&Pi Zinc Sulfide Alpha C o ~ n i i ~ N. Irving Sax and David Rosi, Division of 4n alpha counting with efficiencies in excess of 90% is obtainable by the use of Mylar and plastic-dispersed zinc sulfide film [N. A. Hallden and J. H. Harley, ANAL. CHEM.32, 1861 (1960)] and a scintillation spectrometer (Tri-Garb, Packard Instrument Co., La Grange, A successful technique consists of coating two pieces of the plastic film on the dull surface with the sample, placing them face to face, and inserting into the well of the spectrometer. Based upon a National Bureau of Standards standard for radium-226 and a standardized solution of plutonium-239, high 4 r efficiencies are obtained. The instrument used in this laboratory accommodates plastic films with a total area of 40 sq. em, It has been determined experimentally that when OUTINE
Laboratories and Research, New York State Department of Health, Albany,
the sample to be counted is spread out on this area, up to 50 mg. of carrier weight can be employed with very little decrease in efficiency. The spreading technique consists of transferring the barium radium sulfates precipitate with the aid of 95% ethyl alcohol and a transfer pipet from a centrifuge cup to the zinc sulfide-impregnated film and drying under an infrared heat lamp. Care is taken to obtain an even deposit on the film. A series of six or seven transfers is sufficient to make the total transfer and deposit it evenly on the film. By setting the discriminators of the spectrometer to exclude extraneous pulses, background levels can be maintained a t approximately 0.1 count per minute, enabling the measurement of 10-14 gram of radium per sample. Increases
N. Y.
in the sensitivity of the system may be obtained by allowing for a 3@day accumulation of radon daughter products. Further development of this counting technique is proposed. This laboratory is experimenting with a powder technique for counting radium as the barium radium sulfates precipitate. In this procedure, which seems to produce high 4.tr efficiencies, silver-activated zinc sulfide is mixed with the mixture of sulfates as a slurry in 95% ethyl alcohol, transferred to the inside of a glass or plastic counting vial, and allowed to dry. The thin even coating thus produced on the inside bottom of the counting vial works very well. Furthermore, the use of B counting vial automatically keeps the sample from either being contaminated or contaminating the outside.
Free Diffusion Sa Paul
D. Gam and Jo
!I Telephone Laboratories, Inc., Murray Hill, N. J.
the authors pointed out certain advantages in restricting deconiposition products in thermogravimetric studies [ANAL. CHEW 32, 1563 (1960)l. Subsequently, the advantages of the opposite extreme became apparent. This paper describes the results obtained when diffusion of gas to and from the sample is as nearly unrestricted as is experimentally convenient. That the traditional crucibles are useless in thermogravimetry is essentially true. In a few cases crucibles are acceptable. In vacuum thermogravimetry any open vessel is satisfactory. In controlled atmosphere work, where the atmosphere is solely ECENTLY,
TEMPERATURE IN DEGREES CENTIGRADE
errnol decomposition of lead carbonate
the gas involved in the reaction, the geometry of the container is immaterial. In displacement problems such as the “blowing” of polyethylene the crucible will probably be a convenience. In the work reported here the sample is heated in the form of a thin layer on a flat surface. While this arrangement is not yet ideal, the wall of the traditional crucible is no longer a barrier. Under these conditions the atmosphere has ready access to the sample as well as the product gases to the atmosphere. Again the decomposition will be more rapid than in a crucible but a t a lower temperature, because the partial pressure of the decomposition product will be more nearly that of the ambient atmosphere. The only region in which the partial pressure will build up appreciably is within the bulk of the sample, but, with the thin layer of powder, diffusion to the atmosphere is easy and the decomposition will be comparatively unhampered. The sample holder is simple: a disk with a slight lip, supported by a rod from the center. The results are equally simple. Ordinary reversible decompositions occur, as we could predict, a t B significantly lower temperature than when the products are confined purposely or inadvertently.
Lead carbonate (Figure I) behaves rather differently, No inflection for the basic lead carbonate appears. Instead, the material decomposes first to the dioxycarbonate. Before decomposition to the ovide is complete, the oxygen from the atmosphere, not being kept out by the carbon dioxide, oxidizes PbO to Pb304. FinalIy the sample reyerts to PbO. Again, because the oxygen is not restricted, this decomposition occurs a t a lower temperature than in the deep vessel. The curves from the previous paper are included for comparison. The cobalt oxalate hydrate, too, behaves essentially as predicted (Figure 2) ~
Figure 2. Decomposition of cobalt oxalate hydrate
