12C DECAY OF HIGHLY EXCITED STATES IN 24Mg

HAL Id: jpa-00214865

https://hal.archives-ouvertes.fr/jpa-00214865

Submitted on 1 Jan 1971

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

12C DECAY OF HIGHLY EXCITED STATES IN 24Mg

M. Levine, D. Schwalm, M. Littman

To cite this version:

M. Levine, D. Schwalm, M. Littman. 12C DECAY OF HIGHLY EXCITED STATES IN 24Mg.

Journal de Physique Colloques, 1971, 32 (C6), pp.C6-219-C6-220. �10.1051/jphyscol:1971647�. �jpa- 00214865�

JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 11-12, Tome 32, Novembre-Dkcembre 1971, page (26-219

12C DECAY OF HIGHLY EXCITED STATES IN 24Mg

(*)

M. J. LEVINE, D. SCHWALM (**), and M. G. LIlTMAN Brookhaven National Laboratory, USA

R&um6. - La dksintkgration du 24Mg en deux ions 12C a et6 ktudiee en utilisant la reaction 12C(160, a)24Mg pour peupler les etats a haute 6nergie d'excitation du 24Mg.

Abstract. - The decay of 24Mg into two lac-particles has been studied using the reaction 12C(160, 01)24Mg to populate highly excited states in 24Mg.

Recently the highly excited states of 24Mg have been the subject of intensive investigation by several laboratories [I] [2]. This new interest arises from the strong selection of a few sharp states demonstrated by the 1 6 0 ( 1 2 ~ , a)24Mg reaction [I]. This phenomenon can arise from the states in question either having high angular momentum or consisting of unusual configurations, such as cluster states. This region of excitation in 24Mg has also been studied [2] using the 12C(160, a)24Mg reaction ; in this reaction many more states were populated. It has been suggested that these reactions might be characterized by 'Be and 12C transfer, respectively.

In order to determine the nature of the reaction, it becomes necessary to find out as much as possible about the states populated. To this end, other laboratories [3] have measured the 12C(160, a) 24Mg(a

+

'ONe) correlation to determine the spins of the unbound states in 24Mg which a decay to the ground state of 'ONe, and to determine branching ratios for a decay to the various low-lying states in 'ONe.

In this contribution we describe an extension of these efforts to search for the decay by 12C-emission of very highly excited states (20-28 MeV) in 24Mg.

The experimental arrangement consisted of a AE-E telescope located at 0-degree relative to the incoming beam to detect the a particles in the 12C(160, a)24Mg reaction, and a pair of position-sensitive detectors to detect the particles emitted in the decay of the unbound states in 24Mg. The 56.0 MeV 160 beam passed through the 50 ygm/cm2-thick carbon target and was stopped in a set of tantalum foils, thin enough to allow the a particles of interest to pass into the telescope. The solid angle subtended by the telescope was approximately 11 msr, and each of the

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

(* *) Max-Kade Fellow.

4 mm x 50 mm position-sensitive detectors covered a range of laboratory angles from 20 to 65 degrees.

Triple coincidence was required between the 0-degree telescope and both position-sensitive detectors, and five descriptors for each event were recorded on magnetic tape for later analysis. These descriptors consisted of the a energy at 0 degree, and the energy and position from each position-sensitive detector.

The search for weak decay branches such as the 12C decay mode requires mass identification to separate the desired events from the intense a decay. This was obtained using the measured energy and angle of each of the decay products, which uniquely determine the mass ratio of the decay products, m,/m6. The angular calibration of the position-sensitive detectors was determined to an accuracy of < 1 degree using a movable grid during a separate run, while the energy calibration was carried out using the

12c

and 160 ions from "C

+

¹⁶⁰ elastic scattering as well as a' s from a source.

