Radioactive beams from gas catchers: The CARIBU facility

Available online at www.sciencedirect.com

NIM B Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 266 (2008) 4086–4091 www.elsevier.com/locate/nimb

Radioactive beams from gas catchers: The CARIBU facility G. Savard a,b,*, S. Baker a, C. Davids a, A.F. Levand a, E.F. Moore a, R.C. Pardo a, R. Vondrasek a, B.J. Zabransky a, G. Zinkann a a

Physics Division, Argonne National Laboratory, Argonne, IL 60439, USA Department of Physics, University of Chicago, Chicago, IL 60637, USA

b

Available online 7 June 2008

Abstract Gas catchers allow the transformation of radioactive recoils from various sources into a good optical quality low-energy radioactive beam that is then available for experiments at low-energy or for further acceleration. The CARIBU project uses such a large gas catcher to create beams of neutron-rich isotopes from a Californium source for post-acceleration through the ATLAS superconducting linac to open new research opportunities for nuclear structure physics and astrophysics. The RF gas catcher developed at Argonne has now demonstrated operation at the high intensity required for this application. Ó 2008 Elsevier B.V. All rights reserved. PACS: 29.25.Rm; 29.25.t Keywords: Radioactive beam; Gas catcher

1. Introduction Our understanding of the nuclear landscape and of the astrophysical phenomena involving short-lived nuclei has made significant progress over the last two decades with a combination of powerful stable beam facilities and the advent of the first generation radioactive beam facilities [1]. They have provided access to a wealth of short-lived nuclei and the ability to perform nuclear reactions with them at an energy regime similar to that at which these reactions occur in stars. This progress has however been hampered by some technical difficulties. Stable beam facilities produce short-lived nuclei by reactions that generally populate the proton-rich side of the valley of stability more favorably. First generation radioactive beam facilities suffered from severe limitations in the beam varieties, intensities and energies at which these beams are available. Lowenergy and Coulomb barrier energy beams in particular are *

Corresponding author. Address: Physics Division, Argonne National Laboratory, Argonne, IL 60439, USA. E-mail address: [email protected] (G. Savard). 0168-583X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2008.05.091

usually produced by ISOL [2] techniques that are efficient only for a subset of species with favorable chemical properties. The large amount of experimental information that has been gathered as a result on stable and proton-rich nuclei has led to very successful models that reproduce the properties of the bulk of the nuclei in this region. There is still information to be gathered on the proton-rich side of stability, especially very far from stability and in the reactions involving these nuclei, but overall we understand the main features of the nuclei in these regions. On the other side of the valley of stability the picture is quite different. Because Coulomb repulsion plays a smaller role in these nuclei, the neutron-rich nuclei in this region are often characterized by a large excess of weakly bound neutrons that can form skins [3] or halos [4] where the effective interactions can be significantly different from that of the stable or proton-rich nuclei. The associated lower density can lead to a weakened spin–orbit interaction [5] that would modify the shell structure in this region. The difficulty in accessing the nuclei in this region has not allowed sufficient information to be gathered to support or refute this scenario.

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Knowledge on the properties of these nuclei is not only critical for nuclear structure studies but also for their astrophysical implications [6] in the r-process responsible for the formation of half of the isotopes heavier than Iron. This requires larger access to these regions than presently available. In the coming future, the large radioactive ion beam facilities currently being proposed will allow us to answer these questions fully. In the following we describe an ongoing upgrade of the ATLAS [7] superconducting accelerator at Argonne, the CAlifornium Rare Isotope Beam Upgrade (CARIBU) project, targeted to greatly improve access to the neutron-rich nuclei in this region and allow us to perform initial experiments paving the way for the next generation facilities. 2. Initial program and required beams In order to prepare a targeted initial program to study the changes in shell structure expected for the neutron-rich isotopes and the important properties of these nuclei, it is useful to determine the key experimental information that needs to be gathered. The expected changes in nuclear structure should be most drastically reflected in the single-particle structure and affect also the pairing interaction and the collective behavior of these nuclei. Of particular interest to the astrophysical r-process involving these nuclei will be masses, lifetimes, beta-delayed neutron emission and neutron-capture cross-sections of neutron-rich nuclei, and in addition the fissionability of the very heaviest neutron-rich nuclei. Initial goals of the CARIBU physics program are to address these questions. The beams and beam properties required for this task can be determined by looking at the experimental requirements to obtain this information. The single-particle structure information on neutronrich nuclei is best obtained from single-particle transfer reactions; this is how the same information was best obtained on stable nuclei [8] when the first heavy-ion accelerators became available. Neutron states for example would be best probed by (d,p) reactions in reverse kinematics at a few MeV above the Coulomb barrier with radioactive beams at 8 MeV/u. For states with higher angular momentum, the (a,3He) reaction at 10 MeV/u provides a better matching. Hole-states can be probed similarly with the complementary reactions. The energies are high enough for the incoming and outgoing particles to be well above the Coulomb barrier so that characteristic angular distributions and valid spectroscopic information can be obtained. With standard silicon detector arrays an angular distribution can be measured with a radioactive beam intensity of about 104 particles per second. New devices such as the HELIOS [9] solenoidal spectrometer currently under construction at Argonne should allow such measurements to be performed with even lower intensity beams. Changes in the pairing strength can be obtained from two nucleon transfer reactions such as the (p,t) and (t,p) reactions to look at the strengths in the excited 0+ states,

