12C(16O, α)24Mg EXCITATION FUNCTIONS

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12C(16O, α)24Mg EXCITATION FUNCTIONS

L. Greenwood, T. Braid, K. Katori, J. Stoltzfus, R. Siemssen

To cite this version:

L. Greenwood, T. Braid, K. Katori, J. Stoltzfus, R. Siemssen. 12C(16O, α)24Mg EX- CITATION FUNCTIONS. Journal de Physique Colloques, 1971, 32 (C6), pp.C6-199-C6-200.

�10.1051/jphyscol:1971640�. �jpa-00214858�

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JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 1 1-12, Tome 32, Novembre-Dkcembre 1971, page C6-199

2C(1 0 ,

a)24Mg

EXCITATION FUNCTIONS

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L. R. GREENWOOD, T. H. BRAID, K. KATORI, J. C. STOLTZFUS, and R. H. SIEMSSEN Argonne National Laboratory, Argonne, Illinois 60439

R6sum6. - Les fonctions d'excitation de la reaction 12C(160, a)24Mg pour dix 6tats finals du 24Mg ont kt6 mesurees a haute resolution par pas de 54 keV, de ^E,.,. ⁼¹⁹MeV A E,.,. = 25 MeV.

L'observation de fortes fluctuations de la fonction d'excitation indique que la rkaction proc6de principalement par formation de noyau compose.

Abstract. - High-resolution excitation functions have been measured in 54-keV steps from Ec.m. = 19 to 25 MeV for the 12C(160, ^c()reaction to ten states in 24Mg. The observed strong fluctuation of the excitation function indicates a predominantly compound-nucleus process.

Because of the interest in the mechanism leading to the selective population of prominent states at high excitation in the ''C(160, a)24Mg reaction, [I] high- resolution excitation functions were measured in 54 keV (c. m.) steps over the energy range

E,.,. = 19-25 MeV.

The observed strong fluctuation of the excitation functions indicates a predominantly compound-nucleus process. A similar study with poorer energy resolution has recently been reported by Gastebois et al. [2].

Self-supporting carbon targets, ^N5 pg/cm2 thick, were enclosed in a cold sleeve to minimize carbon buildup (which still amounted to 6 40

%)

and the data were corrected for this buildup by repeating measurements of a selected point. The alphas from ¹⁶⁰ bombardment were momentum analyzed by an Enge split-pole spectrograph at 7.50 lab. and detected in the focal plane by a proportional counter 25 cm long and with position resolution of

-

1 mm (FWHM). The experimental resolution width, due primarily to the target thickness, was about 50 keV.

The data were analyzed with the peak-fitting computer code Autofit [3]. The combination of a large continuous background and a high density of rapidly fluctuating levels presented some difficulties for small cross sections ( 6 1 mb/sr). Absolute cross sections were calculated, using the nominal target thickness, solid angle, and integrated charge, with an estimated accuracy of 30

%.

The errors indicated on the figure are statistical only. The average cross sections appear to be larger than those of Middleton et al. [I] and of Gastebois et al. [2].

The transitions to the 14.14-and 16.55-MeV levels are not resolved from adjacent levels. The 16.55- 16.59-MeV excitation function has sizable contributions from the 6' 16.59-MeV level below

E,.,. z 21 MeV;

(*) Work performed under ^theauspices of the U. S . Atomic Energy Commission.

however, this level practically disappears at higher energies.

The spectra (Fig. 1) measured at El,, = 48.75 and 49.25 MeV (a difference of 220 keV c. m.) are strikingly different. This rapid change with energy is typical of a compound-nucleus process. The excitation functions (Fig. 2 and 3) for transitions to 24Mg states between 14 and 17.5 MeV excitation exhibit strong fluctuations and some appear to show gross structure.

DISTANCE l c m l

FIG. 1. - lZC(160, ~()24Mg spectra at Elab = 48.75 and

49.25 MeV.

The observation of gross structure does not neces- sarily exclude a purely statistical process [4]. Over limited energy ranges we find some cross correlations (not very striking) between different excitation functions, notably between those for the transitions to the 16.07-and 16.20-MeV states. Our measurements at O,,, = 7.50 show no significant correlations between the excitation functions for transitions to the 15.15 and 16.55-MeV states, though Gastebois et al. reported strong ones at O,,, = 15 and 22.50.

Preliminary autocorrelation studies of the excita-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971640

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C6-200 L, R. GREENWOOD, T. H. BRAID, K. KATORI

E,=14.14 MeV

19 20 21 22 23 24 25

EC,LI, ( M e V )

FIG. 2. - Excitation functions for the transitions to some states at 14-16 MeV in 24Mg. Spin and parity assignments are

from Ref. [6] and [7].

tion functions of transitions to the 15.15-, 16.07-, 16.55, 16.59-, and 16.85 MeV levels yield coherence widths between 110 and 130 keV, in good agreement with other measurements [5] and R(0) = 0.09, 0.13, 0.04, and 0.14, respectively. The direct part Y, of the cross section can be determined from the relation

R(0) = (11Neff) (1 - :y

1 ,

E x = 1 6 . 2 0 M e V

-r

10 1 6 . 2 9 M e V

(8+. 9; 10')

\ -

0 -

\ b - u

17.19 M e V

20 21 2 2 2 3 24 2 5

EC,-M,( ^{M e V )}

FIG. 3. - Excitation functions for the transitions to some states at 16-17 MeV in 24Mg. Spin and parity assignments are from

Ref. [6] and [7].

where the present measurements were done. Calcuia- tions to check this expectation are now under way.

Except for the 16.55-16.59-MeV doublet, Y, = 0 can be obtained only for N,,, = 7-1 1. Since this range seems very reasonable for 1, 2 6 h, Nef, =

4

N:;t"

if indeed Y,

-

= 0.

where NeFf is the number of independent cross sec- In summary, we find that 12C(160, E ) ' ~ M ~ excita- tions. For the 12C(160, E ) ~ ~ M ~ reaction, Neff = 1 tion functions fluctuate strongly with energy and thus at O0 and Nef, = 2 If

+

1 at 900. Because of the high indicate strong compound-nucleus contributions. Pen- orbital angular momenta involved, we expect Neff to ding further analyses, the data are consistent with increase rapidly from 1 to its maximum as the angle a compound-nucleus mechanism for the population of increases and thus to be large even a t O,.,. 120 the high-spin states in 24Mg.

References

[I] MIDDLETON (R.) et al., Phys. Rev. Letters. 1970, 24, ^[4]SHAW (R. W.), Phys. Rev., 1969, 184, 1040.

1436. _{[ 5 ]}HALBERT (M. L.) et al., Phys. Rev., 1967, 162, 899.

[2] GASTEBOIS (J.) et al., Lettere A1 Nuovo Cirnanto, 1971,

2. 90. [6] GOBBI (A.) et al., Phys. Rev. Letters, 1971, 26, 1271.

- 7 - -.

[31 COMFORT ^(J. R.), ANL Physics Division ^Informal [7] BALAMUS (D. P.), ~ h y s . Rev. Letters, 1971, 26, ^1271.

Report No. PHY-1970B (August 1970).