spontaneous fission yields from uranium and thorium

6
PHYSICAL REVIEW VOLUME 92, NUMBER 4 NOVEMBER 15, 1953 Spontaneous Fission Yields from Uranium and Thorium* GEORGE W. WETHERILLI Department of Physics, University of Chicago, Chicago, Illinois (Received August 13, 1953) Relative spontaneous fission yields from uranium and thorium have been determined by extracting xenon krypton from geologically old uranium and thorium minerals and measuring the isotopic abundances of these gases in a mass spectrometer. Arguments are presented for believing that the anomalous isotopic abundances observed are caused by spontaneous fission rather than by some other fission process. The spontaneous fission yield curve peaks were found to be much sharper than those associated with other fission processes. Evidence was found for fine structure in the fission yield curve at mass 132, possibly con- nected with preferential formation of spontaneous fission fragments containing 50 protons and 82 neutrons. Evidence for neutron induced fission in pitchblende was found. I. INTRODUCTION T HE first attempt to find spontaneous fission was made by Libby 1 who set a lower limit of 10 +14 years for this process. Spontaneous fission was first ob- served in uranium by Flerov and Petrzhak in 1940. 2 More recently the partial half-life for spontaneous fission of U 238 has been determined by Segre to be 8.0X 10 15 years and that of Th 232 to be 1.4X 10 18 years. 3 Segre also measured the U 235 spontaneous fission half- life and found it to be 1.7X10 17 years. Since this is longer than the U 238 half-life and the normal ratio of U 238 to U 235 is about 138, the contribution of U 235 to spontaneous fission in natural uranium is very small. These long half-lives make it very difficult to isolate and observe spontaneous fission products from uranium and thorium by conventional radiochemical techniques, and as yet no results have been obtained by this method. An alternative technique is to extract from uranium and thorium minerals the stable fission product isotopes which have been accumulating in the mineral through- out geological time. An observable change in the total quantity and in the isotopic abundances of fission product elements will be produced if the uranium or thorium concentration is sufficiently high, the mineral sufficiently old, and the concentration of the normal isotopes sufficiently low. The change in isotopic abun- dance may then be used to calculate relative spon- taneous fission yields. Because of their low terrestrial abundances 4,5 the inert gases are particularly suitable for an investigation of this kind. The spontaneous fission from one gram of * This work was supported in part by a grant from the National Science Foundation. Some of the work was done at the Argonne National Laboratory. > f U. S. Atomic Energy Commission predoctoral fellow. 1 W. F. Libby, Phys. Rev. 55, 1269 (1939). 2 K. A. Petrzhak and G. N. Flerov, J. Exptl. Theoret. Phys. (U.S.S.R.) 10, 1013 (1940); Chem. Abs. 35, 4677 (1941). 3 E. Segre, Phys. Rev. 86, 21 (1952). 4 K. Rankama and T. G. Sahama, Geochemistry (University of Chicago Press, Chicago, 1950), p. 771. 5 The abundance of normal krypton and xenon in minerals has not been previously reported. In the investigation reported in this paper about 10~ 10 cc at S.T.P./g of mineral were found. The normal krypton was about four times as abundant as the normal uranium produces about 10~ 7 cc at S.T.P. of Xe 136 in 300 million years. In a six-percent uranium mineral having this age the ratio of fission product Xe 136 to normal Xe 136 should be about sixty. Thus in radio- active minerals the total amount of xenon and krypton as well as the isotopic distribution should be very different from that which is found in ordinary minerals. In 1947 Khlopin, Gerling, and Baronovskaya 6 found that pitchblende contained much more xenon than is usually found in minerals and that the quantity of xenon found was in rough agreement with the assump- tion that the xenon was produced by spontaneous fission. In 1950 MacNamara and Thode 7 reported measurements of the isotopic abundances of xenon and kypton extracted from pitchblende. These writers stated that the abnormal isotopic abundance which they found were the result of spontaneous fission of uranium and in this way calculated spontaneous fission yields of krypton and xenon. Their hypothesis is not altogether tenable however since the discovery of about 10~ n gram of plutonium per gram of uranium in pitchblende 8 - 9 indicates that the neutron flux in this mineral is sufficiently high to cause appreciable neutron fission of uranium, thus partially masking the effects of spontaneous fission. As yet no theoretical calculations of the shape of the spontaneous fission yield curve have appeared in the literature. Fong 10 has calculated the shape of the slow neutron fission yield curve for U 235 and states that the same theory is applicable to spontaneous fission. Ac- cording to Frankel, 11 fission asymmetry is understand- able on the basis of barrier penetration. Since the re- duced mass of asymmetrical fragments is smaller, they should penetrate the barrier more readily. Hill and Wheeler 12 state that their collective model of the 6 Khlopin, Gerling, and Baronovskaya, Bull. acad. sci. U.R.S.S. Classe sci. chim. 599 (1947); Chem. Abs. 42, 3664 (1948). 7 MacNamara and H. G. Thode, Phys. Rev. 80, 471 (1950). 8 C. A. Levine and G. T. Seaborg, J. Am. Chem. Soc. 73, 3278 (1951). 9 Peppard, Studier, Gergel, Mason, Sullivan, and Mech, J. Am. Chem. Soc. 73, 2529 (1951). 10 Peter Fong, Phys. Rev. 89, 332 (1953). 11 S. Frenkel, J. Phys. (U.S.S.R.) 10, 553 (1946). 12 D. L. Hill and J. A. Wheeler, Phys. Rev. 89, 1102 (1953). 907