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essentially extending the long chains of reaction on stable nuclei to neutron-rich regions. When pairing breaks down, the excited 0+ states should be populated with much larger strengths than the few percent expected by BCS theory. Optimum energy depends on the Q-value of the reaction but is typically up to 10 MeV/u on the nuclei of interest. Similarly, a glimpse at the collective properties of these neutron-rich nuclei can be obtained from Coulomb excitation experiments in a gamma-ray detector array such as GAMMASPHERE. The available information on fission products is limited to studying products from spontaneous fission sources inside gamma-ray arrays and recent measurements with re-accelerated beams at HRIBF [10] and REX-ISOLDE [11]; extending these measurements to a much wider array of fission-product radioactive beams would vastly improve the state of knowledge. The information required for input to the r-process calculations, mainly masses, lifetime and beta-delayed neutron emission probability, is best obtained with low-energy or stopped beams. The masses yield the neutron-capture Q-values which determine the location of the r-process path, lifetimes determine the timescale, and beta-delayed neutron emission probability influence the final isotope distribution observed. Masses can be measured with exquisite accuracy using on-line Penning trap spectrometers at a beam intensity as low as a few ions/min. Lifetimes are required at a typically 10% accuracy and again this is best done with implanted low-energy ions and can be obtained with a few ions/min, as can be beta-delayed neutron emission probability using a suitable detector system. An additional piece of information required, the neutron-capture cross-sections on neutron-rich nuclei, cannot easily be measured directly. Surrogate reactions such as (d,p) can however yield sufficient information to confirm whether the neutron capture rates are fast enough for the assumption of statistical equilibrium to be valid. Finally, when the neutron to seed ratio is high enough, it is expected that the rprocess will terminate with fission of very heavy neutronrich nuclei that will recycle some of the material back towards mid-mass nuclei. The fission barriers and fission distributions for heavy neutron-rich nuclei are essentially unknown. Some initial information moving towards this region could be obtained from fusion–fission studies using very neutron-rich beams at Coulomb barrier energy on stable targets. Beam intensities need to be larger here, of the order of 106 ions/s or more. Summarizing the needs listed above, one concludes that a first generation of experiments to obtain essential cursory information to determine if nuclear structure is significantly modified in neutron-rich nuclei and provide critical input for r-process calculations can be obtained with neutron-rich beams at energy from rest up to 10–15 MeV/u and with intensities varying from a few ions/min up to 106 ions/s. The experiments that must be done require current state-of-the-art equipment that is available at a facility like ATLAS. Providing thorough answers requires much more and will have to await the next generation very large

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facilities, but for an initial foray in this region highlighting the main features to be expected this is sufficient. The CARIBU project aims at providing these initial capabilities. 3. The CARIBU project Providing these beams requires a source capable of delivering a wide array of neutron-rich isotopes at moderate intensities. Spontaneous fission sources can provide the required flux of isotopes but these have to be transformed into a usable beam. Using a 1 Ci 252Cf source in a large gas catcher of the type developed at Argonne for the RIA project would deliver the required low-energy neutron-rich beams. The ATLAS superconducting linac will be used to post-accelerate these beams to the required energy and most required equipment for these initial experiments is already available on the floor of the existing experimental areas. An overall layout of the ATLAS facility with the proposed CARIBU upgrade is shown in Fig. 1. The tasks to be accomplished are: (1) the production of the shortlived neutron-rich isotopes, (2) the extraction, beam formation and mass separation of the selected isotopes, (3) the post-acceleration and delivery to experiments, and (4) insure the availability of the required instruments to perform the experiments. The approach to perform each task is described below.

