The 2 MV tandem pelletron accelerator of the Louvre Museum

Nuclear Instruments and Methods in Physics Research B45 (7990) 296-301 ~or~-~~~and

296

THE 2 MV TOTEM

PE~LE~O~

G. AMSEL I), M. MENU

ACC~E~TOR

‘), J. MOULIN

OF THE LOUVRE

‘) and J. SALOMON

~SEU~

*

2,

I) Groupe de Physique des Solides, Tour 23, University Paris WI, 75251 Paris Cedex OS, France 21Laborutoire de Recherche des Mu&es de France, Pa&s du Imvre, 75041 Paris Cedex 01, France

The IEA facility of the Louvre Museum, code AGLAE, is based on the 6SDII-2 2 ML’ tandem Pelletron accelerator of NEC. A number of details of this machine have been specially designed with NEC to match the specific needs required for museum studies, i.e.: p, & 311e, ‘He, “N beams from a single rf source, purely electrostatic focusing, halo-free submillimeter beam impacts, high-energy as well as low-energy operation at relatively high currents for resommce depth profiting of light nuclei including ’ W, the possibility to automatize the machine operation as far as possible, development towards r4C AMS, and a microbeam extension. Special features of this machine include electrostatic steerers at the injection and electrostatic dog-leg steerers at the high-energy side, double slits at both sides of the accelerator, triplets instead of doublets for beam symmetry, turbomolecular recirculation of the stripper gas at the terminal, a new design of the stripper system itself, and a double feedback system with two capacitive pickoff plates for optimum energy spread and stability. The reasons of these special features will be explained and the characteristics of the machine as observed until now will be presented.

The purpose of this paper is to give a technical description of the 2 h4V tandem accelerator system set up in the Research Laboratory of the French Museums at the site of the Louvre. The motivations to equip a museum with such a heavy analytical toal and of the choice of a tandem type machine are presented in ref. [l]. This accelerator system is the core of the ion beam analysis facility AGLAE which is itself described in detail in ref. [Z]. A number of technical details of this Pelletron type 6SDH-2 machine have been worked out in collaboration with National Electrostatics Co. to inatch the special dem~ds set by its analytical use in the field of art and archeology. In addition to FIXE, which plays a central role in elemental and trace analysis, all the methods of IBA will be put to benefit for the characterization of museum abjects: RBS, NRA and PIGME with the particles p, d, ‘He, 4He and “N. It is most important to be able to switch quickly from one ion to another. Moreover, a microbeam facility will be coupled to the accelerator in a second stage, and the option for AMS had also to be taken into account. These analytical needs set the technical requirements on the machine, which we consider now in some more detail. Resonance depth profiling by NRA plays a major role in our dating studies, in particular for 19F 131, ‘“Na and ‘H. Such techniques

(together with microbeam and AMS operation) imply high energy stability and resolution. Moreover, the energy range must go from 300 keV for protons ( 2JNa and ‘aF resonances) to g MeV for “N+++ beams. Thus high enough currents on target must be available for NRA for potentials from 1.50 kV to 2 MV, while low currents are needed for FIXE type work. In addition, automatic electrostatic energy scarming [4] required by resonance profiling sets severe conditions on the focusing properties of the system. On the other hand halo-free beam impact points on target are of prime necessity when analyzing heterogeneous objects, thus setting strong conditions on ion beam optics in general, as also required by microbeam and AMS operation. In the latter technique high transmission is also most important. There is a last strong requirement on beam optics and transport: the parts of the beams which are lost in the beam transport system should be stopped at the injection, or at least as far as possible from the target, to minimize y-ray background. This is especially important for p, d and “N beams. Finally, as this facility will be used systematically for routine work, automatic operation of the accelerator had to be foreseen, In part&&r, fully electrostatic focusing and steering elements were preferred for the easy change of the energies over wide ranges.

2. Injection system * Work supported by the French Ministry of Research and Teclmology, the Ministry of Culture and GDRXG of the CNRS. 0168-583X/90/$03.50 (North-Holland}

Q Elsevier Science Publishers B.V.