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P H Y S I C A L R E V I E W V O L U M E 9 2 , N U M B E R 4 N O V E M B E R 1 5 , 1953

Spontaneous Fission Yields from Uranium and Thorium* GEORGE W. WETHERILLI

Department of Physics, University of Chicago, Chicago, Illinois (Received August 13, 1953)

Relative spontaneous fission yields from uranium and thorium have been determined by extracting xenon krypton from geologically old uranium and thorium minerals and measuring the isotopic abundances of these gases in a mass spectrometer. Arguments are presented for believing that the anomalous isotopic abundances observed are caused by spontaneous fission rather than by some other fission process. The spontaneous fission yield curve peaks were found to be much sharper than those associated with other fission processes. Evidence was found for fine structure in the fission yield curve at mass 132, possibly con­nected with preferential formation of spontaneous fission fragments containing 50 protons and 82 neutrons. Evidence for neutron induced fission in pitchblende was found.

I. INTRODUCTION

THE first attempt to find spontaneous fission was made by Libby1 who set a lower limit of 10+14

years for this process. Spontaneous fission was first ob­served in uranium by Flerov and Petrzhak in 1940.2

More recently the partial half-life for spontaneous fission of U238 has been determined by Segre to be 8.0X 1015 years and that of Th232 to be 1.4X 1018 years.3

Segre also measured the U235 spontaneous fission half-life and found it to be 1.7X1017 years. Since this is longer than the U238 half-life and the normal ratio of U238 to U235 is about 138, the contribution of U235 to spontaneous fission in natural uranium is very small.

These long half-lives make it very difficult to isolate and observe spontaneous fission products from uranium and thorium by conventional radiochemical techniques, and as yet no results have been obtained by this method. An alternative technique is to extract from uranium and thorium minerals the stable fission product isotopes which have been accumulating in the mineral through­out geological time. An observable change in the total quantity and in the isotopic abundances of fission product elements will be produced if the uranium or thorium concentration is sufficiently high, the mineral sufficiently old, and the concentration of the normal isotopes sufficiently low. The change in isotopic abun­dance may then be used to calculate relative spon­taneous fission yields.

Because of their low terrestrial abundances4,5 the inert gases are particularly suitable for an investigation of this kind. The spontaneous fission from one gram of

* This work was supported in part by a grant from the National Science Foundation. Some of the work was done at the Argonne National Laboratory. >

f U. S. Atomic Energy Commission predoctoral fellow. 1 W. F. Libby, Phys. Rev. 55, 1269 (1939). 2 K. A. Petrzhak and G. N. Flerov, J. Exptl. Theoret. Phys.

