Nuclear Instruments and Methods in Physics Research A 440 (2000) 772}776 Test of He-based neutron polarizers at NIST G.L. Jones*, T.R. Gentile, A.K. Thompson, Z. Chowdhuri, M.S. Dewey, W.M. Snow, F.E. Wietfeldt Indiana University, Bloomington, IN 47408, USA National Institute of Standards and Technology (NIST), Gaithersburg, MD 20899, USA Abstract Neutron spin "lters based on polarized He are useful over a wide neutron energy range and have a large angular acceptance among other advantages. Two optical pumping methods, spin-exchange and metastability-exchange, can produce the volume of highly polarized He gas required for such neutron spin "lters. We report a test of polarizers based on each of these two methods on a new cold, monochromatic neutron beam line at the NIST Center for Neutron Research. 2000 Elsevier Science B.V. All rights reserved. Keywords: Polarized He; Neutron; Polarizer; Spin "lter 1. Introduction and polarizer principles Neutron polarizers and analyzers have a number of applications in both materials science and funda- mental physics. The current methods for polarizing neutrons are either de"cient in one or more respects or limited to a small range of neutron energies [1]. A dense volume of su$ciently polarized He can provide an excellent neutron polarizer that avoids these de"ciencies. Examples where polarized He could be useful include using a He analyzer for small angle neutron scattering (SANS) [2] which requires large angular acceptance. Neutron beta asymmetry measurements rely on absolute know- ledge of the neutron polarization at the 0.1% level. Polarized He may provide analyzers that achieve * Corresponding author. Tel.: #1-315-859-4697; fax: #1- 315-859-4696. E-mail address: gjones@hamilton.edu (G.L. Jones) this accuracy without the systematic errors asso- ciated with super-mirror analyzers [3,4]. A new determination of the weak nucleon}nucleon coup- ling constant, H , plans to make use of a He polarizer's broad energy range, angular acceptance, and ability to change the polarization direction without changing static magnetic "elds [5]. Polarized He makes an excellent neutron spin "lter because of the large spin dependence of the neutron capture cross section. Because the cross section for the absorption of neutrons parallel to the He nuclear spin is negligible, a su$cient col- umn density of 100% polarized He would trans- mit all of these neutrons and none with the antiparallel spin. The neutron polarization P is given by P " ¹ !¹ ¹ #¹ "tanh(()N ¸P ) (1) where ¹ (¹ ) is the transmission of neutrons with spin parallel (antiparallel) to the He spin, 0168-9002/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 0 8 1 - 5