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Study of resonant reactions with radioactive ion beams
Nuclear Instruments and Methods in Physics Research B 172 (2000) 647±654
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Study of resonant reactions with radioactive ion beams A. Galindo-Uribarri a,*, J. Gomez del Campo a, J.R. Beene a, C.J. Gross b,a, J.F. Liang a, S.D. Paul b, D. Shapira a, D.W. Stracener a, R.L. Varner a, E. Chavez c, A. Huerta c, M.E. Ortiz c, E. Padilla d, S. Pascual d a
Physics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6371, USA b Oak Ridge Institute for Science and Education, Oak Ridge, TN 37831, USA c Instituto de Fisica, UNAM, 04510 Mexico, D.F., Mexico d Instituto de Ciencias Nucleares, UNAM, 04510 Mexico, D.F., Mexico
Abstract A fast and ecient method to study (p,p) and (p,a) resonances with radioactive beams in inverse kinematics is described. It is based on the use of thick targets and large area double-sided silicon strip detectors (DSSDs) to detect the recoiling light-charged particles and to determine precisely their scattering angle. The ®rst nuclear physics experiments with the technique have been performed recently at the Holi®eld Radioactive Ion Beam Facility (HRIBF) at Oak Ridge with stable beams of 17 O and radioactive beams of 17 F. The high-quality resonance measurements obtained demonstrate the capabilities of the technique. Pure 17 F beams from HRIBF were produced by fully stripping the ions and separating the interfering and more abundant 17 O ions by the beam transport system. The removal of interfering isobars is one of the various common challenges to both accelerator mass spectrometry (AMS) and radioactive ion beam (RIB) production. Experiments done with RIBs will bene®t from the use of the most ecient techniques for production, isobar separation, transport and detection. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 25.40.Ny; 29.40.Wk Keywords: Radioactive ion beams; Resonance scattering; Silicon microstrip detectors
1. Introduction The advent of radioactive ion beam (RIB) facilities will provide exciting opportunities to pursue new frontiers in nuclear structure and * Corresponding author. Tel.: +1-423-574-6124; fax: +1-423574-8902. E-mail address: [email protected] (A. GalindoUribarri).
reactions physics. Radioactive projectiles remove the limitations to the naturally occurring N =Z ratios of stable beams. It is now becoming possible to systematically study properties of nuclei over a wide range of N =Z. Of particular interest are loosely bound nuclei near the limits of nuclei existence which are expected to exhibit modi®ed shell structures and new collective modes. Compared to conventional stable heavy-ion beams, RIB intensities will be several orders of magnitude lower,
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 2 2 0 - 2
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A. Galindo-Uribarri et al. / Nucl. Instr. and Meth. in Phys. Res. B 172 (2000) 647±654
particularly for those further from stability. Certain elements will be easier to produce as RIBs than others. New instrumentation, improved techniques and a closer interaction between experimenters and accelerator operations sta are required to fully exploit these exotic beams. In the ®rst part of this paper, we describe the production of pure beams of 17 F at Holi®eld Radioactive Ion Beam Facility (HRIBF) suitable for the study of resonant scattering experiments and discuss common problems encountered in accelerator mass spectrometry (AMS) and RIBs. In the second part, we describe a very ecient technique for the study of resonances using exotic beams. High-quality resonance measurements obtained with stable beams of 17 O and radioactive beams of 17 F demonstrate the capabilities of the technique. 2. Production of pure beams of
17
F
Radioactive beams of 17 F are now available for low-energy nuclear physics research from HRIBF at Oak Ridge National Laboratory (ORNL). The K 105 Oak Ridge Isochronous Cyclotron (ORIC) and the HRIBF 25-MV Tandem accelerator were recently recon®gured into a ®rst-generation ISOL-type RIB facility. A detailed description of HRIBF can be found in [1]. The HRIBF Tandem accelerator is the highest operating voltage electrostatic accelerator in the world. A record voltage of 32 MV was attained without accelerating tubes installed; tandem voltages up to 25.5 MV have been used for experiments. The ISOL method requires of two accelerators. The ®rst one (cyclotron) produces radioactive atoms by bombarding thick targets with high-intensity beams of light ions. The second machine (tandem) accelerates the radioactive species to energies suitable for nuclear or astrophysics studies. At HRIBF the two accelerators are coupled by a HV platform and a negative-ion transfer line. The HV platform contains the target-ion source assembly, a mass separator (M=DM 1000) and, if required, a charge exchange canal. RIB production is by light-ion fusion±evaporation reactions for proton-rich nuclei, and proton-induced ®ssion for neutron-rich nuclei.
