- Email: [email protected]
A fast-valve system for characterizing effusive-flow properties of vapor-transport systems: RIB applications
Nuclear Instruments and Methods in Physics Research B 267 (2009) 1187–1192
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
A fast-valve system for characterizing effusive-flow properties of vapor-transport systems: RIB applications J.-C. Bilheux, G.D. Alton * Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6372, USA
a r t i c l e
i n f o
Article history: Received 21 April 2008 Received in revised form 18 December 2008 Available online 3 January 2009 PACS: 02.50.Ng 05.10.Ln 47.45.Dt 47.60.+i 47.70.Nd 51.10.+y 68.43.Vx
a b s t r a c t Decay losses, associated with the times required for particles to diffuse from ISOL production targets and to effusively-flow to an ion source, must be reduced to as low as practically achievable levels in order to deliver useful beam intensities of short-lived isotopes for research at ISOL based Radioactive Ion Beam (RIB) facilities. We have developed a fast-valve system and complementary 3-D Monte–Carlo code which can be used separately or in combination to assess the effusive-flow properties of vapor-transport systems, independent of size, geometry and chemical properties of the transport species. In this report, we describe the fast valve and present time spectra and characteristic time data for noble gases flowing through serial- and parallel-coupled vapor-transport systems similar in geometry but longer than those used for RIB generation at the HRIBF with and without target coating matrices. Ó 2009 Published by Elsevier B.V.
Keywords: Effusive flow Molecular flow Vapor-transport system Monte–Carlo simulation ISOL target Radioactive Ion Beam
1. Introduction Several nuclear structure and nuclear astrophysics research facilities utilize accelerated Radioactive Ion Beams (RIBs) for studying reactions inaccessible in stable projectile/target combination experiments. The preponderance of RIB facilities, including the Holifield Radioactive Ion Beam Facility (HRIBF) [1,2], utilize the Isotope-Separator-On-Line (ISOL) method for production of shortlived species, in which case, isotopes of interest are formed within solid or liquid target matrices. Experimentally useful beam intensities, produced by this method, are often difficult to generate, since they must be diffused from the interior of the production target-material, effusively transported to an ion source, ionized, extracted, mass analyzed, and accelerated to research energies in a time commensurate with their lifetimes. Since decay losses associated with the integrated times required for the independent diffusion and effusive-flow processes to take place are principal means whereby short half-life radioactive species are lost between
* Corresponding author. Tel.: +1 865 574 4751; fax: +1 865 574 1268. E-mail address: [email protected] (G.D. Alton). 0168-583X/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.nimb.2008.12.019
initial formation and utilization, it is imperative to minimize delay times associated with these independent processes. Diffusion-release/effusive-flow processes have been previously studied experimentally [3–5], analytically [6] and numerically [7,8] at laboratories for Radioactive Ion Beam applications. The speeds at which the diffusion and effusive-flow processes occur are related to how well production targets [3,10–14] and vaportransport systems [3,8,9] are designed as well as the maximum temperatures to which they can be raised without compromising the ionization efficiency of the source. The diffusion process usually dominates delay-times associated with the ISOL production technique. For example, the time required for diffusion-release of 132Sn atoms from HRIBF-scale, 10 lm thick, fission reaction UC2/RVCF fibrous targets [12], maintained at 2100 K, is 8 s [10,11], while the effusive-flow time for this species through highly permeable target/vapor-transport systems, typically used at the HRIBF, is less than 15 ms. However, at next-generation RIB facilities, the two processes may be comparable because of the much larger scale target/vapor-transport systems required. Following diffusion release, time information for the effusiveflow process can be determined on an absolute basis by experimental measurement. For this purpose, we have conceived,
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J.-C. Bilheux, G.D. Alton / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1187–1192
designed and developed a first-of-kind fast-valve system that can quantify effusive-flow times as short as 100 ls for any chemically active or inactive species through any target system, independent of size, geometry and materials of construction, with or without target material in the respective target-material reservoir. As complement to the fast-valve system, we have developed a sophisticated 3-D Monte–Carlo code that closely replicates experimental effusive-flow data without target material in the vapor-transport system of interest [8]. This limitation is set by long CPU time requirements and complexity of programs necessary for accurately simulating the multitude of shapes and sizes of potential targets for RIB generation. This code has been used to determine the effusive-flow characteristics of a new concept vapor-transport system that reduces transport times over those of conventional HRIBF systems by >2 orders of magnitude [9]. In this article, the fast-valve system and principles of operation are described and experimental effusive-flow time data are presented for noble gases (He, Ne, Ar, Kr and Xe) flowing through serial- and parallel-coupled vapor-transport systems similar in geometry but longer than those used at the HRIBF. 2. Effusive-flow theory The vacuum conditions that exist in target/vapor-transport/ion source systems are such that molecular-flow (Knudsen) conditions prevail, i.e. the mean free path kMFP >> a, where a is a characteristic dimension of the vapor-transport system. We can appeal to the kinetic theory of gases (see, e.g. [14]) and derive a generalized expression appropriate for the number of particles N left in an arbitrary geometry and size system after an evacuation time t given by:
