40Ca STATES EXCITED BY FOUR NUCLEON TRANSFER REACTIONS

(1)

HAL Id: jpa-00214885

https://hal.archives-ouvertes.fr/jpa-00214885

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 published 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.

40Ca STATES EXCITED BY FOUR NUCLEON TRANSFER REACTIONS

A. Tellez, R. Ballini, I. Fodor, J. Fouan

To cite this version:

A. Tellez, R. Ballini, I. Fodor, J. Fouan. 40Ca STATES EXCITED BY FOUR NUCLEON TRANSFER REACTIONS. Journal de Physique Colloques, 1971, 32 (C6), pp.C6-287-C6-288.

�10.1051/jphyscol:1971667�. �jpa-00214885�

(2)

JO~JRNAL DE PHYSIQUE Colloque C6, supplthent au no 11-12, Tome 32, Novembre-Dicembre 1971, page C6-287

40Ca STATES EXCITED BY FOUR NUCLEON TRANSFER REACTIONS

A. TELLEZ, R. BALLINI, I. FODOR (*) and J. P. FOUAN DCpartement de Physique NuclBaire, C . E. N. Saclay, France

Rbumk. - ^LCtransfert de quatre nuclCons sur 3 6 A r , donnant des etats excites de 4oCa, est obser vB dans les reactions (7Li, t) and ( 1 6 0 , 1 2 C y ) .

Abstract.

-

Four nucleon transfers on 36Ar leading to excited states of 4oCa are observed trhough thereactions (7Li, t) and ( 1 6 0 , I z C y ) .

The 4Ca first Of state a t 3.35 MeV is well interpret- ed [I] by 4 p-4 h configuration with T = 0 intermediate isospin. This is described in the weak coupling scheme as 44Ti x 36Ar and this structure is expected to yield some rotational character.

In such case, four nucleon transfer reactions as ( ' ~ i , t) and (160, I 2 c ) should strongly excite the members of the t( band >> based on the 0' level, in particular the 6' term, if it exists (as in the case of 160).

In fact, the 36Ar(7Li, t)$OCa experiment [2] showed that even parity states of 40Ca are preferentially excited. However, the cross sections are weak (e. g. a few microbarns per steradian for the 0: state) and the use of a gas cell target limited the energy resolution to 250 keV FWHM, thus preventing precise identifica- tion of the excited levels in many cases. The (160, 12C) reaction on the same gas cell leads to an even poorer particle energy resolution but its much larger cross section and the low gamma background produced by the 160 beam makes it suitable for a coincidence experiment ; in such an experiment, good gamma energy resolution can be achieved with a germanium detector and excited levels can be identified through their known gamma decay modes.

In our experiment, the 0.3 pA beam of 55 MeV 160(6') ions was stopped in the gas cell (entrance window : 0.7 pm thick nickel foil, cooled exit window :

10 pm tantalum foil) which was operated at a pressure of 0.4 atmosphere of isotopically separated 36Ar on a total length of 4 cm. Outgoing 12C were detected with a solid state counter telescope (AE : 7 pm ; E = 1 500 pm) near to the beam axis (+ 100) in coincidence with gamma rays detected a t 900 (80 cm3 germanium detector). Large solid angle with suitable counting rate, and minimum Doppler effect can thus be achieved. In these conditions I2C were unambiguously identified down to an energy corresponding to an excitation of 40Ca of approximately 10 MeV. Every relevant event

parameters were recorded on a magnetic tape, and the replay procedure used with different windows on the particle energy showed that it was possible to obtain the gamma spectra corresponding to selected 40Ca levels.

The first 2+ and 4" states at 3.90 and 5.28 MeV, members of the above mentioned c< band D, are as expected the most strongly excited levels. It should be noted that the 3.35 MeV Of member of this group decays by pair production and thus cannot be measured in our experiment.

The 5.21 MeV Of, 5.25 and 5.62 MeV 2+ states are not observed (i. e. the corresponding cross section is less than 1/10 that of the 3.90 MeV 2: level). This suggests that the structure of these states differs from that of the first 2+, for example 2 p-2 h or 4 p-4 h with intermediate T # 0 as recently calculated [I]. (It ought to be mentioned that in our experiment the 2' state yield is strongly inhibited by angular correlation effects. But this attenuation is identical for all 2' -+ Of transitions.)

