Assignment of Jπ = 6+ to two states of 16O via the 12C(α, 8Be)8Be reaction

Nuclear Physics A227 (1974) 349-356; Not

@ North-Holland Publishing Co., Amsterdam

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

ASSIGNMENT

OF J” = 6+ TO TWO STATES OF 160

VIA THE ‘%(a, D. R. JAMES,

*Be)*Be REACTIONt

J. L. ARTZ tt, M. B. GREENFIELDttt

and N. R. FLETCHER

Department of Physics, Florida State University, Tallahassee, Florida 32306 Received 16 April 1974 Abstract: Excitation functions of the reaction %(cr, *Be)*Be have been measured for approximate c.m. angles of 90”, 69”, and 48”, the latter two angles corresponding to P4(cos O,.,.) M 0 and Ps(cos O,.,.) z, 0. Angular distributions were measured at bombarding energies corresponding to the two prominent resonances in I60 at E, = 21.3 and 22.31 MeV. A Jr assignment of 6+ is made for each of these states. E

NUCLEAR

REACTIONS ‘%(a(, *Be), E = 18.5-23 MeV; measured a(E, 0). I60 resonances deduced J,n.

1. Introduction

The reaction 12C(a, 8Be)8Be has recently received considerable attention by experimental investigators I- “). Th e impetus for these investigations has been the search for energy levels in the I60 compound system which may be interpreted as members of a highly deformed rotational band in 160, the intrinsic state of which has a moment of inertia similar to that of a linear chain of four adjacent a-particles. The original ideas concerning the existence of such a rotational band are due to Morinaga “). Considerable evidence supporting these ideas is provided by the work of Chevallier et al. ‘), who identified the J” = 2’, 4+, and 6+ members and by extrapolation located the O+ band head at 16.75 MeV excitation in ’ 60. Unsuccessful searches for the J x = 8” member of the rotational band have been reported ‘, 4), although possible locations have been indicated “). The resonance yield of the 12C( ct, 8Be)8Be reaction is greatly simplified because only natural parity states are allowed, and since the final state consists of two identical particles, only J” = (even)+ states are allowed. When the cross section is characterized by compound nucleus formation, the angular distributions may be described by an equation of the form W(O) = 1 C a,e’+‘P,(cos @I”, I=0 t Work supported in part by the National Science Foundation Grants no. NSF-GP-25974 and NSF-GU-2612. tt Present address: Department of Physics, University of Minnesota, Minneapolis, Minnesota. ttt Present address: Department of Physics, Florida A & M University, Tallahassee, Florida. 349

350

D. R. JAMES et al.

where I must be even. The JR value of a resonant state may be determined by measuring the energy dependence of the Legendre coefficients, a,, and assigning J equal to the Z-value of that coefficient which resonates. If a particular Z-value dominates the cross section then such a time consuming procedure is unnecessary. In such a case, spin values may be assigned by noting the presence or absence of resonances in yield curves measured at angles such that P,(cos e,.,,) = 0 for different Z-values. The existing data in the energy region E, = 20-30 MeV show a gradual trend from an I = 6 dominance in the cross section at the lower energies toward an 1 = 8 dominance at the upper energies “). These Z-values correspond roughly to the kr value of the incident beam being absorbed near the nuclear surface. This is a strong indication that the reaction mechanism is compound although the upper portion of this energy range does not show isolated resonances. At much higher energy, E, w 6070 MeV, an I = 10 partial wave absorption should be observed if surface absorption is still dominant. This is not apparent in the data 6), which indicates a transition from a compound mechanism to a direct mechanism over the energy range E, = 20-60 MeV. The present work reports on the r2C ( 01, *Be)*Be reaction in the energy region E, = 19-23 MeV, where compound nucleus resonances dominate the cross section. A new energy calibration technique is used which yields accurate resonance energies where earlier there existed discrepancies 3*4). Spin assignments are made on the combined evidence of excitation functions and angular distributions. The *Be detection method is also discussed. The present paper supersedes the preliminary report given in ref. “). 2. Experimental procedure Foil targets of natural carbon, w 20-60 pg/cm’, were bombarded by a-particles from the Florida State super FN tandem accelerator. The *Be reaction products are identified by detection of the decay a-particles in coincidence in the two halves of a special detector designed to improve the efhciency of detection and hence increase the counting rates. This detector was fabricated’ in the form of a silicon surface barrier detector having an area of 200 mm2 and a central dead strip 1 mm wide between electrically independent semicircular halves. Having such large detecting areas close together greatly enhances the detection efficiency over other methods lP“). Even more efficient detection systems employing rectangular detectors in multi-detector arrays are now being used in this laboratory and elsewhere 4*‘). The three body reaction kinematics as it pertains to the *Be detection by the method of coincident cl-particles is discussed by Cramer et al. ‘). The efficiency of the present system is discussed in detail elsewhere “). The angular distribution data for ‘%(a, *Be)*Be at E, = 21.3 MeV have provided an excellent check on the energy dependence of the efficiency calculation since the symmetry in the cross section about 0,.,. = 90” demt ORTEC Inc., Oak Ridge, Tennessee.

