ANALYTICAL CHEMISTRY
224
and thelithium aluminum hydride solution was outgassed. When outgassing was complete, stopcock 1 was closed and the water sample allowed to melt. The lithium aluminum hydride was quickly added to the water sample (stopcock 2 closed; stopcock 1 open), and the evacuated system wm filled with the generated hydrogen-tritium mixture. The sample of hydrogen was then forced into the ionization chamber via a modified Toepler pump to nearly 1 atmosphere pressure. Finally, stopcock 3 vas opened and the contents of the ionization chamher were adjusted to a t mospheric pressure by the ,nercury levelsin the side a ~ m and pump chamber. The in the ionization chamber wBs determined using a vibrating reed eleotrometer. The drift ohtnined NBS corrected to standard temperature and pressure.
oarbonate. The barium carbonate, after dryin N&S plated on aluminum disks by the method described by 8alvin et ai. ( 1 ) . The activity was determined using 8. Xueleometer windowlesstype counter. Several plates were prepared for each sample and the average of several determinations NBS used in calculating the final activity in each case.
~h~ m ~ ~ and ~ u ~ ofy filling s,,stem and the of the ionization chambers showed no bothersome memory, and pumping for 30 minutes n-a8 nufficicntto dear the sy8tem for the nest analysis.
(I) Calvin. >I.. el 01.. "Isotovic Cnrhoo." New Pork. John Wiley &
CARBON ASSAY
The carbon dioxide generated during the combustion vas collected in sodium hydroxide solution and precipitated as barium
Greater accuracy, if desired, c m be obtained in the caw oi weak samples ifthe carbon 14 is nssayed as carbon dioxide in an ionisntion chamher instead of using harium earbonat,e plates. LlTER4TURE CITEII
Sons,1949.
(2)
be^. m.G.. Reid, J. c.. and Yanku,ioh, P. E., .~XAL.CHEM., 19,828 (1947).
(3) Rritehevaky, D.. Biggs. M. K., and Fmomim. Radiation Laboratory, Ptch. 644 (1950). (4) Siri. 17.. unpublished research. RECEITED
J U I ~14. i o i l .
work enonsored by
K. K.. Univ. Calif.
u. s. .itomie
~ n e r Comx ~
mission.
Simple Photometric Method for Use with RCA Type EMB Electron Microscope W. G . KIRCHGESSNER, Rausch & Lomb Optical Co., Rochester, R: P. S THE photographic process the supcess or failure of a print the tonal values in t.he original euhject. CouEequenbly, in electron microgrnphy, il correct CXDORure should hc such that the lieht intensities from the unshndowed area of the suhject heiug photographed fall on of the D - log curve. T~ this the straightline
1depends upon the sat.isfactary rendering
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condition satisfactorily a photometer is needed, but hecause of the low intensit.\- values encountered in this txwe of uork.. ordinary photoelecdric meten are inadequate. In view of this limitation, method for objt,aining erposure dab v~~ Bought and .ZR a 8implo ~ n devised, 9 advantages Over previously suggested designs (1-8).
".
The amplifier unit is a Kelch Densichron photoelectric control unit mounted on a bracket att.ached to the side of the microscope. Light rndiiLt,ionsfrom the fluorescent viewing screen in the microscope concentrated by ta-o aspheric condensing lenses (equiv-
nlont \ rill tho *athnrla nf nhntndootrir d l ...~.." fnrnrr = 7.K _"mm _______, ". tho r__l"l".-l"___ I_..
(Cetron CE-ZSV) which is attached to the lens mount. By this xrangement the photoelectric cell receives only light that is reflected from the screen, and adequate needle deflection on the isobtained for thr low intensity values generally-
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Kith t,he unit attached ns desoribed, calibration m s made hy rorrel8,ting t,he relationship betxeen photographic densities. evposurc t,inles, and meter readings. Experiment.ally this n.a.8 done by csposing a series of test negatives for different unit,s of timc a t a standard light intensity as determined hy selection of some constant Densichron meter value. Control of this variable is maintained hy adjustment, of the condenser lens current on the microscope. With tho test negatives exposed to B series of known exposures and proceesed under identical and controlled rwnditionr, the responses of the photographic emulsions usetl \,--ere measured opt,icall~-t p means of ii polwizing photometer. From there d8t.a ideal exposure conditions BIP quickly defined lor various emulsions and thus the amount of light reaching the photoeleet.ric cell hecomes a direct mcasuro of the exposure time. I n thc application of this met,hod ~everaitests have been made to determine its limitations. From the series of data. collected, an average mean deviation of 5 ~ 6 . 0 %W-RS ohtained under the oonditions of ordinary use. Beenure of the deterioration of the fluorescent material on the screen the unit should be calibrated periodically in order to a8SU1-e sat,isfact.oryperformance. The following advantages may he claimed for the device: consist,ent uniformity of negatives, rmulting in the elimination of repeat exposures; and flesihility, in that the unit is easily ' mhled from readily availsblc parte and may be quickly det:whed for usr in 0 t . 1 lnhorstary ~ prohlemp. ACKNOW LEnCMENT
Ack~onlodgmentis made to Winifrcd Steinorth for assist ance in accumulsting the data in connection with this work. Figure 1. Photometer Unit in Position on Electron Miemscope
As illustrated in Figure 1, the photometer consists essentially of three comoonent Darts: the lens system C. the "hotaclectric probe, P , and the amplifier unit, A .
LmERATURE CITED (1) Bishop, F. IT., Electronics. 23, 11W11 (September 1950). (2) Cuokow, F. W., J . Applied Phys.. 17, GO (1946) (abstract). (3) Hamm. F..4.. I M . . 22,111 (1951)(nhstract). R e c n r ~Jone . ~ 7,1951
