SPES AS A SPECIALIZED NATIONAL ISOL

Transcription

1 SPES Project Study SPES AS A SPECIALIZED NATIONAL ISOL 12.1 Neutron Production Rates and Secondary Beam Intensities 12.2 R & D for the SPES Program The main accelerator Stopping target for neutron production Target/ion source for radionuclides production Production of multiple charged ions

2 Chapter 12 SPES Project Study Chapter 12 SPES AS A SPECIALIZED NATIONAL ISOL FACILITY AT LNL Since the decision for the European regional site of the ISOL type facility will not be taken before two or three years from now, we think that the LNL position could be strengthened in the European scenario through the construction of a neutron facility mainly dedicated to the study for production of exotic species. It will consist of a high intensity (5 ma), low energy linac (20 MeV) and a high power (100 kw) neutron target which could represent an optimum test bench for a high power target-source system for the next generation ISOL facility and for the ADS programs. Such high power target may represent also a source of high neutron fluxes for applied and interdisciplinary physics. Because of its modularity, the primary accelerator described in chapter 5 may be constructed in a minimal version allowing the possibility to be further implemented to accelerate beams up to 100 MeV/u. As shown in Fig. 12.1, this implies the construction of a smaller building housing the driver and the production target area (neutron area). The primary accelerator still consists of the sequence of RFQ-ISCL but is limited in energy (20 MeV). The beam power to be considered is ranging between 10 and 100 kw. For the remaining part the facility is conceptually planned as described above; thus the secondary beams are selected by the isobaric separator and delivered for experiments at low-energy (traps, astrophysics, ) or pre-accelerated and injected in the ALPI linac for a further acceleration to higher energies (~5 MeV/amu for the 132 Sn). The limited energy of the primary accelerator will imply a lower neutron production rate; in any case the estimated neutron yield is larger than n/s/sr at 0 0, high enough to satisfy the demand for an advanced RIB facility. The low energy primary beam may only produce low energy neutrons (average energy ~ 7 MeV); this implies that the nuclei with masses located in the central part of the fission spectrum will be poorly produced. This phase of the project has to be considered as an intermediate step for the full project and may be realized in a period of five years without re-acceleration. An additional period of time of two years (and additional funds of the order of magnitude of those already available) should allow to complete the program and operate a national ISOL complex producing and accelerating a number of high quality, high intensity neutron-rich exotic nuclei in the range of masses from 80 to 160. Taking advantage of the ALPI peculiar performance, the delivered beams could be competitive and complementary to those available in the European facility. From the beginning it will result in a medium intensity neutron facility addressed to the national community and dedicated to the RIB production as well as to the BNCT applications and to neutron physics Neutron Production Rates and Secondary Beam Intensities High energy neutron sources based on high current continuous wave (CW) deuteron or proton accelerators and thick targets of light nuclei provide the most suitable intensities for the aims of the project. The experimental data on deuteron and proton induced neutron source reactions were reviewed by Lone [Lon77] and Barschall [Bar78]. At deuteron energies above a few MeV, the d + Be and d + Li reactions have the highest cross sections for production of neutrons in the forward direction. The high energy neutrons measured from thick beryllium and lithium targets exhibit spectral shapes and angular distributions which are characteristic of the deuteron stripping process [Hel47]. Figure

