Energy and half-life of beryllium¹⁰ radioactivity

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Title:
Energy and half-life of beryllium¹⁰ radioactivity
Series Title:
United States. Atomic Energy Commission. MDDC ;
Physical Description:
5 p. : ill. ; 27 cm.
Language:
English
Creator:
McMillan, Edwin M ( Edwin Mattison ), 1907-
University of California
U.S. Atomic Energy Commission
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Technical Information Division, Oak Ridge Directed Operation
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Oak Ridge, Tenn
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Subjects / Keywords:
Beryllium   ( lcsh )
Radioactivity -- Measurement   ( lcsh )
Radioactive substances   ( lcsh )
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federal government publication   ( marcgt )
bibliography   ( marcgt )
technical report   ( marcgt )
non-fiction   ( marcgt )

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Bibliography:
Bibliography: p. 5.
Restriction:
"Date Declassified: June 16, 1947"
Statement of Responsibility:
by Edwin M. McMillan.
General Note:
Manhattan District Declassified Code

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University of Florida
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oclc - 277770313
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AA00009300:00001


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) MDDC 1046


UNITED STATES ATOMIC ENERGY COMMISSION










ENERGY AND HALF-LIFE OF BERYLLIUM10 RADIOACTIVITY



by
Edwin M. McMillan


Umversity of California









This document consists of 5 pages
Date Declassified: June 16, 1947


This document is for official use
Its issuance does not constitute authority
for declassification of classified copies
of the same or similar content and title
and by the same author (s).




Technical Information Division, Oak Ridge Directed Operation
Oak Ridge, Tennessep


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ENERGY AND HALF-LIFE OF BERYLLIUMI0 RADIOACTIVITY


By Edwin M. McMillan


INTRODUCTION

The activity of Belo is of special interest because of theoretical difficulties in explaining its very
long half-life. Its existence has been shown by the work of McMillan and Ruben' and Levinger and
Meiners.2 Isotopic assignment has been confirmed directly by Pierce and Brown,' and the half-life
measured by Hughes, Eggler, and Huddleston.'
In this paper is reported some work made possible by the kindness of Professor A. L. Hughes,
of Washington University, in sending the author a beryllium target that had received a very strong
deuteron bombardment in the St. Louis cyclotron (about 200,000 microampere hours of ll-Mev deu-
terons at 45 incidence). The part of the surface struck by the intense central core of the beam had
spelled off; the portion used consisted of a square inch of surface immediately adjacent tu the spelled
region.

Even though this target was about two years old when received, it still carried a total activity
about 50 times as strong as the Be'o activity. The bulk of the contamination activity appeared in the
filtrate from the NHOH precipitation and had an absorption curve identical with that of Na", so it
probably was due to this substance made from traces of Mg in the target. The chemical procedure
used was simpler than that used by McMillan and Ruben because of the relatively much weaker con-
tamination, and also the fact that experience has shown the simplified procedure to be quite adequate
for purifying Be. The Be was dissolved in conc HCI, precipitated as hydroxide by NILOH, and con-
verted to the basic acetate by dissolving the hydroxide in glacial acetic acid. The basic acetate was
then dissolved in chloroform, washed several times by shaking with water, and then dissolved in dil
HNO,. Finally, the nitrate was ignited to BeO. The activity measurements were made with a G-M
counter having a mica window of 2.5 mg cm2 thickness.


EXCITATION CURVE

The surface of the target was milled off in six layers, each 0.005 inch thick, and the resulting
samples separately purified. Equal portions of these were taken by pipetting out equal volumes of
the saturated chloroform solutions of the basic acetate, and the relative activities of these (counted
in the form of the acetate) were measured. Nearly all the activity was found in the first three layers.
The corresponding energies were computed by assuming the stopping power of Be per electron to be
the same as that of air, and taking account of the 45" incidence. These energies are not highly accu-
rate, particularly because of the uncertainty in the initial energy and the angle. The results were:

No. of layer Energy range Relative activity

3 2.3- 6.3 Mev 0.46
2 6.3 8.9 Mev 1.0

1 8.9-11.0 Mev 0.88


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A check was made with a second pair of pipetted samples, to show that the drop in activity of the
first layer compared to the second is significant. A discussion of these results Is given in a later
paragraph.


ABSORPTION CURVE AND BETA ENERGY

The material from the first three layers was combined and converted to BeO (360 mg were ob-
tained). This was used for ill the other measurements. Two absorption curves were taken. Figure 1
shows a curve taken with a thin layer of BeO (46.5 mg spread over a 2.5-cm diameter circle),
mounted on paper to minimize backscattering. It was plotted without subtracting the counter back-
ground, in order to avoid running off the bottom of the logarithmic paper. On the same plot is shown
a curve for Na", taken on the same apparatus, but with the background subtracted. Since the relative
gamma-ray effect of the latter happens to be nearly equal to the background in the former, a good
comparison of the shapes can be made. It is seen that the Belo curve shows a slight upward convexity
that is not exhibited by the Nan curve. This is not pronounced enough to indicate a line of electrons,
but shows that the shapes of the beta-ray energy distributions are slightly different.
The other absorption curve, taken to locate the end-point, is shown in Figure 2. This time a
thick sample containing all the material was used. The residual counts beyond the end-point, which
amount to 1 10,000 of the initial beta counts corresponding to the amount of material in the sample,
are of the order to be expected from x rays ("bremsstrahlung') generated in the absorbers by beta
impact. Therefore, there is good evidence against the occurrence of a gamma ray in the decay of
Be". The end-point can be located rather accurately from Figure 2; it can be given as 185 5
mg/cm2 Al. The corresponding upper limit energy, using the best present range-energy relation,
is 560 10 kev, which agrees with the rougher value obtained by McMillan and Ruben.'