3.1. Radioactive beam production The gas catcher system developed initially at Argonne for injecting short-lived isotopes into the CPT Penning trap mass spectrometer enables the stopping of fast recoil ions into a gas catcher and their rapid and efficient extraction to form a good optical property low-energy beam. This technique is essentially universal [12] and was applied successfully on isotopes of over 40 different chemical species ranging from the volatile to the most refractory. The technique was extended to larger gas catcher with the RIA gas catcher prototype developed and tested first on-line at ATLAS [13] and then moved and tested successfully at high energy [14] behind the FRS fragment separator at GSI. CARIBU will use such a gas catcher to stop fission recoils from a 1 Ci 252Cf source and extract them as a low-energy beam. 252Cf has a 3.1% spontaneous fission branch and a 2.64 yr lifetime so that a strong enough source can be produced that will still have acceptably low energy loss for the recoils. The CARIBU gas catcher required to stop these recoils is similar in volume to the RIA gas catcher prototype with a length of 80 cm, an inner diameter of 50 cm and an Helium gas pressure of 200 mbar. It will provide access to all species produced in the Californium fission with a mean delay time in the gas catcher of typically 20 ms. This includes the species that are not amenable to the standard ISOL approach. The fission peaks populated by Californium fission are also offset from those obtained with

Fig. 1. Layout of the ATLAS accelerator facility with the new CARIBU building and ion source.

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Uranium fission used in all facilities trying to produce neutron-rich isotopes so that a complementary set of nuclei becomes accessible. The source will be produced at the HIFR reactor in Oak Ridge, deposited on a flat 2 mm thick tantalum backing, similar to that used in [15], and transported to Argonne in a commercial transport cask, where it will be mounted on a shielded plug for installation into the gas catcher. Typically 50% of the recoils from the fission source will be stopped in the gas catcher which extracts roughly 40% of those as beam so that a total of 20% of the total activity from the fission source is converted into beam. A plot of the expected intensity for the low-energy beams is shown in Fig. 2. The gas catcher is heavily shielded to maintain a radiation dose rate below 1 mrem/h at 30 cm from the surface with the source inserted. The gas catcher in CARIBU must operate at high ionization density. About 2  1010 alpha particles and 109 fission recoils per second from the Californium source will deposit energy in the gas catcher. Operation with such a high incoming ion rate had to be demonstrated. A test was setup using a new high-intensity beamline at ATLAS to generate the ionization. A high-intensity 58Ni beam at 260 MeV hitting a rotating target of 58Ni creates radioactive recoils that are focused by a 68 cm diameter bore superconducting solenoid on a large gas catcher modified for high-intensity operation. A movable beamstop at zero degrees stops the bulk of the primary beam and selects the fraction of the large angle scattered beam particles that is transported and focused together with the reaction products. The reaction products are thermalized in the gas catcher and extracted by a combination of DC fields, RF fields and gas flow. The radioactivity extracted from the gas catcher is detected by a silicon detector after transport through a radio-frequency quadru-

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pole (RFQ) and is counted as a function of the primary beam intensity. The measured efficiency, defined as the ratio of radioactive ions extracted as beam from the gas catcher to those that stop in the gas catcher volume, is plotted as a function of the incoming ion intensity in Fig. 3. The high-intensity gas catcher uses a novel RF focusing structure over the full body of the device that allows efficient operation at much higher incoming ion current than previously demonstrated using DC only approaches [16] or only limited RF focusing [17]. The demonstrated operation characteristics are sufficient for CARIBU and approach the RIA operation regime [18]. 3.2. Beam formation and purification The gas catcher and Californium source are located inside a shielded cask on a large high-voltage platform together with RFQs, an isobar separator and beam transport systems. The ions extracted from the gas catcher are focused in two RFQ sections forming a differential pumping section where the helium gas also extracted from the gas catcher is removed. The ions are cooled in this section and then accelerated electrostatically by a 50 kV potential to form a beam that is then matched to a high-resolution isotope separator. The excellent emittance of the beams extracted from the RFQs [19] allows one to build a high-resolving power isotope separator that still fits on a high-voltage platform. The isotope separator [20] is designed to obtain a mass resolution of 20,000 using a matching section to prepare the beam in a vertical slit geometry, an electrostatic doublet to magnify the beam in the dispersive plane, two high-homogeneity magnetic dipoles (with an electrostatic multipole in between) providing a total deflection angle of 120°, followed