Long-lived species such as 18 F (T1=2 110 m) or Ni (T1=2 6:1 d) can be produced in a batch mode, where the target is ®rst bombarded with light ions for a period comparable with the half-life and then transfered mechanically to the sample position of a Cs-sputter ion source. Production of 17 F (T1=2 64:5 s) beam is considerably more challenging. Due to its relevance for nuclear physics and astrophysics considerable eort has been devoted to its production. Two directions have been pursued at HRIBF to produce the negative beams of 17 F required for acceleration in the tandem. In one case, positive beams of molecular ions have been produced using the electron beam plasma ion source (EBPIS) [2] and then dissociated and charge exchanged to form 17 Fÿ , while in the other case, 17 Fÿ has been produced directly in a kinetic-ejection negative-ion source (KENIS) [3]. Fluorine-17 is produced by the 16 O(d,n)17 F reaction in a ®brous, refractory HfO2 target. Atomic ¯uorine does not transport readily through the EBPIS source. For the positive molecular-ion production mode, small quantities of Al vapor were introduced in the target region. A small yield of 17 F and a few other molecular ¯uorides were observed, but the dominant 17 F yield out of the source was Al17 F , which was then dissociated and charge exchanged in a Cs vapor canal. This resulted in a beam with a large energy spread, a poor emittance, but a very low 17 O contamination of 1%. A more promising approach is the use of the KENIS. The mechanism for negative-ion formation is secondary negative-ion emission based on velocity-dependent surface ionization [4]. The new negative-ion source has shown to be highly ecient in simultaneously dissociating and negatively ionizing the atomic ¯uorine constituents. With a 2lA deuteron beam from the ORIC, mass analyzed 17 ÿ F beams exceeding 8 106 ions/s have been extracted on-line from the negative-ion source and injected into the tandem accelerator. Beams of up to 5 105 17 F ions/s have been delivered to several RIB experiments at the HRIBF. The source operates very stably and reliably, and has demonstrated a continuous operational lifetime in excess of one month. Larger currents will be possible with a target assembly designed to sustain ®ve times larger driver beam currents. 56
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The main disadvantage of the negative-ion source is the production of 17 O at a level which is typically an order of magnitude larger than the 17 F. The emittance of the negative-ion source is about 70p mm mrad at 20 keV. This comparatively poor beam quality limits the performance of the high-resolution mass separator (nominal resolving power M=DM 20; 000) in the negativeion injection line to a value comparable to the isobaric mass dierence between 17 F and 17 O (M=DM 5600). Consequently 17 O and 17 F are injected together into the tandem. HRIBF provides a variety of equipment choices for beam transport and analysis and also instruments such as a gas-®lled Enge split-pole spectrograph [5], useful in the measurements of heavy recoils and in the rejection of unwanted particles. Gas-®lled magnets (GFM), a common tool in AMS to aid in isobar separation, have been applied with RIBs that have considerable contamination from a stable isobar (18 O:18 F of 1000:1) for some nuclear physics experiments at the ATLAS facility at Argonne National Laboratory. Details and limitations of this technique are discussed in [6]. For our study of resonances, we used the KENIS and produced pure beams of 33 MeV 17 F by fully stripping to 9 with a second carbon foil inserted on the beam path after the tandem. The 17 9 F was easily separated from the 17 O ions with the use of slits and the tandem energy-analyzing magnet, which has a mass resolving power of up to one part in 12,000. With this technique, a suppression factor of many orders of magnitude is easily obtained without adversely aecting beam quality. The yield of each charge state is velocity dependent. The charge state fraction of 9 ions of F stripping in a 20 lg=cm2 carbon foil at 33 MeV is about 15% [7]. The lowest practical energy to produce pure beams of 17 F by fully stripping is about 1.5A MeV. As the energy is increased, the fraction of F ions in the 9 charge state increases. Multiple scattering and energy loss in the second thin foil do not signi®cantly aect the beam. This technique of fully removing all the electrons is also used in AMS to separate the interfering isobaric ions by the beam transport system. Isobar suppression factors larger than 107 have
649
been achieved [8]. As it will be clear in the second part of this paper, the thick-target technique has the advantage that we can investigate resonances with beam energies signi®cantly above that needed to excite them allowing us to use fully stripped ions. The use of pure beams simpli®es considerably the experimental setup. Fluorine-17 beams of excellent quality can be produced at the HRIBF by using the direct negative-ion source, relaxing the low-energy isobar separation conditions, and using a second foil for fully stripping for energies between 1.5A MeV and the maximum energy of about 11A MeV. Below 1.5A MeV, one could make use of the positive-ion source or separate the isobars in the target area with a tool such as a GFM, or use coincidences with the recoils detected on a gas counter [9]. Alternative techniques to enhance the wanted species based on atomic physics are highly desirable [10]. While some of these techniques might be of very limited use in AMS, one or two orders of magnitude of suppresion of the unwanted species can be very important for RIB physics.
3. Study of resonances using thick targets and DSSDs The spectroscopy of very light nuclei (A 20) has experienced renewed interest [11,12] primarily due to the appearance of RIBs. The use of resonance scattering with thin targets using normal kinematics has been a very useful spectroscopic tool in nuclear physics for many years. Elastic scattering of protons (polarized and unpolarized) from light nuclei has provided important information on the spin-parity and isospin assignments of the resonant states involved. These measurements have consisted mainly of excitation functions at a few angles. Even for these ``conventional'' techniques, the measurement of excitation functions of cross-sections and analyzing powers with small energy steps are very time consuming [13]. The most ecient methods are desirable for the study of resonances using the limited RIB intensities available.
