Explosively clad uranium targets

NUCLEAR

INSTRUMENTS

AND

METHODS

167

(1979)

85-89:

©

NORTH-IIOLLANI)

PIIBLISHING

CO.

EXPLOSIVELY CLAD URANIUM TARGETS H E L M U T FOLGER

Gesellscha/i./fit Sctmerionct?/hrschun,~, D-f100 Darmstadt, Germany and ULF RICHTER

D3namit Nobel A.G., D-5909 Burbach-Wfir~endor/i Germam"

B o m b a r d m e n t s of thick u r a n i u m ['oils with high-energetic, high-intensity heavy ion beams require water-cooled target s y s t e m s if all reaction products have to be collected in the u r a n i u m layer. It was found that about 0.2 m m thick u r a n i u m tbils can be explosively clad to 1.5 m m thick copper or 1.0 m m thick nickel backings and used for this purpose. The high-velocity collision process is explained in principle, and the procedure lbr the preparation of targets is described, Metallographic samples, SEM-images, EDS-spectra, and EDS-line-profiles are investigated to gain information about the intermediate target-to-backing structures. Irradiations of u r a n i u m - c o p p e r targets with b e a m s of 7.5 M e V / u ]a6Xe and 238U and intensities of 3 × 1012 and 2 . 5 x 1 0 II particles.s I respectively, are mentioned.

1. Introduction In high-energetic heavy ion bombardments large ranges of accelerated particles have to be expected in target materials. 7.5 M e V / u 238U ions, for example, have ranges of about 55 m g . c m -2 in uraniuml). This defines the minimum target thickness required to stop the beams and to collect all reaction products in the forward direction. If heavy ion beams of the given energy are produced with current densities of 2 . 5 × 1 0 ~ particles, s 1, a heat of about 70 W is released in beam spots which are often around l cm 2. In this case uranium targets have to be cooled during an irradiation. An effective cooling can be obtained in a direct contact between uranium and a water-cooled copper plate. A convenient procedure for preparing such target systems would involve high vacuum evaporation. But uranium layers being evaporated in thicknesses of up to 100 mg • cm -2 to 1.5 mm thick copper plates 2) are not very resistive against corrosion. A better behaviour is found with uraniu m - c o p p e r samples brought into intimate contact with each other by explosive cladding. This process utilizes the energy liberated during a detonation, and it was B6gl 3) who first proposed to apply this technique to target preparations for high-energetic heavy ion beams at the UNILAC of GSI. In explosive cladding two thoroughly cleaned plates, a flyer plate loaded with explosive and a base plate, are mounted one above the other. A detonation is initiated, and the plates are welded

together due to a high-velocity impact. It is assumed that in some colliding processes a jet of surface material, resulting from shear stress forces, carries traces of surface contaminants out of the colliding bond zone. The whole procedure cannot be performed in a c o m m o n laboratory, it obviously should be an open-air experiment. In the following a general view on explosive cladding will be given, while more detailed information is found in literature 4 6). In an experimental part the preparation of uranium targets with copper or nickel backings is described, then some investigations of the collision area in a uranium-to-copper bond are discussed. Finally applications of explosively clad targets are given.

2. Basics of explosive cladding If two metallic layers have to be connected by explosive cladding, one of them is used as a base plate, whereas the other, called the flyer plate, is positioned at a distance of some millimeters above it (see fig. 1). An explosive is loaded directly to the upper plate, then a detonator starts the process detonotlon products wavy ,Nerfoc(

%' -~., ~--~r::'~r~" .....

explos,ve flying plote

V~/~/~';'//////H

ground plate

v~

b

Fig, l, Schematic of explosive cladding. IV.

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H. F O L G E R A N D U. R I C H T E R

which proceeds over the plate with a velocity vd of 1400-7500 m . s -~, depending on the properties of the explosive selected. Pressures of 3 0 - 3 0 0 kbar are built up during a detonation within 10 6s. They cause the flying plate to contact the ground plate at impact velocities Vp of 1 0 0 - 1 0 0 0 m . s l determined by the initial geometrical configuration and the explosive load. In an experimental arrangement with parallel plates as shown in fig. 1, the collision velocity vc is equal to the detonation velocity ud. The collision angle /~ herein is related to the plate and collision velocities by the equation: Op/Oc=Sill, fl. It is chosen to be about 10 ° in many cases, but also angles of 2°-25 ° are used in special applications. The type of bonding as a result of a high-velocity collision between two metals mainly depends on the properties and surface qualities of the colliding metals, on the plate velocity, and on the collision angle. Mostly a more or less wavy interface is obtained following the direction of the detonation, as indicated in fig. 1. But also plane collision regions, zones with solidified melt, or continuous intermediate layers are obtained.

