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SPES AS A SPECIALIZED NATIONAL ISOL

12.1 Neutron Production Rates and Secondary Beam Intensities

12.2.1 The main accelerator

12.2.2 Stopping target for neutron production

12.2.4 Production of multiple charged ions

12.2 R & D for the SPES Program

12.2.3 Target/ion source for radionuclides production

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Chapter 12

SPES AS A SPECIALIZED NATIONAL ISOL FACILITYAT LNL

Since the decision for the European regional site of the ISOL type facility will not be takenbefore two or three years from now, we think that the LNL position could be strengthened in theEuropean scenario through the construction of a neutron facility mainly dedicated to the study forproduction 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 highpower target-source system for the next generation ISOL facility and for the ADS programs. Suchhigh power target may represent also a source of high neutron fluxes for applied and interdisciplinaryphysics.

Because of its modularity, the primary accelerator described in chapter 5 may be constructedin a minimal version allowing the possibility to be further implemented to accelerate beams up to100 MeV/u. As shown in Fig. 12.1, this implies the construction of a smaller building housing thedriver and the production target area (neutron area). The primary accelerator still consists of thesequence of RFQ-ISCL but is limited in energy (20 MeV). The beam power to be considered isranging between 10 and 100 kW. For the remaining part the facility is conceptually planned asdescribed above; thus the secondary beams are selected by the isobaric separator and delivered forexperiments at low-energy (traps, astrophysics,…) or pre-accelerated and injected in the ALPI linacfor a further acceleration to higher energies (~5 MeV/amu for the 132Sn). The limited energy of theprimary accelerator will imply a lower neutron production rate; in any case the estimated neutronyield is larger than 1014 n/s/sr at 00, 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 bepoorly produced. This phase of the project has to be considered as an intermediate step for the fullproject and may be realized in a period of five years without re-acceleration. An additional period oftime of two years (and additional funds of the order of magnitude of those already available) shouldallow to complete the program and operate a national ISOL complex producing and accelerating anumber of high quality, high intensity neutron-rich exotic nuclei in the range of masses from 80 to160. Taking advantage of the ALPI peculiar performance, the delivered beams could be competitiveand complementary to those available in the European facility. From the beginning it will result in amedium intensity neutron facility addressed to the national community and dedicated to the RIBproduction as well as to the BNCT applications and to neutron physics.

12.1 Neutron Production Rates and Secondary Beam Intensities

High energy neutron sources based on high current continuous wave (CW) deuteron or protonaccelerators and thick targets of light nuclei provide the most suitable intensities for the aims of theproject. The experimental data on deuteron and proton induced neutron source reactions were reviewedby 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. Thehigh energy neutrons measured from thick beryllium and lithium targets exhibit spectral shapes andangular distributions which are characteristic of the deuteron stripping process [Hel47]. Figure 12.2

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show typical example of neutron yield and average neutron energy as a function of the deuteronenergy, both for beryllium and lithium targets. Figure 12.3 shows a comparison of the thick targetneutron spectral distributions at 00 from proton, deuteron and alpha-particle induced reactions. At acomparable projectile energy the intensity and the average energy of neutrons at 00 from a thickberyllium target are highest for the deuterons. The dose rates from the d + Be reaction on thicktargets 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 theproton reactions at the same projectile energy [Lon77]. Figure 12.4 shows typical examples of theangular distributions which become more forward peaked as the deuteron energy increases. Theintensity of the neutrons emitted in the forward direction increases rapidly with the deuteronbombarding energy.

Independently of the stopping target, deuterons seem to be the most appropriate projectilesfor the production of neutrons. Stopping targets as beryllium and lithium both present characteristicswhich 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 to100 kW. With minimal changes to the RFQ designed for the high intensity proton driver the samedevice allows to accelerate also deuterons. The RFQ is followed by a short section of the ISCLlinac. 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 anglebetween 00 and 200 is about 9x10-3 (n/d) so that an intensity of 4x1014 (n/s) is expected, for 100 kWpower in the stopping beryllium/lithium target. The average energy of the emitted neutron results tobe 7.7 MeV. In Table 12.1 we report the expected beam intensities for a primary deuteron beamenergy of 20 MeV and 100 kW power. The stopping target is beryllium and the production target isuranium carbide with a thickness of 100 gcm-2. The intensity evaluations are done by using theexperimental fission yield values taken from Ref. [Eng93]; yields for typical isotopes are shown inFig. 12.5.

