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Radioactive ion beams—global scenario and national efforts
PII:
Radiat. Phys. Chem. Vol. 51, No. 4±6, pp. 497±505, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain S0969-806X(97)00185-0 0969-806X/98 $19.00 + 0.00
RADIOACTIVE ION BEAMSÐGLOBAL SCENARIO AND NATIONAL EFFORTS A. CHAKRABARTI Variable Energy Cyclotron Centre, 1/AF Bidhan Nagar, Calcutta 700 064, India AbstractÐThe availability of Radioactive Ion Beams (RIBs) will signal the dawn of a new era in heavy-ion science. In this context, a brief overview of the scienti®c motivation and the technological challanges to be overcome for building RIB facilities are discussed in this article. The activities in this direction being presently carried out globally are presented brie¯y with special emphasis on Indian eorts. # 1998 Published by Elsevier Science Ltd. All rights reserved
INTRODUCTION
exotic compound nuclei with zs = 0.96 and 1.01, respectively. It can be clearly seen that exotic Yb isotopes are produced with more than two orders of magnitude cross-sections if RIBs are used. Also there is a tremendous gain in the signal to noise ratio. For example, the production cross-section for 150 Yb which is a new and a very exotic nucleus, is about 300 times more with RIB (z = 0.96) as compared to the stable beam situation (z = 0.91). Also, in the stable projectile case the relative production cross-section of 150Yb is about four orders of magnitude less compared to the most favoured channel whereas, in the RIB case 150Yb is produced with the maximum cross-section resulting in an enhancement of signal to noise ratio by a factor of nearly 104. The advantages of RIBs lie mainly in the huge increase in the number of available species as accelerated beams from the particle accelerators. At present the number is less than 200 and with RIBs one expects this number to increase beyond 2000 and more. The technology needed for developing a RIB facility is extremely tricky. There is no blue print so to say and new innovations together with the continuation of extensive R&D in the ®elds of accelerators, ion-sources and instrumentation provide the only route to this challenging task. RIBs therefore represent the frontier in both basic sciences and accelerator technology and at the same time have immense potential for new applications (Fig. 2). The ®rst few decades of the 21st century are surely going to witness a vast growth in this ®eld of research leading to new fundamental discoveries of great relevance to the understanding of the atomic nucleus, origin of elements, evolution of stars and new fruitful applications in condensed matter physics, chemistry, biology and medicine.
RIBs in heavy-ion science are expected to lead to answers to many important questions including the birth of atoms, the dynamics of gravitational collapse of the supernovae, the mechanisms of neutron stars, the limits of particle stability, the existence and life time of super-heavy nuclei and the role of isospin in determining the nuclear structure. It would also allow many basic and applied research studies in the ®elds of condensed matter physics, chemistry, biology, medicine and agriculture. The intense research activities in the ®eld of nuclei away from the b-stability over the last two decades have clearly established the fact that these so called exotic nuclei could have drastically dierent properties as compared to their counterparts close to the stability. This is amply demonstrated by the failure of r = roA1/3 law for light neutron rich nuclei (Tanihata et al., 1985) the failure of mass models to predict the limits of particle stability and the disappearance of the naive link between shell closure and the spherical symmetry with the observation of large deformations in many closed shell nuclei away from stability. One needs, therefore, to study many more new nuclei and many already synthesised nuclei in much more detail for a proper understanding of nuclear structure and dynamics. To produce these new nuclei one needs accelerated secondary radioactive beams which, in combination with stable targets would be able to produce a large number of new species with orders of magnitude higher cross-sections. To illustrate the advantage of radioactive beams the calculated production cross-sections for Yb (Z = 70) isotopes in compound nuclear reactions are shown in Fig. 1. for (a) most neutron de®cient stable projectile-stable target combinations forming a compound system of exoticity z = 0.91 and (b,c) for two RI Beams leading to formation of more 497
498
A. Chakrabarti
Fig. 1. The production cross-section of various products with Z = 70 for various exoticities (exoticity parameter z = 1 on drip line and >1 beyond the drip line) of the compound nuclei formed in RIB induced compound nuclear evaporation reactions. NUCLEAR STRUCTURE AND NUCLEAR ASTROPHYSICS
Our present understanding of nuclear force and the atomic nucleus as a system comprising of a number of nucleons (neutrons and protons) bound together by strong nuclear force is far from being complete. The studies of nuclei far from the line of b-stability in the last two decades clearly brought out the inadequacy of the nuclear models in predicting the properties of these new nuclei. This is because dierent nuclear con®gurations and the order in which the shells are to be ®lled are found to change drastically as one moves away from the line of b-stability, that is for nuclei having N/Z ratio dierent from that on or near the b-stability band. The reason for this inadequacy is not dicult to understand. The parameters of the shell model were determined by comparision with too small a set of experimental data comprising mainly of b-stable
Fig. 2. Potential of radioactive ion beams in science and technology.
nuclei. These certainly need to be modi®ed and re®ned with the help of systematic data on nuclei away from the line of b-stability, usually referred to as exotic nuclei. As an example, the introduction of the spin-orbit interaction was necessary to reproduce the experimentally observed magic numbers for b-stable nuclei and the role of isospin, if any, was not considered on a proper footing. However, as there is already some evidence of the neutron and proton having dierent density distributions in nuclei away from b-stability, it may now be necessary to treat the isospin dependence dierently. The main diculty in testing such new options is the lack of systematic data on nuclei away from the line of b-stability. Since RIBs would make available such data for more than one thousand new nuclei and for hundreds of nuclei which are at present only partially studied, RIBs hold the only hope for unveiling the secrets of the atomic nuclei. It is important to note that these new data and the revelations resulting thereof hold the key to understanding of stellar evolutions, the origin of elements and their abundances. RIBs allow studies of the isospin degree of freedom which previously could not be attempted in a systematic way with stable projectiles. A customary look at the nuclear chart (Fig. 3) reveals that what has been studied so far is just a very small subset allowing only a glimpse of what an atomic nucleus should look like and what its properties should be. Also these new beams give rise to an opportunity for systematic study of the way the properties of highly asymmetric nuclear matter which is of great importance not only in understanding the atomic nuclei but also in understanding the other kinds of
Radioactive ion beams
499 Super Heavy Elements S p =0
Superallowed β – decay, isospin violations etc. Z=N 82 Charge exchange reactions, transfer, subbarrier fusion reactions
Z
S n =0
p,2p radioactivity, βp,β2p decays, GT strengths etc.
