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Mass distribution of 3MeV neutron induced fission of 232Th
J. inorg, nucl. Chem., 1971, Vol. 33, pp. 3643-3647. PergamonPress. Printedin Great Britain
MASS
DISTRIBUTION OF 3MeV NEUTRON INDUCED FISSION O F 232Th
D. L. S W I N D L E ,
D. T. M O O R E , J. N. B E C K and P. K. K U R O D A
D e p a r t m e n t of Chemistry, University of A r k a n s a s . Fayetteville, A r k a n s a s 72701
(First received 8 February 1971 ; in revised form 4 March 1971 ) A b s t r a c t - - T h e 3 M e V neutron induced fission yields of 232Th in the mass regions o f A = 77-93 and `4 = 141-156 were radiochemically determined by isolating fission products belonging to 15 m a s s chains. T h e absolute activities for the nuclides were determined by/3- and y-counting and the cumulative fission yields were calculated relative to 99Mo. T h e s e data were used to define the extreme wings of the m a s s yield curve for Z32Th. By comparison of c o m p l e m e n t a r y fission fragments it was determined that 2-4 s e c o n d a r y neutrons are emitted per fission of 232Th at 3 MeV. A qualitative comparison is made concerning the general features of the 2:~Th m a s s yield curves from 14.8 M e V and 3 M e V neutron induced fission. INTRODUCTION
LOW-ENER6Y fission of ~32Th has been studied in the past using pile neutrons [ 1-3] and using mono-energetic neutrons of 3 MeV, produced in the D(D, n)3He reaction [4-6]. Since these early studies had established the peak positions of the mass yield curve, the purpose of this investigation was to define the outer-most wings of the mass yield distribution curve for 2~2Th fission induced by 3 MeV neutrons. The left wing (A = 77-81 ) was established through separation of arsenic and selenium fractions and the right wing (A = 141-156) was defined by separation of the rare earths, both as a group and individually. EXPERIMENTAL T h e ~:~2Th used in this work was reagent-grade thorium nitrate (Bakers' analyzed). A b o u t 20 g of thorium nitrate were irradiated with the 2.95 + 0.08 M e V neutrons [7] produced in the D (D, n):~He reaction using the University of A r k a n s a s ' Cockcroft-Walton positive ion accelerator. T h e irradiation times varied from I to 4 hr depending on the half-life of the nuclide to be studied. T h e neutron flux was typically I - 5 x l0 s n e u t r o n s / s e c , cm ~. Determinations were carried out at least 3 times to assure better statistical results.
Radiochemical procedures Arsenic. T h e arsenic fraction was radiochemically separated using the procedure of W a r d et al.[8]. 1. R. H. lyer, C. K. M a t h e w s , N. Ravindron, K. Rengan, D. V. Singh, M. V. R a m a n r a h and H. D. Sharma, J. inorg, nucl. Chem. 25, 465 (1963). 2. A. T u r k e v i c h a n d J . B. Niday, Phys. Rev. 84, 52 (1951). 3. M. Bresesti, G. Burei, P. Ferrari and L. Moretto, J. inorg, nucl. Chem. 29, 1189 (1967). 4. K. M. Broom, Phys. Rev. 133,874 (1964). 5. S.J. Lyle, G. R. Martin and Md. M. R a h m a n , Radiochem. Acta 9, 90 (1968). 6. S.J. Lyle and J. Sellers, Radiochem..4 cta 12, 43 (1969). 7. R . W . Fink, Nucleonics 17, 94 (1959). 8. T. E. Ward, D. L. Swindle, R. J. Wright and P. K. Kuroda, Radiochem..4 cta 14, 70 (1970). 3643
3644
D . L . S W I N D L E et al.
The basic decontamination steps were: (a) extraction of As a+ into CC14 from _>- 10 N HCI solutions; (b) back-extraction into water; and (c) precipitation of As as the metal for B-counting. Selenium. The procedure reported by McGee et al.[9] was employed and the Se fraction was extracted into a b e n z e n e - l % phenol mixture from 4 N HBr solutions. The Se was finally reduced to the metal for B-counting. Rare earths. After the bulk of the thorium was removed by extraction into di(2-ethyl hexyl) phosphoric acid in n-heptane[10], the rare earths were precipitated as the fluorides. Following several scavenging steps, the individual rare earths were separated from one another by a Dowex 50W x 8 cation exchange resin column [ 11]. A gradient elution technique was used which employed a-hydroxyisobuteric acid of 0"45 M and 0.20 M concentration. The entire chemical procedure including total elution usually required 18 hr. The individual rare earths were precipitated as oxalates for B-counting.