Table 12.1 — Some examples of expected beam intensities for a production deuteron beam of 20 MeV and100 kW power. The stopping target is beryllium and the production target is uranium carbide with a thicknessof 100 gcm-2. The experimental fission yields are taken from Ref. [Eng93]. The overall efficiency values aretaken 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]

Zn72 46 h 3.6 x 10-6 2 x 102 8 8.6 x 104 4 x 103

Zn78 1.5 s 3.26 x 10-3 2 x 105 0.48 4.7 x 106 2.3 x 105

Ge82 4.6 s 1.24 4 x 107 0.084* 3 x 1010 1.5 x 109

Kr91 8.6 s 2.62 1.6 x 108 29 2.3 x 1011 1010

Kr94 0.2 s 0.74 4.4 x 107 6.4 1.4 x 1010 7 x 108

Rb97 170 ms 8.9 x 10-2 5.3 x 106 11 2.9 x 109 1.5 x 108

Cd124 1.3 s 2.1 x 10-2 1.3 x 106 0.25* 1.5 x 107 7.5 x 105

Sn128 6.5 s 0.2 1.2 x 107 0.37* 2.2 x 109 108

Sn132 40 s 1.88 1.1 x 108 2.4 1.3 x 1010 6.5 x 108

Xe142 1.22 s 1.53 9.2 x 107 26 1.2 x 1011 6 x 109

Xe144 1.2 s 0.123 7.4 x 106 26 9.6 x 109 4.8 x 108

Cs144 1 s 1.74 1.0 x 108 38 2 x 1011 1010

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12.2 R&D for the SPES Program

During the development of the conceptual design for the SPES facility, numerous areas inwhich additional research and development (R&D) seemed appropriated have been identified. Theseareas are briefly described below. The R&D on the different items actually in progress is done incollaboration with other Laboratories and University groups and is moving in a European context.

12.2.1 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 thesequence of RFQ-ISCL the most suitable scheme for proton/deuteron linacs in the energy range of10 - 100 MeV/u, a similar structure is proposed to accelerate the deuteron beam up to 20 MeV ofenergy. In case of future upgrading, this solution gives the possibility to extend the linac energy upto 100 MeV/u.

The most challenging aspect of the RFQ construction design is the mechanical engineeringof the structure. The machine has to operate in CW mode and has to be able to dissipate more than80 kW/m, generated mainly on vane surfaces. In addition to this the mechanical tolerances of thestructure are very tight, in the range of hundredth of mm, both for rf and beam dynamics reasonsand they have to be kept during the operation in spite of the thermal stresses. An aluminium 3 meterlong prototype of the RFQ has been built and actually is under rf measurements. The prototype isdesigning for 352 MHz frequency and is split into three one-meter-long-RFQs, as shown in Fig.12.6.To improve both longitudinal and transverse stability these parts are joined with resonant couplingto form a three-meter-long RFQ.

The interest for a superconducting linac covering the traditional ISCL energy range has recentlygrown, in connection with various high intensity linac studies. An Independently phasedSuperconducting Cavity Linac (ISCL) similar to those used for low energy heavy ions in manynuclear physics laboratories, but at much higher beam intensity and in a wider beta range, seemsthe most suitable for this aim. Development of cavities for this kind of applications has been donemainly at ANL [Del92], and other studies can be found in literature [Tan96]. The high powercoupler design and the beam losses control are specific problem related to the high beam power.

12.2.2 Stopping target for neutron production

It was mentioned above that a beryllium or lithium target-converter serves as an efficientneutron source for isotope production by fission in the secondary production target. This solutionwill allow to solve the technological problems related to the power dissipation (100 kW) from theproduction target by decoupling the intense particle beam from the secondary target in terms of theenergy absorption and consequent target heating. Beryllium and lithium are suitable for such anapplication, but both kind of targets present design problems that demand a carefully investigation.

Lithium targets. Because of the low melting temperature of lithium (1790C) and the lowthermal conductivity (45 W/m0C) metal targets are not stable at high beam power. The removal ofthe huge heat (100 kW) from the target imposes a big challenge on the target design. However, upto megawatt beam power, a fast flowing lithium jet target provides an attractive alternative to othertechniques. Design studies [Gra77] of such a target for MW operation show that for a flow rate of15 m/s the temperature of the lithium in the catch basin will rise to about 300 0C for liquid enteringthe target area at a temperature of about 200 0C. For operating temperature up to 400 0C, stainless

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steel can be used for piping, valves, pumps and other components in the lithium loop with verylittle corrosion. Considerable experience already exists of the long term operation of liquid lithiumsystems [Tre80] [Sil98].

