Charged particle fission of 232Th

J. inorg,nucl, (hem., 197 I, Vok 33, pp. 1535to 1541. PergamonPress. Printedin Great Britain

CHARGED

PARTICLE

FISSION

OF

232Th

G. R. C H O P P I N and A. J. T O F E Department of Chemistry. Florida State University, Tallahassee, Florida 32306

(Received 13 November 1970) Abstract-The mass distributions in the fission of ~:~2Th by protons of 11-3 MeV and deuterons of 11.5 MeV have been measured using Ge(Li) y-ray spectroscopy. The total cross sections were determined to be 93 M.B. and 135 M.B. respectively for the proton and deuteron fission. The peak-tovalley ratios of the kinetic energy spectra of the fission fragments were measured for fission induced by protons and deuterons from 9 to 12 MeV. INTRODUCTION

WHITE much work has been done with neutron fission of ~32Th, relatively little information has been obtained at low energies (< 30 MeV), with protons and deuterons. Radiochemically determined mass yield curves have been reported by Tewes and James[l] for 6-3, 9.3, 13.3 and 17.8 MeV proton energies. The number of investigations of low energy deuteron fission seems to be limited to the data of Ramaniah and Wahl[2] at 9.5 MeV, and that of Alexander and Coryell[3] at 13-6 MeV. All of the studies involved the separation of the elements from the gross fission products by a variety of relatively slow radiochemical flow schemes. The use of lithium-drifted germanium detectors with their excellent resolving power has opened a new approach to the study of nuclear fission flields by providing considerable advantages in speed and convenience over standard radiochemical techniques. Measurements of the y-ray spectra of gross fission products with Ge(Li) detectors[4, 5] have shown that the photopeaks of a large number of radionuclides are well enough resolved to permit identification. Gordon et al.[6] were able to determine the yield of approximately twenty fission product nuclides by this technique. It was demonstrated that the use of Ge(Li) detectors provided satisfactory accuracy in fission yield measurements while allowing the determination of many nuclides simultaneously and eliminating many of the problems associated with inter-experiment normalizations. In the present work, group separations were performed prior to the y-ray measurements in order to reduce the complexity of the y-spectra and to minimize the limitation of analyzer dead-time resulting from excessive gross count rates. H. A. Tewes and R. A. James, Phys. Rev. 88,869 (1952). M. V. Ramaniah and A. C. Wahl,J. inorg, nucl. Chem. 24, 1185 (1962). J. M. Alexander and C. D. Coryell, Phys. Rev. 708, 1274 (1957). N. C. Rasmussen, J. A. Sovka, and S. Mayman, Nucleus Materials Management p. 289. I A E A , Vienna (1966). 5. R. M. Paar, Nucleonics 23, 56 (1965). 6. G. E. Gordon, J. W. Harvey and H. Nakahara, Nucleonics 24, 6:2 (1966). I. 2. 3. 4.

1535

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G.R.

CHOPPIN

and A. J. T O F E

EXPERIMENTAL

Targets and chemical procedure Circular targets of natural thorium foil were sandwiched between high purity a l u m i n u m catcher foils and irradiated on the F.S.U. T a n d e m V a n de Graaff Accelerator. T h e conditions of target irradiation were very similar to those previously reported in detail from this laboratory[7, 8]. After dissolution of the foils in the presence of carriers for the nuclides to be isolated, simple radiochemical separations were used to obtain group separations from the gross fission products. T h e details of the separation of the lanthanide group and m o l y b d e n u m were given elsewhere[9]: the iodine procedure was that of G l e n d e n i n and Metcalf[10]. Ge(Li) speetrometo' T h e Ge(Li~ detector used in this study was a Ortec true coaxial type with an active volume of 7.1 cm :~, and a drifted depth of 10.5 mm. T h e spectrometer s y s t e m consisted of a low noise preamplifier ( I 18A) m o u n t e d directly to the cryostat, an active filter amplifier (440) with a 2/xsec time constant, and a T M C 1024 multichannel analyzer. Because the analog-to-digital converter was limited to 1 0 2 4 c h a n n e l s , and because the efficiency was higher at lower energies, we limited the energy region studied to 50 to 600 keV in most cases. D u e to the complexity of the spectral data, it was imperative that the data reduction be accomplished in an efficient and reproducible manner. Line shape analysis[11] of low energy spectra from Ge(Li) s p e c t r o m e t r y has s h o w n that the full energy photopeak is essentially gaussian, and that more accurate extraction of intensities are possible with a least-square fitting procedure. A gaussian fitting program, F S V M E T [ 1 2 ] , using a variable m e t h o d of minimization, has been found to be very suitable for the evaluation of complex spectra. A n extremely useful feature of the program was the ability to impose constraints upon the fitting parameters in such a m a n n e r as to hold one or more of the parameters constant. By requiring c o n s t a n t widths in regions of overlapping spectra, it was possible to effectively strip the spectra since the full width at half m a x i m u m had been accurately determined as a function of energy for our Ge(Li) detector[13]. Although a few isolated peaks can be evaluated by simply m e a s u r i n g the area u n d e r the photopeak, the majority of the photopeaks did require computer analysis. T h e complexity is clearly illustrated in the determination of the yield of the 522.60 keV peak of ~:~I. T h e c o m p u t e r analysis of the c o m p l e x photopeak, as s h o w n in Fig. 1, indicates that the peak is c o m p o s e d of photopeaks at 522-60 keV, 526.54 keV and the 529.91 keV for ~:~I, ~:~5Iand ~:~:~I respectively. T h e energy and counting efficiency calibrations followed the s a m e procedures described previously [9].