3. Experimental 0 . 1 5 - 0 . 2 5 m m thick uranium foils had to be bonded to 1.5 m m thick copper or 1.0 m m thick nickel plates. However, the relatively thin uranium target material of a size of about 1 0 0 × 1 0 0 m m 2 was not strong enough to be used as a self-supporting flyer plate in a cladding process. So it was attached to a double adhesive tape fixed to a 6 m m thick base plate of 1 4 0 × 1 4 0 m m 2 of steel. The thick copper or nickel target backings of the same dimensions, m o u n t e d at a distance of about 1 m m above the ground plate, were then explosively clad to the supported uranium foils. This procedure has already been described by PrtimmerT). About 1.8 g • cm 2 of an explosive were used for one shot

at a detonation velocity of 2000 m . s ~. Later, the bonded u r a n i u m - c o p p e r or uranium-nickel pieces were separated from their steel supports. The targets of 25 m m diameter were punched out. The uranium surfaces of all samples appeared to be oxidized and had to be cleaned before such layers could be used in an experiment. The different thicknesses of explosively clad uranium and copper or nickel plates which were applied as targets are given in table 1. Depleted uranium with less than 0.4% of 235U, 0.15 and 0.25 m m thick, was obtained in annealed condition from N U K E M , Hanau, G e r m a n y . 0.17 m m thick foils of natural uranium, not being annealed, were supplied by Goddfellow Metals, Cambridge, England. All the foils showed good cladding behaviour.

4. Investigations of a uranium-to-copper bond 4 . 1 . METALLOGRAPHIC EXAMINATION

A specimen for a micrograph was prepared by grinding and polishing the bonded zone of a 0.17 m m thick uranium foil and a 1.5 m m thick copper layer. This was difficult to be performed, as the hardness values of both metals differ very much. It can be seen from fig. 2 that a wavy interface occured as a result of impact, having a wavelength of about 0.08 to 0.12 m m and an amplitude of up to about 0.02 m m The latter value corresponds to a layer of 38 mg • cm 2 of uranium, and it has to be considered in experiments where the homogeneity of the bonded area is of importance. The overall thickness of the uranium layer

V: 125 gu

TABLE 1

Thicknesses of explosively clad U Cu and U - N i targets.

Backing substance 1.5 mm 1.5 mm 1.5 mm 1.0 mm

Cu Cu Cu Ni

Target material 0.25 mm 0.17 mm 0.15 mm 0.15 mm

U U U U

//

I

0.17mm I

Fig. 2. Micrograph of a wavy interface in Cu-to-U bond.

EXPLOSIVELY-CLAD

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melt. Structures of this type are formed in general if melted material is included between colliding plates. The chemical composition of the material in this area is subject of the following considerations.

a

:.,i,;
O,01rn m

4.3. ENERGY DISPERSIVE X-RAY SPECTRA The bonding zone was investigated using a silicon diode in an energy dispersive spectrometer (EDS) being connected to the above mentioned scanning electron microscope. Three different X-ray spectra have been obtained, as can be seen in fig. 4. The gold M-lines are present in all spectra, because the bonded sample being embedded in plastic had received an electrically conductive surface of sputtered gold before this examination. The copper layer generates copper K- and L-lines besides this gold peak (upper spectrum), the uranium region shows uranium M-lines and besides gold no contaminant either (center). In the spectrum of the intermediate layer (below) uranium and copper X-rays can be seen. It was not possible to obtain a quantitative information about the ratio of the components in the intermediate layer, because the analysing beam spot was too large for this purpose. It can be

I

_

CuI

~

x~L/

o

0,01 rnm

C%=

2'

¢.m

Fig. 3, SEM images of collision regions in Cu-to-U bonds without and with solidified melt.

-

z

--

was measured to be d = ( 0 . 1 7 5 _ + 0 . 0 1 5 ) m m (_+8.6%). The target surface, however, varied in its thickness only about 0 . 0 0 4 m m . It had to be polished before an experiment in order to obtain a clean, oxide-free target layer. 4.2• SCANNINGELECTRONMICROPROBES Additional details of the collision zone were observed by means of a scanning electron microscope (SEM). Fig. 3a gives a view of a direct bond between copper (left) and uranium (right) with no apparent intermediate layer. A different image of this region is given in fig. 3b where a spot of about 0 . 0 4 0 × 0 . 0 0 6 m m 2 can be identified as solidified

U%i,I,

UMa

j' I'

LU O=C

r~

I

5 ENERGY

10 keV

Fig. 4. EDS spectra of layers of Cu, U, and solidified melt. IV.

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PREPARATION

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88

H. F O L G E R r

A N D U. R I C H T E R

I

100

.=,0

~,<~1>0-0~

UM-hne

I

~

I

Cuz_hn e

o::

0 0000

'T 0025 0050 DISTANCEmm

0.075

Fig. 5. EDS line profile analysis across a bonded zone of Cuto-U without and with region of solidified melt.

assumed, however, that a solidified uranium-copper melt - probably UCus - will be present. 4.4.

