Independent yields of indium and gallium in the proton induced fission of natural uranium

~,, :

~:

/

/ , ,~

:'~'

, ,,I

~') ;~,'

'~1~

!tI It

])¢l~}H!lilll

tq¢~

I'l'lllle;[ ill ( i l e i l l ]l~'illlJn

INDEPENDENT YIELDS OF INDIUM AND GALLIUM IN THE PROTON INDUCED FISSION OF NATURAL URANIUM'I" K, (', CHAN,:I B, P PATHAK, L. NIKK1NEN, L. LESSARD§ ~md 1. S. GRANTI I oqt, r R,diatitm lmhtmdory. McGill University. Montreal. Quebec, Canadtt

(Received 23 Fehrm,ry It~77) Ahstract~ -Independent yields of In ~md (;it isotopes from the proton induced fission of ~ U are presented for the IllSt lime, Proton energies between 30 and 9t~.fiMeV have been used. Isotopic distributions were measured using an .ndine m~ss spectrometer with an improved ion source. The widths and most probable masses for the In th'dributions ~trecompared with those from an asymmetriccharge split, Rb-Cs. measured by the same method. The c.mpnrison shows evidence of direct reaction contribution to the latter case. Furthermore, the On-Pro charge split. like Rb ~(?s.is found to be well described by the postulate of Equal Charge Displacementbut not by the postulate .f Unchmged Uh~r~zedDistribution.

INTRODUCTION hl !ccent yours, the fission of ~SU induced by I0-I00 k,h:V protons hus been investigated by radiochemical[ I*,I :rod muss spectrometric methodsl7-9l, Previously, ~m°Imc muss spectrometers, with ion sources making use ,,f the very short diffusion times of the ulkakis in ,~r;tphile cutcher foils and of the highly selective thermal ioJli/~dion of the,,e slime elements on suitable surfaces. Imve em~bled the precise measurement of the relative mdcpendent yeild,, of not only the longer lived isotopes, which arc measured by rudiochemical methods, but also ,~f tile shortlivcd and stuble ones for Rb and Cs[7.81. It ',v:l,, found thut, for the Rb-Cs charge split studied, the rcstdls wore hesl interpreted in terms of the postulate of i:,qn;ll Charge Displucement (ECD), contrary to the expeeled trend towards Unchanged Charge Distribution ~t ('O/ at higher excitution energies. However, the li,litati.n ,:,f tile experimentul method to the ~tlkttlis was u !',Hher severe one. und it wits felt necess,try to extend it Io other elenlcnt,',. In the present work. we used an improved version of the ion source which allowed us to mca~ttrt:, for the first time. In and Ga isotopic distrihalloa,,. We Ihu,~ obtained information on symmetric IN,,ion (in the case of In)and it very usymmelric charge ~,piit iin the ¢u~¢ of tlu. which is nearly the comidvmenlury frugment to Pm previously studied by rudiocht2nliclll methods [41). The purpose of this work is to ~ompm'e the characteristics of the isotopic distributions ~f hi and (hi with those of the already known disIrihutions of Rh .'rod Cs, in order to get some insight into Ihc rc;tctitm mechanism mad the mechanisms of charge divi~,ion in fis,~ion induced by protons of tip to 100 MeV cnor~,

W.~k ',upported I~ythe Nilliona]Research Councilof Canada. Ihe,,cnt adttre~,~,: Chalk River National l,ahoratories. Chalk I~,i,.ej. I )ntmm. Camith~, ~lh'e~,~ml;tddl'es,c Univer,,it0de MOlltr~td. Montrt~a],Qut.~bec. iPre',,:nt .ddress: Department of Physics, Schuster Labor~lu,T'3 I nAcl,,Jtv of Manehe,,ter. Enghmd,

