Overview of the Rare Isotope Accelerator project

Overview of the Rare Isotope Accelerator project

Nuclear Instruments and Methods in Physics Research B 204 (2003) 765–770 www.elsevier.com/locate/nimb Overview of the Rare Isotope Accelerator projec...

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Nuclear Instruments and Methods in Physics Research B 204 (2003) 765–770 www.elsevier.com/locate/nimb

Overview of the Rare Isotope Accelerator project q Bradley M. Sherrill National Superconducting Cyclotron Laboratory, Department of Physics, Michigan State University, East Lansing, MI 48824, USA

Abstract This article provides an overview of the Rare Isotope Accelerator, RIA, concept. RIA represents a major advance in rare isotope capabilities for the US and the world nuclear science communities and has been given the highest priority for major new construction by the nuclear science community in the US. RIA combines many aspects of current rare isotope facilities. The concept attempts to provide a versatile facility that can take advantage of a variety of production methods and deliver rare isotope beams at a variety of energies. Ó 2003 Elsevier Science B.V. All rights reserved.

1. Introduction Nuclei with a large excess of neutrons or protons are known to exhibit new features and teach us about aspects of nuclear structure not previously understood. The same nuclei often play an important role in determining the energy generation and nucleosynthesis in astrophysical processes. In the past it was impossible to study many of the interesting nuclei because they could not be produced in sufficient quantities or our experimental techniques were not sufficiently sensitive. However, technology has advanced to the point where it is now possible to build facilities, such as the Rare Isotope Accelerator, RIA, which can produce a wide range of the interesting isotopes. In light of the opportunities associated with the study of nuclei far from stability, the Nuclear q A large umber of people have contributed to the RIA concept. An attempt was made to adequately acknowledge everyone in the text and references. E-mail address: [email protected] (B.M. Sherrill).

Science Advisory Committee, NSAC, has recently recommended the construction of the RIA as the highest priority for major new construction [1]. A summary of the scientific opportunities of RIA are presented in the Long Range Plan Report. A more detail description of the scientific justification for RIA can be found in the RIA Science White Paper [2]. Some the potential applications to other fields are describe in the RIA Applications Workshop 2000 proceedings [3]. Many details of the RIA design and justification can be found at the ANL and NSCL websites [4]. The goal of RIA is to provide the most intense source of rare isotopes for experimental studies. In order to do this, the facility allows for the optimization of the production mechanism for each desired isotope. These mechanisms can be projectile fragmentation or fission; target spallation, fragmentation and fission; and fusion. This requires the RIA driver be capable of accelerating all ions, up to uranium, to at least 400 MeV/nucleon. Production of the highest intensity secondary beams also requires an intense primary accelerator.

0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00500-7

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Using superconducting linac technology it is possible to deliver very high beam intensities of more than 1014 ions/s for light ions and over 1013 uranium ions/s using current ECR ion source technology. The RIA concept also requires a flexible array of target stations, including in-flight separation and ISOL. A full description of the RIA concept is provided in [5]. Included in the RIA concept is the capability of doing experiments with rare isotopes at a variety of energies from a few keV up to 100s of MeV/nucleon. The higher energy beams are an important and critical addition since they allow for an increase in luminosity and allow for experiments with beams with extremely low intensity (even down to ions/week). This expands the scope of RIA to a wider range of the drip lines and to most of the r-process nuclei. The RIA concept evolved over a long process in the US and Canada that began with the 1989 NSAC Long Range Plan [6]. Enthusiasm for rare isotopes began to grow in the US in 1989 when there were a number of workshops devoted to the discussion of accelerated rare isotope beams, including a key meeting in Los Alamos [7]. The roots for the RIA project lie in the efforts in the mid 1980s to develop radioactive ion beam facilities including the Parksville conference in Canada, which has lead to the ISAC facility [8], and the implementation (around 1990) of an in-flight facility at the NSCL. In the mid 1990s ORNL and ANL developed concepts for advanced ISOL based rare isotope research facilities. Smaller scale, ISOL based radioactive ion beam facilities ISAC at TRIUMF and HRIBF at ORNL were build and operated in the later 1990s. The current RIA concept is a merging of many of these ideas and incorporates ISOL and in-flight separation. An NSAC subcommittee under the chairmanship of H. Grunder was constituted in 1998 to consider the optimal method for the production of accelerated rare isotope beams in the US. This committee worked for more than one year evaluating the various options. Its deliberations lead to the current RIA concept [5]. The result, RIA, is a facility which promises to allow most of the rprocess nuclei to be studied as will as nuclei along nearly the entire proton drip line and the neutron drip line up to possibly Z ¼ 40.

