1182

LETTERS TO THE EDITOR

sideration and of the ratio of the momentum transfer,the calculation of the decrease of the force is practically
impossible. We can compute only whether the measured
decrease of the total force can be caused by a decrease of
the gas pressure.
For this purpose we shall take the forces at 15 amp.
measured by Tanberg in vacuum. and by Robertson at
1 mm Hg for a copper arc indicated in Fig. 4 of Robertson's
paper. We assume that the transfer of the momentum of
the stream to the gas diminishes the gas pressure to onehalf. The measured forces in vacuum and at 1 mm Hg
are 270 dynes and 160 dynes, respectively. The reduction
is 110 dynes and the reduced gas pressure must then be
effective on 0. 168 cm'. It naturally must be not larger
than the area of the face of the cathode spot but may belarger than the area of the cathode spot. The face of thecathode in the experiments of Robertson had an area of
0.316 cm2. The area of the cathode spot is 0.00105 cm'
at 15 amperes if we calculate it with the ratio of 0.007
mm' amp. measured in vacuum by Tanberg and Berkey.
At 1 mm Hg the current density is rather higher and thearea of the cathode spot therefore smaller. We see thatthe calculated area lies between the two limits. The
explanation seems therefore to be possible.
Lamar4 also has pointed out that the absence of the force
on the cathode in an arc at higher gas pressures may be
due to the compensating effect of holding back the gaspressure from the cathode spot. But, according to his
opinion this effect occurs as the result of convection
currents set up around the cathode if this is of relatively
small dimensions and therefore can not occur if the cathodehas a large surface area.
Now, if my explanation of the effect is right the compensating effect is not confined to small cathode areas
because the reduction of the gas pressure in front of thecathode spot and its near surroundings does not augment
the gas pressure on areas of the cathode which lie further
away from the cathode spot and because the high velocity
stream is only slowed down by the gas and is not reflectedback to the cathode. On the contrary for the full development of the compensating effect the area of the cathodemust be larger than a certain minimum.
In conclusion I should like to thank Dr. F. Luedi forhis suggestion to explain the results obtained by Robertson.
R. RIscH
Physical Laboratory,
Brown, Boveri and Company,
Baden, Switzerland,
January 18, 1940.

Phys. Rev. 35, 1080 (1930); Kobel, Phys. Rev. 36, 1636
(1930); Tanberg and Berkey, Phys. Rev. 38, 296 (1931).
2 Risch, Helv. Phys. Acta 4, 122 (1931); Risch and Luedi, Zeits. f,
Physik 75, 812 (1932); Mason, Trans. A. I. E. E. p. 245 March, (1933).
' Robertson, Phys. Rev. 53, 578 (1938).
4 Cornpton,
Proc. Nat. Acad. Sci. 18, 711 (1932).

This activity was induced appreciably only by fastneutrons obtained by bombarding lithium with 3-Mev.
deuterons from our cyclotron. The experimental procedure
was as follows.
A few grams of uranium oxide, U30s, carefully purified
and freed from its disintegration products were exposed to
fast neutrons for more than fifty hours. After the exposure,a uranium fraction (U~Os) was separated and purified fromall possible elements produced by fission as well as from its
own disintegration products. The most care was given to
the removal of lanthanum from the sample, the procedure
taking as long as one day. The activity of the irradiateduranium was compared with that of a nonirradiated sample,
in order to subtract the growing p-activity due to disintegration products of uranium. The difference thus obtained shows a 6.5-day period. This activity is probably
due to U"' produced from U ' through loss of a neutron,as in the case of the production of UY from thorium. ' If
this is the case, we have here a member of the missing
radioactive family 4n+1.
The sign of the p-rays was shown to be negative andconsequently we suspected the production of a radioactive
element of atomic number 93, the chemical properties ofwhich are probably homologous to rhenium. From &hedecay curve it is clear that its period must be very long,
if it exists. To search for such an element, the irradiateduranium oxide, which was freed from fission products aswell as its own disintegration products as above mentioned,
was left for about 7 days, and was then dissolved in nitricacid. The solution, after an addition of perrhenic acid, wastreated with ammonium sulphide and then acidified withsulphuric acid, The precipitated rhenium sulphide, after
the removal of contaminated sulphur by carbon bisulphide,
was examined for p- and ~-activities. Neither of them could
be found within the error of our experiments. We may thusthat theconclude, as in the case of 23-minute uranium,
6.5-day uranium decays also into a very long-lived 93
element. The detailed accounts of the experiments will begiven elsewhere.
The above investigations were carried out as a part of
the work of the Atomic Nucleus Sub-Committee of theJapan Society for the Promotion of Scientific Research.
We acknowledge the assistances given by our laboratory
colleagues in connection with the irradiation of samples
and by Messrs. N. Saito and N. Matuura regarding thechemical separations.