All the power supplies of this system, i.e. probe, extractor, double-gap lens, einzel lens, etc., are stabilized to better than 1K3. They can be controlled by a

G. Amsel et al. / The 2 MV tandem Pelletron accelerator of the Louvre Museum O-10 V signal for future fully automatic operation. The supplies in the ion source at the high voltage are light link controlled. Thus all the components of the system can be easily adjusted from the console where the real applied values are displayed.

291

may then be made of C or Al [6]. This might solve the problem of switching quickly from H to He beams and back. A large improvement of He- injection conditions was obtained by coupling an additional 400 l/s turbopump near the source system, for reducing He- stripping in the residual vacuum.

2.1. Source system The classical Alphatross rf source of NEC delivers all the necessary ions: it has been shown recently that this source is also able to produce hydrogen beams, in particular from molecular ions. No duoplasmatron was hence installed. The ions are extracted through metallic canals, Ta or Al, insulated with a boron nitride support. The geometry and nature of the canal may depend on the ions produced: this might be a problem for quickly switching from one beam to another, a problem still to be solved. The probe voltage as well as the gas pressure in the quartz bottle depend on the ion species for optimal transmission through the canal. 2.1.1. H beams The source produces up to 500 PA of positive ions from H, gas: H+, HT and H:. H- ions are obtained after traversal of the 6 cm long charge exchange cell. The Rb furnace is kept at 250” C, as measured by a thermocouple. The cell is kept at 135 o C by a convenient air cooling [5]; in the near future this temperature will be controlled directly by a second thermocouple. A freon-cooled baffle placed downstream prevents the Rb vapours from diffusing towards the high-voltage electrode region. Nevertheless these electrodes were made, as a precaution, of stainless steel, a less reactive material than duraluminium. A freon-cooled baffle upstream will be added to increase the lifetime of the extraction canal. The molecular ions break up in the Rb vapour with cross sections depending on their velocity: the optimum probe voltage might hence be different for ‘H- and Dions. The latter have energies equal to the probe potential (typically 2.5 keV for protons) divided by the number of H atoms in the extracted ion: these energies are small, typically 800 eV for ‘H- ions produced from Hl ions. So a special extraction electrode was added, a simple gap lens, accelerating and focusing the H- ions by an additional 5 keV. With this setup ‘H- currents up to 20 PA have been injected into the accelerator, using an Al canal. It should be noted that 200 nA of ‘H- ions may be injected with the Rb oven switched off, an operation mode convenient for low-current PIXE work. 2.1.2. He beams The operation is classical; however, C or Ta canals must be used, a problem for quickly switching to Hbeams. Injected 4He- currents of 2 yA have been obtained. It seems that a Ta diaphragm inside the quartz bottle could protect the extraction canal which

2.1.3. N beams NH, could not be used as source gas due to severe corrosion problems. A mixture of H,-N, turned out to yield NH; beams up to 1 y A. Reliable optimization of the N,-H, mixture is obtained by coupling a quadrupole mass spectrometer to the source gas inlet through a microleak at high potential. The connection is established through a nylon tubing at a pressure below lop4 Torr to avoid breakdown. 2.2. The source gas handling system A gas injection system with seven bottles and three metering valves has been designed. Some modifications were required later to minimize the loss of expensive gases like 3He and 15N, when switching gases. 2.3. Low-energy beam handling system Fig. 1 presents a scheme of this section of the accelerator system. Acceleration and focusing are through a 30 kV double-gap lens. This system allows us to optimally focus the negative beam without changing the energy. The relatively low maximal acceleration potential was chosen, as a larger potential would require a corresponding einzel lens and this would prevent the use of the lower injection potentials needed for AMS. A +30” inflection magnet inflects 30 keV ions up to mass 33 into the Pelletron (ME/Z’ = 1; p = 0.406

Fig. 1. Layout of the injection system: (1) rf source, (2) rubidium oven, (3) freon-cooled baffle, (4) extractor, (5) double-gap lens, (6) turbopumps 400 and 100 l/s, (7) injection magnet supported by an adjustable frame, (8) beam profile monitor, (9) X-Y steerer, (10) double slit, (11) Faraday cup, and (12) Einzel lens. III. EQUIPMENT

