REVISTA MEXICANA DE FISICA S53 (6) 59–63 DICIEMBRE 2007

The radioactive ion beams facility in brasil RIBRAS

R. LichtenthalerInstituto de Fısica, Universidade de Sao Paulo,

C.P.66318, 05389-970 Sao Paulo, Brazil.

Recibido el 27 de febrero de 2007; aceptado el 30 de marzo de 2007

A double superconducting solenoid system is installed at the Pelletron Laboratory of the University of Sao Paulo, Brazil. This system allowsthe production of secondary beams of8Li, 6He,7Be and other light exotic nuclei. The first results using this facility are presented.

Keywords:Radioactive beams; superconducting solenoids.

Se ha instalado un sistema de dobles solenoides superconductores en el laboratorio del Peletron de la Universidad de Sao Paulo, Brasil. Estesistema permite la produccion de haces secundarios de8Li, 6He, 7Be y otros nucleos exoticos ligeros. Se presentan los primeros resultadosobtenidos con esta instalacion.

Descriptores: Haces radioactivos; solenoides superconductores.

PACS: 21.10.Gv; 25.60.-t; 26.35.+c; 29.38.-c; 25.70.Bc; 24.10.Ht

1. Introduction

The study of reactions induced by light weakly bound andlight exotic projetiles is one of the main interests in NuclearPhysics nowadays. Unstable nuclei like6He, 8Li, 8B andothers present strong cluster structure, even in their groundstate, with small separation energies and a large probabilityof breaking-up when colliding with other nuclei. In particu-lar the6he and8B projetiles are the ones with lower bindingenergies and, in addition, the6He has a halo formed by thetwo neutrons which increases the nuclear radii, lowering ofthe Coulomb barrier. Many experimental and theoretical ef-fort has been spent on the study of the collisions with theseloosely bound projectiles, however there are still interestingopened questions concerning the structure and the reactionmechanism involving exotic projectiles. In particular, the roleof the low binding energies and of the halo on the fusion pro-cess is still not yet understood. An increase of the probabilityof the fusion and of the transfer reactions due to the extra neu-trons or protons in these light nuclei could have consequencesin astrophysics as well as in other fields like the productionof superheavy elements.

This motivated the group of the Pelletron Laboratory ofthe University of Sao Paulo to install the first South Amer-ica Radioactive Ion beams device (RIBRAS) [1] (see Fig. 1).This facility extends the capabilities of the original8 MV Pel-letron accelerator by producing secondary beams of light un-stable nuclei. The most important components in this systemare the two new superconducting solenoids. The solenoidshave 6.5 T maximum central field (5 T.m axial field integral)and a 30 cm clear warm bore, which corresponds to a max-imum angular acceptance in the range of2 ≤ θ ≤ 15 degin the laboratory system. The actual acceptance is between2 ≤ θ ≤ 6 deg and is defined by the Faraday cup and theentrance collimator of the first solenoid. The solenoids werebuilt by Cryomagnetics INC and were designed to operatein connection with the Linac post-accelerator, presently un-

der construction. With the Linac, the energy of the primarybeam will be about2− 3 times larger than the maximum en-ergy of the present Pelletron Tandem of3 − 5 MeV.A. Thepresence of two magnets is very important to produce puresecondary beams. The first solenoid makes an in-flight selec-tion of the reaction products emerging from the primary tar-get in the forward angle region. As the first magnet transmitsall ions with the same magnetic rigidityBρ =

√2mE/Q

the radioactive secondary beam can be rather contaminated.With two solenoids, it is possible to use differential energyloss in an energy degrader foil, located at the crossover pointbetween the magnets. This degrader foil will allow the sec-ond solenoid to select the ion of interest by moving the lightcontaminants out of its bandpass.

2. Primary Production Target

The RIBRAS beam line is presently mounted in the experi-mental room of the Pelletron accelerator Laboratory.

The production system consists of a gas cell, mounted ina ISO chamber followed by a tungsten Faraday cup whichsuppress the primary beam and measures its current. The gascell was mounted with a2.2µm Havar entrance window and a9Be vacuum tight exit window12− 16µm thick which playsthe role of the primary target and the window of the gas cell.The gas inside the cell has the purpose of cooling the Beril-ium foil heated by the primary beam and can also be used asproduction target. In case we want to use a gas target to pro-duce secondary beams, the Berilium foil can be replaced byanother Havar foil and the pressure inside the cell can be in-creased up to several Bars. About35 centimeters behind thegas cell there is a tungsten Faraday cup to stop the primarybeam and measure its intensity. The Faraday cup consists of atungsten rod of2.4cm diameter with a cilindrical hole of1cmdiameter per2cm depth to collect the primary beam particles.In front of the Faraday cup there is a circular colimator whichis polarized to a negative voltage of about−250 Volts in

60 R. LICHTENTHALER

FIGURE 1. The experimental set-up, with the production target, the W beam stopper, the magnets and the scattering chamber, with secondarytarget and detectors. See text for details.

