Sources of Gamma Radiation in a Reactor Core

J. Nucl. Energy, Part B: Reactor Technology, 1959, Vol 1, pp. 98

to

104. Pergamon Press Ltd. Printed in Northern Ireland

SOURCES OF GAMMA RADIATION IN A REACTOR CORE MATTs Roos AB Atomenergi, Drottning Kristinas vag 47, Stockholm, Sweden (Received 8 April 1959)

Abstract-In a thermal reactor the gamma ray sources of importance for shielding calculations and related aspects are (1) fission, (2) decay of fission products, (3) capture processes in fuel, poison and other materials, (4) inelastic scattering in the fuel and (5) decay of capture products. The energy release and the gamma ray spectra of these sources have been compiled or estimated from the latest information available, and the results are presented in a general way to permit application to any thermal reactor, fuelled with a mixture of 235 U and 238 U. As an example the total spectrum and the spectrum of radiation escaping from a fuel rod in the Swedish R3-reactor are presented.

matches the spectrum of SKLYAREVSKII et al. only IN reactor shielding studies and related aspects it is of at the softest gamma line (0·03 MeV), falling a factor importance to know the energy released as gamma 5 below at 0·20 MeV and a factor 10 lower than radiation and its spectral distribution. So far detailed MAIENSCHEIN's spectrum in the region from 0·30 MeV calculations of the total spectrum of gamma radiation to 0·60 MeV. The energy released within 5 x Io-s sec of fission from a reactor core have been hampered by a very limited knowledge of the sources. However, the large and within the energy range 0·3-10 MeV (extrapolated volume of relevant information published during 1958, from 7·3 MeV to 10 MeV) is reported by MAIENespecially concerning the main sources, now facilitates SCHEIN to be 7·2 ± 0·8 MeVIfission and within the range 0·015-0·260 MeV by SKLYAREVSKII to an estimate based on fewer guesses than before. The gamma-ray spectra of different sources can most be about 0·24 ± 0·05 MeVIfission. In addition, conveniently be compared when expressed in units of MAIENSCHEIN found delayed gamma rays in the energy release per fission per energy interval, or MeV I region between 5 X JQ- 8 sec and I0- 6 sec after fission, fission X MeV, and the integrated spectra thus in MeV I and in the energy range 0·1-2 MeV. The intensity is fission. However, only the spectra of prompt fission reported to be (5·7 + 0·3) per cent of the prompt and fission product radiations can be expressed in radiation, or about 0·4 MeVIfission. Thus the total these units without loss of generality, whereas in energy released within I0- 6 sec is about capture processes, for instance, the number of captures 7·9 MeVIfission. (2.1) cannot be related to the number of fissions without The spectrum shown in Fig. 1 is obtained by joining reference to a specific reactor. In order to show the relative importance of the different sources, therefore, the spectra of MAIENSCHEIN and SKL YAREVSKII and in the last section we apply the general results to a is, to account for the unknown spectrum of delayed particular reactor, the Swedish R3 (MARGEN et a!. gamma rays, normalized to 7·9 MeVIfission. 1958), for which we give the total spectrum and the 3. FISSION PRODUCT GAMMA RAYS spectrum of radiation escaping from a fuel rod. Several reports (BLOMEKE and TODD 1957; KNABE 2. PROMPT FISSION GAMMA RAYS and PUTNAM 1958; MAIENSCHEIN et a/. 1958; The spectrum of y-rays emitted within 5 X lo-s sec MILLER 1957; PERKINS and KING 1958; PRAWITZ of fission has been measured in the energy ranges from et a/. 1958; SCOLES l958a,b, STEHN and CLANCY 0·3 MeV to about 7·3 MeV by MAIENSCHEIN et a! 1958) have recently been published on they-radiation (1958), from 0·015 MeV to 0·800 MeV by VOITOVET- of products of thermal fission of 235 U at various coolSKII eta!. (1957) and from about 0·020 MeV to about ing times after irradiation times of various durations. 0·260 MeV by SKLYAREVSKII et a/. (1957). It seems The reports of BLOMEKE and TODD, MILLER, PERKINS possible to join the spectra of MAIENSCHEIN and and KING, PRA WITZ et a/. and SCOLES are based on SKLYAREVSKII in the region between 0·26 MeV available chemical data and thus do not extend to and 0·30 MeV, whereas the spectrum of VonovETSKII very short cooling times. As the most short-lived 1. INTRODUCTION

98

99

Sources of gamma radiation in a reactor core

isotopes emit comparatively hard y-radiation, it is recognized that the extrapolation of decay curves down to zero cooling time might give results which are too small by a factor of four. It would be possible, however, to obtain better agreement by taking into account new nuclear data on very short-lived isotopes, as reported for instance by O'KELLEY eta!. (1958). MAIENSCHEIN eta!. (1958) have measured they-ray spectra at various cooling times down to about I sec after irradiation for time intervals sufficiently short to be considered instantaneous. These values appear to I

I

5

I I

4

-

! I

!