N ¼ N0 exp½t=sC ¼ N 0 exp½fC=Vgt ¼ N0 exp½ðm1=e =L1=e Þt;
ð1Þ
where, at t = 0, N = N0; sC = V/C is the characteristic time spent by a particle of velocity v1/e in the system of volume V and vacuum-conductance C; and L1/e is the average distance traveled by the particle during evacuation under effusive-flow conditions. For chemically active species, the characteristic effusive-flow time sC for particles that interact with the walls of the transport system [3]
sC ffi ½Nb1=e s0 expðHad =kB TÞ þ L1=e =m1=e ¼ ½Nb1=e s0 expðHad =kB TÞ þ C=V;
L1=e ¼ 1:397mV=C ¼ V=A;
ð4Þ
where C = 1.397mA and A is the effective area of the transport system of volume V. Since L is independent of species, we need only minimize L1/e (i.e. V/A) for a particular noble gas element to arrive at a vapor-transport system with optimized geometry that minimizes effusive-flow times and thereby, maximizes achievable RIB intensities for any chemically active or inactive species of a given half-life. Of course, sC, L1/e and V/A each depend on the geometry and size of the vapor-transport system and details of the target design (e.g. the permeability properties of the target material). Thus, for chemically inactive atoms/molecules, if the characteristic time is measured for a species of given mass, M, at a specified vaportransport system temperature, T, the time spectra or characteristic time, sC, is known for all temperatures for the particular specie or, if we measure the characteristic time, sC, for a species of a given mass, M, at a specified vapor-transport system temperature, T, the time spectra or characteristic time is known for all species at this temperature. Alternatively, if the volume V of the transport system is known for this group of elements, then the vacuum-conductance C can be computed for the system. 3. The fast-valve system A commercially available fast-valve (Fig. 1), initially designed to close in 10 ms [15], was modified to achieve a closing time of 100 ls. The minimum closing time was chosen to be much less than the characteristic time for transport of He (the lightest atomic species of interest to the RIB research community). An Al2O3 tube is used to connect the output of the fast-valve to the target/vapor-transport system under evaluation in order to electrically and thermally isolate it from the target/vapor-transport/ion source system. The fast-valve system consists of an electro-pneumatic actuator that drives a shutter for closing gas flow to the target-material reservoir. The shutter system is designed to minimize compression of feed gas that lies in its path during closing that could otherwise affect the time distribution of particles moving through the system. Measured signals are observed to decay as perfect exponentials.
4. Experimental apparatus and procedures
ð2Þ
where Nb1/e is the 1/e number of collisions that a particle makes with the target and walls of the transport system during transit through the system at temperature T; s0 is taken as s0 ffi 3.4 1015 s [3]; Had is the absolute enthalpy of adsorption; L1/e is the 1/e distance traveled per particle and m1/e is the 1/e particle velocity for a Maxwell–Boltzmann velocity distribution function. (m1/e = 1.397v where m = (8kBT/pM)1/2 is the average velocity of a particle within a Maxwell–Boltzmann velocity distribution.) The m1/e = 1.397v relation can be easily derived by integrating the normalized Maxwell–Boltzmann distribution function f(v) until the R integral reaches the 1/e value, i.e. N/N0 = AN f(v)v2dv = 0.6322 (normalization constant, AN) which occurs at a particle velocity v1/e = 1.397v. As noted in Eq. (2), sC depends on the chemistry (enthalpy of adsorption) between the transport-vapor and materials of construction of the system, the mass of the species in question and the characteristic distance traveled per particle during transit through the system under evaluation. Thus, for chemically inactive atoms/molecules, (e.g. noble gas elements) in which case Had ffi 0, the first term in Eq. (2) is ffi0 and hence
sC ffi L1=e =m1=e ¼ L1=e =ð1:397mÞ ¼ V=C:
A practically useful relation between L1/e and C results from Eq. (3):
ð3Þ
The effusive-flow measurements, described in this report, were conducted on an off-line test facility equipped with the essential components required for determining the effusive-flow times of gases through systems of interest (e.g. ion source, beam transport lenses, beam steerers, Faraday cups, a high resolution 90° magnetic analyzer for mass selection, a calibrated standard leak, data acquisition instrumentation required to record intensity versus time data and ancillary power supplies for powering the source and mass analysis system, etc.). The Electron Beam Plasma Ion Source (EBPIS) [16], used in the present experiments, is similar to the source used at CERN [17]. Test gases are fed into the ionization chamber of the source through a calibrated leak so that the flow of each gas is known. A fraction of the gas atoms/molecules is ionized, extracted from the source, accelerated to 20 keV, focused by means of an Einzel lens at the object slits of a 90° magnetic analyzer and then directed through the magnet to the image plane of the analyzer into a Faraday cup, mounted behind mass selection slits, where the signal from the chosen isotope is detected. Following closure of the fast-valve, signal versus time data are recorded. The time to reach 1/e of its initial value is taken as the characteristic effusive-flow time.
J.-C. Bilheux, G.D. Alton / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1187–1192
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Fig. 1. Schematic drawing of the fast-valve and gas input system for feeding gases into vapor-transport systems to the ion source. The inset illustrates the mechanics of valve closure.
