Isotopically enriched rolled Cr foils

Nuclear Instruments and Methods in Physics Research A 362 (1995) 94-97

NUCLEAR INSTRUMENTS 8 METHODS IN PHVSICS RESEARCH

ELSEVIER

Isotopically enriched rolled Cr foils G. Manente a,*, D. Blunt b aIstituto Nazionale di Fisica Nucleare, Laboratori Nazionali di Legnaro, Legnaro (Padova), Italy b Schuster Laboratory,

University of Manchester, Manchester Ml3 9PL, UK

Abstract Isotopically enriched self-supporting Cr targets in the thickness range of 0.3 to 2 mg/cm2 were prepared. After reduction of Cr,O,, the material was further cleaned and solidified by a simple resistance heating technique. The resulting Cr bead can easily be rolled to the minimum desired thickness of 300 p,g/cm2.

1. Introduction The study of exotic proton-rich nuclei close to ‘?Sn is of great interest for investigating the validity of the nuclear shell model in general and single particle energies and the residual proton-neutron interaction in particular. With this aim, the high spin states of the light Sn isotopes produced in the 58Ni + 50Cr reaction at 210 MeV have been studied. The beam was provided by the XTU tandem accelerator of the Legnaro National Laboratory (LNL), and y-rays have been detected using the GASP array composed of 40 high-efficiency Ge detectors. In order to get mass and charge identification, a Si-ball has been used, coupled to the GASP array. It is composed of 40 AE detectors of 130 p,m thickness covering 92% of the total solid angle of the recoil mass spectrometer. Self-supporting “Cr targets of 500 pg/cm’ have been asked of the LNL Target Laboratory. Thicker targets (1.2 mg/cm2) to stop the recoiling nuclei have been requested as well for a y-y coincidence measurement. Targets with such thicknesses have to be prepared by rolling, even though Cr is brittle; a fact which complicates the rolling procedure.

2. Experimental setup Since Cr sublimes and because we already had the separated isotope in powdered elemental form (a Russian product), we tested to obtain a bead from natural metallic Cr powder (M4N Cerac product) by using the first system for rare-earth reduction, which was designed after Ref. [l] and installed in the laboratory many years ago.

l

Corresponding author. Fax +39 49 641 925, e-mail ruggero

@legnaro.infn.it.

The collector consists of a water-cooled Cu block 60 X 30 X 30 mm3 which can be positioned by corrugated stainless steel tubing very close to a Ta crucible. The evaporation source is a seamless 6.4 mm outer-diameter Ta tube, 0.2 mm wall thickness, 50 mm long with a 1 mm orifice at the top, squeezed at the ends and tightened horizontally to two normal feedthroughs for resistance heating of a standard evaporation-condensation system (Fig. 1).

3. Cleaning-condensing

procedure

About 100 mg of “Cr (URSS 132) powder, previously cleaned by a H, flux in our furnace at 1520 K for 2.5 hours, were pressed (20 kN) into a 5 mm diameter pellet, introduced into a Ta tube crucible and heated. We realised very soon that Cr forms an alloy with the Ta (as can be seen by the Cr-Ta alloy phase diagram [2]). Compromise conditions were found by heating to 1800 K for 10 min to obtain not really a bead as hoped for, but only some small droplets condensed around the inner parts of the orifice and towards the tube ends which remain cool because of the proximity to the water-cooled feedthroughs. Some droplets were removed, by bending the cut tube, and rolling. Several “Cr foils were obtained: 0.33, 0.48, 1.2, 1.5 and 2.0 mg/cm’, even though only the 0.48 mg/cm’ target without pinholes and one of 1.2 mg/cm2 were chosen for the experiment. The thin target showed no disturbing Ta impurity in the experiment, but a dominant Ta impurity established afterwards by a SEM measurement was found in the thicker target. So, by starting from these observations, it was thought to use a MO tube, instead of a Ta one, to avoid contamination. To have a significant Cr evaporation rate, a crucible wall temperature of more than 1800 K would be required. But at such temperatures Cr-Ta alloying takes place [2]. Higher temperatures

0168-9002/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0168-9002(95)00315-O

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G. Manente, D. Blunt / Nucl. In&r. and Meth. in Phys. Rex A 362 (1995) 94-97

supplyJ

To power

Fig. 1. Sketch of the Cr evaporation-condensation

(1900-2000 K> may be reached without having alloy formation by using a MO source. In this way, it should be possible to reach a faster evaporation rate to obtain a real Cr bead without any MO contamination. Unfortunately, by using the MO tubes (6 mm outer diameter, 0.2 mm wall thickness, 50 mm long) we met a lot of problems. First of all, the tubes were too brittle to be squeezed. Second, the material was very dirty, probably from oxidation inside

To

tube

6.4

0 x 0.2

x 50

MO liner

setup.

and out. The answer we got from the factory producing the tubes was that it is quite impossible to obtain a ductile seamless MO tube in this thickness range. However, they explained how to squeeze the tube without cracking it, by the use of a warm air blower. One heats the extremity of the tube up to 600 K and squeezes it while still warm. Many attempts were made to define the right parameters to obtain a significant collection efficiency of condensed

