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The Chalk River high-temperature helium-jet ion source
Nuclear Instruments andMethods in Physics Research B70(1992) 245-253 North-Holland
HOH B
Beam Interactions with MaterialsiAtoms
The Chalk River high-temperature helium jet ion source V.T. Koslowsky, M.J . Watson, E. Hagberg, J .C. Hardy, W.L. Perry, M .G . Steer and H. Schmeing AECL Research, Chalk RiverLaboratories, Chalk River, Ontario, KOJ IJO Canada
P.P . Unger and K.S. Sharma
Physics Department, University of Manitoba, Winnipeg, Manitoba, R3T 2N2 Canada
The Chalk River helium-jet ion source has been upgraded to accommodate a helium transfer-gas flow of 6 std I/m andoperate at 2000°C. It now provides yields exceeding 1000 atoms/s forshort-lived isotopes of some refractory elements. 1. Introduction
Development work on the Chalk River heliumjet ion source, first described [1] at EMIS-11, has been strongly influenced by the requirements of our studies of 0+- 0 ' superallowed ß emitters [2]. These studies were primarily aimed at accurate measurements of the half-lives of 42Sc, 46V, soMn and s4 Ca, performed with isotopically pure samples. Higher ion source operating temperatures and higher helium flow rates were required to achieve good yields of these sub-second isotopes that are characterized by high heats of desorption [3]. In this paper, we report our experiences with a high temperature helium jet ion source suitable for short-lived isotopes of some refractory elements . 2. Experimental setup 2.1 . System layout At the recently completed TASCC Facility [4), a heavy-ion beam from either the 15 MV tandem accelerator or the superconducting cyclotron can be directed to the isotope separator [5]. The beam can either enter the ion source and strike an internal target or impinge on targets external to the ion source in a separate helium-flushed target chamber, approximately 2 m away from the ion source. In the latter case, the beam enters the target chamber through a thin metallic-foil window. To prevent inadvertent venting of the beam lines and accelerators in the event of a window rupture, the beam line adjacent to the separator is equipped with shock-attenuating baffles and a highspeed valve. These devices can confine any helium Elsevier Science Publishers B.V.
shock-waves to the nearest 2 m of accelerator beam line . The isotopes 42Sc, 46V, SOMn and 54CO were produced by bombardment of a stack of 15 isotopically enriched targets with several microamperes of protons from the tandem accelerator . Product recoil ranges in 1 to 2 atm of helium are about 2 mm for these reactions so the targets were spaced 5 mm apart. When heavy-ion reactions or proton-induced fission are used to produce radioisotopes, larger volume target chambers are usually required in order to thermalize the more energetic reaction products . Gas streamlines in glass models of our chambers were filmed and studied to ensure that eddy currents, which were shown to trap the recoils for periods of several seconds, were kept to aminimum. In the target chamber, thermalized charged reaction products attach themselves to NaCI aerosol particles that are continuously added to the helium stream as it passes over a hot (600°C) bed of granulated NaCl, which is located in a quartz tube and heated by a tubular oven . The helium gas exits the target chamber via one or several stainless-steel capillaries leading through a manifold to a single teflon transport capillary (0.5 to 1.5 mm internal diameter and 1.5 m long); this is joined in turn to a 1-m-long stainless-steel capillary of comparable inner diameter that is attached to the ion-source assembly . 2.2. Helium removal and the side jet Most of the helium conducted by the capillary is removed by a skimming process before it reaches the ion source. The capillary exit is in a pumped region and faces a flat plate, through which an orifice (1 mm diameter), leading to the ion source, has been bored. IV. IONGUIDES/HE JETS
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V. T. Koslowsky et al. / The Chalk River heliumjet ion source
Upon streaming out of the nozzle, most of the helium gas expands into a wide cone and is pumped away by a Roots pump (nominal pumping speed 2000 I/s) . Most of the activity-laden aerosol particles are relatively heavy and leave the nozzle in a narrow cone, pass through the orifice and enter the interior of the ionsource body. With this arrangement 97.5% of the helium can be skimmed off while more than 90% of the activity passes through the orifice [6]. Losses due to decay during transit from the target chamber to the ion source can only be reduced by increasing the carrier flow rate. Formerly, our helium flow rates were limited to about 1 std 1/m by the helium load tolerated by the ion source (about 0.4 std cm'/s), the vacuum required to avoid an electrical discharge do the extraction electrode, and the gas pressure tolerated by the spark arrestor [1] which isolated the Roots pump at ground potential from the ionsource extraction potential. These limitations have been overcome by operation of the Roots pump and associated forevacuum pumps at extraction potential (30-70 kV), thereby avoiding the need for the spark arrestor, and by installing a "side-jet" [6], which further limits the gas conductance into the ion-source body without significantly altering the aerosol transmission through the orifice. The sidejet consists of an additional stream of pure helium that is blown across the main jet, as shown in the inset of fig . 1 . It is most effective when its throughput is comparable to that of the transport capillary. Its effect on ion-source pressure is shown in fig. 1 . Its use was
1
2
3
4
HELIUM JET GAS FLOW (std. I /m)
Fig. 1 . Helium partial pressure in the ion-source chamber -hen the helium jet is operated with gas flows ranging from 0 to 6 std 1/m . Operation of the side-jet at half the primary heliumjet flow reduces the pressure by a factor of about four without altering the aerosol-particle transmission into the ion-source . Ion-source operation is unaffected up to a helium partial pressure of 2 x 10 -Z Pa. The partial pressure of argon (support gas) measured in the ion-source chamber is 1 .4X 10 -3 Pa . The insert shows the optimum sidejet configuration that was empirically determined.