The collective 3- state at 3.74 MeV is excited while the 4.49 MeV 5- is not (the latter being identified by its 0.75 MeV transition to the 3- level). This can be seen

(*) On leave from Institute for Nuclear Physics, Budapest FIG. 1.

-

The coincident gamma spectrum corresponding to

Hungary. excitation energies up to 5 MeV in 4oCa.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971667

(3)

A. TELLEZ, R. BALLINI, I. FODOR AND J. P. FOUAN

36

COUNTS ~r ('~i,t)~~~a EL; = 26.75 MeV 8 t = 3 O 0 l a b .

A 4

'500-

fJ ,.

d

y l'

500-

*

250 400 550 700 800 900 1000 CHANNEL

FIG. 2. - Triton spectrum from the 36Ar(7Li, t)40Ca reaction. The energy calibration was obtained from a (36Ar -f 1 6 0 )

(7Li, t) (40C-i

+

20Ne) experiment.

on figure 1 which shows the gamma spectrum in coincidence with a I2C energy range corresponding to excitation of 40Ca up to 5 MeV. This could be explained by Coulomb excitation of the target nucleus into a collective state, together with four nucleon transfer.

The 5 - level is rather pure lp-1 h and probably should not be excited through this channel. On the contrary, above 7 MeV excitation, some levels decaying to this 5 - state are excited.

The existence or non existence of a 6+ state lying between 6 and 8 MeV excitation and decaying to the 4+

state at 5.28 MeV with a large probability (250 e2 fm4, if we take for the theoretical quadrupole moment of the band the value obtained in [I]), is an important test of the rotational character of the 0+, 2' and 4+

sequence at 3.35, 3.90 and 5.28 MeV. In previous

@, p' y) experiments [3], three levels with unknown spin exhibited strong transitions to the 4+, 5.28 MeV states, the 6.93, 7.40 and 7.68 MeV ones. The 7.40 and 7.68 MeV states are not produced in the (7Li, t) reaction, where a large peak appears around 6.93 MeV (Fig. 2) but cannot be associated with a single level, due to the rather poor energy resolution. In fact a recent (p, p'y) Doppler shift attenuation measurement showed that

many states were present in this region : the already known 6.91 and 6.95 MeV and the 6.93 MeV which consists of at least two levels (closer than a few keV apart). The gamma transitions associated to the 6.93 MeV multiplet are the following : two transitions to the 3.74 MeV 3- state and a 1.65 MeV transition to the 5.28 MeV 4+ level. The lifetime obtained from this last transition is 80 $- 30 fs which would yield a B(E 2) = 800

+

300 e2 fm4 supposing a 100 % decay to the 4'. On the other hand in the 36Ar(160, 12Cy)40Ca experiment, this 1.65 MeV gamma line has been observed (although not as strongly as the 4: ⁴2: transition) in coincidence with the I2C energy range corresponding to the 6.9 MeV excitation range.

In conclusion our experimental data show that four nucleon transfer on 36Ar proceeds through a rather complex reaction mechanism. The use of gamma coincidence technique showed its ability as a tool to identify some excited levels.

The 3.35, 3.90 and 5.28 MeV states generally inter- preted with a 4 p-4 h << rotational ⁾⁾structure are well excited in these processes but the question of higher spin members of such band remains open.

References

[I] GERACE (W. J.) and GREEN (A. M.), NucZ. Phys., [21 TELLEZ (A.), FOUAN (J. P.), BALLINI (R.), CABE (J.) 1969, A 123, 241. and DELAUNAY (J.), J. Physique, 1970, C 2, 172.

FEDERMAN (P.) and PIITEL (S.), NucZ. Phys., 1969, [3] TELLEZ (A.), Thesis, Universitk de Paris-Sud, Orsay,

A 139, 108. 1971.

ZUKER (A. P.), Padua Conference, 15-21 April 1971.