Lauer, K.F.

Verdingh, V.

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This reprint is intended for restricted distribution only. It reproduces, by kind permis.sion of the publisher, an articl,e from ,,NUCLEA'R INSTRUMENTS AND METHODS" ,. No. 21 - 1963, 161-166. For further copies please apply to North-HdUand Publi,shing Co., P.O. Box 103 - Amsterdam, 'lj_ederland. Dieser Sonderdruck ist fur eine beschrankte V erteilung bestimmt. Die Wiedergabe des vorliegenden in ,,NUCLEAR INST~UMENTS 'A,ND METHODS,,, Nr. 21 - 1963, 161-166 erschienenen Aufsatzes erfolgt mit freundlicher Genehmigung des Herausgebers! Bestellungen weiterer Exemplare 0sind an North-HoUand Publishing Co., P.O. Box 103 - Amsterdam, Nederland, zu richten. Ce tire-a-part est exclusivement destine a une diffusion restreinte. Il reprend, avec l' ai~ble autorisation de l' editeur, un article publie dans « NUCLE4R INSTRUMENTS AND METHODS », N° 21 -1963, 161-166. Tout autre exemplaire de 'cet article doit ~tre· demande a North-Holland Publishing Co.,. P.O. Box JOB-Amsterdam, Nederland. Questo estratto e destinato esclusivamente 'ad una dilfusione limitata. ,. ' Esso e, stato riprodotto, , per gentile concessione dell'Ediwe, . d'a « NVCLEAR INSTRUMENTS AND METHODS », No 21 -1963, 161-166. Ulteriori copie dell' articolo de,{Jbono essere tichieste a NoHh-HoUand Publishing Co., P.O. Box 103-Amsterdam, Nederland. UCLEAR INSTRUMENTS AND METHODS 21 (1963) 161-166; NORTH-HOLLAND PUBLISHING CO. PREPARATION BY ELECTRO-SPRAYING OF THIN URANIUM, PLUTONIUM AND BORON SAMPLES FOR NEUTRON CROSS SECTION MEASUREMENTS IN 4n GEOMETRY K. F. LAUER and V. VERDINGH Central Bureau for Nuclear Measurements, Euratom, Geel, Belgium Received 3 July 1962 b.e preparation of thin films is an important requirement for .any measurements in nuclear physics. If the required surface large, the problem is furthermore complicated. A machine . Introduction Precise measurements of nuclear properties often eed 4n counting. A typical sample for such work, rhich we had to prepare is the following: A foil of 00 - 500 µg element/cm2 which is self-supporting verasurface ofatleast 25 cm 2. The element of inter-,t should be pure, that means it should be a metal :1.ther than a compound. For most of the elements 1ere are serious problems involved in the prepa-:1.tion, handling and storage of such thin films of 1etals. There are also only very few methods that allow ) prepare such a target. Most of the methods that ould be used for preparing thin films require a thin acking. This backing has to be so thin, that its effect is egligible on the particles to be counted. This means that the backing would have to be as 1in as possible and mostly thinner than 50 µg/cm 2 • The choice of the elements in the backing is also mited by the type of experiment to be performed. ubstances especially suitable for e.g. neutron meas-rements are carbon, organic materials, alumina or .lica. . Backings If we use such a thin supporting foil, a ring is eeded to support this thin foil itself. Fig. 1 shows 1e kind of arrangement we have chosen. ve tried the following supporting foils: a) Carbon foils prepared by decomposition of 1ethyl iodide on nickel foils 1). and a technique are described which allow to prepare samples with a diameter of up to 5 cm. The method has been applied to uranium, plutonium and boron . It proved to be extremely difficult to prepare thin carbon foils larger than 2 cm diameter, so that all preliminary experiments were made on smaller carbon foils (exploded view) _Gold lay~ Foil \ Screen -am,Ls,•@M Denosit ~«~LJ3j_rig_ Fig. 1. Exploded view of backing and screen. b) Thin nickel foils were prepared on copper substrates by electrolysis. Similar foils are available on the market. Here it was possible to produce big foils and by electrolytic dissolution of the copper backing we were able to obtain self supporting nickel foils of 5 cm diameter, but these foils were rather inhomo-geneous and unstable with time because of the rapid oxydation of these thin layers. c) The vyns and cellulose foils were prepared in thicknesses from 10 to 100 µg/cm 2 • The plastic foils were made conducting by metallizing with Al or Au by vacuum evaporation. Here again the thick-ness of the metal layer may vary from 10 to 100 µg/cm 2 . By varying the plastic and the metal thickness, foils with very different physical properties can be 1) E. Kashy, R. R. Perry and J. R. Risser, Nucl. Instr. and Meth. 4 (1959) 167. 