Fig . 2. Schematic front view of the current Chalk River helium-jet ion source . Most of the components on the left have been omitted for clarity. The extraction-slit location is shown dashed in the center of the schematic. It is 3 cm long and 1 mm wide. The points of interest are : (1) a 2 mm anode entrance orifice, through which aerosol-particles are injected; (2) the cylindrical anode cylinder; (3) the hollow locating pin and support-gas entrance ; (4) the water-cooled copper side shield; (5) the upper tungsten hairpin filament; (6) the upper molybdenum support yoke - the lower and upper yokes support the anode cylinder and slide in grooves machined into the two side shields ; (7) the replaceable tungsten partition separating the plasma region, below, from the upper heater ; (8) one of two LaB. side heaters ; (9) one of six electrical feedthroughs; (10) the water-cooled copper back-support plane, 15 cm in diameter; (11) one of four nimonic springs that accommodate the expansion of the anode cylinder; (12) a boron nitride plug; and (13) the lower tungsten hairpin filament. formerly precluded because the additional gas load could not be tolerated by the spark arrestor. Because of these alterations, helium-flow rates up to 6 std 1/m can now be utilized for the transport of reaction products without any increase in ion-source pressure . The effectiveness of the side-jet depends critically on alignment. If the position of the side jet deviates from the ideal by more than one quarter of a capillary diameter, it become ineffective . The optimum geometry was determined empirically and is shown in the inset of fig. 1. Provided the helium flow remains laminar, a flow rate of ä std 1/m will sweep-out the "large" 15-capillary target chamber (2 .7 cm') in less than 100 ms when at pressures below 4 atm. The capillary transit time is about 20 ms under these conditions . 2.3. The ion source The ion-source configuration presently in use is shown schematically in fig. 2. As in the earlier heliumjet ion source [1], we have maintained the slit geometry
V. T. Koslowsky et al. / The Chalk River helium jet ion source
of the Bernas-Nier ion source [5], which offers high gas throughput and good source efficiency. The aerosol particles (containing reaction products) enter the source through the 2 mm orifice at the rear of the anode cylinder . The large ratio of extraction-slit to entranceorifice areas is desirable to prevent backstreaming of the reaction products . The lower hairpin filament is heated with a do current and biased between -30 and -90 V relative to the anode. It is fabricated from 1.5 mm diameter tungsten rod. The arc discharge occurs between the lower filament (cathode) and anode when the support gas, usually argon, is bled into the source. The source requires an axial magnetic field of 200 to 300 G, which is provided by an external electromagnet. To facilitate higher-temperature operation (coldspot temperature at 1800°C rather than 1300°C [1]) the following design features have been incorporated (see fig. 2): (1) The water cooling of the copper back-plane that supports the ion-source components has been improved by an increase in the cooling-channel size and relocation of the channels to positions closer to the source of heat . (2) The use of soft solder has been eliminated in the fabrication of the ion-source housing. Permanent joints are either brazed or welded. (3) Twowater-cooled side shields shade the peripheral regions of the source that contain the electrical feedthroughs. Since the side shields are in close proximity to the hot anode, they were fabricated from copper components welded together to form the cooling-water channels . (4) The side shields also support the anode cylinder. In the present design (see fig. 3), the anode is supported at each end by a spring-loaded molybdenum yoke that slides freely in groovesmachined into the top and bottom of the side shields. The edges of the yoke, which are in contact with the side shields, are bevelled to permit slight rotations and rocking of the yoke without binding. This arrangement permits the anode to expand when heated . The springs are fabricated from nimonic-90 wire . Unlike other spring steels, tensioning springs fabricated from nimonic-90 wire do not anneal when the ion source is at a high operating temperature . Alignment of the anode is accomplished with one hollow locating pin that seats into a hole at the rear of the anode about midway down its length and by locating the yoke with keys that permit vertical expansion of the anode cylinder . Both the pin and keys are fastened to the copper back plane. (5) The support-gas connection to the anode formerly consisted of a graphite/ tantalum joint which, after a few days of use, would embrittle and crack. The joint has been eliminated now that the gas is introduced through the centre of the locating pin described above. The pin is fabricated from a molybdenum alloy.
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PIN Fig . 3. Schematic illustration of the approach used to support the anode cylinder. The lower and upper yokes are identical and free to slide in grooves machined into the side shields. This permits the anode cylinder to expand when heated. Lateral stability is provided by the key in the yoke and the hollow locating pin, which centres the anode cylinder andalso acts as an inlet for the support gas. Both the locating pin and key are fastened to the copper back plane . (6) Anode cylinders of tantalum, tungsten wad graphite have been used on-line and off-line. Tantalum anodes are only suitable for operating temperatures below 1600°C. Above these temperatures, the anode distorts and walls can be destroyed within 24 hours by the electron-bombardment heating. Tungsten and graphite have stood up very well at high temperature. Most of our operating experience has been with tungsten as it is less fragile and outgases less when heated . (7) A supplementary heater, consisting of a 1.5mm-diameter tungsten hairpin filament, powered with ac current and biased to -600 V with respect to the anode, has been inserted into thewell at the top of the anode cylinder. The filament tip is located 2 mm from the partition. Electron-bombardment heating provides up to 1 kW of heating power to the upper portion of the anode cylinder. (8) The partition dividing the anode cylinder into an upper and a lower section is now removable and replaceable, since electron bombardment from the top heater eventually destroys this surface. It also permits easier and cheaper fabrication of the anode cylinders. (9) Two additional electrical feedthroughs have been added to introduce power to the anode sideheaters. All six feedthroughs (two each for top, side and lower heaters) are identical and water cooled. (10) The side heaters, a novel feature of this ion source, augment the upper supplementary heater described above. The side heaters consist of sintered LaB6 pellets mounted in close proximity to the hot anode cylinder . They are radiatively heated by the IV . ION GUIDES/HE JETS
V. T Koslowsky et al. / The Chalk Ricerheliumjet ion source
248
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.
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Fig. 4. Photograph of the assembled source with the outer cover removed.
Fig. 5 . Photograph of the helium-jet ion source, ready to be inserted into its vacuum housing. The helium skimmed off is pumped away through a manifold which surrounds the open area near the base flange .
V. T. Koslowsky et al. / The Chalk River heliumjet ion source Table 1 Typical operating conditions of the helium-jet ion source Cathode (lower filament) Arc Magnetic field Support gas/flow rate Extracted current (total) 3eAr current Helium load Top heater filament electron bombardment Side heater electron bombardment
5V 50-90 A 30-60 V 5-10 A 200-300 G argon/0.02 std cm3/s 5-10 mA 5-10 pA 0.4 std cm3 /s 6V 600 V
80 A 0.5-2 A
1 kV
0.25-1 A
anode and biased to about -1 kV with respect to the anode. When they reach 1400 to 1500°C, the pellets easily provide an electron emission current of up to 1 A, thus providing several hundred additional watts of i ,eating power . Typical operating conditions of the source are given in table 1 . Figs . 4 and 5 show photographs of the source face and helium-jet ion-source assembly. The beam is extracted through the 30-mm-long by 1-mmwide slit in the anode and accelerated across the 40 to 70 kV extraction potential .