50
r process
28
Hyperdeformation Predicted Superdeformation Predicted β – stable nuclei Limits of experimentally synthesised nuclei
20 rp Process Stellar Evolution Big Bang
8 0 0
8
20
28
50
126
82
N Fig. 3. The chart of Nuclides.
nuclear matter existing elsewhere in the cosmos, such as the inside a neutron star. The creation of matter in the universe started minutes after the big bang when temperature dropped to about 5 109 Kelvin and deutron became stable against photodisintegration. Thereafter, the element synthesis went on for only a few minutes determined by the neutron's half-life. During this period elements upto Li (predominantly He were synthesised (also a few heavier elements according to the Inhomogeneous Big Bang model). With the temperature dropping and the dissapperance of neutrons from the scene, element synthesis did not take place for hundreds of millions of years, until the nebulae and the stars were born. Thereafter, the elements were cooked, as it were, in dierent stellar sites and the present day abundance of various elements in the universe is a consequence of the details of the element synthesis process in various astrophysical sites. The details of this process depend upon the physical properties of the sites and the properties of nuclei involved in the chain. Since the characteristics of dierent stellar sites vary widely, the routes followed for nucleosynthesis should also vary widely and thus both long-lived and very short-lived nuclei take part in this process. It is for this reason that the properties of exotic nuclei are so important for explaining the observed elemental abundances and for understanding the properties of the various astrophysical sites, starting from quiet stars like the Sun to violently explosive objects like novae, supernovae, X and gray bursts. From the nuclear physics point of view one needs to determine/know properties such as masses, half-
lives and the decay modes of various exotic species and the cross-sections for capture (of n, p or a) reactions of these species at energies of astrophysical interest which are roughly in the range of 1 to 350 keV (in the CM frame). Thus, to understand the evolution of various kinds of stellar objects and the observed elemental abundance the input of nuclear physics is most crucial. Much of this input, namely, the reaction rates and the properties of nuclei close to the proton and neutron drip-lines awaits the successful development of RIBs. RIBs therefore hold the key to understanding of both the atomic nuclei and the macroscopic cosmos. PRODUCTION AND ACCELERATION OF RIB
It is a challenging task to produce a large number (wide range of Z and A) of accelerated secondary beams with intensities sucient to meet experimental requirements and satisfying all the necessary beam qualities of high purity, and good optical property and over a wide energy range from about 1 MeV/u (for determination of crosssections of interest to nucleosynthesis) to about 100 MeV/u (for the production of new species using dierent kinds of nuclear reactions). The current consensus is that a RIB facility should either be of the ``projectile-fragment separator'' (PFS) type or the ``thick-target ISOL-post accelerator'' type (Fig. 4). The ®rst type of facility is suited to delivery of RIBs with energies from about 30 MeV/nucleon to several hundreds of MeV/u (LBL, GANIL, RIKEN, GSI, MSU) while the second type
500
A. Chakrabarti
Fig. 4. The two types of RIB facilities.
(CERN-ISOLDE, Louvain-la-Neuve, Dubna, Triumf, GANIL) are more suited to delivery of RIBs upto about 30 MeV/nucleon. Such a classi®cation is of course somewhat arbitrary since the deceleration of projectile fragments or the further acceleration of ISOL-post Accelerator beams are in principle possible and have been planned. To build a RIB facility one needs to extract the maximum possible beam intensity for the primary projectile from the primary accelerator, use as thick a target as possible and be able to extract and convert most of the reaction products of interest into a pure and well de®ned beam at the desired energy. However, at the present level of accelerator technology, (ion-source research, etc.) a number of technological hurdles are to be overcome at all the stages
of a RIB facility, starting from the mother accelerator which produces such species to the ion-source and various separation and post acceleration stages which ionise, purify, trasmit and accelerate the required beam. The salient features of the two types of RIB facilities and a comparison between the two approaches are given in Table 1. THE INTERNATIONAL SCENARIO
There are quite a few RIB facilities which are either already built or which are presently being built at dierent laboratories around the globe. The basic features of some of these RIB facilities are listed in Table 2. The facilities at Louvain-la-Neuve
Table 1. S. No.
PF type RIB Facility
(1)
Needs no acceleration but needs decelaration for direct determination of cross-sections of astrophysical interest. No limit on the half-life of the RIB. Beam intensity of RIBs generally lower by about two orders of magnitude compared to ISOL method because of lower primary beam intensities of high energy heavyions and thinner targets. The purity of the beam and the other beam qualities such as the emittance and the momentum spread are much worse than that achievable in an ISOL type facility.
(2) (3)
(4)
S. No.
ISOLÐPost Acc. type RIB facility
(1)
Needs acceleration of low energy ISOL beams.
(2) (3)
RIBs with half-life <10 ms is dicult. Higher beam intensities in general but the intensity depends on the chemical nature of the elements. For elements which are dicult to ionise the intensity is usually smaller compared to the PFS method. Isobar separation is usually the problem and demands mass resolving powers as high as 20,000 or more.
