214 
High-voltage Research at M. I. T. 
The present medical work with sup- 
ervoltage X-rays is an outgrowth of a 
development program for nuclear 
research which began at M. J. T. in 
1932. At that time Robert J. Van de 
Graaff, then just removed from Prince- 
ton where he had developed the 
electrostatic principle of voltage gener- 
ANNUAL REPORT SMITHSONIAN INSTITUTION, 1948 
Today the original Round Hill gen- 
erator, moved to a steel building on 
the Institute’s property in Cambridge, 
is In continuous use in nuclear studies 
with electrons or positive ions of 2- 
million-volts energy. The steady 
stream of charged particles produced 
by the electrostatic accelerator is hom- 
ogeneous and accurately controllable 
EQUIVALENT VOLTAGE IN MILLIONS 
oS) 
a 
22 3 MILUON VOLT 
X-RAY SPECTRUM 
é | 
RELATIVE INTENSITY 
o 
Q020 
WAVELENGTH 
0.028 
IN ANGSTROM UNITS 
2 MILUON VOLT 
X-RAY SPECTRUM 
500,000_ VOLT 
X-RAY SPECTRUM 
0036 Q044 052 
Figure 2.—When high-energy electrons are stopped on impact with a gold target, their 
kinetic energy is converted into penetrating electromagnetic radiation. 
The effect of 
increasing the voltage is to move the continuous X-ray spectrum in the direction of shorter 
wave lengths and to increase efficiency of X-ray production. The curves show that the 
spectrum of 3-million-volt X-rays includes wave lengths shorter than those emitted by 
radium in equilibrium with its products, as shown by the line spectrum. Such radiation is, 
therefore, superior to radium in its penetrating, nonscattering properties. 
ation for which he is acclaimed, was 
constructing the large air-insulated 
electrostatic generators at Round Hill, 
New Bedford, for nuclear research. 
Within the Department of Electrical 
Engineering investigations were begin- 
ning on the insulation of electrostatic 
generators in high vacuum and on the 
possibility of developing more com- 
pact sources of high constant potentials. 
in energy and free from extraneous 
background radiation. These char- 
acteristics are virtually indispensable 
in certain important precision studies 
of nuclear structure and are not at- 
tained in the cyclotron, betatron, 
synchrotron, or other indirect par- 
ticle-accelerators which are inherently 
capable of producing far greater ener- 
gies. A new compressed-gas insulated 