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3.1. Method We have demonstrated a technique that allows one to measure good quality excitation functions with RIBs. Several resonances can be studied simultaneously using a single bombarding energy. The use of a thick target with inverse kinematics is discussed in [14]. Inverse resonance scattering was studied with a thick-gas target in 10 C p [11] and a thick-solid target in 13 C p [15] with a small silicon detector. In this work, we combine the use of inverse kinematics, thick targets, large acceptance detectors with high-granularity and highenergy resolution double-sided silicon strip detector (DSSDs), and a microchannel plate (MCP) timing detector to suppress the background associated with RIBs. Fig. 1 shows a schematic illustration of the setup used. Starting with an inverse reaction, the beam passes through the MCP and stops in a target with a uniform concentration of the light nuclei of interest (e.g., H). The technique requires a target thick enough to stop the beam ions or degrade its energy below the region of interest, but thin enough to let the recoil particles out. The recoiling light-charged particles are eciently detected with a high-granularity detector covering a very large solid angle to determine precisely their
Fig. 1. A schematic drawing of the experimental geometry. A time signal is derived from a carbon-foil MCP detector, which serves as a beam monitor and is used to remove unwanted bparticles from the radioactive decay of the beam. A large area silicon DSSD detects the recoiling light-charged particles and determines precisely their scattering angle.
scattering angle and recoiling energy. The measured energy of the light-recoiling particle on a pixel at an angle H can be transformed into centerof-mass spectra after taking into consideration the speci®c energy losses and the two-body kinematics and can be integrated over the azimuthal angle. With this technique measurements can be done at 180° in the center-of-mass that cannot be done with normal kinematics. A proof-of-principle of the technique was carried out using stable beams. Excited states of 18 F were studied with the inverse reaction p(17 O; p17 O. Excitation functions of cross-sections obtained for three bombarding energies are shown in Fig. 2. For comparison, an excitation function measured by Sens et al. [16] with the ``standard'' technique (normal kinematics and thin-target) at an equivalent backward angle is included. It should be noticed that the lower resonances are clearly visible at all the energies even if the incident beam energy is signi®cantly above that needed to excite them. Note the similar shape of the excitation functions obtained in the common range of energies. 3.2. Targets It is well known that plastic targets have the disadvantage that they allow only a limited amount of beam current because of the destruction of the foil and the loss of hydrogen. It is also important to produce H targets as uniform as possible in composition across its thickness. We used polypropylene foils (CH2 n mounted on a large target frame so that we could frequently vary the beam spot, when bombarded with stable beams. For lower intensity RIB beams this is not a problem. The beam spot was typically 1±2 mm diameter. We monitored the count rate on the DSSD by taking the OR of all the strip detector signals. This gives us a clear indication of any loss of hydrogen and also helps to monitor the beam. With the 17 O beam, we moved the target ladder to a new spot about every 10 min making sure that the rate on the detector remained constant. We are currently investigating the use of other targets with a high H content. The use of ZrH
A. Galindo-Uribarri et al. / Nucl. Instr. and Meth. in Phys. Res. B 172 (2000) 647±654
651
ally, a ZrH foil was bombarded for 2 h with 3 pnA of 17 O without moving the beam spot. For this long run, the count rate remained constant indicating that the specimen retained its hydrogen. Targets with optimal thickness will be prepared for use in future experiments. 3.3. DSSD
Fig. 2. Excitation functions at HCM 178° obtained in inverse scattering for 14, 20 and 33 MeV bombarding energies for the p(17 O; p 17 O reaction using the thick-target technique described in this paper. For comparison, an excitation function in a similar energy range for HCM 162° obtained in normal kinematics 17 O p; p 17 O is shown (adapted from [16]).
targets looks very promising as they can be loaded to very high H concentrations (Zr:H 1:1.6) and are very stable at high-beam currents. Foils of ZrH with thicknesses of the order of 20 lm were bombarded with 20 MeV 17 O ions for various beam currents up to an intensity of 10 pnA. The count rate of the recoiling protons in the DSSD scaled linearly with the beam current. Addition-
One detector that is ideal for the study of resonances with RIBs is a DSSD. With proper instrumentation they can deliver energy, time and position information even if several particles strike the detector simultaneously. A large 300-lm thick DSSD [17], 5 cm 5 cm in size with 16 strips per side was used to detect the recoil protons. The DSSD was mounted 12 cm behind the target. It covered the center-of-mass angles from 180° to 160°. This strip arrangement results in a total of 256 pixels, each eectively acting as an individual detector. The X±Y position resolution of 3 mm is determined by the size of each pixel. A system has been developed for the individual readout of each of the 32 segments. The system comprises two preampli®er units combined with a customized shaper ampli®er with a CAMACcontrolled variable gain. The preampli®er module contains 16 channels of charge-sensitive preampli®ers, each with energy and timing outputs. The shaped signals were digitized with peak sensing CAMAC ADCs.
