Excitation functions and α-decay from compound nuclei in Ar induced reactions on 144Sm and 166Er

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Nuclear Physics A220 (1974) 84--92; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

EXCITATION FUNCTIONS A N D ~-DECAY FROM C O M P O U N D NUCLEI IN Ar INDUCED REACTIONS ON 144Sm AND 166Er H. GAUVIN, R. L. H A H N t, B. LAGARDE, Y. LE BEYEC and M. LEFORT

Laboratoire Chimie Nucldaire, Institat Physique Nucldaire, BP no. 1, 91406 Orsay, France Received 19 September 1973 (Revised 19 November 1973)

Abstract: Excitation functions for the reactions 40Ar-}-t44Sm--->Pt and *°Ar+166Er--~Po radionuclides, in which the observed nuclei have two protons less than the product o f the complete fusion o f projectile and target, have been found to be characterized by two peaks. Independent of whether the target was Sm or Er, and independent of the mass of the product, the two peaks were consistently found to be separated by ~ 40 MeV of excitation energy. Consideration o f other experiments with 4°Ar ions, and with lighter projectiles, leads us to the conclusion that the two peaks are due to different compound-nuclear de-excitation processes: the first peak involves the evaporation of an or-particle, (Ar, ~(x--2)n), and the second peak, the emission o f nucleons only, (Ar, 2pxn). El

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N U C L E A R REACTIONS t44Sm(Ar, 2pxn)Pt, 166Er(Ar, 2pxn)Po, E = 200-300 MeV, measured or(E). Deduced evaporation mechanism.

1. Introduction

As part of our continuing study of the nuclear reactions induced by 4°At ions and of the resulting radioactive nuclides 1), we measured the excitation functions for the reactions 4°Ar+ 144Sm ~ Pt nuclides, with two protons less than the product of the complete fusion of projectile and target nucleus. These curves were found to be qualitatively different from those g~nerally obtained for (Ar, xn) and (Ar, pxn) reactions in that they characteristically displayed two peaks for each product nuclide observed. The energy separation between the two peaks was found to be essentially constant, corresponding to 40 MeV of excitation energy. To test the generality of these results, the similar reactions 4°At + t 6 6 E r ~ Po nuclides, which are well-characterized, were studied. The same behavior in the excitation functions was observed for the Po as well as the Pt products. In this paper, we discuss these results, how they were obtained, and their possible interpretation. The conclusion is reached that the two-peaked excitation functions * Permanent address: Transuranium Research Laboratory, Oak Ridge National Laboratory, Oak Ridge, Tenn., USA. 84

144Sm(Ar, 2pxn)Pt, 166Er(Ar,2pxn)Po

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represent different evaporation chains in compound nucleus decay leading to the same final nucleus. The first peak is thought to involve ~-particle evaporation, (Ar, ~ x - 2)n), and the second peak, evaporation of nucleons only, (Ar, 2pxn).

2. Experimental procedure Targets of isotopicaUy separated* 1448m (95 %) and 166Er (96 %) were prepared by electrodeposition on Ni backing foils 1 mg/cm2 thick, and were bombarded by the 300 MeV 4°At beam obtained from the accelerator ALICE at Orsay. Foils of A1 were used to degrade the energy of the beam so that excitation functions could be measured. The values of the energy loss in these foils were calculated from the rangeenergy tables of Northcliffe and Schilling 2). The product nuclei that recoiled out of the target were collected by the gas-jet technique and the radioactivity assayed by ~-particle spectrometry. Our particular apparatus has been previously discussed in detail 3). In this work, a peculiarity of the collection of recoil nuclei by the gas-jet method proved to be an advantage. As mentioned in a previous work 4), radioactive Hg nuclei produced in ~4#Sm(4°Ar, xn) reactions could not be detected in the system, presumably for reasons that involve the chemistry of Hg and the collection mechanisms in gas-jet devices. Similarly, the noble gas Rn from : 66Er(4°Ar, xn) reactions is not normally collected unless special precautions are taken, as has been noted by Hyde et al. s). Thus in spite of their large formation cross sections and large ~-decay branching ratios, the Hg and Rn products of the (Ar, xn) reactions were not seen in these experiments. Their absence simplified the analysis of the ~-spectra measured. But this absence has a more profound result, for one may conclude that none of the Pt or Po activities observed were due to the radioactive decay of Hg and Rn, respectively. Such a conclusion is certainly warranted for Rn, which is pumped out of the system with the helium gas. And it is most probably true for the Hg also, since one would have to consider a very complicated process that would not allow the Hg activities to be detected, and yet would give a high efficiency for the collection and detection of the products of the Hg or-decay. More significantly, if the observed Pt activities arise from ~-decay of Hg, the Pt excitation functions will have tht esholds that are identical with those for 144Sm(Ar, xn) reactions. As will be shown later, this was not the case in our experiments.