Y. NIsH INA

T. YAsAKII Tanberg,

H. EzoE
Nuclear Research Laboratory,
Institute of Physical and Chemical Research,

K. KIMURA
M. I KAWA
Chemical Institute,
Faculty of Science,
Imperial University

Tokyo, Japan,
May 3, 1940.

Induced (I-Activity of Uranium by Fast Neutrons

In the course of experiments on the fission of uranium
by fast neutrons, ' besides fission products the uranium
fraction showed a p-activity with a period 6.5 days.

of Tokyo,

I Y. Nishina, T. Yasaki, H. Ezoe, K. Kimura and M. Ikawa, Nature
144, 547 (1939); Nature, in press (1940).
2 Y. Nishina,
T. Yasaki, K. Kimura and M. Ikawa, Nature 142, 874(1938).

I E. Segrh, Phys, Rev. 55, 1104 (1939).

LETTERS TO THE EDITOR
Anomalous Dispersion and Absorption of Electric W'avesin Solutions of Amino-Acids and Dipeptides

The dielectric constants and absorption coefficients ofaqueous solutions of several amino-acids and dipeptides,
listed in Table I, have been measured for electric waves
of X=25.5 cm (v=1.18)(10' cycles). The method used
was a modification of Drude's second method. The waves
were generated as described earlier. ' The table shows theresults obtained: Ae'/c is the so-called dielectric increment
per mole and h~"/c is the absorption coefficient increment
per mole; e' and &" being the real and the imaginary partsi~". The incrementsof the dielectric constant ~ = ~' —d ~0/c of the static dielectric constant per mole were takenfrom the literature. 2 s The table shows also the relaxation
times r calculated from our data for the dielectric increment by the use of Debye's theory formulae with the
Wyman's formula 7 =a+bp in place of the ClausiusMossoti relation (p volume polarization; u andWyman's constants). The relaxation times ~o calculated
for spherical molecules from molecular volumes are given
for comparison in the last column.
Attention may be drawn to the following points: (a) the
anomalous dispersion and absorption were increasing withincreasing molecular size; (b) the dielectric increment was
independent of concentration within the limits of error;(c) the absorption coefficient increment tended to increase
this could be quantitatively
with the concentration;accounted for as being due to the change in relaxation
time with the change in viscosity to which e" is very
sensitive when frequency corresponds to the edge of thedispersive region.
As is well known the observed static dielectric incredipolar ions in a polarments of simple straight-chain
solvent have been accounted for statistically on theassumption of free rotation about the various single bondsof the chain, irrespective of whether polarization in astatic field is accomplished by deformation or by orientation of the entire molecule. '~ At high frequencies, however,the dielectric properties of the solution sho'uld depend
upon the mechanism of polarization, each mode of deformation or orientation having its own relaxation time.
The behavior of the higher peptides of glycine strongly
suggests polarization by deformation of a Rexible chain,since no anomalous dispersion has been observed at
frequencies corresponding to the rotation of the molecules
as a whole. s The present data for the simple dipeptides,on the other hand, -suggest orientation of these molecules,
and are further consistent with the assumption of anasymmetry factor of about 2. The amino-acids, with the—

TABI.E I. Dielectric constants, absorption

SUBsTANcE

leap/cGlycinea-Alanine
P-Alanine
Glycylglycine
Glycylalanine
Alanylglycine

22. 8
23.3

34.8
71.1

72.3

71.5

b-

coegcients, and relaxation limes.

CONCENTRATION
RANGE

Ap /c

0.2—2.0
0.4—0.8
0.4—0.8
0.2%,4
0.2—0.4
0.2

21.3 14.3
17.4 13.8
25.7 19.1
28.7 41.2
20.2 39.5
18.9 40.3

Dp/c r &(10" r p &(10"

2.60
6.55
6.72
14.94
20.28
21.28

6.61
8.50
8.50

10.82
12.71
12.71

1185

very doubtful exception of glycine, also give evidence ofrelaxation times not greatly different from those corresponding to molecular rotation. A fuller discussion of theexperiments will be published elsewhere.