KNABE and PuTNAM report an energy release of 6·6 MeV/fission based on MAIENSCHEIN's data, which have been corrected for an extension of the energy range down to 0·1 MeV and up to 5·5 MeV. This figure corresponds to the time range I sec-lOS sec after fission. STEHN and CLANCY (1958) have made an extensive survey over several measurements on (J- and y-activites at very short cooling times (this survey includes some of the results reported by MAIENSCHEIN), and they conclude that a reasonable value for the total y-energy released would be about 7·0 MeV/ fission. (3.1) From the standpoint of shielding this value is slightly more conservative than the value of MAIENSCHEIN. It thus seems reasonable to adopt it at present.

I

[-

V\

>

I

"' ::;:

I 2

I

\

I

\

\

MeV

f\,_ 0

2.-Spectrum of gamma rays emitted by 235 U thermal fission products, after 5 x 10 7 sec irradiation time and zero cooling time. Normalized to 7·0 MeV/fission. FIG.

""'

--...

23456789 MeV

I.-Spectrum of gamma rays emitted promptly (within J0- 6 sec) after fission. Normalized to 7·9 MeV/fission.

The fission product y-ray spectrum at zero cooling time and infinite irradiation time, Fig. 2, has been constructed in the following way.

give the best starting point at present in an effort to evaluate the spectrum at zero cooling time and infinite irradiation time. (There is essentially no change in the spectrum between an irradiation time of a few hundred days and infinity, for most reactors.) The energy released between 0·3 MeV and 5·0 MeV from I sec to 108 sec after fission is reported to be 5·9 ± 0·8 MeVf fission. (MAIENSCHEIN has used the chemical data of PERKINS and KING for extrapolation from 1·8 x 103 sec to 108 sec.) Extrapolating in MAIENSCHEIN's curve from I sec down to zero, one obtains an increment of about 0· 3 MeV /fission. An estimate of the contribution from energies < 0·3 MeV can be based on the lowest energy group of PERKINS and KING which extends down to 0·1 MeV. This gives an additional increment of about 0·3 MeVjfission bringing the total energy release up to 6·5 + 0·8 MeVjfission.

The energy release in each of the six energy groups is obtained by extrapolating the photon-intensity time distributions of MAIENSCHEIN from I sec to zero, integrating from zero to 1800 sec, adding a contribution for the time between 1800 sec and 5 x I 0 7 sec (it was necessary to calculate this separately from the data of BLOMEKE and Tooo, for each isotope present and each gamma line, because the similar calculations published by PERKINS and KING have an energy grouping different from MAIENSCHEIN and are thus difficult to compare), multiplying by the group width and by the average energy of the group. The average energy was estimated from the continuous belt measurements of MAIENSCHEIN. The sum of the six groups, covering the energy range 0·3-5·0 MeV is found to be 6·5 MeV/fission. This figure can be compared with the value 5·9 ± 0·8 MeV/ fission of MAIENSCHEIN plus the estimated contribution of energy released within I sec of fission, 0·3 MeV/fission. The difference 6·5 - 5·9- 0·3 = 0·3 MeV/fission is probably attributable to the fact that the photon intensity time-distributions are uncorrected for the spectrometer response function, whereas the total energy release curve has an approximate correction. To the histogram of the six energy groups we finally add the

FIG.

