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UNITED STATES ATOMIC ENERGY COMMISSION
TH1EE DISINTEGRATION OF Rb8
M. E. Bunker
L. M. ]Langer
]R. J. D. Moffat
October 31, 1950
[TID Issuance D~ate]
Los Alamos Scientific Laboratory
IITechnical Information Division, ORE, Oakr Ridge, Tenness*
Reproduced direct from copy
as submitted to this office.
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THE DISIN~TEGR~ATION OF RbB88
M. E. Bunker, L. MI. Langer** and R. J. D. Moffat+*
]Los Alamos Scientific Laboratory, Los Alamos, New ~Mexico
The radioactive disintegration of Rb88 (17. 8 min. ) has been
investigated in a magnetic lens spectrometer. Three beta ray groups
are observed having maximum energies of 5. 13 f 0. 03, 3. 29 t 0. 10,
and 2. 04 f 0. 15 Mev. with intensities of 66, 19, and 15 percent re-
spectively. Trhe 5. 13 Mev group has a forbidden shape indicating a
spin change of 2 units and a parity change. Gamma, rays are found
with energies of 0. 90, 1. 86, and 2. 8 Mev. A unique decay scheme
is presented giving levels in Sr88 at 1. 86 and 2. 8 Mlev. These levels
are substantiated by a previous investigation of the disintegration of
Y88 to Sr88. Spina and parities of Rb88, Sr88, and Y88 are discussed
in the light of the one particle shell model.
*This document is based on work performed under government
contract number W-7405-Eng-36 for the Los Alamos Scientific Laboratory
of the University of California.
**Indiana University, Bloomington, Indiana.
The radioactivity of Rh88 has been studied by various investi-
gators. Glasoe and Steigman1 have identified a 17. 8 minute beta ac-
G. N. Glasoe and J. Steigman, Phys. Rev. 58, 1 (194 0)
tivity with Rb88 and have meaosured the beta-ray end point as 4. 6 Mev
by absorption in alumninumn. G. L. Weil2, by cloud chamber measure-
G. L. Weil, Phys. Rev. 62, 229 (1942).
ments, observed two beta-ray groups with energies of 5. O and 2. 5 Mev.
The period was measured as 17. 5 minutes. Thus log ft for the high
energy group was determined asmv7. 0 suggesting a forbidden transition.
The gamma-rays of the product nucleus Sr have been investi-
gated by several groups 3-.There is general agreement on ganmm-ray
G. S. Goldhaber, Phys. Rev. 59, 937 (1941).
Richardson, Phys. Rev. 60, 188 (1941).
J. R. Downing, M. Deutsch, A. 'Roberts, Phys. Rev. 60, 470 (1941).
G. R. Gamertsfelder, Phys. Rev. 66, 288 (1944).
W. C. Peacock and J. W. Jones, Plut. Proj. Rep. CNL-14, Feb. 1948.
energies of approximately 0. 908, 1. 86, and 2. 8 Mev. These investiga-
tors have used sources of Y88 which decays to Sr88 by positron emis-
sion and K electron capture with a half life of 108 days.
A further investigation of the Rb88 beta radiation employing
higher resolution seemed desirable, since the previous work indicated
complexity of the spectrum and the possibility of a unique shape.
Expe rimental Method
The investigation of the beta and gamma radiation from Rb88
(17. 8 min. )was performed with the aid of a large, magnetic lens
apectrometer An end window Griger tube with a 3. 6 mg/cml mica
]L. At. ]Langer, Phys. Rev. 77, 50 (1950).
window and 0. 5 inch diameter aperture was employed as a detector.
This aperture, together with sources of similar diameter, yielded
6 percent resolution.
Two circular beta ray sources were prepared from finely
powdered Rb SO4. The first was 26 mg/cml on a 0. 0002 inch Al
backing and covered with a thin Zapon film. The second source was
30 mg/cm2 on a backing of 0. 7 mg/cmt nylon, which had been coated
with aquadag to maintain it at ground potential. A thin Zapon cover
was again employed.
For study of the gamma radiation, a strong RbNO3 source was
irradiated and enclosed in a 0. 125 inch walled copper cylinder which
stopped all primary beta radiation. A 0. 75 inch diameter. 56 mg/cml
uraniumn radiator covered the front end of the cylinder.
The sources were irradiated with slow neutrons from a nuclear
reactor. Times of irradiation and of the spectrometer runs were
chosen so that there resulted no interference from other activities.
The radioactive decay of a dummy source was monitored during the
spectrometer runs and the decay period was checked in the spectro-
meter at various energies. The half life was measured as
17. 8 T 0. 2 minutes.
Figure 1 is a momentum plot of the electron spectrum of Rb88
The obviously complex shape has been resolved into three beta-ray
groups on the basis of a Fermi analysis of the data. Figure 2 shows
the conventional F'ermni plot. In addition to the rise at low energy,
there is at high energies a distinct deviation from the straight line
characteristic of an allowed transition. The forbidden factor
a wZ = 1+(W W) applicable to once forbidden transitions
with spin change of two units, has been applied to the data and yields
the straight line shown in Figure 3. By appropriate subtractions, the
spectrum has been resolved into three groups. The two inner groups
are assumed to have the allowed shape. This analysis results in
maximum beta-ray energies of 5.13 f 0.03, 3.29 f 0.10, and
2.04 t 0.15 Mev, with corresponding intensities of 66, 19, and 15
percent. The resulting momentum distributions are shown in Figure 1.