J. B. BATEMAN
G. POTAPHN Ko
Norman Bridge Laboratory of Physics,
California Institute of Technology,
Pasadena, California,
May 17, 1940.

' G. Potapenko, Phys. Rev. 39, 625, 628 (1932).

and T. I.. McMeekin, J, Am. Chem. Soc. 55, 915 (1933).
J.
and Wyman, J. Am, Chem. Soc. 58, 463 (1936).
' W. Kuhn, Zeits.
f. physik. Chen&ie A175, 1 (1935).

~
Wyman
3 GreensteinP

J. Wyman, J, Phys. Chem. 43, 143 (1939).

Radioactive Element 93

Last year a nonrecoiling 2.3-day period was discovered'in uranium activated with neutrons, and an attempt' wasmade to identify it chemically, leading to the conclusionthat it is a rare earth. Impressed by the difficulties raisedby this identification, the authors independently decided
that the subject was worth further investigation. In
Berkeley it was found that: (1) If a layer of (NH4)2U&07
with about 0. 1 mm air equivalent stopping power, placedin contact with a collodion film of 2 mm air equivalent,is activated by neutrons from the cyclotron, the 2.3-day
period appears strongly in the uranium layer, and not at
all in the collodion, which shows a decay curve parallel to,and 1/7 as strong as, that of a paper "fission catcher"behind it. One day after bombardment the uranium layer
has five times the activity of the fission catcher. Thisshows that the 2.3-day period has a range of (0.1 min
air and an intensity larger than all the long period fissioii
products together. (2) When a thin layer of uranium isbombarded with and without cadmium around it, thefission product intensity is changed by a large factor,
while the 2.3-day period and the 23-minute uranium perio(lare only slightly changed, and their ratio remains constant.
Also absorption of resonance neutrons by uranium changes
these two periods in the same ratio, suggesting a genetic
relation between them, and the consequent identificationof the longer period with element 93. In Washington it
was found that the 2.3-day period probably does not
behave consistently as a rare earth, since attempts toconcentrate it chemically with the rare earths froIn
activated uranium failed, although it is known to have an
intensity large compared with that of the rare earth
fission products.

At this stage of the investigation

one of the authors(P.H. A. ) carne to Berkeley on a visit, and a combined
attack was made. With pure 2.3-day substance from thinuranium layers, the chemical properties were investigated,
and a very characteristic difference from the rare earths
was soon found; namely, the substance does not precipitate
with HF in the presence of an oxidizing agent (bromate
in strong acid). In the presence of a reducing agent (SO2)
it precipitates quantitatively with HF. Cerium was used
as a carrier. This property explains the erratic nature ofprevious chemical experiments in which the oxidizing
power of the solution was not controlled. Further chemical

LETTERS TO THE EDITOR

1186
1000—

I

I

I

I

I

I

500—

IOO—

50-

We sought for these by preparing a strong sample (11millicuries) of purified 93 and placing it near a linear
amplifier in a magnetic field to deflect the beta-particles.
From this experiment we conclude that, if alpha-particles
are emitted, their half-life must be of the order of amillion years or more; the same experiment showed thatif spontaneous fission occurs its half-life must be evengreater. We wish to express our gratitude to the Rockefeller Foundation and the Research Corporation, whose
financial support made this work possible.
EDWIN MCMILLAN
Radiation Laboratory,
Department of Physics,
University of California,
Berkeley, California.

IO-

PHILIP HAUGE ABELSON

0

I

I

I

I

40

80

I20

I60

T IMF.

IN

MINUTESI

I

200 240

Department of Terrestrial Magnetism,
Carnegie Institution of Washington,
Washington, D. C. ,May 27, 1940.
I~

E. McMillan, Phys. Rev. SS, 510 (1939).
E. Segrb, Phys. Rev. 55, 1104 (1939).

F1G. 1. Growth of 2.3-day 93»0 from 23-minute U»0. The points
indicate the activities of successive Auoride extractions, plotted at thetimes of extraction. Decay measurements were made a day later on thefirst six fractions, and the resulting slopes are shown on the plot.