100

MATIS

estimated 0·3 MeV/fission in the region <0·3 MeV. The spectrum is obtained by fitting the histogram with a continuous curve in such a way, that within each group the shape of the spectrum resembles that of the corresponding part of the spectrum from the continuous belt measurements, and the curve is normalized to 7·0 MeV/fission. 5

4

>

Q)

~ 3

'> Q)

:::!<

21--

\ ....__, / '

1\ \

•asu

INELASTIC SCATTERING GAMMA RAYS

The differential inelastic scattering cross-sections a(Ey, Em 8) of the energy levels Ey < 1·75 MeV of 2asu have been measured at = 90° for neutrons of energies En< 2 MeV by CRANBERG and LEVIN (1958). The integral inelastic scattering cross-sections a(Ey, En) of the energy levels Ey, which for most energies En are equal to 4rra(Ey, En, rr/2), have been calculated by MANDEVILLE and KAVANAGH (1958), who have completed the cross-section table of CRANBERG and LEVIN by some theoretically deduced values. Although the values of a(Ey, En) are given only for a few neutron energies En, a(Ey, En) can be plotted for each Ey as a function of Em if the total inelastic scattering cross-section

e

2 a(Ey, En) y

\

2

5.

a(En) =

1\

I

0

i

Roos

ul

h 3

4

MeV

FIG. 3.-23 8 U(n, y) 239 U gamma-ray spectrum unnormalized.

4. URANIUM CAPTURE GAMMA RAYS

The (n, y)-spectrum of 238 U has been investigated by BARTHOLOMEW and HIGGS (1958). In the low energy region ScHULTZ et al. (1957) have presented measurements on natural uranium, but their paper gives the intensity only on a relative scale, and is therefore difficult to relate to the 238 U spectrum of BARTHOLOMEW and HIGGS. In Fig. 3 we have reproduced the spectrum of BARTHOLOMEW and HIGGS, after normaliz239 ing it to the last-neutron binding energy of U, 4·70 MeVfcapture.

(4.1)

The gamma-ray spectrum from capture in 235 U has not, to our knowledge, been measured. It has been conjectured by BERTINI et al. (1956) to use the same spectral distribution as for prompt fission gamma rays. On the other hand GROSHEV et al. (1958) have investigated the general shape of the unresolved part of (n, y)spectra for different compound nuclei of the same F or even-even nucI e1. l"k . 1 e 236 U , proton-neutron panty. they show that the continuous spectra start roughly at 1·5 MeV below the binding energy, increase to a maximum at about 2 or 3 MeV and then decrease to zero. On this very approximate basis one can construct a spectrum for 235 U(n, y)2 36 U and normalize it to the binding energy (4.2) 6·42 MeVfcapture.

is known. Compilations of data on a(En) (CRANBERG and LEVIN; HUGHES and SCHWARTZ, 1958) cover the energy range En< 2·5 MeV; at 3·5 MeV it is possible to obtain a point at 3·1 barns from a comparison of the integrals of the spectra of inelastically scattered 2·5 MeV and 3·5 MeV neutrons, as reported by FEnsov (1957). Thus it seems that the total cross-section a(En) levels off to about 3 barns* at energies En> 2 MeV. The plots of a(Ey, En) and ~(En) are shown in Fig. 4. For En< 2 MeV the intensity of each gamma line (of energy Ey) is represented by the integral

v . EY

r2MeV

Jo

N(En)Pc[r~(Ey, En)] dEm

(5.1)

where v . N(En) = uncollided fission neutron spectrum,

=

probability that a neutron of energy En collides with a 238 U-atom and excites the Ey-level, r = radius of the fuel element. and The intensity is obtained in MeV/fission; however, both ~(Ey, En) and r are dependent on the choice of reactor and fuel elements, and the integration will therefore be left to the last section. The hardest gamma line of CRANBERG and LEVIN is actually not one line but a number oflines arising from several energy levels between 1·4 and 1·75 MeV. At still higher energies the level density increases and the statistical theory should become applicable. From the review article of KINSEY (1957), assuming a constant cross-section a(En) = a in the statistical region, the

PJr~(Ey,

En)]

* In his recent compilation HowERTON (1958) suggests that the cross-section curve does not rise above 2· 5 barns.

Sources of gamma radiation in a reactor core

101

Substitution of (5.4) in (5.5), and integration over the integral in c: gives

3

~

E 0

1·060MeV 1·4-1·75MeV

.0

En,

MeV

FIG. 4.-Total inelastic scattering cross-section a(En) and inelastic scattering cross-sections a(Ey, En) of gamma energy levels Ey.

spectrum of inelastically scattered neutrons can be expressed as F(c:, En) de:= const.

where

(

w(En)

WEn+ Q)

. E exp ( -c:/8) de:, (5.2)

Q = binding energy of a neutron in the compound nucleus,

w(En) = const. E;; 514 exp (2aVEn), the level density at the bombarding energy En,

() = -

d

dEn

In w(En)

