- Email: [email protected]
Production of foil targets containing radioactive 22Na
Nuclear Instruments and Methods in Physics Research A 364 (1995) 70-73
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH ~eltonA
ELSEVIER
Production of foil targets containing radioactive
22Na ~
S. Schmidt a,,, C. Rolfs a, W.H. Schulte a, E. Kugler b, j. Lettry b, H. Ravn b, R. Abela c, R.W. Kavanagh d lnstitut flftr Physik mit lonenstrahlen, Ruhr-Universitiit, Bochum, Germany b The ISOLDE Collaboration, CERN, Geneva, Switzerland c Paul-Scherrer-lnstitut, Villigen, Switzerland d California Institute of Technology, Pasadena, CA, USA
Received 26 January 1995 Abstract The production and properties of thin foil targets doped with the radioactive isotope 22Na by means of ion implantation are reported. Targets with activities of 0.3-0.7 mCi were produced and one was successfully used in the study of the 22Na(3He, d)Z3Mg reaction, leading to an improved understanding of the Z2Na production mechanisms in hot stellar burning scenarios.
1. Introduction In nuclear astrophysics, cross sections and resonance strengths of charged-particle-induced capture reactions at low projectile energies (near the particle threshold) are frequently needed. Direct measurement of these quantities is often impossible due to the exponentially decreasing penetrability through the Coulomb barrier towards low energies. In the case of narrow resonances near the particle threshold, the relevant resonance parameters can be deduced from indirect studies of particle transfer reactions. The 22Na(p, ~)23Mg capture reaction, a key reaction in the hot Ne-Na hydrogen burning cycle [1], is such an example: direct measurements could be carried out to resonances as low as E R = 286 keV [2], while several resonances are still expected at lower energies. Since their resonance strengths are determined essentially by the proton partial widths, or equivalently their proton spectroscopic factors, this information can be deduced from studies of the proton-transfer reaction 22Na(3He, d)23Mg. For such studies thin foil targets containing radioactive 22Na (T1/z = 2.6 yr) are needed with the following requirements: i) small thickness (d < 1 ~m) to minimize energy loss of the projectiles and ejectiles, ii) low background in the deuteron spectra produced by the target material and
Supported in part by the Deutsche Forschungsgemeinschafl (Ro 429/21-3). ' Corresponding author. Tel. + 49 234 700 3583, fax + 49 234 7094 172, e-mail [email protected].
contaminations, iii) high enrichment of 22Na in the foil target (i.e. 22Na surface density n t ~ 1017 atoms/cm 2, corresponding to an activity of 23 mCi/cm2), and iv) mechanical and irradiation stability. Several methods for depositing 22Na nuclides on suitable backings have been described [3-7]. Deposition by evaporation of 22Na or by drying an aqueous solution of commercially available Z2NaC! onto the carrier material has the disadvantages of mechanical instability, unavoidable target contaminations, and target inhomogeneities. A method known to produce stable and isotopically clean targets is the implantation of energetic 22Na ions into a suitable carrier material [2,8,9]. This paper reports on the analysis of suitable host foil materials, the production of radioactive 22Na nuclides in a suitable matrix, and the procedure of 22Na implantation into the host foils at the ISOLDE mass separator at CERN.
