,
12—8.
GAMMA RAY WAVELENGTHS
277
kilovolts, are: uranium X2, 2300; radium B, 650; radium C,
3150; radium E, 1300; actinium C", 1500; mesothorium 2, 2050;
thorium B, 360; thorium C, 2250; and thoriu'm_ C”, 1800.
Sargent has shown that there is ‘a relationship between the
-
maximum energy of the disintegration electrons and the decay
constant, which is similar to the Geiger-Nuttall law for alpha
particles (equation 12—5).
_
The beta particles which form the line spectrum are photo_
electrons ejected from the parts of the same or of other atoms,
-
Which lie outside the nucleus, as Will appear after the discussion
of gamma rays.
12—8.
Gamma
Ray Wave—lengths.—The wave—length of
gamma rays may be measured directly by the method of glancing
_
angle reection from a crystal, as With X—rays. Gamma rays from
the source X in gure 12—15 pass through a longand marrow slit
(S) made of two lead plates which are electrically charged _to
.—
"’
S
P
_
’
C
FIG. 12—15.
Measurement of gamma ray wave—lengths.
-
remove all betarays. The rays are diffracted by a rock salt crystal '
‘
C to the photographie plate (P) where they producea series of
lines, one for each wave-length.
\_
,
'
The wave-length‘s of gamma rays have also
been measured
._
studying the beta rays which they eject from thin non—radioactive
foils placed around the emitting material. A beta ray spectrograph
_
like that in gure 12—12 is used. The energy ((%) of the beta
__
'
”in the line spectrum is measured With this apparatus and
added to the energy (W) required to remove the electron from an .
X‘raY level in the atoms of the foilQ As is known from
"the EmStem
'.
‘
photoeleCtric equation, the sum of ôÎÈ- and W 13 equal to the
energy 711}, in a photon of the gamma ray. Thus,
_
-and
=
5/y_{_
)\,, obtained by the two methods jus:
described are in agreement. They range between
4.66 X units
x
,
Àn X unit is equal to 10"11 cm. or one one-thousandth of an angstrom;