=

En

-=-'---

aVEn

+ 5/4

the nuclear temperature introduced by Weisskopf, and a is a constant, which for 238 U has the value 5·25 MeV-1 ' 2 . If we normalize the spectrum (5.2) by the requirement (5.3)

we obtain the fraction F( c:, En) de: dEn of all bombarding neutrons of energy between En and En dEn which are scattered into the energy interval between c: and

+

E

+de:,

The total energy released as gamma radiation is then given by an integral like (5.1), where En- E is substituted for Ey, and where PJrL-) now is a constant, since a was assumed constant in the region of interest (En> 2 MeV):

In our case Enf() ~ 1 (with a minimum value = 8·67 at 2 MeV) so that (5.6) can be simplified to

v. PJrL-) (oo N(En)(28 +En) dEn-

(5.7)

J2MeV

The integral has the value 1·61 MeV per inelastically scattered neutron. When v = 2·47 neutrons/fission, the expression (5.7) takes the value 4·0 PJr L-) MeV/fission.

(5.8)

The spectral distribution of this radiation is unknown. If all the excitation energy were radiated as groundstate transitions, the spectrum would be given by the integrand in (5.7). Since this is not the case, the spectrum is considerably softer, with a peak somewhere between I and 2 MeV. As a rough estimate we assume that 3 Pc(rL-) MeV/fission has the distribution of the integrand in (5.7), and the remaining quarter is distributed in the region <2 MeV in such a way that the spectrum becomes zero at zero energy and is continuous at 2 MeV. 6. GAMMA RAYS FROM CAPTURE IN POISON, CONSTRUCTION MATERIALS AND MODERATOR

After short irradiation times almost all the poison in the reactor fuel is 135Xe, for which both the capture gamma spectrum and the total energy release per capture (the binding energy of 136Xe) are unknown. After a year's irradiation time in a flux of 1012 - 1014 nfcm2 sec the 135Xe-fraction in the poison is still over 50 per cent, so that it is not worthwhile to investigate the spectra of the other poison components in any detail (BLOMEKE and TODD, 1957). The binding energy of the two last neutrons in 136Xe is 14·5 MeV, which suggests that we adopt the value 8 MeVfcapture (6.1) as the approximate total energy release.

MATTS Roos

102

The capture gamma spectra of other absorbers present in the construction materials, the coolant or the moderator can be found in the recent compilations of GROSHEV et a/. (1959), BARTHOLOMEW and HIGGS (1958) or DELOUME (1958). 7. GAMMA RAYS FROM DISINTEGRATION OF CAPTURE PRODUCTS

The main capture products are 239 U, 236 U and 136Xe, of which only 239 U is radio-active. In addition there might be radio-active capture products in the construction materials, the coolant and the moderator. 239 U disintegrates to 239 Np by emission of one photon of 0·074 MeV. 239 Np disintegrates in turn to 239 Pu in a complicated way, by emission of soft ( <0·35 MeV) gamma rays. From the decay schemes suggested by STROMINGER and HOLLANDER (1958) and STROMINGER eta/. (1958 a,b) and by DZELEPOV and PEKER (1957) one obtains a total energy release of approximately 0·4 MeV per disintegration, including the single line of 2a9u.

At saturation (times large compared with the halflives of 239 U, 23·5 m and 239 Np, 2·33 d) every neutron capture in 238 U is followed by one disintegration of a 239 U nucleus and one disintegration of a 239 Np nucleus, so that the net energy release is 0·4 MeVJcapture in

There are 0·66 thermal captures in 238 U per fission, 0·19 thermal captures in 235 U per fission, 0·27 resonance captures in 238 U per fission, 0·079 thermal captures in 135Xe per fission, per fission and 0·029 thermal captures in Zr per fission. 0·012 thermal captures in D The contribution from inelastic scattering in 238 U is found by calculating the collision probability Pc(rL.), which was introduced in (5.1). Pc(rL.) has been computed for uniform source strength distributions and different source geometries by PLACZEK et al. (1953). For a homogenized fuel element of radius 5·62 em the total energy released in inelastic scattering is found to be 0·8 MeV/fission. * In Table 2 we collect all gamma sources together and compare them with a previous calculation by BRAUN (1957) on a similar reactor. The largest difference is found to arise from the 238 U capture, where BRAUN has used a binding energy of 7·5 MeV instead of 4·7 MeV, influenced by the value of the average binding energy per nucleon (which is approximately 7·5 MeV for 238 U) and by a gamma line at 7·5 MeV reported by KENNEY and MATTINGLY (1956), but not found later by BARTHOLOMEW and HIGGS (1958). TABLE 2.-TOTAL GAMMA ENERGY RELEASE Energy release (MeV/fission)

