The 37Cl(3He, α)36Cl reaction and the 36S ground state analog

Nuclear Physws AU2 (1968) 195--203; ~ North-Holland Publishing Co., Amsterdam ,G

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N o t to be reproduced by photoprint or microfilm without written permission from the publisher

T H E aTCI(3He, 00s6Cl R E A C T I O N A N D T H E S6S G R O U N D S T A T E A N A L O G LARS BROMAN t, C. M. FOU and BARUCH ROSNER tt

Department of Physics, University of Pennsylvania Philadelphia, USA ttt Received 6 November 1967 Abstract: The 37Cl(SHe, ~)~6C1reaction has been studied using a 15 MeV SHe beam. Some 34 states up to 12.2 MeV excitation energy were observed. Angular distributions were measured for 11 states and compared with DWBA predictions in order to yield spectroscopic informatlon. The z6S ground state analog was identified at 4.33 MeV excitation energy from which the Coulomb displacement energy for the pair ~6Cl-Z6Swas calculated to be 6.25 MeV.

E I

NUCLEARREACTIONS:37Cl(3He'°O'E=15MeV;measured~r(E~'O)"

I

z6C1 deduced levels, ~, 1, S. Enriched target. 1. Introduction

The nucleus 36C1 has been extensively studied; some 30 investigations are reviewed in the 1962 compilation by E n d t and Van der Leun 1). The post-1962 experiments include studies of the 35C1(d, p)36C1 reaction 2), the 3SCl(n, 7)36C1 reaction 3-8), the 39K(n, ~z)36C1 reaction 9, 10) and the 36Ar(n, p)36C1 reaction 11). N o study of the 37C1(3He, cQ36C1 reaction has, however, so far been reported. This reaction, in contrast to the other mentioned reactions, is capable o f exciting the T = 2 states in 36C1 f r o m which the value for the C o u l o m b displacements energy (AEc) for the 36 C1- 36 S isobaric pair m a y be calculated. It m a y be o f interest to determine if a similar independence of the C o u l o m b displacement energies on the mass n u m b e r occurs for nuclei in which b o t h neutrons and protons occupy the d~ subshell similar to the effect recently f o u n d by Sherr 12) for the lf~ shell. 2. Measurements and results

The 37C1(3He, c036C1 reaction was initiated by a 15 MeV 3He beam f r o m the University of Pennsylvania T a n d e m Accelerator. The target material BaCI2 enriched to 99.3 % in 37C1, was obtained f r o m O a k Ridge N a t i o n a l Laboratories. This comp o u n d was preferred to NaC1 since the (3He, ~) reaction cross sections on barium at t Present address: Department of Physics, Chalmers University of Technology, Gtteborg, Sweden. *t On leave of absence from the Department of Physics, Techmon, Israel Institute of Technology, Halfa, Israel. ttt Work supported by the U. S. National Science Foundatmn. 195

196

L. BROMANet

al.

15 MeV bombarding energy are quite small due to its relatively high atomic number. The targets were then produced by evaporating the material onto 25/tg/cm z carbon backing. All targets had a thickness of about 75 pg/cm 2 as determined by weighing, which is equivalent to an effective chlorine thickness of about 26 pg/cm z. Since the barium chloride is hygroscopic, the targets were stored and transferred in a dry box so that they would not break when inserted into vacuum. The outgoing alpha particles were detected in two different ways. For most angles an array of four solid-state detectors mounted in a 61 cm scattering chamber ~3) was used. With the four detectors coupled to a 4096-channel T M C analyser via separate preamplifiers, four simultaneous alpha spectra could be recorded. The current was monitored by a current integrator coupled to a Faraday cup behind the target. The spectra from different runs were normalized from the integrated intensity of 3He particles elastically scattered of 37C1 as measured by an additional solid-state detector serving as a monitor at 45 ° to the beam direction. The relative total efficiency of each detector was determined from a calibration run using a 24~Am alpha source. The solid angles were defined by tantalum collimators in front of each detector. In order to reduce the background from pile-up effects, narrower collimators and pile-up rejection systems were used for the most forward angles. The spectra obtained by the solidstate detectors were stored on magnetic tape and then analysed and printed using an off-line computer. In spite of the precautions, no measurements closer than 25 ° towards the beam were possible with reasonable current due to the strong elastic peak from the barium. In order to measure the yields at forward angles, three alpha spectra at 25 °, 15 ° and 11 ° were recorded on 50 #m K-1 Ilford Nuclear Research plates in the focal plane of a 65 cm magnetic spectrograph of the Browne-Buechner type 14). In addition, these spectra have a much wider useful energy range (negative Q-values), better energy resolution and negligible background. Thus, they were used to provide a more accurate determination of the aipha-group energies. As in the previous runs, a solid-state detector was used in the target chamber to monitor the elastically scattered 3He particles from the barium for normalization. The 25 ° spectrum was measured with both techniques in order to have an accurate normalization of the alpha groups in the two detecting systems. The photographic plates were developed and scanned in 1 m m strips every m m over the whole energy range and every ~1 m m over the peaks. The 15 ° spectrograph spectrum is shown in fig. 1 where groups assigned to excited states in 36C1 are numbered 0-33. The only contaminants are from oxygen and carbon, and groups from the 160(3He, e)150 and 12C(3He, ~)11C reactions are labelled with the final nucleus symbol. The results from the spectrograph runs are summarized in table I, where excitation energies of states in 36C1 fed in the 37C1(3He, c0 reaction are listed. Only states clearly seen at least at two angles are given in the table. The errors in our excitation energy values are about 15 keV below Ex = 3 MeV and 25 keV above. The measured excitation energies for states seen in both the (3He, a) reaction and the 35C1(d, p) reaction 1 , 2 ) a r e usually well within the quoted errors