2. Experimental set-up and equipment 2.1. Carrier foil
Particular efforts were needed in the search for a suitable carrier foil for the implantation with Z2Na. The properties of different host materials of varying thicknesses were investigated by simulations using the stable isotope 23Na. The host foils were implanted with 23Na ions at an energy of 60 keV, equivalent to the beam energy provided by the ISOLDE facility. The elemental composition of the foils was determined by RBS measurements with a 2-MeV a beam and particle detectors placed at an
0168-9002/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0168-9002(95)00222-7
71
S. Schmidt et aL /NucL Instr. and Meth. in Phys. Res. A 364 (1995) 70-73
observation angle of 160 ° with respect to the beam axis. The stability of the implanted targets was investigated with a 4He beam (I---- 150 nA 4 H e + - ) at an energy of E = 10 MeV (4 MV tandem at Bochum) over periods of up to 9 h. Under these conditions the energy deposition in the foils is higher than under bombardment with 3He at an energy of E = 30 MeV and a beam current of 1 = 100 nA as proposed for the 22Na(3He, d)23Mg experiment. Finally, deuteron spectra of the (3He, d) reaction on the 23Na-implanted foils were recorded at an energy E r = 10 MeV (4 MV tandem at Bochum) using a A E - E telescope. The above procedures were applied to two appropriate host materials, Ni and C foils. Nickel was known for its high capacity for sodium from previous thick backing implantations [8], The mean range of 60 keV Na ions in nickel is 45 i.zg/cm 2. It was found that the maximum implantation dose for a nickel foil of thickness 180 i.tg/cm -~ was n t = 8 X 1016 cm 2. This maximum dose decreased at smaller thicknesses. At higher doses the tension created by the implantation caused breakage of the foil. However, for the present applications nickel had the disadvantage of producing a high deuteron background in the (3He, d) reaction studies on the 23Na implanted foils due to the high level density of the copper isotopes produced by the (3He, d) reaction on nickel. This disadvantage is essentially removed for carbon foils due to the much lower level density of the corresponding nitrogen isotopes. Additionally, the capability of such foils to store implanted sodium was found to be even higher than for nickel foils. The maximum implantation dose was n, > 3 × 1017/cm 2 for carbon foils of thickness 75 i x g / c m ~, the minimum thickness to keep the foils stable during implantation. The mean range of 60-keV Na ions in carbon is 21 i x g / c m 2. Both foil types were checked for stability against He bombardment as described above. Within an uncertainty range of 10% no loss of 23Na nuclides embedded in the targets was detected after the 4He bombardments. The RBS spectra (Fig. 1) clearly show a peak of ct particles scattered from the implanted 23Na atoms. Besides the implanted atoms and the foil material, a small amount of copper was detected as a foil contamination on both foil surfaces, deposited probably during the foil fabrication or by sputtering from a copper diaphragm in front of the targets during the implantation. Considerations of deuteron background, target stability, and sodium capacity led to the conclusions that carbon foils were the proper host material for the present investigations. 2.2. 22Na fabrication
A well established method for the production of 2:Na nuclides is the activation of aluminum by a high energy proton beam via spallation reactions [10]. As discussed in Ref. [9], such an activated aluminum sample is a favourable input material to an ion source since the 22Na nuclides will
t ......... 103
I .........
i .........
F .........
I RBS-spectrum of =No
~
i .........
i
ImplantedC-foil]
g
I O0
200
500
400
500
chonnel number Fig. 1. RBS-spectrum of a 23Na implanted (surface density: nt 1 × 1016 atoms/cm 2) carbon foil. The peaks are labelled by the scattering element symbols.
evaporate out of the bulk material when it is heated to the aluminum melting temperature of 660°C. Because the ion source of the mass separator may be quenched by excessive amounts of foreign vapours it is essential that the aluminum sample is vacuum outgassed and of high purity (99.999%, manufacturer: Johnson Matthey GmbH, Karlsruhe). The disk-shaped sample (8 mm thickness, 15 mm diameter) was embedded between two graphite holders. The activation was carried, out at the PSI in Villigen (Switzerland) at a proton energy of 70 MeV and a beam current of about 40 IxA. Using the known 22Na creation cross sections [10] a 22Na activity of A = 50 mCi is expected after an irradiation time of 8.4 days. The actual activation was accomplished over a number of shorter irradiation periods. A total 22Na activity of A = 6 mCi was accumulated inside the aluminum sample. Probably a large fraction of the overall activity, which was estimated to be 30 mCi for the given activation periods, was lost due to evaporation and diffusion into the graphite holders due to the heat created by the proton beam. 2.3. Ion source