6+ STATES OF -0

351

DET

L IAFA -II

HDLYHLGSCI-IPS

u

I~SDA~

I

Fig. 1. Block diagram of the electronics. PA = preamplifier; DLA = delay line amplifier; DISC = discriminator; DLY = delay; TAC = time to amplitude converter; TSCA = timing single channel analyzer; DGG = delay and gate generator; LGSC = linear gate and slow coincidence; SDA = summing and delay amplifier; AFA = active filter amplifier; PS = pulse stretcher.

onstrated in the data represents the detection of ‘Be particles of drastically different energies due to kinematic effects. A block diagram of the electronics employed is shown in fig. 1. The coincidence timing is established by use of delay line amplifiers (DLA) and discriminators which supply start and stop pulses to the time to amplitude converter yielding a time resolution of 5 15 ns. The actual difference in flight time of a-particles and the small current pulses in the detectors are the main contributors to the time resolution. A *Be energy spectrum is formed by a direct sum and storage of the time gated and gain balanced a-particle energy pulses. Sample summed energy spectra are shown in fig. 2. The fitted portion of the spectrum shows the sum of a background contribution and the peak corresponding to *Be particles in the ground state. Detection of *Be* (2.9 MeV) is strongly disfavored energetically and kinematically. Coincidences arising from other reactions have been shown by Chevallier et al. ‘) to contribute in other energy regions of the spectra. The irregularities seen in the low energy continuum in fig. 2 are caused by a-particle cascade decay and singles events recorded as coincidences due to radiation damage in the detector. These spectrum irregularities did not interfere with extraction of the *Be yield. The monitor detector served as a check on current integration as it measured the elastic scattering from a thin gold deposit on the target. The absolute cross section for “C (3a *Be)*Be was determined to an accuracy of 10 y0 through the use of a target thickness measurement utilizing the Rutherford cross section and the data from the elastic scattering of 160 from 12C at 20 MeV and forward angles.

352

CHANNEL

t

I

I

NUMBER I

I

1-v

E, = 21.25 MeV t3,,

q

9o”

c 158

, 200 CHANNEL

250

300

350

400

NUMBER

Fig, 2. Summed a-particle energy spectra for coincident a-particles. The yield is extracted with the aid of a background fitting procedure.

The energy calibration used for accurately quoting excitat~ou energy c~~res~ond~n~ to the resonance near 22.3 MeV was established by use of the p(‘Li, n)‘Be threshold reaction “>. The use of a ‘Lis2 beam establishes a Bp value at threshold which corresponds to 22.901 MeV a-particles, thus placing a primary energy calibration point close to the energy region of interest.

6’ STATES OF I60

353

1.5

_ 1.0

E

c”OS P b

a

0.0 3.5

20

19

23

w 1

19

I

L

I

I

20

21

22

23

BOMBARDING

ENERGYfMeV)

Fig. 3, Energy dependenceof the 12C(txar, *Be)*Be ~erenti~ angles.

cross section at thm differentreaction

D. R. JAMES et al.

354

3. Results and discussion

The results of cross section measurements of the reaction 12C(a, *Be)*Be for bombarding energies of E, = 19-23 MeV are illustrated in fig. 3. A number of maxima occur throughout the energy region which show a fairly good energy correspondence to known excited states in 160 . These excitation energies, level widths, and resonance energies are tabulated in table 1. The values listed in the present work were not accurately determined except for the resonance at 22.309 +0.007 MeV which was measured in a 12C(cr, @)12Creaction ‘). TABLE1 Resonance energies in I60 12C(a, sBe)sBe present work &.(MeV) 8_,

= 900

e,.,. = 690

Previous work “)

UMeV)

r b, (keV)

19.92

19.92

22.10

X 100

20.3

20.3

(20.05) 20.3

(22.2) 22.4

M 100 x 100

21.3 21.3 (21.85)

21.3

21.3

23.1 23.1 (23.5)

6 50 2 500 < 100

22.31

22.31

23.893 “)

10.006 22.75

22.75

E,(MeV)

I’(keV)

22.04 22.07 22.13 22.52 22.68 23.11 23.15 23.40 23.54 23.75 23.89 23.93 24.05 24.05

60 340 < 150 375 60 w 20 % 500 < 40 300 120 M 25 165 % 80 450

e,.,. = 480

22.75

24.2

26 zt4 5 500

*) Excitation energies in the appropriate energy region in 160, taken from the compilation of ref.‘O) Levels of known T > 0 have been omitted. b, The level widths are approximate values for purposes of assisting in level identification only. These values do not reflect specific width measurements. “) Values from a recent more accurate measurement of this resonance g).

