low for many purposes. It has found 
limited application in exciting fluores- 
cence X-ray spectra from the lighter 
elements. A considerable gap exists, 
therefore, in available energies. 
Fortunately there is a rather wide 
choice of pure beta-ray emitters, rang- 
ing from the strontium-90-yttrium-90 
chain, emitting 0.6-Mev and 2.2-Mev 
betas with an effective half-life of 20 
years, down to materials such as phos- 
phorus-32 with a 1.7-Mev beta and a 
14-day half-life or calcium-45 with a 
0.25-Mev beta and 152-day half-life. 
In all, there are approximately ten beta 
emitters useful for the production of 
X-rays by the methods to be described. 
Mechanisms 
Two mechanisms are active in pro- 
ducing X-radiation when the betas 
from such isotopes are allowed to inter- 
act with target nuclei. The first in- 
volves simple acceleration of charge 
during the beta absorption process and 
results in bremsstrahlung radiation. 
The second radiation source is princi- 
pally the result of ionization in the K 
shell of target atoms with subsequent 
emission of characteristic or A-radia- 
tion. Characteristic radiation may be 
excited directly in an interaction be- 
tween a beta particle and a target 
atom or indirectly when the target 
atom absorbs bremsstrahlung radiation 
formed elsewhere in the target material. 
For thin targets, the efficiency of 
bremsstrahlung production is propor- 
tional to the square of the atomic num- 
ber of the target material and to the 
first power of the beta-ray energy. 
For energies of the order of the P%* beta 
and for a lead target, quantum effi- 
ciencies of the order of 10% have been 
measured when only X-rays in excess 
of 50 kev are considered. The effec- 
tiveness of such a mechanism therefore 
compares reasonably well with the 
branching ratio in the decay schemes of 
materials such as thulium. The effi- 
ciency of direct excitation of fluores- 
cence radiation by beta bombardment 
had not been studied in detail. It is 
complicated by indirect excitation pre- 
viously mentioned. This aspect of 
the performance of the sources is under- 
going further measurement. 
Source Design 
Various configurations can be used 
for beta-ray excited X-ray sources. 
Among others, liquids can be mixed 
142 
2—REFLECTION-TARGET SOURCE 
Collimator may 
be placed here 
Copper-plated 
face of holder- 
soft solder 
after assembly 
Normalized Intensity 
/ 
MOK 
EOCC.O,1, 0 OO, 
KK 
Ne 
ise 
X-rays 
\ Plastic absorber 
for scottered «ldi 
aa A Shielding 
ium target 
4 
Normalized Intensity 
\ 
Magnesium 
source holder 
0.25 in. 
approximately 
+Unfilter 
——— 
filter 
oa 
= in. plastic 
Spectra for 
S 
various @ 
energies 
Normalized Intensity 
—* 
Reflection-target source, with Sr®°-Y°° betas, provides spectra at bottom left for 
various thicknesses of lead target and top right for barium target; spectra for various 
beta energies, bottom right, were obtained with lead target, plastic filters, and 
Sr9°-¥9° source 
3—MONOENERGETIC X-RAY SOURCE 
Essentially monoenergetic 
X-roys at auxiliary target 
K-line energy 
KX OTS 
e 
Seeleey KS 
See 
QR 
x 
m~m 1.5 in 
Source provides almost completely monoenergetic radiation with energy dependent 
only upon auxiliary target material 