ENERGY DISPERSIVE SPECTROMETER SCANS

The concentration of uranium and copper, respectively, was analysed in traverses across two different bond zones by measuring the intensities of the corresponding X-ray lines with the electron dispersive spectrometer. ]'he traces of the analysing beams can be seen in fig. 5 as black bars, whereas the intensities of the uranium M-lines and the copper L-lines are plotted against the distance on the sample in fig. 5. The bonded zone without an intermediate layer shows a relatively sharp discontinuity in the intensities of both lines (fig. 5, above), whereas profile steps in the region of a solidified melt can be seen over a distance of about 0.01 mm (fig. 5, below). From the results of the investigations of the collision area in uranium-to-copper bonds it may be concluded that these two metals can be explosively clad to one another. Sometimes a more or less wavy interface is produced. This may contain inclusions of solidified melt with no reasonable amounts of contaminants.

5. Applications Explosively clad u r a n i u m - c o p p e r targets were especially required for two types of high-intensity heavy ion irradiations where the primary beam as well as all reaction products had to be collected in the uranium target layers for further radiochemical investigations. a) In search of long-lived superheavy elements in the reaction of 7.5 M e V / u ~36Xe with >SU ~), two

uranium targets of 3 0 0 m g . c m 2 with watercooled copper backings were bombarded with 3 × 101l particles, s -1. Integral doses of about 1017 particles could be accumulated in one day with no significant changes of the uranium layers. Later the targets were processed utilizing independent techniques of solution chemistry and gas phase chemistry to measure reaction products. b) The isotope distributions in the reaction of 7.5 M e V / u 23'U with 238U 9) were studied at uranium beam intensities of up to 2.5×10011parti cles • s - l . After irradiations uranium target layers of 300 m g - c m 2 still were in good condition. They were dissolved from their copper backings, and the reaction products were separated chemically into 25 fractions for long-time radiochemical measurements. Higher beam energies and intensities will be charged to explosive clad samples in future experiments. It is necessary therefore to test the durability of a series of different target systems in off-line experiments, simulating thermal heat effects by means of an electron beam gun. Predictions with respect to irradiation damages cannot be made hereby.

6. Concluding remarks It has been shown that uranium foils of 300 m g . c m 2 thickness can be bonded to 1.5 mm thick copper or 1.0 m m thick nickel backings in a high-velocity collision. Explosive cladding therefore is an excellent method to combine metals of very different chemical and physical properties. Microscopical views and X-ray investigations of the bonded area demonstrate the formation of a wavy interface between the metals with some inclusions of solidified uranium-copper melt. No contaminants are observed, insofar. The preparations of thinner uranium-copper systems by explosive cladding are the aim of a projected series of bondings. In most accelerator experiments foils of only some /xg. cm -2 of a substance are demanded. From this point of view it is quite unusual to prepare targets by initiating detonations as described in this paper. But it is evident that thick target layers with water-cooled backings are required in high-intensity heavy ion irradiation experiments in order to accumulate all reaction products. For the preparation of such target systems explosive cladding is one possible and very efficient way. This type of target may as well be installed as a beam stopper with specific properties.

EXPLOSIVELY-CLAD

The authors are indebted to B. Genswtirger of GSI for her important SEM and EDS work. They also wish to thank H. L6w of Dynamit Nobel A.G. and J. Klein of GSI for their helpful assistance during the investigations. The cooperation with the Nuclear Chemistry group of GSI and the Nuclear Chemistry group of the University of Mainz is gratefully acknowledged.

References 1) L.C. Northcliffe and R.F. Schilling, Nucl. Data Tables A7 (1970) 437.

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2) H. Folger and J. Klemm, in Proc. 6th Ann. Conf. of the Int. Nucl. Target Development Society, University of California, LBL-Report-7950 (1978) p. 69. 3) W. B6gl, private communication (1975). 4) G. R. Cowan and A.H. Holtzman, J. Appl. Phys. 34 (1963) 928. 5) U. Richter, Maschinenmarkt 79 (1973). 6) R. A. Prtimmer, Thin Solid Films 45 (1977) 205. 7) R. A. PrOmmer, Internal Report for GSI (Feb. 1976). 8) H. G~iggeler, W. Brtichle, H. Ahrens, H. Folger, G. Franz, J.V. Kratz, M. Sch~idel, I. Warnecke, G. Wirth, N. Trautmann, G. Herrmann, N. Kaffrell, P. Peuser, G. Tittel, M. Weber and M. Zendel, Z. Physik A 286 (1978) 419. 9) M. Sch~idel, J. V. Kratz, H. Ahrens, W. Brtichle, G. Franz, H. G~iggeler, I. Warnecke, G. Wirth, G. Herrmann, N. Trautmann and M. Weis, Phys. Rev. Lett. 41 (1978) 469.

IV. S P E C I A L T A R G E T P R E P A R A T I O N METHODS