249
within 30 ms with a customized valve. Care was taken to ensure that the conductance and flow conditions of both paths were matched . Currents were measured with a Faraday cup and electrometer and recorded with a chart recorder. The results are shown in fig. 6 for a 2.5-m-long, 0 .8-mm-internal diameter capillary. The helium flow was 4 std 1/m . Curves B and C represent the response for 23Na+ and 35 01 +, respectively . Their temporal response is independent of ion-source operating conditions. The test was repeated with a SCC1 3 aerosol . The a5Sc+ and 35CI+ responses are also shown in fig . 6. Curve E shows the 45Sc+ response with no supplementary ion-source heating; it becomes more rapid (by about 120 ms) and intensifies (by about a factor of 3 .5) when the top heater provides an additional 1 kW of heat (see curve A). Interestingly, the corresponding 35CI beam burst (curve D) is unchanged by ion-source operating conditions and is identical to the 35C1 response from NaCl (curve C). This suggests that the ion-source temperature does not strongly affect the break-up of the aerosol particle but strongly affects the ionization efficiency and dwell time of the ioi is in the source. The shape of the curves is partially determined by the 50 ms rise time of the electrometer used. The 30
3. Operating experience 3.1. Time response
uM
An important operating characteristic of an on-line ion source is the time that elapses between the production of a radioactive atom and its extraction from the ion source . For a heliumjet ion source, this dwell time can be analyzed in terms of three components : the target-chamber dwell time, the capillary transit time, and the ion-source dwell time. The capillary transit time can be estimated from the measured helium flow, initial gas pressure and capillary dimensions . For a 2 m length, its contribution to the total dwell time is usually tens of milliseconds and can be neglected . We have investigated the two remaining components several ways . First, we made off-line investigations of the time-response of different elements to ion-source operating conditions by injecting aerosol formed from various salts. For example, we measured the mass separated 23Na+ and 35CI + currents as a function of time when a 100 ms burst of NaCl aerosol was injected into the ion source. In our experimental setup, either pure helium or NaCl-laden helium could be directed to the ion source while the other was evacuated to a vacuum pump . The two gas streams could be interchanged
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â
0.1
0.2 0.3 TIME (s)
0.4
0.5
Fig. 6. Mass-separated beam currents of stable Na, CI and Sc isotopes as a function of time, produced by a momentary injection of NaCl-or ScCl 3-laden helium into the helium-jet ion source (see section 3 .1 for details). The burst of aerosol begins at time zero and occurs for 0.1 s. Curve A is the 45Sc+ response with supplementary heat; curve B is the 23Na+ response with or without supplementary heat; curve C is the 35CI+ response from NaCI aerosol - it is also unaffected by the degree of ion-source heating; curve D is the 35CI+ response from SCCI 3 aerosol - also unaffected by the degree of ion-source heating; and curve E is the 45Sc+ response with no supplementary ion-source heating. The estimated transit time through the capillary and the measured delay of the customized gas switch that was used to inject the aerosol, to talling 50 ms, has been subtracted from the measured response. IV. ION GUIDES/HE JETS
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V.T. Koslowskyet al. / TheChalk Rimerhelium jet ionsource the integrated intensities of the primary and secondary beams. In cases A and C, the ion source was operated without supplementary heaters, and with conservative arc voltage/current settings of 40 V and 5 A. When the source was operated at 70 V and 8 A the response advanced in time, as shown in curve B. The overall
system efficiency lies between 1% and 2% for all three cases. When the primary In beam was left on continuously, the steady-state secondary beam current reached 3% of the primary beam current. Clearly, the targetchamber dwell time dominates any ion source or transit dwell time. Fig. 7. The mass-separated ii51v yield (stable isotope) as a function of time when a 47 nA "'In" beam-pulse from the Chalk River tandem accelerator is thermalized in the target chamber of the helium-jet ion source. Gas flow and targetchamber pressure were 1 std I/m and 1 atm, respectively . The pulse begins at time zero and occurs for 0.2 s. Curve A is the response with a 12 .5-cm-long, 1 .5-cm-diameter target chamber. Curves B and C are the responses for a 4-cm-long 4-cm-diameter chamber. Curve B was obtained with the ionsource arc parameters at 70 V and 8 A, substantially higher than the operating condition of 40 V and 5 A which yielded curves Aand C.
ms switching time and 20 ms capillary transit time have already been subtracted from the measured values. The cause of the remaining 100 ms delay seen in fig . 6 is not known. It could be due to turbulence caused during the 30 ms switching interval, since this is comparable in length to the capillary transit time, or due to an ion-source dwell time . Second, we investigated the effects of the target chamber, which can dramatically alter the temporal response shown in fig . 6. In a separate set of experi115ln5+ ments, a 48 MeV beam from the Chalk River tandem accelerator was directed into a number of
large-volume target chambers. The In beam was thermalized in 1 atm of helium and transported to the helium jet ion source with a 1 std 1/m gas flow. The temporal response of the mass-115 separated (secondary) beam was monitored with a Faraday cup and electrometer after a 200 ms pulse of the primary In beam . The results are shown in fig. 7, timed from the beginning of the primary beam pulse. As before, the capillary transit time has been subtracted from the measured time. Curve A is the response for a 1.:.7-cmlong, 1.5-cm-diameter target chamber equipped with five equally spaced gas inlets and seven equally spaced
gas outlets. Curve C is the response for a 4-cm-long, 4-cm-diameter target chamber equipped with three equally spaced inlets around the cylinder and seven equally spaced outlets on the back flange . The overall system efficiency was evaluated through comparison of
.3.2. Ion source temperatures Yields of isotopes of Sc, V and Co have shown a strong dependence on the degree of supplementary heating . This has led to the present ion-source design, one that is better suited than the original for high-temperature operation, as noted in section 2. We have measured the operating temperature of the ion-source components in a test stand that is equipped with a large quartz window permitting a direct view of the ion-source face . Temperature measurements were made with a disappearing-filament pyrometer . This
method provides a good relative temperature scale but absolute temperature determinations are limited in accuracy to ±200°C by the ill-defined emissivity and absorption of the ion-source components and quartz window. Temperature measurements briefly performed with a two-colour pyrometer, less sensitive to these effects, suggest that our quoted absolute temperatures may be two to three hundred degrees low. The results are shown in fig. 8. With no supplementary heat, the
upper portion of the anode cylinder operates at about 1300°C whereas the lower portion may reach 1600°C. With 1300 W of electron-bombardment power from the tipper filament, the upper portion will reach2000°C. An additional 1000 W from the side heaters raises the
mid-region of the anode cylinder to 1900°C. resulting in a relatively uniform temperature distribution along the length of the anode cylinder. Under these conditions, the suspension system accommodates about a 0.8
mm expansion of the anode cylinder. We checked the transverse displacement of the anode by viewing the extraction slit with a telescope, and found it not to move (within ±0.1 mm) as the source was repeatedly heated and cooled. 3.3. Yields Since the yield of radioactivity of a helium-jet ion source depends critically on the aerosol transport mechanism, much effort by many groups has gone into understanding this process [7-9]. We have elected to
V.T Koslowsky et al. / TheChalk Riverheliumjet ionsource
25 1
4eV,
for which results are shown in fig . 10. The periodic table in fig. 11 shows those elements that have been extracted from the Chalk River heliumjet ion source . Many benefit from higher ion-source operating temperatures . Attempts to extract isotopes 1°C, 14 0 and ZsP failed. The yield values are listed in table 2.
Table 2 Radioactive yields with the Chalk River helium-jet ion source 1400
1600
1800
TEMPERATURE 1 °Q
2000
Fig. 8. Anode-cylinder temperature profile vs heating power. Curve A represents the temperature profile with no supplementary heating. Curve B is the temperature profile with supplementary top heating and curve C is the profile with both top and side heating . Power dissipation from the various heat sources are listed in the accompanying table.