3 SPES Project Study Chapter 12 show typical example of neutron yield and average neutron energy as a function of the deuteron energy, both for beryllium and lithium targets. Figure 12.3 shows a comparison of the thick target neutron spectral distributions at 0 0 from proton, deuteron and alpha-particle induced reactions. At a comparable projectile energy the intensity and the average energy of neutrons at 0 0 from a thick beryllium target are highest for the deuterons. The dose rates from the d + Be reaction on thick targets are higher than those from the p + Be reaction at the same projectile energy [Lon77]. Also, the angular distributions of neutrons from the deuteron reactions are narrowed than those from the proton reactions at the same projectile energy [Lon77]. Figure 12.4 shows typical examples of the angular distributions which become more forward peaked as the deuteron energy increases. The intensity of the neutrons emitted in the forward direction increases rapidly with the deuteron bombarding energy. Independently of the stopping target, deuterons seem to be the most appropriate projectiles for the production of neutrons. Stopping targets as beryllium and lithium both present characteristics which are suitable for the task. For some applications a uranium stopping target is suitable too, even if it shows different angular and energetic distributions. In this context, for the SPES program, the most suitable driver may consists of a deuteron accelerator of 20 MeV energy and power up to 100 kw. With minimal changes to the RFQ designed for the high intensity proton driver the same device allows to accelerate also deuterons. The RFQ is followed by a short section of the ISCL linac. The total length of this 20 MeV deuteron driver may be contained in about 20 meters. The yield of neutrons (per incident deuteron) with energy above 1 MeV emitted in an angle between 0 0 and 20 0 is about 9x10-3 (n/d) so that an intensity of 4x10 14 (n/s) is expected, for 100 kw power in the stopping beryllium/lithium target. The average energy of the emitted neutron results to be 7.7 MeV. In Table 12.1 we report the expected beam intensities for a primary deuteron beam energy of 20 MeV and 100 kw power. The stopping target is beryllium and the production target is uranium carbide with a thickness of 100 gcm -2. The intensity evaluations are done by using the experimental fission yield values taken from Ref. [Eng93]; yields for typical isotopes are shown in Fig Table 12.1 Some examples of expected beam intensities for a production deuteron beam of 20 MeV and 100 kw power. The stopping target is beryllium and the production target is uranium carbide with a thickness of 100 gcm -2. The experimental fission yields are taken from Ref. [Eng93]. The overall efficiency values are taken from Ref. [Rav94]. * Value taken from Ref. [Fog92]. Isotope Half-life Fiss. Yield Prod.Rate Overall Eff. Beam Int. ALPI Int. [%] [At./pµC] [%] [Ions/s] [Ions/s] Zn h 3.6 x x x x 10 3 Zn s 3.26 x x x x 10 5 Ge s x * 3 x x 10 9 Kr s x x Kr s x x x 10 8 Rb ms 8.9 x x x x 10 8 Cd s 2.1 x x * 1.5 x x 10 5 Sn s x * 2.2 x Sn s x x x 10 8 Xe s x x x 10 9 Xe s x x x 10 8 Cs s x x

4 Chapter 12 SPES Project Study 12.2 R&D for the SPES Program During the development of the conceptual design for the SPES facility, numerous areas in which additional research and development (R&D) seemed appropriated have been identified. These areas are briefly described below. The R&D on the different items actually in progress is done in collaboration with other Laboratories and University groups and is moving in a European context The main accelerator The main accelerator has been designed in the framework of the TRASCO project, the INFN- ENEA feasibility study for a waste transmutation Accelerator Driven System (ADS). Being the sequence of RFQ-ISCL the most suitable scheme for proton/deuteron linacs in the energy range of MeV/u, a similar structure is proposed to accelerate the deuteron beam up to 20 MeV of energy. In case of future upgrading, this solution gives the possibility to extend the linac energy up to 100 MeV/u. The most challenging aspect of the RFQ construction design is the mechanical engineering of the structure. The machine has to operate in CW mode and has to be able to dissipate more than 80 kw/m, generated mainly on vane surfaces. In addition to this the mechanical tolerances of the structure are very tight, in the range of hundredth of mm, both for rf and beam dynamics reasons and they have to be kept during the operation in spite of the thermal stresses. An aluminium 3 meter long prototype of the RFQ has been built and actually is under rf measurements. The prototype is designing for 352 MHz frequency and is split into three one-meter-long-rfqs, as shown in Fig To improve both longitudinal and transverse stability these parts are joined with resonant coupling to form a three-meter-long RFQ. The interest for a superconducting linac covering the traditional ISCL energy range has recently grown, in connection with various high intensity linac studies. An Independently phased Superconducting Cavity Linac (ISCL) similar to those used for low energy heavy ions in many nuclear physics laboratories, but at much higher beam intensity and in a wider beta range, seems the most suitable for this aim. Development of cavities for this kind of applications has been done mainly at ANL [Del92], and other studies can be found in literature [Tan96]. The high power coupler design and the beam losses control are specific problem related to the high beam power Stopping target for neutron production It was mentioned above that a beryllium or lithium target-converter serves as an efficient neutron source for isotope production by fission in the secondary production target. This solution will allow to solve the technological problems related to the power dissipation (100 kw) from the production target by decoupling the intense particle beam from the secondary target in terms of the energy absorption and consequent target heating. Beryllium and lithium are suitable for such an application, but both kind of targets present design problems that demand a carefully investigation. Lithium targets. Because of the low melting temperature of lithium (179 0 C) and the low thermal conductivity (45 W/m 0 C) metal targets are not stable at high beam power. The removal of the huge heat (100 kw) from the target imposes a big challenge on the target design. However, up to megawatt beam power, a fast flowing lithium jet target provides an attractive alternative to other techniques. Design studies [Gra77] of such a target for MW operation show that for a flow rate of 15 m/s the temperature of the lithium in the catch basin will rise to about C for liquid entering the target area at a temperature of about C. For operating temperature up to C, stainless 93

