On the reactions of protons with 231Pa and 232Th

1

2.55

)

Nuclear Physics A185 (1972) 252-262; Not to be

@

North-Holland

Publishing

Co., Amsterdam

reproduced by photoprint or microfilm without written permission from the publisher

ON THE REACTIONS

OF PROTONS

WITII 231Pa AND 232Tb

R. L. HAHN t, K. S. TQTH and M. F. RQCHE tt Oak Ridge National Oak Ridge,

Laboratory,

Tennessee 37830

Received 20 September 1971 (Revised 22 November 1971) Abstract: Yields of n-radioactive products from reactions induced in 231Pa targets by protons from 35 to 63 MeV were measured by recoil-collection techniques. The results, as similarly observed by Lefort and co-workers for the reactions of 232Th+p at energies $ 85 MeV, indicate that the yields of reactions involving charged-particle emission are comparable to or larger than those involving only neutron emission. In addition, the experimental yield curves exhibit high-energy tails. Results for both the 231Pa and 23ZTh targets are compared with the predictions of nuclearreaction calculations that take into account the competition between fission and particle emission. Bearing in mind the assumptions inherent in the calculations and the fact that no parameters were fitted to the data, we found that the compound-nucleus model, with fission, did not account for the experimentally observed trends. The intra-nuclear cascade model, including fission competition in the compound-nuclear de-excitation phase, on the other hand, predicted excitation functions that were reasonably consistent with the results for (p, xn) and (p, pxn) reactions. Neither model was successful in accounting for the (p, cc3n) data. In the case of 231Pa results, it was found that recoil-range effects had to be included in the nuclear-reaction calculations.

E

NUCLEAR REACTIONS 231Pa(p, 4n)2z8U, (p, 5n)227U, (p, p4n)227Pa, @, p5n) 226Pa. (n. Ct3n)225Th. E = 35-63 MeV: measured relative yields of recoil nuclei.

In our studies of cl-particle emitting nucleides of neptunium “) and uranium “) produced in reactions with 5 65 MeV protons, the yields of (p, pxn) and (p, axn) reactions were often found to be larger than those of the desired (p, xn) products. Similar effects have been extensively reported by Lefort et al. 3-8) for proton energies 5 155 MeV, as well as by others at much higher energies. (Ref. “) discusses some of this high-energy work; also see ref. ’ “) for a comprehensive review.) To investigate this phenomenon further, we measured yields for several products of the reactions of 231Pa+p at relatively low energies, 35 to 63 MeV. Because recoilcollection techniques were used in these experiments, the reaction yields reported here do not represent absolute cross sections, but depend upon recoil-range effects in the 231Pa targets. Nowever, excitation functions radiochemically determined by T Research sponsored by the US Atomic Energy Commission under contract with the Union Carbide Corporation. tt Present address: Argonne National Laboratory, Argonne, Illinois 60439. 252

REACTIONS

2.53

OF PROTONS

Lefort and co-workers 3- 6*8) for reactions of 232Th+p are very similar to our results, indicating that the 231Fa+p data are not very sensitive to effects associated with recoil range. Theoretical excitation functions, obtained from nuclear-reaction calculations that include the competition between fission and spallation as described in the preceding paper g>, are here compared with data from the reactions of protons with 2311’a and 232Th. C~c~lations are presented both for reactions that proceed via compoundnucleus formation and decay and via the intra-nuclear cascade. 2. Experimental procedure In this section, we very briefly describe some of the features of the experiment. Many of the details of technique and apparatus have been previously reported in the literature 1f“>. The reactions of 2311?awith protons were studied on-line at the Oak Ridge Isoc~onous Cyclotron. The target assembly used in the experiments provided for the colIection of recoil nuclei ejected from a target followed by their rapid assay. Combining this recoil-collection method with a-particle spectrometry overcame several problems, such as the handling of 231Pa targets with a-decay rates of M 106-lo7 disintegrations/mm, the elimination of fission-product backgrounds, and the preparation of thin counting sources, that would have been encountered had standard radiochemical techniques been used. Special care was given to separating 2311?afrom its decay daughters ‘I) prior to the fabrication of targets. After the separation, the concentration of radioactive daughter atoms from the 231Pa decay chain was M lo-’ relative to 231Pa. Targets of protactinium were then prepared by following a published procedure ‘, “). Table 1 lists the a-decay characteristics for the products investigated. The a/total branching ratios listed are all experimental values I3114) except for the case of 2271J.