5. Serial- and parallel-coupled vapor-transport systems A three-dimensional drawing of the integrated ion source/vapor-transport system is displayed in Fig. 2. The mechanical designs for serial- and parallel-coupled vapor-transport systems are briefly described below. 5.1. The serial-coupled vapor-transport system Fig. 2(a) shows a cut-plane through the long axis of the serialcoupled target-material reservoir. The target-material holder is a 15.38 mm I.D., 192 mm long tube coupled to the ion source by means of a vertically-oriented, 8.5 mm I.D. tube, weld attached to the mid-point of the reservoir. The tube is bent horizontally, 25 mm above the reservoir, and continues along the axis of the source until it reaches the cathode plate of the source. The cathode plate is weld attached to the end of the vapor-transport tube. Atoms/molecules, traveling along the tube pass through 8, 1 mm diameter holes, located in the cathode plate, into the ionization chamber of the source. Independently controlled electrical currents are used to heat the material reservoir and transport tube to high temperatures as required for affecting the respective diffusion and effusive-flow processes. Cathode plates are brought to thermionic emission temperatures by adjusting the current through the vapor conduit. 5.2. The parallel-coupled vapor-transport system A cut-plane through the long axis of the parallel-coupled vaportransport system is displayed in Fig. 2(b). For this system, the target-material tube holder is the central tube of a coaxial assembly and again is 192 mm in length and 15.38 mm I.D. but now has
58% transparency slotted walls. The target holder is weld attached to the end plates of a solid wall, 24.4 mm I.D. tubular chamber (also 192 mm in length). The annular space surrounding the targetmaterial reservoir provides a parallel route for radioactive particles to travel following diffusion release from all regions of the columnar target during on-line operation and hence the system is referred to as a parallel-coupled system. The target-material reservoir is again heated by passing current through the coaxial tube assembly. The cathode design and vapor conduit tubes and methods used for their heating are identical for both systems.
6. Effusive-flow data The experimentally observed data, presented in this article, were measured by flowing He, Ne, Ar, Kr and Xe through the all Ta serial- and parallel-coupled vapor-transport systems described previously. Highly permeable Reticulated-Vitreous-Carbon-Foam (RVCF) material [18] is used as matrices for deposition of target materials at the HRIBF [2,3,9–13]. A scanning electron micrograph (SEM) of 2 RVCF material used in the present experiments is displayed in Fig. 3. As noted, the material is low-density and highly permeable as required for fast effusive-flow of particles following diffusion release from thin-layer target material deposits on the surfaces of such matrices. RVCF compressed in the z-direction by a factor of 2 (2 RVCF), was used to simulate the presence of target material in each of the two vapor-transport systems described in this article and thereby to assess the affect of this material on the effusive-flow times of gases flowing through a given transport system. In other studies, higher density forms of RVCF have been used for this purpose (see, e.g. [3]). To simulate the presence of highly permeable target material in each of the systems, their
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Fig. 2. Isometric view of conventional vapor-transport systems, similar in geometry but longer than those used at the HRIBF, coupled to the EBPIS. (a) Serial-coupled and (b) parallel-coupled.
15.38 mm I.D. reservoirs were filled with closely stacked, 2 mm thick, 15 mm diameter 2 RVCF disks to form 192 mm long targets. Time spectra for the noble gases (i.e. He, Ar, Kr and Xe) gases were measured through each of the vapor-transport systems with and without 2 RVCF in the respective reservoirs of both vaportransports systems using the EBPIS.
target-material reservoir material arepdisplayed in Fig. 6. Characffiffiffiffiffi teristic sC data for noble gases versus M flowing through the system with and without 2 RVCF in the target-material reservoir are displayed in Fig. 7.
6.1. Serial-coupled vapor-transport system
Comparisons of measured pffiffiffiffiffi characteristic effusive-flow-times sC for noble gases versus M for the serial (Fig. 2(a)) and parallel (Fig. 2(b)) coupled vapor-transport systems with and without 2 RVCF in their respective reservoirs are displayed in Fig. 8. As noted for the serial-coupled system, gases effuse faster through the system without 2 RVCF in the reservoir than with material in the reservoir. This affect is directly attributable to the slight impedance to flow introduced by the 2 RVCF. The opposite is true for the parallel-coupled system. For this system, the characteristic times for the noble gases are decreased whenever 2 RVCF is added to the parallel-flow configuration due to the presence of the parallel open channel surrounding the slotted target-material reservoir that particles can take during transit through the system rather than through the material itself. The increase in
Time spectra for noble gases flowing through the serial-coupled vapor-transport system without 2 RVCF in the target-material reservoir material are displayed in Fig. 4. As noted, the spectra are pure exponentials, as predicted from theory (e.g. Eq. p(1)). ffiffiffiffiffi Characteristic effusive-flow time sC for noble gases versus M flowing through the serial-coupled vapor-transport system with and without 2 RVCF in the target-material reservoir are displayed in Fig. 5. 6.2. Parallel-coupled vapor-transport system Analogous time spectra for noble gases flowing through the parallel-coupled vapor-transport system without 2 RVCF in the
6.3. Data comparisons
J.-C. Bilheux, G.D. Alton / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1187–1192
1191
Fig. 6. Time spectra for noble gases flowing through the parallel-coupled vaportransport system (Fig. 2b) without 2 RVCF in the target-material reservoir. Ion source: EBPIS.
Fig. 3. Scanning electron micrograph (SEM) of uncoated Reticulated-VitreousCarbon-Foam (RVCF) compressed in one direction (2 RVCF). 2 RVCF is used as matrices for coating target materials at the HRIBF.
pffiffiffiffiffi Fig. 7. Characteristic effusive-flow times sC versus M for noble gases flowing through the parallel-coupled vapor-transport system (Fig. 2b) with and without 2 RVCF material in the reservoir. Experimental data: s, d, D, N; Theory: —; Ion source: EBPIS.