Isotope 23Mg 24A1 27Si 31S 34C1 38' K 3s K 39 Ca 42 sc
take a more pragmatic approach and have empirically determined the operating parameters that provide the maximum radioactive yield, and checked for correlations of radioactive yield with our NaCl aerosol yield . The results are shown in fig. 9. Curves A and B show the yield of 3s K(t 1/2 = 7 .6 min) as a function of helium transport-gas flow . They were taken with the 15capillary target chamber, mentioned above, and 1 mm (curve A) and 0 .8 mm (curve B) inner diameter capillary tubes. The capillary length was 2 .5 m. The activity was produced by bombardment of a stack of 15 KF targets with protons from the tandem accelerator. The dashed line, C, represents the 23Na+ mass-separated beam current observed with the 0 .8 mm capillary. Curves D and E represent the aerosol yield at the end of the capillary as measured independently with a crystal thickness monitor for the same 1 mm and 0.8 mm diameter capillaries. Aerosol yields below 2 std 1/m were too low to be reliably measured with the crystal thickness monitor. The shaded regions indicate the degree of scatter in repeated measurements. The aerosol, 23 Na + and 3sK yields are all observed to improve with increasing capillary diameter and flow up to about 3 std 1/m . The aerosol yield continues to increase with further increases in flow but the 3s K yield decreases. The mechanisms that lead to this effect are not understood. For this target chamber, the optimum experimental conditions appear to lie with the 1-mmdiameter capillary and a flow rate of 3 std I/m . The effect of anode temperature on yield is most dramatic for the production of isotopically separated
46 V S.Mn 54 Co 5scu 63G a 6"As 70As
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','Ba
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Pd
t 112 11 .3 s 2.07s 4.16s 2.58s 1.53s 0.92s 7.6 min 0.86s 0.68s 0.42s 0.28s 0.19S 3.2s 31 s 2.53 min 53 mir. 4.74 min 1.5 min 57 .7 min 69 .2 min 10.3 min 18 min 14 s 64 s 32 s 1 .75 min 24 s 12 s
10 s
37 s 2s 26 s 4s 2.4s
Yield [atoms/s] 22000 a 1000 a 8000 a 250a 10000 , 3000 b 300000 b 450 a 40000 a 10000 , 30000 a 7000 a 5500 a 1200 ` 400 d 700d 300d 250d 3000 d 2200 d 220d 600d 33 000 d 27000 d 3000 d 250d 5000 d 1000 d 150` 350 ` ` 50 500 , 7000 t 1000 t
a (p,
n) reactions; yield normalized to a primary beam current of 10 WA. (p, pn) reaction; yield normalized to a primary beam current of 10 WA ; ion source operated in surface-ionization mode. ` 32S primary beam ; compound-nucleus evaporation product; yield normalized to 0.1 particle WA. d 160 primary beam; compound-nucleus evaporation product; yield normalized to 0.3 particle WA. ` 12C primary beam; compound-nucleus evaporation product; yield normalized to 0.3 particle WA. r Proton induced fission of nat. U; yield normalized to a primary beam current of 2 WA. b
IV. ION GUIDES/HE JETS
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V.T. Koslowskyet al. / The Chalk River helium jetion source
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1400
1800
1600
AVERAGE ANODE TEMPERATURE ICl Fig. 10 . Yield of 46V as a function of average anode temperature. Fig. 9. Yield vs helium flow for: (A) 3sK with a 1-mm-diameter capillary; (B) 3BK with a 0.8-mm-diameter capillary; (C) 'Na(from the aerosol) with a 0.8-mm-diameter capillary ; (D) NaCl with a 1-mm-diameter capillary as measured with a crystal thickness monitor ; and (E) NaCI yield with a 0.8-mmdiameter capillary as measured with a crystal thickness monitor . All capillaries were 2.5 m long. Radioactive yields increase roughly in proportion to the NaCl yield when the capillary size is increased but the trends deviate at flows above 3 std 1/m . The varying flow rates were achieved by changing thetarget-chamber pressure .
O
3.4. Flourination
Attempts to enhanceyields of radioisotopes through the addition of trace quantities of CFa have not been entirely successful . In two on-line experiments, yields of t2'Ba (extracted as Ba +, BaF+ or BaF2) and 42 SC (extracted as Sc', ScF+, ScF2 or ScF3) have not exceeded those obtained from the sources with the supplementary heaters operating . However, in the case of Ba, yields of other isobaric elements were drastically
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Fig. 11 . Periodic table indicating elements that have been investigated on-line . Production rates of many are enhanced by additional heating of the anode cylinder.
V.T. Koslowsky et al. / The Chalk River heliumjet ion source
253
reduced resulting in purer Ba samples . Yields of molecular ions decrease if both CF, and supplementary heat is used . The use of CF, shortens ion-source component life, and typically requires insulators and heat shields to be replaced as often as the filament. After several hours of routine operation, high-voltage discharges become intense and frequent, signalling the need to replace components.
support structure and improve the cooling and shielding of peripheral components. Future efforts will likely concentrate on the extension of the lower-filament lifetime, presently about 12 hours, and checks of the suitability of this ion source for mass measurements [10] of nuclei far from stability; this application demands long-term stable operation of the source .
4. Conclusions
References
The helium jet ion source has proven itself to be a useful tool for the study of a broad range of short-lived isotopes. Recent developments have increased the gas-transfer flow and reduced the transfer time by: (1) removing the restrictive spark arrestor and operating the helium pumping system at extraction potential, and (2) commissioning the sidejet which further limits the helium gas conductance into the ion source . Yields can now be optimized as a function of flow and targetchamber pressure without detrimentally affecting ionsource performance . The ion-source release time, although dependent on element and operating temperature, is measured to be less than 300 ms. The ion source has been equipped with supplementary heaters, which raise the temperature of the midto-upper portion of the anode cylinder by several hundred degrees . This has a dramatic impact on the yields of short-lived isotopes such as 42 Sc, 46 V and 54 Co, which often cannot be extracted without supplementary heating. To permit reliable, continuous operation of the ion source at elevated temperatures, the ion source has been redesigned to improve on the anode cylinder's
[II H. Schmeing, J .S. Wills, E . Hagberg, J .C. Hardy, V.T. Koslowsky and W .L . Perry, Nucl . Instr. and Meth . B26 (1987) 321 . [2) J.C . Hardy, I .S. Towner, V .T. Koslowsky, E . Hagberg and H . Schmeing, Nucl . Phys. A509 (1990) 429 . [31 R. Kirchner, Nucl . Instr. and Meth. B26 (1987) 204. [41 H . Schmeing et al., 12th Int. Conf. on Cyclotrons and their Applications, Berlin, Germany, 1989, eds. B. Martin and K. Ziegler (World Scientific, Singapore, 1989) pp. 88-92. [51 H . Schmeing, 1.C . Hardy, E. Hagberg, W.L. Perry, J.S . Wills, J . Camplan and B. Rosenbaum, Nucl. Instr. and Meth . 186 (1981) 47. [61 H. Schmeing, V .T. Koslowsky, M. Wightman, J .C. Hardy, J.A. MacDonald, T. Faestermann, H.R. Andrews, J .S. Geiger and R.L. Graham, Nucl . Instr . and Meth . 139 (1976) 335 . [71 H . Wollnik, H .G. Wilhelm, G . R66ig and H. Jungclass, Nucl . Instr. and Meth. 127 (1975) 539. [81 W . Wiesehahn, G . Bischoff and J . O'Auria, Nucl. Instr. and Meth . 129 (1975) 187. [91 W .J . Wiesehahn, G. Bischoff and J.M . D'Auria, Nucl. Instr . and Meth . 124 (1975) 221 . [101 K .S. Sharma, H. Schmeing, H.C. Evans, E. Hagberg, J.C . Hardy and V.T. Koslowsky, Nucl. Instr. and Meth. A275 (1989) 123 .