Counting procedures A Beckman low-background B-counting system was employed for the activity measurements of the various fission products. The background of this system was about 0.5 cpm. The samples were mounted using the "filter-stick" method which distributed the precipitate evenly over a known area. Corrections for self-scattering and self-absorption (f~sa factors) were determined for each nuclide according to the method of Nervik and Stevenson [23]. A bsolutefission yield of ~Mo As in many other fission product yield studies, 99Mo was chosen as the reference nuclide. In order to convert the relative yields of the other nuclides into absolute yields, the agMo absolute fission yield was determined. Previously, this had been achieved [4] by using the 197Au(n, T)1aSAu reaction (cross section 49 mbarn) as a monitor. In the present investigation indium foil was utilized as a neutron flux monitor. This method is based on the reaction, 115In (n, n')llsmln which has a threshold at 0.360 MeV and an activation cross section of 360 mbarns at neutron energies of 2.8-3-0 MeV [ 19]. Some advantages of this method are: (a) Due to the larger cross section and the shorter half-life of the product nuclide, much more H~mln activity would be produced in any given bombardment than for ~aSAu. This would certainly lead to better statistical results in the counting data. (b) The 4"5 hr ~15mlnactivity can be easily observed by counting the 334 keV internal transition representing 95 per cent of the total decay. (c) Indium foil is much cheaper than gold and can be disposed of after each experiment, thus possible contamination can be eliminated in future experiments. The 232Th sample was placed between double layers of In foil (95% HSln) and irradiated with 3 MeV neutrons for I hr. After allowing the short-lived Mo isotopes to decay completely, aaMo was separated by the radiochemical procedure reported elsewhere[22]. By absolute y-spectroscopy on both the Mo sample and the In monitor the absolute activity of each nuclide was obtained and decay corrected back to the end of irradiation. The absolute yield of ~Mo was calculated from considerations and equations discussed by Husain [13]. The actual equation used was
Y99= "4 99N s f °'s ( 1 - e-x st ) AsNThfO'Th F ( 1 -- e -~'~t)
where ,499 = absolute activity of 99Mo at the end of irradiation As = absolute activity of~lSmln at the end of irradiation 9. 10. 11. 12. ! 3.
T. McGee, J. Lynch, and G. G. J. Boswell, Talanta 15, 1435 (1968). D. F. Peppard, G. W. Mason, J. L. Maier and W. J. Driscoll, J. inorg, nucl. Chem. 19, 334 (1957). K. Wolfsberg,Anal. Chem. 34, 518 (1962). M. Thein, M. N. Rao and P. K. Kuroda, J. inorg, nucl. Chem. 30, 1145 (1968). L. Husain, Ph.D. Thesis, University of Arkansas, pp. 12-13 (1968).
Mass distribution N, = NTn = f= o-, = ~rT~r = Y~9 =
3 64 5
number of 11'~1natoms in the monitor foil the number of Z32Thatoms in the sample the 3 MeV neutron flux (neutrons/cm 2 . sec) the cross section for the (n, n' ) reaction on "5In, taken as 360 mbarn [19] neutron induced fission cross section of ~32Th at 3 MeV taken as 170 mbarn [4, 20], and the absolute fission yield of 99Mo.