(U.S.S.R.) 10, 1013 (1940); Chem. Abs. 35, 4677 (1941). 3 E. Segre, Phys. Rev. 86, 21 (1952). 4 K. Rankama and T. G. Sahama, Geochemistry (University of

Chicago Press, Chicago, 1950), p. 771. 5 The abundance of normal krypton and xenon in minerals has

not been previously reported. In the investigation reported in this paper about 10~10 cc at S.T.P./g of mineral were found. The normal krypton was about four times as abundant as the normal

uranium produces about 10~7 cc at S.T.P. of Xe136 in 300 million years. In a six-percent uranium mineral having this age the ratio of fission product Xe136 to normal Xe136 should be about sixty. Thus in radio­active minerals the total amount of xenon and krypton as well as the isotopic distribution should be very different from that which is found in ordinary minerals. In 1947 Khlopin, Gerling, and Baronovskaya6 found that pitchblende contained much more xenon than is usually found in minerals and that the quantity of xenon found was in rough agreement with the assump­tion that the xenon was produced by spontaneous fission. In 1950 MacNamara and Thode7 reported measurements of the isotopic abundances of xenon and kypton extracted from pitchblende. These writers stated that the abnormal isotopic abundance which they found were the result of spontaneous fission of uranium and in this way calculated spontaneous fission yields of krypton and xenon. Their hypothesis is not altogether tenable however since the discovery of about 10~n gram of plutonium per gram of uranium in pitchblende8-9 indicates that the neutron flux in this mineral is sufficiently high to cause appreciable neutron fission of uranium, thus partially masking the effects of spontaneous fission.

As yet no theoretical calculations of the shape of the spontaneous fission yield curve have appeared in the literature. Fong10 has calculated the shape of the slow neutron fission yield curve for U235 and states that the same theory is applicable to spontaneous fission. Ac­cording to Frankel,11 fission asymmetry is understand­able on the basis of barrier penetration. Since the re­duced mass of asymmetrical fragments is smaller, they should penetrate the barrier more readily. Hill and Wheeler12 state that their collective model of the

6 Khlopin, Gerling, and Baronovskaya, Bull. acad. sci. U.R.S.S. Classe sci. chim. 599 (1947); Chem. Abs. 42, 3664 (1948).

7 MacNamara and H. G. Thode, Phys. Rev. 80, 471 (1950). 8 C. A. Levine and G. T. Seaborg, J. Am. Chem. Soc. 73, 3278

(1951). 9 Peppard, Studier, Gergel, Mason, Sullivan, and Mech, J. Am.

Chem. Soc. 73, 2529 (1951). 10 Peter Fong, Phys. Rev. 89, 332 (1953). 11 S. Frenkel, J. Phys. (U.S.S.R.) 10, 553 (1946). 12 D. L. Hill and J. A. Wheeler, Phys. Rev. 89, 1102 (1953).

907

908 G E O R G E W . W E T H E R I L L

FIG. 1. Apparatus for the extraction of inert gases from minerals. A. Magnetic valve; B. Stirring rod; Ci. Tubes con­taining 5 g of activated charcoal; C2. Tubes containing 5 g of activated charcoal; C3. Tube containing 5 g of activated char­coal; C4. Tube containing 10 g of activated charcoal; D. Nickel furnace; E. Electric heater winding; F. Xe128 tracer; G. Mag­nesium perchlorate trap; H. Potassium hydroxide trap; J. Cupric oxide trap; K. Magnesium perchlorate trap; L. Calcium vapor furnace; M. Quartz-pyrex graded seal; N. Neon sample tube; O. Argon, krypton, xenon sample tube; P. McCleod gauge; Q. McCleod exhust; R. Tungsten bulb; S. Copper oxide trap; T. Magnesium perchlorate trap.

nucleus is not inconsistent with fission asymmetry, but no quantitative statements regarding this are made in their paper.

In this investigation spontaneous fission yields are obtained for uranium and thorium by extracting xenon and krypton from euxenite [(Y, Er, Ce)203U02Nb205-Ti02H2] and monazite [(Ce, La, Nd, Pr)P04], re­spectively, and analyzing these gases in a mass spec­trometer. Reasons are given for believing that the yields obtained actually represent spontaneous fission rather than some competing fission process.