2. Details of the RIA design A schematic layout of the RIA facility is shown in Fig. 1. The RIA primary accelerator is a CW superconducting linac capable of accelerating all elements from hydrogen to uranium. Such a linac allows for very efficient acceleration and is capable of delivering up to 400 kW of beam power at 400– 1000 MeV/nucleon. This corresponds to 2:4  1013 uranium ions/s, provided ion source performance can increase to match this intensity or multiple ion sources are used. The high efficiency is achieved by accelerating multiple charge states from an ECR and accepting multiple charge states after the various stripping points in the linac [9]. In this way, very little beam is lost in the acceleration process and the system is an order of magnitude or more efficient than cyclotron or synchrotron schemes that rely on stripping. Many of the details and current status of items discussed in this section can be found in the contribution of Savard in this volume [10]. The beams can be delivered to four general experimental areas. One for in-flight studies with beams of greater than 50 MeV/nucleon; another for slow or stopped beams, where, for example, ion trap studies can be performed. In addition, two experimental areas for re-accelerated beams will be available. One for energies up to around 1 MeV/ nucleon for direct and radiative capture measure-

Fig. 1. Schematic drawing illustrating the RIA concept. An efficient superconducting linac provides high intensity primary beams for the production of rare isotopes by a variety of methods, as outlined in the text. It is planned that the facility will be able to provide multiple beams for simultaneous users. Secondary beams from 20 kV to 400 MeV/nucleon will be available.

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ments for astrophysics and the other for lowenergy direction reaction studies, such as inverse kinematic (d,p) measurements, with energies up to 12 MeV/nucleon. The addition of the high energy beam area, which was made possible by the incorporation of heavy ions in the RIA acceleration scheme, expands the scientific reach and, in particular, allows studies where only ions/week are available [11]. This makes possible study of nearly all r-process nuclei and drip line nuclei over a wide range. The sum of all the experimental capabilities will provide RIA with a broad scientific reach. Details of the RIA driver conceptual design have been given elsewhere [12,13]. Table 1 lists a few representative beams and their maximum intensities assuming current ECR ion source performance. The high intensity for the last three beams is achieved using multiple charge state acceleration. In the case of the last row in the table, 238 U, it was assumed that both q ¼ 28 and q ¼ 29 were accelerated out of the ion source [14]. Multiple charge state acceleration will also be useful when rare stable isotopes (or long lived radioactive) isotopes are accelerated. Effectively, there are minimal stripping losses and hence compared to schemes that rely on two stripping stages, RIA is an order of magnitude more efficient. The output of the linac can be multiplexed onto a number of production stations. For light ion production, two thick-target ISOL stations are envisioned. For rare isotope production with heavy ions, two in-flight fragment separators will be used. One will be similar to the current NSCL

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A1900 [15] and be used to deliver beams to an inflight area for experiments with beams of energy greater than 50 MeV/nucleon. This separator should have a momentum acceptance of around 5% and an angular acceptance of 8–10 msr. These parameters are determined to allow a reasonable amount of selectivity in the separation process, yet maintain a high efficiency for fragment collection. At 400 MeV/nucleon, this separator will have between 10% and 100% collection efficiency for fission and fragmentation products. A second, higher acceptance, fragment separator will deliver beams to a stopping station were the ions can be caught in gas (or some other appropriate catcher), stopped, and quickly extracted and re-accelerated. The stopping process provides an additional selectivity, hence this separator can be designed with a lower resolution and a higher acceptance. Its design goals are a momentum acceptance of 15% and a solid angle of 10 msr. The intrinsic momentum resolving power need only be in the range of 1 part in 1000. In order to reduce the effects of the large momentum spread of the fragments and to allow the ions to be stopped in a small gas volume, the high acceptance separator will be followed by a dispersive stage that can be used to compress the range of fragments [16]. If the optical abberations to the spot size can be kept below 1 mm (corresponding to 1 part in 1000 in resolution) the stopping efficiency will be limited by range straggling of the ions. In this case the stopping efficiency for most ions with Z larger than 20 or so can be near 100% in a 1 m gas cell.

Table 1 Representative, possible RIA driver beam list Beam A

Isource (plA)

Energy (MeV/u)

Iout (plA)

Power (kW)

1 3 2 18 40 86 136 238

556 232 416 54 29 15 12 3

899 717 600 551 554 515 476 403

445 186 333 40 18 9 6 1.6

400 400 400 400 400 390 400 150

The final three beams require multiple charge state acceleration, as described in the text to reach the quoted intensity assuming current ECR ion-source performance. The uranium intensity assumes two charge states from the ECR are accelerated.

