256 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1959 
by separation in the eka-thulium position by the ion-exchange 
method would have to be accomplished. These requirements necessi- 
tated new techniques, and also some luck; fortunately, both were 
forthcoming. The new technique involved the separation of element 
101 from the einsteinium in the target by the recoil method. ‘The 
einsteinium was plated onto a gold foil in an invisibly thin layer. 
The helium-ion beam was sent through the back of the foil so that 
the atoms of element 101, recoiling because of the momentum of the 
impinging helium ions, could be caught on a second thin gold foil. 
This second gold foil, containing recoil atoms, yet relatively free of 
the target einsteinium, was dissolved and the chemical separations 
were performed. Very sensitive methods were available for the detec- 
tion of isotopes decaying by alpha-particle emission or by spontaneous 
fission. These methods were so efficient that as little as one or two 
atoms of element 101 per experiment could be detected. 
The earliest experiments were confined to looking for short-lived 
alpha-emitting isotopes that might be due to element 101. However, 
no alpha activity was observed that could be attributed to element 101, 
even when the time between the end of the bombardment and the 
beginning of the alpha-particle analysis was reduced to 5 minutes. 
The experiments were continued, and in one of the subsequent 
bombardments, a single large pulse due to spontaneous fission was 
observed. Chemical experiments indicated that the spontaneous fis- 
sion counts, when they did appear, came in chemical fractions cor- 
responding approximately to element 100 or 101. In the definitive 
experiments, three successive 38-hour bombardments were made, and, 
in turn, their transmutation products were completely and quickly 
separated by the ion-exchange method. A total of five spontaneous 
fission counts was observed in the elution position corresponding to 
element 101, while a total of eight spontaneous fission counts was also 
observed in the element 100 position. No such counts were observed 
in any other position. The spontaneous fission activity in both the 
element 101 and 100 fractions decayed with a half-life of about 3 
hours. This and other evidence led to the view that the isotope has 
the mass number 256 and decays by electron capture, with a half-life 
of the order of an hour, to the isotope Fm?** which is responsible for 
the spontaneous fission decay. 
On the basis of this evidence, the group, consisting of A. Ghiorso, 
B. G. Harvey, G. R. Choppin, S. G. Thompson, and the author, an- 
nounced the discovery of element 101. We gave the new element 
the name mendelevium in recognition of the pioneering role of Dmitri 
Mendeleev, who was the first to use the periodic system of the elements 
to predict the chemical properties of undiscovered elements. Subse- 
quent experiments using larger amounts of einsteinium in the target 