EXPERIMENTAl. The details of the un.line mass spectrometer and the dat. collection system have been presented in tm earlier publication [~], The ion source used in this work is an improved version of the one described in Ref, [8], and will be presented in detail in another publication[101, In this paper, it is sufficient to descrihe the new ion source (Fig, I) as consisting of a graphite oven surrounded by a tantalum heat shield and connected to a "chimney" made with suitable materials (tantalum for Rb and Us, rhenium for in and Ga) in which the ionization takes place, The "chimney" is terminated as it slit facing the optics of the mttss spectrometer, The oven contains the target material, U,OM,dee posited on 23 graphite disks of diameter 12.7ram. thickness 28.6 mg/cm-~, and separated by graphite rings 0.3 mm thick, The average target thickness deposited on each disk is 3,4 mg/cm"~, The total target thickness, graphite and U~O,, is reh|tively high, This is necessary because of the expected low ionization efficiency rot In and Ga. The fission fragments are stopped in the graphite disks, ~nd after diffusingout are ionized selectively for extraction through the "chimney", The oven and ionizer are heated independently by Joule effect using two different power supplies, to temperatures adjusted to give optimum conditions of efficiency, diffusion times, and low m~tural contamination levels, Although surface ionization is very selective for ]n and (;it, the identification of the ]n isotopes has been further confirmed, Jffter mass separation, by measuring the half-lives of the radioactive ones, and by extracting sources of k,~ ~4]n and studying their radioactive deci~ys, The idemifieation of the Ga isotopes is based on the chemical similarities of Ga and In, The m=tss discri. ruination effect of the system wits calibrated using known isotopic abundances of naturLd barium and tin, The observed y=ields of the fission fragments have been corrected for this effect. The irradiations were done in the external beam of the McGill synchrocyclotron, which delivered 5(1 nanoamperes of protons at 100 MeV. The energy of the beam was varied by using Beryllium degraders in the external beam line. The duration of ouch pr,oum beam pulse w~s 0,2see.. and the time interred between two consecutive beam pulses was 2.0 sec, The rates of diffusion of the fission fragments were studied by measuringthe yields dt-tring and after irradiation. The diffusion for In could be described a~ ~L composite of two exponential decay modes with half.lives of 50 msec. and 250 reset., respectively, The diffusion rate of (hi showed a single decay mode with a half-life of 230 reset. The relative independent yields for different isotopes are proportional to the differences between the in-beam and off-beam counting rates, after correctinS for diffusion times, decays of the radinacrive isotopes and mass discrimination effects, No corrections

1916

K. C. CHAN et al.

EXITS L ~ IONIZINGJ ~v~11

HEATSHIELD U

1

GRAPHITE

L_JU TARGET ASSEMBLY

Fig. 1. A schematic diagram of the ion source. were made for the cumulative yields due to the shortlived precursors (Cd in the case of In) for lack of information on the relevant half-lives. But they were believed to be small, because their cross-sections, in the heavy mass side of the isotopic distributions where their effects would be felt, are much smaller than those of In, and also because the diffusion times, especially in the case of In, were very short. A way of checking the presence of such an effect is to vary the time difference between in-beam and off-beam measurements. No significant difference in the isotopic distributions was observed with three different time intervals. RESULTS AND DISCUSSION Indium The relative independent yields of the In isotopes are given in Table 1 and shown in Fig. 2. The error bars represent the relative errors because of counting statistics only. The yields measured at three different intervals during the irradiation cycle were used in the analysis. The values reported in this paper are an average obtained from these three sets of data. The absolute cross-sections were first evaluated by normalizing the yield of ~t7In with the experimental

values reported in Ref. [5]. These values are subject to an error of 30%. Since the yields of '17In are not observed at some of the proton energies used in this work, the normalization was done by "using the deduced yield at mass 117 from a Gaussian fit described in the following paragraph. An independent normalization was done, by deducing the absolute cross-sections of I t7In and '2°In from the Sb isotopic yields determined by radiochemical methods[l]. In this second method, the same N / Z dependence was assumed for the In and Sb isotopes. The cross-sections obtained by normalizing the data at "Tin and '2°In where essentially the same. However, the values obtained by using the Sb cross-sections are consistently lower by a factor of two compared to those obtained by normalizing directly to the "7In cross-sections. Since the total cross-sections for In were found to be a factor of two higher than that for Cs if normalization was done with the H7In cross-sections quoted in Ref. [5], which is quite unreasonable, the Sb values where preferred to normalize the In cross-sections in this paper. The most probable mass was determined, at each

