PHYSICS: DUANE AND SHIMIZU 
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chemical isotopes, also appear to be identical to a very high degree of pre- 
cision, and recently Siegbahn and Stenstrom found no difference between 
their emission spectra in what is called the L series of X-rays. 
The object of the research reported in this note has been to investigate the 
X-ray absorption spectra of chemical isotopes. In general each chemical ele- 
ment, except perhaps those of low atomic numbers, has several critical ab- 
sorption frequencies, one connected with its K series and three connected with 
its L series of X-rays. These critical absorption frequencies mark points in 
the X-ray spectrum where sharp changes in the absorption of X-rays by 
the chemical element occur. The chemical element absorbs X-rays of higher 
frequency than the critical frequency to a much greater extent than it does 
X-rays of lower frequency. 
In measuring the critical absorption frequencies of lead isotopes we have 
used the X-ray spectrometer described in the Physical Review for December 
1917, page 624. A calcite crystal reflected the X-rays whose wave-length is 
given by the equation 
X = 2 a sin ^ = 6.056 sin d X 10-^ cm. 
(where 6 is the grazing angle of incidence) into an ionization chamber. The 
ionization method of detecting the reflected beam of X-rays is far superior to 
the photographic method, both because, if properly used, it requires no cor- 
rection for the penetration of the X-rays into the crystal, and also because it 
gives an estimate of the magnitude of the absorption. 
