A comparison of reactions induced by medium-energy 3He and 4He ions in heavy target nuclei

2.A.l: [ 2.D

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NuclearPhysicsAll9 (1968) 131--145;~)North-HollandPublishing Co., Amsterdam Not to be reproduced by photoprint or microfilmwithout writtenpermissionfrom the publisher

A COMPARISON

OF REACTIONS INDUCED

BY M E D I U M - E N E R G Y 3He A N D 4He I O N S I N H E A V Y T A R G E T N U C L E I N. E. SCOTT t, J. W. COBBLE and P. J. DALY

Chemistry Department, Purdue University, Lafayette, lndiana 47907 tt Received 17 June 1968 Abstract: Induced activity methods have been used to measure the cross sections of many reactions induced by 10-33 MeV SHe ions and 10--44 MeV 4He ions on a number of target nuclei in the mass range A = 180-210. It is shown that (4He, xn) reactions account for more than 90 of the optical-model total reaction cross section in this mass region, and that these reactions proceed predominantly by compound nuclear mechanisms. On the other hand, (3He, xn) reactions account for only about half of the 8He ion total cross section. Using the independence hypothesis, it is argued that at least 40 ~ of all reactions induced by 3He ions in these nuclei involve charged-particle emission, and that direct reaction mechanisms are dominant. With the possible exception of the (3He, n) reaction, the (3He, xn) results are consistent with compound nuclear reaction mechanisms. E

NUCLEAR REACTIONS lSlTa(4He,xn), xseW(4He,xnyp), EaHe = 10-44 MeV, measured tY(E4He). lSXTa,207pb, 20spb(ZHe,xn), aSlTa, aseW, lSSRe, lSVRe,2°°Bi(3He,xnyp), E3He = 10-33 MeV, measured tr(E3He). Natural and enriched targets. 1. Introduction

M a n y experimenters have used i n d u c e d activity m e t h o d s to study nuclear reactions i n d u c e d by 5-50 MeV 4He ions. M e a s u r e m e n t s of excitation functions a n d recoil ranges have established clearly that most 4He i o n - i n d u c e d reactions proceed almost exclusively by c o m p o u n d nucleus f o r m a t i o n a n d decay. I n the case of h i g h - Z comp o u n d nuclei, charged-particle emission is severely inhibited by C o u l o m b effects and, accordingly, reactions of the type (4He, x n ) make the d o m i n a n t c o n t r i b u t i o n s to the total reaction cross section of 4He ions incident o n heavy target nuclei. I n contrast, 3He i o n - i n d u c e d reactions have been explored to a m u c h lesser extent. M o s t interest has centred o n the stripping a n d pick-up reactions which are favoured by the relatively weak b i n d i n g of the aHe nucleus; these processes have been widely exploited in reaction spectroscopy studies. Some induced-activity experiments have been performed using target nuclei of low a n d intermediate masses, a n d evidence for b o t h c o m p o u n d nucleus a n d direct reaction mechanisms has been reported 1,2). Experimental data o n the d o m i n a n t reactions i n d u c e d by 3He ions in heavy target nuclei a p p e a r to be completely lacking. * From the Ph.D. thesis of N. E. Scott; present address: Bettis Atomic Power Laboratory, West Mifflin, Pa. ** Work supported by the U.S. Atomic Energy Commission. 131

132

N.E. SCOTTet

al.

The experiments reported here involved a comparative study of the excitation functions of the principal reactions induced by SHe and *He ions incident on heavy target nuclei. Compound nuclei formed at the same excitation energy (and with not very different angular momentum distributions) in SHe ion and 4He ion bombardments would be expected to decay rather similarly. As compound nucleus processes are known to be dominant in most of the 4He ion-induced reactions, clear-cut evidence about the principal mechanisms involved in the various reactions induced by 3He ions can be deduced from excitation function measurements. In particular the magnitude of the contributions of the stripping, pick-up and other direct processes to the SHe ion total reaction cross section can be established.