II. EXPERIMENTAL METHODS

In order to measure the isotopic abundances of the inert gas contained in a mineral, it is necessary to prepare the gas samples in a form suitable for analysis in a mass spectrometer. To accomplish this it is neces­sary that the gases be removed from the mineral being investigated, that they be purified' of chemically re­active gases such as oxygen, nitrogen, hydrogen, and hydrocarbons. It is also necessary to remove the large quantity of helium which is always found in radio­active minerals in order that the sample be sufficiently small for analysis. These requirements were met by use of the greaseless high vacuum apparatus shown in Fig. 1.

After all parts of this apparatus were thoroughly degassed by heating, the system was isolated from the pumps and the finely divided mineral sample was dropped into the nickel electric furnace (D) by raising the magnetic valve (A) with an alnico magnet. In this furnace the mineral was fused with NaOH, releasing the gases occluded in the mineral. As the gases were being released, a known quantity of Xe128 (F) was added

in order to measure the quantity of xenon released. The first magnesium perchlorate trap (G) removed the water which was evolved from the mineral. The CuO trap (J), operated at 400°C, reacted with H2 and CO oxidizing these gases to H20 and C02, respectively. The KOH trap (H) reacted with C02 to form K2C03

and water. This reaction is greatly facilitated by the presence of water vapor, but it was not necessary to add water since a small amount always comes through the first trap. The water produced by these reactions was taken up by the second magnesium perchlorate trap (K). Most of the remaining chemically reactive gases were removed by volatilizing calcium in the calcium furnace (L), where such compounds as CaO, Ca3N2 and CaC2 were formed. After this treatment the charcoals (Ci, C2, C3) were brought to — 195°C with liquid nitrogen and the helium and neon pumped off with a Toepler pump through V5 into the "gas sepa­rator" and retained for further studies. When no further decrease in pressure could be effected by the use of the calcium furnace, the residual gases were transferred by use of the charcoals into the final purifier. Here the remaining hydrocarbons were cracked on the hot tungsten filament of the'tungsten bulb (R). The hydrogen produced was oxidized by the CuO trap (S) and the water formed was absorbed by the magnesium perchlorate (T). After the total quan­tity of gas had been reduced to between 10~2 and 10~3

cc at S.T.P. the gas was transferred to C2 and the samples sealed off. This sample contained the argon, krypton, and xenon extracted from the mineral.

The gas samples were analyzed in a 60°, 12-in. radius of curvature mass spectrometer especially de­signed for work with small gas samples. High sensi­tivity was obtained by the use of an electron multiplier in the ion collector, and a resolving power of two thousand was achieved by the use of 0.002-in. defining slits in the ion source and an adjustable defining slit in the collector, together with differential pumping be­tween the source region and the analyzer. This differ­ential pumping permitted the high pressure necessary in the source for an intense ion beam without intro­ducing pressure scattering in the analyzer region. Background impurities were kept low by eliminating the usual greased joints and stopcocks both in the high-vacuum part of the mass spectrometer and in the sample system supplying the gas to be analyzed to the machine. Both the analyzer and source vacuum system employed two liquid nitrogen traps in series. The main mercury diffusion pump was backed by an ejector stage mercury diffusion pump which in turn was backed by a mechanical pump. Another liquid nitrogen trap was located between the mechanical pump and this diffusion pump in order to prevent oil from the mechanical pump diffusing into the high vacuum. The entire high-vacuum portion of the instrument was fre­quently baked at over 300°C for several days by the use of nichrome heaters wound directly onto the glass

F I S S I O N Y I E L D S F R O M U A N D T h 909

and metal tubing. This precaution was necessary in order to reduce the background to the level required by the investigation.

The sample tubes containing the gases adsorbed on charcoal were sealed onto the sample system of the mass spectrometer, and the gas introduced by the use of conventional glass breakoffs. The krypton and xenon were desorbed successively by changing the temperature of the charcoal. The quantities of xenon and krypton analyzed were of the order of 10-7 cc S.T.P. With the resolving power of two thousand which was obtainable with this instrument it was possible to resolve the hydrocarbon impurities from the inert gas which was being studied. This was espe­cially important at masses 82 and 130 where small peaks had to be measured in order to correct for normal krypton and xenon in the samples.