161 162 K. F. LAUER AND V. VERDINGH obtained. In general: the thicker the plastic film the more mechanically strongis the foil. The thicker the metal foil becomes the more the optical trans-parency of the foil diminishes, whilst the electrical conductivity of the foil increases. 3. Choice of Method 3.1. SOLUTION EVAPORATION AND PAINTING Of the five methods taken into consideration we eliminated solution evaporation and painting be-cause of the great difficulties encountered when using these methods for the preparation of homo-geneous targets with large surfaces. 3.2. VACUUM EVAPORATION The thermal conditions during vacuum deposi-tion are such that the extremely thin plastic films are easily damaged and so one has to place the target at a considerable distance from the heated substance. Therefore, the recovery factor for vacu-um evaporation is extremely low, so that the vacu-um evaporation of rare and a-active isotopes be-comes a quite problematic method. This especially as it is very difficult to obtain a homogeneous layer without losing considerable amounts of material. Besides of this, evaporation in glove boxes, which is necessary in the case of a-active radio isotopes, is a difficult technique. There is also the danger re-2) D. Carswell and J. Milstead, J. Nucl. Eng. 4 (1957) 51. 3) E. Bruninx and G. Rudstam, Nucl. Instr. and Meth. 13 (1961) 131. sulting from the pyrophoric nature of evaporat Pu and U. 3.3. ELECTROLYSIS The next method we considered for the prei: ration of samples was electrolysis. This method poses also a lot of practical proble1 and the electrolysis of a reasonable volume of sol tion on such an extremely fragile and thin foil again a very difficult technical procedure. In table 1 the behaviour of different supporti materials is compared. We can see that for the electrolytic method on two types of foils could be used and this only und very specific conditions. The results of these expe ments are to be published later. Even though it possible to electrolyse on such a thin foil, the tee nology becomes difficult with increasing size foils. It is extremely difficult to obtain by electr lysis a layer which is larger than 3 cm in diamett 3.4. ELECTRO SPRAYING One of the few methods remaining is the elect spraying method of Carswell and Milsted2). Tl experimental conditions for preparing samples small surface area for /$-counting have been recen ly described by Bruninx and Rudstam 3). 4. Apparatus 4.1. HIGH TENSION SUPPLY The high tension supply is very simple and can 1 TABLE 1 Carbon Nickel Vyns Vyns + Al Vyns + Au Cellulose Cellulose+ Al Cellulose + Au - Not applicable. + Applicable. Solution evaporation (+) + (+) + + + + + ( +) Only applicable under special conditions. Methods and supports used Painting Electrolysis Vacuum Electro evaporation spraying (+) (+) + + + (+) + + + - + (+) + - + + + (+) + + + - + (+) + - + + + (+) + + PREPARATION BY ELECTRO-SPRAYING OF THIN SAMPLES 163 uilt easily from commercially available electronic :>mponents. The circuit diagram and components sed are shown in fig. 2 and its legend. This in-tallation allows us to work with up to 13 kV. One T2 4-------= II .