IV. ION GUIDES/HE JETS
HOH B
Beam Interactions with MaterialsiAtoms
The Chalk River high-temperature helium jet ion source V.T. Koslowsky, M.J . Watson, E. Hagberg, J .C. Hardy, W.L. Perry, M .G . Steer and H. Schmeing AECL Research, Chalk RiverLaboratories, Chalk River, Ontario, KOJ IJO Canada
P.P . Unger and K.S. Sharma
Physics Department, University of Manitoba, Winnipeg, Manitoba, R3T 2N2 Canada
The Chalk River helium-jet ion source has been upgraded to accommodate a helium transfer-gas flow of 6 std I/m andoperate at 2000°C. It now provides yields exceeding 1000 atoms/s forshort-lived isotopes of some refractory elements. 1. Introduction
Development work on the Chalk River heliumjet ion source, first described [1] at EMIS-11, has been strongly influenced by the requirements of our studies of 0+- 0 ' superallowed ß emitters [2]. These studies were primarily aimed at accurate measurements of the half-lives of 42Sc, 46V, soMn and s4 Ca, performed with isotopically pure samples. Higher ion source operating temperatures and higher helium flow rates were required to achieve good yields of these sub-second isotopes that are characterized by high heats of desorption [3]. In this paper, we report our experiences with a high temperature helium jet ion source suitable for short-lived isotopes of some refractory elements . 2. Experimental setup 2.1 . System layout At the recently completed TASCC Facility [4), a heavy-ion beam from either the 15 MV tandem accelerator or the superconducting cyclotron can be directed to the isotope separator [5]. The beam can either enter the ion source and strike an internal target or impinge on targets external to the ion source in a separate helium-flushed target chamber, approximately 2 m away from the ion source. In the latter case, the beam enters the target chamber through a thin metallic-foil window. To prevent inadvertent venting of the beam lines and accelerators in the event of a window rupture, the beam line adjacent to the separator is equipped with shock-attenuating baffles and a highspeed valve. These devices can confine any helium Elsevier Science Publishers B.V.
shock-waves to the nearest 2 m of accelerator beam line . The isotopes 42Sc, 46V, SOMn and 54CO were produced by bombardment of a stack of 15 isotopically enriched targets with several microamperes of protons from the tandem accelerator . Product recoil ranges in 1 to 2 atm of helium are about 2 mm for these reactions so the targets were spaced 5 mm apart. When heavy-ion reactions or proton-induced fission are used to produce radioisotopes, larger volume target chambers are usually required in order to thermalize the more energetic reaction products . Gas streamlines in glass models of our chambers were filmed and studied to ensure that eddy currents, which were shown to trap the recoils for periods of several seconds, were kept to aminimum. In the target chamber, thermalized charged reaction products attach themselves to NaCI aerosol particles that are continuously added to the helium stream as it passes over a hot (600°C) bed of granulated NaCl, which is located in a quartz tube and heated by a tubular oven . The helium gas exits the target chamber via one or several stainless-steel capillaries leading through a manifold to a single teflon transport capillary (0.5 to 1.5 mm internal diameter and 1.5 m long); this is joined in turn to a 1-m-long stainless-steel capillary of comparable inner diameter that is attached to the ion-source assembly . 2.2. Helium removal and the side jet Most of the helium conducted by the capillary is removed by a skimming process before it reaches the ion source. The capillary exit is in a pumped region and faces a flat plate, through which an orifice (1 mm diameter), leading to the ion source, has been bored. IV. IONGUIDES/HE JETS
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V. T. Koslowsky et al. / The Chalk River heliumjet ion source
Upon streaming out of the nozzle, most of the helium gas expands into a wide cone and is pumped away by a Roots pump (nominal pumping speed 2000 I/s) . Most of the activity-laden aerosol particles are relatively heavy and leave the nozzle in a narrow cone, pass through the orifice and enter the interior of the ionsource body. With this arrangement 97.5% of the helium can be skimmed off while more than 90% of the activity passes through the orifice [6]. Losses due to decay during transit from the target chamber to the ion source can only be reduced by increasing the carrier flow rate. Formerly, our helium flow rates were limited to about 1 std 1/m by the helium load tolerated by the ion source (about 0.4 std cm'/s), the vacuum required to avoid an electrical discharge do the extraction electrode, and the gas pressure tolerated by the spark arrestor [1] which isolated the Roots pump at ground potential from the ionsource extraction potential. These limitations have been overcome by operation of the Roots pump and associated forevacuum pumps at extraction potential (30-70 kV), thereby avoiding the need for the spark arrestor, and by installing a "side-jet" [6], which further limits the gas conductance into the ion-source body without significantly altering the aerosol transmission through the orifice. The sidejet consists of an additional stream of pure helium that is blown across the main jet, as shown in the inset of fig . 1 . It is most effective when its throughput is comparable to that of the transport capillary. Its effect on ion-source pressure is shown in fig. 1 . Its use was
1
2
3
4
HELIUM JET GAS FLOW (std. I /m)
Fig. 1 . Helium partial pressure in the ion-source chamber -hen the helium jet is operated with gas flows ranging from 0 to 6 std 1/m . Operation of the side-jet at half the primary heliumjet flow reduces the pressure by a factor of about four without altering the aerosol-particle transmission into the ion-source . Ion-source operation is unaffected up to a helium partial pressure of 2 x 10 -Z Pa. The partial pressure of argon (support gas) measured in the ion-source chamber is 1 .4X 10 -3 Pa . The insert shows the optimum sidejet configuration that was empirically determined.