5 SPES Project Study Chapter 12 steel can be used for piping, valves, pumps and other components in the lithium loop with very little corrosion. Considerable experience already exists of the long term operation of liquid lithium systems [Tre80] [Sil98]. Beryllium targets. Probably beryllium is more suitable than lithium as a target material because the melting point and thermal conductivity ( C and 130 W/m 0 C) are much higher than those of lithium, and furthermore beryllium is chemically more inert at high temperatures. For high power neutron sources a rotating beryllium target (a sheet of beryllium metal thick enough to stop the incident particles) can be useful. Calculations [Log75] indicate that an internally cooled beryllium wheel, 32 cm in diameter and rotating at 6000 rpm could be used with power up to a few MW and a beam diameter of 1 cm. Tests on a prototype target for use in accelerator-based boron neutron capture therapy (BNCT) shown that 30 kw of power could be effectively removed from a beryllium target [Bla98] Target/ion source for radionuclides production In order to obtain experimental information on the neutron-rich nuclei produced by neutroninduced fission of uranium isotopes an experimental program has been approved by INFN and is actually in progress at the Laboratori Nazionali di Legnaro. It consist in the construction of an isotope separator for on-line operation coupled to the 7 MV CN Van de Graaff accelerator. The layout of the isotope separator is very similar to presently existing low-current separators and consists of a 60 0 magnet with 1.3 m bending radius, a mass resolving power M/ M ~800 and dispersion ~1500/M mm (see Fig. 12.7a,b). The idea is to produce neutrons by bombarding thick beryllium targets with deuterons accelerated up to an energy of 7 MeV and current of 3 µa. The bombardment of 9 Be thick target with deuterons has been extensively used in the past as neutron sources [Lon81]. The expected total neutron yield at 7 MeV deuteron energy is 5x10 9 neutrons/sr/µa at 0 0, with an average energy <E n > = 3.2 MeV. The fast neutrons can be slowed down to the thermal energy by means of a suitable designed moderator; a flux of 2x10 8 n th cm -2 s -1 can be obtained [Ago97]. The radioactive isotopes are produced in the fission process induced by, both thermal or fast, neutrons on targets of 235 U and 238 U. The on-line test stand to study the production yields, the release efficiencies, the delay times in the production targets and for testing the ion source efficiencies and emittances. A high-temperature integrated target-ion-source system has been designed for the on-line mass separator. This system includes, besides the target, a 1+ charge state surface ionization source operating at 20 kv. The targets are composed of 235 U and 238 U (typically several grams) in the form of uranium carbide dispersed in a matrix of porous graphite. In order to have a rapid diffusion in the graphite matrix and a good selectivity of the fission products the target-ion-source system can be operated up to C. The expected production rates at the target level for several different isotopes is estimate ranging between atoms/s. Figure 12.8 shows a picture of the integrated targetion source system developed in the framework of this program Production of multiple charged ions To accelerate radioactive beams efficiently, ions charge greater than +1 may be advantageous. Efficient production of such ions would also reduce the cost of the acceleration stages. A possibility would be inject ions from a conventional single-charge state ion source into an Electron Beam Ion Source (EBIS) or Trap (EBIT) and subject them to bombardment by an intense electron beam to boost their charge state. The ions are then extracted as short pulses and accelerated in the post- 94