Peak-to-valley procedure Targets of 232Th (ca. 100p, g) were prepared on the Florida State University electromagnetic isotope separator by direct deposition on thick carbon foils. T h e s e targets were b o m b a r d e d with protons and deuterons from 9 to 12 M e V in increments of 250 keV in a 12-inch scattering chamber. T h e physical characteristics and experimental techniques have been d i s c u s s e d earlier[14]. T h e emitted fission fragments were detected with two silicon surface barrier detectors. T h e detectors had an active area of 50 mm'-', a resistivity of 425 f~cm and a thickness of 500/x. T h e detectors were placed 2.54 cm from the center of the thorium target and at angles of 90 ° and 270 ° with respect to the incident beam. T h e thorium target was at a 45 ° angle to the incident beam. T h e pulses were analyzed on a 7. E. F. Meyer, Ph.D. Dissertation, Florida State University (1964). 8. K. R. C h a p m a n , Tech. Rep. No. 11, T a n d e m Accelerator Laboratory, Florida State University (1968). 9. G. R. C h o p p i n and A. T. Kandil, J. inorg, nucl. Chem. 3 3 . 8 9 7 (1971). 10. W. W. Meinke, The Radiochemistry o f Fluorine, Chlorine, Bromine and Iodine. (Procedure 6), National A c a d e m y of Science-National R e s e a r c h Council Report, N A S - N S - 3 0 0 5 (1960). I 1. D. P. Donnelly, H. W. Baer, J. J. R u d y and M. L. Wiedenbeck, Nucl. Inst. Meth. 22, 333 (1963). 12. E. B. Shera, Los A l a m o s Scientific Laboratory, Modified for Florida State University C o m p u t e r by D. Benson. 13. A . J . Tofe, Ph.D. Dissertation, Florida State University ~1969). 14. E. J. Feldl, J. R. Meriwether, G. R. C h o p p i n and J. D. Fox, Nucl. Inst. Meth. 22, 333 (1963).

1537

Fission of 2:~2Th

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CHANNEL NUMBER Fig. 1. C o m p u t e r analysis of photopeaks of H-"~:~:~~:~:'Iin the 525 keV region. T M C 1024 channel analyzer and the resultan! double humped spectra of the kinetic energy of the fission fragments were used to obtain the peak-to-valley ratio. RESULTS

AN[)

DISCUSSION

The cross sections for the nuclides determined in the 11.3 MeV proton fission are given in Table 1 and the resultant mass yield curve is shown in Fig. 2. Recent work by Ferguson[15] on the fission of 23"Th at 8.0, 10.5 and 12.5 M e V proton energies, by double energy correlation experiments, is in good agreement with our mass yield curve. Ferguson's cross section values were used to help define the heavy mass wings since this mass region seems to be essentially independent of energy between 8-0 and 12.5 MeV. The integrated fission cross section for 11.3 MeV protons is 93 M.B. which is in good agreement with the value of 100 M.B. reported for solid state detector m e a s u r e m e n t s [ 16]. T h e width at half height 15. R. L. F e r g u s o m Private communication. 16. G. R. Choppin, J. R. Meriwether and J. D. Fox, Phys. Ret,. 131, 2149 ( 1963 ).

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C H O P P I N and A. J. T O F E

Table 1. Total cross section proton Nuclide ~zy .~3y Nb anNb ~aMo 141Ce 142La 143Ce 147Nd t49Nd ~5~Pm

Half-life 3'53 hr 10.1 hr 23 hr 67 hr 32.5 days 1.4 hr 33 hr 11.1 days 1.8 hr 28 hr

Cross section 3.37 4-65±0-12 0-417 --_0.083 0-0627___0.0031 2.595±0.030 4-22±0.30 4-23±0.56 2-54 1-40 0.41±0.11 0.032±0.007

for the heavy mass wing of the yield curve is 12.0±0.5 mass units which agrees satisfactorily with the values reported previously of 13 [ 15] and 12.5 [ 17]. The cross sections of a larger number of nuclides (Table 2) were determined for the fission by 11-5 MeV deuteron. The total fission cross section from the mass yield curve in Fig. 3 is 135 M.B. This value is substantially larger than the value of 66 M.B. reported from radiochemical measurements[17] but is lower than the value of 200 M.B. obtained by solid state detectors[16, 18]. The error 101

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z o 11.3 MeV protons o This work o Ref. 17 a Ref. 15 emA Reflection points

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16

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I

I

I

I

100

120

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160

MASS NUMBER

Fig. 2. Mass yield curve for fission of ~32Th by 11.3 MeV protons. 17. S. H. Freid, J. L. Anderson and G. R. Choppin, J. inorg, nucl. Chem. 30, 3155 (1968). 18. G. L. Bate, R. Chaudhry and J. R. Huizenga, Phys. Rev. 131,722 (1963).