Alpha-decay Parent of a-decay chain

TABLE1 characteristics of reaction products “)

Half-life (min)

or/total

Prominent energies (MeV) and nucleides b,

9.3

0.95

1.1 “) 38.3 1.8 8.0

1 d, 0.85 0.74 0.9

&.78(2”2Po) 8.67(*‘%n) X.ol(2~sAt) 8.78(2z4At), 7.84(218Fr) 8*38(213Po)

“) See refs. 2*13*14) for details of the decay chains. b, These cr-particles, emitted by the nucleides noted in parentheses, were used to determine the yields of the parents of the respective decay chains. “) See ref. 2). d) Predicted value from cc-decay systematics 15) is % 0.86. Because of uncertainties in the systematits, a value of x/total = 1 was adopted.

R. L. HAHN et OZ.

254

(cf 63 MeV =AC

ZOOMev

60

?--At

40 20

E =I

0 ~

0 ,.

(b)

4’

-- ”

(0)

44

MeV

60

20 400 : 0

20

40

60

80 100 CHANNEL

120

140

i60

180

~

200

NUMBER

Fig. 1. Alpha-particle spectra from reactions of 231Pa+p at 44 and 63 MeV. Parts (a) and (c) were taken shortly after the bombardments; parts (b) and (d), at later times.

From decay systematics “1, t h e a- d eta y mode is predicted to be dominant for 227U, with cl/total w 0.86; however, because of uncertainties in the calculated partial half-lives, we adopted a/total = 1. 3. Resalts

Care was taken in the experiments to determine that 227Pa, “‘Pa. and ” 5Th were indeed products of rmclear reactions involving charge-particle emission, and were not produced either by ol-decay or by reactions on impurities in the target, Fig. I shows some typical cl-spectra obtained for the reactions ” IPa + p. At the bombar~ng energy of 44 MeV, the spectrum (a) taken during the First 2.5 min after bombardment was dominated by the decay chain of the (p, 5n) product, 227U; in (b), taken from 9 to 20 min after bombardment, the “‘U decay chain from the (p, 4n) reaction was readily apparent. Note, however, that peaks due to the decay chains of 227Pa, the (p, p4n) product, and 225Th, the (p, a3n) product, also contributed appreciably to spectrum (b). At the higher bombarding energy of 63 MeV, neither of the (p, Xn) products, “%I or 227U, was seen to any great extent. In spectrum (c) of fig. 1, taken shortly after the bombardment, the (p, ~5x1) product, “‘Pa and its decay daughters predominated, while the decay chains of 227Pa, 225Th, and possibly “?Th, were seen at later counting times in s~trum (d)_

23iPct +p 102 i

(p,p4nFPa T

’

30

1’

11 40

11

11

50 Ep (MeVI

11

11

I

60

Fig. 2. ~x~e~~e~t~ yields observed for the reactions of =mx+p. Note that wse I-&a% yields have not been corrected for recoil-range effects; see text for dkmsion of this point.

256

R. L. HAHN et al.

The experimental yields of the cl-active products observed in the reactions of 231Pa+p at various bombarding energies Ep are shown in fig. 2. These yields have been corrected for differences in beam intensity, bombardment time, and a/total branching ratios (see table 1). However, no corrections have been applied to the data to account for the fact that the 231Pa targets were thick compared to the ranges of the recoil nuclei 16-r8 ). Thus, the data in fig. 2 are not simply proportional to cross section, but also depend upon recoil range I’), which in turn varies with bombarding energy. Note, however, that these relative yields were reproducible with different targets at selected bombarding energies. The error bars on the points represent statistical uncertainties due only to experimental counting rates. The main conclusion that we draw from fig. 2 is that the yields of the (p, p4n), (P, p5n) and (P, CI3n ) reactions are comparable to the (p, 4n) and (p, 5n) yields. To demonstrate that this conclusion is consistent with trends observed by other experimenters, and that our yield curves are not drastically altered by recoil-range effects, we reproduce our data as smooth curves in fig. 3 along with radiochemically determined excitation functions 3-“) for similar reactions of protons with ’ OgBi and I

i0

30

2ogBitp

50

70

=Th

90

30

50

tp

70

I

I

23’Pa t p

I

I

90

Ep (MeV)

Fig. 3. Comparison of relative yields from reactions induced by protons on 2ogBi [ref. 7], 232Th [refs. 3 --6* 8, ] and 231Pa (this work). For each target, the yields are normalized so the peak of the (p, 5n) curve is set equal to unity.

