Production of neutron-rich isotopes by one- and two-step processes in ISOL targets M. Portillo a, * ,J.Nolen a ,I.Gomes a , V.N. Panteleev b , D.V. Fedorov b , A.E. Barzakh b , V.I. Beznosjuk b , F.V. Moroz b , S.Yu. Orlov b , Yu.M. Volkov b a Physics Division, Building 203, Room H138, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA b IRIS, Petersburg Nuclear Physics Institute RAS, 188350, Gatchina, Leningrad district, Russia Received 3 October 2001; received in revised form 11 January 2002 Abstract Theresultsarepresentedforanexperimentthatcomparesthedifferencebetweenaone-andtwo-stepreactionsetup using 1 GeV protons. The rates of production from an on-line isotope separator target containing UCx are measured forisotopesintheneutronmassregionofRbandCs.Somedetailsaboutthemeasuredresultsandpredictionsbythe Monte Carlo models are discussed. The effects of the delayed release on the extracted efficiency are generalized using analytical models for application to a wide range of nuclear decay lifetimes. Ó 2002 Elsevier Science B.V. All rights reserved. 1. Introduction The two-step reaction process for the produc- tion of rare isotopes in isotope separation on-line (ISOL) targets offers some advantages over the one-step configuration. In the one step, a primary beam of high energy particles, such as protons, directlyimpingesonthetarget.Most,ifnotall,of the power is then dissipated directly into the tar- get. Estimates on the amount of production nec- essary for intense beams of rare isotopes require that that at least 100 lA of protons at energies ranging from 0.5 to 1 GeV be directed on target [1]. Studies in the past have predicted that con- ventional targets will not be able to sustain the typeofpowerdensitiesimposedundertargetcon- ditions required for releasing rare isotopes. Espe- cially in the case of producing neutron deficient isotopes, issues pertaining to target lifetime and reliability require that alternative methods of pro- duction be considered [2,3]. The Monte Carlo models that have been uti- lized here suggest that isotopes that lie on the neutron rich side of the valley of stability result largelyfromneutron-inducedfissionreactionswith heavyelements.Inthecaseof238U,theregionof the neutron spectrum which contributes the most lies between 1 and 5 MeV and is commonly re- ferred to as the ‘‘fast neutron’’ region [4]. Facilities which produce isotopes on-line very often use high energy protons which generate fast neutrons. There is a tendency to associate the direct reaction of these protons to the production Nuclear Instruments and Methods in Physics Research B 194 (2002) 193–206 www.elsevier.com/locate/nimb * Corresponding author. Tel.: +1-630-252-4039; fax: +1-630- 252-9647. E-mail address: portillo@phy.anl.gov (M. Portillo). 0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII:S0168-583X(02)00672-9