298

G. Amsel et al. / The 2 MV tandem Pelletron accelerator of the Louvre Museum

m; e = 30”; dispersion at 0.32 m = 0.214). This magnet is supported by a homemade adjustable frame, for optimal alignment. The components between the magnet and the entrance of the accelerator are shown in fig. 1. The double slit allows us to reduce, if necessary, the on-target beam while keeping gamma-ray or neutron background from the high-energy beam to a minimum; it should also allow us to reject the non-useful parts of the low-energy beam before acceleration. The einzel lens has a specially large aperture of 63.5 mm diameter to reduce spherical aberrations and enhance transmission. A 1000 l/s turbomolecular pump is coupled to the entrance of the accelerator: residual vacuum is 5 X low8 Torr with the source off and lop6 Torr with beam. A quadrupole mass spectrometer at the same location acts both as a leak detector for the whole injection system and for controlling the N, stripper gas pressure better than with a vacuum ion gauge.

TERMINAL

LOW HIGH ENERGY COLUMN

Fig. 2. Layout of the stripping system and terminal: (1) solid foil stripper port (unused), (2) field-free drift regions (76 mm), (3) low-impedancepumpinggaps (70 mm), (4) high-impedance entrance and exit tubes (length 51 nun, diameter 8 mm), (5) gas reinjection chamber, (6) recirculating turbomolecular pump, (7) stripper tube (length 496 mm, diameter 8 mm), (8) Metering valve for stripper gas inlet.

3. The 6SDH-2 Pelletron 3.1. The machine This nonstandard 2 MV machine is equipped with two chains carrying up to 150 pA each in the upcharge-downcharge mode. The linear potential distribution along the column is through 72 resistors at each side of the terminal, breakdown protected, 555 MO each. The total column current at 2 MV is thus 50 l.tA on each side. This large value reduces random electrode potential fluctuations due to uncontrolled secondaryelectron currents hitting the electrodes, which could lead to beam instabilities. Secondary-electron effects in the tube are minimized by ferrite suppressors, arranged so as to keep the beam direction unperturbed. Short-circuiting rods allow us to work at low potentials down to 150 kV so as to keep the accelerating field and the characteristics of the corresponding input lens reasonably constant. Care was taken to avoid “potential gaps” so that around any value of the potential an at least &lo% potential variation is possible without changing the short-circuiting rod positions. 3.2. Stripping system A new gas stripper design has been worked out in collaboration with NEC in order to enhance the transmission of the machine and reduce beam energy tails. A schematic of the stripping system is shown in fig. 2. The N, stripping gas is recirculated by a 200 l/s turbomolecular pump, working at half speed. Short tubes are placed at both ends of the stripper system for increasing pumping impedances so as to reduce the pressure in the accelerator tubes to a minimum The gaps between the

stripper proper and these tubes allow for an efficient recirculation of N,. A special feature of our terminal is a field-free drift region on both sides of the stripper system, each 3 in. long. This geometry was introduced for reducing possible beam energy tails which show up when charge exchange takes place in a region where the electric field is high. In fact the Maxwellian pressure gradient in this region is rather low due to the high accelerator tube impedance. However, a high Nz density is present in the vicinity of the entrance and exit tubes of the stripper system as a consequence of the direct molecular jet emitted by their apertures. This density decreases as the inverse square of the distance to the apertures, at least for distances large with respect to their diameter. In the present case the field-free drift region is around ten times longer than this diameter and the jet density along the beam path is reduced about hundredfold once the beam undergoes acceleration. NEC reports a noticeable improvement in beam quality from our specially designed stripper system. 3.3. Accelerator tank The tank has been built in France to comply with the local regulations for high-pressure vessels. This tank can be pressurized to 9 bar. It is covered with a 12 mm lead shielding. Hence the radiation level is negligible in normal operation. Five ports with 5 in. inner diameter have been installed at the terminal level. Large-diameter capacitive pickoff plates were chosen so as to increase the level of the voltage fluctuation signal. These plates are positioned along a diameter in order to cancel possible parasitic signals due to column vibrations. Considering

G. Amsel et al. / The 2 MV tandem Pelletron accelerator of the Louvre Museum

EL

299

so as to block any propagation of high-frequency electromagnetic shockwaves induced by possible breakdowns. Each of the three main sections of the system is grounded separately (see ref. [2]).