REACTIONS OF PROTONS

257

232Th. All of the data are normalized so that the peak of the (p, 5n) curve for each target is set equal to unity. For the 2“Bi reactions ‘), where the influence of fission is relatively minor, the (p, an) cross sections are similar in magnitude (the (p, 5n) peak value is 610 mb); the (p, pxn) peak cross sections are w 0.25 of the (p, xn) values; and the (p, a3n) cross section is quite small. For the heavier targets, effects due to the spallation-fission competition are evident. All of the excitation functions have decreased in magnitude for the 232Th reactions 3-6, “) (the (p, 5n) peak value here is 55 mb), and the (p, 6n) cross sections are much smaller than the (p, 5n) values. Relative to the (p, xn) curves, the yields of the reactions involving charged-particle emission have increased by a factor of 2 3 as compared to the 2og13i case; the (P, pm) and (P, CI3n ) reactions are not as adversely affected by the increased competition from fission as are the (p, xn) reactions. These qualitative differences that arise due to the increased probability of fission in going from 83Bi to gOTh are even more apparent for 2z:Pa. 4. DiscussiQn In this section, we consider the mechanisms of proton-induced reactions on easily fissionable nuclei by comparing our calculations “) with the data for 232Th and 231Pa. To obtain adequate statistics in the Monte Carlo computations, 1000 cases were run at each energy with the compound-nucleus model, and 2000 cases each with the intra-nuclear cascade model. In most instances, statistical uncertainties in the calculated cross sections are S 10 %, while for the lowest cross-section values, they are 5 30%. 4.1. THE 232Th RESULTS

As discussed above, the 232Th results 3- 69“) c1ear ly sh ow effects due to spallationfission competition. Accordingly, in fig. 4 we present the thorium data-points, connected by smooth curves, and calculated excitation functions that include such effects. The curves on the left side of the figure are based on the assumption that the interaction of projectile and target nucleus always leads to compound nucleus 233Pa. The curves on the right are the result of calculations in which the intra-nuclear cascade is the initial process. The compound-nucleus model obviously does not succeed in reproducing even the qualitative features of the data. The high-energy tails on the (p, xn) excitation functions are not matched by the sharp fall-off of the calculated curves, and the predicted magnitudes of the Cp,pxn) reactions are at least two orders of magnitude smaller than the isolated experimental values at 83 MeV. Assuming that the reaction proceeds via the intra-nuclear cascade improves the agreement between theory and experiment. The (p, xn) data, including the highenergy tails, are reasonably followed by the calculations, and the (p, pxn) curves are now low only by a factor of w 3-4 at 83 MeV.

258

R. L. HAHN et al.

-DATA

OF LEFORT 0

ET AL. FOR 232Th

(p,5n)

A (p,6n) l

227Pa

( p, p4n)228Th

A (p,pSn 0

---

t p

228Po

1227Th

(p, d3n)

226Ac

COMPOUNDT-bWLEAR

-( (

d 30

40

50

60

70

80

30

40

50

60

70

80

E,(MeV)

Fig. 4. Results of nuclear-reaction calculations, including fission competition ‘), compared with the data from refs.3-6* 8, for 232Th. The dashed curves on the left-hand side are based on the assumption that the reaction proceeded via compound-nucleus formation and decay; those on the right side result from the intra-nuclear cascade model.

We consider this improvement in going from the compound-nucleus to the cascade model to be significant. The fits to the data are certainly not quantitative, but the intra-nuclear cascade, with fission included, does succeed in reproducing the main features of the (p, xn) and (p, pxn) data. This observation, that the intra-nuclear cascade model, which was originally intended to apply to high-energy nuclear reactions lo, ‘O), appears preferable to the compound-nucleus model in explaining data obtained at low bombarding energies, has also been noted in refs. 21F““). We also point out that our calculational methods do not allow for parameter fitting; the level density parameter a was set equal to &A when the computations were begun, and all other quantities are fixed in the codes “). Neither model, however, is able to account for the large experimental (p, a3n)