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All the production areas will allow for the possibility for recovery of unused, longer-lived isotopes for off-line studies or re-acceleration. The multiple production stations will also allow the beam to be multiplex and beams simultaneously delivered to several experiments. The RIA post accelerator is described elsewhere in this volume [17]. The goal is to provide efficient acceleration of the secondary ions with minimal stripping for all ions with A up to 240. The RIA project will also include a variety of experimental equipment. Initial workshops and considerations have outlined the necessary experimental equipment [11,18]. The details of this equipment will be worked out as the RIA design and construction proceeds. Fig. 2 illustrates the predicted yields for reaccelerated rare isotopes. The production scheme was optimized for each isotope and the data were compiled by Jiang et al. [19]. ISOL-type production rates are scaled from ISOLDE data and based on Monte Carlo simulations of target performance [20]. Fragmentation yields are based on a modified EPAX 2 parametrization [21] and fission yields are from Junghans [22]. The most intense beams (e.g. cesium), represented by the light areas, are pro-

Fig. 2. Expected yields of accelerated rare isotopes from the RIA facility as compiled by Jiang et al. The optimum production mechanism for each isotope was used. Secondary beam intensities of over 1013 ions/s are possible when target spallation and heated targets can be used, as indicated by the light colored areas. For reference, a possible neutron drip line and possible paths of the rp- and r-processes are shown.

duced with light ion production in heated thick targets. In many cases more than 1013 secondary ion/s are expected. Over a wide range of production, in-flight projectile fragmentation and fission are the preferred methods. As shown in the figure, most of the expected r-process nuclei can be produced and studied. It may also be possible to study nuclei up to the drip line at Z ¼ 40, however, the most sensitive secondary experiments will be needed. However, since the figure is for the most part based on estimated cross sections, it is likely there will be differences in the actual production rates for the most extreme nuclei. A key consideration is the optimum production energy for the secondary ions. A qualitative argument for the optimum energy is given by the yield calculations shown in Fig. 3. Higher primary beam energy implies a higher production yield, but also higher cost. A cost optimum occurs in the 400–600 MeV/nucleon range. However, the issues are much more complex. A significant drawback to production at lower energy is the contamination

Fig. 3. Illustration of the rational behind the choice of 400 MeV/nucleon for the maximum uranium linac energy. Higher primary beam energy implies a higher production yield, but also higher cost. Independent of other considerations, an optimum occurs in the 400–600 MeV/nucleon range.

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from other isotopes, not separated by the fragmentation separation process, and the losses due to charge states for higher Z ions. It is hoped that future advances in detector technology will help in this regard. Fig. 4 illustrates the influence of the fragment separator acceptance on the choice of the optimum production energy. The curves show the calculated yields, using EPAX2, for the production of 78 Ni from 86 Kr. Secondary reactions of the products in the target are neglected. This is an approximate technique for taking into account secondary production of 78 Ni from reactions of other fragments. As shown in the figure, the yield increases with fragment separator momentum acceptance. A facility that produces ions at 200 MeV/nucleon can have the same yield as a facility with 600 MeV/ nucleon that uses a much smaller acceptance fragment separator. The problem with a larger acceptance is the potentially very large number of contaminant ions. Generally, this would prevent the use of a large (10% or greater) acceptance. However, in the case of RIA, the gas stopping station provides an additional range selection. Simulations indicate that for the most part, only the ions of interest will stop in the gas, hence a large acceptance can be tolerated and the ionization affect on the cell will be minimal.

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3. Summary and outlook RIA has the promise to provide a wide array of rare isotopes for study. A guiding principle in the attempt to make available the highest possible quantities of a desired isotope. This requires that a number of production mechanisms be available and hence the driver accelerator be able to provide a wide range of primary beams and energies. The driver accelerator should also have a very efficient acceleration scheme. The multiple charge state acceleration possible with the RIA linac is perhaps the most efficient scheme for acceleration up to 400 MeV/nucleon. The result is a facility that should be able to produce most of the r-process nuclei for study and may make nuclei out to the neutron drip line up to Z ¼ 40. A key feature for RIA is the ability to deliver secondary beams at all energies up to 400 MeV/nucleon. The multiple production and experimental stations also allow for simultaneous multi-user operation. The basic RIA concept has been outlined in this paper. A number of key research and development issues remain to be addressed, but there are no known serious technical issues that would prevent the concept from working. A summary of the current research into the RIA concept is given in

Fig. 4. Illustration of the effect of increasing fragment separator acceptance as a function of energy. Shown are the calculated 78 Ni yields from fragmentation of 86 Kr under various conditions. An in-flight separation facility at 400 MeV/nucleon using a large acceptance fragment separator can have a similar yield to a higher energy facility with a smaller acceptance separator.

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the contribution of Savard in this volume. As the research and development continues, RIA is awaiting an official go ahead from the US Department of Energy. It is hoped that once this is given, planning and construction can proceed expeditiously. If so, the facility may be operational shortly after 2010.

Acknowledgements A very large number of people have contributed to the ideas presented in this paper. The general RIA concept came from the Grunder Committee deliberations. I would particularly like to acknowledge the input and current efforts of the RIA design teams at Argonne National Lab and the National Superconducting Cyclotron Laboratory. A large number of laboratories in the US and around the world are contributing to RIA research and development. In the US these include ANL, BNL, LLNL, LANL, NSCL, ORNL and Texas A&M.

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[8]

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