experiments showed that in the reduced state with athorium carrier it precipitates with iodate, and in theoxidized state with uranium as sodium uranyl acetate.
It also precipitates with thorium on the addition of H202.
It precipitates in basic solution if carbonate is carefully
excluded. These properties indicate that the two valence
states are very similar to those of uranium (U++++ andUO2++ or U~OT ), the chief difference from that element
being in the value of the oxidation potential between thetwo valences, such that the lower state is more stable in
the new element. It is interesting to note that the newelement has little if any resemblance to its homolog
rhenium; for it does not precipitate with H~S in acidsolution, is not reduced to the metal by zinc in acidsolution, and does not have an oxide volatile at red heat.
This fact, together with the apparent similarity to uranium,
suggests that there may be a second "rare earth" groupof similar elements starting with urq, nium.
The final proof that the 2.3-day substance is the daughterof itsof the 23-minute uranium is the demonstration
growth from the latter. For this experiment activateduranium was purified twice by precipitation as sodium
uranyl acetate, which was dissolved in HF and saturated
with SO2. Then equal quantities of cerium were added attwenty-minute intervals and the precipitates filtered out.
The first precipitate, made immediately after purification,
contaminations andcarried all the fluoride-precipitable
was discarded; its weakness indicated a very good purification. The activities of the others are plotted in Fig. 1.
A preliminary study of the radiations from 93"' showsthat it emits continuous negative beta-particles with an
upper limit of 0.47 Mev, and a weak complex spectrum of
low energy gamma-rays (&0.3 Mev) and probably x-rays.
The question of the behavior of its daughter product94'39 immediately
arises. Our first thought was that itshould go to actinouranium by emitting an alpha-particle.

Masses of Stable Nuclei from Ne" to Fe"
The observation of the proton groups in (dp) and (ap)reactions recently made in this laboratory, together witha knowledge of beta-ray maximum energies makes it
possible to give reasonably correct values for isotopic
masses as far as Fe6' with a few exceptions. These values are
given in Table I. The values from Ne" to A's are derived
entirely from transmutation data, and mass-spectrographvalues are given in parentheses for comparison where they
are available. Above K" the accuracy is less and reliancehas to be placed on mass-spectrographmeasurements ofTi", Fes ~ and Crs'.
Such mass values are useful in checking theories ofnuclear structure and to facilitate a bird' s-eye view of thetrend of mass values a quantity called here the "massdeviation" is plotted against the mass number. This issimply the deviation from a whole number plotted on a
sloping scale which enables the whole range of elements toA
be plotted in a small space. Precisely define it is Mp —+0.001(A —20) where Afar is the mass of an element of
TABLE I. Isotopic masses.
Negro

Ne21
Neman

Na&~

Mg64

Mg»
M g26
AP1
Sj68

Si»

Sj$0
P31
$66
$66
$64
C166
A36

CP7

A»

K6&

19.99881 (Bainbridge)
21.00018 (20.99968)
21.99864 (Ba1nbndge)

22.99680
23.99189
24.99277
25.99062
26.98960
27.98639 (27.9866)28.98685 (28.9866)29.98294
30.98457 (30.9843)
31.98306 (31.9823)
32.98260
33.97974
34.9810735.97852
36.97829 (36.9779)
37.97544
38.97518+ Considerably

A«
Ca4o

+41
Ca46
Ca46
Ca44
Sc46

Ti«

Ti4'
Ti46
Ti4&

Tj60
V61
Cr66

Cr6'
Cr64

Fe'4
Mn66
Fe66

Fe67

,

39.97504 (Bainbridge)
39.974540.973941.971142.972344.970145.967847.9651 (Dempster)
48.9664
49.9632 (Dempster) 4'
50.9587
51.9575 (Aston)
52.9572

53.9601 (Dempster) +54.965355.9608 (Dempster)
56.9609

less accurate than the other values given.

