PANEL IIShort summary

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Submitted on 1 Jan 1979

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PANEL IIShort summary

J. Peterson, D. Damien, J. Fuger, G. Lander, J. Spirlet, O. Vogt, A.

Wojakowski

To cite this version:

J. Peterson, D. Damien, J. Fuger, G. Lander, J. Spirlet, et al.. PANEL IIShort summary. Journal de

Physique Colloques, 1979, 40 (C4), pp.C4-152-C4-153. �10.1051/jphyscol:1979450�. �jpa-00218846�

JOURNAL DE PHYSIQUE

Colloque C4, supplbment au no 4, Tome 40, avril 1979, page C4-152

PANEL I1 Short summary

Discussion Leader : J. R. Peterson

Panelists : D. Damien, J. Fuger, G. Lander, J. C . Spirlet, 0. Vogt and A. Wojakowski

Introduction. - In his introduction, the discussion leader defined potential problem areas :

1. Isotope and nuclide availability.

Weighable amounts of actinides are available through Es ; the 10" atoms of the next actinide, Fm, do not allow any solid state studies.

There are mg quantities of 253Es, '"Cf and 248Cm, g quantities of 252Cf, kg quantities of '"Cm, but the distribution of these materials to researchers all over the world is complicated or impossible due in part t o regulations of shipping and/or selling.

Half-life is not always a sufficient criterion for the choice of a nuclide: the handling risk of 248Cm (T,,, = 5 x lo5 y) is determined by the neutron emis- sion, that of 243Am

8

lo3 y) by the gamma radiation of "Wp (consider the effects of daughter

activities).

2. Preparation techniques.

They have to be amenable to the quantity and form of the actinide and t o the proposed measure- ment. The measurement technique has to be adapted to the quantity of sample available.

3. Analysis and characterization.

Do we always study the actinide or sometimes the impurity ? Are we carrying out too many experi- ments on improperly characterized samples ? Is it possible to obtain adequate analyses of sample stoi- chiometry and/or purity ?

Can we repeat measurements on more than one sample or investigate the same sample by different, independent techniques ?

4. Coordination of sample preparation and physical measurements.

Appropriate timing is a special problem for the rare, short-lived actinides to minimize deterioration of the sample by decay, radiation damage or chemi- cal reaction with the environment during storage.

Discussion.

The discussion started on the basis of a list of requirements for certain physical measu- rements (Lander) :

REQUIREMENTS FOR PHYSICAL MEASUREMENTS

Technique Magnetization (using squid) Specific heat NMR Transport Mossbauer XPS-UPS X-ray

neutrons dHvA

sample sample mass purity

- -

mg high kg high

high mg high mg v. high mg medium mg medium mg medium

medium mg high

best best No No best usually No

best best best perfect Polycrystalline material or medium purity samples were considered as sufficient for quite a few types of measurements, whereas stoichiometry (better : ratio of non-metal to metal) is usually more impor- tant and often insufficiently defined (Lander).

Perfect single crystals are required for dHvA measurements, especially for actinides : impurity levels in the ppm range, low dislocation density.

Resistivity ratios of at least 100 are needed for fields below 100 kG irrespective of chemical nature of sample (Arko). Such requirements for perfect crystals are difficult to meet, as single crystals grown at temperatures above 2 000 "C contain, in general, many dislocations (Vogt) ; perfect crystals are restricted to U and Th, as other actinide samples are affected by self-irradiation (Fuger). Single crys- tals are indeed important for spectroscopy (photoemission, optical), as they facilitate in situ cleaning of surfaces or reduction of surface roughness (Naegele) ; stoichiometry of, e.g., oxides is one of the predominant parameters in spectrosco- py (Manes).

As to the definition of weighable amounts, a question arose concerning precision on the micro- gram scale (Huray)

there are classical quartz mi- crobeam techniques to determine masses within 20-30 nanograms (Fuger), but sensitivity and sensibi- lity have to be distinguished (Weigel).

Answering the statements (Lander) that 3 mg samples of U P were used for neutron diffraction, and that stoichiometric UN contains only about 6 weight % N , it was stressed that mg samples would require high neutron fluxes, N/U ratios might be

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979450

SHORT SUMMARY C4-153 determined by neutron diffraction, since the scat-

tering amplitude of N is larger than that of U (M. H.

Mueller). In some cases stoichiometry may be varied during the measurement by controlling the pressure of the gaseous non-metal component (Vogt). Moss- bauer measurements with lighter actinides are not affected by availability problems ; 10-100 mg quanti- ties are commonly used (Np), although mg quantities might be sufficient ; for heavier, rare actinides Mossbauer spectroscopy is unlikely to have the sensitivity to play a major role (Dunlap).

Efforts should be made by physicists to reduce the quantity of materials they require for different tech- niques : for transport properties, 100 pg ; for neu- tron diffraction, 1 mg (lander). Such techniques are available for resistivity measurements, but not yet widely used (Fournier).

High-energy electron diffraction should be added as an important tool for detecting non-stoichiometry in actinides (Bertaut). Electron diffraction has been used for analysis of actinide samples ; unless high energies are available, this technique requires thin films (200 A). A major problem is the need for constant calibration of the electron wavelength by internal standards (Haire).

Electrical conductivity is strongly dependent on sample perfection and stoichiometry ; there are contact problems, and interpretation is difficult ;

information is obtained only for states at the Fermi level (Schoenes).

Single crystals, besides being important for surfa- ce preparation, permit : angle-resolved photoemis- sion, spin-polarized photoemission, and magneto- optical measurements ; information on energy dis- persions and exchange splittings at defined sites of the Brillouin zone can be expected (Schoenes).

Synchrotron radiation for energy dispersive XRD might be useful for structure analysis in very short times (Peterson).

By use of a microeffusion apparatus employing special geometry, UHV and inert handling, it has been ljossible to determine the vapour pressure thermodynamics of '"Cf with a 2 mg sample (Ward).

Techniques are being developed to measure satu- ration vapour pressures and vapour densities of actinide halides with mg samples (Weigel).

Concluding remarks (Peterson). - Not only the theoreticians are being challenged by the complexi- ties of actinide systems ; those involved in sample preparation and characterization face an equally difficult task.

May we both find ever greater success in the

future !