Treatment of data The gross B-decay curves were resolved by standard graphical methods and all activities were decay corrected back to the end of irradiation. The total number of atoms (Ni) of a radioactive species produced in an irradiation of time (t) was determined by Ni
:
Ai - (1--e ~')
t
where A~ is the activity (dpm) of A at the end of irradiation, t is the duration of the irradiation and ~, is the decay constant ofA. The absolute fission yield of 99M0, as described earlier, was determined to be 3-0 _+0"05 per cent. All relative fission yields were calculated by the following equation: Yg, Yi = ~ " Ni where Yi is the fission yield of nuclide i, Y99 is the absolute fission yield of ~°Mo, Ngo is the number of atoms of 99M0 and N~ is the number of atoms of nuclide i produced in the irradiation. The equal charge displacement hypothesis as modified by Pappas[14] was used to calculate fractional chain yields for all nuclides measured in this work. However, no significant change in the cumulative yield of any nuclide was affected. The composite decay of the cerium fraction [141Ce+143Ce(143pr)+144Ce(144pr)] was handled in the same manner as described by Thein et al.[12] except that ~44Ce(285 d) was not observed due to its long half-life and the low neutron flux. The extremely low count rate of the Eu fraction made the corresponding fission yields unreliable and thus are not reported. Similar conditions required a large error limit on the 77As cumulative yield. The branching ratio for the mass chain 77 has been discussed previously [21]. RESULTS AND DISCUSSION T h e c u m u l a t i v e f i s s i o n y i e l d s f o r 15 m a s s c h a i n s a r e g i v e n in T a b l e 1, a n d s e r v e to d e f i n e t h e e x t r e m e w i n g s o f t h e m a s s y i e l d c u r v e o f 23ZTh. T h e e r r o r l i m i t s r e s u l t f r o m s m a l l u n c e r t a i n t i e s in d e c a y c u r v e fitting, c h e m i c a l y i e l d d e t e r m i n a t i o n , n e u t r o n flux m e a s u r e m e n t , e t c . T h e m a s s y i e l d c u r v e f o r 232Th f i s s i o n i n d u c e d b y 3 M e V n e u t r o n s is s h o w n in F i g . 1. T h e c u r v e a l s o i n c l u d e s d a t a b y B r o o m [ 4 ] a n d L y l e et al.[5] a n d d e f i n e s the general features of the mass yield curve. T h e n u m b e r (v) o f s e c o n d a r y n e u t r o n s e m i t t e d b y a p a r t i c u l a r f r a g m e n t p a i r was determined from the relation v = Mc-
(MH+Mr~)
where Mc = mass of the compound nucleus upon excitation by an incoming neutron, and M n a n d M r = m a s s e s o f h e a v y a n d light f i s s i o n f r a g m e n t s r e s p e c t i v e l y . 14. A.C. Pappas, M.I.T. Rep. No. 63 (September 1953).
D. L. S W I N D L E et al.
3646
Table 1. Cumulative chain yields in the 3 M e V neutron induced fission of'-'32Th Mass Nuclide chain measured 77 78 79 81 91 93 99 141 143 145 147 149 151 153 156
rTAs ~SAs 79As slSe 91y a3y aaMo 141Ce ~43Ce mPr 147Nd ~4apm 15apm 153Sm ~Sm
Half-life
Fission yield (%)
39 hr 0'010 -- 0.005 91 min 0.036___0.004 9 min 0.075 +-0"008 18 min 0'50 ___0.05 59d 6.9--_0"7 10"1 hr 7"35 ±0.07 66"7 hr 3"00 _ 0.05 32-5 d 6"6 ±0.7 33 hr 6.0 - 0"6 5.9 hr 4"7 ± 0"5 11"1 d 2"2---0"2 53hr 1.0±0.1 28hr 0.10±0.02 47 hr 0.030 ---0.004 9.4 hr 0.0013 ±0.0005
MM was obtained by taking a measured relative yield from the experimental data and ML was then found by interpolation between two measured yields on the complementary wing. Values for u determined by this method were consistently 2.3-2.4 secondary neutrons emitted per fission of "3ZTh. These results are in good agreement with the previously reported work of Lyle et a/.[5]. It must be noted however, that this method of determining v is not too accurate in the regions of high yields where most of the fissions take place. Figure 1 also shows the distribution of mass in the 14.8 MeV neutron induced fission of 2a2Th, as well as symmetric fission yields from reactor neutron induced fission of Z3ZTh reported by Iyer et a/.[1]. The curve representing 14.8 MeV studies was constructed from experimental data reported by Broom[15], Ganapathy and Kuroda[16], Mo and Rao[17], Thein et al.[12], and Swindle et al.[21]. Figure 1 shows clearly that as the energy of the incident particle is increased, the ratio of the probability of symmetric fission to asymmetric fission increases and thus the peak to valley ratio decreases. The broadening or "splaying out" of the outer wings at 14.8 MeV indicates the increased probability of highly asymmetric fission. This same phenomenon in the case of 235U has been previously discussed by Hyde [ 18]. 15. 16. 17. 18. 19. 20. 21. 22. 23.