LETTERS TO
The Seven-Day Uranium Activity

U"~ by fast neutron

The production of radioactive
irradiation of uranium has been reported recently by
Nishina eI al. ' The same activity has been observed independently here. It was 6rst found in samples of uranium
placed immediately behind a beryllium target bombarded
by 16-Mev deuterons. These showed an activity of half-life
7.0&0.2 days which could not be separated chemicallyfrom uranium. If the purification was done immediately
after activation, the 23-minute uranium and its 2.3-day
daughter' also appeared, but if the puriFication was done
after a few hours, only the 7-day period was seen. The ratio
after a short bombardment of the initial intensities of the
7-day activity to the 2.3-day activity was about s.
Since the 7-day period was not seen in earlier experiments of the author in which uranium layers thin enoughto allow the fission products to recoil out were exposed toneutrons from a beryllium target bombarded by 8-Mev
deuterons, it seemed likely that it is produced by a fastneutron reaction. In confirmation of this it was found thatthe intensity (with 16-Mev H' on Be) is greatly reduced
if the sample is surrounded with paraffin during bombardment and is not affected by cadniium shields. It was also
found possible to observe the 7-day period in a thin uranium
layer placed inimediately behind the target, and an analysisof the resulting decay curve gave the same ratio of intensities (7-day to 2.3-day) as the chemically separated
fractions from a bulk sample.
The 7-day beta-particles have an absorption curvecharacteristic of a simple continuous distribution with anend point at 0.26 Mev. There are also soft gamma-rays
with at least two components, but no annihilation radiation, in agreenient with the observation of Nishina et al.
that the electrons are negative. Dr. A. C. Helmholz, usinga magnetic spectrograph, could Find no internal conversion
lines. Since this excludes the possibility of X capture, thedaughter product must be element 93. An attempt to findsuch an activity was made. An 80-microcurie sample of
7-dRy uranlulT1 wRs allowed to decay fol 26 days, aild thenelement 93 was extracted from it, by the method offluoride precipitation in reducing and oxidizing media, 2with cerium carrier. The resulting product showed nomeasurable activity of either beta- or alpha-rays, so theperiod of this substance must be very long. (It can hardly
be very short, since the active uranium has no high energy
beta-particles and no alpha-particlesof range greaterthan those from Uql. )I wish to express my gratitude to the Rockefeller Foundation and the Research Corporation, whose 6nancial
support made this work possible.
EDWIN MCMILLAN
Radiation Laboratory,
Department of Physics,
University of California,
Berkeley, California,
June 28, 1940.

' Y. Nishina, T. Vasaki, H. Ezoe„K, Kimura and M. Ikawa, Phys.
Rev. SV, 1182 (1940).
2 E. McMillan and P. H. Abelson,
Phys. Rev. SV, 1185 (1940).

The Energy Released in the Reaction Li'(P, e)He' andthe Mass of He'
Two groups of workers in this laboratory have completedof the energy released in the reaction
determinations
absorption
Lis(p, a) He3 using different variabl-pressure
cells calibrated in different ways. The variable-pressure
cells used were constructed with two compartments, theforward one for retarding the alpha-particles, and the rear
one for collecting the ionization produced by them at theend of their range. A linear ampli6er was used to detect
the ionization pulses. The region from which ions werecollected was defined by a perforated screen transverse to
the axis of the cell, but the ionization compartment wasalways at the same pressure as the retarding compartment.
This introduces no complications into the interpretationof the results provided that the alpha-particles, at allpressures used, come to the end of their range in the gasof the ionization compartment. Thus there was only one
window for the particles to traverse, placed over a perforated screen at the entrance to the retarding compartment.
In one experiment, such. a cell was placed to collectparticles from thin targets of lithium and beryllium, bombarded with 370-kev protons. The range difference between
the alpha-particles from Be (p, u)Lie and Li (p n)He~ wasfound to be 1.83 mm. Using the precisely known energy ofthe beryllium particles, from our electrostatic analyzer
measurements, and the slope of the Cornell (1938) rangeenergy curve in this region, we find that Q for Lie(p, a)He3
is 3.95~0.06 Mev.
The other determination was an outgrowth of experirnents on the range-energy curve of alpha-particles in theenergy region 0.8-2.0 Mev. A strong source of polonium
was placed near the entrance of our electrostatic analyzer,
and absorption screens introduced between it and theanalyzer to absorb all but the last 0.5—1.0 cm of range.
A small fraction of the resulting particles, homogeneousin energy, was deHected through the electrostatic analyzer
and allowed to enter an absorption cell of the previously
described type. By varying the defIecting voltage, thecut-off pressures of the cell for alpha-particles from 1.09
to 1.73 Mev have been determined. This same cell, withall its auxiliary apparatus, was then placed opposite athin lithium target and, the cut-off for the alpha-particles
froni Li'(p, u)He3 determined. The result indicated anenergy release of 3.94~0.05 Mev.
%e have used our previous values for reaction energies
in computing the mass of He3 which is included in thefollowing list, based on 1.00813+0.00002, 2.01473 +2, and
4.00386&7 for the masses of H, D, and He4, respectively.