3. Results The excitation functions for the production of Pt nuclides, with masses from 177 to 174, and of Po nuclides, with masses from 199 to 194, from 4°Ar bombardments ? Separated isotopes were obtained from Isotopes Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, and from AERE, Harwell, England.

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of 144Sm and 166Er, respectively, are shown in figs. 1-3. It is seen that all of these excitation functions have the same, general, two-peaked structure. Because of the characteristic kinetic energy difference between the two peaks of ~ 40-50 MeV, and because the highest energy beam on target was ~ 280 MeV, the second peak is not observed in fig. 3 for the products 196p0 to 194p0, which have their first peaks at energies ~ 250 MeV. Corrections were made for a-branching ratios of the various isotopes detected, in order to obtain the yields in figs. 1-3. In the case of 194, 195po ' they were assumed to be 100 ~ . F o r 196-199po and 174-177pt ' values were taken from the literature 6). Several possible explanations for this double-peaked structure may be envisaged and then discounted: (i) The two peaks may be due to (Ar, 2pxn) reactions on two different isotopes present in the targets. However, the isotopic enrichment of the f 1 4 4Iq ~

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144Sm(Ar, 2pxn)Pt, 166Er(Ar, 2 p x n ) P o

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144Sm and 166Er targets was of the order of 95 %. Also, as shown in figs. 4 and 5, the excitation functions for the reactions 144sm(g°Ar, p4n)179Au and 166Er (4°Ar, p4n)2°tAt measured in this same work, clearly do not exhibit the two peaks that would be due to isotopic contamination of the targets. The open circles and arrows in figs. 4 and 5 indicate that no counts were observed for either 1VgAu or 2°1At at bombarding energies where the second peak appears in figs. 1 and 2. (ii) The first of the two peaks observed may be due to Pt and Po nuclides coming from 0~-decay of Hg and Rn activities respectively. As discussed in sect. 2, we reject this possibility because of the absence of Hg or Rn a-peaks in the measured spectra. Moreover, the thresholds for the (Ar, xn) reactions required to lead by a-decay to the observed Pt and Po nuclides are much lower than those measured in this work. For example, based on previous experiments in this region of the periodic table t' 4, 7), the threshold for the production of t76pt by the reaction x44Sm(Ar, 4n)lS°Hg followed by ~ decay is expected to be ~ 165 MeV (lab system). In fig. l, it is clear that the ob-

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served threshold for 176pt is of the order of 200 MeV. (iii) The first of the two peaks observed may 'be due to (At, px'n)a, eaetitms, . f e t t ~ by .eleetron-c,apture decay. However, again the expected thresholds do not agree with those observed in figs. 1 and 2. For example, based on a measured threshold 4) of ~ 230 MeV for the (Ar, p6n) t77Au reaction, one expects the threshold to be ~ 240 MeV for the 176Au reaction (Ar, p7n) 176Au ~ 176pt" As mentioned above, the measured threshold for 176pt is ~ 200 MeV. It would then seem that the two peaks observed in these reactions (figs. 1 and 2) are due not to radioactive decay nor to isotopic impurities, but to nuclear reactions on ~44Sm and ~66Er that lead to Pt and Po nuclides. In the next section, we discuss various interpretations of the double-peaked excitation functions. i

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4. Discussion

Similar two-peaked excitation functions for reactions leading to products having two protons less than the composite nucleus (agglomeration of projectile and target) have been previously reported by several authors s- 13), in studies involving projectiles much lighter than *OAr. In irradiations of Na and Mg targets with protons or deuterons 8- 9) over a large range of bombarding energies, the excitation functions for light products such as 18F exhibited two peaks. These were interpreted as being due to multiple reaction paths leading to the same product. Further evidence of this sort lo) came from the interactions of ~ 0-I00 MeV protons with 59Co where the excitation function for the S9Co(p, 3p3n)54Mn reaction was found to have two peaks, with the second peak appearing at ~ 85 MeV bombarding energy. It was thought 10) that the first peak involved or-particle emission, probably from the compound nucleus, while the second peak most likely involved the emission of nucleons or deuterons in a direct, cascade process. In later studies of proton interactions 11) with heavier targets such as 2°9Bi and 232Th, two-peaked excitation functions were observed between 40 and 70 MeV with, however, the magnitude of the first peak being quite small relative to that of the second. These peaks were attributed to direct reactions, the first peak being interpreted as due to the knock-out of a (preformed) or-particle plus x neutrons from the nucleus, and the second peak as the emission of 2 protons plus x + 2 neutrons in a cascade-plus-evaporation process. With more complex projectiles, such as l°B or 12C, the experimental results lead to a different interpretation 12. 13). For example, in ref. 13), projected recoil ranges as well as cross sections were measured at several bombarding energies. The range data indicated that the first peak in the excitation function was due to a transfer reaction, such as (10B, ~t) where 6Li is transferred from the projectile to the target. The second peak, associated with range values expected for full-momentum transfer, was ascribed to the evaporation of an or-particle from the compound nucleus. In contrast to these studies with lighter heavy ions, it is difficult in the present work with 4°Ar as the projectile, to attribute the first peak in the observed excitation functions (figs. 1 and 2) to transfer processes. There are several arguments which lead us to discount the possibility of transfer reactions. Perhaps the most direct evidence comes from the work of Galin et aL 14) w h o studied the de-excitation of Te compound nuclei formed by 14N and 4°Ar ions. Measuring the energy and angular distributions of the light charged particles (p, d, t, ct and Li) emitted in these reactions, they found a large direct-reaction component in the I*N induced reactions; for example, the observed angular distributions showed a very pronounced peaking at forward angles. But the analogous data for the 4°At reactions were quite different. The angular distributions of protons and or-particles were symmetric about 90 ° (c.m. system) for charged particles of all energies, indicating almost no direct-reaction c o m p o n e n t .