Fig . 2. Schematic front view of the current Chalk River helium-jet ion source . Most of the components on the left have been omitted for clarity. The extraction-slit location is shown dashed in the center of the schematic. It is 3 cm long and 1 mm wide. The points of interest are : (1) a 2 mm anode entrance orifice, through which aerosol-particles are injected; (2) the cylindrical anode cylinder; (3) the hollow locating pin and support-gas entrance ; (4) the water-cooled copper side shield; (5) the upper tungsten hairpin filament; (6) the upper molybdenum support yoke - the lower and upper yokes support the anode cylinder and slide in grooves machined into the two side shields ; (7) the replaceable tungsten partition separating the plasma region, below, from the upper heater ; (8) one of two LaB. side heaters ; (9) one of six electrical feedthroughs; (10) the water-cooled copper back-support plane, 15 cm in diameter; (11) one of four nimonic springs that accommodate the expansion of the anode cylinder; (12) a boron nitride plug; and (13) the lower tungsten hairpin filament. formerly precluded because the additional gas load could not be tolerated by the spark arrestor. Because of these alterations, helium-flow rates up to 6 std 1/m can now be utilized for the transport of reaction products without any increase in ion-source pressure . The effectiveness of the side-jet depends critically on alignment. If the position of the side jet deviates from the ideal by more than one quarter of a capillary diameter, it become ineffective . The optimum geometry was determined empirically and is shown in the inset of fig. 1. Provided the helium flow remains laminar, a flow rate of ä std 1/m will sweep-out the "large" 15-capillary target chamber (2 .7 cm') in less than 100 ms when at pressures below 4 atm. The capillary transit time is about 20 ms under these conditions . 2.3. The ion source The ion-source configuration presently in use is shown schematically in fig. 2. As in the earlier heliumjet ion source [1], we have maintained the slit geometry
V. T. Koslowsky et al. / The Chalk River helium jet ion source
of the Bernas-Nier ion source [5], which offers high gas throughput and good source efficiency. The aerosol particles (containing reaction products) enter the source through the 2 mm orifice at the rear of the anode cylinder . The large ratio of extraction-slit to entranceorifice areas is desirable to prevent backstreaming of the reaction products . The lower hairpin filament is heated with a do current and biased between -30 and -90 V relative to the anode. It is fabricated from 1.5 mm diameter tungsten rod. The arc discharge occurs between the lower filament (cathode) and anode when the support gas, usually argon, is bled into the source. The source requires an axial magnetic field of 200 to 300 G, which is provided by an external electromagnet. To facilitate higher-temperature operation (coldspot temperature at 1800°C rather than 1300°C [1]) the following design features have been incorporated (see fig. 2): (1) The water cooling of the copper back-plane that supports the ion-source components has been improved by an increase in the cooling-channel size and relocation of the channels to positions closer to the source of heat . (2) The use of soft solder has been eliminated in the fabrication of the ion-source housing. Permanent joints are either brazed or welded. (3) Twowater-cooled side shields shade the peripheral regions of the source that contain the electrical feedthroughs. Since the side shields are in close proximity to the hot anode, they were fabricated from copper components welded together to form the cooling-water channels . (4) The side shields also support the anode cylinder. In the present design (see fig. 3), the anode is supported at each end by a spring-loaded molybdenum yoke that slides freely in groovesmachined into the top and bottom of the side shields. The edges of the yoke, which are in contact with the side shields, are bevelled to permit slight rotations and rocking of the yoke without binding. This arrangement permits the anode to expand when heated . The springs are fabricated from nimonic-90 wire . Unlike other spring steels, tensioning springs fabricated from nimonic-90 wire do not anneal when the ion source is at a high operating temperature . Alignment of the anode is accomplished with one hollow locating pin that seats into a hole at the rear of the anode about midway down its length and by locating the yoke with keys that permit vertical expansion of the anode cylinder . Both the pin and keys are fastened to the copper back plane. (5) The support-gas connection to the anode formerly consisted of a graphite/ tantalum joint which, after a few days of use, would embrittle and crack. The joint has been eliminated now that the gas is introduced through the centre of the locating pin described above. The pin is fabricated from a molybdenum alloy.
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PIN Fig . 3. Schematic illustration of the approach used to support the anode cylinder. The lower and upper yokes are identical and free to slide in grooves machined into the side shields. This permits the anode cylinder to expand when heated. Lateral stability is provided by the key in the yoke and the hollow locating pin, which centres the anode cylinder andalso acts as an inlet for the support gas. Both the locating pin and key are fastened to the copper back plane . (6) Anode cylinders of tantalum, tungsten wad graphite have been used on-line and off-line. Tantalum anodes are only suitable for operating temperatures below 1600°C. Above these temperatures, the anode distorts and walls can be destroyed within 24 hours by the electron-bombardment heating. Tungsten and graphite have stood up very well at high temperature. Most of our operating experience has been with tungsten as it is less fragile and outgases less when heated . (7) A supplementary heater, consisting of a 1.5mm-diameter tungsten hairpin filament, powered with ac current and biased to -600 V with respect to the anode, has been inserted into thewell at the top of the anode cylinder. The filament tip is located 2 mm from the partition. Electron-bombardment heating provides up to 1 kW of heating power to the upper portion of the anode cylinder. (8) The partition dividing the anode cylinder into an upper and a lower section is now removable and replaceable, since electron bombardment from the top heater eventually destroys this surface. It also permits easier and cheaper fabrication of the anode cylinders. (9) Two additional electrical feedthroughs have been added to introduce power to the anode sideheaters. All six feedthroughs (two each for top, side and lower heaters) are identical and water cooled. (10) The side heaters, a novel feature of this ion source, augment the upper supplementary heater described above. The side heaters consist of sintered LaB6 pellets mounted in close proximity to the hot anode cylinder . They are radiatively heated by the IV . ION GUIDES/HE JETS
V. T Koslowsky et al. / The Chalk Ricerheliumjet ion source
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..
.
I
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. . 01 l'O ! 1 1i J ;2 01 7 S III' i 'lll' ,IIIIIIf~III !n'l' .II ! .''
13 !Jln
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Fig. 4. Photograph of the assembled source with the outer cover removed.
Fig. 5 . Photograph of the helium-jet ion source, ready to be inserted into its vacuum housing. The helium skimmed off is pumped away through a manifold which surrounds the open area near the base flange .
V. T. Koslowsky et al. / The Chalk River heliumjet ion source Table 1 Typical operating conditions of the helium-jet ion source Cathode (lower filament) Arc Magnetic field Support gas/flow rate Extracted current (total) 3eAr current Helium load Top heater filament electron bombardment Side heater electron bombardment
5V 50-90 A 30-60 V 5-10 A 200-300 G argon/0.02 std cm3/s 5-10 mA 5-10 pA 0.4 std cm3 /s 6V 600 V
80 A 0.5-2 A
1 kV
0.25-1 A
anode and biased to about -1 kV with respect to the anode. When they reach 1400 to 1500°C, the pellets easily provide an electron emission current of up to 1 A, thus providing several hundred additional watts of i ,eating power . Typical operating conditions of the source are given in table 1 . Figs . 4 and 5 show photographs of the source face and helium-jet ion-source assembly. The beam is extracted through the 30-mm-long by 1-mmwide slit in the anode and accelerated across the 40 to 70 kV extraction potential .