(4)
Radioactive ion beams
501
Table 2. RIB facilities at various International Labs Project and Pri. Acclerator Louvain-laNeuve; K = 30 Cyc. Louvain-laNeuve AREANAS K = 110 Cyc. CERN PRIMA 1 GeV Syn. EB 88 LBL K = 30 Cyc. ISAC TRIUMF K = 500 Cyc. RIKEN RI beam PF; K = 540 Cyc. MSU NSCL PF;K1200 Cyc. GSI/SIS PF; JHP 1 GeV LINAC SPIRAL GANIL Cyc. INS K = 68 Cyc.
Production beam, Ep, Ip p;30; 500 p,d,a; 80; 25 p;1000; 3 p;30; 500 p;440; 10±100 A RU;100/u; 1±10 A RU;100/u; 1±10 A RU;100/u; 1±10 p;1000; 10±100 3 He±Ar; 95/u;8 p;40;ÿ
Post Acclerator
RIB Mass No., Er, Ir
K = 110 cyc. LINAC
11±19; 0.6; 5 109 6±31; 1.5; 1 109
LINAC
6±30; 1.0; 1 109 10±15; 1.0; 1 109 60; 1.5;107 ÿ1012 240;100;108
K = 140 Cyc. LINAC RIPS A1200 FRS LINAC K = 265 Cyc. LINAC
240;100;10
8
Status 13
N,19Ne available Project -do-do-do-do-do-
8
-do-
60; 6.5;10 ÿ1012 100; 6.0; Ð 11±19; 0.8;ÿ
-do-
240;1000;10 7
-do-do-
Ep, Ip: Energy (MeV) and Intensity (mA) of production beam. Er, Ir: Energy (MeV/u) and Intensity (pps) of RIB. RIB: Radioactive Ion Beam, 1 cm. Cyc.: Cyclotron, 1 cm. Syn.: Syncrotron, 1 cm. PF: Projectile Fragmentation separator.
and GSI are operational. The ``SPIRAL'' at GANIL is likely to be operational by the beginning of the year 1999. The RI Beam Factory project at RIKEN, see Yano et al. (1995), aims at building a ``dream'' RIB facility capable of delivering more than 1000 RI Beams and providing scope for headon as well as merging collisions of heavy-ions with the help of two intersecting storage rings.
THE RIB PROJECT AT VECC, CALCUTTA.
Among the existing accelerator facilities in India, the VEC cyclotron is the only suitable primary accelerator for the RIB development, owing to its capability to deliver several mA of light ion-beams like p, a in the energy range from 15 to 60 MeV. The variable energy and more than one light-ion option oer the possibility to optimise the production cross-section and with about 30 mA beam current, a large number of RI Beams can be obtained with intensities in the range of 105± 109 pps. It has been decided to develop a RIB facility at VECC to deliver ions up to A = 100 and with energy of about 1.0 MeV/u. The R&D activities for the said facility is being pursued by the VECC group in collaboration with two other Institutes: SINP, Calcutta, and The Institute of Physical and Chemical Research, RIKEN, Japan.
The RIB facility: A general layout of the planned RIB facility is shown in Fig. 5. The radioactive atoms will be produced inside a thick target of thickness close to 1 gm/cm2. The radioactive gas diusing out of the thick target will be transferred to an ion-source where the radioactive atoms will be ionised and the desired beam will be selected after extraction and isobaric separation. The low energy (1.5 keV/u) pure RI beam will be then accelerated in a RFQ linac upto about 80 keV/u and subsequently the beam will be accelerated upto about 1.0 MeV/u using a LINAC. The major thrust areas for R&D and their present status 1. The release of activities from the thick target and their transfer to the ion-source: a thick target chamber has been designed where the target can be heated electrically for faster and more ecient diusion. However, transfer of activities from the target chamber to the ion-source by the generally employed technique of diusion is only ecient for the atoms of a few elements, being mostly the gases. It has been found, in a recent experiment at VEC, that ecient transfer of activities is possible with the Hejet technique even in this high temperature environment. More R&D on this aspect is being carried out at present.
502
A. Chakrabarti
PRIMARY BEAM
TARGET
REFRACTORY COMPOUNDS, TARGET T~ 2500K THICKNESS ~mm.
TRANSFER LINE
NUCLEAR SPECTROSCOPY
1.5 keV/u
ONLINE ECRIS
ECR ION SOURCE, SINGLE STAGE, ONLINE, 6.4 GHz
ISOTOPE SEPARATOR
MASS RESOLVING POWER=8000
SWITCHING MAGNET
RADIO FREQUENCY QUADRUPOLE
MATERIAL SCIENCE, CONDENSED MATTER ETC.
80 keV/u
f=35 MHz; E(in) =1.5 keV/u; E(out) =80keV/u.
SWITCHING MAGNET
LINAC
NUCLEAR ASTRO– PHYSICS
PROTON <40MeV; 10–40 uA ALPHA <80MeV; 10–40 uA.
E (out) =1.0 MeV/u, q/A>0.05. ~5 MeV/u finally
1.0 MeV/u * ESTIMATED INTENSITY 1.0E5 TO 1.0E9 PPS.
Fig. 5. The conceptual layout of the VEC-RIB facility.