Table 1. A summary of the fission yields for In. The cross-sections are in units of mb. The most probable mass,
Cross-section* in mb

(A) Width VT

Mass number

97.6

113 114 115 116 117 118 119 120 121 122 123 124 125 1~ 127 128 1N To~

0.1±0.3 0.4±0.3 0.6±0.4 2.4±0.2 6.1±0.3 11.9±0.4 18.3±0.6 22.2±0.5 23.7±0.4 19.3±0.4 13.5±0.3 7.9±0.3 4.3±0.4 1.6±0.3 1.2±0.4 0.6±0.3 0.3±0.2 124.2±1.5 1~.68 ±0.01

Proton energy (MeV) 76.4 59.8

48.3

30.4

0.8±0.4 0.2±0.3 1 . 1 ± 0 . 1 0.4±0.1 4.0±0.2 1.8±0.1 8.6±0.3 5.2±0.2 2.9±0.5 16.1±0.3 12.3±0.4 8.7±0.5 6.1±2.8 21.9±0.3 19.8±0.4 16.8±0.7 9.9±3.0 ~.8±0.3 26.2±0.4 25.7±0.4 19.0±3.0 ~.5±0.3 23.4±0.4 25.9±0.5 27.3±2.9 14.7±0.2 18.1±0.3 21.4±0,4 30.7±3.0 $.6±0.2 10.7±0.4 13.1±0.5 28.7±2.8 4.7±0.2 5.9±0.3 7.1±0.4 ~.1±4.6 2.2±0.2 2.4±0.2 3.2±0.4 10.9±2.9 0.8±0.2 0.9±0.2 1.3±0,4 9.3±4.2 3.7±2.6 2.3±2.3 128.7±0.9 127.3± 1.1 126.1± 1.5 168.0±10.5 121.~ 121.~ 121.~ 123.07 ±0.01 ±0.01 ±0.03 ±0.~

1.~3

1.469

1.~9

1.~6

1.512

5,~ 9.96

4.~ 9.36

4.43 8.~

4.~ 7.83

3.83 5.~

tThe quoted cross sections (especially at 48.3 and 30.4 MeV) are subject to systematic errors, as at lower incident proton energies, the isotopic distributions of fission fragments are widened because of attenuation in the target.

Independent yields of indium and gallium

in the proton beam while traversing the target which in turn introduces a spread in centroid positions. This spread will effectively increase the widths observed. Using the energy dependence of the centroid from this experiment, we have estimated this spread and calculated the corrected widths given in Table 1. Qualitatively, the In distributions behave like the other ones with increasing proton energy. The centroids of the distributions move towards the neutron deficient side and the widths increase. The differences are nevertheless noteworthy: the energy dependence of the N/Z values of the most probable masses in the case of In is stronger than for the more asymmetric Rb-Cs mass split. Furthermore, the widths of the In distributions appear 'to be much narrower than what one would expect, i.e. In is closer to Cs in mass, but the In widths are nearly equal

energy, by finding the centroid of the isotopic distribution. A Gaussian shape was fitted to the distribution. Figure 2 shows the quality of agreement between the experimental values and the Gaussian expression. With the best fits, the experimental cross-sections appear to be slightly higher for the heavier isotopes and lower for the lighter ones, indicating a slight skewness in the experimental distributions. An exponential of a third order polynomial was also fitted to the data, but no significant improvement in the quality of fit was obtained. Figure 3 shows the energy dependences of the most probable masses in units of N/Z, and of the widths in mass number units, together with those of the isotopic distribution characteristics for Rb and Cs from previous measurements using the same method[7, 8]. There is a broadening of the In widths due to the energy dispersion i

i

i

i

i

i

i

i

i

i

i

i

i

i

i

i

i

.

r

I

i

i

1917

i

i

i

i

i

r

i

r

i

' ' ' ~2~o:z .....