-=-'---~ II 1 2 ~---'= II =--~ II II II II II DY86 Fig. 2. Circuit diagram of the high tension supply. high-tension transformer 100 k.Q 50pF 500pF + condenser consisting of metallic plates at a distance of ± I cm from the secondary coil of the high tension transformer } heating current for filament } bias adjust 220-280 V. lvantage of the kind of generator we have used ems to be that the unit is not stabilized. If there some secondary effect at the capillary, reducing .e flow of liquid, the resulting decrease in current ·oduces a rise in tension which again tends to Llance the decreased flow rate. On the other hand, if e flow rate becomes too high, this means when it :ars the dropping point then the increasing current wers at the same time the output voltage, which again balancing somewhat the overfeed of the pillary. :. CAPILLARIES Normally the capillaries used for spraying are nple straight capillary tubes holding only limited aounts of solution (fig. 3). In our experiments we .d to prepare samples, containing much larger aounts of isotopes. A greater volume of solution LS to be used. Because of the change in height of e liquid column it was impossible to obtain a nstant liquid pressure and hence constant raying conditions. It, therefore, proved to be necessary to construct a container which allowed a free flow of liquid for a considerable length of time. In case of an appreciable evaporation from the upper or the lower part of the capillary one of the well-+ T Fig. 3. Normal straight capillary. known troubles with normal thin capillaries is that they tend to clog. This is mainly due to crystals forming within and even more at the surface of the solution and then lodging themselves in the finest part of the capillary. We have developed different types of capillaries with a small reservoir for the solution. Because of the above mentioned clogging effect the first type of capillary we tried (fig. 4) proved to be not very useful. The next type we constructed was a slight modification and the addition of the + T Fig. 4. Straight capillary with liquid container. swans neck improved the performance of the cap-illary considerably (fig. 5). The introduction of the fine wire into the tip of the capillary was very tedious and the position was not easily reproduc-164 K. F. LAUER AND V. VERDINGH ible. The capillary we are using presently is shown in fig. 6. The hypodermic needle that is cemented to the glass part has two advantages. First the bore of the needles we are using is known with a good + T Fig. 5. Capillary with swans neck. precision. The ideal opening in our case proved to be 0.05 - 0.08 mm. As can be seen from figs. 3 - 6 the electrical contact with the solution is made via a wire. The thickness of this wire has to be adjusted in such a way that the final opening of the capillary is just in the above mentioned optimum region. In the case one does not dispose of the appropriate Pt-wire, it is often possible to use a tungsten wire that has been reduced to the desired thickness by heating it in a flame and stripping off the formed Wire+ Screw Metal cap_ + Hypodermic needle T Fig. 6. Capillary with hypodermic needle tip. oxide. In this way we have also produced conical wires that, used together with a self-drawn glass capillary, allows to have a very reproducible and 4) J.C. Charlton, RCC-R1!9 (1961). small opening as the wire functions like a needl valve. The second advantage of the metallic capilla1 we are now using is that the spraying is very steac and the standing drop formed at the tip is n disturbing the even spraying over a consideral:: length of time. This is not the case when gla capillaries are used. The wire is fixed to a screw and can be easi lowered and raised, thus allowing to adjust tl flow rate. One is also able to clean the capilla itself during spraying by turning the screw sever times up and down. In view of very high puri solutions, that should not be contaminated, would be preferable to replace the hypoderrr: needle by a small piece of Pt-capillary that could· sealed directly into the glass or quartz container. 5. Preparation of Solutions We prepared foils with uranium, plutonium boron deposits. 5.1. URANIUM AND PLUTONIUM The U and the Pu were sprayed in the aceta form as experiments had shown that these co1 pounds are practically not hygroscopic and are s, uble in methyl alcohol. The preparation was started with the aci< nitrate solution. This nitrate solution is evaporated in a platim dish and the residue then calcined for 4 hours 250° to 300° C. The so formed mixed oxide is th easily dissolved on heating in aqueous acetic ac The solution is very carefully concentrated using apparatus described by Charlton4), then carefu recrystallized and the acetate is dissolved direc in methyl alcohol. 