2. Experimental procedure 2.1. SELECTION OF TARGET NUCLEI The choice of heavy-element target nuclei was determined primarily by the existence of reliable decay data for the reaction products of interest. One suitable target nucleus Xa!Ta was used to determine excitation functions for (aHe, xn) and (*He, xn) reactions. However, there are some reactions involving charged-particle emission that could not be studied using tantalum targets and instead cross sections for the equivalent reactions in various tungsten and rhenium isotopes were measured. Sets of (SHe, xn) excitation functions were also determined using enriched 2°Tpb and 2°8pb targets, as the results could be readily compared with data for (4He, xn) reactions in lead target nuclei which had already been reported 3, 4). It was particularly difficult to find a target nucleus suitable for reliably estimating cross sections for the (SHe, pn) stripping reaction; the reaction 209Bi (3He ' pn)210po was finally chosen for study. 2.2. TARGET PREPARATION Tantalum, tungsten and rhenium targets consisted of foils stamped from thin sheets of the metals using a die of known diameter. In bombardments of enriched l S6W, samples of 186WO3 powder were wrapped in high purity aluminium foil. Lead targets were prepared by electrodeposition onto nickel backing foils from dilute solutions of isotopically enriched Pb(NO3)2 in 3 7o sulfamic acid solution. Bismuth targets were prepared by vacuum evaporation onto aluminium backing foils. Target thicknesses were determined in all cases by weight and area measurements, and they ranged from 80 to 7000 # g - c m -2. 2.3. BOMBARDMENT CONDITIONS Most of the bombardments were performed using the external 33 MeV SHe ion and 44 MeV 4He ion beams of the 152 cm cyclotron at Argonne National Laboratory. In one case, a bombardment with 65 MeV 4He ions from the Berkeley 224 cm cyclotron was performed. Typically, a target stack of 8-10 foils was held in a water

I-Ie I N D U C E D

REACTIONS

13 3

cooled holder and irradiated with 0.5-1.0/~A of helium ions for 1 h. Integrated beam intensities were determined by measuring the charge collected on the target holder using a precision current integrator. The energy of the beam incident on a stack of targets was determined by aluminium range measurements immediately after each bombardment. The bombarding energy at any position in a stack was calculated from range energy relationships for a He a n d * H e ions based on the results of Bichsel et al. 5) for proton ranges in aluminium. The energy resolution of the incident cyclotron beam was about 0.5 MeV F W H M . When the beam was degraded to 20 MeV mean energy in a typical stack, the energy spread was about 1.5 MeV F W H M . 2.4. CHEMICAL PROCEDURES Standard radiochemical procedures 6,7) were followed whenever chemical separation of product elements was necessary. Rhenium was precipitated as tetraphenyl arsonium perrhenate and tantalum and tungsten as the respective oxides. Following the lead bombardments, polonium was quantitatively extracted a) by spontaneous deposition on silver foils from 1 M HC1. In the case of the bismuth targets, astatine was extracted into diisopropyl ether before the polonium separation. The 21°p0 accruing from the decay of 21 OAt was later extracted on to silver foils. 2.5. RADIOACTIVITY MEASUREMENTS Most of the product activities were determined by y-spectroscopy using a 11.6 cm a Ge(Li) detector in conjunction with a 1024-channel pulse-height analyser. In a few cases, high detection efficiency rather than good energy resolution was desirable, and a shielded 7.6 cm x 7.6 cm NaI(Tl) crystal was employed. Some of the polonium activities were measured by a-spectroscopy using a gold-plate silicon surface-barrier detector and a 400-channel analyser. The energy and efficiency characteristics of all detectors were determined using standard radioactive sources. Table 1 lists the characteristic radiations used in identifying the product activities and the branching ratios assumed in calculating the cross sections for their production. 2.6. TREATMENT OF ERRORS In the following presentation of experimental results, the error bars on individual excitation functions represent relative errors, while those on sums of excitation functions include both relative and absolute errors. The relative errors arose from uncertainties of _+5 ~o in chemical yield measurements and 3-10 ~ in radioactivity measurements. In a few cases, larger errors arising from such factors as uncertainties in photopeak integration or in decay curve resolution are included. Additional errors in absolute magnitude amounting to an estimated 4-10 ~ are added for uncertainties in detector efficiency calibrations and beam current measurements. The energy error bars which are shown arise from the initial beam energy spread and from straggling.