2. On-Line Electron Cyclotron Resonance (ECR) ion-source: Fig. 6 shows the schematic assembly of the VEC RIB-ECRIS which will be a single stage 6.4 GHz ECR with CAPRICE (Compacte A Plusieurs Resonances Ionisantes Cyclotron Electroniques) (Bourg et al., 1995) geometry operated in high B mode. Two solenoid coils provide axial con®nement and the radial con®nement is produced by a SmCo5 hexapole permanant magnet. The magnetic ®eld con®guration has been designed using the POISSON Group code (Reference Manual for POISSON/SUPERFISH Group of Codes, 1987). An iron yoke has been introduced to enhance the ®eld. Option is kept to displace the extraction end solenoid with respect to the sole-
noid at the injection end. This will give the scope for optimizing the plasma zone length and ®eld con®guration at the extractor end. It is hoped that the ECR will be able to produce in the on-line ion beams of q/A1 0.0625 with high enough eciencies. However, in the online, the residual gas pressure of atoms from the hot target makes it dicult to maintain the required amount of vacuum inside the ionsource and getting high eciency for charge states >1+ becomes dicult. It has been decided to work on a new two ion-source concept where the atoms will be ®rst ionised in a cavity ioniser and then inside the ECR. The detailed design of this new concept is being done at present. Also the possibility of injecting
, , ,, , , ,, , , ,, ,, , , , , ,, , ,,
,, , , , Radioactive ion beams
Cooling
Cooling
6.4 GHz
Ta Transfer PRIMARY Tube REACTION PRODUCTS Target holder
Target
503
IRON CORE
SOLENOID COIL
HEXAPOLE
PULLER ELECTRODE
500 L/S TMP
INSULATOR TUBE
Fig. 6. Schematic diagram of the on-line ECR ion source for VEC-RIB facility.
atoms into the ECR by He-jet (Chakrabarti et al., 1988) after successive skimmer stages is under active consideration. 3. The design of a very high resolution ISOL system: the requirement of a pure beam needs mass SCHEMATIC OF THE HIGH RESOLUTION ISOTOPE SEPARATOR RIBs From ECR Ion Source
Ion Optical Layout : QQQ–D–QQ; Antimirror Symmetric. Q: quadrupole, D:dipole
First Focus
DESIGN GOAL : MASS RESOLVING POWER OF 10000 FOR 100% TRANSMISSION AND AN ION SOURCE EMITTANCE OF 70 mm–mrad.
Second Focus
Mass Separated RIB of interest
RFQ LINAC Fig. 7. The high resolution ISOL beam optical layout.
resolving power greater than 10,000 (for the separation of isobars) without any appreciable loss in the beam intensity. This is an extremely dicult task. With two dipole magnets in the dispersion additive mode, a design value of about 5000 mass resolving power has so far been achieved (Fig. 7). For higher resolving power one de®nitely needs to add more dipole stages but stability of power supplies becomes a very important consideration for achieving mass resolution higher than 5000 (momentum resolution 10000). 4. The design of a Radio Frequency Quadrupole (RFQ): The ®nal RFQ for RIB is rather too long to attempt to build directly. The design of this RFQ has been completed and for the required ®nal energy of 80 keV/u, the vane length has to be as long as 3.5 metres which is too dicult to machine without any R&D on shorter vane lengths. It has been decided, therefore, to divide the RFQ development into three phases:
(a) Design and development of a cold model for RF structure studies; (b) Design and development of a 1.6 m long RFQ capable of accelerating the ions upto about 35 keV/u and ®nally (c) The design and development of the 3.6 m long ®nal RFQ.The design of the 1.6 m long RFQ has been completed. The variation of dierent parameters along the length of the RFQ and a schematic diagram of the same
504
A. Chakrabarti
(a) B
a
Φs
E
m 2.0
1.6 5
–30
B
0.6 1.4
1.8 –40
4 1.2
–50
Φ
0.4
–60
1.0
)
3
eV
1.4
E(M
m 2
0.8
1.6
s
0.2
–70 1.2 –80
0.6
a
1
–90
0.0
1.0
0.4
, ,,, , , , , , , , –20
0
20
40
60
80
100
120
140
CELL NUMBER
CAVITY
(b)
VANE
POST
BASE PLATE
Fig. 8. (a) RFQ vane parameters along its length. (b) Schematic diagram of the RFQ assembly.
are shown in Fig. 8. The fabrication of the half-scale cold model for rf-structure studies is being taken up at present. SUMMARY
A modest level of R&D activity has been initiated in the last two years at VECC for the development of a RIB facility. The basic design of many subsystems are either completed or are being completed. However, the more dicult tasks of fabrication, testing, installation and R&D studies for further improvement are the major challenge ahead.
AuthorÐplease cite the following reference in the text or delete from the reference list: The SPIRAL Radioactive Ion Beam Facility; GANIL R 94 02. AcknowledgementsÐThe RIB activities at VECC presented in this manuscript result from joint collaborative venture for RIB development between scientists of VECC, Calcutta and RIKEN, Japan.
REFERENCES
Tanihata, I. et al. (1985) Measurement of Interaction Cross Sections and Nuclear Radii in the Light p-shell Regions. PRL 160, 2676.
Radioactive ion beams Yano, Y., Goto, A., Katayama, T. and RIBF Group. (1995) RIKEN RI Beam Factory Project; 10th Symp. Accelerator Science and Technology, Hitachinaka. Bourg, F., Geller, R. et Jacquat, B. (1995) Source d'Ions Lourds Multicharges Caprice 10 G Hz Pour Tous les Elements Metalliques et Gazeuk. Proceedings of the 12th International Workshop on ECR Ion Sources (Edited by
505
M. Sekiguchi and T. Nakagawa). Nucl. Inst. Meth., Vol. A254, p. 13, INS-J-182. Reference Manual for POISSON/SUPERFISH Group of Codes. (1987) Los Alamos Acc. Code Group, MS H829, LA-UR-87-126. Chakrabarti, A. et al. (1988) Helium jet recoil transport setup for chemistry and nuclear application at VECC. Nucl. Inst. Meth., A263, 421.