A high efficiency ion source was needed due to the limited amount of available 22Na nuclides: the tungstensurface ionizer from the ISOLDE mass separator at CERN (for details see Ref. [11]) can achieve in principle a 56% ionization efficiency for sodium [9]. It has recently been shown [12] that 32% ionization efficiency may also be obtained if NaCI is used as ion-source-charge material, provided it is sufficiently slowly evaporated into the ion source. Before the implantation run the unloaded ion source was heated above the operating temperature in order to outgas possible contaminations. The activated aluminum sample was then loaded into the source, carried by an AIzO s boat. AI203 has a high melting point (2000°C) and
72
S. Schmidt et aL / NucL lnstr and Meth. in Phys. Res. A 364 (1995) 70-73
barely shows any tendency to react with molten aluminum. The reservoir could be heated indirectly by the ionizer and directly by resistance heating. The ionization occurs inside a Q50.5 mm tungsten tube and is maximized for sodium at an operating temperature of about 2000°C. Besides 22Na ions, other ions (basically A1 and other alkalis) were also created in the source from the aluminum host material and contaminations. While the amount of 23Na ions was found to be of the same magnitude as the number of activated 22Na ions, a large quantity of AI + ions was observed in the mass spectrum, increasing with rising reservoir temperature. An excessive current could have led to significant reduction of the ionization efficiency for sodium. Therefore the reservoir temperature had to be kept near the melting point of aluminum. 2.4. Mass separator
The general purpose separator (GPS) of the new CERN-ISOLDE on-line mass-separator facility at the PSbooster [13] was used in an off-line mode for the 22Na implantations. It operates at an acceleration voltage of 60 kV. Various tests with stable beams proved the beam transport efficiency to be up to 95% with the above mentioned ion source. The mass resolution of up to M / A M = 2400 [13], was largely sufficient to separate 22Na ions especially from aaNa and 27Al ions (Section 2.3). Except for the analysing magnet, only electrostatic elements were used in the beam transport system; consequently, only the adjustment of the magnet had to be changed while switching between different ion masses.
ladder that could carry up to four different targets. One of the positions was used for a 1.5 × 3 mm 2 diaphragm that served as a beam focusing aid. Two positions were occupied by 75 t~g/cm 2 carbon foils. Since the investigation of the :2Na(3He, d)ZaMg reaction was planned at the " B e schleunigerlaboratorium der Universitiit und der Technischen Universit~it Miinchen" (with a Q3D spectrograph), where only small activities could be handled, it was necessary to minimize the usable implanted area by covering the carbon foils by apertures of the same size as the focusing diaphragm in order to reduce the activity of the implanted targets. In addition to the foil ladder, a holder for thick nickel-tantalum backings was provided. These backings were implanted on an area defined by a O 7 mm diaphragm. All targets - - carbon foils and N i - T a backing - - could be implanted during one run without breaking the vacuum. The target holders were electrically insulated to allow for beam current measurements. A secondary electron suppression voltage was applied to a Q~3 mm aperture in front of the foil ladder to reduce the effect of secondary electron currents on the charge integration. The beam current measurements in the implantation chamber were calibrated by replacing the thick-target holder by a precise Faraday cup. The beam optics at the ISOLDE facility were adjusted to a focus inside the 1.5 × 3 mm 2 diaphragm using a 23Na beam that was created by loading the ion source with a small amount of NaCI.
3. Implantation p r o c e d u r e
2.5. Implantation chamber
A special chamber was built (Fig. 2) to facilitate an efficient and homogeneous 22Na implantation. The ion beam passed through an electromagnetic X-Y-deflector which could be used for scanning the beam homogeneously across the target. A liquid nitrogen cooling trap was employed to minimize target contaminations by cracked oil vapours. The pressure inside the chamber was about 5 × 10 -7 mbar. The carbon foils were mounted on a
X-Y
ion beam
aperture
aperture •
| foil assembly
Fig. 2. Schematic diagram of the implantation set-up.