Fig. 4. Time spectra for noble gas elements flowing through the serial-coupled vapor-transport system (Fig. 2a) without 2 RVCF in the target-material reservoir material. Temperature: T = 1360 K; Ion source: EBPIS.
effusive-flow time for the parallel-coupled system over the complementary serial-coupled system with and without RVCF is directly attributable to the larger volume of the parallel-coupled system through which (larger L) particles must travel from the target to the ion source. These studies clearly show that 2 RVCF only slightly increases the effusive-flow times in the serial-coupled vapor-transport system while decreasing the times through the parallel-coupled system. 7. Discussion
pffiffiffiffiffi Fig. 5. Characteristic effusive-flow time sC for noble gases versus M flowing through the serial-coupled vapor-transport system (Fig. 2a) with and without 2 RVCF in the target-material reservoir. Experimental data: s, d, D, N; Theory: —; Ion source: EBPIS.
The fast-valve effusive-flow measurement system, described in this article, has proved to be a very efficient, cost effective and universal means for obtaining fundamentally important information on the effusive-flow properties of vapor-transport systems, independent of size, geometry or materials of construction and chemical properties of vapor-transport species. This information permits delineation of the time dependences the two principal processes (diffusion and effusive-flow) that limit the lifetimes of species that can be processed with intensities adequate for conducting research by giving time specificity to the effusive-flow process. To illustrate the utility of the fast-valve system, we provide effusive-flow time spectra and characteristic effusive time data for noble gases flowing through serial-coupled and parallel-coupled vapor-transport systems, similar in size to those used at the CERN-ISOLDE facility
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species, the fast-valve, separately or in combination with Monte– Carlo codes, such as described in [7,8], can be used to arrive at a system design which minimizes effusive-flow times and thereby, maximize achievable RIB intensities for any chemically active or inactive species of a given half-life. Acknowledgements The authors are indebted to students and staff members of the Advanced Concept Research and Development Group, who through their diligent efforts, contributed to the content of this paper through execution of effusive-flow measurements used to validate the fast-valve system described in this report. This work was supported by the US Department of Energy under contract DE-AC0500OR22725 with UT Battelle. References pffiffiffiffiffi Fig. 8. Comparison of characteristic effusive-flow time sC versus M for noble gases flowing through the serial- (Fig. 2a) and parallel-coupled (Fig. 2b) vapor-transport systems with 2 RVCF material in the reservoir. Experimental data: s, d, D, N; Theory: —; Temperature: 1300 K.
(see, e.g. [17]). These data show that gases effuse faster through the serial-coupled system, system without 2 RVCF in the reservoir than with material in the reservoir. The opposite is true for the parallel-coupled system. For this system, the characteristic times for the noble gases are decreased whenever 2 RVCF is added to the parallel-flow configuration due to the presence of the parallel open channel surrounding the slotted target-material reservoir that particles can take during transit through the system rather than through the material itself. The increase in effusive-flow times for the parallel-coupled system over those for the complementary serial-coupled system, with and without 2 RVCF in the targetmaterial reservoir, is directly attributable to the larger volume, and consequently, longer distance L through which particles must travel from the target to the ion source. These studies clearly show that higher temperatures reduce the effusive-flow times through either system and that 2 RVCF only slightly increases the effusive-flow times in the serial-coupled vapor-transport system while decreasing the times through the parallel-coupled system. Since the distance traveled through a given system L is independent of
[1] G.D. Alton, J.R. Beene, J. Phys. G; Nucl. Part. Phys. 24 (1998) 1347. [2] G.D. Alton, J.R. Beene, R.L. Auble, D. Stracener, Indian J. Phys. 76S (2002) 9. [3] J.-C. Bilheux, Doctor of Physics Thesis, entitled, Design Studies For High-Power Fast Diffusion-Release Targets and Fast Vapor-Transport Systems for Radioactive Ion Beam Generation, Universitè de Versailles, Saint-Quentin-enYvelines, Versailles, France, 2003. [4] Y. Kawai, G.D. Alton, Y. Liu, Proceedings of the 2005 Particle Accelerator Conference, IEEE Catalog Number: 05CH37623, 2005, pp. 1422. [5] R. Kirchner, Nucl. Instr. and Meth. Phys. Res. B 70 (1992) 186. [6] J.R.J. Bennett, AIP Conference Proceedings, vol. 473, 1998, pp. 512. [7] B. Mustapha, J.A. Nolen, Nucl. Instr. and Meth. Phys. Res. B 286 (2003) 204. [8] Y. Zhang, G.D. Alton, J. Vac. Sci. Technol. A 23 (2006) 1558; G.D. Alton, Y. Zhang, Nucl. Instr. and Meth. Phys. Res. A 539 (2005) 540. [9] G.D. Alton, J.R. Beene, Y. Liu, Nucl. Instr. and Meth. Phys. Res., Sect. A 438 (1999) 190. [10] G.D. Alton, Y. Zhang, Y. Kawai, Appl. Rad. Isotopes 64 (2006) 1574. [11] G.D. Alton, H.H. Moeller, ORNL-6957, September 1998, hribf021.pdf. [12] G.D. Alton, J.-C. Bilheux, A.D. McMillan, Nucl. Instr. and Meth. Phys. Res. Sect. A 521 (2004) 108. [13] Y. Kawai, J.-C. Bilheux, D.W. Stracener, G.D. Alton, Appl. Rad. Isotopes 63 (2005) 659. [14] R.D. Present, Kinetic Theory of Gases, McGraw-Hill Book Company., 1958. [15] VAT, Vakuumventile AG, CH-9469 Haag, Switzerland. [16] G.D. Alton, Proceedings of the 2001 Particle Accelerator Conference, IEEE: 01CH37268C 59, 2001. [17] S. Sundell, H. Ravn, The ISOLDE collaboration, Nucl. Instr. and Meth. Phys. Res. B 70 (1992) 160. [18] Reticulated-Vitreous-Carbon-Foam (RVCF) Used in These Investigations and for Generic Target Material Coating Matrices at the HRIBF is a Product of ERG, Oakland, CA 94608.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
A fast-valve system for characterizing effusive-flow properties of vapor-transport systems: RIB applications J.-C. Bilheux, G.D. Alton * Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6372, USA
a r t i c l e
i n f o
Article history: Received 21 April 2008 Received in revised form 18 December 2008 Available online 3 January 2009 PACS: 02.50.Ng 05.10.Ln 47.45.Dt 47.60.+i 47.70.Nd 51.10.+y 68.43.Vx
a b s t r a c t Decay losses, associated with the times required for particles to diffuse from ISOL production targets and to effusively-flow to an ion source, must be reduced to as low as practically achievable levels in order to deliver useful beam intensities of short-lived isotopes for research at ISOL based Radioactive Ion Beam (RIB) facilities. We have developed a fast-valve system and complementary 3-D Monte–Carlo code which can be used separately or in combination to assess the effusive-flow properties of vapor-transport systems, independent of size, geometry and chemical properties of the transport species. In this report, we describe the fast valve and present time spectra and characteristic time data for noble gases flowing through serial- and parallel-coupled vapor-transport systems similar in geometry but longer than those used for RIB generation at the HRIBF with and without target coating matrices. Ó 2009 Published by Elsevier B.V.