249
within 30 ms with a customized valve. Care was taken to ensure that the conductance and flow conditions of both paths were matched . Currents were measured with a Faraday cup and electrometer and recorded with a chart recorder. The results are shown in fig. 6 for a 2.5-m-long, 0 .8-mm-internal diameter capillary. The helium flow was 4 std 1/m . Curves B and C represent the response for 23Na+ and 35 01 +, respectively . Their temporal response is independent of ion-source operating conditions. The test was repeated with a SCC1 3 aerosol . The a5Sc+ and 35CI+ responses are also shown in fig . 6. Curve E shows the 45Sc+ response with no supplementary ion-source heating; it becomes more rapid (by about 120 ms) and intensifies (by about a factor of 3 .5) when the top heater provides an additional 1 kW of heat (see curve A). Interestingly, the corresponding 35CI beam burst (curve D) is unchanged by ion-source operating conditions and is identical to the 35C1 response from NaCl (curve C). This suggests that the ion-source temperature does not strongly affect the break-up of the aerosol particle but strongly affects the ionization efficiency and dwell time of the ioi is in the source. The shape of the curves is partially determined by the 50 ms rise time of the electrometer used. The 30
3. Operating experience 3.1. Time response
uM
An important operating characteristic of an on-line ion source is the time that elapses between the production of a radioactive atom and its extraction from the ion source . For a heliumjet ion source, this dwell time can be analyzed in terms of three components : the target-chamber dwell time, the capillary transit time, and the ion-source dwell time. The capillary transit time can be estimated from the measured helium flow, initial gas pressure and capillary dimensions . For a 2 m length, its contribution to the total dwell time is usually tens of milliseconds and can be neglected . We have investigated the two remaining components several ways . First, we made off-line investigations of the time-response of different elements to ion-source operating conditions by injecting aerosol formed from various salts. For example, we measured the mass separated 23Na+ and 35CI + currents as a function of time when a 100 ms burst of NaCl aerosol was injected into the ion source. In our experimental setup, either pure helium or NaCl-laden helium could be directed to the ion source while the other was evacuated to a vacuum pump . The two gas streams could be interchanged
m r~ 2 WF rm ~â1
â
0.1
0.2 0.3 TIME (s)
0.4
0.5
Fig. 6. Mass-separated beam currents of stable Na, CI and Sc isotopes as a function of time, produced by a momentary injection of NaCl-or ScCl 3-laden helium into the helium-jet ion source (see section 3 .1 for details). The burst of aerosol begins at time zero and occurs for 0.1 s. Curve A is the 45Sc+ response with supplementary heat; curve B is the 23Na+ response with or without supplementary heat; curve C is the 35CI+ response from NaCI aerosol - it is also unaffected by the degree of ion-source heating; curve D is the 35CI+ response from SCCI 3 aerosol - also unaffected by the degree of ion-source heating; and curve E is the 45Sc+ response with no supplementary ion-source heating. The estimated transit time through the capillary and the measured delay of the customized gas switch that was used to inject the aerosol, to talling 50 ms, has been subtracted from the measured response. IV. ION GUIDES/HE JETS
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V.T. Koslowskyet al. / TheChalk Rimerhelium jet ionsource the integrated intensities of the primary and secondary beams. In cases A and C, the ion source was operated without supplementary heaters, and with conservative arc voltage/current settings of 40 V and 5 A. When the source was operated at 70 V and 8 A the response advanced in time, as shown in curve B. The overall
system efficiency lies between 1% and 2% for all three cases. When the primary In beam was left on continuously, the steady-state secondary beam current reached 3% of the primary beam current. Clearly, the targetchamber dwell time dominates any ion source or transit dwell time. Fig. 7. The mass-separated ii51v yield (stable isotope) as a function of time when a 47 nA "'In" beam-pulse from the Chalk River tandem accelerator is thermalized in the target chamber of the helium-jet ion source. Gas flow and targetchamber pressure were 1 std I/m and 1 atm, respectively . The pulse begins at time zero and occurs for 0.2 s. Curve A is the response with a 12 .5-cm-long, 1 .5-cm-diameter target chamber. Curves B and C are the responses for a 4-cm-long 4-cm-diameter chamber. Curve B was obtained with the ionsource arc parameters at 70 V and 8 A, substantially higher than the operating condition of 40 V and 5 A which yielded curves Aand C.
ms switching time and 20 ms capillary transit time have already been subtracted from the measured values. The cause of the remaining 100 ms delay seen in fig . 6 is not known. It could be due to turbulence caused during the 30 ms switching interval, since this is comparable in length to the capillary transit time, or due to an ion-source dwell time . Second, we investigated the effects of the target chamber, which can dramatically alter the temporal response shown in fig . 6. In a separate set of experi115ln5+ ments, a 48 MeV beam from the Chalk River tandem accelerator was directed into a number of
large-volume target chambers. The In beam was thermalized in 1 atm of helium and transported to the helium jet ion source with a 1 std 1/m gas flow. The temporal response of the mass-115 separated (secondary) beam was monitored with a Faraday cup and electrometer after a 200 ms pulse of the primary In beam . The results are shown in fig. 7, timed from the beginning of the primary beam pulse. As before, the capillary transit time has been subtracted from the measured time. Curve A is the response for a 1.:.7-cmlong, 1.5-cm-diameter target chamber equipped with five equally spaced gas inlets and seven equally spaced
gas outlets. Curve C is the response for a 4-cm-long, 4-cm-diameter target chamber equipped with three equally spaced inlets around the cylinder and seven equally spaced outlets on the back flange . The overall system efficiency was evaluated through comparison of
.3.2. Ion source temperatures Yields of isotopes of Sc, V and Co have shown a strong dependence on the degree of supplementary heating . This has led to the present ion-source design, one that is better suited than the original for high-temperature operation, as noted in section 2. We have measured the operating temperature of the ion-source components in a test stand that is equipped with a large quartz window permitting a direct view of the ion-source face . Temperature measurements were made with a disappearing-filament pyrometer . This
method provides a good relative temperature scale but absolute temperature determinations are limited in accuracy to ±200°C by the ill-defined emissivity and absorption of the ion-source components and quartz window. Temperature measurements briefly performed with a two-colour pyrometer, less sensitive to these effects, suggest that our quoted absolute temperatures may be two to three hundred degrees low. The results are shown in fig. 8. With no supplementary heat, the
upper portion of the anode cylinder operates at about 1300°C whereas the lower portion may reach 1600°C. With 1300 W of electron-bombardment power from the tipper filament, the upper portion will reach2000°C. An additional 1000 W from the side heaters raises the
mid-region of the anode cylinder to 1900°C. resulting in a relatively uniform temperature distribution along the length of the anode cylinder. Under these conditions, the suspension system accommodates about a 0.8
mm expansion of the anode cylinder. We checked the transverse displacement of the anode by viewing the extraction slit with a telescope, and found it not to move (within ±0.1 mm) as the source was repeatedly heated and cooled. 3.3. Yields Since the yield of radioactivity of a helium-jet ion source depends critically on the aerosol transport mechanism, much effort by many groups has gone into understanding this process [7-9]. We have elected to
V.T Koslowsky et al. / TheChalk Riverheliumjet ionsource
25 1
4eV,
for which results are shown in fig . 10. The periodic table in fig. 11 shows those elements that have been extracted from the Chalk River heliumjet ion source . Many benefit from higher ion-source operating temperatures . Attempts to extract isotopes 1°C, 14 0 and ZsP failed. The yield values are listed in table 2.
Table 2 Radioactive yields with the Chalk River helium-jet ion source 1400
1600
1800
TEMPERATURE 1 °Q
2000
Fig. 8. Anode-cylinder temperature profile vs heating power. Curve A represents the temperature profile with no supplementary heating. Curve B is the temperature profile with supplementary top heating and curve C is the profile with both top and side heating . Power dissipation from the various heat sources are listed in the accompanying table.