For the implantation run, the AI20 3 boat was filled with the activated aluminum sample - - cut into small pieces - - and was then loaded into the ion source. After evacuation the ionizer temperature was slowly increased, heating indirectly also the reservoir. When the first e2Na+-beam current was observed on the target ( I t = 60 pA) the ionizer temperature was set to the operating value of 2000°C. The 22Na current subsequently increased to I t = 80 nA. During the following three hours the current dropped to I, = 25 nA, making direct reservoir heating necessary. By this direct heating the mean current could be kept at this level for another eight hours before the 22Na material was exhausted. Two carbon foils and one nickel-tantalum backing were implanted. The X - Y scanning unit was used only with the N i - T a backing. For the carbon foils the measured beam diameter was of the same order as the size of the apertures in front of the foils, ensuring a good homogeneity of the implanted carbon foils without scanning the beam. The total 2:Na activity deposited on the targets was determined by the integrated beam current and by measurements with a calibrated dosimeter. Activities of A = 0.7 and 0.3 mCi on the carbon foils and 0.5 mCi on the
S. Schmidt et al. / Nucl. Instr. and Meth. in Phys. Res. A 364 (1995) 70-73
N i - T a backing were measured. An activity of A = 0.5 mCi was found on the apertures inside the implantation set-up. Thus, a total activity of A = 2 mCi had been guided into the implantation chamber. With a beam transport efficiency of 95% (Section 2.4) this number leads to an ionization efficiency of 35% which is 21% less than the optimum value of 56% achieved in previous experiments [91.
4. Summary The successful production of the shortest lived radioactive foil targets used up to now was the basic condition for the investigation of the 22Na(3He, d)23Mg reaction, which is of high interest for the understanding of the reaction flow in the N e - N a burning cycle and for the production of 22Na under hot stellar burning conditions. Carbon was found to be the most appropriate host material for the foil implantation allowing high implantation doses and keeping mechanical stability at the same time. Using the ISOLDE facility at CERN we have produced two 22Na implanted foils with 22Na activities of A = 0.3 and 0.7 mCi (equivalent to target densities of n t = 3.3 × 1016 and 7.6 × 10 l~ cm-2), respectively, and one implanted N i - T a backing with an activity of A = 0.5 mCi (nt = 5.7 × 1015 era-2). The measurements of the 22Na(3He, d)Z3Mg reaction using the 0.7 mCi foil target have been described in Ref. [14]. The results from these measurements led to a considerable reduction of the uncertainties of the stellar 22Na(p, ~/)23Mg reaction rates in the temperature region most interesting for astrophysical scenarios. The experiments using the eeNa implanted N i - T a backing (22Na(n, p)22Ne and 22Na(p, "y)23Mg) are in preparation.
Acknowledgements The authors would like to thank K. Brand, H.H. Bukow, D. Frischke, H.J. Meijer, and L. Wielunski for their help in
73
the preparation of the target foils, as well as J. Jegge and P. Schmelzbach for their generous support in the 22Na activation. We appreciate very much the help of D. Forkel, O. Johnson, and M. M6hle during the course of the 22Na implantation.
References [1] C.E. Rolfs and W.S. Rodney, Cauldrons in the Cosmos (University of Chicago Press, 1988). [2] S. Seuthe, C. Rolls, U. Schr/Sder, W.H. Schulte, E. Somorjai, H.P. Trautvetter, F.B. Waanders, R.W. Kavanagh, H. Ravn, M. Arnould and G. Paulus, Nucl. Phys. A 514 (1990) 471. [3] G.R. Massuomi, P.J. Schultz, W.H. Lennart and J. Ociera, Nucl. Instr. and Meth. B 30 (1988) 592. [4] H. Huomo, R. Jones, J. Hurst, A. Vehanen, J. Throwe, S.G. Usmar and K.G. Lynn, Nucl. Instr. and Meth. A 284 (1989) 359. [5] M. Wiescher, J. G&res, K.L. Kratz, B. Leist, K.H. Chang, B.W. Filippone, L.W. Mitchell, M.J. Savage and R.B. Vogelaar, Nucl. Instr. and Meth. A 267 (1988) 242. [6] H.C. Griffin, T.D. Steiger, J. Van House, M. Skalsey, R. Conti, A. Rich and P.W. Zitzewitz, Hyperfine Interactions 44 (1988) 147. [7] J.M. D'Auria, M. Dombsky, L. Buchmann