2 i

i

i

i

120

1

i

i

~

124

i

i

r

128

75.71

10

GA

1.o

4:;;--

i

J

'I !', ;i:L!' ,

,

116

i

t

i

120

124

i

i

128

i

J

116

t

L

l

I

[

I

120 MASS

I

I

I

I

124

75

77

79

NUMBER

Fig. 2. Fission yields of In and Ga. The most probable masses (arrow) and widths (horizontal bar) are expressed in mass units and are obtained using Gaussian fits. The widths have not been corrected for energy dispersion in target. I

I

I

I

[

I

I

-

i

t

'
7 .~N 1.51

CsI

~'-

1.41

1.4 4

30

50

70

90 PROTON

L

I

L

_i

30 ENERGY

i

50

J

70

~ _ _ 1 _

~

90

(Me"/)

Fig. 3. (a) The energy dependence of the most probable masses of In and Ga compared with Rb and Cs. They are expressed in terms of N/Z units. The curve is representing the N/Z's of the fissioning nuclei, with prefission neutrons taken into account, also shown: It is lowered vertically (dashed curve) to compare with In and Cs. Fig 3. (b) The energy dependence of the widths of the isotope distributions for In and Ga compared to Rb and Cs. They are expressed in terms of mass numbers. The widths for in have been corrected for energy dispersion in target.

K. C, CHAN et al,

1918

to the Rb ones, Similar results have been found for the proton induced fission of Thorium [11], One would expect UCD (as well as ECD) to be valid for symmetric fission. After taking into account prefission neutron emission (the values have been obtained from Ref, [2]), we obtained the energy dependence of the N/Z value of the fission fragments assuming UCD (c/, UCD curve in Fig, 3). As expected, the trend of the UCD curve agrees very well with that of In (symmetric fission), but not with those of Rb and Cs (asymmetric fission), In fact, if UCD is valid for In, the difference between the UCD and In curves will give the number of neutrons emitted from the In fragment, This is equivalent to calculating the total number of neutrons emitted as: 93 u.r = 239- ~ (A>t. where (A)~. is the most probable mass of In, The above formula should hold to a good approximation even if UCD is not valid, because In and its complementary fragment, Ru, have almost the same mass and their most probable masses should have approximately the same N/Z, The ~T values obtained using this method are given in Table 1 and shown in Fig. 4 where they are compared with the values that have been extracted for the Rb-Cs charge split. From the comparison, one finds that vr for symmetric fission agrees with the value obtained for the Rb-Cs split up to 40 MeV and diverges more and more with increasing proton energies, At 100 MeV, one finds that two more neutrons are emitted from symmetric fission than for the Rb-Cs pair, In terms of energy, it means that, either it takes up to 2.8 MeV more energy to boil off a neutron from a Rb-Cs pair than from a symmetric pair, or the average excitation energy of the nuclei fissioning into a Rb-Cs pair is lower than in the case of symmetric fission, The evidence from the comparison of the widths of the different isotopic distributions points towards the second possibility. The significantly larger widths of the Cs distributons have been interpreted as suggestive of the existence of fissioning nuclei of very low excitation energies and I