4. Fluorine-17 experiment An experiment to study resonant states in 18 Ne testing the technique presented in this work was succesfully done at the HRIBF. A post acceleration stripper was used to produce a 17 F9 beam with intensities on the order of 104 ions/s with no contamination from the 17 O isobar. Fluorine-17 ions at a bombarding energy of 33 MeV were stopped in a 40-lm CH2 n target. With the use of the DSSD, an excitation function was measured covering the energy range of 400 keV to 1.4 MeV in the center of mass.
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4.1. Background removal RIBs also bring with them their own problems such as low intensities and potentially high backgrounds caused by the radioactivity of the beam. Detection systems must be able to cope with this background problem. On an early test run, permanent magnets placed beside the target were used to produce a strong magnetic ®eld, which partially swept the positrons from the detection area. The strength of the ®eld was not sucient to eliminate the positrons because of the proximity of the DSSD to the target. In the ®nal experiment con®guration, a MCP was placed at the entrance to the scattering chamber to monitor the beam and to provide a timing signal used to gate the proton detector. The MCP can stand a large counting rate of at least 106 ions/s. This coincidence technique eliminated the positron background which results from the decay of 17 F ions stopped in the target (see Fig. 3). a-particles from a very low level of contamination present on the surface of the scattering chamber are visible on
the high-energy side of the singles spectrum. This background was also easily eliminated with the coincidence requirement. 4.2. Results Fig. 4 shows an excitation function obtained in a single exposure of about 16 h, covering the center of mass energy range of 400±1400 keV in 5 keV steps. The resonance recently observed [9] at center of mass energy 600 keV and spin parity J p 3 and a width of 18 keV has been con®rmed. Additional resonances at energies of 1184 and 1231 keV were observed having characteristics consistent with 2 and 3ÿ spin parity assignements, respectively. The data were analyzed using the Rmatrix code MULTI described in [18]. The strongest resonances observed (3 at 600 keV and 2 at 1184 keV) were ®tted with an lp 0, which corresponds to an s1=2 proton coupled to the d5=2 proton of the 17 F ground-state con®guration. Further experiments with improved statistics to explore higher lying resonances are being planned.
Fig. 3. Comparison of recoil proton spectra from the reaction p(17 F,p)17 F taken in singles (dots) and in coincidence (crosses) with the MCP detector. Vertical arrows show the position of resonances studied in this work. The coincidences with the MCP eectively eliminates the background from positrons (low energy) originating from the 17 F deposited in the target and from the alphas (high energy) coming from contamination in the chamber.
A. Galindo-Uribarri et al. / Nucl. Instr. and Meth. in Phys. Res. B 172 (2000) 647±654
Fig. 4. Resonant scattering excitation function for
5. Conclusion A fast and ecient method to study resonances with radioactive beams and light nuclei in inverse kinematics has been demonstrated. It is based on the use of thick targets and large area DSSDs. The ®rst nuclear physics experiment with this technique has been performed at HRIBF. Beams of 17 F with excellent purity and optical quality can be produced at the HRIBF in its full energy range by using a variety of equipment choices for beam production, transport and analysis. Note added during the preparation of this manuscript. In recent experiments at HRIBF, a beam of almost 108 17 Fÿ ions/s was produced by the KENIS, resulting in a pure 17 F9 beam of 2 106 ions/s after acceleration by the tandem.
Acknowledgements We wish to thank the HRIBF Operations and Facility Support Personnel for the production of the 17 F beams. This work has bene®tted from discussions with A.E. (Ted) Litherland (IsoTrace)
17
653
F+p elastic scattering at HCM 178° (Eb 33 MeV).
and D. Khatamian (AECL), who has also provided the ZrH targets. Oak Ridge National Laboratory is managed by Lockheed Martin Energy Research Corp. for the US Department of Energy (DOE) under contract DE-AC05-96OR22464. This research was also supported by DOE through Contract Nos. DE-AC05-76OR00033 with Oak Ridge Institute for Science and Education.
References [1] R. Auble, in: J.L. Duggan, I.L. Morgan (Eds.), Proceedings of the Applications of Accelerators in Research and Industry, AIP Press, New York, CP475, 1999, p. 292. [2] G.D. Alton, Nucl. Instr. and Meth. A 382 (1996) 207. [3] G.D. Alton, Y. Liu, C. Williams, S.N. Murray, in: K.W. Shepard (Eds.), Heavy Ion Accelerator Technologies: Eighth International Conference, AIP Press, New York, 1999, p. 330. [4] J.K. Norskov, B.I. Lundqvist, Phys. Rev. B 19 (1979) 5661. [5] J.F. Liang et al., Nucl. Instr. and Meth. A 435 (1999) 393. [6] K.E. Rehm et al., Nucl. Instr. and Meth. A 370 (1996) 438. [7] K. Shima, N. Kuno, M. Yamanouchi, H. Tawara, At. Data Nucl. Data Tables 51 (1992) 173.