(7.1)

238 U.

The approximate spectrum is given in Table 1 below. TABLE 1.-GAMMA ENERGY RELEASE FROM DISINTEGRATION OF 239 U I

Energy range (MeV) -

·~--

i'

Energy release (MeV/capture)

--------

0·05-0·10 0·10-0·15 0·15-0·20 0·20-0·25 0·25-0·30 0·30-0·35

-----------

"

0·109 0·046 0 0·091 0·125 0·027

8. TOTAL GAMMA SPECTRA. APPLICATION TO THE SWEDISH R3-REACTOR

The R3-reactor is a U0 2-fuelled, D 2 0-moderated and D 2 0-cooled reactor to be operated at 125 MW. The fuel elements contain 43·5 per cent U0 2 by volume, 46·4 per cent D 20 at about 220°C and 10·1 per cent Zr, and they are arranged in the moderator on a square lattice with a lattice pitch of 27 em (MARGEN et a/., 1958).

Prompt fission Fission products 238 U capture 235 U capture Inelastic scattering 135 Xe capture Other capture 239 U decay Other decay Total

7·9 7·0 4·37 1·22

7·8 7·2

0·8* 0·63 0·27** 0·37

0·9

8·2

l·Ot 0·4t

22-6

25·5

** Zr and D 20 t AI

The resulting total spectrum is shown in Fig. 5. The spectrum of radiation escaping from a fuel rod can be found by multiplying the total spectrum by the energydependent escape probability PescCE) for photons of energy E. P 88 c(E) has been investigated by STORY (1957) for a uniform source strength distribution and *Using the cross-section value of HowERTON (1958) mentioned in the footnote on p. 100, this figure changes to 0·7 MeV/fission.

103

Sources of gamma radiation in a reactor core

> ~

>

"'

::;

I

2

3 MeV

FiG. 5. A-Spectrum of total gamma energy released in the R3

core (scale on left). B-Spectrum of gamma radiation escaped from the R3 fuel elements (scale on right). N.P. The peaks at 4 MeV and 6·25 MeV are due to capture in 238 U and D, respectively.

for a step-function approximation to the real source strength distribution in cylindrical rods. However, it can be shown in the energy range covered by STORY, that a simplified calculation taking into account only the first absorbing collision and assuming a uniform source strength distribution gives a result which lays well between the maximum and minimum curves of STORY. The escape probability would thus be given by

Pesc(E)

=

1-

Pc[r,uen(E)],

(8.1)

where Pc is the collision probability of PLACZEK, and .Uen(E) is the energy absorption coefficient for photons of energy E in the homogenized rod of radius r. The integral of the spectrum of escaping radiation, shown in Fig. 5, is found to be 7·6 MeVjfission, which can be compared with the figure 8·6 MeVjfission of BRAUN. The fraction of gamma energy escaping from the fuel rod is thus approximately 1/3. Acknowledgements-For valuable discussions and helpful suggestions the author is indebted to Messrs. J. S. STORY of Harwell and J. BRAUN and N. G. SJOSTRAND of AB Atomenergi. REFERENCES BARTHOLOMEW G. A. and HIGGS L. A. (1958) Compilation of Thermal Neutron Capture Gamma Rays. AECL-669. BERTINI H. W., COPENHAVER C. M., PERRY A. M. and STEVENSON R. B. (1956). ORNL-2113. BLOMEKE J. 0. and TODD M. F. (1957) Uranium-235 Fission-

Product Production as a Function of Thermal Neutron Flux, Irradiation Time, and Decay Time. ORNL-2127. BRAUN J. (1957) Gamma Volume-sources in the Reactor Core Report AEF 88 AB Atomenerji. In Swedish. CRANBERG L. and LEVIN J. S. (1958) Phys. Rev. 109, 6, 20632070.