Keywords: Effusive flow Molecular flow Vapor-transport system Monte–Carlo simulation ISOL target Radioactive Ion Beam
1. Introduction Several nuclear structure and nuclear astrophysics research facilities utilize accelerated Radioactive Ion Beams (RIBs) for studying reactions inaccessible in stable projectile/target combination experiments. The preponderance of RIB facilities, including the Holifield Radioactive Ion Beam Facility (HRIBF) [1,2], utilize the Isotope-Separator-On-Line (ISOL) method for production of shortlived species, in which case, isotopes of interest are formed within solid or liquid target matrices. Experimentally useful beam intensities, produced by this method, are often difficult to generate, since they must be diffused from the interior of the production target-material, effusively transported to an ion source, ionized, extracted, mass analyzed, and accelerated to research energies in a time commensurate with their lifetimes. Since decay losses associated with the integrated times required for the independent diffusion and effusive-flow processes to take place are principal means whereby short half-life radioactive species are lost between
* Corresponding author. Tel.: +1 865 574 4751; fax: +1 865 574 1268. E-mail address: [email protected] (G.D. Alton). 0168-583X/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.nimb.2008.12.019
initial formation and utilization, it is imperative to minimize delay times associated with these independent processes. Diffusion-release/effusive-flow processes have been previously studied experimentally [3–5], analytically [6] and numerically [7,8] at laboratories for Radioactive Ion Beam applications. The speeds at which the diffusion and effusive-flow processes occur are related to how well production targets [3,10–14] and vaportransport systems [3,8,9] are designed as well as the maximum temperatures to which they can be raised without compromising the ionization efficiency of the source. The diffusion process usually dominates delay-times associated with the ISOL production technique. For example, the time required for diffusion-release of 132Sn atoms from HRIBF-scale, 10 lm thick, fission reaction UC2/RVCF fibrous targets [12], maintained at 2100 K, is 8 s [10,11], while the effusive-flow time for this species through highly permeable target/vapor-transport systems, typically used at the HRIBF, is less than 15 ms. However, at next-generation RIB facilities, the two processes may be comparable because of the much larger scale target/vapor-transport systems required. Following diffusion release, time information for the effusiveflow process can be determined on an absolute basis by experimental measurement. For this purpose, we have conceived,
1188
J.-C. Bilheux, G.D. Alton / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1187–1192
designed and developed a first-of-kind fast-valve system that can quantify effusive-flow times as short as 100 ls for any chemically active or inactive species through any target system, independent of size, geometry and materials of construction, with or without target material in the respective target-material reservoir. As complement to the fast-valve system, we have developed a sophisticated 3-D Monte–Carlo code that closely replicates experimental effusive-flow data without target material in the vapor-transport system of interest [8]. This limitation is set by long CPU time requirements and complexity of programs necessary for accurately simulating the multitude of shapes and sizes of potential targets for RIB generation. This code has been used to determine the effusive-flow characteristics of a new concept vapor-transport system that reduces transport times over those of conventional HRIBF systems by >2 orders of magnitude [9]. In this article, the fast-valve system and principles of operation are described and experimental effusive-flow time data are presented for noble gases (He, Ne, Ar, Kr and Xe) flowing through serial- and parallel-coupled vapor-transport systems similar in geometry but longer than those used at the HRIBF. 2. Effusive-flow theory The vacuum conditions that exist in target/vapor-transport/ion source systems are such that molecular-flow (Knudsen) conditions prevail, i.e. the mean free path kMFP >> a, where a is a characteristic dimension of the vapor-transport system. We can appeal to the kinetic theory of gases (see, e.g. [14]) and derive a generalized expression appropriate for the number of particles N left in an arbitrary geometry and size system after an evacuation time t given by:
N ¼ N0 exp½t=sC ¼ N 0 exp½fC=Vgt ¼ N0 exp½ðm1=e =L1=e Þt;
ð1Þ
where, at t = 0, N = N0; sC = V/C is the characteristic time spent by a particle of velocity v1/e in the system of volume V and vacuum-conductance C; and L1/e is the average distance traveled by the particle during evacuation under effusive-flow conditions. For chemically active species, the characteristic effusive-flow time sC for particles that interact with the walls of the transport system [3]
sC ffi ½Nb1=e s0 expðHad =kB TÞ þ L1=e =m1=e ¼ ½Nb1=e s0 expðHad =kB TÞ þ C=V;
L1=e ¼ 1:397mV=C ¼ V=A;
ð4Þ