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[8] P.W. Kubik, G. Korschinek, E. Nolte, Nucl. Instr. and Meth. B 1 (1984) 51. [9] D.W. Bardayan et al., Phys. Rev. Lett. 83 (1999) 45. [10] I. Tomski, A.E. Litherland, Private communication. [11] L. Axelsson et al., Phys. Rev. C 54 (1996) 1511. [12] K.E. Rehm et al., Phys. Rev. C 52 (1995) 460. [13] S.E. Darden, S. Sen, G. Murillo, M. Fernandez, J. Ramirez, A. Galindo, P.L. Jolivette, B.P. Hichwa, Nucl. Phys. A 429 (1984) 218.
[14] K.P. Artemov et al., Sov. J. Nucl. Phys. 52 (1990) 408. [15] A. Huerta Hernandez et al., Nucl. Instr. and Meth. B 143 (1998) 569. [16] J.C. Sens, F. Rietsch, A. Pape, R. Armbruster, Nucl. Phys. A 199 (1973) 232. [17] Micron Semiconductor Ltd., Lancing, Sussex BN15 8UN, England. [18] R.O. Nelson, E.G. Bilpuch, G.E. Mitchell, Nucl. Instr. and Meth. A 236 (1985) 128.
www.elsevier.nl/locate/nimb
Study of resonant reactions with radioactive ion beams A. Galindo-Uribarri a,*, J. Gomez del Campo a, J.R. Beene a, C.J. Gross b,a, J.F. Liang a, S.D. Paul b, D. Shapira a, D.W. Stracener a, R.L. Varner a, E. Chavez c, A. Huerta c, M.E. Ortiz c, E. Padilla d, S. Pascual d a
Physics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6371, USA b Oak Ridge Institute for Science and Education, Oak Ridge, TN 37831, USA c Instituto de Fisica, UNAM, 04510 Mexico, D.F., Mexico d Instituto de Ciencias Nucleares, UNAM, 04510 Mexico, D.F., Mexico
Abstract A fast and ecient method to study (p,p) and (p,a) resonances with radioactive beams in inverse kinematics is described. It is based on the use of thick targets and large area double-sided silicon strip detectors (DSSDs) to detect the recoiling light-charged particles and to determine precisely their scattering angle. The ®rst nuclear physics experiments with the technique have been performed recently at the Holi®eld Radioactive Ion Beam Facility (HRIBF) at Oak Ridge with stable beams of 17 O and radioactive beams of 17 F. The high-quality resonance measurements obtained demonstrate the capabilities of the technique. Pure 17 F beams from HRIBF were produced by fully stripping the ions and separating the interfering and more abundant 17 O ions by the beam transport system. The removal of interfering isobars is one of the various common challenges to both accelerator mass spectrometry (AMS) and radioactive ion beam (RIB) production. Experiments done with RIBs will bene®t from the use of the most ecient techniques for production, isobar separation, transport and detection. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 25.40.Ny; 29.40.Wk Keywords: Radioactive ion beams; Resonance scattering; Silicon microstrip detectors
1. Introduction The advent of radioactive ion beam (RIB) facilities will provide exciting opportunities to pursue new frontiers in nuclear structure and * Corresponding author. Tel.: +1-423-574-6124; fax: +1-423574-8902. E-mail address: [email protected] (A. GalindoUribarri).
reactions physics. Radioactive projectiles remove the limitations to the naturally occurring N =Z ratios of stable beams. It is now becoming possible to systematically study properties of nuclei over a wide range of N =Z. Of particular interest are loosely bound nuclei near the limits of nuclei existence which are expected to exhibit modi®ed shell structures and new collective modes. Compared to conventional stable heavy-ion beams, RIB intensities will be several orders of magnitude lower,
0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 2 2 0 - 2
648
A. Galindo-Uribarri et al. / Nucl. Instr. and Meth. in Phys. Res. B 172 (2000) 647±654
particularly for those further from stability. Certain elements will be easier to produce as RIBs than others. New instrumentation, improved techniques and a closer interaction between experimenters and accelerator operations sta are required to fully exploit these exotic beams. In the ®rst part of this paper, we describe the production of pure beams of 17 F at Holi®eld Radioactive Ion Beam Facility (HRIBF) suitable for the study of resonant scattering experiments and discuss common problems encountered in accelerator mass spectrometry (AMS) and RIBs. In the second part, we describe a very ecient technique for the study of resonances using exotic beams. High-quality resonance measurements obtained with stable beams of 17 O and radioactive beams of 17 F demonstrate the capabilities of the technique. 2. Production of pure beams of
17
F
Radioactive beams of 17 F are now available for low-energy nuclear physics research from HRIBF at Oak Ridge National Laboratory (ORNL). The K 105 Oak Ridge Isochronous Cyclotron (ORIC) and the HRIBF 25-MV Tandem accelerator were recently recon®gured into a ®rst-generation ISOL-type RIB facility. A detailed description of HRIBF can be found in [1]. The HRIBF Tandem accelerator is the highest operating voltage electrostatic accelerator in the world. A record voltage of 32 MV was attained without accelerating tubes installed; tandem voltages up to 25.5 MV have been used for experiments. The ISOL method requires of two accelerators. The ®rst one (cyclotron) produces radioactive atoms by bombarding thick targets with high-intensity beams of light ions. The second machine (tandem) accelerates the radioactive species to energies suitable for nuclear or astrophysics studies. At HRIBF the two accelerators are coupled by a HV platform and a negative-ion transfer line. The HV platform contains the target-ion source assembly, a mass separator (M=DM 1000) and, if required, a charge exchange canal. RIB production is by light-ion fusion±evaporation reactions for proton-rich nuclei, and proton-induced ®ssion for neutron-rich nuclei.