Radiat. Phys. Chem. Vol. 51, No. 4±6, pp. 497±505, 1998 # 1998 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain S0969-806X(97)00185-0 0969-806X/98 $19.00 + 0.00
RADIOACTIVE ION BEAMSÐGLOBAL SCENARIO AND NATIONAL EFFORTS A. CHAKRABARTI Variable Energy Cyclotron Centre, 1/AF Bidhan Nagar, Calcutta 700 064, India AbstractÐThe availability of Radioactive Ion Beams (RIBs) will signal the dawn of a new era in heavy-ion science. In this context, a brief overview of the scienti®c motivation and the technological challanges to be overcome for building RIB facilities are discussed in this article. The activities in this direction being presently carried out globally are presented brie¯y with special emphasis on Indian eorts. # 1998 Published by Elsevier Science Ltd. All rights reserved
INTRODUCTION
exotic compound nuclei with zs = 0.96 and 1.01, respectively. It can be clearly seen that exotic Yb isotopes are produced with more than two orders of magnitude cross-sections if RIBs are used. Also there is a tremendous gain in the signal to noise ratio. For example, the production cross-section for 150 Yb which is a new and a very exotic nucleus, is about 300 times more with RIB (z = 0.96) as compared to the stable beam situation (z = 0.91). Also, in the stable projectile case the relative production cross-section of 150Yb is about four orders of magnitude less compared to the most favoured channel whereas, in the RIB case 150Yb is produced with the maximum cross-section resulting in an enhancement of signal to noise ratio by a factor of nearly 104. The advantages of RIBs lie mainly in the huge increase in the number of available species as accelerated beams from the particle accelerators. At present the number is less than 200 and with RIBs one expects this number to increase beyond 2000 and more. The technology needed for developing a RIB facility is extremely tricky. There is no blue print so to say and new innovations together with the continuation of extensive R&D in the ®elds of accelerators, ion-sources and instrumentation provide the only route to this challenging task. RIBs therefore represent the frontier in both basic sciences and accelerator technology and at the same time have immense potential for new applications (Fig. 2). The ®rst few decades of the 21st century are surely going to witness a vast growth in this ®eld of research leading to new fundamental discoveries of great relevance to the understanding of the atomic nucleus, origin of elements, evolution of stars and new fruitful applications in condensed matter physics, chemistry, biology and medicine.
RIBs in heavy-ion science are expected to lead to answers to many important questions including the birth of atoms, the dynamics of gravitational collapse of the supernovae, the mechanisms of neutron stars, the limits of particle stability, the existence and life time of super-heavy nuclei and the role of isospin in determining the nuclear structure. It would also allow many basic and applied research studies in the ®elds of condensed matter physics, chemistry, biology, medicine and agriculture. The intense research activities in the ®eld of nuclei away from the b-stability over the last two decades have clearly established the fact that these so called exotic nuclei could have drastically dierent properties as compared to their counterparts close to the stability. This is amply demonstrated by the failure of r = roA1/3 law for light neutron rich nuclei (Tanihata et al., 1985) the failure of mass models to predict the limits of particle stability and the disappearance of the naive link between shell closure and the spherical symmetry with the observation of large deformations in many closed shell nuclei away from stability. One needs, therefore, to study many more new nuclei and many already synthesised nuclei in much more detail for a proper understanding of nuclear structure and dynamics. To produce these new nuclei one needs accelerated secondary radioactive beams which, in combination with stable targets would be able to produce a large number of new species with orders of magnitude higher cross-sections. To illustrate the advantage of radioactive beams the calculated production cross-sections for Yb (Z = 70) isotopes in compound nuclear reactions are shown in Fig. 1. for (a) most neutron de®cient stable projectile-stable target combinations forming a compound system of exoticity z = 0.91 and (b,c) for two RI Beams leading to formation of more 497
498
A. Chakrabarti
Fig. 1. The production cross-section of various products with Z = 70 for various exoticities (exoticity parameter z = 1 on drip line and >1 beyond the drip line) of the compound nuclei formed in RIB induced compound nuclear evaporation reactions. NUCLEAR STRUCTURE AND NUCLEAR ASTROPHYSICS
Our present understanding of nuclear force and the atomic nucleus as a system comprising of a number of nucleons (neutrons and protons) bound together by strong nuclear force is far from being complete. The studies of nuclei far from the line of b-stability in the last two decades clearly brought out the inadequacy of the nuclear models in predicting the properties of these new nuclei. This is because dierent nuclear con®gurations and the order in which the shells are to be ®lled are found to change drastically as one moves away from the line of b-stability, that is for nuclei having N/Z ratio dierent from that on or near the b-stability band. The reason for this inadequacy is not dicult to understand. The parameters of the shell model were determined by comparision with too small a set of experimental data comprising mainly of b-stable
Fig. 2. Potential of radioactive ion beams in science and technology.
nuclei. These certainly need to be modi®ed and re®ned with the help of systematic data on nuclei away from the line of b-stability, usually referred to as exotic nuclei. As an example, the introduction of the spin-orbit interaction was necessary to reproduce the experimentally observed magic numbers for b-stable nuclei and the role of isospin, if any, was not considered on a proper footing. However, as there is already some evidence of the neutron and proton having dierent density distributions in nuclei away from b-stability, it may now be necessary to treat the isospin dependence dierently. The main diculty in testing such new options is the lack of systematic data on nuclei away from the line of b-stability. Since RIBs would make available such data for more than one thousand new nuclei and for hundreds of nuclei which are at present only partially studied, RIBs hold the only hope for unveiling the secrets of the atomic nuclei. It is important to note that these new data and the revelations resulting thereof hold the key to understanding of stellar evolutions, the origin of elements and their abundances. RIBs allow studies of the isospin degree of freedom which previously could not be attempted in a systematic way with stable projectiles. A customary look at the nuclear chart (Fig. 3) reveals that what has been studied so far is just a very small subset allowing only a glimpse of what an atomic nucleus should look like and what its properties should be. Also these new beams give rise to an opportunity for systematic study of the way the properties of highly asymmetric nuclear matter which is of great importance not only in understanding the atomic nuclei but also in understanding the other kinds of
Radioactive ion beams
499 Super Heavy Elements S p =0
Superallowed β – decay, isospin violations etc. Z=N 82 Charge exchange reactions, transfer, subbarrier fusion reactions
Z
S n =0
p,2p radioactivity, βp,β2p decays, GT strengths etc.