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N . E. S C O T T e t

al.

TABLE 1 Decay characteristics of the product nuclei Nucleide

Half-life

y-ray energy (keV)

Assumed branching ratio

Ref.

Xa°Re lalRe lSaRe la2Re ISaRe aa4Re XS6Re lSSRe SabRe X7STa lS°Ta lsYW lssW lS6Ir 2°7p0

2.45 rain 19.9 h 12.9 h 64 h 71 d 38 d 90 h 16.7 h 24 h 2.1 h 8.1 h 24 h 69 d 16 h 5.7 h

902 365 1122 1122 162 904 137 155 245 214 103 686 290 297 993

1.0 0.65 0.38 0.20 0.23 0.40 0.095 0.095 0.04 0.84 0.044 0.33 0.008 0.40 0.60

a) b) e) e) e) d) a) a) a) a) d) a) a) e) e)

Nucleide

Half-life

~-energy (keV)

Assumed branching ratio

Ref.

2°6Po

2°sPo 21°Po a) Ref. 9).

8.8 d 2.9 y 138 d b) Present work.

5224 5115 5305

0.05 0.99 1.00 e) Ref. lo).

o) a) o) a) Ref. n).

3. Experimental results 3.1. REACTIONS INDUCED BY 4He IONS I n the d e t e r m i n a t i o n s of l SlTa(4He, x n ) l S S - ~ R e excitation functions, chemical separations were n o t necessary because of the high energy resolution of the G e ( L i ) detector. As the a b u n d a n c e per decay of the intense 365 keV ?-ray of l SiRe could n o t be inferred with certainty from existing decay data, it was necessary to carry out the following determination. The intensity of the 365 keV activity in a freshly separated r h e n i u m sample was accurately measured. Two weeks later the daughter tungsten activities were separated f r o m the sample a n d counted. As the decay scheme of ~81W is well established 12), its activity was m e a s u r e d absolutely a n d used to calculate the n u m b e r of l SiRe nuclei in the original sample when the 365 keV activity h a d been measured. A b r a n c h i n g ratio of 0.65_+ 0.05 365 keV ?-rays per decay of 1a 1Re was thus established. The half-life of 1 s 1Re was redetermined to be 19.9 + 0.7 h. Two 182Re isomers having half-lives of 12.9 h a n d 64 h are p r o d u c e d in the ~s ~Ta (4He, 3n) reaction, a n d p r o d u c t i o n cross sections for each isomer were determined.

He INDUCEDREACTIONS

135

The decay of the 1122 keV y-ray activity c o m m o n to b o t h isomers was followed for a period of 19 d, a n d the decay curve was then resolved into its two components. The m a x i m u m cross section for the p r o d u c t i o n of the higher spin 64 h isomer was f o u n d to occur approximately 1.5 MeV higher in b o m b a r d i n g energy t h a n the m a x i m u m for the 13 h isomer production. Qualitatively, this result is readily explained as the TABLE2 lalTa(4He, xn)taS-~Re cross sections 4He energy (MeV)

(4He, n) (mb)

(4He, 2n) (rob)

17.1 19.2 21.0 22.8 24.5 26.1 27.3 27.6 28.7 29.1 30.5 31.5 31.9 33.3 34.0 34.6 35.3 35.9 36.6 37.1 37.6 38.3 38.8 39.5 40.7 41.0 41.8 43.2 44.2 47.8 51.2 55.0 59.7 64.4

11.3 44.2 44.7 28.2 22.9 17.1

1.6 49.6 222.6 417.7 696.9 869.1

15.0

869.6

13.8 12.9

773.9 559.5

12.1 10.3

382.6 262.1

10.0

196.8

7.9

138.3

(4He, 3n) (mb)

(4He, 4n) (mb)

18.1 158.7 718.8 1130.6 1158.8 17.9 1266.3 91.0 1215.0 6.6

110.4

205.2 997.2

6.3 6.0

95.0 76.4

5.9

79.4

377.2 573.4 896.5 757.4 554.6 362 192 139 64

1247 1286 1001 632 323 108

c o m p o u n d nucleus average a n g u l a r m o m e n t u m increases with increasing 4He i o n b o m b a r d i n g energy. The total cross sections for the p r o d u c t i o n of l SZRe a n d the results for the other l S l T a ( 4 H e , x n ) reactions are shown in table 2 a n d in fig. 1. Cross sections for a n u m b e r of charged-particle emission reactions induced by 4He ions in 1S6W were also measured, a n d the results are included in table 3. The