TOP

4. High-energy beam handling 4.1. The beam exit section

Fig. 3. Cross section of the tank at the terminal level, showirxg the positions of the generating voltmeter (GVM), the capacitive pickoff plates (CPO) and the corona system (COR).

the future AMS

work,

applications

of

a large-diameter

the

system

generating

and

especially

voltmeter

was

chosen (from General Ionex). Its blades are guilded for avoiding charge accumulation in oxidized regions. The GVM is installed vertically at the top of the tank; the amplitude of the terminal vibrations is minimum in this direction due to the construction techniques used for the column. With respect to the GVM, we find at 45” the first CPO, at 135” the corona system and at 225” the second CPO, an optimal configuration to reduce cross talk between the different elements (see fig. 3). Finally the coupling of the accelerator tank to both sides of the system is through insulating ceramic tubes

SCALE

(cm)

7

/

100 I

Fig. 4 shows the general layout of this section. In this part of the system the vacuum is obtained by two cryogenic pumps: one (1500 l/s from Cryophysics) just after the machine, pumping the high-energy (HE) tube, and the other (900 l/s from Leybold Heraeus) just upstream of the switching magnet. The use of aluminium gaskets allows us to reach lo-’ Torr in this section, without beam. Electrostatic “dog-leg” X-Y steerers allow us to displace or realign the beam at the exit of the machine if necessary. One of these X-Y steerer pairs is placed inside the tank. The alignment of the system should be good enough so that the steerers are used only exceptionally. 2.5 m downstream from the HE tube, double slits were positioned to remove the divergent parts of the beam. These slits should allow us to reduce the analyzed beam far away from the target when low currents are needed so as to reduce gamma-ray or neutron background in the experimental area. An electrostatic quadrupole triplet is used to focus the beam through the magnet chamber into the beam line of interest. Such a lens should favor circularly symmetrical beam spots. It might appear during further tests that the locations of the double slits and the triplet should be changed for optimal optical properties.

200

I

Fig. 4. Layoyt of the high-energy beam handling system: (1) X-Y dog-leg steerers, (2) double slit, (3) electrostatic quadrupole triplet, (4) Faraday cup, (5) electrostatic energy scanning plates, (6) analyzing-switching magnet, (7) 1500 and 900 l/s cryopumps, (8) feedback beam position sensing slit. III. EQUIPMENT

G. Amsel et al. / The 2 MV tandem Pelietron accelerator of the Louure Museum

300

Table 1 Characteristics of the analyzing magnet Angle p [“I fml

c f”l

ME/Z2 Pfev

4.3. Potential stabilization

magnet

230

1.13 at 4 m

6.7

56

1.73 at 3 m

0.889 16.7

25

1.85 at 2 m

2.700

0

30

1.337

45

As beam energy resolution and long-term stability are essential for AGLAE, a double feedback loop mixing the CPO’s signal with either the slit or the GVM signal was specially designed 1781. NEC claims that the ripple recorded with the CPO is reduced by a factor of 2-3 when the “fast feedback loop” is switched in. Peak to peak the ripple is then in the 1O-4 range typically.

Dispersion a) Waist b, location after [ml

tuna1 15

between 4.0 and 8.0 m between 3.0 and 6.0 m between 1.5 and 2.5 m

4.4. Automatic

‘) A change AE/E of the beam energy induces, dowstream from the switching magnet at this position, a displacement Ax of the beam, given by the formula: Ax =1/2. Dispersion: AE,/E. b, These waist locations are achieved by tuning the triplet located

upstream

from the switching

magnet.

4.2. Magnet

The analy~g-switching magnet with a 30 mm gap was built by Bruker, according to our specifications. The vacuum chamber, equipped with seven ports ( I!Z45 * , +30” , k 15 O, 0 o ), is lined with tantalum for reducing gamma-ray and also deuteron-induced neutron ernission. It should be noted that during electrostatic energy scanning the beam moves around its central trajectory: the pole and vacuum chamber dimensions are such that even for its extreme positions the beam remains in a constant-field region. The poles are shaped to focus the beam both in the horizontd and vertical plane. Table 1 presents the main characteristics of the magnet and typical waist locations in the different beam lines. The bipolar Bruker current supply of the magnet is stabilized to 10e6 for maximal current (100 A). The field may be controlled with a Hall probe and a feedback loop BH15 unit from Bruker with field stability 2 X 10-5.