300

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Fig. 1. Alpha-particle spectrum from the 37C1(3He, c0anC1reaction taken with a broad range magnetic spectrograph at (9 = 15 °.

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L. BROMAN et al.

up to an excitation energy of 5 MeV. Above 5 MeV excitation, the level density from the (d, p) study is so high that definite identifications are impossible for most states.

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Fig. 2. Alpha-particle spectrum from the ~TCl(aHe, ~)36C1 reaction observed at O = 70 ° using surface barrier solid-state detectors.

The angular distributions of the alpha groups using solid-state detectors in the scattering chamber were recorded at 5 ° intervals between 25 ° and 90 ° . As an example, the high-energy part of such a spectrum taken at 70 ° is shown in fig. 2. The lowenergy part of these spectra was less useful due to increasing background and level density, thus only the transitions to the ground state and first ten excited states were further analysed. 3. The D W B A analysis Angular distributions were obtained for transitions leading to the ground state and ten excited states in a 6C1" They are shown in fig. 3. The points represent the experimental data and the solid lines the angular distributions calculated by the D W B A

37C1(3He, cQ36C1REACTION

199

method using the code J U L I E t . Different sets of parameters were tried, the ones giving best fit to the data and used in the analysis are listed in table 2. TABLE 1 Energy levels m ~nC1 determined f r o m t h e spectrograph exposures Level number

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Excitation a) energy (MeV) 0.000 0.793 1.165 1.608 1.970 2.497 2.682 2.905 3.492 3.736 4.333 4.56 4.59 4.74 4.94 5.73 6.00

Level number

Excitation a) energy (MeV)

17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

6.15 6.19 6.45 6.49 6.54 6.68 6.75 6.84 6.89 7.09 7.16 7.64 8.89 8.95 11.24 11.44 12.23

a) Errors are 15 keV for E x < 3 M e V a n d 25 keV for E x > 3 MeV.

The relationship between the measured and calculated (3He, 00 cross sections is d~ d~ex p

- 1.63 ~

StJtrlj(O),

(1)

lj

where ~0 (O) is the reduced cross section evaluated by the DWBA program for pickup of a neutron from the (lj) shell orbital and S tj the spectroscopic factor for the (lj) state. The In values and the extracted relative spectroscopic factors for the investigated transitions are listed in table 3. For the (3He, ~) reaction, the zero-range distorted wave theory using the normalization factor of eq. (1) underestimates the cross section by at least an order of magnitude. As the determination of the target thickness involves also a large source of error in the experimental cross section, the absolute spectroscopic factors are, therefore, non-significant, and we have normalized our factors so that the sum for the ground state, first, second and tenth excited states is four. t W e are indebted to Dr. R. M. Drisko for h a v i n g m a d e the J U L I E code available for us.

200

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Fig. 3. Angular distribution o f alpha-particle groups observed in the 87C1(3He, c~)8~C1 reaction leading to the first 11 states m zeC1. The solid lines represent the results o f D W B A calculations assuming the In values and excitation energies indzcated in the figure. TABLE 2 Optical-potential parameters used in the D W B A analysis V (MeV)

W (MeV)

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b) Ref. 18).

TABLE 3 Angular m o m e n t a and relative spectroscopic factors In the zvcI(ZHe, cQ reaction for transitions to some states in 86C1 Level

Excitat]on energy (MeV)

In

S (rel.)