K.M. Broom, Phys. Rev. 126, 627 (1962). R. Ganapathy and P. K. Kuroda, J. inorg, nucl. Chem. 28, 2071 (1966). Tin Moand M. N. Rao, J. inorg, nucl. Chem. 30, 345 (1968). E. K. Hyde, Nuclear Properties of the Heavy Elements Vol. III. pp. 104. Prentice-Hall, N e w Jersey (1964). G. Weber and M. Guillaume, Radiochem. Radioanal. Lett. 3, 97 (1970). A. Gerezin, G. A. Stolarov, Y. V. Nikolskii and 1. E. Chelnokov, At. Energ. 5,659 (1958). D. L. Swindle, R. J. Wright, T. E. Ward and P. K. Kuroda, J. inorg, nucl. Chem. 33, 651 (197 i). K. M. Broom, Ph.D. thesis, University of Arkansas, pp. 25-27 (1963). W. E. Nervik and P. C. Stevenson, Nucleonics 10, 18 (1952).
M a s s distribution
3647
/
5.0
/
//
!
-" \x. //
1.0
I
~t
I
V 0.5
D _J UJ
>Z 0 o3 O3 h
0.1
0.05
0.01
0.005
0.002
70
OO
90
100 tl0 120 150 140 150 MASS NUMBER
t60
Fig. 1. M a s s yield distribution of 232Th. O - T h i s work; O - B r o o m ; A - L y l e et al.; ..... / X - lyer et al. (Reactor neutron fission of Z32Th); . . . . . 14'8 M e V neutron fission of 232Th; × - Mirror points calculated using v = 2.4.
lyer e t a/.'s[1] discovery of a third peak in the reactor neutron induced fission of 232Th suggests the existence of a similar peak in the symmetric fission region of Z32Th induced by 3 MeV neutrons, although we were unable to confirm this experimentally in the present work. Acknowledgements--The a u t h o r s would like to thank the United States Atomic Energy C o m m i s s i o n for support of this work through contract n u m b e r At-(40-1)-3235. We are also grateful to Mr. D. Coffield and Mr. M. Myers, for operation of the accelerator and help in drafting, respectively. Special t h a n k s go to Drs. T. E. W a r d and J. L. M e a s o n for their valuable advice.
MASS
DISTRIBUTION OF 3MeV NEUTRON INDUCED FISSION O F 232Th
D. L. S W I N D L E ,
D. T. M O O R E , J. N. B E C K and P. K. K U R O D A
D e p a r t m e n t of Chemistry, University of A r k a n s a s . Fayetteville, A r k a n s a s 72701
(First received 8 February 1971 ; in revised form 4 March 1971 ) A b s t r a c t - - T h e 3 M e V neutron induced fission yields of 232Th in the mass regions o f A = 77-93 and `4 = 141-156 were radiochemically determined by isolating fission products belonging to 15 m a s s chains. T h e absolute activities for the nuclides were determined by/3- and y-counting and the cumulative fission yields were calculated relative to 99Mo. T h e s e data were used to define the extreme wings of the m a s s yield curve for Z32Th. By comparison of c o m p l e m e n t a r y fission fragments it was determined that 2-4 s e c o n d a r y neutrons are emitted per fission of 232Th at 3 MeV. A qualitative comparison is made concerning the general features of the 2:~Th m a s s yield curves from 14.8 M e V and 3 M e V neutron induced fission. INTRODUCTION
LOW-ENER6Y fission of ~32Th has been studied in the past using pile neutrons [ 1-3] and using mono-energetic neutrons of 3 MeV, produced in the D(D, n)3He reaction [4-6]. Since these early studies had established the peak positions of the mass yield curve, the purpose of this investigation was to define the outer-most wings of the mass yield distribution curve for 2~2Th fission induced by 3 MeV neutrons. The left wing (A = 77-81 ) was established through separation of arsenic and selenium fractions and the right wing (A = 141-156) was defined by separation of the rare earths, both as a group and individually. EXPERIMENTAL T h e ~:~2Th used in this work was reagent-grade thorium nitrate (Bakers' analyzed). A b o u t 20 g of thorium nitrate were irradiated with the 2.95 + 0.08 M e V neutrons [7] produced in the D (D, n):~He reaction using the University of A r k a n s a s ' Cockcroft-Walton positive ion accelerator. T h e irradiation times varied from I to 4 hr depending on the half-life of the nuclide to be studied. T h e neutron flux was typically I - 5 x l0 s n e u t r o n s / s e c , cm ~. Determinations were carried out at least 3 times to assure better statistical results.