Table 2 Currents

for different

ion species during

‘H ‘H ‘He 4Heb)

Z 1 1 2 1

energy scanning system

An automatic, electrostatic hysteresis-free potential scanning system equips the accelerator for resonance profiling [4]. Two pairs of vertical plates placed at both sides of the analyzing magnet may be biased by two k 6.5 kV voltage supplies. These supplies are computer-controlled through a O-10 V reference voltage [2]. It was shown in ref. [4] that such a system acts as a linear amplifier with gain G such that AV = GU, where U is the applied potential difference between the plates. This device allows us to scan the beam energy with high precision and repeatability for resonance depth profiling in a range of + 8% around any central potential &. Typical values of the gain G are for protons: 53.2 at 15O and 15.2 at 45O.

5. Conclusion The Louvre accelerator system is the first designed for and dedicated to the characterization of art and archaeological objects. The system was installed during the summer of 1988, in collaboration with NEC engineers. The performance, during the tests, for some ion species and the main features of the system are summarized in Appendix 1. As the accelerator system has been set up only recently, it is presently undergoing

testing

E

I measured a)

I specified b,

(ii) r4N ‘) Injected NH;

I,., analyzed d,

BeVJ

[ PAI

[PAI

WI

Ml

300 4000 6000 300

1.2 6.0 0.75 0.50

1.0 5.0 0.50 0.50

1400 1400

390 900

(i) ‘H and 4He Particle

system

Z 3 2

Particle current d,

Energy ‘)

Ml

[MeVl

130 450

7.77 5.77

a) The beam currents in particle PA, were measured on the 45O extension, in a Faraday cup located 2 m downstream from the switching magnet, through a 1 mm hole. b, 4He specifications at low energies could not be reached in a 1 mm hole, but in 2 mm. ‘) “N beams were not yet tried. d, Conditions as for the other particles but through a 2 mm hole. The specification was 200 particle nA into 1 mm-for the most probable charge state. @ For 2 MV acceleration potential.

G. Amsel et al. / The 2 MV tandem Pelletron accelerator of the Louvre Museum

intensive testing and running NEC’s engineers.

in, with the cooperation

of

We thank J. Ligot, Director of LRMF, for his constant support of this project. The efficient and friendly cooperation with J. Ferry, R. Rathmell and J.B. Schroeder from NEC was and is still a key factor in the success of this undertaking. J. Chaumont and E. Cottereau were most helpful in general discussions and J. Camplan for the design of the analyzing magnet. The help of E. d’Artemare, P. Kostka and B. Simon for the actual setting up of the accelerator system was most appreciated.

Appendix 1 Beam currents achieved for various ions and conditions are presented in table 2. 14N specifications are not yet reached. We hope that the new N,-H, gas mixture control system discribed above will allow us to optimize 14N as well as 15N beam currents in the near future.

301

During all these test we measured a short-term modulation of the currents better than _+20%. The lifetime of this source was tested to be more than 170 hours for a ‘H beam.

References [l] M. Menu, these Proceedings (Ninth Int. Conf. on Ion Beam Analysis, Kingston, Ontario, Canada, 1989) Nucl. Instr. and Meth. JS-45(1990) 597. [2] M. Menu, T. Cahigaro, J. SaIomon, G. Amsel and J. Moulin ibid. p. 610. [3] Ph. Walter, M. Menu and J.C. Vi&ridge, ibid, p. 119. [4] G. Amsel, E. d’Artemare and E. Girard, Nucl. Instr. and Meth. 205 (1983) 5. [5] R.J. Girnius, L.W. Anderson and E. Staab, Nucl. Instr. and Meth. 143 (1977) 505. [6] M. Strathman, private communication. [7] G.L. Miller, private communication. [8] G. Amsel and B. Maurel, Nucl. Instr. and Meth. 218 (1983) 183.

III. EQUIPMENT