The data were recorded on a Brown chart recorder. Because the peak heights decay as the gas supply is depleted, it was necessary to correct for this decrement. An exponential correction was used. In some cases it was necessary to correct for the change in isotopic com­position with time, since the lighter isotopes flow through the leak more rapidly than the heavy ones, causing the gas remaining in the reservoir behind the leak to become enriched in the heavier isotopes. Molecular flow was maintained at all times, so it was not necessary to correct for fractionation of the isotopes in flowing through the apparatus. Since the sensitivity of the electron multiplier varies slightly with mass it was necessary to correct for this discrimination. The discrimination was determined by running samples of normal gas and comparing the results with the accepted values of the isotope ratios as given by Bainbridge and Nier.13

III. EXPERIMENTAL RESULTS

A. Spontaneous Fission Yields from Uranium

Spontaneous fission yields in the region of krypton and xenon were obtained by processing the mineral euxenite as described in Sec. II. The results of the isotopic analysis, after making corrections for normal

TABLE I. Yields of xenon and krypton isotopes from the spontaneous fission of U238.

Mass

136 134 132 131 129

Xenon Yield* i

109.6-g sample

6.00 4.99 ±0.07 3.57 ±0.06 0.455±0.02

<0.012

(percent) 33.8-g sample

6.00 4.91 ±0.06 3.51 ±0.07 0.474±0.02

<0.02

Mass

86 84 S3

K r y p t o n Yieldb (percent)

109.6-g sample

0.75 ±0.11 0.119±0.040 0.036±0.015

a Xe136 yield taken as 6.00 percent. b Xe/Kr ratio determined by calibration of mass spectrometer with

stable isotope tracers.

13 K. T. Bainbridge and A. O. Nier, Relative Isotopic Abundances of the Elements, Preliminary Report No. 9, Nuclear Science Series, National Research Council, 1950 (unpublished).

TABLE II. Neutron sources in Madagascar euxenite.

Neutron Yield source (Neutrons/g sec)

Cosmic raysa 10~4

Spontaneous fission b 8.8 X 10~4

(a,n) reactions0 10~3

a See footnote 15. b See footnote 16. c See footnote 17.

krypton and xenon, are shown in Table I. The normal corrections were made by assuming the Kr82 and Xe130

peaks to represent normal krypton and xenon re­spectively. By analogy to neutron fission the inde­pendent yields at masses 82 and 130 should be less than 10~4 percent and may be neglected. The ratio of fission xenon to normal xenon was 23.4; the ratio of fission krypton to normal krypton was 0.58. The errors shown in Table I represent the mean deviations of the peak heights. Xenon measurements on two samples of euxenite are reported; the large sample is believed to give the most accurate data because in this case the peaks were sufficiently large to effectively eliminate background errors. Krypton results are re­ported for the large sample only. Data were obtained from a number of other samples all of which showed the same yields within their larger experimental errors; the yields presented in Table I represent the most accurate results which were obtained. The results are normalized by assigning a six percent yield to mass 136, thus allowing the results to be compared with U236 slow neutron fission yield curve. This comparison is shown in Figs. 2 and 3.

The limit at mass 129 represents the accuracy of the normal correction to the peak at mass 129. The large errors at masses 83 and 84 are caused by the error in the normal correction at these masses; the error at mass 86 represents the accuracy of the Xe/Kr ratio which in turn depends on the accuracy of calibration of the stable isotope tracers which were used to calibrate the mass spectrometer.