Radiochemical procedures Arsenic. T h e arsenic fraction was radiochemically separated using the procedure of W a r d et al.[8]. 1. R. H. lyer, C. K. M a t h e w s , N. Ravindron, K. Rengan, D. V. Singh, M. V. R a m a n r a h and H. D. Sharma, J. inorg, nucl. Chem. 25, 465 (1963). 2. A. T u r k e v i c h a n d J . B. Niday, Phys. Rev. 84, 52 (1951). 3. M. Bresesti, G. Burei, P. Ferrari and L. Moretto, J. inorg, nucl. Chem. 29, 1189 (1967). 4. K. M. Broom, Phys. Rev. 133,874 (1964). 5. S.J. Lyle, G. R. Martin and Md. M. R a h m a n , Radiochem. Acta 9, 90 (1968). 6. S.J. Lyle and J. Sellers, Radiochem..4 cta 12, 43 (1969). 7. R . W . Fink, Nucleonics 17, 94 (1959). 8. T. E. Ward, D. L. Swindle, R. J. Wright and P. K. Kuroda, Radiochem..4 cta 14, 70 (1970). 3643
3644
D . L . S W I N D L E et al.
The basic decontamination steps were: (a) extraction of As a+ into CC14 from _>- 10 N HCI solutions; (b) back-extraction into water; and (c) precipitation of As as the metal for B-counting. Selenium. The procedure reported by McGee et al.[9] was employed and the Se fraction was extracted into a b e n z e n e - l % phenol mixture from 4 N HBr solutions. The Se was finally reduced to the metal for B-counting. Rare earths. After the bulk of the thorium was removed by extraction into di(2-ethyl hexyl) phosphoric acid in n-heptane[10], the rare earths were precipitated as the fluorides. Following several scavenging steps, the individual rare earths were separated from one another by a Dowex 50W x 8 cation exchange resin column [ 11]. A gradient elution technique was used which employed a-hydroxyisobuteric acid of 0"45 M and 0.20 M concentration. The entire chemical procedure including total elution usually required 18 hr. The individual rare earths were precipitated as oxalates for B-counting.
Counting procedures A Beckman low-background B-counting system was employed for the activity measurements of the various fission products. The background of this system was about 0.5 cpm. The samples were mounted using the "filter-stick" method which distributed the precipitate evenly over a known area. Corrections for self-scattering and self-absorption (f~sa factors) were determined for each nuclide according to the method of Nervik and Stevenson [23]. A bsolutefission yield of ~Mo As in many other fission product yield studies, 99Mo was chosen as the reference nuclide. In order to convert the relative yields of the other nuclides into absolute yields, the agMo absolute fission yield was determined. Previously, this had been achieved [4] by using the 197Au(n, T)1aSAu reaction (cross section 49 mbarn) as a monitor. In the present investigation indium foil was utilized as a neutron flux monitor. This method is based on the reaction, 115In (n, n')llsmln which has a threshold at 0.360 MeV and an activation cross section of 360 mbarns at neutron energies of 2.8-3-0 MeV [ 19]. Some advantages of this method are: (a) Due to the larger cross section and the shorter half-life of the product nuclide, much more H~mln activity would be produced in any given bombardment than for ~aSAu. This would certainly lead to better statistical results in the counting data. (b) The 4"5 hr ~15mlnactivity can be easily observed by counting the 334 keV internal transition representing 95 per cent of the total decay. (c) Indium foil is much cheaper than gold and can be disposed of after each experiment, thus possible contamination can be eliminated in future experiments. The 232Th sample was placed between double layers of In foil (95% HSln) and irradiated with 3 MeV neutrons for I hr. After allowing the short-lived Mo isotopes to decay completely, aaMo was separated by the radiochemical procedure reported elsewhere[22]. By absolute y-spectroscopy on both the Mo sample and the In monitor the absolute activity of each nuclide was obtained and decay corrected back to the end of irradiation. The absolute yield of ~Mo was calculated from considerations and equations discussed by Husain [13]. The actual equation used was
Y99= "4 99N s f °'s ( 1 - e-x st ) AsNThfO'Th F ( 1 -- e -~'~t)
where ,499 = absolute activity of 99Mo at the end of irradiation As = absolute activity of~lSmln at the end of irradiation 9. 10. 11. 12. ! 3.