and J.S. Vincent, Nucl. Instr. and Meth. A 288 (1990) 354. [8] S. Seuthe, H.W. Becker, A. Krauss, A. Redder, C. Rolfs, U. SchriSder, H.P. Trautvetter, K. Wolke, S. Wiistenbecker, R.W. Kavanagh and F. Waanders, Nucl. Instr. and Meth. A 260 (1987) 33. [9] H. Ravn, W.H. Schulte, C. Rolls, F.B. Waanders and R.W. Kavanagh, Nucl. Instr. and Meth. B 58 (1991) 174. [10] A. Griitter, Nucl. Phys. A 383 (1982) 98. [111 T. Bj0rnstadt, E. Hageb~, P. Hoff, E. Kugler, H.L. Ravn, S. Sundell and B. Vosicki, Phys. Scripta 34 (1986) 578. [12] J.R. Poulsen, M. Eldmp, J. Lettry and the ISOLDE collaboration, Nucl. Instr. and Meth. B, in press. [131 E. Kugler, D. Fiander, B. Jonson, H. Hass, A. Przewloka, H.L. Ravn, D.J. Simon, K. Zimmer and the ISOLDE Collaboration, Nucl. Instr. and Meth. B 70 (1992) 41. [141 S. Schmidt, C. Rolls, W.H. Schulte, H.P. Trautvetter, R.W. Kavanagh, S. Faber and G. Graw, submitted to Nucl. Phys. A.
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH ~eltonA
ELSEVIER
Production of foil targets containing radioactive
22Na ~
S. Schmidt a,,, C. Rolfs a, W.H. Schulte a, E. Kugler b, j. Lettry b, H. Ravn b, R. Abela c, R.W. Kavanagh d lnstitut flftr Physik mit lonenstrahlen, Ruhr-Universitiit, Bochum, Germany b The ISOLDE Collaboration, CERN, Geneva, Switzerland c Paul-Scherrer-lnstitut, Villigen, Switzerland d California Institute of Technology, Pasadena, CA, USA
Received 26 January 1995 Abstract The production and properties of thin foil targets doped with the radioactive isotope 22Na by means of ion implantation are reported. Targets with activities of 0.3-0.7 mCi were produced and one was successfully used in the study of the 22Na(3He, d)Z3Mg reaction, leading to an improved understanding of the Z2Na production mechanisms in hot stellar burning scenarios.
1. Introduction In nuclear astrophysics, cross sections and resonance strengths of charged-particle-induced capture reactions at low projectile energies (near the particle threshold) are frequently needed. Direct measurement of these quantities is often impossible due to the exponentially decreasing penetrability through the Coulomb barrier towards low energies. In the case of narrow resonances near the particle threshold, the relevant resonance parameters can be deduced from indirect studies of particle transfer reactions. The 22Na(p, ~)23Mg capture reaction, a key reaction in the hot Ne-Na hydrogen burning cycle [1], is such an example: direct measurements could be carried out to resonances as low as E R = 286 keV [2], while several resonances are still expected at lower energies. Since their resonance strengths are determined essentially by the proton partial widths, or equivalently their proton spectroscopic factors, this information can be deduced from studies of the proton-transfer reaction 22Na(3He, d)23Mg. For such studies thin foil targets containing radioactive 22Na (T1/z = 2.6 yr) are needed with the following requirements: i) small thickness (d < 1 ~m) to minimize energy loss of the projectiles and ejectiles, ii) low background in the deuteron spectra produced by the target material and
Supported in part by the Deutsche Forschungsgemeinschafl (Ro 429/21-3). ' Corresponding author. Tel. + 49 234 700 3583, fax + 49 234 7094 172, e-mail [email protected].
contaminations, iii) high enrichment of 22Na in the foil target (i.e. 22Na surface density n t ~ 1017 atoms/cm 2, corresponding to an activity of 23 mCi/cm2), and iv) mechanical and irradiation stability. Several methods for depositing 22Na nuclides on suitable backings have been described [3-7]. Deposition by evaporation of 22Na or by drying an aqueous solution of commercially available Z2NaC! onto the carrier material has the disadvantages of mechanical instability, unavoidable target contaminations, and target inhomogeneities. A method known to produce stable and isotopically clean targets is the implantation of energetic 22Na ions into a suitable carrier material [2,8,9]. This paper reports on the analysis of suitable host foil materials, the production of radioactive 22Na nuclides in a suitable matrix, and the procedure of 22Na implantation into the host foils at the ISOLDE mass separator at CERN.