136

N.E. SCOTTet al.

reaction i S6w(4I..ie ' p)1S9Re was studied using a natural tungsten foil target as the presence of the lighter stable W isotopes could not interfere with the measurements. The other reactions listed were studied using a target of WO3 enriched to 97.1 ~o in ls6W, and the cross sections were calculated from the ratios of the ~STw, ~ssw, a S6Re and l SSRe activities to the l S9Re activity. Excitation functions for (4He, xn) reaction in 2°Tpb and 2°spb have previously been measured by John 3), and the excitation function for the 2°spb(4He, n)211po reaction has been reported by Spiess *). We measured cross sections at three bombarding energies for the 20 spb(4He ' 3n)209po reaction and obtained results which are essentially in agreement with those of John a).

15

25

3.5

4b

~0

~.~

4He Ion Bombarding Energy (MeV) Fig. 1. Excitation functions for XatTa(4He,xn)laS-~Re reactions. 3.2. REACTIONS INDUCED BY SHe IONS Cross sections for the (3He, xn) reactions in 181Ta were determined using methods similar to those described earlier for the (4He, xn) reactions, except that chemical separation of rhenium was necessary to avoid i n t e r f e r e n c e from tungsten activities in determining the (SHe, xn) excitation functions. The results are shown in table 4 and infig. 2. Excitation functions for the (3He, xn) reactions in 2°7pb and 2°spb were determined using ~-spectroscopy to measure 2°7p0 activities and ce-spectroscopy for

He INDUCED REACTIONS

137

2°6p0, 2°Spo a n d 21Op0 activities. Yields o f 103 y 2 O9po were n o t in general m e a sured because o f the l o n g half-life. T h e 2°7pb targets c o n t a i n e d 5.5 ~o 2 ° s p b a n d cross sections d e t e r m i n e d in this w o r k for the heavier i s o t o p e were used to correct the 2°Tpb(3He, x n ) data. T h e results are s u m m a r i z e d in tables 5 and 6. M a n y different target nuclei were used in studying the charged-particle emission reactions i n d u c e d b y aHe ions, a n d the cross sections are listed in table 3. In the m e a s u r e m e n t s involving the reaction 2 O9Bi(aHe, pn)2t Opo ' it was necessary to correct TABLE 3 Cross sections for reactions involving charged-particle emission Reaction 2°~Bi(~He, pn)~Xopo

lS~Re(SHe, 0c)lS6Reand 185Re(aHe, 2p)iSeRe XS~Re(SHe,~)tS4gRe lSlTa(SHe, or)Is°raTa

ls6W(3He, 2p)lSTW lSlTa(ZHe, ~t2n)lTSTa

1seW(nile, p)lSeRe lssW(3He, pn)lS4Re ls6W(SHe, p2n)~SeRe ls6W(4He, p)lSgRe lseW(4He, pn)lSSRe xs6W(4He, p3n)lS6Re lseW(4He, 2pn)asvW

Bombarding energy (MeV) 24.7 27.0 29.1 31.0 32.9 34.7 17.3 24.8 30.6 17.3 24.8 30.6 25.5 28.1 30.4 32.7 19.6 28.2 31.6 25.5 28.1 30.4 32.7 31.6 31.6 31.6 43.4 43.4 43.4 43.4

Cross section (mb) 87.0 108.5 142.0 117.0 117.5 94.8 7.4 32.2 48.2 4.3 13.9 27.4 25.6 24.2 21.2 23.8 6.5 31.3 36.6 3.8 5.7 7.9 9.5 22.7 96 240 21.4 55.7 5.9 1.6

for 210po f o r m e d f r o m the decay o f 21 OAt a n d 21 OBi" A b o u t 6 h after the b o m b a r d ment, the b i s m u t h target was dissolved, astatine was extracted, a n d then p o l o n i u m was quantitatively s e p a r a t e d f r o m the t a r g e t solution. Several days later the a m o u n t s o f grow-in 210po in the astatine extract a n d in the t a r g e t solution were separately measured. I t was therefore possible to estimate the a m o u n t o f 21°po f o r m e d in the target by r a d i o a c t i v e decay before the initial chemical separation.