where C = 1.397mA and A is the effective area of the transport system of volume V. Since L is independent of species, we need only minimize L1/e (i.e. V/A) for a particular noble gas element to arrive at a vapor-transport system with optimized geometry that minimizes effusive-flow times and thereby, maximizes achievable RIB intensities for any chemically active or inactive species of a given half-life. Of course, sC, L1/e and V/A each depend on the geometry and size of the vapor-transport system and details of the target design (e.g. the permeability properties of the target material). Thus, for chemically inactive atoms/molecules, if the characteristic time is measured for a species of given mass, M, at a specified vaportransport system temperature, T, the time spectra or characteristic time, sC, is known for all temperatures for the particular specie or, if we measure the characteristic time, sC, for a species of a given mass, M, at a specified vapor-transport system temperature, T, the time spectra or characteristic time is known for all species at this temperature. Alternatively, if the volume V of the transport system is known for this group of elements, then the vacuum-conductance C can be computed for the system. 3. The fast-valve system A commercially available fast-valve (Fig. 1), initially designed to close in 10 ms [15], was modified to achieve a closing time of 100 ls. The minimum closing time was chosen to be much less than the characteristic time for transport of He (the lightest atomic species of interest to the RIB research community). An Al2O3 tube is used to connect the output of the fast-valve to the target/vapor-transport system under evaluation in order to electrically and thermally isolate it from the target/vapor-transport/ion source system. The fast-valve system consists of an electro-pneumatic actuator that drives a shutter for closing gas flow to the target-material reservoir. The shutter system is designed to minimize compression of feed gas that lies in its path during closing that could otherwise affect the time distribution of particles moving through the system. Measured signals are observed to decay as perfect exponentials.
4. Experimental apparatus and procedures
ð2Þ
where Nb1/e is the 1/e number of collisions that a particle makes with the target and walls of the transport system during transit through the system at temperature T; s0 is taken as s0 ffi 3.4 1015 s [3]; Had is the absolute enthalpy of adsorption; L1/e is the 1/e distance traveled per particle and m1/e is the 1/e particle velocity for a Maxwell–Boltzmann velocity distribution function. (m1/e = 1.397v where m = (8kBT/pM)1/2 is the average velocity of a particle within a Maxwell–Boltzmann velocity distribution.) The m1/e = 1.397v relation can be easily derived by integrating the normalized Maxwell–Boltzmann distribution function f(v) until the R integral reaches the 1/e value, i.e. N/N0 = AN f(v)v2dv = 0.6322 (normalization constant, AN) which occurs at a particle velocity v1/e = 1.397v. As noted in Eq. (2), sC depends on the chemistry (enthalpy of adsorption) between the transport-vapor and materials of construction of the system, the mass of the species in question and the characteristic distance traveled per particle during transit through the system under evaluation. Thus, for chemically inactive atoms/molecules, (e.g. noble gas elements) in which case Had ffi 0, the first term in Eq. (2) is ffi0 and hence
sC ffi L1=e =m1=e ¼ L1=e =ð1:397mÞ ¼ V=C:
A practically useful relation between L1/e and C results from Eq. (3):
ð3Þ
The effusive-flow measurements, described in this report, were conducted on an off-line test facility equipped with the essential components required for determining the effusive-flow times of gases through systems of interest (e.g. ion source, beam transport lenses, beam steerers, Faraday cups, a high resolution 90° magnetic analyzer for mass selection, a calibrated standard leak, data acquisition instrumentation required to record intensity versus time data and ancillary power supplies for powering the source and mass analysis system, etc.). The Electron Beam Plasma Ion Source (EBPIS) [16], used in the present experiments, is similar to the source used at CERN [17]. Test gases are fed into the ionization chamber of the source through a calibrated leak so that the flow of each gas is known. A fraction of the gas atoms/molecules is ionized, extracted from the source, accelerated to 20 keV, focused by means of an Einzel lens at the object slits of a 90° magnetic analyzer and then directed through the magnet to the image plane of the analyzer into a Faraday cup, mounted behind mass selection slits, where the signal from the chosen isotope is detected. Following closure of the fast-valve, signal versus time data are recorded. The time to reach 1/e of its initial value is taken as the characteristic effusive-flow time.
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Fig. 1. Schematic drawing of the fast-valve and gas input system for feeding gases into vapor-transport systems to the ion source. The inset illustrates the mechanics of valve closure.
