with the beta emitter material, or solid 
powders can be intermixed much in the 
manner that neutron sources are con- 
structed, or a core of beta-ray emitter 
can be surrounded by a casing of target 
material. Still another configuration 
that lends itself readily to changing the 
energy of the X-rays emitted by the 
source employs a reflection target 
wherein the beta emitter is separated 
from a target surface in almost direct 
analogy to an X-ray tube. 
A typical X-ray transmission target 
(casing target) configuration and a 
typical reflection target configuration 
are shown in Figs. 1 and 2, together 
with spectra obtained using Sr°°-Y%° as 
the betaemitter. Various target thick- 
nesses were used to illustrate the effect 
of conversion of bremsstrahlung radia- 
tion to characteristic radiation in each 
case. The curves are all normalized to 
the same peak intensity, and the data 
were obtained with the usual sweep- 
type differential-pulse-height analyzer. 
The measured energy spectrum is of 
course smeared by the finite resolution 
of the sodium-iodide scintillation-coun- 
ter detector. The actual spectrum 
would exhibit a very sharp spike at 
73.8 kev in the case of lead. 
Since the intensity of the brems- 
strahlung continuum varies rapidly 
with beta-ray energy, it is evident that 
to secure monoenergetic X-ray sources 
low-energy betas should be employed. 
Figure 2 displays the result of filtering 
the betas from Sr%-Y through 
various thicknesses of Lucite. It is 
noteworthy that the ratio of the K-line 
intensity to the continuum increases 
with decreasing beta-ray energy; also 
the integrated intensity decreases with 
decreasing energy. The bottom curve 
of this series shows the lead fluorescence 
excited by the bremsstrahlung present 
due to the beta source structure itself 
and demonstrates another interesting 
source configuration. 
To obtain a source of monoenergetic 
radiation, the wavelength of which may 
be varied over wide limits, it is only 
necessary to provide a source such as 
illustrated in Fig. 1 with a second tar- 
get so positioned that it can be bom- 
barded by the X-radiation from the 
source. A typical configuration is 
shown in Fig. 3. 
Applications 
The wide range of energy spectra 
available from the several source de- 
signs, in combination with the wide 
TYPICAL RADIOGRAPH using 10-mc Sr®- 
Y°° source, lead target, and type K film; 
objects are aluminum step wedge (0.050- 
in. steps), resistor, and electronic tube 
range of effective half-lives, opens a 
number of areas of possible utility.* 
The sources may be used for radiog- 
raphy of thin sections or light materials. 
For radiography, it appears feasible to 
obtain carrier-free activity such that 
1 curie of strontium can be incorporated 
in a 1.5-mm focal spot. For emitters 
of shorter half-life, the specific activi- 
ties would be correspondingly higher. 
Fairly good definition and compara- 
tively short exposure times will be 
possible with a full-scale strontium 
source now under construction. Radi- 
ographs of full density should be ob- 
tainable through 44 in. of aluminum at 
4-min exposure times and source-to- 
film distances of about 1 ft using type 
K film. Intensification screens and 
faster films would reduce exposures 
correspondingly. The sources alone 
are not sufficiently intense for medical 
radiography, but it appears entirely 
possible that compact image intensifiers 
(either electronic or solid-state types) 
may be used to advantage, thereby 
making such applications feasible. 
The possibility of placing the source 
within the object to be radiographed 
may be advantageous. The possible 
utility of the sources in medical 
therapy has not been examined. 
Hither the continuous spectrum from 
a source emitting primarily brems- 
strahlung or a line-spectrum source of 
known properties may be used in thick- 
ness gaging over a fairly wide range of 
thicknesses. There appear to be cer- 
tain advantages in using X-ray sources 
in preference to beta-ray sources in 
* Patents have been applied for on various 
phases of this work. 
some gaging applications, but this 
aspect of the problem needs further 
exploration. The continuous spectrum 
is ideally suited to density measure- 
ments in liquids, gas-liquid mixtures, 
and slurries. High-speed density de- 
terminations to better than 1% accu- 
racy have been achieved with auto- 
matic instrumentation. 
The line spectra obtainable from 
these sources are admirably suited to 
absorptiometry, particularly for coat- 
ing control or for additive concentra- 
tion control, as well as for chemical 
analysis. The energy of the incoming 
radiation may be set at the critical 
absorption edge of the element of 
interest and thus render the system 
preferentially sensitive to variations in 
this element. Such a technique ap- 
pears promising, for example, in silver- 
emulsion thickness control where anti- 
mony radiation may be used to probe 
silver content of the emulsion as it is 
deposited without producing sufficient 
exposure to fog the film. 
With either the X-rays from such 
sources or beta rays directly, X-ray 
fluorescence analysis in certain simple 
systems can be achieved. Here the 
low intensity of the sources, compared 
to X-ray machines, may be at least 
partially compensated for by the use of 
Ross balanced-filter techniques to ob- 
tain large effective apertures and still 
retain the required energy selection 
properties of the detector. 
In most of the applications examined, 
with the exception of radiography, the 
sources have employed millicurie 
amounts of beta emitter to achieve 
adequate counting rates in the detec- 
tors, which are usually scintillation 
counters exhibiting close to 100% effi- 
ciency in the energy range of interest. 
The yield per millicurie of beta emitter 
will, of course, vary with the details of 
the source configuration. For a typi- 
cal transmission-target design, a nomi- 
nal value of 149 mc of X-rays per milli- 
curie of beta particles may be used to 
estimate the required activities. For 
reflection targets, this figure must be 
reduced by about one order of magni- 
tude, i.e., 10 pce/me. These rough 
figures apply to bremsstrahlung spectra 
for emitters such as P*? and Y°%. The 
ratio of bremsstrahlung to fluorescence 
radiation may be estimated from the 
curves previously discussed. 
Further details of developments using 
the techniques sketched here will be 
reported at a future time. 
143 
