162

Characteristics of various electron beam sources

George E. Thomas and John P. GreeneArgonne National Laboratory, Argonne, Illinois 60439, USA

Peter Maier-KomorTechnische Universität München, W-8046 Garchmg, Germany

Robert H. LeonardFlorida State University, Tallahassee, Florida 32306, USA

Various changes have been made in two electron beam sources to make them more suitable for target preparation. The beam spotsize has been made more flexible and the accessibility to some of the component parts has been improved

1. Introduction

The basic principles of operation of an electronbeam gun for target preparation and the history of thesystem as we know it today have been presented atprevious conferences of the INTDS [1-3]. The systemswhich will be described in this contribution use Temes-cal (Temescal Division of the BOC Group, Berkeley,CA) electron beam sources with a 270' deflection an-gle. Theminimum spot size at the evaporant is typically6 mm diameter, which makes the evaporation of verysmall amounts of isotopic material very difficult. Fur-thermore, cleaning the pocket is often a long and tedi-ous task because the gun has to be partially disassem-bled .

In order to reduce the beam spot to 1-2 mm diame-ter several changes have been made . In the followingchapter, two types of guns will be discussed : one havinga single pocket hearth (Temescal model SFIH-270-0)and one with a four pocket hearth (Temescal modelSTIH-270-1) . Each has been modified to improve itsperformance .

2. Single pocket gun

2.1 . Producing a smaller beam spot size

Several improvements of the original configurationof the single pocket electron gun (Temescal model

* Work supported by the U.S . Department of Energy, NuclearPhysics Division, under contract W-31-109-ENG-38 .

0168-9002/91/$03 .50 © 1991 - Elsevier Science Publishers B.V . (North-Holland)

Nuclear Instruments and Methods in Physics Research A303 (1991) 162-164North-Holland

SFIH-270-0) have been made at the Technical Univer-sity of Munich . First, in order to achieve optimumfocusing, the standard ac power supply was replaced bya do power supply, because an alternating magneticfield at the filament results in an undesirable wideningof the beam spot in the x-direction . Thinner tungstenfilaments (0.5 mm diameter) with higher resistance wereused to reduce the filament current and, thus, the re-lated but unwanted beam deflection . Also, a reductionto the number of filament windings reduced the lengthof the spot size in the y-direction .

Another change in the single pocket gun was theconstruction of a new copper block with a removablecrucible having the bottom of the pocket 3.5 mm belowthe optimum beam spot position, in order to focus thebeam on the top surface of a 3 .5 mm diameter droplet.In order to reduce sparking, the gun was prepared tooperate at 5 kV. Shunts were used to adjust the perma-nent magnetic field, and pole extensions were used tofurther focus the beam . This was done by adjusting theshunt to operate at 5 kV without additional magneticfield from the electromagnet.

Thus modified, the gun was tested at Argonne Na-tional Laboratory to determine the quality of the beamusing a new power supply (Eraton model EB-8 fromInnotec, Sun Valley, CA 93065) . It was found that withthe new power supply the beam properties were almostidentical to those measured previously at the TechnicalUniversity of Munich (see fig . 1) . The beam propertiesof this gun were also compared to those of the fourpocket gun discussed to the following section .

3. Four pocket gun

3.1 . Construction and operation limitations

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The four pocket electron beam gun (Temescal, modelSTIH-270-1) has a larger distance between the polepieces of the bending magnet, which makes it difficultto obtain a small beam spot size . In a first attempt, afilament sirmlar to the one used successfully with thesingle pocket gun was installed. A substantial improve-ment in the spot size was observed (see fig . 1) . It wasnot possible, however, to obtain a beam spot size which

Fig. 1 . Beam spot shapes from electron beam-eroded carbon discs . (1) Beam shape obtained with the single pocket gun and theERATRON EB-8 power supply (see section 2.1) . (2) Beam shape obtained with the original four pocket gun. (3) Beam shape

obtained with the modifications described m section 3 .1 . (4) Beam shape obtained with the modifications described msection 3.2.

would fit within a circular area smaller than about 3mm diameter .