In order to make sure that the yields presented in Table I actually represent spontaneous fission, it is necessary to evaluate the relative importance of other possible causes of fission in the mineral, and compare the rate of these other fission processes with the spontaneous fission rate in the mineral. Since the sample of euxenite studied contains six percent uranium the spontaneous fission rate is about 4X10~4 fission /g sec. Fission by alpha particles may be excluded be­cause the penetration coefficient of the uranium coulomb barrier for alpha particles is less than 10~10. Similarly, the effect of cosmic-ray mesons may be neglected since according to the measurement of George and Evans14 the number of /x mesons stopped per gram of absorber at a depth of 20 meters of water

14 E. P. George and J. Evans, Proc. Phys. Soc, (London) A64, 193 (1951).

910 G E O R G E W . W E T H E R I L L

SMOOTHED OUT U 2 3 5 (n, f ) YIELD CURVE—]

E U 2 3 8 SPONTANEOUS FISSION POINTS §

.01 40 60 80 100 120

MASS 140 160

P'IG. 2. Comparison of U238 spontaneous fission and U235 slow neutron fission yields.

equivalent underground is less than 10~6 per second. The 7r-meson intensity is much less. The importance of neutron fission can be estimated by considering the yields of various possible sources of neutrons in the mineral as shown in Table II.15-17

The neutron production rate is therefore about 2X10~3 neutron/g sec. The mineral sample studied was analyzed chemically and found to contain about one percent gadolinium and one percent dysprosium. Because of the large thermal neutron cross sections of these elements about one in three thousand of the neutrons captured in the mineral will be captured in U235. Since many of the neutrons will escape from the mineral without being thermalized, the rate of neutron fission of U235 is less than 7XK)-7/gsec. Because U238

will not fission by capture of slow neutrons, only fast neutron fission of this isotope need be considered. Since the euxenite occurs as small lumps of less than one cc volume embedded in large masses of rock, most of the fast neutrons produced in the mineral will escape into the surrounding rock. Using a fast neutron cross

15 Korff and Hamermesh showed that the cosmic ray neutron intensity is about 10~4 neutrons/g sec at a depth of six meters of water equivalent from the top of the atmosphere. I t should be much less underground. S. A. Korff and B. Hamermesh, Phys. Rev. 69,155 (1946).

16 The value *>=2.2 was used. See footnote 3. 17 The yield of neutrons from the (a,n) reactions was estimated

using the results of measurements of the amount of excess Ne21

and Ne22 produced in these minerals by the (a,n) reaction on O18

and F19 respectively. These results and this calculation will be described in a separate paper.

section of 2.9X10~25 cm2, the number of fast neutron fissions induced in U238 per gram per second is less than 10"~6. Thus the effect of fast neutron fission can also be neglected. The only gamma rays of sufficient energy to cause fission are the neutron capture gammas; the number of these is approximately equal to the number of neutrons. Since the photofission cross sections are smaller than the fast neutron fission cross sections, and the total cross section for absorption of the gammas is comparable to that for the fast neutrons, the effect of photofission must also be negligible. For these reasons one may have some confidence that the fission yields obtained truly represent spontaneous fission of U238.

From the quantity of lead and uranium in this mineral the age is calculated to be about 600 million years. However the quantity of helium and spontaneous fission xenon which was found in the mineral indicates an age of only about 50 million years. This low gas age might lead one to suspect that the mass-129 yield ob­tained is too low, the Xe129 yield being held up by the 1.7Xl07-year I129. This is probably not the case how­ever, because as long as the I129 remains in equilibrium with the uranium the measured Xe129 yield will be correct, even though the inert gases may leak from the mineral either slowly over the entire lifetime of the mineral or all at once 50 million years ago. Since the I129 is produced as a "hot atom," it will react quickly with one of the common elements in the mineral and should not be removed by leakage along with the inert gases. In the unlikely event that the I129 was separated from the mineral by a sudden outgassing process 50 million years ago, the upper limit for the Xe129 yield in Table I should be raised to 0.02 percent. This does not affect the general features of the spontaneous fission yield curve.

It may be noted that the spontaneous fission yields, shown in Figs. 2 and 3, are even more asymmetric than

7

i 6 UJ

u * 5

ou 235 SLOW NEUTRON FISSION

• i U 2 3 8 SPONTANEOUS FISSION

126

FIG. 3. Com­parison of U238

spontaneous fis­sion and U235 slow neutron fission showing the dif­ferences in the fine structure in the region of xenon.

