First experiments with the FUSION detector

doi:10.1016/S0168-9002(99)00683-X

Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

Volume 437, Issues 2–3, 21 November 1999, Pages 490-500

https://doi.org/10.1016/S0168-9002(99)00683-X Get rights and content

Abstract

The Fusion Utility for Secondary Ions (FUSION) has been built to measure the fusion–fission cross section induced by radioactive beams at energies around the barrier. A set of 10 parallel plate avalanche counters measures the fission fragments which are stopped therein. Twenty plastic scintillators, surrounding the inner shell of PPACs, detect the eventual residue from the projectile. This detector could work in different geometry and allow measurements of low cross section with very weak beams. The measurements at GANIL of the fission cross sections at barrier energies for ^9,11Be on U are reported.

Introduction

Fusion phenomena at low energy are related to the possibility of two colliding nuclei to overcome the repulsive Coulomb barrier which exists between them. This reaction which can only be understood quantum mechanically depends strongly on the structure of the collision partners [1], [2], [3], [4], [5]. A global understanding of fusion was proposed in the framework of coupled channel calculations which allow consistent calculations of elastic, inelastic and fusion reactions.

The two remaining major problem were pointed out by Nagarajan in his concluding remarks of the Heavy Ion Fusion Conference held in Padova (May 1994):

Is there a correlation between transfer cross section and fusion enhancement? Does break-up inhibit fusion or not?

Radioactive nuclear beams offer a good way to answer these questions by allowing a test of the importance of various effects such as nucleon transfer, break-up or exotic structures like neutron skin or halos.

Several contradictory predictions [8], [9], [10], [11], [12], [13] have been proposed for the fusion cross-section of the neutron halo of nuclei, such as ¹¹Li, ^11,14Be [6], [7]. The presence of this halo can cause a significant enhancement of the sub-barrier fusion cross-sections both because of a lowering of the barrier associated with a stronger nuclear interaction and of an extra enhancement arising from excitation of the predicted soft E1 dipole modes [14]. This enhancement factor might reach 1 or 2 orders of magnitude [8], [9]. The dissociation of the projectile is also predicted to have an influence, with some theories suggesting a strong hindrance to fusion at the barrier [10] while others claim that this will further enhance the fusion probability [12].

Section snippets

Choice of the systems

We have planned to measure at GANIL(Caen, France), using the LISE spectrometer [15], the fusion and transfer cross-section for ^7,9,10,11Be+²³⁸U at sub-barrier energies. The ¹¹Be presents a one neutron halo, the ⁷Be is expected to have a large transfer channel influence and the ^9,10Be will serve as references. The projectile choice is defined by the secondary beam intensities. At GANIL, secondary beams are produced by fragmentation. The fragmentation cross sections for ¹¹Li and ¹⁴Be are too weak

Signature of the events

The range and the velocity of the particles are the parameters which allow the best separation between fission fragments and residual projectiles. The fission fragments have a number of protons between 35 and 60 and a kinetic energy distribution between 60 and 150 MeV. A residue from the projectile will have a charge close to the charge of the projectile, in our case 4, and an energy below 50 MeV for the sub-barrier studies. The ranges of these two species are different by one order of

General information

The FUSION detector (Fusion Utility for Secondary Ions) has been developed at Saclay (France) for this purpose. This detector could be placed in two different configurations according to the expected counting rates. The first one has the structure of a box of detection, with one or three targets inside. The second one, the in-line configuration, is an alternating sequence of targets and detectors. It could run with five targets at the same time. Each configuration is an arrangement of a first

Fission measurement

A first test was made with a californium source on a thin support. The californium could fission spontaneously and, due to the thin support, the two fission fragments could be detected in coincidence in the PPACs. The charge, position, and time-of-flight difference between the set of front PPACs and the back ones were measured. Plastic scintillators were never fired by fission fragments. This is a validation of our gross range selection. The charge deposited in the PPACs, the time of flight and

First measurements

The first experiment done with the FUSION set-up was the study of ^9,11Be+²³⁸U systems. The configuration used for the first experiment was the box with only one target in order to start in optimized conditions.

In order to determine a cross section for a specific reaction it is necessary to take care of impurities in the secondary beam. For each run, the beam purity was higher than 95% and in general closer to 98%, due to selection of the secondary beam in the LISE spectrometer with a degrader

Conclusion

A detector adequate to measure fusion cross-section at very low counting rate has been developed with very satisfying tests. It could be used in two different configurations as function of the expected counting rate. One, three or five targets could be used at the same time. Relative fission cross-sections were obtained for ⁹Be+²³⁸U and ¹¹Be+²³⁸U using the box configuration of the detector. No clear answers could be provide on the effect of weakly bound nucleons on the fusion process close to

Acknowledgements

We would like to thank R. Verna and J.D. Hinnefeld for their careful reading of the script.