Long-lived species such as 18 F (T1=2 110 m) or Ni (T1=2 6:1 d) can be produced in a batch mode, where the target is ®rst bombarded with light ions for a period comparable with the half-life and then transfered mechanically to the sample position of a Cs-sputter ion source. Production of 17 F (T1=2 64:5 s) beam is considerably more challenging. Due to its relevance for nuclear physics and astrophysics considerable eort has been devoted to its production. Two directions have been pursued at HRIBF to produce the negative beams of 17 F required for acceleration in the tandem. In one case, positive beams of molecular ions have been produced using the electron beam plasma ion source (EBPIS) [2] and then dissociated and charge exchanged to form 17 Fÿ , while in the other case, 17 Fÿ has been produced directly in a kinetic-ejection negative-ion source (KENIS) [3]. Fluorine-17 is produced by the 16 O(d,n)17 F reaction in a ®brous, refractory HfO2 target. Atomic ¯uorine does not transport readily through the EBPIS source. For the positive molecular-ion production mode, small quantities of Al vapor were introduced in the target region. A small yield of 17 F and a few other molecular ¯uorides were observed, but the dominant 17 F yield out of the source was Al17 F , which was then dissociated and charge exchanged in a Cs vapor canal. This resulted in a beam with a large energy spread, a poor emittance, but a very low 17 O contamination of 1%. A more promising approach is the use of the KENIS. The mechanism for negative-ion formation is secondary negative-ion emission based on velocity-dependent surface ionization [4]. The new negative-ion source has shown to be highly ecient in simultaneously dissociating and negatively ionizing the atomic ¯uorine constituents. With a 2lA deuteron beam from the ORIC, mass analyzed 17 ÿ F beams exceeding 8 106 ions/s have been extracted on-line from the negative-ion source and injected into the tandem accelerator. Beams of up to 5 105 17 F ions/s have been delivered to several RIB experiments at the HRIBF. The source operates very stably and reliably, and has demonstrated a continuous operational lifetime in excess of one month. Larger currents will be possible with a target assembly designed to sustain ®ve times larger driver beam currents. 56
A. Galindo-Uribarri et al. / Nucl. Instr. and Meth. in Phys. Res. B 172 (2000) 647±654
The main disadvantage of the negative-ion source is the production of 17 O at a level which is typically an order of magnitude larger than the 17 F. The emittance of the negative-ion source is about 70p mm mrad at 20 keV. This comparatively poor beam quality limits the performance of the high-resolution mass separator (nominal resolving power M=DM 20; 000) in the negativeion injection line to a value comparable to the isobaric mass dierence between 17 F and 17 O (M=DM 5600). Consequently 17 O and 17 F are injected together into the tandem. HRIBF provides a variety of equipment choices for beam transport and analysis and also instruments such as a gas-®lled Enge split-pole spectrograph [5], useful in the measurements of heavy recoils and in the rejection of unwanted particles. Gas-®lled magnets (GFM), a common tool in AMS to aid in isobar separation, have been applied with RIBs that have considerable contamination from a stable isobar (18 O:18 F of 1000:1) for some nuclear physics experiments at the ATLAS facility at Argonne National Laboratory. Details and limitations of this technique are discussed in [6]. For our study of resonances, we used the KENIS and produced pure beams of 33 MeV 17 F by fully stripping to 9 with a second carbon foil inserted on the beam path after the tandem. The 17 9 F was easily separated from the 17 O ions with the use of slits and the tandem energy-analyzing magnet, which has a mass resolving power of up to one part in 12,000. With this technique, a suppression factor of many orders of magnitude is easily obtained without adversely aecting beam quality. The yield of each charge state is velocity dependent. The charge state fraction of 9 ions of F stripping in a 20 lg=cm2 carbon foil at 33 MeV is about 15% [7]. The lowest practical energy to produce pure beams of 17 F by fully stripping is about 1.5A MeV. As the energy is increased, the fraction of F ions in the 9 charge state increases. Multiple scattering and energy loss in the second thin foil do not signi®cantly aect the beam. This technique of fully removing all the electrons is also used in AMS to separate the interfering isobaric ions by the beam transport system. Isobar suppression factors larger than 107 have
649
been achieved [8]. As it will be clear in the second part of this paper, the thick-target technique has the advantage that we can investigate resonances with beam energies signi®cantly above that needed to excite them allowing us to use fully stripped ions. The use of pure beams simpli®es considerably the experimental setup. Fluorine-17 beams of excellent quality can be produced at the HRIBF by using the direct negative-ion source, relaxing the low-energy isobar separation conditions, and using a second foil for fully stripping for energies between 1.5A MeV and the maximum energy of about 11A MeV. Below 1.5A MeV, one could make use of the positive-ion source or separate the isobars in the target area with a tool such as a GFM, or use coincidences with the recoils detected on a gas counter [9]. Alternative techniques to enhance the wanted species based on atomic physics are highly desirable [10]. While some of these techniques might be of very limited use in AMS, one or two orders of magnitude of suppresion of the unwanted species can be very important for RIB physics.