5. Serial- and parallel-coupled vapor-transport systems A three-dimensional drawing of the integrated ion source/vapor-transport system is displayed in Fig. 2. The mechanical designs for serial- and parallel-coupled vapor-transport systems are briefly described below. 5.1. The serial-coupled vapor-transport system Fig. 2(a) shows a cut-plane through the long axis of the serialcoupled target-material reservoir. The target-material holder is a 15.38 mm I.D., 192 mm long tube coupled to the ion source by means of a vertically-oriented, 8.5 mm I.D. tube, weld attached to the mid-point of the reservoir. The tube is bent horizontally, 25 mm above the reservoir, and continues along the axis of the source until it reaches the cathode plate of the source. The cathode plate is weld attached to the end of the vapor-transport tube. Atoms/molecules, traveling along the tube pass through 8, 1 mm diameter holes, located in the cathode plate, into the ionization chamber of the source. Independently controlled electrical currents are used to heat the material reservoir and transport tube to high temperatures as required for affecting the respective diffusion and effusive-flow processes. Cathode plates are brought to thermionic emission temperatures by adjusting the current through the vapor conduit. 5.2. The parallel-coupled vapor-transport system A cut-plane through the long axis of the parallel-coupled vaportransport system is displayed in Fig. 2(b). For this system, the target-material tube holder is the central tube of a coaxial assembly and again is 192 mm in length and 15.38 mm I.D. but now has
58% transparency slotted walls. The target holder is weld attached to the end plates of a solid wall, 24.4 mm I.D. tubular chamber (also 192 mm in length). The annular space surrounding the targetmaterial reservoir provides a parallel route for radioactive particles to travel following diffusion release from all regions of the columnar target during on-line operation and hence the system is referred to as a parallel-coupled system. The target-material reservoir is again heated by passing current through the coaxial tube assembly. The cathode design and vapor conduit tubes and methods used for their heating are identical for both systems.
6. Effusive-flow data The experimentally observed data, presented in this article, were measured by flowing He, Ne, Ar, Kr and Xe through the all Ta serial- and parallel-coupled vapor-transport systems described previously. Highly permeable Reticulated-Vitreous-Carbon-Foam (RVCF) material [18] is used as matrices for deposition of target materials at the HRIBF [2,3,9–13]. A scanning electron micrograph (SEM) of 2 RVCF material used in the present experiments is displayed in Fig. 3. As noted, the material is low-density and highly permeable as required for fast effusive-flow of particles following diffusion release from thin-layer target material deposits on the surfaces of such matrices. RVCF compressed in the z-direction by a factor of 2 (2 RVCF), was used to simulate the presence of target material in each of the two vapor-transport systems described in this article and thereby to assess the affect of this material on the effusive-flow times of gases flowing through a given transport system. In other studies, higher density forms of RVCF have been used for this purpose (see, e.g. [3]). To simulate the presence of highly permeable target material in each of the systems, their
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Fig. 2. Isometric view of conventional vapor-transport systems, similar in geometry but longer than those used at the HRIBF, coupled to the EBPIS. (a) Serial-coupled and (b) parallel-coupled.
15.38 mm I.D. reservoirs were filled with closely stacked, 2 mm thick, 15 mm diameter 2 RVCF disks to form 192 mm long targets. Time spectra for the noble gases (i.e. He, Ar, Kr and Xe) gases were measured through each of the vapor-transport systems with and without 2 RVCF in the respective reservoirs of both vaportransports systems using the EBPIS.
target-material reservoir material arepdisplayed in Fig. 6. Characffiffiffiffiffi teristic sC data for noble gases versus M flowing through the system with and without 2 RVCF in the target-material reservoir are displayed in Fig. 7.
6.1. Serial-coupled vapor-transport system
Comparisons of measured pffiffiffiffiffi characteristic effusive-flow-times sC for noble gases versus M for the serial (Fig. 2(a)) and parallel (Fig. 2(b)) coupled vapor-transport systems with and without 2 RVCF in their respective reservoirs are displayed in Fig. 8. As noted for the serial-coupled system, gases effuse faster through the system without 2 RVCF in the reservoir than with material in the reservoir. This affect is directly attributable to the slight impedance to flow introduced by the 2 RVCF. The opposite is true for the parallel-coupled system. For this system, the characteristic times for the noble gases are decreased whenever 2 RVCF is added to the parallel-flow configuration due to the presence of the parallel open channel surrounding the slotted target-material reservoir that particles can take during transit through the system rather than through the material itself. The increase in
Time spectra for noble gases flowing through the serial-coupled vapor-transport system without 2 RVCF in the target-material reservoir material are displayed in Fig. 4. As noted, the spectra are pure exponentials, as predicted from theory (e.g. Eq. p(1)). ffiffiffiffiffi Characteristic effusive-flow time sC for noble gases versus M flowing through the serial-coupled vapor-transport system with and without 2 RVCF in the target-material reservoir are displayed in Fig. 5. 6.2. Parallel-coupled vapor-transport system Analogous time spectra for noble gases flowing through the parallel-coupled vapor-transport system without 2 RVCF in the
6.3. Data comparisons
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Fig. 6. Time spectra for noble gases flowing through the parallel-coupled vaportransport system (Fig. 2b) without 2 RVCF in the target-material reservoir. Ion source: EBPIS.
Fig. 3. Scanning electron micrograph (SEM) of uncoated Reticulated-VitreousCarbon-Foam (RVCF) compressed in one direction (2 RVCF). 2 RVCF is used as matrices for coating target materials at the HRIBF.
pffiffiffiffiffi Fig. 7. Characteristic effusive-flow times sC versus M for noble gases flowing through the parallel-coupled vapor-transport system (Fig. 2b) with and without 2 RVCF material in the reservoir. Experimental data: s, d, D, N; Theory: —; Ion source: EBPIS.
Fig. 4. Time spectra for noble gas elements flowing through the serial-coupled vapor-transport system (Fig. 2a) without 2 RVCF in the target-material reservoir material. Temperature: T = 1360 K; Ion source: EBPIS.
effusive-flow time for the parallel-coupled system over the complementary serial-coupled system with and without RVCF is directly attributable to the larger volume of the parallel-coupled system through which (larger L) particles must travel from the target to the ion source. These studies clearly show that 2 RVCF only slightly increases the effusive-flow times in the serial-coupled vapor-transport system while decreasing the times through the parallel-coupled system. 7. Discussion
pffiffiffiffiffi Fig. 5. Characteristic effusive-flow time sC for noble gases versus M flowing through the serial-coupled vapor-transport system (Fig. 2a) with and without 2 RVCF in the target-material reservoir. Experimental data: s, d, D, N; Theory: —; Ion source: EBPIS.