2. Experimental set-up and equipment 2.1. Carrier foil
Particular efforts were needed in the search for a suitable carrier foil for the implantation with Z2Na. The properties of different host materials of varying thicknesses were investigated by simulations using the stable isotope 23Na. The host foils were implanted with 23Na ions at an energy of 60 keV, equivalent to the beam energy provided by the ISOLDE facility. The elemental composition of the foils was determined by RBS measurements with a 2-MeV a beam and particle detectors placed at an
0168-9002/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0168-9002(95)00222-7
71
S. Schmidt et aL /NucL Instr. and Meth. in Phys. Res. A 364 (1995) 70-73
observation angle of 160 ° with respect to the beam axis. The stability of the implanted targets was investigated with a 4He beam (I---- 150 nA 4 H e + - ) at an energy of E = 10 MeV (4 MV tandem at Bochum) over periods of up to 9 h. Under these conditions the energy deposition in the foils is higher than under bombardment with 3He at an energy of E = 30 MeV and a beam current of 1 = 100 nA as proposed for the 22Na(3He, d)23Mg experiment. Finally, deuteron spectra of the (3He, d) reaction on the 23Na-implanted foils were recorded at an energy E r = 10 MeV (4 MV tandem at Bochum) using a A E - E telescope. The above procedures were applied to two appropriate host materials, Ni and C foils. Nickel was known for its high capacity for sodium from previous thick backing implantations [8], The mean range of 60 keV Na ions in nickel is 45 i.zg/cm 2. It was found that the maximum implantation dose for a nickel foil of thickness 180 i.tg/cm -~ was n t = 8 X 1016 cm 2. This maximum dose decreased at smaller thicknesses. At higher doses the tension created by the implantation caused breakage of the foil. However, for the present applications nickel had the disadvantage of producing a high deuteron background in the (3He, d) reaction studies on the 23Na implanted foils due to the high level density of the copper isotopes produced by the (3He, d) reaction on nickel. This disadvantage is essentially removed for carbon foils due to the much lower level density of the corresponding nitrogen isotopes. Additionally, the capability of such foils to store implanted sodium was found to be even higher than for nickel foils. The maximum implantation dose was n, > 3 × 1017/cm 2 for carbon foils of thickness 75 i x g / c m ~, the minimum thickness to keep the foils stable during implantation. The mean range of 60-keV Na ions in carbon is 21 i x g / c m 2. Both foil types were checked for stability against He bombardment as described above. Within an uncertainty range of 10% no loss of 23Na nuclides embedded in the targets was detected after the 4He bombardments. The RBS spectra (Fig. 1) clearly show a peak of ct particles scattered from the implanted 23Na atoms. Besides the implanted atoms and the foil material, a small amount of copper was detected as a foil contamination on both foil surfaces, deposited probably during the foil fabrication or by sputtering from a copper diaphragm in front of the targets during the implantation. Considerations of deuteron background, target stability, and sodium capacity led to the conclusions that carbon foils were the proper host material for the present investigations. 2.2. 22Na fabrication
A well established method for the production of 2:Na nuclides is the activation of aluminum by a high energy proton beam via spallation reactions [10]. As discussed in Ref. [9], such an activated aluminum sample is a favourable input material to an ion source since the 22Na nuclides will
t ......... 103
I .........
i .........
F .........