TABLE I. Maximum production rates at RIBRAS using only 1solenoid

Production reaction secondary beam purity

(pps)9Be(7Li,8 Li)8Be 0.3× 106 40%9Be(7Li,6 He)10B 0.3− 0.6× 105 10%

3He(7Li,7 Be)t 0.9× 105

3He(6Li,7 Be)d 2× 105 1%

order to suppress the secondary electrons emitted from thecup. As the Faraday cup is electrically isolated from thechamber, the beam current can be measured by a meter andthe total charge can be integrated providing a measurement ofthe number of particles of primary beam incident in the pri-mary target. Direct measurements of the secondary beam canbe performed by placing a silicon telescope at zero degrees inthe cross over point between the two solenoids. In this casethe primary beam has to be reduced up to less than0.5nAe.

3. 8Li, 6He and 7Be secondary beams.

The 8Li and 6He secondary beams are produced bythe neutron pickup and proton stripping reactions9Be(7Li,8Li), Qreac = +0.3674 MeV and 9Be(7Li,6He),Qreac = −3.389 MeV respectively. The primary7Li beam isproduced by the NEC multi-cathode snics ion source with anintensity of typically 100 pnA (particle nano-Amperes). The8 MV Pelletron accelerate the7Li beam up to the maximumenergy ofEl = 32 MeV. Thus the maximum energies of the8Li and 6He secondary beams are of about30 MeV for the8Li and27 MeV for the6He considering the energy losses inthe primary target of 16µm of 9Be and the kinematics of theproduction reactions.

FIGURE 2. ∆E − E spectrum for the3He(6Li,7 Be) and3He(7Li,7 Be) reactions.

Rev. Mex. Fıs. S53 (6) (2003) 59–63

THE RADIOACTIVE ION BEAMS FACILITY IN BRASIL RIBRAS 61

FIGURE 3. ∆E − E spectrum for the9Be(7Li,8Li) reaction.

The production rates for6He measured at 4 energieslower than the maximum energy11 ≤ E6he ≤ 16MeVstill show productions rates of about0.2 × 105 to 0.4 × 105

pps. The production rates are compared to DWBA predic-tions for the reaction9Be(7Li,6He)10B and the agreement isquite good. In the DWBA calculations we used spectrocopicfactors obtained from a fit of existing data at higher energies.

Two reactions were used to produce the7Be beam bothusing a3He gas target.3He(7Li,7Be)t Q = −0.88 MeVand 3He(6Li,7Be)d Q = +0.11 MeV. There is a factor2 in the production rate between these two reactions the3He(6Li,7Be)d being the higher. The proton rich7Be sec-ondary beam has clearly more contaminants compared to theneutron rich6He and8Li and this is due to the fact thatthe proton rich beams normally have lower magnetic rigid-ity compared to the neutron rich beams and in general lowerthan the primary. As a consequence the energy degraded par-ticles from the primary beam fall in the same band pass ofthe secondary beam decreasing the purity of the secondarybeam. In table 1 we present the purity of the secondary beamscalculated asNsec/Ntotal for spectra measured at zero de-grees with the primary beam dimmed. Despite the low purityobserved mainly for the7Be, the particles of the secondarybeam are well separated from the other contaminants (seeFigs. 2 and 3) making possible the elastic scattering measure-ments.

4. First results

In Figs. 4, 5, and 6 we present an angular distributions of theelastic scattering of6He+27Al [2], 7Be+51V at 26 MeV and8Li+58Ni compared to optical model calculations using theSao Paulo optical potential [3]. The Sao Paulo optical poten-

FIGURE 4. Angular distributions of6He+27Al [2] compared tooptical mode calculations using the Sao Paulo potential. The errorbars in the scattering angles are due to the secondary beam diver-gence plus the acceptance of the colimators of the detectors.

FIGURE 5. Angular distribution of7Be+51V at 26MeV comparedto optical mode calculations using the Sao Paulo potential.

tial is double folding for the real part of the potential. Theimaginary part is taken as proportional to the real part withthe same geometry. There 2 possible parameters to be varied,

Rev. Mex. Fıs. S53 (6) (2003) 59–63

62 R. LICHTENTHALER

the difuseness (a) of the mass distributions of the projectileand target used in the double folding and the normalization ofthe imaginary potential (Ni). The normalization of the realpart was kept fixed. The solid curves of Figs. 6 and 7 havebeen obtained adjustingaproj andNi to fit to the data. Thebest fit provided the total reaction cross section for these sys-tems. In order to compare the reaction cross sections amongdifferent systems it is necessary to extract the geometrical andCoulomb barrier effects. In Fig. 6 we compare the reducedreactions cross sections for the exotic systems7Be+51V and6He+27Al with other normal and weakly bound systems.The reduction of the cross sections and energies have beencalculated by the formulas:σred = σ/(A1/3

p + A1/3t )2 and

Ered = Ec.m ∗ (A1/3p + A

1/3t )/(ZpZt) proposed by P.R.S.

Gomes [5,6].