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Cited by (12)

Energy levels of light nuclei A=11
2012, Nuclear Physics A
Subbarrier fusion in the systems <sup>11,10</sup>Be + <sup>209</sup>Bi
2004, Nuclear Physics A
The subbarrier fusion cross sections with radioactive ion beams in the systems ^10,11Be+²⁰⁹Bi have been measured and compared also with the stable system ⁹Be+²⁰⁹Bi. All three cross sections are similar within experimental accuracy; this is somehow unexpected since, in particular, the fusion process with ¹¹Be should be influenced by its halo structure and small binding energy and could behave in a different way from the other two Be isotopes, in particular from ¹⁰Be, well bound. Extensive calculations were undertaken within the formalism of continuum discretized coupled channel (CDCC) that considered only the coupling to two-body breakup, into n+core, of the projectile with excitation to continuum up to 10 MeV. The results are in agreement with the ^11,10Be experimental data except at low energy where the cross sections could be influenced by target/projectile collective excitations. The CCFULL theoretical approach, compared to CDCC, seems to reproduce better the subbarrier region and worse the above barrier one. The ⁹Be cross sections are all underestimated by the CDCC approach while are well reproduced by CCFULL calculations. This suggests that, in this last case, the schematization of the breakup process into 2 only fragments maybe is not adequate and/or other excitations, properly handed by the CCFULL code, could be relevant.
Fusion at the barrier with light radioactive ion beams
2001, Nuclear Physics A
The experimental results recently obtained for fusion reactions at energies close to the Coulomb barrier with light radioactive (loosely bound) beams are reviewed and critically discussed. There have been two conflicting views on the effect of the loose binding of the projectile on the fusion cross section. On the one hand one expects an enhancement of the fusion cross section due to the loose binding while, on the other hand, the easy breakup of the projectile is expected to inhibit the fusion cross section. We critically discuss these two aspects of loose binding by comparing the experimental results for a number of radioactive beams.
The data for ¹⁷F (where the last neutron binding energy $S_{n} =0.601$ MeV), neither show breakup effects nor enhancement when compared with the fusion of the nucleus ¹⁹F. The data for a ⁶He beam ( $S_{2n} =0.975$ MeV) show enhancement, very strong in one case, and the strong breakup (BU) $+$ transfer cross section may be related to this. The fusion data obtained with the halo nucleus ¹¹Be ( $S_{n} =0.504$ MeV) do not show enhancement below the barrier, compared with ⁹Be, and show moderate effects above the barrier. Moreover, ⁹Be ( $S_{n} =1.665$ MeV) shows a very strong BU $+$ transfer cross section as in the case of ⁶He, but this apparently has no direct effect on fusion. The data with the neutron-rich ³⁸S, ^29,31Al radioactive beams do not differ much from data with the stable ³²S, ²⁷Al, respectively. In conclusion we observe: (i) effects of loose binding in neutron skin nuclei but not with other halo nuclei, (ii) a reduction of the fusion cross section above the barrier due to breakup effects.
The various theoretical approaches developed up to now based on “simple” assumptions give opposite predictions for fusion, with some expecting enhancement of the cross section while others expect a hindrance of the cross section. However, most of these approaches do not correctly treat the transitions to the continuum which are expected to be induced easily in these weakly bound nuclei. It is necessary to develop a more complete and possibly “simple” theoretical treatment that incorporates the continuum coupling which will play an important role in subbarrier fusion.
Reactions and single-particle structure of nuclei near the drip lines
2001, Nuclear Physics A
The techniques that have allowed the study of reactions of nuclei situated at or near the neutron or proton drip line are described. Nuclei situated just inside the drip line have low nucleon separation energies and, at most, a few bound states. If the angular momentum in addition is small, large halo states are formed where the wave function of the valency nucleon extends far beyond the nuclear radius. We begin with examples of the properties of nuclear halos and of their study in radioactive-beam experiments. We then turn to the continuum states existing above the particle threshold and also discuss the possibility of exciting them from the halo states in processes that may be thought of as “collateral damage”. Finally, we show that the experience from studies of halo states has pointed to knockout reactions as a new way to perform spectroscopic studies of more deeply bound non-halo states. Examples are given of measurements of $l$ values and spectroscopic factors.
The nature and reactions of halo nuclei
2001, Nuclear Physics A
Fusion induced by radioactive ION beams
2005, International Journal of Modern Physics E

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^☆: Experiment performed at GANIL, CAEN, France.

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Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment

First experiments with the FUSION detector☆

Abstract

Introduction

Section snippets

Choice of the systems

Signature of the events

General information

Fission measurement

First measurements

Conclusion

Acknowledgements

Phys. Rep.

Phys. Lett.

Phys. Lett.

Phys. Lett.

Nucl. Instr. and Meth.

Phys. lett.

Nucl. Phys.

Nucl. Phys.

Nucl. Instr. and Meth.

Phys. Rep.

Phys. Lett.

Phys. Rev. Lett.

Phys. Rev.

Ann. Rev. Nucl. Part. Sci.

Rep. Prog. Phys.

Phys. Rev. Lett.

Nucl. Phys.

Phys. Rev.

Phys. Rev.

Energy levels of light nuclei A=11

Subbarrier fusion in the systems <sup>11,10</sup>Be + <sup>209</sup>Bi

Fusion at the barrier with light radioactive ion beams

Reactions and single-particle structure of nuclei near the drip lines

The nature and reactions of halo nuclei

Fusion induced by radioactive ION beams