Beryllium targets. Probably beryllium is more suitable than lithium as a target material becausethe melting point and thermal conductivity (1278 0C and 130 W/m0C) are much higher than thoseof lithium, and furthermore beryllium is chemically more inert at high temperatures. For high powerneutron sources a rotating beryllium target (a sheet of beryllium metal thick enough to stop theincident particles) can be useful. Calculations [Log75] indicate that an internally cooled berylliumwheel, 32 cm in diameter and rotating at 6000 rpm could be used with power up to a few MW anda beam diameter of 1 cm. Tests on a prototype target for use in accelerator-based boron neutroncapture therapy (BNCT) shown that 30 kW of power could be effectively removed from a berylliumtarget [Bla98].

12.2.3 Target/ion source for radionuclides production

In order to obtain experimental information on the neutron-rich nuclei produced by neutron-induced fission of uranium isotopes an experimental program has been approved by INFN and isactually in progress at the Laboratori Nazionali di Legnaro. It consist in the construction of anisotope separator for on-line operation coupled to the 7 MV CN Van de Graaff accelerator. Thelayout of the isotope separator is very similar to presently existing low-current separators and consistsof a 600 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 berylliumtargets with deuterons accelerated up to an energy of 7 MeV and current of 3 µA. The bombardmentof 9Be 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 5x109 neutrons/sr/µA at 00, with anaverage energy <En> = 3.2 MeV. The fast neutrons can be slowed down to the thermal energy bymeans of a suitable designed moderator; a flux of 2x108 nth cm-2 s-1 can be obtained [Ago97]. Theradioactive isotopes are produced in the fission process induced by, both thermal or fast, neutronson targets of 235U and 238U. The on-line test stand to study the production yields, the releaseefficiencies, the delay times in the production targets and for testing the ion source efficiencies andemittances.

A high-temperature integrated target-ion-source system has been designed for the on-linemass separator. This system includes, besides the target, a 1+ charge state surface ionization sourceoperating at 20 kV. The targets are composed of 235U and 238U (typically several grams) in the formof uranium carbide dispersed in a matrix of porous graphite. In order to have a rapid diffusion in thegraphite matrix and a good selectivity of the fission products the target-ion-source system can beoperated up to 2500 0C. The expected production rates at the target level for several different isotopesis estimate ranging between 104 – 106 atoms/s. Figure 12.8 shows a picture of the integrated target-ion source system developed in the framework of this program.

12.2.4 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 possibilitywould be inject ions from a conventional single-charge –state ion source into an Electron Beam IonSource (EBIS) or Trap (EBIT) and subject them to bombardment by an intense electron beam toboost their charge state. The ions are then extracted as short pulses and accelerated in the post-

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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 radiofrequency 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 ElectronBeam Ion Source (EBIS). The main new feature of TIS, with respect to an EBIS (or EBIT), is theadding of radial ion confinement of the rf quadrupoles to the potential well of the electron beamspace charge. In this situation residual gas ions can be trapped together with the wanted ions. Thenall the residual ions can easily fill the potential well and then reduce the ion containment capabilityfor the wanted ions. In order to avoid this drawback, in TIS, a pulsed electron beam will be used insuch a way that the transversal ion containment is maintained only by the rf quadrupoles. In thisway by adding to the rf field a continuous quadrupole field it is possible to select the ion type to becontained in the pseudo-potential well (as done in usual mass spectrometers). Meanwhile, the slowelectrons, generated form successive ionizations, are swept away because the rf period is much lessthan the electron transit time of the quadrupole field. For this reason multipactoring phenomenashould be avoided. Figure 12.9 shows a general view ion source prototype and the rf quadrupoleelectrodes for the plasma confinement, respectively. Experimental results on plasma confinementare shown in Fig. 12.10.

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) 1967.[Del92] J.R. Delayen et al., Proc. of the 1992 Linac Conference, Ottawa, AECL-10728 (1992)

695.[Eng93] T.R. England and B.F. Rider, Fission Product Yields, Los Alamos, LA-UR-94-3106;

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) 1003.[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 10-40 MeV, BNL-NCS-50681 (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 1998.[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.

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Fig. 12.1 - Layout of the facility; in green the neutron area for R

IBs production and other applications

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Fig. 12.2 – Thick target neutron yield and average neutron energy as a function of the deuteron energy, bothfor beryllium and lithium targets [Lon77].

Fig. 12.3 – Comparison of the neutron spectral distributions from p, d, α bombardment of beryllium[Lon77].

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Fig. 12.5 – Experimental fission yields of typical isotopes, for fast neutron inducing fission of 238U [Eng93].

Fig. 12.4 – Angular distribution of the neutrons from d + Be reaction with a thick target at differentdeuteron energies [Meu75].

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Fig. 12.9 – A general view of the TIS prototype (top) and of the rf quadrupoles electrodes for the plasma confinement (bottom).

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Fig. 12.10 – Plasma confinement of the rf quadrupole electrodes as seen by a CCD camera placed at the end of the quadrupole axis.