The fast-valve effusive-flow measurement system, described in this article, has proved to be a very efficient, cost effective and universal means for obtaining fundamentally important information on the effusive-flow properties of vapor-transport systems, independent of size, geometry or materials of construction and chemical properties of vapor-transport species. This information permits delineation of the time dependences the two principal processes (diffusion and effusive-flow) that limit the lifetimes of species that can be processed with intensities adequate for conducting research by giving time specificity to the effusive-flow process. To illustrate the utility of the fast-valve system, we provide effusive-flow time spectra and characteristic effusive time data for noble gases flowing through serial-coupled and parallel-coupled vapor-transport systems, similar in size to those used at the CERN-ISOLDE facility
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species, the fast-valve, separately or in combination with Monte– Carlo codes, such as described in [7,8], can be used to arrive at a system design which minimizes effusive-flow times and thereby, maximize achievable RIB intensities for any chemically active or inactive species of a given half-life. Acknowledgements The authors are indebted to students and staff members of the Advanced Concept Research and Development Group, who through their diligent efforts, contributed to the content of this paper through execution of effusive-flow measurements used to validate the fast-valve system described in this report. This work was supported by the US Department of Energy under contract DE-AC0500OR22725 with UT Battelle. References pffiffiffiffiffi Fig. 8. Comparison of characteristic effusive-flow time sC versus M for noble gases flowing through the serial- (Fig. 2a) and parallel-coupled (Fig. 2b) vapor-transport systems with 2 RVCF material in the reservoir. Experimental data: s, d, D, N; Theory: —; Temperature: 1300 K.
(see, e.g. [17]). These data show that gases effuse faster through the serial-coupled system, system without 2 RVCF in the reservoir than with material in the reservoir. The opposite is true for the parallel-coupled system. For this system, the characteristic times for the noble gases are decreased whenever 2 RVCF is added to the parallel-flow configuration due to the presence of the parallel open channel surrounding the slotted target-material reservoir that particles can take during transit through the system rather than through the material itself. The increase in effusive-flow times for the parallel-coupled system over those for the complementary serial-coupled system, with and without 2 RVCF in the targetmaterial reservoir, is directly attributable to the larger volume, and consequently, longer distance L through which particles must travel from the target to the ion source. These studies clearly show that higher temperatures reduce the effusive-flow times through either system and that 2 RVCF only slightly increases the effusive-flow times in the serial-coupled vapor-transport system while decreasing the times through the parallel-coupled system. Since the distance traveled through a given system L is independent of
[1] G.D. Alton, J.R. Beene, J. Phys. G; Nucl. Part. Phys. 24 (1998) 1347. [2] G.D. Alton, J.R. Beene, R.L. Auble, D. Stracener, Indian J. Phys. 76S (2002) 9. [3] J.-C. Bilheux, Doctor of Physics Thesis, entitled, Design Studies For High-Power Fast Diffusion-Release Targets and Fast Vapor-Transport Systems for Radioactive Ion Beam Generation, Universitè de Versailles, Saint-Quentin-enYvelines, Versailles, France, 2003. [4] Y. Kawai, G.D. Alton, Y. Liu, Proceedings of the 2005 Particle Accelerator Conference, IEEE Catalog Number: 05CH37623, 2005, pp. 1422. [5] R. Kirchner, Nucl. Instr. and Meth. Phys. Res. B 70 (1992) 186. [6] J.R.J. Bennett, AIP Conference Proceedings, vol. 473, 1998, pp. 512. [7] B. Mustapha, J.A. Nolen, Nucl. Instr. and Meth. Phys. Res. B 286 (2003) 204. [8] Y. Zhang, G.D. Alton, J. Vac. Sci. Technol. A 23 (2006) 1558; G.D. Alton, Y. Zhang, Nucl. Instr. and Meth. Phys. Res. A 539 (2005) 540. [9] G.D. Alton, J.R. Beene, Y. Liu, Nucl. Instr. and Meth. Phys. Res., Sect. A 438 (1999) 190. [10] G.D. Alton, Y. Zhang, Y. Kawai, Appl. Rad. Isotopes 64 (2006) 1574. [11] G.D. Alton, H.H. Moeller, ORNL-6957, September 1998, hribf021.pdf. [12] G.D. Alton, J.-C. Bilheux, A.D. McMillan, Nucl. Instr. and Meth. Phys. Res. Sect. A 521 (2004) 108. [13] Y. Kawai, J.-C. Bilheux, D.W. Stracener, G.D. Alton, Appl. Rad. Isotopes 63 (2005) 659. [14] R.D. Present, Kinetic Theory of Gases, McGraw-Hill Book Company., 1958. [15] VAT, Vakuumventile AG, CH-9469 Haag, Switzerland. [16] G.D. Alton, Proceedings of the 2001 Particle Accelerator Conference, IEEE: 01CH37268C 59, 2001. [17] S. Sundell, H. Ravn, The ISOLDE collaboration, Nucl. Instr. and Meth. Phys. Res. B 70 (1992) 160. [18] Reticulated-Vitreous-Carbon-Foam (RVCF) Used in These Investigations and for Generic Target Material Coating Matrices at the HRIBF is a Product of ERG, Oakland, CA 94608.