70 x 30 x 0.05

ftntshed

Fig. 2. Procedure of the crucible preparation

crucible

rlfh

fhe Cr peiie,

for the Cr evaporation

II. TARGET/STRIPPER

FOIL PREPARATIONS

96

G. Manente, D. Blunt/Nucl.

Instr. and Meth. in Phys. Res. A 362 (1995) 94-97

material and also to find the right hole dimension, the right distance of the cold collector block, etc., but with very poor results. The average collection efficiency was about 10%. The fundamental mistake was found in the squeezed part of the tube. With this simple procedure, we did not succeed in squeezing a sufficient length at the MO tube ends. Due to the higher thermal conductivity of MO compared to Ta, we were not able to condense appreciable material on the Cu collector block and we always found a lot of material close to the squeezed parts of the tube which were close to the water-cooled current feedthroughs. We then thought to use a MO sheet inserted inside the Ta tube. That was done by rolling a MO sheet to about 70 X 30 X 0.05 mm3, in which a 1 mm hole was drilled. Rolling it 1.5 turns (like rolling a cigarette) the liner was inserted inside the Ta tube and the two holes (MO liner: 1 mm and Ta tube: 2 mm) were aligned and fixed by squeezing one end. A Cr pellet (3 mm diameter, 10 kN pressure) was introduced and the second end was squeezed also (Fig. 2). This procedure must be done with care in order to maintain the small apertures aligned. So after having tightened the crucible onto the current feedthroughs, the Cu block was placed very close (0.2 mm) to the upper part of the tube crucible (Fig. 1) and the bell jar could be evacuated. By starting from a vacuum of 3 X 10m4 Pa, the crucible was heated up to 1100-1300 K and left for some minutes to degas. Then the cooling water of the Cu block was opened and the temperature slowly increased up to about 2000 K and was kept for 2 min. We collected 80-90% of the material in the form of a shiny bead (right side of Fig. 1). The Cr loss was mainly located in the tube ends near the water-cooled current feedthroughs.

Energy 1;6

2ooo,

1500

2;o

,

Lr

I

v) 3

(MeV) 1;8

1000

8

x 20 MO

TO

r----I i

I

500

2

',

00 400

450

500 Channel

550

600

Fig. 3. RBS spectrum of a 300 kg/cm’ 50Cr foil obtained by the distillation method using a Ta crucible with a MO liner. The analysis was performed by using a-particles of 2 MeV, normal incidence

and 160” scattering

angle.

(instead of 1700 K for 6 hours). After H, reduction, the powder was grey, and after pressing (10 kN for 3 mm diameter) showed a metallic luster. We real&d also that by treating a larger amount of Cr,O, powder, the H, reduction only partially took place (the upper part of the well-distributed powder in an alumina boat became grey, while the rest remained green: not reduced). This means that the very long H, process is needed and must be performed. Once having obtained the shiny pellet, the same distillation procedure as described above was performed and similar results were achieved. Good collection efficiency of up to 90% was obtained only when using the thin MO liner insert.

4. Pressing procedure Another important step is to press the resulting bead several times in different positions inside a “soft” stainless steel sandwich, increasing the pressure step by step. The bead will not break because it is protected in its imprint. A disk as large as possible is obtained for the rolling. A “Cr foil of 400 pg/cm’ was produced without any pinholes, and also one of 300 kg/cm* (but with some pinholes) without Mo-Ta contamination, as can be seen from the spectrum of Fig. 3.

5. Reduction procedure By startmg from Cr,O, instead of metallic Cr powder, we took advantage of the work of Friebel et al. [3], where the best reduction results were obtained by treating the compound with a H, flux for many hours at 1700 K. Typically 100 mg of Cr,O, were first ground in a mortar to obtain a tiner powder and then inserted in the Hz tubular furnace at a temperature of 1570 K for 8 hours

6. Conclusion The best evaporation-condensation method for preparing a bead of Cr sufficiently pure for rolling is as follows: First the described reduction of Cr,O, by H, must be done. Then the pressed Cr pellet should be evaporated from a thin MO foil liner in a squeezed Ta tube crucible. Cr has a low diffusion rate in MO [2] and the relatively low mass of the MO liner means a fast increase of its Cr concentration, even if only about 70 mg of Cr is involved. Leakage of Cr from the Cr-Mo alloy into the Ta-tube can be neglected because the contact face of the MO liner with the Ta tube is small. With an evaporation temperature of 2000 K, which is sufficiently below 2090 K [2], a melting of the Cr-rich MO alloy can be avoided. Impurities of Cr,O, in the condensed Cr bead turn out to be negligible, because even at 2200 K only a relatively small vapor pressure of 1 Pa would exist. MO is a poor reductant for Cr,O,. That is why no MO impurities caused by the highly volatile MOO, ruin the ductility of the condensed Cr bead.

G. Manente, D. Blunt / Nucl. Instr. and Meth. in Phys. Res. A 362 (1995) 94-97

References

Acknowledgements The authors wish to thank R. Pengo for his suggestions, G. Dona and M. Negrato for assistance in the mechanical work, S. Zandolin and V. Rigato for the analysis of the films and the spectra preparation. A particularly deserved thanks goes to Eng. P. Favaron for the solution of the

technical and metallurgical metallic materials.

97

problems

connected

with the

[l] H.J. Maier and W. Kutschera, Nucl. Instr. and Meth. 167 (1979) 91. [2] T.B. Massalski, Binary Alloy Phase Diagrams, American Society for Metals (August 1987) pp. 836 and 865. [3] H.U. Friebel, D. Frischke, R. Grossman and H.J. Maier, Proc. 6th World Conf. of the INTDS, Berkeley, CA 1977, LBL-7950, p. 105.

II. TARGET/STRIPPER

FOIL PREPARATIONS