Dy 108

Gd 106

Sm

92

104

Nd 102

Ce 100

Ba

98

Xe

94

96

92

Te 90

Sn

88

Cd

84 86

Pd

80

82

78

Ru 76

Mo

74

Zr

>10

72 70

Sr 66 Kr 64 Se 60 58

Ge 56 54

Zn 52

62

68

Extracted fission Product yield (ions/second)

r-process path

10 10

5

-10 6

4

-10 5

3

4

10 -10 03 10 -10 3

Ni 28

30

32

34

36

38

40

42

44

46

48

6

50

Fig. 2. Calculated mass separated yield for the various neutron-rich isotopes available at low-energy at CARIBU.

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G. Savard et al. / Nucl. Instr. and Meth. in Phys. Res. B 266 (2008) 4086–4091 ANL high-intensity gas catcher test: extraction efficiency vs incidentionrate

extraction efficiency (%)

40 beamstop mid-location2 beamstop mid-location beamstop near beamstop far back exponential falloff DC catcher ref [16] RF catcher ref [17]

35 30 25 20 15 10 5 0 1.0E+01

1.0E+03

1.0E+05

1.0E+07

1.0E+09

incidention rate (ion/sec) Fig. 3. Extraction efficiency measured with the modified full-RF RIAprototype gas catcher. Increasing ionization regimes are obtained by moving the zero degree beamstop away from the production target so that a larger fraction of the scattered beam enters the gas catcher, together with the reaction products, in position ‘‘beamstop far back” than in position ‘‘beamstop near”. Also shown are published results for other gas catcher of similar size and stopping power using DC fields only [16] or a combination and DC fields with RF fields only in the extraction region [17].

by another electrostatic dipole and a matching section to return the beam to a cylindrical shape. It is essentially one stage of the two-stage separator [21] designed at Argonne for the RIA facility. After mass separation, the beam enters a switchyard from which it can be directed to either a lowenergy experimental area where diagnostics for yield optimization are available or low-energy experiments can be performed, or to a charge-breeder system to prepare these beams for post-acceleration. A layout of the CARIBU building with the high-voltage platform and the required equipment is shown in Fig. 4. 3.3. Post-accelerator Obtaining Coulomb barrier energy beams from these low-energy beams requires an efficient post-accelerator.

CARIBU will use an ECR charge breeder to increase the charge state of the radioactive ions so that they can then be fed directly into the ATLAS superconducting linac. Charge breeding efficiency [22–24] in excess of 10% for gaseous species and around 5% for solids have been demonstrated and have been used in our yield calculations. The back end of the ECR-1 ion source at ATLAS has been modified to provide access for injecting the 1+ beams inside the ECR plasma when operating the source as a charge-breeder. The ECR-1 high-voltage platform will operate at the same voltage as the CARIBU platform. The ECR-1 ion source and the gas catcher/RFQ system will be tied together at 50 kV above that potential with a tunable lower voltage supply fine tuning the relative potential of the two to optimize capture of the mass separated 1+ ion beam into the ECR source plasma region. The chargebred ions are then extracted from the normal extraction end of the source and mass separated on the platform before being accelerated, bunched and injected into the ATLAS linac in a fashion similar to that used for stable beams. The ATLAS accelerator is currently completing an energy upgrade where the last cryostat is being replaced by a RIA-type cryostat and eight new resonators that will increase the total energy of the machine to cover the range required for the CARIBU physics program without the need for stripping. Little other additional modifications of the facility are required except for the addition of lowintensity diagnostics, both scanners and Faraday cups, to ease the tuning of the weak radioactive beams. 3.4. Instrumentation The ATLAS facility is already equipped with a suite of state-of-the-art experimental equipment to allow most of the experimental program described above to be performed. A silicon detector array has already been used in combination with the FMA mass analyser to perform a (d,p) reaction with a 58Ni radioactive beam. Such a setup

Fig. 4. Layout of the new CARIBU building. The high-voltage platform holding the source shielded cask, the gas catcher and RFQs, the isobar separator and the switchyard is shown on the right. The open area on the left is a low-energy experimental area for diagnostics and mass and lifetime measurements. The beamline going downward from the switchyard leads to the ECR charge-breeder and the ATLAS accelerator.