The comparative half lives for the transitions are respectively log
ft = 7. 3, 7. 0, and 6. 2. It is of interest to note that the integral of
the allowed transition probability over the energy, must here be modi-
2 2~ h rout( 2 -W
fied by the factor a = W 1+ (W~ ).Tepodc W-W
is then a better indication of the comparative half life ~, and for tran-
F. B. Shull and E. Feenberg, Phys. Rev. 75, 1768 (1949).
sitions with a parity change and spin change of two units should have a
value 10 .~ For the high energy group from Rb ,. (Wf 1)ft = 0. 24 x 10 .
Figure 4 shows the observed distribution of photo and Compton electrons
from the uraniumn radiator. There is evidence of a K photo peak corresponding
to a gamma-ray of 0. 90 Mev. In addition, a gamma-ray energy of 1. 85 Mnev
is determined by means of the second photo peak and also from the Compton
edge. The high energy Compton tail gives a gamma-ray energy of 2. 8 Mev.
The three beta-ray groups of 5. 13, 3. 29, and 2. 04 Merv, together
with the observed gamma-ray energies of 0. 90, 1. 86, and 2. 8 Mev,
suggest the decay scheme shown in Figure 5. The 1. 86 Mnev gamman-ray
fits well the energy difference between the two highest energy beta groups.
The level in Sr8 indicated by the Z. 04 Mev beta-ray is somewhat higher
than the gamma-rays of 0. 90 and 2. 80 Mev would substantiate. Levels
inSr have been determined by Peacock and Jones7 in studying the
decay of Y88 by K capture and positron emission. They found gamma-rays
at 0. 908, 1. 853, and 2. 76 Mev. These values are in excellent agree-
ment with the gamma-rays from Rb .~ Since the value of 2. 04 Mev is
the result of two subtractions in the Fermi analysis, it is possible that
the error is greater than 0. 15 Mev and that the softest beta-ray group
actually feeds the 2. 8 Mev level. A search for a gamma-ray of about
0. 3 Mev which would bridge the energy difference gave negative results.
The spectrum shape of the high energy beta-ray group indicates
that the transition is once forbidden with a spin change of two units, and
since the product nucleus Sr is even-even and hence presumably has
even parity and spin 0, odd parity and spin 2 may be assigned to the ground
state of Rb .Furthermore, even parity may be assigned to the 1. 86 and
2. 8 Metv levels in Sr88, since the ft values (log it r 7. 0, 6. 2) of the two
inner beta-ray groups from Rb88 suggest once forbiddenness and hence a
parity change. Assignment of even parity to these levels contradicts the
parity assignments made by Peacock and Jones7 on the basis of internal
conversion coefficients. However, more recent calculations of the in-
ternl conversion coefficientsl0 permit a reinterpretation of the data of
Mi. E. Rose, Tables privately circulated.
Peacock and Jonesa. Their measurement of the 1. 853 Mev gamma-ray is
consistent with magnetic dipole radiation and would indicate spin 1 and
even parity for the 1. 8 Mev level. The reported conversion of the Y88
0. 908 Mellv gamma-try is less than the theoretical electric dipole value
and must be assumred to be in error. Peacock and Jones' assignment
of spin 2 for the 2. 8 Mev level in Sr88 is still justified on grounds of the
relative intensities of positron emission and K electron capture in Y88.
Consideration of the one-particle spin-orbit coupling model,
extended to odd-odd nucleil2, suggests a spin of 4 and odd parity for the
M. G. Mayer, Phys. Rev. 78, 16 (1950).
L. W. Nordheim, Phys. Rev. 78, 294 (1950)
ground s~tat of Y88. This is the result of pb for the 39th proton and
1g/ for the 49th neutron. These assignments agree both with the order
of energy levels obtained theoretically and with the measured spins of
Y89 and Sr87, for which Z anrd N respectively are the same as for Y .
The shell model prediction for the ground state of Rh88 is less
certain. The 37th proton and 51st neutron are predicted as p and
g./ respectively. The spin of Rbs with 37 protons and Sbl with 51
protons have been measured and substantiate these assignments. How-
85 67 121
ever, Rb and Zn exhibit fg/ for the 47th nucleonl while Sb and
Zr show dg/ for the 51st nucleon. Thus four possible combinations
result in odd parity and permit a spin of two units. The p3/2 7/2
combination predicts odd parity and spin 2 uniquely and seems most
The authors wish to express their gratitude to Dr. J. D. Knight
for chemical preparation of the source material and to Mr. R. E. Carter
for help with some of the measurements. The entire staff at this labors-
tory have been most cooperative during this investigation.
4 5 6 8 9 10 1
Figure 2. Conventional Fermi plot. The curve is bent
towards the energy axis as it approaches the
end point. Values of F were determined from
the tables of IVosakowaki.
I 2 3 4 5 6 7 8 9 10 1
figure 3. Forbidden Fermi plot of the data and results of
two successive subtractionsl. It is assumed that
the spectra of the inner groups are of the allowed
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 11000
Hp (GAUSS -CM)
Figure 4, Compton and photo electrons ejected from a uranium radiator by the gamma rays following the decay of Rb88
I T ,EVEN
Figure 5. Disintegration s~chme of Rb88 and Y88 and energy levels of Sr .
END OF DOCUMENT
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