50
r process
28
Hyperdeformation Predicted Superdeformation Predicted β – stable nuclei Limits of experimentally synthesised nuclei
20 rp Process Stellar Evolution Big Bang
8 0 0
8
20
28
50
126
82
N Fig. 3. The chart of Nuclides.
nuclear matter existing elsewhere in the cosmos, such as the inside a neutron star. The creation of matter in the universe started minutes after the big bang when temperature dropped to about 5 109 Kelvin and deutron became stable against photodisintegration. Thereafter, the element synthesis went on for only a few minutes determined by the neutron's half-life. During this period elements upto Li (predominantly He were synthesised (also a few heavier elements according to the Inhomogeneous Big Bang model). With the temperature dropping and the dissapperance of neutrons from the scene, element synthesis did not take place for hundreds of millions of years, until the nebulae and the stars were born. Thereafter, the elements were cooked, as it were, in dierent stellar sites and the present day abundance of various elements in the universe is a consequence of the details of the element synthesis process in various astrophysical sites. The details of this process depend upon the physical properties of the sites and the properties of nuclei involved in the chain. Since the characteristics of dierent stellar sites vary widely, the routes followed for nucleosynthesis should also vary widely and thus both long-lived and very short-lived nuclei take part in this process. It is for this reason that the properties of exotic nuclei are so important for explaining the observed elemental abundances and for understanding the properties of the various astrophysical sites, starting from quiet stars like the Sun to violently explosive objects like novae, supernovae, X and gray bursts. From the nuclear physics point of view one needs to determine/know properties such as masses, half-
lives and the decay modes of various exotic species and the cross-sections for capture (of n, p or a) reactions of these species at energies of astrophysical interest which are roughly in the range of 1 to 350 keV (in the CM frame). Thus, to understand the evolution of various kinds of stellar objects and the observed elemental abundance the input of nuclear physics is most crucial. Much of this input, namely, the reaction rates and the properties of nuclei close to the proton and neutron drip-lines awaits the successful development of RIBs. RIBs therefore hold the key to understanding of both the atomic nuclei and the macroscopic cosmos. PRODUCTION AND ACCELERATION OF RIB
It is a challenging task to produce a large number (wide range of Z and A) of accelerated secondary beams with intensities sucient to meet experimental requirements and satisfying all the necessary beam qualities of high purity, and good optical property and over a wide energy range from about 1 MeV/u (for determination of crosssections of interest to nucleosynthesis) to about 100 MeV/u (for the production of new species using dierent kinds of nuclear reactions). The current consensus is that a RIB facility should either be of the ``projectile-fragment separator'' (PFS) type or the ``thick-target ISOL-post accelerator'' type (Fig. 4). The ®rst type of facility is suited to delivery of RIBs with energies from about 30 MeV/nucleon to several hundreds of MeV/u (LBL, GANIL, RIKEN, GSI, MSU) while the second type
500
A. Chakrabarti
Fig. 4. The two types of RIB facilities.
(CERN-ISOLDE, Louvain-la-Neuve, Dubna, Triumf, GANIL) are more suited to delivery of RIBs upto about 30 MeV/nucleon. Such a classi®cation is of course somewhat arbitrary since the deceleration of projectile fragments or the further acceleration of ISOL-post Accelerator beams are in principle possible and have been planned. To build a RIB facility one needs to extract the maximum possible beam intensity for the primary projectile from the primary accelerator, use as thick a target as possible and be able to extract and convert most of the reaction products of interest into a pure and well de®ned beam at the desired energy. However, at the present level of accelerator technology, (ion-source research, etc.) a number of technological hurdles are to be overcome at all the stages
of a RIB facility, starting from the mother accelerator which produces such species to the ion-source and various separation and post acceleration stages which ionise, purify, trasmit and accelerate the required beam. The salient features of the two types of RIB facilities and a comparison between the two approaches are given in Table 1. THE INTERNATIONAL SCENARIO
There are quite a few RIB facilities which are either already built or which are presently being built at dierent laboratories around the globe. The basic features of some of these RIB facilities are listed in Table 2. The facilities at Louvain-la-Neuve
Table 1. S. No.
PF type RIB Facility
(1)
Needs no acceleration but needs decelaration for direct determination of cross-sections of astrophysical interest. No limit on the half-life of the RIB. Beam intensity of RIBs generally lower by about two orders of magnitude compared to ISOL method because of lower primary beam intensities of high energy heavyions and thinner targets. The purity of the beam and the other beam qualities such as the emittance and the momentum spread are much worse than that achievable in an ISOL type facility.
(2) (3)
(4)
S. No.
ISOLÐPost Acc. type RIB facility
(1)
Needs acceleration of low energy ISOL beams.
(2) (3)
RIBs with half-life <10 ms is dicult. Higher beam intensities in general but the intensity depends on the chemical nature of the elements. For elements which are dicult to ionise the intensity is usually smaller compared to the PFS method. Isobar separation is usually the problem and demands mass resolving powers as high as 20,000 or more.
