Studies of neutron-rich isotopes with the CPT mass spectrometer and the CARIBU project

doi:10.1016/j.ijms.2006.01.047

International Journal of Mass Spectrometry

Volume 251, Issues 2–3, 1 April 2006, Pages 252-259

https://doi.org/10.1016/j.ijms.2006.01.047 Get rights and content

Abstract

The heavy neutron-rich isotopes area is the least explored region of the nuclear landscape. Although sensitive techniques exist to gather the required information on these isotopes, they just have not been made available in sufficient quantity and with the right properties for many of the most basic studies. Recent measurements at the Canadian Penning trap (CPT) mass spectrometer, using isotopes produced from the fission of ${}^{252}{Cf}$ stopped in the CPT gas catcher system, have allowed the mass of a number of neutron-rich isotopes to be determined. This approach is being further pursued in the CARIBU project with the installation of a new dedicated heavily shielded source and gas catcher that will yield four orders of magnitude higher neutron-rich isotope yield at low energy for mass measurements and Coulomb barrier energy for nuclear structure studies.

Introduction

Our understanding of nuclear structure has evolved in stages, frequently driven by technological advances. Initial mass measurement capabilities [1] indicated the presence of stable isotopes of the different elements. Light-ion induced reactions [2] then allowed the investigation of these stable nuclei and the resultant explosion of new information stimulated the development of the shell model [3] and collective models [4]. Accelerated heavy ions [5] allowed us to move away from the valley of stability and progress to very high spin. The curvature of the valley of stability allowed roughly a 1000 new proton-rich isotopes to be studied. Again, this wealth of information stimulated theory and a new generation of mean field models and techniques for cranking the mean field to understand the effects of fast rotation. We are in a new phase. In theory, the development of ab initio methods [6] has moved our understanding of the structure of light nuclei onto an entirely new quantitative plane with strong predictive power and high precision. In experiment, the challenge of very neutron-rich nuclei with completely new topologies such as neutron halos and skins has been glimpsed at, and accelerated radioactive beams are seen as the practical way to make progress.

The neutron-rich “terra incognita” in which thousands of isotopes lie, and about which we know little, has already been shown to be full of surprises. At the dripline, where binding is the weakest, extensive “halos” [7] of low density neutron matter have been found in light nuclei. In several cases the dripline was found to extend further than expected. Nearer stability, strong modification to the normal sequence of single-particle states [8] has been observed, leading to new shell gaps and new shapes. There are also strong indications, from the isotope production in the r-process for example, that the pronounced shell structure we are familiar with close to stability is altered in weakly bound neutron-rich systems. Standard nuclear reactions tend to populate the proton-rich side of the nuclear chart and, as a result, the neutron-rich region of the nuclear chart has remained mostly uncharted. Exploring the far reaches of this region is a key component of the low-energy scientific programs of planned future facilities such as RIA and FAIR. And while the full capabilities of those facilities will be required to thoroughly explore this region, interesting forays in this new territory would yield extremely useful information provided intense neutron-rich isotope beams at low energies and Coulomb barrier energies were available.

In recent years, mass measurements of short-lived neutron-rich isotopes have been performed at the Canadian Penning trap (CPT) spectrometer at Argonne. These isotopes were obtained from a ${}^{252}{Cf}$ source with the fission fragments stopped in the gas catcher system developed at the CPT, thermalized there and subsequently injected into the measurement trap system. While this allows to reach isotopes more neutron-rich than available before and illustrates the universality of the approach, it is still limited in scope by the strength of the fission source that can be used safely in the existing CPT injection system. Overcoming this limitation requires a dedicated properly shielded source holder and gas catcher system. This is the approach chosen in the CARIBU californium source upgrade to ATLAS where a four orders of magnitude stronger source than that used at the CPT will be used to provide an array of neutron-rich radioactive beams, including isotopes that have not been amenable to ISOL techniques before, at sufficient energy and intensity to provide information on the key nuclear properties and help delineate some of the parameters required for the future research programs.

The sections below give a description of the CPT spectrometer and present some of the results on neutron-rich heavy isotopes obtained this far. This is followed by a brief description of the CARIBU project and the capabilities it will provide for mass measurements and other studies on neutron-rich isotopes.

Section snippets

The CPT mass spectrometer

A schematic view of the CPT mass spectrometer system is shown in Fig. 1. The instrument can be divided into three main parts by function: (i) the production of ions, (ii) the preparation of ions for measurement, and (iii) the measurement of the mass of the selected ions.

Initial results on neutron-rich isotopes

A survey of the ions produced from the heavy ${}^{252}{Cf}$ fission fragment peak indicated that most of the activity was collected as doubly charged ions. The cyclotron frequencies of 26 of these doubly charged nuclides, namely ${}^{141 - 147}{Ba}^{2 +}$ , ${}^{143 - 148}{La}^{2 +}$ , ${}^{145 - 151}{Ce}^{2 +}$ , and ${}^{148 - 153}{Pr}^{2 +}$ , were measured. Molecular ions composed of C, H, O, and N atoms are extracted from the gas catcher together with the radioactive ions and were used as calibrants.

An estimate of the total system efficiency can be made by

The CARIBU project

The CARIBU californium source upgrade project to the ATLAS facility at Argonne aims to provide beams to increase the present knowledge of the neutron-rich region of the chart of nuclides. The task involves enabling experiment with beams of short-lived neutron-rich isotopes at low and Coulomb barrier energy regimes. Capabilities exist for radioactive beams in this region at other facilities but limitations in the species that can be extracted and the energy to which they can be accelerated

Acknowledgements

This work was supported by the U.S. Department of Energy, Nuclear Physics Division, under Contract W-31-109-ENG-38 and grants from the Natural Sciences and Engineering Research Council of Canada. The authors are also indebted to the Office of Basic Energy Sciences, U.S. Department of Energy, for the use of the ${}^{252}{Cf}$ source, through the transplutonium element production facilities at the Oak Ridge National Laboratory. The help of Dr. Irshad Ahmad for preparation of the source is also

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Studies of neutron-rich isotopes with the CPT mass spectrometer and the CARIBU project

Abstract

Introduction

Section snippets

The CPT mass spectrometer

Initial results on neutron-rich isotopes

The CARIBU project

Acknowledgements

Phys. Lett. B

Nucl. Instrum. Meth. Phys. Res. B

Nucl. Instrum. Meth. Phys. Res. A

Phys. Lett. A

Nucl. Instrum. Meth. Phys. Res. B

Nucl. Phys. A

Nucl. Phys. A

Nucl. Phys. A

Phil. Mag.

Proc. R. Soc. A

Phys. Rev.

Phys. Rev.

Phys. Rev. Lett.

Annu. Rev. Nucl. Part. Sci.

Prog. Theor. Phys. Suppl.

Hyperfine Interact.