the nucleon distribution can be determined more precisely (for ~2000 nuclei) through elastic-proton-scattering experiments using a storage ring and an internal gas target. Studies of the central density of nuclei with extreme N/Z ratio are of great interest for studying the saturation properties of asymmetric nuclear matter. The RI Beam Factory is capable of storing electron beams in one of the rings of the MUSES and storing unstable nuclei in the other ring. Then electron scatterings on unstable nuclei are possible. Although the luminosity restricts the determination of higher-order moment of the density distributions, it is possible to determine the rootmean-square radii of a considerable number of unstable nuclei (600 nuclides). But this method has difficulties when it is applied to exotic nuclei because the number of stored nuclei is very much limited due to small production rates and short lifetimes. Isotope shift measurement is 4xl~ I also possible by storing ions of unstable nuclei having several "~ c~ electrons (particularly lithium-like c~ ions). Since one deals with "~ 6x104[: electrons in inner shells in such cases, a large isotope shift is ~ 0 expected.too An ordinary laser beam ~E~ 04f is low in energy for this . 1.5xl measurement. However x-rays of the optimum energy can be ~ 2x1031. generated and used in measurement if one uses an undulator with X I I I ~ I I continuously variable pitch in the ,~ electron storage ring. As this ~ 3x103 I" method uses the light absorption ~'~ ~, 210U process by atoms, it has a large ~ 0 106 107 cross-section of 10 l0 - l012 b and 105 X ray energy (eV) \ 8 2 ions! thus it can be applied to unstable nuclei with low intensity. By the Fig. 3 Simulated x-ray resonance spectra of Li same reason, it can be applied like ions of U isotopes. The measurement can be directly to secondary beams made even with only 82 stored ions. without storing them, provided that the electron beams are strong enough. In addition, a nuclear spin and a nuclear moment can also be determined by measuring the hyperfine structure of the spectrum at the same time. The simulated resonance spectra are shown in Fig. 3.
27u 1
oE
'
' = 5ij l ^A '
ol
F
I
I
I
I
I
i
'213U 1
I
I
I
Experiments at MUSES
The MUSES is a unique experimental system which provides various collision modes between any two among heavy-ion, RI beam, electron, and internal gas jet target. The most interesting among then are the electron vs RI beam and the x-ray vs RI beam scatterings as already presented above.
257c
1. Tanihata /Nuclear Physics A588 (1995) 253c-258c
The other interesting use is the internal gas target for proton and other light particle collisions. This mode is important not only for RI beams but also for stable heavy ions. Because of the cooling of the beam as well as the inverse kinematical conditions, an experimental resolution can be extremely good. Because of an easiness of the target, highly resolution study can be made for all elements. Other unique use of the MUSES is the merging beam mode. This mode provides a method to use the secondary beam for low energy reaction studies. In general, the secondary beam method of RI beam production is considered not to be suited for low energy reaction studies mainly due to its unavoidable energy spread. However, in the merging mode, two high-energy beams can be used to have a low energy collision in their center of mass. The luminosities of various modes of operation for some selected beams are shown in Table I. Nuclei with neutron skins or halos are expected to give special effects on fusion reactions. When nuclei approach each other, the cores usually repel each other by the Coulomb force and this repulsion forms the Coulomb barrier which reduces the fusion cross-section. On the other hand, fusion may occur more easily in the reaction of nuclei with neutron skins or halos because fusion may start when the cores are still far away. However, nuclei with skins or halos are weakly bound and thus the reaction Q values are high. This leads to the opposite prediction that the nucleus cannot fuse without emitting many nucleons because of the high excitation of the final state. Enhancement or not, the change in the fusion cross-section due to the presence of a neutron layer is gaining attention.
Table I. Luminosities with RI b e a m s
# from RIPS
Luminosity (cm-2 s -1)
T1/2 (s)
equivalent/s
RI+ gas jet
RI + e
RI +A
(merging) 6He llBe 150
1013 1013 1013
17F
1013
39Ca
1012
49Sc 56Ca
5.1012 5.104
0.8
2.1026
6.1023
13.8
9.1029 2.1031 9.1030
3.1027 6.1028
1.6.1019 9.1022 2.1024
3.1028
9.1023
7.1028
2.1026
7.1021
1.5.1033 2.1021
4.1030 6.1018
1.5.1026 7.1021
5.1031 1.5'1029 6.1025 2.1028 2.1026 6.1023 1.7 x 1029 cm2.s
5.1024 2.1021 2.1019
122 64.7 0.86 3400 1? 5.105
56Ni 2.1011 64Ge 5.109 63 132Sn 4.107 40 HI+ HI colliding mode at 1.2A GeV: HI+ HI merging mode : 1.1 x 1026.s