nitric acid alone, the main y-ray a t 59.6 k.e.v. is shown, mith a minor peak at 30 k.e.v. When the main peak alone is considered, the increase of atomic number of the absorbing element from sodium to silver has a principal effect of decreasing the relative height of this peak. Beginning u-ith iodine, the presence of x-rays created by the photoelectric effect becomes noticeable, K x-rays of the absorbing element being detected and contributing to the total activity measured in gross gamma counting. This photoelectric effect increases with atomic number to thulium, which has a K-absorption edge of 59.2 k.e.v., just slightly below the 59.6 k.e.77. y-ray emitted by americium. With further increase in atomic number the photoelectric effect on K-shell electrons is absent, and the only effect noticed on the y-ray spectrum is again a diminution of the main photopeak. This is shown by the curve representing absorption by mercury (lowest curve in Figure 3 ) , Relative sensitivities for various elements in absorptimetric analysis should be evident from published values of absorption coefficients. Curve A in Figure 4 is a plot of mass absorption coefficients obtained from data by Grodstein (2j for 60-k.e.v. x-rays (open squares) and from Compton and Allison ( 1 ) for x-rays having a wavelength of 0.2 A,, or energy of 62 k.e.v. (open circles). The dashed portion of the curve is an extrapolation for data not obtainable from the literature. Curve B shows evperiniental values of I,/I for
19 -15
9 u
Figure 4. Absorption coefficients and specific absorptionof elements
I--
I'
wz
,I
2
zf2i K O 2 -I 15
H Z \
l-40
lo
11
1 ,A g.!
'I .
(8)
. .1
,
10
100 grams per liter of various elements plotted on a different scale. Although the present experimental arrangement does not allow a n absolute calculation of absorption coefficients, individual elements should exhibit values of I , / I which depend on the relative magnitude of their absorption coefficients. Curves A and B should therefore be similar in appearance, indicating that the experimental sensitivities are proportional to those evpected on theoretical grounds. This is seen to be the case for elements n ith low and high atomic numbers. Hon-ever, with elements having atomic numbers 48 to 68 the photoelectric effect is operative. The counting rate I is increased by the generated x-rays and the values of I , / I are decreased in proportion to the quantity of x-rays generated. For this reason curve B devi-
20
30
40
50
60
70
80
90
a 8
z
3
s
ATOMIC NUMBER OF ABSORBING ELEMENT
ates from curve A for these elements, and the actual sensitivities for their absorptimetric analysis is lower than expected. LITERATURE CITED
(1) Compton, 8. H., Allison, S.
I