ISOTOPIC ABUNDANCE,SPECIFIC ACTIVITY, AND HALF-LIFE

Hughes et al' found the half-life by comparing the specific activity with the isotopic abundance
computed from the neutron flux, the time of exposure, and the cross section for the reaction
Bee(n,)Be0o. In our case the pertinent reaction is Bee(d.plBel0. whose cross section has not been
measured. On the other hand, the isotopic abundance was great enough that it could be measured
directly on a Nier mass spectrometer. This measurement was made by A. K. Pierce under the
direction of Dr. B. J. Moyer. The material was introduced into the spectrometer ion source in the
form of the anhydrous chloride sealed in a glass capillary, which was broken after evacuation. Good
(BeD)+ and (Beo)+ peaks were obtained, separated by over ten times the total peak width. The galva-
nometer deflections at the mass 10 peak were around 2 cm. Since the ratio of the mass 9 to the mass
10 current was large, different galvanometer sensitivities had to be used, and the ratio of currents
was determined by duplicating the deflections with known currents. Seven measurements with two
different fillings of the source gave a mean 9,/10 ratio of 15,700 1 2,700, the error being estimated
from internal consistency only. To check against the possibility of the mass 10 peak being due to
(Be'HP)+, (Be1o)+, or (Ne2o)+', another run was made using a sample prepared in the same way from
inactive Be. This time the mass 10 peak was much smaller but still observable, with a 9/10 ratio
of 100,000. If this is taken as a correction on the value given, the ratio Bea, Belo is 18.600 3,000.
The specific activity was measured by comparing the counting rate of a uranium standard (cali-
brated by Dr. C. Tobias of the Medical Physics Department against an accurately prepared standard)
with the counting rates of samples of the active BeO. Three samples were prepared, consisting of
12.0, 13.7, and 23.0 mg of the BeO spread over an area of 3 cm' on filter paper. These were counted
with no metal backing in order to minimize backscattering. The absorption correction for the window
plus the air path plus half the sample thickness, evaluated from the initial slope of the absorption
curve, was about 20%. The mean value obtained for the specific activity was 16.6 1.7 disintegra-
tions per second per mg BeO, or 46 + 5 disintegrations per second per mg Be, the error given being









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80 420 44
MG PER SO CM Al ABSORBER


Figure 1. Absorption of beta particles in aluminum. (Circles and solid line: Be"' absorption data,
plotted without subtracting counter background. Dashed line: Na" absorption data, with background
subtracted, for comparison of shape.)








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oo
END-POINT WITHIN THIS RANGE



I0 -Tr n-

in
I--
z
O 60
S60 BACKGROUND




40 -

I I l I i I
450 460 170 480 490 200 210 220
MG PER SO CM AI ABSORBER




Figure 2. Detail of Be' absorption curve near the end-point. (Absorption of counter window plus air
path included. Errors indicated are standard errors computed from counting stati.,tics.)


considerably greater than the spread in the three values, in order to allow for the various incertain-
ties involved in all absolute beta-intensity measurements, particularly when the sample and the stand-
ard have different energies.

The half-life is easily computed from the above data. The decay constant Ais given by (disinte-
gration rate per gram) '(number of active atoms per gram). or:

= 46,000 x 9 x 18.600 = (1.28: 0.24) x 10-"sec-.
6.02 x 10'?

The half-life is then (2.5 0.5) x 100 years, in agreement with the value of 2.9 x 106 years given by
Hughes et al," within the accuracy of the determinations.


DISCUSSION OF YIELD

Dr. Serber has made an estimate of the yield expected in the reaction Be'(d,p)Be'0. First the
total cross section for capture of the deuteron to form the intermediate nucleus was estimated from
the classical expression: a = n R'2(1 EB F), where R = nuclear radius (taken as the sum of the beryl-
lium and deuteron radii, or 5.5 x 10lScm.) and EB = barrier height (taken as 1.67 Mev). Then the
division between the (d,p) and (d,n) processes was computed, including a penetration factor for the
former and otherwise supposing division in the ratio of available volumes in phase space; the (d,a)
process was neglected as being probably less important. Finally the competing reaction (d,2n) with
an estimated threshold at 5 Mev was included, being given a probability proportional to the volume in
phase space and with a coefficient adjusted to match the data in section on Excitation Curve. This
process accounts for the decrease in cross section beyond 8 Mev.


_ ___









MDDC 1046 [ 5


From this theory the absolute yield at 11 Mev is estimated to be 0.0016 active atoms per deu-
teron, and the observed yield of 21,000 dis /sec, or 1.6 x 10i active atoms in the part of the target
used would correspond to 46,000 microampere-hours bombardment of that part, which seems reason-
able. Furthermore, the data of reference 1 com-ined with the measured half-life give an observed
yield of 0.0039 active atoms per deuteron at 16 Mev. which can be compared with the value of 0.0026
computed from the above theory at this energy. The difference is well within the accuracy of the
rather rough theory. It is interesting to note that the theory gives a neutron yield of 0.008 per deu-
teron at 16 Mev, in exact agreement with the value measured in the Crocker Radiation Laboratory
by H. Yockey. This discussion of the excitation curve and the yield shows that there is no great pe-
culiarity in the formation of Be'O to match its abnormal behavior in beta decay.



REFERENCES
1. McMillan, E. M. and S. Ruben, Phys. Rev. 70:123 (1946).
2. Levinger, J. and E. Meiners, Phys. Rev. 71:586 (1947).
3. Pierce, A. K. and F. W. Brown, III, Phys. Rev. 70:779 (1946).
4. Hughes, D. J., C. Eggler, and C. M. Huddleston, Phys. Rev. 71:269 (1947).




UNIVERSITY OF FLORIDA

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