T. McGee, J. Lynch, and G. G. J. Boswell, Talanta 15, 1435 (1968). D. F. Peppard, G. W. Mason, J. L. Maier and W. J. Driscoll, J. inorg, nucl. Chem. 19, 334 (1957). K. Wolfsberg,Anal. Chem. 34, 518 (1962). M. Thein, M. N. Rao and P. K. Kuroda, J. inorg, nucl. Chem. 30, 1145 (1968). L. Husain, Ph.D. Thesis, University of Arkansas, pp. 12-13 (1968).
Mass distribution N, = NTn = f= o-, = ~rT~r = Y~9 =
3 64 5
number of 11'~1natoms in the monitor foil the number of Z32Thatoms in the sample the 3 MeV neutron flux (neutrons/cm 2 . sec) the cross section for the (n, n' ) reaction on "5In, taken as 360 mbarn [19] neutron induced fission cross section of ~32Th at 3 MeV taken as 170 mbarn [4, 20], and the absolute fission yield of 99Mo.
Treatment of data The gross B-decay curves were resolved by standard graphical methods and all activities were decay corrected back to the end of irradiation. The total number of atoms (Ni) of a radioactive species produced in an irradiation of time (t) was determined by Ni
:
Ai - (1--e ~')
t
where A~ is the activity (dpm) of A at the end of irradiation, t is the duration of the irradiation and ~, is the decay constant ofA. The absolute fission yield of 99M0, as described earlier, was determined to be 3-0 _+0"05 per cent. All relative fission yields were calculated by the following equation: Yg, Yi = ~ " Ni where Yi is the fission yield of nuclide i, Y99 is the absolute fission yield of ~°Mo, Ngo is the number of atoms of 99M0 and N~ is the number of atoms of nuclide i produced in the irradiation. The equal charge displacement hypothesis as modified by Pappas[14] was used to calculate fractional chain yields for all nuclides measured in this work. However, no significant change in the cumulative yield of any nuclide was affected. The composite decay of the cerium fraction [141Ce+143Ce(143pr)+144Ce(144pr)] was handled in the same manner as described by Thein et al.[12] except that ~44Ce(285 d) was not observed due to its long half-life and the low neutron flux. The extremely low count rate of the Eu fraction made the corresponding fission yields unreliable and thus are not reported. Similar conditions required a large error limit on the 77As cumulative yield. The branching ratio for the mass chain 77 has been discussed previously [21]. RESULTS AND DISCUSSION T h e c u m u l a t i v e f i s s i o n y i e l d s f o r 15 m a s s c h a i n s a r e g i v e n in T a b l e 1, a n d s e r v e to d e f i n e t h e e x t r e m e w i n g s o f t h e m a s s y i e l d c u r v e o f 23ZTh. T h e e r r o r l i m i t s r e s u l t f r o m s m a l l u n c e r t a i n t i e s in d e c a y c u r v e fitting, c h e m i c a l y i e l d d e t e r m i n a t i o n , n e u t r o n flux m e a s u r e m e n t , e t c . T h e m a s s y i e l d c u r v e f o r 232Th f i s s i o n i n d u c e d b y 3 M e V n e u t r o n s is s h o w n in F i g . 1. T h e c u r v e a l s o i n c l u d e s d a t a b y B r o o m [ 4 ] a n d L y l e et al.[5] a n d d e f i n e s the general features of the mass yield curve. T h e n u m b e r (v) o f s e c o n d a r y n e u t r o n s e m i t t e d b y a p a r t i c u l a r f r a g m e n t p a i r was determined from the relation v = Mc-
(MH+Mr~)
where Mc = mass of the compound nucleus upon excitation by an incoming neutron, and M n a n d M r = m a s s e s o f h e a v y a n d light f i s s i o n f r a g m e n t s r e s p e c t i v e l y . 14. A.C. Pappas, M.I.T. Rep. No. 63 (September 1953).