Isotope 23Mg 24A1 27Si 31S 34C1 38' K 3s K 39 Ca 42 sc
take a more pragmatic approach and have empirically determined the operating parameters that provide the maximum radioactive yield, and checked for correlations of radioactive yield with our NaCl aerosol yield . The results are shown in fig. 9. Curves A and B show the yield of 3s K(t 1/2 = 7 .6 min) as a function of helium transport-gas flow . They were taken with the 15capillary target chamber, mentioned above, and 1 mm (curve A) and 0 .8 mm (curve B) inner diameter capillary tubes. The capillary length was 2 .5 m. The activity was produced by bombardment of a stack of 15 KF targets with protons from the tandem accelerator. The dashed line, C, represents the 23Na+ mass-separated beam current observed with the 0 .8 mm capillary. Curves D and E represent the aerosol yield at the end of the capillary as measured independently with a crystal thickness monitor for the same 1 mm and 0.8 mm diameter capillaries. Aerosol yields below 2 std 1/m were too low to be reliably measured with the crystal thickness monitor. The shaded regions indicate the degree of scatter in repeated measurements. The aerosol, 23 Na + and 3sK yields are all observed to improve with increasing capillary diameter and flow up to about 3 std 1/m . The aerosol yield continues to increase with further increases in flow but the 3s K yield decreases. The mechanisms that lead to this effect are not understood. For this target chamber, the optimum experimental conditions appear to lie with the 1-mmdiameter capillary and a flow rate of 3 std I/m . The effect of anode temperature on yield is most dramatic for the production of isotopically separated
46 V S.Mn 54 Co 5scu 63G a 6"As 70As
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"S n 118cs 120cs
','Ba
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Pd
t 112 11 .3 s 2.07s 4.16s 2.58s 1.53s 0.92s 7.6 min 0.86s 0.68s 0.42s 0.28s 0.19S 3.2s 31 s 2.53 min 53 mir. 4.74 min 1.5 min 57 .7 min 69 .2 min 10.3 min 18 min 14 s 64 s 32 s 1 .75 min 24 s 12 s
10 s
37 s 2s 26 s 4s 2.4s
Yield [atoms/s] 22000 a 1000 a 8000 a 250a 10000 , 3000 b 300000 b 450 a 40000 a 10000 , 30000 a 7000 a 5500 a 1200 ` 400 d 700d 300d 250d 3000 d 2200 d 220d 600d 33 000 d 27000 d 3000 d 250d 5000 d 1000 d 150` 350 ` ` 50 500 , 7000 t 1000 t
a (p,
n) reactions; yield normalized to a primary beam current of 10 WA. (p, pn) reaction; yield normalized to a primary beam current of 10 WA ; ion source operated in surface-ionization mode. ` 32S primary beam ; compound-nucleus evaporation product; yield normalized to 0.1 particle WA. d 160 primary beam; compound-nucleus evaporation product; yield normalized to 0.3 particle WA. ` 12C primary beam; compound-nucleus evaporation product; yield normalized to 0.3 particle WA. r Proton induced fission of nat. U; yield normalized to a primary beam current of 2 WA. b
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V.T. Koslowskyet al. / The Chalk River helium jetion source
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5 4
z
3
r c a H m Q O W
0
1400
1800
1600
AVERAGE ANODE TEMPERATURE ICl Fig. 10 . Yield of 46V as a function of average anode temperature. Fig. 9. Yield vs helium flow for: (A) 3sK with a 1-mm-diameter capillary; (B) 3BK with a 0.8-mm-diameter capillary; (C) 'Na(from the aerosol) with a 0.8-mm-diameter capillary ; (D) NaCl with a 1-mm-diameter capillary as measured with a crystal thickness monitor ; and (E) NaCI yield with a 0.8-mmdiameter capillary as measured with a crystal thickness monitor . All capillaries were 2.5 m long. Radioactive yields increase roughly in proportion to the NaCl yield when the capillary size is increased but the trends deviate at flows above 3 std 1/m . The varying flow rates were achieved by changing thetarget-chamber pressure .
O
3.4. Flourination
Attempts to enhanceyields of radioisotopes through the addition of trace quantities of CFa have not been entirely successful . In two on-line experiments, yields of t2'Ba (extracted as Ba +, BaF+ or BaF2) and 42 SC (extracted as Sc', ScF+, ScF2 or ScF3) have not exceeded those obtained from the sources with the supplementary heaters operating . However, in the case of Ba, yields of other isobaric elements were drastically
YIELD INVESTIGATED ; NOT OBSERVED
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Ni
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=1
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Fig. 11 . Periodic table indicating elements that have been investigated on-line . Production rates of many are enhanced by additional heating of the anode cylinder.
V.T. Koslowsky et al. / The Chalk River heliumjet ion source
253
reduced resulting in purer Ba samples . Yields of molecular ions decrease if both CF, and supplementary heat is used . The use of CF, shortens ion-source component life, and typically requires insulators and heat shields to be replaced as often as the filament. After several hours of routine operation, high-voltage discharges become intense and frequent, signalling the need to replace components.
support structure and improve the cooling and shielding of peripheral components. Future efforts will likely concentrate on the extension of the lower-filament lifetime, presently about 12 hours, and checks of the suitability of this ion source for mass measurements [10] of nuclei far from stability; this application demands long-term stable operation of the source .
4. Conclusions
References
The helium jet ion source has proven itself to be a useful tool for the study of a broad range of short-lived isotopes. Recent developments have increased the gas-transfer flow and reduced the transfer time by: (1) removing the restrictive spark arrestor and operating the helium pumping system at extraction potential, and (2) commissioning the sidejet which further limits the helium gas conductance into the ion source . Yields can now be optimized as a function of flow and targetchamber pressure without detrimentally affecting ionsource performance . The ion-source release time, although dependent on element and operating temperature, is measured to be less than 300 ms. The ion source has been equipped with supplementary heaters, which raise the temperature of the midto-upper portion of the anode cylinder by several hundred degrees . This has a dramatic impact on the yields of short-lived isotopes such as 42 Sc, 46 V and 54 Co, which often cannot be extracted without supplementary heating. To permit reliable, continuous operation of the ion source at elevated temperatures, the ion source has been redesigned to improve on the anode cylinder's
[II H. Schmeing, J .S. Wills, E . Hagberg, J .C. Hardy, V.T. Koslowsky and W .L . Perry, Nucl . Instr. and Meth . B26 (1987) 321 . [2) J.C . Hardy, I .S. Towner, V .T. Koslowsky, E . Hagberg and H . Schmeing, Nucl . Phys. A509 (1990) 429 . [31 R. Kirchner, Nucl . Instr. and Meth. B26 (1987) 204. [41 H . Schmeing et al., 12th Int. Conf. on Cyclotrons and their Applications, Berlin, Germany, 1989, eds. B. Martin and K. Ziegler (World Scientific, Singapore, 1989) pp. 88-92. [51 H . Schmeing, 1.C . Hardy, E. Hagberg, W.L. Perry, J.S . Wills, J . Camplan and B. Rosenbaum, Nucl. Instr. and Meth . 186 (1981) 47. [61 H. Schmeing, V .T. Koslowsky, M. Wightman, J .C. Hardy, J.A. MacDonald, T. Faestermann, H.R. Andrews, J .S. Geiger and R.L. Graham, Nucl . Instr . and Meth . 139 (1976) 335 . [71 H . Wollnik, H .G. Wilhelm, G . R66ig and H. Jungclass, Nucl . Instr. and Meth. 127 (1975) 539. [81 W . Wiesehahn, G . Bischoff and J . O'Auria, Nucl. Instr. and Meth . 129 (1975) 187. [91 W .J . Wiesehahn, G. Bischoff and J.M . D'Auria, Nucl. Instr . and Meth . 124 (1975) 221 . [101 K .S. Sharma, H. Schmeing, H.C. Evans, E. Hagberg, J.C . Hardy and V.T. Koslowsky, Nucl. Instr. and Meth. A275 (1989) 123 .
IV. ION GUIDES/HE JETS