3. Study of resonances using thick targets and DSSDs The spectroscopy of very light nuclei (A 20) has experienced renewed interest [11,12] primarily due to the appearance of RIBs. The use of resonance scattering with thin targets using normal kinematics has been a very useful spectroscopic tool in nuclear physics for many years. Elastic scattering of protons (polarized and unpolarized) from light nuclei has provided important information on the spin-parity and isospin assignments of the resonant states involved. These measurements have consisted mainly of excitation functions at a few angles. Even for these ``conventional'' techniques, the measurement of excitation functions of cross-sections and analyzing powers with small energy steps are very time consuming [13]. The most ecient methods are desirable for the study of resonances using the limited RIB intensities available.
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3.1. Method We have demonstrated a technique that allows one to measure good quality excitation functions with RIBs. Several resonances can be studied simultaneously using a single bombarding energy. The use of a thick target with inverse kinematics is discussed in [14]. Inverse resonance scattering was studied with a thick-gas target in 10 C p [11] and a thick-solid target in 13 C p [15] with a small silicon detector. In this work, we combine the use of inverse kinematics, thick targets, large acceptance detectors with high-granularity and highenergy resolution double-sided silicon strip detector (DSSDs), and a microchannel plate (MCP) timing detector to suppress the background associated with RIBs. Fig. 1 shows a schematic illustration of the setup used. Starting with an inverse reaction, the beam passes through the MCP and stops in a target with a uniform concentration of the light nuclei of interest (e.g., H). The technique requires a target thick enough to stop the beam ions or degrade its energy below the region of interest, but thin enough to let the recoil particles out. The recoiling light-charged particles are eciently detected with a high-granularity detector covering a very large solid angle to determine precisely their
Fig. 1. A schematic drawing of the experimental geometry. A time signal is derived from a carbon-foil MCP detector, which serves as a beam monitor and is used to remove unwanted bparticles from the radioactive decay of the beam. A large area silicon DSSD detects the recoiling light-charged particles and determines precisely their scattering angle.
scattering angle and recoiling energy. The measured energy of the light-recoiling particle on a pixel at an angle H can be transformed into centerof-mass spectra after taking into consideration the speci®c energy losses and the two-body kinematics and can be integrated over the azimuthal angle. With this technique measurements can be done at 180° in the center-of-mass that cannot be done with normal kinematics. A proof-of-principle of the technique was carried out using stable beams. Excited states of 18 F were studied with the inverse reaction p(17 O; p17 O. Excitation functions of cross-sections obtained for three bombarding energies are shown in Fig. 2. For comparison, an excitation function measured by Sens et al. [16] with the ``standard'' technique (normal kinematics and thin-target) at an equivalent backward angle is included. It should be noticed that the lower resonances are clearly visible at all the energies even if the incident beam energy is signi®cantly above that needed to excite them. Note the similar shape of the excitation functions obtained in the common range of energies. 3.2. Targets It is well known that plastic targets have the disadvantage that they allow only a limited amount of beam current because of the destruction of the foil and the loss of hydrogen. It is also important to produce H targets as uniform as possible in composition across its thickness. We used polypropylene foils (CH2 n mounted on a large target frame so that we could frequently vary the beam spot, when bombarded with stable beams. For lower intensity RIB beams this is not a problem. The beam spot was typically 1±2 mm diameter. We monitored the count rate on the DSSD by taking the OR of all the strip detector signals. This gives us a clear indication of any loss of hydrogen and also helps to monitor the beam. With the 17 O beam, we moved the target ladder to a new spot about every 10 min making sure that the rate on the detector remained constant. We are currently investigating the use of other targets with a high H content. The use of ZrH
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ally, a ZrH foil was bombarded for 2 h with 3 pnA of 17 O without moving the beam spot. For this long run, the count rate remained constant indicating that the specimen retained its hydrogen. Targets with optimal thickness will be prepared for use in future experiments. 3.3. DSSD
Fig. 2. Excitation functions at HCM 178° obtained in inverse scattering for 14, 20 and 33 MeV bombarding energies for the p(17 O; p 17 O reaction using the thick-target technique described in this paper. For comparison, an excitation function in a similar energy range for HCM 162° obtained in normal kinematics 17 O p; p 17 O is shown (adapted from [16]).