The result of the mass ratio analysis is shown in figure 1. The a-'ONe coincidences leading to the low- lying states of

ON^

can be seen up to the 4.25- and 4.97-MeV states (unresolved). The truncation of the spectrum at this point is due to the particle-decay threshold in " ~ e which prevents the recoiling 20Ne- ion from fulfilling our coincidence requirement.

Furthermore, the existence of 12C-12C coincidences is clearly demonstrated, while we did not observe a peak corresponding to 8Be-160 decay. The 'Be-160 should appear at m,/m6 = 8/16, since both a's from the decay of 8Be will be detected at the same angle.

The relative intensities of the four peaks, 'ONe(g. s.), 20Ne(1.63), 'ONe(4.25

+

4.97), 12C, are roughly given by 1, 2.7, 1.4, 0.28. The upper limit for the ratio of 12C(2+)

+

12C(g. s.) decays to I2C(g. s.)

+

12C(g. s.) decays is approximately 0.1.

Figure 2 shows the a (0 degree) spectra corresponding to the four observed decay channels, where the cc energy has been converted into excitation energy in

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971647

C6-220 M. J. LEVINE, D. SCHWALM AND M. G. LITTMAN

DECAYS TO ON^ 1.63 MeV

,

^100-

0 ^{" " " "}

2 2 0 0

2 DECAYS TO ON^ (4.25.4.97) MeV

0 , ' , I , I , t , I

10 12 14 16 18 2 0 2 2 2 4 2 6 2 8

" M ~ EXCITATION ENERGY (MeV)

FIG. 2. - Spectra are displayed in terms of excitation energy in 24Mg, corresponding to different decay modes.

1200 8 0 0

,

^{4 0 0}

0 8 0 0

9

6 0 0 4 0 0

-3l.w -20.m -ls.m -1o.m

m r r r v l ^{L L} 0 ^{2 0 0}

c r o

FIG. 1. -The ratio of the masses of the decay particles is ^{4 0 0} displayed against the energy Q2 = E S

+

^E6 - Eex(24Mg). % 3 0 0 z 2 0 0

In this contour plot background is suppressed, and because the ₁₀₀ spectrum is symmetric about rnslrns = 1, the portion of the o

spectrum corresponding to ms/rns > 2 has been omitted from 10 12 14 1 6 18 2 0 22 2 4 2 6 B

the figure. 24 Mg EXCITATION ENERGY (MeV)

24Mg. For comparison, figure 3 shows spectra cor- responding to the a-decay channels where only a two-fold coincidence was required between the telescope and one position-sensitive detector. These events were recorded simultaneously with the triple coincidences. The higher cut-off in the triples spectra is due to the geometry. The obtained energy resolution for these spectra is about 300 keV. The entire region of excitation covered by the triple coincidence spectra is notable for its lack of sharp structure. The spectrum

FIG. 3. - Spectra obtained from two-fold coincidences are displayed in terms of excitation energy in 24Mg.

obtained in coincidence with 12C decays shows a broad peak (I' c 700 keV) at 25.2 MeV, and possibly one at 23.6 MeV. A cursory comparison of this spectrum with 12C-12C elastic scattering data [4]

some striking similarities. However, any shows conclusions based on this spectrum must be postponed until the analysis of the 12C angular correlation, now in progress, has been completed.

References

[I] MIDDLETON (R.), GARRETT (J. D.) and FORTUNE (H.T.), [3] BALAMUTH (D. P.), HOLDEN (J. E.), No6 (J. W.) and Phys. Rev. Letters, 1970, 24, 1436. ZURM~~HLE (R. W.), Phys. Rev. Letters, 1971, 26, [2] STOKSTAD (R. G.), GOBBI (A.), MAURENZIG (P. R.) 1271, and references therein.

and WIELAND (R.), Bull. Am. Phys. Soc., 1970, [4] BROMLEY (D. A.), KUEHNER (J. A.) and ALMQVIST (E.),

15, 1677. Phys. Rev. Letters, 1960, 4, 365.