N.E. SCoTr et aL

138

4. Theoretical computations 4.1. TOTAL REACTION CROSS SECTIONS O p t i c a l - m o d e l total reaction cross sections f o r b o t h aHe and 4He ions incident on l SlTa, 2°Tpb an d 2 °8 p b targets were calculated using the A B A C U S - I I c o m p u t e r p r o g r a m m e o f A u e r b a c h l a). T h e radial v a r i a t i o n o f the n u cl ear p o t en t i al V(r) TABLE4 lSlTa(SHe, xn) ls~-'Re cross sections nile energy (MeV) 14.0 15.0 15.9 16.0 17.0 17.1 17.7 18.0 18.8 19.0 19.3 20.0 20.4 21.0 22.0 22.7 23.0 24.0 25.0 26.0 27.0 27.4 28.0 28.6 28.9 29.0 30.0 31.0 31.2

(aHe, n) (mb)

(SHe, 2n) (mb)

(aHe, 3n) (mb)

0.55

(aHe, 4n) (mb)

0.6 2.3

2.6 8.0 25.8 0.07 0.09

6.4 58.5

0.26 103 0.29

9.9 165

0.41 0.50

295 335

7.7

385 400 390 365 325

33.1 82.8 153.2 246.3 343.6

282

447.1

235 192 156

545.4 616.9 672.8

14.2 11.4 0.79 0.81 13.4

1.01 12.7

was represented by a W o o d s - S a x o n form.

V(r) = ( V + i W ) / ( 1 + exp [(r-ro)/d]),

(1)

where V and W are real a n d im a g i n a r y nuclear well depths, r o the well radius an d d a diffuseness parameter. F o r 4He ions, the p a r a m e t e r s o f H u i z e n g a and Igo 14) were used with r o calculated f r o m r o = 1.17 A ~ + 1.77 fm.

(2)

139

He INDUCED REACTIONS

I(:X:X~

10(

g

o (D

0"110

I

I

15

20

I

25

I

50

35

3He Ion Bombording Energy(MeV) Fig. 2. Excitation functions for lSITa(3He, xn) TM ~Re reactions. TABLE 5

2°vPb(3He, xn)~l°-~Po cross sections 3He energy (MeV) 16.3 17.8 19.3 20.6 22.1 23.3 24.5 25.7 26.8 27.9 29.0 30.0 31.0

(3He, 2n) (rob)

(SHe, 3n) (mb)

7.1 13.5 18.2 17.5 15.9

2.43 8.0 31.5 80.8 162.9 214.9

19.8 21.5 16.2 17.2 16.3

350.4 372.9 379.2 293.2 239.8 176.4

(3He, 4n) (mb)

3.9 5.6 11.2 40.9 96.5 195.8 266.6 347.2 431.2

140

N.E. SCOTT e t al.

For 3He ions, cross sections were calculated using a set of parameters suggested by Bassel 15), which give reasonable agreement with experimental scattering data. The real potential was evaluated from V = - 160 + 0.152 E - 3 5 ( N - Z ) / A MeV,

(3)

and the well radii were calculated from r o (real) = 1.22 A ¢ f m ,

r o (imag) = 1.5 A ¢ fm.

(4)

TABLE 6 2°apb(3He, xn)Sn-xPo cross sections 8He energy (MeV)

(nile, n) (mb)

(ZHe, 3n) (rob)

18.5 20.0 21.3 22.6 23.8 25.0 26.1 27.2 28.3 29.3 30.4 31.4

0.29 0.47 0.46 0.70

14.1 52.5 72.0 156.5

(SHe, 4n) (nab)

1.5 15.8

283.5 259.5 294.3 213.9 235.5 181.0 163.9

35.5 96.8 131.6 238.1 344.3 479.6

U (MeV)

W (MeV)

1.02 1.19 2.0

TABLE 7

Optical-model parameters Projectile

Target

tHe tHe tHe SHe SHe aHe

181Ta ~0~Pb s°aPb lSlTa 2oTpb s0spb

--50

--21.3

-- 50

-- 23.2

--50 a) a) a)

--23.3 --20.0 --21.0 --21.0

a) Calculated using eq. (3).