targets looks very promising as they can be loaded to very high H concentrations (Zr:H 1:1.6) and are very stable at high-beam currents. Foils of ZrH with thicknesses of the order of 20 lm were bombarded with 20 MeV 17 O ions for various beam currents up to an intensity of 10 pnA. The count rate of the recoiling protons in the DSSD scaled linearly with the beam current. Addition-
One detector that is ideal for the study of resonances with RIBs is a DSSD. With proper instrumentation they can deliver energy, time and position information even if several particles strike the detector simultaneously. A large 300-lm thick DSSD [17], 5 cm 5 cm in size with 16 strips per side was used to detect the recoil protons. The DSSD was mounted 12 cm behind the target. It covered the center-of-mass angles from 180° to 160°. This strip arrangement results in a total of 256 pixels, each eectively acting as an individual detector. The X±Y position resolution of 3 mm is determined by the size of each pixel. A system has been developed for the individual readout of each of the 32 segments. The system comprises two preampli®er units combined with a customized shaper ampli®er with a CAMACcontrolled variable gain. The preampli®er module contains 16 channels of charge-sensitive preampli®ers, each with energy and timing outputs. The shaped signals were digitized with peak sensing CAMAC ADCs.
4. Fluorine-17 experiment An experiment to study resonant states in 18 Ne testing the technique presented in this work was succesfully done at the HRIBF. A post acceleration stripper was used to produce a 17 F9 beam with intensities on the order of 104 ions/s with no contamination from the 17 O isobar. Fluorine-17 ions at a bombarding energy of 33 MeV were stopped in a 40-lm CH2 n target. With the use of the DSSD, an excitation function was measured covering the energy range of 400 keV to 1.4 MeV in the center of mass.
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4.1. Background removal RIBs also bring with them their own problems such as low intensities and potentially high backgrounds caused by the radioactivity of the beam. Detection systems must be able to cope with this background problem. On an early test run, permanent magnets placed beside the target were used to produce a strong magnetic ®eld, which partially swept the positrons from the detection area. The strength of the ®eld was not sucient to eliminate the positrons because of the proximity of the DSSD to the target. In the ®nal experiment con®guration, a MCP was placed at the entrance to the scattering chamber to monitor the beam and to provide a timing signal used to gate the proton detector. The MCP can stand a large counting rate of at least 106 ions/s. This coincidence technique eliminated the positron background which results from the decay of 17 F ions stopped in the target (see Fig. 3). a-particles from a very low level of contamination present on the surface of the scattering chamber are visible on
the high-energy side of the singles spectrum. This background was also easily eliminated with the coincidence requirement. 4.2. Results Fig. 4 shows an excitation function obtained in a single exposure of about 16 h, covering the center of mass energy range of 400±1400 keV in 5 keV steps. The resonance recently observed [9] at center of mass energy 600 keV and spin parity J p 3 and a width of 18 keV has been con®rmed. Additional resonances at energies of 1184 and 1231 keV were observed having characteristics consistent with 2 and 3ÿ spin parity assignements, respectively. The data were analyzed using the Rmatrix code MULTI described in [18]. The strongest resonances observed (3 at 600 keV and 2 at 1184 keV) were ®tted with an lp 0, which corresponds to an s1=2 proton coupled to the d5=2 proton of the 17 F ground-state con®guration. Further experiments with improved statistics to explore higher lying resonances are being planned.
Fig. 3. Comparison of recoil proton spectra from the reaction p(17 F,p)17 F taken in singles (dots) and in coincidence (crosses) with the MCP detector. Vertical arrows show the position of resonances studied in this work. The coincidences with the MCP eectively eliminates the background from positrons (low energy) originating from the 17 F deposited in the target and from the alphas (high energy) coming from contamination in the chamber.
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Fig. 4. Resonant scattering excitation function for
5. Conclusion A fast and ecient method to study resonances with radioactive beams and light nuclei in inverse kinematics has been demonstrated. It is based on the use of thick targets and large area DSSDs. The ®rst nuclear physics experiment with this technique has been performed at HRIBF. Beams of 17 F with excellent purity and optical quality can be produced at the HRIBF in its full energy range by using a variety of equipment choices for beam production, transport and analysis. Note added during the preparation of this manuscript. In recent experiments at HRIBF, a beam of almost 108 17 Fÿ ions/s was produced by the KENIS, resulting in a pure 17 F9 beam of 2 106 ions/s after acceleration by the tandem.
Acknowledgements We wish to thank the HRIBF Operations and Facility Support Personnel for the production of the 17 F beams. This work has bene®tted from discussions with A.E. (Ted) Litherland (IsoTrace)
17
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F+p elastic scattering at HCM 178° (Eb 33 MeV).
and D. Khatamian (AECL), who has also provided the ZrH targets. Oak Ridge National Laboratory is managed by Lockheed Martin Energy Research Corp. for the US Department of Energy (DOE) under contract DE-AC05-96OR22464. This research was also supported by DOE through Contract Nos. DE-AC05-76OR00033 with Oak Ridge Institute for Science and Education.
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