F I S S I O N Y I E L D S F R O M U A N D T h 911

neutron fission yields, the most striking effect being at mass 129 where the yields differ by a factor of about one hundred. Furthermore, the yield at mass 131 is about 50 percent lower that that reported by Mac-Namara and Thode,7 and the yield at mass 129 is lower than that reported by these investigators by a factor of at least seven. The krypton results are also significantly different. This is the effect which might be expected if MacNamara and Thode had observed a mixture of neutron-induced and spontaneous fission. The sharpness of the spontaneous fission yield peaks is in agreement with Glendenin's and Steinberg's18 work on curium. In experiments on high-energy fission it is found that the fission becomes a more symmetric when the excitation energy of the nucleus is increased.19

In spontaneous fission where there is no excitation, it might be expected that the fission would tend to be even less symmetric than in thermal neutron fission, as was found in this investigation.

It should also be noted that it is difficult to place the mass-132 point on a smooth curve containing the other points. This is an indication of fine structure similar to that found in neutron fission by MacNamara, Collins, and Thode,20 and by Glendenin, Steinberg, Inghram, and Hess.21 This "spike" at mass 132 may indicate preferential formation of spontaneous fission fragments containing 50 protons and 82 neutrons. It may be pointed out that the barrier penetration theory of Frenkel22 predicts a peak to trough ratio of only about ten, whereas the experimental value as reported in this paper is greater than 500.

B. Spontaneous Fission of Thorium

Spontaneous fission of thorium is much more difficult to detect because the half-life for the process is 168 times as long as for the spontaneous fission of uranium.3

For this reason it is necessary to obtain a mineral sample with a high ratio of thorium to uranium and

TABLE III . Comparison of spontaneous fission xenon from monazite with U238 spontaneous fission yields.

Mass

136 134 132 131 129

Monazite yielda

(percent)

6.00 5.12 ±0.10 3.63 ±0.08 0.509±0.02 0

U238 yield (percent)

6.00 4.99 ±0.07 3.57 ±0.06 0.455±0.02

<0.012

a Xe136 yield taken as 6.00 percent. Xe129 taken to be 100 percent normal xenon.

18 E. P, Steinberg and L. E. Glendenin, U. S. Atomic Energy Commission Declassified Document, A.E.C.D. 3520, 1953 (un­published).

19 R. W. Spence, U. S. Atomic Energy Commission Unclassified Document BNL-C-9, 1949, Brookhaven Chemistry Conference No. 3 (unpublished).

20 MacNamara, Collins, and Thode, Phys. Rev. 78, 129 (1950). 21 Glendenin, Steinberg, Inghram, and Hess, Phys. Rev. 84, 860

(1951). 22 S. Frenkel, J. Phys. (U.S.S.R.) 10, 553 (1946).

FIG. 4. Spontane- ^ Q L /f / eous fission yields > * | I J from The232 in the Af / region of xenon. JI f- A'

Oh — - ^ I I I 1 I 1 I

128 130 132 134 136 MASS

which has other desirable characteristics, the most im­portant of which are great age, small leakage, and high concentration of thorium. The best mineral which could be obtained for this investigation was monazite [(Ce, La, Nd, Pr)P04]. This mineral usually contains about six percent thorium and a few tenths percent uranium. Although a higher thorium to uranium ratio would be very desirable, some information concerning thorium spontaneous fission yields can be obtained from a mineral of this kind.

The most accurate data for xenon was obtained from a sample of Ceylon monazite sand, containing 7.79 per­cent thorium and 0.301 percent uranium and having an age of 530 million years. The xenon isotopic abundances found in this mineral are shown in Table III. If one assumes a ratio of the thorium to uranium yield at mass 136 together with the thorium and uranium concentra­tions given above and the known spontaneous fission half-lives, it is possible to determine within rather wide limits the thorium spontaneous fission yields at other mass numbers. The results of this calculation are shown in Fig. 4. It may be noted that again there is evidence for fine structure at mass 132. Thus, while these results for thorium are not very precise, they show that thorium spontaneous fission yields are similar to those from uranium.