In all cases, a Coulomb potential corresponding to a uniform charge distribution inside a sphere having a radius equal to the real well radius was used. A summary of the parameters used in calculating the total reaction cross sections is given in table 7. 4.2. MONTE CARLO CALCULATIONS

Relative cross sections for individual reactions were calculated using the Monte Carlo evaporation programme of Dostrovsky et al. 16), which is based on the statistical theory of nuclear reactions. Although this method of computation involves

He INDUCED REACTIONS

141

several approximations including the use of a spin-independent level density formulation, it has been quite successful in reproducing the gross features of excitation functions for reactions proceeding by compound nuclear mechanisms. In the present calculations, the formula

W(E) = C exp (2[a(E-6)] ~)

(5)

was used to obtain the level density W ( E ) at an excitation energy E. In eq. (5), C is a constant, a the level density parameter for which the value ~o A was used and 6 the pairing energy which was adopted from Cameron's tables 17). Where possible, experimental nuclear masses z o) were used, but otherwise the semi-empirical masses calculated by Hillman 18) were substituted. Typically, 500 evaporation events were sampled at a particular excitation energy for each compound nucleus of interest in the present study. 5. Discussion 5.1. QUALITATIVE FEATURES

Compared to 4He, the SHe nucleus has an extremely low binding energy (7.7 MeV). Whereas a compound nucleus formed in 4He ion bombardment has an excitation energy approximately equal to the bombarding energy, the excitation energy of a compound nucleus formed in 3He ion bombardment generally exceeds the bombarding energy by about 10 MeV. In the target systems studied here, the classical Coulomb barriers for He ions are close to 20 MeV. Consequently, if a SHe ion which is sufficiently energetic to exceed the classical barrier forms a compound nucleus, the resulting excitation energy will exceed 30 MeV, and the evaporation of three or more nucleons will be favoured. The small magnitudes of the (SHe, n) and (SHe, 2n) cross sections in all the cases studied are easily understood on this basis. Although the excitation functions for these reactions do not exhibit the shape characteristic of reactions involving nuclear evaporation (e.g. the l alTa(4He, 2n) reaction), the difference may be due to barrier penetration effects. However, for the three (SHe, n) reactions studied, the excitation functions increase monotonically with increasing bombarding energy, thus suggesting that direct reaction mechanisms are making some contributions in these cases. 5.2. COMPARISONS WITH TOTAL REACTION CROSS SECTIONS

The experimental cross sections for each of the individual (*He, xn) reactions in 1SlTa were added, and in fig. 3 the sums are compared with the optical-model total reaction cross section. At every energy of bombardment, the sum of the (*He, xn) cross sections effectively exhausts the predicted reaction cross section. A similar result for (*He, xn) reactions in 197Au has been obtained by Vinciguerra et al. 19). The *He ion-induced reactions involving charged-particle emission which are listed in table 3 contribute less than 5 ~ to the total reaction cross section at 43.4 MeV; however, cross sections of some possible reactions were not measured. Nonetheless,

142

N.E. SCOTT et al.

it is clear that (*He, xn) reactions account for at least 90 ~o of all reactions induced by 4He ions on target nuclei in the 180-200 mass region. In fig. 4, the sums of the (SHe, xn) cross sections for the three target systems studied are compared with the optical-model reaction cross sections. The apparent leveling off of the 2°Tpb points and the fluctuations in the 2°Spb data are not considered significant and are probably due to experimental errors. Despite these un-

1000

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t/)

o I(X3

41-1e lon Bombarding Energy (IVIeV) Fig. 3. A comparison of the sum of (4He, x n ) cross sections in section.

