PARTICULAR MULTINUCLEON TRANSFER PROCESS IN THE 7Li + 197Au REACTION FOR GRAZING CONDITIONS

HAL Id: jpa-00214876

https://hal.archives-ouvertes.fr/jpa-00214876

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.

PARTICULAR MULTINUCLEON TRANSFER PROCESS IN THE 7Li + 197Au REACTION FOR

GRAZING CONDITIONS

J. Quebert, W. Sztark

To cite this version:

J. Quebert, W. Sztark. PARTICULAR MULTINUCLEON TRANSFER PROCESS IN THE 7Li +

197Au REACTION FOR GRAZING CONDITIONS. Journal de Physique Colloques, 1971, 32 (C6),

pp.C6-255-C6-258. �10.1051/jphyscol:1971658�. �jpa-00214876�

JOURNAL DE PHYSIQUE Colloque C6, supplkment au no 11-12, Tome 32, Novembre-Dicembre 1971, page C6-2%

PARTICULAR MULTINUCLEON TRANSFER PROCESS

IN THE 7Li + 1 9 7 A ~ REACTION FOR GRAZING CONDITIONS (*) J. L. QUEBERT and H. SZTARK

Centre d'Etudes Nucldaires de Bordeaux-Gradignan, France

RbumB. - L'bmission preponderante de particules alpha et de tritons dans la reaction 7Li ^f 197Au ne provient pas d'une dissociation directe du projectile. Les particules sont kmises a partir d'un systtme en rotation quasi lie forme par la cible et la structure en agrkgats la moins l i b du projectile. Ce mecanisme se produit essentiellement lorsque le projectile est dans le voisinage de la cible avec une vitesse relative negligeable.

Abstract. - The preponderant emission of alpha particles and tritons in 7Li ^f 197Au cannot be accounted for by a projectile break-up hypothesis. The particles are emitted from a quasi-bound rotating system formed by the target and the less bound cluster-structure of the projectile. This mechanism essentially occurs when the projectile is in the vicinity of the target with a negligible relative velocity.

Introduction. - The emitted particles in 6Li inter- actions with heavy targets correspond essentially to the weakly-bound a-d component of the projectile [I-41. The continum spectra have no fine structure ; they are several MeV wide and symmetrical in shape around their mean energy. They have been attributed to a Coulomb or nuclear break-up of the projectile t5-71. However, neither the angular distributions nor the spectra shapes and the mean energies can be properly accounted for.

We present some results about 7Li + Ig7Au, studied between 27 MeV and 45 MeV. Most of the emitted particles are alphas and tritons, corresponding also t o the less bound structure in 7Li. However, we shall show that the break-up hypothesis is not valid. We identify a and t spectra as respectively associated to a ( t + target) and a n (a + target) system. This mechanism is maximum for kinematic conditions roughly corresponding to a grazing incidence.

Experimental results. - Most of the experiments have been done a t the Lawrence Radiation Laboratory of Berkeley (USA), using the HILAC machine. They have been carried on in Saclay (France), with the SPNBE Tandem Van de Graaff. Three kinds of studies have been done : observation of direct particle spectra ; particle-gamma coincidences ; particle-particle coinci- dences.

1. PARTICLE SPECTRA. - Particles have been identi- fied a t 32 MeV (7Li Coulomb barrier). Alpha and triton emissions are preponderant compared with p, d

(") Work partly performed under the auspices of the US

Atomic Energy Commission.

~ ~ Alpha " and tritcn spectra " ~ ~ ' " " ~ ~

W=W, EINC

=

.

alphas

.. . .

r trltonr

. . .

' 10 u.

.-'. . .

, . .

: ....*..

o

,

,

,

0 5 10 15 20 25 E tHeV,

FIG. 1. - Relative alpha and triton spectra at E7Li

32 MeV and 'Y

90°.

and 6 ~ e . We can note in figure 1, that the alpha spectrum is more intense than the triton one, reflecting roughly the ratio between total cross-sections. We can see also the lack of structure and the symmetrical shapes of the spectra. Figure 2 shows the behaviour of the total particle spectrum against the incident energy a t 900 to the beam. The alpha spectrum, for instance, is clearly lowering in intensity, after having reached a maximum, while its mean energy and its width are slightly increasing. So, the alpha differential cross-section has a maximum which is moving according to the detection angle. Moreover, a t 32 MeV the angular distributions present a maximum a t the same angle. An example is shown in figure 3 about alpha particles. These two results point out a good testing of the grazing angle law. The good symmetry of da/d9 around the grazing angle indicates a surface mechanism and could be explained by a tunneling