3.2 . Modified emitter system

At Florida State University, an emitter system asshown in fig . 2 was developed for a four pocket electrongun. There, it was used successfully in the production ofthin films of stable and isotopically enriched materials.This modification was adapted for use with the ANLfour pocket gun and an Innotec power supply to make

Fig . 2. Schematic drawing of the modified enutter system . (1) Anode with a 2-3 mm diameter hole, (2, 3) beam former made m twopieces, (4) filament clamps, (5, 6) cathode blocks, and (7) filament . Tungsten wire 0.5 mm thick .

IV . VAPORDEPOSITED TARGETS

16 4

Table 1Films prepared with the modified electron gun

the gun more useful for the evaporation of small quanti-ties of material.

The filament consists of three turns of 0.5 mmdiameter tungsten wire with support legs at both sides.The coil part of the filament is offset from the supportlegs so that it is close to the beam former plate (a smallcollimator with a diameter of 2.0 mm). By rotating thefilament, the vertical alignment can be changed. Withthe filament mounted horizontally, the beam formertotally blocks all electrons emitted from the support legsand the bottom side of the filament, and blocks part ofthe diffused beams. The anode (part 1 in fig . 2), has thesame size hole as the beam former and is made oftantalum (parts 4, 5, and 6 are clamped to the horizon-tally mounted filament). Using this emitter system, andadjusting both the shunt and the pole pieces, the beamspot size was about 1.0 mm x 4.0 mm (see fig . 1) . Thewidth seems to be the critical parameter when usingsmall amounts of material.

In addition, a new hearth assembly was designed andconstructed . It allows removal of the top half for clean-ing purposes . The hearth is machined from Cu with foursmall dimple pockets in order to keep 50-100 mgsamples in place during evaporation .

4. Results and conclusions

The results from evaporations of several differentisotopes are listed in table 1. The particular isotope,accelerating voltage, beam current and evaporation rateare given. It should be noted that some of the tradition-

G.E. Thomas et al. / Characteristics of electron beam sources

Comments/remarks

laser mirrorsrelease agentcold fusion exp.

backing foils

release agent

release agent

40 lag cm-Z +20 Wg cm - z C

30 hg cm-Z C "spot" targets30 Irg cm-z C "spot" targets

ally more difficult elements, such as the rare earths,zirconium, molybdenum and others, have beenevaporated with little difficulty using this setup.

The commercial Temescal electron beam gun systemis not well suited for evaporating small quantities ofsource material . Design changes to this single pocketsystem have been made which make the electron gunusable for the evaporation of small amounts of material .Similar modifications can also improve the performanceof the four pocket electron beam gun. This gun has theadditional advantage that as many as four differentmaterials may be evaporated without breaking thevacuum . The new Eratron EB-8 power supply, whichwas tested, proved to be very stable and reliable forboth the single and the four pocket gun.

Acknowledgements

The authors would like to thank Dr . D.S . Gemmelland Dr . I . Ahmad for their continuing support of thetarget making facility and its associated projects .

References

[1] P. Maier-Komor, Proc . 3rd World Conf . of the INTDS,Chalk River, Canada, 1974, AECL-5503, p. 70.

[2] P. Maser-Komor, Proc . 4th World Conf . of the INTDS,Argonne, IL, USA, Report ANL/PHY/MSD-76-1, p. 207.

[3] G.E. Thomas, Nucl . Instr . and Meth . A282 (1989) 124.

E[kV]

1[mA]

Imag[A]

Rate[hg cm -z s-1 ]

Zr 10 160W 9.8 115NaCI 5.5 10 -0.34 13 .0Cr 6 16Mo 10 90 -0.95 11 .1C 10 70 -0.81 6.3Au 10 20 -0.96 80BaCl 2 5 .2 20 0 8Y 8 30 -0.71 9CSI 5 9 -0.37Gd 10 40 23Fe 10 25 -0.75 3.6Nd 10 23 -0.77 1.3Pt 10 85 -0.71 3.0Os 10 37 -0.75 2.2

Substrate T[K]

quartz disksglass slidesPd diskglass slides 623glass slides

glass slides

glass slidesstretched AuC foilscollodionC foilsC foils