5.2. BORON In case of the boron samples a direct dissoluti of boric acid (H 3B03 ) in methyl alcohol or ace1 is possible. The product sprayed from this solut: corresponds exactly to the above formula. 6. Spraying Machine The spraying machine has been constructed our workshop and gives us the possibility to co· PREPARATION BY ELECTRO-SPRAYING OF THIN SAMPLES 165 ,ith our capiUary a surface of up to 8 x 9 cm2 • A :hematic view is given in fig. 7. The type of overage obtained can be seen from fig. 8 which ives sweep diagrams of the machine. It can be seen Fig. 7. Schematic view of the spraying machine. Fig. 8. Sweep diagram of the machine. om this, that coupled with each back and forward ,veep there is a small perpendicular movement of 1e carriage. The complete installation is fixed nder a glove box in such away that the major parts i the mechanical drive, including the motor, are 1tside of the active area. Fig. 9 shows the m-allation before mounting in the glove box . . Description of Layers Obtainable 1. MACROSTRUCTURE Fig. 10 shows the type of layer obtained with ~anylacetate. Fig. 9. Complete installation before mounting in the glove box. 7.2. HOMOGENEITY The homogeneity was measured in the case of U and Pu samples by low geometry counting of the layer using collimating screens for picking out the wanted part of the surface to be measured. Fig. 10. Aspect of an uranium acetate layer. 166 K. F. LAUER AND V. VERDINGH 7.3. STABILITY Uranium acetate Uranium acetate can be kept under normal laboratory conditions without any measurable change in aspect and weight occurring. Under vacuum there is no weightloss. The method has been also used to prepare layers of uranium oxides on thick metallic supports (Pt, etc.). By applying successive coats andintermittent calcination of the acetate to the oxide, layers of several mg uranium oxide/cm 2 have been prepared. These relatively thick layers are perfectly stable, adherent and have the same homogeneity as the acetate layers they were prepared from. Plutonium acetate The behaviour of the plutonium acetate is identi-cal with the one of the uranium acttate. Boric acid The boric acid is stable under normal conditions. Under vacuum the boric acid has such an apprecia-ble vapour pressure that it slowly evaporates. It is, therefore, only possible to use such boric acid layers under atmospheric or higher pressures. Efficiency The efficiency of the method depends on the ratio of the area of the layer to be prepared and the area covered by the machine used. We are using metallic diaphragms and the product sprayed onto these diaphragms is easily recovered. The loss occurrir at the tip of the capillary by cleaning away crysta is also very small and all the material lost in sue a way during one preparation is recovered chem cally and can be reused after retreatment. Co1 sidering the above mentioned points we can sc: that our efficiency is better than 95 %- In case , extremely diluted solutions the losses by adsorptic are becoming important and there the efficiency i of course, much less. 8. Conclusions The described method allows us to prepare laye of chemical compounds on any kind of conductir surface. The thickness of these layers is not limitt to any special value and the surface area of such layer is only limited by the machine used for tl preparation. The method has been applied 1 uranium, plutonium and boron compounds. The layers prepared varied from zero to ov, several milligrams of compound per square cent meter. Thehomogeneityoftheprepared layers was abo1 1 %- The method is applicable to any compour that can be dissolved in an appropriate solven Acknowledgements We are indebted to Mr. Moret and Mr. Van Saer for the design of the spraying apparatus. Theplast foils have been prepared using a technique d veloped by Mr. Spernol. 

PREPARATION BY ELECTRO-SPRAYING OF THIN URANIUM, PLUTONIUM AND BORON SAMPLES FOR NEUTRON CROSS SECTION MEASUREMENTS IN 4π GEOMETRY. EUR 278.e

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