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is quite suitable for initial single- and two-nucleon transfer reaction studies. GAMMASPHERE is still a state-of-theart gamma-ray detector array for the Coulomb excitation work. The CPT Penning trap mass spectrometer will be moved to the low-energy experimental area just off the CARIBU high-voltage platform and will be used to perform the mass measurements required by the astrophysics part of the program. A tape station exists for initial diagnostics and lifetime measurements. In addition, a new reaction spectrometer, the HELIOS [9] solenoidal spectrometer, is under construction. It will use a large bore superconducting solenoid to refocus the light particles emitted in transfer reaction to the beam axis where they will be detected by a cylindrical silicon detector array. This approach provides a larger solid angle coverage, clean identification, and a better energy resolution since the kinematic compression of the energy scale in the laboratory is not present in this approach. A new tape station will also be constructed with better detector coverage and faster transport times. Finally, the possibility of adding a colinear laser spectroscopy system is being considered. 4. Current status The CARIBU upgrade project was funded in January 2006. The new building is now completed, the high-voltage platform is in place, the gas catcher and its shielding under construction, the magnets for the isobar separator ordered and the modifications to the ECR source completed. The project is on schedule to provide low-energy neutron-rich beams during 2008 and re-accelerated beams in early 2009. The facility will provide low-energy beams with intensities up to above 107 ions/s and re-accelerated beams at up to 106 ions/s. The second ECR source at ATLAS will remain in normal operation so that stable beam are still available from ATLAS after the start of operation of CARIBU. The additional capabilities brought about by the CARIBU upgrade should lead to significant progress

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in our understanding of the changes in the nuclear structure of neutron-rich nuclei and their implications for the r-process. This work was supported by the US Department of Energy, Office of Nuclear Physics, under contract no DEAC02-06CH11357. References [1] I. Tanihata, these Proceedings, Nucl. Instr. and Meth. B 266 (2008) 4067. [2] H.R. Ravn, B.W. Allardice, On-line mass separators, in: D.A. Bromley (Ed.), Treatise on Heavy-Ion Science, Plenum Press, New York, 1989, ISBN 0-306-42949-7. [3] I. Hamamoto, X.Z. Zhang, Phys. Rev. C 52 (1995) R2326. [4] I. Tanihata et al., Phys. Rev. Lett. 55 (1985) 2676. [5] J. Dobaczewski et al., Phys. Rev. Lett. 72 (1994) 981. [6] K.-L. Kratz et al., Astrophys. J. 403 (1993) 216. [7] . [8] J.P. Schiffer et al., Phys. Rev. 115 (1959) 427. [9] A.H. Wuosmaa, J.P. Schiffer, B.B. Back, C.J. Lister, K.E. Rehm, Nucl. Instr. and Meth. A 580 (2007) 1290. [10] D. Radford et al., Phys. Rev. Lett. 88 (2002) 22501. [11] Th. Kroell et al., Eur. Phys. J. 150 (2007) 127. [12] G. Savard et al., Nucl. Instr. and Meth. B 204 (2003) 582. [13] W. Trimble et al., Nucl. Phys. A 746 (2004) 415C. [14] M. Petrick, et al., these Proceedings, Nucl. Instr. and Meth. B 266 (2008) 4493. [15] R.A. Anderl et al., in: Proceedings of the 27th Conference on Remote Systems Technology, 1979, p. 347. [16] L. Weissman et al., Nucl. Instr. and Meth. A 540 (2005) 245. [17] A. Takamine et al., Rev. Nucl. Instr. 76 (2005) 103503. [18] G. Savard et al., in preparation. [19] F. Herfurth et al., Nucl. Instr. and Meth. A 469 (2001) 254. [20] C.N. Davids, et al., these Proceedings, Nucl. Instr. and Meth. B 266 (2008) 4449. [21] M. Portillo, J.A. Nolen, T.A. Barlow, in: P. Lucas, S. Webber (Eds.), Proceedings of the 2001 IEEE Particle Accelerator Conference, Chicago, IL, June 18–22, 2001, p. 3015. [22] T. Lamy et al., Rev. Sci. Instr. 73 (2) (2002) 717. [23] T. Lamy et al., in: Proceedings EPAC 2002, Paris, France, 3–7 June 2002, p. 1724. [24] F. Wenander, Nucl. Phys. A 746 (2004) 40c.