6 Chapter 12 SPES Project Study accelerator. Pulsed operation of the post-accelerator would significantly reduce the operating cost. The Trapped Ion Source (TIS) under development at INFN-BARI, is a new type of source capable, in principle, of producing very highly charged ions and, at the same time, it is a radio frequency quadrupole linear trap suitable to study the interaction of the trapped ions with electrons, high-energy particles or laser beams [Var99]. In practice, it is a modified version of an Electron Beam Ion Source (EBIS). The main new feature of TIS, with respect to an EBIS (or EBIT), is the adding of radial ion confinement of the rf quadrupoles to the potential well of the electron beam space charge. In this situation residual gas ions can be trapped together with the wanted ions. Then all the residual ions can easily fill the potential well and then reduce the ion containment capability for the wanted ions. In order to avoid this drawback, in TIS, a pulsed electron beam will be used in such a way that the transversal ion containment is maintained only by the rf quadrupoles. In this way by adding to the rf field a continuous quadrupole field it is possible to select the ion type to be contained in the pseudo-potential well (as done in usual mass spectrometers). Meanwhile, the slow electrons, generated form successive ionizations, are swept away because the rf period is much less than the electron transit time of the quadrupole field. For this reason multipactoring phenomena should be avoided. Figure 12.9 shows a general view ion source prototype and the rf quadrupole electrodes for the plasma confinement, respectively. Experimental results on plasma confinement are shown in Fig REFERENCES [Ago97] S. Agosteo et al., Rad. Prot. Dosim. 70 (1997) 559. [Bar78] H.H. Barschall, Ann. Rev. Nucl. Part. Sci. 28 (1978) 207. [Bla98] B.W. Blackburn et al., Med. Phys. 25 (1998) [Del92] J.R. Delayen et al., Proc. of the 1992 Linac Conference, Ottawa, AECL (1992) 695. [Eng93] T.R. England and B.F. Rider, Fission Product Yields, Los Alamos, LA-UR ; ENDF-349 (1993). [Gra77] P. Grand and A.N. Golan, Nucl. Instrum. Meth. 145 (1977) 49. [Hel47] A.C. Helmholtz et al, Phys. Rev. 72 (1947) [Log75] C.M. Logan et al., Conf. on Radiation Test Facilities for the CTR Surface and Materials Program, P.J. Persiani Editor, ANL/CTR-75-4 (1975) 410. [Lon77] M.A. Lone, Symp. On Neutron Cross Sections MeV, BNL-NCS (1977) 79.M.A. Lone et al., Nucl. Instrum. Meth. 143 (1977) 331. [Lon81] M.A. Lone et al., Nucl. Instrum. Meth. 189 (1981) 515. [Meu75] J.P. Meulders et al., Phys. Med. Biol. 20 (1975) 235. [Sil98] G.I. Silvestrov, Liquid Metal Jet Targets for Intense High Energy Beams, BINP [Tan96] Y. Tanabe et al., Proc. EPAC96, Sitges, Institute of Physics Publishing Bristol and Philadelphia (1996) 569. [Tre80] A. LaMar Trego, HEDL-SA-1919FP, Hanford Engineering Devel. Lab. (1980). [Var99] V. Variale et al., First Test Results on the Trapped Ion Source, PAC99. 95

7 SPES Project Study Chapter 12 Fig Layout of the facility; in green the neutron area for RIBs production and other applications 96

8 Chapter 12 SPES Project Study Fig Thick target neutron yield and average neutron energy as a function of the deuteron energy, both for beryllium and lithium targets [Lon77]. Fig Comparison of the neutron spectral distributions from p, d, α bombardment of beryllium [Lon77]. 97

9 SPES Project Study Chapter 12 Fig Angular distribution of the neutrons from d + Be reaction with a thick target at different deuteron energies [Meu75]. Fig Experimental fission yields of typical isotopes, for fast neutron inducing fission of 238 U [Eng93]. 98

10 Chapter 12 SPES Project Study Fig A picture of the RFQ prototype developed in the framework of the TRASCO project. (3m long structure (3.5λ), 350 MHz, construction tollerance 0.04 mm resonator copuling between segment following LAN technique) 99

11 SPES Project Study Chapter 12 Fig. 12.7a The target/ion source system installed at the CN accelerator. 100

12 Chapter 12 SPES Project Study Fig b - The low-current isotope seoarator installed at the CN accelerator. 101

13 Fig A picture of the integrated target-ion source system developed for the on-line separator experiment. SPES Project Study Chapter

14 Chapter 12 SPES Project Study Fig A general view of the TIS prototype (top) and of the rf quadrupoles electrodes for the plasma confinement (bottom). 103

15 SPES Project Study Chapter 12 Fig Plasma confinement of the rf quadrupole electrodes as seen by a CCD camera placed at the end of the quadrupole axis. 104