181 Ta

with the optical-model total cross

certainties, it is evident that the sums of the (SHe, xn) cross sections fall far short of the calculated reaction cross sections. At the highest energy, the experimental sums for lSlTa, 2°7pb and 2°spb targets amount to 59 ~o, 50 ~o, and 45 ~o, respectively, of the optical-model cross sections. In contrast to the (*He, xn) reactions, it appears that (SHe, xn) processes account for only about half of all the reactions induced by 3He ions. When the cross sections for the reactions (3He, p), (3He, pn),

143

He INDUCED REACTIONS

(3He. p2n), (3He, 2p), (3He, ~) and (aHe, ~2n) at 31.6 MeV are added to the sum of the 1a lTa(3He, xn) cross sections at the same energy, the result is in agreement with the optical-model cross section within the experimental error of ___20 ~ .

200C

I(X)C o

E vSOC g

/ /oo+Oo( /+/oo,

20/'pb + 3He

ISITo + 3He

208pb + 5He

oOt °°

O3 L) 20(

o

I00

50

25

I

35

3He !on Bombording Energy (MeV)

Fig. 4. A comparison of the sums of (3He, xn) cross sections in ~SlTa,2°TPband ~°sPbwith the opticalmodel total cross sections. 5.3. REACTION MECHANISMS The results of these studies are in agreement with previous conclusions that (4He, xn) reactions are predominantly compound nuclear in mechanism. The predictions of the Monte-Carlo evaporation calculations are in excellent agreement with experiment for the (4He, xn) reactions in 1SlTa and in 2°Tpb (fig. 5). At the highest excitation energies, the calculations predicted that not more than 2 ~ of all the interactions would involve charged-particle emission. Although the measurements showed that charged-particle emission occurs somewhat more frequently, the calculations are not sufficiently rigorous to allow firm conclusions about reaction mechanisms to be drawn. In the case of the 3He ion-induced reactions however, it is clear that most of the charged-particle emission reactions do not involve compound nucleus formation. At the energies under consideration, the (4He, xn) results establish that only neutrons are emitted in the decay of at least 90 ~ of the compound nuclei formed. F r o m the independence hypothesis and the (3He, xn) results, it then follows that not more than 60 ~ of all aHe ion-induced reactions involve compound nucleus formation; at least 40 ~ of aHe induced reactions proceed by some form of direct mechanism.

N. ~. SCOTTet al.

144

it remains to be discussed whether even the (aHe, xn) reactions are mainly compound nuclear in mechanisms. Evaporation calculations for the three target systems studied were quite successful in predicting the shapes and magnitudes of the individual 1.0

o= \

/

cr(4Heixn) T..o.(4He,xn)

Experimental ------C.,.="

o., / \ OK)

/'~

,

22 26 30 Excitation Energy (MeV)

18 " ~ -

)

,/'/~

17,

14

/I

,

34

38

Fig. 5. A comparison of the experimental (IHe, xn) excitation functions for 2°Tpb with the predictions of the Monte-Carlo evaporation calculations. The cross section for an individual reaction at a particular energy is presented as a fraction of the s u m of all the (4He, xn) cross sections at that energy.

, Experimental lated

- -- -- Calcu

0.8

0.6 o-(3He,xn) T.o(3He,xn) 0.4-

/\\ // \ \ \\+.e.2o, / / ",3~,3=

0.2

0

15

20

25

30

35

40

Excitation Energy (MeV) Fig. 6. A comparison of the experimental QHe, xn) excitation functions for ~°sPb with the predictions of the Monte-Carlo evaporation calculations.

He INDUCED REACTIONS

145

excitation functions when the cross sections for c o m p o u n d nucleus f o r m a t i o n were set equal to the s u m of the (3He, x n ) cross sections. Typical results are shown i n fig. 6 for the 20Spb(3He ' x n ) reactions. As these reactions involve the same c o m p o u n d nucleus as the 2°Tpb(4He, x n ) reactions illustrated in fig. 5, the i n d e p e n d e n c e hypothesis can be applied with a d d i t i o n a l confidence. The M o n t e - C a r l o e v a p o r a t i o n results are consistent with a c o m p o u n d n u c l e a r m e c h a n i s m for (3He, x n ) reactions in this mass region. W e are grateful to Dr. N. T. Porile for m a n y helpful discussions a n d to Dr. R. H. Bassel for suggesting optical potential parameters for 3He ions.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) 17) 18) 19) 20)

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