I

I

"rn

1

I0 -

masses close to the target nucleus mass, mainly because of the observed shoulders on the heavy mass side of the Cs distributons at higher energies[8,12], This is in agreement with the observed energy dependence of the most probable masses and uT values discussed in the preceding paragraph, There is also supporting evidence at an even higher (160MeV) proton energy[13]. Since asymmetric fission is preferred in low excitation energy fission events, these nuclei with low excitation energies will not contribute significantly to the In distributions, which is in agreement with the observed narrower In widths, In fact, the excitation energies of the fissioning nuclei responsible for the broadening of the Cs distributions are expected to be lower than 30 MeV, since the In cross-sections observed in the experiment are found to remain high down to 30 MeV proton energies, These nuclei can be produced by two mechanisms: a long evaporation chain preceding fission or a direct reaction event, The latter mechanism is preferred for the following reasons: (1) If the first case is true, the total number of neutrons emitted should remain high for both In and Rb-Cs, It is observed to be lower in the case of Rb-Cs and the difference increases at higher proton energies, (2) The curves shown in Fig. 4 indicate that the Rb-Cs ~'r value diverges from the In case at higher energies in a manner which is strongly reminiscent of the expected energy dependence of the direct reaction cross-section relative to the compound nucleus formation cross-section. (3) The wfdening of the Cs isotopic distribution is becoming more apparent at higher proton energies when a shoulder appears on the heavy mass side of the distribution, It seems indeed that the broadening is due to the persistence of neutron rich fissioning nuclei at high bombarding energies, which appears to be incompatible with the first mechanism. Gallium Because of the low cross-sections of the Ga isotopes, we could obtain an isotopic distribution at only one energy. A Gaussian shape has been fitted to the measured isotopic distribution, from which the most probable mass and width were extracted. The absolute cross-sections were determined by normalizing to the experimental value of 72Ga from Ref. [3]. The quoted accuracy is better than 30%.

Table 2. A summary of the fission yields of Ga isotopes, The meanings of the quantities (A), (N/Z), and ~r are explained in the caption of Table 1

~.S

Mass numbers Cross-sections (mb) 6

20

I

1 60

I

I

I00

Proton energyj MeV

Fig. 4. The total numbers of neutrons emitted VTfor the cases of In and Rb-Cs, at different bombardingenergies.

(A) /N/Z) Width VT

74 75 76 77 78 79 80 Total

Proton energy 97,6 MeV 1.7± 0.4 2.1z0.3 2.3±0.3 2.1 ± 0.3 1.2±0.2 0,3±0.4 0.2 - 0.5 9.8 ± 1.0 75.71±0.83 1.442 4.50- 0.03 8.91 ~ 0,8

Independent yields of indium and gallium Although we have data for only one proton energy, interesting conclusions can be drawn from a comparison with the radiochemically determined isotopic distributions of Pm reported in Ref, [4]. Using their measured vahtes for the cross-sections of the Pm isotopes at proton energies from 20 to 85 MeV, we can extrapolate lhe width and most probable mass up to 98.6 MeV. They ,tre 15(I.79 and 4.26, respectively in mass units. Since Ga ~md Pm ~re nearly complementary fragments, one can cxtn~ct the total number of neutrons emitted. A v, of 8.9t is obtained, intermediate between those for the Rb-C!s pair (8.32) and In (9.96), It is of interest to determine which mechanism (UCD or ECD) can best describe the observed most probable masses. If one ;~ssumes [21 J,p,.~n~,,,,/u.n = 0.5 and the ratio of post-fission neutrons1141 for Pm and Ga, u~,,,/uc~,,, to be 2.0, one can t~bt:tin the N/Z values for Ga and Pro, prior to postfission neutron emission, as 1.490 and 1.539, respectively. These qu,'mtities are shown in Ref. [8] to be insensitive to the exact value of ~,p,.~......./u.r. In order to get the same N/Z ior Ga and Pro, as required by the UCD postulate, a value ~t' ~',,,,,h,,,,*~- I is needed. This is in contradiction with experimental results1141. On the other hand, these N/Z vtdues are in good agreement with the ECD postulate. t!sing the most stable charges from Coryell[15], we find ~hat Pm and Ga are 3.00 and 2.92 charge units away from the most probable charge, respectively. It is. therefore, concluded that the Ga-Pm pair is similar to the Rb-Cs charge split, in that the ECD postulate is valid for both of them. The width of the Ga distribution is in general ~greement with Hagebo's 1161 results. The narrow widths ,ff Ga anti Pm led both Hagebo and Galinierld] to propose either a fast fission or a low energy deposition by the incident particles, it is hard to reconcile the latter statement with the ~ found. According to the statistical Iheo~y, the narrow widths can also be due to a low ~uclear temperature at the scission point. Further investigation is needed to distinguish between those different possibilities. CONCLUSION In this paper, we presented for the first time measurements of accurate isotopic distributions by a mass spectrometer of In and Ga isotopes produced in the fission of ' ~ U by medium energy protons. They are found to be very nearly Gaussian in shape, The most probable mass of In has a stronger energy dependence than that of Rb-Cs, Assuming UCD, the u-r calculated fur in is also significantly higher. Furthermore, the