0 1 2 3 4 5 6 7 8 9 10

g.s. 0.793 1.165 1.608 1.970 2.497 2.682 2.905 3.492 3.736 4.333

2 2 2 0 0 0 2 2 0 3 2

1.30 1.91 0.44 0.22 0.50 0.45 0.44 0.45 0.18 0.06 0.35

a) New In assignment. b) In = 1 in the 35C1(d, p)snc1 reaction 8). e) Suggested T = 2 86S g.s. analog.

Remark

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a) o)

202

t. BROMANet aL 4. Discussion

The spectroscopic information obtained from the present 37C1(3He,a)36C1 reaction study as shown in table 3 deserves some further discussion. The sum of the spectroscopic factors of the ground state, the 0.79, 1.165 and 4.333 MeV states has been normalized to four. This normalization is based upon the assumption that these four states are formed by coupling a d~ neutron hole to a d~ proton. Since the neutron shell is closed at 37C1according to the shell-model prediction, the sum rule predicts the sum to be equal to four - the total number of available d~ neutrons. However, possible admixture of configurations with a s~ neutron hole coupled to a d~ proton cannot be ruled out for the 2 + and 1 + states in 36C1. Therefore, our normalization factor is only a rough estimate. All the/-values determined here are consistent with the results from 35Cl(d ' p)36CI reaction except the 2.905 MeV state. The present assignment is l = 2 instead of 1 = 1. N e w / - v a l u e assignments were made for 2.682, 3.492 and 4.333 MeV states. Most noteworthy in the alpha spectrum is the strong transition to the state at 4.333 MeV excitation, which was barely observed in the 35C1(d, p) reaction. This is the energy region where the analog to the 36S ground state is expected to be located. Since the (d, p) reaction excites only T = 1 states in 36C1,this state, which has also the correct angular m o m e n t u m transfer, could be a possible candidate for the lowest 0 +, T = 2 state. Furthermore, the spectroscopic factor S = 0.35 experimentally obtained in the transition to the 4.333 MeV state is in good agreement with the theoretically predicted value S = 0.25 for this T = 2 state 15). I f this identification is correct, we obtain for the Coulomb displacement energy for the 36CI-368 isobaric pair the value AEc --- 6250___25 keV. For the 34C1-34S case, Freeman et aL 16) obtained AEc = 6264+5 keV. Thus, the dependence of the Coulomb energy on the mass number is seen to be much weaker than A -~, which would require a decrease of 123 keV going from A = 34 to A = 36. However, at this high excitation energy strong l = 2 transitions may occur corresponding to neutron pick-up from the d~ orbit. Consequently, the J= = 0 +, T = 2 assignment to the 4.333 MeV state is only a tentative one and further investigations, such as the 34S(3He, p)36C1 study, may clarify this point. The authors wish to express their thanks to Professors R. Middleton and W. E. Stephens for their interest in this work. The careful preliminary analysis of the data by D. Blumenthal is very much appreciated. References

1) 2) 3) 4) 5)

P. M. Endt and C. Van der Leun, Nuclear Physics 34 (1962) 1 A. M. Hoogenboom, E. Kashy and W. W. Buechner, Phys. Rev. 128 (1962) 305 I. Lovas and Z. Zamorl, Nuclear Physics 39 (1962) 605 J. E. Draper and C. O. Bostrom, Nuclear Physics 47 (1963) 108 L. Meyer, Nuclear Physics 52 (1964) 213

zrCl(ZHe, ~x)z~C1REACTION

203

6) W. Rudolph and H. V. Gersch, Nuclear Physics 71 (1965) 221 7) G. van Mlddelkoop and P. Spilling, Nuclear Physics 77 (1966) 267 8) F. Becvar, J. Honzatko and Z. Kosina, Report UJV 1705, Listopad, Czechoslovakia (1965) unpublished 9) R. Bass and F. M. Saleh, Phys. Lett. 3 (1963) 296 10) R. Bass and F. M. Saleh-Bass, Nuclear Physics A95 (1967) 38 11) E. A. Davis, T. W. Bonnet, D. M. Worley, Jr., and R. Bass, Nuclear Physics 55 (1964) 643 12) R. Sherr, Phys. Lett. 24B (1967) 321 13) R. W. Zurrnuhle, Nucl. Instr. 36 (1965) 168 14) C. P. Browne and W. W. Buechner, Rev. Sci. Instr. 27 (1956) 899 15) J. B. French and M. H. McFarlane, Nuclear Physics 26 (1961) 168 16) J. M. Freeman, J. G. Jenkin, G. Murray and W. E. Burcham, Phys. Lett. 16 (1966) 959 17) R. H. Bassel, private communication 18) L. McFadden and G. R. Satchler, Nuclear Physics 84 (1966) 177