He'

3.01685 ~10

Lio

6.01684~11
7.01814~11

Li'

8.00765 +15
Be' 9.01484 ~13
Bes

The previously accepted value for the energy release in
the reaction Li (P, n)He' was 3.72+0.08 Mev.
SAMUEL K. ALLIsoN
LEoNARD C. MILLER
GILBERTPERLOW

J.

University of Chicago,
Chicago, Illinois,
June 19, 1940.

LEsTER S. SKAeos
NIcHQLAs

M. SMITH, IR.

PH YSICALREVIE%'VOLUME73, NUMBER

MA V

c, 1948

Nuclear Properties of 93'" *Department

ARTHUR C. WAIL** AND GLENN T. SEABQRG
of Chemistry and Radiation Laboratory, University of California, Berkeley, California(Received January 19, 1948)

The isotope Np"', produced as the decay product of the previously known 6.8-day U"',has been isolated v ith carrier material and some of its nuclear properties measured. The U"'
was formed by means of the reaction U~'(n, 2n)U"'. Np"' decays with the emission of alphaparticles with a half-life of about 3 X10' years. It has been shown that the upper limit for the
cross section for the 6ssion of Np"' with slow neutrons is less than 1 percent of that of U"'.

HE bombardment of uranium with fastneutrons produces" the beta-emitting 6.8day U'" formed in the reaction U"'(n, 2n)U'".

The chemical procedure, in outline, is asfollows. The uranyl nitrate hexahydrate is subjected to an ether extraction process in order to
We have investigated 93'", the daughter of U'", remove the bulk of the uranium, and rare earth
the primary purpose of our investigation being
fluoride is precipitated from the water phase.
to measure the fission properties of this isotope. This rare earth fluoride contains the 94, 93, andOur measurements,
performed upon a sample
thorium, the uranium and the protactinium
weighing about 0.6 microgram, show that 93"' (when zirconium carrier is present) remaining
does not undergo appreciable fission with slow
in solution. This rare earth fluoride is dissolved
neutrons. The experiments also show that 93'" in sulfuric acid, and from a cold potassium
is an alpha-emitting
isotope with a half-life of bromate (KBrO~) solution the rare earth fluoride
3X10'
about
years.
is again precipitated. In this step the 94, in its
For the preparation of the sample of 93'" reduced form, and the thorium and the rare
about 1200 grams of uranyl nitrate hexahydrate,earth fission products are precipitated as inUO2(NOq)2. 6H20, were placed directly behindsoluble fluorides, and the 93 remains in solution
the beryllium target of the 60-inch Berkeley in its oxidized form. After reduction with sulfur
cyclotron and exposed to the fast neutrons
dioxide, the 93 in the solution is precipitatedproduced in a 15,000-microampere hour bom- with a small amount of rare earth fluoride. Of
bardment of the beryllium with deuterons. After course, it is necessary that a number of cycles
the lapse of several months, so that substanbe performed in order to remove completely
tially all of the U"' had decayed to 93'", a from the final sample the 94, uranium, protacchemical separation was performed on this bomtinium, and thorium, and in our procedure there
barded uranyl nitrate in order to isolate the were three separations from 94, six separations
93'" into a thin rare earth fluoride precipitate from uranium, five separations from protac(approximately 0.3 mg/cm'), special care being
tinium, and five separations from thorium. The
exercised to remove all of the 94, uranium, pro- last 94-containing rare earth precipitate wastactinium, and thorium. This procedure has been mounted for alpha-counting and was found todescribed in principle in another report. 'contain practically no 94 (&30 counts per minute) while the final sample containing the 93'"
*This article was mailed, as a secret report, fromhad a counting rate of about 300 alphas per
Committee inBerkeley, California to the Uranium
Washington, D. C. on April 14, 1942. The experimental
minute. In order to keep track of the yield of 93
work was done during 1941 and the early part of 1942.
through this rather lengthy chemical procedure
The original article is published here essentially withoutchange. The abstract was written at the time of publication
there was added at the beginning some 2.3-daysince the original report contained no summary.
93"' as tracer for the 93, and it was found that* Now at Department of Chemistry, Washington University, St. Louis, Missouri.
the yield of 93 in the final sample amounted to' E. M. McMillan, Phys. Rev. 58, 178 (1940).
~ Y. Nishina,
T. Yasaki, H. Ezoe, K. Kimura, and M. about 50 percent of that originally added to the
Ikawa, Phys. Rev. 5'F, 1182 (1940).
'G. T. Seaborg and A. C. Wahl, J. Am. Chem. Soc. bombarded uranium nitrate.
In order to prove that in this chemical pro(March, 1948).
940