DELOUME F. E. (1958) Gamma Ray Energy Spectra from Thermal Neutron Capture. APEX-407. DZELEPOV V. S. and PEKER L. K. (1957) Decay Schemes of Radioactive Isotopes. AECL-457. FEnsov N. I. (1957) Atomnaya Energiya 3, 9, 211. GROSHEV L. V., DEMIDOV A.M., LUTSENKO V. N. and PELEKHOV V.I. (1958) Proceedings of the Second International Conference on the Peaceful Uses of Atomic Energy, Geneva, P/2029. United Nations, New York. GROSHEV L. V., DEMIDOV A.M., LUTSENKO V. N. and PELEKHOV V. I. (1959) Atlas of Spectra of Gamma Rays Produced by Thermal Neutron Capture. Pergamon Press, London. HoWERTON R. J. (1958) Semi-empirical Neutron Cross-sections, 0·5-15 MeV (II) 1, UCRL-5351. ( HuGHES D. J. and SCHWARTZ R. B. (1958) Neutron Cross Sections (2nd Ed.). BNL-325. KENNEY R. W. and MATTINGLY J. T. (1956) Thermal Neutron. Capture Gamma-ray Spectrum from 238 U. UCRL--4735. KINSEY B. B. (1957) Encyclopedia of Physics Vol. XL, pp. 202449. Springer, Berlin. KNABE W. E. and PUTNAM G. E. (1958) The Activity of the Fission Products of 235 U. APEX-448. MAIENSCHEIN F. C., PEELLE R. W., LOVE T. A. and ZOBEL W. (1958) Energy Spectra of Fission-associated Gamma Radiation EAES Shielding Symposium in Cambridge; Proceedings of

the Second International Conference on the Peaceful Uses of Atomic Energy, Geneva P/670; ORNL-2609. MANDEVILLE C. E. and KAVANAGH D. L. (1958) The Scatteril{lf of Neutrons by 238 U. CWR-4028. MARGEN P. H., CARRUTHERS H., HARGO B., LINDBERG G. and PERSHAGEN B. (1958) R3-A Natural Uranium Fuel Heavy Water Moderated Reactor for Combined Electricity Production and District Heating. AB Atomenergi report R3-100. MILLER C. F. (1957) Gamma Decay of Fission Products from the Slow-neutron Fission of 235 U. USNRDL-TR-187. O'KELLEY G. D., EICHLER E. and JoHNSON N. R. (1958) Proceedings of the Second International Conference on the Peaceful Uses of Atomic Energy, Geneva, P/672. United Nations, New York. PERKINS J. F. and KING R. W. (1958) Nucl. Sci. Engng. 3, 726-746. PLACZEK G., CASE K. M. and DE HoFFMANN F. (1953) Introduction to the Theory of Neutron Dif.litsion Vol. I. Los Alamos.

104

MATTS Roos

PRAWITZ J., Low K. and BJ6RNERSTEDT R. (1958) Proceedings

STORM E., GILBERT E. and IsRAEL H. (1958) Gamma Ray

of the Second International Conference on the Peaceful Uses of Atomic Energy, Geneva, P/149. United Nations, New York. SCHULTZ H. L., BOCKELMAN C. K., DRAPER J., FENSTERMACHER C. A. and RosLER L. (1957) Gamma Rays Following Resonant Neutron Capture. TID-7547. ScOLES J. F. (1958a) Fission Product Gamma Ray Spectra. Con-

Absorption Coefficients for Elements I through 100 derived from the Theoretical Values of the NBS. LA-2237. STORY J. S. (1957) Escape of Gamma Radiation from Uranium Rods in a Pile and Heat Evolved in the Moderator. AERE

vair-Fort Worth FZM-1042. SCOLES J. F. (1958b) Calculated Gamma Ray Spectra from 235 U Fission Products. NARF-58-37T. SKLYAREVSKII V. V., FOMENKO D. E. and STEPANOV E. P. (1957) Soviet Physics 5, 2, 220-225 (Russian reference JETP, 32, 256-262). STEHN J. R. and CLANCY E. F. (1958) Proceedings of the Second

International Conference on the Peaceful Uses ofAtomic Energy Geneva, P/1071. United Nations, New York.

T/R 2218. STROMINGER D. and HoLLANDER J. M. (1958) Decay Schemes. UCRL-8289. STROMINGER D., HOLLANDER J. M. and SEABORG G. T. (1958)

Rev. Mod. Phys. 30, 2, II. SULLIVAN W. H. (1957) Trilinear Chart of Nuclides. Oak Ridge. VOITOVETSKII V. K., LEVIN B. A. and MARCHENKO E. V. (1957) Soviet Physics 5, 2, 184-188 (Russian reference JETP, 32, 263-268).