Ellen, Try NCRP No. 51, Appendix B.6 or ANSI N43.3.-1993, Annex A, Table A1. Hope this helps! v/r Michael mford@pantex.com ************************************************************************ The RADSAFE Frequently Asked Questions list, archives and subscription information can be accessed at http://www.ehs.uiuc.edu/~rad/radsafe.html