REACTIONS

OF PROTONS

259

cross sections. Evaporation of a-particles from the compound nucleus, although allowed for in our model, has a small probability; and only neutrons and protons are emitted in the cascade. Thus, the predicted cross sections for the (p, a3n) reaction are too small even to appear in fig. 4. Similar results, obtained by the Orsay group in studies of (p, czxn) and (p, paxn) reactions on bismuth 7*““) and (p, axn) reactions on thorium 5), have led them to postulate the direct knock-out in such reactions of a-particle clusters from the nuclear surface. 4.2. THE 231Pa RESULTS

Much of the above discussion about compound-nucleus predictions for 232Th also applies to 231Pa*, the results of such calculations for the reactions of 231Pa+p are shown in fig. 2 of the previous paper “) and are not repeated here. Suffice it to say that although the theoretical (p, pxn) yields from compound-nucleus evaporation are not negligible relative to the (p, xn) yields, they are still substantially smaller than the experimental values; the cascade model gives better agreement with experiment. Detailed comparison of the cascade calculations, in which fission effects have been included, is made with the relative yield curves for 231Pa on the left side of fig. 5. /

,

I

-DATA

I FROM

---INTRA-NUCLEAR

/ THIS

CASCADE

, WORK

I

I

FOR

,

I

,

23’Po+p

---INTRA-NUCLEAR CASCADE. CORRECTED FOR RANGE- AND ANGULARDISTRIBUTION EFFECTS

i0*

~~~’

/‘,“\, ‘\ / / \ v’ \

/ I

Id’ 30

40

I

I 50

1’ \ I

I 60

I 30 Ep (MeV)

40

I

I 50

I

, 60

40-l

Fig. 5. Intra-nuclear cascade predictions compared with the yields of 231Pa+p from this work. The dashed curves on the left side of the figure are from the usual cascade calculations 9, while those on the right side are from calculations that include effects associated with the escape of the recoil nuclei from the target (see text for a detailed discussion). Both sets of calculations are normalized to the data at the peak of the experimental (p, 5n) curve.

R. L. HAHN

260

et al.

The calculated values have been normalized to the smoothed experimental curves at the peak of the (p, 5n) excitation function, and it is seen that the calculated (p, pxn) yields are now predominant at bombarding energies 2 50 MeV. Again, the cascade calculation cannot even crudely reproduce the (p, a3n) yield curve. Perhaps the most striking feature in fig. 5, in contrast to the situation for 232Th, is the fact that the high-energy tails on the calculated (p, xn) curves are much more pronounced than on the experimental curves. As a possible explanation of this latter difference, it is again noted that the 231Pa data include effects associated with the collection of recoil nuclei. Because the range of the recoil nucleus depends upon its energy and angle I’), which in turn depend upon the details of the nuclear-reaction mechanism, any corrections made to the data would be model dependent. It was thus decided not to attempt to correct the data, but to include the recoil effects in the nuclear-reaction calculations “). A brief discussion of the modifications required in the codes follows; more details are given in ref. ‘“). The Monte Carlo programs, which calculate the step-wise emission of particles in compound-nucleus or cascade reactions, were altered to calculate the velocity and angle imparted to the nucleus by each such emission step. These calculations then gave for each product nucleus a distribution of the number of nuclei ni with recoil kinetic energy Ei moving at laboratory angle Bi with respect to the beam direction. The theory of Lindhard, Scharff and Schistt 17, ’ “) (LSS), with its linear dependence of recoil range upon recoil energy, was used to calculate average recoil ranges R iEzi;;d onto the beam direction) in the protactinium targets according to the

W

=

CiniPi

(PIR)

cos%i . C *i