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971658

C6-256 ^{J. L.} QUEBERT AND H. SZTARK

According to a transfer hypothesis, the residual nuclei 'OOHg* and 201T1* are unstable against neutron emission. The alpha and triton mean energies allow to determine the mean excitation energy and the width in both residual states. At the Coulomb barrier the following parameters are got :

'OOHg : E*

22.6 MeV, r ^{= 6 MeV ;}

'OIT1 : EX

17.4 MeV, r ^{= 2.8 MeV.}

As Oglobin [8] pointed it out, the excitation energies correspond t o the following expression :

E" = B,(m + ^T) + ^A(m + ^{T) - A(R)}

where B, is roughly the Coulomb barrier (nz + ^T), m, the transferred cluster, T, the target, R, the residual nucleus and A, the mass excess.

In other respects, the mean kinetic energies in the a + ' O O ~ g * a n d t + '01~1* channels can be calculated very simply with the expressions :

E ,

= B,(a + ''OHg*) and ⁼ B,(t + 201T1*).

However the experimental values are about 2 MeV lower than the calculated B,. Experimentally,

and et are slightly increasing with the incident energy.

The first hypothesis involves E* should strongly increase with the incident energy. So, the second calculation is more credible and the results remind of a kind of evaporation process, associated to a surface interaction. We shall discuss more about this process.

2. PARTICLE-GAMMA COINCIDENCES. - This multi- dimensional experiment has been done in Berkeley, using a PDP 7 computer. Particles were recorded a t

FIG. 2. - Total particle spectrum against the incident energy. 1620 to the beam with a ring counter and rays at 900, with a Ge(Li) detector. We can see in figure 4,

,"

"

.-

=

-

-

u

-

L m

I I I

Alphas (19MeV In c.M.S) d

0

- _{d w}

.

o 50

150 e

FIG. 3. - Angular distribution of 19 MeV a particles (C. M. S.).

The drawn curves are not theoretical. A similar result is got with tritons (same grazing angle and same shape).

multinucleon transfer to the target. The deduced %

distance of maximum interaction is D o = 12.7 fm and -

the width is AD -- 4 fm. It is interesting to that

F I ~ . 4. - ResultS from particle-y coincidences. The coincidence

this mechanism becomes negligible when D reaches

and t soectra are similar to the direct ones. The y lines corres-

the classical contact value D, = 10 fm. ponding to each kind of particle are shown on the right.

PARTICULAR MULTINUCLEON TRANSFER PROCESS

the particle spectrum associated to some 1 9 7 A ~ ,

8Hg and 199T1 (?) lines. We explain the last two p~oductions by the 197Au(t, 2 n)19*Hg* and 197Au(a, 2 n)199~1*

reactions. This result confirms a transfer process as well as the following experiment :

3. PARTICLE-PARTICLE COINCIDENCES. - At first this experiment was undertaken to get informations about the 7Li break-up process by looking for possible a-t coincidences. In fact it will show we cannot anymore consider a break-up hypothesis and will confirm ( t + T) and (a + T) residual interactions.

Two small detectors were set-up side by side in a normal plan to the beam. The distance from the detectors to the target centre could be modified so as to change the coincidence geometry. Figure 5 shows

FIG. 5. - Particle-particle coincidences.

is defined by the target centre and the front sides of the detectors. We note four broad peaks at IS0 and an important angular correlation

effect.

some normalised results. We note a maximum counting rate a t Q1, = 180. In this case, four peaks are distin- guished ; they are in coincidence in twos as the figure shows it. We explain this result with a Dalitz plot in figure 6 : Three kinds of interactions can be considered to analyse the total spectrum : (a + t)*, (oblique density) ; (t + Au)*, (vertical density) ; (a + Au)*, (horizontal density). In each coincidence geometry case, a part of this diagram is recorded as indicated in the figure. So, the detected spectra in both counters are a mixing of projections on each axis.

The first case corresponds to a break-up, in which the 712- (4.63 MeV) state in 7Li has been particularly involved, as well as other excitations above the a + t

FIG. 6. - Dalitz plot for 7Li + 197Au +

+ ^t + 197Au.

The exact proportions are not respected. The (a + T)* and (t + T)* interactions are schematically drawn to understand

the projected spectra.

threshold. However the theoretical results give rise to a mixing of four peaks as it follows : t-a-t-a, in disagreement with the experimental results. This is strongly against the 7Li break-up hypothesis. On the other hand we find a good agreement with (t + T)"

and (a + T)* states similar to those got by direct

FIG. 7. - Theoretical analysis of the coincidence spectra at

@12 =

180. The full line is the total spectrum and the peaks in coincidence are joined by the same kind of line.

spectra analysis a t the Coulomb barrier. The strong angular correlation, only, is not yet explained, for we need more complete experimental results.