Fission of 2S~Th

1539

Table 2. Total cross section deuteron

Nuclide ,oy

Cross section (M.B.)

Half-life 3.2 hr 64.2 hr 50 min 3.52 hr 10.10 hr 90hr 35 days 23 hr 67 hr 28 min 12"6 hr 8.05 days 2.3 hr 21 hr 52 min 6.7 hr 33 hr 2"85 days 11.1 days 1-73 hr

,Jmy ~2y ~3y "SNb "nNb ~Mo ~Sl ~:~"1 1:~11 ~:~1 ~:~1 1341 ~351 14sCe 144Ce 147Nd ~4"~Nd

0.020 0'543 4.005 5"70-+1'14 0"188 0-0049 1.94-+0-05 0"182 0.246___0-036 1.005 _+0.030 0.549 3"448 -+ 0.05 4.151 -+ 0-022 3"44 -+ 0.05 0-743 +- 0-01)8 5-33 4-64 0"113+-0"01)3

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MASS NUMBER

Fig. 3. Mass yield curve for fission of ~:~Th by 11.5 MeV deuterons.

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G.R.

C H O P P I N and A. J. T O F E

limits in the measured nuclides in Tables 1 and 2 represent the differences in replicate determinations of two or more prominent photopeaks. Where no errors are listed, the value is a result of only one photopeak analysis. The heavy wing of the mass yield curve for deuteron fission has a width at half height of 1 8 . 5 ± 0 . 5 mass units. This is in excellent agreement with the value of 1 8 +- 1 mass units determined by Nemilov[19] in his measurement of the mass and kinetic energy distribution of fragments from fission of 2~2Th by 12 MeV deuterons. The fission of 23°Th by 25.7 MeV alpha particles in which the compound nucleus is formed with approximately the same excitation energy gives a value for the width of 17 mass units. In the mass yield curves of Figs. 2 and 3, data in the valley are limited. The peak-to-valley ratios of the kinetic energy spectra were measured as a function of energy to provide more information on this aspect of the yield curve. The results are given in Fig. 4. The standard deviations of the peak-to-valley ratios were obtained from the equation: ty = N p / N v ( 1 / N p +

1 / N v ) 1/2

where N e is the number of pulses at the max of the high energy peak and Nv the number of pulses in the valley. Some recent proton data by Ferguson [15] is included for comparison with our measurements. e

0 Ref. 15

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F] This work (Deuteron)

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Fig. 4. The peak-to-valley ratio in the fission of 232Th by protons and deuterons of 9-512 MeV. 19. Yu. A. Nemilov, V. V. Pavlov, Yu. A. Selitskii, S. M. Solwer and V. P. Fismont, J. nucl. Phys. USSR 1,633 (1965).

Fission of ea2Th

1541

The peak-to-valley ratio calculated from our proton mass yield curve (Fig. 2) is 5.0. This value is in excellent agreement with our value from the kinetic energy spectrum of 4-95+0.15 for 11.3 MeV protons. Thus, we feel that the reproducibility of the peak-to-valley measurement as well as the total fission cross section and half width conclusively define the 11.3 MeV proton mass yield curve.

The peak-to-valley ratio for the mass yield curve (Fig. 3) for fission by 11-5 MeV deuterons is 7 . 4 + 0 . 2 . The value from the kinetic energy spectrum for the peak-to-valley ratio is 3.5 + 0.1. While this value is much lower than the radiochemical ratio, it is in good agreement with the ratio of 3 . 1 + 0 - 2 obtained by Nemilov. This lower value, perhaps, can be explained by the differences in the angles for the collection of the fission fragments. In our radiochemical study the fragments were collected over the total solid angle, while in our work and in Nemilov's semiconductor experiments, the detectors were at 90 ° and the fission fragments were observed only at that angle. The excitation energy at 1l-5 to 12,0 MeV is not only sutficient for fission of the initial compound nucleus but even after the emission of one or two neutrons, there is still sufficient excitation energy for fission to occur. Since the fission of nuclei with the greatest excitation energy will be more isotropic and more symmetric, the contribution of symmetric fission at an angle of 90 ° is higher than the contribution observed over the total solid angle resulting in a lower peak-to-valley ratio. This explanation f o r . t h e disagreement in the peak-to-valley ratio measurements in the deuteron fission would seem to be contradicted by the good agreement of the peak-to-valley ratio measurements in proton fission. However, the decrease in the 12 MeV particle beam in passing through the radiochemical targets is 1.46 MeV and 1.06 MeV for protons and deuterons respectively: this corresponds to a change in the peak-to-valley ratio of 1.5 for protons and essentially no change for deuterons. Perhaps, this variation in the proton case provides compensation for the effect of the detection angle which is not possible in the deuteron fission. Acknowled,~,ements-The financial assistance of the U.S.A.E.C. through contract AT-(40-1 )-I 709 is gratefully acknowledged. The NSF also supported this research through a grant to the FSU ('omputer Center.