A calculation of the mass-136 fission yield based on the age of the mineral, the quantity of gas obtained as measured with Xe128 tracer, and the known spontaneous fission rates of uranium and thorium, gives a result of 3.3 percent. Thus the quantity of gas obtained agrees in order of magnitude with the assumption that the anomalous abundances are caused by spontaneous fission. A more exact measurement of the absolute spontaneous fission yield could not be made because of uncertain effects cause by leakage of gas from the mineral, difficulty in equilibrating the evolved gas with the tracer, and possible errors in the calibration of the tracer.

Krypton results were obtained from another sample of monazite of Brazilian origin. These are shown in Table IV, and it may be seen that again there is no great difference from the euxenite results.

An estimation of the importance of other causes of fission in monazite can be made as was done earlier in the case of euxenite. In this connection it should be pointed out that for the greatest part of their life the monazite sand grains were a minor constituent of a

912 G E O R G E W . W E T H E R I L L

TABLE IV. Comparison of spontaneous fission krypton from monazite with U238 spontaneous fission yields.

Monazite yielda U238 yielda

Mass (percent) (percent)

86 0.87 ±0.12 0.75 ±0.11 84 0.180±0.040 0.119±0.040 83 0.036±0.025 0.036±0.015

a Normalized to Xe136=6.00 percent. Xe/Kr ratio determined by cali­bration of mass spectrometer with stable isotope tracers.

granite and fast neutrons produced in the monazite would have been lost in the surrounding rock. Further­more it should be noted that this mineral contains large quantities of the rare earth elements which will be effective in absorbing slow neutrons. Again the con­clusion is that other causes of fission are negligible com­pared to spontaneous fission.

C. Neutron Fission in Pitchblende

Xenon and krypton extracted from a twenty-gram sample of Belgian Congo pitchblende containing 44 percent uranium indicated the presence of neutron fission as well as spontaneous fission. The yields of fission xenon and krypton from the sample are shown in Table V.

It may be noted that the fission yields in the pitch­blende fall off much more slowly on the sides of the fission yield peaks than, is the case in euxenite and monazite. By using the U238 spontaneous fission yields as determined from the euxenite sample and the U235

slow-neutron fission yields of Thode and Graham23 it is possible to claculate the relative contributions of neutron-induced and spontaneous fission in this mineral. The result of this calculation is that about 35 percent of the fission is neutron-induced. If an estimate of the neutron production rate is then made in the same way as was done in the case of euxenite, it is found that about ten percent of the neutrons produced in the

23 H, G. Thode and R. L. Graham, Can. J. Research A25, 1 (1947).

pitchblende induce a neutron fission. The pitchblende sample used was 650 million years old; since the half-life of U235 is only 700 million years, considerably more neutron fission may have occurred in a pitchblende of similar constitution but with an age of 2100 million years, since at the time of formation of this mineral the U235 abundance was about six percent. This may be re­lated to the fact that the oldest pitchblende or uraninite deposits which have been found have an age of about 2100 million years, whereas the earth itself is con­siderably older.

IV. ACKNOWLEDGMENT

The writer wishes to express appreciation to C. Patterson for instruction in the chemical techniques

TABLE V. Comparison of fission yields of xenon and krypton from pitchblende with U238 spontaneous fission yields.

Mass

136 134 132 131 129 86 84 S3

Belgian Congo pitchblende yield

(percent)

6.00 5.62 ±0.05 3.69 ±0.04 1.18 ±0.02 0.190±0.004 1.26 ±0.18 0.401±0.06 0.166±0.03

U238 spontaneous fission yield

(percent)

6.00 4.99 ±0.07 3.57 ±0.06 0.455±0.02

<0.012 0.75 ±0.11 0.119±0.040 0.036±0.015

used in lead analyses which were made to determine mineral ages, and for the use of some purified reagents, to G. Til ton for several uranium analyses, to O. Joensuu for emission spectrometric lead analyses, to D. F. Peppard and J. P. Marble who supplied mineral samples, and to R. J. Hayden who constructed the mass spectrometer which was used in this investigation and who prepared the xenon and krypton tracers which were used.

Special thanks must be given to M. G. Inghram whose counsel and encouragement guided this work from its inception to its completion.