I RBS-spectrum of =No
~
i .........
i
ImplantedC-foil]
g
I O0
200
500
400
500
chonnel number Fig. 1. RBS-spectrum of a 23Na implanted (surface density: nt 1 × 1016 atoms/cm 2) carbon foil. The peaks are labelled by the scattering element symbols.
evaporate out of the bulk material when it is heated to the aluminum melting temperature of 660°C. Because the ion source of the mass separator may be quenched by excessive amounts of foreign vapours it is essential that the aluminum sample is vacuum outgassed and of high purity (99.999%, manufacturer: Johnson Matthey GmbH, Karlsruhe). The disk-shaped sample (8 mm thickness, 15 mm diameter) was embedded between two graphite holders. The activation was carried, out at the PSI in Villigen (Switzerland) at a proton energy of 70 MeV and a beam current of about 40 IxA. Using the known 22Na creation cross sections [10] a 22Na activity of A = 50 mCi is expected after an irradiation time of 8.4 days. The actual activation was accomplished over a number of shorter irradiation periods. A total 22Na activity of A = 6 mCi was accumulated inside the aluminum sample. Probably a large fraction of the overall activity, which was estimated to be 30 mCi for the given activation periods, was lost due to evaporation and diffusion into the graphite holders due to the heat created by the proton beam. 2.3. Ion source
A high efficiency ion source was needed due to the limited amount of available 22Na nuclides: the tungstensurface ionizer from the ISOLDE mass separator at CERN (for details see Ref. [11]) can achieve in principle a 56% ionization efficiency for sodium [9]. It has recently been shown [12] that 32% ionization efficiency may also be obtained if NaCI is used as ion-source-charge material, provided it is sufficiently slowly evaporated into the ion source. Before the implantation run the unloaded ion source was heated above the operating temperature in order to outgas possible contaminations. The activated aluminum sample was then loaded into the source, carried by an AIzO s boat. AI203 has a high melting point (2000°C) and
72
S. Schmidt et aL / NucL lnstr and Meth. in Phys. Res. A 364 (1995) 70-73
barely shows any tendency to react with molten aluminum. The reservoir could be heated indirectly by the ionizer and directly by resistance heating. The ionization occurs inside a Q50.5 mm tungsten tube and is maximized for sodium at an operating temperature of about 2000°C. Besides 22Na ions, other ions (basically A1 and other alkalis) were also created in the source from the aluminum host material and contaminations. While the amount of 23Na ions was found to be of the same magnitude as the number of activated 22Na ions, a large quantity of AI + ions was observed in the mass spectrum, increasing with rising reservoir temperature. An excessive current could have led to significant reduction of the ionization efficiency for sodium. Therefore the reservoir temperature had to be kept near the melting point of aluminum. 2.4. Mass separator
The general purpose separator (GPS) of the new CERN-ISOLDE on-line mass-separator facility at the PSbooster [13] was used in an off-line mode for the 22Na implantations. It operates at an acceleration voltage of 60 kV. Various tests with stable beams proved the beam transport efficiency to be up to 95% with the above mentioned ion source. The mass resolution of up to M / A M = 2400 [13], was largely sufficient to separate 22Na ions especially from aaNa and 27Al ions (Section 2.3). Except for the analysing magnet, only electrostatic elements were used in the beam transport system; consequently, only the adjustment of the magnet had to be changed while switching between different ion masses.
ladder that could carry up to four different targets. One of the positions was used for a 1.5 × 3 mm 2 diaphragm that served as a beam focusing aid. Two positions were occupied by 75 t~g/cm 2 carbon foils. Since the investigation of the :2Na(3He, d)ZaMg reaction was planned at the " B e schleunigerlaboratorium der Universitiit und der Technischen Universit~it Miinchen" (with a Q3D spectrograph), where only small activities could be handled, it was necessary to minimize the usable implanted area by covering the carbon foils by apertures of the same size as the focusing diaphragm in order to reduce the activity of the implanted targets. In addition to the foil ladder, a holder for thick nickel-tantalum backings was provided. These backings were implanted on an area defined by a O 7 mm diaphragm. All targets - - carbon foils and N i - T a backing - - could be implanted during one run without breaking the vacuum. The target holders were electrically insulated to allow for beam current measurements. A secondary electron suppression voltage was applied to a Q~3 mm aperture in front of the foil ladder to reduce the effect of secondary electron currents on the charge integration. The beam current measurements in the implantation chamber were calibrated by replacing the thick-target holder by a precise Faraday cup. The beam optics at the ISOLDE facility were adjusted to a focus inside the 1.5 × 3 mm 2 diaphragm using a 23Na beam that was created by loading the ion source with a small amount of NaCI.