Here p is a dimensionless quantity detined by LSS and related to the range R; calculations were performed with the parameter ’ “) k = 0.18. Provision was also made in the calculations to include the possibility that the recoils were collimated because our experimental apparatus accepted recoils emitted only within M 34” of the beam axis. These calculations gave, for any selected reaction, the fraction of recoil nuclei (5 1) that escaped from the target and impinged upon the catcher foil, and were used to obtain reaction yields that could be compared with the 231Pa data. The yields calculated with the intra-nuclear cascade model, corrected for recoil effects, are compared with the data in the right half of fig. 5. The calculations are normalized to the data at the (p, 5n) peak, and it is seen that including recoil effects somewhat improves the agreement between experiment and theory. However, the poor agreement between calculated and experimental (p, a3n) curves noted previously appears to be made even worse by the inclusion of recoil effects in the calculations.

REACTIONS

OF PROTONS

261

5. Conclusions

Nuclear-reaction yields obtained for the interactions of protons with 231Pa (from this work) and with 232Th [from refs. 3-6* “)I are characterized by the facts that (i) the yields for reactions involving the emission of charged particles, especially protons, are comparable to or larger than those involving neutron emission only, (ii) the yield curves have high-energy tails and (iii) these effects become pronounced at relatively low proton bombarding energies, 2 45 MeV. Theoretical analyses of these reactions were performed with both the compound-nucleus and intra-nuclear cascade models in which effects due to fission-spallation competition are included “). Rearing in mind the assumptions inherent in the calculations and the fact that no parameters were fitted to the data, we found that, although the calculations allowed charged-particle evaporation to occur in competition with neutron emission and fission, the compound-nucleus model, with fission, did not account for the experimentally observed trends. The intra-nuclear cascade model, including fission competition in the compound-nuclear de-excitation phase, on the other hand, predicted excitation functions that were reasonably consistent with the results for (p, xn) and (p, prm) reactions. Neither model was successful in accounting for the (p, a3n) data. We wish to thank 0. W. Hermann for his able cooperation in modifying and running the nuclear-reaction codes. The capable work of J. L. Roberts and 6. G. Brantley in purifying and electro-depositing the 231Pa targets is greatly appreciated. Finally thanks are due to the ORIC operations crew for their help and cooperation. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17)

R. L. Hahn, M. F. Roche and K. S. Toth, Nucl. Pbys. All3 (1968) 206 R. L. Hahn, M. F. Roche and K. S. Toth, Phys. Rev. 182 (1969) 1329 M. Lefort, G. N. Simonoff and X. Tarrago, Nucl. Phys. 25 (1961) 216 C. Brun and G. N. Simonoff, J. de Phys. 23 (1962) 12 H. Gauvin, M. Lefort and X. Tarrago, Nucl. Phys. 39 (1962) 447 H. Gauvin, J. de Phys. 24 (1963) 836 Y. Le Beyec and M. Lefort, Nucl. Phys. A99 (1967) 131 B. Gatty, M. Lefort and X. Tarrago, J. de Phys. 29 (1968) 45 R. L. Hahn, Nucl. Phys. A185 (1972) 241 E. K. Hyde, The nuclear properties of the heavy elements, vol. III, Fission phenomena (PrenticeHall, Englewood Cliffs, New Jersey, 1964) F. L. Moore and J. R. Stokely, private communications C. Ferradini, J. Chim. Phys. 53 (1956) 714 C. M. Lederer, J. M. Hollander and I. Perlman, Table of isotopes, 6th ed. (Wiley, New York, 1967) E. K. Hyde, I. Perlman and G. T. Seaborg, The nuclear properties of the heavy elements, vol. II, Detailed radioactivity properties (Prentice-Hall, Englewood Clitfs, New Jersey, 1964) K. A. Keller and H. Miinzel, Nucl. Phys. A148 (1970) 615 J. Lindhard, M. Scharff and H. E. Schiott, Mat. Fys. Medd. Dan. Vid. Selsk. 33 (1963) no. 14 P. G. Steward, University of California report no. UCRL-18127, 1968, unpublished

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18) L. C. NorthchfTe and R. F. SehiIIing, Nucl. Data A7 (1970) 233

19) J. M. Alexander, Studies of nuclear reactions by recoil techniques, in Nuclear chemistry, Collective vol. f, ed. L. YatTe (Academic Press, New York, 1968) p. 273 20) R. Serber, Phys. Rev. 72 (1947) 1114 21) V. V. Verbinski and W. R. Burrus, Phys. Rev. 177 (1969) 2671 22) R. G. Alsmiller, Jr. and 0. W. Hermann, Nucl. Sci. Eng. 40 (1970) 254 23) R. Bimbot and M. Lefort, J. de Phys. 27 (1966) 25 24) R. L. Hahn, Oak Ridge National Laboratory report no. ORNL-TM-3179,1971, unpublished 25) H. E. Schiatt, Mat. Fys. Medd. Dan. Vid. Selsk. 35 (1966) no. 9