144Sm(Ar, 2pxn)Pt, 166Er(Ar, 2pxn)Po

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If one can extrapolate these results to the heavier targets used here, the conclusion would be that the Pt and Po nuclides observed are due to compound-nucleus evapora40 tion of charged particles and not to an (18Ar, 00 reaction, in which ~2S is transferred to the target. Other results on transfer reactions support this idea. Thus, in experiments in which the masses and atomic numbers of the light products of 4°Ar bombardment of Th were determined at the grazing angle 15, 16), it was found that the cross sections were largest for few nucleon transfers leading to products such as CI and S, and decreased uniformly as the number of transferred nucleons increased. Although the lightest products observed in these experiments were Mg and A1, it is reasonable to expect the trend of decreasing cross section to continue as the very light products, such as He, are reached. In addition, recent studies with 4°Ar of multi-nucleon transfers in the rare-earth region, in which the radioactive, heavy product nuclei were detected, indicate the same trend, namely that the cross section decreases rapidly as the size (i.e., Z and A) of the transferred aggregate increases 17). In the same work cited 17), it was also found that the excitation functions for complex transfer reactions show a monotonically rising variation with increasing bombarding energy, not at all like the welldefined peaks observed in figs. 1 and 2. If the two-peaked excitation functions are not due to direct processes, about the only possibility left is that they arise from compound-nuclear processes. The main clue to their possible interpretation is that the difference in energy between the first and second peaks is essentially constant, independent of whether the target is Sm or Er, and independent of the mass of the product nuclide. Expressed in terms of excitation energy, this difference is ~ 40 MeV. Because the binding energy of an s-particle is 28 MeV, we conclude that the first peak in the excitation function for a given reaction involves the evaporation of an 0e-particle from the compound nucleus, and the second peak, at an excitation energy some 40 MeV higher, involves instead the successive evaporation of the 2 n + 2 p that would constitute an s-particle; that is, the peaks are due to (Ar, ~ ( x - 2 ) n ) and (Ar, 2pxn) reactions. This interpretation of a-particle evaporation is consistent with the results of ref. 14), which showed enhanced cross sections for s-particle evaporation in Ar induced reactions, relative to reactions induced by 14N. The very large values of angular momentum 18) brought into the compound nucleus by 4°Ar are expected to increase the probability of emission of 0~-particles, which can remove more angular momentum from the nucleus than can lighter particles. Further support for the involvement of compound-nuclear processes comes from comparison of the ratios of the magnitudes of the first and second peaks in the excitation functions for the Pt and Po isotopes. For ~76pt, this ratio is ,~ ½, indicating that the emission of 0t-particles is quite probable [in the p-induced reactions on heavy targets discussed earlier 11), the analogous ratio is around 1]. For the corresponding product, 198po, of the (Ar, 0~4n) reaction on t66Er, this ratio is ~ 4. This result does not mean, however, that the absolute probability of a-particle evaporation has in-

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creased in going from Pt to Po (in fact, the increasing C o u l o m b barrier should reduce the a-particle emission). Instead, we interpret this large ratio change to be due to the increased p r o b a b i l i t y of fission at each step in the de-excitation of the Po nuclides relative to Pt, thus, the cross section for the successive emission of light nucleons in the (At, 2p6n) ~98Po reaction becomes m u c h smaller t h a n that of the c o r r e s p o n d i n g (At, a4n) reaction. I n s u m m a r y then, the two peaks in the excitation functions are t a k e n to be due to two different e v a p o r a t i o n chains in c o m p o u n d - n u c l e u s decay that can lead to the same final product: the first peak being due to the (Ar, a ( x - 2 ) n ) reaction, a n d the second peak to the (Ar, 2 p x n ) reaction. The e n h a n c e d p r o b a b i l i t y for a-particle emission, relative to results obtained with lighter ions, would seem to be due to the large a n g u l a r m o m e n t u m values b r o u g h t to the c o m p o u n d nucleus by 4°Ar. The possibility of direct transfer processes c o n t r i b u t i n g substantially to the observed excit a t i o n functions is t h o u g h t to be quite small.

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