(4)
Radioactive ion beams
501
Table 2. RIB facilities at various International Labs Project and Pri. Acclerator Louvain-laNeuve; K = 30 Cyc. Louvain-laNeuve AREANAS K = 110 Cyc. CERN PRIMA 1 GeV Syn. EB 88 LBL K = 30 Cyc. ISAC TRIUMF K = 500 Cyc. RIKEN RI beam PF; K = 540 Cyc. MSU NSCL PF;K1200 Cyc. GSI/SIS PF; JHP 1 GeV LINAC SPIRAL GANIL Cyc. INS K = 68 Cyc.
Production beam, Ep, Ip p;30; 500 p,d,a; 80; 25 p;1000; 3 p;30; 500 p;440; 10±100 A RU;100/u; 1±10 A RU;100/u; 1±10 A RU;100/u; 1±10 p;1000; 10±100 3 He±Ar; 95/u;8 p;40;ÿ
Post Acclerator
RIB Mass No., Er, Ir
K = 110 cyc. LINAC
11±19; 0.6; 5 109 6±31; 1.5; 1 109
LINAC
6±30; 1.0; 1 109 10±15; 1.0; 1 109 60; 1.5;107 ÿ1012 240;100;108
K = 140 Cyc. LINAC RIPS A1200 FRS LINAC K = 265 Cyc. LINAC
240;100;10
8
Status 13
N,19Ne available Project -do-do-do-do-do-
8
-do-
60; 6.5;10 ÿ1012 100; 6.0; Ð 11±19; 0.8;ÿ
-do-
240;1000;10 7
-do-do-
Ep, Ip: Energy (MeV) and Intensity (mA) of production beam. Er, Ir: Energy (MeV/u) and Intensity (pps) of RIB. RIB: Radioactive Ion Beam, 1 cm. Cyc.: Cyclotron, 1 cm. Syn.: Syncrotron, 1 cm. PF: Projectile Fragmentation separator.
and GSI are operational. The ``SPIRAL'' at GANIL is likely to be operational by the beginning of the year 1999. The RI Beam Factory project at RIKEN, see Yano et al. (1995), aims at building a ``dream'' RIB facility capable of delivering more than 1000 RI Beams and providing scope for headon as well as merging collisions of heavy-ions with the help of two intersecting storage rings.
THE RIB PROJECT AT VECC, CALCUTTA.
Among the existing accelerator facilities in India, the VEC cyclotron is the only suitable primary accelerator for the RIB development, owing to its capability to deliver several mA of light ion-beams like p, a in the energy range from 15 to 60 MeV. The variable energy and more than one light-ion option oer the possibility to optimise the production cross-section and with about 30 mA beam current, a large number of RI Beams can be obtained with intensities in the range of 105± 109 pps. It has been decided to develop a RIB facility at VECC to deliver ions up to A = 100 and with energy of about 1.0 MeV/u. The R&D activities for the said facility is being pursued by the VECC group in collaboration with two other Institutes: SINP, Calcutta, and The Institute of Physical and Chemical Research, RIKEN, Japan.
The RIB facility: A general layout of the planned RIB facility is shown in Fig. 5. The radioactive atoms will be produced inside a thick target of thickness close to 1 gm/cm2. The radioactive gas diusing out of the thick target will be transferred to an ion-source where the radioactive atoms will be ionised and the desired beam will be selected after extraction and isobaric separation. The low energy (1.5 keV/u) pure RI beam will be then accelerated in a RFQ linac upto about 80 keV/u and subsequently the beam will be accelerated upto about 1.0 MeV/u using a LINAC. The major thrust areas for R&D and their present status 1. The release of activities from the thick target and their transfer to the ion-source: a thick target chamber has been designed where the target can be heated electrically for faster and more ecient diusion. However, transfer of activities from the target chamber to the ion-source by the generally employed technique of diusion is only ecient for the atoms of a few elements, being mostly the gases. It has been found, in a recent experiment at VEC, that ecient transfer of activities is possible with the Hejet technique even in this high temperature environment. More R&D on this aspect is being carried out at present.
502
A. Chakrabarti
PRIMARY BEAM
TARGET
REFRACTORY COMPOUNDS, TARGET T~ 2500K THICKNESS ~mm.
TRANSFER LINE
NUCLEAR SPECTROSCOPY
1.5 keV/u
ONLINE ECRIS
ECR ION SOURCE, SINGLE STAGE, ONLINE, 6.4 GHz
ISOTOPE SEPARATOR
MASS RESOLVING POWER=8000
SWITCHING MAGNET
RADIO FREQUENCY QUADRUPOLE
MATERIAL SCIENCE, CONDENSED MATTER ETC.
80 keV/u
f=35 MHz; E(in) =1.5 keV/u; E(out) =80keV/u.
SWITCHING MAGNET
LINAC
NUCLEAR ASTRO– PHYSICS
PROTON <40MeV; 10–40 uA ALPHA <80MeV; 10–40 uA.
E (out) =1.0 MeV/u, q/A>0.05. ~5 MeV/u finally
1.0 MeV/u * ESTIMATED INTENSITY 1.0E5 TO 1.0E9 PPS.
Fig. 5. The conceptual layout of the VEC-RIB facility.