I~1~,~

widths of the Cs distributions are larger than those t~,f In. These observations are compatible with and indicative of the existence of direct reaction contributions leavinlg the fissioning nucleus with very low excitation energy and with a mass close to the target mass, The Ga distribution was compared with the radiochemical measurements ~f the Pm distributions. The ECD postulate, which was previously found to describe rather well the observed Rb-Cs distributions, has been found to be also v,'tlid for the more asymmetric Pm-Ga split. The width and t~, value at 98.6 MeV for the latter split may be indicative ot rather high deposition energy events with probably low nuclear temperature at the scission point. REFERENCES

I. L. D. Miller and L. Yaffe, J. lnorg. Nucl. Chem. 35. 18(15 (1973). 2. M. Diksie, D. K. McMillan and 1. Yaffe. J. lmm,,. Urn'/ Chem. 36, 7 (1974). 3. A. H. Khan, G. B. Saha and L. Yaffe, Can. J. Chem. 46, 3565 (1968). 4..I.L. Galinier, Ph.D. Thesis, Dept. uf Chemistry, McGill University, Montreal, Quebec, Canada (1975). 5. S. Sarkar, Ph.D. Thesis, Dept. of Chemistry, McCiillI;nivcr sity, Montreal, Quebec, Canada (1975). 6. P. P. Benjamin, D. A. Marsden, N. T. Porile and I. Ya||'e, Can, J. Chem. 47, 301 (1969). 7. B. L. Tracy, J. Chaumont, R. Klapisch, J. M. Nitschke, A. M. Postanzer, E. Roeckl and C. Thibault, Phys. Re~:. t'.J.; 222 (1972). 8. J. K. P. Lee, G. Pilar, B. L. Tracy and I,. YJe..l hu)rg. Nucl. Chem. 37, 2035 (1975). 9. R. Klapisch, J. Chaumont, C. Philippe, 1. Amarel, R. Fergeau, M. Salom(~and R. Bernas, Nucl. In.sir. Methods 571.21~ (1967). 10. L. Lessard, McGill University, to be published. Alsn, t~iull, o] APS 21 762 (1976), 11. L. Nikkinen, Foster Radiation Laboratory, McGi[I University, Montreal, Canada private communication. 12. M. de Saint-Simon, W. Reisdorf, L. Remsberg, L. Lessard, C. Thibault, E. Roeckl, R. Klapisch, I. V. Kuzenetsov, Yu. Ts. Oganessian and Yu. E. Penionchkevitch, Labomtoire Rend Bemas du Centre de Spectromdtrie Nucldaire et de Spectrom~trie de Mass--B.P. No. 1-914(16,Orsay France. to be published. 13. J. Chaumont, Ph.D. Thesis, Facult6 des Sciences. I)rsav (1970), unpublished. 14. E. Cheifetz and Z. Fraenkel, Phys. Rev. Lett. 21, 36 tl968L E. Cheifetz, Z. Fraenkel, J. Galin, M. Lefort, .1. Peter and X Tarrago, Phys Rev. C2, 256 (1970). 15. C. D. Coryell, Ann. Rev. Nucl. Sci. 2, 305 (1953). 16. E. Hagebo, A. C. Pappas and P. Aagaard..I. lm~rg. Nucl. Chem. 26, 1639 (1964).