Discussion and concIusion. - The above experi-

mental results allow us to write :

C6-258 J. L. QUEBERT AND H. SZTARK The following properties of this particular mecha-

nism can be pointed out :

- The mechanism occurs for kinematic conditions where the target and the projectile are touching gently.

- The emitted particle, not involved in the residual interaction, evidences an angular distribution compa- tible with a surface absorption mechanism.

- The mean energy of the same particle is slightly increasing with the incident energy, as well as the width of its continuum spectrum.

The analysis of the spectra shapes led us to find out an empiric law to calculate the mean energy E,, available in the channel (n + ^{R), n} being the consi- dered emitted particle and R, the residual system.

We found :

+ 0 . 2 7 ( ~ , - Bb(P + T ) ) ; m is the interacting particle, T, the target, B,, the Coulomb barrier, Bb, the Coulomb potential at the interaction distance (Do), E,, the a + t binding energy in 7 ~ i and E ~ , the c. o. m. incident energy. To give a general trend of this equation, let us consider a quasi- bound rotating system : (a + t + I g 7 ~ u ) . The emitted particle n has a velocity : v = v, + v,, where v, comes from the Coulomb repulsion and v, from the rotation of the system. The particle energy is then :

with p = (v,, v,), B, = Coulomb barrier in (n + R) and e, = rotational energy. We suppose that

is linearly related to the projectile energy (E, - B:), when the interaction takes place, and t o the binding energy E, :

= k.(+ - B,') - E,. So, the mean energy is

EM

= Bc +

^{= Bc - Eb} + k.(Ep - B:)

(k = 0.27 - 217 in the 7 ~ i + lg7Au case). We can see

the effect of 2 JB,

q, which is widening the natural shape of the spectrum through

and so, through the incident energy.

Similar spectra have been recorded with other targets [2,3]. The 6Li results also, display such a kind of spectra with a and d, even with light targets [9].

A similar process could happen with 1°B(a ; 6 ~ i ) and 12C(a ; 'Be ⁼ 2 a), that we have studied also. In each case, at the Coulomb barrier, we are able to calculate the particle mean energy, assuming this particle is the non-interacting one (However in 6Li + 6Li ⁺ a +

(d + 6Li) + 3 a [9], the remaining a particle is not detected because of its small energy). k, only, is more difficult to determine, because of the lack of experi- mental results on a large spread of incident energy. It seems to be the same for both emitted particles and depends slightly on the projectile identity. We found the following trend : k - ^{2/P ; P =} projectile mass.

In conclusion it seems there is an experimental evidence of an intermediate surface mechanism with lithium ions. Experimental results are against the break-up hypothesis, involving a particular excitation in the projectile. It is obvious that deformation are strongly involved in this process, particularly when the relative velocity between the target and the projectile is negligible.

Acknowledgments.

One of us (J. L. Q.) is greatly indebted to Dr R. M. Diamond and t o Dr F. S. Ste- phens for their help and the opportunity to participate in this experiment in their team at the Berkeley Law- rence Radiation Laboratory. He is also grateful to Dr E. Cotton, in Saclay, for the possibility of using the Tandem machine. He would like to express his appreciation to Drs Bassani, J. L. Leigh, K. M. Maier, L. Marquez, K. NakaI and to N. Saunier for discussion and help. At last he thanks Mrs R. Garrett for processing the data, the HILAC crew of Berkeley and the Tandem crew of Saclay for stable beams.

References [I] ANDERSON (C. E.), Proc. 2nd Conf. React. Complex

Nuclei, Gatlinburg, 1960.

[2] OLLERHEAD (R. W.), CHASMAN (C.) and BROMLEY (D. A.), Phys. Rev., 1964, B74, 134.

[3] SPETH (E.), Dissertation, Heidelberg, 1970.

[4] BETHGE (K.) and MEIER-EWERT (K.), Phys Rev.

Letters, 1967, 18, 1010.

[5] NAKAMURA (K.), Phys. Rev., 1966, 152, 955.

[6] GLUCKSTERN (R. L.) and BREIT (G.), Proc. 2nd Conf.

React. Complex Nuclei, Gatlinburg, 1960.

[7] SPETH (E.), PFEIFFER (K. 0.) and BETHGE (K.), Phys.

Rev. Letters, 1970, 24, 1493.

[8] OGLOBIN (A. A.), NucI. React. Heavy ions, Heidel- berg, 1969.

[9] QUEBERT (J. L.), COSTE (M.) et MARQUEZ (L.), Nuclear

Physics, 1964, 54, 257.