2. On-Line Electron Cyclotron Resonance (ECR) ion-source: Fig. 6 shows the schematic assembly of the VEC RIB-ECRIS which will be a single stage 6.4 GHz ECR with CAPRICE (Compacte A Plusieurs Resonances Ionisantes Cyclotron Electroniques) (Bourg et al., 1995) geometry operated in high B mode. Two solenoid coils provide axial con®nement and the radial con®nement is produced by a SmCo5 hexapole permanant magnet. The magnetic ®eld con®guration has been designed using the POISSON Group code (Reference Manual for POISSON/SUPERFISH Group of Codes, 1987). An iron yoke has been introduced to enhance the ®eld. Option is kept to displace the extraction end solenoid with respect to the sole-
noid at the injection end. This will give the scope for optimizing the plasma zone length and ®eld con®guration at the extractor end. It is hoped that the ECR will be able to produce in the on-line ion beams of q/A1 0.0625 with high enough eciencies. However, in the online, the residual gas pressure of atoms from the hot target makes it dicult to maintain the required amount of vacuum inside the ionsource and getting high eciency for charge states >1+ becomes dicult. It has been decided to work on a new two ion-source concept where the atoms will be ®rst ionised in a cavity ioniser and then inside the ECR. The detailed design of this new concept is being done at present. Also the possibility of injecting
, , ,, , , ,, , , ,, ,, , , , , ,, , ,,
,, , , , Radioactive ion beams
Cooling
Cooling
6.4 GHz
Ta Transfer PRIMARY Tube REACTION PRODUCTS Target holder
Target
503
IRON CORE
SOLENOID COIL
HEXAPOLE
PULLER ELECTRODE
500 L/S TMP
INSULATOR TUBE
Fig. 6. Schematic diagram of the on-line ECR ion source for VEC-RIB facility.
atoms into the ECR by He-jet (Chakrabarti et al., 1988) after successive skimmer stages is under active consideration. 3. The design of a very high resolution ISOL system: the requirement of a pure beam needs mass SCHEMATIC OF THE HIGH RESOLUTION ISOTOPE SEPARATOR RIBs From ECR Ion Source
Ion Optical Layout : QQQ–D–QQ; Antimirror Symmetric. Q: quadrupole, D:dipole
First Focus
DESIGN GOAL : MASS RESOLVING POWER OF 10000 FOR 100% TRANSMISSION AND AN ION SOURCE EMITTANCE OF 70 mm–mrad.
Second Focus
Mass Separated RIB of interest
RFQ LINAC Fig. 7. The high resolution ISOL beam optical layout.
resolving power greater than 10,000 (for the separation of isobars) without any appreciable loss in the beam intensity. This is an extremely dicult task. With two dipole magnets in the dispersion additive mode, a design value of about 5000 mass resolving power has so far been achieved (Fig. 7). For higher resolving power one de®nitely needs to add more dipole stages but stability of power supplies becomes a very important consideration for achieving mass resolution higher than 5000 (momentum resolution 10000). 4. The design of a Radio Frequency Quadrupole (RFQ): The ®nal RFQ for RIB is rather too long to attempt to build directly. The design of this RFQ has been completed and for the required ®nal energy of 80 keV/u, the vane length has to be as long as 3.5 metres which is too dicult to machine without any R&D on shorter vane lengths. It has been decided, therefore, to divide the RFQ development into three phases:
(a) Design and development of a cold model for RF structure studies; (b) Design and development of a 1.6 m long RFQ capable of accelerating the ions upto about 35 keV/u and ®nally (c) The design and development of the 3.6 m long ®nal RFQ.The design of the 1.6 m long RFQ has been completed. The variation of dierent parameters along the length of the RFQ and a schematic diagram of the same
504
A. Chakrabarti
(a) B
a
Φs
E
m 2.0
1.6 5
–30
B
0.6 1.4
1.8 –40
4 1.2
–50
Φ
0.4
–60
1.0
)
3
eV
1.4
E(M
m 2
0.8
1.6
s
0.2
–70 1.2 –80
0.6
a
1
–90
0.0
1.0
0.4
, ,,, , , , , , , , –20
0
20
40
60
80
100
120
140
CELL NUMBER
CAVITY
(b)
VANE
POST
BASE PLATE
Fig. 8. (a) RFQ vane parameters along its length. (b) Schematic diagram of the RFQ assembly.
are shown in Fig. 8. The fabrication of the half-scale cold model for rf-structure studies is being taken up at present. SUMMARY
A modest level of R&D activity has been initiated in the last two years at VECC for the development of a RIB facility. The basic design of many subsystems are either completed or are being completed. However, the more dicult tasks of fabrication, testing, installation and R&D studies for further improvement are the major challenge ahead.
AuthorÐplease cite the following reference in the text or delete from the reference list: The SPIRAL Radioactive Ion Beam Facility; GANIL R 94 02. AcknowledgementsÐThe RIB activities at VECC presented in this manuscript result from joint collaborative venture for RIB development between scientists of VECC, Calcutta and RIKEN, Japan.
REFERENCES
Tanihata, I. et al. (1985) Measurement of Interaction Cross Sections and Nuclear Radii in the Light p-shell Regions. PRL 160, 2676.
Radioactive ion beams Yano, Y., Goto, A., Katayama, T. and RIBF Group. (1995) RIKEN RI Beam Factory Project; 10th Symp. Accelerator Science and Technology, Hitachinaka. Bourg, F., Geller, R. et Jacquat, B. (1995) Source d'Ions Lourds Multicharges Caprice 10 G Hz Pour Tous les Elements Metalliques et Gazeuk. Proceedings of the 12th International Workshop on ECR Ion Sources (Edited by
505
M. Sekiguchi and T. Nakagawa). Nucl. Inst. Meth., Vol. A254, p. 13, INS-J-182. Reference Manual for POISSON/SUPERFISH Group of Codes. (1987) Los Alamos Acc. Code Group, MS H829, LA-UR-87-126. Chakrabarti, A. et al. (1988) Helium jet recoil transport setup for chemistry and nuclear application at VECC. Nucl. Inst. Meth., A263, 421.