FIGURE 6. Angular distributions of8Li+58Ni compared to opticalmode calculations using the Sao Paulo potential.

FIGURE 7. Reduced reaction cross sections for the6He+27Aland 7Be+51V compared to other normal and weakly bound sys-tems. The experimental data for the stable systems16O, 9Be, and7,6Li+27Al are from Refs. 4 and 5.

FIGURE 8. Reduced reaction cross sections for the8Li+58Niand 7Be+51V compared to other normal and weakly bound sys-tems. The experimental data for the stable systems16O, 9Be, and7,6Li+64Zn are from Ref. [6].

The detection system consisted of silicon telescopes witha thin∆E(20µm) and thicker E silicon detectors. The tele-scopes are mounted on a rotating plate inside the scatteringchamber after the first solenoid.

One can observe that the smallest reduced cross section isfor the tightly bound projectile16O + 27Al [4], for which thebreak-up process is not expected to occur. The values ofσred

R

for the weakly bound stable nuclei6,7Li and 9Be on27Al [5]are similar to each other and are larger than that for the16O.The values ofσred

R for the halo nucleus6He, which has thesmallest break-up threshold energy and has the largest uncer-tainties, is seemingly similar toσred

R of the weakly bound sta-ble nuclei. This indicates that for light systems the effects ofthe halo on the reaction cross sections could be much smallerthan the effect observed with the same projectiles on heaviertargets as the64Zn [5]. In this case the reaction cross sectionsfor the6He+64Zn system were considerably higher than thecross sections for the weakly bound projectiles6,7Li and 9Beon 64Zn. One possible explanation for this fact could be theeffect of the Coulomb breakup which would be more impor-tant for the64Zn target when compared to the lighter27Al [2].Certainly higher quality data are needed to observe the effectof the Borromean nature of6He on the reaction cross sectionat low energies and light targets.

The reaction cross section for the7Be+51V system fallsin the systematics for the weakly bound7Li projectile. Thisis expected since the7Be is the mirror of7Li and has no halo.

5. Scientific Program with RIBRAS

The proposals of experiments for RIBRAS approved in thelast PAC of the Sao Paulo Pelletron Laboratory for the year

Rev. Mex. Fıs. S53 (6) (2003) 59–63

THE RADIOACTIVE ION BEAMS FACILITY IN BRASIL RIBRAS 63

of 2005-2007 consisted basically of elastic scattering stud-ies with exotic projectiles on several targets and transfer re-actions involving exotic projectiles. Measurements of elas-tic scattering angular distributions using projetiles like6He,8Li and 7Be are in progress with RIBRAS. A few systemsare being studied at the moment,6He+27Al, 6He+120Sn,8Li+120Sn and8Li+58Ni at energies around the Coulombbarriers. The main interests are in the effect of nuclear andCoulomb breakup on the elastic scattering and the thresholdanomaly behaviour in systems with weakly bound projec-tiles. Elastic scattering measurements with these radioactiveion beams are feasible with the present intensities of the Pel-letron primary7Li beam which are between30 − 100pnA.With the present intensities direct measurements of reactionslike α(8Li , n)11B, p(8Li , α) are possible. These reactionsplay an important role in the inhomogeneous model of theprimordial nucleosynthesis as a possible bridge to overcomethe A = 8 mass gap for the synthesis of heavier elements.Indirect measurements of relevant astrophysical factors canalso be performed using transfer reactions via the Asymp-

totic Normalization Coefficient (ANC) determination. Thisis the case for instance of the reaction9Be(8Li,9 Li)8Bewhich could provide the ANC for the8Li + n →9 Li cap-ture reaction. This is an example of a reaction that can notbe measured directly since there are no targets of8Li or neu-trons. Other heavier secondary beams like17,18F can also beproduced at RIBRAS opening the possibility of direct mea-surements of reactions of the CNO and hot CNO cycles likep(17F, α)14O andp(18F, α)15O. The study of transfer andcapture reactions of astrophysical interest will be possible tobe performed in the near future using both solenoids.

The experimental data and theoretical analysis presentedin this paper are the result of the RIBRAS collaboration ofRef. 2.

Acknowledgments

The authors thank to the Fundacao de Amparo a Pesquisado Estado de Sao Paulo (FAPESP), Auxılio Pesquisano.97/9956-5 and Projeto Tematico no.2001/06676-9.

1. R. Lichtenthaler, A. Lepine-Szily, V. Guimaraes, G. F. Lima,and M. S. Hussein.Europ. Phys. Jou.25 (2005) 733.

2. E.A. Benjamimet al., Phys. Lett. B647(2007) 30.

3. L.C. Chamonet al., Phys. Rev. C66 (2002) 014610.

4. E. Crema, Masters degree thesis USP, 1979 , unpublished

5. P.R.S. Gomeset al., Phys. Lett. B601(2004) 20.

6. P.R.S. Gomeset al., Phys. Rev. C71 (2005) 034608.

Rev. Mex. Fıs. S53 (6) (2003) 59–63