3. Implantation p r o c e d u r e
2.5. Implantation chamber
A special chamber was built (Fig. 2) to facilitate an efficient and homogeneous 22Na implantation. The ion beam passed through an electromagnetic X-Y-deflector which could be used for scanning the beam homogeneously across the target. A liquid nitrogen cooling trap was employed to minimize target contaminations by cracked oil vapours. The pressure inside the chamber was about 5 × 10 -7 mbar. The carbon foils were mounted on a
X-Y
ion beam
aperture
aperture •
| foil assembly
Fig. 2. Schematic diagram of the implantation set-up.
For the implantation run, the AI20 3 boat was filled with the activated aluminum sample - - cut into small pieces - - and was then loaded into the ion source. After evacuation the ionizer temperature was slowly increased, heating indirectly also the reservoir. When the first e2Na+-beam current was observed on the target ( I t = 60 pA) the ionizer temperature was set to the operating value of 2000°C. The 22Na current subsequently increased to I t = 80 nA. During the following three hours the current dropped to I, = 25 nA, making direct reservoir heating necessary. By this direct heating the mean current could be kept at this level for another eight hours before the 22Na material was exhausted. Two carbon foils and one nickel-tantalum backing were implanted. The X - Y scanning unit was used only with the N i - T a backing. For the carbon foils the measured beam diameter was of the same order as the size of the apertures in front of the foils, ensuring a good homogeneity of the implanted carbon foils without scanning the beam. The total 2:Na activity deposited on the targets was determined by the integrated beam current and by measurements with a calibrated dosimeter. Activities of A = 0.7 and 0.3 mCi on the carbon foils and 0.5 mCi on the
S. Schmidt et al. / Nucl. Instr. and Meth. in Phys. Res. A 364 (1995) 70-73
N i - T a backing were measured. An activity of A = 0.5 mCi was found on the apertures inside the implantation set-up. Thus, a total activity of A = 2 mCi had been guided into the implantation chamber. With a beam transport efficiency of 95% (Section 2.4) this number leads to an ionization efficiency of 35% which is 21% less than the optimum value of 56% achieved in previous experiments [91.
4. Summary The successful production of the shortest lived radioactive foil targets used up to now was the basic condition for the investigation of the 22Na(3He, d)23Mg reaction, which is of high interest for the understanding of the reaction flow in the N e - N a burning cycle and for the production of 22Na under hot stellar burning conditions. Carbon was found to be the most appropriate host material for the foil implantation allowing high implantation doses and keeping mechanical stability at the same time. Using the ISOLDE facility at CERN we have produced two 22Na implanted foils with 22Na activities of A = 0.3 and 0.7 mCi (equivalent to target densities of n t = 3.3 × 1016 and 7.6 × 10 l~ cm-2), respectively, and one implanted N i - T a backing with an activity of A = 0.5 mCi (nt = 5.7 × 1015 era-2). The measurements of the 22Na(3He, d)Z3Mg reaction using the 0.7 mCi foil target have been described in Ref. [14]. The results from these measurements led to a considerable reduction of the uncertainties of the stellar 22Na(p, ~/)23Mg reaction rates in the temperature region most interesting for astrophysical scenarios. The experiments using the eeNa implanted N i - T a backing (22Na(n, p)22Ne and 22Na(p, "y)23Mg) are in preparation.
Acknowledgements The authors would like to thank K. Brand, H.H. Bukow, D. Frischke, H.J. Meijer, and L. Wielunski for their help in
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the preparation of the target foils, as well as J. Jegge and P. Schmelzbach for their generous support in the 22Na activation. We appreciate very much the help of D. Forkel, O. Johnson, and M. M6hle during the course of the 22Na implantation.
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