The yield measured

at 8,.,. = 90” is dominated by the large resonance near displayed also are those at E, = 22.31 and 22.75 MeV. The data shown by the open circles are from an early measurement done to overlap with the data of Martin and Ophel “). Since all resonances excited in this reaction will have their maximum yield at 8,.,. = 90”, the overlap of resonances tends to smooth out the yield curve from E, = 20-23 MeV except for these prominent resonances mentioned. It should be noted that the previously reported “) cross sections were = 25 % greater than that reported by Martin and Ophel “) at 8,.,. = 90” and E, = 19 MeV and about 30 % greater than the results of Scheibling et al. “). The present values are exactly one half of those previous values and more recent E, = 21.3 MeV although prominently

results from the Strasbourg group are now in good agreement with the present work. The yield curves measured at 6,.,_ B 69” and 0,.,. M 48”, where P&OS SQSm.)w 0 and &(ccs L,.) w 0, respectively, show considerably more structure since many resonances which would overlap have been eliminated by the choice of angle. There may exist many resonances in the energy range B, = 19-21. MeV other than. those listed in table 1. The unfolding of this energy region requires one to rne~~r~ angufar distributions at many closely spaced bombarding energies, and this is a study currently in progress. It can be inferred from the yields in fig. 3 that the resonances at E, = 21.3 and 22.31 MeV correspond to states in 1%I which have J” = 6+ since they do not appear ~rorni~~n~y at SC_,*= 48”. The arrows in fig. 1 indicate the energies at ksr ~str~butions were measured. The results of angular distribution measurements at E, = 21.3 MeV and 22.31 MeV are shown in fig. 4. The solid curve is merely an arbitrarily normalized 4

E,=22.31Me'4

0.0

Fig. 4.

90 B cm.

120

distributions measured at the resonance energies of Ea = 21.3 and 22.31 MeV. The solid curves represent a jP&os &JZ distribution.

IPs(cos 0F_m.)12Freon. The close co~es~o~dence ~twee~ the data and the I = 6 Legendre function demonstrates that nearly all of the cross section at these energies results from a J” = 6+ compound state, As expected, the cross section in each anguIar distribution is greater than the jPsj2 curve at 13,+,. near 90” since all other partial cross sections will also maximize at that angle.

356

D. R. JAMES et al.

The autl ors wish to acknowledge the participation of G. R. Morgan in the data taking portion of this investigation. We also appreciate the cooperation of the ORTEC Corporation in fabricating detectors to our specifications. References 1) P. Chevallier, F. Scheibling, G. Goldring, I. Plesser and M. W. Sachs, Phys. Rev. 160 (1967) 827 2) P. Martin and T. R. Ophel, Nucl. Phys. Al94 (1972) 491 3) D. R. James, J. L. Artz, M. B. Greenfield and N. R. Fletcher, Proc. Int. Conf. on nuclear physics, ed. J. de Boer and H. J. Mang, vol. 1 (North-Holland, Amsterdam, 1973) p. 164 4) F. Brochard, P. Chevallier, D. Disdier, G. Rudolf and F. Scheibling, ibid., p. 204 5) H. Morinaga, Phys. Rev. 101(1956) 254 6) G. J. Wozniak, N. A. Jelley and J. Cemy, Phys. Rev. Lett. 31 (1973) 607 7) J. G. Cramer, K. A. Eberhard, N. R. Fletcher, E. Mathiak, H. H. Rossner and A. Weidinger, Nucl. Instr. 111 (1973) 425 8) M. B. Greenfield, J. L. Artz and N. R. Fletcher, Bull. Am. Phys. Sot. 17 (1972) 489; J. L. Artz, M. B. Greenfield and N. R. Fletcher, Phys. Rev., to be published 9) N. R. Fletcher, D. R. James, G. R. Morgan and G. A. Norton, Nucl. Instr., to be published 10) F. Ajzenberg-Selove, Nucl. Phys. Al66 (1971) 1