D. L. S W I N D L E et al.
3646
Table 1. Cumulative chain yields in the 3 M e V neutron induced fission of'-'32Th Mass Nuclide chain measured 77 78 79 81 91 93 99 141 143 145 147 149 151 153 156
rTAs ~SAs 79As slSe 91y a3y aaMo 141Ce ~43Ce mPr 147Nd ~4apm 15apm 153Sm ~Sm
Half-life
Fission yield (%)
39 hr 0'010 -- 0.005 91 min 0.036___0.004 9 min 0.075 +-0"008 18 min 0'50 ___0.05 59d 6.9--_0"7 10"1 hr 7"35 ±0.07 66"7 hr 3"00 _ 0.05 32-5 d 6"6 ±0.7 33 hr 6.0 - 0"6 5.9 hr 4"7 ± 0"5 11"1 d 2"2---0"2 53hr 1.0±0.1 28hr 0.10±0.02 47 hr 0.030 ---0.004 9.4 hr 0.0013 ±0.0005
MM was obtained by taking a measured relative yield from the experimental data and ML was then found by interpolation between two measured yields on the complementary wing. Values for u determined by this method were consistently 2.3-2.4 secondary neutrons emitted per fission of "3ZTh. These results are in good agreement with the previously reported work of Lyle et a/.[5]. It must be noted however, that this method of determining v is not too accurate in the regions of high yields where most of the fissions take place. Figure 1 also shows the distribution of mass in the 14.8 MeV neutron induced fission of 2a2Th, as well as symmetric fission yields from reactor neutron induced fission of Z3ZTh reported by Iyer et a/.[1]. The curve representing 14.8 MeV studies was constructed from experimental data reported by Broom[15], Ganapathy and Kuroda[16], Mo and Rao[17], Thein et al.[12], and Swindle et al.[21]. Figure 1 shows clearly that as the energy of the incident particle is increased, the ratio of the probability of symmetric fission to asymmetric fission increases and thus the peak to valley ratio decreases. The broadening or "splaying out" of the outer wings at 14.8 MeV indicates the increased probability of highly asymmetric fission. This same phenomenon in the case of 235U has been previously discussed by Hyde [ 18]. 15. 16. 17. 18. 19. 20. 21. 22. 23.
K.M. Broom, Phys. Rev. 126, 627 (1962). R. Ganapathy and P. K. Kuroda, J. inorg, nucl. Chem. 28, 2071 (1966). Tin Moand M. N. Rao, J. inorg, nucl. Chem. 30, 345 (1968). E. K. Hyde, Nuclear Properties of the Heavy Elements Vol. III. pp. 104. Prentice-Hall, N e w Jersey (1964). G. Weber and M. Guillaume, Radiochem. Radioanal. Lett. 3, 97 (1970). A. Gerezin, G. A. Stolarov, Y. V. Nikolskii and 1. E. Chelnokov, At. Energ. 5,659 (1958). D. L. Swindle, R. J. Wright, T. E. Ward and P. K. Kuroda, J. inorg, nucl. Chem. 33, 651 (197 i). K. M. Broom, Ph.D. thesis, University of Arkansas, pp. 25-27 (1963). W. E. Nervik and P. C. Stevenson, Nucleonics 10, 18 (1952).
M a s s distribution
3647
/
5.0
/
//
!
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Fig. 1. M a s s yield distribution of 232Th. O - T h i s work; O - B r o o m ; A - L y l e et al.; ..... / X - lyer et al. (Reactor neutron fission of Z32Th); . . . . . 14'8 M e V neutron fission of 232Th; × - Mirror points calculated using v = 2.4.
lyer e t a/.'s[1] discovery of a third peak in the reactor neutron induced fission of 232Th suggests the existence of a similar peak in the symmetric fission region of Z32Th induced by 3 MeV neutrons, although we were unable to confirm this experimentally in the present work. Acknowledgements--The a u t h o r s would like to thank the United States Atomic Energy C o m m i s s i o n for support of this work through contract n u m b e r At-(40-1)-3235. We are also grateful to Mr. D. Coffield and Mr. M. Myers, for operation of the accelerator and help in drafting, respectively. Special t h a n k s go to Drs. T. E. W a r d and J. L. M e a s o n for their valuable advice.