NUCLEARPROPERTIES OF 93'''

cedure no uranium, protactinium, or thorium
remained in the final sample, a chemical separation, of a similar type, was performed on nonirradiated uranyl nitrate using U~' as tracer for
uranium, Pa"' as tracer for protactinium, andUXI as tracer for thorium. This "b)ank experiment" proved that in our chemical procedure
there remains in the fina1 sample much less thana microgram of each of these elements.
In order to know how much 93"7 was formedin the bombardment
described above, it wasnecessary, of course, to determine the yield of
U"' in the bombardment of uranyl nitrate hexahydrate under similar conditions. A sample of
700 grams of UO2(NOq)2 6HqO was placed directly behind the beryl1ium target and subjected
to a 500-microampere hour bombardment. This
sample was carefully purified from all of thefission products and from element 93 by the
ether extraction method; the ether phase, containing the bulk of the uranyl nitrate and theU23~, was washed 8 or 10 times with water inorder to insure a very thorough purification.
Aliquot amounts, determined by weighing theuranyl nitrate containing the 6.8-day U'", weretaken from the ether phase and the intensity of
the activities was measured with a Lauritsenquartz fiber electroscope calibrated in an absolutemanner for the radiation from the 6.8-day U'".
This calibration was accomplished by preparing
thin electrolytically deposited aliquots of theuranium and counting the beta-particles fromthe 6.8-day U'~ with a thin-window GeigerMiiller counter of ca.librated e%ciency; theweight of uranium in these small aliquots wasdetermined by counting its natural alpha-particles with an ionization chamber of calibrated
efFiciency. (Correction for the self-absorption in
the samples and absorption in the Geiger-Miiller
counter window of the 6.8-day U"' beta-particles
was made by using the value for the half thickness of the absorption of these beta-particles,
which we have determined as 6 mg Al/cm'. )From the measurementson the calibrated
Lauritsen electroscope of the chemically purified
aliquots from the 500-microampere hour bombardment, we found that the yield of 6.8-day
U"~ was 0.0054 microcurie per gram of
UOg(NOg)2

6HgO

per microampere hour of deuterons on beryllium.
From this yield it follows that there was formedin the 15,000-microampere hour bombardmentof 1200 grams of UO2(NO~)~. 6H20 about 97 millicuries of U"', which corresponds to about 1.2
microgram. Since substantially all of this decayed
to 93'" before the chemical separation was performed and since the yield in the chemical procedure for the isolation of 93 was about 50 percent, as mentioned above, this means that there
was present in our final sample about 0.6 rnicrogram of 93'". We consider this estimate of theamount to be accurate to within a factor of
about 2. The alpha-counting rate of this sample,
as mentioned above, was about 300 counts per
minute, and, from this value, together with thecalibrated efFiciency (45 percent) of the ionizationchamber, it is calculated that the half-life ofalpha-emitting 93"~ is about 3X10' years.
This sample was placed on one electrode of an
ionization chamber connected to a linear amplifier and recording system which was adjusted
so as to record the pulses due to fissions. As aneutron source we used 300 milligrams of radiummixed with beryllium powder. The ionizationchamber and the 300-mg radium-beryllium
neutron source were completely surrounded byparafFin. Under these conditions the 0.6 microgram 93'" sample gave four fission counts inabout 56 hours of counting time. When theionization chamber was surroundedby cadmium, there were recorded two counts in 56
hours of counting time. Thus, the counting rate
due to slow neutrons was two counts in 56 hours
or 0.035 fission counts per hour. For a calibration
experiment the 0.6-microgram sample of 9323~
was replaced by 200 micrograms of electrolytically deposited uranium, whose weight wasdetermined by counting its alpha-particles with
an ionization chamber of calibrated e%ciency.
The easily measurable fission rate of this uranium sample, containing 1.4 micrograms of U"',
when compared to that for 9323~ showed that theslow neutron fission cross section of 93'" is lessthan one percent4 of that for U~'.
4 Note added at time of publication.
More recent work
by A. Ghiorso, D. W. Osborne, and L. B. Magnusson has
demonstrated that Np"7 undergoes slow neutron fission
with a cross section a few thousandths of a percent of thatof U~Bf'. (Science 104, 386 (1946).)